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Title:Electronic devices based on semiconducting transition metal dichalcogenides
Author(s):Zhao, Zijing
Advisor(s):Zhu, Wenjuan
Department / Program:Electrical & Computer Eng
Discipline:Electrical & Computer Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):TMDs, electronic devices
Abstract:Semiconducting transition metal dichalcogenides (TMDs) possess great potential as the channel material for next-generation electronic and optoelectronic devices due to their atomically thin body without surface dangling bonds and tunable bandgaps. This thesis investigates various transistors based on exfoliated and synthesized TMD materials. As a base for the results reported in this thesis, the fabrication processes are developed with key steps and parameters presented in detail. The characterization methods are established and elaborated, including the home-built photocurrent measurement setup and Labview programs. The electrical properties of CVD synthesized monolayer MoS2 and WSe2 are investigated. The mobilities are extracted from back gate transistors, where the contact resistance is excluded with 4-point measurement for MoS2, and the histogram of mobilities is summarized for WSe2. Three different device structures based on exfoliated few-layer MoTe2 are proposed and fabricated: back gate transistor, suspended transistor, and double gate transistor. The contrast-thickness correlation and Raman spectra of few-layer MoTe2 are obtained. Room-temperature ohmic contact and low electron barrier are found for Cr contacted MoTe2. Schottky barrier limited behavior is revealed through variable temperature measurement. The mobility of a 10 nm thick MoTe2 reaches 8 cm2/V·s. The bandgap of MoTe2 is extracted using optical and electrical methods with consistency. The intrinsic mobility of MoTe2 is revealed by suspended transistors. Room temperature mobility at 108 cm2/V·s is recorded, which is twice as large as the supported device due to reduced phonon scattering. The low-temperature mobility is found limited by charged impurities, which diverges significantly from supported devices due to the absence of dielectric screenings. In addition, Raman spectroscopy shows enhanced out-of-plane vibration in the suspended channel. Additional control of the doping density and vertical electric field is introduced in double gate transistors. Polarity switching and polarity pinning in double-gated devices are understood by separating the electrostatic effect on contact and channel. The intrinsic conductance under the vertical electric field is studied, and the photocurrent mechanism is presented.
Issue Date:2020-05-11
Rights Information:Copyright 2020 Zijing Zhao
Date Available in IDEALS:2020-08-26
Date Deposited:2020-05

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