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Im, HoHyun, MS.,
Department of Chemical and Biological Engineering, Korea University
Advisor: Professor Bong, Ki Wan

Two-dimensional (2D) materials such as graphene, black phosphous and transition metal dichalcogenides (TMDCs) have been recieved tremendous attention due to their atomic-sacle thickness and unique electrical and optical properties. Among 2D materials, transition metal dichalcogenides (TMDCs) are considered as a promising semiconductor material that can overcome the physical limitation of sillicon-based semiconductors. Specifically, tungsten diselenide (WSe2) having theoretically good electrical properties, one of TMDCs, is being researched extensively.
However, accomplishing theoretical high performance is challenging due to the Fermi level pinning effect. One method of overcoming the Fermi level pinning effect is to dope the semiconductor material. Unfortunatly, conventional doping method such as diffusion and ion implantation is inapplicable because regulating the penetration depth and avoiding damage to the lattice caused by ion energy are difficult. Therefore, novel doping methods suitable for WSe2 are required.
In this thesis, an oxidation-based doping approach for WSe2 is presented. After the top few layers of WSe2 are oxidized, WOX (x<3) is created, which has a higher work function than WSe2. The underlying WSe2 exhibits p-doping due to the difference of work functions between WSe2 and WOX.
First, Pt/hBN/WSe2 metal-insulator-semiconductor capaitor with inverted configuration is fabricated to demonstrate the p-doping effect of oxidation-based doping method. The oxidation through UV/ozone treatment is self-limiting process because WOX (x<3) acts as a diffusion barrier. The hole carrier concentration increases as the oxidation time increases. And, It is saturated after fully oxidation (180 min).
Second, WSe2-based metal-insulator-semiconductor field-effect transistor (MISFET) with a novel dielectric material is fabricated using self-aligned gate technique. HSQ is patterned on the channel region of FET as a novel gate dielectric material. Self aligned-gate allows selective oxidation of only source/drain regions of the FET without an additional mask process. Depending on oxidation time, electrical characteristics of fabricated devices were measured. As the duration of the remote O2 plasma treatment increases, the performance of the devices improves.
By offering a novel approach to dielectric materials and device structure, these findings can contribute to the practical fabrication process and performance enhancement of electronic devices based on 2D materials.

Contents
Abstract	i
Contents	iii
List of figures	v
List of tables	ix
Chapter 1. Introduction	1
1.1 Motivation	1
1.2 Literature survey	3
1.2.1 Doping of tungsten diselenide (WSe2)	3
1.2.2 Top-gated WSe2 field-effect transistors	12
1.3 Outline	18
Chapter 2. Theoretical background	20
2.1 Two-dimensional (2D) materials	20
2.1.1 Transition metal dichalcogenides (TMDCs)	21
2.1.2 Tungsten diselenide (WSe2)	26
2.1.3 Insulator (hBN)	30
2.2 Semiconductor contact	31
2.2.1 Metal-semiconductor contact	32
2.2.2 Schottky and Ohmic contact	36
2.3 Semiconductor devices	37
2.3.1 Metal-insulator-semiconductor capacitor	37
2.3.2 Metal-insulator-semiconductor field-effect transistor	47
Chapter 3. Capacitance-voltage characteristics of Pt/hBN/WSe2 metal-insulator-semiconductor capacitor doped by charge-transfer process	51
3.1 Introduction	51
3.2 Experimental details	54
3.3 Results and discussion	57
3.4 Conclusion	77
Chapter 4. Novel fabrication method for WSe2-based field-effect transistors with HSQ as gate dielectric	78
4.1 Introduction	78
4.2 Experimental details	81
4.3 Results and discussion	84
4.4 Conclusion	98
Chapter 5. Conclusion	99
Chapter 6. Future work	101
References	102