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MSE PhD Proposal – Philip Campbell
Friday, November 20 at 10 a.m.
Room 295 Love
Committee:
Dr. Eric Vogel, Co-Advisor, MSE
Dr. Jud Ready, Co-Advisor, GTRI
Dr. Seung Soon Jang, MSE
Dr. Matthew McDowell, MSE
Dr. P. Douglas Yoder, ECE
Title: Synthesis of Large-Area 2D Materials for Vertical Heterostructures
Transition-metal dichalcogenides have recently emerged as a class of two-dimensional materials relevant for use in electronic devices. Similar to graphene, these materials form layered crystals which lack out-of-plane bonding, allowing exfoliation to create atomically thin flakes of the materials on a substrate. Unlike graphene, however, TMDs have an intrinsic bandgap, making these materials more appealing for digital applications. Further, the electronic and optical properties of TMDs vary with the number of layers, with significant widening of the bandgap and a transition from an indirect to a direct bandgap in the monolayer limit.
Several synthesis methods for TMDs have been explored, ranging from chemical vapor deposition (CVD) to thin film alloying methods. CVD methods have been shown to produce large grain sizes, but it is difficult to create a large-area CVD growth process. Conversely, thin-film based methods result in wafer-scale coverage albeit with small grain sizes on the order of tens of nanometers. A common drawback to both methods is the high synthesis temperature required, ranging from roughly 550 – 1050 °C. Additionally, the mobility values measured for synthetic materials are typically lower than 10 cm2 V-1 s-1, which is significantly lower than the theoretical mobility limit for TMDs.
2D vertical heterostructures composed of TMDs have a number of interesting applications, including digital logic, analog communications systems, and optical applications. However, the quality of currently available synthetic materials is not sufficient to realize many of these applications. Further, the impact of defects and layer-to-layer interactions on the electronic behavior of heterostructures is not well understood. This work proposes the use of experimental and theoretical studies to characterize the atomic-scale defect structure in synthetic TMDs and relate this defect structure to the anticipated performance of TMD-based heterostructures. To produce high-quality MoS2 at low temperatures, plasma-enhanced CVD and thin film-based synthesis processes will be explored. These materials will be characterized using Raman spectroscopy and X-ray photoelectron spectroscopy. Electrical measurements will be used to link the physical defect structure of the material to the electronic properties. Finally, the synthetic materials will be used to fabricate vertical heterostructure devices, whose performance will be simulated and experimentally measured to understand the nature of layer-to-layer interaction and defects on device performance
