Characterization Of Disorder And Its Effects On Two-Dimensional Transition Metal Dichalcogenides
Date of Award
Doctor of Philosophy
Electrical and Computer Engineering
Steven Durbin, Ph.D.
Damon Miller, Ph.D.
Joan Redwing, Ph.D.
Band gap engineering, chemical vapor deposition, disorder, semiconductors, transition metal dichalcogenides, two-dimensional materials
Since the discovery of the unique electrical properties of graphene in 2004, there has been an increased interest in two-dimensional (2D) materials. However, graphene is known as a zero-gap semi-metal, thus spurring further research in order to find suitable 2D semiconductor materials. Transition metal dichalcogenides (TMDs) are a class of layered materials that have recently attracted attention. In particular, MoS2, MoSe2, WS2, and WSe2 have direct band gaps in their monolayer form and favorable electrical properties, making them promising materials for both electronic and optoelectronic applications. Like many 2D materials, studies in semiconductor TMD materials began with mechanically exfoliated sheets and quickly expanded to more scalable methods such as metal-organic chemical vapor deposition (MOCVD) to produce high-quality devices. Through these studies, improvements in both the material growth process and device performance have been achieved.
Although 2D TMD materials have a direct band gap in their monolayer form, as layers are added, the band gap is observed to narrow and become indirect. This has formed the basis of many studies of band gap tuning techniques, such as heterostructures, strain engineering, and twistronics. However, adjusting the band gap by tuning the degree of disorder in the lattice of TMD materials has yet to be explored. Disorder, in this context, refers to an atomic lattice characterized by antisite defects. In this work, a lattice is viewed from the perspective of structural motifs as opposed to deviations from periodicity. The Bragg-Williams order parameter S is used to quantify the degree of ordering in the lattice, and several common material analysis techniques can be used to extract the order parameter. It has recently been experimentally established, using materials such as ZnSnN2, ZnO, InxGa1xN, silicon, and graphene, that tuning the degree of ordering in a film can reliably tune the band gap, and this can be directly related to the corresponding number and types of structural motifs present in a specific sample.
This work focuses on characterizing the effects that different process parameters have on the degree of disorder in monolayer TMD materials grown via MOCVD. The order parameter is determined using Raman spectroscopy and scanning electron microscopy, and the expected continuous linear relationship between the measured band gap and S2 is observed for these materials. An interpretation of why each crystal growth process parameter has an impact on the degree of disorder is given. Furthermore, other film attributes, such as the monolayer coverage, thickness of the sample, and domain properties, are explored in the context of both improved film quality and disorder.
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York, Krystal Rose, "Characterization Of Disorder And Its Effects On Two-Dimensional Transition Metal Dichalcogenides" (2022). Dissertations. 3830.