Handbook of Porous Silicon [1 ed.] 9783319057439, 9783319057446, 9783319057453

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Leigh Canham Editor

Handbook of Porous Silicon

1 3Reference

Handbook of Porous Silicon

Leigh Canham Editor

Handbook of Porous Silicon With 247 Figures and 139 Tables

Editor Leigh Canham pSiMedica Ltd Malvern, UK

ISBN 978-3-319-05743-9 ISBN 978-3-319-05744-6 (eBook) ISBN 978-3-319-05745-3 (print and electronic bundle) DOI 10.1007/978-3-319-05744-6 Springer Zug Heidelberg New York Dordrecht London Library of Congress Control Number: 2014954093 # Springer International Publishing Switzerland 2014 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. Exempted from this legal reservation are brief excerpts in connection with reviews or scholarly analysis or material supplied specifically for the purpose of being entered and executed on a computer system, for exclusive use by the purchaser of the work. Duplication of this publication or parts thereof is permitted only under the provisions of the Copyright Law of the Publisher’s location, in its current version, and permission for use must always be obtained from Springer. Permissions for use may be obtained through RightsLink at the Copyright Clearance Center. Violations are liable to prosecution under the respective Copyright Law. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. While the advice and information in this book are believed to be true and accurate at the date of publication, neither the authors nor the editors nor the publisher can accept any legal responsibility for any errors or omissions that may be made. The publisher makes no warranty, express or implied, with respect to the material contained herein. Printed on acid-free paper Springer is part of Springer Science+Business Media (www.springer.com)

Preface

The semiconductor silicon has often been referred to as the most studied and most influential material made by mankind. The stone, bronze, and iron “ages” became the “silicon age” with the emergence of integrated circuits. Micromachining it enabled complex microdevices with moving parts to be realized. Nanostructuring it via porosification has yielded some amazing phenomena, most of which are covered in this handbook. Scientific interest in porous silicon has grown significantly over the last 25 years fundamentally because it is a form of silicon that has both highly tunable and remarkable properties. These properties can be dramatically different from those of solid silicon and have enabled opportunities to arise in diverse fields that started in electronics but now include microsystems, optoelectronics, optics, acoustics, energy conversion, diagnostics, nutrition, medical therapy and, cosmetics. By having almost 100 complementary reviews, this handbook strives to be the most comprehensive textbook on the material ever published. I hope it will be useful to my many friends and research colleagues already very active with porous silicon research but also to those new to the field. The expertise required to create this handbook represents the combined intellect of more than 80 scientists from more than 30 countries whose pioneering work has shaped this multidisciplinary scientific field of endeavor. The handbook is organized into five parts that cover fabrication, properties, characterization, processing, and applications. Each part has an introductory review where I have tried to show the important links with more focused reviews elsewhere and to highlight and provide references to important topics that did not receive dedicated reviews. Nonetheless, I apologize in advance for the many omissions of important work that “fell through the cracks.” Within each part, the reviews are also grouped and ordered by complementary topics. For example, the first part on fabrication starts with the various techniques but then progresses to fabrication of different types of porosity with the given technique(s) and then the different physical forms of porous silicon. In a similar manner, the properties part of the handbook groups together complementary reviews on specific optical, magnetic, emissive, and chemical properties and so on. There are many people to thank for such a bold enterprise. It has taken years of both planning and execution, so first and foremost, I would like to thank both the v

vi

Preface

contributors and the publisher, Springer, for their patience! I would like to thank Karin Bartsch, Coordinating Editor at Springer, for her tireless efforts at dealing with so many authors, myself in particular. Rasidha Sulthana also did a great job in managing the reformatting, type-setting, and checking of proofs for 94 review articles. I would like to thank Sonia Ojo and Lydia Mueller, Senior Editors at Springer, for their continued faith and support of this lengthy project. Finally, thank you, the reader, for your interest in this material. I hope this book facilitates your research in the future and perhaps stimulates you to join our vibrant and expanding “porous silicon community.” The “old dog” of semiconductors continues to exhibit new tricks. Malvern August 2014

L. Canham

Contents

Part I Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1

........................

3

Porous Silicon Formation by Anodization . . . . . . . . . . . . . . . . . . . . . . . Armando Loni

11

..................

23

Porous Silicon Formation by Stain Etching . . . . . . . . . . . . . . . . . . . . . . Kurt W. Kolasinski

35

Routes of Formation for Porous Silicon Leigh Canham

Porous Silicon Formation by Galvanic Etching Kurt W. Kolasinski

Porous Silicon Formation by Metal Nanoparticle-Assisted Etching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Claude Le´vy-Cle´ment

49

Porous Silicon Formation by Photoetching . . . . . . . . . . . . . . . . . . . . . . Sadao Adachi

67

Porous Silicon Formation by HNO3/HF Vapor Etching . . . . . . . . . . . . Brahim Bessaı¨s

75

Porous Silicon Formation by Porous Silica Reduction . . . . . . . . . . . . . Leigh Canham

85

.................

93

Macroporous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Noureddine Gabouze and Franc¸ois Ozanam

103

Mesoporous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Alexey Khokhlov and Rustem Valiullin

115

Microporous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Leigh Canham

129

Porous Silicon Formation by Mechanical Means Jaroslaw Jakubowicz

vii

viii

Contents

Pore Volume (Porosity) in Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . Leigh Canham

135

Ultrathin Porous Silicon Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Brahim Bessaı¨s

143

Porous Silicon Multilayers and Superlattices . . . . . . . . . . . . . . . . . . . . Vivechana Agarwal

153

Porous Silicon Membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Leigh Canham

163

MACE Silicon Nanostructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ciro Chiappini

171

..........................

187

Polymer - Porous Silicon Composites Ester Segal and Maksym A. Krepker Part II

Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

199

..........................

201

Thermal Properties of Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . Nobuyoshi Koshida

207

Mechanical Properties of Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . Leigh Canham

213

Mesopore Diffusion Within Porous Silicon . . . . . . . . . . . . . . . . . . . . . . Jo¨rg K€arger and Rustem Valiullin

221

Refractive Index of Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Honglae Sohn

231

........................

245

.....................................

255

.........................

263

Diamagnetic Behavior of Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . Klemens Rumpf and Petra Granitzer

281

Ferromagnetism and Ferromagnetic Silicon Nanocomposites . . . . . . . Petra Granitzer and Klemens Rumpf

287

Tunable Properties of Porous Silicon Leigh Canham

Optical Birefringence of Porous Silicon Minoru Fujii and Joachim Diener Color of Porous Silicon Leigh Canham

Electrical Transport in Porous Silicon Sanjay K. Ram

Paramagnetic and Superparamagnetic Silicon Nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Petra Granitzer and Klemens Rumpf

297

Contents

ix

Photoluminescence of Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . Bernard Gelloz

307

.........................

321

Thermoluminescence of Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . Valeriy Skryshevsky

335

Optical Gain in Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Katerˇina Herynková and Ivan Pelant

345

Electroluminescence of Porous Silicon Bernard Gelloz

Chemical Reactivity and Surface Chemistry of Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Michael J. Sailor

355

Biocompatibility of Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Suet P. Low and Nicolas H. Voelcker

381

Biodegradability of Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Qurrat Shabir

395

Part III

403

Characterization

.................................

Characterization Challenges with Porous Silicon . . . . . . . . . . . . . . . . . Leigh Canham

405

Microscopy of Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Rau´l J. Martı´n-Palma and Vicente Torres-Costa

413

...........................

423

Gas Adsorption Analysis of Porous Silicon . . . . . . . . . . . . . . . . . . . . . . Armando Loni

431

X-Ray Diffraction in Porous Silicon Jeffery L. Coffer

NMR Cryoporometry and Estimation of Pore Sizes in Mesoporous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Rustem Valiullin

439

...............

449

Magnetic Characterization Methods for Porous Silicon . . . . . . . . . . . . Klemens Rumpf and Petra Granitzer

455

Chemical Characterization of Porous Silicon . . . . . . . . . . . . . . . . . . . . Mihaela Kusko and Iuliana Mihalache

463

Characterization of Porous Silicon by Infrared Spectroscopy . . . . . . . Yukio H. Ogata

473

Characterization of Porous Silicon by Calorimetry Jarno Salonen

x

Contents

Cell Culture on Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nicolas H. Voelcker and Suet P. Low

481

Electronic Band Structure in Porous Silicon . . . . . . . . . . . . . . . . . . . . . Julia Tag€ uen˜a-Martı´nez and Chumin Wang

497

........................

505

Effects of Irradiation on Porous Silicon Roberto Koropecki and Roberto Arce Part IV

Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

521

Typical Processing Steps with Porous Silicon . . . . . . . . . . . . . . . . . . . . Leigh Canham

523

...........................

531

Colloidal Lithography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Luca Boarino and Michele Laus

541

Imprinting Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Judson D. Ryckman and Sharon M. Weiss

551

Drying Techniques Applied to Porous Silicon . . . . . . . . . . . . . . . . . . . . Leigh Canham

559

Homoepitaxy on Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Barbara Terheiden

567

Heteroepitaxy on Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reza Sabet Dariani

581

Oxidation of Macroporous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ekaterina V. Astrova

589

Sintering of Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Izabela Kuzma-Filipek

599

Photolithography on Porous Silicon Adrian Keating

Porous Silicon and Conductive Polymer Nanostructures Via Templating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Farid A. Harraz

611

Melt Intrusion in Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Armando Loni

623

..................

629

Gas and Liquid Doping of Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . Riccardo Rurali

639

Functional Coatings of Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . Fre´de´rique Cunin

647

Porous Silicon and Electrochemical Deposition Yukio H. Ogata and Kazuhiro Fukami

Contents

xi

Electroencapsulation of Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . Matti Murtomaa and Jarno Salonen

665

Photoluminescent Nanoparticle Derivatization Via Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Benjamin R. Horrocks

671

...............

683

Milling of Porous Silicon Microparticles . . . . . . . . . . . . . . . . . . . . . . . . Armando Loni

695

Ohmic and Rectifying Contacts to Porous Silicon . . . . . . . . . . . . . . . . . Jayita Kanungo and Sukumar Basu

705

Processing of Macroporous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ekaterina V. Astrova

715

Part V

731

Silicon-Carbon Bond Formation on Porous Silicon Lawrence A. Huck and Jillian M. Buriak

Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Porous Silicon Application Survey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Leigh Canham

733

RF Electrical Isolation with Porous Silicon . . . . . . . . . . . . . . . . . . . . . . Ga€el Gautier

741

.........................

753

Porous Silicon Gettering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Izabela Kuzma-Filipek and Hariharsudan Sivaramakrishnan Radhakrishnan

767

Porous Silicon Micromachining Technology . . . . . . . . . . . . . . . . . . . . . Giuseppe Barillaro

779

Porous Silicon Functionalities for BioMEMS . . . . . . . . . . . . . . . . . . . . Julien Schweicher and Tejal A. Desai

787

...............

797

Porous Silicon Photonic Crystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . J. Octavio Estevez and Vivechana Agarwal

805

Porous Silicon Optical Waveguides . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sharon M. Weiss and Xing Wei

815

Porous Silicon Diffraction Gratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . Adrian Keating

823

.............................

835

Thermal Isolation with Porous Silicon Androula G. Nassiopoulou

Porous Silicon for Microdevices and Microsystems Luca De Stefano and Ilaria Rea

Porous Silicon Phononic Crystals Paul Snow

xii

Contents

..................................

845

Porous Silicon Optical Biosensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Giorgi Shtenberg and Ester Segal

857

Porous Silicon-Based Mass Spectrometry . . . . . . . . . . . . . . . . . . . . . . . Yannick Coffinier and Rabah Boukherroub

869

Porous Silicon Immunoaffinity Microarrays . . . . . . . . . . . . . . . . . . . . . Belinda Adler, Hong Yan, Simon Ekstro¨m, and Thomas Laurell

887

Porous Silicon in Brachytherapy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Leigh Canham and Drew Ferguson

901

Drug Delivery with Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Jarno Salonen

909

Porous Silicon and Tissue Engineering Scaffolds . . . . . . . . . . . . . . . . . Jeffery L. Coffer

921

Porous Silicon in Photodynamic and Photothermal Therapy . . . . . . . . Victor Yu Timoshenko

929

Porous Silicon in Immunoisolation and Bio-filtration . . . . . . . . . . . . . . Julien Schweicher and Tejal A. Desai

937

Porous Silicon and Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tayyar Dzhafarov

945

Porous Silicon and Micro-Fuel Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . Ga€el Gautier

957

Porous Silicon and Li-Ion Batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nae-Lih Wu

965

Energetics with Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Monuko duPlessis

975

Porous Silicon and Functional Foods . . . . . . . . . . . . . . . . . . . . . . . . . . . Leigh Canham

985

Porous Silicon for Oral Hygiene and Cosmetics . . . . . . . . . . . . . . . . . . Leigh Canham

999

Porous Silicon Gas Sensing Giuseppe Barillaro

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1009

Contributors

Sadao Adachi Division of Electronics and Informatics, Faculty of Science and Technology, Gunma University, Gunma, Kiryu-shi, Japan Belinda Adler Faculty of Engineering, Department of Biomedical Engineering, Lund University, Lund, Sweden Vivechana Agarwal Centro de Investigacion en Ingenieria y Ciencias Aplicadads, Universidad Autonoma del Estado de Morelos, Col. Chamilpa, Cuernavaca, Mexico Roberto Arce Grupo de Semiconductores Nanoestructurados, Instituto de Fı´sica del Litoral, IFIS Litoral (UNL-CONICET), Santa Fe, Argentina Ekaterina V. Astrova Ioffe Physical Technical Institute, St. Petersburg, Russia Giuseppe Barillaro Dipartimento di Ingegneria dell’Informazione, Università di Pisa, Pisa, Italy Sukumar Basu IC Design & Fabrication Center, Department of Electronics & Telecommunication Engineering, Jadavpur University, Kolkata, India Brahim Bessaı¨s Research and Technology Centre of Energy, Borj-Cedria Technopark, Hammam-Lif, Tunisia Luca Boarino Nanofacility Piemonte, Istituto Nazionale di Ricerca Metrologica, Torino, Italy Rabah Boukherroub Parc de la Haute Borne, Institut de Recherche Interdisciplinaire (IRI), Groupe NanoBiointerfaces, Villeneuve d’Ascq Ce´dex, France Jillian M. Buriak Department of Chemistry, University of Alberta, Edmonton, AB, Canada Leigh Canham pSiMedica Ltd., Malvern Hills Science Park, Malvern, Worcester, UK Ciro Chiappini Department of Materials, Imperial College London, Faculty of Engineering, London, UK xiii

xiv

Contributors

Jeffery L. Coffer Department of Chemistry, Texas Christian University, Fort Worth, TX, USA Yannick Coffinier Parc de la Haute Borne, Institut de Recherche Interdisciplinaire (IRI), Groupe NanoBiointerfaces, Villeneuve d’Ascq Ce´dex, France Fre´de´rique Cunin Institut Charles Gerhardt Montpellier, Montpellier, France Reza Sabet Dariani Department of Physics, Alzahra University, Tehran, Iran Tejal A. Desai Department of Bioengineering and Therapeutic Sciences, University of California, San Francisco, CA, USA Luca De Stefano IMM-CNR Institute for Microelectronics and Microsystems – Unit of Naples, National Research Council, Naples, Italy Joachim Diener Physik-Department, Technische Universit€at M€unchen, Garching, Germany Monuko duPlessis Carl and Emily Fuchs Institute for Microelectronics, Department of Electrical, Electronic and Computer Engineering, University of Pretoria, Pretoria, South Africa Tayyar Dzhafarov Department of Solar and Hydrogen Energy Converters, Institute of Physics, Azerbaijan National Academy of Sciences, Baku, Azerbaijan Simon Ekstro¨m Faculty of Engineering, Department of Biomedical Engineering, Lund University, Lund, Sweden J. Octavio Estevez CIICAp-UAEM, Av. Universidad, Col. Chamilpa, Cuernavaca, Morelos, Mexico Drew Ferguson OncoSil Medical Ltd, Malvern Hills Science Park, Malvern, Worcestershire, UK Minoru Fujii Department of Electrical and Electronic Engineering, Graduate School of Engineering, Kobe University, Kobe, Japan Kazuhiro Fukami Department of Materials Science and Engineering, Kyoto University, Kyoto, Japan Noureddine Gabouze Centre de Recherche sur la Technologie des Semiconducteurs et l’Energie (CRTSE), Algiers, Algeria Ga€el Gautier Universite´ de Tours, GREMAN, Tours, France Bernard Gelloz School of Engineering, Nagoya University, Nagoya, Japan Petra Granitzer Institute of Physics, Karl-Franzens-University Graz, Graz, Austria

Contributors

xv

Farid A. Harraz Nanostructured Materials and Nanotechnology Division, Central Metallurgical Research and Development Institute (CMRDI), Cairo, Egypt Promising Centre for Sensors and Electronic Devices (PCSED), Advanced Materials and Nano–Research Centre, Najran University, Najran, Saudi Arabia Katerˇina Herynková Institute of Physics, Academy of Sciences of the Czech Republic, Prague 6, Czech Republic Benjamin R. Horrocks School of Chemistry, Newcastle University, Newcastle upon Tyne, UK Lawrence A. Huck Department of Chemistry, University of Alberta, Edmonton, AB, Canada Jaroslaw Jakubowicz Poznan University of Technology, Institute of Materials Science and Engineering, Poznan, Poland Jayita Kanungo IC Design & Fabrication Center, Department of Electronics & Telecommunication Engineering, Jadavpur University, Kolkata, India Jo¨rg K€ arger Faculty of Physics and Earth Sciences, University of Leipzig, Leipzig, Germany Adrian Keating The Microelectronics Research Group, School of Mechanical and Chemical Engineering, The University of Western Australia, Crawley, WA, Australia Alexey Khokhlov Faculty of Physics and Earth Sciences, University of Leipzig, Leipzig, Germany Kurt W. Kolasinski Department of Chemistry, West Chester University, West Chester, PA, USA Roberto Koropecki Grupo de Semiconductores Nanoestructurados, Instituto de Fı´sica del Litoral, IFIS Litoral (UNL-CONICET), Santa Fe, Argentina Nobuyoshi Koshida Tokyo University of Agriculture and Technology, Tokyo, Japan Maksym A. Krepker Department of Biotechnology and Food Engineering, Technion – Israel Institute of Technology, Haifa, Israel Mihaela Kusko Laboratory of Nanobiotechnology, National Institute for Research and Development in Microtechnologies (IMT – Bucharest), Bucharest, Romania Izabela Kuzma-Filipek IMEC, Heverlee, Belgium Thomas Laurell Faculty of Engineering, Department of Biomedical Engineering, Lund University, Lund, Sweden

xvi

Contributors

Michele Laus Dipartimento di Scienze e Innovazione Tecnologica (DISIT), Università del Piemonte Orientale “A. Avogadro”, INSTM, UdR Alessandria, Alessandria, Italy Claude Le´vy-Cle´ment Institut de Chimie et des Mate´riaux Paris-Est, CNRS, Thiais, France Armando Loni pSiMedica Ltd, Malvern, Worcestershire, UK Suet P. Low Mawson Institute, University of South Australia, Adelaide, SA, Australia Rau´l J. Martı´n-Palma Departamento de Fı´sica Aplicada, Universidad Auto´noma de Madrid, Cantoblanco, Madrid, Spain Iuliana Mihalache Laboratory of Nanobiotechnology, National Institute for Research and Development in Microtechnologies (IMT – Bucharest), Bucharest, Romania Matti Murtomaa Department of Physics and Astronomy, University of Turku, Turku, Finland Androula G. Nassiopoulou NCSR Demokritos, Institute of Nanoscience and Nanotechnology (INN), Terma Patriarchou Grigoriou, Athens, Greece Yukio H. Ogata Institute of Advanced Energy, Kyoto University, Uji, Kyoto, Japan Franc¸ois Ozanam Physique de polytechnique, Palaiseau, France

la

Matie`re

Condense´e,

CNRS-Ecole

Ivan Pelant Institute of Physics, Academy of Sciences of the Czech Republic, Prague 6, Czech Republic Hariharsudan Sivaramakrishnan Radhakrishnan IMEC, Heverlee, Belgium Sanjay K. Ram Department of Physics and Astronomy, Aarhus University, Aarhus C, Denmark Interdisciplinary Nanoscience Center – iNANO, Aarhus University, Aarhus C, Denmark Ilaria Rea IMM-CNR Institute for Microelectronics and Microsystems – Unit of Naples, National Research Council, Naples, Italy Klemens Rumpf Institute of Physics, Karl-Franzens-University Graz, Graz, Austria Riccardo Rurali Institut de Cie`ncia de Materials de Barcelona (ICMAB-CSIC), Campus de Bellaterra, Bellaterra, Spain

Contributors

xvii

Judson D. Ryckman Department of Electrical Engineering and Computer Science, Vanderbilt University, Nashville, TN, USA Michael J. Sailor Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA, USA Jarno Salonen Department of Physics and Astronomy, Laboratory of Industrial Physics, University of Turku, Turku, Finland Julien Schweicher Department of Chemistry, Universite Libre de Bruxelles, Bruxelles, Belgium Ester Segal Department of Biotechnology and Food Engineering, The Russell Berrie Nanotechnology Institute, Technion – Israel Institute of Technology, Haifa, Israel Qurrat Shabir pSiMedica Ltd, Malvern, Worcester, UK Giorgi Shtenberg The interdepartmental Program of Biotechnology, Department of Biotechnology and Food Engineering, Technion – Israel Institute of Technology, Haifa, Israel Valeriy Skryshevsky Institute of High Technologies, Taras Shevchenko National University of Kyiv, Kyiv, Ukraine Paul Snow Department of Physics, University of Bath, Bath, UK Honglae Sohn Department of Chemistry, Chosun University, Gwangju, Republic of Korea Julia Tag€ uen˜a-Martı´nez Instituto de Energı´as Renovables, Universidad Nacional Auto´noma de Me´xico, Temixco, Morelos, Me´xico Barbara Terheiden Department of Physics, University of Konstanz, Constance, Germany Victor Yu Timoshenko Physics Department, M.V. Lomonosov Moscow State University, Moscow, Russia Vicente Torres-Costa Departamento de Fı´sica Aplicada, Universidad Auto´noma de Madrid, Cantoblanco, Madrid, Spain Rustem Valiullin Faculty of Physics and Earth Sciences, University of Leipzig, Leipzig, Germany Nicolas H. Voelcker Mawson Institute, University of South Australia, Adelaide, SA, Australia Chumin Wang Instituto de Investigaciones en Materiales, Universidad Nacional Auto´noma de Me´xico, Me´xico, Me´xico

xviii

Contributors

Xing Wei Department of Electrical Engineering and Computer Science, Vanderbilt University, Nashville, TN, USA Sharon M. Weiss Department of Electrical Engineering and Computer Science, Vanderbilt University, Nashville, TN, USA Nae-Lih Wu Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan Hong Yan Faculty of Engineering, Department of Biomedical Engineering, Lund University, Lund, Sweden

Part I Fabrication

Routes of Formation for Porous Silicon Leigh Canham

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Schematic Route Map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Specific Fabrication Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Porous silicon has been fabricated by both “top-down” techniques from solid silicon and “bottom-up” routes from silicon atoms and silicon-based molecules. Over the last 50 years, electrochemical etching has been the most investigated approach for chip-based applications and has been utilized to create highly directional mesoporosity and macroporosity. Chemical conversion of porous or solid silica is now receiving increasing attention for applications that require inexpensive mesoporous silicon in powder form. Very few techniques are currently available for creating wholly microporous silicon with pore size below 2 nm. This review summarizes, from a chronological perspective, how more than 30 fabrication routes have now been developed to create different types of porous silicon.

Introduction Porous silicon, solid silicon with voids therein, can be generated by diverse means. Although “top-down” techniques utilizing electrochemical etching techniques have dominated the academic literature over the last 50 years, from 1960 to 2010, there L. Canham (*) pSiMedica Ltd., Malvern Hills Science Park, Malvern, Worcester, UK e-mail: [email protected] # Springer International Publishing Switzerland 2014 L. Canham (ed.), Handbook of Porous Silicon, DOI 10.1007/978-3-319-05744-6_1

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have since been many other routes demonstrated: both “top-down” routes from solid silicon and “bottom-up” routes from silicon atoms and silicon-based molecules. The purpose of this review is to capture for the reader, in one brief document, all those fabrication techniques the author is aware of and highlight their potential applicability, depending on desired structures, targeted application area, and acceptable levels of cost. In the following chapters of this handbook, eight of these are then chosen to be reviewed in detail.

Schematic Route Map Figure 1 illustrates the traditional route whereby porous silicon is created from solid silicon, which itself is derived from solid silica. A number of techniques such as anodization (see handbook chapter “▶ Porous Silicon Formation by Anodization”), vapor etching (“▶ Porous Silicon Formation by HNO3/HF Vapor Etching”), glancing angle deposition, lithographic etching, and photoetching (“▶ Porous Silicon Formation by Photoetching”) are suitable for Si wafer-based processing. Others can be used on both wafer and powder silicon feedstocks, such as stain etching (handbook chapter “▶ Porous Silicon Formation by Stain Etching”), galvanic etching (“▶ Porous Silicon Formation by Galvanic Etching”), and MACE (“▶ Porous Silicon Formation by Metal Nanoparticle-Assisted Etching”and “▶ MACE Silicon Nanostructures”). Most of these techniques create highly directional porosity and therefore properties that can be highly anisotropic (see handbook chapters “▶ Electrical Transport in Porous Silicon”; “▶ Mechanical Properties of Porous Silicon” and “▶ Optical Birefringence of Porous Silicon”). Until quite recently, etching of highly porous structures from solid silicon was reliant on acidic fluoride chemistry; however, alkali-based etches have now been shown to be at least capable of macropore generation under restricted conditions. Porosifying controlled areas of a silicon wafer enables porous silicon to be integrated with silicon circuitry or MEMS devices within chip-based products. Although porous silicon particles (microparticles and nanoparticles) can be derived from anodized wafers (see handbook chapters “▶ Milling of Porous Silicon Microparticles” and “▶ Photoluminescent Nanoparticle Derivatization Via Porous Silicon”), this route is only viable for low-volume high-value product areas, as in some medical therapy applications (see handbook chapter “▶ Drug Delivery with Porous Silicon”). If highly porous structures are required at high volumes, etching techniques will typically have to remove large quantities of solid silicon as waste, unless recycled. For lower-value, high-volume porous silicon products that are not silicon chip-based (see handbook chapters “▶ Porous Silicon and Functional Foods” and “▶ Porous Silicon for Oral Hygiene and Cosmetics”), there is therefore increasing interest in fabrication routes that utilize existing highly porous feedstocks or silicon-based molecules that are themselves waste products from solid silicon manufacturing. These increasingly use chemical conversion of, for example, silica, silane, or silicon tetrachloride (see Fig. 2). The chemical conversion can be

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Fig. 1 Routes to porous silicon via solid silicon

Fig. 2 Routes to porous silicon using chemical conversion

promoted thermally, mechanically, or electrochemically. Here the morphology of porosity can reflect that of the starting solid feedstocks (see handbook chapter “▶ Porous Silicon Formation by Porous Silica Reduction”) or how the silicon nanoparticles are assembled into a porous body via sintering (see handbook chapter “▶ Porous Silicon Formation by Mechanical Means”).

Specific Fabrication Techniques Table 1 illustrates the variety of processes (currently more than 30) now available to create porous silicon, arranged in approximately the chronological order they have been introduced. Historically, it was high levels of mesopores (see handbook chapter on “▶ Mesoporous Silicon”) that were created first via anodization (1) and stain

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Table 1 A multitude of routes to form porous silicon. The techniques highlighted in black are reviewed in detail in the handbook. Also highlighted in black are those techniques reported to generate macroporous and microporous silicon. The literature has to date been dominated by mesoporous silicon fabrication Fabrication technique Anodization Stain etching Anodization

Class of technique Etching (wet) Etching (wet) Etching (wet)

Class of porosity Mesoporous Mesoporous Macroporous

Anodization

Etching (wet)

Microporous

Spark erosion

Etching (dry)

Mesoporous

Photoetching

Etching (wet)

Mesoporous

Laser ablation Hydrothermal etching Metal ion-assisted chemical etching (MACE) Galvanic etching Plasma deposition Vapor etching Laser-induced plasma

Thermal Etching (wet) Etching (wet)

Mesoporous Mesoporous Mesoporous

Etching (wet) Deposition Etching (wet) Etching (dry)

Mesoporous Mesoporous Mesoporous Macroporous

Glancing angle deposition Melt gasification

Deposition

Mesoporous

Thermal

Macroporous

Plasma hydrogenation Dealloying Laser-induced silane decomposition Magnesiothermic reduction of silica

Deposition Etching (wet) Deposition

Mesoporous Mesoporous Mesoporous

Conversion reaction

Mechanochemical reduction Milling/sintering

Conversion reaction Mechanical

DRIE-UV lithography Femtosecond laser ablation Ultrathin film annealing Anodization (alkali) Electrodeposition

Etching (dry) Thermal

Macroporous Mesoporous

Thermal Etching (wet) Deposition

Mesoporous Macroporous Mesoporous

Early paper on technique Uhlir (1956) Archer (1960) Theunissen et al. (1972) Canham and Groszek (1992) Hummel and Chang (1992) Noguchi and Suemune (1993) Savin et al. (1996) Chen et al. (1996) Dimova-Malinovska et al. (1997) Ashruff et al. (1999) Kalkan et al. (2000) Saadoun et al. (2002) Kabashin and Meunier (2002) Beydaghan et al. (2004)

Year 1956 1960 1972 1992 1992 1993 1996 1996 1997

1999 2000 2002 2002 2004

Nakahata and Nakajima (2004) Abdi et al. (2005) Fukatani et al. (2005) Voigt et al. (2005)

2005 2005 2005

Mesoporous Microporous Mesoporous

Bao et al. (2007)

2007

Zheng et al. (2007)

2007

Macroporous

Jacubowicz et al. (2007) Woldering et al. (2008) Mahmood et al. (2009)

2007

Fang et al. (2010) Abburi et al. (2010) Krishnamurthy et al. (2011)

2004

2008 2009 2010 2010 2011

(continued)

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Table 1 (continued) Fabrication technique Carbothermal reduction of silica Sacrificial template Sodiothermic reduction of silica Magnetron sputtering Micromachining and wet etching Platinum NP-assisted etching (PaCE) Ion implantation Templated silicon tetrachloride reduction Rochow reaction-based etching

Class of technique Conversion reaction Deposition Conversion reaction Deposition Etching (wet)

Class of porosity Macroporous

Early paper on technique Yang et al. (2012)

Year 2012

Mesoporous Mesoporous

Huang et al. (2013) Wang et al. (2013)

2013 2013

Mesoporous Macroporous

Godhino et al. (2013) Deng et al. (2013)

2013 2013

Etching (wet)

Mesoporous

Li et al. (2013)

2013

Irradiation Conversion reaction Etching (dry)

Macroporous Mesoporous

Stepanov et al. (2013) Dai et al. (2014)

2013 2014

Macroporous

Zhang et al. (2014)

2014

etching (2) of electronic-grade crystalline silicon. Depending on wafer resistivity and anodization conditions, it was subsequently shown that both macropores (see chapter “▶ Macroporous Silicon”) and micropores (see chapter “▶ Microporous Silicon”) could also be realized via the anodization route. In the 1990s a multitude of different techniques for creating mesoporous luminescent silicon were identified. All the etching techniques tend to create “open” porosity where pores are accessible from the external surfaces of the structure. Specific techniques to create “closed” porosity include melt gasification (Nakahata and Nakajima 2004) and milling/sintering (Jakubowicz et al. 2007). The most popular conversion reaction is currently the magnesiothermic reduction of porous silica, as introduced by Sandhage and co-workers in 2007 (Bao et al. 2007). This has been utilized with both synthetic silicas and biogenic silicas extracted from plants (see handbook chapter “▶ Porous Silicon Formation by Porous Silica Reduction”). The major challenge in scalability for mesoporous silicon via this route is control of the strong exothermic nature of the reaction to avoid sintering. Indeed, carbothermal reduction (Yang et al. 2012) requires much higher temperatures and is more amenable to macroporous silicon fabrication. Sodiothermic reduction (Wang et al. 2013) can be carried out at very low temperatures but is probably less scalable because of the high cost and reactive nature of sodium metal. Similar restrictions are also applicable to the recent study using NaK alloy (Dai et al. 2014). Aluminothermic reduction (Zheng et al. 2007) looks much more attractive in this regard since aluminum is a very inexpensive metal. Note that there are currently very few techniques to make wholly microporous silicon (see handbook chapter “▶ Microporous Silicon”) where the average pore diameter is under 2 nm. For virtually all top-down techniques, the porous silicon created is polycrystalline. For some bottom-up techniques such as sputtering/

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dealloying (Fukatani et al. 2005), electrodeposition (Krishnamurthy et al. 2011), or sodiothermic reduction (Wang et al. 2013), it is reported to be amorphous. Choice of fabrication technique for both mesoporous and macroporous silicon is very much dictated by application area, which in turn has differing requirements on porosity levels, pore morphology, skeleton purity, physical form, cost, and volume.

References Abburi M, Bostrom T, Olefjord I (2010) Electrochemical texturing of multicrystalline silicon wafers in alkaline solutions. In: Proceedings of the 24th European photovoltaic solar energy conference, Hamburg, pp 1779–1783 Abdi Y, Derakhshandeh J, Hashemi P, Mohajerzadeh S, Karbassian F, Nayeri F, Arzi E, Robertson MD, Radamson H (2005) Light emitting nano-porous silicon structures fabricated using a plasma hydrogenation technique. Mater Sci Eng B124–125:483–487 Archer RJ (1960) Stain films on silicon. J Phys Chem Solids 14:104–110 Ashruf CMA, French PJ, Bressers PMMC, Kelly JJ et al (1999) Galvanic porous silicon formation without external contacts. Sens Actuat A 74:118–122 Bao Z, Weatherspoon MR, Shian S, Cai Y, Graham PD, Allan SM, Ahmad G, Dickerson MB, Church BC, Kang Z, Abernathy HW III, Summers CJ, Liu M, Sandhage KH (2007) Chemical reduction of three-dimensional silica micro-assemblies into microporous silicon replicas. Nat Lett 446:172 Beydaghyan G, Kaminska K, Brown T, Robbie K (2004) Enhanced birefringence in vacuum evaporated silicon thin films. Appl Optics 43(28):5343–5349 Canham LT, Groszek AJ (1992) Characterization of microporous silicon by flow calorimetry – comparison with a hydrophobic silica molecular sieve. J Appl Phys 72:1558 Chen Q, Zhou G, Zhu J, Fan C, Li X-G, Zhang Y (1996) Ultraviolet light emission from porous silicon hydrothermally prepared. Phys Lett A 224:133–136 Dai F, Zai J, Yi R, Gordin ML, Sohn H, Wang D (2014) Bottom-up synthesis of high surface area mesoporous crystalline silicon and evaluation of its hydrogen evolution performance. Nat Commun 5:3605 Deng T, Chen J, Wu CN, Liu ZW (2013) Fabrication of inverted pyramid silicon nanopore arrays with three step wet etching. ECS J Solid State Sci Technol 2(11):419–422 Dimova-Malinovska D, Sendova-Vassileva M, Tzenov N, Kamenova M (1997) Preparation of thin porous silicon layers by stain etching. Thin Solid Films 297:285–290 Fang DZ, Striemer CC, Gaborski TR, McGrath JL, Fauchet PM (2010) Methods for controlling the pore properties of ultra-thin nanocrystalline silicon membranes. J Phys Cond Mater 22:454134 Fukatani K, Ishida Y, Aiba T, Miyata H, Den T (2005) Characterization of nanoporous Si thin films obtained by Al-Si phase separation. Appl Phys Lett 87:253112 Godhino V, Caballero-Hernandez J, Jamon D, Rojas TC, Schierholz R, Garcia-Lopez J, Ferrer FJ, Fernandez A (2013) A new bottom-up methodology to produce silicon layers with a closed porosity nanostructure and reduced refractive index. Nanotechnology 24:275604 Huang X, Gonzalo-Rodriguez R, Rich R, Gryczynski Z, Coffer JL (2013) Fabrication and size dependent properties of porous silicon nanotube arrays. Chem Commun 49(51):5760–5762 Hummel RE, Chang S-S (1992) Novel technique for preparing porous silicon. Appl Phys Lett 61(16):1965–1967 Jakubowicz J, Smardz K, Smardz L (2007) Characterisation of porous silicon prepared by powder technology. Physica E38:139–143 Kabashin AV, Meunier M (2002) Fabrication of photoluminescent Si-based layers by air optical breakdown near the silicon surface. Appl Surf Sci 186:578–582 Kalkan AK, Bae S, Li H, Hayes DJ, Fosash SJ (2000) Nanocrystalline Si thin films with arrayed void-column network deposited by high density plasma. J Appl Phys 88(1):555–561

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Krishnamurthy A, Rasmussen DH, Suni II (2011) Galvanic deposition of nanoporous Si onto 6061 A1 alloy from Aqueous HF. J Electrochem Soc 158(2):D68–D71 Li X, Xiao Y, Yan C, Song JW, Talvev V, Schweizer SL, Pielkieska K, Sprafke A, Lee JH, Wehrspoon RB (2013) Fast electroless fabrication of uniform mesoporous silicon layers. Electrochim Acta 94:57–61 Mahmood AS, Sivakumar M, Venkatakrishnan K, Tan B (2009) Enhancement in optical absorption of silicon fibrous nanostructure produced using femtosecond laser ablation. Appl Phys Lett 95:034107 Nakahata T, Nakajima H (2004) Fabrication of lotus-type porous silicon by unidirectional solidification in hydrogen. Mater Sci Eng A 384:373 Noguchi N, Suemune I (1993) Luminescent porous silicon synthesized by visible light irradiation. Appl Phys Lett 62:1429–1431 Sadadoun M, Mliki N, Kaabi H, Daoudi K, Bessais B, Ezzaouia H, Bennaceur R (2002) Vapouretching-based porous silicon: a new approach. Thin Solid Films 405:29–34 Savin DP et al (1996) Properties of laser ablated porous silicon. Appl Phys Lett 69(20):3048–3050 Stepanov AL, Trifonov AA, Osin YN, Valeev VF, Nuzhdin VI (2013) Fabrication of nanoporous silicon by Ag + ion implantation. Nanosci Nanoeng 1(3):134–138 Theunissen MJJ (1972) Etch channel formation during anodic dissolution of n-type silicon in aqueous hydrofluoric acid. J Electrochem Soc 119:351–360 Uhlir A (1956) Electrolytic shaping of germanium and silicon. Bell Syst Tech J 35:333–347 Voigt F, Bruggemann R, Unold T, Huisken F, Bauer GH (2005) Porous thin films grown from sizeselected silicon nanocrystals. Mater Sci Eng 25(5–8):584–589 Wang JF, Wang KX, Du FH, Guo XX, Jiang YM, Chen JS (2013) Amorphous silicon with high specific surface area prepared by a sodiothermic reduction method for supercapacitors. Chem Commun 49:5007–5009 Woldering LA, Tjerkstra RW, Jansen HV, Setija ID, Vos WL (2008) Periodic arrays of deep nanopores made in silicon with reactive ion etching and deep UV lithography. Nanotechnology 19:145304 Yang X, Zhang P, Shi C, Wen Z (2012) Porous graphite/silicon micro-sphere prepared by in-situ carbothermal reduction and spray drying for lithium ion batteries. ECS Solid Lett 1(2):M5–M7 Zhang Z, Wang Y, Ren W, Tan Q, Chen Y, Li H, Zhong Z, Su F (2014) Scalable synthesis of interconnected porous silicon/carbon composites by the Rochow reaction as high performance anodes of lithium ion batteries. Angew Chem Int Ed Engl 53(20):5165–5169 Zheng Y, Yang J, Wang J, NuLi Y (2007) Nano-porous Si/C composites for anode material of lithium ion batteries. Electrochim Acta 52:5863–5867

Porous Silicon Formation by Anodization Armando Loni

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Anodization of Silicon Wafers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Anodization Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Rinsing and Drying of Porous Layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Layer Uniformity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nonaqueous Electrolytes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrolyte Additives: Surfactants, Oxidizers, and Others . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tunable Porosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Process Variability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrode Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Health and Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

12 12 13 14 15 16 16 17 18 19 19 20

Abstract

The key aspects of porous silicon manufactured by anodization are reviewed, with the following subjects being addressed: anodization of different wafer types, wafer cell design, post-anodization handling requirements (rinsing/drying/storage), parameters affecting layer uniformity, the use of nonaqueous electrolytes and electrolyte additives (surfactants, oxidizers, and other types), methods for tuning porosity, process control and natural variability, different electrode materials, and the requirements for maintaining health and safety.

A. Loni (*) pSiMedica Ltd, Malvern, Worcestershire, UK e-mail: [email protected] # Springer International Publishing Switzerland 2014 L. Canham (ed.), Handbook of Porous Silicon, DOI 10.1007/978-3-319-05744-6_2

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Introduction While the vast majority of published research on porous silicon is based upon layers produced by “bench-top” processing of wafer pieces and small-diameter wafers, this historical work underpins more recent developments in equipment and processing methodologies for commercial applications, where scaled-up manufacture, wafer-scale integration, and batch-to-batch reproducibility are key. Three comprehensive books on porous silicon have been published, wherein detailed information can be found related to silicon anodization (Canham 1997; Lehman 2002; Sailor 2012a). The topics covered include dissolution chemistries and the dependences of porosity, pore morphology, and pore size distribution on various parameters (e.g., wafer type/doping, electrolyte composition, current density, time); additionally, different types of electrochemical cells are discussed (Lehman 2002; Sailor 2012a), as well as some of the more practical aspects related to anodization (Sailor 2012a; e.g., wafer preparation, equipment and instrumentation, health and safety). The reader is referred to these references for essential background reading.

Anodization of Silicon Wafers The porosification of the surface of a silicon wafer is generally referred to as “anodization” and occurs when the wafer is anodically biased in a fluoride-based electrolyte solution. The most commonly used electrolyte component is hydrofluoric acid (HF), with ammonium fluoride (Kuhl et al. 1998) being less common. The degree of anodization is defined by the layer formation rate and porosity and, together with pore morphology, depends on wafer type and resistivity, the applied current density and time, and the electrolyte composition (HF concentration, with or without additives). Secondary parameters include electrolyte temperature and pH. The surface of a silicon wafer, as received from the manufacturer, will always be covered with a native oxide film. The oxide layer will be removed when the wafer is immersed in HF, although the cleanliness of the underlying surface can influence the anodization process and certain applications may therefore require wafer pre-cleaning prior to anodization (Sailor 2012b). A heavily doped wafer (p+,++, n+,++) can be readily anodized in a variety of HF-based electrolytes to form mesoporous silicon. A lightly doped wafer (p
, n
, and majority carrier concentration 90 %) would normally exhibit crazing and pore collapse on air exposure (with loss of pore volume), unless kept wet before utilizing supercritical drying (Canham et al. 1994a). Chapter “▶ Drying Techniques Applied to Porous Silicon” of this handbook focuses on such issues. In practice, it can be extremely difficult to completely remove residual solution from within micro-/mesopores due to inherently strong capillary forces – hence, the importance of repeated rinsing and washing, at the very least, to minimize the concentration of any toxic component.

Layer Uniformity The physical properties of a porous silicon sample can vary depending on, for example, the degree of lateral variation in layer thickness and/or porosity, the interface roughness between porous layers or at the layer/substrate boundary, and any vertical porosity gradation. As mentioned previously, careful design of the anodization equipment can minimize most sources of inhomogeneity. A uniform current presented across the whole area of the exposed wafer surface is essential, and for this reason, it is best to incorporate a counter electrode of a size (and shape) that is similar to the wafer. Additionally, electrical contact to the wafer must be uniform across the whole of the rear surface, especially important for low conductivity wafers. The use of a large graphite backing/guard plate has been shown to improve uniformity across the wafer, to some extent (Hossain et al. 2002). If light assistance is required during anodization, then electrode shadowing must be minimized on the front face of the wafer (e.g., by using a mesh or spiral arrangement). Electrical fringe effects can affect the anodization uniformity due to different relative current densities in comparison with the center of a wafer. If the wafer is partially immersed in the electrolyte, for example, the meniscus region will form with a higher current density, as will the thin edge/perimeter of the wafer that is in contact with the electrolyte; protecting such areas with a suitably defined electrolyte-resistant mask can improve uniformity. If the wafer is patterned on the surface, the current density and charge flow differ at the mask edges and undercutting can result (Guendouz et al. 2000). The existence of chemical concentration gradients and the accumulation of hydrogen gas bubbles at the surface of the wafer (and also forming within the porous region itself), as well as the occurrence of a nonuniform temperature

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distribution caused by heating, are all problematic when anodizing in a static (non-flowing) electrolyte. While ultrasonic agitation can be used to liberate gas bubbles during anodization (Takai and Itoh 1986; Liu et al. 2003), a continuous (laminar) flow of electrolyte is recommended as a simpler means of flushing the hydrogen away from the wafer surface (eventually to be vented to the outside ambient), thus improving layer uniformity. A continuous flow also helps to dissipate heat buildup in the electrolyte and can minimize chemical concentration gradients within the electrolyte and at the pore fronts, both of which will affect the anodization process and properties of the resulting layers. For porous silicon multilayer structures (see chapter “▶ Porous Silicon Multilayers and Superlattices” of this handbook), improved interface uniformity has been achieved by incorporating a short period of zero bias between individual layers, thereby allowing the hydrogen to diffuse from within the pores and the fluoride to be replenished at the pore tips (Takai and Itoh 1986). The “pulsed etching” (Billat et al. 1997) and “stop-etching” (Hou et al. 1996) techniques are based on a similar principle, with the latter (Khokhlov 2008) reportedly narrowing the pore size distribution for layers produced with high current density (although the role of static chemical leaching during the relatively longer periods under zero bias was not addressed) and the former finding use in the anodization of p–n junctions patterned on silicon wafers (McGinnis et al. 1999).

Nonaqueous Electrolytes Aside from the more common aqueous HF-based electrolytes, nonaqueous organic electrolytes in combination with a suitable fluoride source have been used, primarily for the production of macroporous layers both on silicon wafers (Propst and Kohl 1994; Rieger and Kohl 1995; Ponomarev and Levy-Clement 1998, 2000; Flake et al. 1999; Thakur et al. 2012a) and in freestanding form (Thakur et al. 2012b). Examples include acetonitrile, propylene carbonate, and dimethylformamide, with anhydrous HF (up to 2 M), tetrafluoroborate, and lithium fluoroborate being used as fluoride sources. The inclusion of a supporting electrolyte, such as tetrabutylammonium perchlorate (up to 0.25 M), has been shown to offer additional flexibility with regard to the resulting pore morphologies attained (Ponomarev and Levy-Clement 2000). In contrast to aqueous electrolytes, nonaqueous electrolytes facilitate silicon dissolution without hydrogen evolution.

Electrolyte Additives: Surfactants, Oxidizers, and Others The addition of a surfactant (wetting agent) to the electrolyte helps to prevent evolving hydrogen bubbles from “sticking” to the porous silicon surface. Alcohol (e.g., methanol, ethanol) is the most common, with, among others, formic acid (Baranov et al. 2000), acetic acid (Baranov et al. 2000; Semai et al. 2009), and sodium laurel sulfate (Ogata et al. 2000); commercially available surfactants such

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as NCW-1001 (Ogata et al. 2000), Mirasol (Lehman and Foll 1990), Triton X-100 (Chao et al. 2000), and DECON (Kordas et al. 2001) have also been used. While alcohol-based surfactants are used in substantial quantities (>15 % v/v), the cationic or anionic surfactants (Chao et al. 2000; Sotgiu et al. 1997) are used in concentrations as low as 10
4 M (typically  5 % v/v) and sometimes in combination with alcohol. An alcohol-based surfactant will instigate chemical leaching of the porous silicon layer during anodization (discussed below), particularly so with long anodization times and high alcohol content, and this can result in a porosity gradient within the layer (more porous at the surface). Without surfactant, however, an anodized layer can be nonuniform in thickness, with substantial interface roughness at the porous silicon substrate (Halimaoui 1993) or between different layers. Adding hydrochloric acid to the electrolyte changes the pH and can lead to favorable changes in the properties of the anodized material such as, for example, enhanced and stable photoluminescence (Zangooie et al. 1998; Belogorokhov and Belogorokhova 1999). Various types of chemical oxidizer have been incorporated into HF-based electrolytes. CrO3 has been used (Foll et al. 2000; Christophersen et al. 2000; Ouyang et al. 2005) to produce macroporous silicon, while potassium permanganate (KMnO4) also acts to increase pore size (Ogata et al. 2000; Harraz et al. 2008). The inclusion of hydrogen peroxide (H2O2) has been shown to produce layers with monohydride passivation (Yamani et al. 1997), although its role in the formation of layers with wider pores has been the subject of greater interest (Ogata et al. 2000; Ge et al. 2010). Reduced interface roughness, previously observed (Setzu et al. 1998; Servidori et al. 2000) in samples anodized at low temperature and attributed to an increased electrolyte viscosity, has been replicated (for some conditions) by adding a small fraction of glycerol to the electrolyte (Kan et al. 2005). In situ functionalization of porous silicon during anodization has been achieved by incorporating the HF-compatible organic molecule 1-heptyne (C7H12) in the electrolyte, in concentrations up to 0.9 M (Mattei and Valentini 2003).

Tunable Porosity If an alcohol surfactant is incorporated in the electrolyte, then the porosity of a layer will depend on the dilution used, higher porosities being obtained with higher alcohol content. As mentioned previously, the in situ chemical leaching of a layer (with or without electrical bias) in HF-alcohol electrolytes can be used to good effect to increase porosity (Halimaoui 1994; Herino et al. 1987) and pore size (Herino et al. 1987; Herino 1997). Porosity can be more easily “tuned” through choice of the applied current density and anodization time (Herino et al. 1987; Herino 1997), and this is the basis of forming complex multilayer or graded structures. When comparing different anodization processes, the current densities (assuming the wafer areas are the same)

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Table 1 Layer parameter dependency on current density and time (detached mesoporous silicon layers produced with 600 p-type wafers, 0.005–0.02 O cm, using 1:1 40 % HF:methanol electrolyte; porosity determined gravimetrically; surface area and pore volume/diameter determined by nitrogen gas adsorption – author’s data) Current density (mA/cm2) 33 99 99

Anodization time (min) 90 30 60

Porosity (%) 66 78 86

Depth (μm) 161 125 234

Surface area (m2/g) 296 434 495

Pore volume (ml/g) 0.759 1.435 2.267

Average pore diameter (nm) 10 10 18

and anodization times should be chosen such that the overall charge flow is comparable for each process – this facilitates “like-for-like” comparison of the resulting properties. If an alcohol surfactant is present, however, it can be difficult to completely preclude the effects of chemical leaching (Herino et al. 1987), and this should be considered when making process comparisons. From Table 1, it can be tentatively surmised that, for a fixed charge flow, the porosity difference between 33 mA/cm2 (90 min) and 99 mA/cm2 (30 min) is primarily due to the enhanced electrochemical dissolution of the layer at the higher current density; when the current density is fixed (99 mA/cm2), the higher porosity for the 60 min process is primarily due to chemical leaching of the layer. As expected, the different porosities are accompanied by variations in surface area, pore volume, and average pore diameter. Porosity can also be tuned by applying a magnetic field during anodization. A magnetic field applied perpendicular to the wafer surface, with the electrolyte temperature maintained at 0  C, can regulate the supply of holes, the resulting porosity being dependent on the magnetic field strength (Nakagawa et al. 1996).

Process Variability When comparing individual wafers or layers produced sequentially, or under different anodization conditions, the condition of the electrolyte must be considered, as this leads to process variation. The accumulation of silicon in the electrolyte from each dissolution process (e.g., in the form of H2SiF6), combined with the steady depletion of fluorine, is evinced by a gradual increase in electrolyte conductivity and pH; this natural evolution of electrolyte composition with continuous use is particularly important for high-throughput production, where associated variations within a fixed volume of electrolyte include a gradual reduction in layer thickness and an increase in porosity (reduced yield). Indeed, very high porosities can be achieved with a “well-used” electrolyte. The point that defines the end of the useful life of the electrolyte depends on the highest porosity or yielded weight of porous silicon that is acceptable for the intended product or application. In order to maintain the desired blend, the electrolyte

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can be replenished by auto-dosing (with fresh HF and/or surfactant) on a continual basis; this requires a feedback control loop whereby some indicator of the condition of the electrolyte is continuously monitored, e.g., HF concentration (Nehmann et al. 2012). However, the working volume of the equipment and the need to dispose of large quantities of waste electrolyte in a safe manner both limit the extent to which auto-dosing can be used. When a number of wafers are anodized simultaneously, the amount of electrical power supplied to the electrolyte is much greater than that required for a single wafer; this leads to electrolyte heating, which, depending on current density, anodization time, and wafer batch size, can be significant. The chemical dissolution process is temperature dependent (Garman et al. 2001; Balagurov et al. 2006), while the evaporation rate of any volatile surfactant will also increase at elevated temperature. It has been shown, also, that photoluminescence wavelength and intensity and layer crystallinity are dependent on the electrolyte temperature used for anodization (Ono et al. 1993). The control of electrolyte temperature during anodization is therefore important in many respects. Any heat generated during anodization will be dissipated to some extent by continuously pumping the electrolyte within the equipment, although the use of a cooling coil (e.g., in the electrolyte reservoir) or integrated chilling bath can be more effective.

Electrode Materials Platinum metal is commonly used as an electrode material, due to its relatively high chemical resistivity. It has been shown (Pourbaiz et al. 1959; Llopis and Sancho 1961; Kodera et al. 2007), however, that anodically biased platinum slowly corrodes in the presence of acidic solutions. While the corrosion rate is extremely small, if the same counter electrode is used continuously, a buildup of platinum (and its oxide) occurs in the electrolyte; this is deposited within the equipment, as evinced by a dark film lining the inside of the components; deposition also occurs within the porous silicon layers (typically 80 min in concentrated HF solutions by a 3-μm-thick AZ 6130 photoresist film. The morphological, electrical, and mechanical effects of etched poly-Si structures have been extensively investigated by Stoldt and coworkers (Miller et al. 2005, 2007, 2008; Becker et al. 2010a, b, 2011). Micromachined p-type polysilicon in contact with a Au layer demonstrated heterogeneous cracking or porosity across the poly-Si surface as a result of etching. This resulted in greatly increased electrical resistance and decreased the characteristic frequency of mechanical resonators. Because the mechanical properties of MEMS depend strongly on their structure, changes that occur during postprocessing must be taken into account. Miller et al. (2007) measured the decrease in the frequency of mechanical resonance that occurred as a function of immersion time in HF for microcantilevers as well as “comb drives” in contact with Au. Time-dependent variation was also observed in

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the modulus and hardness measured during indentation testing. They observed that grain delineation accompanied the formation of a nanoscale porous layer in the near surface region of the poly-Si. The por-Si layer exhibited decreased stiffness, which changed the effective thickness of the beams. Galvanic etching can greatly influence the material properties, design, performance, lifetime, tribology, manufacture, and required operating environment of microscale and nanoscale devices. Morphology, resistive probe, surface wetting, and electrochemical characterization of single-crystalline and polycrystalline Si subjected to galvanic corrosion in concentrated 48 wt% HF, 23 wt% HF diluted with water, and a 20:1 solution of 23 wt% HF with Triton X-100 were performed by Miller et al. (2008). The measured current density of micromachined Si was compared with (100) wafers using polarization characterization, identifying the por-Si formation regime. Porosity ranged from 20 % to 47 % but in some cases was as high as 70 %. The porosity generally increased as the surface area ratio of Au to Si increased. Chronopotentiometry, resistive probe, and microtensile characterizations were used to identify regimes of rapid initiation, subsequent steady-state corrosion, and the final catastrophic failure of the microtensile specimens. Corrosion current depended exponentially on the amount of metal present. This implied that the corrosion rate was limited by the surface area of the metal cathode. Addition of surfactant led to higher current densities and more uniform layers. Subsequently, this group (Becker et al. 2010b) used focused ion beam (FIB) milling of microscale silicon-on-insulator (SOI) devices to determine the depth uniformity of the galvanically formed por-Si as a function of the geometry of the device. They developed a finite-element-method simulation to model the galvanic corrosion process. The model reproduced the current-limited condition resulting from the finite surface area of metal relative to Si and predicted the uniform etch rate across the device for surfactant-enhanced HF solutions as observed after FIB milling. Becker et al. (2010a) have optimized the process to produce thick, high-surfacearea por-Si films for applications as an explosive material (see chapter entitled “▶ Energetics with Porous Silicon”). Films up to ~150 μm thick with specific surface area ~700 m2 g1 and pore diameters ~3 nm were fabricated. Subsequently, this group optimized their galvanic process for even greater specific area (Becker et al. 2011). A 170-nm-thick Pt layer was sputtered on the back of the Si to form the cathode and an H2O2/HF/ethanol solution acted as the etchant. The H2O2 concentration was 2.4 vol% of 30 % H2O2 in water. The HF to ethanol ratio was either 20:1 or 3:1. The surface area ratio of Pt to Si was ~5. In the 20:1 solution, films with specific surface areas of ~840 m2 g1 and porosities of 65–67 % were obtained. Somewhat higher specific areas (890–910 m2 g1) and porosities (79–83 %) were observed for the 3:1 solution. Galvanic cells can be formed on Si powders just as they can for c-Si and poly-Si. Nielsen et al. (2007) used galvanic displacement to deposit Pt on Si powder grains, which were dispersed in an aqueous mixture of HF and H2PtCl6 for 15 min. The grains were then removed and dispersed in a 1:3:2 mixture of HF/H2O2/methanol to affect galvanic etching. They used this procedure to produce dispersions of

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photoluminescent Si nanoparticles in the size range of 3–6 nm by subsequent sonication of the grains in isopropanol. Only a thin porous layer was produced and the core of the originally 80-μm-diameter particles remained unaffected. Nanoparticles could be generated in a similar fashion by substituting a Si wafer for the Si powder. Nakamura et al. investigated both Ag/por-Si (2010) and Pt/por-Si (2011) composite powders. Metallurgical grade Si powder was added to the 18 % HF, and then AgNO3 was added to the mixture held at 30
C. Nanometer-sized Ag particles deposited singly or as aggregates on the Si powder surface. Higher concentrations of AgNO3 resulted in decreased Si volume and increased Ag layer thickness. The powders were photoluminescent. The PL intensity was weaker but more stable than that of conventionally stain-etched por-Si powders. Pt layers were deposited on a Si surface together with the formation of a porous structure. It was found that the oxidation state of Pt layers strongly depended on the conditions for the preparation of Pt/PSi composite powders. Galvanic cell formation and Si etching are also strongly linked to metal deposition on Si surfaces (Ogata and Kobayashi 2006; Allongue and Maroun 2006; Gorostiza et al. 2000, 2003; daRosa et al. 2008). Applications to micro- and nanoscale devices in fields ranging from electronic devices to chemical sensors, including schemes developed for the metallization and nanopatterning of semiconductor substrates with high selectivity and with optimal interfacial properties, have been discussed by Carraro et al. (2007). To understand the rate as well as pH and concentration dependence of deposition, it is essential to apply mixed-potential theory (Gorostiza et al. 2000, 2003; daRosa et al. 2008). The morphology of the deposit is of great interest in metalassisted etching (Peng et al. 2003; Huang et al. 2011). Large-area nanostructured noble-metal films of Ag, Pt, and Au can be deposited (Song et al. 2005) with various morphologies on Si. The morphology of Ag films, which is different from that of Pt and Au, depends sensitively on the deposition conditions. The roughness of Cu films was found to decrease with increasing HF to CuSO4 concentration ratio. Concurrently, the Si surface became less oxidized and lateral connectivity between Cu nuclei increased (daRosa et al. 2008). Au nanostructures, clusters, or dendrites with leaflike or branch-like structure can be prepared by adjusting the temperature and the concentration of HAuCl4, ultrasonic agitation, and addition of ethanol (Wang et al. 2006).

Conclusion Galvanic and metal-assisted etching follow the same electrochemistry. The differences between them are related to the initial structure of the metal electrocatalyst that is responsible for making the electrochemical etching spontaneous. While studies of galvanic etching are dwarfed by the number devoted to metal-assisted etching, it will continue to be an essential process to understand for any application that involves the formation of a metal/silicon interface that is subsequently exposed to HF, such as in the fabrication of MEMS devices. Thick uniform films with surface areas as high as 910 m2 g1 have been achieved with optimized galvanic etching (Becker et al. 2011).

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References Allongue P, Maroun F (2006) Metal electrodeposition on single crystal metal surfaces: mechanisms, structure and applications. Curr Opin Solid State Mater 10:173–181 Archer RJ (1960) Stain films on silicon. J Phys Chem Solids 14:104–110 Ashruf CMA, French PJ, Bressers PMMC, Sarro PM, Kelly JJ (1998) A new contactless electrochemical etch-stop based on a gold/silicon/TMAH galvanic cell. Sensor Actuat A 66:284–291 Ashruf CMA, French PJ, Bressers PMMC, Kelly JJ (1999) Galvanic porous silicon formation without external contacts. Sensor Actuat A 74:118–122 Ashruf CMA, French PJ, Sarro PM, Kazinczi R, Xia XH, Kelly JJ (2000) Galvanic etching for sensor fabrication. J Micromech Microeng 10:505–515 Becker CR, Currano LJ, Churaman WA, Stoldt CR (2010a) Thermal analysis of the exothermic reaction between galvanic porous silicon and sodium perchlorate. ACS Appl Mater Interfaces 2:2998–3003 Becker CR, Miller DC, Stoldt CR (2010b) Galvanically coupled gold/silicon-on-insulator microstructures in hydrofluoric acid electrolytes: finite element simulation and morphological analysis of electrochemical corrosion. J Micromech Microeng 20:085017 Becker CR, Apperson S, Morris CJ, Gangopadhyay S, Currano LJ, Churaman WA, Stoldt CR (2011) Galvanic porous silicon composites for high-velocity nanoenergetics. Nano Lett 11:803–807 Bressers PMMC, Plakman M, Kelly JJ (1996) Etching and electrochemistry of silicon in acidic bromine solutions. J Electroanal Chem 406:131–137 Carraro C, Maboudian R, Magagnin L (2007) Metallization and nanostructuring of semiconductor surfaces by galvanic displacement processes. Surf Sci Rep 62:499–525 Chabal YJ, Harris AL, Raghavachari K, Tully JC (1993) Infrared spectroscopy of H-terminated silicon surfaces. Int J Mod Phys B 7:1031–1078 Chasiotis I, Knauss WG (2003) The mechanical strength of polysilicon films: part 1. The influence of fabrication governed surface conditions. J Mech Phys Solids 51:1533–1550 Chattopadhyay S, Li X, Bohn PW (2002) In-plane control of morphology and tunable photoluminescence in porous silicon produced by metal-assisted electroless chemical etching. J Appl Phys 91:6134–6140 Clark IT, Aldinger BS, Gupta A, Hines MA (2010) Aqueous etching produces Si(100) surfaces of near-atomic flatness: strain minimization does not predict surface morphology. J Phys Chem C 114:423–428 daRosa CP, Maboudian R, Iglesia E (2008) Copper deposition onto silicon by galvanic displacement: effect of silicon dissolution rate. J Electrochem Soc 155:E70–E78 Dudley ME, Kolasinski KW (2008) Wet etching of pillar covered silicon surface: formation of crystallographically defined macropores. J Electrochem Soc 155:H164–H171 Gorostiza P, Anbu Kulandainathan M, Dı´az R, Sanz F, Allongue P, Morante JR (2000) Charge exchange processes during the open-circuit deposition of nickel on silicon from fluoride solutions. J Electrochem Soc 147:1026–1030 Gorostiza P, Allongue P, Dı´az R, Morante JR, Sanz F (2003) Electrochemical characterization of the open-circuit deposition of platinum on silicon from fluoride solutions. J Phys Chem B 107:6454–6461 Harada Y, Li X, Bohn PW, Nuzzo RG (2001) Catalytic amplification of the soft lithographic patterning of Si. Nonelectrochemical orthogonal fabrication of photoluminescent porous Si pixel arrays. J Am Chem Soc 123:8709–8717 Hines MA (2003) In search of perfection: understanding the highly defect-selective chemistry of anisotropic etching. Annu Rev Phys Chem 54:29–56 Hines MA, Faggin MF, Gupta A, Aldinger BS, Bao K (2012) Self-propagating reaction produces near-ideal functionalization of Si(100) and flat surfaces. J Phys Chem C 116:18920–18929 Huang Z, Geyer N, Werner P, de Boor J, Go¨sele U (2011) Metal-assisted chemical etching of silicon: a review. Adv Mater 23:285–308

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Huh M, Yu Y, Kahn H, Payer J, Heuer A (2006) Galvanic corrosion during processing of polysilicon microelectromechanical systems – the effect of Au metallization. J Electrochem Soc 153:G644–G649 Kahn H, Deeb C, Chasiotis I, Heuer AH (2005) Anodic oxidation during MEMS processing of silicon and polysilicon: native oxides can be thicker than you think. J Microelectromech Syst 14:914–923 Kelly JJ, Philipsen HGG (2005) Anisotropy in the wet-etching of semiconductors. Curr Opin Solid St M 9:84–90 Kelly JJ, Xia XH, Ashruf CMA, French PJ (2001) Galvanic cell formation: a review of approaches to silicon etching for sensor fabrication. IEEE Sensors J 1:127–142 Koker L, Kolasinski KW (2000) Photoelectrochemical etching of Si and porous Si in aqueous HF. Phys Chem Chem Phys 2:277–281 Koker L, Kolasinski KW (2001) Laser-assisted formation of porous silicon in diverse fluoride solutions: reactions kinetics and mechanistic implications. J Phys Chem B 105:3864–3871 Kolasinski KW (2010) Charge transfer and nanostructure formation during electroless etching of silicon. J Phys Chem C 114:22098–22105 Kolasinski KW, Gogola JW (2012) Electroless etching of Si with IO3 and related species. Nanoscale Res Lett 7:323 Kolasinski KW, Gogola JW, Barclay WB (2012) A test of Marcus theory predictions for electroless etching of silicon. J Phys Chem C 116:21472–21481 Lehmann V (2002) Electrochemistry of silicon: instrumentation, science, materials and applications. Wiley-VCH, Weinheim Li X, Bohn PW (2000) Metal-assisted chemical etching in HF/H2O2 produces porous silicon. Appl Phys Lett 77:2572–2574 Liu YF, Xie J, Zhao H, Luo W, Yang JL, An J, Yang FH (2012) An effective approach for restraining electrochemical corrosion of polycrystalline silicon caused by an HF-based solution and its application for mass production of MEMS devices. J Micromech Microeng 22:035003 Meltzer S, Mandler D (1995) Study of silicon etching in HBr solutions using a scanning electrochemical microscope. J Chem Soc Faraday Trans 91:1019–1024 Miller DC, Gall K, Stoldt CR (2005) Galvanic corrosion of miniaturized polysilicon structures morphological, electrical, and mechanical effects. Electrochem Solid State Lett 8:G223–G226 Miller DC, Hughes WL, Wang ZL, Gall K, Stoldt CR (2007) Mechanical effects of galvanic corrosion on structural polysilicon. J Microelectromech Sys 16:87–101 Miller DC, Becker CR, Stoldt CR (2008) Relation between morphology, etch rate, surface wetting, and electrochemical characteristics for micromachined silicon subject to galvanic corrosion. J Electrochem Soc 155:F253–F265 Muhlstein CL, Stach EA, Ritchie RO (2002) A reaction-layer mechanism for the delayed failure of micron-scale polycrystalline silicon structural films subjected to high-cycle fatigue loading. Acta Mater 50:3579–3595 Nakamura T, Hosoya N, Tiwari BP, Adachi S (2010) Properties of silver/porous-silicon nanocomposite powders prepared by metal assisted electroless chemical etching. J Appl Phys 108:104315 Nakamura T, Tiwari BP, Adachi S (2011) Direct synthesis and enhanced catalytic activities of platinum and porous-silicon composites by metal-assisted chemical etching. Jpn J Appl Phys 50:081301 Nielsen D, Abuhassan L, Alchihabi M, Al-Muhanna A, Host J, Nayfeh MH (2007) Current-less anodization of intrinsic silicon powder grains: formation of fluorescent Si nanoparticles. J Appl Phys 101:114302 Noguchi N, Suemune I (1993) Luminescent porous silicon synthesized by visible light irradiation. Appl Phys Lett 62:1429–1431 Noguchi N, Suemune I (1994) Selective formation of luminescent porous silicon by photosynthesis. J Appl Phys 75:4765–4767

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Ogata YH, Kobayashi K (2006) Electrochemical metal deposition on silicon. Curr Opin Solid State Mater Sci 10:163–172 Peng K, Yan Y, Gao S, Zhu J (2003) Dendrite-assisted growth of silicon nanowires in electroless metal deposition. Adv Func Mater 13:127–132 Pierron ON, Macdonald DD, Muhlstein CL (2005) Galvanic effects in Si-based microelectromechanical systems: thick oxide formation and its implications for fatigue reliability. Appl Phys Lett 86:211919 Song YY, Gao ZD, Kelly JJ, Xia XH (2005) Galvanic deposition of nanostructured noble-metal films on silicon. Electrochem Solid State Lett 8:C148–C150 Splinter A, St€urmann J, Benecke W (2001a) Novel porous silicon formation technology using internal current generation. Mater Sci Eng C 15:109–112 Splinter A, Sturmann J, Benecke W (2001b) New porous silicon formation technology using internal current generation with galvanic elements. Sensor Actuat A 92:394–399 Sun NN, Chen JM, Jiang C, Zhang YJ, Shi F (2012) Enhanced wet-chemical etching to prepare patterned silicon mask with controlled depths by combining photolithography with galvanic reaction. Ind Eng Chem Res 51:793–799 Turner DR (1960) On the mechanism of chemically etching germanium and silicon. J Electrochem Soc 107:810–816 Wang CH, Sun DC, Xia XH (2006) One-step formation of nanostructured gold layers via a galvanic exchange reaction for surface enhancement Raman scattering. Nanotechnology 17:651–657 Xia XH, Ashruf CMA, French PJ, Kelly JJ (2000) Galvanic cell formation in silicon/metal contacts: the effect on silicon surface morphology. Chem Mater 12:1671–1678

Porous Silicon Formation by Stain Etching Kurt W. Kolasinski

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Etchant Composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Rational Formulation of Stain Etchants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practical Advice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Spontaneous electroless etching of silicon surfaces with hydrofluoric acid and chemical oxidant-based solutions is often referred to as “stain etching” due to the color change it imparts. The field is comprehensively reviewed with regard to the etching mechanisms and the range of chemical oxidants explored to date.

Introduction The topic of this review is the etching of silicon by a fluoride solution in which etching is initiated by hole injection from a sufficiently strong oxidant. This spontaneous electroless etching process is commonly called stain etching when it leads to the formation of a porous silicon film (por-Si). Fuller and Ditzenberger (1957), Turner (1960), and Archer (1960) were unaware of the porous nature of the films they formed but named the process after the colored stains (Fig. 1) that appeared on their substrates. Because some of the most effective oxidants contain a metal ion, it has sometimes also been denoted metal-assisted etching. This term is, however, ambiguous and easily confused with a related catalytic etching process in K.W. Kolasinski (*) Department of Chemistry, West Chester University, West Chester, PA, USA e-mail: [email protected] # Springer International Publishing Switzerland 2014 L. Canham (ed.), Handbook of Porous Silicon, DOI 10.1007/978-3-319-05744-6_4

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Fig. 1 Four Si(100) substrates stain etched for different times to produce homogeneous smooth films of different depths, hence different colors caused by thin film interference in the porous silicon layers

which a metal nanoparticle acts to catalyze hole injection from an oxidant. I will call this catalytic etching process metal-assisted etching. It is possible to use oxidants that are capable of stain etching (spontaneous electroless etching) in conjunction with metal catalyst particles. Interestingly, as reviewed elsewhere in this volume (see chapter “▶ Porous Silicon Formation by Metal NanoparticleAssisted Etching”), the most frequently used oxidant for metal-assisted etching (HOOH) has such poor hole injection kinetics (Kooij et al. 1998) that it does not lead to significant por-Si formation in the absence of the metal catalyst. Stain etching is spontaneous, but what differentiates it from chemical etching is that free charge transfer is involved in stain etching. Hydroxide exhibits both chemical and electrochemical etching pathways (Allongue et al. 1993a, b). Etching in acidic fluoride is exclusively electrochemical (Kolasinski 2008), with an extremely low etch rate in the absence of a bias, light or dissolved oxygen (Kolasinski 2003). Chemical etching can be used to produce nearly perfectly flat and hydrogen-terminated Si surface (Burrows et al. 1988; Chabal et al. 1993; Hines et al. 2012). Chemical etching is initiated by the action of OH
. It took over 30 years from the first reports of stain etching until the realization that it can produce photoluminescent por-Si films (Sarathy et al. 1992; Fathauer et al. 1992). A sketchy report of electroluminescence in a stain layer was attributed by Gee (1960) to amorphous Si. However, the film was likely a por-Si layer created by a 0.1 % HNO3 + 49 % HF solution. It is in some respects unfortunate that Turner discovered stain etching by using HNO3 as the oxidant. The HNO3 + HF system has the advantage that the solution composition can be varied to cover both an electropolishing regime, in which very high etch rates are observed accompanied by the formation of flat surfaces, as well as a por-Si formation regime. Unfortunately, the reduction of NO3
and the myriad other nitrogen-containing species that are

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formed as by-products are extremely complex. Only recently have Acker and co-workers been able to establish the role of these various species in the electropolishing regime (Acker et al. 2012; Hoffmann et al. 2011; Steinert et al. 2007, 2008; Henssge and Acker 2007; Acker and Henssge 2007; Steinert et al. 2005, 2006). This complexity (i.e., high sensitivity to composition, temperature, extent of reaction, various dissolved gases, etc.) also leads to irreproducibility in the ability of HNO3 + HF solutions to form photoluminescent por-Si films. The formation of thick films is rather difficult with this system. One application of stain etching in HF + HNO3 solutions has been in the production of “black silicon,” that is, broadband antireflection coatings, which are particularly useful in solar cell applications (Menna et al. 1995). This application is reviewed elsewhere in this handbook (chapter “▶ Porous Silicon and Solar Cells”). Control of the porosity profile, and therefore refractive index profile, can be particularly useful for tuning the reflectivity of the por-Si film (Schirone et al. 1997, 2000; Striemer and Fauchet 2002). Stain etchants can be used not only on crystalline Si but also multicrystalline Si solar cells to reduce reflectivity (Gonzalez-Diaz et al. 2009; Bilyalov et al. 2003). Porous Si layers have long been used in micromachining (Steiner and Lang 1995), and etching of Si in HF + HNO3 or NaNO2 solutions has also found uses in these applications (Melnikov et al. 2008; Yamamura and Mitani 2008). Stain-etched layers have been doped with Er3+ and Yb3+ to produce layers that may help to extend and enhance the wavelength response of Si solar cells (Diaz-Herrera et al. 2009, 2011). They can also be transformed into superhydrophobic surfaces (Liu et al. 2009). This property is reviewed elsewhere in the handbook (chapter “▶ Tunable Properties of Porous Silicon”). Much of the focus of this review will concentrate on HF + oxidants-other-thanNO3
solutions. One prime motivation for this is that some of these solutions produce por-Si in a much more reliable, reproducible, and controllable manner. In particular, Loni et al. (2011) have demonstrated that Fe3+ can be used to create high-surface area por-Si from low-cost metallurgical grade Si. Similarly, Sato and co-workers have used HNO3 + acetic acid + HF solutions (with added methanol and ultrasonic agitation) to produce Si nanocrystals with variable visible photoluminescence (Sato et al. 2009a, b). This property is reviewed elsewhere in this handbook (chapter “▶ Photoluminescence of Porous Silicon”).

Etchant Composition The fluoride in a stain etchant is usually supplied by HF, although solutions with strong fluorine-containing acids such as HBF4 and HSbF6 have also been reported ¨ nal et al. 2001). NH4HF2 can be used to replace HF as long (Parbukov et al. 2001; U as the solution is acidified (Mills and Kolasinski 2004; Nahidi and Kolasinski 2006). An unusual example is that of CeF4 dissolved in concentrated H2SO4 (Kolasinski et al. 2012; Dudley and Kolasinski 2009a). In this case, the oxidant and fluoride are supplied by the same species. This stain etchant leads to extremely

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uniform and highly luminescent por-Si layers but is limited to very slow etch rates because of the low solubility of CeF4. By far the most commonly used oxidant is HNO3, either to etch Si uniformly (Yamamura and Mitani 2008; Robbins and Schwartz 1959, 1960, 1961; Schwartz and Robbins 1976; Jenkins 1977; Kooij et al. 1999; Kulkarni and Erk 2000; Svetovoy et al. 2006) or to form por-Si (Turner 1960; Archer 1960; Beckmann 1965; Beale et al. 1986; Shih et al. 1992, 1993; McCord et al. 1992; Kidder et al. 1992; Dubbelday et al. 1993; Steckl et al. 1994; Chandler-Henderson et al. 1994; Liu et al. 1994, 2007; Kalem and Rosenbauer 1995; Anaple et al. 1995; Amato 1995; Schoisswohl et al. 1995; Jones et al. 1995; Di Francia et al. 1995; Di Francia and Citarella 1995; Winton et al. 1996, 1997; Velasco 2003; GuerreroLemus et al. 2003; González-Dı´az et al. 2006; Zeng et al. 2005; Melnichenko et al. 2005; Luchenko et al. 2007; Balaguer and Matveeva 2010; Mogoda et al. 2011; Lippold et al. 2011, 2012; Terheiden et al. 2011). Other sources of nitrogen oxo ions such as NaNO2 (Archer 1960; Melnikov et al. 2008; Kelly et al. 1994; Campbell et al. 1995; Vázsonyi et al. 2001; Abramof et al. 2006, 2007), NO2 (Archer 1960; Yoshioka 1969), NOBF4 (Kelly et al. 1994; Safi et al. 2002), or NOHSO4 (Patzig et al. 2007) have been used, but these are essentially running off of the same chemistry. A variation on standard HNO3-based stain etching was demonstrated by Chen et al. (1996, 2001; Li et al. 1999). They applied a hydrothermal treatment to produce por-Si. In the first report, they dissolved 0.3 mol L
1 LiF in 10.0 mol L
1 HNO3 then heated this solution with the Si substrate to 140  C inside of a stainless steel vessel for 2 h. The resulting ~ 70 % porous material was heavily oxidized, photoluminescent, and ~10 μm thick. Subsequently they switched to 0.3 mol L
1 Fe(NO3)3 in 40 wt% HF heated to 142  C for 45 min. This treatment leads to the incorporation of iron into the por-Si. Anisotropic etching has been reported in solutions composed of H2SiF6(aq) + HNO3(aq) (Famini et al. 2006). Acker and co-workers (Acker et al. 2012; Hoffmann et al. 2011; Steinert et al. 2005, 2006, 2007, 2008; Henssge and Acker 2007; Acker and Henssge 2007; Weinreich et al. 2007; Jadzinsky et al. 2007) have significantly advanced our understanding of silicon etching in HF + (nitrate, etc.) solutions. The temperature and composition of the solutions are critical. A number of different N-containing species form in solution. The concentrations of these as well as the interactions of etch products with etching are found to be temperature dependent. Both N(III) species and dissolved gases play roles in the mechanism. Denoting (3NO+ · NO3
) as [N4O62+], they found a linear relationship between etch rate and [N4O62+] concentration and concluded that NO+ is a reactive species in the rate-limiting step. High concentrations of this intermediate led to uniform etching (electropolishing). The rate of stain etching is enhanced by the presence of defects. This characteristic has been used to create a hybrid amorphous porous/defect-followed mesoporous structure (Woo et al. 2012a, b). Gerischer and L€ubke (1988) found that oxidants with an electrochemical potential more positive than the potential that corresponds to the Si valence band maximum can inject holes into the valence band. These included MnO4
, Br2, IrCl62
, and I2.

Porous Silicon Formation by Stain Etching

39

However, since these experiments were carried out in  1 M NH4F solutions, no por-Si was formed. Corrosion was noted for all these oxidants but, curiously, not for Fe3+. As detailed in the Table 1, a number of other oxidants have been tried or may still be capable of producing stain etchants. Of the alternatives listed in Table 1, by far the most interesting for producing photoluminescent, nanocrystalline thick films are Fe3+, supplied by FeCl3 · 6H2O, and VO2+, supplied by V2O5. The former is used at a concentration of roughly 1 M, the latter closer to 0.1 M (Kolasinski et al. 2010, 2012; Dudley and Kolasinski 2009a). Reproducible por-Si formation is possible with both oxidants when concentrated HF (aq) (49 wt% ¼ 29 mol L
1 in HF) is used as the acidic fluoride source. Lowering the concentration of fluoride lowers the etch rate. The etch rate is also directly proportional to the concentration of either Fe3+ or VO2+; however, unlike HNO3, there is no threshold concentration below which stain etching does not occur. The formation of por-Si films in Fe3+ + HF solutions under illumination with a Xe lamp has also been studied (Xu and Adachi 2007). Kolasinski and Barclay (2013a) have demonstrated that the stoichiometry of stain etching is distinct from that of anodic por-Si formation. This occurs because the injection of a conduction band electron in the current doubling step is used to reduce H+ to H2 in anodic etching, whereas this electron is consumed by the oxidant in stain etching. Therefore, two moles of oxidant are required to etch one mole of Si (twice as much as expected), but much less H2 is formed (with concomitant reduced bubble formation and improved film homogeneity). Several candidates for por-Si formation lead to poor results, such as HCrO4
, MnO4
, and S2O82
. The cation that accompanies these oxidants (usually Na+, K+, or NH4+) can precipitate in the form of a hexafluorosilicate and interfere with film formation (Koker et al. 2002). For example, Hoshino and Adachi produced photoluminescent films with 5–7 wt% K2Cr2O7 in 50 % HF solution; however, they also observed the deposition of K2SiF6 as did Mogoda et al. (2011). Usually, HF + CrO3 solutions lead to uniform etching or only roughening of the Si surface (Nahidi and Kolasinski 2006; Kooij et al. 1999; Kelly et al. 1994; van den Meerakker and van Vegchel 1989a, b; Nahm et al. 1997; Gabouze et al. 2003). There are several reports of the appearance of thin (Fathauer et al. 1992; Beale et al. 1986) or patchy porous layers (Kelly et al. 1994) as well. Just as for H2O2, the kinetics of hole injection from ClO4
are so slow as to preclude significant etching (Dudley and Kolasinski 2009a, b). This suggests that the presence of a metal catalyst might lead to enhanced etching for ClO4
just as it does for H2O2. Potential oxidants that have yet to be attempted include Co3+, Ru(CN)63
, and UO2+. Co3+ complexes are explosive (Smirnov et al. 2004). In comparison, the expense of Ru(CN)63
and the potential radioactivity of UO2+ seem appealing. Metals such as Cu, Ag, Au, Rh, Pd, Pt, Hg, and Tl that will plate out onto Si (Ogata and Kobayashi 2006; Allongue and Maroun 2006) have not been included in Table 1. The halogens and halogenates are extremely reactive with Si when mixed with HF solutions (Kolasinski and Gogola 2012). In particular, the iodate ion IO3
produces extremely high etch rates when ethanol is added to the etchant to avoid the precipitation of I2. However, the halogens are able to react thermally with

Yes Yes

Yes Yes

Hydrothermal etching Rapid, uniform, thick layers, visible PL

0.957 0.991

NO3
VO2+

Yes Yes

Slow etch rate, no visible PL Irreproducible, visible PL

Yes Yes

0.8665 0.957

Yes

Yes

Comments Destroys por-Si Not yet attempted Uniform, thick, visible PL

Not yet attempted

porSi No

Etch Yes

0.86

E /V 0.5355 0.612 0.771

Ru (CN)63
IrCl62
NO3


Oxidant I2 UO2+ Fe3+

Kolasinski et al. (2012) Turner (1960), Archer (1960), Beckmann (1965), Beale et al. (1986), Shih et al. (1992, 1993), McCord et al. (1992), Kidder et al. (1992), Dubbelday et al. (1993), Steckl et al. (1994), Chandler-Henderson et al. (1994), Liu et al. (1994, 2007), Kalem and Rosenbauer (1995), Anaple et al. (1995), Amato (1995), Schoisswohl et al. (1995), Jones et al. (1995), Di Francia et al. (1995), Di Francia and Citarella (1995), Winton et al. (1996, 1997), Velasco (2003), GuerreroLemus et al. (2003), González-Dı´az et al. (2006), Zeng et al. (2005), Melnichenko et al. (2005), Luchenko et al. (2007), Balaguer and Matveeva (2010); Mogoda et al. (2011), Lippold et al. (2011, 2012), Terheiden et al. (2011) Chen et al. (1996, 2001), Li et al. (1999) Kolasinski et al. (2010, 2012), Dudley and Kolasinski (2009a), Kolasinski and Barclay (2013a, b), Kolasinski (2010), Kolasinski and Yadlovskiy (2011), Kolasinski and Gogola 2011)

Loni et al. (2011), Nahidi and Kolasinski (2006), Kolasinski et al. (2010, 2012), Dudley and Kolasinski (2009a), Kolasinski (2010)

Reference Gerischer and L€ ubke (1988), Kolasinski and Gogola (2012)

Table 1 Oxidants that may be or have been proven to be capable of initiating etching by hole injection into silicon (E values taken from Haynes 2010)

40 K.W. Kolasinski

Yes Yes Yes

1.3583 1.47 1.482

1.507

1.72

1.776

1.92 2.01

Cl2 ClO3
BrO3


MnO4


Ce4+

H2O2

Co3+ S2O82


Yes

Yes

Yes

Yes

No Yes Yes

1.189 1.195 1.350

ClO4
IO3
HCrO4


Yes

1.0873

Br2

No

No

Yes

Yes

No No No

No Yes Yes

No

Not yet attempted

Dissolved in conc H2SO4, visible PL only after a few days, uniform, slow Significant etching only with metal catalyst

Patchy and irreproducible

Destroys por-Si Destroys por-Si Destroys por-Si

Minimal etching maybe By-products destroy por-Si Patchy, irreproducible

Destroys por-Si

Kolasinski and Gogola (2012)

Kolasinski and Gogola (2012), Meltzer and Mandler (1995), Bressers et al. (1996), Zhang et al. (2006) Dudley and Kolasinski (2009b) Kolasinski and Gogola (2012) Fathauer et al. (1992), Nahidi and Kolasinski (2006), Jenkins (1977), Beale et al. (1986), Kelly et al. (1994), Safi et al. (2002), van den Meerakker and van Vegchel (1989a, b), Gabouze et al. (2003), Sirtl and Adler (1961), Secco d’Aragona (1972), Hoshino and Adachi (2007) Kolasinski and Gogola (2012) Kolasinski and Gogola (2012) Nahm et al. (1997), Kolasinski and Gogola (2012), Seo et al. (1993) Nahidi and Kolasinski (2006), Mogoda et al. (2011), Kelly et al. (1994), Nahm et al. (1997) Kolasinski et al. (2012), Dudley and Kolasinski (2009a), Kolasinski (2010) Ashruf et al. (1999), Li and Bohn (2000), Rao et al. (2007), Nielsen et al. (2007), Huang et al. (2011), Li (2012)

Porous Silicon Formation by Stain Etching 41

42

K.W. Kolasinski

Si and do so in a manner that destroys por-Si films. On the other hand, Xu and Adachi (Xu and Adachi 2006) investigated etching in KIO3 + HF solutions. While they observed no por-Si formation in the dark, with illumination at 600 nm from a Xe lamp, they reported the formation of a photoluminescent film. The addition of surfactants to stain etchants has occasionally been attempted. Sometimes this resulted in rather thin films (Vázsonyi et al. 2001). Acetic acid (Robbins and Schwartz 1960; Jenkins 1977) or ethanol can be used in this respect. Caution: Ethanol reacts with the oxidant, particularly HNO3. Such solutions should never be placed in a closed container. Whereas ethanol addition is commonly used during anodic formation of por-Si to reduce the deleterious effects of bubbles, its use in stain etching is rather limited. The apparent reduction of bubble formation and sticking to the substrate is more closely related to a strong reduction in the etch rate, rather than a superlative surfactant action (Kolasinski et al. 2010).

Rational Formulation of Stain Etchants Through experiments on a broad range of oxidants and a better understanding of the fundamentals of the charge transfer process, Kolasinski developed a set of guidelines for the formulation of effective stain etchants (Nahidi and Kolasinski 2006; Kolasinski et al. 2010, 2012; Dudley and Kolasinski 2009a; Kolasinski and Gogola 2012; Kolasinski 2005, 2010, 2014). • The oxidant must be able to inject holes into the Si valence band at an appreciable rate; thus, its electrochemical potential should be more positive than approximately +0.7 V. • The fluoride solution must be acidic to avoid OH
catalyzed etching. • Oxide formation needs to be slow or nonexistent. • Sufficiently high fluoride concentration compared to the oxidant concentration avoids electropolishing caused by the buildup of oxide. • The oxidant and all products must be soluble. Not only must the oxidant be soluble so as to collide with the surface and inject holes, but also metals or other products should not plate out on the surface. • Film homogeneity is enhanced if the oxidant’s half-reaction does not evolve gas. • The net etching reaction from hole injection to Si atom removal (including the reactions of any by-products) has to be sufficiently anisotropic (attacking all kinds of sites but only at the bottom of the pore) to support pore nucleation and propagation.

Practical Advice • Cleanliness, especially regarding removal of organic hydrophobic contaminants, is essential. Teflon beakers and crystals have to be degreased before etching. Ultrasonication in acetone then ethanol then water is usually sufficient.

Porous Silicon Formation by Stain Etching

43

• Keep cleaned crystals in distilled water and avoid Langmuir-Blodgett film deposition. Do not ruin a perfectly good clean crystal by pulling it through the surface film that naturally forms on water exposed to the atmosphere. Before removing a crystal from water, always break the surface layer by agitating the top of the water with forceps. The semiconductor industry does this routinely by using overflowing baths. • Get a tank of argon and use it liberally. Samples should be dried in streaming Ar. Argon is heavier than air and, therefore, preferable to N2 because it forms a buffer layer above solutions that helps avoid Langmuir film formation. Bubbling Ar through etchants not only sparges them of dissolved O2 and CO2 but also stirs the etchant and helps remove H2 when large-scale etching is performed. Filling the headspace in HF bottles and distilled water containers with Ar helps to reduce dissolved O2. Covering etchants, particularly Fe3+-based etchants, with a buffer layer of Ar helps to avoid precipitation for long etch times. • For uniform thick films, slow etching using VO2+ or Fe3+ with low H2 production is preferred. Critical point drying is a must (see chapter “▶ Drying Techniques Applied to Porous Silicon”). • V2O5 dissolves in concentrated HF(aq) but not water. To make diluted solutions, always dissolve first in concentrated HF then add water.

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Nahm KS, Seo YH, Lee HJ (1997) Formation mechanism of stains during Si etching reaction in HF-oxidizing agent-H2O solutions. J Appl Phys 81:2418 Nielsen D, Abuhassan L, Alchihabi M, Al-Muhanna A, Host J, Nayfeh MH (2007) Current-less anodization of intrinsic silicon powder grains: formation of fluorescent Si nanoparticles. J Appl Phys 101:114302 Ogata YH, Kobayashi K (2006) Electrochemical metal deposition on silicon. Curr Opin Solid State Mater Sci 10:163–172 Parbukov AN, Beklemyshev VI, Gontar VM, Makhonin II, Gavrilov SA, Bayliss SC (2001) The production of a novel stain-etched porous silicon, metallization of the porous surface and application in hydrocarbon sensors. Mater Sci Eng C 15:121–123 Patzig S, Roewer G, Kroke E, Ro¨ver I (2007) NOHSO4/HF – a novel etching system for crystalline silicon. Z Naturforsch B Chem Sci 62:1411–1421 Rao S, Mantey K, Therrien J, Smith A, Nayfeh M (2007) Molecular behavior in the vibronic and excitonic properties of hydrogenated silicon nanoparticles. Phys Rev B 76:155316 Robbins H, Schwartz B (1959) Chemical etching of silicon. I. The system HF, HNO3, and H2O. J Electrochem Soc 106:505–508 Robbins H, Schwartz B (1960) Chemical etching of silicon. II. The system HF, HNO3, H2O and HC2H3O2. J Electrochem Soc 107:108–111 Robbins H, Schwartz B (1961) Chemical etching of silicon. III. A temperature study in the acid system. J Electrochem Soc 108:365–372 Safi M, Chazalviel J-N, Cherkaoui M, Belaı¨di A, Gorochov O (2002) Etching of n-type silicon in (HF + oxidant) solutions: in situ characterisation of surface chemistry. Electrochim Acta 47:2573–2581 Sarathy J, Shih S, Jung K, Tsai C, Li K-H, Kwong D-L, Campbell JC, Yau S-L, Bard AJ (1992) Demonstration of photoluminescence in nonanodized silicon. Appl Phys Lett 60:1532–1534 Sato K, Tsuji H, Hirakuri K, Fukata N, Yamauchi Y (2009a) Controlled chemical etching for silicon nanocrystals with wavelength-tunable photoluminescence. Chem Commun 3759–3761 Sato K, Tsuji H, Fukata N, Hirakuri K, Yamauchi Y (2009b) Facile preparation of red luminescent silicon nanocrystals via controlled chemical etching. Chem Lett 38:558–559 Schirone L, Sotgiu G, Califano FP (1997) Chemically etched porous silicon as an anti-reflection coating for high efficiency solar cells. Thin Solid Films 297:296–298 Schirone L, Sotgiu G, Montecchi M (2000) Towards the morphology control of stain etched porous silicon. J Porous Mater 7:405–408 Schoisswohl M, Cantin JL, von Bardeleben HJ, Amato G (1995) Electron paramagnetic resonance study of luminescent stain etched porous silicon. Appl Phys Lett 66:3660–3662 Schwartz B, Robbins H (1976) Chemical etching of silicon IV. Etching technology. J Electrochem Soc 123:1903–1909 Secco d’Aragona F (1972) Dislocation etch for (100) planes in silicon. J Electrochem Soc 119:948–951 Seo YH, Nahm KS, Lee KB (1993) Mechanistic study of silicon etching in HF-KBrO3-H2O solution. J Electrochem Soc 140:1453–1458 Shih S, Jung KH, Hsieh TY, Sarathy J, Campbell JC, Kwong DL (1992) Photoluminescence and formation mechanism of chemically etched silicon. Appl Phys Lett 60:1863–1865 Shih S, Jung KH, Qian RZ, Kwong DL (1993) Transmission electron-microscopy study of chemically etched, porous Si. Appl Phys Lett 62:467–469 ¨ tzgrubenentwicklung Sirtl E, Adler A (1961) Chroms€aure-Flußs€aure als spezifisches System zur A auf silizium. Z Metallkd 52:529–531 Smirnov AV, Ilyushin MA, Tselinskii IV (2004) Synthesis of cobalt(III) ammine complexes as explosives for safe priming charges. Russ J Appl Chem 77:794–796 Steckl AJ, Xu J, Mogul HC (1994) Crystallinity and photoluminescence in stain-etched porous Si. J Electrochem Soc 141:674–679 Steiner P, Lang W (1995) Micromachining applications of porous silicon. Thin Solid Films 255:52–58

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Steinert M, Acker J, Henssge A, Wetzig K (2005) Experimental studies on the mechanism of wet chemical etching of silicon in HF/HNO3 mixtures. J Electrochem Soc 152:C843–C850 Steinert M, Acker J, Krause M, Oswald S, Wetzig K (2006) Reactive species generated during wet chemical etching of silicon in HF/HNO3 mixtures. J Phys Chem B 110:11377–11382 Steinert M, Acker J, Oswald S, Wetzig K (2007) Study on the mechanism of silicon etching in HNO3-rich HF/HNO3 mixtures. J Phys Chem C 111:2133–2140 Steinert M, Acker J, Wetzig K (2008) New aspects on the reduction of nitric acid during wet chemical etching of silicon in concentrated HF/HNO3 mixtures. J Phys Chem C 112:14139–14144 Striemer CC, Fauchet PM (2002) Dynamic etching of silicon for broadband antireflection applications. Appl Phys Lett 81:2980–2982 Svetovoy VB, Berenschot JW, Elwenspoek MC (2006) Precise test of the diffusion-controlled wet isotropic etching of silicon via circular mask openings. J Electrochem Soc 153:C641–C647 Terheiden B, Hensen J, Wolf A, Horbelt R, Plagwitz H, Brendel R (2011) Layer transfer from chemically etched 150 mm porous Si substrates. Materials 4:941–952 Turner DR (1960) On the mechanism of chemically etching germanium and silicon. J Electrochem Soc 107:810–816 ¨ nal B, Parbukov AN, Bayliss SC (2001) Photovoltaic properties of a novel stain etched porous U silicon and its application in photosensitive devices. Opt Mater 17:79–82 van den Meerakker JEAM, van Vegchel JHC (1989a) Silicon etching in CrO3-HF solutions I. High [HF]/[CrO3] ratios. J Electrochem Soc 136:1949–1953 van den Meerakker JEAM, van Vegchel JHC (1989b) Silicon etching in CrO3-HF solutions II. Low [HF]/[CrO3] ratios. J Electrochem Soc 136:1954–1957 Vázsonyi E´, Szilágyi E, Petrik P, Horváth ZE, Lohner T, Fried M, Jalsovszky G (2001) Porous silicon formation by stain etching. Thin Solid Films 388:295–302 Velasco JG (2003) Open-circuit study of stain etching processes leading to the formation of porous silicon layers. J Electrochem Soc 150:C335–C341 Weinreich W, Acker J, Graber I (2007) Determination of total fluoride in HF/HNO3/H2SiF6 etch solutions by new potentiometric titration methods. Talanta 71:1901–1905 Winton MJ, Russell SD, Wolk JA, Gronsky R (1996) Processing independent photoluminescence response of chemically etched porous silicon. Appl Phys Lett 69:4026–4028 Winton MJ, Russell SD, Gronsky R (1997) Observation of competing etches in chemically etched porous silicon. J Appl Phys 82:436–441 Woo TK, Kim SI, Kim SE, Ahn HS (2012a) Three-dimensional dual-porous structure developed by preferential/stain etching on grind-damaged (001) Si: formation and optical properties. J Electrochem Soc 159:P1–P7 Woo TK, Kim SE, Ahn HS (2012b) Experimentally derived catalytic etching kinetics for defectutilized dual-porous silicon formation. J Phys Chem C 116:7040–7049 Xu YK, Adachi S (2006) Light-emitting porous silicon formed by photoetching in aqueous HF/KIO3 solution. J Phys D Appl Phys 39:4572–4577 Xu YK, Adachi S (2007) Properties of light-emitting porous silicon photoetched in aqueous HF/FeCl3 solution. J Appl Phys 101:103509 Yamamura K, Mitani T (2008) Etching characteristics of local wet etching of silicon in HF/HNO3 mixtures. Surf Interface Anal 40:1011–1013 Yoshioka S (1969) Investigation of the chemical properties of stain films on silicon by means of infrared spectroscopy and omegatron mass analysis. Philips Res Rep 24:299–321 Zeng FG, Zhu CC, Fu XN, Wang WW, Zhao ZM (2005) Preparation of co-passivated porous silicon by stain etching. Mater Chem Phys 90:310–314 Zhang L, Ma XZ, Lin MX, Lin Y, Cao GH, Tang J, Tian ZW (2006) A comparative study on electrochemical micromachining of n-GaAs and p-Si by using confined etchant layer technique. J Phys Chem B 110:18432–18439

Porous Silicon Formation by Metal Nanoparticle-Assisted Etching Claude Le´vy-Cle´ment

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-Step Metal-Assisted Etching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-Step Metal-Assisted Etching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reviews . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

49 50 50 58 59 59

Abstract

Essential aspects in the fabrication of porous silicon and nanostructures by metal-assisted particle etching are presented. Basic processes using 1-step or 2-step method are described as well as mechanism of metal-assisted chemical etching. Influence of various parameters such as nature of metal, temperature, etching solution composition, intrinsic properties of silicon substrate on the morphology of porous silicon, or nanostructures is discussed. Applications of silicon nanostructures obtained by metal-assisted etching are briefly introduced, showing the promising potential of this etching method whose main properties are simplicity, low cost, easy process control, reproducibility, and reliability for fabrication of silicon nanostructures including silicon nanowires.

Introduction Metal-assisted chemical etching (MAE or MACE) is based on a localized oxidation and dissolution of silicon in HF in the presence of an oxidizing agent, whereas the metal (generally noble metal) catalytically enhances the etching process C. Le´vy-Cle´ment (*) Institut de Chimie et des Mate´riaux Paris-Est, CNRS, Thiais, France e-mail: [email protected] # Springer International Publishing Switzerland 2014 L. Canham (ed.), Handbook of Porous Silicon, DOI 10.1007/978-3-319-05744-6_5

49

C. Le´vy-Cle´ment

50

(Dimova-Malinovska et al. 1997; Li and Bohn 2000; Xia et al. 2000; Harada et al. 2001; Chattopadhyay et al. 2002; Peng et al. 2002; Hadjersi et al. 2004; Qiu et al. 2005; Tsujino and Matsumura 2005a, b; Yae et al. 2003). The Si underneath the metal is etched much faster than that without metal coverage. As a result the metal sinks into the Si substrate generating pores into Si substrate or Si nanowires (SiNWs) corresponding to the un-etched Si between pores (Chapter “▶ MACE Silicon Nanostructures” on SiNWs). The metal, especially noble metals, may be dissolved in HF (1-step MAE method) or deposited on silicon, as nanoparticles or thin films, before the etching process in HF solution containing an oxidizing agent (2-step MAE method).

1-Step Metal-Assisted Etching The metal deposition and etching are carried out in the same chemical solution (Fig. 1). A typical solution is 0.02 M AgNO3 + 4.6 M HF. This method leads to the formation of porous Si (Qiu et al. 2005; Peng et al. 2003; Peng and Zhu 2004) or pillar-like or craterlike microstructures (Peng et al. 2003) or arrays of silicon nanowires (SiNWs) standing vertically on the Si substrate (Peng et al. 2002, 2003, 2004, 2005a, 2006a; Peng and Zhu 2003, 2004; Cheng et al. 2008; Benoit et al. 2008). Metal ions are dissolved in HF. When the redox potential of the ions is more positive than the valence band of Si, a galvanic reaction occurs in which the ions are reduced to metal as particles, dendrites, and film, while the Si is oxidized and dissolved in HF (case of noble metals) following the reaction: 

4 Mþ ðaqÞ þ Si0 ðsÞ þ 6F ! 4 MðsÞ þ SiF6 2 ðaqÞ

(1)

The formation of dendrites in the case of Ag and Au is at the origin of the formation of SiNWs. AgNO3 (Peng et al. 2002, 2003, 2004, 2006a; Peng and Zhu 2004; Cheng et al. 2008; Benoit et al. 2008; Smith et al. 2013), KAuCl4 or HAuCl4 (Peng et al. 2002; Qiu et al. 2005; Peng and Zhu 2003, 2004), and K2PtCl6 or H2PtCl6 (Peng et al. 2003) have been used, as well as other nitrate metal salts (Cu, Ni, Mn, Fe, Co, Cr, Mg) (Peng et al. 2002, 2003; Table 1). It is a very easy method to produce Si nanostructures especially nanowires, but there is little possibility to control their dimension and homogeneity as well for the Si microstructures.

2-Step Metal-Assisted Etching The metal particles are deposited on the Si surface generally by electroless metal deposition (EMD) or chemical vapor deposition or sputtering, prior to the etching in the HF solution in the presence of an oxidizing agent. During the etching, dissolution of Si underneath the metal particles is strongly enhanced, and pores are formed while the particles sink into the Si pores (Li and Bohn 2000; Harada et al. 2001; Chattopadhyay et al. 2002; Tsujino and Matsumura 2005a, b;

Porous Silicon Formation by Metal Nanoparticle-Assisted Etching

51

In situ metal particle deposition Si Substrate

+ Si localized dissolution

Si nanopores or nanowires + Metal nanoparticles

Fig. 1 Schematic of 1-step metal-assisted etching

Yae et al. 2003; Fig. 2). The etching can be done in various HF solutions containing an oxidizing agent, typically H2O2 (Li and Bohn 2000; Tsujino and Matsumura 2005a, b, 2006a, 2007; Peng et al. 2008; Huang et al. 2007; Chartier et al. 2008; Lee et al. 2008; Megouda et al. 2009a). Other oxidizing agents such as Fe(NO3)3 (Peng et al. 2005b, 2006b), Mg(NO3)2 (Peng et al. 2006b), Na2S2O8 (Hadjersi et al. 2004, 2005a; Douani et al. 2008; Hadjersi 2007), KMnO4 (Hadjersi et al. 2004, 2005a; Douani et al. 2008), K2Cr2O7 (Douani et al. 2008; Hadjersi et al. 2005b; Waheed et al. 2010), KBrO3 or KIO3 (Waheed et al. 2010), Co(NO3)2 (Megouda et al. 2009b), molecular O2 dissolved in H2O (Yae et al. 2003, 2005, 2006, 2008a, b, 2010a, 2012; Masayuki et al. 2011), and electrical holes, h+ by anodization (Zhao et al. 2007; Chouroua et al. 2010; Huang et al. 2010a), are also used. The deposited metals under the form of nanoparticles or colloidal particles or patterned thin film are most generally noble metals such as Ag (Hadjersi et al. 2004, 2005b; Tsujino and Matsumura 2005a, 2006a, 2007; Peng et al. 2005b, 2006b, 2008; Huang et al. 2007; Chartier et al. 2008; Lee et al. 2008; Douani et al. 2008; Hadjersi 2007; Waheed et al. 2010; Yae et al. 2005; Yang et al. 2008; Asoh et al. 2007a), Au (Li and Bohn 2000; Qiu et al. 2005; Peng et al. 2008; Lee et al. 2008; Megouda et al. 2009a; Yae et al. 2005; Zhao et al. 2007; Bauer et al. 2010), Pt (Li and Bohn 2000; Xia et al. 2000; Chattopadhyay et al. 2002; Tsujino and Matsumura 2005a, b; 2006a; Lee et al. 2008; Yae et al. 2005, 2006, 2012), Pd (Hadjersi et al. 2004; Tsujino and Matsumura 2005a; Waheed et al. 2010; Yae et al. 2005, 2008a, b; 2010a, 2012; Masayuki et al. 2011), Pd-Pt (Li and Bohn 2000; Asoh et al. 2008, 2009), or Rh (Yae et al. 2012), but other metals such as Al (Dimova-Malinovska et al. 1997), Bi (Megouda et al. 2009b), Cu (Tsujino and Matsumura 2005a; Peng et al. 2008; Mitsugi and Nagai 2004), Ni (Zhao et al. 2007), and Fe can also be used. The chemical or electrochemical reactions occur preferentially near the noble metal. By analogy to electrochemical formation of porous Si, the role of the cathode is attributed to the metal and that of the anode to the silicon underneath the metal particle. The oxidant is reduced at the metal particle with production of electrical holes (Li and Bohn 2000; Peng et al. 2008; Tsujino and Matsumura 2007; Chartier et al. 2008). As an example when the oxidizing agent is H2O2, the reaction at the cathode is the following: H2 O2 þ 2Hþ ! 2H2 O þ 2hþ

(2)

At the interface between Si and the metal particle, the holes, h+, are injected into the valence band of Si, which is dissolved as SiF62. Depending on H2O2 concentration, the localized Si dissolution occurs as either a two-hole or four-hole process

5M

0.01 0.2 M

0.2 M

0.08–0.15 M

K2PtCl6

CuNO3

NiNO3 MnNO3 FeNO3 CoNO3 CrNO3 Mg (NO3)2

12 M

12 M 5M

0.02

KAuCl4

HF M 4.5–5

5M 5M 5M NH4F 5M 5M

Metal ion M 0.02 M

0.015 M 0.02 M Idem Idem

Catalyst AgNO3

140 60 min

50

50 10–30 min 50 50

50 120 15

Temp  C/ etching time 50 20–60 min

Au dendrites Continuous Pt grain films Continuous Cu grain films No metal deposition

Ag dendrites Idem Idem Continuous Ag grain film Au nanowhiskers

Metal deposit Ag dendrites

Porous Si consisting of μm pillars, cones, craters pits nanostructures

Disordered shallow pits

SiNWs Disordered shallow pits

Disordered shallow pits

Si morphology SiNWs 10–30 μm long 30–300 nm wide Regular porous Disordered porous μm stalagmites Uniformly etched

Table 1 Various morphologies produced on single crystal Si by the 1-step metal-assisted etching method

Peng et al. (2002, 2003)

Peng et al. (2003)

Peng et al. (2003)

Peng and Zhu (2003, 2004) Peng et al. (2003)

Qiu et al. (2005)

Peng et al. (2003) Peng and Zhu (2004) Peng et al. (2003) Peng et al. (2003)

References Peng et al. (2002, 2003, 2006a), Cheng et al. (2008)

52 C. Le´vy-Cle´ment

Porous Silicon Formation by Metal Nanoparticle-Assisted Etching Selective Dissolution of silicon

Metal deposition Si Substrate

53

Metal Nanoparticles

Pore formation

Fig. 2 Schematic of 2-step metal-assisted etching

with direct dissolution of Si in a divalent or tetravalent state, respectively. H2 evolution occurs during metal-assisted etching and has been first attributed to H+ proton reduction (Li and Bohn 2000), but then latter it has been attributed to an anode reaction due to the strong similitude between stain etching in HF/HNO3 and metal-assisted etching (Chartier et al. 2008). A mixed reaction composed of divalent and tetravalent dissolution for the dissolution of Si in MAE, which takes also into account the H2 evolution, is proposed for the anodic reaction: Si þ 6HF þ nhþ ! H2 SiF6 þ nHþ þ




 4  n =2 H2 "

(3)

The overall reaction being Si þ 6HF þ n=2H2 O2 ! H2 SiF6 þ nH2 O þ




 4  n =2 H2 "

(4)

The morphology of the Si nanostructures depends on various etching conditions such as the metal used (chemical nature and initial morphology), etching solution composition, etching time, temperature, and Si properties (crystallography, orientation doping density, conductivity type) (Table 2). The main results using metal-assisted etching are summarized: – The Si dissolution rate increases from Ag, Au, Pt, Pd, to Rh in the presence of HF/H2O2 or HF/O2 (Yae et al. 2012; Asoh et al. 2009). – The morphology of metal-assisted etched structures is defined by the shape of the metal catalyst. Well-separated metal particles usually result in well-defined pores (Tsujino and Matsumura 2005a, b; Chartier et al. 2008; Bauer et al. 2010). In the case of etching solutions also used in stain etching, it is possible that porous Si forms in the regions without noble metal. – When the metal particles (20–200 nm) are randomly distributed on Si, a disordered network of pores is formed into Si surface in the case of Ag, Au, and Pt, after etching in HF/H2O2 (Tsujino and Matsumura 2005a), HF/O2 (Yae et al. 2005) and HF/Fe(NO)3 (Peng et al. 2006b). Metal particles are observed at the bottom of the pores. Various porous Si structures are found depending on the nature of the metal and etching conditions (Fig. 3). The pore diameter is the same as that of Ag or Au particles, whereas for Pt the pores are cone shaped of micrometer size and lined with mesoporous Si (Tsujino and Matsumura 2005a, b). For Pd a polishing occurs when the etching is done in 7.3 M HF/molecular O2 (Yae et al. 2005, 2010a). When the distances between noble metals in the case of Ag and Au are small, the

H2O2 0.9 M

0.6 % H2O2

Idem

Idem H2O2 3.3 M

Idem

H2O2 5 M

Pt

Ag EMD

Ag colloı¨dal particles Pt EMD Au ϕ ¼ 3 nm, 6 mm pitch,

Au/Pd (3–20 nm) Pt

Au

Particle Ag EMD 30–100 nm

Oxidizing agent H2O2 2.72 M

7.9 M ρ ¼ 61 %

Idem

10.8 M ρ ¼ 77 %

HF M 28.9 M ρ ¼ 91 % 28.9 M ρ ¼ 97 % 10 %

5 min

30 s

30 s

50 30 min

30 min

Temp  C/ etching time 25 30 min

n+Si, p-Si p+-Si p-Si

p-Si, 7–14 p+-Si (100)

p-Si, (100) (113), (110) 7–14 Ω cm (100)

(100) (111)

Si type resistivity Ω cm p-Si (100)

Idem, etching depth ¼ 10 nm Columnar structures, 1 μm deep Macropores ϕ ¼ 100 nm Similar to Ref. (Li and Bohn 2000), but bigger structures

Narrow straight cylindrical nanoholes 35 μm long Tortuous channels Nanopores ϕ ¼ 30 nm, ⊥ Si surface, interconnected, etching depth ¼ 350 nm

SiNWs vertical SiNWs slanted to Si surface

μm spherical cavities + 3 μm thick porous Si Cylindrical holes inclined to Si surface

Nanostructure Pores 5–40 μm long, covered with microporous Si

Table 2 Examples of various Si morphologies obtained by 2-step metal-assisted etching, the symbol f is used for diameter

Chattopadhyay et al. (2002)

Li and Bohn (2000)

Peng et al. (2008)

References Tsujino and Matsumura (2005a)

54 C. Le´vy-Cle´ment

Fe(NO)3 0.2 M

K2Cr2O7, 0.05 M KMnO4 0.05 M Na2S2O8 0.05 M Co(NO3)2 0.03 M

Ag EMD Pt EMD

Ag (20 nm) Vacuum evaporation

Bi (20 nm) Vacuum evaporation

O2 (5 ppm)

Pt electrodeposited 0.05–0.2 μm

22.5 M

22.5 M

5M

7.3 M

50 15 min 60 min

10 min 30 min 30 min

25 24 H 50 50 min

p-Si (100) 100 Ω cm

n- and p-Si 1 Ω cm p-Si (111) p-(100) 3–6 Ω cm n-Si (100) 1.6 Ω cm

Etch pits ϕ ¼ 9 μm contain pillars and porous Si

Etch pits 100 nm deep

Winding macropores ϕ ¼ 0.36 μm, covered with microporous Si Megouda et al. (2009b)

Douani et al. (2008)

Cylindrical macropores ϕ ¼ 0.4 and 2 μm covered with microporous Si Etch pits ϕ ¼ 0.15 μm

Peng et al. (2006b)

Yae et al. (2005)

SiNWs array Separated deep pores

Macropores ϕ ¼ 1 μm covered with microporous Si

Porous Silicon Formation by Metal Nanoparticle-Assisted Etching 55

C. Le´vy-Cle´ment

Fig. 3 (continued)

56

Porous Silicon Formation by Metal Nanoparticle-Assisted Etching

57

structures evolve from pores into wall-like or wirelike structures vertically standing with diameter between 100 and 200 nm and several microns long (Peng et al. 2005b, 2006b, 2008). – The etching solution composition has a strong influence on the metal-assisted etched Si morphology. In the case of the HF/H2O2 solution, a unified notation with ρ ¼ [HF]/([HF] + [H2O2]) allows to compare the conditions and resulting morphology published in the literature (Chartier et al. 2008). Different regimes of Si dissolution are distinguished leading to the formation of cylindrical pores for low oxidizing agent concentration (ρ > 70 %), to cone-shaped pores with walls lined with porous Si for medium oxidizing agent concentration (20 % < ρ < 70 %), and to craters (ρ < 20 %) and polishing for high H2O2 concentration (ρ < 7 %) (Chartier et al. 2008). For 20 % < ρ < 100 % a duplex structure is formed at the surface of the Si wafer. The top layer is micro-mesoporous, and once dissolved in NaOH, a macrotexturized surface (from pores to craters) is observed. At ρ  80 % the maximum etching rate is reached and n equals 3 in the overall reaction 4 meaning that the etching process occurs as a mixed 2 and 4 electrons. Switching from cylindrical pores to helical is accomplished by changing the solution concentration. The generation of helical holes is attributed to the microscopically difference in etching rate of silicon on a Pt particle (Tsujino and Matsumura 2005b). – When Ag, Au, and Pd/Pt metal particles are deposited in ordered arrays through a mask as a patterned thin film (~20–40 nm thick), quasi-ordered silicon micro-/ nanostructures (pores) are formed in silicon, whatever is the kind of deposited noble metal (Huang et al. 2007; Yae et al. 2010a; Asoh et al. 2007a, b, c, 2008, 2009; Bauer et al. 2010; Peng et al. 2007; Ono et al. 2007, 2009; Le´vy-Cle´ment ä Fig. 3 SEM images (plan view and cross section). (a) Ag particles deposited by EMD in 5  104 M AgNO3 + 0.14 M HF during 5 min (Chartier et al. 2008); (b) porous Si, Ag deposited on (Li et al. 2012) p-type Si etched in HF/H2O2 (ρ ¼ 80 %) for 30 s (Chartier et al. 2008); (c) straight and curved cylindrical pores with Ag particles at their bottom, same as (b) at higher magnification; (d) cone-shaped macropores lined with microporous Si, Ag deposited on p-Si etched in HF/H2O2 (ρ ¼ 20 %) for 20 s (Chartier et al. 2008); (e) array of SiNWs, Ag deposited on Si etched in HF/H2O2 (ρ ¼ 70 %) for 5 min (Le´vy-Cle´ment and Chartier 2013); (f) typical helical pore, Au deposited on Si etched in HF/H2O2 (ρ ¼ 97 %) for 5 min (from Tsujino and Matsumura 2005b, with permission); (g) cone-shaped pore with Au aggregates at the bottom, Au EMD deposited on Si etched in HF/H2O2 (ρ ¼ 97 %) for 1 h (from Lee et al. 2008, with permission); (h) macroporous Si, Ag deposited on Si etched in HF/H2O2 (ρ ¼ 70 %) for 15 min, after dissolution of microporous Si in diluted KOH. Si etched thickness is 8 μm (Le´vyCle´ment and Chartier 2013); (i) cone-shaped pores with a Pt particle at the bottom, Pt deposited on Si etched in HF/H2O2 (ρ ¼ 70 %) for 15 min (Le´vy-Cle´ment and Chartier 2013); (j) ordered array of macropores with uniform diameter, Pt-Pd-coated Si etched in HF/H2O2 for 1 min; polystyrene sphere honeycomb mask periodicity is 3 μm (from Asoh et al. 2009, with permission); (k) array of SiNWs (60 nm ϕ and 4.5 μm long), 40 nm Ag film deposited with polystyrene sphere mask (ϕ ¼ 100 nm) on Si etched in HF/H2O2 (ρ ¼ 91 %) for 10 min (Le´vy-Cle´ment and Chartier 2013); (l) porous Si, Ag deposited on Si etched in HF/Na2S2O8 (22.5 M/0.15 M) for 10 min (from Hadjersi 2007, with permission)

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et al. 2011; Boarino et al. 2011; Pacholski 2011; Geng et al. 2011; Hildreth et al. 2009, 2013; Rykaczewski et al. 2011; Scheeler et al. 2012; Geyer et al. 2012, 2013; G€uder et al. 2013; Wang et al. 2013a). This observation is the basis of the fabrication of a rich variety of ordered Si structures with different length scales, as topographic lines, concentric rings, square arrays, etc., can be created at a micro- and nanoscale after etching the silicon and subsequent removal of the mask. This method is widely used to form regular arrays of macropores and SiNWs with controllable diameter, length, and density (Huang et al. 2007; Peng et al. 2007). – In situ formation of Au or Ag nanoparticles occurs by adding an Au colloidal solution (Branz et al. 2009; Li et al. 2012) or AgNO3 (Lu and Barron 2013) in the HF/H2O2/H2O solution allowing to transform the 2-step method directly in a 1-step metal-assisted etching. The H2O2 plays a dual role in reducing the Au3+ and Ag+ ions into nanoparticles onto Si wafer and facilitating the Si etching. Mesopores of 20–100 nm diameter and 500 nm long are obtained when adding a sonication process (Lu and Barron 2013). A high concentration of H2O2 in the solution leads to electropolishing and the formation of ultrathin Si wafer (Bai et al. 2013). The advantages of using metal-assisted etching are numerous. The method is easy to handle and is suitable for batch fabrication of porous Si devices. Porous Si layers can be formed on highly resistive Si. Compared to stain-etched layers, those obtained by metal-assisted etching have better uniformity and much higher thickness. MAE can be used to make high surface-to-volume ratio structures, especially when associated with a lithography method to deposit patterned metal catalyst. Control of the orientation of Si nanostructures (e.g., pores, nanowires) relative to the substrate is achieved. There is no obvious limitation on the size of features fabricated by MAE, and fabrication of straight and well-defined pores or wires with diameters as small as 5 nm or as large as 1 μm can be obtained.

Applications Metal-assisted etching is frequently used to prepare photoluminescent porous Si (Dimova-Malinovska et al. 1997; Li and Bohn 2000; Harada et al. 2001; Chattopadhyay et al. 2002; Hadjersi et al. 2004, Hadjersi et al. 2005b, c; Hadjersi 2007; Megouda et al. 2009b; Zhao et al. 2007; Gorostiza et al. 1999; Chattopadhyay and Bohn 2006; Hadjersi and Gabouze 2007; Lipinski et al. 2009). MAE displays little crystallographic dependence and can be performed on crystalline or multicrystalline Si substrates, and the various Si morphologies and nanomicrostructures obtained are promising for photovoltaic applications in several areas: antireflective coating (Yae et al. 2003, 2005, 2006; Peng et al. 2005a, b; Benoit et al. 2008; Lu and Barron 2013; Tsujino and Matsumura 2006b; Chaoui et al. 2008; Nishioka et al. 2008, 2009; Srivastava et al. 2010; Cao et al. 2011; Kim et al. 2011, 2012; Geng et al. 2012; Wang et al. 2013b; Li et al. 2013a), texturization

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for multicrystalline wafers (Tsujino and Matsumura 2006a; Waheed et al. 2010; Branz et al. 2009; Li et al. 2012; Wan et al. 2008; Koynov et al. 2006, 2007; Lipin´ski 2008; Bastide et al. 2009; Yuan et al. 2009; Lin et al. 2010; Toor et al. 2011; Oh et al. 2012; Srivastava et al. 2012; Tang et al. 2013; Shi et al. 2013a, b; Hsu et al. 2012), porous emitter (Hadjersi and Gabouze 2008; Li et al. 2013b), advanced solar cells based on cylindrical macropores (Peng et al. 2010) or Si nanorods or nanowires (Garnett and Yang 2008), and layer detachment technique to prepare solar cells based on low-quality substrates or on ultrathin Si layers (Shiu et al. 2011; Lin et al. 2012). Silicon nanostructures obtained by MAE has been successfully used in lithium rechargeable batteries (Ripenbein et al. 2010; McSweeney et al. 2011; Liu et al. 2011), nanocapacitors (Chang et al. 2010), diffusion membrane applications (Cruz et al. 2005; Chen et al. 2011), formation of nanostructured adhesive metal film on porous Si surface (Yae et al. 2010b, 2011a, b), production of Si powders (Loni et al. 2011), porous Si nanowires with photocatalytic properties (Qu et al. 2009, 2010), and biosensing chips (Xiao et al. 2013). The catalytic properties of Pt have been used to develop a slicing method showing the possibility to produce Si wafers from an ingot (Salem et al. 2010) or to form through a hole in Si (Sugita et al. 2011). MAE combined with patterning of metal catalyst thin film deposition is widely used to form regularly organized nanostructures on Si (Yae et al. 2010a; Asoh et al. 2007a, b, c, 2008, 2009; Bauer et al. 2010; Peng et al. 2007; Ono et al. 2007, 2009; Pacholski 2011; Scheeler et al. 2012; Chattopadhyay and Bohn 2004; Hung et al. 2010; Lee et al. 2011) or arrays of vertically standing Si nanowires with controlled diameter and controlled distances between them (Huang et al. 2007, 2010b; Peng et al. 2007; Le´vy-Cle´ment et al. 2011; McSweeney et al. 2011; Qu et al. 2009, 2010; Zhang et al. 2008; Chang et al. 2009; de Boor et al. 2010).

Reviews Huang et al. recently did a comprehensive and systematic review of metal-assisted chemical etching mechanism and process parameters (Huang et al. 2011). The review by Ono is focused on combination of patterning methods of metal deposition and metal-assisted etching (Ono and Asoh 2012), whereas the review by Li is related to high aspect ratio structures with an emphasis on photovoltaic applications (Li 2012). Other papers contain a mini-review on metal-assisted etching (Kolasinski 2005; Qiu and Chu 2008; Korotcenkov and Cho 2010).

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Porous Silicon Formation by Photoetching Sadao Adachi

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photoetching Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . n-Si/Electrolyte Interface and Photoetching Reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PS Layers Formed by Photoetching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

67 68 68 70 73 73

Abstract

The literature on the photoetching technique of preparing photoluminescent mesoporous silicon films using both hydrofluoric acid-based and alkali electrolytes is reviewed. The benefits of using an incoherent light source and specific oxidizing agents are highlighted. The technique is particularly useful for creating thin porous regions in n-type Si wafers, SOI wafers, micromachined wafers, or those that contain electronic circuitry.

Introduction Visible photoluminescence (PL) from porous silicon (PS) observed at room temperature has inspired sustained research into its potential application in Si-based optoelectronic devices and its theoretical basis (Canham 1990). This property is reviewed in the handbook chapter “▶ Photoluminescence of Porous Silicon.” Most PS layers are prepared by anodic etching on p-type Si substrates, a technique in which

S. Adachi (*) Division of Electronics and Informatics, Faculty of Science and Technology, Gunma University, Gunma, Kiryu-shi, Japan e-mail: [email protected] # Springer International Publishing Switzerland 2014 L. Canham (ed.), Handbook of Porous Silicon, DOI 10.1007/978-3-319-05744-6_6

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metal is often deposited on the rear surface of the Si substrate in order for it to be used as an ohmic back contact (see handbook chapter “▶ Porous Silicon Formation by Anodization”). However, the requirement for a back contact electrode is a limitation of this method; for example, it is difficult to form a PS layer on a silicon-on-insulator (SOI) structure or on Si integrated circuits. A photoetching method, on the other hand, requires no electrodes and allows the formation of a visible luminescence layer on not only single-crystalline Si substrates but also SOI structures.

Photoetching Setup An experimental setup used for the formation of PS by photoetching is shown in Fig. 1 (Xu and Adachi 2006). The sample surface is illuminated by a Xe lamp through an optical filter that blocks wavelengths shorter than 600 nm. The use of an optical filter is to block the heat rays from the Xe lamp. A laser, a W lamp, or another light source may be used instead of a Xe lamp. The use of an incoherent light source such as a Xe or W lamp enables the formation of a large and homogeneous PS layer. Typically, an n-type Si wafer is immersed in an etchant solution of HF. The addition of an oxidant (e.g., H2O2 or I2) to the HF solution results in the stable formation of PS layers in a short time period.

n-Si/Electrolyte Interface and Photoetching Reaction Figure 2 shows the energy band diagrams for n-Si electrodes in pure HF (pH ¼ 2.3) and HF/oxidant solutions without and with light illumination (Xu and Adachi 2006). The electron affinity (χs) of Si is 4.05 eV. At zero pH, the redox coupling is defined as the normal hydrogen electrode with a potential of 4.5 eV with respect to vacuum. This potential shifts towards more positive values with the increase in pH (+0.059 eV/pH). Thus, the electron energy of the pure HF solution with respect to vacuum is 4.36 eV (χl). The Fermi levels (EF and EF,redox) on both sides of the n-Si/electrolyte interface are brought to the same energy level by a transfer of electrons from the Si substrate to the electrolyte (Fig. 2a). The half-reaction for the oxidizing agent KIO3 is þ   ΙΟ 3 þ 6Η þ 6e ¼ I þ 3H2 O



E ¼ 1:085 eV




where e represents the electron and Eo is the standard reduction potential with respect to the standard hydrogen electrode. The redox potential (Eabs) with respect to vacuum for the HF/KIO3 redox system is then given by Eabs ¼ 4.5  Eo ¼ 5.6 eV (Fig. 2c). It is to be noted that the larger the Eo value is in the positive (negative) scale, the stronger is the oxidation (reduction) agent (Adachi and Kubota 2007; Xu and Adachi 2007; Tomioka et al. 2007).

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Fig. 1 Experimental setup used for porous silicon formation by photoetching in an HF/oxidant solution

Fig. 2 Energy band diagram for n-Si immersed in pure HF solution (a, b) and those in HF/KIO3 solution (c, d). In (b), porous silicon (PS) is formed stably on the back side in opposition to the illuminated surface. In (d), PS is formed only on the illuminated surface

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The absorption of photons results in the generation of electron–hole pairs. The holes at the n-Si/electrolyte interface can participate in PS formation. In the case of the pure HF solution (Fig. 2b), the photoexcited holes are hard to drift towards the surface by the very small downward band bending, or possibly by the almost-flat band. Thus, efficient PS formation cannot be expected in pure HF solution. When the Si wafer is dipped in the HF/oxidant solution (Fig. 2d), on the other hand, many photoexcited holes move towards the n-Si/electrolyte interface at the front surface, resulting in the formation of PS with good reproducibility (Xu and Adachi 2006, 2007; Adachi and Kubota 2007; Tomioka et al. 2007). Reproducibility has been observed to be problematic in the formation of PS by photoetching. In an extreme case, no PS layer was formed on the front surface, although surprisingly PS was formed on the surface of the sample that was not exposed to illumination (i.e., on the back surface) (Andersen et al. 1995). The effectiveness of surface cleaning by sulfuric peroxide mixture (SPM) treatment or by KOH etching before PS formation has been reported in Tomioka et al. (2007) and Andersen et al. (1995).

PS Layers Formed by Photoetching A summary of PS formation by photoetching is presented in Tables 1 and 2 (Xu and Adachi 2006, 2007; Adachi and Kubota 2007; Tomioka et al. 2007; Andersen et al. 1995; Noguchi and Suemune 1993; Zhang et al. 1993; Cheah and Choy 1994; Jones et al. 1996; Kolasinski et al. 2000; Yamamoto and Takai 2000, 2001; Mavi et al. 2001, 2006; Marotti et al. 2003; Cho et al. 2006; Xu and Adachi 2008; Matsui and Adachi 2012; Zheng et al. 2005; Adachi and Tomioka 2005). To the best of our knowledge, there has not been reported any good plan-view high-resolution scanning electron microscopy images of the photosynthesized PS layers showing the morphology of their typical structures. In (Xu and Adachi 2007), the atomic force microscopy images were reported to show many irregularly shaped hillocks and voids distributed randomly over the entire PS surface. The observed root-mean-squares roughnesses were a few nanometers. Lateral patterning of PS layers has been performed using photoassisted electrochemical etching rather than pure photoetching (Baranauskas et al. 1995; Diesinger et al. 2003). Lateral modification of the porosity has also been obtained by photochemical dissolution of the anodic PS layers under illumination with a beam made of interference fringes (Ferrand and Romestain 2001). A metal-insulator-semiconductor-type electroluminescent (EL) device has been fabricated from PS layers synthesized by photoetching in an HF/I2 solution (Adachi and Kubota 2008). An insulating layer was formed on the PS layer by chemical oxidation in an acidic solution. Spectral output of the EL device was in the red-yellow region peaking at 2 eV.

Anhydrous and hydrous HF 32 % HF

40 % HF

2HF:1HNO3:4H2O

48 % HF

6HF:1H2O2

100HF: (17–250) H2O2 40 % HF

n (0.4  0.7)

n (2), p (2)

n

n (4.5  6.4)

n (35  45)

n (0.22  0.38, 35  45) n (10)

n (5  8)

Solution 50 % HF

Type (Ω cm) n (0.01  15)

1.9  2.0 1.9  2.0

Nd:YAG laser (1,064 nm), Ar laser (514 nm)

1.8  1.95

1.8  2.3

1.9  2.2

1.7  1.9

1.7  1.9

2.0

PL peak energy (eV) 1.8

HeNe laser

Gas and solid-state lasers HeNe laser

Gas, dye, and solid-state lasers

Ar laser, Xe lamp (465–780 nm) HeNe laser

W lamp (undispersed)

Light source HeNe laser, Xe lamp

Table 1 Photoetching for porous silicon formation in acidic solutions Comments No PS formation when excited at λ ¼ 300–400 nm. No PS formation on p-Si PS is formed only on the metal-backed Si substrates. PS layer thickness: 300  500 nm PL peak energy depends on photoetching wavelength PS is easily formed on the back surface of the sample PS is formed on both n-Si and p-Si. PL peak energy depends on excitation (PL) wavelength The shorter the photoetching wavelength, the higher the PL peak energy. The higher the photoetching laser power, the higher the PL peak energy The shorter the photoetching wavelength, the higher the PL peak energy Blue luminescence (420 nm) after dipping in 1C2H5OH:1H2O for 148 h PL intensity is shown to strongly depend on etching solution composition and time A two-peak (1.91 and 2.02 eV) structure in the PL spectrum (Ar laser). A single PL peak at 2.0 eV (Nd:YAG laser)

(continued)

Kolasinski et al. (2000) Yamamoto and Takai (2000) Yamamoto and Takai (2001) Mavi et al. (2001)

Jones et al. (1996)

Cheah and Choy (1994) Andersen et al. (1995)

References Noguchi and Suemune (1993) Zhang et al. (1993)

Porous Silicon Formation by Photoetching 71

Solution 1HF:1H2O2

49 % HF

40 % HF

HF:KIO3:H2O

HF:I2:H2O

HF:FeCl3:H2O

HF:H2O2:H2O

HF:KIO3:H2O

8HF:1H2O2

Type (Ω cm) n (1  5)

n (4  6)

n (3  5)

n (1  3)

n (1  3)

n (1  3)

n (1  3)

n (1  3)

n (10  20)

Table 1 (continued)

Xe lamp ( 600 μm achieved obtained for J < JPSL; arrays with pore diameters 100 μm–250 nm can be obtained Well-developed macropores oriented

Not much investigated Prone to pore branching and strange morphologies, but regular macropores arrays can be obtained Not much investigated Obtained at current densities 40 wt%) hydrofluoric acid. This last parameter has been particularly important in the literature with regard micropore generation. Bomchil et al. (1983) demonstrated in 1983, via X-ray tomography, that the addition of a simple surfactant like ethanol to HF-based solutions produced anodized layers of much higher uniformity, due to lowering of the effects of hydrogen gas generation. This was subsequently verified and adopted by most research groups worldwide to this day. One consequence of this is that virtually all material made by wafer anodization over the last 30 years is primarily mesoporous, not wholly microporous. One has to search the early literature to find examples where very concentrated (>40 wt%) hydrofluoric acid was used that generates the smallest pores. Although the more recent literature contains many examples where authors refer to fabrication of “microporous” silicon, an examination of their fabrication or characterization data reveals this is somewhat a misnomer, according to the IUPAC criteria above. In extreme cases, even macropores (>50 nm diameter) have been referred to as “micropores.” Such studies are not referenced here although “microporous silicon” appears in their titles. In this review we first examine the literature for strong evidence of micropores (2 nm width) by high-resolution transmission electron microscopy (see handbook chapter “▶ Microscopy of Porous Silicon”) and the absence of any hysteresis in a type I isotherm of nitrogen adsorption and desorption in a material of very high surface area (see handbook chapter “▶ Gas Adsorption Analysis of Porous Silicon”). Although the first Uhlir studies (1956) included work with 24–48 % HF, it is not clear whether or not they processed p-silicon with the most concentrated electrolyte. Watanabe and Sakai were probably therefore the first to create highly microporous silicon, albeit without realizing it (Watanabe and Sakai 1971). Other workers who conducted early studies on the properties of layers anodized in p-wafers using highly concentrated HF included Unagami (1980) and Koshida et al. (1985) who correctly referred to a “microporous structure” (Koshida

Microporous Silicon

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Fig. 1 (a) TEM image and inset diffraction pattern from microporous silicon fabricated by anodization of p-wafers in concentrated (50 %) aqueous hydrofluoric acid

a

Fig. 2 (a) Typical isotherm from mesoporous silicon with hysteresis (b) isotherm from microporous silicon fabricated by anodization of p-wafers in concentrated (50 %) aqueous hydrofluoric acid (Canham and Groszek 1992)

Amount adsorbed (10–3 mol)

0.3 0.2 0.1

mesoporous Si

0

b

0.2

0.1

0

microporous Si

0

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/Po)

et al. 1986). The 1988 review of Bomchil et al. (1988) clarified the trends in poresize distribution with anodization parameters for a range of mesoporous layers. They indicated the first gas adsorption evidence for some pores being below 2 nm in specific cases but commented that the technique was no longer accurate in this poresize regime. Table 1 summarizes more recent studies where ultrasmall pore size was quantified and the techniques used to estimate size. Note how, where measured, the surface areas can have very high values (>800 m2/g).

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Table 1 Estimates of micropore (60 % (oxidized) >60 % (mesopores) >90 % (macropores in composites) >30 % (mesopores) >60 % (macropores) >50 % (mesoporous wire array) >30 % (macropores)

>50 % (mesopores) >50 % (mesopores)

Study examples (Boarino et al. 1999; Nassiopoulu and Kaltsas 2000) (Salonen et al. 2005; Canham 2007; Chiappini et al. 2010; Canham 2014) (Coffer et al. 2005; Sun et al. 2007)

(Cho 2010; Wu et al. 2012; Ge et al. 2012)

(Tang et al. 2010; de Boor et al. 2012)

(Qu et al. 2010) (Deloiuse and Miller 2004)

Applications of Porosity Table 3 provides ten examples of applications where medium to high porosity is targeted, and guidelines as to likely minimum values are needed. To achieve ultrahigh porosities (>90 %) in silicon via wet etching techniques, great care has to be taken during drying (see handbook chapter “▶ Drying Techniques Applied to Porous Silicon”).

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Tompkins HG, Irene EA (2005) Handbook of ellipsometry. William Andrew/Springer, New York/ Heidelberg Torres-Costa V, Paszti F, Climent-Font A, Martin-Palma RJ, Martinez-Duart JM (2005) Porosity profile determination of porous silicon interference filters by RBS. Phys Stat Solidi (c) 2:3208–3212 Wu H, Chan G, Choi JW, Ryu III, Yao Y, McDowell MT, Lee SW, Jackson A, Yuang Y, Hu L, Cui Y (2012) Stable cycling of double-walled silicon nanotube battery anodes through solidelectrolyte interphase control. Nat Nanotechnol. doi:10.1038/NNANO.2012.35 Yonehara T, Sakaguchi K, Sato N (1994) Epitaxial layer transfer by bond and etch back of porous silicon. Appl Phys Lett 64:2108–2110 Zharkii SM, Karabutov AA, Pelivanov IM, Podymova NB, Timoshenko VY (2003) Laser ultrasonic study of porous silicon layers. Semiconductors 37(4):468–472

Ultrathin Porous Silicon Films Brahim Bessaı¨s

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Preparation of Ultrathin PS Films for XRR Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . XRR Analyses of Ultrathin PS Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

143 145 147 149

Abstract

Investigations of the structure and morphology of ultrathin PS films are reviewed, of relevance to the technological control of miniaturized PS-based devices. Several characterization tools with high reliability and precision are available; however, many of them are destructive or could affect the ultrathin PS structure. Grazing incidence X-ray reflectivity (XRR) is a powerful tool to probe the structural and morphological characteristics of ultrathin PS films. Homogeneity, thickness, surface and interface roughness, porosity, and density of ultrathin PS films were accurately determined using XRR. Nonetheless, prior to XRR measurements, ultrathin PS films should be submitted to complementary nondestructive morphological and optical examinations (thickness, roughness, oxidation, etc.).

Introduction Since the discovery of the room temperature photoluminescence (PL) in 1990 (Canham 1990), porous silicon (PS) has raised a strong interest owing to its potential applications in electronic and optoelectronic devices (see handbook chapter “▶ Porous Silicon Application Survey”). The physical properties and performance B. Bessaı¨s (*) Research and Technology Centre of Energy, Borj-Cedria Technopark, Hammam-Lif, Tunisia e-mail: [email protected] # Springer International Publishing Switzerland 2014 L. Canham (ed.), Handbook of Porous Silicon, DOI 10.1007/978-3-319-05744-6_14

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144 Table 1 Methods used for the characterization of thin PS films Technique Gravimetry SEM TEM AFM SE

XPS

XRR

UV–vis–NIR

Parameters Average porosity, thickness Thickness, pore morphology Pore and crystallite size distribution Surface roughness Optical index, absorption, porosity gradient Chemical composition Porosity, density, thickness, interface roughness Thickness, absorption coefficient

Comments Lower accuracy with thin layers Destructive Destructive Accuracy in tapping mode Nondestructive

Destructive at high X-ray dose, formation of Si oxides Quasi-nondestructive, possible reduction of the porosity Homogeneity, optical absorption, aging, etc.

References (Pickering et al. 1984; Brumhead et al. 1993) (Riley and Gerhardt 2000; Dittrich et al. 1995) (Berbezier and Halimaoui 1993; Gardelis et al. 2008) (von Behren et al. 1995; Striemer et al. 2007) (Pettersen et al. 1998; Wongmanerod et al. 2001) (Hardeman et al. 1985; Lees et al. 2003) (Sama et al. 2001; Ennejah et al. 2011) (Pickering et al. 1984; von Behren et al. 1995; Butturi et al. 1997)

of these devices often crucially depend on their interfacial structures. Thereby, characterization of the structure of ultrathin films became increasingly important. With the continuing minimization of thin film devices, characterization techniques with high reliability and precision are required (Isao and Boquan 1999). Nanoengineered PS films have many advantages in lab-on-chip devices, separations/filtrations, and nanoelectromechanical and nanofluidic systems applications (Whitby and Quirke 2007; Tong et al. 2004; Hinds et al. 2004). The handbook chapter “▶ Porous Silicon for Microdevices and Microsystems” focuses on such uses. For most applications, the main important performance goals of membranelike PS films are high permeation rate and good selectivity, which are achieved by high porosity, narrow pore size distribution, and small thickness (Kavalenka et al. 2012). The thickness of the films is a crucial parameter that influences most of the film properties; hence, most of technological applications require thin films of precise and definite thickness. In this context, the control of the structure and morphology of ultrathin PS films (porosity, crystallite sizes, homogeneity, etc.) is of prime importance to achieve powerful miniaturized PS-based devices. In Table 1, we summarize the main methods used for the characterization of ultrathin PS films. The technique of grazing incidence X-ray reflectivity represents a quasinondestructive method for the determination of thicknesses less than 250 nm (with ˚ of one or several ultrathin PS films), electronic densities, a precision of about 1–3 A and surface and interfacial roughnesses and porosities (Chamard et al. 2001). The value of the porosity of the PS film is very sensitive to the preparation conditions together with the employed determination method. XRR measurements are highly

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successful when the investigated films are deposited on polished substrates (a condition also needed for SE investigations) (Tompkins and Irene 2005; Ennejah et al. 2011). Furthermore, XRR is a well-adapted technique for measurements of thin PS films (Eymer et al. 1999; Keissing 1931), if one can visualize Keissing interferential fringes (Parratt 1954). The latter could only be observed if the PS film has a flat and homogeneous thickness not exceeding 250–300 nm and a surface roughness less than 3 nm, which approximately corresponds to 1 % of the maximum allowed thickness (Isao and Boquan 1999; Ennejah et al. 2011; Eymer et al. 1999). One may guess that these conditions are not so simple to obtain with ultrathin PS films; this is why sets of PS samples should be prepared in order to optimize the electrochemical etching (EE) conditions (Guilinger et al. 1995; Ennejah et al. 2011; Eymer et al. 1999). These special precautions could be satisfied by varying the anodization time at a rather low current density (Ennejah et al. 2011). Accordingly, XRR can be considered as a powerful characterization method. In order to take benefit of the effectiveness of XRR, one needs to estimate the chemical species, the homogeneity, the surface roughness, and the thickness of the PS films by performing infrared spectroscopy, SE, AFM, and UV–vis–NIR characterizations. Therefore, care should be taken to protect the ultrathin PS films against oxidation prior to XRR investigations. The aim of this review is to focus on the early-stage formation kinetic of EE-based ultrathin PS films using the glazing angle XRR.

Preparation of Ultrathin PS Films for XRR Analysis Homogeneous ultrathin PS films may be prepared by electrochemical etching (EE) of p-type monocrystalline Si wafers, in HF-based solutions during short periods and optimized current densities allowing to get various thicknesses (Ennejah et al. 2011; Guilinger et al. 1995) and an average porosity of about 50–75 %. For a general review of this fabrication technique, see handbook chapter “▶ Porous Silicon Formation by Anodization.” In order to observe Keissing fringes, the ultrathin PS thickness should be lower than 250 nm and the surface roughness as low as 3 nm (Ennejah et al. 2011). An example consists of lowering the surface reflectivity of pyramidal-textured Si solar cells by forming ultrathin nanoporous films on the pyramidal facets (Kim et al. 2009; Fig. 1a) using the EE technique. New ways of structuring rather uniform ultrathin nano-PS films having various pore shapes consist of HNO3/HF vapor etching of silicon (Fig. 1b) or crystallizing a thin amorphous silicon film (Kavalenka et al. 2012; Striemer et al. 2007; Saadoun et al. 2002; Fig. 1c). For PS films prepared from the EE method, one should optimize anodization conditions and check and/or estimate the silicon oxide content (SiOx) in the PS films before performing XRR characterization. In fact, freshly prepared PS films are generally covered with Si–H bonds; nevertheless formation of Si–O bonds always takes place at the early stages of PS formation (Ennejah et al. 2011). On the other hand, one should have a rough idea on the homogeneity of the PS films by checking their diffuse and specular reflectivity (Fig. 2a, b).

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Fig. 1 (a) EE-based nanoporous pyramidal plane applied in pyramidal-textured Si solar cells (Kim et al. 2009). (b) SEM cross-sectional view of a gold-coated thin PS film prepared from HNO3/HF vapor etching (Ben Jaballah et al. 2005). (c) Cross-sectional SEM image of porous nanocrystalline (pnc) Si membrane imaged on the surface of a metalized silicon wafer revealing the cylindrical nature of the pores (Kavalenka et al. 2012)

b 60

8

Specular Reflectivity (%)

Diffuse Reflectivity (%)

a 10s 40s 50s

7 6 5 4 3 400

600 800 1000 Wavelength (nm)

1200

10s 40s 50s 120s

50 40 30 20 10 400

600 800 1000 Wavelength (nm)

1200

Fig. 2 Variation of (a) diffuse and (b) specular optical reflectivity of ultrathin PS films with anodization time (Ennejah et al. 2011)

One may notice that for anodization time ranging from 10 to 50 s, the diffuse reflectivity is lower than the specular one. As anodization time (i.e., thickness) increases, the interference fringes (Fig. 2b) become closer to each other indicating at a first sight the formation of relatively smooth and homogeneous PS films.

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To evaluate the surface roughness, AFM microscopy characterizations were made on the same samples (Ennejah et al. 2011). By examining the AFM surface topography of ultrathin PS films in tapping mode, Ennajeh et al. (2011) found that the PS surfaces have a root-mean-square (rms) roughness not exceeding 1.5 nm (Ennejah et al. 2011), whatever the anodization time may be, indicating an etching time independence of the surface roughness. In fact, short anodization times do not allow deep etching of early formed surface structures, suggesting an almost invariable roughness. For current densities not exceeding 5 mA/cm2 and etching times ranging from 10 to 1,200 s, the rms does not exceed 1.5 nm (Ennejah et al. 2011). Spectroscopic ellipsometry (SE) measurements of thickness, optical index, and optical absorption require modeling the PS films. In that case, the Bruggeman (1935) model may be adopted, if one considers the PS film formed of two phases (e.g., silicon and vacuum) having different volume percentages. The PS thickness evaluated from SE varies from tens of nanometers to a few hundreds of nanometers, with a quasi-constant porosity value (Ennejah et al. 2011).

XRR Analyses of Ultrathin PS Films For X-rays, the matter refraction index is slightly lower than unity. The reflection of the X-rays occurring at the interface separating two media could be estimated from the Snell–Descartes law, which reveals total reflection below a critical angle depending on the electronic density of the material (Abramof et al. 2006). This critical angle has a very small value in the grazing X-ray technique (Abramof et al. 2006). The analysis of the reflected X-ray intensity and shape in the total reflection region (i.e., angles smaller than the critical one) (Fig. 3a) provides information on the surface structure from ten to several hundred angstroms deep. One may point out two critical angles θcPS and θcSi related to reflection at the interfaces air/PS and PS/silicon, respectively (Ennejah et al. 2011; Fewster 1996). One may also notice that θcPS is smaller than θcSi due to lower density of the PS films. In the total reflection zone, the reflected X-ray intensity is sensitive to electron density variation of the characterized material and is independent of the amorphous or crystalline structure or the crystallites orientation at the surface (Chamard et al. 2002). If the surface of the sample presents two or more homogeneous media, the reflection curve shows peculiarities associated to the electronic property of each media and to the different interface behaviors. For angles higher than the critical one, the X-rays are absorbed by the material and the reflected intensity drops abruptly. In the case of stratified homogeneous media having smooth boundary interfaces, one may observe reflected rays presenting Keissing fringes (Fig. 2b), which give information about film thickness and density variation between media and interface states. In the conditions of Ennejah et al. (2011), the Keissing fringes vanish at anodization time exceeding 50 s (Fig. 3b). The porosity of the PS films can be estimated from the relation (Buttard et al. 1998):

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b

107 θcSi θcSP

106

10 s 40 s 100 s

105

0.1

0.2

0.3

0.4

0.5

Grazing incidence angle 2θ (°)

0.6

X-ray reflected Intensity (counts/s)

X-Ray reflected intensity (counts/s)

a

106 10 20 30 40 50 100

105

s s s s s s

104

103

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

Grazing incidence angle 2θ (°)

Fig. 3 (a) XRR measurements on thin PS films etched for 10, 40, and 100 s in the total reflection region, (b) XRR measurements of thin PS films in the X-ray penetration region (Ennejah et al. 2011)

Pð%Þ ¼

h


 i 1  ðθcSP =θcSi Þ2  100

Figure 4 shows the variation of porosity, thickness, and roughness, determined from XRR patterns versus anodization time. In fact, pore initiation proceeds spontaneously on smooth silicon substrate yielding 1010/cm2 tiny etch pits after only a few seconds of anodization; many of these pits merge to form approximately 106/cm2 to 108/cm2 stable pores, which then continue to grow into the substrate (Buttard et al. 1998; Zhang 1991), leading to a final stable porosity. The porosity of ultrathin PS films, determined from Keissing fringes, grows almost linearly for short anodization time (Fig. 4a); then it tends to a stable porosity value (Ennejah et al. 2011). The thickness of EE-prepared ultrathin PS films (Fig. 4b), determined from XRR, exhibits a linear growth for short and long anodization times; the estimated values (Fig. 4b) are almost similar to that determined from SE or electronic microscopy (Guilinger et al. 1995) and even gravimetry for rather thick PS films (Guilinger et al. 1995). Riley and Gerhardt (2000) observed the same linear thickness behavior for thin p-type PS (Fig. 4b), although a nonlinear growth was depicted on n+-type PS films at short anodization times while using gravimetric analysis (Brumhead et al. 1993). It seems to be obvious that in case of homogeneous EE, the porosity of p-type PS films remains stable and etching is time independent, while the thickness grows linearly within etching time (Bessaı¨s et al. 2000). From XRR, one may evaluate the PS surface roughness and the PS/Si interface roughness (Fig. 4c). The PS surface roughness fluctuates in the range of 0.4–1 nm and is always less than unity; it tends to a constant value (equivalent to that determined from AFM for a virgin silicon wafer) with increasing anodization time (Guilinger et al. 1995; Ennejah et al. 2011), results in agreement with that obtained from AFM and TEM analysis (Guilinger et al. 1995). The interface roughness could be determined in the XRR specular geometry, where only the direction perpendicular to the sample surface is investigated, allowing to probe the interface state evolution associated to progressive pore nucleation.

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a

b 200 Thickness (nm)

Porosity (%)

70 60 50 40

160 120 80 40

0

20

40 60 80 Anodization time (s)

Interface roughness (nm)

c

100

10

20 30 40 Anodization time (s)

50

7 6 5 4 roughness at SP/Si interface roughness at PS surface

3 1.0 0.5 10

20 30 40 Anodization time (s)

50

Fig. 4 (a) Porosity, (b) thickness, and (c) interface roughness versus anodization time (Ennejah et al. 2011)

References Abramof PG, Beloto AF, Ueta AY, Ferreira NG (2006) X-ray investigation of nanostructured stain-etched porous silicon. J Appl Phys 99:024304 Ben Jaballah A, Hassen M, Hajji M, Saadoun M, Bessais B, Ezzaouia H (2005) Chemical vapor etching of silicon and porous silicon: silicon solar cells and micromachining applications. Phys Stat Sol (a) 202(8):1606 Berbezier I, Halimaoui A (1993) A microstructural study of porous silicon. J Appl Phys 74:5421 Bessaı¨s B, Ben Younes O, Ezzaouia H, Mliki N, Boujmil MF, Oueslati M, Bennaceur R (2000) Morphological change in porous silicon nanostructures: non-conventional photoluminescence shift and correlation with optical absorption. J Lumin 90:101 Bruggeman DAG (1935) Calculation of various physics constants in heterogenous substances I. Dielectricity constants and conductivity of mixed bodies from isotropic substances. Annalen Der Physik 24(7):636 Brumhead D, Canham LT, Seekings DM, Tufton PJ (1993) Gravimetric analysis of pore nucleation and propagation in anodised silicon. Electrochim Acta 38:191 Buttard D, Dolino G, Bellet D, Baumbach T, Rieutord F (1998) X-ray reflectivity investigation of thin p-type porous silicon layers. Sol State Commun 109:1

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Butturi MA, Carotta MC, Martinellia G, Passaria L, Youssef GM, Chiorino A, Ghiotti G (1997) Effects of ageing on porous silicon photoluminescence: correlation with FTIR and UV-vis spectra. Solid State Commun 101:11 Canham LT (1990) Silicon quantum wire array fabrication by electrochemical and chemical dissolution of wafers. Appl Phys Lett 57:1046 Chamard V, Bastie P, Le Bolloch D, Dolino G, Elkaı¨m E, Ferrero C, Lauriat J-P, Rieutord F, Thiaudie`re D (2001) Evidence of pore correlation in porous silicon: an x-ray grazing-incidence study. Phys Rev B 64:245416 Chamard V, Setzu S, Romestain R (2002) Light assisted formation of porous silicon investigated by X-ray diffraction and reflectivity. Appl Surf Sci 191:319 Dittrich T, Rauscher S, Timoshenko VY, Rappich J, Sieber I, Flietner H, Lewerenz HJ (1995) Ultrathin luminescent nanoporous silicon on n-Si: pH dependent preparation in aqueous NH4F solutions. Appl Phys Lett 67:1134 Ennejah N, Aouida S, Bessais B (2011) Ultra thin porous silicon films investigated by X-ray reflectometry. Phys Status Solidi C 8:1931 Eymer J, Fournel F, Rieutord F, Buttard D, Moriceau H, Aspar B (1999) X-ray reflectivity of ultrathin twist-bonded silicon wafers. Appl Phys Lett 75:3509 Fewster PF (1996) X-ray analysis of thin films and multilayers. Rep Prog Phys 59:1339 Gardelis S, Nassiopoulou AG, Petraki F, Kennou S, Tsiaoussis I, Frangis N (2008) Morphology, structure, chemical composition, and light emitting properties of very thin anodic silicon films fabricated using short single pulses of current. J Appl Phys 103:103536 Guilinger TR, Kelly MJ, Chason EH, Headley TJ, Howard AJ (1995) Nondestructive measurement of porous silicon thickness using X-ray reflectivity. J Electrochem Soc 142(5):1634 Hardeman RW, Beale MIJ, Gasson DB, Keen JM, Pickering C, Robbins DJ (1985) Porous silicon films: preparation and examination with surface and optical methods. Surf Sci 152–153:1051 Hinds B, Chopra N, Rantell T, Andrews R, Gavalas V, Bachas L (2004) Aligned multiwalled carbon nanotube membranes. Science 303:62 Isao K, Boquan L (1999) Structural characterization of thin films by x-ray reflectivity. Rigaku J 16(2):31 Kavalenka MN, Striemer CC, Fang DZ, Gaborski TR, McGrath JL, Fauchet PM (2012) Ballistic and non-ballistic gas flow through ultrathin nanopores. Nanotechnology 23:145706 Kiessig H (1931) Ann Phys Leipz 10:769 Kim BS, Lee DH, Kim JSH, An G-H, Lee K-J, Myung NV, Choa Y-H (2009) Silicon solar cell with nanoporous structure formed on a textured surface. Commun Am Ceram Soc 92:2415 Lees IN, Lin H, Canaria CA, Gurtner C, Sailor MJ, Miskelly GM (2003) Chemical stability of porous silicon surfaces electrochemically modified with functional alkyl species. Langmuir 19:9812 Parratt LG (1954) Surface studies of solids by total reflection of X-rays. Phys Rev 95:359 Pettersen LA, Hultman L, Arwin H (1998) Porosity depth profiling of thin porous silicon layers by use of variable angle spectroscopic ellipsometry: a porosity graded-layer model. Appl Opt 37:4130 Pickering C, Beale MIJ, Robbins DJ, Pearson PJ, Greef R (1984) Optical studies of the structure of porous silicon films formed in p-type degenerate and non-degenerate silicon. J Phys C Solid State Phys 17:6535 Riley DW, Gerhardt RA (2000) Microstructure and optical properties of submicron porous silicon thin films grown at low current densities. J Appl Phys 87:2169 Saadoun M, Mliki N, Kaabi H, Daoudi K, Bessais B, Ezzaouia H, Bennaceur R (2002) Vapouretching-based porous silicon: a new approach. Thin Solid Films 405:29 Sama S, Lequien CS, Milita S, Romestain R, Servidori M, Setzu S, Thiaudiere D (2001) Porous silicon characterization by x-ray reflectivity: problems arising from using a vacuum environment with synchrotron beam. J Phys D Appl Phys 34:841 Striemer CC, Gaborski TR, McGrath JL, Fauchet PM (2007) Charge and size-based separation of macromolecules using ultrathin silicon membranes. Nature 445:749

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Tompkins HG, Irene EA (2005) Handbook of ellipsometry. William Andrew Publishing/Springer Tong H, Jansen H, Gadgil V, Bostan C, Berenschot E, Van Rijn C, Elwenspoek M (2004) Silicon nitride nanosieve membrane. Nano Lett 4:283 von Behren J, Tsybeskov JL, Fauchet PM (1995) Preparation and characterization of ultrathin porous silicon films. Appl Phys Lett 66:1662 Whitby M, Quirke N (2007) Fluid flow in carbon nanotubes and nanopipes. Nat Nanotechnol 2:87 Wongmanerod C, Zangooie S, Arwin H (2001) Determination of pore size distribution and surface area of thin porous silicon layers by spectroscopic ellipsometry. Appl Surf Sci 172:117 Zhang XG (1991) Mechanism of pore formation on n-type silicon. J Electrochem Soc 138:3750

Porous Silicon Multilayers and Superlattices Vivechana Agarwal

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Type of Multilayers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Patterning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Functionalization and Stabilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Major Challenges/Problems in Fabricating Thick Multilayered Structures . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

153 154 157 157 158 159 160

Abstract

Electrochemical etching of silicon can generate porous silicon where porosity is modulated with depth. The overall fabrication technique, experimental tips for improving uniformity, typical porosity profiles, and methods of patterning and stabilization are reviewed. Due to its ease of fabrication, such multilayers have been extensively fabricated and applied in different fields, such as photonics, phononics, sensing etc.

Introduction After the pioneering work of G. Vincent (1994) in 1994, followed by M.G. Berger et al. (1994) demonstrating the possible fabrication of porous silicon (PS) superlattices, in the last few years, porous silicon multilayers have found broad range of applications. For example, the formation of tunable one-dimensional photonic devices such as Bragg mirrors, rugate filters, V. Agarwal (*) Centro de Investigacion en Ingenieria y Ciencias Aplicadads, Universidad Autonoma del Estado de Morelos, Col. Chamilpa, Cuernavaca, Mexico e-mail: [email protected] # Springer International Publishing Switzerland 2014 L. Canham (ed.), Handbook of Porous Silicon, DOI 10.1007/978-3-319-05744-6_15

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microcavities, and waveguides (see handbook chapter “▶ Porous Silicon Optical Waveguides”) and their use as optical bio- and chemical sensors (handbook chapter “▶ Porous Silicon Gas Sensing”), optoelectronics (handbook chapter “▶ Electroluminescence of Porous Silicon”), solar cells (handbook chapter “▶ Porous Silicon and Solar Cells”), etc. As an excellent example, in 2001, G. Lammel (Lammel et al. 2001) demonstrated a PS multilayer-based MEMS infrared gas spectrometer. In general, the main advantage of using porous silicon for making multilayers is the cost-effective, easy, and fast fabrication of any combination of layers with the required optical thicknesses to obtain the desired optical properties. In the next sections the formation of the multilayered structure and its classification based on the type of silicon wafer and pore morphology is given. Additionally, patterning and stabilization of porous silicon multilayers is followed by a brief overview of the major considerations while making thick multilayers.

Fabrication A number of methods have been used for fabricating PS multilayers (ML): (i) constant current density on a periodically doped Si wafer (Frohnhoff and Berger 1994), (ii) variation of illumination density on uniformly doped n-type Si wafer (Frohnhoff et al. 1995), (iii) variation of current density with a uniformly doped silicon wafer (Vincent 1994; Berger et al. 1997) (some examples given in Table 1), and (iv) glancing angle deposition technique (Kaminska et al. 2003; Robbie et al. 2004). Variation of current density in an electrochemical etching an uniformly doped silicon wafer is the most extensively used method, based on the fact that the etching process is self-limiting (Smith and Collins 1992) and mainly occurs at the pore tips, i.e., at the interface between the electrolyte and the silicon substrate, without affecting the already formed porous layer. Hence, the PS multilayer is formed by controlling the current density along the depth of the Si substrate (see Fig. 1). In other words, for the formation of the first layer, a constant current density provides the holes, and the dissolution reaction begins from the defects on the surface of the Si wafer. The pores are formed and their walls are eroded until they are emptied of holes. This passivates the already formed PS from further attack, and the reaction proceeds at the tip of the pore, maintaining the entire etched Si skeleton. The formation of the subsequent porous layers, by changing the current density, is due to the overlap of the depletion regions formed on the pore walls in a Schottky diode picture of the Si-electrolyte interface. Lehmann and Gosele (1991) proposed the quantum model, in which the decrease of the carrier concentration in the porous layer is related to an increased bandgap, which acts as a barrier for the hole diffusion from the bulk Si to the pore walls. On the other hand, for obtaining the multilayers of required optical properties, the exact variation of etching rate and porosity (or refractive indices) as a function of current density is needed (an example adapted from the reference

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Table 1 Some examples of different types of PS multilayer structures formed with the uniformly doped silicon substrate (taking “micro” 50 nm)

P

Type of structures Microporous

Mesoporous

Meso- and macroporous

N

Meso + macro

Macroporous

Micro-, meso-, macroporous

Some fabrication parameters p or p- type, approx. porosity range 55–75 %, electrolyte: HF + ethanol p + or p++ with resistivity > σ|| at any voltage. PF mechanism was used to explain the dependence of the conductivity on the electric field in the PS. According to Forsh et al. (Forsh et al. 2004, 2005), an increase in thermal emission of carriers across the potential barriers at the boundaries of NCs can be due to the electric field-induced enhancement of thermal ionization of impurity atoms and reduction in fluctuations of the potential profile (barriers at boundaries of NCs). Previously, the PF mechanism of conduction was observed in sandwiched configuration PS device prepared from (100)-oriented p-type c-Si wafers (Ben-Chorin et al. 1994). The temperature-dependent conduction shows the widening of the gap between the conductivities in both directions at lower temperatures, which means the Ea in (110) direction (Ea)⊥ is smaller than Ea in (001) direction (Ea)||. Apparently, the material has a certain distribution of potential barriers by height. As the length of the percolation path (constituted by Si-NCs) in the perpendicular (110) direction is shorter than that in the parallel (001) direction owing to the shape anisotropy of NCs, the average height of potential barriers in the perpendicular direction will also

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be lower than that in the parallel direction. This will lead to higher values of σ ⊥ and lower values of its (Ea)⊥ compared to σ ||. Earlier work on both freestanding nanosized PS and mesoporous anodized from (100) c-Si show almost isotropic behavior (Kocka et al. 1996a). It was argued that the transport is controlled by the more or less homogeneous (isotropic) “tissue” part of PS, in which c-Si “islands” are embedded. However, in 2006 Borini et al. demonstrated anisotropic behavior in the conductivity of (100) mesoporous by measuring temperature-dependent conductivity of their sample using two different electrode configurations (Boarino et al. 2009; Borini et al. 2006). The authors observed that the electronic transport parallel to the sample surface (σ ||) is strongly inhibited at room temperature but not along the perpendicular direction (σ ⊥). This behavior was well correlated with the typical microstructure of the mesoporous where, due to the presence of branched columnar morphology, the σ || pathways are poorly interconnected, with several bottlenecks in which potential barriers are built up. Thus, the transport is strongly inhibited in the longitudinal (parallel to the sample surface direction), while in the transverse direction (perpendicular to the sample surface) the bottlenecks can be easily bypassed following the alternative pathways available. It was also shown (Borini et al. 2006) that such electrical anisotropy can be reversibly removed by heating the samples (increasing temperature from 20  C to 100  C) when σ || increases almost six orders of magnitude equaling σ ⊥. The rise of temperature allows the charge carriers to overcome the nanoconstrictions (Coulomb blockade due to charges trapped in the nanoconstrictions), opening the longitudinal percolative pathways. The increase in temperature can remove the Coulomb blockade of a fraction of NCs, until the percolation threshold is reached and exceeded.

Conductivity Versus Porosity in Porous Silicon Effective Medium Theory Approach Among many other microstructural parameters, porosity is one such physical parameter, which is generally used to describe the degree of porous nature of a PS layer. Porosity has been well researched with the fabrication methods and environment. Therefore, if this physical parameter could be correlated with electrical conductivity of the PS using any analytical way, it could serve an important role in tailoring the microstructure to obtain desired device properties. However, not much work has been done to explore this correlation. Effective medium approximation (EMA) as proposed by Bruggeman was used to some extent to determine a correlation between the effective conductivity of the PS layer and porosity of the layer: n X σ i  σ eff vi ¼0 σ i þ 2σ eff i

(6)

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According to Dutta et al., up to certain low values of porosity, this theory worked well but failed to explain the effective conductivities of mid to highly porous layer (Dutta et al. 2002). The major challenge in the pore shape is the possibilities of different pore branching formations during PS growth (Saha et al. 1998). PS layers with “identical porosity” might have different surface-to-volume ratios, leading to different effective conductivities. They followed generalized EMA (GEMA), which accounts for the general form of the shape of the inclusions, where they included the extent of pore branching by “uniformity factor” (Saha et al. 1998): n X i

1=t

1=t

σ i  σ eff vi ¼0 φp 1=t 1=t σi þ 2σ eff 1  φp

(7)

where φp is the percolation volume fraction and t is a nonlinearity correction factor. So for spherical inclusions with φp ¼ 2/3 and t ¼ 1, the GEMA reduces to Bruggeman’s EMA. The theoretically calculated values match well with the experimental values up to the porosity range 70 %. But beyond this range of porosity, the calculated value underestimates the effective conductivity. Similar problem of mismatch between calculated and experimental results of effective conductivity was also observed by Bouaicha et al. in the porosity range exceeding 65 % (Bouaicha et al. 2006): vox

σ QD=QW  σ eff σ ox  σ eff σ v  σ eff þ vv þ vQD=QW ¼0 σ ox þ 2σ eff σ v þ 2σ eff σ QD=QW þ 2σ eff

(8)

where ΔE

σ QD=QW ¼ σ Si e4kT

(9)

They assumed in their theoretical calculation that the nanoporous silicon is formed by three phases: vacuum, oxide, and c-Si nanocrystallites (quantum dots (QD) for nanoporous or quantum wire (QW) for mesoporous structure) having the same mean-size dimension. The contribution of the latter phase in the total electrical conductivity was developed analytically by using the quantum confinement theory. This assumption worked well when the porosity was within 30–65 %, and beyond that theoretical values were too low compared to the experimental ones. However, for large porosities (greater than 65 %), where the PS structure exhibit visible luminescence, they could successfully obtain a perfect agreement between the theory and the experiment for all porosities when they considered that the base medium is vacuum in which silicon crystallites are incorporated (Khardani et al. 2006). This means that for the case of high porosities, the role of porosity is substituted by the quantum dot volume fraction in the fitting procedure. They successfully extended this work to obtain a good correlation between effective conductivity of mesoporous Si material and their corresponding porosity.

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In 2008, however, they further modified their work by considering the Si-NCs as being formed by multiple-sized crystalline dots (John and Singh 1994) embedded in silicon dioxide and vacuum (Bouaicha et al. 2008): vox

N X σ ox  σ eff σ v  σ eff σ QDi  σ eff þ vv þ vQDi ¼0 σ ox þ 2σ eff σ v þ 2σ eff σ QDi þ 2σ eff i¼1

(10)

As a result, they obtained a good agreement between theory and experiment for all porosities. In this case (Eq. 9), all values of ΔE are considered including those < ΔE0. This avoids the tendency of the medium to be an insulator for higher porosities unlike what happens when the PS medium is considered to have three phases with single mean-sized QD.

Percolation Theory Approach A different approach to correlate porous silicon conductivity with material porosity was described in Ref. Aroutiounian and Ghulinyan (2003). In this work, the conductivity was shown to be mainly crystalline for porosities much lower than the percolation threshold at 57 %, while a fractal behavior was observed at porosities near percolation threshold. For higher values of porosities, the conductivity was described as a quasi-one-dimensional hopping. The report concluded that in PS with increasing porosity, at lower temperatures, the dimension of the channels of electrical current flow decrease from 3 to 1, as described by the Mott law for amorphous semiconductors. However, the model results described in this work show some deviation from the experimental results. In spite of workers having presented models to fit their experimental data for a range of porosity, none of the works have attempted to fit the data of others with their models. If these models were tested to fit a wider range of published data, one could hope to find a more comprehensive model that could find wider application to make porosity a useful parameter in predicting the electrical behavior of PS.

Attempts on Classification of Electrical Properties of Porous Silicon The study of a heterogeneous material as PS could benefit greatly from a classification system that would allow a more systematic understanding and correlation of its properties. A wide variety of PS has been classified into some broad microstructural groups based on porosity. However, the same agreement has not been reached in correlating electrical properties of these PS groups to parameters related to porosity of the material. This is because electrical current which flows through the Si network depends largely on the size of Si structure and its surroundings and is not directly linked to pore size.

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In 1997 a comprehensive review of the properties of porous silicon was published by Canham that included a classification proposed by Ben-Chorin (Canham 1997). Ben-Chorin classified PS into two broad and distinct classes, “low porosity” and “nanosized porous Si” material, and explained the plausible electrical transport behavior of these two groups. The “low porosity” material is prepared from highly doped c-Si wafers (resistivity < 1 Ωcm), and quantum confinement does not play any role in transport. The nanosized PS is prepared from low-doped c-Si wafers and mostly under illumination. Crystallite size in such material is usually 100) is not trivial, and so a continuous homogeneous filling of such pores has not been achieved yet (Dolgyi et al. 2012). In the case of electrodeposition, an aqueous metal-salt solution is used as electrolyte. In Table 1 a summary of employed electrolytes can be found. For a review dedicated to this impregnation technique, see the chapter in this handbook “▶ Porous Silicon and Electrochemical Deposition.” During the process, two kinds of metal deposition are occurring. One possible reaction is electroless metal deposition: 2M2þ þ Si þ H2 O ! 2M þ SiO2 þ 4Hþ , and furthermore electrodeposition takes place under cathodic conditions: M2þ þ 2e ! M Gaseous hydrogen is formed, if the current density exceeds a certain value (Bandarenka et al. 2012) which can influence and hamper the precipitation of the metal structures. 2Hþ þ 2e ! H2

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Fig. 2 Scanning electron micrographs (BSE) showing different arrangements of deposited Ni nanostructures within the pores of the porous silicon (Granitzer et al. 2008b). The pore diameter in all templates is around 60 nm, and the length is around 30 μm. First row: the spatial distribution of the deposited metal varies between 2/3 and 1/3 of the depth of the porous layer. Second row: the porous layers are filled between surface and bottom of the pores, but the shape of the precipitated Ni structures differs (from left to right: wires, ellipsoids, particles). Third row: zoomed areas of row two

In general, the deposition of metals into porous silicon is a cathodic process and reduces the metal-salt ions to metal (e.g., Ni2+ + 2e ¼ Ni). Imperfections and variations of the pore shape such as dendritic branches can also lead to inhomogeneities of the metal deposits, and thus a homogeneous filling of the pores is extremely difficult to obtain. In the case of high aspect ratio pores, it is important to ensure that the exchange of electrolyte is sufficient along the entire pore length. If it is insufficient, pores can be blocked and a continuous growth of a metal wire along the whole length is inhibited. The degree of pore filling depends on the applied current density as well as on the pulse duration of the current (Granitzer et al. 2009). By choosing the deposition parameters in an adequate way, the geometry and spatial distribution of the ferromagnetic deposits are tunable, and thus tailoring of the magnetic properties of the composite is possible. The metal structures can be deposited in a broad size range (spheres according to the pore diameter, ellipsoids with a long axis of 100–500 nm and needles up to a few micrometers in length). In Fig. 2 an overview of various Ni fillings within porous silicon can be seen.

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Magnetic Properties of the Composite Characteristics of ferromagnetism are a spontaneous magnetization and the occurrence of a hysteresis (Bozorth 1993). The mechanisms responsible for the formation of magnetic domains are exchange interaction and anisotropy effects (Bertotti 1998). If the size of the ferromagnetic substance is reduced to below a certain qffiffiffiffiffiffiffiffi critical radius r d ¼ 36 μKM1 2 (K1 is anisotropy constant, MS is the saturation 0

S

magnetization), the domain structure is reduced to one single domain. Such nanosized structures and especially the arrangement of magnetic nanostructures on surfaces or in three dimensions give rise to novel properties which depend not only on the geometry of the structures but also on the interactions between them. Employing templates with well-separated pores for the incorporation of magnetic nanostructures improves the coercivity and squareness of a hysteresis compared to magnetic thin films, and the magnetic easy axis is turned from the in-plane to the outof-plane direction. The deposition of ferromagnetic thin films on porous silicon results in an in-plane magnetization (Dai et al. 2007). There are three possible parameters of the samples which can be modified to determine the magnetic behavior, namely, the kind of metal deposited within the pores, the morphology of the porous template, and the electrochemical conditions which can be tuned during the deposition procedure resulting in different geometries of the deposits. All these features influence the magnetic properties, whereas the morphology of the porous silicon template mainly influences the dipolar coupling between adjacent pores and thus the magnetic anisotropy between easy and hard axis magnetization (Granitzer et al. 2012b). With increasing distance between the pores, the magnetic interaction between metal structures of adjacent pores decreases. The magnetostatic energy Em of a couple of two parallel dipoles of length l at a distance r is given by ! p2 1 1  pffiffiffiffiffiffiffiffiffiffiffiffiffiffi , Em ¼ 2πμ0 r r 2 þ l2 where p is the pole strength (p ¼  μ0  π d2 m/4) (Samwel et al. 1992). A further reason for enhanced magnetic anisotropy is the elongated shape of the deposits and pore walls with a minimum of side branches which strongly influence the stray fields (Bryan et al. 2012). In the case of nanowire arrays, the magnetic anisotropy is mainly composed of shape anisotropy and magnetocrystalline anisotropy, whereas in the case of Ni wires the shape anisotropy is the dominating factor (Vega et al. 2011). Preliminary investigations showed that the Ni wires embedded in porous silicon are polycrystalline, and thus the magnetocrystalline anisotropy is negligible. Different loading of the pores can be achieved by modifying the deposition parameters (Rumpf et al. 2010) leading to nanostructures of distinct geometry and in various spatial distributions along the pores (Rumpf et al. 2008). Figure 3 shows three magnetization curves of porous silicon samples with equal morphology but different geometry of the Ni deposits, varying between spherical particles (~60 nm), ellipsoids (~300 nm), and wires (~1 μm). The coercivity varies

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Fig. 3 Hysteresis loops of Ni particles ( full line), Ni ellipsoides (dotted line), and Ni wires (dashed line) deposited within the pores of porous silicon, whereas the morphology has been the same. The coercivity as well as the remanence decreases with increasing elongation of the metal nanostructures. The magnetic field has been applied perpendicular to the sample surface

Table 2 Variation of the coercivity and squareness with modification of the pore diameter Pore diameter PS 60 nm (Vega et al. 2011) PS 35 nm (Granitzer et al. 2012b) AAO 80 nm (Vazquez et al. 2004) AAO 30 nm (Vazquez et al. 2004)

HC,II (Oe) 280 660 360 600

HC,⊥ (Oe) 180 190 250 260

(MR/MS)II 0.48 0.89 0.35 0.70

(MR/MS)⊥ 1.5 3.47 1.44 2.3

PS porous silicon, AAO anodic aluminum oxide

between 280 Oe (wires) and 500 Oe (spherical particles) and the squareness (MR/MS) between 0.2 (wires) and 0.52 (particles), MR being the magnetic remanence and MS the saturation magnetization. Magnetic remanence, especially the squareness, provides information about coupling mechanisms between metal nanostructures (Stoner and Wohlfarth 1948). Ferromagnetic structures with a random distribution of orientations offer a squareness of 0.5. In comparison structures with the easy axis aligned parallel to the applied magnetic field exhibit a squareness of 1 whereas with the easy axis perpendicular to the magnetic field a value of 0 (Coey 2009). A decrease of the squareness is caused by a diminution of the magnetic remanence. The smaller values determined from specimens with embedded Ni wires within porous silicon compared to the ones with Ni particles arise from demagnetizing effects. If the morphology of the template is modified and the geometry of the deposits is equivalent (wires), the magnetic characteristics also change as summarized in Table 2. In all samples the shape anisotropy is the dominating factor because of the high aspect ratio. The differences in the magnetic behavior are mainly due to magnetic interactions and the thickness of the Ni wires. Comparing the magnetization of porous silicon/metal nanocomposites with oxide heterostructures consisting of CoFe2O4 nanopillars in a BiFeO3 matrix (Chen et al. 2013), the magnetic moment is about one order of magnitude higher

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in the case of the porous silicon composite. Other systems with two-dimensional arrangements of magnetic nanostructures (Enders et al. 2010) need big areas or a magnetic material with high magnetization. One of the characteristics of the porous silicon system is the three-dimensional arrangement of the ferromagnetic nanostructures embedded within high aspect ratio pores which yields to a high magnetic moment. There is also a report on Ni deposition within porous silicon which describes the formation of Ni silicide during the electrodeposition process (Dolgiy et al. 2013) which reduces the magnetic moment of the specimen.

Conclusion Intrinsic ferromagnetism of nanostructures silicon is very weak, but the nanocomposites consisting of deposited magnetic nanostructures offer a strong ferromagnetic behavior. Porous silicon with its tunable morphology is an adequate template material for metal deposition to gain 3D arrays of deposited nanostructures. In the case of ferromagnetic metal precipitation, the magnetic properties of the nanocomposite can be tailored on the one hand by the deposition procedure and on the other hand by the modification of the template morphology. The obtained system exhibits a quasi “ferromagnetic semiconductor” material which offers its ferromagnetic properties at room temperature. Due to the silicon substrate, it can be readily integrated in today’s microtechnology, and thus the system is promising for various applications in sensor technology, magneto-optics, and also the emerging field of spintronics.

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Jeske M, Schultze JW, Tho¨nissen M, M€ under H (1995) Electrodeposition of metals into porous silicon. Thin Solid Films 255:63 Koda R, Fukami K, Sakka T, Ogata YH (2012) Electrodeposition of platinum and silver into chemically-modified microporous silicon electrodes. NRL 7:330 Kopnov G, Vager Z, Naaman R (2007) New magnetic properties of silicon/silicon oxide interfaces. Adv Mater 19:925 Koshida N, Koyama H (1992) Visible electroluminescence from porous silicon. Appl Phys Lett 60:347 Laiho R, L€ahderanta E, Vlasenko L, Vlasenko M, Afanasiev M (1993) Magnetic properties of light-emitting porous silicon. J Lumin 57:197 Lehmann V, Gr€uning U (1997) The limits of macropore array fabrication. Thin Solid Films 297:13 Lehmann V, Stengl R, Luigart A (2000) On the morphology and the electrochemical formation mechanism of mesoporous silicon. Mater Sci Eng B 69–70:11 Ogata YH, Kobayashi K, Motoyama M (2006) Electrochemical metal deposition on silicon. Curr Opin Solid State Mater Sci 10:163 Rumpf K, Granitzer P, Krenn H (2008) Porous silicon/metal hybrid system with ferro and paramagnetic behavior. IEEE Trans Magn 44:11 Rumpf K, Granitzer P, Po¨lt P (2010) Influence of the electrochemical process parameters on the magnetic behavior of a silicon/metal nanocomposite magnetic thin films. ECS Trans 25:157 Rumpf K, Granitzer P, Hilscher G, Po¨lt P (2011) Interacting low dimensional nanostructures within a porous silicon template. J Phys Conf Series 303:12048 Rumpf K, Granitzer P, Hilscher G, Albu M, Po¨lt P (2012) Magnetically interacting low dimensional Ni-nanostructures within porous silicon. Microelectron Eng 90(c):83 Samwel EO, Bissel PR, Lodder JC (1992) Internal field corrections in perpendicular columnar structured alumite films. J Magn Magn Mater 115:327 Stoner EC, Wohlfarth EP (1948) A mechanism of magnetic hysteresis in heterogeneous alloys. Phil Trans Royal Soc A Phys Math Eng Sci 240:599 Vager Z, Naaman R (2004) Bosons as the origin for giant magnetic properties of organic monolayers. Phys Rev Lett 92:087205 Vazquez M, Pirota K, Hernandez-Velez M, Prida VM, Navas J, Sanz R, Batallan F, Velazquez J (2004) Magnetic properties of densely packed arrays of Ni nanowires as a function of their diameter and lattice parameter. J Appl Phys 95:6642 Vega V, Prida VM, Garcia JA, Vazquez M (2011) Torque magnetometry analysis of magnetic anisotropy distribution in Ni nanowire arrays. Phys Status Solidi A 208:553 Yae S, Hirano T, Matsuda T, Fukumuru N, Matsuda H (2009) Metal nanorod production in silicon matrix by electroless process. Appl Surf Sci 255:4670 Zacharatos F, Nassiopoulou AG (2008) Copper-filled macroporous Si and cavity underneath for microchannel heat sink technology. Phys Status Solidi A Appl Res 205:2513

Paramagnetic and Superparamagnetic Silicon Nanocomposites Petra Granitzer and Klemens Rumpf

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Superparamagnetic Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Infiltration of Iron Oxide Nanoparticles Into Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Magnetic Behavior of the Composite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biomedical Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

298 298 299 299 302 303 303

Abstract

In this chapter the paramagnetic properties of nanostructured silicon are outlined and furthermore the magnetic properties of a composite material consisting of porous silicon with infiltrated superparamagnetic iron oxide nanoparticles are discussed. The magnetic behavior of the system depends on the nanoparticle size as well as on the magnetic coupling between them. Both influence the so-called blocking temperature, the transition between superparamagnetic behavior and blocked state. A particle size-related assessment shows that the blocking temperature increases with increasing particle size if the distances between the particles are equal. The blocking temperature can be decreased by weakening the magnetic interaction between the particles. Due to the good biocompatibility of both porous silicon and iron oxide nanoparticles, the composite system is of interest for biomedical applications in the fields of therapy and diagnosis.

P. Granitzer (*) • K. Rumpf Institute of Physics, Karl-Franzens-University Graz, Graz, Austria e-mail: [email protected] # Springer International Publishing Switzerland 2014 L. Canham (ed.), Handbook of Porous Silicon, DOI 10.1007/978-3-319-05744-6_31

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Introduction In the case of paramagnetic materials, each atom possesses a permanent dipole moment due to a remaining electron spin or orbital magnetic moments. Without external magnetic field, the magnetic moments offer a random orientation resulting in no net magnetization. If a magnetic field is applied, the free rotating dipoles align preferentially in field direction, but there is no interaction between individual dipoles. Silicon, especially with hydrogen-terminated silicon surfaces, is in general diamagnetic and free of paramagnetic defects (see handbook chapter “▶ Diamagnetic Behavior of Porous Silicon”). Oxidation leads to interface defects (Si/SiO2), so-called dangling bonds. They are paramagnetic in the neutral charge state which corresponds to a one-electron spin S ¼ 1/2 configuration (Bardeleben and Cantin 1997). These paramagnetic defects (Pb center) are superimposed on the diamagnetic contribution. Due to the large surface area in nanostructured porous silicon, much higher concentrations of defects can occur, contrasting with bulk silicon. The interface between Si/SiO2 has been investigated by many authors according to the dangling bond of the silicon atom, the Pb center which is the dominant paramagnetic defect in porous silicon (Cantin et al. 1997). Their concentration depends on the position of the Fermi level and it can also be modified by the H-termination. Oxidation of the surface can increase the Pb center defect concentration. Two further paramagnetic defects in the oxide layer have been observed, the oxygen vacancy defect E’ (Pointdexter et al. 1981) and the EX defect in SiO2 (Pointdexter et al. 1981). EPR studies give information about these defects from analysis of g-tensors (Cantin et al. 1995) and hyperfine data (von Bardeleben et al. 2005). A special kind of paramagnetism is the so-called superparamagnetism which is dedicated to small magnetic particles which fall below a critical radius of single domain particles. After the incorporation of such particles into the pores of porous silicon, the composite system offers specific magnetic properties which will be discussed in the next section of this review.

Superparamagnetic Nanoparticles If a ferro-/ferrimagnetic material is fabricated in a nanoscopic size range and goes below a critical value, the particle becomes superparamagnetic (SPM) which means that the thermal energy dominates over the anisotropy energy and the whole particle behaves like a paramagnetic spin (Bertotti 1998). Below this so-called superparamagnetic “blocking radius,” the particles lose their remanence. The transition takes place at the so-called blocking temperature TB ¼ KV/25kB (K is the anisotropy constant; V is the particle volume). SPM nanoparticles are of interest for magnetic applications such as high-density data storage in fabricating particles with high anisotropy constant and thus overcome the superparamagnetic limit (Frey et al. 2009). Due to magnetic interactions

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between them, new properties arise such as soft magnetic alloys (Suzuki 1999) and hard magnetic materials with an improved energy product (Skomsky and Coey 1993). In biomedicine SPM nanoparticles are also employed for diagnostics and therapeutics, e.g., magnetic resonance imaging (Yallapu et al. 2011), hyperthermia (Lee et al. 2011), and cancer therapy (Lee et al. 2011). Especially iron oxide nanoparticles play an important role due to their low toxicity. Such magnetic nanoparticles are often prepared by chemical synthesis, and in general they are coated with a shell of a few nanometers which often is a metal-oxide (Wei et al. 2010), a silica (Joo et al. 2009), or an organic surfactant (Shukla et al. 2009) to prohibit agglomeration and to stabilize them. Thus, magnetic exchange interaction is excluded and only dipolar coupling can take place. A further approach to stabilize the particles is their incorporation into a nonmagnetic or weakly magnetic matrix as, for example, polymers (Munoz-Bonilla et al. 2012), silica (Lee et al. 2008), zeolites (Lukatskaya et al. 2009), or porous silicon (Granitzer et al. 2010). Furthermore, the use of a matrix adds another degree of freedom and allows tuning of the properties, depending on the particle arrangement within the matrix.

Infiltration of Iron Oxide Nanoparticles Into Porous Silicon Because of its adjustable morphology, porous silicon (Lehmann 2002) is useful as a host matrix. For example, iron oxide nanoparticles have been deposited onto hydroxyl functionalized porous silicon samples resulting in a self-organizing dendrite-like arrangement on the surface which offers a new composite material (Balakrishnan et al. 2006). The magnetite nanoparticles which are superparamagnetic show dendrite-like formation caused by a diffusion-limited aggregation model (Witten and Sander 1981). If Fe3O4 nanoparticles are infiltrated into the pores of mesoporous silicon, the achieved system leads to a composite material (Fig. 1) showing a ferromagnetic-like behavior at low temperatures (T < TB) and superparamagnetism at higher temperatures (T > TB) (Granitzer and Rumpf 2011). This transition temperature (blocking temperature TB) can be influenced by the particle size but also by the distance between the particles. In principle the distance between the particles within the pores can be tuned either by the thickness of the organic coating or by the filling density. A further degree of freedom is to vary the distance between particles of adjacent pores by the morphology of the matrix.

Magnetic Behavior of the Composite The superparamagnetic behavior of the Fe3O4/porous silicon system above a blocking temperature TB is examined by temperature-dependent magnetization measurements (Granitzer et al. 2010). For more detail on this technique and

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Fig. 1 Scanning electron image of magnetite nanoparticles within porous silicon (Granitzer and Rumpf 2011). The size of the particles is 8 nm and the oleic acid shell is about 2 nm

others, see handbook chapter “▶ Magnetic Characterization Methods for Porous Silicon.” In the case of 8 nm particles, zero field-cooled (ZFC)/field-cooled (FC) investigations performed at an applied field of 500 Oe show a rather high blocking temperature TB at 170 K which indicates magnetic interactions between the particles. The strength of the coupling can be modified by varying the packing density of the particles within the pores. This effect can be achieved by changing the concentration of the particle solution (Granitzer et al. 2011; Fig. 2b). By varying the particle size, TB is also strongly influenced which can be seen in Fig. 2a. Furthermore, a shift of the blocking temperature to lower temperatures with higher applied fields is observed. This behavior of superparamagnetic particles is known to be proportional to H2/3 at high fields and proportional to H2 for lower fields (Goya and Morales 2004). The blocking temperatures determined for different applied magnetic fields are summarized in Table 1. Superparamagnetic behavior of porous silicon loaded with a magnetic material could also be achieved by the deposition of Ni within luminescent stain-etched porous silicon (Nakamura and Adachi 2012). Furthermore, porous silicon with adsorbed parabenzoquinone molecules resulted in a nanocomposite which exhibited paramagnetic properties (Antropov et al. 2012). However, this could be due to the occurrence of additional dangling bonds due to nanostructuring, rather than the loading of the porous silicon with organic molecules. Iron oxide nanoparticles are of particular interest due to their magnetic behavior (Gubin 2009) but also because of their low toxicity which renders

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Fig. 2 (a) Shift of the blocking temperature with the particle size in using the same particle concentration. (b) Change of TB with the concentration of the particle solution. In the latter case the particle size is 8 nm for all concentrations. In all cases a template with equal morphology has been used

them applicable in biomedicine (Roca et al. 2009). The infiltration of iron oxide nanoparticles into porous silicon which also is biocompatible (Canham 1995) results in a system of interest for magnetically guided drug delivery (Gu et al. 2010).

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Table 1 Shift of the blocking temperature with increasing applied magnetic field to lower temperatures. The size of the infiltrated particles is 8 nm. The concentration of the magnetite solution was fixed at 8 mg Fe/ml Magnetic field [Oe] 5 500 1,000

Blocking temperature [K] 170 75 50

Biomedical Applications Due to the low toxicity of both materials (porous silicon and iron oxide particles), the system is of interest for biomedical applications. The handbook chapter “▶ Biocompatibility of Porous Silicon” reviews the important in vivo data on the former material. Drug delivery with porous silicon is under investigation employing not only particles but also films, chip implants, and composite materials. Nonetheless, microparticles play a key role because they are compatible to most existing drug delivery concepts (Anglin et al. 2008). On the one hand nanostructured PS tablets can be utilized to carry a cocktail of drugs or nutrients which will be delivered in a predetermined time within the body and on the other hand percutaneous implants of PS with radioactive content can provide radiation to tumor cells (Anglin et al. 2008). Serda et al. reports on a multistage delivery system applicable for biological imaging. The loading of PS microparticles with superparamagnetic (SPM) iron oxide nanoparticles (NPs) is presented as well as the examination of their cellular uptake by macrophages. Furthermore the influence of 3-aminopropyltriethoxysilane on the PS surface and retention of the iron oxide NPs is investigated (Serda et al. 2010). For biosensing applications there are various approaches such as the fabrication of arrays of micro test tubes and microbeakers consisting of macroporous silicon with incorporated iron oxide nanoparticles (Ghoshal et al. 2011). Furthermore, optical interferometer biosensors based on porous silicon are used for immunoassaying by combing superparamagnetic particles with an interferometer porous silicon platform (Ko et al. 2012). The handbook chapters “▶ Porous Silicon Immunoaffinity Microarrays” and “▶ Porous Silicon Optical Biosensors” provide overviews of these application areas. The combination of porous silicon with Fe3O4 particles and additional loading with a molecular payload is of interest for controlled transport in applying an external magnetic field. The loaded molecules (enzymes) can be transported and subsequently released in an appropriate solution (Thomas et al. 2006). The in vitro cytotoxicity of porous silicon particles loaded with magnetite nanoparticles (see the related handbook chapter “▶ Cell Culture on Porous Silicon”) has been investigated in using the cell viability of human liver cancer cells and rat hepatocytes (Kinsella et al. 2013). In vivo studies have been carried out on a carcinoma rat model to figure out the biodistribution properties of the composite material (Kinsella et al. 2013). Furthermore, after in vitro cytotoxicity tests, in vivo assays have been carried out on New Zealand rabbits showing low inflammatory

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response and no necrosis effect in the tissues of the treated eyes or in the fibrous tissue formed around the agglomerated explanted flakes (Munoz Noval et al. 2013). The composite system has been investigated in relation to the magnetic preconditions for biomedical applications which means no magnetic remanence at room temperature (Rumpf et al. 2013). The magnetic moment is attempted to be high as possible due to closely packed particle loading. To date, experiments demonstrating transport of porous silicon loaded with iron oxide nanoparticles of different size (between 5 and 8 nm) in water have been successful. Size-dependent magnetization measurements and subsequent cytotoxicity evaluation of Fe3O4/porous silicon showed that the nanocomposite is an encouraging material for drug delivery (Granitzer et al. 2013).

Conclusions Porous silicon offers in general a diamagnetic behavior, although due to surface modification and oxidation, paramagnetic defects occur. The magnetic properties of the material can be extended, for example, by the infiltration of superparamagnetic nanoparticles. The achieved nanocomposite shows a transition between superparamagnetic behavior and blocked state, depending on the size of the loaded particles and their particle-particle distance which allows the tuning of the magnetic properties. Concerning the applicability the system is of interest not only for magnetic but also for biomedical applications due to the biocompatibility of both materials.

References Anglin EJ, Cheng L, Freeman WR, Sailor MJ (2008) Porous silicon in drug delivery devices and materials. Adv Drug Deliv Rev 60:1266 Antropov IM, Semisalova AS, Konstantinova EA, Perov NS, Kozlov SN (2012) Effect of parabenzoquinone adsorption on the magnetic properties of nanostructured silicon. Semiconductors 46:1119 Balakrishnan S, Gun’ko YK, Perova TS, Moore RA, Venkatesan M, Douvalis AP, Brouke P (2006) Dendrite-like self-assembly of magnetite nanoparticles on porous silicon. Small 2:864 Bardeleben HJ, Cantin JL (1997) Paramagnetic defects in porous silicon. In: Canham LT (ed) Properties of porous silicon. INSPEC, London Bertotti G (1998) Hysteresis in magnetism. Academic, New York Canham LT (1995) Bioactive silicon structure fabrication through nanoetching techniques. Adv Mater 7:1033 Cantin JL, Schoisswohl M, von Bardeleben H-J, Hadj Zoubir N, Vergnat M (1995) Electronparamagnetic-resonance study of the microscopic structure of the Si(001)-SiO2 interface. Phys Rev B 52, R11599 Cantin JL, Schoisswohl M, von Bardeleben HJ (1997) Structural and optical properties of porous silicon nanostructures, Chap 14. In: Amato G, Delerue C, von Bardeleben HJ (eds). Structural and optical properties of porous silicon nanostructures. Gordon and Breach Science Publishers Amsterdam Frey NA, Peng S, Cheng K, Sun S (2009) Magnetic nanoparticles: synthesis, functionalization, and applications in bioimaging and magnetic energy storage. Chem Soc Rev 38:2532 Ghoshal S, Ansar AAM, Raja SO, Jana A, Bandyopadhyay NR, Dasgupta AK, Ray M (2011) Superparamagnetic iron oxide nanoparticle attachment on array of micro test tubes and

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microbeakers formed on p-type silicon substrate for biosensor application. Nanoscale Res Lett 6:540 Goya GF, Morales MP (2004) Field dependence of blocking temperature in magnetite nanoparticles. J Metastab Nanocryst Mater 20:673 Granitzer P, Rumpf K (2011) Magnetic nanoparticles embedded in a silicon matrix. Materials 4:908 Granitzer P, Rumpf K, Venkatesan M, Roca AG, Cabrera L, Morales MP, Poelt P, Albu M (2010) Magnetic study of Fe3O4 nanoparticles incorporated within mesoporous silicon. J Electrochem Soc 157:K145 Granitzer P, Rumpf K, Venkatesan M, Cabrera L, Roca AG, Morales MP, Poelt P, Albu M, Ali K, Reissner M (2011) Magnetic behaviour of a magnetite/silicon nanocomposite. J Nanopart Res 13:5685 Granitzer P, Rumpf K, Tian Y, Akkaraju G, Coffer JL, Poelt P, Reissner M (2013) Size-dependent assessment of Fe3O4-nanoparticles loaded into porous silicon optical. ECS Trans 50:77 Gu L, Park J-H, Duong KH, Ruoslahti E, Sailor MJ (2010) Magnetic luminescent porous silicon microparticles for localized delivery of molecular drug payloads. Small 22:2546 Gubin SP (ed) (2009) Magnetic nanoparticles. Wiley-VCH, Weinheim Joo SH, Park JY, Tsung CK, Yamada Y, Yang P, Somorjai GA (2009) Thermally stable Pt/mesoporous silica core–shell nanocatalysts for high-temperature reactions. Nat Mater 8:126 Kinsella J, Ananda S, Andrew J, Grondek J, Chien M-P, Scandeng M, Gianneschi N, Ruoslahti E, Sailor M (2013) Enhanced magnetic resonance contrast of iron oxide nanoparticles embedded in a porous silicon nanoparticle host. Proc SPIE 2013:8594 Ko PJ, Ishikawa R, Takamura T, Sohn H, Sandhu A (2012) Porous silicon based protocol for the rapid and real-time monitoring of biorecognition between human IgG and protein A using functionalized superparamagnetic beads. IEEE Trans Magn 48:2846 Lee J, Lee Y, Youn JK, Na HB, Yu T, Kim H, Lee S-M, Koo Y-M, Kwak JH, Park HG, Chang HN, Hwang M, Park J-G, Kim J, Hyeon T (2008) Simple synthesis of functionalized superparamagnetic magnetite/silica core/shell nanoparticles and their application as magnetically separable high-performance biocatalysts. Small 4:143 Lee J-H, Jang J-T, Choi J-S, Moon S-H, Noh S-H, Kim J-W, Kim J-G, Kim I-S, Park K-I, Cheon J (2011) Exchange-coupled magnetic nanoparticles for efficient heat induction. Nat Nanotechnol 6:418 Lehmann V (2002) Electrochemistry of silicon, instrumentation, science, materials and applications. Wiley-VCH, Weinheim Lukatskaya MR, Vyacheslavov AS, Lukashin AV, Tretyakov YD, Zhigalina OM, Eliseev AA (2009) Cobalt-containing nanocomposites based on zeolites of MFI framework type. J Magn Magn Mater 321:3866 Munoz Noval A, Garcia R, Ruiz Casas D, Losada Bayo D, Sanchez Vaquero V, Torres Costa V, Martin Palma RJ, Garcia MA, Garcia Ruiz JP, Serrano Olmedo JJ, Munoz Negrete JF, del Pozo Guerrero F, Manso Silvan M (2013) Design and characterization of biofunctional magnetic porous silicon flakes. Acta Biomater 9:6169 Munoz-Bonilla A, Marcelo G, Casado C, Teran FJ, Fernandez-Garcia M (2012) Preparation of glycopolymer-coated magnetite nanoparticles for hyperthermia treatment. J Polym Sci Part A Polym Chem 50:5087 Nakamura T, Adachi S (2012) Properties of magnetic nickel/porous silicon composite powders. AIP Adv 2:32167 Pointdexter EH, Caplan PJ, Deal BE, Razouk R (1981) Interface states and electron spin resonance centers in thermally oxidized (111) and (100) silicon wafers. J Appl Phys 52:879 Roca AG, Costo R, Rebolledo AF, Veintemillas-Verdaguer S, Tartaj P, Gonzalez-Carreno T, Morales MP, Serna CJ (2009) Progress in the preparation of magnetic nanoparticles for applications in biomedicine. J Phys D Appl Phys 42:224002 Rumpf K, Granitzer P, Poelt P, Reissner M (2013) Specific loading of porous silicon with iron oxide nanoparticles to achieve different blocking temperatures. Thin Solid Films 543:56

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Photoluminescence of Porous Silicon Bernard Gelloz

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photoluminescence Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photoluminescence of Individual Nanocrystals from Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . Photoluminescence of Porous Silicon Layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

The photoluminescence of mesoporous silicon and silicon nanocrystals has received enormous study over the last 25 years. The spectroscopic nature and efficiency of various emission bands from the near-infrared to the ultraviolet are briefly reviewed, as are mechanistic studies on individual nanocrystals. Improvements in surface passivation and size control of silicon nanocrystals have led to impressive photoluminescence quantum efficiencies in the visible range.

Introduction The demonstration in 1990 that porous silicon could emit efficient tunable visible photoluminescence (PL) at room temperature and attributed to quantum-size effects in crystalline silicon (Canham 1990) has induced considerable worldwide research activities in order to (i) identify the various PL bands and their respective properties and emission mechanisms, (ii) optimize the PL efficiency, (iii) optimize the PL stability, and (iv) tailor the PL spectrum (peak wavelength and FWHM). B. Gelloz (*) School of Engineering, Nagoya University, Nagoya, Japan e-mail: [email protected] # Springer International Publishing Switzerland 2014 L. Canham (ed.), Handbook of Porous Silicon, DOI 10.1007/978-3-319-05744-6_32

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This chapter reviews briefly the specificities of porous silicon PL measurements, the PL of individual silicon nanocrystals from porous silicon, and the PL of porous silicon layers.

Photoluminescence Measurement The PL of porous silicon is quite broad (see Fig. 1a for spectra examples) and inhomogeneous in nature and may involve several phenomena taking place in the silicon nanostructure as well as in the tissue material surrounding it. The chemistry of the various interfaces (e.g. Si/air, Si/SiOx) in porous silicon may also play a very significant role in the luminescence. Thus, extreme care is necessary in the PL measurement procedure and its interpretation (Pelant and Valenta 2012). When using a single broadband grating, care should be taken to eliminate contribution of the short wavelength part to the long wavelength part of the spectrum due to the second order of grating diffraction. Fine structures in the PL have sometimes been observed. At room temperature, these structures (peaks modulating the otherwise generally Gaussian-shaped PL spectrum) were mostly attributed to thin film interference (Kim et al. 2003; Hooft et al. 1992). For the visible range, such interference effect can mostly be observed for layer thicknesses ranging from about 0.5 to 3 μm. At low temperature (20 %). It is limited by the blinking effect, characterized by periods where an excited nanocrystal does not emit any light for an extended period of time and which has been seen earlier in various other semiconductor nanodots. A dot whose surface is not perfectly passivated does not emit light. The surrounding matrix could affect the PL lifetime as well as the emission spectral tunability at the high-energy end of the visible range (Kusova et al. 2010).

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Fig. 1 (a) Room-temperature PL spectra from porous silicon layers with different porosities kept under Ar atmosphere. (b) Experimental and theoretical PL energies as a function of crystallite size. The upper line is the free exciton bandgap and the lower line is the lowest transition energy in the presence of a Si ¼ O bond. The solid and open dots are the peak PL energies of PSi samples kept in Ar and air, respectively (Reprinted figures with permission from Wolkin et al. (1999) as follows: M. V. Wolkin, J. Jorne, P. M. Fauchet, G. Allan, and C. Delerue, Phys. Rev. Lett., 82, 197–200, 1999. Copyright (1999) by the American Physical Society). Image: PL under UV excitation of (c) a porous silicon layer (initial porosity 68 %) treated by HWA (Gelloz et al. 2005; Gelloz and Koshida 2005), (d) the same type of porous layer detached from the substrate and then treated by HWA, and (e) the same type of porous layer heavily thermally oxidized (nanocrystal density negligible) and then treated by HWA (blue PL) (Gelloz and Koshida 2009b; Gelloz et al. 2009)

Photoluminescence of Porous Silicon Layers Depending mostly on the degree of quantum confinement and on the chemical state of its surface, porous silicon could luminesce from the near-infrared (
1.5 μm) to the near-UV as a result of distinct emission bands having different origins (Table 2; Cullis et al. 1997; Bisi et al. 2000). The near-infrared band has not been as extensively

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Table 1 Summary of studies and results related to the photoluminescence of single silicon nanocrystals System studied Si nanopillars oxidized to produce luminescent silicon cores (Valenta et al. 2002; Sychugov et al. 2005) Silicon nanodot with oxide shell prepared by gas phase pyrolysis (Martin et al. 2008)

Individual silicon nanocrystals within porous silicon grains (Valenta et al. 2008; Martin et al. 2004; Credo et al. 1999; Mason et al. 1998, 2001; Gargas et al. 2008)

Organically capped nanosilicon (Kusova et al. 2010)

Comments Efficiency: 35 %. FWHM ~ 130 meV (single band). Emission polarized in arbitrary directions. Blinking kinetics appear to be different from that of porous silicon particles (Sychugov et al. 2005) FWHM ~ 100 meV; satellites at ~150 meV attributed to coupling to LO or TO Si–O–Si phonon modes of SiO2. Strong electron–phonon coupling to Si–O–Si phonons limiting the tunability of light emission versus size at high emission energies Efficiency: 88 % (Credo et al. 1999; Mason et al. 1998); 10–20 % for 1 exciton per nanodot; decreases as the 0.7th power of excitation at higher excitation (Valenta et al. 2008). FWHM ~ 120 meV (Valenta et al. 2008; Credo et al. 1999; Mason et al. 1998); splitting of ~160 meV (attributed to the stretching vibration of the Si–O–Si bridge bond) (Credo et al. 1999; Mason et al. 1998). Efficiency limited by blinking; in blinking, OFF state is due to Auger recombinations and exciton–exciton scattering (Valenta et al. 2008). Blinking obeys a power law statistics (Martin et al. 2004). Ellipsoid shapes of silicon dots; degree of polarization strongly anisotropic; depends on anodization conditions (Gargas et al. 2008) Efficiency: 20 % (for an ensemble of colloidal nanodots). FWHM >100 meV; satellite peak at ~150 meV. Decay time: 10 ns at 550 nm

studied as the visible bands. It was observed in both partially oxidized porous silicon and oxygen-free samples. It has been related to both quantum-size effect and surface states (Koch et al. 1993). The UV band has been observed only in oxidized porous silicon. It has been related to oxide luminescence, with the silicon nanocrystals playing a potential role in the photoexcitation process (Qin et al. 1996). The F-band, with emission peaks around 415–470 nm, FWHM of
0.38–0.5 eV, and quantum efficiencies of 0.1 % at best, has been reported in various thermally or chemically oxidized PSi samples (Koyama and Koshida 1997; Qin et al. 1997; Cullis et al. 1997; Bisi et al. 2000; Canham 1997). The PL lifetime is in the nanosecond range. Many reports have attributed this band to oxide-related defects or contamination (occurring upon storage and prolonged exposure to air) by organic chromophores (e.g., carbonyl groups) (Loni et al. 1995). Recently, another origin has been suggested: direct bandgap core luminescence (Γ-Γ transitions) (Prokofiev et al. 2009). Using layers of oxide-embedded silicon nanocrystals obtained by sputtering, de Boer et al. (2010) have clearly identified this direct bandgap band and referred to it as the “Hot PL band” because it can be observed only under high excitation. Indeed, getting a reasonable probability of radiative Γ-Γ transitions, a constant generation of hot carrier at the Γ15-point (direct gap valley) by Auger

From near-infrared to yellow 590–1,300

surface including oxygen atoms

From yellow to blue 425–630

From near-infrared to blue 400–1,300

hydrogenterminated surface

Hot PL band

Sband

Label IR band

Spectral range/ typical peak wavelength (nm) Near IR 1,100–1,500

Table 2 Porous silicon luminescence bands

Typical: 1–10 % (Billat 1996; Bsiesy et al. 1991; Gelloz and Koshida 2000; Gelloz et al. 1998a,b); record: 23 % (Gelloz et al. 2005) 0.01 %

μs range

ps range

–

Best efficiency

A few ns to ~150 μs

Typical lifetime at RT 10 ns–10 μs

–

Yes

Yes

Directly electrically excitable No

(continued)

Generally proposed origin/ comments States at the silicon surface (Koch et al. 1993; Fauchet et al. 1995; Canham 1995) Quantum confinement in Si nanocrystals; indirect bandgap transitions; blueshift upon size reduction; (Cullis et al. 1997; Bisi et al. 2000; Wolkin et al. 1999) Fast relaxation of excitation to surface states related to Si ¼ O species sets a limit to the size effect and a minimum emission wavelength of ~590 nm. Quantum confinement in Si nanocrystals; direct bandgap transitions; observed only under high excitation; redshift upon size reduction (de Boer et al. 2010)

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Long-lived band

UV band

Label F-band

Table 2 (continued)

UV: ~350 (Qin et al. 1996) 270–290 (Gelloz et al. 2014) Blue-green 450–540

Spectral range/ typical peak wavelength (nm) Blue-green 420 ~ 470

1–8 s up to ~ 200 K; ~1 s at RT

ps–ns

Typical lifetime at RT ~10 ns

2 % at 300 K; 8.5 % at 4 K (Gelloz and Koshida 2012) (including the F-band emission)

Best efficiency 0.1 %

–

No

Directly electrically excitable No

Oxide-related species; emission energy strongly dependent on excitation energy (Gelloz and Koshida 2012, 2009a; Kovalenko et al. 1999; Wadayama et al. 2002; Kux et al. 1995)

Generally proposed origin/ comments Oxide-related defects (Cullis et al. 1997; Bisi et al. 2000; Koyama and Koshida 1997; Qin et al. 1997)/contamination by organic groups (Loni et al. 1995) The Hot PL band may contribute to the green part (Prokofiev et al. 2009) Oxide-related defects (Qin et al. 1996; Gelloz et al. 2014)

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recombination of multiple excitons, is necessary in order to compete with the otherwise very fast (1–10 ps) non-radiative relaxation toward the Δ-valley (indirect gap valley). The Hot PL band characteristics can be found in Table 2. This Hot PL band has only been clearly identified in a partially oxidized system (de Boer et al. 2010). It could be a part of the so-called F-band, in particular the green part, but is unlikely to be the F-band itself, most importantly because (i) it is red-shifted as the crystalline core decreases, reaching the yellow range of the visible spectrum, thus not matching any more the blue emission of the F-band, and (ii) a fast blue band (peak wavelength
420 nm, independent of nanocrystal core size; lifetime
10 ns) was still observed as an independent oxide-related band (likely to be part of the F-band) in addition to the Hot PL band (de Boer et al. 2010). This second point is also consistent with the observation of a fast (nanosecond range) efficient blue emission peaked at
410 nm (FWHM
50 nm) in heavily oxidized porous silicon not containing any significant amount of nanocrystals, thus supporting the attribution of the blue part of the F-band to oxide-related species (Gelloz and Koshida 2009b, 2012). This latter fast blue band has been observed together with a long-lived band (blue-green), which has been studied in details recently (Gelloz and Koshida 2009b, 2012). It is mostly characterized by (i) a long decay time (several seconds) in the range 4–180 K, (ii) a thermally activated quenching from about 180 K (activation energy
0.2 eV), and (iii) a spectrum strongly dependent on the excitation energy. The best PL external quantum efficiencies of this blue band was 2 % at room temperature and 8.5 % at 4 K (Gelloz and Koshida 2012). An emission band showing some similarities with this long-lived band has been observed previously by a few other groups (Kovalenko et al. 1999; Wadayama et al. 2002; Kux et al. 1995). Two of them (Kovalenko et al. 1999; Wadayama et al. 2002) got an emission band peaked roughly around 540 nm for similar excitation wavelengths (337 and 325 nm). Kovalenko et al. (1999) got a decay time of 0.5 s (shorter than that observed by Gelloz et al. (Gelloz and Koshida 2009b, 2012)) at 15 K, in “as prepared” (but probably slightly aged) as well as in oxidized porous silicon, and attributed the emission to quantum confinement in very small silicon nanocrystals. Wadayama et al. (2002) reported an emission decaying very slowly (>1 s) at room temperature in porous silicon having been subjected to rapid thermal oxidation (1,000  C) and quenching in liquid nitrogen. The fast cooling in liquid nitrogen was the necessary step for the very slow band to be observed, which was not understood. Kux et al. (1995) also observed this band in oxidized porous silicon (using rapid thermal oxidation) and suggested it could be related to SiOH groups. Gelloz et al. (Gelloz and Koshida 2009b, 2012) attributed the origin of this band to molecular-like species in or at the surface of the oxide tissue in oxidized porous silicon. Mechanisms based on molecular species are supported by the fact that it could also be observed in fully oxidized porous silicon, with no silicon nanocrystals left in the nanostructure, and also in pure porous glass. Moreover, a similar band has been observed recently in both silica and Al2O3 nanoparticles. The origin of the band was not completely clear but was hypothesized to be luminescence from photogenerated OH radicals (Anjiki and Uchino 2012).

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The most studied and technologically interesting band is the so-called S-band (S for slow; decay times are rather long compared to those of direct bandgap semiconductors). It is electrically excitable and its properties (e.g., emission spectrum, efficiency, chemical activity) can be in principle engineered. Its main characteristics are summarized in Table 3. It originates mostly from exciton recombinations in Si nanocrystals as indicated by polarization memory of PL, PL saturation under high Table 3 Some characteristics of the S-band at 300 K Typical values/ characteristics 1,300–400 nm

Property Peak wavelength

External quantum efficiency

FWHM

Bare layer

Porous silicon microcavity

Decay

Layers: 1–10 % (Billat 1996; Bsiesy et al. 1991; Gelloz and Koshida 2000; Gelloz et al. 1998a, b); record: 23 % (Gelloz et al. 2005) Powders: 10–16 % (Park et al. 2009; Xia et al. 2012; Tu et al. 2012) (Values for red-orange PL) 150–180 nm

13 nm (emission: 740 nm) (Squire et al. 1998) 10–40 meV (emission: 1.5–2.2 eV) (Araki et al. 1996) Decay time: ns to ~150 μs from blue to red; increases at low temperature due to singlet–triplet exciton state splitting (Cullis et al. 1997; Bisi et al. 2000) Multiexponential decay

Comments Porosity dependent; generally related to nanocrystal sizes; can be strongly influenced by surface states (Wolkin et al. 1999) Key factors: nanocrystal density (related to porosity), surface passivation (Gelloz et al. 2005); exciton localization (Billat 1996; Bsiesy et al. 1991; Gelloz and Koshida 2000; Gelloz et al. 1998a, b) Inhomogeneous broadening (mostly size effect) (Cullis et al. 1997; Bisi et al. 2000) Photonic crystal effect (Pavesi et al. 1996; Araki et al. 1996; Gong et al. 2010; Squire et al. 1998) Depends on quantum confinement strength (effect on wavelength) and surface passivation (limited by non-radiative recombinations) (Cullis et al. 1997; Bisi et al. 2000) Multiexponential shape attributed to exciton migration, carrier escape from Si dots, distribution of dot shape emitting at the same energy (Cullis et al. 1997; Bisi et al. 2000), or more recently to the blinking effect (Dunn et al. 2009) (continued)

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Table 3 (continued)

Resonant excitation

Typical values/ characteristics  0.2; decreases with increasing detection wavelength (Cullis et al. 1997; Bisi et al. 2000) Phonon replica at 56 and 19 meV

Nonresonant excitation

Multiple-peak structures; peak separation ~ 64 meV

Property Degree of polarization (nonresonant photoexcitation)

Fine structure at low temperature

Excitation density

Stability/surface chemistry

Output scales linearly with excitation density until Auger effects become predominant. (Cullis et al. 1997; Mihalcescu et al. 1995; Bisi et al. 2000) Poor/Si–H bonds (as-anodized) Good/Si–H bonds replaced by more stable Si–C bonds (Buriak 2002)

Improved/Si–H bonds replaced by more stable Si–Ag bonds (Sun et al. 2005) Good/oxide shell

Comments Can be anisotropic (effect of crystallite shape)

Consistent with TO and TA Si phonons (Cullis et al. 1997; Calcott et al. 1993) Consistent with TO Si phonons (Xu and Adachi 2010) Critical excitation level: more than one exciton per silicon nanocrystal. See Table 1 for more details

Si–H bonds are easily oxidized, even in air Replacing hydrogen by long organic chains also enhance stability by steric hindrance effect (Buriak 2002; Boukherroub et al. 2000)

Si–SiO2 interfacial defects should be minimized. Best techniques: hightemperature oxidation (Cullis et al. 1997; Takazawa et al. 1994; Petrovakoch et al. 1992; Yamada and Kondo 1992), high-pressure water vapor annealing (Gelloz et al. 2005; Gelloz and Koshida 2005, 2007, 2006), chemical oxidation (Xia et al. 2012; Tu et al. 2012)

excitation due to Auger recombinations (Mihalcescu et al. 1995), and resonant excitation and hole-burning experiments (evidencing phonon-mediated recombinations and singlet–triplet exciton state splitting) (Cullis et al. 1997; Bisi et al. 2000). Very high confinement energy (>0.7 eV) results in the breakdown of k-conservation rules and direct recombinations become possible (Kovalev et al. 1998).

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Figure 1a shows the progressive shift of the PL band of porous silicon having its surface terminated by silicon–hydrogen bonds (oxygen-free) across the visible region obtained by changing the anodization conditions (Wolkin et al. 1999). Generally, its intensity increases and its peak wavelength decreases when the porosity increases. The efficiency usually decreases in the order n-type, p-type, n+-type, and p+-type porous silicon due to differences in nanostructures. However, surface states, in particular those introduced by oxygen via Si¼O bonds, can greatly influence the peak wavelength, as shown in Fig. 1b. In zones II and III, localized states lie inside the bandgap, setting a limit to the emission blueshift expected from the size reduction and pinning the emission peak wavelength at about 590 nm for nanocrystals below ~2.5 nm in size (Wolkin et al. 1999). White PL can be obtained using partially oxidized porous silicon emitting two-peak PL spectra resulting from the superposition of the blue-green emission of the F-band (or of both the F-band and the long-lived band) and the yellow-redemitting S-band (Gelloz and Koshida 2005; Gelloz et al. 2009; Dohnalova et al. 2010). While most data relate to porous silicon layers, recently a lot of effort was devoted to the fabrication of porous silicon powders, in particular for bio-imaging purposes. Some powders were obtained by breaking down anodized porous layers followed by various types of oxidation steps used to tune the emission spectra and improve the efficiency and stability (Park et al. 2009; Xia et al. 2012; Tu et al. 2012). Porous silicon layers have also been made using electroless techniques. The technique is used with thin films of microcrystalline silicon produced by PECVD and leads to PL intensities (efficiency 1–10 %) comparable with that obtained from anodized crystalline silicon (Solomon et al. 2008). Surface passivation is essential for high emission quantum efficiency to reduce the competitive defect-related non-radiative recombinations and good stability. Silicon nanocrystals can be passivated by an oxide shell (Cullis et al. 1997; Takazawa et al. 1994; Petrovakoch et al. 1992; Yamada and Kondo 1992; Gelloz et al. 2005; Gelloz and Koshida 2005, 2007, 2006; Xia et al. 2012; Tu et al. 2012) (one of best technique is high-pressure water vapor annealing (Gelloz et al. 2005; Gelloz and Koshida 2005, 2007, 2006)) or by replacing Si–H bonds of as-anodized porous silicon by stronger and more stable bonds, such as Si–C bonds (Buriak 2002; Boukherroub et al. 2000). A striking effect of surface termination has recently been confirmed by Dohnalova et al. (2013), who reported fast (nanosecond range) direct bandgap-like blue emission using silicon colloids (prepared via a chemical route, not from porous silicon) with carbon surface termination. The phonon-less transitions were explained by drastic modifications of electron and hole wavefunctions (these transitions are not direct bandgap Γ-Γ transitions).

Conclusion Recent developments such as fast phonon-less transitions from carbon-terminated nanocrystals (Dohnalova et al. 2013), fast direct bandgap transitions (Prokofiev et al. 2009; de Boer et al. 2010), and very high values of luminescence quantum

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efficiencies of silicon nanocrystals in layers (in porous silicon, 23 % (Gelloz et al. 2005; Gelloz and Koshida 2005), and in other assemblies, 18–100 % (Ledoux et al. 2000) and 60 % (Jurbergs et al. 2006)) show that the luminescence of nanocrystalline silicon is progressively paving its way toward applications.

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Electroluminescence of Porous Silicon Bernard Gelloz

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electroluminescence in Electrolytic Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electroluminescence of Solid-State Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

321 322 323 324 329

Abstract

The performance of porous silicon visible light-emitting diodes is reviewed and compared to those of silicon nanocrystals prepared by other fabrication routes. Efficiencies up to 1 % have been achieved with porous silicon but almost up to 10 % with silicon nanocrystal-polymer composites. Challenges with simultaneously achieving high efficiency, output power, stability, and modulation speed are highlighted. Further improvements could be realized from narrowing the skeleton size distribution and improving the electronic transport and surface passivation of porous silicon.

Introduction Single silicon nanocrystals have been claimed to have very high electroluminescence (EL) yields (up to 20 % (Valenta et al. 2004)). However, a practical LED requires nanocrystals either dispersed in a matrix (like a polymer, or a wide-gap material) or as parts of a silicon nanostructure (such as a porous silicon layer). Because the “S-band” (see chapter “▶ Photoluminescence of Porous Silicon”) is electrically excitable, B. Gelloz (*) School of Engineering, Nagoya University, Nagoya, Japan e-mail: [email protected] # Springer International Publishing Switzerland 2014 L. Canham (ed.), Handbook of Porous Silicon, DOI 10.1007/978-3-319-05744-6_34

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it was hoped that porous silicon EL could bring a revolution in optoelectronics. However, after two decades of intensive research, the challenge of getting, for the same device, high efficiency, high output, and sufficient stability is still open. In the past few years, the interest in using porous silicon for EL has considerably decreased. Very recently, encouraging high LED efficiencies (up to 8.6 % (Cheng et al. 2011) and 1 % (Maier-Flaig et al. 2013)) have been reported using colloidal silicon nanoparticles exhibiting very high photoluminescence external quantum efficiencies (40–60 %) (Jurbergs et al. 2006; Mastronardi et al. 2012). However, recent new advances in porous silicon luminescence efficiency (Gelloz et al. 2005b; Gelloz and Koshida 2005) and stability (Gelloz et al. 2003b, 2005b, 2006) show that porous silicon remains as well a very promising candidate for EL purposes. This chapter reviews the many types of devices that have been proposed and emphasizes the best results.

Electroluminescence in Electrolytic Systems Well-controlled EL has been observed in porous silicon contacted by a liquid electrolyte during anodic oxidation (Halimaoui et al. 1991; Billat 1996) or using redox species able to inject carriers inside silicon nanocrystals (Table 1). The apparently Table 1 EL characteristics of porous silicon contacted by a liquid electrolyte Experimental conditions Anodic polarization. Photogeneration of holes required for porous silicon from lightly doped n-type silicon Anodic polarization

Cathodic polarization. Photogeneration of electrons required for porous silicon from lightly doped p-type silicon (Gelloz and Bsiesy 1998; Peter and Wielgosz 1996)

Carrier injection Holes from substrate; electrons from oxidation of silicon (Chazalviel and Ozanam 1992)

Holes from substrate; electrons from oxidation of methyl viologen (Kooij et al. 1995) or formic acid (Green et al. 1995) Electrons from substrate; holes from reduction of S2O82 (Bressers et al. 1992; Canham et al. 1992; Peter et al. 1996; Romestain et al. 1995; Noguchi et al. 1999)

EL properties EL triggered by injection of holes into luminescent nanocrystals; blue shift of EL during oxidation. EL limited in time because Si nanocrystals are irreversibly oxidized (Halimaoui et al. 1991; Bsiesy et al. 1991; Hory et al. 1995; Cantin et al. 1996; Billat 1996) EL triggered by injection of holes into luminescent nanocrystals; not stable due to simultaneous oxidation of silicon under anodic polarization

EL triggered by injection of electrons into luminescent nanocrystals; EL spectrum is voltage tunable (Romestain et al. 1995). Reversible; stability affected by ion consumption, slow oxidation of silicon surface, and possibly evolution of the location of charge exchanges (Saren et al. 2002; Noguchi et al. 1999). Reaction kinetics affects the EL characteristics (Gelloz and Bsiesy 1998; Gelloz et al. 1996)

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high EL efficiency was explained by efficient carrier injection at low voltages (~1–2 V) within the whole, or a large part, of the porous silicon skeleton (Gelloz et al. 1996), without significant voltage drop across the porous silicon layer itself (Gelloz et al. 1999, 2003a; Gelloz and Bsiesy 1998). Related voltage-tunable and energy-selective photoluminescence quenching (resulting from carrier injection and non-radiative Auger recombinations) was also reported (Romestain et al. 1995; Peter et al. 1996).

Electroluminescence of Solid-State Porous Silicon In solid-state devices, the voltage is applied between the substrate backside and a top semitransparent electrode (usually gold or indium tin oxide). Due to the high resistivity of porous silicon, carrier transport and injection in Si nanocrystals is challenging (see chapter “▶ Electrical Transport in Porous Silicon”). As a result, the EL mechanism in porous silicon mainly involves impact processes (Oguro et al. 1997), even in porosified pn junctions. A porous silicon structure has been specially designed to favor this excitation mechanism (Gelloz et al. 2005a). Porous silicon EL is generally affected by poor efficiency (mainly because of low carrier injection efficiency, leakage, and high applied voltages), poor stability (deterioration of surface passivation, or oxidation (Zhang et al. 1996), during operation and storage), and low output intensity (a few Cd/m2 at best (Gelloz and Koshida 2004a)). Many strategies have been implemented in order to improve the EL efficiency and stability (Gelloz and Koshida 2003, 2004b, 2008). The LED top electrode (usually deposited directly onto porous silicon), also the optical window for EL output, is also a source of stability concerns. Most devices use either ultrathin gold (a few nanometer thick in order to have a reasonable transparency to visible light) or indium tin oxide (ITO) electrodes. ITO shows better stability than gold (Simons et al. 1997), in particular due to better air permeability and better mechanical stability (ITO electrodes can be much thicker than gold ones). Fluorinated tin oxide electrodes exhibited much better stability (three orders of magnitude in lifetime) than ITO (Macedo et al. 2008). The EL uniformity and intensity of large area porous silicon LEDs have been reported to have significantly improved by using a double-layer metallization with a thin semitransparent metal layer at the bottom and a thick metal grid on top. All EL characteristics can be enhanced by inserting a buffer layer between porous silicon and the top electrode. The function of such buffer layer is generally to improve the mechanical and electrical stability, lower leakage, and improve the physical isolation of porous silicon from the environment (capping effect). Thin amorphous carbon films (Hong-Jian et al. 2002; Gelloz and Koshida 2004a), polysilicon (Fauchet et al. 1997), and merely a compact low porosity layer (Gelloz and Koshida 2000) have all been proved very effective. Notice that capping LED with silicon oxide is a good way to improve stability. The enhancement depends on the oxide quality (Koshida et al. 2001).

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EL switching speed is limited, in conventional diodes, to the MHz regime by the luminescence lifetime (Cox et al. 1999). In all types of LED architectures, tunneling through oxide barriers and capacitive effects (which is the greatest limitation in electrolytic systems (Gelloz et al. 2003a)) could also limit the switching speed. However, using a MOSFET configuration, direct modulation of EL beyond radiative recombination rates (up to, but not limited to, 100 MHz) has been demonstrated taking advantage of fast non-radiative Auger recombinations (subnanosecond decay rates). EL was obtained using dc gate excitation, whereas its quenching was obtained from an ac source-to-drain injection (Carreras et al. 2008). The first porous silicon devices included as-anodized layers (Table 2). Then, porosified pn junctions (Table 3), partially oxidized porous silicon (Table 4), impregnation of porous silicon by another conductive material (Table 5), and surface modification (Gelloz et al. 2003b) were investigated. The most noticeable results were obtained using thin pn junctions (about 0.2 % in efficiency) (Loni et al. 1995; Lalic and Linnros 1996a), thermal (Tsybeskov et al. 1995; Hirschman et al. 1996) or electrochemical (Gelloz and Koshida 2000; Gelloz et al. 1998a, b) oxidation (ECO) (from 0.1 % to 1 % in efficiency), and high-pressure water vapor annealing (HWA) (very good stability) (Gelloz et al. 2006). ECO drastically reduces the leakage current while optimizing carrier injection into the nanocrystals. Figure 1 shows a picture of EL obtained for such a device (Gelloz and Koshida 2000). HWA provides a very good surface passivation with stable, good quality, and relaxed oxide. The EL spectrum is generally identical to that of the photoluminescence except for devices where the EL mechanism involves other luminescence bands rather than the S-band. One-peak voltage-tunable EL between 2 and 5 V was observed using a thin and efficient device (Gelloz and Koshida 2004a). As for the photoluminescence, the EL spectrum can be narrowed down to a few nanometers in FWHM using microcavities (Pavesi et al. 1996; Araki et al. 1996; Chan and Fauchet 1999). Integration issues have also been investigated (Barillaro et al. 2001). An EL device was integrated and driven by a bipolar transistor (Hirschman et al. 1996).

Conclusion The recent data of EL quantum efficiencies (up to 8.6 % (Cheng et al. 2011)) of devices including silicon nanocrystals from colloidal solutions are very promising and for now higher than any data obtained from porous silicon (maximum of about 1 % (Gelloz and Koshida 2000)). Also, the size uniformity of silicon nanocrystals obtained by other methods is generally much better than that obtained in porous silicon. Stability remains an issue in all types of devices so far, with one exception, a device including a porous silicon layer treated by HWA, which showed no degradation in several months, though its efficiency was low (Gelloz et al. 2006). So far, the best EL devices based on porous silicon, in terms of efficiency, were those where porous silicon was treated by ECO (optimized PL efficiency and

0.1

n+ (L) Oxide-free, from p(D)

L + H2 exposure for 12 h

1

n(L)

Au or ITO Au Au

Minutes

>5 h

10–10 5–1.5  10

p+n(L)

p+n+(L)

Au

ITO

ECO

3

3–1

Au

2 min exposure in 10 % HNO3

5-

p and p+n (L) p+n(L)

1 min L

Au

EL threshold V-mA/cm2 6 h 80 h Hours Seconds

650

690

670–780

650

Emission peak (nm) 640 700 600 630

1.1–0.08

0.8–0.07

0.2-

0.01-

Highest efficiency EQE-EPE (%) 0.1810 20.180.16–0.016

References Chen et al. (1993) Steiner et al. (1993) Loni et al. (1995) Linnros and Lalic (1995) Peng and Fauchet (1995) Lalic and Linnros (1996a, b) Nishimura et al. (1998) Gelloz et al. (1998a)

Table 3 EL characteristics of most devices based on porosified pn junction. D and L mean that anodization was conducted in the dark and under illumination, respectively. EQE and EPE are external quantum efficiency and external power efficiency, respectively

326 B. Gelloz

ITO ITO

HWA ECO + HWA

Anneal in N2 or in 10 % O2 in N2 ECO ECO ECO ECO

p+p(D)

Al-poly Si ITO ITO Al/n+ ITO

n+(L) n+(L) p(L) n+(D) n+(L) n +p n+

Posttreatment H2O2 oxidation

Structure n(UV)

Contact Au

4

4

4

2.2–7  10 2–10 2–10

4

3.5–4  10 3–10 4

1.5–2

EL threshold V-mA/cm2

Hours > 1 week Days, EQE is stable Stable Stable

1 month

Stability >7 h

700 820

680

640 640

620–770

Emission peak (nm) 460–550

10 3–

1.07–0.37

0.51–0.05 0.21–0.02

0.1-

Highest efficiency EQE-EPE (%)

References Fauchet et al. (1997), Tsybeskov et al. (1995, 1996) Fauchet et al. (1997), Tsybeskov et al. (1995, 1996) Gelloz et al. (1998a) Gelloz et al. (1998b) Pavesi et al. (1999) Gelloz and Koshida (2000, 2004a) Gelloz and Koshida (2006) Gelloz et al. (2006)

Table 4 EL characteristics of most devices based on partially oxidized porous silicon. D and L mean that anodization was conducted in the dark and under illumination, respectively. EQE and EPE are external quantum efficiency and external power efficiency, respectively

Electroluminescence of Porous Silicon 327

Structure p(D)

n+(D)

n(L)

n(L:UV)

n(L:UV) n(L:UV)

n(L:UV)

n(L:UV)

Contact Au

Au

Au

Au

Au Au

Au

Au

Al electroplating

Sb electroplating

Ga electroplating Sn electroplating

Posttreatment Polypyrrole electrodeposition PANI chemical deposition Polyaniline chemical deposition In electroplating

0.1

0.1

500

3–400

EL threshold V-mA/cm2 2– 690 nm, (4) λ > 730 nm, (5) λ < 580 nm (Blonskyy et al. 2003)

Possible Use of PS Thermoluminescence for Dosimetry of Ionizing Radiation The generally accepted requirements imposed on materials for TL dosimeters are the following: • • • • •

High TL signal per units absorbed dose (i.e., high luminescence yield Y ) Wide interval in which the luminescence intensity is linear with absorbed dose Low dependence of the TL response on the energy of the incident radiation Low fading (i.e., ability to store dosimetric information for a long time) Simple TL glow curve (i.e., one isolated peak is observed)

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V. Skryshevsky

The TL dosimetric materials should also be chemically inert, mechanically strong, and radiation resistant (Moscovitch and Horowitz 2007; Chen and McKeever 1997). Today, commercial TL dosimeters have been designed using alkali-earth haloids LiF:Mg, Ti (TLD-100, TLD-600, TLD-700), CaF2:Dy (TLD-200), CaF2: Mn (TLD-400), oxides Al2O3:C (TLD-500), BeO, SiO2, sulfates MgSO4, CaSO4, AS sulfides (A ¼ Mg, Sr, Ca, Ba), and others materials (Rivera 2011; Moscovitch and Horowitz 2007; Kortov 2007; Chen and McKeever 1997). The typical radiation range of such dosimeters is 0.01 μGy–100 Gy. The majority of the commercial TL dosimeters have small fading (less than 5 % annually). Spectra of commercial luminescent dosimeters exhibit emission at 380–480 nm, TL peak is maximum at 180–260
C, making it easy to read (Kortov 2007). From the simple band model of luminescence, it follows that fading is small if the electron traps, which produce luminescence excitation centers, have a sufficiently large energy depth. The temperature of glow peak maximum (180–260
C) corresponds to the relatively deep centers (Ea ¼ 0.8–1.2 eV). So, the requirement of small fading leads to TL dosimetric materials being wide band gap dielectrics (Kortov 2007). The main motivation to use the PS thermoluminescence for dosimetry of ionizing radiations lies in the following: (i) high PL quantum yield of PS (tens of percent) (see handbook chapter “▶ Photoluminescence of Porous Silicon”), (ii) TL spectra of nano-PS as well as commercial dosimeters exhibits emission in the visible region, and (iii) PS is a biocompatible material (see handbook chapter “▶ Biocompatibility of Porous Silicon”) that allows to create a vivo dosimetry systems of high spatial resolution. However, low-energy depth of the traps in the PS (less than 0.4 eV) results in low temperature of TL peaks (less than 250 K, Figs. 1, 2, 3, and 4) and high fading comparing with commercial dosimeters that restricts the PS application. Despite this, further study is warranted, taking into account that various forms of silica (Trukhin et al. 2007; McKeever 1984) and quartz crystals (McKeever 1984; de Carvalho et al. 2010; Wintle 1997; Koul 2008) reveal above room temperature TL peaks up to 400–500
C. For non-sensitized natural quartz, the increase in TL intensity near 100
C and 325
C is observed with particle size decreasing, and it is explained by the increase in the specific surface area (Fig. 5) (de Carvalho et al. 2010). Also, there is at present interest in the TL response and dosimetric utility of commercially available doped optical fibers. Optical fibers based on Ge-doped (GeD2) silica are characterized by storage stability and reliability and provide higher luminescence yield in comparison to the undoped fiber or to those containing impurities like Al, F, P and some commercial TLD. As can be seen from Fig. 6, the TL of the GeD2 fiber has higher yield compared to two commercially available TL dosimeters TLD500 and TLD700, commonly used as in vivo radiation dosimeters for patients undergoing radiotherapy (Benabdesselam et al. 2013). The dosimetric peak of GeD2 is located at 530 K, throughout the temperature range; one can observe only one blue-violet emission (at around 3.1 eV). This well-known emission has been ascribed to the luminescence of the twofold Ge center (¼Ge:). TL response of GeD2 is linear with absorbed dose up to 900 Gy with 5 % fading

Thermoluminescence of Porous Silicon

341

Fig. 5 Glow curve of quartz particles different sizes sensitized with 50 Gy of γ-radiation and heat treatments at 400
C. The particle size: 300  425 μm ( filled square), 75  150 μm (open circle), 38  75 μm ( filled triangle), and less than 38 μm (open rhomb) (de Carvalho et al. 2010)

Fig. 6 TL superposition of the GeD2 optical fiber, TLD500 and TLD700 under the same conditions of irradiation and readout (Benabdesselam et al. 2013)

342

V. Skryshevsky 8

PMT Current (10−9 A)

7 6 5

219°C 271°C

103°C

4

155°C

3 2 1 50

100

150

200

250

300

350

400

Temperature (°C)

Fig. 7 TL glow curve of PS irradiated by X-rays at room temperature and heated to 400
C. The solid line is experimental data, and the dashed lines are fits of the standard TL expression to the data (Cooke et al. 1997)

during 8 h (Benabdesselam et al. 2013). Clinical applications based on TL properties of Ge-doped optical fibers are also described (Abdul Rahman et al. 2012). The most sensitive TL material was found to be 4.0 mol% aluminum-doped silica, providing 3.5 times the luminescence yield of TLD100 and 5.4 times that of germanium-doped silica with TL peaks at 160
C, 225
C, and 310
C. The photon dose response of aluminum-doped silica was observed to be linear over the range of investigated dose, 0.5–10.0 Gy (Yusoff et al. 2005). For 6-, 9-, and 12-MeV electron energies, the minimum detectable doses were in the range 3–5, 30–50, and 800–1,400 μGy for TLD-100 chip, Ge-doped, and Al-doped fibers, respectively (Hashim et al. 2009). Since PS is easily oxidized, it should be checked as an attractive dosimeter material. It was found that TL glow curve of PS irradiated by X-rays at Troom and heated in 50–400
C interval exhibits wide curve with peaks at 103
C, 155
C, 220
C, and 270
C (Fig. 7). Each of the peaks emits similar emission of red-orange PL spectra (Cooke et al. 1997). It is assumed that these high-energy peaks are associated with radiation-induced defects E0 ( Si •) and nonbridging oxygen hole centers (NBOHCs) ( Si  O •) that generated in insulating SiOx layer which covers the PS surface. Another potential application is using PS as a template for various scintillation materials. For example, the dosimeter based on TL of InxTl1xI nanostructures synthesized in PS voids and exposed to hard γ-radiation (60Co) was studied (Franiv et al. 2004). It was shown that the integrated emission intensity depends on the size of nanocrystals and luminescence yield of TL linearly increases with the increasing dose absorbed in the crystal (Fig. 8).

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343

Fig. 8 Glow curve of InxTl1xI in mordenite (1) and PS (2–5) matrices and exposed to hard γ-rays for different time (Hours): 0.5 (2), 2.0 (3), 6.0 (4), 12.0 (5). Irradiation source: 60Co, 1.25 Mev, 8 mR/h (Franiv et al. 2004)

Conclusion Along with alternative methods, TL is a useful tool to study the energy levels of electron traps in PS, since it is contactless and sensitive to the formation mode of the luminescent material. Currently, TL in PS has not been developed for radiation dosimeter applications, due to the low depth of the traps and strong fading. Nevertheless, with the progress in the creation of a competitive commercial TL dosimeters based on doped silica, observations of high temperature peaks of TL in oxidized PS, the biocompatibility, and other properties of PS, it makes sense to further study this material for dosimetry of ionizing radiations in vivo. Also of interest is the use of PS as a template for other scintillation materials.

References Abdul Rahman AT, Hugtenburg RP, Abdul Sani SF, Alalawi AIM, Issa F, Thomas R, Barry MA, Nisbet A, Bradley DA (2012) An investigation of the thermoluminescence of Ge-doped SiO2 optical fibres for application in interface radiation dosimetry. Appl Radiat Isot 70:1436–1441 Anastasiadis C, Triantis D (2000) Thermally stimulated detrapping in porous silicon. Mater Sci Eng B 69–70:149–151 Benabdesselam M, Mady F, Girard S (2013) Assessment of Ge-doped optical fibre as a TL-mode detector. J Non-Cryst Solids 360:9–12 Blonskyy IV, Brodyn MS, Vakhnin AY, Kadan VM, Kadashchuk AK (2001) Thermoluminescent study of porous silicon. Phys Lett A 279:391–394 Blonskyy IV, Kadan VM, Kadashchuk AK, Vakhnin AY, Zhugayevych AY, Crervak IV (2003) New mechanism of charge carriers localization in silicon nanowires. Phys Low Dimens Struct 7/8:25 Brodovoy OV, Skryshevsky VA, Brodovoy VA (2002) Recombination properties of electronic states in porous silicon. Solid State Electron 46(1):83–87 Chen R, McKeever SWS (1997) Theory of thermoluminescence and related phenomena. World Scientific, Singapore Ciurea ML, Baltog I, Lazar M, Iancu V, Lazanu S, Pentia E (1998) Electrical behaviour of fresh and stored porous silicon films. Thin Solid Films 325(1–2):271–277

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Ciurea ML, Draghici M, Lazanu S, Iancu V, Nassiopoulou A, Ioannou V, Tsakiri V (2000) Trapping levels in nanocrystalline porous silicon. Appl Phys Lett 76:3067–3069 Cooke DW, Bennett BL, Farnum EH, Hults WL, Muenchausen RE, Smith JL (1997) Thermally stimulated luminescence from x-irradiated porous silicon. Appl Phys Lett 70:3594 de Carvalho lB Jr, Guzzo PL, Sullasi HL, Khoury HJ (2010) Effect of particle size in the TL response of natural quartz sensitized by high dose of gamma radiation and heat-treatments. Mater Res 13(2):265–271 Franiv AV, Bovgyra OV, Savchyn OV (2004) Thermostimulated luminescence spectra of InxTl1xI nanostructures synthesized in porous silicon. Funct Mater 11(4):742–745 Hashim S, Al-Ahbabi S, Bradley DA, Webb M, Jeynes C, Ramli AT, Wagiran H (2009) The thermoluminescence response of doped SiO2 optical fibres subjected to photon and electron irradiations. Appl Radiat Isot 67:423–427 Kortov V (2007) Materials for thermoluminescent dosimetry: current status and future trends. Radiat Meas 42:576–581 Koul DK (2008) 110
C thermoluminescence glow peak of quartz-a brief review. Pramana J Phys 71(6):1209–1229 Kovalev D, Heckler H, Averboukh B, Ben-Chorin M, Schwartzkopff M, Koch F (1998) Hole burning spectroscopy of porous silicon. Phys Rev B 57(7):3741–3744 McKeever SWS (1984) Thermoluminescence in quartz and silica. Radiat Prot Dosim 8(1/2):81–98 Moscovitch M, Horowitz YS (2007) Thermoluminescent materials for medical applications: LiF: Mg, Ti and LiF:Mg, Cu, P. Radiat Meas 41:S71–S77 Pincˇik E, Bartosˇ P, Jergel M, Falcony C, Bartosˇ J, Kucˇera M, Kákosˇ J (1999) The metastability of porous silicon/crystalline silicon structure. Thin Solid Films 343–344:277–280 Rivera T (2011) Synthesis and thermoluminescent characterization of ceramics materials. In: Sikalidis C (ed) Advances in ceramics – synthesis and characterization, processing and specific applications. InTech, Rijeka, Croatia pp 127–164 Skryshevskii YA, Skryshevskii VA (2001) Thermally stimulated luminescence in porous silicon. J Appl Phys 89(5):2711–2714 Tale IA (1981) Trap spectroscopy by the fractional glow technique. Phys Status Solidi A66:65–75 Tretyak OV, Skryshevsky VA, Vikulov VA, Boyko YV, Zinchuk VM (2003) Surface electronic states in metal-porous silicon-silicon structures. Thin Solid Films 445(1):144–150 Trukhin AN, Troks J, Griscom DL (2007) Thermostimulated luminescence and electron spin resonance in X-ray- and photon-irradiated oxygen-deficient silica. J Non-Cryst Solids 353:1560–1566 Wintle AG (1997) Luminescence dating: laboratory procedures and protocols. Radiat Meas 27(5/6):769–817 Yusoff AL, Hugtenburg RP, Bradley DA (2005) Review of development of a silica-based thermoluminescence dosimeter. Radiat Phys Chem 74:459–481

Optical Gain in Porous Silicon Katerˇina Herynková and Ivan Pelant

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Stimulated Emission in Silicon Nanocrystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photoluminescence of Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Optical Gain in Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

345 346 347 349 350 352

Abstract

Realization of a silicon-based laser, integrated with optical and electronic components on a single chip, remains one of the main challenges in optoelectronic technology. Achievement of optical amplification in silicon nanocrystals is a complex problem depending on many factors; however, it is theoretically possible. The chapter discusses the potential of luminescent porous silicon for lasing and lists observations of positive optical gain in various types of silicon nanocrystals. Several different approaches lead to observation of small positive optical gain in porous silicon-based samples; however, a real injection silicon-based laser has not yet been demonstrated.

Introduction Realization of an efficient silicon-based light source or laser, integrated with optical and electronic components on a single chip, remains one of the main challenges in silicon-based optoelectronic technology. Positive net optical gain K. Herynková (*) • I. Pelant Institute of Physics, Academy of Sciences of the Czech Republic, Prague 6, Czech Republic e-mail: [email protected]; [email protected] # Springer International Publishing Switzerland 2014 L. Canham (ed.), Handbook of Porous Silicon, DOI 10.1007/978-3-319-05744-6_36

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(i.e., amplification of light incident on the material) due to the presence of stimulated emission and population inversion is, together with a positive feedback, a necessary prerequisite for lasing. Featuring efficient room-temperature luminescence, porous silicon and other systems containing silicon nanocrystals belong among possible candidates. The history of searching for optical gain in (bulk) silicon reaches back to the era of the first theoretical prediction of lasing in semiconductors (Bernard and Duraffourg 1961) and successful construction of the first worldwide semiconductor laser in the beginning of the 1960s (Hall et al. 1962; Nathan et al. 1962). Since then, there has been an intensive discussion whether it is possible to observe light amplification also in indirect band gap materials such as silicon or germanium (Adams and Landsberg 1968). Even though the indirect band gap materials show very weak luminescence due to the involvement of the third particle – phonon – in the radiative recombination process, there is no “nonexistence” theorem stating that lasing is impossible in indirect semiconductors (Landsberg 1967). Nevertheless, W.P. Dumke in his theoretical work in 1962 (Dumke 1962) ruled out both silicon and germanium as he concluded that a small positive optical gain for band-to-band transitions achievable in these indirect semiconductors is always overcompensated by stronger positive free carrier absorption (FCA) where a photon absorbed by a material excites a free carrier from a filled state to an unoccupied state in the same band. It is true that to date no experiment has found stimulated emission in bulk silicon, even though some recent theoretical works pointed out that Dumke probably did not formulate his calculations fully correctly. These works also particularized the conditions for at least theoretical achievement of optical amplification in bulk silicon (Trupke et al. 2003; Chen et al. 2006).1

Stimulated Emission in Silicon Nanocrystals Discovery of strong visible luminescence of porous silicon (a system of interconnected silicon nanoparticles) by L. Canham (1990) gave a new motivation for the search of optical gain in silicon. In silicon nanocrystals, the conditions for achieving optical amplification might be generally more favorable compared to the bulk material. The main reasons are as follows: (i) Strong spatial localization of the carriers increases the overlap of their wavefunctions and consequently the rate of radiative recombination. Moreover, the complementary overlap of the electron and the hole wavefunctions in k-space due to Heisenberg’s uncertainty relations should result

1

In bulk germanium, first attempts to obtain real germanium lasing structure failed too (Chynoweth AG et al (1963) Bell telephone laboratories technical memorandum no 63-1151-12, 63-1131-8); however, 40 years later J. Michel et al. successfully demonstrated positive optical gain taking advantage of heavy Ge n-doping along with tensile-strain-induced modification of the germanium electronic band structure (Liu J et al (2009) Opt Lett 34:1738).

Optical Gain in Porous Silicon

347

in gradual appearance of the quasi-direct electron-hole recombination without phonon assistance. (ii) The quantum confinement effect in quantum dots opens their bandgap and raises consequently the energy of the emitted photon hν coming from the radiative recombination of an electron and a hole across the band gap. As the cross section of FCA is, according to the Drude model, proportional to 1/ν2, the undesirable FCA effect is strongly suppressed in nanocrystals compared to bulk silicon. Smaller nanocrystals with higher band gap have in general better conditions for stimulated emission. (iii) The stronger wavefunctions overlapping, however, has also a negative impact because of enhancing non-radiative Auger recombination which can deplete possible population inversion. Nevertheless, the three- or four-particle (when an extra phonon is involved) Auger process may be strongly inhibited using moderate excitation fluencies when less than two electron-hole pairs per quantum dot are excited (M’ghaieth et al. 1999). More complexity to the problem of the optical gain in nanocrystals is added by the size distribution of the silicon nanocrystals in real samples and a substantial role of the surface due to increased surface/volume ratio (Wolkin et al. 1999). Isolated theoretical analyses were not able to give a satisfactory answer to the problem of light amplification in silicon so far (Dal Negro et al. 2003; Degoli et al. 2009); therefore, the experimental approach is usually chosen to decide whether optical gain is present in the particular sample or not. The first successful observation of positive optical gain in silicon nanocrystals dispersed in a silicon dioxide matrix was reported in 2000 by the Trento group (Pavesi et al. 2000), and similar results were achieved also by several other groups shortly afterwards (see Table 1 for the list of the relevant publications). In order to illustrate a variety of possible alternative approaches, Table 1 lists studies of optical gain in silicon nanocrystals (Si-ncs) prepared by other techniques than via porous silicon.

Experimental Methods The direct evidence of stimulated emission and optical gain can be provided either by pump and probe optical transmission measurements where a weak probe beam is amplified by the sample in which the population inversion was generated by an intense pump beam (Pelant and Valenta 2012) or by the so-called variable stripe length (VSL) technique (Shaklee and Leheny 1971; Shaklee et al. 1973). In the VSL technique, the photoluminescence propagating in a narrow laser-excited stripe (playing the role of the probe beam) is amplified by stimulated emission; this is the so-called amplified spontaneous emission, ASE. Positive optical gain is then evidenced by i) exponential growth of the luminescence intensity with the stripe length, ii) an accompanying spectral narrowing for longer stripes, and iii) superlinear increase of the emission intensity with increasing pump power. The VSL method, however, is prone to experimental gain-like artefacts (Dal Negro et al. 2004b; Valenta et al. 2003), e.g., waveguide leaky (substrate) modes may be easily misinterpreted as the optical gain (Luterová et al. 2006; Valenta et al. 2002). Therefore, extreme care

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Table 1 Optical gain studies in various kinds of silicon nanostructures (apart from porous silicon). All data refer to room temperature Material Si-ion-implanted quartz or thermal SiO2 Si-ncs prepared by PECVD

Gain coefficient 100 cm 1

Detection wavelength 800 nm

52 cm

1

760 nm

Si-ncs/SiO2 multilayers in SiO2

50 cm

1

730 nm

Si/SiO2 lattice by reactive deposition on fused quartz Superlattices with monodispersed Si-ncs Si-ion-implanted SiO2 films Si-ion-implanted SiO2 films Si-ion-implanted SiO2 layers Nitride-passivated Si-ncs grown by PECVD Si-ncs embedded in silicon nitride grown by PECVD Si-rich silica waveguides with Er3+ ions Si-ncs-based laser cavity Si-ncs by reactive deposition on fused quartz

6 cm

1

720 nm

26 cm

1

750 nm

12 cm

1

760 nm

No value given No gain

450 nm

No gain

52 cm

1

1.12 dB/cm

438 cm/MW

Publication Pavesi et al. (2000) Dal Negro et al. (2003, 2004a) Ruan et al. (2003); Fauchet et al. (2005) Khriachtchev et al. (2001) Cazzanelli et al. (2004a) Luterova et al. (2005) Luterová et al. (2002) Elliman et al. (2003) Chen et al. (2007)

700 nm

Monroy et al. (2011)

1.53 μm

Navarro-Urrios et al. (2006)

1.54 μm

Koshel et al. (2011) Sirleto et al. (2012)

Comment

Only fast ns component having gain Only fast ns component having gain Small size dispersion of Si-ncs

Comparison of oxygen/nitride passivation of Si-ncs

cw Raman gain

has to be given to the experiment, and it was suggested to complete the VSL measurement by the so-called shifted-excitation-spot (SES) measurement (Valenta et al. 2002) in order to exclude possible experimental artefacts. However, it should be mentioned that some concerns persist in the research community all the time about correctness of interpretation of the whole family of experimental results indicating the presence of optical gain in silicon nanostructures.

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Photoluminescence of Porous Silicon Porous silicon emits bright room-temperature luminescence in the visible region (Fig. 1). The emission spectrum depends on the preparation conditions and in general contains two emission bands: The main, “slow” (S) or red-emission band, is peaked at 600–850 nm and decays slowly on 10–100 μs scale. The microscopic origin of the S-band is usually explained as a radiative recombination in the nanocrystal core supplemented with some oxygen-related surface recombination channel (Wolkin et al. 1999). The second, “fast” (F) or blue-emission band, is located between 420 and 500 nm and has a very fast decay in nanosecond time scale. No consensus exists so far about its origin: It has been interpreted as due either to some structural defects in the silicon nanocrystal oxide shell (Tsybeskov et al. 1994), to an intrinsic emission in the core of small silicon nanocrystals (Valenta et al. 2008), or to direct-band-gap electron-hole recombination at the Γ-point of the energy band structure of silicon nanocrystals (Prokofiev et al. 2009). For more data on the efficiency and spectral characteristics of the visible photoluminescence, the reader can consult the dedicated chapter “▶ Photoluminescence of Porous Silicon” in this handbook.

Fig. 1 (a) Example of room-temperature photoluminescence spectra of porous silicon samples. (b) and (c): temporal evolution of the F and S bands, respectively (Dohnalová et al. 2009) (Reprinted with permission from IOP Publishing)

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K. Herynková and I. Pelant

Optical Gain in Porous Silicon In as-prepared standard porous silicon layers on silicon wafers, positive optical gain has not been achieved. This is likely due to two facts: (i) huge light scattering loss in the spongy structure and (ii) strong absorption of pump light in the opaque underlying wafer, local heating, and rapid burning (eruption) of the thin and fragile porous layer. Nevertheless, positive optical gain was reported in specially prepared samples made of porous silicon. The first successful approach, used by Luterová et al. (2004) and Dohnalová et al. (2008, 2009), used silicon nanocrystals embedded into a transparent SiO2 matrix (which has lower absorption in the UV region than bulk Si) in order to avoid excess heating owing to intense pump laser pulses. Here,

Fig. 2 (a) The S-band: VSL for low (0.4 MW/cm2, black symbols; 10 times magnified) and high (1.5 MW/cm2, red symbols) excitation laser intensity. Exponential increase of the high excitation curve yields a net gain coefficient of 41 cm 1. (b) The F-band: Two-dimensional map of the normalized amplified spontaneous emission (ASE) spectra as a function of the stripe length shows remarkable narrowing. (c) The F-band: Dependence of the ASE on the excitation density in a log-log scale for three different stripe lengths, 0.1 mm (red), 1 mm (black), and 4.8 mm (blue), reveals a superlinear increase. (d) Microscopic mechanism – selective absorption scheme (F, excited state (F-band); S, metastable state (S-band); G, ground state shared by F and S states) (After Dohnalová et al. (2008, 2009). Reprinted with permission from IOP Publishing)

Optical Gain in Porous Silicon

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Table 2 Reports on optical gain in porous silicon. If not explicitly stated otherwise, all studies were done at room temperature Material Reconstituted films from colloidal solution of ultrasmall silicon nanoparticles Dried films from colloidal solution of ultrasmall silicon nanoparticles Heavily oxidized porous silicon

Gain coefficient 103–105 cm 1

Detection wavelength F-band, 390 nm

Publication Nayfeh et al. (2001)a

Comment

7  104 cm 1

S-band, 610 nm

Nayfeh et al. (2002)a

Observation of laser oscillations

F-band, 450–460 nm S-band, 650 nm S-band, 620 nm F-band, 430 nm

Cazzanelli et al. (2004b)

Polarizationresolved gain

Luterová et al. (2004) Dohnalová et al. (2010) Dohnalová et al. (2009, 2010) Zˇ´ıdek et al. (2011)

Femtosecond excitation Time-resolved measurements

74 cm

1

Porous silicon grains in sol-gel SiO2 matrix Oxidized porous silicon in sol-gel SiO2 matrix Oxidized porous silicon in sol-gel SiO2 matrix

25 cm

1

Porous silicon with different surface cappings

100 cm

Dye-impregnated oxidized porous silicon waveguides Nanopatterned Si

9 cm 1 (40 dB/cm)

Carbon-enriched nanopatterned Si Porous silicon planar waveguides co-doped by Er and Yb ions

5 cm

1

22 cm

1

1

S-band, 590 nm

700 nm

Oto´n et al. (2006)

1.278 μm

Cloutier et al. (2005)

No value given

1.278 μm

Rotem et al. (2007)

6.4 dB/cm

IR, 1.53 μm

Najar et al. (2007, 2009)

260 cm

1

Femtosecond excitation, ultrafast (> mesoporous > macroporous (Sailor 2012). As mentioned above, silicon oxides will dissolve in strong base following Eq. 10. The product shown in Eq. 10 is the species [SiO2(OH)2]2, the doubly deprotonated form of silicic acid, Si(OH)4. While it is the dominant species that exists in highly basic (pH > 12) water (pKa of [SiO(OH)3] ¼ 12), in neutral or acidic solution, silicic acid exists in its fully protonated form, Si(OH)4 (pKa of Si(OH)4 ¼ 10) (Greenwood and Earnshaw 1984). Thus at neutral, physiologic pH the dissolution of SiO2 proceeds as given in Eq. 15. SiO2 þ 2 H2 O ! SiðOHÞ4ðaqÞ

(15)

When the solution concentration of Si(OH)4 is sufficiently large, the reaction of Eq. 15 runs in reverse, and silicic acid condenses back into a solid (Eq. 16). In the course of this condensation reaction, various “polysilicic acids” with the general formula [SiOx(OH)42x]n, where 2 <  < 0, are present in solution (Greenwood and Earnshaw 1984). In neutral or acidic solutions, these oligomers can condense to the point of precipitation. SiðOHÞ4ðaqÞ ! SiO2 þ 2 H2 O

(16)

The reaction of Eq. 16 is the chemistry that occurs during the “sol–gel” process, used to prepare colloids, films, or monoliths of porous silica from solution

Chemical Reactivity and Surface Chemistry of Porous Silicon

365

precursors (Brinker and Scherer 1990). This reaction explains why elemental silicon does not corrode appreciably at pH values 500  C, significant carbonization occurs (Salonen et al. 2004), leading to black films with a sooty appearance (Ruminski et al. 2010). This more extensive pyrolysis removes the residual hydrogen from the hydrocarbon layer, and the high carbon content material is referred to as “thermally carbonized porous Si,” or TCPSi. The acetylene precursor used in these reactions can be replaced with an organic polymer (polyfurfuryl alcohol) to generate similar materials (Kelly et al. 2011). Although less well defined than material prepared by the hydrosilylation route, the pyrolyzed porous Si formulations are chemically stable (Salonen et al. 2004; Bimbo et al. 2010; Jalkanen et al. 2009; Limnell et al. 2007; Tsang et al. 2012), leading to strong interest in these formulations for bioimplant, drug delivery

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(Salonen et al. 2005a, b, 2008; Lehto et al. 2005; Limnell et al. 2006; Heikkila et al. 2007; Kaukonen et al. 2007), and biosensor (Ruminski et al. 2010; Jalkanen et al. 2009; Tsang et al. 2012; Salonen et al. 2006; Bjorkqvist et al. 2004b, 2005) applications.

H Si Si

Si

Si Si

H

H

H +H

C

Si

C

H

485 °C

C

C

Si C

C

H

H Si C

C

ð22Þ

C

Modification strategies that generate functional nanostructures from pyrolyzed, carbon-containing porous Si have been demonstrated (Jalkanen et al. 2009; Salonen et al. 2006; Bjorkqvist et al. 2004b, 2005; Sciacca et al. 2010). The radical coupling method of Iijima (Iijima and Kamiya 2008), developed to functionalize carbon fibers and diamond-like carbon, has been found to work on hydrocarbonized porous Si as well (Sciacca et al. 2010). For hydrocarbonized porous Si, the reaction involves a radical initiator (benzoyl peroxide) and a dicarboxylic acid linker (Eq. 23). The chemistry allows permeation of aqueous solutions and attachment of specific biological capture probes.

ð23Þ

Porous Silicon Nitrides After silicon oxide, silicon nitride is probably the most important modification of silicon in the semiconductor industry. A common etch stop and dielectric layer used in integrated circuit manufacture, the fracture toughness and durability of silicon nitride (Si3N4) has also spurred its use in high performance parts such as bearings and gas turbine blades. The larger index of refraction of Si3N4 relative to SiO2 (2.0 vs. 1.5, respectively) makes silicon nitride an attractive alternative to

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silicon oxide for optical devices made from porous Si. The industrial processes used to prepare silicon nitride involve direct reaction of Si with N2 at high temperature (for the bulk material) and CVD or plasma-enhanced CVD deposition from N2, NH3, and silane precursors (for thin films). Similarly, porous Si can be nitrided by heating in NH3 or N2 ambients (Bjorkqvist et al. 2003; Morazzani et al. 1996) or by plasma-assisted CVD. A typical thermal preparation involves heating in pure N2 at 1,100  C for 12 min (Bjorkqvist et al. 2003). Such high temperatures tend to decrease surface area, pore diameter, and pore volume in the resulting material (Bjorkqvist et al. 2003). A low temperature (600  C) process has been developed using a rapid thermal processor in a pure N2 ambient (James et al. 2010). Although the lower temperature preserves the pore structure, it introduces a significant quantity of silicon oxide (Lai et al. 2011). The stability of porous Si samples modified with silicon nitride is improved compared with as-formed porous Si, though it is comparable to samples prepared with a thermally grown silicon oxide (Bjorkqvist et al. 2003; James et al. 2009, 2010; Lai et al. 2011).

Attachment of Biomolecules to Functionalized Porous Si Biologically active molecules have been covalently attached to porous Si surfaces for a variety of medically relevant applications: antibodies have been attached to porous Si to impart molecular selectivity for biosensing (Serda et al. 2010; Tinsley-Bown et al. 2000; Gu et al. 2012; Lowe et al. 2010; Andrew et al. 2010; Wu et al. 2009; Rossi et al. 2007; Meskini et al. 2007; Bonanno and DeLouise 2007; Park et al. 2006; Starodub et al. 1996); therapeutics have been attached to porous Si for slow release drug delivery (Wu et al. 2008, 2011a, b; Chhablani et al. 2013; Sailor and Park 2012); and sugars and polyethers have been attached to porous Si as antifouling coatings in bioimplants (Schwartz et al. 2005; Kilian et al. 2007a, b; Godin et al. 2010). The strategy generally involves a linker molecule that connects the biologically active species to the porous Si surface. Many suitable surface attachment chemistries for such linkers have been described in this chapter. To attach a biological species to an immobilized linker, an activating step is usually employed to allow formation of a covalent bond between the species and the linker. The book Bioconjugate Techniques by Hermanson (1996) is a leading reference on the chemical reactions that can be employed to form these bonds. The most common approach for porous Si involves the use of carbodiimide coupling reagents such as 1-ethyl-3[3-dimethylaminopropyl]carbodiimide hydrochloride, or EDC. EDC is employed to couple a primary amine (NH2) group to a carboxyl group, forming an amide bond. Thus the chemistry is amenable to a porous Si surface that has been modified with carboxyl (CO2H) or amine (NH2) species as long as the target molecule has a corresponding amine or carboxyl group. One example is given in Eq. 24 (Wu et al. 2008).

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ð24Þ

Comparison of Si–C vs. Si–O Chemistries For situations where the functionalized material must eventually dissolve, such as in vivo drug delivery, chemistries involving Si–O bonds represent an attractive alternative to Si–C chemistries. The time needed for highly porous SiO2 to dissolve in aqueous media can be on the order of hours to days – appropriate for many shortterm drug delivery applications. In contrast, the Si–C chemistries of porous Si generally display significantly longer degradation times, which can be as long as several months or even years (Canham et al. 1999, 2000; Cheng et al. 2008). This is particularly true of the pyrolysis-derived (carbonized and hydrocarbonized) materials (Eq. 22) (Salonen et al. 2008). The well-defined degradation mechanism for porous SiO2 generates silicic acid (Si(OH)4) as the end product (Eq. 15), which is a naturally occurring species that is present in body tissues at a mean concentration of ~5 ppm (Van Dyck et al. 2000). In contrast, samples prepared by pyrolysis of organic materials may contain polycyclic aromatics or other potentially toxic species that could be difficult to identify or characterize. For both the thermal oxide and carbon pyrolysis reactions, the degree of stability in aqueous media can be tuned by longer thermolysis times or higher temperatures.

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Biocompatibility of Porous Silicon Suet P. Low and Nicolas H. Voelcker

Contents Biocompatibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodegradability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cytotoxicity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fate of Porous Silicon Particles in the Body . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . In Vivo Behavior of Porous Silicon Implants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Toward In Vitro and In Vivo Biosensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Porous Silicon for Tissue Engineering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Localized Drug Delivery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Vaccine Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

The biocompatibility of porous silicon is critical to its potential biomedical uses, both in vivo within the human body for therapy and diagnostics, and in vitro for biosensing and biofiltration. Published data from cell culture and in vivo studies are reviewed, and a number of emerging applications for bioactive or biodegradable silicon are discussed.

Biocompatibility The term “biocompatibility” is defined as “the ability of a material to perform with an appropriate host response in a specific situation” (Williams 2008). A biocompatible material can be inert, where it would not induce a host immune response and have S.P. Low (*) • N.H. Voelcker Mawson Institute, University of South Australia, Adelaide, SA, Australia e-mail: [email protected]; [email protected] # Springer International Publishing Switzerland 2014 L. Canham (ed.), Handbook of Porous Silicon, DOI 10.1007/978-3-319-05744-6_38

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little or no toxic properties. A biocompatible material can also be bioactive, initiating a controlled physiological response. For porous silicon, bioactive properties were initially suggested based on the observation that hydroxyapatite (HA) crystals grow on microporous silicon films. HA has implications for bone tissue implants and bone tissue engineering (Canham 1995). An extension of this work showed that an applied cathodic current was able to further promote calcification on the surface (Canham et al. 1996). More recently, Moxon et al. showed another example of bioactive porous silicon where the material promoted neuron viability when inserted into rat brains as a potential neuronal biosensor, whereas planar silicon showed significantly fewer viable neurons surrounding the implant site (Moxon et al. 2007).

Biodegradability A comprehensive review on pSi biodegradability is covered in chapter “▶ Biodegradability of Porous Silicon”. We discuss this here as the degradation rate, and products can influence its biocompatibility in biomedical applications. Porous silicon is instable in aqueous solutions and degrades into orthosilicic acid (Si(OH)4) (Allongue et al. 1993) as a result of oxidative hydrolysis (Scheme 1). Silicic acid is a nontoxic small molecule and the common form of bioavailable silicon in the human body (Carlisle 1972, 1982). Silicic acid does not accumulate within the human body and has been shown to be absorbed readily by the gastrointestinal tract of humans and is rapidly excreted via the urinary pathway (Reffit et al. 1999). Although silicic acid at concentrations of 2 mM has been reported to be cytotoxic to fibroblasts and macrophages (Tanaka et al. 1994), high concentrations of silicic acid up to 100 mM have been tested in vitro on cells with no apparent affect on their viability (Mayne et al. 2000). The rate of dissolution can be controlled by the porosity of porous silicon (Anderson et al. 2003) and by its surface chemistry (Canham et al. 1999, 2000). Silicon with medium porosity (62 % porosity)

Scheme 1 Proposed mechanism for porous silicon degradation in aqueous solutions, adapted from Allongue et al. (1993). (a) A Si-H-terminated surface immersed in H2O. (b) The Si-H bond undergoes hydrolytic attack and is converted to Si-OH and produces a hydrogen molecule. (c) The Si-OH at the surface polarizes and weakens the Si-Si backbonds, which are then attacked by H2O, producing HSi(OH)3. (d) In solution, the HSi(OH)3 molecule is quickly converted to Si(OH)4 releasing a second hydrogen molecule

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shows slow degradation, whereas higher porosity silicon (>80 % porosity) showed exponential release of silicic acid over time (Anderson et al. 2003). Surface modification has been applied to the porous silicon surface to impart protection against hydrolytic attack and has the dual role of being able to change the surface chemistry (Low et al. 2006). By applying different surface modifications, porous silicon degradation rates can be tuned anywhere from minutes to months (Godin et al. 2010). This makes porous silicon as an ideal transient material for localized drug delivery or cell delivery purposes. The degradation rate of porous silicon increases with increasing pH (Anderson et al. 2003), and the local tissue pH therefore has to be taken into consideration when designing porous silicon for a certain biomaterial application. Different methods for surface modification and subsequent effect on cells are covered in chapters “▶ Functional Coatings of Porous Silicon” and “▶ SiliconCarbon Bond Formation on Porous Silicon”. In brief, the functional groups presented on the surface of porous silicon allow for the attachment of biological factors and proteins in culture medium, which in turn influence cell attachment. Several in vitro culture studies have shown that surface modification of the porous silicon surface can modulate cell attachment and growth (Low et al. 2006; Yang et al. 2010). Neuroblastoma (Low et al. 2006; Yang et al. 2010; Gentile et al. 2012; Khung et al. 2006), human embryonic kidney cells (Sweetman et al. 2011), B50 cells (Mayne et al. 2000; Bayliss et al. 1997, 1999), and primary mesenchymal cells (Clements et al. 2011; Noval et al. 2012) are a few cell types that have been successfully cultured on porous silicon surfaces.

Cytotoxicity As discussed above, the degradation products of porous silicon have been shown to be relatively harmless and have opened the use of this material in biological environments. The interaction of porous silicon and cells is covered in chapter “▶ Cell Culture on Porous Silicon”, but silicic acid is not the only degradation product that may induce cytotoxicity. It has been recently demonstrated that porous silicon is capable of producing reactive oxygen species (ROS) (Belyakov et al. 2007; Kovalev et al. 2004). ROS have important physiological roles such as signalling molecules to regulate cell proliferation, apoptosis, and differentiation (Finkel and Holbrook 2000). ROS generation by porous silicon is directly related to the surface chemistry (Kovalev et al. 2002, 2004), and therefore, porous silicon particles are more susceptible to generating ROS. Recent investigations have demonstrated that untreated particles generate ROS at concentrations that lead to cell death, whereas simple surface stabilization with oxidation was able to mitigate this effect (Low et al. 2010; Santos et al. 2010). The size of porous silicon particles is also an important factor. Particles smaller than 3 μm have been demonstrated to be cytotoxic to monocytes (Ainslie et al. 2008); loss in Caco-2 cell metabolic activity was seen with particles between 1.2 and 25 μm in size (Santos et al. 2010). In contrast, particles below 500 nm were

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demonstrated to be nontoxic to lymphoma cells (De Angelis et al. 2010), and particles smaller than 1 μm did not cause any cytotoxic effects with macrophage and endothelial cells (Godin et al. 2012).

Fate of Porous Silicon Particles in the Body The retention of porous silicon in the body has recently been shown to be transient. Intravenous injection of porous silicon nanoparticles into mice leads to its accumulation within the liver and the spleen, demonstrating rapid removal from the circulatory system (Bimbo et al. 2010; Park et al. 2009). Another study intravenously injected oxidized and aminosilane-functionalized porous silicon microparticles into mice. The study revealed that surface chemistry and charge affected microparticle distribution within the body (Tanaka et al. 2010a). In this study, porous silicon microparticles after intravenous administration were also found to accumulate within the liver and spleen. The enzyme lactate dehydrogenase (LDH) is often used as an indicator of tissue damage, and this study found LDH levels were only increased after multiple administrations of the particles and cytokine levels remained stable. There was no difference in LDH levels between particles with different surface chemistries. Other studies have shown that intravenously administered particles that accumulated within the liver and spleen degraded over a period of 4 weeks, and cells within these organs retained their normal morphology (Park et al. 2009). Tanaka et al. showed in a mouse model that injected porous silicon particles loaded with siRNA accumulated primarily within the liver and spleen (Tanaka et al. 2010b). Clearance or degradation of the particles within these organs occurred within 3 weeks for the spleen but significantly longer in the liver, indicating that degradation kinetics of the porous silicon particles was organ dependent. No tissue injury or inflammatory cytokines were detected for the organs investigated, and the morphology of the cells within these organs remained unchanged. This study again demonstrated that porous silicon did not cause any adverse tissue effects when injected into the body. For drug delivery applications, ingestion of porous silicon is of particular interest. Porous silicon is reasonably stable at low pH (Anglin et al. 2008), showing degradation kinetics that are suitable for drug delivery to the intestine. An investigation into the distribution of ingested porous silicon microparticles showed that they passed through the gastrointestinal tract without signs of uptake of particles within the lining of the gastrointestinal tract (Bimbo et al. 2010). Another study utilized 18F-radiolabeled thermally hydrocarbonized porous silicon particles to investigate the accumulation and retention of ingested particles in a rat model. Once again, the particles were stable in stomach acid, remained in the GI tract, and did not cross the intestinal cell layer (Sarparanta et al. 2011). Another article by the same authors attached the protein hydrophobin class II to the thermally carbonized particles. This conveyed mucoadhesive properties to the particles, allowing them to attach to gastric cells. The particles were stable in gastric fluid, and in vivo

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experiments showed that the particles were retained within the stomach cavity for a period of time. However, upon entering the intestinal tract, the particles lost their adhesive properties and were quickly expelled (Sarparanta et al. 2012). Particle size can also be used to control the distribution throughout the body and can be a form of targeted drug delivery. Porous silicon particles larger than 519 nm in diameter are unable to cross the placenta into a fetus and can help prevent fetal exposure to administered drugs (Refuerzo et al. 2011).

In Vivo Behavior of Porous Silicon Implants A range of in vitro cell culture studies have been carried on porous silicon to demonstrate the suitability of this material as a support for mammalian cells. Implants, on the other hand, can experience a variety of tissue and host immune responses, such as generalized cytotoxic effects, microvascularization, and hypersensitivity to the implant. To date, there have been limited numbers of studies on the effects of porous silicon in vivo. In 2000, Rosengren et al. implanted unmodified porous silicon into the abdominal wall of rats with flat silicon and titanium as controls. An inflammatory response was observed with a resultant fibrous capsule forming around the implant with minimal cell death at the cell-porous silicon interface, and it was noted that the tissue response was similar for porous silicon, flat silicon, and titanium (Rosengren et al. 2000). Fibrous encapsulation of an implant is a common tissue response to a foreign body (Ratner and Bryant 2004). The factor determining the outcome of the implantation is the thickness of the capsule and the degree of inflammation around the capsule, often measured by the frequency of inflammatory cells such as macrophages and foreign body giant cells. Excessive capsule thickness or inflammation can cause pain or discomfort around an implant and can ultimately result in implant rejection. For active pSi implants, such as devices for drug delivery, this encapsulation may also influence the rate of drug release. With the aim of developing a drug delivery vehicle, hydrosilylated and thermally oxidized porous silicon microparticles were injected into the vitreous of rabbit eyes (Cheng et al. 2008). It was noted that the particles settled into the inferior vitreous cavity over a few days. Degradation of the hydrosilylated particles took considerable time (>4 months) in comparison to untreated particles which degraded within a period of 3–4 weeks. No adverse effects were observed in ocular tissues including the retina and the lens. Furthermore, normal ocular pressure was maintained. Implants of porous silicon membranes under the rat conjunctiva demonstrated similar results (Fig. 1). A small inflammatory response was initially observed, but histological examination of the implant site showed that a thin fibrous capsule formed around the porous silicon membrane with only a small fraction of inflammatory cells surrounding the implant site (Low et al. 2009). The capsule around was significantly thinner than the fibrous capsule surrounding the surgical sutures used to hold the membranes in place. Tissue erosion and vascularization were absent, indicating that porous silicon was highly biocompatible within tissues of the eye.

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Fig. 1 Thermally oxidized and amino-silanized porous silicon membranes containing cultured limbal cells were implanted under the conjunctiva of rats. (a) Images of the implant site after 0, 3, 6, and 9 weeks showing gradual dissolution of the membrane. (b) Histological analysis of the implant site with the porous silicon (PS). Small amount of inflammatory cells (IC) are found and the formation of a fibrous capsule around the porous silicon membrane (F) (Low et al. 2009)

Outside of the eye, the bioactive properties of pSi implants were investigated in contact to nerve tissue. Porous silicon films on bulk silicon supports were implanted into the sciatic nerve of a rat. Nerve tissue could hence grow on the porous region or the flat region. The authors observed that the formed fibrous capsule formation was significantly thinner on the porous silicon region in comparison to the flat silicon region. They postulated that the porous nature allowed for the implant to anchor strongly to the tissue and thus prevent sheer forces that may influence the formation of fibrous capsules. They also determined that a greater percentage of axons formed on the porous silicon, further highlighting the bioactivity of porous silicon in terms of promoting neural cell formation (Johansson et al. 2009).

Toward In Vitro and In Vivo Biosensors Porous silicon is rousing interest in the biosensor community because of several unique intrinsic material properties. First and foremost are the optical properties which include photoluminescence, thin-film reflectance, and photonic effects (Jane et al. 2009). Second, the material has a high surface area (allowing higher binding density as compared to flat surfaces), the ability to introduce size exclusion layers (filtering out undesired molecules), and a well-developed surface chemistry with a range of options for bioreceptor immobilization. The photoluminescence properties of porous silicon have been exploited since the initial discovery of this effect (Canham 1990). Quenching of the photoluminescence signal has been utilized to detect proteins and enzyme activity (Letant et al. 2004), selectively capture streptavidin biomolecules (Letant et al. 2003), selectively detect myoglobin from a serum solution (Starodub et al. 1999), and capture DNA (Chan et al. 2000).

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However, the simplest form of a porous silicon biosensor utilizes thin-film interference effects, resulting in a characteristic Fabry-Perot fringe pattern. Changes in the position of the fringe pattern indicate the binding or loss of molecules within the porous layer (Brecht and Gauglitz 1995). This technique has been utilized to detect down to femtomolar concentrations of proteins and DNA binding to the porous silicon surface (Lin et al. 1997; Steinem et al. 2004; Szili et al. 2011). Porous silicon structures with alternating layers of low and high porosity show 1D photonic effects with sharp stop bands. Depending on the interface between the layers, these structures are termed Bragg mirrors or rugate filters (Pavesi and Dubos 1997). The binding to or release of molecules from the porous layer leads to shifts in the spectral peak (Guillermain et al. 2007). The photonic properties of porous silicon have been used by the Sailor group for the in situ monitoring of cell viability. This concept has been coined the “smart Petri dish.” A light source is aimed at an incident angle which is reflected away from the detector. Cells attached to the porous silicon surface scatter some of the light back to the detector, leading to a small spectral peak. Change in cell morphology as a result of cell death leads to an increase in light scattering and therefore an increased detector signal. This allows the label free and in situ monitoring of cell viability without the need for adding dyes into the cell culture medium (Schwartz et al. 2006). The described optical effects could also be used for implanted biosensors which combine the aspects of biocompatibility and biodegradability with the optical effects which are retained upon implantation. Monitoring of a sensor implanted underneath the skin can be accomplished by merely irradiating the sensor with a light source and collecting the reflected spectra. A drawback is the fouling of the sensor when placed into a complex biological environment which will interfere with the sensor readout. The Gooding group utilized hydrosilylation and subsequent conjugation of oligoethylene oxide (OEG) moieties to produce a non-fouling layer, which effectively prevented the adhesion of proteins while still maintaining reflectivity, even when placed into human blood plasma (Kilian et al. 2007). This feat bodes well for the possibility of in situ monitoring in biological fluids and in vivo.

Porous Silicon for Tissue Engineering The biocompatible, bioactive, and biodegradable properties of porous silicon render this material a suitable scaffold for tissue reengineering. For example, with relevance to neural engineering, it has been demonstrated that porous silicon is able to support the growth of neuronal cells that still maintain their action potential capabilities (Ben-Tabou de Leon et al. 2004). Another study has found that porous silicon with an average pore diameter of 300 nm is able to guide axonal growth (Johansson et al. 2005, 2008) suggesting a potential application for porous silicon in nerve regeneration. The early studies identifying bioactive porous silicon in the ability to form HA crystals have generated interest in its use as a bone matrix alternative. Porous silicon-substituted HA structures implanted into the femur displayed better bone

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integration around the implant compared to a nonporous HA implant (Porter et al. 2006). This was attributed to the porous silicon degradation, revealing voids for bone ingrowth and allowing dynamic remodeling. The biodegradable nature of porous silicon makes it an ideal carrier for cell therapy applications. Here, stem cells on a suitable carrier are implanted into a host, for example, in order to regenerate tissue function. After delivery of the cells into the host, it is desirable that the carrier be degradable in vivo. As a potential treatment for ocular surface disease, human limbal stem cells isolated from the cornea have been expanded on porous silicon membranes as carriers and used to demonstrate cell outgrowth from membranes in an animal model. The stem cell migrated from the porous silicon membrane into the surrounding tissue, and histological analysis of the porous silicon membranes after 8 weeks showed low inflammatory response and absence of vascularization of the implant and significant implant degradation (Low et al. 2009).

Localized Drug Delivery Porous silicon has also been shown to be an effective platform in sustained drug delivery exploiting the large drug loading capacity that stems from the large internal surface. A study looked at five different orally received drugs and their compatibility with a porous silicon delivery vehicle. The drugs were loaded into thermally carbonized and thermally oxidized porous silicon particles. Water-soluble drugs usually display fast drug release profiles, while poorly soluble drugs show slow release kinetics (Salonen et al. 2005). Porous silicon particles were able to moderate the drug release kinetics, on the one hand reducing the release rate of water-soluble drugs while on the other hand enhancing release kinetics for poorly soluble drugs (Wang et al. 2009). Coupled with its stability at low pH (Anglin et al. 2008), porous silicon makes an ideal carrier for oral drug delivery into the small intestine. A tenfold increase in permeation of insulin across intestinal cell layers was achieved when delivered with porous silicon particles over traditional soluble permeation solutions (Foraker et al. 2003), suggesting that apart from moderating drug release, porous silicon can enhance drug absorption by the body. Another significant advantage of using porous silicon particles is to be loaded with a high payload of drug so that a single dose of the drug delivery system suffices for continuous therapeutic effects, avoiding repeated drug administrations. Here, amino-functionalized porous silicon particles have been used to deliver siRNA to successfully silence a gene for an oncoprotein in vivo where the silencing effect was maintained for several weeks, whereas traditional applications required multiple administrations (Tanaka et al. 2010b). The porous silicon particles have also been used for sustained peptide delivery in rats. In this case, surface modification was used to modulate peptide release from the particles. Thermally carbonized, thermally oxidized, and undecylenic acid-conjugated thermally carbonized particles were loaded with a peptide. They found that as peptide release was partially related to the degradation kinetics of pSi,

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the thermally oxidized particles, which degraded the fastest, had the greatest release over a 2-week period both in vitro and in vivo, whereas the more stable thermally carbonized particles released less peptide (Kovalainen et al. 2012). This was also demonstrated with a ghrelin antagonist; sustained release was achieved with this peptide over 17 h when loaded into pSi particles, and without the particles, the peptide lost its activity within 4 h (Kilpel€ainen et al. 2009). These studies demonstrate the advantages of using pSi as a carrier vehicle for protein delivery. Bioactivity of proteins can be preserved, and the lifetime of a loaded protein can be extended, leading to a sustained drug delivery profile.

Vaccine Development A further application for biocompatible porous silicon relates to vaccination using antigen-loaded particles. Porous silicon particles were conjugated to antigens that specifically target the toll-like receptors on dendritic cells (DC). This stimulated phagocytosis by dendritic cells, maturing the cells to become antigen-presenting cells (Fig. 2) (Meraz et al. 2012). The activated DCs increased proinflammatory cytokines IL-1β, TNF-α, and IL-6, and when the activated DCs were injected into mice, they migrated into the lymphatic system where they activated T cells by upregulation of cell surface receptors and presenting the antigen along with major histocompatibility (MHC), all of which play a role in mediating an active immune response. This study demonstrated effective stimulation of the immune system with antigen-loaded porous silicon which is highly relevant to the development of vaccines for various diseases.

Fig. 2 Pseudocolored SEM images of dendritic cells at low (top) and high (below) magnification. (a) Cells only; (b) dendritic cells phagocytosing porous silicon particles and (c) porous silicon particles loaded with liposaccharide antigen being taken up by the dendritic cells (Meraz et al. 2012)

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Summary Since the discovery that porous silicon can stimulate the formation of HA crystals in simulated body fluid, the use of porous silicon in biomaterial applications has soared. Apart from bioactivity, properties such as in vitro and in vivo biocompatibility, biodegradability, high surface area, tunability of pore size and porosity, and finally ease of surface modification have contributed to this increasing interest. These properties open exciting avenues for neural, ocular, and bone tissue engineering and also for drug and vaccine delivery. Combining the biocompatibility with the material’s optical properties of porous silicon enables diagnostic applications such as smart tissue cultureware and implantable biosensors.

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Biodegradability of Porous Silicon Qurrat Shabir

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanism of Biodegradation and Degradation Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Kinetics of Degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

In the biomaterials field there is an increasing interest in medically biodegradable materials. The medical biodegradability of mesoporous silicon is now established both in vitro and in vivo. The review highlights the techniques used to date to characterize this phenomenon, the degradation kinetics, and the various factors that can influence the kinetics of dissolution into orthosilicic acid.

Introduction There is growing interest and acceptance in replacing permanent prostheses by temporary ones in the human body. These would in effect help the body to heal itself and require biomaterials to have “biodegradability” within physiological environments (Ratner et al. 2004). Currently four different terms are found in the literature to signify that a material or device will eventually disappear after having been introduced into a living organism: biodegradation, bioerosion, bioabsorption,

Q. Shabir (*) pSiMedica Ltd, Malvern, Worcester, UK e-mail: [email protected] # Springer International Publishing Switzerland 2014 L. Canham (ed.), Handbook of Porous Silicon, DOI 10.1007/978-3-319-05744-6_39

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PLGA/PLA Grafts, sutures, implants, depots

CERAMICS

Hydroxyapatite/Tri calcium phosphate Orthopaedic devices &tissue engineering scaffolds

METALS

SEMICONDUCTORS

Mg / Fe alloys

Mesoporous Silicon

Coronary Stents Paediatric implants

Brachytherapy Tuneable drug delivery

Fig. 1 Biodegradable materials of different classes and their medical uses

and bioresorption (Ratner et al. 2004). Unfortunately no agreed distinctions exist; we will use the biodegradability term here. Figure 1 shows examples of biodegradable materials and their uses. The in vitro discovery in 1995 that high porosity mesoporous silicon (pSi) can be rapidly biodegradable, unlike solid crystalline silicon (Canham 1995), and subsequent in vivo demonstrations of biodegradability and biocompatibility (Bowditch et al. 1999; Park et al. 2009; Sarparanta et al. 2012, 2014) have been very significant in this regard. The huge surface area (e.g., 100–500 cm2/cm3) of pSi coupled with its nanostructured skeleton promotes solubility in water and biological media. An increasing level of research is being conducted on the use of both porous silicon and silica materials in drug and nutrient delivery (see handbook chapters “▶ Drug Delivery with Porous Silicon” and “▶ Porous Silicon and Functional Foods”). A biodegradable porous matrix offers the dual advantages of sustained release at target sites in the body and gradual biological elimination after administration (Ahuja and Pathak 2009). The performance of porous silicon in this regard should be compared with that of biodegradable polymers that can “microencapsulate” drugs (Park et al. 2005) and mesoporous biodegradable silicas that can also entrap them (Finnie et al. 2009). This review discusses the hydrolysis mechanism underlying mesoporous silicon biodegradability; the factors affecting typical kinetics of that biodegradability, together with techniques used to date to tune those kinetics and timescales achieved.

Mechanism of Biodegradation and Degradation Products In vitro degradation studies of porous silicon have shown release of orthosilicic acid from both anodized films and microparticles using molybdate blue assays or ICP analysis (Anderson et al. 2000; Anglin et al. 2008; Chiappini et al. 2010). Mesoporous silicon membranes/microparticles in simulated body fluids change color, becoming transparent as biodegradation proceeds to completion (Fig. 2). These results are more evident as the released silicic acid forms a blue-colored complex with molybdenum blue and the color intensity increases

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Fig. 2 In vitro biodegradation of an 83 % porosity mesoporous silicon membrane

Fig. 3 Molybdenum blue assay for orthosilicic acid released from mesoporous silicon membranes at different time points

with time (Fig. 3). Both in vitro (Canham 1995) and in vivo studies (Bowditch et al. 1999; see Fig. 4) have used electron microscopy to reveal mesoporous film corrosion and disappearance. The first in vivo study (Bowditch et al. 1999) of implanted discs used a combination of electron microscopy and monitoring of disc weights (2014). Porous silicon in aqueous conditions undergoes hydrolysis to form orthosilicic acid and the reaction is catalyzed by OH-; hence the rate of dissolution increases

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Fig. 4 In vivo biodegradation of a 30 % porosity mesoporous silicon layer in the subcutaneous site of the guinea pig. Plan view HREM images of porosified silicon disc surfaces (a) prior to implantation, (b) after 4 weeks in vivo, and (c) after 12 weeks in vivo

with increasing pH. Dissolution of unoxidized silicon can be described with a simplified two-step process: Si þ 2H2 O ! SiO2 þ 2H2 SiO2 þ 2H2 O ! Si ðOHÞ4 The oxidative first step involves electronic carrier (hole) injection and is dependent on both electronic bandgap and doping of the semiconductor. Complete hydrolysis of the oxide phase then generates orthosilicic acid, which is the natural bioavailable form of silicon, freely diffusible in human tissues, and readily excreted via the kidneys (Jugdaohsingh et al. 2002; Refitt et al. 1999). The biocompatibility of porous silicon is reviewed in detail elsewhere in this handbook (“Biocompatibility of porous silicon”) so is not discussed here.

Kinetics of Degradation The kinetics of biodegradation is affected by physical parameters like degree of crystallinity, porosity, surface area, and pore size distribution. A striking example is the difference in solubility between amorphous and polycrystalline silicon (Shabir et al. 2011). The kinetics is also tunable by pore wall surface chemistry which affects wettability by body fluid and resistance to initial hydrolysis. Mesoporous silicon

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Table 1 Biodegradation kinetics with differing pSi structures, surface chemistries, and biological environments Surface chemistry Native oxide (autoclaved)

pSi structure and physical parameters Low porosity layer on n + Si discs (30 % porosity and 30 μm thickness)

Biological fluid/body site Blood plasma (subcutaneous site)

Degradation kinetics >3 months half-life

Native oxide

Multilayer microparticles (~67 % porosity) Multilayer microparticles (~67 % porosity) Multilayer microparticles (~67 % porosity) Microparticles (40 nm APD) Microparticles (40 nm APD) Nanoparticles (126 nm diameter, 7.5 nm APD) Nanoparticles (80–120 nm diameter, ~5 nm APD) Microparticles (20–50 μm, 446 m2/g, 1.5 ml/g, 10.7 nm APD) Microparticles (20–50 μm, 367 m2/g, 0.84 ml/g, 7.6 nm APD) Films and microparticles

Vitreous humor (eye)

1 week halflife

Reference Bowditch et al. (1999) Canham (2014) Cheng et al. (2008)

Vitreous humor (eye)

5 weeks half-life

Cheng et al. (2008)

Vitreous humor (eye)

16 weeks half-life

Cheng et al. (2008)

Phosphate buffered saline Phosphate buffered saline Phosphate buffered saline Phosphate buffered saline

100 % after 2 days 100 % after 3–4 days 100 % after 4h 3 h half-life

Godin et al. (2010) Godin et al. (2010) Park et al. (2009) Hon et al. (2012)

Phosphate buffered saline

80 % after 96 h

Tzur-Balter et al. (2013)

Phosphate buffered saline

~5 % after 300 h

Tzur-Balter et al. (2013)

Phosphate buffered saline

Microparticles

Simulated body fluids

Difficult to quantify but slow kinetics Difficult to quantify but slow kinetics

McInnes et al. (2009, 2012) Henstock et al. (2014)

Thermal oxidation Hydrosilylation

Silicon native oxide PEGylation Silicon native oxide Rapid thermal oxidation (800C) Silicon native oxide Hydrosilylation (dodecyl groups) Composites with PLLA Composites with polycaprolactone

is often manufactured by electrochemical etching techniques (see handbook chapter “▶ Routes of Formation for Porous Silicon”) resulting in hydrogen-terminated surfaces (Si-Hx). For drug delivery applications, a less reactive surface is crucial, and the hydrogen termination of the freshly etched pSi is normally replaced (Li et al. 2009). By converting the reactive groups into a more stable oxidized, hydrosilylated, or (hydro) carbonized form, the pSi surface can be modified in terms of hydrophilicity and resistance to hydrolysis (Canham et al. 1999). Such changes in pore wall chemistry have been shown to significantly change biodegradation kinetics, as summarized in Table 1.

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The hydrolysis of silica-based surfaces is also strongly pH dependent (Iler 1979). Comparison of hydrolysis rates at pH 2 and 9 shows an increase in excess of three orders of magnitude in the alkali fluid. Significant differences are therefore expected and observed between physiological environments of varying pH. Examples of relevance to medical uses are the low pH condition inside lysozymes (Gu et al. 2012) and the low pH microenvironment in polymer-pSi composites due to polymer biodegradation products (Henstock et al. 2014; McInnes et al. 2009, 2012). Another is the widely varying stabilities observed in foodstuffs and beverages for oral consumption (Canham 2007). The implications of the latter case are discussed in the handbook chapter “▶ Porous Silicon and Functional Foods.”

Conclusions There has been growing interest in development of nanostructured porous silicon-based medical therapy over the past few years. Porous silicon dissolves in body fluids into orthosilicic acid, a benign bone nutrient bioavailable from the diet. To make pSi more compatible with loaded drugs and nutrients, various strategies are used to make the nanostructured surfaces less reactive, resulting in slower biodegradation in body fluids. As expected, nanoparticles completely biodegrade much faster than microparticles and films of similar morphology. Composites of biodegradable polymers and porous silicon are likely to exhibit much slower biodegradation rates of the semiconductor component. There is much potential to tailor the silicon surfaces in terms of chemistry, pore size, pore structure, and porosity making it a versatile carrier system for controlled release applications.

References Ahuja G, Pathak K (2009) Porous carriers for controlled/modulated drug delivery. Indian J Pharm Sci 71(6):599–607 Anderson SHC, Elliott H, Wallis DJ, Canham LT, Powell JJ (2000) Dissolution of different forms of partially porous silicon wafers under simulated physiological conditions. Phys Stat Solidi (a) 197:331–335 Anglin EJ, Cheng L, Freeman WR, Sailor MJ (2008) Porous silicon in drug delivery devices and materials. Adv Drug Deliv Rev 60(11):1266–1277 Bowditch AP, Waters K, Gale H, Rice P, Scott EAM, Canham LT, Reeves CL, Loni A, Cox TI (1999) In-vivo assessment of tissue compatibility and calcification of bulk and porous silicon. Mat Res Soc Symp Proc 536:149–154 Canham LT (1995) Bioactive silicon structure fabrication through nanoetching techniques. Adv Mater 7:1033–1037 Canham LT (2007) Nanoscale semiconducting silicon as a nutritional food additive. Nanotechnology 18:185704 Canham LT (2014) Porous silicon for medical use: from conception to clinical use, Chap 1. In: Santos HA (ed) Biomedical uses of porous silicon. Woodhead publishing, UK. pp 3–20

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Canham LT, Reeves CL, Newey JP, Houlton MR, Cox TI, Buriak JM, Stewart MP (1999) Derivatized mesoporous silicon with dramatically improved stability in simulated human blood plasma. Adv Mater 11(18):1505–1507 Cheng L, Anglin E, Cunin F, Kim D, Sailor MJ, Falkenstein I, Tammewar A, Freeman WR (2008) Intravitreal properties of porous silicon photonic crystals: a potential self-reporting intraocular drug-delivery vehicle. Br J Ophthalmol 92(5):705–711 Chiappini C, Liu X, Fakhoury JR, Ferrari M (2010) Biodegradable porous silicon barcode nanowires with defined geometry. Adv Funct Mater 20(14):2231–2239 Finnie KS, Waller DJ, Perret FL, Krause-Heuer AM, Lin HQ, Hanna JV, Barbe CJ (2009) Biodegradability of sol–gel silica microparticles for drug delivery. J Sol Gel Sci Technol 49:12–18 Godin B, Gu J, Serda RE, Bhavane R, Tasciotti E, Chiapinni C, Lu X, Tanaka T, Decuzzi P, Ferrari M (2010) Tailoring the degradation kinetics of mesoporous silicon through PEGylation. J Biomed Mater Res 94(4):1236–1243 Gu L, Ruff LE, Qin Z, Corr M, Hedrick SM, Sailor MJ (2012) Multivalent porous silicon nanoparticles enhance the immune activation potency of agonistic CD40 antibody. Adv Mater. doi:10.1002/adma.201200776 Henstock JR, Ruktanonchai UR, Canham LT, Anderson SI (2014) Porous silicon confers bioactivity to polycaprolactone composites in vitro. J Mater Sci 25(4):1087–1097 Hon NK, Shaposhnik Z, Diebold ED, Tamanoi F, Jalali B (2012) Tailoring the biodegradability of porous silicon nanoparticles. J Biomed Mater Res 100(12):3416–3421 Iler RK (1979) Chemistry of silica: solubility, polymerization, colloid and surface properties and biochemistry. Wiley, New York Jugdaohsingh R, Anderson SH, Tucker KL, Elliott H, Kiel DP, Thompson RP, Powell JJ (2002) Dietary silicon intake and absorption. Am J Clin Nutr 75(5):887–893 Li HL, Zhu Y, Xu D, Wan Y, Xia L, Zhao X (2009) Vapour-phase silanization of oxidised porous silicon for stabilizing composition and photoluminescence. J Appl Phys 105:114–307 McInnes SJP, Thissen H, Choudbury NR, Voelcker NH (2009) New biodegradable materials produced by ring opening polymerisation of poly(L-lactide) on porous silicon substrates. J Coll Interf Sci 332:336–344 McInnes SJ, Irani Y, Williams KA, Voelcker NH (2012) Controlled drug delivery from composites of nanostructured porous silicon and poly(L-lactide). Nanomedicine 7(7):995–1016 Park JH, Ye M, Park K (2005) Biodegradable polymers for microencapsulation of drugs. Molecules 10:146–161 Park J-H, Gu L, von Maltzahn G, Ruoslahti E, Bhatia SN, Sailor MJ (2009) Biodegradable luminescent porous silicon nanoparticles for in vivo applications. Nat Mater 8(4):331–336 Ratner BD, Hoffman AS, Schoen FJ, Lemons JE (eds) (2004). Biomaterials science: an introduction to materials in medicine, 2nd edn. Elsevier, US. p 851 Refitt DM, Jugdaosingh R, Thompson RPH, Powell JJ (1999) Silicic acid: its gastrointestinal uptake and urinary excretion in man and effects on aluminium excretion. J Inorg Biochem 76:141–147 Sarparanta M, Bimbo LM, Rytkonen J, Makila E, Laaksonen TJ, Laaksonen P, Nyman M, Salonen J, Linder MB, Hirvonen J, Santos HA, Airaksinen AJ (2012) Intravenous delivery of hydrophobin-functionalized porous silicon nanoparticles: stability, plasma protein adsorption and biodistribution. Mol Pharm 9:654–663 Shabir Q, Pokale A, Loni A, Johnson DR, Canham LT, Fenollosa R, Tymczenko M, Rodrı´guez I, Meseguer F, Cros A (2011) Medically biodegradable hydrogenated amorphous silicon microspheres. Silicon 2011:173–176 Tzur-Balter A, Rubinskia A, Segal E (2013) Designing porous silicon-based microparticles as carriers for controlled delivery of mitoxantrone dihydrochloride. J Mater Res 28(2):231–239

Part III Characterization

Characterization Challenges with Porous Silicon Leigh Canham

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Skeleton Dimensionality and Nanostructure Packing Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pore Size Distribution and Morphology: Euclidean or Fractal? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanical Strength at High Porosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Metastable Silicon Hydride Surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Effects of Very Low Thermal Conductivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Concluding Comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Mesoporous silicon is a complex nanostructure whose optoelectronic properties and morphology have received intense study over the last 25 years. Its properties often depend on both its skeleton size distribution and the chemical nature of its high internal surface area. This review collates some of the lessons learned with regard characterization, highlighting potential issues that need to be considered and artifacts that can arise. These have in the past both complicated data interpretation and even caused problems in reproducing published data.

Introduction High porosity mesoporous silicon is a complex nanostructure whose optoelectronic properties and morphology have received intense continuous study over the last 25 years, following the publication of its dramatic luminescence properties in 1990. There are a series of reviews that historically chart progress in understanding and L. Canham (*) pSiMedica Ltd., Malvern Hills Science Park, Malvern, Worcester, UK e-mail: [email protected] # Springer International Publishing Switzerland 2014 L. Canham (ed.), Handbook of Porous Silicon, DOI 10.1007/978-3-319-05744-6_40

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Densely packed interconnected nanostructures

Difficult microscopy

Inaccurate size metrology

Hierarchical porosity

Morphological complexity

Complex theoretical modelling

Low mechanical strength

Degradation upon liquid removal

Properties change during processing

Unstable surface chemistry

“Aging” during storage

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Fig. 1 Problem areas for characterization of mesoporous and microporous silicon

exploitation of its luminescent, optical, and electrical properties (Fauchet et al. 1995; Hamilton 1995; Cullis et al. 1997; Bisi et al. 2000; Boarino et al. 2009; Torres-Costa and Martin-Palma 2010; Chao 2011; Golovan and Timoshenko 2013; Pacholski 2013). This review highlights the five problem areas shown in Fig. 1 with regard to characterization that have often hindered progress in optoelectronic applications. They also have relevance to the many other application areas under more recent development (see handbook chapter “▶ Porous Silicon Application Survey”). The objective is to alert the reader to some general issues, prior to other handbook reviews that analyze in detail the insight gained from specific techniques, like gas adsorption, microscopy, calorimetry, infrared spectroscopy, and so on. In this regard it is also complementary to the handbook review “▶ Effects of Irradiation on Porous Silicon” which focuses on changes that can occur as a result of photon beam or particle beam irradiation.

Skeleton Dimensionality and Nanostructure Packing Density The length scale at which dramatic quantum size effects occur in silicon lies in the range 1–3 nm (see Fig. 1 in handbook review “▶ Electronic Band Structure in Porous Silicon”). Silicon quantum dots or wires of this size push the spatial resolution of many microscopy techniques, and when very small size variations cause large changes in properties, accurate metrology becomes increasingly important and difficult. This is not helped if those nanostructures are very close together,

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interconnected to varying degrees, and arranged in complex three-dimensional networks. The first challenge therefore relates to those properties that are tunable by size metrology of the silicon skeleton: how do we accurately extract the silicon nanocrystallite size distribution throughout its volume? The handbook review “▶ Microscopy of Porous Silicon” discusses such issues and progress to date.

Pore Size Distribution and Morphology: Euclidean or Fractal? The size of mesopores (2–50 nm) and particularly micropores (0–2 nm) also makes their accurate metrology via microscopy difficult. The handbook review “▶ Gas Adsorption Analysis of Porous Silicon” discusses how mesopore size can be evaluated over the entire sample volume accessible, but it is not an accurate technique for micropores. However porous silicon is normally not exclusively riddled with only one class of pore. There is now significant evidence that many electrochemical etching regimes, for example, produce more than one class of porosity in a given structure; macropore walls can themselves be mesoporous; structures can contain both mesopores and micropores. In extreme cases, so-called “hierarchical” porosity is present with pore diameters covering a huge range of length scales. This type of wide-ranging length scale of porosity, together with a “fractal” geometrical arrangement of pores (see Fig. 2), is a topic only briefly mentioned elsewhere in this handbook (see, e.g., the chapter “▶ Mesoporous Silicon”), and so it will be highlighted here. Extremely regular arrays of large macropores with smooth sidewalls and Euclidean geometry are a regular feature of the porous silicon literature (see Fig. 3a), as are highly directional mesopore arrays with minimal pore branching under specific anodization conditions (Canham 1990; Ouyang et al. 2005). Very different fractallike etching patterns were first revealed from electrochemically etched macropores, also under certain conditions (Harsanyi and Habermeier 1987). Seminal investigations of the anodization formation mechanisms by Smith and co-workers (Chuang et al. 1989; Smith and Collins 1992; Tondare et al. 2008) then provided strong microscopic evidence for the broader occurrence of fractal pore morphology.

Fig. 2 Fractal-like pore arrangement in a porous silicon membrane (Lysenko et al. 2004)

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Fig. 3 LHS: Euclidean macropore array in p-type silicon (Kim et al. 2009); RHS: Fractal-like oxide replica of the pore volume in n-type silicon (Tondare et al. 2008)

At least with their specific etching conditions (n-substrates with both mesopores and macropores), macropore walls were clearly also mesoporous at decreasing length scales (see Fig. 3b). An increasing number of properties of primarily mesoporous silicon are now being modeled using a fractal geometry for the porous silicon surface. Examples include the exterior surface roughness of layers (Happo et al. 1998) and their hydrophobicity (Cao et al. 2008; Gentile et al. 2011), optical absorption (Derlet et al. 1995), electrical transport (Ben-Chorin et al. 1995; Axelrod et al. 2002), gas transport (Lysenko et al. 2004), hydride content (Nychyporuk et al. 2005), vapor adsorption within pores (Moretti et al. 2007), and low temperature thermal conductivity (Valalaki and Nassiopoulou 2014). In contrast, much of the theoretical modeling of the band structure of mesoporous silicon has been based on idealized nanoscale silicon building blocks (quantum wires and dots) or Euclidean geometry-based subtractive models which introduce periodic porosity via supercells (see handbook chapter “▶ Electronic Band Struc ture in Porous Silicon”).

Mechanical Strength at High Porosity Crystalline silicon is a strong but brittle material. The introduction of porosity often lowers hardness, stiffness, and fracture strength (see handbook chapter “▶ Mechanical Properties of Porous Silicon”), and if the structure becomes too weak, it cannot often survive common material processing techniques without alteration. Examples include air drying (see handbook chapter “▶ Drying Techniques Applied to Porous Silicon”), reduction of particle size via communition (see handbook chapter “▶ Milling of Porous Silicon Microparticles”), and oxidation of layers. The properties of electrochemically etched layers can depend not only on etch parameters but how the material was dried. The properties of microparticles can be sensitive to how they were milled.

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In addition, for materials with very low strength, characterization techniques normally deemed quite gentle can become destructive. One example is the widespread use of gas adsorption to measure pore size which produces irreversible changes in ultrahigh porosity silica “aerogels” (Scherer et al. 1995) and is likely to run into similar issues with characterization of silicon “aerocrystals” (Canham et al. 1994).

The Metastable Silicon Hydride Surface Processing silicon in hydrofluoric acid solutions has been an invaluable route to generating all classes of porous silicon, but the resulting silicon hydride bonds are metastable in ambient air and gradually replaced by native oxide growth. The effects on the chemical composition of highly porous stain-etched silicon were recorded by infrared spectroscopy in the 1960s (Beckmann 1965), and effects of air storage on many properties (photoluminescence, refractive index, electrical resistivity) emphasized much later for microporous silicon (Canham et al. 1991). If structures are not chemically “passivated” immediately after etching then the properties will evolve with storage time until ambient oxidation is complete. This can take many months (Canham 1997). In addition, many characterization techniques utilize energetic beams that can accelerate this process if carried out in ambient air or even vacuum (see handbook chapter “▶ Effects of Irradiation on Porous Silicon”). In some cases, such as ion beam analysis in vacuo of chemical composition, “capping” of layers is required to provide reliable data (Giaddui et al. 1998).

The Effects of Very Low Thermal Conductivity The incredibly low thermal conductivity of high porosity silicon is reviewed in two chapters of this handbook: “▶ Thermal Properties of Porous Silicon” and “▶ Thermal Isolation with Porous Silicon.” It can be utilized in specific applications but also introduce challenges with many characterization techniques and processes. Particular care must be taken with characterization techniques that utilize high energy excitation in localized volumes. Examples include micro Raman spectroscopy, cathodoluminescence spectroscopy, and photoluminescence microscopy. The effects of exothermic reactions are also often amplified by poor energy dissipation, such as in thermal passivation of mesoporous silicon powders (Loni and Canham 2013). Massive temperature rises in the silicon skeleton can occur, so much so that some degree of sintering or even localized melting occurs during analysis. This becomes more pronounced for porous silicon membranes and powders which are removed from the bulk silicon heat sink and more pronounced when data is collected in vacuum, removing gaseous heat transport. Figure 4 provides an example where Raman analysis at gas pressures was used to demonstrate silicon nanoparticle temperatures up to 800  C under focussed photoexcitation.

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Fig. 4 Temperature of silicon nanoparticles under micro Raman analysis, as a function of gas pressure (Costa et al. 1998)

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A particularly striking example of this is the visible light emission from some porous silicon and silicon nanoparticle structures originally ascribed to “photoluminescence” but later revealed to be blackbody thermal radiation by careful experimentation (Costa et al. 1998; Roura and Costa 2002). Some very spectrally broad “cathodoluminescence” spectra published are also likely to be primarily thermal radiation. Much of the nonlinear optical properties reported for porous silicon over the period 1992–2002 may also need reinterpretation accounting for thermal effects, as discussed by Roura and co-workers (Roura and Costa 2002). This topic does not have a dedicated review in this handbook, but is an important issue, mentioned at the end of a recent review on the topic (Golovan and Timoshenko 2013).

Concluding Comments High porosity mesoporous silicon is a fascinating nanostructure with low dimensionality that has a number of novel properties. It also can possess chemical instability, mechanical weakness, and low thermal transport. These latter properties can necessitate very careful characterization in order to avoid data misinterpretation and unwanted changes to the nanostructured material.

References Axelrod E, Givant A, Shappir J, Feldman Y, Saar A (2002) Dielectric relaxation and transport in porous silicon. Phys Rev B 65:165–429 Beckmann KH (1965) Investigation of the chemical properties of stain films on silicon by means of infrared spectroscopy. Surf Sci 3(4):314–332

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Ben-Chorin M, Moller F, Koch F, Schirmacher W, Eberhard M (1995) Hopping transport on a fractal: ac conductivity of porous silicon. Phys Rev B 51(4):2199–2213 Bisi O, Ossicini S, Pavesi L (2000) Porous silicon: a quantum sponge structure for silicon based optoelectronics. Surf Sci Reports 38(1–3):1–126 Boarino L, Borini S, Amato G (2009) Electrical properties of mesoporous silicon: from a surface effect to coulomb blockade and more. J Electrochem Soc 156(12):K223–K226 Canham LT (1990) Silicon quantum wire array fabrication by electrochemical and chemical dissolution of waters. Appl Phys. Lett 57(10):1046–1048 Canham LT (1997) Properties of porous silicon, EMIS datareview series no 18. IEE Press, London Canham LT, Houlton MR, Leong WY, Keen JM (1991) Atmospheric impregnation of porous silicon. J Appl Phys 70(1):422–431 Canham LT, Cullis AG, Pickering C, Dosser OD, Cox TI, Lynch TP (1994) Luminescent anodized silicon aerocrystal networks prepared by supercritical drying. Nature 368:133–135 Cao L, Price TP, Weiss M, Gao D (2008) Super water- and oil-repellent surfaces on intrinsically hydrophilic and oleophillic porous silicon films. Langmuir 24(5):1640–1643 Chao Y (2011) Optical properties of nanostructured silicon. Compr NanoSci Technol 1:543–570 Chuang SF, Collins SD, Smith RL (1989) Porous silicon microstructure as studied by transmission electron microscopy. Appl Phys Lett 55:1540–1543 Costa J, Roura P, Morante JR, Bertran E (1998) Blackbody emission under laser excitation of silicon nanopowder produced by plasma-enhanced chemical-vapour deposition. J Appl Phys 83(12):7879–7885 Cullis AG, Canham LT, Calcott PDJ (1997) The structural and luminescence properties of porous silicon. J Appl Phys 82(3):909–965 Derlet PM, Choy TC, Stoneham AM (1995) An investigation of the porous silicon optical absorption power law near the band edge. J Phys Condens Matter 7:2507–2523 Fauchet PM, Tsybeskov L, Peng C, Duttagupta SP, von Behren J, Kostoulas Y, Vandyshev JMV, Hirschman KD (1995) Light-emitting porous silicon: materials science, properties and device applications. IEEE J Sel Topics Quant Electron 1(4):1126–1139 Gentile F, Battista E et al (2011) Fractal structure can explain the increased hydrophobicity of nanoporous silicon films. Microelectron Eng 88:2537–2540 Giaddui T, Earwaker LG, Forcey KS, Loni A, Canham LT (1998) Improved capping layers for suppression of ambient ageing in porous silicon. J Phys D Appl Phys 31:1131–1136 Golovan LA, Timoshenko VY (2013) Nonlinear optical properties of porous silicon nanostructures. J Nanoelectron Optoelectron 8:223–239 Hamilton B (1995) Porous silicon. Semicond Sci Technol 10:1187–1207 Happo N, Fujiwara M, Iwamatsu M, Horii K (1998) Atomic force microscopy study of self-affine fractal roughness of porous silicon surfaces. Jpn J Appl Phys 37:3951–3953 Harsanyi J, Habermeier HU (1987) Fractal micropatterns generated by anodic etching. Microelectron Eng 6(1–4):575–580 Kim JH, Kim KP, Lyu HK, Woo SH, Seo HS, Lee JH (2009) Three dimensional macropore arrays in p-type silicon fabricated by electrochemical etching. J Korean Phys Soc 55(1):5–9 Loni A, Canham LT (2013) Exothermic phenomena and hazardous gas release during thermal oxidation of mesoporous silicon powders. J Appl Phys 113:173505 Lysenko V, Vitiello J, Remaki B, Barbier D (2004) Gas permeability of porous silicon nanostructures. Phys Rev E 70:017301 Moretti L, De Stefano L, Rendina I (2007) Quantitative analysis of capillary condensation in fractal-like porous silicon nanostructures. J Appl Phys 101:024309 Nychyporuk T, Lysenko V, Barbier D (2005) Fractal nature of porous silicon nanocrystallites. APS J Phys Rev B 71:115–402 Ouyang H, Christopherson M, Fauchet PM (2005) Enhanced control of porous silicon morphology from macropore to mesopore formation. Phys Stat Solidi (a) 202(8):1396–1401 Pacholski C (2013) Photonic crystal sensors based on porous silicon. Sensors 13:4694–4713

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Roura P, Costa J (2002) Radiative thermal emission from silicon nanoparticles: a reversed story from quantum to classical theory. Eur J Phys 23:191–203 Scherer WG, Smith DM, Stein D (1995) Deformation of silica aerogels during characterisation. J Non Cryst Solids 186:309–315 Smith RL, Collins SD (1992) Porous silicon formation mechanisms. J Appl Phys 71, R1 Tondare VN, Gierhart BC, Howitt DG, Smith RL, Chen SJ, Collins SD (2008) An electron microscopy investigation of the structure of porous silicon by oxide replication. Nanotechnology 19:225–301 Torres-Costa V, Martin-Palma RJ (2010) Application of nanostructured porous silicon in the field of optics. A review. J Mater Sci 45:2823–2838 Valalaki K, Nassiopoulou AG (2014) Thermal conductivity of highly porous silicon in the temperature range 4.2 to 20K. Nano Res Lett 9, 318

Microscopy of Porous Silicon Rau´l J. Martı´n-Palma and Vicente Torres-Costa

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Transmission Electron Microscopy Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Scanning Electron Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Atomic Force Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Other Microscopies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

The use of microscopy in the structural characterization of mesoporous silicon has been widespread with the most popular techniques being several electron microscopies, Raman, and atomic force microscopy. The general field is reviewed including other techniques such as luminescence and multiphoton microscopy to study the optoelectronic properties, and acoustic microscopy to study the mechanical properties.

Introduction The complex structure of porous silicon (PS) provides this material with many interesting physicochemical properties, among which visible luminescence can be highlighted. Additionally, the possibility of controlling its morphology on the micro- and nanoscales makes PS a very versatile material for its use in many different applications in a broad diversity of fields (Canham 1997; Lehmann 2002; Sailor 2011, and many other

R.J. Martı´n-Palma (*) • V. Torres-Costa Departamento de Fı´sica Aplicada, Universidad Auto´noma de Madrid, Cantoblanco, Madrid, Spain e-mail: [email protected] # Springer International Publishing Switzerland 2014 L. Canham (ed.), Handbook of Porous Silicon, DOI 10.1007/978-3-319-05744-6_41

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examples throughout this book), including optics (Torres-Costa and Martı´n-Palma 2010) and biomedicine (Martı´n-Palma et al. 2010). But the particular structure of porous silicon is itself a matter of study, given its inherent complex surface, bulk, and interface morphology. Besides, there are many different forms of porous silicon, which makes the analysis of its structure even more complex. In all, detailed structural characterization is a key factor to improve our understanding of the physical mechanisms responsible for the very different behavior of PS with respect to bulk Si and to improve the overall behavior of PS-based structures. Within this context, researchers have performed over the years studies on the morphology of porous silicon, with notable implications in applied research and the further development of devices. Aiming at precisely determining its intricate structure, a host of different microscopy characterization techniques have been used over the years. From these, (high-resolution) transmission electron microscopy (HR)TEM, (high-resolution) scanning electron microscopy (HR)SEM, and atomic force microscopy (AFM) can be highlighted. Other experimental techniques discussed in this chapter include cathodoluminescence (CL), scanning tunneling microscopy (STM), photoluminescence (PL), micro-Raman spectroscopy, confocal scanning beam microscopy, near-field scanning optical microscopy (NSOM), multiphoton microscopy, and acoustic microscopy. The main advantage of most microscopy techniques is that they allow the direct characterization of the morphology at the micron- and nanoscale and thus are not based in indirect results to determine, for instance, the typical nanocrystal or pore size. Also, many of these techniques are nondestructive and can be combined for the study of a given sample. In the following sections, a non-exhaustive review of the most notable uses of different microscopies applied to the study of porous silicon is presented.

Transmission Electron Microscopy Studies Transmission electron microscopy (TEM) and its high-resolution version (HRTEM) (Goodhew et al. 2001) have been used by several researchers for the characterization of porous silicon at the nanoscale, which allow resolving very small regions. Furthermore, the combination of TEM/HRTEM with image processing constitutes an extremely powerful technique to accomplish detailed morphological analysis at the nanoscale. Moreover, the combined use of TEM with energy-dispersive X-ray spectroscopy (EDX or EDS) or energy-filtering TEM (EFTEM) allows the determination of the chemical composition of the structure of PS with a very high degree of precision. HRTEM studies have allowed determining the lattice parameter, crystallite size, as well as orientation of the silicon nanocrystals present in PS (Cole and Harvey 1992; Cullis and Canham 1991; M€under et al. 1992; Lehmann et al. 1993; Frohnhoff et al. 1995; Martı´n-Palma et al. 2002, 2004; Pascual et al. 2005; Wijesinghe et al. 2009). Pore size and nanocrystal arrangement and distribution have been found to enormously depend on the particular fabrication technique and parameters.

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Fig. 1 (Left) Morphology of nanostructured porous silicon, consisting in an amorphous matrix with Si crystallites embedded in it with no preferential orientation. This material shows a polycrystalline diffraction pattern. (Middle) Image processing allowed studying the structure of the individual Si nanocrystals. (Right) The size of the individual Si crystallites, interplanar distance (d ), lattice parameter (a), total lattice expansion (Δa), and relative lattice expansion (Δa/aSi) were directly determined

Fig. 2 (Left, middle) Interface between PS and bulk Si. (Right) Dislocations are usually found at the PS/Si interface

Under the appropriate fabrication conditions, the electrochemical etch of Si results in PS layers composed of Si nanocrystals with no preferential orientation embedded in an amorphous matrix (Martı´n-Palma et al. 2002), as shown in Fig. 1. Furthermore, the lattice parameter of the Si nanocrystals which compose PS has been found to increase with respect to that of bulk Si. This effect is also shown in Fig. 1. Additionally, TEM techniques allow the analysis of cross sections of porous silicon. As an example, HRTEM has been commonly used to study the transition region between PS and the underlying Si substrate. In this regard, HRTEM crosssectional images (Fig. 2) show that both the thickness and roughness of the

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transition region between PS and the substrate are generally small. As such, the PS/Si interface has a very small effect on, for example, the optical behavior in the visible wavelength regime of PS optical devices. In Fig. 2, a strong strain contrast between the PS layer and the Si substrate is observed, which is likely caused by high stress at the PS/Si interface (Martı´n-Palma et al. 2004). It has been found that the stress at the PS/Si interface is caused by a high density of dislocations and latticematching occurs through pairs of edge dislocations (Fig. 2). TEM has also been used to analyze the in-depth porosity profile of PS-based multilayer stacks. An example is shown in Fig. 3 (left).

Scanning Electron Microscopy Scanning electron microscopy (SEM) and high-resolution SEM (HRSEM) (Goodhew et al. 2001) have been extensively used to study the morphology of porous silicon. It is virtually impossible to provide the reader with a complete list of works in this field. In this regard, many SEM images of high quality can be found in books, reviews (see, e.g., Canham 1997; Lehmann 2002; Sailor 2011), and even the Web! In addition to analyze the surface morphology, SEM allows studying cross sections of porous silicon. Figure 3 shows two examples, consisting in a crosssectional SEM image of a PS multilayer stack (middle) and a microcavity (right). From Fig. 3, it is observed that PS-based multilayers have indeed a notable lateral and in-depth homogeneity, with well-defined and sharp interfaces between adjacent layers. This results in good optical behavior (see chapters “▶ Color of Porous Silicon,” “▶ Optical Gain in Porous Silicon,” “▶ Porous Silicon Photonic Crystals,” “▶ Porous Silicon Multilayers and Superlattices,” and “▶ Refractive Index of Porous Silicon”). Figure 4 shows top SEM images of a complex porous silicon-based structure, consisting in square wells with dimensions in the micron range whose walls are composed of multilayer vertical structures with feature sizes in the nanometer range. Additionally, microporous silicon is observed at the bottom of the wells. Effectively, these structures behave as three-dimensional photonic crystals

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Fig. 4 SEM top views of a complex PS structure, consisting in a square pattern on the micron scale and a multilayer stack on the nanometer scale (see the walls of the vertical structures)

(Recio et al. 2012 and chapters “▶ Porous Silicon Photonic Crystals” and “▶ Porous Silicon Phononic Crystals”). As in the case of TEM/HRTEM, image processing can be applied to the analysis of images acquired by SEM/HRSEM with the objective of determining several parameters of interest. These include porosity, specific surface area, pore size, and pore size distribution (Ludurczak et al. 2009).

Atomic Force Microscopy Atomic force microscopy (AFM) (Eaton and West 2010) is a technique which has been widely used for the analysis of the surface structure of PS at the nanoscale. Surface roughness is a factor of major importance for many different practical uses, including optical and biomedical applications (chapters “▶ Cell Culture on Porous Silicon,” “▶ Drug Delivery with Porous Silicon,” “▶ Porous Silicon and Tissue Engineering Scaffolds,” “▶ Porous Silicon in Brachytherapy,” “▶ Porous Silicon in Photodynamic and Photothermal Therapy,” “▶ Porous Silicon Optical Biosensors,” “▶ Porous Silicon Optical Waveguides,” and “▶ Porous Silicon Phononic Crystals”). In the case of optics, low roughness at the external surface and the substrate interface is mandatory to achieve good quality optical layers. To illustrate the use of this technique, Fig. 5 shows AFM images of low porosity and high porosity PS surfaces. These surfaces may be considered optically flat for practical purposes in the whole visible range (diffuse reflection at the blue end of the visible spectrum is lower than 0.3 %). A related technique, namely, scanning tunneling microscopy (STM), has also been used to acquire topographic as well as photon emission maps of the surface of porous silicon (Dumas et al. 1993). In this technique, a STM tip is used as a local source of electrons to excite cathodoluminescence (CL), resulting in an image resolution of around 1 nm. In this line, CL mapping of porous silicon can be achieved by modifying a commercial SEM system, basically by adding a collecting

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mirror and a photomultiplier to the SEM (Bruska et al. 1996), aiming at understanding the relationship between the structural and electronic properties in this material. A variation of STM, named scanning tunneling microscopy light emission (STM-LE), allows the measurement of the visible spectra from individual protrusions on the surface of PS (Ito et al. 1995). Additionally, photoassisted STM has been used to study the surface of porous silicon (Pavlov and Pavlova 1997). In this particular case, electron-hole pairs are excited by light and STM is used to measure the corresponding tunneling current of excited carriers.

Other Microscopies Although TEM/HRTEM, SEM/HRSEM, and AFM have been widely and routinely used to analyze its morphology and overall physicochemical behavior, porous silicon has been studied using a number of other microscopy techniques. Among them, photoluminescence can be highlighted given that initially the most attracting property of porous silicon was its light-emitting capabilities in the visible wavelength regime at room temperature. As an example of the myriad of studies in this area, luminescence from individual Si nanocrystals in PS has been spatially isolated and detected (Mason et al. 1998). For this study, a combination of single-particle spectroscopy and shear force microscopy was used. In a subsequent study, the distribution of individual chromophores in porous silicon was analyzed by combining the previous techniques with fluorescence microscopy (Mason et al. 2001). The experimental results link the number and size of quantum dots in PS with its photoluminescence emission. In this line, photoluminescence and reflected light images of porous silicon can be acquired by means of a confocal scanning beam macroscope/microscope (Ribes et al. 1995). It is worth noting that confocal imaging allows reconstructing 3D profiles of porous silicon. Raman microscopy has also been widely used to determine the structure and overall properties of porous silicon at the nanoscale. In particular, parameters like crystallite size, temperature, and stress in porous silicon have been studied by a

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combination of micro-Raman and microphotoluminescence spectroscopies (Manotas et al. 1999), as shown in Fig. 6. Besides PS layers and multilayers, more elaborate structures such as microcapsules have also been studied by microRaman spectroscopy, aiming at evaluating stress and crystallinity (Naumenko et al. 2012). Additionally, near-field scanning optical microscopy (NSOM) has been successfully applied to the imaging of topography and locally induced photoluminescence of porous silicon (Rogers et al. 1995). The experimental results are, as in previous cases, consistent with the quantum confinement model.

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Multiphoton microscopy is another technique applied to the study of the nonlinear optical response in porous silicon (Palestino et al. 2009). As an example of its use, two-photon-excited fluorescence (TPEF) emission and second harmonic generation (SHG) from glucose oxidase (GOX) adsorbed on porous silicon were detected simultaneously. And, finally, acoustic microscopy, comprising high-frequency microechography, acoustic signature V(z), and acoustic imaging, can be used to investigate the elastic properties of PS in a nondestructive manner (Da Fonseca et al. 1995). Among these properties, thickness, longitudinal wave velocity, density, acoustic impedance, velocities of surface acoustic modes, presence of elastic gradients, and surface and subsurface roughness and defects can be measured, estimated, or, at least, qualitatively determined.

Concluding Remarks The complex structure of porous silicon makes it a very versatile material which can be used in a wide variety of fields. At the same time, characterizing the morphology of PS is a complex task. Several microscopy techniques have allowed over the years to precisely determine the morphology and many other properties of PS, with typical feature sizes spanning the micro- to nanometric length scales.

References Bruska A, Chernook A, Schulze S, Hietschold M (1996) Cathodoluminescence and writing of optical patterns on porous silicon by scanning electron microscopy. Appl Phys Lett 68(17):2378 Canham LT (1997) Properties of porous silicon. Institution of Engineering and Technology, London Cole MW, Harvey JF (1992) Microstructure of visibly luminescent porous silicon. Appl Phys Lett 60(22):2800–2802 Cullis AG, Canham LT (1991) Visible light emission due to quantum size effects in highly porous crystalline silicon. Nature 353:335 Da Fonseca RJM, Saurel JM, Foucaran A, Massone E, Taliercio T, Camassel J (1995) Acoustic microscopy investigation of porous silicon. Thin Solid Films 255:155 Dumas P, Gu M, Syrykh C, Gimzewski JK, Makarenko I, Halimaoui A, Salvan F (1993) Direct observation of individual nanometer-sized light-emitting structures on porous silicon surfaces. Europhys Lett 23(3):197 Eaton P, West P (2010) Atomic force microscopy. Oxford University Press, Oxford Frohnhoff S, Marso M, Berger MG, Tho¨nissen M, L€ uth H, M€ under H (1995) An extended quantum model for porous silicon formation. J Electrochem Soc 142(2):615 Goodhew PJ, Humphreys J, Beanland R (2001) Electron microscopy and analysis, 3rd edn. Taylor & Francis, New York Ito K, Ohyama S, Uehara Y, Ushioda S (1995) Visible light emission spectra of individual microstructures of porous Si. Appl Phys Lett 67(17):2536 Lehmann V (2002) Electrochemistry of silicon: instrumentation, science, materials and applications. Wiley-VCH, Weinheim

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Lehmann V, Jobst B, Muschik T, Kux A, Petrova-Koch V (1993) Correlation between optical properties and crystallite size in porous silicon. Jpn J Appl Phys 32(5):2095 Ludurczak W, Garel O, Berthoumieu Y, Babot O, Donias M, Dufour-Gergam E, Niang F, Pellet C, Toupance T, Verjus F (2009) Image processing for the characterization of porous silicon nanostructure. Phys Status Solidi C 6(7):1675 Manotas S, Agullo´-Rueda F, Moreno JD, Martı´n-Palma RJ, Guerrero-Lemus R, Martı´nez-Duart JM (1999) Depth-resolved microspectroscopy of porous silicon multilayers. Appl Phys Lett 75(7):977 Martı´n-Palma RJ, Pascual L, Herrero P, Martı´nez-Duart JM (2002) Direct determination of grain sizes, lattice parameters, and mismatch of porous silicon. Appl Phys Lett 81:25–27 Martı´n-Palma RJ, Pascual L, Landa A, Herrero P, Martı´nez-Duart JM (2004) High resolution transmission electron microscopic analysis of porous silicon/silicon interface. Appl Phys Lett 85(13):2517–2519 Martı´n-Palma RJ, Manso-Silván M, Torres-Costa V (2010) Review of biomedical applications of nanostructured porous silicon. J Nanophoton 4:042502-1-20 Mason MD, Credo GM, Weston KD, Buratto SK (1998) Luminescence of individual porous Si chromophores. Phys Rev Lett 80(24):5405 Mason MD, Sirbuly DJ, Carson PJ, Buratto SK (2001) Investigating individual chromophores within single porous silicon nanoparticles. J Chem Phys 114(18):8119 M€ under H, Andrzejak C, Berger MG, Klemradt U, L€ uth H, Herino R, Ligeon M (1992) A detailed Raman study of porous silicon. Thin Solid Films 221(1–2):27 Naumenko D, Snitka V, Duch M, Torras N, Esteve J (2012) Stress mapping on the porous silicon microcapsules by Raman microscopy. Microelectron Eng 98:488 Palestino G, Martin M, Agarwal V, Legros R, Cloitre T, Zimányi L, Gergely C (2009) Detection and light enhancement of glucose oxidase adsorbed on porous silicon microcavities. Phys Status Solidi C 6(7):1624 Pascual L, Martı´n-Palma RJ, Landa-Cánovas AR, Herrero P, Martı´nez-Duart JM (2005) Lattice distortion in nanostructured porous silicon. Appl Phys Lett 87(25):251921-1-3 Pavlov A, Pavlova Y (1997) Investigation of the surface topography of light emitting nanostructures of porous Si and the related photovoltaic effect by photoassisted scanning tunnelling microscopy. Thin Solid Films 297:132 Recio G, Dang ZY, Torres-Costa V, Breese MBH, Martı´n-Palma RJ (2012) Highly flexible method for the fabrication of photonic crystal slabs based on the selective formation of porous silicon. Nanoscale Res Lett 7:449 Ribes AC, Damaskinos S, Dixon AE, Carver GE, Peng C, Fauchet PM, Sham TK, Coulthard I (1995) Photoluminescence imaging of porous silicon using a confocal scanning laser macroscope/microscope. Appl Phys Lett 66(18):2321 Rogers JK, Seiferth F, Vaez-Iravani M (1995) Near field probe microscopy of porous silicon: observation of spectral shifts in photoluminescence of small particles. Appl Phys Lett 66(24):3260 Sailor MJ (2011) Porous silicon in practice. Weinheim, Wiley Torres-Costa V, Martı´n-Palma RJ (2010) Application of nanostructured porous silicon in the field of optics. A review. J Mater Sci 45(11):2823–2838 Wijesinghe TLSL, Li SQ, Breese MBH, Blackwood DJ (2009) High resolution TEM and tripleaxis XRD investigation into porous silicon formed on highly conducting substrates. Electrochim Acta 54:3671

X-Ray Diffraction in Porous Silicon Jeffery L. Coffer

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Analysis of Strain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Analysis of pSi Multilayers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Analysis of Loaded/Infiltrated pSi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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X-ray diffraction (XRD) is a useful, complementary tool in the structural characterization of porous silicon (pSi), providing information not readily available from direct visualization techniques such as electron microscopies. This review outlines key considerations in the use of diffraction techniques for analyses of this material in both thin film form and freestanding porous Si nano or microparticles. Examples of the range of content in the analysis of pSi are provided, including formation mechanisms, layer thickness, extent of pSi oxidation, and degree of crystallinity. Such properties influence practical properties of pSi such as its biodegradability. We also focus on selected key properties where XRD has been particularly informative: (a) strain, (b) the structural analysis of pSi multilayers, and (c) an analysis of pSi loaded with small molecules of fundamental or therapeutic interest.

J.L. Coffer (*) Department of Chemistry, Texas Christian University, Fort Worth, TX, USA e-mail: [email protected] # Springer International Publishing Switzerland 2014 L. Canham (ed.), Handbook of Porous Silicon, DOI 10.1007/978-3-319-05744-6_42

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Introduction X-ray diffraction (XRD) (Pecharsky and Zavalij 2009) is a complementary tool in the structural characterization of porous silicon (pSi), providing useful information not readily available from direct visualization techniques such as electron microscopies. This review outlines key considerations in the use of diffraction techniques for analyses of this material in thin film form attached to its underlying Si substrate, along with recent results applied to freestanding porous Si nano or microparticles. In terms of instrumentation, a typical x-ray powder diffractometer used in the analysis of pSi is illustrated in Fig. 1. Spectra of pSi in powder form with good signal-to-noise ratios can be obtained using a Cu Kα source operating at 25–30 kV on sample sizes of 10–25 mg. XRD has been utilized for a diverse range of scientific content in the analysis of pSi, ranging from formation mechanisms (Chamard et al. 2001) to layer thickness (Guilinger et al. 1995). Other examples include the use of XRD as an informative probe of the extent of pSi oxidation (Ogata et al. 2001; Buttard et al. 1996a; Pap et al. 2005) as well as the degree of crystallinity (Lehmann et al. 1993); experimental modification of these two parameters strongly influences other unique properties of pSi such as its biodegradability (Shabir 2014; Shabir et al. 2011). Representative examples of the range of information obtained from XRD on pSi are outlined in Table 1.

Fig. 1 Typical x-ray powder diffractometer (Phillips XL 300). Key components are identified, including the transformer/power supply, x-ray source, goniometer, and detector

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Table 1 Areas of pSi research served by XRD Fundamental topic/application Formation mechanism Layer thickness Oxidation Crystallinity/amorphization Thermal expansion Wetting by liquids Structural form (film, membrane, particle) Interfacial roughness Homoepitaxy Strain

Porosity gradients/multilayers Loading/infiltration

Reference Chamard et al. (2001) Guilinger et al. (1995) Ogata et al. (2001), Buttard et al. (1996a), Pap et al. (2005) Lehmann et al. (1993), Deb et al. (2001) Faivre et al. (2000) Bellet and Dolino (1994) Milita et al. (2001), Buttard et al. (2002), Russo et al. (2011) Lomov et al. (2000) Liu et al. (2003) Barla et al. (1984), Young et al. (1985), Bensaid et al. (1991), Bellet et al. (1992), Lehmann et al. (1993), Bellet and Dolino (1996), Lopez-Villegas et al. (1996), Buttard et al. (1999), Abramof et al. (2006), Wijesinghe et al. (2009) Buttard et al. (1996b, 1998) Henschel et al. (2008, 2009), Berwanger et al. (2009), Henschel et al. (2010), Wang et al. (2010), Ge et al. (2013)

We subsequently focus below on three key properties where XRD has been particularly informative: (a) strain, (b) the structural analysis of pSi multilayers, and (c) an analysis of pSi loaded with small molecules of fundamental or therapeutic interest.

Analysis of Strain The most detailed scrutiny has emerged from studies of pSi samples obtained from anodization of p and p+ wafers; traditional high-resolution diffraction (Ogata et al. 2001; Pap et al. 2005), along with double (Buttard et al. 1996a; Young et al. 1985; Lopez-Villegas et al. 1996) and triple diffraction measurements (Wijesinghe et al. 2009), has been evaluated. In the standard diffraction experiments of this type of pSi, two features are observed in the 26–31o region: a sharp peak associated with the reflection (~28o) and a broad diffuse peak (Ogata et al. 2001; Young et al. 1985). Experiments to date suggest that the relative contributions of each are a function of HF electrolyte concentration (Ogata et al. 2001) and wafer resistivity (Ogata et al. 2001; Buttard et al. 1996a). Importantly, with thermal annealing up to 450  C, the intensity of the sharp feature disappears (Fig. 2) (Ogata et al. 2001). Subsequent detailed concurrent XRD, transmission electron microscopy (TEM), and electron diffraction have shown that the broad diffuse peak is not associated with amorphous material, but likely rather a consequence of a random distribution of nanopores (Bensaid et al. 1991). This is not without some controversy, however, as some groups propose the possibilities of strained microcrystallites (Lehmann et al. 1993) or pSi oxidation contributing to this phenomenon.

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Fig. 2 XRD spectra demonstrating a shift of (111) peak of a p+ pSi film after annealing at (a) as prepared, (b) 350  C, (c) 400  C, and (d ) 450  C (Adapted from Ogata et al. 2001)

Pragmatically, it should also be noted that for a freestanding, oriented pSi membrane, the Bragg condition is only satisfied for the peak near 70o (Ogata et al. 2001). Quantitative analysis of strain (typically by standard Hall-Williamson plots (Williamson and Hall 1953)) for a number of different sample types has led to an establishment of the following useful trends for pSi: • Strain is present in the form of lattice expansion perpendicular to the sample surface (Barla et al. 1984; Young et al. 1985; Bellet and Dolino 1996; LopezVillegas et al. 1996), with typical Δa/a values in the range of 1.49  10 3 to 2.2  10 3 (Lopez-Villegas et al. 1996). • Significantly, strain increases with increasing porosity (Barla et al. 1984; Bellet and Dolino 1996). • Strain increases with surface oxidation brought about by thermal annealing (and correlates with dangling bond concentration) (Ogata et al. 2001; Buttard et al. 1996a; Pap et al. 2005). These trends provide complementary insights into changes in the fundamental structure of pSi that accompany its common manipulation in the laboratory.

Analysis of pSi Multilayers The extensive number of studies of pSi-based biosensors fabricated in multilayer form (of alternating porosities) has provided strong motivation for analyses by XRD (Thust et al. 1996; Lin et al. 1997; Dancil et al. 1999; Chan et al. 2000). Related handbook chapters are “▶ Porous Silicon Multilayers and Superlattices,”

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and “▶ Porous Silicon Optical Biosensors.” Results for this type of pSi sample have been reported from double diffraction experiments, with data typically presented in the form of so-called rocking curves. One of the most detailed investigations has been reported by Bellet and coworkers for a structure of alternating porosities of 36 and 60 % for 10 periods (Buttard et al. 1996b, 1998). The most unique result of such investigations is the appearance of prominent satellite peaks in the ω/2Θ plots and their analysis by detailed simulations. Excellent agreement between experiment and simulation is found when a linear gradient transition layer (in terms of both porosity and lattice parameter) is employed between layers, with a width of 14 nm for this layer providing optimal results (Buttard et al. 1996b, 1998). For pSi superlattice multilayers of high quality, it is also possible to correlate the observed fringes in the low-order satellite peaks with the total number of periods in the pSi film (Buttard et al. 1998). This is viewed as strong evidence of lateral homogeneity of the entire superlattice film thickness overall.

Analysis of Loaded/Infiltrated pSi One of the interesting fundamental questions associated with a nanoporous material concerns the effect of the nanopore on the structure and associated properties of a loaded or infiltrated substance. For the case of pSi films still attached to its underlying Si substrate, this question has been addressed for a range of simple organic molecules, ranging from alkanes (such as hexane (Henschel et al. 2009)) to alcohols of various sizes (from as small as ethanol (Henschel et al. 2010) to a 19 carbon linear chain alcohol (Henschel et al. 2008; Berwanger et al. 2009)). In such studies, the size of the pore clearly has a strong influence on the structure of the infiltrated species. For example, mesoporous silicon with a 15 nm pore diameter results in the formation of lamellar bilayer structure, as evidenced by the appearance of a discrete Bragg reflection associated with this type of structure. In contrast, mesopores with a 10 nm diameter lack these layering reflections (Henschel et al. 2008). In the long term, it is believed that investigations of this sort may provide useful information concerning the use of mesoporous silicon matrices for inducing nucleation for the crystallization of protein solutions. Recent developments have also shown the ability of freestanding porous Si particles to act as stand-alone carriers for drug delivery (Anglin et al. 2008; Salonen et al. 2008), in vivo imaging (Park et al. 2009), and sensing (Sailor and Link 2005). In addition to ideally providing information regarding relative particle size (from a Scherrer analysis of linewidth) (Patterson 1939), for drug-loaded pSi materials, x-ray diffraction can also reveal details regarding the influence of pore structure on the crystallinity of the drug upon infiltration. For example, recent reports have shown that the crystalline antibacterial drug triclosan can be readily infiltrated into mesoporous SI via a straightforward meltloading procedure (Wang et al. 2010). These studies have shown that a significant broadening and/or loss of triclosan-associated peak intensity in the 20–30 range of 2θ takes place, depending on pSi porosity and loading method. Such changes are

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associated with nanostructuring or amorphatization of the loaded drug within the mesopores. To illustrate this effect, typical XRD patterns for crystalline triclosan and a pSi sample of 81 % porosity exposed to molten triclosan at 90  C for 35 min are illustrated in Fig. 3. Significant loss of intensity in the crystalline triclosan reflections near 24 and 25 are clearly observed.

Conclusions The above x-ray diffraction studies of pSi clearly demonstrate the level of sensitive structural information that this technique can provide. Given the increasing importance of this matrix in biosensing and drug delivery, along with emerging areas in energy relevant to battery technology (e.g., Li storage and cycling (Ge et al. 2013)), ample motivation for expanded use of XRD is in place.

References Abramof PG, Beloto AF, Ueta AY, Ferreira NG (2006) X-ray investigation of nanostructured stain-etched porous silicon. J Appl Phys 99:024304 Anglin EJ, Cheng L, Freeman WR, Sailor MJ (2008) Porous silicon in drug delivery devices and materials. Adv Drug Deliv Rev 60:1266–1277 Barla K, Herino R, Bomchil G, Pfister JC, Freund A (1984) Determination of lattice parameter and elastic properties of porous silicon by X-ray diffraction. J Cryst Growth 68:727–732 Bellet D, Dolino G (1994) X-ray observation of porous silicon wetting. Phys Rev B 50:17162–17165 Bellet D, Dolino G (1996) Diffraction studies of porous silicon. Thin Solid Films 276:1–6 Bellet D, Dolino G, Ligeon M (1992) Studies of coherent and diffuse X-ray scattering by porous silicon. J Appl Phys 71:145–149

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Gas Adsorption Analysis of Porous Silicon Armando Loni

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Main Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Application to Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Origin of Hysteresis in “Non-interconnected” Mesopores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Determination of Effects Associated with Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

432 432 433 434 434 436 436

Abstract

Pore volume and surface area of porous silicon are key parameters to consider when developing applications that rely on the capacity to carry a payload, such as drug delivery, or that are dependent on the degree of “reactivity,” such as sensing or energetics. The ability to define and tune surface areas and pore size distributions is a necessity for clinical use of the material. Herein, the historical assessment of these physical parameters by gas adsorption is reviewed, the methodology behind the measurements is described, the limitations are highlighted, and data related to its use in determining the effects associated with different anodization parameters and post-anodization processing is presented.

A. Loni (*) pSiMedica Ltd, Malvern, Worcestershire, UK e-mail: [email protected] # Springer International Publishing Switzerland 2014 L. Canham (ed.), Handbook of Porous Silicon, DOI 10.1007/978-3-319-05744-6_43

431

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A. Loni

Introduction Increasingly, porous silicon is being evaluated for the delivery of therapeutic agents, such as hydrophobic drugs, proteins, and peptides (see chapter “▶ Drug Delivery with Porous Silicon”). Surface area is particularly important for the optimization of large-molecule monolayer adsorption, while pore diameter is particularly important when loading proteins. Pore volume is related to porosity (see chapter “▶ Pore Volume (Porosity) in Porous Silicon”) and is generally described in terms of open volume (ml) per unit weight (g) of material, while surface area is defined by the exposed internal surface (m2) per unit weight of material; these parameters can be measured using gas adsorption-desorption analysis (Gregg and Sing 1982). As well as surface area and pore volume, information on pore size and shape can also be surmised.

Main Principles The gas adsorption-desorption technique relates to the adsorption of nitrogen (or, less commonly, carbon dioxide, argon, xenon, and krypton), at cryogenic temperatures, via adsorption and capillary condensation from the gas phase, with subsequent desorption occurring after complete pore filling. An adsorption-desorption isotherm is constructed based upon the relationship between the pressure of the adsorbate gas and the volume of gas adsorbed/desorbed. Computational analysis of the isotherms based on the BET (Brunauer-Emmett-Teller) (Brunauer et al. 1938) and/or BJH (Barrett-Joyner-Halenda) (Barrett et al. 1951) methods, underpinned by the classical Kelvin equation, facilitates the calculation of surface area, pore volume, average pore size, and pore size distribution. A variety of instruments designed specifically for gas adsorption-desorption analysis are commercially available. Common to all is the sample preparation and measurement methodology (International Organization for Standardization 2006a): a portion of the porous material to be analyzed (typically > 150 mg) is placed in a glass sample tube, dried/degassed (taking care to avoid thermal modification of the structure), and weighed; after attaching to the instrument, the sample tube is evacuated and the free space volume measured by dosing with helium; after evacuating the helium, the tube is immersed in cryogenic fluid (77 K for nitrogen adsorbate) and the adsorbate gas dosed to the tube in incremental volumes, with the pressure (P) being measured in situ relative to the saturation vapor pressure (PSV) of the gas; multilayer adsorption onto the pore walls occurs initially, followed by capillary condensation as the relative pressure is increased; dosing continues until the isotherm reaches a plateau, signifying that the pores are completely filled (P/PSV ¼ 1); thereafter, the pressure is incrementally reduced such that the liquid starts to desorb, the porous structure eventually becoming empty once again (P/PSV ¼ 0). Surface area is obtained relatively quickly from the adsorption portion of the isotherm (in the region of low relative vapor pressure) and follows the complete

Gas Adsorption Analysis of Porous Silicon

a

H1

H2

433

b H2

H2 Pore Blocking

Vad

Vad

Cavitation

Amount odsorbed

Delayed Condensation

0.2 H3

0.4

0.6

H4

0.8

1.0

Delayed Condensation

0.2 Meniscus

0.4

0.6

0.8

1.0

W>Wc

W90 %) may be subject to pore collapse during air-drying (see chapter “▶ Drying Techniques Applied to Porous Silicon”), and this would yield a comparatively lower pore volume (as well as lower surface area) and therefore porosity. With regard to thermal oxidation, while the classical model implies there is a continual shrinkage of pore size due to the associated volumetric expansion of the crystal lattice (Sailor 2012), in practice, the average pore size actually increases; this is accompanied by a shift in the pore size distribution and significant reductions in both surface area and pore volume, probably due to sintering (see chapter “▶ Sintering of Porous Silicon”) (Loni and Canham 2013).

Gas Adsorption Analysis of Porous Silicon

435

Pore Size Distribution : Anodised & Oxidised pSi

Cumulative Pore Volume (ml/g)

2.5

2

1.5

1

0.5

0 0

100

200

300

S1: 66% porosity

400 500 600 700 Average Pore Diameter (Å) S2: 86% porosity

800

900

1000

S2: Oxidised (800°C, 85 min)

Fig. 2 Pore size distributions for low- and high-porosity porous silicon (also showing the effect of thermal oxidation) Table 1 Parameters obtained from gas adsorption-desorption analysis after low- and highcurrent-density anodization and after static thermal oxidation in air (porosity calculated gravimetrically and from total pore volume; author’s data) Description S1: anodized (66 % gravimetric) S2: anodized (86 % gravimetric) S2 then oxidized (800  C, 85 min)

BET surface area (m2/g) 287

Pore volume (ml/g) 0.789 (64 % porosity)

Average pore size (nm) 10.9

495

2.267 (84 % porosity)

18.3

238

1.199 (75 % porosity)

20.1

Nitrogen annealing has been shown to increase pore size through coalescence (Bjorkqvist et al. 2006), similar to oxidation, and also results in an overall reduction in pore volume (and therefore payload capacity) (Limnell et al. 2007), while chemical derivatization of pore walls has been shown to have very little effect on the surface area and average pore size (Buriak et al. 1999). The technique has also proved useful in the characterization of optical grating-type waveguides (Radzi et al. 2012) as well as in the study of low-temperature solid-state interactions in nanoporous silicon (Khokhlov 2008) and in the characterization of other forms of porous silicon including etched silicon powders (Loni et al. 2011), silicon microassemblies (Bao et al. 2007), and silicon aerogels (Chen et al. 2012).

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Limitations Gas adsorption-desorption analysis becomes problematic for materials with low surface area (Yanazawa et al. 2000; Suzuki and Oosawa 1997) and with pore sizes >100 nm (Klobes et al. 2006). For mesoporous silicon with porosity less than 50 % (equivalent pore size 0.1 %, heavier elements >1 %); typical accuracy 10 % Destructive method Quantification needs very similar standards Calibration is critical

Heide (2011), Watts and Wolstenholme (2003)

Watts and Wolstenholme (2003), Thompson et al. (1985)

Vickerman et al. (1989)

No chemical state ID No chemical structure No organic analyses Only a lateral resolution of ~ 1 mm The chemical composition of PS can change during ion beam (RBS) analysis While it is very sensitive for heavy elements (ppm), it has a low sensitivity for light elements

Kimura (2006)

Chemical Characterization of Porous Silicon

465

Table 2 Compositional analysis contributing to PL mechanism elucidation Type of PS Asprepared

Investigation techniques XPS

AES

SIMS

RBS

Passivated surface PS

EDX

XPS

AES RBS

Oxidized PS

XPS

AES

SIMS

RBS

Motivation for using technique The presence of a multiphase system, amorphous and crystalline, directly evidenced by XPS, is responsible for visible PL The electron structure of the chemical bonds and defects that might influence PL, like the oxygen role Composition of the silicon surface during porosification in fluorohydrogenate ionic liquids Thickness of thin films and elemental composition (quantitative AES) The presence of H and H2 bonds on the Si surface atoms; the character of the confined energy states in PS vs. crystalline silicon Reveals that some oxidation occurs as part of the etching process or as a result of exposure to atmospheric oxygen PS porosity – beam effect strongly depends on the porosity of the sample The porosity profile of the PS multilayer structure – five low/high porosity bilayers Detailed information on the nature of the surfaces, the extended defects, the amount of hydrogen passivation, and their oxidation state The effect of plasma fluorination used for surface passivation on PL – tuning of optical and dielectric properties Quantitative depth profiling of the Si, O, N, and S elements – concentration distribution The existence of Si, O, and C has been evidentiated in different PS samples, which is consistent with the XPS results Study the various oxidation states present before and after aging of PS and the effect upon the optical properties Fraction of oxidized layer (SiO2 present) over the cross section of PS samples induced by electrochemical oxidation in H2SO4 Comparison of SiO2/Si (bulk) and SiO2/PS interfaces shows characteristic oxygen depletion in the last case after thermal oxidation Refractive index change and the PL blue shift observed are explained as features of aging-induced oxidation

Ref. Perez et al. (1992)

Domashevskaya et al. (1998) Raz et al. (2010)

Galiy et al. (1998) Dorigoni et al. (1996) Collins et al. (1992) Ko´tai et al. (2000) Torres-Costa et al. (2004) Yu et al. (2005)

Pan et al. (2004)

Xiong et al. (2001) Feng et al. (2006) Thogersen et al. (2012) Salem et al. (2006) Cwil et al. (2006)

Beresna et al. (2007)

PS modification

RBS

Lanthanide-doped mesoPS Er, Fe ions incorporated in PS Infrared PL of Er-doped PS Impregnation with different elements (Ni, Cu, Au, Pt, In, Fe) Compact low resistivity silicide contacts on Si

Long-time stabilization of CHxmodified PS PL properties Thin metallic layer analyses

SIMS

Metal (Ag, Cu, Au) deposition

Passivated luminescent PS based on organic monolayers covalently attached Selenization treatment

Ultrathin metallic films (Au/Ti/Ni thermal evaporation, Ti electron beam evaporation) Electroplated PS with Fe and Co; Fe, Co, and Ni galvanic deposition from sulfate salts

XPS

Optoelectronics

AES

Analysis

Applications

Henley et al. (2000), Herino (2000), Ramos et al. (2000)

Mahmoudi et al. (2007), Stewart and Buriak (2001), Kleps et al. (2000), Gaponenko (2001), Bondarenko et al. (2003)

Vdovenkova et al. (1997, 1999), Hamadache et al. (2003), Domashevskaya et al. (2012)

Reference Boukherroub et al. (2001), Lin et al. (2011), Hong et al. (2010), Harraz et al. (2002)

Results and comments Control of different organic molecules (alkenes and nonconjugated dienes) attachment process to the surface through Si-C bonds Passivation and origin of the PL intensity enhancement achieved by Se bonding on Si surface Clarify the reaction mechanism and quantify the metal deposition; PL enhancement by Au film deposition is attributed to the oxidation inhibiting effect of the Au film Chemical composition and electronic structure (electron concentration in the conduction band close to the surface of PS and at Si-Ti Si-Au interfaces) The particle size was calculated using static dielectric constant; Chemical composition of PS surface and layer depth profile; modifications of line shapes evidence metal reaction Surface passivation by coating with CHx layer or Lewis acid-mediated hydrosilylation Penetration of the metallic layer into the PS pores and effect on EL Optical activation of lanthanides within the whole area Analysis of in-depth Er and Fe concentrations The Er depth distribution in the PS is not influenced by annealing up to 1,000  C, although it strongly influences the oxygen content of the Si skeleton and the Er IR PL intensity The deposition homogeneity through the whole thickness of the layer and the effect on the PS optical properties

Table 3 Utilization of the compositional analysis for novel applications development

466 M. Kusko and I. Mihalache

Biomedicine

Energy

Sensors

PS particles for drug delivery

Pt incorporation within the calcium phosphate layers on PS

SIMS

Functionalization with catalysts

Photoelectrochemical system for water splitting

NO gas sensor based on PS Electrocatalytic activity of metalporous Si nanoassemblies PS surface passivation for crystalline Si solar cell PS nanostructures as hydrogen reservoirs

EDX

SIMS

XPS

RBS EDX

Surface functionalization by alkynes and alkenes

SIMS

DNA hybridization sensing

PS surface functionalization with organic layers for biosensing

EDX XPS

Clarifies that the amount of hydrogen produced by Si-H bond reaction with OH is limited by the Si-O and Si-F bonds formed during electrochemical etching Predicts the treatments necessary to accelerate the photooxidation of nano-silicon by water for hydrogen production PS matrix impregnated with transition metal-mediated dehydrogenate silanes (zirconocene and titanocene) Calibration of impregnation process of PS particles with Fe2O3 Diffusion studies of Pt complex-based antitumor compounds

XPS confirms the preliminary functionalization steps and presence of thick protein film covering the PSi surface Mechanism of grafting PEG monolayers; optimization of silane deposition and further surface modification with aldehyde and PEG Presence of reactive functional groups grafted on the PS surface (amine from APTS molecules or carboxylic functions from acrylic acid) Monitorization of Lewis acid-mediated hydrosilylation, consistent surface functionalization down the length of the pore Using mesoPS for in situ synthesis significantly increases the quantity of DNA probes attached Study of DC-sputtered Au distribution in the pores EDX confirms the presence and the uniformity distribution of Pt particles on/into PS layers Finding of an efficient surface passivation of a PS surface

Kleps et al. (2010), Li et al. (2000)

Miu et al. (2010), Bilyalov et al. (2000), Zhan et al. (2011), Bahruji et al. (2009), Li and Buriak (2006)

Jia et al. (2010), Guo et al. (2009), Arroyo-Hernandeza et al. (2003), Fang et al. (2006), Buriak et al. (1999), Lawrie et al. (2009), Baratto et al. (2000)

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spectroscopy (XPS), Auger electron spectroscopy (AES) on one hand, and secondary ion mass spectroscopy (SIMS) and Rutherford backscattering spectroscopy (RBS) on the other hand have been generally used to analyze the porous layers, mainly to determine their compositional profile. Each of them has advantages and disadvantages which represent critical factors in deciding the most appropriate one for achieving the information of interest and are briefly presented in Table 1.

Chemical Composition of Electrochemically Etched Porous Silicon The as-prepared PS layers were firstly investigated with these complementary methods in order to achieve a complete image of the large internal area chemical map where about 20 % of the silicon atoms are located, looking forward to accomplish a general model for porosification process and also a mechanism for the most renowned property of this material, photoluminescence (see chapter “▶ Photoluminescence of Porous Silicon”). Table 2 provides example of the compositional studies of freshly prepared and passivated/oxidized PS. While the associated analyses have been dedicated mainly to understand the role of the surface states on PL, including the time instability, and to find physicalchemical methods for stabilization and/or enhancement, the extension of the application areas towards nanocomposites and the attachment of complex molecules has given more importance to these methods (see, e.g., chapters “▶ Ferromagnetism and Ferromagnetic Silicon Nanocomposites,” “▶ Porous Silicon Optical Biosensors,” “▶ Porous Silicon Immunoaffinity Microarrays”). Table 3 contains examples of applications where the corresponding assay analysis methods represented a key process to certify formation of more complex structures on the surface through both introduction of functional groups and implantation/deposition of metallic ions and the consequent development of hybrid devices based on the PS matrix. It is notable that the compositional analyses performed to study the surface oxidation process and its influence on the luminescence properties demonstrated also the higher chemical reactivity of the PS layers possessing higher porosity, mainly because higher porosity means a larger internal surface. Acknowledgements The authors thank M. Simion, A. Bragaru, and T. Ignat for assistance in conducting literature searches and L. Canham for valuable suggestions.

References Arroyo-Hernandeza M, Martın-Palma RJ, Perez-Rigueiro J, Garcıa-Ruiz JP, Garcıa- Fierro JL, Martınez-Duart JM (2003) Biofunctionalization of surfaces of nanostructured porous silicon. Mater Sci Eng C 23:697 Bahruji H, Bowker M, Davies PR (2009) Photoactivated reaction of water with silicon nanoparticles. Int J Hydrog Energy 34:8504–8510

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Jia P, Yan Chun T, Ning X, Wei L, ShouJun X, JianNing L (2010) Covalently derivatized NTA microarrays on porous silicon for multi-mode detection of his-tagged proteins. Sci China Chem 53:1–10 Kimura K (2006) Rutherford backscattering spectroscopy – encyclopedia of analytical chemistry. Wiley, New York Kleps I, Angelescu A, Miu M (2000) Preparation and characterisation of metallic thin films for electroluminescent devices based on porous silicon. In: Nanostructured films and coatings. NATO Science Series, Kluwer, Dordrecht, pp 337–345 Kleps I, Ignat T, Miu M, Craciunoiu F, Trif M, Simion M, Bragaru A, Dinescu A (2010) Nanostructured silicon particles for medical applications. J Nanosci Nanotechnol 10: 2694–2700 Ko´tai E, Pászti F, Szilágyi E (2000) Investigation of beam effect on porous silicon. Nucl Instrum Methods Phys Res B 161–163:260–263 Lawrie JL, Xu Z, Laibinis PE, Molinari M, Weiss SM (2009) DNA oligonucleotide synthesis in mesoporous silicon for biosensing applications. Frontiers in pathogen detection: from nanosensors to systems edited by Fauchet PM, Proc SPIE 7167:71670R Li YH, Buriak JM (2006) Dehydrogenative silane coupling on silicon surfaces via early transition metal catalysis. Inorg Chem 45:1096–1102 Li X, St. John J, Coffer JL, Chen Y, Pinizzotto RF, Newey J, Reeves C, Canham LT (2000) Porosified silicon wafer structures impregnated with platinum anti-tumor compounds: fabrication, characterization, and diffusion studies. Biomed Microdev 2:265–272 Lin L, Sun X, Tao R, Feng J, Zhang Z (2011) The synthesis and photoluminescence properties of selenium-treated porous silicon nanowire arrays. Nanotechnology 22:075203 Mahmoudi B, Gabouze N, Guerbous L, Haddadi M, Beldjilali K (2007) Long-time stabilization of porous silicon photoluminescence by surface modification. J Lumin 127:534–540 Miu M, Kleps I, Danila M, Ignat T, Simion M, Bragaru A, Dinescu A (2010) Electrocatalytic activity of platinum nanoparticles supported on nanosilicon. Fuel Cells 10(2):259–269 Pan LK, Ee YK, Sun CQ, Yu GQ, Zhang QY, Tay BK (2004) Band-gap expansion, core-level shift, and dielectric suppression of porous silicon passivated by plasma fluorination. J Vac Sci Technol B 22:583 Perez JM, Villalobos J, McNeill P, Prasad J, Cheek R, Kelber J, Estrera JP, Stevens PD, Glosser R (1992) Direct evidence for the amorphous silicon phase in visible photoluminescent porous silicon. Appl Phys Lett 61:563 Ramos AR, Conde O, Paszti F, Battistig G, Vazsonyi E, da Silva MR, da Silva MF, Soares JC (2000) Ion beam synthesis of chromium silicide on porous silicon. Nuclear Instrum Methods Phys Res B 161–163:926–930 Raz O, Shmueli Z, Hagiwara R, Ein-Eli Y (2010) Porous silicon formation in fluorohydrogenate ionic liquids. J Electrochem Soc 157:H281–H286 Salem MS, Sailor MJ, Harraz FA, Sakka T, Ogata YH (2006) Electrochemical stabilization of porous silicon multilayers for sensing various chemical compounds. J Appl Phys 100:083520 Stewart MP, Buriak JM (2001) Exciton-mediated hydrosilylation on photoluminescent nanocrystalline silicon. J Am Chem Soc 123:7821–7830 Thogersen A, Selj JH, Marstein ES (2012) Oxidation effects on graded porous silicon antireflection coatings. J Electrochem Soc 159:D276–D281 Thompson M, Baker MD, Christie A, Tyson JF (1985) Auger electron spectroscopy (chemical analysis). Wiley, New York Torres-Costa V, Pászti F, Climent-Font A, Martı´n-Palma RJ, Martı´nez-Duart JM (2004) RBS characterization of porous silicon multilayer interference filters. Electrochem Solid State 7: G244–G246 van der Heide P (2011) X-ray photoelectron spectroscopy: an introduction to principles and practices. Wiley, Chichester Vdovenkova T, Strikha V, Vikulov V (1997) Auger electron spectroscopy study of the electronic structure of porous silicon–metal interfaces. J Electron Spectrosc Relat Phenom 83:159

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Characterization of Porous Silicon by Infrared Spectroscopy Yukio H. Ogata

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sample Preparation and Instrumentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hydrogen-Terminated Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Oxidation of Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IR Measurement Using Methods Other Than the Transmission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

474 474 474 475 476 478 478

Abstract

The surface of electrochemically etched porous silicon is passivated with hydrogen just after preparation. The surface is gradually oxidized under ambient atmosphere, and the rate depends upon the ambient condition. The chemical and physical changes affect the properties of porous silicon-based devices. Proper understanding of the surface is important, and infrared (IR) spectroscopy is an effective and easy tool for monitoring and/or characterizing the surface state. Silicon is almost transparent to IR light, and hence the convenient transmission measurement is applicable to films and membranes of porous silicon. The measurement technique is first described, and then assignments of absorption bands in the spectra are given for the hydrogen-terminated and oxidized surface. The prevention of oxidation and the functionalization of porous silicon surface are important for many practical uses, where IR measurements can be used to monitor the surface. In addition, methods other than the transmission mode are briefly introduced.

Y.H. Ogata (*) Institute of Advanced Energy, Kyoto University, Uji, Kyoto, Japan e-mail: [email protected] # Springer International Publishing Switzerland 2014 L. Canham (ed.), Handbook of Porous Silicon, DOI 10.1007/978-3-319-05744-6_48

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Introduction A silicon surface is stabilized with hydrogen termination with the hydrofluoric acid (HF) treatment or during the porosification in solution containing fluoride (Trucks et al. 1990; Searson and Zhang 1990; Gerischer et al. 1993). The surface states, SiHx, influence the properties. The passivated surface undergoes oxidation on putting it in an environment, where some oxidants are present; the oxidation rate depends upon the type of oxidant or the oxidizing ability. The oxidation also affects the properties. Silicon surface is often modified with organic species in order to stabilize the surface against its oxidation and more to give new functionality (Sailor 2011). Understanding of these chemical states of the silicon surface is indispensable for the study on porous silicon. There are many analytical methods to access the chemical information. Among them, infrared spectroscopy (IR) is a powerful tool to analyze the atomic bonding in a molecule (G€unzler and Gremlich 2002; Tolstoy et al. 2003; Settle 1997; Stuart 2004) and is widely used. In this chapter, only basic IR response of porous silicon itself is described, but the technique is widely utilized in such as characterization/quantification of materials loaded in pores like proteins or drug molecules, special inhomogeneity, oxide nature, free carrier concentrations, and degree of derivatization. The reader may find some examples in the other chapters.

Sample Preparation and Instrumentation IR measurement is usually based on the transmission mode. It is the easiest way of the measurement. Silicon is almost transparent against infrared light, whereas it absorbs visible light to some extent. Moderately or lightly doped silicon enables the transmission measurement of a porous silicon layer. Porous silicon has a large surface area (Herino et al. 1987), and the transmission IR provides a good quality of the spectrum. A silicon wafer after dipping in dilute HF can be used as the reference for the IR measurement. Highly doped silicon or degenerate silicon (n+ or p+ Si) exhibits some absorption, especially at high-frequency region due to the high impurity concentration. Even in the case, the transmission IR becomes possible if the layer is detached from the substrate by applying high current after formation of a porous silicon layer and the free-standing porous silicon layer is measured. These days, almost all IR measurement uses Fourier transform IR (FTIR), which is superior to the dispersive IR in the sensitivity, sampling time, and resolution. Measurable wavenumber region depends upon the detector used: the most popular detectors are TGS (triglycine sulfate), 350–7,800 cm
1, and semiconductor-type detector MCT (HgCdTe), the low detection limit around 650 cm
1.

Hydrogen-Terminated Porous Silicon Silicon is tetravalent, and the crystal has a tetrahedral structure. Hydrogen molecules can bond with silicon in the crystal as SiHx (x ¼ 1–3). An example of the spectra prepared from p-type silicon (100) is shown in Fig. 1a. Three absorption

Characterization of Porous Silicon by Infrared Spectroscopy

a

b p-Si (100) PS oxidized

p-Si (100) PS as-prepared

SiHxstr

Absorbance

SiH2, SiH deform

Absorbance

475

SiH2 scis

Meas.

Si-O-Si

Si-O-Si

OnSi-Hx Cal.

2500

2000

1500

1000

Wavenumber / cm–1

500

2500

2000

1500

1000

500

Wavenumber / cm–1

Fig. 1 IR spectra of porous silicon prepared from p-type silicon (100): (a) as prepared and the results of vibrational analysis by the molecular orbital calculation, (b) oxidized in dry air at 333 K for 50 days

bands are clearly observed in three frequency regions: 2,090–2,150 cm
1, ~920 cm
1, and 620–700 cm
1. A broad band in the 1,000–1,300 cm
1 region is sometimes found in some spectra. This is caused by post-oxidation during the sample drying procedure. The peak assignment had been performed comparing with the spectra of the related materials, amorphous hydrogenated silicon (a-Si:H) (Lucovsky et al. 1979; Knights et al. 1978) and HF-treated silicon (Burrows et al. 1988; Chabal and Raghavachari 1984). Porous silicon and a-Si:H resemble with each other except the crystallinity. Their FTIR spectra are basically similar. HF-treated silicon is of course monocrystalline and measured by the reflection method. The spectrum is exactly that of porous silicon. The only difference between them is the azimuth. The monocrystalline surface is uni-oriented, and the orientation gives different spectra between p and s polarizations when the incident beam is polarized. Pores in microporous silicon are randomly oriented, and the polarized beam gives similar spectra. On the one hand, mesoporous silicon with a few branching and macroporous silicon are expected to show the difference when using differently polarized incident beam. The assignment of the absorption bands appearing in porous silicon is investigated experimentally and theoretically (Unagami 1980a; Gupta et al. 1988; Kato et al. 1988; Ito et al. 1990; Ogata et al. 1995a, 1998; Gupta et al. 1991). The theoretical or computational analyses such as ab initio molecular orbital calculation can be the powerful tool for the assignment. An assignment of the vibration modes is given in Table 1. The exact values of wavenumbers vary depending upon the measurements, and hence, the small variations should not be reproachable.

Oxidation of Porous Silicon Silicon is a very less noble element, and it is prone to be oxidized under ambient atmosphere. The oxidation can be easily followed by FTIR (Kato et al. 1988; Gupta et al. 1991; Unagami 1980b; Borghesi et al. 1994; Ogata et al. 1995b;

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Table 1 Absorption bands appearing in as-prepared porous silicon Wavenumber (cm
1) 2,142 2,108 2,087 916 667 626

Assignment SiH3 stretching SiH2 stretching SiH stretching SiH2 scissors bending SiH2 wagging SiH bending

Remark

Dimer (SiH2)n bending also participates

References Kato et al. (1988), Ogata et al. (1995a) Kato et al. (1988), Ogata et al. (1995a) Kato et al. (1988), Ogata et al. (1995a) Gupta et al. (1988), Ogata et al. (1995a) Ogata et al. (1998) Ogata et al. (1998)

Lucovsky 1979). Figure 1b gives an IR spectrum undergoing oxidation (Ogata et al. 1995b). Many absorption bands appear due to the oxidation, and SiHx-related bands still remain in some cases. The oxidation rate depends on the surrounding environment. The sample put in dry air at ambient temperature is fairly stable against oxidation, while the oxidation proceeds quite fast in saturated humidity atmosphere and at high temperature (Ogata et al. 1995b). The possible assignment to the vibrational modes is given in Table 2. The features are classified into three frequency regions: (a) a broad absorption around 3,600 cm
1 resulting from the O-H stretching vibrations, (b) absorptions slightly higher than the SiHx stretching (2,080–2,150 cm
1) attributed to the back-bond oxidation, and (c) a strongest absorption at 1,000–1,200 cm
1 and many absorptions lower than 1,000 cm
1 caused by Si-O-Si and deformational vibrations related to OnSiHx. Theoretical calculation helps the assignment (Kato et al. 1988; Ogata et al. 1995b; Lucovsky 1979; Lucovsky et al. 1983) but partially. The assignment of vibrational modes becomes complicated because many species are possible and the unoxidized SiHx states remain. Some vibrations of OnSiHx, especially the deformational modes appearing in the frequency lower than 1,000 cm
1, are difficult to be assigned with the computational analysis because of the presence of many modes and the coupling. Some experimental verification is always necessary for the assignment.

IR Measurement Using Methods Other Than the Transmission The surface of porous silicon is often modified with some materials in order to stabilize it against oxidation or to give a new functionality (Sailor 2011; Buriak 2002; Boukherroub et al. 2002; Salonen et al. 2002). Modification with organic materials is followed by FTIR. Porous silicon has a huge surface area, and the high surface area enables a very high sensitivity of the adsorbed molecules on the surface. It must be noted that the surface is sometimes involuntarily oxidized. The FTIR can be performed using a porous layer formed on a silicon substrate

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Table 2 Absorption bands appearing in oxidized porous silicon (except SiHx relating vibrations) Wavenumber (cm
1) 3,660 3,600 2,256

Assignment O-H stretching O-H stretching O3Si-H stretching

2,200

O2Si-H2 stretching

2,160

OSi-H3 stretching

1,050

Si-O-Si stretching OnSiHx deformation

870

800 708 470

OnSiHx deformation OnSiHx deformation Si-O-Si outof-plane rocking

Remark Sharp peak from an isolated hydroxyl group Broad absorption from physisorbed water

Broad absorption with a shoulder at 984 cm
1 Si-O-Si symmetric stretching is also possible in the region 700–900 cm
1

References

Gupta et al. (1991), Borghesi et a. (1994), Ogata et al. (1995b) Gupta et al. (1991), Borghesi et al. (1994), Ogata et al. (1995b) Gupta et al. (1991), Borghesi et al. (1994), Ogata et al. (1995b) Gupta et al. (1988), Lucovsky et al. (1983) Gupta et al. (1991), Lucovsky et al. (1983) Gupta et al. (1991), Lucovsky et al. (1983) Gupta et al. (1991), Lucovsky et al. (1983) Lucovsky et al. (1983)

with the transmission mode and also the pulverized sample. For the latter, the pulverized sample is mixed with KBr and then pressurized to make a pellet. The diffused light is analyzed (diffuse reflectance FTIR). The absorption bands appear in the same frequencies as the transmission spectroscopy, but the intensity is different because of the difference in the optical path. The qualitative comparison with the transmission spectroscopy needs the conversion with the use of the Kubelka-Munk function. The attenuated total reflection (ATR) is used to analyze the state of the very surface collecting the evanescent light. This method uses a tight contact of the sample with a prism with high refractive index such as ZnSe and KRS-5 (mixed crystal of TlI and TlBr). The use of silicon replacing the prism for the ATR enables the multiple internal reflection infrared spectroscopy (MIR-FTIR) (Rao et al. 1991; Kimura et al. 2001). Two edges of a silicon sample are beveled with 45 and polished. IR beam comes into the sample from the beveled edge and goes out to an FTIR analyzer from the other edge after multiple reflections at the both surfaces. The top surface is exposed to an electrolyte, and the spectrum provides the in situ and almost instantaneous information of the surface. The method has been effectively utilized for the investigation of temporal change on silicon or porous silicon,

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Table 3 Porous silicon-related absorption bands other than Si-H and Si-O Wavenumber (cm
1) 2,850–2,950

870

Assignment Adventitious carbon contamination from ambient air Si-Nx bending

812 770

Si-Fx stretching Si-CHx stretching or rocking

680 610

Si-C stretching Si-Si lattice vibration

Remark Appearing in aged sample

References Ogata et al. (1995b)

Observed using ATR-FTIR Very weak

James et al. (2010)

TO + TA combination

Ogata et al. (1995a) Salonen et al. (2002), Canaria et al. (2002) Canaria et al. (2002) Ogata et al. (1995a), Johnson and Loudon (1964)

such as the early stage of the porosification (Kimura et al. 2001) and the surface change during the potential or current oscillation during anodic polarization (Chazalviel et al. 1998; Kimura et al. 2003). Finally, it may be useful to give some IR absorption bands found in porous silicon other than listed in Tables 1 and 2 (Table 3).

Conclusion IR is a powerful and easy-to-use technique to obtain the surface chemical state of porous silicon. The convenience results from the transparency of silicon for IR light and the high surface area. The basic features begin from the knowledge of the bondings to hydrogen, Si-H, and to oxygen Si-O. The model calculations sometimes provide useful information in the assignment. It is true for the stretching vibrations, which are isolated and appear in the frequency region of 2,050–2,300 cm
1, while vibrational modes appearing at the low frequencies attributed to the deformational modes are often coupled and crowded, and hence the theoretical analysis is often difficult. Additional experimental work often helps the assignment. The assignment still remains some discussion, but the major understanding had been achieved in the last century.

References Borghesi A, Guizzetti G, Sassella A et al (1994) Induction-model analysis of Si-H stretching mode in porous silicon. Solid State Commun 89:615–618 Boukherroub R, Wojtyk J, Wayner D et al (2002) Thermal hydrosilylation of undecylenic acid with porous silicon. J Electrochem Soc 149:H59–H63

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Buriak J (2002) Organometallic chemistry on silicon and germanium surfaces. Chem Rev 102:1271–1308 Burrows V, Chabal Y, Higashi G et al (1988) Infrared-spectroscopy of Si(111) surfaces after HF treatment – hydrogen termination and surface-morphology. Appl Phys Lett 53:998–1000 Canaria C, Lees I, Wun A et al (2002) Characterization of the carbon-silicon stretch in methylated porous silicon – observation of an anomalous isotope shift in the FTIR spectrum. Inorg Chem Commun 5:560–564 Chabal Y, Raghavachari K (1984) Surface infrared study of Si(100)-(2x1)H. Phys Rev Lett 53:282–285 Chazalviel J, da Fonseca C, Ozanam F (1998) In situ infrared study of the oscillating anodic dissolution of silicon in fluoride electrolytes. J Electrochem Soc 145:964–973 Gerischer H, Allongue P, Kieling V (1993) The mechanism of the anodic-oxidation of silicon in acidic fluoride solutions revisited. Ber Bunsen-Ges Phys Chem 97:753–756 G€unzler H, Gremlich H-U (2002) IR spectroscopy: an introduction. Wiley-VCH, Weinheim Gupta P, Colvin V, George S (1988) Hydrogen desorption-kinetics from monohydride and dihydride species on silicon surfaces. Phys Rev B 37:8234–8243 Gupta P, Dillon A, Bracker A et al (1991) FTIR studies of H2O and D2O decomposition on porous silicon surfaces. Surf Sci 245:360–372 Herino R, Bomchil G, Barla K et al (1987) Porosity and pore-size distributions of porous silicon layers. J Electrochem Soc 134:1994–2000 Ito T, Yasumatsu T, Watabe H et al (1990) Effects of hydrogen-atoms on passivation and growth of microcrystalline Si. MRS Symp Proc 164:205–210 James TD, Parish G, Musca CA et al (2010) N2-Based thermal passivation of porous silicon to achieve long-term optical stability. Electrochem Solid-State Lett 13:H428–H431 Johnson F, Loudon R (1964) Critical-point analysis of phonon spectra of diamond silicon and germanium. Proc R Soc Lond A 281:274–290 Kato Y, Ito T, Hiraki A (1988) Initial oxidation process of anodized porous silicon with hydrogenatoms chemisorbed on the inner surface. Jpn J Appl Phys 27:L1406–L1409 Kimura Y, Kondo Y, Niwano M (2001) Initial stages of porous Si formation on Si surfaces investigated by infrared spectroscopy. Appl Surf Sci 175:157–162 Kimura Y, Nemoto J, Shinohara M et al (2003) In-situ observation of chemical states of a Si electrode surface during a galvanostatic oscillation in fluoride electrolytes using infrared absorption spectroscopy. Phys Status Solidi A 197:577–581 Knights J, Lucovsky G, Nemanich R (1978) Hydrogen-bonding in silicon-hydrogen alloys. Phil Mag B 37:467–475 Lucovsky G (1979) Chemical effects on the frequencies of Si-H vibrations in amorphous solids. Solid State Commun 29:571–576 Lucovsky G, Nemanich R, Knights J (1979) Structural interpretation of the vibrational-spectra of a-Si:H alloys. Phys Rev B 19:2064–2073 Lucovsky G, Yang J, Chao S et al (1983) Oxygen-bonding environments in glow-discharge deposited amorphous silicon-hydrogen alloy-films. Phys Rev B 28:3225–3233 Ogata Y, Niki H, Sakka T et al (1995a) Hydrogen in porous silicon - vibrational analysis of SiHx species. J Electrochem Soc 142:195–201 Ogata Y, Niki H, Sakka T et al (1995b) Oxidation of porous silicon under water-vapor environment. J Electrochem Soc 142:1595–1601 Ogata Y, Kato F, Tsuboi T et al (1998) Changes in the environment of hydrogen in porous silicon with thermal annealing. J Electrochem Soc 145:2439–2444 Rao A, Ozanam F, Chazalviel J (1991) Insitu Fourier-transform electromodulated infrared study of porous silicon formation - evidence for solvent effects on the vibrational linewidths. J Electrochem Soc 138:153–159 Sailor MJ (2011) Porous silicon in practice. Wiley-VCH, Weinheim Salonen J, Laine E, Niinisto L (2002) Thermal carbonization of porous silicon surface by acetylene. J Appl Phys 91:456–461

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Searson P, Zhang X (1990) The anodic-dissolution of silicon in HF solutions. J Electrochem Soc 137:2539–2546 Settle F (1997) Handbook of instrumental techniques for analytical chemistry. Prentice Hall, Upper Saddle River Stuart BH (2004) Infrared spectroscopy: fundamentals and applications. Wiley, Hoboken Tolstoy VP, Chernyshova I, Skryshevsky VA (2003) Handbook of infrared spectroscopy of ultra thin films. Wiley-VCH, New York Trucks G, Raghavachari K, Higashi G et al (1990) Mechanism of HF etching of silicon surfaces – a theoretical understanding of hydrogen passivation. Phys Rev Lett 65:504–507 Unagami T (1980a) Formation mechanism of porous silicon layer by anodization in HF solution. J Electrochem Soc 127:476–483 Unagami T (1980b) Oxidation of porous silicon and properties of its oxide film. Jpn J Appl Phys 19:231–241

Cell Culture on Porous Silicon Nicolas H. Voelcker and Suet P. Low

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Early Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Surface Modification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Topography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gradients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

481 482 483 488 490 493 493

Abstract

Cell culture is a powerful in vitro characterization technique to optimize the properties of a biomaterial for in vivo biomedical use by conversely revealing potential sources of cytotoxicity. A comprehensive literature survey of the range of cell types cultured on porous silicon is given, together with a discussion of how surface chemistry, topography, and porosity gradients affect cell behavior.

Introduction Cell culture is often utilized to determine the biocompatibility of materials and precedes or even replaces in vivo animal and human testing. The behaviour of cells such as attachment, proliferation, morphological changes, metabolic changes, cytotoxicity, protein expression, and RNA expression are all important factors that have to be taken into account when designing a biomaterial (Freshney 2005; Masters 2000). In this regard, many materials are being investigated for their suitability for

N.H. Voelcker (*) • S.P. Low Mawson Institute, University of South Australia, Adelaide, SA, Australia e-mail: [email protected] # Springer International Publishing Switzerland 2014 L. Canham (ed.), Handbook of Porous Silicon, DOI 10.1007/978-3-319-05744-6_50

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Fig. 1 Cells being cultured on porous silicon particles that have been compressed into a disk form as part of a cell delivery platform (Low 2008)

the culture of cells or even to study cellular interactions. Porous silicon is a popular choice for biosensor, bio-microelectromechanical systems (bioMEMS), biomaterials and tissue engineering applications. The porous structure, degradability, electrical conductivity, overall biocompatibility (see chapter “▶ Biocompatibility of Porous Silicon”), and ease of surface modification make this a fascinating platform to investigate cell culture interactions. For example, porous silicon disks are being developed for delivery of therapeutic ocular cells (Fig. 1).

Early Studies A variety of mammalian cells have been successfully cultured onto porous silicon surfaces. The first publications on this topic by Bayliss et al. demonstrated that attachment of Chinese hamster ovary (CHO) cells proceeded on porous silicon surfaces to a similar extent as on bulk silicon (Bayliss et al. 1997a, b). This was also confirmed with the neuronal cell line B50 (Bayliss et al. 2000). Cell viability in these studies was determined using two colorimetric assays, the MTT based on enzymatic reduction of a tetrazolium salt to a purple formazan and the neutral red uptake assay. B50 and CHO cells were cultured on bulk silicon, porous silicon, glass, and polycrystalline silicon. Both viability assays suggested that the neuronal cells showed preference for porous silicon above the other surfaces, while CHO cells showed the lowest viability on the porous silicon surface (Bayliss et al. 1999, 2000). The surfaces of the porous silicon used in these early studies were not modified post-etching, and it was not until a study utilized porous silicon surfaces with an oxide layer for cell culture that surface chemistry was found to play a crucial factor (Chin et al. 2001). Rat hepatocytes were cultured onto ozone-oxidized porous silicon that was further modified by fetal bovine serum and collagen type I coating. Here, the hepatocytes showed a preference for the collagen-coated surface (Chin et al. 2001). Viability assays such as MTT, XTT, MTS, or Alamar Blue are commonly used to determine the suitability of a material as a support for the attachment and growth of cells. These assays are based on the reduction of the tetrazolium dyes by cellular enzymes to formazan dyes with characteristic color. In 2006, it has come to light

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that porous silicon, even with an oxide layer, interferes with these assays by reducing tetrazolium dyes (Laaksonen et al. 2007; Low et al. 2006). Passivating the surface against hydrolytic attack reduces but does not completely remove the interfering behavior (Laaksonen et al. 2007). Dye uptake viability assays such as neutral red which make use of the ability of viable cells to incorporate the dye in the lysosomes were found to be not compatible with porous silicon either, since the neutral red dye can also ingress into the porous layer (Low et al. 2006). These findings suggest that viability assays for cells in contact with porous silicon need to be carefully evaluated for compatibility.

Surface Modification Surface modification of porous silicon has been used to protect the surface against hydrolytic attack in aqueous medium and stabilize or slow down surface degradation. It can also be used to promote or prevent mammalian cell adhesion (Low et al. 2006; Faucheux et al. 2004). The changes in surface chemistry have long been known to affect the attachment and proliferation of anchorage-dependent mammalian cells on materials featuring otherwise almost identical topography, where cell attachment can be inhibited on very hydrophobic or hydrophilic surfaces (Groth and Altankov 1996; Yanagisawa et al. 1989). This has been mainly attributed to the amount of serum proteins (containing attachment factors) that is pre-adsorbed to the surface (Faucheux et al. 2004), which in turn can mediate cell attachment (Webb et al. 2000). Freshly etched porous silicon (Si–H) is rather hydrophobic, whereas ozoneoxidized surface (Si–OH) is very hydrophilic. Attachment of proteins in cell culture medium has been known to bind to moderately hydrophilic surfaces, leading to greater cell attachment on those surfaces (Webb et al. 1998). Water contact angles, qualitatively describing surface wettability for freshly etched and surface-modified porous silicon surfaces, are shown in the table below (Table 1). Arguably, the simplest method to stabilize the porous silicon surface is oxidation. A popular technique is to use ozone to rapidly generate a Si–OH capped surface with a thin oxide layer. Alternatively, thermal treatment in air (400–800  C) is used to generate thicker oxide layers (Pap et al. 2004). Surface hydroxyl groups Table 1 Sessile drop water contact angle measurements for unmodified and surface-modified porous silicon etched under the same conditions (Low et al. 2006) Surface modification of porous silicon Freshly etched Amino silanized Collagen coated Polyethylene glycol silanized Fetal bovine serum coated Ozone oxidized

Contact angle >99 56 32 26 10 1,000  C – (100–750 Torr), H2 and vacuum

Structural reorganization present with thin oxide layer Transformation of pores into completely empty spheres resembling SOI, called ESS (empty space in silicon) Enhanced surface diffusion, enhanced structural changes close to the top surface in reduced pressure conditions and preconditioning with H2 Dendritic microstructure with higher initial porosity shows slower restructuring during heat treatment

Authors

Depauw et al. (2008, 2009) Kuzma-Filipek et al. (2007) Lehman et al. (2002) Depauw et al. (2009) Mizushima et al. (2000) Kuribayashi et al. (2004)

Kuzma-Filipek (2010) Kuribayashi et al. (2004) Sato et al. (2000)

Ott et al. (2004) Zheng et al. (1997)

(Depauw et al. 2008, 2009; Brendel et al. 2010). Similar studies of void shape evolution and SON (silicon-on-nothing) structure formation from masked anisotropic reactive ion etching resulting in square arrays of holes during H2 anneal were shown in Sudoh et al. (2009). Other less costly techniques using purely electrochemical macroporous silicon were also proposed (Lehmann et al. 1992; Solanki et al. 2004). Sintered porous silicon is also known to improve optical properties by scattering light at the 10–100 nm voids, which was investigated by numerous of authors

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(Wolf et al. 2008; Ghannam et al. 2006). Additionally while constructing multilayer porous designs, these promote constructive interference by multiple reflections at the layer interfaces, which are preserved after high-temperature treatment. For those reasons it is incorporated in thin film solar cells as optical boosters (Zettner et al. 1998; Duerinckx et al. 2006; Kuzma-Filipek et al. 2008; Krichen et al. 2009). Authors in Duerinckx et al. (2006) and Kuzma-Filipek et al. (2007) additionally emphasize that the high-temperature driven reorganization of the nm-sized pores into voids surrounded with silicon matrix preserves the conductivity of the majority of carriers in the devices (Kuzma-Filipek et al. 2007, 2008; Duerinckx et al. 2006). It was shown in Kuzma-Filipek et al. (2007) that the reorganization of porous silicon by sintering at 1,130  C in pure H2 is assuring no resistive losses for the current flow.

Conclusions Numerous applications in, e.g., biomedical or microelectronics require application of sintered porous silicon, and this is for either controlled molecule release, enhancement of mobility, conductivity, or optical properties of porous silicon. In the layer transfer processes (LTP), sintering is a critical step to form a sacrificial porous layer used for active layer detachment. Sintering process of porous silicon is well described by classical theory of sintering and is driven by the minimization of the surface energy, linked to the internal surface area. The annealing temperature and annealing ambient atmosphere are triggers to activate both volume and surface transport mechanisms, with the emphasis to the later one. Elongated annealing time can lead to similar evolution of the pore structure as increasing temperature.

References Brendel R (2001) Review of layer transfer processes for crystalline thin-film silicon solar cells. Jpn J Appl Phys 40:4431–4439 Brendel R, Ernst M (2010) Macroporous Si as an absorber for thin film solar cells. Phys Stat Sol RRL 4(1–2):40–42 Brendel R, Auer R, Artmann H (2001) Textured monocrystalline thin-film Si cells from the porous silicon (psi) process. Prog Photovolt Res Appl 9:217–221. doi:10.1002/pip.377 Buttard D, Dolino G, Faivre C (1999) Porous silicon strain during in situ ultrahigh vacuum thermal annealing. J Appl Phys 85(10):7105–7111 Canham LT (1997) Porous silicon formation by anodisation. In: Canham LT (ed) Properties of porous silicon. INSPEC, London, pp 12–22 Canham LT, Reeves C, Anderson M, Cox TI, Tinsley-Bown AM, Perkins EA, Squirrell DJ, Hollings M, Hutchinson A, Nicklin S, Wun A, Sailor MJ (2000) Tuning the morphology of porous silicon for immunoassays. In: Conference proceedings 2nd international conference on porous semiconductors- science and technology, Madrid, pp 547–554 Claassen WAP, Bloem J (1980) The nucleation of CVD silicon on SiO2 and Si3N4 substrates. J Electrochem Soc 127:194

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Coblenz WS (1990) The physics and chemistry of the sintering of silicon. J Mater Sci 25: 2754–2764 Depauw V, Richard O, Bender H, Gordon I, Beaucarne G, Poortmans J, Mertens R, Celis J-P (2008) Study of pore reorganisation during annealing of macroporous silicon structures for solar cell application. Thin Solid Films 516:6934–6938 Depauw V, Gordon I, Beaucarne G, Poortmans J, Mertens R, Celis J-P (2009) Large-area monocrystalline silicon thin films by annealing of macroporous arrays: Understanding and tackling defects in the material. J Appl Phys 106:033516 Duerinckx F, Kuzma-Filipek I, Van Nieuwenhuysen K, Beaucarne G, Poortmans J (2006) Reorganized porous silicon Bragg reflectors for thin-film silicon solar cells. IEEE Electr Dev Lett 27:837–839 Eaglesham DJ, White WE, Feldman LC, Moriya N, Jacobson DC (1993) Equilibrium shape of Si. Phys Rev Lett 70:1643 Gao T, Gao J, Sailor M (2002) Tuning the response and stability of thin film mesoporous silicon vapour sensors by surface modification. Am Chem Soc 18(25):9953–9957 Gardelis S, Bangert U, Harvey AJ, Hamilton B (1995) Double-crystal X-ray diffraction, electron diffraction, and high resolution electron microscopy of luminescent porous silicon. J Electrochem Soc 142:2094 Geguzin Ya, E (1984) The Physics of sintering (in Russian), Nauka, Moscow, pp 307. 2nd edition. Ghannam M, Abouelsaood A, Kuzma I, Duerinckx F, Poortmans J (2006) Optical modeling of capped multilayer porous silicon used as a back. In: Reflector in thin-film silicon solar cells proceedings of the IEEE 4th world conference on photovoltaic energy conversion, Hawaii, pp 1362–1364 Greskovich C, Rosolowksi JH (1976) Sintering of covalent solids. J Am Ceram Soc 59(7–8): 336–343 Halimaoui A, Campidelli Y, Larre A, Bensahel D (1995) Thermally induced modifications in the porous silicon properties. Phys Stat Sol B 190:35 Herino R (1997) Pore size distribution in porous silicon. In: Canham LT (ed) Properties of porous silicon. INSPEC, London, pp 89–96 Herino R, Perio A, Barla K, Bomchil G (1984) Microstructure of porous silicon and its evolution with temperature. Mater Lett 2:519–523 Kim HJ, Depauw V, Duerinckx F, Beaucarne G, Poortmans J (2006) Large area thin –film free standing monocrystalline Si solar cells by layer transfer. In: Proceedings to 4th WCPEC conference, Waikoloa Krichen M, Zouari A, BenArab A (2009) A simple analytical model of thin films crystalline silicon solar cell with quasi-monocrystalline porous silicon at the backside. Microelectron J 40: 120–125 Kuchler G, Scholten D, M€ uller G, Krinke J, Auer R, Brendel R (2000) Fabrication of textured monocrystalline Si films using the porous silicon (PSI) process. In: 16th European photovoltaic solar energy conference, London, pp 1695–1698 Kuribayashi H, Hiruta R, Shimizu R, Sudoh K, Iwasaki H (2004) Investigation of shape transformation of silicon trenches during hydrogen annealing. Jpn J Appl Phys, Part 2 43:L468–L470 Kuzma-Filipek I (2010) Advanced epitaxial silicon solar cells on low cost silicon substrates by means of porous silicon internal reflectors. PhD dissertation, KU Leuven, May 2010, ISBN:978-94-6018-205-1 Kuzma-Filipek I, Duerinckx F, Van Nieuwenhuysen K, Beaucarne G, Poortmans J, Mertens R (2007) Porous silicon as an internal reflector in thin epitaxial cells. Phys Stat Sol (A) 204(5): 1340–1345 Kuzma-Filipek I, Duerinckx F, Van Kerschaver E, Van Nieuwenhuysen K, Beaucarne G, Poortmans J (2008) Chirped porous silicon reflectors for thin-film epitaxial silicon solar cells. J Appl Phys 104:073529 Labunov V, Bondarenko V, Glinenko L, Dorofeev A, Tabulina L (1986) Heat treatment effect on porous silicon. Thin Solid Films 137:123–134

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Labunov V, Bondarenko VP, Borisenko VE, Dorofeev AM (1987) High-temperature treatment of porous silicon. Phys Stat Sol (a) 102:193 Lehman V (2002) Electrochemistry of silicon: Instrumentation, science, materials and applications. Wiley – VCH, Weinheim, p 117 Lehmann V et al. (1992) German patent DE 42 02 455 C1 Martini R, Depauw V, Gonzalez M, Vanstreels K, Nieuwenhuysen KV, Gordon I, Poortmans J (2012) Mechanical properties of sintered meso-porous silicon; a numerical model. Nanoscale Res Lett 7:597 Mizushima I, Sato T, Taniguchi S, Tsunashima Y (2000) Empty-space-in-silicon technique for fabricating a silicon-on-nothing structure. Appl Phys Lett 77(20):3290–3292 Moller HJ, Welsch G (1985) Sintering of ultrafine silicon powder. J Am Ceram Soc 68(6):320–325 Muller G (2002) Restrukturierung von porosem silizium durch temperaturbehandlung. PhD thesis, Institute of Applied Physics, University of Erlangen-Nurnberg M€ uller G, Brendel R (2000) Simulated annealing of porous silicon. Phys Stat Sol (a) 182:313–318 M€ uller G, Nerding M, Ott N, Strunk HP, Brendel R (2003) Sintering of porous silicon. Phys Stat Sol (A) 197:83–87 Ogata YH, Yoshimi N, Yasmuda R, Tsuboi T, Sakka T, Otsuki A (2001) Structural change in p-type porous silicon by thermal annealing. J Appl Phys 90(12):6487–6492 Ott N, Nerding M, Mueller G, Brendel R, Strunk HP (2003) Structural changes in porous silicon during annealing. Phys Stat Sol A 197:93–97 Ott N, Nerding M, Mueller G, Brendel R, Strunk HP (2004) Evolution of the microstructure during annealing of porous silicon multilayers. J App Phys 95(2):497–503 Reuter M, Brendle W, Tobail O, Werner JH (2009) 50 μm thin solar cells with 17.0 % efficiency. Solar Energy Mater Solar Cells 93:704–706. doi:10.1016/j.solmat.2008.09.035 Rinke T, Bergmann R, Werner J (1999) Quasi-monocrystalline silicon for thin-film devices. Appl Phys A Mater Sci Proc 68:705 Salonen J, Kaukonen A, Hirvonen J, Lehto V (2008) Mesoporous silicon in drug delivery applications. J Pharm Sci 97(2):632–653 Salonen J, Makila E, Riikonen J, Heikkila T, Lehto VP (2009) Controlled enlargement of pores by annealing of porous silicon. Phys Stat Sol A 206(6):1313–1317 Sato N, Sakaguchi K, Yamagata K, Fujiyama Y, Yonehara T (1995) Epitaxial growth on porous Si for a new bond and etchback silicon on insulator. J Electrochem Soc 142:3116 Sato T, Mitsutake K, Mitzushima I, Tsunashima Y (2000) Micro-structure transformation of silicon: a newly developed transformation technology for patterning silicon surfaces using the surface migration of silicon atoms by hydrogen annealing. Jpn J Appl Phys, Part 1 39:5033–5038 Solanki CS (2004) Study of porous silicon layer transfer for applications in thin-film monocrystalline silicon solar cells. PhD, K.U. Leuven, pp 15–30, 100–101 Solanki C, Bilyalov R, Poortmans J et al (2004) Characterization of free-standing thin crystalline films on porous silicon for solar cells. Thin Solid Films 451:649–654. doi:10.1016/j. tsf.2003.11.157 Song JH, Sailor MJ (1999) Chemical modification of crystalline porous silicon surfaces. Comm Inorg Chem 21:69–84 Stewart MP, Robins EG, Geders TW, Allen MJ, Chol HC, Buriak JM (2000) Three methods for stabilization and functionalization of porous silicon surfaces via hydrosilylation and electrografting reactions. Phys Stat Sol A 182:109–115 Sudoh K, Iwasaki H, Hiruta R, Kuribayashi H, Shimizu R (2009) Void shape evolution and formation of silicon-on-nothing structures during hydrogen annealing of hole arrays on Si (001). J Appl Phys 105:083536 Sugiyama H, Nittono O (1989) Annealing effect on lattice distortion in anodized porous silicon layer. Jap J Appl Phys 28(11):2013–2016 Tayanaka H, Yamauchi K, Matsushita T (1998) Thin-film crystalline silicon solar cells obtained by separation of a porous silicon sacrificial layer. In: Schmid J, Ossenbrink H, Helm P,

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Porous Silicon and Conductive Polymer Nanostructures Via Templating Farid A. Harraz

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Various Classes of Porous Silicon Templates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Filling Porous Silicon Templates with Various Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Selective Dissolution of Porous Silicon Templates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

612 613 614 618 619 620

Abstract

Conductive polymer nanostructures synthesized using porous silicon (PSi) templates are described, with an emphasis on PSi template advantages, pore-filling phenomenon, mechanism of polymerization, and selective removal of PSi to release the polymeric structures. The interaction of pyrrole monomers, as a case study, on the entire surface of PSi under both galvanostatic and potentiostatic deposition modes is presented with discussion on the processing issues associated with the electrochemical deposition process inside the pores. Additionally, various materials infiltrated into PSi templates are briefly described. Examples of free-standing conductive polymer structures formed by selective dissolution of PSi are provided.

F.A. Harraz (*) Nanostructured Materials and Nanotechnology Division, Central Metallurgical Research and Development Institute (CMRDI), Cairo, Egypt Promising Centre for Sensors and Electronic Devices (PCSED), Advanced Materials and Nano–Research Centre, Najran University, Najran, Saudi Arabia e-mail: [email protected] # Springer International Publishing Switzerland 2014 L. Canham (ed.), Handbook of Porous Silicon, DOI 10.1007/978-3-319-05744-6_63

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Introduction Template synthesis is a commonly used tool for the preparation of nanomaterials and has been developed independently in various fields of nanotechnology. Several studies and reviews on various template syntheses are well documented (Hulteen and Martin 1997; Johnson et al. 1999; Whitney et al. 1993; Martin 1996; Moller and Bein 1998; Tian et al. 2003; Raman et al. 1996; Bhushan 2004; Yang et al. 1999). Compared to other sophisticated methods such as molecular beam epitaxy and nanolithography, the template synthesis is an elegant, cost-effective with a high-yield alternative technique (Martin 1994; Hu et al. 1999; Aravamudhan et al. 2007; Chen et al. 2006). Prefabricated nanostructured materials with cylindrical nanopores are often used as shape-defining molds. For the fabrication of one-dimensional polymer nanostructures, three main strategies have been defined including self-assembly (Shimizu 2008), electrospinning (Greiner and Wendorff 2008), and template synthesis. However, significant drawbacks are associated with the former two techniques as the number of materials that could be self-assembled is limited, in addition to the difficulty to achieve diameters below 100 nm with good degree of organization using electrospinning. However, template synthesis is advantageous and has become one of the interesting and active synthetic approach in that field. It acts to direct the growth or organization of nanostructures under conditions that preserve the original shape or symmetry of template. The most common templates are track-etch polymeric membranes (Chakarvarti and Vetter 1998) and porous alumina (Masuda and Fukuda 1995). However, PSi (Lehmann 1993) template has significant advantages over the traditional templates in fabrication of one-dimensional nanostructured arrays with high aspect ratios and narrow diameter distributions. One of the unique properties of PSi is the ability to precisely tune the pore diameter and surface morphology via varying the Si dopant type and level and controlling the applied current and time during the electrochemical etching. The compatibility of PSi with conventional silicon processing technology adds another advantage. A variety of materials including metals, conductive polymers, carbons, oxides, and semiconductors can be deposited within the cylindrical pores of PSi templates by different methods such as electrochemical deposition, sol–gel, and chemical vapor deposition, leading to different morphologies including nanotubes, nanowires or nanorods, nanorings, nanodots and heterogeneous nanostructures, etc. The dimensions of as-formed nanostructures can be easily controlled by regulating the template pores and shapes and the deposition conditions. We focus on conducting polymers; the process flow for fabrication of polymeric nanostructures using PSi templates is depicted in Fig. 1. Since the main objective of this handbook is dedicated to the fabrication, characterization, processing, and applications of PSi, this chapter focuses on utilizing PSi as a template-directing synthesis of polymeric nanostructures. We begin with the preparation of various shapes and morphologies of PSi templates. This is followed by a brief summary and tabulation of the main studies

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Fig. 1 Process flow for fabrication of polymeric nanostructures using porous silicon template

on polymer and other materials incorporation into PSi templates. We then focus on the electrochemical polymerization of pyrrole into PSi templates as a case study and discuss the processing issues associated with its deposition inside the nanoscale pores under both potentiostatic and galvanostatic deposition modes. Selective dissolution of PSi templates to release the deposited polymers is consequently presented. Finally, a summary of the chapter is given.

Various Classes of Porous Silicon Templates With PSi templates, pore diameters from 1 μm are accessible. Typical examples of SEM micrographs of PSi templates, mainly ordered macropores (5 μm), medium-sized pores (120 nm), and mesopores (20 nm), are presented in Fig. 2. The templates were prepared by electrochemical dissolution of Si in HF-based solutions (see handbook chapter “▶ Porous Silicon Formation by Anodization”). For ordered macropores (Kobayashi et al. 2006), p-type Si(100), 10–20 Ωcm, was used. Ordered etch pits were initially created on Si surface by photolithographic pre-patterning technique and subsequent alkaline etching process. The dissolution was conducted, after removal of a SiO2 resist film, in a solution 47 wt% HF/H2O/2-propanol at current density 13 mA cm 2 for 60 min. For medium-sized pores (Ogata et al. 2007), 6 wt% HF + 8 mM KMnO4 + 3,000 ppm NCW-1001(surfactant) was used as anodizing solution with n-type Si(100), 0.0100–0.0180 Ω cm at 25 mA cm 2 for 125 s. For mesopores (Harraz 2006), a p-type Si(100), 0.01-0.02 Ω cm, was anodized in 28 wt% HF/H2O/ethanol solution at 50 mA cm 2 for 30 s.

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Fig. 2 SEM top views (-1) and cross-sectional views (-2) of porous silicon templates with different pore diameters. (a) Ordered macropores: 5 μm. (b) Medium-sized pores: 120 nm. (c) Mesopores: 20 nm (Fukami et al. 2008)

Filling Porous Silicon Templates with Various Materials Porous silicon has been employed as a template for infiltrating various materials. The variety of materials includes but is not limited to metals, oxides, magnetic alloys, and conducing polymers. Table 1 lists the main studies on polymer impregnation into various PSi templates together with the deposition methods and work objectives. The template nanostructures with specific physical properties are expected to demonstrate a broad range of utility in spintronics, magnetic and magneto-optic devices, and biochemical, acoustic, drug delivery, photovoltaic, and sensing applications. Table 2 collects various materials that have been infiltrated into PSi templates. It is worthy to note that a common drawback for most studies listed in Table 2 is related to the observation of pore mouth blockage due to pore plugging effect. In case of polymer deposition and under optimized conditions, such plugging effect could be avoided to a large extent. The use of PSi as a template-directing synthesis of polymeric nanostructures is explored in this section. Conducting polymers are usually synthesized from the appropriate monomers by either chemical or electrochemical oxidative polymerization. Electrochemical polymerization is preferred for better penetration inside the nanopores. Polypyrrole (PPy) is one of the most important and extensively studied conducting polymers (Moreno et al. 1999; Vrkoslav et al. 2006; Lewis et al. 1997; Akundy and Iroh 2001). The deposition of PPy into PSi templates could be achieved by the electrochemical oxidation of pyrrole monomers at constant current or potential in the acetonitrile solution containing tetrabutylammonium perchlorate as supporting electrolyte. Typical potential (E–t) and current transients (i–t) recorded during the deposition of PPy into mesoporous silicon templates are shown in Fig. 3.

Poly( pphenylenevinylene) Polyaniline

Polypyrrole, polyaniline, polythiophene Poly-vinyl-carbazole MEH-PPV

PMMA

Variety of organic and biopolymers PDMS

Mesopores

Mesopores

Mesopores, medium-sized pores, ordered macropores Medium-sized pores Macropores

Macropores

Mesopores

Macropores

Mesopores

Deposited polymer Poly(bithiophene), poly (3-dodecylthiophene) Polypyrrole

PSi template type Mesopores

Solution cast

Wetting in solutions or melts Solution cast

Electrochemical Spin coating

Spin-on coating, immersion Electrochemical

Chemical

Electrochemical

Deposition method Electrochemical

Sensing and drug delivery Gecko for mimetic devices

Improve rectifying performance Pore filling, tuning surface morphology Photovoltaic device Improve photoluminescence Pore filling

Improve electrical contact, gas sensing Light-emitting device

Objectives Pore filling

Table 1 List of the main studies on polymer impregnation into porous silicon templates

Guo et al. (2013)

Li et al. (2003)

Steinhart et al. (2002)

Harraz (2006, 2011), Harraz et al. (2008a, b), Fukami et al. (2008) Nahor et al. (2011) Mishra et al. (2008)

Fan et al. (1998), Halliday et al. (1996)

References Schultze and Jung (1995), Errien et al. (2005) Moreno et al. (1999), Vrkoslav et al. (2006) Nguyen et al. (2003)

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Table 2 List of various materials infiltrated into porous silicon templates PSi template type Macropores

Deposited material LiNbO3

Micropores Macropores

NaNO2 SnSe, Sn, SnO2

Macropores

Ni, Cu,

Mesomacropores Mediumsized pores Mediumsized pores

Au, Ni–Co Co–Pt alloy Fe3O4

Deposition method Impregnation in solution Infiltration Chemical reduction or oxidation Electroless, electrochemical, immersion plating Electrochemical Electrochemical Infiltration

Objectives Silicon photonics

References Zhao et al. (2005a)

Template filling

Murzina et al. (2007) Zhao et al. (2005b)

Pore filling, high aspect ratio metallic structures Pore filling, ferromagnetic Magnetic structures Ferromagnetic characteristic

Zhang et al. (2006), Kobayashi et al. (2006), Harraz et al. (2005) Chourou et al. (2011), Rumpf et al. (2011) Harraz et al. (2013) Harraz (2013)

Fig. 3 Potential transient (galvanostatic deposition: 1 mA) and current transient (potentiostatic deposition: 0.8 V) of PPy in mesoporous silicon templates in 0.1 M pyrrole monomers

The mechanism of polymer infiltration into the pores is of major importance in order to obtain the desired structures and control the final morphology. Different characteristic stages of PPy deposition could be observed. The sharp potential rise through A–B region is related to nucleation of PPy at the pore bottom and partial oxidation of PSi. Constant potential recorded during B–C period is attributed to polymer growth inside the pores (pore filling). At point C, the pores are completely filled with PPy. A potential shift accompanies the stage transition of

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Fig. 4 SEM images of cross-sectional views of porous silicon templates after deposition of polypyrrole into (a) macropores, (b) medium-sized pores, (c) mesopores, and deposition of polyaniline into (d) macropores with inhomogeneous pore filling

C–D. Further polymerization after step D results in a potential leveling. At this stage the polymerization proceeds preferentially on the outer surface of template. One can observe different characteristic stages of potentiostatic deposition (i–t transient) resemble those obtained for (E–t) transient. Two current minima are observed at points B and D, of which the deposition proceeds inside the pores and at the outer surface, respectively. The current decrease during C–D stage in i–t curve is corresponding to a potential increase in E–t curve. It is worthy noted that, the amount of charge consumed for complete pore filling with PPy (qAC) and during the stage transition C–D (qCD) in both galvanostatic and potentiostatic techniques is equivalent each other. The shape of the above transients is essentially dependant on the type of conducting polymer and pore size, depth, and morphology of PSi templates. This means that the time of electrochemical polymerization is different with each template. For the case of macropore filling, the potential shift was not observed after 5 h electropolymerization. After pore filling, the as-formed polymer arrays can be obtained by a selective dissolution and removal of PSi templates in alkaline solution. Cleaved cross-sectional samples shown in Fig. 4 revealed that the porous templates are filled with polymer (bright areas corresponding to polypyrrole deposition). However, image (d) is related to the deposition of polyaniline in random

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macroporous silicon template, of which inhomogeneous pore filling can be observed. This is actually related to the electrochemical polymerization of polyaniline is not optimized with respect to deposition potential or current values and solution chemistry.

Selective Dissolution of Porous Silicon Templates Porous silicon template could be carefully removed via selective chemical etching in tetramethyl ammonium hydroxide (TMAH) solution at 40–90  C. Such dissolution process is crucial and not so easy to control. The etch rate is not the same for different Si templates. It depends on the Si dopant type and level as well as on the pore depth and diameter. The removal of template could be done using KOH solution, but the etching behavior is completely different from the case of TMAH. Ethylene diamine pyrocatechol is also reported as a chemical etch solution at 115  C to successfully remove the silicon (Tondare et al. 2008). Some technical challenges exist during Si template removal, including control of gas evolution during the dissolution, avoiding the corrosion of infiltrated material, and to achieve a high purity of the remaining structures. The released polymeric nanostructures are shown in Fig. 5. Well-ordered, linear micro rods and nanowires of PPy are obtained. The polymers are true replicas of the original pores. Image (f) is related to the released arrays of polyaniline. The formation of nanowires actually is a proof that the deposition of polymer proceeds preferentially at the

Fig. 5 SEM images of polypyrrole nano- and microstructures after selective removal of porous silicon templates. Images (a–e) are related to polypyrrole, image (f) is for polyaniline. (d) is a magnified image of (b). Image (e) corresponds to partially filled pores. Templates used are (a) ordered macropores and (c) mesopores while (b, d, e, f) medium-sized pores

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pore bottom and grows toward the outer surface. The different pore sizes of templates do not appear to hinder the polymerization process. Furthermore, no pore blockage with polymer was observed by applying such electrochemical techniques. Some of the nanowires are oriented in different directions; this observation was not a result of imperfections in either the PSi template or polymerization behavior as the original pores were almost aligned in parallel (Fig. 2). Instead, this “bundling” is most likely an artifact of mechanical forces on the nanowires incurred during the dissolution and drying processes (see handbook chapter “▶ Drying Techniques Applied to Porous Silicon”). It appears now that surface morphology of polymeric micro- and nanostructures could be tuned via the tuning of PSi templates, which is considered a unique property of PSi compared to other porous templates. Additionally, due to the semiconducting behavior of PSi layer as well as the silicon pore walls, one expects that the polymerization proceeds homogeneously from the pore bottom or preferentially at the pore walls, depending on the dopant type of the starting silicon substrate and the mode of electropolymerization process. This facilitates the preparation of different shapes, like wires, rods, or tubes. However, this is not possible in case of porous alumina, for instance. Furthermore, due to the insulating character of porous alumina, one should create an electrical contact on the backside before inserting the template in the electrolytic solution. However, this step is not needed in case of PSi, particularly the case of porous layers prepared using heavily doped substrates. Finally, such hybrid nanostructures of PSi and conducting polymers could be beneficial to current and future technologies based on conducting polymers, including photovoltaic devices and sensing applications.

Conclusions To summarize this chapter, a variety of porous silicon layers (mesoporous, medium-sized pores and macroporous regimes) could offer a versatile approach as a template-directing synthesis of polymeric micro- and nanostructures. The pores are filled homogeneously with polymer using the electrochemical technique. Selective dissolution of the silicon template is possible using an alkaline solution, thereby releasing well-ordered polymeric nanostructures having the same architectures of that of host PSi templates. The current template synthesis technique is advantageous compared to other conventional porous templates, especially the frequently used porous alumina template, since the surface morphology of deposited material could be carefully tuned. Such combined approach (porous silicon template electrochemistry) is expected to demonstrate a broad range of utility for fabricating various micro- and nanostructured materials. Finally, one further expected benefit of PSi template synthesis which is challenging with other approaches is the ability to infiltrate dissimilar materials into a single nanostructure. This actually would allow for coupling between materials that have, for instance, complementary optical and magnetic properties that can interact in a novel way.

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References Akundy GS, Iroh JO (2001) Polypyrrole coatings on aluminum: synthesis and characterization. Polymer 42:9665–9669 Aravamudhan S, Luongo K, Poddar P, Srikanth H, Bhansali S (2007) Porous silicon templates for electrodeposition of nanostructures. Appl Phys A 87:773–780 Bhushan B (2004) Springer handbook of nanotechnology. Springer, Berlin/Heidelberg Chakarvarti SK, Vetter J (1998) Template synthesis-a membrane based technology for generation of nano-micro materials: a review. Radiat Meas 29:149–159 Chen X, Steinhart M, Go¨sele U (2006) Ordered arrays of mesoporous microrods from recyclable macroporous silicon templates. Adv Mater 18:2135–2156 Chourou ML, Fukami K, Sakka T, Ogata YH (2011) Gold electrodeposition into porous silicon: comparison between meso- and macroporous silicon. Phys Stat Sol (c) 8(6): 1783–1786 Errien N, Froyer G, Louarn P (2005) Electrochemical growth of poly(3-dodecylthiophene) into porous silicon layers. Retho Synth Metals 150:255–258 Fan J, Wan M, Zhu D (1998) Studies on the rectifying effect of the heterojunction between porous silicon and water-soluble copolymer of polyaniline. Synth Metals 95:119–124 Fukami K, Harraz FA, Yamauchi T, Sakka T, Ogata YH (2008) Fine-tuning in size and surface morphology of rod-shaped polypyrrole using porous silicon as template. Electrochem Commun 10:56–60 Greiner A, Wendorff J (2008) Functional self-assembled nanofibers by electrospinning. In: Shimizu T (eds) Self-assembled nanomaterials I. Springer-Verlag Berlin Heidelberg, pp 107–171 Guo D-J, Zhang H, Li J-B, Fang S-M, Dai Z-D, Tan W (2013) Fabrication and adhesion of a bio-inspired microarray: capillarity-induced casting using porous silicon mold. J Mater Chem B 1:379–386 Halliday DP, Holland ER, Eggleston JM, Adams PN, Cox SE, Monkman AP (1996) Electroluminescence from porous silicon using a conducting polyaniline contact. Thin Solid Films 276:299–302 Harraz FA (2006) Electrochemical polymerization of pyrrole into nanostructured p-type porous silicon. J Electrochem Soc 153(5):C349–C356 Harraz FA (2011) Impregnation of porous silicon with conducting polymers. Phys Stat Sol (C) 8(6):1883–1887 Harraz FA (2013) Synthesis and surface properties of magnetite (Fe3O4) nanoparticles infiltrated into porous silicon template. Appl Sur Sci 287:203–210 Harraz FA, Kamada K, Sasano J, Izuo S, Sakka T, Ogata YH (2005) Pore filling of macropores prepared in p-type silicon by copper deposition. Phys Stat Sol (a) 202(8):1683–1687 Harraz FA, Salim MS, Sakka T, Ogata YH (2008a) Hybrid nanostructure of polypyrrole and porous silicon prepared by galvanostatic technique. Electrochim Acta 53:3734–3740 Harraz FA, El-Sheikh SM, Sakka T, Ogata YH (2008b) Cylindrical pore arrays in silicon with intermediate nano-sizes: a template for nanofabrication and multilayer applications. Electrochim Acta 53:6444–6451 Harraz FA, Salem AM, Mohamed BA, Kandil A, Ibrahim IA (2013) Electrochemically deposited cobalt/platinum (Co/Pt) film into porous silicon: structural investigation and magnetic properties. Appl Surf Sci 264:391–398 Hu J, Odom TW, Lieber CM (1999) Chemistry and physics in one dimension: synthesis and properties of nanowires and nanotubes. Acc Chem Res 32:435–445 Hulteen JC, Martin CR (1997) A general template-based method for the preparation of nanomaterials. J Mater Chem 7:1075–1087 Johnson SA, Ollivier PJ, Mallouk TE (1999) Ordered mesoporous polymers of tunable pore size from colloidal silica templates. Science 283:963–965

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Kobayashi K, Harraz FA, Izuo S, Sakka T, Ogata YH (2006) Microrod and microtube formation by electrodeposition of metal into ordered macropores prepared in p-type silicon. J Electrochem Soc 153(4):C218–C222 Lehmann V (1993) The physics of macropore formation in low-doped n-type silicon. J Electrochem Soc 140:2836–2843 Lewis TW, Moulton SE, Spinks GM, Wallace GG (1997) Optimization of a polypyrrole based actuator. Synth Met 85:1419–1420 Li YY, Cunin F, Link JR, Gao T, Betts RE, Reiver SH, Chin V, Bhatia SN, Sailor MJ (2003) Polymer replicas of photonic porous silicon for sensing and drug delivery applications. Science 299(5615):2045–2047 Martin CR (1994) Nanomaterials: a membrane-based synthetic approach. Science 266:1961–1966 Martin CR (1996) Membrane-based synthesis of nanomaterials. Chem Mater 8(8):1739–1746 Masuda H, Fukuda K (1995) Ordered metal nanohole arrays made by a two-step replication of honeycomb structures of anodic alumina. Science 268:1466–1468 Mishra JK, Bhunia S, Banerjee S, Banerji P (2008) Photoluminescence studies on porous silicon/ polymer heterostructure. J Lumin 128:1169–1174 Moller K, Bein T (1998) Inclusion chemistry in periodic mesoporous hosts. Chem Mater 10(10):2950–2963 Moreno JD, Marcos ML, Agullo´-Rueda F, Guerrero-Lemus R, Martı´n-Palma RJ, Martı´nez-Duart JM, González-Velasco J (1999) A galvanostatic study of the electrodeposition of polypyrrole into porous silicon. Thin Solid Films 348:152–156 Murzina TV, Sychev FY, Kolmychek IA, Aktsipetrov OA (2007) Tunable ferroelectric photonic crystals based on porous silicon templates infiltrated by sodium nitrite. Appl Phys Lett 90:161120 Nahor A, Berger O, Bardavid Y, Toker G, Tamar Y, Reiss L, Asscher M, Yitzchaik S, Sa’ar A (2011) Hybrid structures of porous silicon and conjugated polymers for photovoltaic applications. Phys Stat Sol (C) 8(6):1908–1912 Nguyen TP, Le Rendu P, Lake´hal M, de Kok M, Vanderzande D, Bulou A, Bardeau JP, Joubert P (2003) Filling porous silicon pores with poly( p-phenylenevinylene). Phys Stat Sol (a) 197:232–235 Ogata YH, Koyama A, Harraz FA, Salem MS, Sakka T (2007) Electrochemical formation of porous silicon with medium sized-pores. Electrochemistry 75:270–272 Raman NK, Anderson MT, Brinker CJ (1996) Template-based approaches to the preparation of amorphous, nanoporous silicas. Chem Mater 8(8):1682–1701 Rumpf K, Granitzer P, Poelt P, Allbu M (2011) Double-sided porous silicon template for metal deposition. Phys Stat Sol (c) 8(6):1808–1811 Schultze JW, Jung KG (1995) Regular nanostructured systems formed electrochemically: deposition of electroactive polybithiophene into porous silicon. Electrochim Acta 40:1369–1383 Shimizu T (2008) Self-assembled nanomaterials I. Nanofibers, vol 219, Advances in polymer science. Springer, Berlin Steinhart M, Wendorff JH, Greiner A, Wehrspohn RB, Nielsch K, Schilling J, Choi J, Go¨sele U (2002) Polymer nanotubes by wetting of ordered porous templates. Science 296:1997–1997 Tian ML, Wang JU, Kurtz J, Mallouk TE, Chan MHW (2003) Electrochemical growth of singlecrystal metal nanowires via a two-dimensional nucleation and growth mechanism. Nano Lett 3:919–923 Tondare VN, Gierhart BC, Howitt DG, Smith RL, Chen SJ, Collins SD (2008) An electron microscopy investigation of the structure of porous silicon by oxide replication. Nanotechnology 19:225301, 4 pp Vrkoslav V, Jelı´nek I, Broncová G, Král V, Dian J (2006) Polypyrrole-functionalized porous silicon for gas sensing applications. Mater Sci Eng C 26:1072–1076 Whitney TM, Jiang JS, Searson PC, Chien CL (1993) Fabrication and magnetic properties of arrays of metallic nanowires. Science 261:1316–1319

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Yang P, Zhao D, Margolese DI, Chmelka BF, Stucky GD (1999) Block copolymer templating syntheses of mesoporous metal oxides with large ordering lengths and semicrystalline framework. Chem Mater 11(10):2813–2826 Zhang X, Tu KN, Xie YH, Tung CH (2006) High aspect ratio nickel structures fabricated by electrochemical replication of hydrofluoric acid etched silicon. Electrochem Solid State Lett 9(9):C150–C152 Zhao L, Steinhart M, Yosef M, Lee SK, Geppert T, Pippel E, Scholz R, Go¨sele U, Schlecht S (2005a) Lithium niobate microtubes within ordered macroporous silicon by template thermolysis of a single source precursor. Chem Mater 17:3–5 Zhao L, Yosef M, Steinhart M, Go¨ring P, Hofmeister H, Go¨sele U, Schlecht S (2005b) Porous silicon and alumina as chemically reactive templates for the synthesis of tubes and wires of SnSe, Sn, and SnO2. Angew Chem Int Ed 45:311–315

Melt Intrusion in Porous Silicon Armando Loni

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Calculation of Payload . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Loading of Freestanding Flakes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Loading of Powders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Loading porous silicon by bringing it into contact with a molten substance facilitates complete pore filling through capillary action, or melt intrusion. If the intrusion is carried out at elevated temperature, the molten active can return to a solid form at room temperature, remaining in the pores. Reviewed herein are the methodologies used for loading freestanding porous silicon flakes and powders.

Introduction Many applications of porous silicon make use of the available void fraction, whereby the pores are filled with some substance of interest, for example, a beneficial active compound (see, e.g., the handbook chapters “▶ Drug Delivery with Porous Silicon” and “▶ Porous Silicon and Functional Foods”). The loading methodology depends, to some extent, on the properties of the active material to be loaded. If the active can be dissolved in a suitable solvent, and assuming the solvent does not interact with the porous silicon, then high payloads can be achieved by A. Loni (*) pSiMedica Ltd, Malvern, Worcestershire, UK e-mail: [email protected] # Springer International Publishing Switzerland 2014 L. Canham (ed.), Handbook of Porous Silicon, DOI 10.1007/978-3-319-05744-6_64

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Table 1 Actives loaded into porous silicon by melt intrusion (author’s data, except where indicated)

D-Panthenol

Function Humectant/moisturizer

Triclosan Ciprofloxacin (Wang et al. 2012) Ethyl butyrate Camellia Peppermint Erythritol

Antibacterial Antibiotic Flavoring/fragrance Fragrance Flavoring Sweetener

Lauric acid (Saffie and Canham L Composite material comprising a porous semiconductor impregnated with an organic substance. US Patent US 8, 088, 401 B2, 3 Jan 2012) Vitamins E and K (Saffie and Canham L Composite material comprising a porous semiconductor impregnated with an organic substance. US Patent US 8, 088, 401 B2, 3 Jan 2012)

Cosmetics additive

Thermal properties relevant to loading Viscous oil; thins at 75
C; useable to 135
C Melts at 55
C Molten at 250
C Thin oil at room temperature Thin oil at room temperature Thin oil at room temperature Melts at 121
C; useable to 250
C Melts at 44
C

Supplement

Viscous oils; thin at 70
C

Active

sequential dosing and drying. For an active with intrinsically low solubility, and that possesses good thermal stability up to its melting point, an alternative loading technique is melt intrusion. Used originally for impregnating porous glass with metal oxide (Bartholomew and Garfinkel 1969) and semiconductor (Huber and Huber 1988) materials and, more recently, to produce metal-porous silica nanocomposite structures (Charnaya et al. 2011) and polymer composites from porous silicon (Steinhart et al. 2002; Li et al. 2003), the melt intrusion technique facilitates, in principle, complete pore filling in a single dose. Melt intrusion can be used in situations where the active to be loaded is either in a concentrated liquidus phase at room temperature or can be rendered molten from a solid at elevated temperature. Some actives can be too viscous at room temperature although the application of heat (while avoiding thermal decomposition) can facilitate pore intrusion. Examples of active compounds that have been loaded into mesoporous silicon by melt intrusion are listed in Table 1.

Calculation of Payload The payload, as determined experimentally, is typically calculated in weight percent, following the formula:

Melt Intrusion in Porous Silicon


 Weight% ¼ WA = WA þ WpSi  100

625

(1)

where WA and WpSi refer to the weights of active (in the pores) and porous silicon, respectively. The maximum theoretical payload corresponding to completely filled pores can be calculated if the porosity of the porous silicon (ρpSi) and the density of the active (ρA) are known; in terms of density (ρ), Eq. 1 can be rewritten to give a maximum predicted payload: h  i Weight% ¼ ρA = ρA þ ρpSi ð100  P =P  100, where P is the porosity (% void) and ρpSi ¼ 2.33 g/cm3. Porosity and pore volume are, of course, related. Pore volume is usually defined in terms of unitary weight of material (ml/g, cm3/g); for porous silicon, therefore, multiplying the measured pore volume by the density of the active gives an upper limit for the weight of active that will completely fill the pores, and the theoretical payload can be written as Weight% ¼ VA ρA =ðVA ρA þ 1Þ  100, where VA is the available pore volume (in ml/g) that can be completely filled with active.

Loading of Freestanding Flakes If a molten active is slowly added to the surface of a porous silicon film attached to its parent wafer, it intrudes into the pore structure at a rate that depends primarily on both the viscosity of the melt and the pore morphology. If the porous silicon is in the form of detached, freestanding flakes, these can be immersed completely in the melt; with two surfaces in contact with the melt, the loading is quicker, the uniformity in both cross section and depth can be better, and payloads can be higher than those achievable with wafers, especially so for very thick layers. As melt intrusion proceeds, air is displaced from the porous silicon flake and air bubbles may be evident. Depending on viscosity and the layer thickness, the pores can be completely filled within a couple of minutes. On removal from the melt, any excess on the surface of the flake can be wiped off with filter paper or a cotton swab – slightly wetting these items, first, with a solvent that is known to dissolve the active, minimizes any residual surface excess; this is particularly useful when measuring payload, since it is the active that is in the pores, alone, that contributes to the calculation. Flakes are therefore useful as model structures, since information such as how payloads vary with porosity can readily be obtained, Fig. 1.

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Fig. 1 Payload dependency on porosity for Triclosan, loaded from melt (Author’s data; mesoporous silicon prepared from 5 to 20 mΩcm p-type wafers by anodization)

Loading of Powders While knowledge of the available pore volume is useful to know, in practice it is not strictly necessary. If the porous silicon is in powder form, the active can be slowly added to the porous silicon and mixed – the mixing action homogenizes the blend and allows empty particles to make contact with residual molten active that has not yet been adsorbed; with subsequent addition and mixing, all of the porous particles eventually become filled by capillary action. Alternatively, the porous silicon powder can be added to the molten active – a slurry is formed, initially, which gradually thickens and “dries” as more powder is added. The end of either process is defined as the “wet point,” where the powder is on the verge of being freeflowing. The wet point is an inherently variable state that depends on individual judgment. Without prior knowledge of the maximum permissible weight of active, and depending on the skill of the formulator, the loading will usually be taken slightly beyond the wet point, leaving residual surface active; in this situation, additional porous silicon powder can be added to effectively adsorb the residue – the downside then is that there will be a small fraction of material that is not fully loaded, although this will be homogenized within the overall blend. Another method for loading powders is to “blend and melt.” If the active compound is in a solid powder form, this can be blended with the porous silicon powder – the composite blend is then heated to beyond the melting point of the active, where it starts to be adsorbed within the porous silicon. Careful manipulation of the ratios (together with uniform blending) can facilitate loading to just before the wet point, such that residual active on the surface of the porous silicon can be minimized. The “blend and melt” technique has been applied to the loading of Ciprofloxacin (Wang et al. 2012), Triclosan (Wang et al. 2010), and Ketoconazole (Tang et al. 2013).

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Discussion As with solution loading, the surface passivation of the porous silicon is important when loading by melt intrusion – if the active is hydrophilic, for example, then the payload is likely to be small for hydride-passivated porous silicon. Some actives can, during loading, chemically oxidize or etch the porous silicon at elevated temperature, while some actives can degrade when in contact with the internally passivated silicon hydride surface. For the majority of applications, the highly reactive hydride surface will be modified before loading, more often than not by thermal oxidation (with, of course, subsequent reductions in pore volume and payload). Payloads achievable through melt intrusion are dependent on the viscosity of the active as well as loading time. The loading time will also be dependent on pore morphology and either layer thickness or particle sizes. Experimental values for payload tend to be lower than theoretical estimates, Fig. 1; this is likely to be related to both the pore morphology – whereby it is more difficult for intrusion to occur within smaller interconnecting pores or branches – and pore depth, if pores become “blocked” before being filled to the pore tip, for example, (Salonen et al. 2005); for very high-porosity material, there may also be some degree of pore collapse on air-drying, and this would also contribute to different experimental payloads. If the active reverts to a solid while confined in the pores (e.g., after cooling), its physical properties are likely to be determined by the nanostructure – the freezing and melting points of pore-confined solids, for example, have been shown to be dependent on pore size and geometry (Charnaya et al. 2011; Jackson and McKenna 1990), and, depending on pore size, the active may remain amorphous or recrystallize. Residual active on the surface of loaded powders is difficult to avoid if the pores are to be completely filled and will depend on the particle size distribution – smaller particles will become fully loaded before larger ones, for example. Residual surface active will not be in a nanostructured form and, if solid at room temperature, will cool to a crystalline state rather than remain amorphous (as it could be in small mesopores). This dual physical nature of the active can be characterized (see “▶ X-ray Diffraction in Porous Silicon”) and should be taken into consideration for any proposed application.

References Bartholomew RF, Garfinkel HM (1969) Preparation of thick crystalline films of tin oxide and porous glass partially filled with tin oxide. J Electrochem Soc 116(9):1205–1208 Charnaya EV, Tien C, Lee MK, Xing DY, Borisov BF, Kumzerov YA Properties of metal-porous silica matrix nanocomposites: crystallization and melting, structure, atomic mobility in melts, superconductivity. In: First international conference on composites and nanocomposites, Kottayam, India 7-9 Jan 2011 Huber CA, Huber TE (1988) A novel microstructure: semiconductor-impregnated porous vycor glass. J Appl Phys 64(11):6588–6590

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Jackson CL, McKenna GB (1990) The melting behaviour of organic materials confined in porous solids. J Chem Phys 93(12):9002–9011 Li YY, Cunin F, Link JR, Gao T, Betts RE, Reiver SH, Chin V, Bhatia SN, Sailor MJ (2003) Polymer replicas of photonic porous silicon for sensing and drug delivery applications. Science 299:2045–2047 Saffie R, Canham L Composite material comprising a porous semiconductor impregnated with an organic substance. US Patent US 8,088,401 B2, 3 Jan 2012 Salonen J, Paski J, Vaha-Heikkila K, Heikkila T, Bjorkqvist M, Lehto VP (2005) Determination of drug load in porous silicon microparticles by calorimetry. Phys Stat Sol (a) 202(8):1629–1633 Steinhart M, Wendorff JH, Greiner A, Wehrspohn RB, Nielsch K, Schilling J, Choi J, Gosele U (2002) Polymer nanotubes by wetting of ordered porous templates. Science 296:1997 Tang L, Saharay A, Fleischer W, Hartman PS, Loni A, Canham LT, Coffer JL (2013) Sustained antifungal activity from a ketoconazole-loaded nanostructured mesoporous silicon platform. Silicon. doi:10.1007/s12633-013-9143-5 Wang M, Coffer JL, Doraj K, Hartman PS, Loni A, Canham LT (2010) Sustained antibacterial activity from triclosan-loaded nanostructured mesoporous silicon. Mol Pharm 7(6):2232–2239 Wang M, Coffer JL, Hartman PS, Loni A, Canham LT Delivery and activity of low solubility antibiotics loaded into porous silicon. In: Abstract P2-76, 8th international conference on porous semiconductors science & technology, 25–30 March 2012, pp 452–453

Porous Silicon and Electrochemical Deposition Yukio H. Ogata and Kazuhiro Fukami

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrodeposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Displacement Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Micro- and Nanostructure Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

630 630 631 632 632 636

Abstract

Porous silicon is a promising template for the preparation of metal nanostructures by electrochemical deposition. Because porous silicon is a semiconductive porous electrode, electrochemical deposition of metals occurs not only at the bottom of pores but also on the pore wall and pore openings. Thus, the control of electrochemical deposition within porous silicon has been a challenging issue. Electrochemical deposition on porous silicon is influenced by illumination conditions. Metal deposition on porous silicon is possible by displacement deposition. Many studies have reported on electrochemical deposition of metal for practical applications. In this chapter, electrodeposition under polarization is firstly reviewed. Secondly, displacement deposition on porous silicon is explained. Finally, the microscopic structure formation by electrodeposition on porous silicon is summarized.

Y.H. Ogata (*) Institute of Advanced Energy, Kyoto University, Uji, Kyoto, Japan e-mail: [email protected] K. Fukami Department of Materials Science and Engineering, Kyoto University, Kyoto, Japan e-mail: [email protected] # Springer International Publishing Switzerland 2014 L. Canham (ed.), Handbook of Porous Silicon, DOI 10.1007/978-3-319-05744-6_65

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a

Y.H. Ogata and K. Fukami

b

c

Fig. 1 Schematic illustrations of typical morphologies obtained in electrodeposition on porous silicon. Illustrations of (a), (b), and (c), respectively, show pores capped with a metal film, pores modified with discrete metal islands, and pores completely filled with metal

Introduction Metal deposition on porous silicon has been studied since the early period of porous silicon research. The major target started from the improvement of the luminescent properties (Canham 1997), the formation of an effective heterojunction between semiconductor and metal for applying the junction to various types of electronic devices at the time. Afterwards, the target spreads to versatile fields such as optical diagnostics/plasmonics, fabrication of metal–silicon hybrid materials for catalyst, microstructure formation of a metal–silicon hybrid material, and low-dimensional structure formation. These applications utilize the characteristic structure of porous silicon: (a) fine porous structure with high aspect ratio; (b) easily oxidizable nature; and (c) neither conductive nor nonconductive, but semiconductive material. Basic understanding on the deposition behavior on a porous silicon surface is indispensable for meeting a variety of requirements depending upon the application (Hyde and Compton 2003; Oskam et al. 1998; Zhang 2001). Since application of a wet process for copper wiring was demonstrated by the IBM group in 1997 (Andrecacos et al. 1998), applicability of the electrochemical deposition to fine structure formation has been widely recognized. Electrochemical deposition enables to prepare various types of porous silicon modified with metal as illustrated in Fig. 1. We confine the topic in this section to the wet processes here. The processes consist of electrodeposition and displacement or electroless deposition.

Electrodeposition Silicon is a semiconductor and it causes specific characteristics which are not expected in the deposition on a conductive substrate. Metal deposition or reduction reaction requires the supply of electrons at the conductive band or the hole injection to the valence band.

Porous Silicon and Electrochemical Deposition

Menþ þ ne ! Me

or

631

Menþ ! Me þ nhþ

(1)

where Me stands for metal. The hole-injection process is only possible when the redox potential or the Fermi level of metal/metal-ion system stays below the energy level of the valence band. It happens in some noble metal systems. Otherwise electrons at the conduction band are required for the progress of metal deposition reaction. n-type silicon meets this requirement, whereas p-type silicon lacks this condition. Illumination helps the progress of deposition on p-type porous silicon. Deposition becomes possible even on p-type silicon when breakdown occurs under very high polarization. Heavily doped silicon or degenerate silicon, n+ or p+ silicon, behaves like conductor, where illumination is not the requirement even it is p-type silicon. It must be noted that cathodic reaction proceeds on a semiconductor surface following its kinetics at the beginning of deposition, but the surface covered with deposited metal behaves as a metal electrode. The two kinetics are usually very different from each other.

Displacement Deposition Silicon is a less-noble material. The redox potential (2) is difficult to measure directly in aqueous solution since silicon is easy to be oxidized and cannot stay as silicon itself. The value estimated from thermodynamic data (Bard et al. 1985) is as follows: Si þ 2H2 O ! SiO2 þ 4Hþ þ 4e

Eo ¼ 0:85 V versus SHE

(2)

Porous silicon readily undergoes oxidation in aqueous solution. Electrons formed by the oxidation reduce metal ions thermodynamically, if the equilibrium potential stays positive compared with the potential of reaction (2). When porous silicon is immersed in an aqueous solution containing noble metal ions such as silver and copper, the metal is deposited on porous silicon. Oxidation of the porous silicon surface is confirmed by IR spectroscopy (Hilliard et al. 1994) and XPS spectroscopy (Jeske et al. 1995). In contrast, displacement reaction does not take place in deposition of less-noble metals such as nickel and iron (Tsuboi et al. 1998; Ronkel et al. 1996). Deposition behavior does not show a significant difference between n-type and p-type silicon substrates (Tsuboi et al. 1999). The potential is determined by a mixed potential of the cathodic and anodic reactions. The rate of reaction (2) slows down in progression since the reaction leaves insulative SiO2 resulting in the decrease of active silicon surface. It leads to the decrease in metal deposition rate. In some cases, holes participate in place of electrons. The deposition and oxidation do not necessarily take place at the same site. Produced charge carriers can migrate in silicon or metal, and they can be consumed at the different sites from the electron source

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(Harraz et al. 2002). The deposition ceases after the silicon surface exposing to electrolyte is completely oxidized. There are competitive reactions to metal deposition. The most influential reaction is hydrogen evolution reaction. The reaction competes with metal deposition reaction. As a result, noble metal is deposited, but less-noble metal is difficult to be deposited. The potential of hydrogen evolution reaction shifts toward negative with increasing solution pH. One can deposit less-noble metals in solution of high pH (Takano et al. 2000; Harraz et al. 2003). Solution containing fluorides is also used for the displacement solution. Presence of fluorides reduces it to a binary system eliminating the oxides (Harraz et al. 2003). The oxidized silicon dissolves or silicon directly dissolves away into the solution.

Micro- and Nanostructure Formation Metal deposition creates a hybrid material of metal and semiconductor and the new material is expected to develop a new function, where microstructuring is crucial. A variety of techniques have been utilized for producing the 2D and 3D structures. They are controlled physically, mechanically, optically, and electrochemically. Some examples of the 2D or position-selective local deposition are summarized in Table 1. The optical control is only possible on p-type silicon and deposition of less-noble metal. Illumination creates charge carriers and the illuminated spot undergoes deposition. Otherwise, excess free electrons and displacement deposition hinder the selective deposition. The advantage or uniqueness arises from the micro- and nanostructures. It is often preferred that deposition copies precisely the porous silicon structure. The hybrid structure or the structure after removing the porous silicon substrate can be utilized in a wide variety of applications. Metal deposition in these structures and the successive silicon dissolution have been utilized to form the 3D structures. Many interesting results have been obtained (Table 2).

Conclusions Electrochemical deposition enables wide applications of porous silicon in many fields such as optics, sensing, microfabrication, and catalysis. Fine tuning in morphology of deposits, which is crucial for applications, has been desired. As reviewed in this chapter, control of metal electrodeposition has greatly improved in the recent decades. However, there still exist many open questions, such as nucleation and growth and mass transfer for 3D structure formation. They are doubtlessly important and seem to be future issues.

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Table 1 2D structure formation of deposits on porous silicon Type Optical

Physical

Deposit Ni, Cu

Technique DP, 532 nm laser irradiation

Ni, Cu

ED, Ar+ laser, scanning

Au

Laser heating with its interference pattern, alloying

Au, Cu

ED, Si2+ ion implantation

Ag, Cu

DP, porous Al2O3 or polystyrene colloidal crystal as mask DP, inkjet printing

Ag

Mechanical

Pd

Au, Ag Co Electrochemical

ED, AFM-scratching through the SiO2 layer DP, nanoindentation

Cu

ED, electrochemical machining with ultrashort voltage pulses

Cu

ED, electrochemical scanning capillary microscope

DP displacement deposition, ED electrodeposition

Remarks Smallest diam. 5 μm with P ion implantation Metal patterning without mask or resist. Suppression of DP is necessary for Cu. 20 μm Laser heating after uniform Au deposition. Etch pattern formation: pit, ring, line High deposition rate at the defected sites. Patterning Natural lithography, 50–100 nm size, starting from flat Si Direct patterning on porous silicon. SERS applications Defects, sub-100 nm width Defects, 30–50 nm resolution Roughened surface accept the nucleation, 500 nm size. Starting from flat Si Pure electrochemical, 200 nm size, using a flat Si

References Kordas et al. (2001), Scheck et al. (2004) Sasano et al. (2003; 2004)

Koynov et al. (2006)

Schmuki and Erickson (2000)

Ono et al. (2007)

Chiolerio et al. (2012)

Santinacci et al. (2003)

Kubo et al. (2005)

Trimmer et al. (2005)

Staemmler et al. (2004)

Mesoporous (normal: 5–50 nm in diameter)

Porous silicon Mesoporous (fine: 3–5 nm in diameter)

DP ED ED ED ED

ED

Fe Fe Ni-Fe/Au

Co-Fe

Ni, Co, NiCo, Fe

ED, DP

Pt

Cu

DP

Au

DP

DP

Pt

Ag, Au

Technique DP

Deposit Pt

Table 2 3D structure formation and deposition control on porous silicon

p-Si, catalytic activity of Pt nanoparticles on CO oxidation, methanol as solvent for the deposition bath p-Si, micro and meso, plasmon resonance studies, ethanol as solvent for the deposition bath p-Si, chemical modifications of the pore wall, effect of the affinity of pore wall with water p-Si, dendrite growth, Si nanowire formation p-Si, n-Si. Deposition behavior at the pore opening p-Si. Chronoamperometry n-Si. Characterization by XPS p-Si, n-Si. 40 nm and 290 nm in diameter XANES investigation. Coverage of the top surface by Fe, but penetration of Co in pores n-Si, shape varied between spheres, ellipsoids, and wires, and their magnetic properties

Remarks n-Si, micro and meso, catalyst for fuel cells

(continued)

Rumpf et al. (2010), Granitzer et al. (2010)

Bandarenka et al. (2012; 2013) Renaux et al. (2000) Ronkel et al. (1996) Aravamudhan et al. (2007) Kashkarov et al. (2009)

Peng and Zhu (2004)

Fukami et al. (2012)

Polisski et al. (2011)

References Brito-Neto et al. (2006), Hayase et al. (2004) Polisski et al. (2008)

634 Y.H. Ogata and K. Fukami

ED

Cu

Au

Electroless deposition

ED

Cu

Cu, Ni-B

ED

Cu, Ni

DP, metal-catalyzed CVD

ED, single solution for porosification and deposition ED, dark for Cu, illumination for Ni

Ni

Au

Technique DP, pore widening of with oxidation and dissolution in HF

Deposit Ni

DP displacement deposition, ED electrodeposition

Others

Porous silicon Macroporous

Table 2 (continued)

p-Si, application of Cu deposited Si nanowires to Li-ion batteries n-Si, medium-sized pores (~100 nm in diameter), surface plasmon resonance study p-Si (111), Si nanowire arrays, vapor–liquid–solid growth p-Si, mesopores formed by metalassisted etching, adhesive metal films n-Si, macropores formed by metalassisted etching

p-Si, n-Si, Cu micro-rods, Ni microtubes

Remarks p-Si, complete replication of pores. Freestanding arrays of high-aspect-ratio Ni pillars n-Si, microneedles with 10 μm diam

Lee et al. (2009)

Yae et al. (2011)

Yasseri et al. (2006)

Mun˜oz-Noval et al. (2013)

Kobayashi et al. (2006), Fang et al. (2007) Foell et al. (2010)

Sato et al. (2005)

References Zhang et al. (2006)

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References Andrecacos PC, Uzoh C, Dukovic JO et al (1998) Damascene copper electroplating for chip interconnections. IBM J Res Dev 42:567–574 Aravamudhan S, Luongo K, Poddar P et al (2007) Porous silicon templates for electrodeposition of nanostructures. Appl Phys A 87:773–780 Bandarenka H, Redko S, Smirnov A et al (2012) Nanostructures formed by displacement of porous silicon with copper: from nanoparticles to porous membranes. Nanoscale Res Lett 7:1–10 Bandarenka H, Prischepa SL, Fittipaldi R et al (2013) Comparative study of initial stages of copper immersion deposition on bulk and porous silicon. Nanoscale Res Lett 8:85 Bard AJ, Parsons R, Jordan J (1985) Standard potentials in aqueous solution. Marcel Dekker, New York Brito-Neto JGA, Araki S, Hayase M (2006) Synthesis and characterization of porous platinum layers deposited on highly doped n-type porous silicon by immersion plating. J Electrochem Soc 153:C741–C746 Canham L (1997) Properties of porous silicon. INSPEC, Malvern Chiolerio A, Virga A, Pandolfi P et al (2012) Direct patterning of silver particles on porous silicon by inkjet printing of a silver salt via in-situ reduction. Nanoscale Res Lett 7:1–7 Fang C, Foca E, Xu S et al (2007) Deep silicon macropores filled with copper by electrodeposition. J Electrochem Soc 154:D45–D49 Foell H, Hartz H, Ossei-Wusu E et al (2010) Si nanowire arrays as anodes in Li ion batteries. Phys Status Solidi RRL 4:4–6 Fukami K, Koda R, Sakka T et al (2012) Platinum electrodeposition in porous silicon: the influence of surface solvation effects on a chemical reaction in a nanospace. Chem Phys Lett 542:99–105 Granitzer P, Rumpf K, Venkatesan M et al (2010) Magnetic study of Fe3O4 nanoparticles incorporated within mesoporous silicon. J Electrochem Soc 157:K145–K151 Harraz FA, Tsuboi T, Sasano J et al (2002) Metal deposition onto a porous silicon layer by immersion plating from aqueous and nonaqueous solutions. J Electrochem Soc 149:C456–C463 Harraz FA, Sasano J, Sakka T et al (2003) Different behavior in immersion plating of nickel on porous silicon from acidic and alkaline fluoride media. J Electrochem Soc 150:C277–C284 Hayase M, Kawase T, Hatsuzawa T (2004) Miniature 250 micrometer thick fuel cell with monolithically fabricated silicon electrodes. Electrochem Solid-State Lett 7:A231–A234 Hilliard J, Andsager D, Abuhassan L et al (1994) Infrared-spectroscopy and secondary-ion massspectrometry of luminescent, nonluminescent, and metal quenched porous silicon. J Appl Phys 76:2423–2428 Hyde M, Compton R (2003) A review of the analysis of multiple nucleation with diffusion controlled growth. J Electroanal Chem 549:1–12 Jeske M, Schultze JW, Thonissen M et al (1995) Electrodeposition of metals into porous silicon. Thin Solid Films 255:63–66 Kashkarov VM, Len’shin AS, Agapov BL et al (2009) Preparation of porous silicon nanocomposites with iron and cobalt and investigation of their electron structure by X-ray spectroscopy techniques. Tech Phys Lett 35:827–830 Kobayashi K, Harraz FA, Izuo S et al (2006) Microrod and microtube formation by electrodeposition of metal into ordered macropores prepared in p-type silicon. J Electrochem Soc 153: C218–C222 Kordas K, Remes J, Leppavuori S et al (2001) Laser-assisted selective deposition of nickel patterns on porous silicon substrates. Appl Surf Sci 178:93–97 Koynov S, Brandt MS, Stutzmann M (2006) Metal-induced seeding of macropore arrays in silicon. Adv Mater 18:633–636 Kubo N, Homma T, Hondo Y et al (2005) Fabrication of patterned nanostructures with various metal species on Si wafer surfaces by maskless and electroless process. Electrochim Acta 51:834–837

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Lee C, Tsuru S, Kanda Y et al (2009) Formation of 100 micrometer deep vertical pores in Si wafers by wet etching and Cu electrodeposition. J Electrochem Soc 156:D543–D547 Mun˜oz-Noval Á, Fukami K, Martı´n-Palma RJ et al (2013) Surface plasmon resonance study of Au nanorod structures templated in mesoporous silicon. Plasmonics 8:35–40 Ono S, Oide A, Asoh H (2007) Nanopatterning of silicon with use of self-organized porous alumina and colloidal crystals as mask. Electrochim Acta 52:2898–2904 Oskam J, Long JG, Natarajan A et al (1998) Electrochemical deposition of metals onto silicon. J Phys D Appl Phys 31:1927–1949 Peng KQ, Zhu J (2004) Morphological selection of electroless metal deposits on silicon in aqueous fluoride solution. Electrochim Acta 49:2563–2568 Polisski S, Goller B, Lapkin A et al (2008) Synthesis and catalytic activity of hybrid metal/silicon nanocomposites. Phys Status Solidi RRL 2:132–134 Polisski S, Goller B, Heck SC et al (2011) Formation of metal nanoparticles in silicon nanopores: plasmon resonance studies. Appl Phys Lett 98:011912 Renaux C, Scheuren V, Flandre D (2000) New experiments on the electrodeposition of iron in porous silicon. Microelectron Reliab 40:877–879 Ronkel F, Schultze JW, Arens-Fischer R (1996) Electrical contact to porous silicon by electrodeposition of iron. Thin Solid Films 276:40–43 Rumpf K, Granitzer P, Albu M et al (2010) Electrochemically fabricated silicon/metal hybrid nanosystem with tailored magnetic properties. Electrochem Solid-State Lett 13:K15–K18 Santinacci L, Djenizian T, Hildebrand H et al (2003) Selective palladium electrochemical deposition onto AFM-scratched silicon surfaces. Electrochim Acta 48:3123–3130 Sasano J, Schmuki P, Sakka T et al (2003) Laser-assisted nickel deposition onto porous silicon. Phys Status Solidi A 197:46–50 Sasano J, Schmuki P, Sakka T et al (2004) Laser-assisted maskless Cu patterning on porous silicon. Electrochem Solid-State Lett 7:G98–G101 Sato H, Homma T, Mori K et al (2005) Electrochemical formation process of Si macropore and metal filling for high aspect ratio metal microstructure using single electrolyte system. Electrochemistry 73:275–278 Scheck C, Liu YK, Evans P et al (2004) Photoinduced electrochemical deposition of Cu on p-type Si substrates. Phys Rev B 69:035334 Schmuki P, Erickson LE (2000) Selective high-resolution electrodeposition on semiconductor defect patterns. Phys Rev Lett 85:2985–2988 Staemmler L, Suter T, Bohni H (2004) Nanolithography by means of an electrochemical scanning capillary microscope. J Electrochem Soc 151:G734–G739 Takano N, Niwa D, Yamada T et al (2000) Nickel deposition behavior on n-type silicon wafer for fabrication of minute nickel dots. Electrochim Acta 45:3263–3268 Trimmer A, Maurer JJ, Schuster R et al (2005) All-electrochemical synthesis of submicrometer Cu structures on electrochemically machined p-Si substrates. Chem Mater 17:6755–6760 Tsuboi T, Sakka T, Ogata YH (1998) Metal deposition into a porous silicon layer by immersion plating: influence of halogen ions. J Appl Phys 83:4501–4506 Tsuboi T, Sakka T, Ogata YH (1999) Effect of dopant type on immersion plating into porous silicon layer. Appl Surf Sci 147:6–13 Yae S, Sakabe K, Fukumuro N et al (2011) Surface-activation process for electroless deposition of adhesive metal (Ni-B, Cu) films on Si substrates using catalytic nanoanchors. J Electrochem Soc 158:D573–D577 Yasseri AA, Sharma S, Jung GY et al (2006) Electroless deposition of Au nanocrystals on Si(111) surfaces as catalysts for epitaxial growth of Si nanowires. Electrochem Solid-State Lett 9:C185–C188 Zhang XG (2001) Electrochemistry of silicon and its oxide. Kluwer Academic/Plenum, New York Zhang X, Tu KN, Xie YH et al (2006) High aspect ratio nickel structures fabricated by electrochemical replication of hydrofluoric acid etched silicon. Electrochem Solid-State Lett 9:C150–C152

Gas and Liquid Doping of Porous Silicon Riccardo Rurali

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Direct Charge Injection and Passive Impurity Reactivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reduced Dielectric Mismatch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

639 640 641 642 643

Abstract

There is now both experimental and theoretical data relating to conductivity changes in porous silicon and other silicon nanostructures arising from the adsorption of specific molecules. The phenomenon is reviewed with emphasis on the potential mechanisms involved and its exploitation with regard sensing applications.

Introduction Since the early years, porous Si (p-Si) attracted great interest for possible applications in the field of sensors, from the former IBM Patent including p-Si into a C-MOS device (Burkhardt and Poponiak 1977) to the first studies on p-Si reactivity to vapors and moistures (Anderson et al. 1990; Schechter et al. 1995; Motohashi et al. 1996; O’Halloran et al. 1999; Foucaran et al. 1997) to the evidence of p-Si luminescence quenching in interaction with nitrogen dioxide (Harper and Sailor 1996). The high surface-to-bulk ratio and the porous structure itself seemed ideally R. Rurali (*) Institut de Cie`ncia de Materials de Barcelona (ICMAB-CSIC), Campus de Bellaterra, Bellaterra, Spain e-mail: [email protected] # Springer International Publishing Switzerland 2014 L. Canham (ed.), Handbook of Porous Silicon, DOI 10.1007/978-3-319-05744-6_66

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suited for the efficient detection of molecules in the liquid or in the gas phase. More recently, though, porous Si has emerged as an effective playground to test in a simple and cost-effective way several ideas applied to the rising field of semiconducting nanowires (Bruno et al. 2007; Rurali 2010). Although chemical sensing is an interesting field per se also for the nanowires community, the similarity between these two materials naturally reoriented the research in porous Si toward applications that naturally lay in realm of (nano)electronics. Molecular doping, after all, is nothing else that a special class of gas sensing, that is, the capability to tune the electronic properties of the material, namely, its carrier density, in a controlled way. This approach is in principle very appealing for nanowires, as the efficiency of conventional impurity doping has been shown to decrease as a result of several mechanisms, such as quantum confinement (Niquet et al. 2010), dielectric mismatch (Bjo¨rk et al. 2009), and surface segregation (Fukata et al. 2011).

Direct Charge Injection and Passive Impurity Reactivation The first and more convincing reports of reproducible n-type doping were related to the adsorption of ammonia (Geobaldo et al. 2003; Chiesa et al. 2003; Garrone et al. 2005). At first a short transient where the adsorption results in a decrease of carrier density is observed. Samples are obtained from highly boron p-type doped Si wafers; thus, the first electrons transferred are recombining with the holes present, compensating the residual p-type character of the material. After that, a systematic increase of the carrier concentration as the NH3 partial pressure is increased is consistently observed (Garrone et al. 2005). Further, electron spin resonance (ESR) measurements and first-principles calculations based on density-functional theory (DFT) convincingly show that the injected charges are electrons and n-type doping is achieved. The theoretical calculations show that the NH3 molecule can easily bind a surface dangling bond via the nitrogen lone pair and then transferring one electron to the Si skeleton (Miranda-Durán et al. 2010; Miranda et al. 2012); see Fig. 1. Interestingly, Konstantinova et al. (2009) reported a dependence of the ammonia doping effect on trace water presence, a mechanism that is not yet fully clear and needs to be explored further. Some of these and other experiments also related the adsorption of NO2 to an increase of hole concentration, indicating the achievement of p-type doping (Garrone et al. 2003, 2005; Boarino et al. 2001; Geobaldo et al. 2001, 2004; Gaburro et al. 2004). Here, however, the situation is not as clear as with NH3. The same DFT calculations that supported the n-type character of NH3 here suggest that NO2 should not provide measurable effect of the conductivity of the porous Si sample. Specifically, both the HOMO and the LUMO of the molecule fall well inside the Si bands, thus failing in providing shallow levels similar to those provided in conventional impurity doping (Miranda-Durán et al. 2010). Nevertheless, it should not be forgotten that upon etching, most of the active boron impurities end up at subsurface sites where they form an electrically inactive complex with a surface dangling bond. It then turns out that what NO2 adsorption

Gas and Liquid Doping of Porous Silicon

641

a

b

c

d

Fig. 1 Adsorption of (a) NH3 and (b) NO2 at a Si dangling bond. NH3 adsorption results in n-type doping, pinning the Fermi level close to the conduction band (c), while the NO2 adsorbed system remains intrinsic, with no half-filled level next to any of the bands (d). The projected DOS of the side panels illustrates the contribution of the atomic species. N and O are shown in blue and red spheres, respectively, while yellow and white spheres represent Si and H atoms (Reprinted with permission from Á. Miranda-Durán et al. (2010) Nano Lett. 10 (9), 3590. Copyright 2010 American Chemical Society)

does is not providing itself a hole, but rather reactivating the passive subsurface boron atoms. It can be said that NO2 is electronegative enough to take the electron of the dangling bond, but not enough to create a hole in the valence band. Clearly, this looks like a less reliable doping mechanism, as it ultimately relies on the concentration of the preexisting doping impurities and on how many of them were passivated at subsurface locations, then amenable to reactivation.

Reduced Dielectric Mismatch Other experiments presented results that, at first sight, are even more puzzling than those related to the adsorption of NO2. Ethanol, acetone, and water are expected to be virtually inert when brought in contact with a Si surface (Chiesa et al. 2005; Timoshenko et al. 2001; Cultrera et al. 2013). No strong chemical interaction is observed and DFT calculations yield fully occupied, mid-gap molecular levels, which are too deep for their electrons to be thermally activated to the host conduction band. Interestingly, here the mechanism is fundamentally different to the cases discussed above, although it shares with NO2 adsorption the fact that the carriers

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responsible for the increase in the conductivity were already present in the material, but deactivated. Let us see how. Before quantum confinement effects can be observed, there is a regime often referred to as dielectric confinement or dielectric mismatch. There, the average thickness of the Si instances that make up the spider web that results from the electrochemical etching process is around 5–12 nm. At these sizes quantum confinement cannot effectively deactivate dopants by making their electronic levels deep, but dielectric confinement can. In a nutshell, when the host material tries to screen an ionized impurity, image charges accumulate at the nanostructured Si surface and change the dielectric problem (Diarra et al. 2007). This new situation is easily shown to lead to deeper, thus electrically inactive, impurity levels. This is going to be the situation of those boron atoms that have not ended up at passive subsurface locations discussed above. Upon pore condensation with, say, ethanol, the effective dielectric constant of the medium surrounding the nanostructured Si changes and increases from 1 (vacuum) to 25 (ethanol). A simple model based on dielectric mismatch is discussed by Timoshenko et al. (2001) shows that in such conditions the activation energy of a B acceptor decreases from 105 to 30 eV. It should be kept in mind that B impurities are present in large concentrations in porous Si samples. Again, like in the case of NO2 adsorption, this is not a genuine doping mechanism where the molecule injects a charge in the host material. Here the concentration and the doping type (n- or p-type) depend on the chemical nature of the impurities that are reactivated. Liquid doping can feature considerably subtler effects and is more difficult to capture with simple model as the adsorption of a few gas molecules (Canham 1986; Beale et al. 1992; Gelloz et al. 1996, 2000). For instance, Canham (1986) observed that the near-infrared PL of bulk n-type silicon was reversibly lowered upon exposure to oxygen and reversibly increased upon exposure to water, but in p-type silicon water had the opposite effect, lowering the PL efficiency. This was interpreted within the Stevenson-Keyes theory as surface potential changes due to adsorbed surface states. Water vapor can be effective in pushing the surface Fermi level toward the conduction band edge, while oxygen is known to induce negative charging on etched Si surfaces. Non-radiative surface recombination velocity was thereby affected and hence PL efficiency. Although these observations were made on bulk and not nanostructured Si, they give a hint of the possible intertwined new effects that can come into play. In the same line, cross-sectional imaging of EL suggested that the conductivity of wet porous silicon could be dramatically higher than that of the same structure dried (Beale et al. 1992).

Conclusions Gas and liquid doping has been reported in porous Si, including porous Si-related systems such as nanowires that hold great promises for the next generation of (nano)electronic devices. In this chapter we have given an overview of the most

Gas and Liquid Doping of Porous Silicon

pyridine

643

ethanol

e-

e-

h+ h+ h+

h+

e-

Fig. 2 Cartoon illustrating the two doping mechanisms: charge transfer from the adsorbates (left) and dopant reactivation upon adsorption (right).

noticeable success cases in this field. It must be kept in mind, nonetheless, that doping is the controlled modification of the conductivity of a semiconducting material and in this sense conventional impurity doping is intended and works. Unfortunately, in many of the cases discussed, the level of control of the increase of free carrier is poor or absent and relies on the characteristic of the undoped material. NO2 adsorption, ethanol pore condensation, and similar mechanisms effectively reactivate latent dopants, but it is the concentration of the latter that will determine the conductivity of the material. It is only through the chemisorption of ammonia and pyridine, among the cases discussed here, that molecular adsorption influences directly the variation of free carriers (Fig. 2). When it comes to sensing, on the other hand, many of these limitations disappear, and we envisage this to be the field where porous Si might be most useful. At a large extent, in a gas sensor, what matters is detecting gas concentration above a certain threshold, and any change in the conductivity giving this information will serve this purpose. It does not really matter now if the carrier concentration increases because of preexistent dopant reactivation or genuine charge injection, as long as a measurable jump in the current shows up.

References Anderson RC, Muller RS, Tobias CW (1990) Investigations of porous silicon for vapor sensing. Sens Actuators A 21–23:835 Beale MIJ, Cox TI, Canham LT, Brumhead D (1992) The depth dependence of photoluminescence and electrolytic electroluminescence in porous silicon films. MRS Proc 283:377 Bjo¨rk MT, Schmid H, Knoch J, Riel H, Riess W (2009) Donor deactivation in silicon nanostructures. Nat Nanotechnol 4:103

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Functional Coatings of Porous Silicon Fre´de´rique Cunin

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Functional Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Characterization Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Functionalities and Application Areas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

This chapter reports on the variety of functional coatings for porous silicon structures, and reviews the methods developed for their respective deposition, the spectroscopic and analytical techniques used for their characterization, and the functionalities imparted to porous silicon related with their specific domains of application.

Introduction Functional coating refers to thin layer or covering of functional material that is applied to a substrate in order to create additional function on the substrate and specifically design its surface properties for a practical application. Functional coatings are developed for applications in various areas: soft functional biocoatings

F. Cunin (*) Institut Charles Gerhardt Montpellier, Montpellier, France e-mail: [email protected] # Springer International Publishing Switzerland 2014 L. Canham (ed.), Handbook of Porous Silicon, DOI 10.1007/978-3-319-05744-6_67

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are designed for medical and biosensing uses (chapters “▶ Drug Delivery with Porous Silicon,” “▶ Porous Silicon Immunoaffinity Microarrays,” and “▶ Porous Silicon in Brachytherapy”), carbon coatings for stable battery anodes (chapter “▶ Porous Silicon and Li-Ion Batteries”), and metals for electrical contact (chapter “▶ Ohmic and Rectifying Contacts to Porous Silicon”) or gas detection (chapter “▶ Porous Silicon Gas Sensing”). They play a key role in improving/ controlling the mechanical and chemical stability of the substrate, which is crucial for sensing applications (chapters “▶ Porous Silicon Diffraction Gratings,” “▶ Porous Silicon Photonic Crystals,” and “▶ Porous Silicon Optical Biosensors”), for the development of micro-optic devices (chapters “▶ Porous Silicon Functionalities for BioMEMS,” “▶ Porous Silicon for Microdevices and Microsystems,” and “▶ Porous Silicon Photonic Crystals”) or for tuning the biodegradability of a medical nano-tool (chapter “▶ Chemical Reactivity and Surface Chemistry of Porous Silicon”). Functional coating appears as continuous or discontinuous film of variable thickness depending on its chemical nature and way of deposition, which covers the outer and/or the inner surface of the porous substrate. Coatings variety with the different deposition techniques attempted are first summarized. Classical and specific characterization techniques for coating layers and composites are reported. Finally, selected useful functionalities and their domain of application are presented. Figure 1 illustrates the variety of functional coatings on structures of porous silicon including films, membranes, and micro- and nanoparticles. The outer and/or the inner surface of the pores can be layered or fully embedded.

Functional Coatings Table 1 lists various coatings on porous silicon structures (chip based, microand nanoparticles), and the methods are developed for their respective deposition.

Characterization Techniques Commonly used spectroscopic or analytical techniques for characterizing surfaces and coating layers on porous silicon are Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy, energy dispersive X-ray spectrometry, fluorescence spectroscopy, UV–Vis absorption/reflectance spectroscopy, thin film optical interference spectroscopy, impedance spectroscopy, optical microscopy, scanning electron microscopy, transmission electron microscopy, atomic force microscopy, ellipsometry, nitrogen adsorption/ desorption analysis, and water contact angle. Spectroscopic and analytical techniques specifically used to characterize coatings on porous silicon are secondary ion mass spectrometry (SIMS) (polymer coatings), cyclic voltammetry and electrochemical measurements (conductive

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Fig. 1 Various functional coatings onto porous silicon surfaces: (a) thermally responsive polymer/porous silicon hybrid for biosensing (Perelman et al. 2010), (b) porous silicon/polymer nanocomposite for biosensing (Li et al. 2005), (c) BSA protein-adsorbed porous silicon surface (Tay et al. 2004), (d) diamond-capped porous silicon film for optical devices (Fernandes et al. 1999), (e) biocompatible polymer/porous silicon composite fiber (Kashanian et al. 2010), (f) block copolymer-coated surface for templating (Qiao et al. 2007), (g) ZnO-deposited porous silicon (Kayahan 2010), (h) SERS active silver-coated porous silicon (Virga et al. 2012) (Reproduced, with permission, from Perelman et al. (2010), Li et al. (2005), Tay et al. (2004), Fernandes et al. (1999), Kashanian et al. (2010), Qiao et al. (2007), Kayahan (2010), Virga et al. (2012))

polymer, metals and metal oxides coatings), magnetic measurements (metals and metal oxides coatings), Raman spectroscopy (carbon coatings), surface plasmon resonance (SPR), and nuclear magnetic resonance (NMR) (lipid coatings and organic layers).

Functionalities and Application Areas Table 2 lists the various functionalities imparted to porous silicon, with relevant coating types and domains of application.

Conclusion Surface coating/capping is an essential step when designing porous silicon-based systems. Initially developed for surface passivation purposes in the early 1990s for stabilizing photoluminescence or refractive index, for example (Loni 1997), surface coating has greatly evolved from passive to much more active duty. Nowadays functional coatings not only passivate the surface but also impart specific

Coating nature Polymers

Organization of the composite (thickness when relevant) Coating in and outside pores Coating outside pores (μm to hundreds of μm) Coating outside pores (tens of μm) Coating outside pores (tens of nm) Coating in and outside pores Coating in and outside pores

Coating outside pores Coating outside pores Coating inside pores

Deposition technique Impregnation/drop coating/contact lamination

Spin coating/dip coating/spray coating

Chemical vapor deposition Electropolymerization

Surface-initiated polymerization

Microemulsion

Self-assembly Covalent grafting of polymers and oligomers onto porous silicon

Table 1 Functional coatings and deposition techniques

Responsive polymers and copolymers Biocompatible or biodegradable Biodegradable copolymers Copolymers Responsive and biocompatible Polymers

Conducting polymers

Study examples Responsive polymers and hydrogels Biocompatible or biodegradable polymers Copolymers Industrial synthetic polymers Copolymers Copolymers

Qiao et al. (2007)a Schwartz et al. (2005), Kilian et al. (2007a), Britcher et al. (2008), Segal et al. (2009), Godin et al. (2010)a, Sciacca et al. (2011)

Fan et al. (2012)a, MP

Abalyaeva and Efimov (2000), Batra et al. (2006), Urbach et al. (2007)a, Chiboub et al. (2010a, b)a, Belhousse et al. (2010)a McInnes et al. (2009)a, Bonanno and DeLouise (2010)a, Wang et al. (2012)a, Vasani et al. (2011), Pace et al. (2012a)a, Yoon et al. (2003)

McInnes et al. (2012)a

Li et al. (2005)MP, Tighilt et al. (2007), Zhang et al. (2011)a

Use with porous silicon Bakker et al. (2003)a, DeLouise et al. (2005)a, Segal et al. (2007)a, Wu and Sailor (2009)a, Alvarez et al. (2009)a, Pastor et al. (2011)MP, Low et al. (2006), Park et al. (2009)NP, Fan et al. (2009)MP, Kashanian et al. (2010)a, MP, Gongalsky et al. (2012)a, MP, Perelman et al. (2010)a

650 F. Cunin

Biomolecules

Carbonb

Metals and metal oxides

In situ polymerization of precursors and thermal decomposition Deposition/ adsorption/selfassembly

Thermal decomposition of acetylene

Chemical vapor deposition

Atomic layer epitaxy

Impregnation of precursors and thermal decomposition/ annealing Sputtering/magnetron sputtering

Immersion plating

Carbon

Proteins, peptides, antibodies, DNA, lipid layers, etc.

Coating in and outside pores

Carbon Diamond, diamondlike carbon Carbon Functionalized carbon

SnO, CoSi2, Si(1-x)Gex

Cu, ZnO, WO3, Al, Ag, ZnO

Ag, Metal oxides (Sn, Fe, Ni) Au

Ag, Au, Pd, Pt, Se, Cu, Bi, W, Mo, Ni, Fe, Al, Ti, Mg, Fe2O3

Coating inside pores

Coating outside pores Coating in and outside pores Coating in and outside pores Coating inside pores Coating outside pores (tens of nm) Coating in and outside pores

Coating outside pores (μm) Coating inside pores Coating inside pores Coating outside pores

(continued)

Tay et al. (2004)a, Steinem et al. (2004)a, Schwartz et al. (2007), Perelman et al. (2008)a, Alvarez et al. (2009)a, Wu et al. (2009)a, Flavel et al. (2011)a,

Salonen et al. (2004), Bjorkqvist et al. (2004, 2009), Kaukonen et al. (2007)MP, Torres-Costa et al. (2008, (2009), Bimbo et al. (2010)NP, Fang et al. (2010), Sciacca et al. (2011), Makila et al. (2012), Jalkanen et al. (2012), Rytkonen et al. (2012)NP Cho (2010)MP, Kelly et al. (2011)a

Mattei et al. (2007), Fernandes et al. (1999)a, Baranauskas et al. (2000), Makara et al. (2003), Aroutiounian et al. (2007)

Ducso et al. (1996), Loni (1997)

Ghosh et al. (2002), Kumar et al. (2012), Sun et al. (2010), Giaddui et al. (1998), Kim and Lee (2008), Kayahan (2010)

Chan et al. (2003)a, Cobianu et al. (1997)a, Moshnikov et al. (2012)a, Behzad et al. (2012)

Lin et al. (2004), Giorgis et al. (2009), Virga et al. (2012), Canham (2006), Dhar and Chakrabarti (1996), Park et al. (2006)a

Functional Coatings of Porous Silicon 651

Organic and inorganic monolayers

Coating nature

Table 1 (continued)

Lipid bilayers

Functional alkanes and alkenes, silane, organics

Coating outside pores (nm) Coating in and/or outside pores

Fusion of lipid vesicles (SUV, MLV, giant, etc.) Covalent grafting (electro, UV, thermo, catalyst, microwave assisted)

Proteins, gelatin Proteins, antibodies, DNA, etc.

Coating outside pores Coating in and outside pores

Study examples

Spin coating Covalent grafting

Deposition technique

Organization of the composite (thickness when relevant) Use with porous silicon

Choi and Buriak (2000), Buriak (2002), Schmeltzer et al. (2002), Stewart and Buriak (2002), Boukherroub et al. (2003), Vrkoslav et al. (2007), Lagrost et al. (2007), Kilian et al. (2008), Pace et al. (2009, 2010), Velleman et al. (2010)a, Guan et al. (2011), Xue et al. (2011)a, Sweetman et al. (2011b), Sweetman and Voelcker (2012)

Bimbo et al. (2011)a, MP, Sweetman et al. (2012)a, Sarparanta et al. (2012)a, NP, Bimbo et al. (2012)a, MP, Kilian et al. (2007b), Mey et al. (2012)a Orosco et al. (2006)a, Gao et al. (2008) Dancil et al. (1999)a, Tinsley-Bown et al. (2000)a, Voicu et al. (2004)a, Kilian et al. (2007c), Bocking et al. (2008), Sam et al. (2010, 2011), Sweetman et al. (2011a)a Worsfold et al. (2006)a, Cunin et al. (2007)a, Pace et al. (2012b)a

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Spin coating Impregnation of precursors and thermal treatment Electrochemical anodization Coating on top of porous silicon

Coating in and outside pores

Coating outside pores Coating inside pores

Coating in and outside pores Coating inside pores

Porous silica

Silica gel Dense silica

Porous silica

Functional alkanes and alkenes Nitrides

Lharch et al. (2003)

Posada et al. (2006) Akusawa and Hara (2007)

Corban et al. (2003), Amato et al. (2005)

Frascella et al. (2009)

Buriak (1999)

b

The surface of porous silicon was oxidized or chemically modified prior to the coating step Si-C (silicon carbide) coatings on porous silicon are not reviewed in this chapter c Silicon dioxide functional coatings are obtained by deposition of additional SiO2 material on the porous silicon substrates. Silicon dioxide layers obtainable by direct controlled oxidation of the porous silicon films (by chemical, thermal, ozone, or aging treatments) are not reviewed in this chapter. Thermal oxidation is reviewed elsewhere in handbook (chapters “▶ Oxidation of Macroporous Silicon”) MP functional coating is realized on porous silicon microparticles NP functional coating is realized on porous silicon nanoparticles

a

Silicon dioxidec

Chemical vapor deposition Chemical vapor deposition

Ultrahigh vacuum

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Table 2 Functionalities of coatings and utilization Functionality Surface chemical/ mechanical stabilization; surface passivation; surface protective coating

Coating nature Silicon dioxide Carbon/diamond Polymers Biomolecules/ organic monolayers Metals

Application area Optical devices Gas sensors

Surface hydrophilicity/ hydrophobicity

Biomolecules/ polymers/organic layers Carbon

Gas sensors, biosensors, controlled drug delivery

Biomimetic surface/ artificial membrane

Lipid bilayers

Biosensors, drug characterization, diagnosis

Biocompatible surface; bioactive surface, cell adhesion promoting surface; cell targeting; biofouling surface; furtivity

Polymers (natural, biocompatible, conductive, etc.)

Cell culture/ bioassays Bioimaging/ biosensing/drug delivery Tissue engineering/ implants

Biomolecules (antibodies, proteins, etc.) Organic layers

Controlled drug delivery Membrane/ separation

Study examples Posada et al. (2006), Fernandes et al. (1999), Fang et al. (2010), Bimbo et al. (2012), Tay et al. (2004), Salonen et al. (2004), Bjorkqvist et al. (2003, 2004, 2009), Torres-Costa et al. (2008, 2009), Buriak (2002) Buriak (2002), Pace et al. (2009, 2010), Sweetman and Voelcker (2012), Salonen et al. (2004), TorresCosta et al. (2008, 2009), Bimbo et al. (2011, 2012), Kovalainen et al. (2013), Liu et al. (2012) Worsfold et al. (2006), Kilian et al. (2007b), Cunin et al. (2007), Pace et al. (2012b), Mey et al. (2012) DeLouise et al. (2005), Schwartz et al. (2005), Low et al. (2006), Whitehead et al. (2008), Park et al. (2009), Alvarez et al. (2009), McInnes et al. (2009), Wu et al. (2009), Godin et al. (2010), Sweetman et al. (2011a, b, 2012, Secret et al. (2012), Flavel et al. (2011), Guan et al. (2011), Sarparanta et al. (2012) (continued)

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Table 2 (continued) Functionality Sensing active surface (enhanced sensitivity, specificity)

Stimuli responsive surface

Coating nature Silicon dioxide Polymers (responsive, natural, biocompatible, conductive, etc.)

Application area Optical devices Sensors, biosensors Controlled drug delivery

Biomolecules (DNA, antibodies, bacteria, peptides, etc.) Metal/metal oxides Carbon/diamond/ functionalized carbon

Surfaceenhanced Raman spectroscopy (SERS)

Polymers (responsive, biocompatible, conductive, etc.) Biomolecules (antibodies, etc.) Silicon dioxide

Smart responsive surfaces and scaffolds Controlled drug delivery/ nanovalves

Study examples Corban et al. (2003), Posada et al. (2006), Urbach et al. (2007), Belhousse et al. (2010), Bonanno and DeLouise (2010), Sciacca et al. (2011), Gongalsky et al. (2012) Dancil et al. (1999), Tinsley-Bown et al. (2000), Steinem et al. (2004), Voicu et al. (2004), Kilian et al. (2007c), Wu et al. (2009), Jane et al. (2009), Sam et al. (2010, 2011), Chan et al. (2003), Lin et al. (2004), Gabouze et al. (2007), Giorgis et al. (2009), Jiao et al. (2010), Virga et al. (2012), Moshnikov et al. (2012), Fernandes et al. (1999), Kaukonen et al. (2007), Kelly et al. (2011), Makila et al. (2012) Batra et al. (2006), Wu and Sailor (2009), Fan et al. (2009), Segal et al. (2009), Kashanian et al. (2010), Perelman et al. (2010), Vasani et al. (2011), Pastor et al. (2011), De Rosa et al. (2011), McInnes et al. (2012), Pace et al. (2012a), Xue et al. (2011), Anglin et al. (2008) (continued)

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Table 2 (continued) Functionality Conductive/magnetic/ superconductive surface

Coating nature Carbon Iron oxide, mixed oxide, niobium Polyaniline

Application area Li batteries Optoelectronic Sensors

Low reflective surface

Porous silicon itself Diamond-like carbon

Optical and optoelectrical devices Solar cells

Luminescence enhancement/ modification/ stabilization

Silicon dioxide, metals/metal oxides, diamond, polymers (conductive, bioresorbable)

Optical devices Sensors, bioimaging

Study examples Kim et al. (2008), Cho (2010), Mattei et al. (2007), Park et al. (2006), Chiboub et al. (2010a, b), Abalyaeva and Efimov (2000), Thakur et al. (2012), Trezza et al. (2008), Dai et al. (2007) Schirone et al. (1997), Lipinski et al. (2003), Panek (2004), Aroutiounian et al. (2007) Posada et al. (2006), Kim et al. (2009), Kim et al. (2012), Wang et al. (2000), Gongalsky et al. (2012), Chen et al. (2013), Halliday et al. (1996)

functionality and activity, e.g., increasing/changing photoluminescence properties, enhancing sensitivity, providing superconductivity, etc. Remarkable accomplishments in functional coatings were also achieved in particular interfacing with biological environments, opening great promises for nanomedicine.

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Electroencapsulation of Porous Silicon Matti Murtomaa and Jarno Salonen

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrospraying . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electroencapsulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Process Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Electroencapsulation is an application of electrospraying in which liquid is atomized into micro- or even nanosized droplets using electrostatic forces alone. The method allows better controllability of the capsulation process than, e.g., the most commonly used spray drying. Due to the applied electrostatic forces, it also enables production of complex capsule structures, like solid shell covered liquid core particles. In this review, we will focus on electroencapsulation processes used to improve the handling, processing, and administration of porous silicon-based drug delivery systems.

M. Murtomaa (*) Department of Physics and Astronomy, University of Turku, Turku, Finland e-mail: [email protected] J. Salonen Department of Physics and Astronomy, Laboratory of Industrial Physics, University of Turku, Turku, Finland e-mail: [email protected] # Springer International Publishing Switzerland 2014 L. Canham (ed.), Handbook of Porous Silicon, DOI 10.1007/978-3-319-05744-6_68

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Introduction Recently, mesoporous silicon (PSi) has proven to be an appealing candidate as a drug carrier (Salonen et al. 2008 and Handbook Chapter “▶ Drug Delivery with Porous Silicon”). However, there are some issues which hinder its use in oral delivery. Firstly, nanoparticles have a strong tendency to form aggregates because of forces such as van der Waals attractive force (Parsegian 2006; Peng et al. 2010). This makes the handling, processing, and dosing very challenging. Secondly, certain combinations of the loaded drug, PSi porosity, and surface chemistry may encourage premature dissolution of the drug in stomach (Salonen et al. 2001). For optimal drug absorption in the body, it is often best that the dissolution of the drug takes place as the drug carriers arrive to the small intestine (Kimura and Higaki 2002; Masaoka et al. 2006). To avoid unwanted metabolism and degradation, and to increase bioavailability, drug carriers need to be protected with a shell which dissolves at the optimal delivery site. One possible method for achieving both improved “mechanical” properties and targeted delivery is electroencapsulation, which allows better controllability of the capsulation process than, e.g., spray drying.

Electrospraying Electroencapsulation is an application of electrospraying (electrohydrodynamic spraying). In electrospraying, liquid is atomized into micro- or even nanosized droplets using electrostatic forces alone (Bailey 1989). As high electric field is applied to the tip of a capillary containing the liquid, high charge density is induced on the liquid surface. The highly charged surface of the droplet experiences strong coulombic force due to applied external electric field and also due to high unipolar charge density within the liquid. These forces cause elongation of the meniscus. As electric field is increased, the diameter of droplets decreases and the dropping frequency increases. This regime is called micro-dripping, and the size of the droplets can be precisely controlled. When the electric field is high enough, the liquid in the tip of capillary forms a so-called Taylor cone and the surface disrupts into small droplets emitting from the tip. The formed droplets have very narrow size and charge distribution. This mode of electrospray is called cone-jet mode (Cloupeau and Prunetfoch 1994; Grace and Marijnissen 1994; Jaworek 2007; Jaworek and Sobczyk 2008). The narrow size distribution is enhanced by coulombic forces which tend to keep the uniformly charged droplets apart (Brandenberger et al. 1999). Also, monodispersity can be enhanced using pulsed or ac voltage superimposed in excitation (dc) voltage (Sample and Bollini 1972; Sato 1984).

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Electroencapsulation In electroencapsulation, two immiscible liquids are atomized simultaneously using two nozzles maintained at opposite polarities (Langer and Yamate 1969). Thus, emitted droplets have opposite polarities and will be attracted by coulombic forces. As droplets collide, they form a capsule where liquid with higher surface tension is covered with liquid having lower surface tension. Simultaneously, the opposite charges cancel each other resulting in electrically neutral structure which can be further processed. Another method for electroencapsulation involves only one nozzle but there are two concentric capillaries carrying immiscible liquids (Loscertales et al. 2002; Lopez-Herrera et al. 2003). If this geometry, electro-coextrusion, is used, the resulting droplets remain highly charged. The core capillary may contain PSi particles in a drug solution and the surrounding capillary polymer solution, for example. In general, the core liquid contains a drug solution or a suspension to be shielded. If a solid capsule is desired, the solution used as shell liquid is chosen so that it can be solidified by, for example, evaporation of solvent or by using anti-solvent method. Electroencapsulation of PSi has been recently reported for the first time by Roine et al. In their work, two oppositely charged sprays were used (Roine et al. 2012). Core liquid contained drug-loaded PSi micro- or nanoparticles immersed in glycerol (particles size