Effects of fundamental processing parameters on the structure and composition of two-dimensional MoS₂ films

Date of Award

2017

Degree Name

Ph.D. in Materials Engineering

Department

Department of Chemical and Materials Engineering

Advisor/Chair

Advisor: Christopher Muratore

Abstract

The unique properties resulting from strongly anisotropic chemical bonds found in the whole family of transition metal dichalcogenide materials (TMDs) have been researched for over 50 years for various applications, with MoS₂ being the most heavily researched. The pace of research has surged with the recent isolation and analysis of 2D materials exfoliated from Van der Waals solids, first reported by Novoselov and Geim. MoS₂ was first identified to have interesting electrical properties in 2D form by Mak et al. in 2010, where it was discovered that at a monolayer, the band structure had shifted to a direct bandgap semiconductor and the band gap shifts from ̃1.3 eV to ̃1.9 eV. Coupled with mechanical flexibility, optical transparency, and many other unique properties, 2D MoS₂ an ideal semiconductor candidate for nanoelectronic applications. The defect engineering that makes silicon-based technology so flexible has yet to be explored in 2D TMD materials. Grain boundaries are currently a structural defect that cannot be eliminated in wafer scale synthesis of these materials, therefore, a better understanding of how to control grain boundary density and understand the effects on properties is an urgent need. Each grain boundary acts as a scatter defect thought to adversely affect carrier mobility and density, negatively affecting device performance. This grain boundary problem is the driving force behind this study to discover the underlying basic principles and impact of sputter plasma deposition parameters, substrate processing conditions, and the nucleation and growth kinetics of 2D MoS₂ films. This dissertation consists of three studies designed to tailor grain boundary density in 2D MoS₂: 1) a study on the impact of intrinsic plasma parameters and characteristics of the pulsed DC magnetron sputtering discharge on the resulting 2D MoS₂ films and their structure/properties, 2) a study on the impact of the partial pressure of the diatomic (S₂) species in sulfur vapor, which dictates the high processing temperatures necessary for nucleation and growth kinetics of 2D MoS₂ and correlation of processing conditions to the chemistry and structure of the deposited films, and 3) a study of the impact of the MoO₃ precursor film grain size, thickness, structure, and morphology on the nucleation and growth kinetics of 2D MoS₂ on composition and structure properties. Novel approaches to reduce grain boundary density and synthesis temperature without compromising stoichiometry in waferscale 2D films were developed based on fundamentals of thin film nucleation and growth. For the first time in 2D MoS₂ literature, the identification of the S₂ species in the sulfur vapor mixture has definitively been determined to be the only reactant to produce 2D MoS₂. This revelation will allow for enhanced control of the 2D MoS₂ reaction kinetics allowing for the growth of larger grain and higher crystallinity 2D films. Another first in 2D MoS₂ literature, was the report of a new synthesis process where 1) room temperature sputtered MoO₃ precursor films are sputtered onto sapphire substrates, 2) annealed to where they react to create a thin Al₂(MoO₄)₃ interface layer, 3) the excess MoO₃ was allowed to sublime away, and 4) then sulfidized at 850°C in a S/Ar environment where the Al₂(MoO₄)₃ slowly decomposed releasing MoO₃ precursor for subsequent reaction with the S₂ vapor species. This process produced highly crystalline, large grain, approximately 1.5 layer thick 2D MoS₂ films.

Keywords

Grain boundaries Density, Molybdenum disulfide, Thin films, Materials Science, Nanoscience, Engineering, 2D MoS2, MoO3, aluminum molybdate, 2D nucleation and growth, sulfur vapor, S2, plasma diagnostics

Rights Statement

Copyright © 2017, author

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