自從石墨烯被成功製備且驗證單原子層材料能夠穩定存在後，對各類二維材料的研究突飛猛進的發展。在這些材料之中，具有半導體特性的過渡性金屬硫屬化合物(Transition Metal Dichalcogenide, TMD)因其獨具的電學與光學性質而獲得了廣泛的注意。值得一提的是，以二硫化鉬(MoS2)製作之光感測電晶體已被驗證具有大於1安培/瓦的高光響應度，被視為下世代光電元件的關鍵材料。然而，其緩慢的響應速度限制了許多方面的應用。
在本篇論文中，我們藉由清大材料所李奕賢教授實驗室所提供之大面積、高品質二硫化鉬薄膜(以化學氣相沉積方式製備)，成功製作出具高響應度之單層二硫化鉬光偵測電晶體。以通道後製(channel last, CL)及通道先製(channel first, CF)技術製作之元件皆在本文中展示。由於其較低的接觸阻抗，以通道先製技術製作之光偵測電晶體具有較好的光電特性。與文獻中在環境下操作的二硫化鉬光偵測器相比，本實驗中展示之光偵測電晶體具有就我們所知最高的光響應度(2.7×〖10〗^3安培/瓦)及相對快速的響應速度(約幾秒)。通道長度調變效應也在本文中被討論，我們觀察到上升時間(Rise time)與光激發電子電洞對受電場分離至源/汲極的傳輸時間有很高的相依性，而下降時間(Fall time)則是和缺陷的複合機制較為相關。
為了消除元件中的持續性光電導(Persistent photoconductivity, PPC)效應，我們在動態量測的上升端以及下降端施加額外的閘極脈衝偏壓，這麼做能夠將元件的反應速度降至100毫秒以下，此為本實驗機台量測速度之極限。這樣的結果可適用於論文中所有的元件。我們提出了一個簡化的能帶模型來闡述閘極脈衝偏壓的機制，指出在閘極氧化層及二維材料介面的電洞陷阱(hole trap)是造成二硫化鉬光感測電晶體響應速度緩慢的重要原因。
本篇論文的觀察指出了改善二硫化鉬光偵測電晶體特性的大方向，為未來相關實驗奠定了有效的分析工具及方法。

Since graphene has been successfully synthesized and show decent stability in two-dimensional (2D) form, the research of various 2D materials are skyrocketing. Among the big family of 2D atomic layers, transition metal dichalcogenides (TMDs) with sizable bandgap have attracted tremendous attention due to its unique electrical and optical properties. Monolayer MoS2, in particular, has been revealed to possess high photoresponsivity larger than 1 A/W in phototransistor type, promising for next generation optoelectronic devices. However, the slow response speed obscures its applications.
In this thesis, high responsivity and fast response time monolayer MoS2 phototransistor array have been successfully fabricated thanks to the high quality, large area TMD films grown by chemical vapor deposition, provided by NTHU Prof. Yi-Hsien Lee’s group. Two different processing steps, channel last (CL) and channel first (CF) fabrication, are demonstrated in our study, presenting superior properties in CF devices on account of lower contact resistance. To the best of our knowledge, our CF device holds highest responsivity (2.7×〖10〗^3 A/W) and relatively fast response speed (~few second) under ambient air compared with other MoS2 phototransistors in literature. In addition, channel length modulation effects are also examined, suggesting that rising speed is dependent with the transportation time of photogenerated e-h pairs being separated and drift to Source/Drain contact, while the falling speed is dominated by the recombination through the trap centers.
To eliminate the persistent photoconductivity (PPC) effect, we applied gate voltage pulse at both rising and falling edge, further reducing the response speed to less than 100ms, limited by the temporal resolution of our measurement. The result shows good consistency in all of our devices. A simplified band bending model has been proposed to explain the gate voltage pulse mechanisms, indicating the photogating effect of hole traps at oxide/MoS2 interface are responsible to the slow response speed.
Our findings provide a general guideline to improve the performance of MoS2 phototransistor array.

摘要 I
Abstract III
Acknowledgement V
Contents VII
Table Lists X
Figure Caption XI
Chapter 1 Introduction 1
1.1 Background 1
1.2 Synthesis of Monolayer TMDs 4
1.2.1 Top-down exfoliation 4
1.2.2 Bottom-up synthesis 5
1.3 Application of TMDs 8
1.4 Motivation 9
Chapter 2 Mechanisms Description 11
2.1 MoS2 transistors 11
2.1.1 operation principles 11
2.1.2 Ultra-thin body (UTB) nature 12
2.1.3 Electrical contacts 14
2.1.4 Hysteresis 15
2.2 Photocurrent generation mechanisms 17
2.2.1 Photoconductive (PC) effect 18
2.2.2 Photovoltaic (PV) effect 19
2.2.3 Photogating effect 20
2.2.4 Photo-thermoelectric (PTE) effect 21
2.2.5 Photo-bolometric (PB) effect 22
Chapter 3 Experimental Apparatus and Device Fabrication 23
3.1 Manufacturing and measurement Instrument description 23
3.1.1 Wet bench 23
3.1.2 Vertical Furnace 25
3.1.3 Plasma Enhanced Atomic Layer Deposition (PE-ALD) 26
3.1.4 Chemical Vapor Deposition of MoS2 monolayer 27
3.1.5 Electron-Beam Lithography 28
3.1.6 Dry etching machine Lam2300 30
3.1.7 Annealing system 31
3.1.8 Electron-Beam Evaporator 32
3.1.9 Sputtering Deposition 33
3.1.10 Measurement system 34
3.2 Material analysis 35
3.2.1 Raman spectroscopy 35
3.2.2 Photoluminescence (PL) spectroscopy 36
3.2.3 In-line Scanning Electron Microscope 37
3.3 Fabrication of monolayer MoS2 phototransistors 38
3.3.1 Channel last device fabrication 39
3.3.2 Channel first device fabrication 41
Chapter 4 Result and Discussion 43
4.1 Monolayer MoS2 grown by CVD process 43
4.2 Annealing after transfer process 44
4.3 Channel last (CL) and channel first (CF) device comparison 45
4.3.1 Transfer and output characteristics 46
4.3.2 Responsivity and response time 53
4.3.3 Contact resistance and Schottky barrier height (SBH) 60
4.3.4 Summary 66
4.4 Gate voltage pulses accelerated response speed 68
4.4.1 Gate voltage pulse at falling edge 68
4.4.2 Gate voltage pulse at rising edge 69
4.4.3 Mechanism description 71
Chapter 5 Conclusion and Future Prospect 75
5.1 Conclusion 75
5.2 Future prospect 76
Reference 78
Publications 88

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