MoS₂/dielectric stack optimization for high performance metal-oxide-semiconductor field-effect transistor

Abstract

With a sizable bandgap and a high degree of electrostatic gate control, ultrathin two-dimensional (2D) molybdenum disulfide (MoS₂) have recently gained tremendous interest for flexible electronic and optoelectronic applications. However, MoS₂ based electronic devices suffer from several issues including lack of scalable methods to synthesize high-quality and large-area MoS₂ thin films, low electron mobility in MoS₂ transistors, and difficulty in integration with high-k dielectrics. This thesis is devoted to developing MoS₂/high-k top-gate transistors with high electron mobility through MoS₂/dielectric stack optimization. First, we investigate epitaxial growth of MoS₂ on lattice-matched GaN. High-quality, unstrained, and few-layer MoS₂ with strict registry to the GaN lattice is achieved. By combining atomic force microscopy (AFM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and first-principles simulation, we qualitatively determine the interlayer coupling and atomic registry in MoS₂/GaN stack. Second, we propose a HF passivation technique to improve the carrier mobility and interface quality of chemical vapor deposited (CVD) monolayer MoS₂ on the SiO₂/Si substrate. This passivation method leads to a more than doubled electron mobility, a reduced gate hysteresis gap, and a low interface trapped charge density, due to satisfied interface dangling bonds and reduced interface trap trapped charges. Finally, we investigate the integration of high-k dielectrics with 2D MoS₂ by atomic layer deposition (ALD). Using a low temperature ALD method, we obtain uniform and conformal ZrO₂ films on MoS₂, with the corresponding top-gate MOSFET exhibiting good device performances comparable to that of HfO₂/MoS₂ transistor. We further study the feasibility of direct ALD growth of high-k oxides on CVD MoS₂. Our investigation reveals that due to the overdeposition of molybdenum trioxide (MoO₃) on the surface of MoS₂, sub-10 nm high-k oxides can be easily deposited on CVD MoS₂. These results may provide important scientific insights for achieving high performance MoS₂ based nanoelectronic devices.

Date of Award	2018
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology