Novel Devices Based on Relaxor Ferroelectric PMN-PT Single Crystals [1st ed.] 9789811543111, 9789811543128

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Springer Theses Recognizing Outstanding Ph.D. Research

Huajing Fang

Novel Devices Based on Relaxor Ferroelectric PMN-PT Single Crystals

Springer Theses Recognizing Outstanding Ph.D. Research

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Huajing Fang

Novel Devices Based on Relaxor Ferroelectric PMN-PT Single Crystals Doctoral Thesis accepted by Tsinghua University, Beijing, China

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Author Dr. Huajing Fang Department of Chemistry Tsinghua University Beijing, China

Supervisor Assoc. Prof. Qingfeng Yan Department of Chemistry Tsinghua University Beijing, China

ISSN 2190-5053 ISSN 2190-5061 (electronic) Springer Theses ISBN 978-981-15-4311-1 ISBN 978-981-15-4312-8 (eBook) https://doi.org/10.1007/978-981-15-4312-8 Jointly published with Tsinghua University Press The print edition is not for sale in China (Mainland). Customers from China (Mainland) please order the print book from: Tsinghua University Press. © Tsinghua University Press, Beijing and Springer Nature Singapore Pte Ltd. 2020 This work is subject to copyright. All rights are reserved by the Publishers, 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 publishers, 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 publishers nor the authors or the editors give a warranty, express or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publishers remain neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Supervisor’s Foreword

As an important ferroelectric material, Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-PT) solid solutions combined the advantages of relaxor ferroelectric PMN and ferroelectric PT predictably. With the modified Bridgman technique, large-size and high-quality PMN-PT single crystals have been successfully grown. The single crystals with the composition near the morphotropic phase boundary (MPB) exhibit ultra high piezoelectric constant of *2500 pC/N and large electromechanical coupling coefficient up to 0.9. In recent years, piezoelectric devices based on the PMN-PT single crystals have received much attention and made a revolution in the electromechanical transduction area. In contrast, the applications related to their excellent ferroelectric and pyroelectric properties of this multifunctional materials are still not mature enough. Therefore, further investigation on extending their ferroelectric and pyroelectric applications is highly desirable. The main aim of this doctoral thesis is to explore the applications of ferroelectric materials such as PMN-PT single crystals in information technology. Several prototype devices have been designed and fabricated which are responsible for the collection, processing and storage of the information data. For example, an optothermal field effect transistor (FET) based on the pyroelectric PMN-PT single crystal and MoS2 single-layer film is introduced in Chap. 2. Infrared light can be used as the input signal instead of the gate voltage to realize remote controlling of the channel current. In Chap. 3, a new photodetector with ultra-wide spectral response in the ultraviolet to terahertz band has been achieved by using the high pyroelectric coefficient of PMN-PT single crystals. The detecting performance is improved through domain engineering and structural optimization of the device. To use the mechanical energy in the environment as the energy source for information writing, a self-powered ferroelectric memory system is demonstrated in Chap. 4. The data can be written in the PMN-PT-based FET by tapping a triboelectric nanogenerator. The research was carried out at Department of Chemistry in Tsinghua University with the cooperation from Department of Applied Physics in The Hong Kong Polytechnic University, Department of Physics in Tsinghua University and Institute of Microelectronics in Tsinghua University. The research contents in this thesis are v

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Supervisor’s Foreword

not only in the frontiers of ferroelectric materials, but also at the multidisciplinary highly cross-cutting edge. Some excited results and design ideas are expected to drive ferroelectric materials to play an important role in a wide range of applications. Beijing, China January 2020

Assoc. Prof. Qingfeng Yan

Acknowledgements

First and foremost, I wish to express my sincere gratitude to my supervisor, Assoc. Prof. Qingfeng Yan, for his constant support, insightful understanding, and guidance throughout my graduate life at Tsinghua University. Also, a special thanks to my late supervisor, Prof. Dezhong Shen. I have been deeply influenced by his noble character and the rigorous academic attitude. I am grateful for having the opportunity to conduct collaborative research at several institutions such as Department of Physics, Institute of Microelectronics in Tsinghua University, and Department of Applied Physics in The Hong Kong Polytechnic University. I wish to thank Professor Qiang Li, Professor Ji-Yan Dai, Professor Jia-Lin Sun, and Professor Tian-Ling Ren for their valuable suggestions and precious time on my research. Without their support, I cannot finish my graduate study. In addition, the support and assistance provided by Professor Liduo Wang, Professor Helen Lai Wa Chan, Assoc. Prof. Yang Chai, and Assoc. Prof. Guifang Dong were immeasurably helpful. Over the past five years of my life at Tsinghua University, my current and former group members helped me a lot since I joined the group. I appreciate their tremendous help and collaborations that make me progress. Finally, I would like to thank my family for their endless love and my friends for encouragement and unwavering support.

vii

Abstract

Owing to the ultrahigh piezoelectric coefficient and electromechanical coupling factors, Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) complex perovskite single crystals have been widely used in the piezoelectric devices such as medical ultrasonic transducers and underwater acoustic transducers. However, as an outstanding representative of relaxor ferroelectrics, the excellent pyroelectric and ferroelectric properties of PMN-PT single crystals have not yet been fully exploited in practical application. It is of great significance to extend their application in areas other than piezoelectric devices. In this dissertation, we explored the applications of these materials in information technology by developing several novel prototype devices based on PMN-PT single crystals. An optothermal field effect transistor (FET) was constructed on the rhombohedral PMN-26PT single crystal. The two dimensional (2D) MoS2 monolayer was chosen as the channel semiconductor materials. The working mechanism of this FET was studied by investigating the relationship between the drain current of MoS2 channel and the infrared illumination. It was found that the FET can be gated with infrared illumination and the modulation process is reversible. The infrared response is attributed to the polarization change of PMN-26PT gate insulator induced by the pyroelectric effect. Hence, the drain current of the FET can be remotely controlled by infrared photons without applying a gate voltage. Such a fusion of pyroelectric effect and the interface engineering of 2D materials provides an effective strategy for the ‘photonics revolution’ of FET. A pyroelectric coefficient as high as 7.5  10−4 C/m2 K was obtained in PMN-28PT single crystal after poling along the [111] direction. The extinction of all angles between 0 to 90° was observed by the polarized light microscope, indicating the ‘1R’ single domain structure. Then, an ultra-broadband photodetector was monolithically integrated on such a [111]-oriented PMN-28PT single crystal by implementation of silver nanowires (Ag NWs) as the transparent top electrode. A systematic study of the photoresponse was carried out. The photodetector generated pyroelectric current signals under the illumination with wavelength ranging from UV to terahertz (THz). Enhanced current signals was found at short wavelengths, which was possibly caused by the excitation of surface plasmons in Ag ix

x

Abstract

NWs. In addition, the photodetector with optimized device architecture also showed a dramatic improvement in operation frequency up to 3 kHz, which was an order of magnitude higher than that of traditional pyroelectric photodetector. A poly(vinylidene fluoride) (PVDF) porous film and a polystyrene (PS) nanospheres array were deposited on PET substrates respectively to fabricated a low cost arch-shaped triboelectric nanogenerator (TENG). The TENG could generate an output voltage as high as 220 V, which was sufficient to initiate the polarization switching in ferroelectric materials. The feasibility of this new poling process was theoretically and experimentally verified. Meanwhile, a bottom gated ferroelectric FET was fabricated on a PMN-35PT single crystal, with pentacene as the channel material. The ON/OFF current ratio of this ferroelectric FET was higher than 103. Inspired by abovementioned results, we further demonstrated an integrated module of self-powered ferroelectric transistor memory through the combination of a TENG and a ferroelectric FET. The stored information could be easily written in the memory system with mechanical energy, which solved the power consumption problem of information writing in ferroelectric nonvolatile memories. Keywords PMN-PT Single Crystals
Pyroelectric
Field Effect Transistor
Photodetector
Ferroelectric Memory

Contents

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Ferroelectric Materials . . . . . . . . . . . . . . . . . . . . . . . 1.1.1 Basic Concept of Ferroelectric Materials . . . . . 1.1.2 Physical Properties of Ferroelectric Materials . 1.2 PMN-PT Relaxor Ferroelectric Single Crystals . . . . . . 1.2.1 PMN-PT Complex Perovskite Structure . . . . . 1.2.2 Research Progress of PMN-PT Single Crystals 1.2.3 The Applications of PMN-PT Single Crystals . 1.3 The Main Research Contents of This Thesis . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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2 An Optothermal Field Effect Transistor Based on PMN-26PT Single Crystal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Introduction of This Chapter . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1 Field Effect Transistors and the ‘Photonics Revolution’ 2.1.2 Interface Engineering of Two-Dimensional Channel Semiconductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 The Design and Fabrication of Optothermal FET . . . . . . . . . . 2.2.1 Structural Design of the Prototype FET . . . . . . . . . . . . 2.2.2 Processing of PMN-26PT Single Crystal . . . . . . . . . . . 2.2.3 Growth and Transfer of Monolayer MoS2 . . . . . . . . . . 2.3 The Device Performance and Working Mechanism of Optothermal FET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.1 Infrared Response . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2 Mechanism Analysis of Optothermal Regulation . . . . . 2.4 Summary of This Chapter . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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3 An Ultrabroadband Photodetector Based on PMN-28PT Single Crystal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Introduction of This Chapter . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.1 Broadband Photodetector . . . . . . . . . . . . . . . . . . . . . . 3.1.2 Response Characteristics of Pyroelectric Detectors . . . . 3.2 Structural Design and Preparation of Ultrabroadband Detector 3.2.1 Structural Design of Photodetectors . . . . . . . . . . . . . . 3.2.2 Processing and Characterization of PMN-PT Single Crystal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.3 Domain Engineering of PMN-PT Single Crystal . . . . . 3.2.4 Preparation of Silver Nanowires Transparent Electrode 3.3 The Performance and Mechanism Analysis of the Photodetector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1 Optimization of Pyroelectric Frequency Response . . . . 3.3.2 Broadband Response in the Ultraviolet-Terahertz Range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.3 The Influence of Surface Plasmons on Pyroelectricity . 3.3.4 Discussion on the Mechanism of Ferroelectric Photovoltaic in the Ultraviolet Band . . . . . . . . . . . . . . 3.4 Summary of This Chapter . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 A Mechanical Energy Writeable Ferroelectric Memory Based on PMN-35PT Single Crystal . . . . . . . . . . . . . . . . . . . . . . 4.1 Introduction of This Chapter . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.1 Ferroelectric Non-volatile Memory and Its Energy Consumption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.2 Self-powered System and Nanogenerator . . . . . . . . . . . 4.2 Arch-Shaped TENG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1 Design and Preparation of TENG . . . . . . . . . . . . . . . . 4.2.2 Energy Conversion Mechanism of TENG . . . . . . . . . . 4.2.3 Electrical Output Performance of TENG . . . . . . . . . . . 4.3 Self-powered Ferroelectric Memory System . . . . . . . . . . . . . . 4.3.1 Properties of PMN-PT Single Crystal . . . . . . . . . . . . . 4.3.2 Exploration of New Poling Methods . . . . . . . . . . . . . . 4.3.3 Ferroelectric FET Memory . . . . . . . . . . . . . . . . . . . . . 4.3.4 Self-powered Ferroelectric Memory System . . . . . . . . . 4.3.5 Storage Density and Microscopic Ferroelectric Domain 4.4 Summary of This Chapter . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Contents

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5 Conclusion and Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 5.1 Research Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 5.2 Prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104

Abbreviations

CVD d33 Ec Eg FET ITO k33 kt MPB p PET PFM PL PLM PMMA PMN-PT Pr PS Ps Psa PVDF PZT SEM Tc TEM TENG TMDs

Chemical Vapor Deposition Piezoelectric Coefficient Coercive Electric Field Band Gap Field Effect Transistor Indium Tin Oxide Longitudinal Electromechanical Coupling Coefficient Thickness Extensional Mode Electromechanical Coupling Coefficient Morphotropic Phase Boundary Pyroelectric Coefficient Polyethylene Terephthalate Piezoresponse Force Microscopy Photoluminescence Polarized Light Microscopy Polymethylmethacrylate Lead Magnesium Niobate–Lead Titanate Remnant Polarization Polystyrene Spontaneous Polarization Saturated Polarization Polyvinylidene Fluoride Lead Zirconate Titanate Scanning Electron Microscopy Curie Temperature Transmission Electron Microscopy Triboelectric Nanogenerator Transition Metal Dichalcogenides

xiii

xiv

Trt XRD er k

Abbreviations

Rhobohedral-to-Tetragonal Ferroelectric Phase Transition Temperature X-ray Diffraction Room Temperature Dielectric Permittivity Wavelength

Chapter 1

Introduction

1.1 Ferroelectric Materials 1.1.1 Basic Concept of Ferroelectric Materials Solid materials can be divided into conductors, semiconductors, and insulators according to the respective conductive properties. Insulators can also be referred to as dielectric materials [1]. There is no free charge inside the ideal dielectric materials, charged particles are bound around the molecules or atoms. As shown in Fig. 1.1, when the dielectric is under the influence of an external electric field, the internal charge distribution will change: the positive and negative charge centers of non-polar molecules will be separated under the induction of the electric field to generate the electric dipole moment (displacement polarization); the intrinsic dipole moment of the polar molecule rotates until it coincides with the direction of the external electric field (orientated polarization). The phenomenon of the elastic displacement and dipole orientation alignment of the charge inside the dielectric is called the polarization of the dielectric [2, 3]. Polarization causes the dielectric to exhibit electrical

Fig. 1.1 The polarization of dielectric materials © Tsinghua University Press, Beijing and Springer Nature Singapore Pte Ltd. 2020 H. Fang, Novel Devices Based on Relaxor Ferroelectric PMN-PT Single Crystals, Springer Theses, https://doi.org/10.1007/978-981-15-4312-8_1

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1 Introduction

properties at the macroscopic scale. A large amount of charges appear on the surface of the dielectric, which is called polarization charge. The degree of polarization of the dielectric by the external electric field can be expressed by the vector sum of the electric dipole moment per unit volume [4], defined as the polarization intensity. (Polarization, P): − → pi P= V

(1.1)

There is a simple linear relationship between the polarization intensity of the common dielectric and the strength of the electric field. Once the external electric field is removed, the polarization of the dielectric immediately disappears. However, there is a special type of dielectric material with non-linear response, which can maintain the polarization state after the external electric field is removed. These ferroelectric materials are a class of materials that have spontaneous polarization and the spontaneous polarization can be switched by an external electric field [5]. A hysteresis loop similar to that depicted in Fig. 1.2 is an important feature of ferroelectric materials [6]. The non-poled ferroelectric material consists of many small regions with different spontaneous polarization directions. The spontaneous polarization directions in each small region is the same. This kind of small region is called the ferroelectric domain, and the boundary of adjacent domains is called the domain wall [7]. When a ferroelectric material is gradually applied to an external electric field, its polarization P increases along the O-A dash line as the electric field E increases. The orientation of almost all of the domains at point A is parallel to the direction of the external electric field such that the polarization is saturated. Increasing the electric field will only cause a linear increase in the displacement polarization of electrons or ions. Extending the linear portion to the longitudinal axis, the intercept is called the saturated polarization (Psa ) of the material. In the process of removing the external electric field, the polarization does not return to the Fig. 1.2 The P-E loop of a typical ferroelectric material

1.1 Ferroelectric Materials

3

origin, but reaches the point B along the A-B curve. The polarization at this time is an important parameter for characterizing the ferroelectric properties of the material, called remnant polarization (Pr ). When the electric field is applied in the opposite direction, the polarization will decrease along the B-C curve to a negative saturation value. The B-C curve intersects with the horizontal axis, and the corresponding absolute value of the electric field strength is called the coercive field (E c ), which can be used to measure the difficulty of poling ferroelectrics.

1.1.2 Physical Properties of Ferroelectric Materials Since the discovery of ferroelectricity in 1920s [8], the research on ferroelectric materials can be described as long-lasting. So far, there are thousands of ferroelectric materials that have been discovered, and the physical origins of ferroelectric properties of different materials are not the same [9]. With the continuous emergence of various new types of ferroelectric materials, the theoretical understanding of ferroelectric materials is gradually deepening and improving. As early as 1940, Mueller’s theory of Rochelle salt proposed a systematic understanding of the relationship between various properties of materials [10]. In 1945, the first displacement ferroelectric without hydrogen bonds, barium titanate (BaTiO3 ), was discovered. During this period, the theory of ferroelectric phenomenology began to sprout and grew. The phenomenological theory summarizes some of the phenomena observed in ferroelectric materials with a few parameters and describes the relationship between these phenomena without in-depth explanation of the underlying causes [11, 12]. By the 1960s, many ferroelectric materials such as lead zirconate titanate (PZT) and lithium niobate (LiNbO3 ), appeared one after another. From the perspective of lattice dynamics, Cochran and Anderson creatively proposed soft mode theory to explain ferroelectric phase transitions. Their viewpoint is that the occurrence of spontaneous polarization corresponds to the softening of an optical transverse mode at the center of the Brillouin zone [13, 14]. After that, the soft mode theory and the Landau phase transition theory caused profound changes in ferroelectric physics. After long-term work in related fields, scientists have gradually clarified the relationship between some physical properties of dielectric materials [15], as shown in Fig. 1.3. Fig. 1.3 Piezoelectric, pyroelectric and ferroelectric material relationships

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1 Introduction

As we can see that a material with ferroelectric properties must have both pyroelectric and piezoelectric properties. Among them, pyroelectric performance describes the relationship between polarization and temperature. When the ferroelectric material is uniformly heated, the intensified thermal vibration tends to disturb the ordered state after poling, so that the polarization is lowered. This change in polarization is manifested macroscopically as the release of charge, which will cause current in an external circuit [16]. The pyroelectric performance can be quantitatively characterized by the pyroelectric coefficient of the material, which is defined as the first derivative of the polarization (P) versus temperature (T ) [17] p=

dP dT

(1.2)

The pyroelectric coefficient is not a scalar but a vector with a unit of C m−2 K−1 . Piezoelectric properties refer to the phenomenon of charge accumulation on a surface that is proportional to external force when a material is subjected to mechanical stress (positive piezoelectric effect); or stress and strain generated when the material is subjected to an applied electric field (inverse piezoelectric effect) [18, 19]. The piezoelectric effect bridges the dielectric properties and elastic properties, reflecting the coupling relationship between the polarization and mechanical stress of the material. The piezoelectric constant d mj is usually used to describe this coupling relationship. The physical meaning is the change rate of the electrical displacement component Dm with the stress component X j when the applied electric field strength is constant [20].  dm j =

∂ Dm ∂Xj

 (1.3) E

The piezoelectric constant is a third-order tensor, and the first-kind piezoelectric constant (piezoelectric strain constant) is expressed with the unit of pC/N. In general, ferroelectric materials have better piezoelectric properties than non-ferroelectric piezoelectric materials, so the most widely used materials in the piezoelectric field are ferroelectric materials. These physical properties of ferroelectric materials determine their broad application areas.

1.2 PMN-PT Relaxor Ferroelectric Single Crystals 1.2.1 PMN-PT Complex Perovskite Structure In general, most inorganic ferroelectric materials perform much better than organic ferroelectric materials. Oxygenated compounds are important families in inorganic ferroelectric materials. According to the division of crystal structure, they mainly

1.2 PMN-PT Relaxor Ferroelectric Single Crystals

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Fig. 1.4 Crystal structure of perovskite compounds

include the following categories: tungsten bronze structure, bismuth layer-structured compounds, pyrochlore structure, lithium niobate structure and perovskite structure [21]. Among them, the ferroelectric properties of the materials with perovskite structure are the best. Figure 1.4 shows a schematic diagram of the crystal structure of a perovskite material with a chemical formula of ABO3 . The cation at the A site occupies 8 vertex positions of a single unit cell, and the cation at the B site occupies the body center of the unit cell accordingly. The oxygen ions are located in the six face centers and enclose the B-site cations to form the [BO6 ] octahedron. When the vibration equilibrium position of the B-site cation is shifted with respect to the center of the oxygen octahedron, an electric dipole moment is generated. And the response of the electric dipole moment to the external electric field is a poling process of the perovskite-type ferroelectric materials. This special crystal structure allows ions at each equivalent lattice position to be replaced by other ions with similar radius and similar properties, which can greatly enrich the types of perovskite structural materials. In the perovskite structure, if there is more than one kind of cation in the A or B position, it is called a composite perovskite structure. The lead magnesium niobatelead titanate system (1 - x)Pb(Mg1/3 Nb2/3 )O3 - xPbTiO3 (hereinafter referred to as PMN-PT) is in the case of B-site complex. The A site in the PMN-PT crystal structure is occupied by Pb2+ ion with a large radius, while the B site is occupied by Mg2+ , Nb5+ or Ti4+ ions with a small radius. From the chemical composition analysis, the perovskite composite PMN-PT can be regarded as a complex solid solution system formed by the perovskite structure of the relaxor ferroelectric PMN and the ordinary ferroelectric PT [22]. Since the crystal structures of the two compositions are similar, PMN and PT can be dissolved with each other in any ratio. However, as the PT composition content increases, the crystal structure of PMN-PT will exhibit a phase transition from the rhombohedral ferroelectric phase (R) to the tetragonal ferroelectric phase (T). Figure 1.5 shows the low temperature phase diagram of the PMN-PT solid solution system [23]. In a certain composition region, the rhombohedral and tetragonal ferroelectric phases in the PMN-PT system will coexist. The

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1 Introduction

Fig. 1.5 The phase diagram of PMN-xPT solid solution system [23]

crystal present the optimal piezoelectric performance, and this composition region is called the morphotropic phase boundary (MPB). It is generally believed that the MPB of PMN-xPT material system at room temperature is between x = 0.30–0.35 [24– 26]. The structure and properties of PMN-PT solid solution materials are closely related to their compositions. Therefore, PMN-PT functional materials with high piezoelectric, ferroelectric, and pyroelectric properties and specific structures can be obtained by composition adjustment.

1.2.2 Research Progress of PMN-PT Single Crystals Due to the periodic structure and the anisotropy of physical properties, single crystals always have the best performances among the materials of the same chemical composition. For example, monocrystalline silicon is superior to polycrystalline silicon and amorphous silicon in both mechanical and electrical properties in the field of photovoltaic technology. In addition, the structure of single crystal materials is much simpler than polycrystalline materials, which is more conducive to the study of the mechanisms of various physical effects [27, 28]. In order to make full use of these advantages of crystals, people began to explore the growth of relaxor ferroelectric single crystals. However, compared with the ceramic preparation process, the growth of single crystal has more influencing factors and is therefore more complicated. Fortunately, in the existing relaxor ferroelectric material system, PMN-PT has the characteristics of quasi-congruent melting [29], and it is easier to grow large single crystals than the incongruent melting solid solution of other chemical compositions. For a long time, researchers have carried out a lot of works about the preparation and performance of PMN-PT relaxor ferroelectric single crystals. For instance, Shrout et al. [30] has grown a PMN-30PT single crystal with a maximum size of about 1 cm in 1990, and measured a d 33 mode piezoelectric coefficient of 1500 pC/N. Park and Shrout [31] has grown high-performance PMN-PT crystals and studied the properties of single crystals near the MPB composition. Dong and Ye [32] has grown

1.2 PMN-PT Relaxor Ferroelectric Single Crystals

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PMN-PT single crystals in 2000, and systematically studied the effects of chemical and thermodynamic parameters on the crystal growth process. In China, many research institutions represented by the Shanghai Institute of Ceramics, Tsinghua University, and Xi’an Jiaotong University have also developed the ability to grow large-size PMN-PT single crystals. Nowadays, large-size PMN-PT single crystals with a growth direction along the [001] crystal orientation have been commercialized [33]. The piezoelectric constant of the MPB composition crystal exceeds 2000 pC/N, and the longitudinal electromechanical coupling coefficient k 33 is as high as 0.9. These properties are far superior to the currently widely used PZT piezoelectric ceramics. Figure 1.6 shows the PMN-PT single crystals and various processed products. The PMN-PT relaxor ferroelectric single crystal is the most outstanding representative of ferroelectric materials. The emergence of PMN-PT crystal and another

Fig. 1.6 PMN-PT crystals and processed products. Reproduced with permission [33] . Copyright 2014, Elsevier

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1 Introduction

relaxor system (1 - x)Pb(Zn1/3 Nb2/3 )O3 - xPbTiO3 (referred to as PZN-PT) is known as the “exciting breakthrough” in the past 50 years in the entire ferroelectric industry [34]. Researches on the performance and structure of PMN-PT single crystals have attracted much attention. For example, Liu et al. [35] has systematically studied the effects of vibration modes, ferroelectric domain structure, temperature and other factors on the energy dissipation behavior of PMN-PT single crystals. Li and coworkers [36] have studied the surface acoustic wave properties of different cut PMN-PT single crystals and found that the energy flow angle of single crystals is significantly smaller than that of ordinary piezoelectric materials, which is beneficial to increase the working bandwidth and reduce energy loss. Wan et al. [37] has accurately studied the basic electro-optic properties of PMN-PT single crystals according to the method of minimum deflection angle. It was found that the refractive index of PMN-PT single crystal has frequency dispersion phenomenon and the Sellmeier dispersion equation of the crystal is summarized. Yan et al. [38] has analyzed the energy changes in the formation of microdomains, calculated the relationship between potential energy density and microdomain size, and discussed the effect of dual polarization mechanism on the dielectric behavior of PMN-PT single crystal. Zeng et al. [39] has observed the nanoscale ferroelectric domain structure and dynamic behavior of PMN-PT single crystals by piezoresponse force microscope (PFM), and obtained the three-dimensional polarization distribution image of domain structure. Another research hotspot for PMN-PT crystals is to increase its ferroelectric phase transition temperature (T rt ). The dielectric constant of the PMN-PT single crystal near the MPB composition has another significant inflection point before the temperature rises to the Curie temperature. At that temperature, the crystal undergoes a phase transition from the rhombohedral ferroelectric phase to the tetragonal ferroelectric phase [40]. The phase transition of the ferroelectric structure makes the coexistence of two phases disappear, and the original excellent piezoelectric activity and electromechanical coupling performance also deteriorate. In other words, the ferroelectric phase transition temperature is the upper limit of the actual use temperature of the PMN-PT single crystal in the piezoelectric device. Since the T rt of PMN-PT near the MPB composition is only 50–80 °C, their applications in highpower devices is restricted [41]. Recent studies have found that the introduction of the third composition in PMN-PT crystal can effectively break the bottleneck of T rt [42–44]. Yamashita et al. [45] has used a top seed method to grow a 24PIN43PMN-33PT single crystal with a T rt of about 120 °C. Zhang et al. has used the Bridgman method to grow a larger size PIN-PMN-PT ternary crystal [46], which has a piezoelectric constant of about 1400 pC/N and a T rt between 100 and 125 °C. Xia et al. [47] has successfully grown a PMN-PT-PZ single crystal with a PZ content of 5%, and increased the T rt to 127 °C while maintaining a piezoelectric constant of 1000 pC/N. The research work of introducing the third composition in the crystal lays the foundation for the practical application of the PMN-PT systems single crystal at higher temperature.

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1.2.3 The Applications of PMN-PT Single Crystals The study of functional materials is application-oriented and can contribute to the transformation of scientific and technological achievements. The application of functional materials must be based on some of their physical or chemical properties. It can be seen from Fig. 1.3 that the PMN-PT single crystals as a ferroelectric material have both piezoelectric properties, pyroelectric properties and ferroelectric properties. Below we introduced the current application research progress of PMN-PT single crystals based on different physical properties.

1.2.3.1

Piezoelectric Devices

Due to the elimination of grain boundaries, PMN-PT single crystals exhibit excellent piezoelectric and electromechanical coupling properties at room temperature. They are expected to play an important role in the field of mechanical energy and electrical energy conversion technology. The successful growth of large-size, high-quality PMN-PT relaxor ferroelectric single crystals has also laid a good material foundation for the development of high-performance piezoelectric devices. At present, there are many successful examples for the application of high piezoelectric performance PMN-PT single crystal, and some have even developed into commercial products. 1. Medical ultrasonic imaging Ultrasonic imaging is currently one of the most effective medical diagnostic techniques, and ultrasonic transducers are a core component in the entire imaging system [48]. The electromechanical conversion properties of the piezoelectric material used in the transducer will ultimately determine the resolution of the ultrasonic imaging. The electromechanical coupling coefficient of PMN-PT single crystal is higher than 0.9, and it has a small attenuation and velocity dispersion in the frequency range of 20–100 MHz [49]. These excellent material properties make it shine in the field of ultrasonic transducers. For example, Peng et al. [50] has prepared a singleelement ultrasonic transducer based on PMN-PT single crystal as a transcranial Doppler probe. The high-frequency focusing transducer can be prepared on PMNPT single crystal by using a dimpling grinder [51]. It exhibits better sensitivity and bandwidth than a single-element transducer of planar configuration. Based on the Krimholz-Leedom-Mattaei (KLM) model, array type transducers fabricated from PMN-PT single crystals can theoretically achieve 130% bandwidth, and experiment bandwidth values as high as 114% have also been reported [52]. IBULE Company (South Korea) has successfully developed a 64-channel ultrasonic transducer array using PMN-33PT single crystal, and its imaging quality is significantly better than the commercially available lead zirconate titanate ceramic transducer [53]. Philips Company began research on the production of single crystal transducers in 1997. The first broadband phase array transducer was named “PureWave” and was launched in 2004. Figure 1.7 shows heart image obtained by the phase array transducers [54].

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1 Introduction

Fig. 1.7 Apical 4 chamber view of cardiac harmonic and color flow images obtained using a single crystal transducer. Reproduced with permission [54]. Copyright 2005, IEEE

The annular array transducer [55] shown in Fig. 1.8 can also be fabricated using PMN-PT single crystal and its 1-3 composites. It can be applied to an endoscopic ultrasonic imaging system to get information about human organs and tissues fast and clearly without mechanical rotation. 2. Hydroacoustic transducer Sound waves are the most effective telematics carrier under water, so submarine, subsea oil exploration and underwater exploration equipment need to transmit information by means of sound waves [56]. The underwater acoustic transducer is an energy conversion element that realizes underwater acoustic emission and reception. In the early 1990s, the research on relaxor ferroelectric single crystals was promoted by the urgent application need for underwater acoustic transducers and medical ultrasonic imaging. The US Naval Undersea Warfare Center (NUWC) developed the underwater acoustic transducers based on PMN-PT single crystal and PZN-PT single crystal [57], and explored their application in underwater unmanned vehicles and torpedoes. Researchers in Penn State University have conducted researches on low-noise Tonpilz hydrophones. Figure 1.9a depicts the cross-sectional structure of a typical Tonpilz hydrophone. It is reported that PMN-PT single crystal can reduce the

Fig. 1.8 The annular array transducers based on PMN-PT crystals. Reproduced with permission [55]. Copyright 2010, IEEE

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Fig. 1.9 a A schematic diagram of Tonpilz hydrophones [59], b photograph of the needle-type hydrophone. Reproduced with permission [61]. Copyright 2004, Elsevier

volume of the transducer and widen the band at the same source level after replacing piezoelectric ceramics [58, 59]. In 2001, researchers in the Shanghai Institute of Ceramics and collaborators have also developed the underwater acoustic transducer based on PMN-PT single crystals [60]. Figure 1.9b is a single-element hydrophone developed by Lau et al. [61] using a [001] oriented PMN-35PT single crystal. The hydrophone has a flat frequency response and good receiving sensitivity in the range of 17–24 MHz. 3. Piezoelectric actuator The actuator that works with piezoelectric effect has the advantages of large drive displacement, fast response speed, low driving voltage, etc., and is currently used in mainstream products of electromechanical systems [62–64]. Kim et al. [65] has designed a bendable actuator for nanopositioning systems based on PMN-PT single crystals. The displacement of the actuator reached 42 μm at 10 V operating voltage, approximately 15 times higher than that of PZT products. Woody et al. has increased the driving force by a stacked piezoelectric actuator consisting of a multilayer PMNPT piezoelectric wafer [66] and analyzed the effects of different dimensions on drive performance. Figure 1.10 shows two flextensional actuators made by TRS Technologies company: the left picture shows the “31” mode of operation, the drive element is a complete single crystal wafer; the right picture shows the “33” mode of

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1 Introduction

Fig. 1.10 Flextensional actuators based on PMN-PT crystals. Reproduced with permission [68]. Copyright 2012, AIP Publishing

operation, the component is stacked by 40 pieces of PMN-PT single crystals along the thickness direction [67, 68]. Lam et al. [69] has designed a fluid ejector using a piezoelectric ring of PMN-35PT single crystal, as shown in Fig. 1.11a. Figure 1.11b shows a cross-sectional schematic of the PMN-PT single-crystal ejector that produces a relatively high axial displacement at low drive voltages, which outperforms the corresponding PZT ceramic injector. 4. Piezoelectric transformer The piezoelectric transformer comprehensively utilizes the positive and inverse piezoelectric effects of the material. The transformer first converts the electrical energy into mechanical vibration near the resonant frequency, and then uses the impedance transformation to achieve buck or boost mode [70–72]. The early Rosentype transformer was developed based on BaTiO3 ceramics. The voltage step-up ratio is limited by the piezoelectric properties of the material and has no practical

Fig. 1.11 The photo and schematic of a fluid ejector based on PMN-PT crystal. Reproduced with permission [69]. Copyright 2005, Elsevier

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application prospects. With the development of PZT-based piezoelectric ceramics, the performance of piezoelectric transformers has gradually improved and has been marketed [73–75]. Compared with electromagnetic transformers, piezoelectric transformers have the advantages of small size, light weight, high conversion efficiency, good output waveform and no electromagnetic interference. They are widely used in cold cathode fluorescent lamps and negative ion generators [76, 77]. The trend of miniaturization and integration of information equipment has placed more stringent requirements on the performance of piezoelectric materials in transformers. The excellent piezoelectric and electromechanical coupling properties of PMN-PT single crystals have attracted more and more attention in this field. Zhuang et al. [78] has developed flat-panel transformers based on PMN-PT and Mn-PMN-PT single crystals with a power density of up to 38 W/cm3 , which is five times that of the same type PZT ceramic transformers. Wang et al. [79] has fabricated a longitudinal piezoelectric transformer using PMN-PT single crystal, which is simpler and smaller than the Rosen type transformer and avoids parasitic vibration and acoustic mismatch. The laminated composite of the longitudinal piezoelectric transformer and the Terfenol-D magnetostrictive rod can further realize the dual resonance converse magnetoelectric effect [80]. It can be seen from Fig. 1.12a that this double resonance effect is derived

Fig. 1.12 a The schematic diagram and b photo of the double resonance inverse magnetoelectric device based on PMN-PT single crystal. Reproduced with permission [80]. Copyright 2011, AIP Publishing. c The AC electroluminescence device with PMN-PT single crystal transformer. Reproduced with permission [81]. Copyright 2007, Springer Nature

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1 Introduction

from the mechanically mediated resonant inverse piezoelectric effect in the PMNPT single crystal and the resonant inverse magnetoelectric effect in the Terfenol-D alloy. Figure 1.12b is a photo of the device. Wang et al. [81] has successfully applied the PMN-PT single crystal transformer into the AC electroluminescence device. As shown in Fig. 1.12c, a voltage of 2 V was input to the transformer and it can drive a luminescent screen with a threshold voltage exceeding 110 V. 5. Mechanical energy harvesting The PMN-PT single crystal near the MPB composition has a very large piezoelectric constant (~2500 pC/N), which is almost four times that of commercial PZT ceramics and 20 times that of barium titanate [82]. Such an excellent piezoelectric activity can greatly increase the conversion efficiency of mechanical energy when PMN-PT single crystal was used to harvest mechanical energy in the environment. Ren and collaborators [83] have used a PMN-29PT single crystal to prepare a high performance energy harvesting device with the output power of 4.94 mW at a periodic 4 N load of 1.4 kHz. They have also reported that a peak voltage of 91 V could be obtained with a proof mass of 0.5 g and a periodic external force of 0.05 N, the prototype device can drive low-power portable electronic products [84]. Lee’s group [85, 86] have studied a flexible piezoelectric energy harvester based on the PMNPT single crystal thick film. The specific preparation process is shown in Fig. 1.13. When the PMN-PT single crystal thick film was transferred to the PET substrate, a current output of up to 0.233 mA can be achieved upon bending. The authors have further demonstrated the application of the energy harvesting device as a rat cardiac pacemaker. 6. Piezoelectric substrates Large-size PMN-PT single crystals have good cutting hardness, so they can be machined into piezoelectric substrates as bulk materials [87]. From the inverse piezoelectric effect, it is known that applying an external electric field to a PMN-PT single crystal generates stress inside the crystal, which in turn causes a mechanical deformation proportional to the electric field. Therefore, PMN-PT single crystals can be

Fig. 1.13 Flexible PMN-PT based piezoelectric energy harvesters. Reproduced with permission [85]. Copyright 2014, John Wiley and Sons

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Fig. 1.14 Experimental setup for Raman and PL measurements of MoS2 on PMN-PT. Reproduced with permission [92]. Copyright 2013, American Chemical Society

considered as an excellent linear electrostrictive material [88]. When some other materials are deposited or attached on the surface of PMN-PT single crystals, stress modulation can be easily achieved by applying an external electric field. This in situ, reversible stress modulation plays an important role in many fields [89–91]. For example, Lau et al. [92] has used the biaxial compression strain generated by the inverse piezoelectric effect of PMN-PT single crystal to modulate the band structure of the two-dimensional molybdenum disulfide, as shown in Fig. 1.14. A significant blue shift of band gap has been observed inside the two-dimensional material, which increased by approximately 0.3 eV per 1% of the strain. Hao’s group [93] have constructed a dual-functional device by depositing ZnS based luminescent materials on a PMN-PT single crystal piezoelectric substrate. The specific design idea is shown in Fig. 1.15. Manganese-doped zinc sulfide is an excellent mechanoluminescence material. The piezoelectric potential generated by external force applied to the wurtzite ZnS lattice triggers the Mn2+ luminescence center. Therefore, when a ZnS:Mn thin film is deposited on the surface of a [001]oriented PMN-PT single crystal, the ZnS:Mn thin film electroluminescence can be indirectly realized by applying an alternating electric field to the PMN-PT. At the same time, the high-frequency alternating electric field applied to the PMN-PT single crystal will output ultrasonic waves to the outside. Therefore, when these two materials are superimposed, functional integration is achieved. In another work, they have extended the idea of stress engineering, using PMN-PT single crystal as the electrostrictive substrate to dynamically adjust the near-infrared photoluminescence properties of SrTiO3 :Ni2+ inorganic film [94]. Single photon sources that emitted on demand are key components of many quantum communication and computing technologies. Recently, Zhang et al. has developed a wavelength-tunable single-photon emission source based on the piezoelectric properties of PMN-PT single crystals. The light source structure is shown in Fig. 1.16 [95]. They have integrated a semiconductor nanofilm diode containing selfassembled quantum dots on a 300 μm thick PMN-PT wafer. Inputting an ultrashort electrical pulse into the diode can trigger a single photon in the nanofilm. Changing

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1 Introduction

Fig. 1.15 The schematic diagram of PMN-PT based dual-functional device. Reproduced with permission [93]. Copyright 2012, John Wiley and Sons

Fig. 1.16 A wavelength-tunable single-photon emission source based on the PMN-PT single crystal. Reproduced with permission [95]. Copyright 2013, American Chemical Society

the bias applied to the PMN-PT substrate can precisely adjust the energy of the single photon. The entire single photon source operates at up to 0.8 GHz. In addition, PMN-PT single crystals have also been applied in other fields such as chemical sensors [96], non-destructive testing [97], etc. The various examples listed above are mainly focused on the piezoelectric effect (or inverse piezoelectric effect) of PMN-PT single crystal, although belonging to different application fields.

1.2 PMN-PT Relaxor Ferroelectric Single Crystals

1.2.3.2

17

Pyroelectric Devices

As an outstanding representative of ferroelectric materials, the excellent pyroelectric properties of PMN-PT single crystals have not received much attention until recent years. Compared with traditional pyroelectric materials such as triglycine sulfate (TGS) and lithium tantalate (LiTaO3 ) crystals, the pyroelectric parameters of PMN-PT single crystals performed well. In recent years, the development of PMNPT-based pyroelectric devices has been very attractive. Luo’s group from the Shanghai Institute of Ceramics, Chinese Academy of Sciences have achieved a series of important research progress on the pyroelectric properties and prototype devices of PMN-PT single crystals [98–102]. Thus it can be seen that PMN-PT single crystals have attractive application prospects in addition to piezoelectric devices. 1. Infrared detector The design idea of using pyroelectric effects to detect heat radiation was originally proposed by Ta in the paper “Action of radiations on pyroelectric crystals” [103]. In 1956, Chynoweth revealed in the barium titanate system that pyroelectric crystals only responded to temperature changes [104], which promoted the study of ferroelectric materials for detecting infrared radiation. With the development of pyroelectric theory, many important pyroelectric materials have been discovered and improved for the preparation of pyroelectric devices. Tang et al. [105] has systematically studied the effects of different compositions, temperature and crystallographic orientation on the pyroelectric properties of PMN-PT single crystals. On the basis of predecessors’ researches, they have determined that rhombohedral PMN-PT single crystal with [111] orientation has the best pyroelectric figure of merit. And based on this, the infrared detectors with compensated elements have been prepared. In addition, Luo’s group [106] has further studied the effect of Mn ion doping on the performance of PMN-PT single crystal pyroelectric devices. It was found that the defect dipole pair generated after doping can pin the domain walls, reduce the dielectric loss of single crystal and improve the pyroelectric performance. The internal structure of the pyroelectric device is shown in Fig. 1.17a. Shao et al. [107] has used ANSYS software for finite element simulation to optimize the electrically calibrated infrared detector structure, as shown in Fig. 1.17b. Xu et al. [108] has improved the electrode layout of the PMN-PT single crystal, which improved the responsivity of the infrared detector by 3.6 times. Wang et al. [109] has designed an 8 × 1 CMOS high SNR reading circuit to provide a viable solution for signal reading of PMN-PT single crystal based uncooled focal plane array detectors. Li et al. [110] has focused on the size effect of the pyroelectric properties of Mn-doped PMN-PT single crystals. Theoretical simulations and device measurements have shown that reducing the asymmetric electrode size and chip thickness can significantly increase the pyroelectric coefficient. The resulting device had a specific detection rate of 3.01 × 109 cm Hz1/2 /W at a 4 Hz chopping frequency, which achieved the international leading level. 2. Heat energy harvesting The pyroelectric effect is not only a signal conversion mechanism, but also a thermal to electric energy conversion mechanism. We know that temperature changes in space

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Fig. 1.17 Infrared detectors based on PMN-PT crystals a Reproduced with permission [106]. Copyright 2011, John Wiley and Sons. b Reproduced with permission [107]. Copyright 2012, Elsevier. c Reproduced with permission [109]. Copyright 2011, John Wiley and Sons.

will create a temperature difference, and the thermoelectric material can convert the temperature difference across the material into a current through the Seebeck effect to achieve power generation [111–114]. When the temperature is evenly distributed in the local space but fluctuates with time, it is necessary to use the pyroelectric effect to convert thermal energy into electrical energy [115–117]. The PMN-PT single crystal after poling has excellent pyroelectric properties, which is also used in the field of heat energy harvesting. As shown in Fig. 1.18, Ravindran et al. [118] has designed a micro thermomechanic-pyroelectric generator (μTMPG) using a rhombohedral PMN-13PT single crystal. After optimization, the output power of the system can reach 39.4 mW. Kandilian et al. [119] has examined the pyroelectric power generation efficiency of PMN-32PT single crystals under the Olsen cycle. The study found that the energy density and power density of PMN-32PT single crystals in a single cycle reached 100 mJ/cm3 and 4.92 mW/cm3 , respectively, which are equivalent to their physical model predictions.

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Fig. 1.18 A micro thermomechanicpyroelectric generator (μTMPG) based on PMN-13PT single crystal. Reproduced with permission [118]. Copyright 2011, AIP Publishing

1.2.3.3

Ferroelectric Devices

The discovery of ferroelectric materials has been around for nearly 100 years. Unfortunately, many commercial devices only utilizes their piezoelectric or pyroelectric properties. Prior to the invention of the ferroelectric random access memory (FeRAM), there were no devices that actually utilized the ferroelectric properties of materials [120]. Until now, there have been few practical products using the ferroelectric performance of PMN-PT single crystals on the market. This is because the nature of the ferroelectric effect is the relationship between the two electrical quantities (electrical displacement and electric field), and does not involve conversion between other physical quantities. In order to realize the application of ferroelectric performance, the current mainstream idea is to establish a relationship between electrical displacement and other physical quantities through macroscopic multiphase recombination. A few works on prototype device design have also used PMN-PT single crystals as ferroelectric functional substrates, for example: The 2-2 type magnetoelectric composite heterostructure is a typical example of ferroelectric materials participating in multiphase composite [121]. The ferromagnetic film directly grown on the ferroelectric PMN-PT single crystal substrate has a good interface bond, which can eliminate the clamping effect during the coupling process [122]. Sun et al. [123] has coated ferrite films on [011] cut PMN-PT single crystals, and observed strong magnetoelectric coupling effects in Fe3 O4 /PMN-PT multiferroic heterojunctions. The electrostatic field induced ferromagnetic resonance field changes to 600 Oe, which corresponds to the magnetoelectric coupling coefficient of 67 Oe cm kV−1 . Zhu et al. [124] has prepared a hexagonal ZnO:Mn thin magnetic semiconductor film on the [111] crystal plane of PMN-PT ferroelectric single crystal. Charge-mediated bistable resistance and ferromagnetic switching modulation phenomena have been observed in the heterojunction at room temperature, providing a new strategy for designing information storage devices. Chen et al. [125] has grown an oxygen-depleted TiO2-δ inorganic semiconductor film on the surface of a [001] oriented PMN-30PT single crystal, as shown in Fig. 1.19. It has been found that the carrier concentration in the TiO2-δ film can be reversibly adjusted as

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Fig. 1.19 Semiconductor/PMNPT heterostructures with tunable electrotransport and magnetic behaviors. Reproduced with permission [125]. Copyright 2016, American Chemical Society

the gate voltage changes the polarization direction of the PMN-PT single crystal, which in turn changes the electrical transport and magnetic properties of the film. The relaxation phenomenon of resistivity has also been found in the process of in situ regulation. The relaxation originated from the competition mechanism of Ti3+ -VO electron trap and PMN-PT surface bound charges. In addition to thin film materials with thickness ranging from tens to hundreds of nm, researchers have attempted to combine two-dimensional materials with several atomic layer thicknesses and PMN-PT single crystals to construct ferroelectric field effect transistors. Jie et al. [126] has studied the regulation behavior of the electrical transport properties of grapheme by directly transferring the single-layer graphene grown by chemical vapor deposition (CVD) onto the surface of PMN-PT single crystal. In the gate voltage scan range of over −100 V to +100 V, the transfer curve of graphene in the channel appears p-type, similar in shape to the hysteresis loop of ferroelectric material, as shown in Fig. 1.20a. In order to further study the effect of ferroelectric polarization on the charge transport of two-dimensional materials, Park et al. has inserted the hexagonal boron nitride (hBN) into the interface between graphene and PMN-PT single crystal, as shown in Fig. 1.20b [127]. In this somewhat complex structure, the intrinsic properties of graphene have been preserved and distinguished from the ferroelectric polarization effects of the PMN-PT single crystal. In a wide gate voltage scan range, the spontaneous polarization of the PMN-PT single crystal is sharply reversed, affecting the asymmetry and antihysteretic behavior of the channel conductance. When the scan range of the gate voltage is higher than the coercive field of the PMN-PT single crystal, the channel anti-lag behavior gradually evolves into a common ferroelectric hysteresis behavior. This work elaborated the complex coupling relationship between channel current saturation, sudden conductance variation, antihysteretic behaviors and polarization switching of ferroelectrics.

1.3 The Main Research Contents of This Thesis

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Fig. 1.20 Ferroelectric FET based on graphene and PMN-PT crystals. a Reproduced with permission [126]. Copyright 2013, American Chemical Society. b Reproduced with permission [127]. Copyright 2015, American Chemical Society

1.3 The Main Research Contents of This Thesis The development of new applications is the only way to industrialize functional materials, and it is also the top priority of materials science. The PMN-PT relaxor ferroelectric single crystal has been “a pearl in the palm” for scientists and engineers in the field of ferroelectrics due to its excellent piezoelectric, ferroelectric and pyroelectric properties. Compared with piezoelectric devices that are relatively mature or even commercialized, the application research of the excellent pyroelectric properties and ferroelectric properties needs to be further strengthened. Designing and developing new devices based on the pyroelectric and ferroelectric properties of PMN-PT single crystals is of great significance for expanding the application of ferroelectric materials in electronics and other related high-tech fields. In this thesis, PMN-PT relaxor ferroelectric single crystal is taken as the research object, and some new applications of ferroelectric materials in the field of information technology are explored. As is well known, the main components of integrated

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1 Introduction

circuit include the following three broad categories: transistors, memories, and sensors/actuators [128]. Based on the excellent pyroelectric and ferroelectric properties of high-quality PMN-PT single crystals, we have designed and fabricated three types of prototype devices. New applications of PMN-PT single crystals have been developed in addition to piezoelectric devices. The specific research content mainly focuses on the following three aspects: 1. An optothermal field effect transistor based on PMN-26PT single crystals Transistors are a very important class of devices in the integrated circuit. They are the hardware foundation for implementing Boolean logic operations. A conventional field effect transistor is a voltage control element, and a gate voltage is a signal source that regulates an output current, and thus electrons are the only information carrier. Photonic technology is infiltrating into various fields of the information industry. The concept of “photonics revolution” is to point out that photons will replace electrons as information carriers of the information technology field in the future. In Chap. 2, the high-quality rhombohedral PMN-26PT single crystal wafer has been used to replace the gate dielectric layer in the conventional transistor. The source, drain and gate electrodes have been deposited on the wafer by the semiconductor process such as photolithography. A single layer of molybdenum disulfide (MoS2 ) grown by chemical vapor deposition has been used as a channel material to construct an optothermal field effect transistor. By using the excellent pyroelectric properties of the [111]-oriented PMN-26PT single crystal, infrared light can be used as the input signal instead of the gate voltage. The working mechanism of the prototype device has been studied by investigating the relationship between infrared light intensity and channel current combined with finite element analysis. The remote control of the channel current in the transistor by infrared light will play an important role in the field of information technology such as infrared detection, infrared communication and thermal imaging. 2. An ultrabroadband photodetector based on PMN-28PT single crystal The sensor is a key component of the information acquisition system. It can convert various physical or chemical signals such as light, heat, force, magnetism and sound into electrical signals according to certain rules. Photodetectors belong to an important class of sensors, they can detect light of different wavelengths depending on the different materials and working principles. In Chap. 3, a new type of photodetector based on PMN-28PT single crystal has been explored. The pyroelectric coefficient of PMN-28PT single crystal has been improved by domain engineering, and the domain structure has been examined by observing the extinction with a PLM. At the same time, silver nanowires have been introduced as a transparent electrode, and the thermal conduction step has been circumvented by modifying the structure of the conventional pyroelectric device, so as to improve the frequency response of the device without loss of responsivity. The spectral detection range of the device has been studied by testing the photoelectric response at different wavelengths, and the response mechanism of the device under short-wavelength radiation has been emphasized.

1.3 The Main Research Contents of This Thesis

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3. A self-powered ferroelectric memory system based on PMN-35PT single crystal The storage of information is a prerequisite for ensuring that users can access the information. High-performance memory plays an increasingly important role in electronic products such as smartphones and computers. Ferroelectric memory has become a new candidate of non-volatile memory due to its advantages of radiation resistance and fast reading/writing. The two stable polarization states of the ferroelectric material (+Pr and −Pr ) can be used to compile “0” and “1” in binary Boolean logic operations. The rapid polarization switching of ferroelectric domains is realized by applying an external electric field, which is the fundamental mechanism for realizing the information writing of ferroelectric memory. But this kind of information writing is a process with power consumption. It would be a revolutionary attempt to use the mechanical energy that exists everywhere in the environment as the energy source for information writing. In Chap. 4, we have developed a triboelectric nanogenerator using a single layer polystyrene (PS) nanospheres array and a polyvinylidene fluoride (PVDF) porous film. The working principle and electrical output performance of the nanogenerator have been studied, and the feasibility of using pulsed high voltage to pole PMN-PT ferroelectric single crystals has been explored. At the same time, the ferroelectric transistor memory cell has been constructed by using the PMN-35PT wafer near the MPB composition as the gate dielectric layer, and its storage performance has been studied. By designing of a reasonable circuit, the energy supply module and the ferroelectric storage unit have been connected to realize a self-powered ferroelectric storage system. Finally, the possibility of increasing the storage density of the self-powered system has been discussed by the poling process with PFM. As shown in Fig. 1.21, the prototype devices designed in this thesis perform their duties in the field of information technology. From the perspective of information flow, these devices are responsible for the collection, processing and storage of information data. In the following chapters, we will introduce the design idea, preparation process, performance characterization and working mechanism of these devices one by one.

Fig. 1.21 The main research contents of this thesis

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References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26.

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56. Mo XP (2006) Innovations for sonar: new technology and designs for underwater acoustic transducers. Physics 35:414–419 57. Xu JY, Jin M (2008) New relaxor ferroelectric crystals-growth, performance and applications [M]. Chemical Industry Press 58. Sherlock NP (2010) Relaxor-PT single crystals for broad bandwidth, high power sonar projectors. PA: The Pennsylvania State University 59. Ye ZG (2008) Handbook of advanced dielectric, piezoelectric and ferroelectric materials— synthesis, characterization and applications. Woodhead, England 60. Meng H, Yu HP, Luo HS et al (2004) PMNT and its application in underwater acoustic transducers. Acoust Electron Eng 1:22–26 61. Lau ST, Lam KH, Chan HLW et al (2004) Ferroelectric lead magnesium niobate-lead titanate single crystals for ultrasonic hydrophone applications. Mater Sci Eng B 111:25–30 62. Xu TB, Tolliver L, Jiang X et al (2013) A single crystal lead magnesium niobate-lead titanate multilayer-stacked cryogenic flextensional actuator. Appl Phys Lett 102:042906 63. Damjanovic D, Newnham RE (1992) Electrostrictive and piezoelectric materials for actuator applications. J Intell Mater Syst Struct 3:190–208 64. Wilkie WK, Inman DJ, Lloyd JM et al (2006) Anisotropic laminar piezocomposite actuator incorporating machined PMN-PT single-crystal fibers. J Intell Mater Syst Struct 17:15–28 65. Kim KC, Kim YS, Kim HJ et al (2006) Finite element analysis of piezoelectric actuator with PMN-PT single crystals for nanopositioning. Curr Appl Phys 6:1064–1067 66. Woody SC, Smith ST, Jiang X et al (2005) Performance of single-crystal Pb(Mg1/3 Nb2/3 )32%PbTiO3 stacked actuators with application to adaptive structures. Rev Sci Instrum 76:075112 67. Jiang XN, Cook W, Hackenberger WS (2009) Cryogenic piezoelectric actuators. Proc SPIE 7439:74390Z 68. Zhang SJ, Li F (2012) High performance ferroelectric relaxor-PbTiO3 single crystals: status and perspective. J Appl Phys 111:031301 69. Lam KH, Chan HLW, Luo HS et al (2005) Piezoelectrically actuated ejector using PMN-PT single crystal. Sens Actuat A 121:197–202 70. Huang YT, Lin ZY (2004) Progress in research on piezoelectric transformers. Electron Compon Mater 23:7–10 71. Hu JH, Li HL, Chan HLW et al (2001) A ring-shaped piezoelectric transformer operating in the third symmetric extensional vibration mode. Sens Actuat A 88:79–86 72. Wang F, Lin SY (2008) Research and development of piezoelectric ceramic transformer. Electron Compon Device Appl 10:75–77 73. Li LT, Deng W, Chai J et al (1990) Lead zirconate titanate ceramics and monolithic piezoelectric transformer of low firing temperature. Ferroelectrics 101:193–200 74. Kozielski L, Lisi´nska-Czekaj A, Czekaj D (2007) Graded PZT ceramics for piezoelectric transformers. Prog Solid State Chem 35:521–530 75. Smith GL, Pulskamp JS, Sanchez LM et al (2012) PZT-based piezoelectric MEMS technology. J Am Ceram Soc 95:1777–1792 76. Fuda Y, Kumasaka K, Katsuno M et al (1997) Piezoelectric transformer for cold cathode fluorescent lamp inverter. Jpn J Appl Phys 36:3050–3052 77. Wang F, Shi W, Luo H (2010) Step-down piezoelectric transformer fabricated with (1-x)Pb(Mg1/3 Nb2/3 )O3 -xPbTiO3 single crystal. Rev Sci Instrum 81:043904 78. Zhuang Y, Ural SO, Gosain R et al (2009) High power piezoelectric transformers with Pb(Mg1/3 Nb2/3 )O3 -PbTiO3 single crystals. Appl Phys Express 2:121402 79. Wang F, Shi W, Tang Y et al (2010) A longitudinal (1-x)Pb(Mg1/3 Nb2/3 )O3 -xPbTiO3 singlecrystal piezoelectric transformer. Appl Phys A 100:1231–1236 80. Leung CM, Or SW, Wang F et al (2011) Dual-resonance converse magnetoelectric and voltage step-up effects in laminated composite of long-type 0.71Pb(Mg1/3 Nb2/3 )O3 -0.29PbTiO3 piezoelectric single-crystal transformer and Tb0.3 Dy0.7 Fe1.92 magnetostrictive alloy bars. J Appl Phys 109:104103

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Chapter 2

An Optothermal Field Effect Transistor Based on PMN-26PT Single Crystal

2.1 Introduction of This Chapter 2.1.1 Field Effect Transistors and the ‘Photonics Revolution’ At the end of the nineteenth century, Boolean discovered that logical thinking can be presented in mathematical expressions [1]. Later, John Atanasoff and Claude Shannon advocated the use of binary circuits to implement Boolean logic operations [2]. The transistor is the underlying component that implements the three basic logic gates AND, OR and NOT. It has been studied for about 90 years, dating back to 1925. At that time, German scientist Julius Lilienfeld had proposed to apply a strong electric field to control the charge carriers on the semiconductor surface [3], the idea of adjusting the channel conductivity by electric field has been applied for a patent. The first transistor that worked fine was developed more than twenty years later by three members in Bell Labs, Walter Brattain, John Bardeen, and William Shockley [4]. The three scientists have won the 1956’s Nobel Prize in Physics for this outstanding contribution. Later, with the emergence of some landmark inventions such as silicon transistors and integrated circuits, field-effect transistors (FETs) have become an essential part of the modern electronics industry. Today, although it has long been widely used in many aspects such as amplifiers and electronic switches, with the continuous development of materials science and electronics, the research on field effect transistors is still in progress [5–8]. Conventional field effect transistors use the electric field effect in the input loop to control the magnitude of the output current in the channel. The typical three-electrode structure is shown in Fig. 2.1 [9]. Between the source electrode and the drain electrode is a channel filled with a semiconductor material. At a fixed bias voltage (U DS ), the current in the channel (I DS ) is determined by the carriers in the semiconductor. The carrier concentration can be controlled with the gate voltage (U GS ) applied between the source electrode and the gate electrode. It can be seen that the FET is a voltage control element (the gate voltage regulates the channel output current). Thus the electron is the only information carrier. Since the electron carrier cannot propagate © Tsinghua University Press, Beijing and Springer Nature Singapore Pte Ltd. 2020 H. Fang, Novel Devices Based on Relaxor Ferroelectric PMN-PT Single Crystals, Springer Theses, https://doi.org/10.1007/978-981-15-4312-8_2

29

30

2 An Optothermal Field Effect Transistor Based on PMN-26PT …

Fig. 2.1 Basic structure of a typical field effect transistor

in free space, it can only flow along the solid loop, the electrical delay effect in the loop limits the speed at which information is transmitted. In order to break through this bottleneck, it is necessary to make a bold innovation on the information carrier, at least in the process of interconnecting logic gates, using an uncharged carrier instead of the electron carrier [10]. As early as the 1990s, photonics technology has penetrated into the field of information industry. The concept of “photonics revolution” proposed at the 2009 World Economic Forum is a far-sighted indication that photons will be widely used as information carriers and core media in the future [11]. Phototransistors can be constructed by introducing photons into the transistor as information carriers [12–15]. In a phototransistor, when the energy of the incident light is higher than the band gap of the channel semiconductor, the incident photons will be absorbed by the semiconductor in the channel to form photogenerated carriers. Thus the current in the channel is modulated by the incident photons, exhibiting photosensitive properties. In this chapter, we have developed a field effect transistor that can be controlled by infrared light using the pyroelectric properties of the relaxor ferroelectric single crystal PMN-PT. The incident photons will interact with the dielectric layer instead of being absorbed by the channel semiconductor. Its working principle is also different from the traditional phototransistors.

2.1.2 Interface Engineering of Two-Dimensional Channel Semiconductor The emergence of graphene has led people into a new field of materials. When the thickness of ordinary bulk materials is reduced to only a few atomic layers, many of their physical properties undergo incredible changes [16]. In addition to graphene, many new two-dimensional material systems have been discovered in recent years [17–19]. Transition metal dichalcogenides (TMDs) are an important and unique class that compensates for the lack of graphene (zero band gap) [20]. Taking molybdenum disulfide (MoS2 ) as an example, it has a typical anisotropic structure. There is a strong covalent bond between adjacent sulfur atoms and molybdenum atoms in each layer, while the interlayer van der Waals force is weak [21, 22]. Therefore, a single layer of the compound can be prepared by mechanical exfoliating the bulk material. The band gap of the MoS2 bulk material is about 1.2 eV, it can be increased to 1.9 eV when the thickness is reduced to the monoatomic layer. This reduction in dimension not only

2.1 Introduction of This Chapter

31

brings about a quantitative change in the band gap, but also causes a change from the original indirect band gap to the direct band gap [23]. Therefore, it is more suitable as a channel material for field effect transistors than the semi-metallic graphene. Since the thickness of two-dimensional materials is only a few atomic layers, another very important feature is that they are easy to conform to the substrate material. The optical and electrical properties of 2D materials are easily affected by the surrounding medium environment including the substrate material [24–26]. When a two-dimensional material is in contact with a functional material, the interface between them has a great influence on the 2D material. Since carriers in 2D materials are confined into a space of 1 nm in the thickness direction [27], many short-range effects occurring at the interface can be clearly manifested. It has been shown in the literature that interface engineering is an effective means to control the carrier transport characteristics of two-dimensional materials [28–30]. For example, Li et al. [28] have reported the effect of self-assembled organic molecules monolayers with different dipole moments on the electrical properties of MoS2 . In this chapter, we have demonstrated the details of an optothermal field effect transistor by interface engineering of single atomic layer MoS2 and PMN-PT single crystals.

2.2 The Design and Fabrication of Optothermal FET 2.2.1 Structural Design of the Prototype FET As shown in Fig. 2.2, the prototype device we designed has a multi-layer structure. A high-quality rhombohedral PMN-26PT single crystal wafer replaces the dielectric layer in the conventional FET, and the channel is filled with a single layer of molybdenum disulfide. The specific design idea is as follows: The rhombohedral PMN-PT single crystal has an excellent pyroelectric performance. To fully exert this performance the device requires a clever structural design, and the selection of the channel semiconductor material is important. Since the pyroelectric effect is a surface effect, the change in temperature only affects the charge density of the top and bottom surfaces of the PMN-PT single crystal. If the two-dimensional material is directly in contact with the surface of the PMN-PT single crystal, interface engineering can be Fig. 2.2 A schematic structure of the optothermal field effect transistor. Reproduced with permission [31]

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2 An Optothermal Field Effect Transistor Based on PMN-26PT …

used to bridge the surface charge density of PMN-PT single crystal with the carrier concentration of 2D material. As a result, the incident infrared light causes a change in the crystal temperature, which in turn causes a change in the polarization of the single crystal (intuitively manifested as a change in the surface charge density of the single crystal). And then, the carrier concentration of MoS2 in the channel must change accordingly. It is worth mentioning that in order to enhance the absorption of incident infrared light, the gate electrode is superimposed with a black carbon electrode in addition to the gold film. On the other hand, the PMN-26PT wafer as a dielectric layer is easily broken when it is polished to a thickness of 50–60 µm, which causes inconvenience to subsequent processing and testing. In order to ensure the quality of the crystal and reduce the occurrence of macro defects in the transistor fabrication process, we have introduced indium tin oxide (ITO) glass as a transparent conductive substrate. Below we discussed the rationality of the structural design of this device from the thermodynamics theory. To describe the thermodynamic state of the elastic dielectric PMN-PT crystal in this system, three pairs of variables are required. They are a pair of scalars (temperature T, entropy S); a pair of vectors (electric field E, electrical displacement D) and a pair of second-order tensors (stress X, strain x). Considering the temperature, electric field and stress as independent variables, the differential form of electric displacement is as follows [32]: dDm = (

∂ Dm ∂ Dm ∂ Dm ) X,E dT ) E,T dX i + ( ) X,T dE n + ( ∂ Xi ∂ En ∂T

(2.1)

where m = 1–3, n = 1–3, i = 1–6. When the applied electric field is zero, the electrical displacement can be simplified to: dDm = (

∂ Dm ∂ Dm ) X dT )T dX i + ( ∂ Xi ∂T

(2.2)

where ( ∂∂DXmi )T is the piezoelectric strain constant, denoted as d mi , and ( ∂∂DTm ) X is the pyroelectric coefficient under constant stress, denoted as pX . Considering the case where temperature, electric field and strain are independent variables, the differential form of electric displacement is as follows: dDm = (

∂ Dm ∂ Dm ∂ Dm )x,E dT ) E,T dxi + ( )x,T dE n + ( ∂ xi ∂ En ∂T

(2.3)

Similarly, when the applied electric field is zero, the electrical displacement can be simplified to dDm = (

∂ Dm ∂ Dm )T dxi + ( )x dT ∂ xi ∂T

(2.4)

2.2 The Design and Fabrication of Optothermal FET

33

Because the applied electric field is zero, the strain is only a function of stress and temperature, so: dxi = (

∂ xi ∂ xi ) X dT )T dX j + ( ∂Xj ∂T

(2.5)

If the PMN-PT wafer is completely freestanding, i.e. dX j = 0, then ∂ xi ) X dT ∂T

(2.6)

∂ Dm ∂ xi ∂ Dm ) X dT + ( )x dT )T ( ∂ xi ∂T ∂T

(2.7)

dxi = ( It can be found from (2.4) dDm = ( That is to say, (

dDm ∂ Dm ∂ Dm ∂ xi )X = ( )x + ( )X )T ( dT ∂T ∂ xi ∂T

(2.8)

where ( ∂∂Dxim )T and ( ∂∂ xTi ) X are piezoelectric stress constant emi and thermal expansion coefficient α i . As shown in Eq. (2.8), the pyroelectric coefficient of a PMN-PT crystal in a completely free state consists of two terms. The first term on the right side of the equation is called the primary pyroelectric coefficient, denoted as px . The second term on the right side of the equation is called the secondary pyroelectric coefficient, which is the product of the piezoelectric stress constant and the thermal expansion coefficient of the material. When the PMN-PT wafer is mounted on the ITO substrate, it is no longer in a completely free boundary condition, but rather a partial clamping model as shown in Fig. 2.3. It is assumed that the thickness direction of the PMN-PT wafer is 3-axis, and the plane in which the wafer is located is composed of orthogonal 1-axis and 2-axis. The PMN-PT wafer is approximately isotropic in the plane. Since the bottom surface of the PMN-PT wafer is fixed on the rigid ITO substrate, it is clamped in the plane composed of 1-axis and 2-axis. Only the deformation in the 3-axis direction is allowed. In this case, the boundary conditions of the sample are as follows: Fig. 2.3 Partial clamping model of PMN-PT single crystal on ITO substrate

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2 An Optothermal Field Effect Transistor Based on PMN-26PT …

⎧ D1 = D2 = 0 ⎪ ⎪ ⎨ x1 = x2 = 0 ⎪ X = X 2, X 3 = 0 ⎪ ⎩ 1 α1 = α2 , α4 = α5 = α6 = 0

(2.9)

Substituting this into Eq. (2.2), dD3 = 2d31 dX 1 + p3X dT

(2.10)

The pyroelectric coefficient in the partial clamping state can be calculated by the derivative equation of (2.10), p3PC = p3X + 2d31 (

dX 1 ) dT

(2.11)

It can be seen from the above equation that the PMN-PT crystal fixed on the rigid ITO substrate is limited by the thermal expansion of the 1–2 plane, which adds an influence of the d31 mode piezoelectric effect. Although the piezoelectric charge generated in this mode has opposite polarity to the pyroelectric charge, there is no significant decrease between the effective p3PC and p3X . This is because the PMN-PT wafer is a ferroelectric material whose pyroelectric effect is caused by the spontaneous polarization in the 3-axis direction as a function of temperature. The second term on the right side of Eq. (2.11) is at least an order of magnitude lower than the first term and can be ignored. So, the device structure we designed in Fig. 2.2 is reasonable. If non-ferroelectric pyroelectric crystals such as wurtzite structure zinc oxide (ZnO) are chosen, it is unreasonable to follow this device structure. If the thermal expansion of zinc oxide in the plane perpendicular to the c-axis is limited, the effective pyroelectric coefficient will be sharply reduced [32], thereby reducing the optothermal regulation performance of the device. Figure 2.4 shows the equivalent circuit of the optothermal field effect transistor. It can be found that the infrared light replaces the gate voltage (U GS ) as an input signal and enables the channel current (I DS ) to be remote controlled. This transition from “electrical control” to “optical control” in field effect transistors would have potential applications in information technology such as infrared detection, infrared communication and thermal imaging.

2.2.2 Processing of PMN-26PT Single Crystal In order to maximize the optothermal regulation performance of the field effect transistor, it is crucial for selecting the pyroelectric single crystal substrate. A notable feature of crystalline materials is their anisotropy in physical properties, and pyroelectric properties are no exception. For the PMN-PT system single crystal, previous research works have shown that the crystals with composition in the rhombohedral

2.2 The Design and Fabrication of Optothermal FET

35

Fig. 2.4 Equivalent circuit of the optothermal FET

phase region can obtain the optimal pyroelectric performance along the [33] orientation [33]. Thus, we selected a rhombohedral PMN-26PT crystal, and oriented the single crystal by an orientation meter. After determining the [111] crystal plane, the single crystal was cut into a wafer with a thickness of about 0.5 mm. The crystal structure of the PMN-PT wafer has been analyzed by X-ray diffractometer (XRD, Rigaku SmartLab), and the diffraction result is shown in Fig. 2.5a. Within the entire scan range, the wafer only has a diffraction peak corresponding to the [111] crystal plane, which not only confirms the accuracy of the orientation, but also lays the foundation for subsequent machining. Raman spectroscopy is also an effective method to characterize the structural information inside a substance. In addition to elastic collisions (Rayleigh scattering), there are also inelastic collisions (Raman scattering) between photons and matter molecules. The difference in the frequency of the two scattered light (Raman

Fig. 2.5 a XRD pattern of PMN-PT single crystal after machining, Reproduced with permission [31]. b Raman spectrum of PMN-PT single crystal

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2 An Optothermal Field Effect Transistor Based on PMN-26PT …

shift) is related to the molecular vibration and rotational energy level of the measured substance, so the Raman shift can be used as the basis for material identification and structural analysis [34]. Since Raman spectroscopy is very sensitive to the local symmetry of materials, it can often be used as a supplement to XRD analysis techniques. We have tested the Raman spectrum of the oriented PMN-PT single crystal at room temperature with a 532 nm laser excitation (25 mW). Figure 2.5b shows the Raman spectrum of the crystal in the range of 200–1000 cm−1 . Three distinct characteristic peaks can be observed near the wavenumbers of 272, 583 and 770 cm−1 , which correspond to the T2g mode, Eg mode and broadened A1g vibration mode of PMN-PT crystals, respectively. Among them, the A1g vibration mode located in the high wavenumber region reflects the stretching vibration of the [BO6 ] octahedron in the perovskite structure [35], and thus it is sensitive to the order of the B-site ions. Further analysis shows that the broadened A1g vibration mode is composed of the I peak at 740 cm−1 and the II peak at 800 cm−1 , and the intensity of the two peaks are comparable, indicating that the PMN-26PT single crystal has a rhombohedral structure. To facilitate the further processing, we cut the [111] oriented wafer into a number of 5 mm × 5 mm squares, as shown in the inset of Fig. 2.5a. The top and bottom surfaces of the PMN-26PT samples were polished, and then gold electrodes were deposited by an ion sputter for poling and pyroelectric performance tests. The single crystal sample has been poled under a DC electric field of 2 kV/mm at room temperature for half an hour. After aging for 24 h, the pyroelectric performance of the poled PMN-PT crystal has been tested using a pyroelectric function module of the TF2000 ferroelectric analyzer (aixACCT Systems). Figure 2.6a shows the change in polarization during the temperature rise from room temperature to 60 °C. It can be seen from the blue curve that the polarization of the single crystal tends to

Fig. 2.6 a The relationship between the polarization of [111] oriented PMN-PT single crystal and temperature, b comparison of pyroelectric coefficients of PMN-PT single crystals and other materials [36]

2.2 The Design and Fabrication of Optothermal FET

37

decrease monotonously throughout the heating process. As mentioned in Formula (1.2) in Chap.1, the pyroelectric coefficient can be obtained by deriving polarization intensity to the temperature. The sample has a pyroelectric coefficient of 7.1 × 10−4 C/m2 K around room temperature. Figure 2.6b compares the performance of different pyroelectric materials at room temperature [36]. The pyroelectric coefficient of the PMN-PT single crystal is higher than those of common pyroelectric materials such as LiTaO3 crystal, PZT ceramic, PVDF polymers, and TGS crystals. Hence, PMN-PT single crystal is an ideal substrate in optothermal field effect transistors. After studying the pyroelectric properties of the crystal, several PMN-26PT single crystals were polished to a thickness of 60 µm by mechanical thinning. The paraffin is used to fix the wafer in the center of the stainless steel grinder without bubbles. The initial stage was reducing the thickness of the wafer from 0.5 to 0.1 mm with 2000 mesh sandpaper. Then, the polishing powder and the precision polishing film were used to polish the PMN-PT wafer to 60 µm thick. Finally, the crystal was removed from stainless steel grinder and washed with acetone to rinse off the residual paraffin. It can be seen from Fig. 2.2 that the highly smooth surface of PMN-PT single crystal is a prerequisite of the interface engineering. Next, we used the atomic force microscope (AFM) to characterize the surface roughness of the PMN-PT single crystal after polishing. When the AFM probe is sufficiently close to the surface of the PMN-PT single crystal, the interatomic force between them is significantly enhanced. The height information of the surface of the PMN-PT crystal sample can be given by detecting the change in the force of the probe during the scanning process. Figure 2.7 shows the topography we scanned in a 5 µm × 5 µm square area, where the different colors represent the actual heights throughout the sample. The height of each point in the statistical experiment results can be calculated. The root mean square roughness of the polished PMN-PT sample surface is 2.289 nm, and the average roughness is only 1.520 nm. These results indicate that the surface of the PMN-PT crystal sample after polishing is quite smooth.

Fig. 2.7 Surface morphology of PMN-PT crystal after polishing

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2 An Optothermal Field Effect Transistor Based on PMN-26PT …

Fig. 2.8 a Photograph of the polished PMN-PT single crystal, b preparation of gold electrodes with different channel lengths

A 50 nm gold electrode was deposited on the bottom surface of the PMN-PT crystal by magnetron sputtering, and a 100 nm carbon black electrode was coated on the gold electrode by thermal evaporation. The PMN-PT single crystal sheet with a thickness of 60 µm is very fragile and not suitable for the subsequent semiconductor processing (photolithography). Therefore, it is necessary to fix the single crystal sheet on the ITO transparent conductive glass. Figure 2.8a is the photo of PMN-PT crystals fixed on the ITO glass. The gold/carbon black electrode is bonded to ITO, and the polished surface of the PMN-PT single crystal is exposed. After the 50 nm gold electrode was uniformly deposited on the top surface of the wafer by magnetron sputtering, the AZ5214 photoresist was spin-coated. After selecting the appropriate photolithographic mask and exposing it under UV light, the designed source and drain electrodes can be covered with photoresist. The excess gold film was removed using a pre-formed KI/I2 gold etching solution for about 30 s. The gold electrode pattern as shown in Fig. 2.8b is obtained by washing the photoresist with acetone. The length of the channel between adjacent gold electrodes varies, depending on the pattern of the selected lithographic mask. In order to enhance the hydrophilic properties of the single crystal substrate, the single crystal has been subjected to an oxygen plasma treatment for 5 min. The plasma cleaning process can further remove organic residues such as photoresist, but it does not cause damage on the surface of the PMN-PT crystal. Figure 2.9 shows the change in contact angle of the PMN-PT single crystal before and after oxygen plasma treatment. The contact angle of the single crystal surface after plasma cleaning is greatly reduced and the surface has exhibited a hydrophilic state. At this point, the processing of the PMN-PT single crystal is completed. For the subsequent part of this chapter, the sample shown in Fig. 2.8b is simply referred to as a single crystal substrate.

2.2.3 Growth and Transfer of Monolayer MoS2 The monolayer molybdenum disulfide (MoS2 ) grown by chemical vapor deposition is selected as the channel material. The specific growth process can be found in related

2.2 The Design and Fabrication of Optothermal FET

39

Fig. 2.9 The contact angle of PMN-PT single crystals a before oxygen plasma treatment, b after oxygen plasma treatment

Fig. 2.10 a Monolayer MoS2 sample on a silicon substrate, b large-area MoS2 sample transferred to the surface of PMN-PT single crystal. Reproduced with permission [31]

literature [37], and will not be described here. Figure 2.10a shows the photograph of a large area MoS2 grown by a CVD method under an optical microscope. The MoS2 monolayer film on the silicon substrate is continuous, the size is larger than 300 µm × 300 µm and the grain boundaries are clearly visible (at the red arrow in the figure). A solution of polymethyl methacrylate (PMMA) was spin-coated on the surface of the silicon wafer and heated at 100 °C for 20 min in the air to completely evaporate the solvent. Then, the PMMA-coated MoS2 monolayer film was peeled off from the silicon substrate by a potassium hydroxide solution. The PMMA/MoS2 monolayer film floating on the liquid surface was transferred to a single crystal substrate after being rinsed twice with deionized water (removing residual alkali solution). It can be seen from Fig. 2.10b that the MoS2 monolayer film was smoothly transferred to the surface of the single crystal substrate, and the morphology has not been destroyed. The PMMA/MoS2 film in the field of view was still continuous, and no macroscopic wrinkles were observed. The MoS2 film covered on the channel and the gold electrode

40

2 An Optothermal Field Effect Transistor Based on PMN-26PT …

Fig. 2.11 Raman spectra of the MoS2 sample. Reproduced with permission [31]

is flat and closely adheres to the substrate, and the grain boundary is clearly visible (as shown by the red arrow in the figure). We used Raman spectroscopy to characterize the quality of a MoS2 monolayer film transferred to the surface of a PMN-PT single crystal. The sampling point was located inside the channel and the excitation source wavelength was 488 nm. As shown in Fig. 2.11, the samples showed two distinct characteristic peaks at 384 and 404 cm−1 , corresponding to the E12g and A1g vibration modes. The E12g vibration mode is derived from the in-plane vibration of molybdenum atoms and sulfur atoms, while the A1g vibration mode is derived from the out-of-plane vibration of two atoms. The specific atomic displacement patterns in the two Raman activity modes are shown in the inset of Fig. 2.11. In addition, the difference in Raman shift of the two characteristic peaks is closely related to the number of layers in the MoS2 sample [38, 39]. The Raman shift difference of 20 cm−1 in the figure indicates that the MoS2 sample on the surface of the PMN-PT is single layer. At the same time, we also studied the luminescence properties of a single layer MoS2 sample transferred to the surface of a PMN-PT single crystal substrate. Figure 2.12 shows the photoluminescence (PL) spectrum of the sample with an excitation wavelength of 488 nm. As shown, the photoluminescence peak was observed at 659 nm. Further, the band gap (E g ) of the single layer MoS2 sample can be calculated by the following Formula [40] Eg =

hc λ

(2.12)

where h is the Planck constant (4.136 × 10−15 eV s), c is the speed of light (3 × 108 m/s) and λ is the wavelength at the photoluminescence peak position. E g is found to be 1.88 eV, which is consistent with the direct band gap of the twodimensional semiconductor materials reported in the previous literature [41]. The results of Raman spectroscopy and photoluminescence spectroscopy indicate that

2.2 The Design and Fabrication of Optothermal FET

41

Fig. 2.12 PL spectra of the monolayer MoS2 sample. Reproduced with permission [31]

the single layer MoS2 sample on the surface of the PMN-PT single crystal has a high quality and is suitable as a channel semiconductor to construct a field effect transistor.

2.3 The Device Performance and Working Mechanism of Optothermal FET 2.3.1 Infrared Response Before characterizing the performance of the optothermal field effect transistor, it is necessary to perform poling treatment on the PMN-PT single crystal. An electric field of 2 kV/mm was applied between the ITO substrate and the source electrode by a DC power source for 15 min, so that the PMN-PT crystal was fully poled. In order to investigate the relationship between the incident light intensity of infrared light and the channel current, we built a test system on the optical platform. A near-infrared laser with a wavelength of 1064 nm was selected as the incident light source. The device was irradiated from the side of the ITO glass, and the spot was focused on the bottom surface of the single crystal covered with the carbon electrode, as shown in Fig. 2.2. The source and drain electrodes of the field effect transistor are taken out by silver wires and connected to the Keithley 2400 source meter. Figure 2.13a shows the current-voltage (I-V) curves measured in the channel with different incident light intensities from 4 to 14 mW/mm2 . It can be seen that the channel current always maintains a good linear relationship with the bias voltage (U DS ) applied to the drain electrode. Increasing the incident light intensity increases the slope of the I-V curve (the conductance in the channel). Under a constant bias of U DS = 0.5 V, the relationship between channel current and incident light intensity is plotted in Fig. 2.13b. It can be seen that the channel current increases monotonically with the

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2 An Optothermal Field Effect Transistor Based on PMN-26PT …

Fig. 2.13 a I-V curves of field effect transistors under different infrared intensities, b the relationship between the channel current and the light intensity at a fixed bias of 0.5 V. Reproduced with permission [31]

increase of incident light intensity, and the channel current grows faster under the stronger illumination conditions. We further studied the dynamic response of the channel current to external infrared light. The channel current was monitored in real time using a Keithley 2400 source meter under periodic modulated light illumination. Figure 2.14 depicts the real-time variation of the channel current at a fixed bias U DS = 0.5 V. When the infrared light (6 mW/mm2 ) irradiated from the ITO glass side to the PMN-PT single crystal, the current in the channel rapidly rose to 16 nA. Within 30 s of continuous illumination, the channel current was substantially stable with no significant change. When the incident infrared light was blocked, the channel current continued to decrease within 2 min, and the falling process in the dark state can be clearly divided into two stages. In the initial 3–5 s of the dark state process, the channel current drops rapidly and then slowly decreases. This is because the change rate of the channel current is closely related to the temperature change rate of the device. In the initial cooling stage, the cooling rate is faster because the temperature difference between the device and the

2.3 The Device Performance and Working Mechanism …

43

Fig. 2.14 Dynamic response of channel current to infrared illumination. Reproduced with permission [31]

surrounding environment is large. When the temperature of device is close to room temperature, the cooling rate tends to be slow.

2.3.2 Mechanism Analysis of Optothermal Regulation The working mechanism of the prototype device can be explained by the schematic diagram in Fig. 2.15. After poling, the dipoles inside the PMN-PT crystal are aligned along the direction of the external electric field. As a result of the dipole alignment, positive bound charges are accumulated at the top surface of the PMN-PT crystal. These bound charges attract electrons inside the monolayer MoS2 . Since the MoS2 prepared by CVD growth is an n-type semiconductor, electrons are the majority carriers [42]. When a certain amount of electrons are trapped by the surface bound charge of the PMN-PT single crystal and cannot move freely, it tends to affect the

Fig. 2.15 Schematic diagram of the mechanism, a initial state at room temperature, b infrared illumination state. Reproduced with permission [31]

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2 An Optothermal Field Effect Transistor Based on PMN-26PT …

conductivity of the two-dimensional material in the channel. As shown in Fig. 2.15a, this is the initial state of the device at room temperature after poling the PMN-PT single crystal. When the incident infrared light is absorbed by the carbon black electrode on the bottom surface of the PMN-PT single crystal, it is converted into heat, so that the temperature of the dielectric layer rises. Due to the pyroelectric effect of PMN-PT single crystal as mentioned in Fig. 2.6a, the polarization of the crystal decreases with increasing temperature. That is to say, the surface bound charge of the PMN-PT single crystal is reduced. As shown in Fig. 2.15b, some of the electrons trapped in the monolayer MoS2 are now released, participating in the charge carriers of the channel. Therefore, the channel current will increase under the same U DS bias. In order to deeply understand the interaction mechanism between the charge carriers of MoS2 and the surface bound charges of PMN-PT crystal, we used finite element analysis (COMSOL Multiphysics) to simulate the electric field distribution near the interface in the channel. The thickness of the monolayer MoS2 two-dimensional material is only 0.6 nm, which is much smaller than the thickness of the PMN-PT dielectric layer (~60 µm). Here, we only consider the influence of the 10 nm depth of the PMN-PT crystal surface in the modeling. This is because the bound charge generated by polarization exists only on the surface of the crystal, and the interface engineering utilizes the short-range effect of the space near the two-dimensional material. It can be seen from the simulation results in Fig. 2.16 that the positive bound charges at the interface creates an electric field in space (ferroelectric field). The color of the section in the figure is the potential distribution, the black arrow indicates the direction of the electric field, and the size of the arrow is logarithmically related to the field strength. The electric field formed in the crystal is vertically downward, just

Fig. 2.16 Local electric field distribution in the channel of FET

2.3 The Device Performance and Working Mechanism …

45

opposite to the direction of polarization, and is called the depolarization field (E d ). If the surface of the crystal was not covered by the electrode and the semiconductor, the intensity of the depolarization field can be calculated by the following formula [43]: Ed =

P ε0 εr

(2.13)

where P is the polarization of the crystal, and ε0 and εr are the vacuum dielectric constant and the relative dielectric constant of the crystal, respectively. When the surface of the crystal in the channel is covered with a semiconductor material, the depolarization field at this time needs to be multiplied by the coefficient caused by the difference in capacitance [44] Ed =

P (Cs /Cox + 1)ε0 εr

(2.14)

where C s and C ox are the capacitance of the channel semiconductor and the dielectric layer, respectively. The local electric field formed in MoS2 is vertically upward, and the field strength is much larger than the depolarization field calculated by (2.14), thus generating a strong field effect on the carriers in the semiconductor. As a result, a certain number of electrons in MoS2 are trapped by the interface and cannot move freely. We compare the physical process of optothermal response with the principle of a conventional gate voltage controlled field effect transistor. In a transistor, the change in channel current at a fixed bias essentially reflects the change in conductivity of the channel semiconductor. The conductivity (κ) of a semiconductor can be expanded into the following formula [45] κ =μ× N ×q

(2.15)

where μ is the carrier mobility, N is the carrier concentration and q is the elementary charge (1.6 × 10−19 C). In conventional field effect transistors, the expression of conductivity can be further expanded as follows [46] κ=

μCox Z L(UG S − Uth )

(2.16)

where C ox is the dielectric layer capacitance, Z is the channel length, and L is the channel width. U GS is the gate voltage and U th is the equivalent threshold voltage. As can be seen from Eq. (2.16), the carrier concentration in the channel is determined by the gate voltage, and the conductivity of the channel is only a function of the gate voltage. Thus a conventional field effect transistor is a voltage control component and the gate voltage is the signal source that regulates the output channel current. In an optothermal field effect transistor, the incident infrared light acts as a gate voltage,

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the incident light intensity controls the conductivity in the channel, and the infrared photons replace the electrons as the information carriers of the input signal. From another point of view, the carrier concentration of the two-dimensional semiconductor material in the device is reversibly changed by interface engineering. The pyroelectric charge induced by the incident infrared light achieves physical doping of the semiconductor material. This physical doping is different from the wellknown chemical doping, such as the incorporation of phosphorus or boron element into the intrinsic silicon, which can significantly increase the carrier concentration [45]. However, this chemical doping is irreversible, and the introduced impurities are difficult to eliminate from the as-prepared material, and the dynamic regulation in Fig. 2.14 cannot be realized. In this chapter, we use PMN-PT ferroelectric single crystal as a functional substrate to achieve reversible regulation of the electrical transport properties of MoS2 . This design idea also provides an effective way for the performance exploration and physical doping of other two-dimensional materials.

2.4 Summary of This Chapter 1. Based on the excellent pyroelectric properties of the rhombohedral PMN-26PT relaxor ferroelectric single crystal, a new optothermal FET device structure has been designed. In order to verify the rationality of the device design, the pyroelectric coefficient under the two-dimensional constrained model was derived from the thermodynamic theory. Compared with the pyroelectric coefficient under the free boundary condition, the two-dimensional constraining will produce the d31 mode piezoelectric effect correction term, but the influence of the correction term in the ferroelectric material is negligible. 2. The [111] oriented PMN-26PT single crystal has been mechanically thinned and polished to a wafer with a thickness of 60 µm. A three electrodes structure of the FET is constructed thereon by semiconductor processing. A large-area MoS2 monolayer film has been grown by CVD method, and this two-dimensional material has been successfully transferred to the surface of PMN-PT single crystal as a channel semiconductor in the transistor. 3. The device performance of optothermal field effect transistor has been characterized. The electric field distribution in the channel has been simulated by finite element analysis technology. The pyroelectric effect of PMN-PT single crystal is the main mechanism of optothermal regulation. Compared to conventional FETs, the regulation signal of the device is no longer the gate voltage, but the infrared light. The photons in the infrared band become the input signal of the transistor, enabling remote control of the channel current. This allows the channel current to be turned on and off without touching the transistor. 4. The surface bound charge of ferroelectric materials is sensitive to external physical stimuli such as electric fields, temperature and pressure. Experiments show that when the two-dimensional semiconductor directly contact with the ferroelectric material, the carriers inside the 2D material are regulated by the bound

2.4 Summary of This Chapter

47

charge of the ferroelectric materials at the interface, and the photoelectric properties of the 2D materials will also change. The design of interface engineering provides a new idea for studying the interface properties and physical doping of two-dimensional materials.

References 1. 2. 3. 4.

5. 6. 7. 8. 9.

10. 11. 12. 13.

14. 15. 16. 17. 18. 19. 20. 21. 22.

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23. Zeng H, Dai J, Yao W et al (2012) Valley polarization in MoS2 monolayers by optical pumping. Nat Nanotechnol 7:490–493 24. Tongay S, Zhou J, Ataca C et al (2013) Broad-range modulation of light emission in twodimensional semiconductors by molecular physisorption gating. Nano Lett 13:2831–2836 25. Çakır D, Sevik C, Peeters FM (2014) Engineering electronic properties of metal-MoSe2 interfaces using self-assembled monolayers. J Mater Chem C 2:9842–9849 26. Xu K, Huang Y, Chen B et al (2016) Toward high-performance top-gate ultrathin HfS2 fieldeffect transistors by interface engineering. Small 12:3106–3111 27. Yu Z, Ong ZY, Pan Y et al (2016) Realization of room-temperature phonon-limited carrier transport in monolayer MoS2 by dielectric and carrier screening. Adv Mater 28:547–552 28. Li Y, Xu CY, Hu P et al (2013) Carrier control of MoS2 nanoflakes by functional self-assembled monolayers. ACS Nano 7:7795–7804 29. Najmaei S, Zou X, Er D et al (2014) Tailoring the physical properties of molybdenum disulfide monolayers by control of interfacial chemistry. Nano Lett 14:1354–1361 30. Duan X, Wang C, Pan A et al (2015) Two-dimensional transition metal dichalcogenides as atomically thin semiconductors: opportunities and challenges. Chem Soc Rev 44:859–8876 31. Fang HJ, Lin ZY, Wang XS et al (2015) Infrared light gated MoS2 field effect transistor. Opt Express 23:31908–31914 32. Wang CL, Li JC, Zhao ML (2009) Physics of piezoelectrics and ferroelectrics [M]. Science Press 33. Tang YX (2007) Novel pyroelectric materials and their applications in infrared devices, Doctoral thesis, Shanghai Institute of Ceramics, Chinese Academy of Sciences 34. Zhang YY, Zhang J (2012) Application of resonance Raman spectroscopy in the characterization of single-walled carbon nanotubes. Acta Chim Sinica 70:2293–2305 35. Luo NN (2015) Design and crystal growth of complex perovskite relaxor ferroelectrics with high Trt. Doctoral thesis, Tsinghua University 36. Shao SP (1994) Pyroelectric effect and its application [M]. Ordnance Industry Press 37. Wang X, Feng H, Wu Y et al (2013) Controlled synthesis of highly crystalline MoS2 flakes by chemical vapor deposition. J Am Chem Soc 135:5304–5307 38. Lee C, Yan H, Brus LE et al (2010) Anomalous lattice vibrations of single-and few-layer MoS2 . ACS Nano 4:2695–2700 39. Li H, Zhang Q, Yap CCR et al (2012) From bulk to monolayer MoS2 : evolution of Raman scattering. Adv Funct Mater 22:1385–1390 40. Duan X, Wang C, Fan Z et al (2015) Synthesis of WS2x Se2-2x alloy nanosheets with composition-tunable electronic properties. Nano Lett 16:264–269 41. Mak KF, Lee C, Hone J et al (2010) Atomically thin MoS2 : a new direct-gap semiconductor. Phys Rev Lett 105:136805 42. Deng Y, Luo Z, Conrad NJ et al (2014) Black phosphorus-monolayer MoS2 van der Waals heterojunction p-n diode. ACS Nano 8:8292–8299 43. Black CT, Farrell C, Licata TJ (1997) Suppression of ferroelectric polarization by an adjustable depolarization field. Appl Phys Lett 71:2041–2043 44. Ko C, Lee Y, Chen Y et al (2016) Ferroelectrically gated atomically thin transition-metal dichalcogenides as nonvolatile memory. Adv Mater 28:2923–2930 45. Neamen DH (2003) Semiconductor physics and devices, 3rd edn. McGraw-Hill higher education 46. Assadi A, Svensson C, Willander M et al (1988) Field-effect mobility of poly(3hexylthiophene). Appl Phys Lett 53:195–197

Chapter 3

An Ultrabroadband Photodetector Based on PMN-28PT Single Crystal

3.1 Introduction of This Chapter 3.1.1 Broadband Photodetector A sensor is a key component of the information acquisition system. As an extension of the human sense organ, it can convert the outside information into useful information for humans according to different principles [1]. Photodetectors are one of the most important sensors. They can realize the conversion between optical information and electrical information. They have a broad application in many practical fields such as modern industrial production, aerospace technology, military and medical care [2]. As we all know, light is a kind of electromagnetic wave that spreads in space. The wavelength range of electromagnetic wave is very wide, ranging from the size of Planck length to the size of entire universe. Light covers only a small wavelength range in the electromagnetic spectrum. Figure 3.1 shows the electromagnetic spectrum (Maxwell’s rainbow). The light with a wavelength between 0.4 and 0.75 μm can be perceived by human eyes. This part of the optical radiation is called visible light [3]. Photodetectors can detect light of different wavelengths depending on the inherent properties and working principle of sensitive materials. They compensate for the limitations of the human eye in spectral resolution capabilities and extend the wavelength range of visual perception. For example, the most commonly used materials for UV detectors are wide band gap semiconductor materials such as zinc oxide (ZnO) and titanium dioxide (TiO2 ) [4–6]. Under the ultraviolet light illumination, electrons are excited to the conduction band from the valence band. At this time, photocurrent can be measured by applying a bias voltage to the semiconductor. The narrow band gap semiconductor materials such as InSb and Hg1-x Cdx Te are more suitable for working in the infrared region [7–9]. Broadband photodetectors can offer the detection over a wide range of the optical radiation spectrum, which provides a large amount of information from the target being detected than a single-band photodetector. Hence, broadband photodetectors can improve the recognition rate of the target [10]. In recent years, the application © Tsinghua University Press, Beijing and Springer Nature Singapore Pte Ltd. 2020 H. Fang, Novel Devices Based on Relaxor Ferroelectric PMN-PT Single Crystals, Springer Theses, https://doi.org/10.1007/978-981-15-4312-8_3

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3 An Ultrabroadband Photodetector Based on PMN-28PT Single Crystal

Fig. 3.1 Electromagnetic spectrum

demands of broadband detectors (especially those requiring high integration such as space-detection technology) have stimulated research interest of scientists. A wide variety of sensors have been developed. According to the working principle, they can be divided into photon detector and photothermal detector. 1. Photon detector The basic principle of a photon detector is to generate electron-hole pairs when the bound electrons in the semiconductor are excited by the radiation photon energy, thereby changing some electrical properties such as conductivity of the material [11]. According to this principle, a broadband photodetector can be prepared by synthesizing a semiconductor material with a high absorption coefficient in a wide wavelength range. For example, Chen et al. [12] prepared a flexible photodetector with UV-Vis-NIR response through the combination of organolead halide perovskites (CH3 NH3 PbI3 ) with a conjugated polymer. Gao et al. [13] used SnS2 nanosheets and PbS quantum dots to prepare a photodetector with spectral selectivity in the wavelength range of 300–1000 nm. The energy band structure of the absorbing layer semiconductor material determines the response wavelength range of the device. Photon detectors always have the inherent cutoff wavelength limit when the photon energy is insufficient to excite the electrons in semiconductor materials. 2. Photothermal detector The photothermal detector utilizes the energy transfer between light and matter. When the electromagnetic radiation is absorbed by the material, the lattice vibration is aggravated. Macroscopically, an increase in temperature will be observed. As long as the radiated light energy is sufficient to cause a temperature change in the sensing material, it can be detected regardless of the wavelength of the radiation [14]. Hence, ultrabroadband response can be realized in photothermal detectors because they do not have any cutoff wavelength limit.

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3.1.2 Response Characteristics of Pyroelectric Detectors A pyroelectric detector is one kind of photothermal detectors. When the temperature of pyroelectric material changes under illumination, the polarization intensity changes to generate a pyroelectric current. Unlike other kinds of photothermal detectors such as thermocouples and thermistors, pyroelectric detectors respond to temperature changes over time rather than temperature values [15]. Therefore, stable background radiation in the environment and noise that does not change with time can be ignored. On the other hand, pyroelectric detectors are a type of detection technology that is expected to achieve higher response speeds because it is not necessary to establish a thermal equilibrium state during the detection process. Compared to broadband photon detectors, pyroelectric detectors have no cut-off wavelength limitation and can achieve ultra-wide spectral response from ultraviolet to terahertz band. However, due to the traditional photo-thermal-electric conversion mechanism, the sensitivity and response speed of pyroelectric detectors need to be improved. The sensitivity of the detector is largely governed by the pyroelectric properties of the sensitive material. In order to increase the sensitivity, it is necessary to select a material with a pyroelectric coefficient as high as possible. Usually, single crystals have a higher pyroelectric coefficient than ceramics and films, so growing high quality single crystals and determining the optimal cut are the basis for high performance pyroelectric detectors. At present, some progress has been made in pyroelectric detectors based on rhombohedral PMN-PT single crystals [16–20]. However, only the response in the infrared region has been reported, and the devices have been designed as the traditional heat conduction mode. The detection signal is greatly attenuated when the chopping frequency of the modulated infrared light rises to about 100 Hz. So the device cannot recognize the incident light with high frequency modulation. In order to improve the response speed of pyroelectric detectors, it is necessary to optimize the structure design of the device. How to avoid delays and energy losses during heat transfer is particularly important. Aiming at the key problems in the field of broadband photo detection, a new type of photodetector based on the optothermal effect and the pyroelectric effect of PMNPT single crystal has been designed [21]. This chapter concentrates on the aspects of structural design, preparation process, performance characterization and mechanism analysis of the ultrabroadband photodetectors.

3.2 Structural Design and Preparation of Ultrabroadband Detector 3.2.1 Structural Design of Photodetectors As shown in Fig. 3.2, the structure of the device is similar to a parallel plate capacitor with face electrodes. PMN-PT single crystal wafer plays the role of sensitive material.

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3 An Ultrabroadband Photodetector Based on PMN-28PT Single Crystal

Fig. 3.2 The structure of ultrabroadband photodetector. Reproduced with permission [21]. Copyright 2016, American Chemical Society

It is the core part of the device and is responsible for converting incident light signals of different wavelengths into electrical signals. The gold electrode is sputtered on the bottom surface of PMN-PT crystal. The top surface acts as a photosensitive surface and cannot block the absorption of optical radiation. Since the metal thin film has a strong reflection of incident light, it is not suitable for a top electrode. Here we used the silver nanowires (Ag NWs) mesh as the transparent electrode to cover the top surface of the PMN-PT single crystal. The Ag NWs transparent electrode not only satisfies the requirements of electrical conductivity, but also ensures the transmission of optical radiation. As a result, the energy of the radiated photon can be directly absorbed by the PMN-PT crystal to produce a pyroelectric current. From the perspective of energy saving, the device is an energy conversion type sensor. The photodetector can operate at zero bias because the external power supply is not required for the generation of pyroelectric current. Compared with traditional sensors such as photoconductive detectors, the pyroelectric device reduces the power consumption during the detection process, and thus has a very attractive application prospect.

3.2.2 Processing and Characterization of PMN-PT Single Crystal In order to build high performance photodetectors, the choice of sensitive materials is critical. The large-sized, high-quality PMN-PT single crystal grown by the modified Bridgman method is an ideal pyroelectric material. In addition to the good pyroelectric performance, the chemical inertness in the atmosphere and the ease of machining also increase the advantages of the crystal in practical applications. A large number of previous studies have shown that the rhombohedral PMN-xPT single crystal (26 ≤ x ≤ 28) along the direction of spontaneous polarization, that is, [111] orientation shows the optimal pyroelectric performance [16]. Therefore, in this chapter we have selected a crystal with a rhombohedral component to prepare a photodetector. First, the PMN-PT single crystal grown by the modified Bridgman method was oriented

3.2 Structural Design and Preparation of Ultrabroadband Detector

53

using a DX type X-ray crystal orientation instrument. Figure 3.3a shows the XRD pattern of the oriented PMN-PT wafer. There is only one diffraction peak at 39.3° over the entire scanning angle range, corresponding to the [111] crystal plane of the PMN-PT crystal. Figure 3.3b shows the [111] crystal plane in the cubic model. The spontaneous polarization of the rhombohedral phase crystal is along the [111] crystal orientation, so there are eight equivalent spontaneous polarization directions in the crystal before poling, as shown by the red arrow in the Fig. 3.3b, which we call the “8R” state. Next, we prepared a [111] orientation sample by sputtering a gold electrode to study the dielectric properties and further calibrate the actual composition of the single crystal. Figure 3.4a shows the temperature dependence of dielectric constant for virgin crystal samples at different test frequencies. The temperature corresponding

Fig. 3.3 a The XRD pattern of [111]-oriented PMN-PT crystal. b The cubic model of “8R” state. Reproduced with permission [21]. Copyright 2016, American Chemical Society

Fig. 3.4 a Temperature dependence of dielectric constant of the PMN-PT crystal at different frequencies. b The relationship between lg(1/ε − 1/εm ) and lg(T − Tm ) at 1 kHz

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to the peak of the spectrum is called T m , at which the crystal completes the phase transition from ferroelectric phase to paraelectric phase. The T m with a test frequency of 1 kHz is usually used to calibrate the composition of PMN-PT crystals [22]. The specific calibration relationship is P T (%) =

Tm + 10 500

(3.1)

Substituting the data measured in Fig. 3.4a, it is found that the content of lead titanate (PT) in the selected wafer is about 28%, that is, the wafer composition is 0.72PMN-0.28PT, hereinafter referred to as PMN-28PT. At the same time, the broadening of the dielectric peak of the PMN-PT crystal can be seen from Fig. 3.4a, which reflects the dispersion phase transition of the relaxor ferroelectric materials. In addition, the frequency dispersion of the dielectric characteristics can also be observed in this relaxor ferroelectric crystal. In order to quantitatively evaluate the relaxation properties of ferroelectric materials, Uchino and Nomura [23] introduced the following formula: 1 (T − Tm )γ 1 − = ε εm C

(3.2)

where C is a constant determined by the type and intrinsic properties of materials, and εm is the dielectric peak value. The index γ is called the relaxation factor and can be used to measure the relaxation properties of a material. Generally, the value of γ is in the interval of [1, 2]. When γ = 1, it is expressed as ordinary ferroelectric. When γ is close to 2, the material exhibits a strong relaxation property. Figure 3.4b shows the linear relationship between lg(1/ε − 1/εm ) and lg(T − Tm ) at 1 kHz. The relaxation factor of the crystal is found to be 1.61 by fitting the experimental data, indicating that the PMN-PT crystal has good relaxation characteristics. Next, we poled a [111]-oriented PMN-28PT single crystal sample at room temperature for 15 min under a DC electric field of 2 kV/mm. In order to verify the full polarization, we measured the piezoelectric constant using a quasi-static d 33 meter (ZJ-4A) at room temperature. The average value is about 90 pC/N, which is consistent with the values reported in the literature [24]. Therefore, the poling process is suitable for the PMN-PT crystal. We would also use the same poling process when preparing the device. To examine the pyroelectric properties of the PMN-28PT crystals, we measured the polarization and pyroelectric coefficient of the sample at different temperature by the pyroelectric module of the TF2000 Analyzer (aixACCT, Germany). The temperature changed from 20 to 180 °C (higher than the Curie temperature of the sample) with a heating rate of 5 °C/min. The black curve in Fig. 3.5 shows the change of the polarization intensity with temperature. As the temperature increases, the polarization intensity of the sample gradually decreases from 35 to 0 μC/cm2 . According to the definition of pyroelectric coefficient, we calculated the first-order derivative of the black curve. As shown in Fig. 3.5, the pyroelectric coefficient of PMN-PT single

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Fig. 3.5 Polarization and pyroelectric coefficient versus temperature. Reproduced with permission [21]. Copyright 2016, American Chemical Society

crystal has three peaks during the heating process, corresponding to the three phase transitions. The phase transition temperature of the tetragonal ferroelectric phase to paraelectric phase is consistent with the T m measured in Fig. 3.4a. The pyroelectric coefficient of PMN-28PT single crystal sample is found to be as high as 7.5 × 10−4 C/m2 K at room temperature. And the blue curve in Fig. 3.5 is nearly flat in a wide temperature range from 20–55 °C. This indicates that the pyroelectric coefficient of PMN-28PT crystal has little change before the phase transition temperature. So, the PMN-28PT crystal is expected to be a sensitive material for high performance pyroelectric detectors.

3.2.3 Domain Engineering of PMN-PT Single Crystal Next, the domain structure of the PMN-PT crystal is studied for a deeper understanding of the excellent pyroelectric performance. Previous works have shown that the domain structure of PMN-PT crystal is closely related to the macroscopic properties. As we known, relaxor ferroelectric single crystals produce high piezoelectric effects. In addition to the intrinsic effects of crystals, there may be some other extrinsic factors (such as domain wall effects) [25]. The PMN-PT single crystal shows a stronger piezoelectric properties with a smaller domain size and a higher domain wall density [26]. Figure 3.6 lists the phase structure, polarization direction, and domain structure of the PMN-PT single crystals [27]. Different macroscopic symmetry and domain structures are formed after poling along different directions. When the crystals are poled along the polar axes, a single domain state can be obtained. When the crystals are poled along the nonpolar axis, a multi-domain state can be obtained, which is referred to as “engineering domain” [28]. Among the nine domain structures listed

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3 An Ultrabroadband Photodetector Based on PMN-28PT Single Crystal

Fig. 3.6 The phase structure and domain structure of PMN-PT crystals [27]

below, the rhombohedral phase PMN-PT crystal with a “4R” domain structure has the best longitudinal piezoelectric performance. In fact, pyroelectric properties of relaxor ferroelectric single crystals are also related to their domain structure. Different from the piezoelectric properties, pyroelectric properties of the single crystal shows less contribution of the domain wall. The angle between the direction vector of the poling electric field and the polar axis of the crystal is defined as θ . The component of pyroelectric coefficient in the direction of electric field peff is [16]: pe f f = p × cos θ

(3.3)

The largest apparent component can be obtained when the angle θ is 0, that is, the pyroelectric crystal is in a single domain state. As indicated earlier, the spontaneous polarization of the rhombohedral PMN-PT crystal is along the [111] direction. There are eight possible domain orientations in the crystal. If the direction of the poling electric field is along the polar axis direction, it is sufficient to get a single domain state of “1R” under a large electric field. In this chapter, we prepared a [111]-oriented PMN-28PT wafer and applied a sufficient DC electric field between the electrodes. Thus, the excellent pyroelectric coefficient obtained from the PMN-28PT wafer is derived from the “1R” domain state, as shown in Fig. 3.6. Direct observation of the domain structure can further confirm our analysis results. At present, there are many methods to observe the domain structure of ferroelectric materials, including various electron microscopy techniques (scanning electron microscope, SEM and transmission electron microscope, TEM, etc.), piezoresponse

3.2 Structural Design and Preparation of Ultrabroadband Detector

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force microscope (PFM), and polarized light microscope (PLM), acid etching technology [29]. Some of these techniques can analyze the domain structure throughout the internal space of the sample, and some can only observe the surface domain. Here we use a polarized light microscope (PLM, XJZ-6) to directly observe the domain structure of the PMN-PT single crystal after poling along the [111] direction. This optical method utilizes the birefringence properties in a ferroelectric single crystal. Birefringence refers to the phenomenon that a beam of light enters an anisotropic dielectric material and is refracted into two beams of light with different propagation directions. The two beams of light are not only perpendicular to each other, but also have different propagation speeds in the medium. One of the beams whose deflection direction conforms to the law of refraction is called ordinary light (o-light), and the corresponding refractive index is recorded as the ordinary refractive index (no ) of the material. The other beam that does not satisfy the law of refraction is called extraordinary light (e-light) [30]. In ferroelectric crystals, the birefringence of the domains is affected by the polarization direction. The domains with different orientations can be displayed in the orthogonal polarizers. This PLM method is simple and effective for observing the ferroelectric domains and dynamic changes of the domains under electric field [31]. Due to the limited magnification of the optical microscope, PLM can only observe domain structures with micro scale. Although its resolution is not as high as that of electron microscopy, it is competent for the observation of macroscopic domain structures in this experiment. A small piece of [111]-oriented PMN-28PT crystal was selected to reduce the thickness to about 100 μm by mechanical thinning. The top and bottom surfaces were polished and covered with silver paste as a temporary electrode. Two silver wires are respectively connected to the top and bottom electrodes for poling. An electric field of 2 kV/mm is applied for 15 min at room temperature. After poling, the silver paste was washed with a high polar organic solvent, N,N-Dimethylformamide (DMF). Thereafter, the crystals were washed successively with acetone and ethanol, and dried for observing domain structure. As shown in Fig. 3.7a, the poled PMN-28PT wafer was placed between two orthogonal polarizers. The incident natural light passed through the polarizer and irradiated onto the PMN-28PT wafer. The domain structure of the sample can be observed from the analyzer by rotating the wafer. Figure 3.7b shows the optical image of a PMN-28PT wafer rotated to different angles. The sample is always extinct over an angular range of 0–90°. This is because the rhombohedral phase PMN-PT is a single optical axis crystal. In the [111]-poled PMN-28PT single domain wafer, the direction of the polarization is the direction of the optical axis. Birefringence does not occur since the light propagates along the polarization axis. Rotating the PMN-28PT wafer around the polarization axis, the extinction of all angles indicates that the domain orientation in the sample is uniform. If there are some other oriented domain in addition to the c domains, some bright areas should be observed during the rotation of the wafer. The PLM results confirmed the single domain state, and lay the foundation for the domain engineering of PMN-28PT single crystal.

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Fig. 3.7 a Observation of single domain PMN-PT crystals using PLM, b Full-angle extinction in a PMN-28PT single crystal after poling (scale bar is 200 μm). Reproduced with permission [21]. Copyright 2016, American Chemical Society

3.2.4 Preparation of Silver Nanowires Transparent Electrode As shown in Fig. 3.2, the electrode is an integral part of the device for poling PMNPT single crystal and extracting electrical signals of the detector. A gold film was sputtered on the bottom surface of the polished single crystal as there is no special

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requirement for the bottom electrode. But the top electrode is very different. In addition to good conductivity, the top electrode cannot block the absorption of incident light with different wavelengths. Among the various types of transparent electrodes, silver nanowires electrode is an excellent candidate. In addition to the high transmission and conductivity, it can be prepared as a flexible or even stretchable transparent electrode [32]. Compared to the patterned metal grid electrodes, silver nanowires transparent electrodes prepared from bottom to top have a mature mass production process and a lower production cost [33]. The silver nanowires transparent electrode is suitable for various solution process methods [34]. In this chapter, due to the size limitation of the PMN-PT crystal (5 × 5 × 0.1 mm3 ), we used the simplest drop casting process to prepare the top electrode. The silver nanowires (Ag NWs, Nanjing XFNANO Materials Tech Co., Ltd.) was first diluted with ethanol and ultrasonicated for 15 min to reduce reunion. The uniformly mixed silver nanowires solution exhibited a gray color with no significant precipitation. An appropriate amount of silver nanowires solution is applied dropwise on the polished surface of the PMN-PT single crystal. A transparent conductive electrode can be obtained after the ethanol is completely evaporated. It should be noted that the sample needs to lay flat and minimize the vibration around it during the evaporation of ethanol. In order to quantitatively study the light transmittance and electrical resistance of the silver nanowires electrode prepared by this method, a reference sample was prepared by the same process on a quartz glass substrate. Figure 3.8a shows the transmission spectrum of a silver nanowires electrode deposited on the quartz substrate. The electrode is highly transparent in the wavelength range of 200–2300 nm (covering the ultraviolet to near-infrared region). The transmittance in the visible light region is higher than 85%. The inset shows a photograph of the silver nanowire electrode, and the logo of Tsinghua University under the quartz is clearly visible. In addition, we tested the sheet resistance of the silver nanowires transparent electrode using a four-probe sheet resistance tester (4 Dimensions 280SJ, USA). The sheet resistance is about 30 /, which is comparable to commercial ITO electrodes. Such an excellent conductivity benefits from two aspects: First, silver is the best conductor in metals, and electrons are easily transported within a single silver nanowire. Second, the morphology of the silver nanowires transparent electrode is shown in Fig. 3.8b. A large number of silver nanowires on the substrate are randomly distributed, and formed a dense network. Electrons can transfer between two intersecting silver nanowires. Based on this, the flow of electrons in the entire two-dimensional plane is smooth, which will be very beneficial to the extraction of electrical signals in the photodetector.

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3 An Ultrabroadband Photodetector Based on PMN-28PT Single Crystal

Fig. 3.8 a The transmission spectrum of a silver nanowires electrode, inset is a photo of silver nanowires on a quartz plate. b SEM image of silver nanowires electrode. Reproduced with permission [21]. Copyright 2016, American Chemical Society

3.3 The Performance and Mechanism Analysis of the Photodetector After preparing the top electrode, the device structure is completely formed. Since the 100 μm thick PMN-28PT single crystal is free-standing, we used a PC board with a hole to package the device. The bottom electrode and the transparent top electrode of the crystal are connected to a DC high voltage power supply for poling. As mentioned earlier in Chap. 2, the total pyroelectric coefficient of the PMN-28PT crystal in the completely free state consists of two parts, the primary pyroelectric coefficient and the secondary pyroelectric coefficient. Typically, the primary pyroelectric coefficient plays a major role in the device. So, we do not consider the effect of the secondary pyroelectric coefficient caused by the piezoelectric effect here. Next, the near-infrared light of 1064 nm is taken as an example to demonstrate the photoelectric response. And the working mechanism of the device is explained in detail. Figure 3.9a shows the current-time curve of the PMN-28PT crystal. In the dark state, the temperature of the PMN-PT crystal is stable at T 1 (near room temperature), and no current flows through the external load. In this equilibrium state, the internal

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Fig. 3.9 a Pyroelectric current caused by infrared light, b working principle of the device. Reproduced with permission [21]. Copyright 2016, American Chemical Society

dipoles of the single domain crystal are aligned and bound charges are present on the top and bottom surfaces perpendicular to the polarization direction. The relationship between the bound charges density (σ) and the spontaneous polarization (Ps ) is [35]:  Ps =

σ Sd =σ Sd

(3.4)

where S represents the overlap area of the top and bottom electrodes, and d is the thickness of the PMN-PT wafer. Formula (3.4) indicates that the spontaneous polarization of the PMN-PT wafer is numerically equal to the bound charges density. At the constant temperature, these bound charges on the surface of the crystal are

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neutralized by an equal amount of opposite charges, which are derived from the diffusion charge in the body or the free charge of the external environment, as shown in Fig. 3.9b-i. For most pyroelectric materials, the process of this charge neutralization is relatively slow, and the required time constant τ is usually between 1 and 1000 s [35]. When the PMN-PT crystal absorbs the incident infrared radiation and causes a temperature rise, the lattice thermal vibration inside the single crystal is intensified. Then, the spontaneous polarization and the bound charge density of the crystal surface decrease. As shown in Fig. 3.9b-ii, the opposite free charges originally used to neutralize the bound charge will be excessive. In order to maintain the neutral state, the excess free charge will be released through the external circuit to form a pyroelectric current. As can be seen from Fig. 3.9a, the pyroelectric current gradually falls back to zero after an instantaneous peak. The continuous illumination causes the temperature of the PMN-PT wafer to rise, but the heat exchange with the surrounding environment causes the temperature of the wafer to decrease. The combined effect of these two factors allows the wafer to reach a new equilibrium state where the wafer temperature stabilizes at T 2 (T 2 > T 1 ). As shown in Fig. 3.9b-iii, there is no current in the external circuit. After obscuring the incident light, the PMN-PT wafer is cooled by natural convection and thermal radiation. As shown in Fig. 3.9b-iv, the spontaneous polarization will rise again during the cooling process. When the opposite free charges are insufficient to neutralize the bound charge, they will be compensated by the external circuit. Thus, we observed a reverse pyroelectric current in the external circuit. If the incident light is chopped periodically, the crystal will cycle through the above four steps, and an alternating pyroelectric current will be generated in the external circuit. The magnitude of the current I pyro can be calculated by the following formula [36]: I pyr o = p × S ×

dT dt

(3.5)

where p is the pyroelectric coefficient mentioned above, S represents the overlapping area of the top and bottom electrodes, and dT /dt is the first derivative of temperature versus time, i.e. the rate of temperature rise or decrease. Figure 3.10 shows the magnitude of the pyroelectric current under different infrared light intensity. Here, the different infrared light intensities are achieved by attenuating the laser with different filter and calibrated with a standard laser power meter (LP-3A type). As shown by the red curve, the pyroelectric current of the device tends to increase linearly with incident light intensity. It increased from 1.5 nA at an initial intensity of 200 mW/cm2 to 50 nA at a light intensity of 3500 mW/cm2 . This is because the temperature change rate dT /dt increase with the incident light intensity. As mentioned above, the pyroelectric coefficient of the PMN-28PT single crystal we used in the temperature range around room temperature did not change much. According to Formula (3.5), under the premise that the pyroelectric coefficient and

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Fig. 3.10 Pyroelectric current and photoresponsivity versus light intensity. Reproduced with permission [21]. Copyright 2016, American Chemical Society

the area of the sample are constant, I pyro is proportional to the temperature change rate dT /dt. Current responsivity (RA ) is a commonly used parameter for evaluating photodetectors. It is defined as the ratio of the output current signal (I) of the photodetector to the incident light intensity (P) [7] RA =

I P

(3.6)

The blue curve in Fig. 3.10 shows the relationship between the current responsivity of the photodetector and the incident light intensity at 1064 nm infrared light. They are positively correlated, and the current responsivity is slightly saturated under strong illumination. In this work, the current responsivity at 3500 mW/cm2 reached a maximum of approximately 90 nA/W. At the same time, we also studied the pyroelectric voltage induced by infrared radiation of different intensity. The near-infrared light with a wavelength of 1064 nm was modulated in a cycle of 20 s, and the open circuit voltage between the gold electrode and the silver nanowires transparent electrode was measured by a precision measuring unit (Agilent B2911A). Figure 3.11 depicts the open circuit voltage output over time. When the infrared light irradiated to the transparent electrode on the wafer for 10 s, the open circuit voltage rises from negative to positive. After shielding the infrared light, the PMN-PT wafer is cooled and the open circuit voltage drops from positive to negative. The voltage variation also exhibits periodicity, and the peak-to-valley value is positively correlated with the incident light intensity.

3.3.1 Optimization of Pyroelectric Frequency Response Frequency response is an important indicator of a sensor. For photodetectors, the output electrical signal is not temporally fully synchronized with the incoming optical

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3 An Ultrabroadband Photodetector Based on PMN-28PT Single Crystal

Fig. 3.11 Pyroelectric voltage at different light intensity. Reproduced with permission [21]. Copyright 2016, American Chemical Society

signal. Due to the inertia of the device, a lag process will inevitably occur. This makes the photodetector to have a certain bandwidth limitation in frequency response. For a modulated optical signal whose frequency exceeds the bandwidth, the photodetector cannot respond, and the detection capability is lost. The upper limit of the frequency response of conventional pyroelectric detectors is about 100 Hz, which limits their application in high frequency optoelectronic technology such as dynamic measurement [36–38]. We can find the reason of this shortcoming from the working principle of the traditional pyroelectric detector. The entire photo-thermal-electric conversion process can be roughly divided into two steps: the first step is the absorption of the optical radiation to rise the temperature of the pyroelectric material. The top electrode of the conventional pyroelectric detector is a highly conductive metal electrode such as a gold film. And a black light absorbing layer such as carbon nanotubes covers above the electrode. The radiant light energy is converted into thermal energy by the light absorbing layer and then the temperature of the pyroelectric material is changed by heat conduction. The second step is the extraction of the electrical signal achieved by the temperature increase in the pyroelectric material. The average action time of the second step is the relaxation time of spontaneous polarization in the pyroelectric PMN-PT crystal, and the value is on the order of picoseconds (10−12 s) [39]. The delay of heat conduction is a key step in the low response rate of conventional pyroelectric devices. In metal electrodes, the electronic thermal conduction is the dominant mechanism. The PMN-PT single crystal acts as a dielectric, and it has almost only phonon heat conduction mechanism. Due to the different thermal conduction mechanisms on both sides of the electrode interface, the heat transfer from the top electrode to the PMN-PT single crystal in the conventional pyroelectric device is slow. In order to solve this problem, we have improved the structure of the conventional pyroelectric detector. The use of transparent silver nanowires as the top electrode

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allows incident light to be directly absorbed by the pyroelectric PMN-PT wafer, circumventing the heat transfer process. Next we tested the frequency response characteristics of this new structured photodetector. As shown in the inset of Fig. 3.12a, the continuous infrared light can be chopped by an optical chopper to prepare an intermittent light pulse. The parameters such as the frequency and period of the light pulse are controlled by the rotational speed of the chopper blades. Figure 3.12a records the pyroelectric currents excited by light pulses of different frequencies. It can be seen that the pyroelectric current does not change much with the increase of the chopping frequency. Even at 3 kHz, the pyroelectric current remains the same order of magnitude with no significant reduction. Figure 3.12b depicts the output current versus time for the device at 2 and 3 kHz chopping frequencies. The pyroelectric current has typical alternating characteristics similar to a sine wave. The shape of the curve changes significantly compared to the result at the low chopping frequency. After replacing the traditional electrode material with a novel silver nanowires transparent top electrode, the energy of the radiated photon is directly absorbed Fig. 3.12 a Pyroelectric current at different chopping frequencies, inset is a schematic diagram of the measurement system. b Pyroelectric current at 2 and 3 kHz chopping frequencies. Reproduced with permission [21]. Copyright 2016, American Chemical Society

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by the pyroelectric crystal, which exacerbates the lattice vibration and improves the photo-thermal-electric conversion rate. Compared with the traditional pyroelectric detectors, the frequency response characteristics have been greatly improved, which helps to expand the application of pyroelectric detectors in high-frequency optoelectronic technology such as motion detection and dynamic imaging.

3.3.2 Broadband Response in the Ultraviolet-Terahertz Range In addition to improvements in frequency response, we also studied the relationship between the pyroelectric current and the wavelength of the incident light. The black curve in Fig. 3.13a shows the pyroelectric current generated by the detector after absorbing the light at different wavelengths. Over the entire wavelength range from

Fig. 3.13 a Pyroelectric current and photoresponse versus the wavelength of the incident light. b Pyroelectric currents at four typical wavelength. Reproduced with permission [21]. Copyright 2016, American Chemical Society

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375 nm to 118.8 μm, the current peak remains in the same order of magnitude. The incident light intensity at different wavelength is kept to about 100 mW. After calibrating the actual light intensity at different wavelengths, the spectral responsivity under different monochromatic light can be further calculated by (3.6). The blue curve in Fig. 3.13a demonstrates the responsivity changing with wavelength, whose trend is basically consistent with the change of the pyroelectric current. Figure 3.13b shows the pyroelectric current output of a photodetector at four typical wavelengths. The four wavelengths are 375 nm, 532 nm, 808 nm, and 118.8 μm, representing the radiation in the ultraviolet, visible, near-infrared, and terahertz bands. The device has a significant response to these optical radiations, indicating its ultra-wide spectrum detection capability. This is very difficult to be achieved in photon detectors. Due to the limitation of the band gap, semiconductors always have a cutoff wavelength, and photons with energy less than the band gap cannot be identified. The generation of pyroelectric signals does not involve the transition of electrons. Therefore, in theory, any photon that can cause photothermal effects can be detected without a cutoff wavelength. It is worth mentioning that in the detection spectrum of ultrabroadband photodetectors, the terahertz (THz) band is a kind of special radiation, which is called the “life ray”. Although terahertz radiation (0.1–10 THz) is difficult to be detected with general photodetectors, it has important significances [40]. Since terahertz radiation is in the transition zone between optics and microwave, it has broad application prospects in the fields of space communication, weapon guidance and dangerous goods detection [41]. The ability to effectively detect terahertz waves has an important scientific value for the development of terahertz technology.

3.3.3 The Influence of Surface Plasmons on Pyroelectricity An interesting experimental phenomenon was observed in Fig. 3.13. Although optical radiations with different wavelength have almost the same intensity, radiation in the short-wavelength region can induce a larger pyroelectric current. This indicates that ultraviolet light and visible light are absorbed to generate a higher temperature change rate. However, under normal circumstances, the radiation in the infrared region should have a relatively higher photothermal effect. We believe that the anomalous phenomenon may be caused by surface plasmons (SPs) excited in the silver nanowires transparent electrode. In the Einstein phonon model, phonons are quasi-particles produced by the quantization of lattice vibration inside solids [42]. The photothermal effect of incident light in a PMN-PT single crystal can be simply regarded as the energy transfer between the photons and phonons after collision. The efficiency of this energy transfer in PMN-PT single crystals is not so high, and the main bottleneck is the weak absorption of incident photons. The presence of silver nanowires on the surface of the single crystal can effectively improve the conversion efficiency of short-wavelength light energy. This is because the diameter of the silver nanowires is much smaller than the

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Fig. 3.14 Effect of the surface plasmons

incident wavelength and surface plasmons are excited by the incident light field. As shown in Fig. 3.14, the natural frequency of free electrons in the silver nanowires is close to the frequency of the short-wavelength incident light, resulting in surface plasmon resonance. This resonance significantly increases the local electromagnetic field around the silver nanowires, thereby enhancing the absorption of incident light at short wavelengths. The surface plasmon generated in the silver nanowires acts as a medium between the photon and the phonon, improving the efficiency of energy transfer. Its function is analogous to the role of the acoustic matching layer in ultrasonic transducers. Using surface plasmons of noble metal nanostructures to enhance the interaction between light and matter is an effective strategy, and its application in solar cells and other optoelectronic devices has been reported [43–45].

3.3.4 Discussion on the Mechanism of Ferroelectric Photovoltaic in the Ultraviolet Band The photoelectric conversion process in photovoltaic devices can be mainly divided into three stages: generation, separation and transport of photogenerated carriers. In recent years, the research about ferroelectric photovoltaics has been in the ascendant in the field of ferroelectric materials. Photovoltaic phenomena has been observed in different ferroelectric oxide systems such as PZT, LiNbO3 and BiFeO3 [46]. Studies have shown that the mechanism of ferroelectric photovoltaics is quite different from that of traditional heterojunctions (including p-n junctions and Schottky junctions). It utilizes an electric field formed by the alignment of the internal dipole polarization to separate photogenerated carriers in the ferroelectric materials [47]. In the PMNPT single crystal based photodetector, it is debatable whether such a ferroelectric

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Fig. 3.15 The bandgap of PMN-PT single crystal, inset is a schematic diagram of ABO3 structure. Reproduced with permission [21]. Copyright 2016, American Chemical Society

photovoltaic mechanism exists, especially in the ultraviolet region. Here, we discussed the response mechanism of PMN-PT single crystal in the ultraviolet band in combination with related experiments. The PMN-PT single crystal can be regarded as a wide band gap semiconductor. Figure 3.15 depicts the relationship between (αhν)2 and photon energy hν obtained from the absorption spectrum of [111]-oriented PMN-28PT wafer. The direct band gap of the wafer can be found by the Tauc equation [48]: (αhν)2 = B(hν − E g )

(3.7)

where α is the absorption coefficient and B is a constant. Extending the linear portion of the curve until it intersects the horizontal axis yields a direct band gap of approximately 3.06 eV. The wide band gap of PMN-PT is derived from the [BO6 ] octahedron in the ABO3 perovskite system, as shown in the inset of Fig. 3.15. The bottom of the conduction band and the top of the valence band are derived from the d orbital of the B-position cation and the 2p orbital of the oxygen ion, respectively [49]. Theoretically, when the incident ultraviolet light energy is greater than 3.06 eV, the transition of valence band electron can be excited to generate an electron-hole pair. Electron-hole pairs need to be effectively separated once they have been generated. The driving force of carrier separation in ferroelectric photovoltaics is derived from the built-in electric field. Figure 3.16 shows the P-E loop measured in a [111]oriented PMN-28PT single crystal using a TF 2000 ferroelectric analyzer (aixACCT, Germany). The saturation polarization of the crystal is about 38.3 μC/cm2 and the residual polarization of the crystal is 35.1 μC/cm2 . The small difference between the two values indicates the existence of single domain, which is in agreement with the results we observed with PLM. If the built-in electric field formed by the residual polarization inside the PMN-PT wafer could effectively separate the electron-hole pairs, the device will output photocurrent under ultraviolet light.

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Fig. 3.16 The P-E loop of [111]-PMN-28PT single crystal. Reproduced with permission [21]. Copyright 2016, American Chemical Society

Figure 3.17a shows the output current measured at 375 nm UV with periodic switching. Careful analysis shows that this current is not caused by the ferroelectric photovoltaic effect. The reasons are as follows: For photovoltaic devices, the current in the dark state should be zero to satisfy the energy conservation. But the PMN-PT ultrabroadband detector shows a very significant forward current in the dark state. We studied the charge transfer in the whole process by integrating the current with time. As shown in Fig. 3.17b, the amount of transferred charge in both light and dark states is approximately −200 nC. This alternating current is a typical pyroelectric current

Fig. 3.17 a Current response under ultraviolet light, b periodic variation of integrated charge. Reproduced with permission [21]. Copyright 2016, American Chemical Society

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in alternating hot and cold conditions. Therefore, the main working mechanism of the device in the ultraviolet band is still the pyroelectric effect. The current generated by the ferroelectric photovoltaic effect is very weak and has not been detected in this work. We believed this may be related to the macroscopic thickness of the PMN-PT wafer, the single domain structure, and the large intrinsic resistance. The specific reasons need to be further studied.

3.4 Summary of This Chapter 1. The rhombohedral PMN-28PT single crystal with excellent pyroelectric performance was prepared, and the pyroelectric coefficient of up to 7.5 × 10−4 C/m2 K was obtained by domain engineering in [111]-oriented sample. The extinction of all angles from 0 to 90° was observed by polarized light microscope, and the existence of the “1R” single domain state after poling was confirmed. 2. A novel ultrabroadband photodetector based on the “1R” single domain PMNPT single crystal has been designed and prepared. By modifying the structure of conventional pyroelectric devices, the performance-optimized device has a frequency response of more than 3 kHz without degradation of responsivity. Compared to traditional pyroelectric devices, the frequency response is an order of magnitude higher. 3. The silver nanowires transparent electrode which has excellent conductivity and high transmittance in the wavelength range of 200–2300 nm was introduced as the top electrode. The photoelectric response tests showed that the device can detect the incident light from the ultraviolet to terahertz band, and realize the ultra-wide spectral response. At short wavelengths, the pyroelectric signals can be further enhanced possibly by the excitation of surface plasmons in the silver nanowires. 4. Theoretical analysis predicts that ultraviolet light with a wavelength of 375 nm (3.3 eV) is sufficient to induce the ferroelectric photovoltaic effect in PMNPT single crystal. However, experiment results have shown that the response mechanism of the PMN-28PT ultrabroadband photodetector in the ultraviolet region mainly depends on the pyroelectric effect. The photocurrent caused by the ferroelectric photovoltaic effect has not been detected in our devices and the specific reasons need to be further explored.

References 1. Yang N (2012) Sensors and test technology (Chuanganqi yu Ceshi Jishu) [M]. Aviation Industry Press 2. Zhu M (2015) Study on the graphene/silicon heterojunction photodetectors. Doctoral thesis, Tsinghua University

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3. Richards A (2001) Alien vision: exploring the electromagnetic spectrum with imaging technology. SPIE Press 4. Bai S, Wu WW, Qin Y et al (2011) High-performance integrated ZnO nanowire UV sensors on rigid and flexible substrates. Adv Funct Mater 21:4464–4469 5. Li XD, Gao CT, Duan H et al (2012) Nanocrystalline TiO2 film based photoelectrochemical cell as self-powered UV-photodetector. Nano Energy 1:640–645 6. Wang XF, Zhang Y, Chen XM et al (2014) Ultrafast, superhigh gain visible-blind UV detector and optical logic gates based on nonpolar a-axial GaN nanowire. Nanoscale 6:12009–12017 7. Kuo CH, Wu JM, Lin SJ et al (2013) High sensitivity of middle-wavelength infrared photodetectors based on an individual InSb nanowire. Nanoscale Res Lett 8:327 8. Downs C, Vandervelde TE (2013) Progress in infrared photodetectors since 2000. Sensors 13:5054–5098 9. Sun ZH, Liu ZK, Li JH et al (2012) Infrared photodetectors based on CVD-grown graphene and PbS quantum dots with ultrahigh responsivity. Adv Mater 24:5878–5883 10. Dang VQ, Han GS, Trung TQ et al (2016) Methylammonium lead iodide perovskite-graphene hybrid channels in flexible broadband phototransistors. Carbon 105:353–361 11. Fang HJ, Li J, Ding J et al (2017) An origami perovskite photodetector with spatial recognition ability. ACS Appl Mater Interfaces 9:10921–10928 12. Chen S, Teng CJ, Zhang M et al (2016) A flexible UV-vis-NIR photodetector based on a perovskite/conjugated-polymer composite. Adv Mater 28:5969–5974 13. Gao L, Chen C, Zeng K et al (2016) Broadband, sensitive and spectrally distinctive SnS2 nanosheet/PbS colloidal quantum dot hybrid photodetector. Light-Sci Appl 5:e16126 14. Hao XJ (2015) Photoelectric sensor and application technology [M]. Publishing House of Electronics Industry 15. Chynoweth AG (1956) Dynamic method for measuring the pyroelectric effect with special reference to barium titanate. J Appl Phys 27:78–84 16. Tang YX (2007) Novel pyroelectric materials and their applications in infrared devices. Doctoral thesis, Shanghai Institute of Ceramics, Chinese Academy of Sciences 17. Zhao XY, Wu X, Liu LH et al (2011) Pyroelectric performances of relaxor-based ferroelectric single crystals and related infrared detectors. Phys Status Solidi A 208:1061–1067 18. Shao X, Ding J, Ma X et al (2012) Design and thermal analysis of electrically calibrated pyroelectric detector. Infrared Phys Techn 55:45–48 19. Xu Q, Zhao XY, Li XB et al (2015) Novel electrode layout for relaxor single crystal pyroelectric detectors with enhanced responsivity and specific detectivity. Sens Actuat A 234:82–86 20. Wang J, Jing Y, Jing W et al (2011) Signal readout for pyroelectric detector based on relaxor ferroelectric single crystals. Phys Status Solidi A 208:1078–1083 21. Fang HJ, Xu C, Ding J et al (2016) Self-powered ultra-broadband photodetector monolithically integrated on a PMN-PT ferroelectric single crystal. ACS Appl Mater Interfaces 8:32934– 32939 22. Tu C, Chien R, Wang F et al (2004) Phase stability after an electric-field poling in Pb(Mg1/3 Nb2/3 )1-x Tix O3 crystals. Phys Rev B 70:220103 23. Uchino K, Nomura S (1982) Critical exponents of the dielectric constants in diffused-phasetransition crystals. Ferroelectr Lett 44:55–61 24. Tang Y, Zhao X, Feng X et al (2005) Pyroelectric properties of [111]-oriented Pb(Mg1/3 Nb2/3 )O3 -PbTiO3 crystals. Appl Phys Lett 86:082901 25. Lin D, Zhang S, Li Z et al (2011) Domain size engineering in tetragonal Pb(In1/2 Nb1/2 )O3 -Pb (Mg1/3 Nb2/3 )O3 -PbTiO3 crystals. J Appl Phys 110:084110 26. Yamamoto N, Itsumi K, Hosono Y (2011) Effects of manganese oxides/gold composite electrode on piezoelectric properties of lead magnesium niobate titanate single crystal. Jpn J Appl Phys 50:09NC05 27. Li F, Zhang SJ, Li ZR et al (2012) Recent development on reloxor-PbTiO3 single crystals: the origin of high piezoelectric response. Prog Phys 32:178–198 28. Davis M, Damjanovic D, Hayem D et al (2005) Domain engineering of the transverse piezoelectric coefficient in perovskite ferroelectrics. J Appl Phys 98:014102

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29. Fu CL (2009) Ferroelectric thin film and its application [M]. Science Press 30. Wang X (2014) Crystal optics [M]. Nanjing University Press 31. Gao JH (2016) Studies of field induced phase transition in PLZST antiferroelectric single crystals near morphotropic phase boundary. Doctoral thesis, Tsinghua University 32. Ko Y, Song SK, Kim NH et al (2015) Highly transparent and stretchable conductors based on a directional arrangement of silver nanowires by a microliter-scale solution process. Langmuir 32:366–373 33. Lee JG, Kim DY, Lee JH et al (2017) Production of flexible transparent conducting films of self-fused nanowires via one-step supersonic spraying. Adv Funct Mater 27:1602548 34. Hu L, Kim HS, Lee JY et al (2010) Scalable coating and properties of transparent, flexible, silver nanowire electrodes. ACS Nano 4:2955–2963 35. Wang QY (2014) Photoelectric sensor application technology [M]. China Machine Press 36. Kulkarni ES, Heussler SP, Stier AV et al (2015) Exploiting the IR transparency of graphene for fast pyroelectric infrared detection. Adv Opt Mater 3:34–38 37. Peng QX, Wu CG, Luo WB et al (2013) An infrared pyroelectric detector improved by cool isostatic pressing with cup-shaped PZT thick film on silicon substrate. Infrared Phys Techn 61:313–318 38. Akai D, Hirabayashi K, Yokawa M et al (2006) Pyroelectric infrared sensors with fast response time and high sensitivity using epitaxial Pb(Zr, Ti)O3 films on epitaxial γ-Al2 O3 /Si substrates. Sens Actuat A 130:111–115 39. Auston DH, Glass AM (1972) Optical generation of intense picosecond electrical pulses. Appl Phys Lett 20:398–399 40. Cai H, Guo XJ, He T (2010) Terahertz wave and its new applications. Chin J Opt Appl Opt 3:209–222 41. Federici JF, Schulkin B, Huang F et al (2005) THz imaging and sensing for security applications—explosives, weapons and drugs. Semicond Sci Technol 20:S266 42. Wei D (2007) Solid state physics [M]. Tsinghua University Press 43. Lee YC, Lin KT, Chen HL (2016) Ultra-broadband and omnidirectional enhanced absorption of graphene in a simple nanocavity structure. Carbon 108:253–261 44. Kang MG, Xu T, Park HJ et al (2010) Efficiency enhancement of organic solar cells using transparent plasmonic Ag nanowire electrodes. Adv Mater 22:4378–4383 45. Liu Y, Cheng R, Liao L et al (2011) Plasmon resonance enhanced multicolour photodetection by graphene. Nat Commun 2:579 46. Yuan Y, Xiao Z, Yang B et al (2014) Arising applications of ferroelectric materials in photovoltaic devices. J Mater Chem A 2:6027–6041 47. Yang SY, Seidel J, Byrnes SJ et al (2010) Above-band gap voltages from ferroelectric photovoltaic devices. Nat Nanotechnol 5:143–147 48. He C, Wang F, Zhou D et al (2006) Determination of optical constants of tetragonal Pb (Mg1/3 Nb2/3 ) O3 -PbTiO3 ferroelectric single crystals. J Phys D Appl Phys 39:4337–4340 49. Wan X, Chan HLW, Choy CL et al (2004) Optical properties of (1-x)Pb (Mg1/3 Nb2/3 )O3 xPbTiO3 single crystals studied by spectroscopic ellipsometry. J Appl Phys 96:1387–1391

Chapter 4

A Mechanical Energy Writeable Ferroelectric Memory Based on PMN-35PT Single Crystal

4.1 Introduction of This Chapter 4.1.1 Ferroelectric Non-volatile Memory and Its Energy Consumption Information storage refers to the preservation of processed information in a certain format and order [1]. This is a prerequisite for ensuring that information can be used in case of need. Ferroelectric memory is a promising memory technology that is radiation resistant, fast access, and has low power consumption. It can storage data for long periods of time in open circuit [2–4]. Just as the residual magnetization of ferromagnetic materials in magnetic memory technology, the residual polarization of the ferroelectric material is the key of ferroelectric memory technology [5]. It can be seen from the P-E loop of ferroelectric materials, the two opposite polarization states can just compile the binary “0” and “1”. In the perovskite structure, polarization originates from the displacement of B site cations in the [BO6 ] octahedral center. As shown in Fig. 4.1, the two opposite polarization states have the lowest free energy and are thermodynamically stable [6]. Since the two polarization states are well maintained at a temperature below the Curie temperature, ferroelectric materials can be used as an excellent non-volatile memory medium. The physical nature of the information storage (write operation) in a ferroelectric memory is the poling process of ferroelectric materials. For inorganic ferroelectric materials, the poling process involves the switching of ferroelectric domains. The required poling voltage depends on the product of the coercive electric field and the thickness of the ferroelectric material. The poling process of organic ferroelectric materials such as polyvinylidene fluoride (PVDF) and its copolymer P(VDF-TrFE) involves the flipping of the dipole moment in the polymer chain [7], which usually requires a much higher voltage. On the other hand, the dielectric properties of ferroelectric materials limit the current flowing through them. This makes the power

© Tsinghua University Press, Beijing and Springer Nature Singapore Pte Ltd. 2020 H. Fang, Novel Devices Based on Relaxor Ferroelectric PMN-PT Single Crystals, Springer Theses, https://doi.org/10.1007/978-981-15-4312-8_4

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Fig. 4.1 Physical mechanism of ferroelectric memory

supply required for poling ferroelectric materials must meet the special requirements of high voltage and low current. The operating power consumption of the write operation in ferroelectric memory is not so large, but it is essential. However, it is difficult to install or replace a battery/cable in some special environments. Hence, it is desirable to have a self-sustaining capability for ferroelectric memory. Finding a new energy supply solution is an important issue to achieve the independent and sustainable operation of ferroelectric memory.

4.1.2 Self-powered System and Nanogenerator The self-powered system is a new technology trend in recent years. Through the integration between different components and modules, the whole system can operate independently without any external electric power supply [8–10]. The energy modules in the system can harvest a variety of energy (such as solar energy, wind energy and thermal energy) from the environment and convert it into electrical energy. Such harvested energy is used to supply sensors and MEMS with different functions. Mechanical energy is widespread in the environment and has the characteristics of weak, low frequency, and irregular. It is difficult to convert them into electric energy efficiently by using traditional electromagnetic generators. In contrast, triboelectric nanogenerators are more suitable for operation at low frequency [11]. This emerging green energy technology combines the well-known principles of triboelectric and electrostatic induction, and its greatest feature is the ability to output high voltages [12]. Figure 4.2 summarizes the development of triboelectric nanogenerators since their inception [13]. In just five years, the energy conversion efficiency of triboelectric nanogenerators has been exploding. Currently, the energy conversion efficiency has exceeded 80% through material optimization and device structure design. In addition to high electrical output, the advantages of wide selection of materials, strong

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Fig. 4.2 Development history of TENG [13]

environmental adaptability, and all weather applicability make triboelectric nanogenerators become a promising energy harvesting technology which can be widely used in self-powered systems [14]. In this chapter, we have designed a cheap and efficient arch-shaped triboelectric nanogenerator and demonstrated its electrical output performance and energy conversion principle [15]. Based on the advantages of high open circuit voltage of triboelectric nanogenerators, a new poling process of ferroelectric materials is explored. Finally, the self-powered ferroelectric memory system is realized by poling the PMN-35PT based FET with mechanical energy.

4.2 Arch-Shaped TENG 4.2.1 Design and Preparation of TENG Figure 4.3 shows the three-dimensional structure of the arch-shaped nanogenerator we designed. The main frame consists of two transparent conductive plastic films. These plastic films have been widely used in the field of flexible photovoltaic devices. Fig. 4.3 Schematic 3D diagram depicting the structure of arch-shaped TENG. Reproduced with permission [15]. Copyright 2015, Royal Society of Chemistry

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They are obtained by depositing a transparent conductive oxide ITO with a thickness of several hundred nanometers on a polyethylene terephthalate (PET) substrate. The PET faces need to be modified after cutting into suitable sizes for triboelectrification. We selected polystyrene (PS) nanosphere arrays and polyvinylidene fluoride (PVDF) porous films to deposit on the two PET surfaces as friction materials. Finally, the two modified ITO/PET plastic films are packaged together with two friction materials face to face. By pre-stressing, the plastic film is bent into an arch shape in a relaxed state. Then, the plastic deformation can be easily achieved by the elasticity of the PET substrate. When external pressure is applied to the arch-shaped plastic film, the two friction layer materials come into contact with each other. After the external force is unloaded, the plastic film is restored into the original arch shape, and the friction layer materials are separated from each other. The elastic modulus of PET plastic is about 2–2.5 GPa. Such a moderate elasticity ensures that the external force to cause the deformation is not too large. And the shape is quickly recovered after unloading the external force. Coupled with the rational design of the arch-shaped structure, the triboelectric nanogenerator can realize the reversible deformation and provides a mechanical basis for the periodic input of mechanical energy. At the same time, the excellent conductivity of the outer ITO electrode makes it easy to derive the induced current, which in turn provides an electrical basis for the device to periodically output electrical energy. The friction layer materials used in the arch-shaped triboelectric nanogenerator have also been carefully designed. The triboelectric effect between two materials is largely dependent on their relative position in the electrostatic sequence and the surface morphology of the materials. It has been found that surface roughening of the friction material at micron or nanometer scale can significantly increase the static electricity [16]. Inspired by this idea, research work on improving the performance of triboelectric nanogenerators by various micro-nano processing methods such as photolithography and plasma etching has been reported [17–19]. However, most of the processes rely on expensive equipment and are time-consuming, which limits their development prospects in practical applications. Self-assembled nanosphere arrays, also known as colloidal crystals, are an effective strategy for preparing nanoscale ordered structures. The advantages of large area, low cost and simple preparation make this technology widely used in the field of micro-nano processing [20]. Previously, we have used the periodic polystyrene nanosphere array as a mask to prepare patterned sapphire substrates [21] and highly ordered ferroelectric polymer dots array [22]. However, the application of such a polystyrene nanospheres array in the field of nanogenerators has not been reported. We first prepared uniform-sized polystyrene nanospheres using the emulsifier-free polymerization method [23]. The 600 nm diameter PS nanospheres can form a twodimensional colloidal sphere monolayer on the liquid surface through the waterair interface self-assembly process. Figure 4.4a shows the morphology of colloidal nanospheres successfully transferred to a PET substrate. It can be seen from the SEM image that the PS nanospheres are in a hexagonal arrangement to form an ordered array. However, the adjacent nanospheres are isolated from each other, which may

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Fig. 4.4 SEM images of the PS nanospheres array a before and b after annealing. Reproduced with permission [15]. Copyright 2015, Royal Society of Chemistry

not be able to withstand repeated deformations in the nanogenerator. In order to enhance the mechanical strength of the PS nanospheres array, in situ thermal annealing process was used to make adjacent nanospheres stick to each other, as shown in Fig. 4.4b. The annealing process can also reinforce the contact between the PS nanosphere and the PET substrate, avoiding the shedding of PS nanospheres. The geometry and morphology of the polystyrene nanospheres do not change significantly after annealing. So, the bonded arrays could serve as the friction layer material. Polyvinylidene fluoride (PVDF) is selected as the other friction layer material because it is far from the PS in the electrostatic sequence. Since fluorine is the most electronegative element in the periodic table, the introduction of fluorine atoms tends to make the fluoropolymer very suitable as a negative friction material in triboelectric nanogenerators [24, 25]. The detailed preparation process of the polyvinylidene fluoride porous film was as follows: First, 3 mL of DMF and 3 mL of acetone were mixed in a flask. Then, 0.25 g PVDF powder (average molecular weight ~ 534,000, Sigma-Aldrich) was added into the flask. The mixture was stirred at room temperature for 2 h to obtain a colorless solution. An appropriate amount of the solution was spin-coated on the ITO/PET substrate and placed in an oven at 80 °C to dry slowly. The volatilization of the mixed solvent can be controlled to cause vapor-induced phase separation. As shown in Fig. 4.5, PVDF forms a largearea disordered network on the PET surface. The inset shows an enlarged view of

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Fig. 4.5 SEM image of PVDF porous film. Reproduced with permission [15]. Copyright 2015, Royal Society of Chemistry

the nanoscale structure on the disordered network. This multilevel rough structure not only increases the contact area with the PS nanospheres array, but also enhances the friction between the two materials.

4.2.2 Energy Conversion Mechanism of TENG Next, we analyzed the energy conversion mechanism of the arch-shaped nanogenerator. The upper and lower ITO/PET plastic films shown in Fig. 4.3 are a convex curved surface and a concave curved surface in the relaxed state. To simplify the physical model, we introduced the differential method to explore the working mechanism of the nanogenerator. It is assumed that the entire surface of the ITO/PET film modified by PVDF porous film in the upper part of the device is composed of an infinite number of infinitesimal element (Xi ), which can be approximated as a plane. Since the structure of the arch-shaped nanogenerator is vertically symmetrical, there must be another infinitesimal element (Yi ) corresponding to Xi in the lower part of the device. The upper and lower two infinitesimal elements constitute a parallel plate capacitor as shown in Fig. 4.6. The entire nanogenerator is considered to be composed of a myriad of parallel plate capacitors. So, we only need to analyze one pair of the infinitesimal elements. When the nanogenerator undergoes the process of triboelectric charging, the two friction surfaces will receive equal amount of opposite charges. The absolute value of the surface charge density is recorded as σ . From the position of the two friction materials in the electrostatic sequence, it can be judged that the PVDF side has negative charges (−σ ), and the PS side has positive charges (+σ ). A parallel plate capacitor consisting of (Xi , Yi ) pairs is investigated, which can be decomposed into three independent capacitors in series. From top to bottom, the dielectric layers of the three capacitors are PET, air and PET, respectively. The thickness of PET is

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Fig. 4.6 Energy conversion mechanism of the arch-shaped TENG

denoted by d 1 , and its relative dielectric constant is denoted as εr1 . The thickness of the friction layer PVDF and PS nanospheres is much smaller than the thickness of the PET substrate. So, the modified PET shows different electrostatic properties on the surface, but the thickness and dielectric constant do not change significantly. The field strength in the PET dielectric layer can be derived from the Gauss theorem in the electromagnetism: EP ET = −

Q Sε0 εr 1

(4.1)

where Q is the amount of charge transferred between the two ITO electrodes, S is the area of the infinitesimal element and ε0 is the vacuum permittivity. When the nanogenerator is pressed, the distance x between the two infinitesimal elements dynamically changes with time, making it a function of time x(t). The internal field strength of the air capacitor is [26] E Air =

− QS + σ (t) ε0

(4.2)

The potential difference between the two ITO electrodes at a certain time can be calculated by: V (t) = E P E T d1 + E Air x(t) + E P E T d1

(4.3)

Substituting (4.1) and (4.2) into (4.3), it can be obtained: V (t) = −

σ x(t) Q 2d1 [ + x(t)] + Sε0 εr 1 ε0

(4.4)

As can be seen from (4.4), the potential difference between the two ITO electrodes is a function of time. When the external force changes the distance between the upper and lower PET substrates, the potential difference between the ITO electrodes

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changes. As a result of electrostatic induction, a portion of the induced charge flows back and forth between the two ITO electrodes to form an alternating current. By this way, the mechanical energy generated by the external force can be converted into electrical energy.

4.2.3 Electrical Output Performance of TENG To examine the output performance of triboelectric nanogenerator, we measured the electrical output signal while inputting mechanical energy to the nanogenerator. The mechanical energy produced by human movement is very common in life and enough to drive our arch-shaped nanogenerators. In theory, the mechanical energy produced by walking is close to 70 W, and energy produced by the slight upper limb motor is about 3 W [27]. Figure 4.7a shows the open circuit voltage measured with a digital multimeter (RIGOL DM3068). A positive voltage of more than 200 V and a negative voltage of several tens of volts is generated in each tapping cycle. Since the human tapping is random, the output voltage of each time is different, but basically it remains the same order of magnitude. Figure 4.7b shows the results after we exchanged the test electrodes of the nanogenerator. The amplitude of the voltage is almost unchanged, but the polarity is reversed. Hence, it can be determined that the output voltage signal is real and effective. In addition to the considerable instantaneous electrical output, the durability of the device has to be considered. The ability to withstand long-term use determines the lifetime of the device. We used a high-power exciter to simulate the action of tapping. The rapid and uninterrupted impact on the nanogenerator generated a longterm AC voltage. As shown in Fig. 4.8, the magnitude of either the forward output voltage or the reverse output voltage remains the same. The open circuit voltage does not show any decay after more than 1000 cycles. So, the arch-shaped nanogenerator has good durability and can be used for a long time. We also examined the relationship between the impact frequency and the electrical output performance of the nanogenerator. Since human tapping is limited by the reaction time and random, it is difficult to achieve precise control. So we still use the high power exciter to simulate the action of tapping. The quantitative characterization of the performance of the nanogenerator can be achieved by adjusting the impact frequency of the exciter. Figure 4.9 depicts the output voltage produced by a nanogenerator at different impact frequencies in the range of 1–9 Hz. The open circuit voltage provided by the nanogenerator is not sensitive to the input frequency of the mechanical energy. This is because the output voltage is composed of short pulses. In the low frequency range, the change in the input frequency of the mechanical energy only changes the time interval between adjacent pulses and does not affect the peak shape of the pulse. It is well known that most of the mechanical energy in daily life such as breeze, waves, and human breathing, are in the low frequency region. Therefore, the triboelectric nanogenerator we designed may have wide adaptability.

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Fig. 4.7 The open circuit output voltage signal of TENG a in the forward connection and b in the reversed connection. Reproduced with permission [15]. Copyright 2015, Royal Society of Chemistry

For a closed loop, the electrical output of the power supply is related to its external load. Therefore, we studied the relationship between the output performance of nanogenerator and the external load in the circuit. The impact frequency and amplitude are kept constant throughout the test. It is known from the previous characterization that the open circuit voltage of the power supply will remain stable. The resistance of the external load (R) is adjusted from 1k to 50 M. The voltage across the load (U) is measured and the current (I) in the loop can be calculated by:

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Fig. 4.8 Durability test of the arch-shaped TENG. Reproduced with permission [15]. Copyright 2015, Royal Society of Chemistry

Fig. 4.9 Relationship between output voltage and frequency. Reproduced with permission [15]. Copyright 2015, Royal Society of Chemistry.

I =

U R

(4.5)

The corresponding electrical output power (P) is: P = I ×U

(4.6)

Figure 4.10 shows the corresponding voltage, current, and output power with dif-

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Fig. 4.10 Relationship between output performance and load. Reproduced with permission [15]. Copyright 2015, Royal Society of Chemistry

ferent external loads. The voltage of the external circuit increases monotonously with the increase of the load resistance, and the current in the loop exhibits an opposite trend. As a product of the two parameters, the output power does not monotonously change. The maximum electrical output power is 1.8 mW at an external load resistance of 5 M. Taking the effective area of friction layer (3 cm × 3 cm) into account, the output power density of the nanogenerator is about 2 W/m2 . In addition to the high electrical output performance, arch-shaped nanogenerators have an advantage that cannot be ignored. It can be seen from the structure diagram that almost all part of the device are made of organic polymer materials. The entire structure is simple and light (only 1.2 g), which is convenient for users to carry. A battery with an open circuit voltage of 1.5 V is about 25 g. In contrast, the output voltage of the nanogenerator at unit mass is quite impressive.

4.3 Self-powered Ferroelectric Memory System After the energy supply module is established, we begin to design a ferroelectric memory. Considering that the ferroelectric single crystal has no grain boundaries and macroscopic defects, we use PMN-PT ferroelectric single crystal to establish a self-powered memory system. It is more advantageous for us to grasp the essential factor and mechanism of the prototype device.

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4.3.1 Properties of PMN-PT Single Crystal The processing of the crystal is similar to that in the previous chapter, but in this chapter we do not need to take advantage of the pyroelectric properties of the crystal. Conversely, the polarization of the ferroelectric material used in the memory cannot be too sensitive to temperature. Thus, we chose the [001] crystal orientation. After the crystal was sliced perpendicular to the [001] crystal orientation, the sample was examined by XRD. The photograph in Fig. 4.11a shows the oriented single crystal and the [001] oriented slice. The wafer is larger than 1 cm × 1 cm and the surface is sputtered with a gold electrode. Figure 4.11b shows the XRD pattern of the wafer. There are two sharp diffraction peaks at 22° and 45°, which correspond to the (001) and (002) crystal planes respectively. Large single crystals may have uneven crystal composition along the growth direction due to the difference in the segregation coefficients of various elements. In general, the content of titanium increases along the growth direction [28]. Before using the wafer, it is necessary to determine its composition. The gold electrode was sputtered on the top and bottom surfaces of the oriented wafer as shown in Fig. 4.11a. And the dielectric-temperature curve was test by an impedance analyzer (Agilent 4294A). As shown in Fig. 4.12, the blue curve is the dielectric constant at 1 kHz. According to the method described in Chap. 3, the content of lead titanate in the wafer is estimated to be about 35% by the T m temperature. So, the wafer we used is 0.65PMN-0.35PT, hereinafter referred to as PMN-35PT. Another inflection point (~70 °C) can be observed in the blue curve, which corresponds to the phase transition temperature of the rhombohedral ferroelectric phase to the tetragonal ferroelectric phase of the PMN-PT single crystal (T rt ). The brown curve in the figure is the dielectric loss at the same test frequency. The dielectric loss of the single crystal

Fig. 4.11 a [001] oriented PMN-PT crystals, b XRD pattern of the [001] oriented PMN-PT single crystal. . Reproduced with permission [15]. Copyright 2015, Royal Society of Chemistry

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Fig. 4.12 The dielectric properties at different temperature

changes with temperature and there is a peak near T m . The dielectric loss at room temperature is only 0.02, which also indicates the high quality of the PMN-PT crystal. Because once cracks and other defects such as inclusions appear inside the crystal, it would cause an increase in dielectric loss, which limits the application of single crystal in electrical devices. The crystal structure of PMN-xPT materials changes with its composition. Raman spectroscopy is very effective in studying the crystal structure. Figure 4.13a shows the Raman spectrum of a PMN-35PT single crystal at room temperature. The test conditions are consistent with those described in Chap. 2. The curve of the crystal appears to be almost indistinguishable from the results of PMN-26PT crystal. Three

Fig. 4.13 a Raman spectroscopy of PMN-PT crystal, b comparison of Raman spectroscopy in high wavenumber regions

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characteristic peaks were observed: T2g mode with wavenumber of 272 cm−1 , Eg mode at 583 cm−1 and A1g vibration mode at 770 cm−1 . As shown in Fig. 4.13b, the peak shape of A1g vibration mode of the PMN-35PT crystal is different from that of PMN-26PT crystal. The left shoulder of the A1g vibration mode in PMN-35PT single crystal is significantly higher than the right side. In PMN-xPT system, the A1g vibration mode is related to the stretching vibration of the [BO6 ] octahedron, and thus is sensitive to the B–O bond [29]. The strength of the Ti–O bond is lower than that of the Mg–O and Nb–O bonds. As the Ti content increases, the Raman shift will move toward the low wavenumber direction due to the weakened B–O bond. The I peak of tetragonal symmetry near 740 cm−1 is enhanced relative to the II peak at 800 cm−1 , which indicates the phase coexisting in the PMN-35PT single crystal. Next, we tested the ferroelectric properties of the PMN-35PT crystal. Figure 4.14 shows the P-E loop of the wafer with important information as follow: First, the coercive electric field (E C )of the crystal is 260 V/mm. We can roughly calculate the voltage required for poling through the thickness. Second, the Pr reaches 29 µC/cm2 , which is sufficient for the ferroelectric memory. Third, the squareness ratio of the hysteresis loop is an important parameter to measure the ferroelectric properties of materials. A good squareness ratio can reduce the probability of misreading operation in the ferroelectric memory. The squareness ratio of the hysteresis loop (Rsq ) can be calculated by the following formula [30] Rsq = (

Pr P1.1Ec )+( ) Psa Pr

(4.7)

where Pr , Psa and P1.1Ec are the residual polarization, the saturation polarization and the polarization at 1.1E c . Rsq of an ideal ferroelectric material should be equal to 2. From the data in Fig. 4.14, the [001] oriented PMN-35PT wafer has a Rsq of about 1.3, which is larger than several materials reported in the literature [31–33], indicating that the single crystal of this component has superior ferroelectric properties. Fig. 4.14 The P-E loop of PMN-35PT single crystal at room temperature. Reproduced with permission [15]. Copyright 2015, Royal Society of Chemistry

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4.3.2 Exploration of New Poling Methods The coercive field strength of the single crystal is 260 V/mm, and the thickness of the sample is about 270 µm. So, the poling voltage of the sample is at least 70 V. Usually, poling voltage is set to 2–3 times of the coercive voltage. In Sect. 4.2.3, we have tested the open-circuit voltage of the arch-shaped nanogenerator. The output voltage exceeds 200 V, which is much higher than the coercive voltage of 270 µm thick sample. However, poling time is another important factor to be considered. Poling of ferroelectric materials involves a dynamic process of domain inversion. The normal poling process uses a DC power supply. The time is enough for all the domains to switch. Therefore, the material can be sufficiently poled under a regulated power supply. However, the output voltage of the triboelectric nanogenerator is composed of pulses. As shown in Fig. 4.15, the pulse width with a voltage exceeding 100 V in the forward output is about 2.7 ms. Whether the single crystal can be poled in such a short period of time requires further investigation of the poling kinetics. In ferroelectric materials, the process of polarization switching is generally considered to be accomplished by two major stages. In the nucleation stage of the new domain, the number of nucleation per unit area in per unit time is called the nucleation rate (ξ ). It is related to the activation field (α) of the material and the strength of the applied field (E). When the field strength is low, the following exponential relationship is satisfied [34] ξ ∝ exp(−α/E)

(4.8)

Obviously, increasing the strength of the applied electric field is beneficial to the nucleation of domain. When the applied electric field exceeds a certain threshold, the nucleation rate will be converted to a power function of the electric field. Studies have shown that the domain expansion rate is also positively correlated with the applied field strength during the domain growth stage [35]. For single crystals, the classical Fig. 4.15 A single pulse output voltage of TENG. Reproduced with permission [15]. Copyright 2015, Royal Society of Chemistry

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Kolmogorov-Avrami-Ishibashi (KAI) model can describe the dynamic process of polarization switching [36]:  P(t) = 2Psa {1 − exp[−(t/t0 )n }

(4.9)

where ΔP(t) represents the switchable polarization fraction at time t, Psa is the saturation polarization, t 0 represents the characteristic time of domain switching, and n describes the geometric dimension of domain growth. The KAI model shows that the degree of poling is exponentially related to the poling time without the effect of domain-domain interaction. Viehland et al. [37] studied the polarization switching kinetics in PMN-PT single crystals and proposed a modified random field heterogeneous nucleation model to describe the nucleation and growth of domains in PMN-PT crystals. Their results show that the polarization switching inside the crystal is sufficient to complete in hundreds of microseconds if the field strength is large enough. Increasing the field strength can further shorten the time of polarization switching. Therefore, the open circuit voltage with a pulse width of several milliseconds in Fig. 4.15 is able to induce the polarization switching in PMN-PT single crystal. Based on the above feasibility analysis, we designed a set of poling circuits as shown in Fig. 4.16. Since the AC voltage of nanogenerator may cause fatigue or even cracking of the PMN-PT crystal, we built a bridge rectifier using four diodes. The bridge rectifier can convert the AC voltage of the nanogenerator into a DC voltage. The crystal sample can be poled with the DC voltage by tapping the nanogenerator. We examined the rectification effect of the circuit by measuring the voltage between the points A and B in the equivalent circuit. Figure 4.17 shows the output voltage by tapping the nanogenerator. Comparing with the results of Fig. 4.7a, the negative voltage generated during the unloading process is converted into a positive voltage. And the peak value of the positive voltage remains about 200 V. By tapping the nanogenerator, PMN-PT single crystals can be poled with the rectified voltage. To verify the poling effect, we characterized the piezoelectric properties of the single crystal. Because the PMN-PT relaxor ferroelectric single crystal near the MPB is poled along the [001] direction, it will inevitably bring about excellent piezoelectric properties. The electromechanical coupling factor is an important parameter for quantitatively describing the piezoelectric properties of a sample. We Fig. 4.16 The bridge rectifier circuit diagram for poling the PMN-PT single crystal directly by the TENG. Reproduced with permission [15]. Copyright 2015, Royal Society of Chemistry

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Fig. 4.17 The output voltage after rectification. Reproduced with permission [15]. Copyright 2015, Royal Society of Chemistry

used a precision impedance analyzer (Agilent 4294A) to test the impedance-phase spectrum of a single crystal sample poled by a nanogenerator. As shown in Fig. 4.18, the thickness mode resonance frequency (f r ) and anti-resonance frequency (f a ) are located at 8.73 MHz and 9.92 MHz, respectively. According to the resonance and anti-resonance peak positions, the thickness mode electromechanical coupling factor (k t ) can be calculated by the following formula: [38] kt2 =

π fr π f a − fr tan( ) 2 fa 2 fa

(4.10)

Here, k t was calculated to be 0.51, slightly lower than the parameter (k t = 0.6) of the sample after poling with a regulated power supply [39]. At the same time, we also measured the piezoelectric constant using a quasi-static d 33 tester (ZJ-4A). The poled single crystal shows a high d 33 of about 1040 pC/N. These results demonstrate that the application of a pulse voltage of a nanogenerator to a single crystal can Fig. 4.18 The thickness mode impedance and phase spectra of the PMN-PT single crystal after poling. Reproduced with permission [15]. Copyright 2015, Royal Society of Chemistry

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induce polarization switching. Although the poling effect is not as good as that of a DC regulated power supply, it can be further optimized by increasing the output of the nanogenerator or reducing the thickness of the crystal sample.

4.3.3 Ferroelectric FET Memory Next, we designed a single-element ferroelectric memory using PMN-PT single crystals. As demonstrated in Chap. 2, the channel current of the field effect transistor is related to the polarization state of the dielectric layer. In this chapter, we pay more attention to the polarization direction of PMN-PT. The surface bound charge of the single crystal decided by the polarization direction has a greater influence on the channel current. The two states can be distinguished if the ON/OFF current ratio corresponding to the two polarization directions is larger than hundreds. Compiling the binary states of the channel current into binary data “0” and “1” respectively is the basic principle of storing data by the ferroelectric field effect transistor [40]. Inset of Fig. 4.19a shows the ferroelectric field effect transistor structure. PMN35PT wafer along the [001] orientation were mechanically thinned to 100 µm and used as a dielectric in the field effect transistor after polishing. A gold electrode was deposited directly on the bottom surface of the PMN-35PT wafer as the gate electrode. Then, we deposited a 50 nm thick pentacene film as a channel material on the top surface of the crystal. The evaporation rate was controlled at 1.2 nm/min to ensure the continuity of the pentacene film. The source and drain electrodes in the transistor were also completed by an evaporation process. A 45 nm gold film was deposited through a mask with a channel size of 60 µm × 1 mm. The performance of the prepared ferroelectric field effect transistor was tested by a semiconductor characterization system (Keithley 4200SCS) at room temperature. Figure 4.19a shows the transfer curve of the ferroelectric transistor. At a fixed channel voltage V ds of -8 V, the channel current changes significantly during the sweep of gate voltage. At a gate voltage of -60 V, the transistor achieved an “ON” state with a channel current exceeding 1 µA. At a gate voltage of +40 V, the transistor achieved an “OFF” state with a channel current of only 1 nA. The channel current ratio in the two states is as high as 1000, which satisfies the basic requirements of the memory. Figure 4.19b shows the I ds -V ds characteristics of the transistor at three typical gate voltages. Applying a 40 V gate voltage is sufficient to close the transistor as shown by the black curve. A gate voltage of −60 V can turn on the transistor, as shown by the blue curve, where the conductance of the channel (the slope of the curve) is greatly increased compared to the black curve. This is consistent with the result of transfer curve. We can also get the following information that the absolute value of the voltage required for writing is less than 60 V in this transistor memory.

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Fig. 4.19 a The transfer curve of the pentacene ferroelectric transistors under a V ds of -8 V. Inset is the structure of FET, b typical I ds -V ds characteristics of a pentacene ferroelectric transistor with a PMN-PT insulator. Reproduced with permission [15]. Copyright 2015, Royal Society of Chemistry

4.3.4 Self-powered Ferroelectric Memory System The research motivation of this chapter is to solve the problem of energy consumption in ferroelectric non-volatile memory, so that the memory can operate normally without external electric power supply. As mentioned above, the write operation in the ferroelectric memory is essentially the polarization switching of the ferroelectric material. Although the voltage required for poling ferroelectric materials is high, the current in the poling process is small due to the large intrinsic resistance of the ferroelectric material. Therefore, the power consumption of the device during the writing of the ferroelectric memory is not so high. If the instantaneous power consumption of the memory is less than the mW level, it can be driven by the nanogenerator to absorb

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Fig. 4.20 Conceptual diagram of self-powered ferroelectric memory system. Reproduced with permission [15]. Copyright 2015, Royal Society of Chemistry

the mechanical energy in the environment. Based on this concept, we designed an integrated self-powered ferroelectric memory system as described by Fig. 4.20. The instantaneous high voltage generated by external force acts as the gate voltage of the ferroelectric transistor memory to pole the PMN-35PT single crystal in the dielectric layer. By changing the circuit connection mode of the nanogenerator with the source electrode and the gate electrode, the polarization direction of the single crystal in the dielectric layer can be switched, and the writing of the binary data “0” or “1” can be realized. In some emergency situations, the user can easily drive the system by tapping the nanogenerator with a finger. The matching circuit design of the self-powered ferroelectric memory system can be illustrated by Fig. 4.21. The electrical output signal of the nanogenerator is first rectified by a bridge rectifier and then loaded between the gate electrode and the source electrode of the ferroelectric transistor through a polarity inversion switch. Data of “0” or “1” is realized by adjusting the polarization direction in the PMN-35PT wafer by the polarity inversion switch. We have read the data in ferroelectric memory to determine whether the mechanical energy successfully wrote the data. The current in the pentacene channel was tested with the suspended gate. The two curves in Fig. 4.22 represent the state of the transistor after writing “0” and “1” respectively. After applying a positive voltage pulse to the transistor gate by the nanogenerator, the channel exhibits a high resistance state which stands for data “0”. Conversely, the red curve exhibits a low

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Fig. 4.21 Supporting circuit for the self-powered ferroelectric memory system. Reproduced with permission [15]. Copyright 2015, Royal Society of Chemistry

Fig. 4.22 Measurement of the I ds upon applying a positive or negative gate voltage pulse. Reproduced with permission [15]. Copyright 2015, Royal Society of Chemistry

resistance state, representing the data “1” written by the nanogenerator. The above results show that the use of mechanical energy can successfully write data into the designed ferroelectric memory system.

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4.3.5 Storage Density and Microscopic Ferroelectric Domain Storage capacity is an important indicator of information storage technology that reflects the number of bytes that can be accommodated in a single memory. To increase the storage capacity of the ferroelectric memory, it is necessary to reduce the size of each memory cell. Hence, the first thing to do is studying the polarization switching behavior of ferroelectric materials in micro-nanoscale. The macroscopic poling method and the test system used in the previous chapters are difficult to study the domain structure and ferroelectric properties of the PMN-35PT single crystal at the micro-nanoscale. So, we used a piezoresponse force microscope (PFM) to study the microdomain structure in single crystal and discussed how to improve the storage density of our self-powered memory systems. As shown in Fig. 4.23a, a small high-frequency AC voltage was applied to the local region of the PMN-35PT single crystal with the PFM probe. The deflection of the PFM probe caused by the piezoelectric effect of the PMN-35PT is detected and imaged with a lock-in technique. The resonance amplitude and phase information can be quantitatively extracted. The amplitude is related to the piezoelectric response capability of the local region, and the phase can reflect the direction of the polarization [41]. Taking the c domain with a polarization vector perpendicular to the surface as an example: if the polarization direction is downward (c+ domain), the piezoelectric response at that point will be synchronized with the excitation voltage applied by the probe, i.e. the phase is 0°. Conversely, if the polarization direction is upward (c− domain), the phase angle is 180°. The domain structure image of the scanned region can be obtained by scanning the surface of the polished PMN-35PT single crystal with a PFM probe, extracting the vibration signals of each point and establishing a spatial map. Figure 4.23b presents the phase information of the [001]-oriented unpoled PMN-35PT crystal

Fig. 4.23 a A schematic diagram of a PMN-PT single crystal test by PFM, b ferroelectric domains of unpoled PMN-35PT single crystal

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with a scan area of 10 µm × 10 µm. The micro domain structure can be clearly observed from the figure. The width of these light and dark phases is about several hundred nanometers and the length is several micrometers. For an unpoled single crystal, a phase transition from the paraelectric phase to the ferroelectric phase occurs as it cools from a high temperature to a low temperature below the Curie point. These domain structures are formed during the phase transition to minimize the total free energy of the system [42]. On the one hand, the formation of 180° domains can reduce the electrostatic energy inside the PMN-PT crystal. While the formation of non-180° domains effectively reduces the strain energy generated by the phase transition. On the other hand, the appearance of too many ferroelectric domains will greatly increase the domain wall energy [43]. A suitable number of domains which compromise the two factors can make the crystal in a stable state with the lowest total energy. We used PFM probes to test the polarization switching behavior of local regions on the surface of PMN-35PT crystals, as shown in Fig. 4.24. It shows a good ferroelectric phase loop (blue) over the −10 to 10 V scan range, indicating the existence of a switchable intrinsic polarization. The red curve shows the amplitude as a function of the bias of the PFM probe, which is similar to the butterfly curve in the bipolar strain test. The absolute value of the coercive voltage during the polarization switching is about 2.5 V, but the positive and negative coercive voltages show a slight asymmetry. The reason of this coercive voltage asymmetry may be the difference of test electrodes (the top electrode is a PFM probe, the bottom electrode is suspended) and the internal bias field present in the crystal, etc. The ability of the micro domain to respond to an applied electric field makes high density ferroelectric memory possible, as long as the PFM probe is used to apply a suitable bias to the crystal. It can be seen from Fig. 4.24 that the DC bias with an absolute value exceeding 5 V is sufficient to cause saturation polarization. In this

Fig. 4.24 PFM results of the PMN-35PT single crystal

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Fig. 4.25 a Rectangular domain written by PFM, b line scan phase data at line A–B

experiment, we use −10 and +10 V bias (far less than the output voltage of nanogenerator) to write the information “0” and “1” respectively. A 6 µm × 6 µm region was selected on the surface of the polished PMN-35PT single crystal, which was equally divided into six adjacent rectangles. Figure 4.25a shows the domain structure after positively or negatively poling. Six rectangular domains can be identified from the image, and adjacent rectangular domains have significant phase differences. The line scan phase data between point A and point B is shown in Fig. 4.25b, the phase angle of each point changes with the position. The phase angle of the purple rectangular domain is substantially 0° and the phase angle of the yellow rectangular domain is about 180o , which respectively represent binary data “1” and “0”. It can also be seen from Fig. 4.25b that the phase angle change at the 180° domain wall is very steep, indicating that the thickness of the domain wall is very thin, much smaller than the width of the domain (~1 µm). So the size of the memory cell can be further reduced to write information in a smaller space. It is worth noting that the signal crosstalk between adjacent domains was observed in Fig. 4.25a. The red dashed line is the dividing line between two poling area. But the edges of the rectangular domains are not ideally neat, which limits the further reduction of the memory unit. However, the signal crosstalk problem can be effectively overcome by the isolation of array element. When the ferroelectric material is discontinuous, the internal electric field and stress in the array elements will not interfere with each other. We have prepared a P(VDF-TrFE) ordered array with a period of about 400 nm using nanoimprint technology. Experiment results showed that this strategy can effectively avoid signal crosstalk between adjacent memory cells [22]. Some researchers have reduced the cycle of ferroelectric memory cells to 140 nm by nanoimprint technology, and achieved storage densities of up to 33 Gbits/in.2 [44]. It can be expected that if the rectified open circuit voltage by the nanogenerator is supplied to the PFM probe, combined with advanced nanofabrication technology, the storage density of the self-powered memory system will be greatly improved.

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4.4 Summary of This Chapter 1. An arch-shaped TENG was designed and fabricated. The selected friction layer materials were PS nanospheres array and PVDF porous film. The energy conversion principle of the nanogenerator is analyzed. Electrical performance tests have shown that an open circuit voltage of more than 200 V can be obtained by tapping. When the load is 5 M in an external circuit, the nanogenerator has a maximum output power of 1.8 mW and a power density of about 2 W/m2 . In addition, the device is insensitive to mechanical energy input frequencies in the low frequency region and exhibits good durability over 1000 cycles. 2. A new method for poling perovskite type ferroelectrics was developed, and the feasibility was demonstrated through theoretical analysis. Experimental results have shown that the pulsed high voltage output from the nanogenerator can effectively pole the PMN-PT single crystal. The [001]-oriented PMN-35PT after poling exhibits excellent piezoelectric properties. The thickness mode electromechanical coupling coefficient is about 0.51 and the piezoelectric constant is up to 1040 pC/N. 3. A ferroelectric field effect transistor was fabricated using a PMN-35PT single crystal. The pentacene film with a thickness of 50 nm was deposited as a channel material. As a memory unit, the ON/OFF current ratio of the transistor is approximately 1000. The transfer curve indicates that the absolute value of the voltage required for the write operation is less than 60 V. 4. The self-powered ferroelectric memory system is realized through the design of supporting circuit. The pulse voltage generated by the TENG is able to complete the information writing in the ferroelectric memory. The distinct bistable ON and OFF current states shown in the I-V curves confirm the excellent memory operation. Therefore, the self-powered memory system can solve the energy consumption problem in the ferroelectric non-volatile memory. 5. The polarization switching behavior and domain structure of PMN-35PT single crystal at micro scale were studied by PFM. The poling with PFM probe was used to demonstrate the possibility of improving the storage density of the memory system.

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