Quantum Dot Photodetectors [30, 1 ed.] 3030742695, 9783030742690

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Lecture Notes in Nanoscale Science and Technology 30

Xin Tong Jiang Wu Zhiming M. Wang   Editors

Quantum Dot Photodetectors

Lecture Notes in Nanoscale Science and Technology Volume 30

Series Editors Zhiming M. Wang, Chengdu, China Greg Salamo, Fayetteville, USA Stefano Bellucci, Frascati RM, Italy

Lecture Notes in Nanoscale Science and Technology (LNNST) aims to report latest developments in nanoscale science and technology research and teaching – quickly, informally and at a high level. Through publication, LNNST commits to serve the open communication of scientific and technological advances in the creation and use of objects at the nanometer scale, crossing the boundaries of physics, materials science, biology, chemistry, and engineering. Certainly, while historically the mysteries in each of the sciences have been very different, they have all required a relentless step-by-step pursuit to uncover the answer to a challenging scientific question, but recently many of the answers have brought questions that lie at the boundaries between the life sciences and the physical sciences and between what is fundamental and what is application. This is no accident since recent research in the physical and life sciences have each independently cut a path to the edge of their disciplines. As both paths intersect one may ask if transport of material in a cell is biology or is it physics? This intersection of curiosity makes us realize that nanoscience and technology crosses many if not all disciplines. It is this market that the proposed series of lecture notes targets. More information about this series at http://www.springer.com/series/7544

Xin Tong • Jiang Wu • Zhiming M. Wang Editors

Quantum Dot Photodetectors

Editors Xin Tong Institute of Fundamental and Frontier Sciences University of Electronic Science and Technology of China Chengdu, Sichuan, China

Jiang Wu Institute of Fundamental and Frontier Sciences University of Electronic Science and Technology of China Chengdu, Sichuan, China

Zhiming M. Wang Institute of Fundamental and Frontier Sciences University of Electronic Science and Technology of China Chengdu, Sichuan, China

ISSN 2195-2159     ISSN 2195-2167 (electronic) Lecture Notes in Nanoscale Science and Technology ISBN 978-3-030-74269-0    ISBN 978-3-030-74270-6 (eBook) https://doi.org/10.1007/978-3-030-74270-6 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 This work is subject to copyright. All rights are solely and exclusively licensed 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. 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. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Preface

Photodetectors are optoelectronic devices that enable conversion of photons into charge carriers to generate electric signals; they have seen widespread use in modern militaries as well as civilian life, which includes night-vision, imaging technique, optical communication, surveillance system and environment monitoring. Photodetectors can be generally classified as X-ray, ultraviolet (UV), visible and infrared (IR) photodetectors based on the detection wavelength, which can be achieved by employing various photo-active semiconductor materials with different band gaps during device fabrication. Nowadays, the majority of the commercial photodetectors used in electronic devices consist of inorganic semiconductors such as silicon and III–V compounds, showing relatively high cost, complicated fabrication processes and limited mechanical flexibility. Thus, it is of significant importance to develop large-area and flexible photodetectors with a reduction in the size, weight, power and cost (SWaP-C) for applications in a new generation of optoelectronics. Quantum dots (QDs) are small-sized semiconductor nanoparticles synthesized by top-down methods such as epitaxial self-assembly growth and chemical colloidal synthesis, showing outstanding light harvesting and emitting properties, which are promising for the fabrication of various optoelectronic devices including photodetectors. Generally, QDs possess a physical size smaller than the Bohr radius of bulk materials, resulting in unique quantum confinement effect that enables the size-dependent band gaps for broad absorption and tunable optical detection. Over the past few decades, several types of QDs including self-assembled QDs, colloidal II-VI/IV-VI/I-III-VI QDs, perovskite QDs, carbon QDs and their hybrids of zero dimensional QDs/two-dimensional (2D) materials with improved optical and electrical properties were developed and employed to achieve high-performance QD photodetectors, representing an exciting field that has a strong impact in terms of both physics and devices, thus paving the way for the developments of next-­ generation photodetectors. This book consists of the latest advances in QD-based photodetectors including the rational synthesis of QDs, the device structure design and fabrication as well as novel optical engineering/manipulating technologies of QD photodetectors. v

vi

Preface

Specifically, QD photodetectors for infrared photodetection is the major topic of the chapter “Progress in Quantum Dot Infrared Photodetectors”. In that chapter “Progress in Quantum Dot Infrared Photodetectors”, QDs preparation, fundamental properties, device types of various QD infrared photodetectors based on self-­ assembled QDs (e.g., InAs, GaAs) and colloidal QDs (e.g., HgTe, PbS) are reviewed. The imaging arrays techniques of QD infrared photodetectors and the comparison of colloidal QD photodetectors and HgCdTe photodiodes are discussed as well. Chapters “Photodetectors Based on Perovskite Quantum Dots” and “Photoconductive Detectors Based on Perovskite Quantum Dots or Nanocrystals: From Lead-­Based System to Lead-­Free System” mainly focus on the investigations of recent emerging metal halide perovskites QDs with remarkable optical and electrical properties and their application in photodetectors. The chapter “Photodetectors Based on Perovskite Quantum Dots” presents the recent development of photodetectors based on pure perovskite QDs sorted by response wavelength and photodetectors fabricated using hybrid perovskite QDs with introduced functional materials. The chapter “Photoconductive Detectors Based on Perovskite Quantum Dots or Nanocrystals: From Lead-­Based System to Lead-­Free System” aims to introduce and compare the lead-based and lead-free perovskite QDs by summarizing their synthesis processes, bandgap engineering, modification methods, optoelectronic properties and photoconductor devices applications. The major challenges that need to be addressed in the future and the outlooks of perovskite QDs/NCs photodetectors were presented. In the chapter “Solution-­ Processable Carbon and Graphene Quantum Dots Photodetectors”, the application of carbon and graphene QDs (C/GQDs) in high-­ performance photodetectors are presented. The controlled synthesis of QDs, device structures, working mechanisms and the use of C/GQDs in hybrid structures with 2D materials for improved photodetectors performance are reviewed. The chapter “Quantum Dot/Graphene Heterostructure Nanohybrid Photodetectors” offers an up-to-date review of the recent progress made in the research of QD/graphene nanohybrid photodetectors. The remaining challenges and perspective in future research and development to make QD/graphene nanohybrids competitive for commercialization are discussed. The chapter “Two-­Dimensional Materials-­Based Quantum Dots for Wavelength-­ Selective, Tunable and Broadband Photodetector Devices” provides a comprehensive summary over the latest scientific and technological progress of wavelength selective, tunable and broadband photodetectors based on 2D materials QDs such as carbon based QDs, transition metal dichalcogenides (TMDCs) QDs and black phosphorus (BP) QDs. In the chapter “Self-­Assembled Quantum Dot Photodetector: A Pathbreaker in the Field of Optoelectronics”, the recent advances, fabrication, applications and various heterostructures based on self-assembled QD photodetectors and their cease-less application in the field of optoelectronics are summarized. The editors would like to thank all the authors for their significant contributions and efforts to this book. We are convinced that this book will be a valuable guideline for future optimizations and developments of semiconductor QDs-based

Preface

vii

photodetectors. We would like to thank Mr. Yimin You for his contribution in providing helpful editorial assistance. We would also like to thank the staff at Springer for their support. Lastly, the editors would acknowledge the financial support from the National Key Research and Development Program of China (2019YFB2203400, 2019YFE0121600), the Sichuan Science and Technology Program (2021YFH0054), the “111 Project” (B20030) and the UESTC Shared Research Facilities of Electromagnetic Wave and Matter Interaction (Y0301901290100201). Chengdu, China  

Xin Tong Jiang Wu Zhiming M. Wang

Contents

 Progress in Quantum Dot Infrared Photodetectors��������������������������������������    1 Antoni Rogalski  Photodetectors Based on Perovskite Quantum Dots������������������������������������   75 Shalong Wang and Jizhong Song  Photoconductive Detectors Based on Perovskite Quantum Dots or Nanocrystals: From Lead-Based System to Lead-Free System����������������������������������������������������������������������������������������  119 Zhifeng Shi and Wenqing Liang  Solution-Processable Carbon and Graphene Quantum Dots Photodetectors������������������������������������������������������������������������  157 Azhar Ali Ayaz Pirzado, Faraz Mahar, Ayaz Ali Hakro, Xiujuan Zhang, and Jiansheng Jie Quantum Dot/Graphene Heterostructure Nanohybrid Photodetectors����������������������������������������������������������������������������  215 Judy Wu, Maogang Gong, Russell C. Schmitz, and Bo Liu  Two-Dimensional Material-Based Quantum Dots for Wavelength-Selective, Tunable, and Broadband Photodetector Devices����������������������������������������������������������  249 Samit K. Ray, Subhrajit Mukherjee, Tamal Dey, Subhajit Jana, and Elad Koren  Self-Assembled Quantum Dot Photodetector: A Pathbreaker in the Field of Optoelectronics����������������������������������������������  289 Abhinandan Patra and Chandra Sekhar Rout Index������������������������������������������������������������������������������������������������������������������  307

ix

Contributors

Tamal Dey  School of Nanoscience and Technology, Indian Institute of Technology, Kharagpur, India Maogang  Gong  Department of Physics and Astronomy, University of Kansas, Lawrence, KS, USA Ayaz  Ali  Hakro  Department of Electronic Engineering, Faculty of Engineering and Technology, University of Sindh, Allama I.I.Kazi Campus, Jamshoro, Sindh, Pakistan Subhajit  Jana  Department of Physics, Indian Institute of Technology, Kharagpur, India Jiansheng Jie  Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou, Jiangsu, P. R. China Elad  Koren  Faculty of Materials Science and Engineering, Technion  – Israel Institute of Technology, Haifa, Israel Wenqing  Liang  School of Physics and Microelectronics, Key Laboratory of Materials Physics of Ministry of Education, Zhengzhou University, Zhengzhou, China Bo  Liu  Department of Physics and Astronomy, University of Kansas, Lawrence, KS, USA Faraz Mahar  Department of Electronic Engineering, Faculty of Engineering and Technology, University of Sindh, Allama I.I.Kazi Campus, Jamshoro, Sindh, Pakistan Subhrajit Mukherjee  Faculty of Materials Science and Engineering, Technion – Israel Institute of Technology, Haifa, Israel Abhinandan Patra  Centre for Nano and Material Sciences, Jain University, Jain Global Campus, Jakkasandra, Ramanagaram, Bangalore, India xi

xii

Contributors

Azhar Ali Ayaz Pirzado  Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou, Jiangsu, P. R. China Department of Electronic Engineering, Faculty of Engineering and Technology, University of Sindh, Allama I.I.Kazi Campus, Jamshoro, Sindh, Pakistan Samit  K.  Ray  Department of Physics, Indian Institute of Technology, Kharagpur, India S.N. Bose National Centre for Basic Sciences, Salt-lake, Kolkata, India Antoni Rogalski  Institute of Applied Physics, Military University of Technology, Warsaw, Poland Chandra Sekhar Rout  Centre for Nano and Material Sciences, Jain University, Jain Global Campus, Jakkasandra, Ramanagaram, Bangalore, India Russell C. Schmitz  U.S. Army RDECOM, CERDEC, Night Vision and Electronic Sensors Directorate, Ft. Belvoir, VA, USA Zhifeng Shi  School of Physics and Microelectronics, Key Laboratory of Materials Physics of Ministry of Education, Zhengzhou University, Zhengzhou, China Jizhong  Song  School of Physics and Microelectronics, Zhengzhou University, Zhengzhou, China Shalong  Wang  School of Physics and Microelectronics, Zhengzhou University, Zhengzhou, China Judy  Wu  Department of Physics and Astronomy, University of Kansas, Lawrence, KS, USA Xiujuan  Zhang  Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou, Jiangsu, P. R. China

Progress in Quantum Dot Infrared Photodetectors Antoni Rogalski

Abstract  This chapter reviews the present status and possible future developments of QDIPs. An emphasis is put on potential developments of both epitaxial and colloidal quantum dot photodetectors. At the beginning, the design and fabrication of QDIPs is shortly described. Next, the detector characterization and fundaments of detection mechanisms for QDIPs are presented. In the past decade, there has been significant progress in development of colloidal quantum dot (CQD) photodetectors. For their potential advantages can be included: cheap and easy fabrications, size-tunable across wide infrared spectral region, and direct coating on silicon electronics for imaging, what potentially reduces array cost and offers new modifications like flexible infrared detectors. Investigation of the performance of QDIPs is compared to other types of infrared photodetectors. A model is based on fundamental performance limitations enabling a direct comparison between different infrared material technologies. The main evaluation is directed toward high operating temperature (HOT) photodetectors. Keywords  Epitaxial and colloidal quantum dot infrared photodetectors · HOT infrared detectors · HgCdTe photodiodes · p-i-n depleted photodiodes · BLIP performance · Photodetector performance limits

1  Introduction Advances in the epitaxial growth of strained heterostructures, such as InGaAs on GaAs, have led to the realization of coherent islands through the process of self-­ organization. These islands behave electronically as quantum boxes, or quantum dots (QDs). Zero-­dimensional quantum-confined semiconductor heterostructures have been investigated theoretically and experimentally for some time [1–3]. At present, nearly defect-free quantum dot devices can be fabricated reliably and reproducibly. Also new types of infrared photodetectors taking advantage of the A. Rogalski (*) Institute of Applied Physics, Military University of Technology, Warsaw, Poland e-mail: [email protected]

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 X. Tong et al. (eds.), Quantum Dot Photodetectors, Lecture Notes in Nanoscale Science and Technology 30, https://doi.org/10.1007/978-3-030-74270-6_1

1

2

A. Rogalski

quantum confinement obtained in semiconductor heterostructures have emerged. Like the QWIPs, the QDIPs are based on optical transitions between bound states in the conduction (valence) band in QDs. Also, like the QWIPs, they benefit from a mature technology with large bandgap semiconductors. Interest in quantum dot research can be traced back to a suggestion by Arakawa and Sakaki in 1982 [1] that the performance of semiconductor lasers could be improved by reducing the dimensionality of the active regions of these devices. Initial efforts at reducing the dimensionality of the active regions focused on using ultrafine lithography coupled with wet or dry chemical etching to form 3D structures. However, it was soon realized that this approach introduced defects (high density of surface states) that greatly limited the performance of such QDs. Initial efforts were mainly focused on the growth of InGaAs nanometer-sized islands on GaAs substrates. In 1993, the first epitaxial growth of defect-free quantum dot nanostructures was achieved by using molecular beam epitaxy (MBE) [4]. First observations of intersublevel transitions in the far infrared were reported in the early 1990s, either in InSb-based electrostatically defined QDs [5] or in structured two-dimensional (2D) electron gas [6]. The first QDIP was demonstrated in 1998 [7]. Great progress has been made in their development and performance characteristics [8–14] and in their applications to thermal imaging focal plane arrays [15–22]. This chapter reviews the present status and possible future developments of QDIPs. An emphasis is put on potential developments of both epitaxial and colloidal quantum dot photodetectors. At the beginning, the design and fabrication of QDIPs is shortly described. Next, the detector characterization and fundaments of detection mechanisms for QDIPs are presented. Investigation of the performance of QDIPs is compared to other types of infrared photodetectors. A model is based on fundamental performance limitations enabling a direct comparison between different infrared material technologies. It is assumed that the thermal generation in the active detector’s region is limiting factor. Special attention is put on trends in performance limits of high operating temperature (HOT) photodetectors. Hitherto, for HgCdTe technology the semiempirical rule Rule 07 (specified in 2007) is used as a reference for other technologies. It is shown that a new metrics called “Law 19” better describe the ultimate photodetector performance.

2  QD Preparation and Fundamental Properties Rapid progress in the development of epitaxial growth techniques has made it possible to grow semiconductor structures at one-monolayer accuracy. The device structure dimensions can be comparable to wavelengths of the relevant electron or hole wave functions, at least in the growth direction. This means that one can do electrical engineering at the quantum-mechanical level. The electron confinement within a sufficiently narrow region of semiconductor material can significantly

Progress in Quantum Dot Infrared Photodetectors

3

change the carrier energy spectrum, and novel physical properties are expected to emerge. These novel properties will give rise to new semiconductor devices as well as to drastically improved device characteristics [1, 23]. Most expected improvements in electronic and optoelectronic device performance originate from the change in the density of states.

2.1  Low-Dimensional Solids: Background In addition to quantum well case, where energy barriers for electron motion exist in one direction of propagation, one can also imagine electron confinement in two directions and, as the ultimate case, in all three directions. The structures of these kinds are now known as quantum wires (QWs) and quantum dots (QDs). Thus, the family of dimensionalities of the device structures involves bulky semiconductor epilayer [three-dimensional (3D)], thin epitaxial layer of quantum well [two-­ dimensional (2D)], elongated tube or quantum wire [one-dimensional (1D)], and, finally, isolated island of QD [zero-dimensional (0D)]. These four cases are shown in Fig. 1. In a crystalline semiconductor, the electrons and holes that determine the transport and optical properties are considered as “quasi-free” with the effective mass m* taking account of the periodic crystal potential. The transport/optical properties are basically determined by the uppermost valence band and lowest conduction band, Bulk (3D)

+ +

+

Ev

Ec

ρ(E)~E1/2

f(E)

ρ(E)~const

n(E)

f(E)

n(E)

E1

Ec

n(E)=ρ(E)f(E) ρ(E)~E1/2

n(E)=ρ(E)f(E)

ρ(E)

n(E)=ρ(E)f(E)

E1

Ec

n(E)=ρ(E)f(E) ρ(E)~δ(E)

ρ(E)

+

E2

E2

E2 E1

ρ(E)

+

ρ(E)

Dot (0D)

Ec

Eg

f(E)

Wire (1D)

Well (2D)

f(E)

n(E)

n(E)

E

E

E

E

(a)

(b)

(c)

(d)

Fig. 1  Schematics of density of states and carrier distribution for (a) bulk, (b) quantum wells, (c) quantum wires, and (d) quantum dots. Quantum dot density of states is independent of temperature

4

A. Rogalski

which are separated in energy by the bandgap, Eg. Confinement of electrons in one or more dimensions modifies the wave functions, dispersion, and density of states. The effective potential associated with spatial variation of the conduction and valence band edges is spatially modulated in so-called compositional superlattices that consist of alternating layers of two different semiconductors. Farther confinement in two or finally three dimensions results in size quantization in the corresponding directions and stronger discretization of the energy spectrum and density of state distribution approaching atomic behavior for three-dimensional confinement. An ideal quantum dot, also known as a quantum box, is a structure capable of confining electrons in all three directions, thus allowing zero dimensions in their degree of freedom. The energy spectrum is completely discrete, similar to that in an atom. The total energy is the sum of three discrete components:



E

0D

E

nx

E

2 2 2 h2 klz h2 kmy h2 knx E    , my lz 2 m 2 m 2 m

(1)

where n, m, and l are integers (1, 2, . . .) used to index the quantized energy levels and quantized wave numbers, which result from the confinement of the electron motion in the x-, y-, and z-directions, respectively. As for the bulk material, the most important characteristic of a QD is its electron density of states in the conduction band given by

 0 D  E   2   Enx  Emy  Elz  E  , n , m ,l

(2)

where δ(E) is the Heaviside step function with δ(E ≥ Enx) = 1 and δ(E  Einter. Instead, the intraband transitions occur when the photon energy matches with the intraband energy gap hν = Eintra. The latter requires degenerate doping, which is a synthetic challenge. Colloidal QD photodetectors typically comprise a single nanocomposite layer deposited on a substrate, and large-area, two-terminal, vertical devices are fabricated using p-(indium-tin-oxide) and n-type (aluminum) contacts, as shown schematically in Fig. 12a. Figure 12b illustrates the capture and transport mechanism of a colloidal dot film. The charge transport mechanisms in colloidal QD nanocomposites exhibit differences compared to epitaxial QDIPs. As is shown in Fig. 13 for the interband device, the bipolar, interband (or excitonic) transitions across the colloidal QD bandgap contribute to the photoresponse of the detector. In addition, since CQDs are electron acceptors and the polymers are typically hole conductors, the photogenerated excitons are dissociated at the QD/polymer interface. Thus, photoconduction through the nanocomposite occurs as electrons hop among QDs and holes transport through the polymer [17]. This incoherent hopping between nanocrystals results in carrier mobilities four to six orders of magnitude lower in QDs than bulk crystals [64]. Progress in doping of nanoparticles has open the way for the use of intraband absorption. However, the intraband devices still suffer from three main limitations: too large dark current, small thermal activation energy, and slow photoresponse. Nevertheless, the intraband transitions are attractive possibility for longer-­ wavelength device availability. Usually, the broadband spectral absorption originated from the interband optical transitions, while narrowband responsivity resulted from intraband transitions. Ramiro et al. [65] have demonstrated intraband absorption and photodetection in heavily doped PbS colloidal quantum dots in the 5–9-μm range, beyond the PbS bulk bandgap. Also intraband transitions have been demonstrated in a mixture of HgSe and HgTe nanocrystals [66]. For example, Fig. 14 presents the size-dependent transition energy of HgTe and HgSe CQDs [61]. The experimental data were measured from optical absorbance. The theoretical lines are calculated using the size-­ dependent interband energy gap according to the two-band Kronig-Penney (k·p) model [67]:



Einter 

Eg 2



Eg 2

 Ep

 2 2 2 mo R 2

(4)

where Eg is the energy gap, Ep is the Kane parameter (~ 20 eV), mo is the free electron mass, and R is the dot radius for the energy of the first transition. Similar estimations have been presented for intraband transitions [61].

16

A. Rogalski Intraband transition

Interband transition hν = Eintra

hν > Einter

Conduction band

Conduction band Einter

Fermi level

Fermi level

Valence band

Eintra

1se to 1pe Valence band

Fig. 11  Illustration of interband and intraband transition processes in colloidal quantum dots n-type contact (Al) CQD/Polymer Nanocomposite

Cathode

Anode

p-type contact (ITO) CQD film

Glass substrate

(a)

(b)

Fig. 12  Colloidal quantum dot photodetector: (a) schematic diagram of device heterostructure in CQD/conducting polymer nanocomposites; (b) an SEM image of a PV QD detector with transport illustration of photogenerated charge LUMO

MEH-PPV

MEH-PPV PbS

hν

HOMO

-

+

MEH-PPV PbS

Ec Ev

Fig. 13  Schematic diagram of energy vs. position for interband transitions in PbS/MEH-PPV colloidal QD-conducting polymer nanocomposites demonstrating photocurrent generation for IR photodetection. (After Ref. [12])

Infrared CQDs are sensitive to moisture and influence of higher temperature. From these reasons, the core-shell structures and polymer matrices require protections to inhibit charge collection and reduce efficiency. It appears that polymer encapsulations often block infrared radiation. Therefore, alternative protection methods are required. It is well established that A2B6 (like HgTe) and A4B6 (like PbS) semiconductors, especially with higher atomic number electrons, due to contribution of ionic bonds

Progress in Quantum Dot Infrared Photodetectors Fig. 14  Interband energy gaps and intraband energy gaps for HgTe and HgSe CQDs, respectively

17

0.6 k· p theory (interbad) k· p theory (intraband)

Energy gap (eV)

0.5

0.4

0.3

0.3 Experimental results HgTe CQDs HgSe CQDs

0.2

0.0

0

2

4

6

8 10 12 14 Diameter (nm)

16

18

20

have weaker bonds (are less stable) than A3B5 materials with strobe covalent bonds. Unfortunately, A3B5 CQDs are less developed than II-VI and IV-VI counterparts.

3  Types of Photodetectors This section gives an overview of emerging CQD-based photodetector performance and comparison with traditionally and commercially available ones in different applications in high operating temperature conditions. Photodetectors are related to the excitation of free carriers as a result of optical transition, including the photoconductive effect, photovoltaic effect, and phototransistor effect.

3.1  Photoconductors Schematic operations of two most popular photodetectors are shown in Fig. 15. The photoconductive detector is essentially a radiation-sensitive resistor with two metal contacts. A photon of energy greater than the bandgap energy is absorbed to produce electron-hole (e-h) pairs, thereby changing the electrical conductivity of the material. The generated e-h pairs are separated by the external electric field, generating a photocurrent.

18

A. Rogalski

VDS

Top contact n-type material

VS

-+ Ohmic contact Photoactive medium i.e. QDs

Ohmic contact

Substrate i.e. glass

VS

-

+ VDS

-

p-type material i.e. CQDs + Transparent contact i.e. ITO Substrate i.e. glass

hν

EF

+

Current

-

+

-

-

+

+

hν

-

-

+

EF + + +

-

p type

Idark Iphoto

+

-

-

n type

p-n junction Current

Illuminated Photocurrent

Dark current

Dark current

Voltage

Photocurrent

Voltage

Illuminated

(a)

(b)

Fig. 15  Schematic of the photoconductive (a) and photovoltaic (b) effects. At constant applied voltage, the electrons and holes traverse to respective electrodes at different speeds, what is shown by the different lengths of arrows in the top figures (electrons are traveled faster)

Assuming that the signal photon flux density Фs(λ) is incident on the detector area A = wl (w, width; l, length), the basic expression describing photoconductivity in semiconductors under equilibrium excitation (i.e., steady state) is

I ph  q A s g,



(5)

where q is the electron charge and Iph is the short circuit photocurrent at zero frequency, that is, the increase in current above the dark current accompanying irradiation. The quantum efficiency, η, can be defined as the number of electron-hole pairs generated per incident photon and describes how well the detector is coupled to the impinging radiation. The second parameter, the photoconductive gain, g, is determined by the properties of the detector (i.e., by which detection effect is used and the material and configuration of the detector) and can be defined as the number of carriers passing contacts per one generated pair. The value of g describes how well

Progress in Quantum Dot Infrared Photodetectors

19

the generated charge carriers are used to generate current response of a photodetector. In general, photoconductivity is a two-carrier phenomenon, and the total photocurrent of electrons and holes is



I ph =

qwt ( ∆nµ e + ∆pµ e ) Vb l



(6)

where μe is the electron mobility, μh is the hole mobility, and Vb is the bias voltage and

n  no  n; p  po  p,

(7)

no and po are the average thermal equilibrium carrier densities, and ∆n and ∆p are the excess carrier concentrations. Taking the conductivity to be dominated by electrons (in all known high-­ sensitivity photoconductors, this is found to be the case) and assuming uniform and complete absorption of the light in the detector, it can be shown that [68]



g

 . l 2 / eVb

(8)

So, the photoconductive gain can be defined as g



 , tt

(9)

where tt is the transit time of electrons between ohmic contacts. This means that the photoconductive gain is given by the ratio of free carrier lifetime, τ, to transit time, tt, between the sample electrodes. The photoconductive gain can be less than or greater than unity depending upon whether the drift length, Ld = vdτ, is less than or greater than interelectrode spacing, l. The value of Ld > l implies that a free charge carrier swept out at one electrode is immediately replaced by injection of an equivalent free charge carrier at the opposite electrode. Thus, a free charge carrier will continue to circulate until recombination takes place. Taking into account Eqs. (5) and (9), the photocurrent



I ph 

q A seVb l2

(10)

is linearly dependent on the photon flux density (i.e., excitation power), the photogenerated carrier lifetime, the electron mobility, and the applied bias. The current responsivity of the photodetector is equal to



Ri 

 qg, hc

(11)

20

A. Rogalski

where λ is the wavelength, h is the Planck constant, and c is the velocity of light.

3.2  Photovoltaic Detectors Photovoltaic (PV) photocurrent generation is based on the separation of electrical carriers by built-in electric field at p-n junction or Schottky barrier (Fig. 16). The green shaded region at the top Fig. 15b is the depletion region, which can be tuned by bias voltage. By diffusion, the electrons and holes generated within a diffusion length from the junction reach the space charge region. To achieve high quantum efficiency, carriers generated outside of the depletion region must avoid recombination while they diffuse to either depletion region or contact. The generated photocurrent shifts the current-voltage characteristics as is shown in the bottom of Fig. 15b. In the case of the photodiode, the photoelectric gain is usually close to 1, so according to Eq. (11), the current responsivity is Ri 



q , hc

(12)

and detectivity can be determined as [68]   q  4 kT D   2q 2 b   hc  Ro A  



1/ 2

,

(13)

where Ro is the dark resistance of the diode at zero bias and Φb is the background flux density. For the last formula, we may distinguish two important cases: • Background-limited performance; if 4kT/RoA  > 2q2ηФb, then 1/ 2

 q  Ro A  D  . 2hc  kT  



(14)

The detectivity in the absence of background photon flux can be also expressed as D 

  q  A   hc  2q  I d  2 I s  

1/ 2

,

(15)

where Id is the dark current and Is is the saturation current. In reverse biases, Id tends to –Is and the expression in brackets tends to Is. Photodiodes are usually operated at zero bias (photovoltaic mode) or under reverse bias (photoconductive mode). The absolute response of photodiode is usually smaller than a photodetector working with the photoconducting or photogating mechanisms, since there is no internal gain. Under reverse-bias operation, the junction capacity is reduced increasing the speed of the photodiode. Strong reverse bias can initiate impact ionization multiplication of carriers, or avalanching (avalanche photodiode). The large internal gain results in detection of extremely low signal power.

3.3  Photogating Effect A particular example of the photoconductive effect is photogating. The photogating effect can be realized in two ways by: • Generation of e-h pairs, when one type of carriers is trapped by the localized states (nanoparticles and defects) • Generation of e-h pairs in trap states and one type of carriers is transferred to 2D materials, whereas the other resides at the same place to modulate the layered materials In both cases, due to long carrier lifetime, the enhancement of sensitivity is at the cost of photoresponse speed. If holes/electrons are trapped in localized states (see Fig. 17), they act as a local gate, effectively modulating the resistance of active materials. In this case, the photocarriers are only limited by the recombination lifetime of the localized trap states, leading to a large photoconductive gain, g. The trap states where carriers can reside for long times are usually located at defects or at the surface of the semiconducting material. This effect is of particular importance for nanostructured materials, like colloidal quantum dots, nanowires, and 2D semiconductors, where the large surface and reduced screening play a major role in the electrical properties.

22

A. Rogalski hν

EF

-

-

-

+

+

-

-

+

Idark Iphoto

Fig. 17  Band alignment under illumination with photons of energy higher than the bandgap generating e-h pairs. Holes are trapped at the band edge and act as a local gate. In consequence, the field effect induces more electrons in the channel, generating a photocurrent which adds to the dark current. If the electron lifetime exceeds the time it takes for the electron to transit device, then the long time of the trapped holes ensures the electrons can circulate through an external circuit many times, resulting in gain

Strong photogating effect is observed in phototransistor. The operation of this device is similar to photoconductor; however its fabrication on a substrate enables applying a gate voltage to tune carrier transport in the active area  – see Fig.  18. Phototransistors basically have the same three-terminal configuration as field-effect transistors (FETs). While in the operational mode of a normal FET, the amount of current flowing (the drain current, ID) in the accumulated channel is controlled by the magnitude of gate voltage (VG) at a given source to drain bias (VDS), for phototransistors, the control of channel conductance can be additionally enabled by the absorption of light. Especially, phototransistor has become a preferred architecture of the hybrid QD-two-dimensional (2D) photodetector due to enhanced modulation of carrier dynamics and concentrations in comparison with standard photoconductor. In the case of the photodiode, the photoelectric effect is usually equal to 1, due to separation of minority carriers by the electrical field of depletion region. However in a hybrid combination of two material photodetectors, photosensitization and carrier transport take place in separately optimized regions: one for efficient light absorption (e.g., colloidal quantum dots) and the second (e.g., in two-dimensional materials like graphene) to provide fast charge reticulation. In this way, ultrahigh gain up to 108 electrons per photon and exceptional responsivities for short-­ wavelength infrared photodetectors have been demonstrated [41, 69]. The simple architecture of hybrid phototransistor, very popular in design of 2D material photodetectors with the fast transfer channel for charge carriers, is shown in Fig.  19. Since, e.g., the graphene in these devices is not responsible for light absorption but only the sensing of charge, absorber choice determines the spectral response. The graphene’s large ambipolar mobility (~103–105  cm2/Vs) acts as a

Progress in Quantum Dot Infrared Photodetectors

23

built-in photogain (i.e., amplifier) mechanism enhancing the detector response. It should be mentioned that the transistor introduces a threshold, so that turn-on occurs only after a certain photocarrier density (hence also power of light) is exceeded. 2D materials with thickness down to atomic layer are more susceptible to local electric fields than conventional bulk materials, and the photogating effect can strongly modulate the channel conductivity by external gate voltage, VG. Improving the optical gain is particularly important since the quantum efficiency is limited because of the weak absorption in 2D materials. This effect is especially seen in longer-wavelength IR spectral region, where the light absorption is weak. In the case of hybrid detector shown in Fig. 19a, the holes are injected into transporting channel, whereas the electrons remain in the photoactive layer. The injected charges

VDS

VS

Ohmic contact

Photoactive medium i.e. QDs

Ohmic contact

Dielectric layer i.e. SiO2

VG

Gate i.e. n-doped Si

Fig. 18  Schematic cross section of phototransistor

(a) Hybrid phototransistor

(c) Photoconductive gain

VDS

Source

Drain Drain VGS

Photoactive layer Source 2D material Insulator

(d) I-VG trace shift under illumination

(b) Closed channel under illumination Drain

Source

Drain current, IDS

Gate

Iph=gm·∆VG Ipg+Ipc

A

Ipg

B

Idark ∆VG Gate voltage, VG

Fig. 19  Photogating effect in 2D material photodetectors: (a) the operation of hybrid phototransistor, (b) closed channel under illumination, (c) photoconductive gain, and (d) I-VG trace under illumination

24

A. Rogalski

can reticulate even several thousand times before recombination, giving contribution in gain under illumination. The photocarrier lifetime is enhanced through both the bandgap structure and defect engineering, and at the same time the trapping mechanisms limit the response time of photodetector even to several seconds. There is a trade-off between the enhancement of sensitivity and photoresponse speed. The photocurrent change by photogating effect can be written as [70, 71]

I ph  gm VG ,

(16)

where gm is the transconductance and ΔVG is the equivalent photoinduced voltage. Figure 19d indicates a shift of the IDS(VG) trace after the light illumination. Generally, both positive and negative photoconductance behaviors are observed in hybrid 2D structures, and working points A and B, related to gm and ΔVG, perform opposite directions.

4  Self-Assembled Quantum Dot Infrared Detectors The quantum-mechanical nature of QDIPs leads to several advantages over quantum well infrared photodetectors (QWIPs) and other types of IR detectors that are available. As in the HgCdTe, QWIP, and type II superlattice technologies, QDIPs provide multiwavelength detection. However, QDs provide many additional parameters for tuning the energy spacing between energy levels, such as QD size and shape, strain, and material composition. The potential advantages in using QDIPs over quantum wells are as follows: • Intersubband absorption may be allowed at normal incidence (for n-type material). In QWIPs, only transitions polarized perpendicularly to the growth direction are allowed, due to absorption selection rules. The selection rules in QDIPs are inherently different and normal incidence absorption is observed. • Thermal generation of electrons is significantly reduced due to the energy quantization in all three dimensions. As a result, the electron relaxation time from excited states increases due to phonon bottleneck. Generation by LO phonons is prohibited unless the gap between the discrete energy levels equals exactly to that of the phonon. This prohibition does not apply to quantum wells, since the levels are quantized only in the growth direction and a continuum exists in the other two directions (hence generation-recombination (g-r) by LO phonons with capture time of a few picoseconds). Thus, it is expected that signal-to-noise (S/N) ratio in QDIPs will be significantly larger than that of QWIPs. • Lower dark current of QDIPs is expected than of QWIPs due to 3D quantum confinement of the electron wave function. Both the increased electron lifetime and the reduced dark current indicate that QDIPs should be able to provide high-temperature operation. In practice, however, it has been a challenge to meet all of the above expectations.

Progress in Quantum Dot Infrared Photodetectors

25

Carrier relaxation times in QDs are longer than the typical 1–10 ps measured for quantum wells. It is predicted that the carrier relaxation time in QDs is limited by electron-hole scattering [72], rather than phonon scattering. For QDIPs, the lifetime is expected to be even larger, greater than 1 ns, since the QDIPs are majority carrier devices due to absence of holes. The main disadvantage of the QDIP is the large inhomogeneous linewidth of the quantum dot ensemble variation of dot size in the Stranski-Krastanov growth mode [73, 74]. As a result, the absorption coefficient is reduced, since it is inversely proportional to the ensemble linewidth. Large, inhomogeneously broadened linewidth has a deleterious effect on QDIP performance. Subsequently, the quantum efficiency QD devices tend to be lower than what is predicted theoretically. Vertical coupling of quantum dot layers also reduces the inhomogeneous linewidth of the quantum dot ensemble; however, it may also increase the dark current of the device, since carriers can tunnel through adjacent dot layers more easily. As in other types of detectors, nonuniform dopant incorporation adversely affects the performance of the QDIP.  Therefore, improving QD uniformity is a key issue in the increasing absorption coefficient and improving the performance. Thus, the growth and design of unique QD heterostructure is one of the most important issues related to achievement of state-of-the-art QDIP performance.

4.1  QDIP Model In further considerations a QDIP model developed by Ryzhii et al. is adapted [75, 76]. The QDIP consists of a stack of QD layers separated by wide-gap material layers (see Fig. 20). Each QD layer includes periodically distributed identical QDs with the density ∑QD and sheet density of doping donors equal to ∑D. Simple estimation indicates that the quantum-dot density is equal δ = 1/s2, where s is the interdot spacing. In the realistic QDIPs, the lateral size of QDs, aQD, is sufficiently large in Infrared radiation

GaAs k-th QD layer

Emitter

(k+1)-th QD layer

Capture

φB

Field-assisted tunneling GaAs

∆E

L s

Thermionic emission

Quantum dots

Collector

(a)

InAs QD

(b)

Fig. 20  Schematic view of (a) the quantum dot structure and (b) conduction band structure of the dot

26

A. Rogalski

comparison with the transverse size, hQD. Consequently, only two energy levels associated with the quantization in the transverse direction exist. Sufficiently large lateral size, lQD, causes a large number of bound states in dots and, consequently, is capable of accepting a large number of electrons, whereas the transverse size is small in comparison with the spacing between the QD layers, L. The lateral spacing between

QDs is equal to LQD    QD  . The average number of electrons in a QD belong1/ 2

ing to the k-th QD layer, , can be indicated by a solitary QD layer index (k = 1, 2, ..., K, where K is the total number of the QD layers). The QDIP active region (the stack of QD arrays) is sandwiched between two heavily doped regions that serve as the emitter and collector contacts. Due to the discrete nature of QDs, the fill factor F should be included for optical absorption in QDs. This factor can be estimated in a simple way as F=



3

V , s

(17)

where V is the quantum dot volume. For self-assembled QDs, a Gaussian distribution has been observed for the electronic and optical spectra. Phillips modeled the absorption spectra for an ensemble of QDs using a Gaussian line shape [73]



  E  E 2  n1  QD g ,   E   o exp   2     ens  ens  

(18)

where αo is the maximum absorption coefficient, n1 is the areal density of electrons in the quantum dot ground state, δ is the quantum dot density, and Eg = E2 – E1 is the energy of the optical transition between ground and excited states in the QDs. It should be noticed that Eq. (18) estimates the absorption coefficient for the necessary presence of electrons in the QD ground state. The optical absorption between the ground and excited levels is found to have a value [77]



o 

3.5  10 5 , in cm 1 

(19)

where σ is the linewidth of the transition in meV. Equation (19) indicates the tradeoff between the absorption coefficient and the absorption linewidth, σ. The expressions σQD and σens are the standard deviations in the Gaussian line shape for intraband absorption in a single quantum dot and for the distribution in energies for the QD ensemble, respectively. The terms n1/δ and σQD/σens describe a decrease in absorption due to absence of available electrons in the QD ground state and inhomogeneous broadening, respectively.

27

Progress in Quantum Dot Infrared Photodetectors Table 3  Typical parameter values of QDIP fabricated from GaAs or InGaAs aQD 10–40 nm

h 4–8 nm

ΣQD (1–10)1010 cm−2

ΣD (0.3–0.6)ΣQD

L 40–100 nm

K NQD 10–70 8

Table 3 contains the reference values of QD parameters. These values are considered for a QDIP fabricated from GaAs or InGaAs. The self-assembled dots formed by epitaxial growth are typically pyramidal to lens shaped with a base dimension of 10–20 nm and height of 4–8 nm with an areal density determined to be 5 × 1010 cm−2 using atomic force microscopy. Similar with QWIP, the main mechanism producing the dark current in the QDIP device is the thermionic emission of the electrons confined in the QDs. The dark current can be given by

J dark = qvn3 D ,

(20)

where υ is the drift velocity and n3D is the three-dimensional density, both for electrons in the barrier [78]. Equation (20) neglects the diffusion contribution. The electron density can be estimated by



 m kT  n3 D  2  b 2   2  

3/ 2

 E exp   a  kT

 , 

(21)

where mb is the barrier effective mass and Ea is the activation energy, which equals the energy difference between the top of the barrier and the Fermi level in the dot. At higher operating temperature and larger bias voltage, the contribution of field-­ assisted tunneling through a triangular potential barrier is considerable [79, 80]. Figure 21 shows, for example, the normalized dark current versus bias for temperature range 20–300 K for QDIP with AlGaAs confinement layers below the QD layer and on top of the GaAs cap layers [9]. In such a case, we have the InAs islands into quantum wells and AlGaAs blocking layers effectively improve the dark current and detectivity. As it is shown, at low temperature (e.g., 20 K), the dark current increased rapidly as the bias was increased, which is attributed to electron tunneling between the QDs. For higher bias |0.2|  ≤  Vbias  ≤  |1.0|, the dark current increases slowly. With further increase in bias, the dark current strongly increases, which was largely due to lowering of the potential barriers. Figure 21 also shows the photocurrent induced by the room-temperature background. It is clear that BLIP temperature varies with bias. In InAs/GaAs QDIPs, the photoconductive gain has typical values in the 1–5. However, the gain strongly depends on QDIP design and detector polarization. Much higher values, up to several thousands, have been observed [8, 74]. The higher gain of the QDIPs in comparison with QWIPs (typically in the range 0.1–50 for similar electric field intensities) is the result of longer carrier lifetimes. The larger photoconductive gain has influence on higher current responsivity.

28

A. Rogalski 103 λp = 6.2 µm

Dark current density (A/cm2)

101 10–1 10–3 10–5 10–7 10

–9

10–11

20K 40K 60K 77K 90K 110K 130K 150K 170K 190K 220K 250K 296K

–2

Background illumination at 20K

–1

0 Bias voltage (V)

1

2

Fig. 21  Dark current density of QDIP with AlGaAs blocking layer including photocurrent induced by room-temperature background. (After Ref. [9])

The photoconductive gain and the noise gain in conventional photoconductive detector are equal to each other. It is not the same in QDIPs since these devices are not homogeneous, nor are they bipolar devices. The photoconductive gain in QWIPs is expressed in terms of the capture probability pc as [81, 82]



g ph 

1  pc / 2 , Npc

(22)

where pc   >  βkT/q should be fulfilled (Rd is the dynamic impedance of the detector and β is an ideality factor usually in the range 1–2). Generally, it is not a problem to fulfil this inequality for SWIR and MWIR FPAs where the dynamic resistance of detector Rd is large, but it is very important for LWIR designs where Rd is low. There are more complex injection circuits that effectively reduce the input impedance and allow lower detector resistance to be used. The above requirement is especially critical for near-room-temperature HgCdTe photodetectors operating in LWIR region. Their resistance is very low due to a high thermal generation. In materials with a high electron to hole ration as HgCdTe, the resistance is additionally reduced by ambipolar effects. Small-size uncooled 10.6-­ μm photodiodes (50 × 50 μm2) exhibit less than 1 Ω zero bias junction resistances that are well below the series resistance of a diode [97, 98]. As a result, the

Progress in Quantum Dot Infrared Photodetectors

35

performance of conventional devices is very poor, so they are not usable for practical applications. The saturation current for 10-μm photodiode achieves 1000 A/cm2, and it is by four orders of magnitude larger than the photocurrent due to the 300-K background radiation. The potential advantage of QDIPs is considerably lower dark current and higher RoA product in comparison with HgCdTe photodiodes (see Fig. 28) [98]. Figure 29 compares the calculated thermal detectivity of HgCdTe photodiodes and QDIPs as a function of wavelength and operating temperature with the experimental data of uncooled HgCdTe and type II InAs/GaInSb SL detectors. The Auger mechanism is likely to impose fundamental limitations to the LWIR HgCdTe detector performance. The calculations have been performed for optimized doping concentration p  =  γ1/2ni. The experimental data for QDIPs are gathered from the literature for detectors operated at 200 and 300 K. Uncooled LWIR HgCdTe photodetectors are commercially available and manufactured in significant quantities, mostly as single-element devices [99]. They have found important applications in IR systems that require fast response. The results presented in Fig. 29 confirm that the type II superlattice is a good candidate for IR detectors operating in the spectral range from the midwavelength to the very long-­ wavelength IR. However, comparison of QDIP performance both with HgCdTe and type II superlattice detectors gives evidence that the QDIP is suitable for high temperature. Especially encouraging results have been achieved for very long-­ wavelength QDIP devices with a double-barrier resonant tunneling filter with each quantum dot layer in the absorption region [100]. In this type of device, photoelectrons are selectively collected from the QDs by resonant tunneling, while the same tunnel barriers block electrons of dark current due to their broad energy distribution. For the 17-μm detector, a peak detectivity of 8.5 × 106 cmHz1/2/W has been measured. Thermal detectors seem to be unsuitable for the next generation of IR thermal imaging systems, which are moving toward faster frame rates and multispectral operation. A response time much shorter than that achievable with thermal detectors is required for many nonimaging applications. Improvement in technology and 103

HgCdTe photodiodes QDIPs

Q

Ds

101

T=200K RoA (300K,FOV=2π)

RoA (Ωcm2)

Fig. 28  RoA product of HgCdTe photodiodes and QDIPs as a function of wavelength. The calculations for HgCdTe photodiodes have been performed for the optimized doping concentration p = γ1/2ni, where    Ai 7 /  Ai1 and ni is the intrinsic carrier concentration

10–1

Hg

Cd

10–3 10

Te

T=250K

T=300K

–5

10–7

5

10 15 Cutoff wavelength (µm)

20

36

A. Rogalski Uncooled: Type II PC on GaAs (NWU) Type II PV on GaSb optically immersed (NWU) T=200K HgCdTe PC optically immersed (VIGO)

1013

D*(cmHz1/2/W)

1012

HgCdTe photodiodes QDIPs T=250K

1011

DB*(300K,FOV=2π)

HgCdTe, 300K optcal immersion

1010

T=300K

109

QDIPs: at 200K (Ref. 87) at 200K (Ref. 96) at 200K (Ref. 86) at 300K (Ref. 87) at 300K (Ref. 96)

108 107

5

10 15 Cutoff wavelength (µm)

20

Fig. 29  Calculated performance of Auger generation-recombination-limited HgCdTe photodetectors as a function of wavelength and operating temperature. BLIP detectivity has been calculated for 2π FOV, the background temperature is TBLIP = 300 K, and the quantum efficiency η = 1. The calculations for HgCdTe photodiodes have been performed for the optimized doping concentration p = γ1/2ni. The experimental data is taken for commercially available uncooled HgCdTe photoconductors (produced by VIGO System) and uncooled type II detectors at the Center for Quantum Devices, Northwestern University (Evanston, Illinois). The experimental data for QDIPs are gathered from the marked literature for detectors operated at 200 and 300 K

design of QDIP detectors make it possible to achieve both high sensitivity and fast response at room temperature.

4.3  Multiband QD Photodetectors QDIP devices capable of detecting several separate wavelengths can be fabricated by vertical stacking of the different QWIP layers during epitaxial growth. The schematic structure is shown in Fig. 30. In the case of structure describing by Lu et al. [101], each of QDIP absorption band consists of ten periods of InAs/InGaAs QD layers sandwiched between the top and bottom electrodes. Figure  31 shows the simplified band diagram of this structure at different bias levels. The bias voltage selection of detection bands originates from the asymmetric band structure. At low bias voltage, the high-energy GaAs barrier blocks the photocurrent generated by LWIR radiation and only responds to the MWIR incidence. On the contrary, as the bias voltage increases, the barrier energy decreases, allowing LWIR signals to be detected at different bias voltage levels.

Progress in Quantum Dot Infrared Photodetectors

37

n+GaAs contacting layer GaAs GaAs InAs QDs GaAs InGaAs InAs QDs GaAs

MWIR LWIR

x10 x10

n+GaAs contacting layer Semiinsulating GaAs substrate

Fig. 30  Schematic structure of the multispectral QDIP device. (After Ref. [101]) Tunneling

Blocked

-

MWIR GaAs barriers

LWIR -

InGaAs barriers

InAs QDs

InAs QDs

(a)

Tunneling LWIR InGaAs barriers InAs QDs

Tunneling

-

MWIR GaAs barriers

InAs QDs

(b)

Fig. 31  Simplified band diagram of the structure shown in Fig. 30 at different bias levels: (a) low and (b) higher bias voltages. (After Ref. [101])

The first two-color quantum dot FPA demonstration was based on a voltage-­ tunable InAs/InGaAs/GaAs DWELL structure [102, 103]. As is described in Sect. 2.2, in this type of structure, InAs QDs are placed in an InGaAs well, which in turn is placed in a GaAs matrix (see Fig. 6). Figure 32 shows the multicolor response from a DWELL detector [32]. This device has demonstrated multicolor response ranging from the MWIR (3–5  μm) based on a bound-to-continuum transition to the LWIR (8–12 μm), which is based on a bound state in the dot to a bound state in the well. A very long-wavelength infrared (VLWIR) response has also been observed and has been attributed to transitions between two bound states in the QDs, since the calculated energy spacing between the dot levels is about 50–60 meV. Moreover, by adjusting the voltage bias of the device, it is possible to modify the ratio of electrons promoted by MWIR, LWIR, and VLWIR absorptions. Typically the MWIR response dominates at low to nominal voltages due to higher escape probability. With increasing voltage, the LWIR and eventually VLWIR responses are enhanced due to the increased tunneling probability of lower states in the DWELL detector (see Fig. 33) [15]. The bias-­dependent shift of the spectral response is observed due to quantum-confined Stark effect. This voltage control of spectral response can be exploited to realize spectrally smart sensors whose wavelength and bandwidth can be tuned depending on the desired application [32, 103–106].

38

Fig. 33  Spectral response from a DWELL detector with response at Vb = + /−1 V and + /−2 V. Note the response in the two MWIR and LWIR bands can be measured using this detector. The relative intensities of the bands can be altered by the applied bias. (After Ref. [15])

0.030

Vb= – 1.4V T = 4.6K

Responsivity (V/W)

0.025 InAs QD

0.020 0.015

InAs QD

0.010

InAs QD

0.005 0

5

10

15 20 25 Wavelength (µm)

0.8

Photoresponse (a.u.)

Fig. 32 Multicolor response from an InAs/ In0.15Ga0.85As/GaAs DWELL detector. The MWIR (LWIR) peak is possibly a transition from a state in the dot to a higher (lower) lying state in the well, whereas the VLWIR response is possibly from two quantum-confined levels within the QD. This response is visible to 80 K. (After Ref. [32])

A. Rogalski

35

LWIR2

+1 V -1 V +2 V -2 V

0.6

30

LWIR1

0.4 MWIR2 MWIR1 0.2

0.0

2

3

4

5

6 7 8 9 10 11 12 Wavelength (µm)

13 14

Typically, the detector structure consists of a 15-stack asymmetric DWELL structure sandwiched between two highly doped n-GaAs contact layers. The DWELL region consists of a 2.2 ML of n-doped InAs QDs in an In0.15Ga0.85As well, itself placed within a GaAs matrix. By varying the width of the bottom InGaAs well from 10 to 60 Å, the operating wavelength of the detector can be changed from 7.2 to 11 μm. The responsivity and detectivity obtained from the test devices at 78 K are shown in Fig.  34 [95]. The measured detectivities were 2.6  ×  1010 cmHz1/2/W (Vb  =  2.6  V) for the LWIR band and 7.1  ×  1010 cmHz1/2/W (Vb  =  1  V) for the MWIR band.

4.4  Focal Plane Arrays Since the early 1990s, fully 2D infrared imaging staring arrays have been entered the production stage. Depending on the array architecture, the process can include over 150 individual fabrication steps. The prevailing hybridization process involves

39

Progress in Quantum Dot Infrared Photodetectors Fig. 34  Peak responsivity for a 15-stack DWELL detector at 78 K obtained using a calibrated blackbody source. Solid squares, MWIR responsivity; solid triangles, LWIR responsivity; open square, MWIR detectivity; open triangles, LWIR detectivity. (After Ref. [95])

Reflection grating

Indium bump

Top contact

Si3N4 dielectric insulation Au metal

QDIP active region

Detector common contact

Selective etch layer Stacked QD layers Infrared radiation

Thinned GaAs substrate Infrared radiation

Fig. 35  Cross section of a detector element in a QDIP array

flip-chip indium bonding between the “top” surfaces of the ROIC and detector array. This technology was adopted to develop quantum dot infrared arrays in the first decade of the twenty-first century. Figure 35 shows the cross-sectional view of an array’s pixel. The most spectacular achievement was demonstration by Gunapala group’s at Jet Propulsion Laboratory (JPL) [105, 106]. This group has fabricated the first long-­ wavelength 640 × 512 pixel QDIP focal plane array (FPA) using DWELL structures with 25-μm pitch. The InAs QDs were grown in the center of a 75-Å In0.12Ga0.88As QW with the ground-state electrons provided by doping the InAs with Si to a density of 5  ×  1017  cm−3. The QWs were separated by 500  Å of undoped GaAs. To obtain higher quantum efficiency, the over number of photosensitive DWELL was increased to 30 stacks. The active DWELL structure grown on semi-insulating 75-mm GaAs substrate is sandwiched between 0.66-μm GaAs top and 0.5-μm bottom contact layers. The array is thinned down to a membrane, what results in quite large indium bump hybrids since the thermal mismatch issues between detector array and silicon are eliminated. In consequence, the operability of hybrid interconnect is high. The DWELL structure absorbs both 45° and normal incident light, and a reflection grating enhanced almost four times the normal quantum efficiency than the one

40

A. Rogalski

without (see Fig.  36a). The array pixels exhibit peak responsivity about 8.5  μm, with peak detectivity 1010 Jones at 77 K. Figure 36b shows the experimentally measured NEDT histogram at an operating temperature of 60  K, bias voltage of −350 mV at 300-K background with f/2 optics. The mean NEDT value is 40 mK (after Ref. [105]). The uncorrected nonuniformity of the 640 × 512 pixels FPA is about 20% (sigma/mean). Figure 37 shows an image taken with QDIP camera at a frame rate of 30 Hz and using a ROIC capacitor having a maximum charge capacity of 11 × 106 electrons (the maximum number of photoelectrons and dark electrons that can be gathered during the integration time of detector pixel). Each detector array is diced for hybridization with an ISC-9803 ROIC (FLIR Indigo Systems). The above presented performance of DWELL-QDIP array lags behind that for the state-of-the-art QWIPs and standard HgCdTe and type II SL photodiodes. The detectivity/NEDT is still limited by the high dark current and the relatively low quantum efficiency. Further performance improvement is possible by optimizing the device structure to decrease the dark current and to increase the dot density or the number of dot stacks. At present stage of infrared detector development, there is an increased emphasis on large-area FPAs with multicolor function and higher operating temperature [107]. Vertical stacking of different QDIP structures during epitaxial growth enables fabrication of multicolor detectors. Varley et  al. [108] have demonstrated a two-­ color, MWIR/LWIR, 320 × 256 FPA based on DWELL detectors. Minimum noise equivalent difference temperature (NEDT) values of 55 mK (MWIR) and 70 mK (LWIR) were measured at 77 K (see Fig. 38). Figure 39 presents images obtained in the MWIR and LWIR using the two-color DWELL camera operated at 60 K.

4

2.5x10

DWELL-QDIP Normal incident

0.60

Number of pixels

Responsivity (A/W)

0.80

With grating (unthined)

0.40 No grating 0.20 0 5

2.0x104 1.5x104 1.0x104 0.5x104

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(a)

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40 60 80 100 120 140 160 180 200 NEDT (mK)

(b)

Fig. 36  640  ×  512 pixel QDIP FPA: (a) normal incidence spectral responsivity of a DWELL-­ QDIP with and without reflection gratings; (b) the experimentally measured NEDT histogram at an operating temperature of 60 K, bias voltage of −350 mV at 300-K background with f/2 optics. The mean NEDT value is 40 mK. (After Ref. [105])

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41

Fig. 37  Image taken with the first 640 × 512 pixel QDIP LWIR FPA imaging system with an f/2-coated germanium optical assembly. (After Ref. [105])

0.30

MWIR LWIR

0.20 NEDT (K)

Fig. 38  NEDT in the MWIR and LWIR bands at 77 K. Irradiance levels for MWIR and LWIR are 3–5 μm (f/2) and 8–12 μm (f/2.3), respectively. (After Ref. [108])

0.10

0.00 1015

1016

1017

1018

Irradiance (photons/cm2s)

Fig. 39  Images obtained in the MWIR and LWIR and observed at 60  K using the two-color DWELL camera. (After Ref. [108])

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5  Colloidal Quantum Dot Infrared Detectors HgCdTe has inspired the development of the three “generations” of detectors considered for the foremost military and civilian applications [68]. The further development will be connected with implementation of fourth-generation staring systems, in which the main features are to be high resolution (with a very large number of pixels – above 108), multicolor functionality, three-dimensional readout integration circuits (3D ROICs), and other integration functions, e.g., better radiation/pixel coupling, avalanche multiplication in pixels, and polarization/phase sensitivity. The first three generations of the imaging systems rely primarily on the planar FPAs. Several methods to overcome these limitations, including the detectors bonding to flexible or curved molds, have been proposed [109]. Evolution of fourth generation is inspired by the most famous visual systems, which are the biological eyes. The solution based on the Petzval-matched curvature allows the reduction of field curvature aberration. In addition, it combines such advantages as simplified lens system, electronic eye systems, and wide field of view [110, 111]. Theoretical estimates carried out by Martyniuk et al. [98] in 2008 indicate that the self-assembled quantum dot infrared photodetectors (QDIPs) are suitable for noncryogenic operation especially in long-wavelength infrared (LWIR) region. In practice however, the reduced performance of QDIP is the result of nonoptimal band structure and technological problems such as QD size and density control. More recently, the attractive alternative to self-assembled epitaxial QDs has been CQDs with better size tunability of optical features and reduction of fabrication cost. CQDs have attracted attention as a candidate material for a range of optoelectronic applications including light-emitting diodes, lasers, optical modulators, solar cells, and photodetectors. The first mass market for these nanoparticles appeared a couple year ago as they started to be as phosphors for TV display [112].

5.1  General Overview In the last decade, a significant progress in fabrication of CQD photodetectors has been observed. In this approach, an active region is constructed based on 3D quantum-­confined semiconductor nanoparticles synthesized by inorganic chemistry. CQDs offer a promising alternative to the single-crystal IR materials (InGaAs, InSb, InAsSb, HgCDTe, as well as T2SLs) – see Fig. 40. These nanoparticles could improve CQD photodetector performance compared to epitaxial QDs due to many aspects gathered in Table 4 [12, 13, 113]. It is expected that the extension of application of CQD-based devices will be significant, especially in the area of IR imaging which is currently dominated by epitaxial semiconductor and hybrid technologies [19]. The colloidal quantum dot and 2D layered material photodetectors fabricated on flexible substrates are promising materials to overcome technical challenges in the development of fourth-generation IR systems [18].

Progress in Quantum Dot Infrared Photodetectors

0

1.0

2.0

Wavelength (µm) 3.0

43

4.0

5.0

PbS Quantum Dots HgTe Quantum Dots Polymers Si InGaAs InAsSb (TE cooled) InSb (cooled) HgCdTe (cooled) T2SLs (cooled)

Fig. 40  The wavelengths range that can be detected by materials commonly used in imaging applications Table 4  Advantages and disadvantages of CQD photodetectors in comparison with single-crystal QD photodetectors Advantages Control of dot synthesis and absorption spectrum by ability of QD size filtering, what leads to highly uniform ensembles Much stronger absorption than in Stranski-Krastanov-grown QD due to close-packed CDs Considerable elimination of strains influencing the growth of epitaxial QDs by greater selection of active region materials Reduction of cost fabrication (using, e.g., such solution as spin coating, inject printing, doctor blade, or roll-to-roll printing) compared to epitaxial growth Deposition methods are compatible with a variety of flexible substrates and sensing technologies such as CMOS (e.g., direct coating on silicon electronics for imaging)

Disadvantages Inferior chemical stability and electronic passivation of the nanomaterials in comparison with epitaxial materials Bipolar, interband (or excitonic) transitions across the CQD bandgap (e.g., electrons hopping among QDs and holes transport through the polymer) contrary to the intraband transitions in the epitaxial QDs Insulating behavior due to slow electron transfer through many barrier interfaces in a nanomaterial Problems with long-term stability due to the large density of interfaces with atoms presenting different or weaker binding High level of 1/f noise due to disordered granular systems

CQDs belong to wider class of low-dimensional materials such as 2D materials, nanowires, and their hybrid structures. The effective active regions of photodetectors are fabricated also by hybridization of 2D materials with CQDs [113]. Figure 41 gathers representative values of responsivity for nanostructured materials versus their detection wavelengths [114]. As is shown, the nanostructured materials are characterized by broadband spectral response. However, large scattering of parameters is due to not mature fabrication of detector structures.

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1014 Epitaxial QDs Colloidal QDs Hybrid CQDs/Graphene Nanowires

1012 1010 Responsivity (A/W)

Hybrid CQDs/Graphene

108 106 104

Nanowires

102 100 10–2

2D graphene/bP/bS 1

2

4 3 5 Wavelength (µm)

6

7 8 9 10

20

Fig. 41  A summary of the spectral responsivity of infrared photodetectors based on different nanostructured materials. (After Ref. [114])

Early studies of CQD-based detectors started from photoconductors. Most of them reported until now are for the NIR to MWIR regions. However, in spite of the simplicity in device architectures, performance of CQDs photoconductors was limited by their dark current, 1/f noise, and difficulty in the precise control of doping concentration. Improved performance was achieved using photovoltaic devices, which in principle decrease 1/f noise and dark currents. Our attention will be directed to photodiodes and more challenging detector structures called hybrid photodetectors (phototransistors with strong photogating effect). In terms of photoresponse, the current responsivity of CQD-based photodiodes typically ranges from a 100 mA/W to about 500 mA/W, what corresponds to external quantum efficiency around 10–50%  – higher in short-wavelength infrared (SWIR) region (see Fig. 42). These values of quantum efficiency are considerably higher than those obtained for epitaxail (self-assembled) QD photodetectors, which are typically about 2%. For comparison, typical quantum efficiencies of commercially available photodiodes [InGaAs, InSb, HgCdTe, and type II superlattices (T2SLs)] are shown on the right side of Fig. 42. The speed of CQD photodetectors is controlled by the CQD surface chemistry and surface passivation. Usually, CQDs synthesis requires the use of long-chain organic ligands to ensure stable growth of QDs. However, long-chain ligands are not suitable for good carrier transport in electronic devices. From this reason, initial long-chain ligands are exchanged by short-chain ones to increase better QDs coupling and increase carrier mobility/conductivity. The carrier hopping among

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101 InSb,InGaAs, HgCdTe PDs Type-II SL PDs

Responsivity (A/W)

η=100% η=70% η=60%

100

[116]

[115] Typical CQD PDs

[117]

η=10% [115]

[118]

[115]

[20]

10–1

[20] Typical epitaxial QDPDs

η=2%

CQD photodiodes PbS (Ref. 39) HgSe HgTe

10–2

2

3

4 5 Wavelength (µm)

6

7

Fig. 42  Room-temperature current responsivity as a function of the wavelength for different infrared detector technologies. The experimental data are taken from literature as marked. (Refs. [20, 48, 115–118]). PDs, photodiodes

quantum dots and the extended lifetimes of trapped photocarriers considerably limit speed of CQD detectors. The excited-state lifetime is typically about 100 ns what with carrier mobilities of 2  ×  10−3  cm2/Vs and applied voltage of 1  V allow to achieve efficient photocurrent generation in MWIR photodiodes [42]. In comparison with photodiodes, the current responsivity of phototransistors can be considerably higher, especially for hybrid photodetectors (see Sect. 3.3). The main feature of the hybrid photodetector (see Fig. 19) is ultrahigh gain originating from the high carrier mobility of the 2D material sheet and the recirculation of charge during the lifetime of the carriers remaining trapped either in the active material (quantum dots or other light-absorbing region to include carbon nanotubes and nanoplates). For example, photoexcited holes in the quantum dots are transferred to the graphene (Gr) layer and drift by bias VDS to the drain, with a typical transit time, τtransit, being inversely proportional to the carrier mobility, while electrons remain trapped (with a typical lifetime, τlifetme) in the quantum dots. Multiple circulation of holes in the graphene channel following a single e-h photogeneration leads to strong photoconductive gain, g = τlifetme/τtransit, indicating the importance of long lifetime and high carrier mobility. Many authors have demonstrated the gain of 108 electrons per absorbed photon and a responsivity of ~107 A/W in short-­ wavelength (SWIR) hybrid phototransistors [41, 119] – see Fig. 43. As this figure shows, the photoconductive gain is in the range between 102 and 108.

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Hybrid photodetectors offer improvements in responsivity; however the majority of these devices have a limited linear dynamic range due to the charge relaxation time, which quickly saturates the available states for photoexcitation, leading to a drop in responsivity with incident optical power. For example, Fig. 44 shows the current responsivity as a function of light intensity for hybrid PbS quantum dot/ graphene photodetector with different size of quantum dots. The plots also confirm higher responsivity for smaller quantum dots. As is shown, for small dots (d = 2.65 nm), the photoresponsivity reaches above 107 A/W under low light incident power. Responsivity of hybrid photodetectors can be considerably better in comparison with commercially available detectors; however considerable drop in operating 109 T = 300 K

108

Konstantatos et al., Nature Commun. 7, 363 (2012) Hybrid: smal PbSQDs/Gr phototransistor [Ref. 42]

Green color - ≤ 1-ns response times Blue color - ≥ 1-sec response times

107

Current responsivity (A/W)

106

Konstantatos et al., Nature Commun. 7, 363 (2012) Hybrid: large PbSQDs/Gr phototransistor (Ref. 42] Huo et al., Adv. Mater. 29, 1606576 (2017) Hybrid: HgTeQDs/MoS2phototransistor(Ref. 123)

105 104 103

Grotevent ., et al Adv. Optical Mater. 1900019 (2019) Hybrid: PbSQDs/Gr phototransistor (Ref. 120)

Hybrid: PbSQDs/WS2

Özdemir et al., ACS Photonics 6, 2381 (2019)[Ref. 121]

102

Luo et al., ACS Nano 13, 9028 (2019)3 Hybrid: PbSQDs/Bi2O2Se phototransistor (Ref. 122) Hybrid: PbSQDs/MoS2

Si APD Ge APD

101

Ideal photofdiode η=100%, g = 1

InGaAs APD InGaAs PD

100 Si PD

10–1 10–2

Ackerman et al., APL 116, 083502 (2020) HgTe CQD photodiode (Ref. 116)

Ge PD

Tang et al., Small 15, 1804920 (2019) HgTe CQD photodiode (Ref. 117)

0.5

1.0

1.5

Tang et al., Laser Photonics Rev. 13, 1900165 (2019). HgTe CQD photodiode (Ref. 118)

2.0 2.5 Wavelength (µm)

3.0

3.5

4.0

Fig. 43  Spectral current responsivity of CQD-based hybrid photodetectors and photodiodes compared with commercial ones. Solid line shows 100% quantum efficiency. Green colors denote ≤1 ns response times, while the blue color denotes ≥1 second response times. The experimental data for photodetectors are labeled with their reference as well as a brief description of the photodetector style. The commercial photodiodes are shown in green. PD, photodiode; APD, avalanche photodiode

Progress in Quantum Dot Infrared Photodetectors

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108 PbS CQD/Graphene hybrid photodetector

Current responsivity (A/W)

107 106

105 104

103 102 10–5

VDS = 5 V λinc = 635 nm

d = 2.65 nm d = 3.62 nm d = 4.96 nm 10–4

10–3

10–2

Power density

10–1

100

101

(mW/cm2)

Fig. 44  Current responsivity as a function of excitation intensity for hybrid PbS quantum dot/Gr photodetector with different size of quantum dots. (Adapted after Ref. [119])

speed (bandwidth) is observed. Generally, their response time (millisecond range and longer) is three orders of magnitude longer in comparison with commercial detectors (microsecond range and shorter). Above drawbacks limit their practical applications. For example, the recovery time of hybrid PbS QD/Gr photodetectors is 50–100 ms, and in addition the recovery process has a slow component above 1 s associated with traps in the quantum dots [41, 119]. Despite the very high responsivity of hybrid CQD/Gr photodetectors, their power consumption, electronic readout circuits, and noise are determined by the semimetallic nature of graphene, what leads to a large dark current [120]. Alternative 2D materials like transition metal dichalcogenides (TMDs) have been considered as a potential replacement of graphene for transistor channels. The use of 2D materials with a bandgap of 1–2 eV is a particular promise offering a lower leakage current. They are considered in recently published papers [22, 121, 122].

5.2  Lead Chalcogenide CQD Photodetectors In lead chalcogenide family [16, 17, 64, 123, 124], the most popular are lead sulfide (PbS) CQDs with absorption tunable in NIR spectral range (1–3 μm). Their technology is compatible with CMOS readout technology used in fabrication monolithic focal plane arrays (FPAs). Figure  45 presents the cross-sectional view of active

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layer of PbS QD deposited by spin coating on Si/SiO2 substrate elaborated by IMEC company. On the bottom, TiN contact is deposited on Si/SiO2 substrate, and next a metal-oxide electron transport layer. The active region contains a 150-nm-thick QD layer deposited from solution as three-layer stack. The perfect crystallinity in the QDs reveals transmission electron microscope picture. On the top, the active region is covered by organic hole transport layers and a semitransparent top contact. Figure  46 shows the current density in dark condition and under illumination (λ = 1450 nm) versus bias voltage. The dark current for reverse-biased photodiodes at −3  V is approximately 3  μA/cm2, and this value scales from detector size of 2 × 2 mm2 down to 50 × 50 μm2. As is shown in Fig. 47a, the absorption peak depends on the nanocrystal size. Using larger size (5.5-nm diameter), the peak absorption occurs at the wavelength of 1440  nm, but using smaller size (3.4  nm), the peak is at 980  nm. This size-­ dependent tunability can be applied in hyperspectral visible image sensors. Despite strong progress in PbS CQD fabrication, their quantum efficiency is quite low with value to 20% compared to 70% for commercial InGaAs photodiodes (see Figs. 42 and 47b). From a performance standpoint, short-wavelength infrared (SWIR) PbS CQDs have achieved detectivities above 1012 Jones at room temperature which is comparable to commercial InGaAs photodiodes (see Fig. 47c). Recently, Nakotte et  al. [121] have reviewed the development of hybrid lead chalcogenide-based photodetectors. These detectors, in various device architectures, utilize different types of 2D materials including graphene and transition metal dichalcogenides. Table 5 lists the high-performance devices together with their figure of merit. The experimentally measured carrier responsivities of hybrid PbS CQD photodetectors are included in Fig. 43.

Semitransparent top contact Quantum dot active layer

Organic hole transport layer Layer 3 (50 nm) Layer 2 (50 nm) Layer 1 (50 nm) Metal-oxide electron transport layer

TiN bottom contact

Fig. 45  Cross-sectional view (left) and TEM picture (right) of a three-layer active stack of 5.5-nm PbS quantum dots. (After Ref. [17])

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Progress in Quantum Dot Infrared Photodetectors 102 Top illumination

Current density (mA/cm2)

101 100 10–1

Jphoto

10–2 10–3

50x50µm2 2x2 mm2

Jdark

10–4 10–5 10–6 –3

–2

–1

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1

3

2

Fig. 46  Dark and photocurrent density curves for different sizes of PbS CQD photodiodes showing no reverse leakage scaling from detector size of 2  ×  2  mm2 down to50  ×  50  μm2. (After Ref. [17]) 10 0.30

15

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PbS CQD

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11

10

10

10

10

9

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(b)

0.6

1.2 1.6 0.8 Wavelength (µm)

2.0

(c)

Fig. 47  PbS CQD photodiode: (a) spectral tunability in dependence on dot size (After Ref. [17]), (b) quantum efficiency and responsivity, (c) detectivity comparison of PbS CQD photodiode with commercial InGaAs photodiode. (After Ref. [16])

Table 5  High-performance hybrid PbS CQD photodetectors

QD/material PbS/graphene PbS/WSe2 PbS/WS2 PbS/MoS2 PbS/Bi2O2Se

Ligand (s) EDTa TBAIb Zn2I/MPAc Zn2I/MPA EDT

Wavelength (nm) 1200 970 1800 1800 2000

Response (rise) time (ms) 100 7 200 32 4

Responsivity (A/W) 8 × 103 2 × 105 1442 202 103

Detectivity (Jones) 109 7 × 1013 1012 2.8 × 1011 1 N/A

EDT ethanedithiol, bTBAI tetrabutylammonium iodide, cMPA mercaptopropionic acid

a

Ref. [125] [126] [127] [127] [128]

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5.3  HgTe CQD Photodetectors The first mercury telluride (HgTe) CQD detector was a photoconductor reported in 2011 [50]. At present the main technological efforts are directed toward photodiodes. Table  6 presents development of HgTe CQD photodetectors over the past decade [22]. HgTe CQDs have demonstrated wide spectral tunability from SWIR [133, 136– 138] up to THz region [139]. For example, Fig. 48 shows the typical infrared absorption of different-sized thin-film HgTe dots in SWIR and MWIR regions with cutoff wavelength at 2.5 μm and 4 μm, respectively. The first BLIP HgTe CQD photovoltaic detector operated in MWIR spectral range (λc = 5.25 μm at 90 K) was fabricated in 2015 by Guyot-Sionnest group [133]. Further improvement in sensitivity and response speed has been achieved by heavily p-type doping with Ag2Te nanocrystals. Figure 49 presents the construction of HgTe CQD photovoltaic detector on composite indium tin oxide (ITO)/sapphire substrate. HgTe CQDs, with the final thickness  ~  400  nm, are deposited Table 6  Performance comparison between HgTe CQD photodetectors

Device structure HgTe CQDs photoconductors HgTe CQDs photoconductors HgTe CQDs photoconductors HgTe/AsS3 phototransistors HgTe CQDs phototransistors MoS2-HgTe CQDs hybrid phototransistors HgTe CQD photodiodes HgTe CQD photodiodes Plasmon resonance-­ enhanced HgTe CQD photodiodes Flexible HgTe CQD photodiodes Hyperspectral HgTe CQD photodiodes Dual-band HgTe CQD photodiodes HgTe CQD photodiodes After Ref. [22]

Spectral range Year (μm) 2011