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研究生:劉仕卿
研究生(外文):Shih-Ching Liou
論文名稱:懸浮多層二硫化鉬元件製作與傳輸特性
論文名稱(外文):Fabrication and transport properties of suspended few-layer MoS2 devices
指導教授:梁啟德
指導教授(外文):Chi-Te Liang
口試委員:包淳偉
口試委員(外文):Chun-Wei Pao
口試日期:2015-07-27
學位類別:碩士
校院名稱:國立臺灣大學
系所名稱:物理研究所
學門:自然科學學門
學類:物理學類
論文種類:學術論文
論文出版年:2015
畢業學年度:103
語文別:英文
論文頁數:69
中文關鍵詞:二硫化鉬懸浮結構場效電晶體電子遷移率次臨界擺幅
外文關鍵詞:MoS2suspendedfield-effect transistormobilitysubthreshold swing
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二維材料領域最近轉向研究具有半導體性的過渡金屬硫族化物(TMD)。因其具有高電流開關比(on/off ratio),高電子遷移率(mobility)以及直接能隙(intrinsic bandgap)等特性,使此種材料對未來光電科技應用上的重要性逐漸增加。在本篇論文中,我們探討懸浮結構的過渡金屬硫族化物—二硫化鉬(MoS2)的傳輸特性。之前的研究結果顯示,以二硫化鉬為材料所製成的場效電晶體(field-effect transistor)其電子遷移率遠小於理論預期且其具有表面缺陷(trap state)。造成這些結果部分原因可能是基板的效應包含了游離帶電雜質(charged impurity)與基板表面懸浮鍵(dangling bond)。在此,為了排除基板效應,我們製作懸浮多層二硫化鉬以探討其本質的傳輸特性。我們提供了兩種不需要化學蝕刻步驟且不會有化學殘留製做懸浮元件的方法。第一種為剝離法(exfoliating method),用機械剝離法將二硫化鉬貼在事先蝕刻好溝槽的基版上。第二種為轉映法(transfer method),用聚二甲基矽氧烷(PDMS)軟片將二硫化鉬轉映在事先定義好的電極上。結果顯示,懸浮的元件會比有基板支撐的元件具有較高的電子遷移率,電導和較低的次臨界擺幅(subthreshold swing)。這些結果也顯示了基板對二硫化鉬傳輸特性有重要的影響。更重要的是,我們提供了一個快速製造懸浮結構的方法可以用來探討二硫化鉬本質的熱膨脹,機械和光電等特性。

Recently, there is great interest in semiconducting transition-metal dichalcogenide (TMD), which is one category of two-dimensional (2D) materials. Due to high on/off current ratio, high mobility, and intrinsic bandgap, TMD-based materials are essential for future electronic and optoelectronic applications. In this thesis, we study the transport properties of suspended molybdenum disulfide (MoS2), which is one of the semiconducting TMD materials. Previous studies of the transport properties in MoS2-based field-effect transistors (FETs) showed that the mobility is significantly lower than theoretical prediction, and the existence of trapping states. Part of the undesirable effects can be attributed to the substrates including charged impurities and dangling bonds on substrate surface. To exclude these substrate effects for approaching intrinsic properties of MoS2, we fabricated suspended few-layer MoS2 devices and studied the transport properties. We employed two different resist-free methods to fabricate suspended MoS2 devices, which require no chemical etching process. First method was to exfoliate MoS2 flakes onto SiO2 substrates with pre-defined trenches and followed by electrode deposition. Second method was to transfer MoS2 flakes by polydimethylsiloxane (PDMS) films onto the pre-defined electrodes. The suspended MoS2 devices exhibit higher mobility, higher conductance, and lower subthreshold swing, as compared to the controlled MoS2 devices which are SiO2-supported. These results suggest that substrate effect play a critical role on carrier transport of MoS2-based FETs. Moreover, we have demonstrated a feasible method to fabricate suspended MoS2 devices for the investigation of its intrinsic thermal, mechanical, optical, and optoelectronic properties.

致謝 iii
摘要 iv
ABSTRACT i
CONTENTS ii
LIST OF FIGURES iv
Chapter 1 Introduction 1
1.1 Transition metal dichalcogenides 1
1.2 Molybdenum disulphide 2
1.3 Paper review 3
1.4 Motivation 7
1.5 Thesis structure 8
Chapter 2 Field-effect transistor characterization 10
2.1 Mobility 10
2.1.1 Field-effect mobility 11
2.1.2 Phonon scattering in single-layer MoS2 12
2.1.3 Charged impurity scattering in single-layer MoS2 13
2.2 Thickness-dependent transport properties 17
2.3 On/off ratios and subthreshold swing 20
2.4 Contact resistance 21
2.4.1 Schottky contact and ohmic contact 21
2.4.2 Ohmic contact in MoS2 FETs 22
2.5 Gas adsorbates effect on MoS2 FETs 24
Chapter 3 Device fabrication and experiment apparatus 26
3.1 Exfoliating MoS2 flakes on pre-etched trenches 26
3.1.1 Define trenches by reactive-ion etching 26
3.1.2 Exfoliating MoS2 on pre-trenched SiO2 substrates 27
3.1.3 Resist-free electrodes deposition 29
3.2 Transfer MoS2 samples onto pre-defined electrodes 30
3.2.1 Fabrication of electrodes by electron-beam lithography 30
3.2.2 Transfer MoS2 samples onto pre-defined electrodes 31
3.3 Experiment apparatus 34
3.3.1 Scanning electron microscope (SEM) 34
3.3.2 Atomic force microscope (AFM) 35
3.3.3 Electrical measurement systems and methods 36
Chapter 4 Results and discussions 38
4.1 Thermal annealing 38
4.2 Results of suspended few-layer MoS2 FETs 39
4.2.1 Device S1 39
4.2.2 Device S2 44
4.2.3 Device S3 49
4.2.4 Device S4 52
4.3 Comparison of suspended devices with SiO2-supported devices 54
4.4 Discussions of transport properties of suspended MoS2 FETs 57
Chapter 5 Conclusions and future work 63
Reference 65



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