Study on modulation of electrical properties in two-dimensional semiconductors

다국어 초록 (Multilingual Abstract)

In this dissertation, high-performance semiconductor devices based on two-dimensional materials are fabricated by various functional modulation process. Edge contact structure of graphene, air-stable ambipolar BP transistor, Ohmic contact in MoS2, WSe2, MoTe2, and BP (black phosphorous) FETs and plasma induced doping, layer-by-layer removal of WSe2 are studied in order to reveal the high-performance nature of 2D materials.
For manufacturing high-performance 2D semiconductors, contact resistance impedes high-performance 2D semiconductors. The formation of “edge-contacted” graphene through the use of a controlled plasma processing technique generates a bond between the graphene edge and the contact metal, which controls the edge structure of the bond and significantly reduces the contact resistance. The contact resistance attained by using pre-plasma processing in “edge-contacted” graphene FETs was of 270 Ω·μm, which is a decrease of 77%. We also demonstrated the fabrication of solution-processed BV polymeric contacts for the preparation of high mobility MoS2, WSe2, MoTe2, and BP (black phosphorous) FETs with significantly lowered contact resistance. Ohmic contacts were achieved and produced 3-, 700-, 3000-, and 17-fold increases in electron mobilities. Carrier transport mechanism at metal−2D interface are investigated.
For manufacturing high-performance 2D logical circuits, n- and p-type performance of 2D materials are necessary. Considering the n- and p-type nature of 2D materials, doping method were studied by chemicals and plasma. Surface charge transfer doping techniques, n-type BV doping, are introduced to 2D MoS2, WSe2, MoTe2, and BP (black phosphorous) FETs. By BV doping and BV interlayer pre-doping, ambipolar and p-type 2D materials based FETs could, therefore, be transformed into n-type FETs. Most importantly, our devices exhibit excellent stability in both ambient and vacuum. High performance p-type WSe2 field-effect transistors were achieved transferred from ambipolar behavior by O2 plasma treatment.
High electronic performance of 2D transistors is strongly dependent on its thickness, down to monolayer FETs. We report layer-by-layer thinning of WSe2 via chemical KOH solution, by removing the surface top layer WOX formed by O2 plasma treatment. Monolayer WSe2 flakes were obtained after several consecutive etching, which indicates the layer-by-layer etching technology is reliable and stable. Finally, enhanced ambipolar performance of WSe2 FETs with 10- and 35-fold increases in electron and hole mobilities, respectively, was achieved from etched WSe2 field-effect transistors.

목차 (Table of Contents)

• Chapter 1. Introduction 1
• 1.1 Two-dimensional materials 1
• 1.2 Limitation in 2D semiconductors: contact resistance 3
• 1.3 2D Semiconductor applications 5
• 1.4 Scope of dissertation 7
• Chapter 2. Reducetion of contact resistance for high-performance 2D FETs 9
• 2.1 Introduction to contact resistance 9
• 2.2 Experimental methods to reduce contact resistance 14
• 2.2.1 Conventional surface contact 15
• 2.2.2 Edge contact of graphene 15
• 2.3 Results and Discussion 18
• 2.3.1 Measurements of Rc from graphene with a controlled edge 18
• 2.3.2 Mechanisms of pre-plasma process leading to low Rc. 25
• 2.3.3 Carrier transport at surface and edge contacts 32
• 2.3.4 Enhancement of carrier transport and mobility at the M-G contact junction 36
• 2.3.5 Ohmic contact established in MoS2, WSe2, MoTe2 and BP transistors 39
• 2.4 Conclusion 50
• Chapter 3. Carrier type modulation in 2D FETs using polymeric materials 52
• 3.1 Introduction 52
• 3.2 Polymer contact process of semiconductors 59
• 3.3 BV doping, h-BN capping and BV interlayer structure 59
• 3.4 Results and discussion 67
• 3.4.1 Mechanism of carrier type modulation 67
• 3.4.2 Air-stable BV doped 2D FETs 72
• 3.4.3 Effect of temperature on ambipolar properties of BP devices 76
• 3.4.4 Homogeneous BP invertor 78
• 3.4.5 The application of ambipolar BP FETs to a photodetector 80
• 3.4.6 BP thickness dependence of BV-doped devices 85
• 3.5 Conclusion 86
• Chapter 4. Layer-by-layer etching of two-dimensional WSe2 88
• 4.1 Introduction 88
• 4.2 Layer-by-layer etching process 92
• 4.3 Results and discussion 95
• 4.4 Conclusion 101
• Chapter 5. Summary 103
• 5.1 Summary 103
• 5.2 Suggested future work 105