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Title Page

Contents

Abstract 19

CHAPTER Ⅰ. INTRODUCTION 21

1.1. Introduction to two-dimensional materials 21

1.2. Developments and challenges of 2D material-based broadband photodetectors 22

1.3. Strategies for 2D material based high-performance broadband photodetectors 24

1.4. Contribution of this dissertation for the development of 2D material based-broadband photodetectors 26

1.5. Reference 30

CHAPTER Ⅱ. THEORETICAL AND EXPERIMENTAL BACKGROUND 33

2.1. Fundamental photocurrent mechanisms in 2D materials 33

2.2. Exponential parameter, α-photoconductive and photogating effects characterization 37

2.3. Schottky barrier height-photovoltaic effect characterization 38

2.3.1. Arrhenius plot 38

2.3.2. Richardson plot 39

2.4. Seebeck coefficient-photothermoelectric effect characterization 40

2.4.1. Electrical conductivity dependent Seebeck coefficient 40

2.4.2. Electrical conductivity dependent Seebeck coefficient measurement 41

2.5. Photocurrent characterization methods 42

2.5.1. The d.c. photocurrent measurement 42

2.5.2. The a.c. photocurrent mapping measurement 43

2.6. Hydrothermal synthesis of Tellurene 44

2.7. Reference 46

CHAPTER Ⅲ. CONTROLLABLE PHOTOGATING AND PHOTOCONDUCTIVE EFFECTS IN ReS₂-2D Te HETEROJUNCTION FOR HIGH-PERFORMANCE BROADBAND PHOTODETECTORS 49

3.1. Introduction 49

3.2. Experiment 51

3.2.1. Device Fabrication 51

3.2.2. Characterization and measurement 52

3.3. Results and discussions 52

3.3.1. Optical and electrical characterizations of ReS₂-2D Te transistor 53

3.3.2. Back-gate controlled photocurrent mechanism in ReS₂-2D Te phototransistor 57

3.3.3. Photocurrent behavior of ReS₂-2D Te phototransistor 60

3.3.4. Power-dependent photoresponse of ReS₂-2D Te phototransistor 63

3.3.5. Performance of ReS₂-2D Te photodetector from visible to NIR range 68

3.4. Conclusion 71

3.5. Reference 72

CHAPTER Ⅳ. UNVEILING PHOTOVOLTAIC AND PHOTOTHERMOELECTRIC EFFECTS IN ReS₂-2D Te HETEROJUNCTION 80

4.1. Introduction 80

4.2. Experiment 83

4.2.1. Device fabrication 83

4.2.2. Characterization and measurement 84

4.3. Results and discussions 86

4.3.1. Electrical properties of ReS₂-2D Te heterojunction 86

4.3.2. Photovoltaic effect in ReS₂ phototransistor 87

4.3.3. Photothermoelectric effect in 2D Te phototransistor 91

4.3.4. Energy band characterization of ReS₂-2D Te heterojunction 96

4.3.5. Photovoltaic and photothermoelectric effects in ReS₂-2D Te phototransistor 98

4.3.6. Enhanced photothermoelectric effect in ReS₂-2D Te phototransistor by forward bias 102

4.3.7. Enhanced photovoltaic effect in ReS₂-2D Te phototransistor by reversed bias 103

4.4. Conclusion 107

4.5. Reference 108

CHAPTER Ⅴ. UNUSUAL PHOTOTHERMOELECTRIC EFFECT IN SEMIMETAL-SEMICONDUCTOR WTe₂-2D Te HETEROJUNCTION 113

5.1. Introduction 113

5.2. Experiment 116

5.2.1. Device Fabrication 116

5.2.2. Characterization and measurement 117

5.3. Results and discussions 118

5.3.1. Optical and electrical properties of WTe₂-2D Te heterojunction 118

5.3.2. Seebeck coefficient measurements of WTe₂ and 2D Te 121

5.3.3. Unusual photothermoelectric effect in WTe₂-2D Te phototransitor at zero drain-source bias 122

5.3.4. Effect of drain-source bias on pre-existed photovoltaic and photothermoelectric voltages 127

5.3.5. Photovoltaic effect on 2D Te assisted by WTe₂ at negative drain-source bias 128

5.3.6. Typical photothermoelectric effect on 2D Te at positive drain-source bias 130

5.4. Conclusion 133

5.5. Reference 134

CHAPTER Ⅵ. SUMMARY OF DISSERTATION 139

List of Tables

Table 2-1. Sketch of different photocurrent mechanisms. 36

Table 4-1. Bias-controlled photocurrent generation mechanism: photovoltaic(PV), photothermoelectric (PTE), photothermal effect (PT), and PV+PTE effects. 106

Table 5-1. Bias-controlled dominant photocurrent mechanism in WTe₂-2D Te: photovoltaic effect (PV), photothermoelectric effect (PTE), and... 132

List of Figures

Fig. 2-1. Charge-injection mechanisms under a) depletion regime, b) Flat band regime, and c) Accumulation regime. 39

Fig. 2-2. Device structure for Seebeck coefficient measurement. 42

Fig. 2-3. a) Photograph and b) Schematic circuitry for the d.c. photocurrent measurement. 43

Fig. 2-4. a) Photograph and b) Schematic circuitry for the a.c. photocurrent mapping measurement. 44

Fig. 2-5. Synthesis process of 2D Te using hydrothermal method. 45

Fig. 3-1. a) 3D schematic and b) optical image of ReS₂-2D Te heterostructure device. c) Thickness profiles of ReS₂ and 2D Te flakes.... 56

Fig. 3-2. a) Schematic of heterostructure device under illuminated condition. The Ids -Vgs characteristic at Vds＝2 V in the dark and at...[이미지참조] 59

Fig. 3-3. Photocurrent as a function of back-gate voltage at Vds＝2 V under various incident laser powers of a) 458 nm and c) 1062 nm lasers. Energy band...[이미지참조] 63

Fig. 3-4. Power-dependent a) Photocurrent, b) responsivity, c) detectivity, and external quantum efficiency under 458 nm illumination at Vds＝2 V. Power-...[이미지참조] 66

Fig. 3-5. The response curve of ReS₂-2D Te photodetector under 458 nm a)and 1062 nm c) laser excitations at Vds＝2 V and Vgs＝0 V. A peak analysis...[이미지참조] 68

Fig. 3-6. Optoelectronic characterization of ReS₂-2D Te photodetector in a wide detection range from visible to NIR regions at Vds＝2 V. a) Responsivity...[이미지참조] 70

Fig. 4-1. a) 2D Te flake on SiO₂ (300 nm)/Si (500 µm) substrate. b) ReS₂ flake on PDMS. c) Superposition of 2D Te and ReS₂ flakes. d) Deposition of electrodes... 84

Fig. 4-2. a) 3D schematic and b) optical image of ReS₂-2D Te heterostructure device. c) Thickness profiles of ReS₂ and 2D Te flakes. The inset shows the... 87

Fig. 4-3. a) Optical image of the ReS₂ FET. The scale bar signifies 5 µm. b) Intensity and c) phase maps of the photocurrent from ReS₂ at Vds＝0 V and Vgs...[이미지참조] 89

Fig. 4-4. a) Temperature-dependent transfer curves of ReS₂ field effect transistor. b) Arrhenius plot of ReS₂ -Au contact. c) EA versus Vgs, where...[이미지참조] 91

Fig. 4-5. a) Optical image of the 2D Te FET. The scale bar signifies 5 µm. b) Intensity and c) phase maps of the photocurrent from 2D Te at Vds＝0 V and...[이미지참조] 92

Fig. 4-6. a) Temperature-dependent output curve of 2D Te device. b) Richardson's plot of 2D Te-metal contact. 93

Fig. 4-7. a) A schematic for the energy band and flow direction of IPV and IPTE. b) Intensity and c) phase line plots from source to drain electrodes (along the...[이미지참조] 95

Fig. 4-8. a) 2D Te device structure for thermoelectric power measurement. Temperature-resistance relations of b) top and c) bottom electrodes. d) back-... 96

Fig. 4-9. a) Schematic of KPFM measurement. b) Optical image and c) KPFM map of ReS₂-2D Te device. d) ΔVCPD profiles extracted from KPFM map in Fig....[이미지참조] 98

Fig. 4-10. Photocurrent intensity maps of the ReS₂-2D Te heterostructure at Vds＝0 V and different Vgs values: a) -40 V; b) 0 V; c) 40 V. The scale bar...[이미지참조] 101

Fig. 4-11. Photocurrent intensity maps of the ReS₂-2D Te heterostructure at Vds＝2 V and different Vgs values: a) -40 V; b) 0 V; c) 40 V. The scale bar...[이미지참조] 103

Fig. 4-12. Photocurrent intensity maps of the ReS₂-2D Te heterostructure at Vds＝-2 V and different Vgs values: a) -40 V; b) 0 V; c) 40 V. The scale bar...[이미지참조] 105

Fig. 5-1. a) Schematic and b) optical image and thickness profiles of the WTe₂-2D Te heterostructure device. c) Raman spectra of WTe₂, 2D Te, and WTe₂... 120

Fig. 5-2. a) 2D Te and d) WTe₂ device structures for thermoelectric power measurements. Temperature-resistance relations of top and bottom electrodes... 122

Fig. 5-3. a) Intensity and b) phase maps of the photocurrent from WTe₂-2D Te heterojunction at Vds＝0 V and Vgs＝40 V. The mapping images were...[이미지참조] 126

Fig. 5-4. The effect of Vds on photocurrent is accompanied by pre-exist photovoltaic and photothermoelectric voltages.[이미지참조] 128

Fig. 5-5. a) Photocurrent intensity map of the WTe₂-2D Te heterostructure at Vds＝2 V and Vgs＝40 V. The mapping images were obtained with a laser power...[이미지참조] 129

Fig. 5-6. Photocurrent intensity maps of the WTe₂-2D Te heterostructure at V ds＝-2 V and different Vgs values: a) -40 V; b) 0 V; c) 40 V. The mapping...[이미지참조] 131

초록보기

 Two-dimensional (2D) heterostructures have garnered intense research interest owing to their advanced characteristics compared to 2D homostructures. Combining the best features of different ingredients in a single structure, 2D heterostructures enhance the performance of 2D material-based devices and enable the discovery of new physical effects at their interfaces. In the field of optoelectronics, different photocurrent mechanisms have been exploited in the heterojunctions, which play crucial roles in working principles of optoelectronic devices. Despite these advantages, achieving high-performance broadband phototransistors remains a challenge for 2D heterostructure-based devices. Furthermore, each heterostructure device typically operates using a single photocurrent mechanism. Hence, innovative strategies are required to combine 2D materials and explore adaptable photocurrent mechanisms, advancing high-performance phototransistors for a broad detection range.

This study introduces various methods and interpretations for understanding photocurrent generation mechanisms in a heterojunction. Using d.c. photocurrent measurement, a high-performance broadband photodetector based on ReS₂-2D Te heterojunction was successfully achieved by effectively controlling photoconductive and photogating effects. With the same structure, photovoltaic (PV) and photothermoelectric (PTE) effects were characterized in ReS₂-2D Te p-n junction through a.c. photocurrent mapping measurement. In addition, our observations reveal different photodetection mechanisms within the heterostructure. Even more, by controlling drain-source and gate biases, we can selectively choose a main detection mechanism of the structure. Thus, the adaptive sensing of the light is possible, which is unprecedented. Another study on the WTe₂-2D Te heterostructure noted distinct nature of photocurrent puddles within the structure. To homogenize the puddles, careful material selection based on thermal and electrical properties is essential because these puddles can interact counteractively. Otherwise, leveraging each photocurrent domain enables adaptive operation with both PV and PTE, enhancing device performance through the energy conversion from multiple sources (light and waste heat), and enabling the detection of long-wavelength light through light-induced heating. Our research reveals diverse photocurrent types coexisting in heterostructures. Distinguishing these mechanisms is crucial for precise control and selection in diverse design applications.

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