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A study on high-performance photodetector based on transition metal dichalcogenides

  • 박영서 (Youngseo Park, 아주대학교 일반대학원)

초록/요약

Two-dimensional layered metal dichalcogenide (TMD), atomically composed of a transition metal and two chalcogen atoms, has attracted great interest in photodetector due to superior characteristics such as adjustable bandgap, strong absorption, and flexibility. Especially, TMD can easily form heterojunction despite lattice mismatch by out-of-plane van der Waals (vdWs) bonding, which opens up the possibility of various energy band alignment. In this thesis, author conducted several studies for high-performance photodetector by using unique characteristics of TMD, which consists of three main topics. First, MoS2 phototransistor was fabricated and analyzed to understand its characteristics. By investigating bias-dependent properties, it is confirmed that responsivity is controlled with bias, and that it is caused by traps. Hence, the interface traps between MoS2 channel and gate oxide were closely investigated by analyzing temperature-dependent properties. Next, the study has been carried out to improve responsivity in MoS2 phototransistor. To this end, author adopts two approaches: cavity effect and additional absorption layer. Al metal gate was adopted in MoS2 phototransistor to confine light across MoS2 channel and gate oxide that serve as cavities. Moreover, absorption peak wavelength is adjusted by the thickness of MoS2 and gate insulator. In a different approach, perovskite was used as additional absorption layer on MoS2. The carriers, photogenerated in the perovskite layer, are transported into the MoS2 channel, which helps the incomplete light absorption in the thin MoS2. Finally, author has studied MoS2/Ge vdWs heterojunction photodetectors to extend the cutoff range. The vdWs force in TMDs enable formation of new heterojunction regardless lattice constant. The MoS2/Ge vdWs heterostructure, composed of large-bandgap MoS2 and small-bandgap Ge, can achieve broadband detection. Moreover, MoS2/Ge photodiode is designed to enable wavelength-selective detection according to the bias. Furthermore, high responsivity and fast response MoS2/Ge vdW heterojunction phototransistor was demonstrated. I believe that these works will be helpful to use photodetectors based on TMDs and understand these properties.

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목차

Chapter 1: Introduction 1
1.1 Overview 1
1.2 Overall objective 2
1.3 Thesis organization 3
Reference 5
Chapter 2: Trap analysis in multilayer MoS2 phototransistors 7
2.1 Introduction 7
2.2 Fabrication process 8
2.3 Bias-dependent responsivity in multilayer MoS2 phototransistors 9
2.3.1 Device structure 9
2.3.2 Electrical characteristics 10
2.3.3 Optoelectronic characteristics 11
2.3.4 Gate bias-dependent responsivity 13
2.3.5 Drain bias-dependent responsivity 14
2.4 Temperature-dependent activated trap charges in multilayer MoS2 phototransistors 18
2.4.1 Device structure 18
2.4.2 Electrical characteristics 19
2.4.3 Temperature-dependent electrical characteristics 20
2.4.4 Interface trap density 23
2.4.5 Thermally activated interface traps 24
2.5 Conclusion 27
Reference 28
Chapter 3: Wavelength-selective responsivity enhancement in metal-gated multilayer MoS2 phototransistor 30
3.1 Introduction 30
3.2 Device structure and fabrication 32
3.2.1 Al metal-gated multilayer MoS2 phototransistor 32
3.2.2 Si-gated multilayer MoS2 phototransistor (control device) 34
3.3 Device characterization and discussion 35
3.3.1 Electrical characteristics 35
3.3.2 Optoelectronic characteristics 37
3.3.3 Transient characteristics 39
3.3.4 Wavelength-selective responsivity 40
3.3.5 Finite difference time domain (FDTD) simulation 41
3.4 Conclusion 43
Reference 44
Chapter 4: Enhanced photoresponse of MoS2 phototransistors with metal-halide perovskite 47
4.1 Introduction 47
4.2 Device structure and fabrication 49
4.2.1 Multilayer MoS2 phototransistor (control sample) 49
4.2.2 Synthesis of organolead halide peroveskite 50
4.2.3 Hybrid perovskite/MoS2 phototransistor 51
4.3 Device characterization and discussion 52
4.3.1 Electrical characteristics 52
4.3.2 Optoelectronic characteristics 54
4.3.3 Comparison of figure of merit 56
4.4 Conclusion 59
Reference 60
Chapter 5: Broadband TMD/Ge van der Waals heterojunction photodiode 62
5.1 Introduction 62
5.2 Device structure and fabrication 64
5.2.1 Fabrication process of Ge/TMD photodetector 64
5.2.2 Ge/MoS2 photodetector structure 65
5.3 Device characterization and discussion 68
5.3.1 Device operation principle of selective photodetector 68
5.3.2 Wavelength-dependent photoresponse characterization 69
5.3.3 Power-dependent photoresponse characterization 70
5.3.4 Noise equivalent power (NEP) 72
5.3.5 Transient characterization 73
5.3.6 Selective VIS/SWIR dual imaging 75
5.4 WS2/Ge vdWs heterojunction 77
5.4.1 Device structure and fabrication 77
5.4.2 Power-dependent photoresponse characterization 77
5.4.3 Wavelength-dependent photoresponse characterization 78
5.4.4 Highly n-doped WS2/Ge heterojunction 79
5.5 CVD-grown MoS2/Ge vdWs heterojunction 80
5.5.1 Device structure and fabrication 80
5.5.2 Photoresponse characterization 82
5.6 Conclusion 83
Reference 84
Chapter 6: Broadband TMD/Ge van der Waals heterojunction phototransistor 87
6.1 Introduction 87
6.2 Device structure and fabrication 89
6.2.1 Device fabrication 89
6.2.2 MoS2/Ge vdWs heterojunction phototransistor structure 91
6.3 Device characterization and discussion 93
6.3.1 Electrical characterizations of each junction 93
6.3.2 Electrical characterizations of MoS2/Ge phototransistor 94
6.3.3 Photoresponse characterization of MoS2/Ge phototransistor 96
6.3.4 Transient characterization of MoS2/Ge phototransistor 97
6.3.5 Noise and sensitivity 99
6.3.6 Photocurrent gain (βph) 101
6.3.7 Broadband imaging 102
6.4 Conclusion 104
Reference 105
Chapter 7: Conclusion and future work 109
7.1 Summary 109
7.2 Suggestion for future work 111
7.2.1 MoS2/Ge/MoS2 double vdWs heterojunction phototransistor 111
7.2.2 MoS2/Ge vdWs heterojunction image sensor 112
Appendices 114
Appendix A Extraction of electrical parameters 114
Appendix B Extraction of optoelectrical parameters 118

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