二維半導體是應用於下一代奈米電子元件的有潛力的材料。二硫化鉬 (MoS2) 以其良好的遷移率、高開/關比和低功耗而受到關注。然而，由於金屬電極和半導體之間的化學無序紊亂和缺陷而導致金屬-半導體界面處的大量接觸電阻 (Rc)，仍然是 MoS2效電晶體 (Field Effect Transistor, FET) 的重大挑戰。此外，傳統的背柵結構使 MoS2 通道受到環境條件的影響，其中水分子和氧氣的吸附增加了滯後現象和界面陷阱密度，這會使元件降低其性能。
在我們的第一項研究中，我們探討了脈衝雷射退火 (Pulsed laser annealing, PLA) 在降低 MoS2 場效電晶體 (FET) 內接觸電阻的有效性。我們採用高波長 (1064nm) 脈衝激光雷射，對各種金屬電極（包括 Ag/Au、Ni/Au 和 Cr/Au）的 MoS2 FET 元件進行退火。結果表明，PLA 後 FET 性能顯著提高。具體而言，以Ag 接觸 MoS2 FET 在 Vd = 1 V 時效應遷移率從 60 cm2/V·s 增加到 135 cm2/V·s，導通電流從 40.5 µA 提高到 96.1 µA，同時接觸電阻顯著降低至 0.29 kΩ µm。 PLA MoS2 元件表現出 107 的高開/關比。分析測試闡明了增強性能背後的化學和物理機制，證明了 PLA 在促進半導體界面金屬擴散方面的有效性。
在我們的第二項研究中，我們使用鹼金屬氟化物作為介電覆蓋層，包括氟化鋰(LiF)、氟化鈉(NaF) 和氟化鉀(KF) 來作爲介電覆蓋層，以減輕氧氣和水暴露對環境的影響。其中，LiF電介質覆蓋層顯著提高了電晶體性能，特別是在汲極電壓為1V 時，場效應遷移率從74 cm2/V·s提高到137 cm2/V·s，電流密度從17 µA/µm提高到32.13 µA/µm，並且次臨界擺幅減少0.8 V/dec。總結來說，我們證實使用LiF做為介電覆蓋層在本質上能改善遷移率、電流密度以及次臨界擺幅的問題，以增強電晶體的性能表現。
關鍵詞：過渡金屬二硫屬化物；二硫化鉬；場效應電晶體；脈衝鐳射；接觸電阻；鹼金屬氟化物封端；摻雜

Two-dimensional semiconductors are promising materials for next-generation electronic devices. Molybdenum disulfide (MoS2) is notable for its good mobility, high on/off ratio, and low power consumption. Nevertheless, the substantial contact resistance (Rc) at the metal-semiconductor interface, resulting from chemical disorders and defects between the metal electrodes and the semiconductors, remains a significant challenge for MoS2-based field-effect transistors (FETs). Moreover, the conventional back-gated architecture subjects the MoS2 channel to ambient conditions, where the adsorption of water molecules and oxygen increases hysteresis and interface trap density, thereby negatively impacting device performance.
In our first study, we explore the effectiveness of pulsed laser annealing (PLA) in reducing contact resistance within MoS2 FETs. Employing a high-wavelength (1064nm) pulsed laser, we anneal MoS2 FETs with various metal electrodes, including Ag/Au, Ni/Au, and Cr/Au. Our results demonstrate a remarkable enhancement in FET performance metrics following PLA. Specifically, Ag-contacted MoS2 FETs exhibit a peak field-effect mobility increase from 60 cm2V-1s-1 to 135 cm2V-1s-1 and an on-current improvement from 40.5 µA to 96.1 µA at Vd = 1 V, accompanied by a significant reduction in contact resistance to 0.29 kΩ µm. PLA MoS2 devices showed the high on/off ratio of 10^7. Analytical tests elucidate the chemical and physical mechanisms underlying the enhanced performance, highlighting the efficacy of PLA in promoting metal diffusion at the semiconductor interface.
In our second study we have used alkali metal fluorides as dielectric capping layers, including lithium fluoride (LiF), sodium fluoride (NaF), and potassium fluoride (KF) dielectric capping layers to mitigate the environmental impact of oxygen and water exposure. Among them, the LiF dielectric capping layer significantly improved the transistor performance, specifically in terms of enhanced field effect mobility from 74 to 137 cm2/V·s, increased current density from 17 µA/µm to 32.13 µA/µm at a drain voltage of Vd of 1 V, and decreased subthreshold swing to 0.8 V/dec. In conclusion, the use of LiF as a dielectric capping layer has proven to be the most effective in enhancing transistor performance, demonstrating substantial improvements in mobility, current density, and subthreshold swing.
Keywords: transition metal dichalcogenide, molybdenum disulfide, field-effect transistor, pulse laser annealing, contact resistance, alkali fluoride capping, doping.
 

Table of Contents


摘要 i
Abstract ii
Table of Contents iv
List of Figures vi
List of Tables xiv
Acknowledgements xv
Chapter 1 Introduction and Literature Review 1
1.1 Introduction 2
1.2 Emergence of 2D Semiconductors 6
1.2.1 2D Field Effect Transistors (2D FET) 7
1.3 Understanding Device Operation 8
1.3.1 I-V Curve 9
1.3.2 Main Indicators for FET 10
1.3.3Contact Geometry 12
1.4 Metal-Semiconductor Junction: Critical Parameters 14
1.4.1 Schottky Barrier Contact and Schottky Barrier Height 14
1.4.2 Fermi Level Pinning 15
Chapter 2 Experimental Methods, Characterizations Tools and their Principles 20
2.1 Synthesis of MoS₂ via Chemical Vapor Deposition (CVD) 21
2.1.1 Growth of MoS2 Triangular Monolayer 21
2.1.2 Growth of 2inch wafer MoS2 Monolayer 23
2.2 Transfer Method of MoS2 onto p+Si/SiO2 Substrate 23
2.2.1 MoS2 Traiangle Flake 23
2.2.2 MoS2 Thin Film 25
2.3 Fabrication of MoS2 Field Effect Transistors 26
2 .4 Structural and morphology characterization 27
2.4.1 Raman Spectroscopy 27
2.4.2 X-Ray Photelectron Spectroscopy (XPS) 28
2.4.3 X-ray Diffraction (XRD) 29
2.4.4 UV-Visible Spectroscopy 29
2.4.5 Scanning Electron Microscopy (SEM) 30
2.4.6 Transmission Electron Microscopy (HR-TEM) 31
2.4.7 Atomic Force Microscopy (AFM) 32
Chapter 3 Advanced Contact Engineering in MoS2 FET: Long Wavelength Pulsed
Laser Annealing for Low Contact Resistance at Metal-Semiconductor Interface… 33
3.1 Introduction 34
3.2 Results and Discusssion 37
3.3 Conclusion 65
Chapter 4 Enhanced Electrical Transport Properties of MoS2 FET by Using
Alkali Metal Fluorides as Dielectric Capping Layers 66
4.1 Introduction 67
4.2 Result and Discusssion 69
4.3 Conclusion 98
Chapter 5 Future Perspective 99
Bibliography 102
Appendix 116

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