過渡性金屬硫屬化合物(Transition Metal Dichalcogenide, TMD)是一種具有二維層狀結構的材料，由於其獨特的電子結構和物理性質，於先進邏輯元件電路或光電元件系統中，具有巨大潛力而成為國際熱門研究。其中，二硫化鉬(Molybdenium Disulfide, MoS2)為地表最豐富且無毒性之二維材料，合成製程已有眾多基礎，成為二維半導體中最具代表的系統，該材料之電晶體及光偵測電晶體，被發現具有大於10^8的開關比及1000安培/瓦的超高光響應度，搭配單層MoS2特殊的能帶結構及表面特性，成為研究二維半導體新穎光電元件之理想系統。
目前，單層TMD二維材料的巨大表面體積比，豐富的物理特性經常決定表面及介面狀態，材料外部之環境因素和內部之缺陷效應，經常限制傳輸性能、引起巨大接觸電阻並存在嚴重費米能級釘扎。本研究期望探討二維半導體之表面及介面問題及解決之道，了解並克服目前於二維材料光電元件的挑戰，有效調控單層二維材料元件的介面及特性，將TMD材料應用於發展新型元件系統及相關應用。綜觀本論文的實驗結果與討論，可分為三個部分：
(1) 堆疊式二維材料元件
(2) 介面特性調控
(3) 先進元件
在本論文中，我們採用CVD合成之TMD(主要為MoS2 和 WSe2)，於第四章，以場效電晶體及光偵測器的特性，說明於優良材料品質的二維材料，其表面、介面及元件製程對元件特性的影響與限制。第五章，深入探討二維材料元件的介面問題，透過發展介面態及缺陷控制的調控，大幅改善二維材料元件效能，並深入討論相關機制。我們發現在基板介面及金屬/半導體接觸介面，能透過有效的調控，有效提升單層二維半導體(MoS2和WSe2)的電子遷移率及光響應度。於第六章，進一步結合材料堆疊、介面調控及元件技術，實現多種新型二維材料元件系統。以脈衝電訊號輸入的介面調控，成功製作可用於類神經網絡運篹的多態元件，同時兼具超高響應及高速的新型光感光偵測器，藉由單層TMD異質堆疊的能帶接面控制，實現波段可控之近紅外光偵測器。
透過這些先進元件的實現，說明單層TMD二維材料的介面調控，具有發展各類新穎元件系統的潛力。

Transition metal dichalcogenides (TMDs) exhibit two-dimensional (2D) lattices and unique electronic structures for rich physical properties, which is potential for applications in logic devices and photodetectors (PDs). In the material family of TMD, molybdenum disulfide (MoS2) is the most popular because of non-toxicity and earth abundant. It is considered as a representative 2D semiconductor with a potential for diverse electronic and optoelectronic devices, including field-effect transistors (FET) and photo-field-effect transistor (photo-FET) with an ON/OFF ratio of over 108 and an ultrahigh responsivity of 1000 A/W. The unique electronic band structure and layered structures make it a promising material for advanced optoelectronic device systems.
With large surface-to-volume ratio, their rich physical properties of the monolayer TMD are usually determined at the interface and surfaces. A reduced transport performance of TMD device are commonly sensitive to environmental factors and internal defects, resulting in a huge contact resistance and serious fermi level pinning. Goal of this thesis is to study the surface and interface issues in the 2D semiconductors for better understanding and overcoming current challenges in novel 2D devices. We aim to effectively engineer the interface and performances of monolayer 2D material devices. Based on the experimental results in this thesis, the discussions are categorized into three parts:
(1) Stacked 2D Device
(2) Interface engineering
(3) Advanced Devices


In this thesis, the CVD-grown monolayer TMD (MoS2 and WSe2) are selected and studied. In Chapter 4, performances of the monolayer devices (electronic FET and optoelectronic PDs) are utilized to study possible dominating factors and limitations, such as sample quality, surface and interface issues, and fabrications. In Chapter 5, engineering of the interface issues in the 2D devices are presented. By controlling interfacial states and defects, performances of the monolayer devices were significantly enhanced. Possible mechanisms are discussed in this chapter. The electron mobility and responsivity of the monolayer (MoS2 and WSe2) devices can be effectively increased through optimization of substrate interface and metal/semiconductor contact interface. In Chapter 6, some prototype advanced devices are demonstrated by combining stacking, interface control, and fabrications of the monolayer 2D.
Realization of the advanced devices demonstrates that the interface control of the monolayer devices enables the exploration of diverse advanced devices based on artificial 2D lattices.

Outline
摘要 1
Abstract 3
Chapter 1. Introduction 5
1-1. Background 5
1-2. Motivation 7
Chapter 2. Literature Review 8
2-1. Uniqueness in Transition Metal Dichalcogenides (TMDs) 8
2-1-1. Lattice Structure 8
2-1-2. Band Structure 10
2-1-3. Optical and valley-dependent properties 13
2-1-4. Electrical Transport 15
2-1-5. Novel devices based on TMD 17
2-2. Synthesis of TMDs 22
2-2-1. Bottom-up Synthesis 22
2-2-2. Scalable Synthesis 26
2-3. Electronics of TMD 27
2-3-1. Transistors 27
2-3-2. Transfer and Output Characteristics 29
2-3-3. Figure Merit of 2D electronics 31
2-3-4. Mobility Limitation 34
2-3-5. Contact Resistance (Rc) 37
2-3-6. Schottky Barrier Height (SBH) 39
2-3-7. Reduced Rc and SBH 42
2-3-8. Hysteresis 47
2-4. Optoelectronics of TMD 49
2-4-1. Optoelectronic Properties of MoS2 49
2-4-2. Detect Wavelength (nm) 49
2-4-3. Responsivity (R) and Response Time (t) 51
2-5. Photocurrent Generation Mechanisms 53
2-5-1. Photoconductive (PC) Effect 54
2-5-2. Photovoltaic (PV) Effect 55
2-5-3. Photogating Effect (PG) 56
2-5-4. Photo-thermoelectric (PTE) 58
2-5-5. Photo-bolometric (PB) effect 59
2-5-6. Trap State and Trap Center 59
Chapter 3. Experiments 62
3-1. Laboratory Equipment 62
3-1-1. Raman Spectroscopy 62
3-1-2. Photoluminescence (PL) 63
3-1-3. Atomic Force Microscope 64
3-1-4. Electron-Beam Lithography 65
3-1-5. Photolithography 65
3-1-6. Electron Gun Evaporator 67
3-1-7. DC Sputtering Deposition 68
3-1-8. Reactive-Ion Etching 69
3-1-9. Annealing System 70
3-1-10. Measurement System 70
3-1-11. Pulse Measurement System 71
3-2. Sample preparation 71
3-2-1. Chemical Vapor Deposition (CVD) 71
3-2-2. Polymethyl Methacrylate (PMMA) Transfer 73
3-3. Device Fabrication of TMD 74
3-3-1. E-beam Lithography (EBL) Process 75
3-3-2. Development Process 75
3-3-3. E-gun Evaporator Process 76
3-3-4. Lift off Process 76
3-4. Measurements 76
3-4-1. Electrical Properties 76
3-4-2. Optoelectronic Properties 78
Chapter 4. Stacked 2D Device 80
4-1. Electrical Transport Properties of TMDs 80
4-1-1. MoS2 80
4-1-2. WS2 82
4-1-3. WSe2 84
4-1-4. MoSe2 85
4-1-5. Doping of TMD 86
4-2. Optoelectronic Properties of MoS2 89
4-2-1. Responsivity 89
4-2-2. Responsivity versus VD and VG 89
4-2-3. Responsivity versus the Wavelength 91
4-2-4. Responsivity versus the Intensity 92
4-3. Slow response of MoS2 93
4-3-1. Response Time (t) 93
4-3-2. PG and PPC effect 95
4-3-3. Reduced PG and PPC 96
4-3-4. Power Dependent Plot 99
4-3-5. Identification of PG effect 100
4-3-6. Reduced PPC effect. 101
4-3-7. Stress and Sense I-V Measurement 104
Chapter 5. Interface Engineering 105
5-1. Interface of Semiconductor 106
5-1-1. Semiconductor / Atmosphere 106
5-1-2. Semiconductor / Substrate 108
5-2. Contact 112
5-2-1. Vertical contacts 113
5-2-2. Lateral / Edge contact 115
5-2-3. Ttransferred Metals 118
5-3. Gate Tuned Interfacial States 119
5-3-1. Gate Voltage Control 119
5-3-2. Pulse Engineering 120
5-3-3. Pulse at Rising and Falling Edge 122
5-3-4. Mechanism Description 124
Chapter 6. Advanced Devices 127
6-1. Steep Subthreshold Slope for Low Power Device 127
6-1-1. Motivation 127
6-1-2. Steep s.s. Device 128
6-2. Near IR Photodetector 129
6-2-1. Motivation 129
6-2-2. Ideal Hetero-interface and Interlayer Transition of Vertically-stacked CVD-grown WS2/MoS2 131
6-2-3. Observation of NIR Response in Vertically-stacked CVD-grown WS2/MoS2 Junctions 132
6-3. Multi-level Sensing and Image Sensor 135
6-3-1. Motivation 135
6-3-2. Multi-level Sensor 135
6-3-3. Image sensor 136
6-4. Advanced Pulsed Photodetectors. 137
6-4-1. Pulsed Photodetectors with high responsivity and high speed. 137
6-4-1. Conclusion of Pulse Control 139
6-4-2. Heterostructures 140
Chapter 7. Conclusion and Future Prospect 144
7-1. Conclusion 144
7-2. Future Prospects. 145
Publication lists 146
References 148

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