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Hagerty, PhillipPhysical Vapor Deposition of Materials for Flexible Two Dimensional Electronic Devices
Master of Science (M.S.), University of Dayton, 2016, Chemical Engineering
Molybdenum Disulfide (MoS2) and Tungsten Disulfide (WS2) are two materials in a larger class of materials known as Transition Metal Dichalcogenides (TMDs) that have begun emerge as semiconducting materials. When their horizontal length scale is reduced from bulk to monolayer they demonstrate surprising combinations of properties including a direct electronic band gap and mechanical flexibility. Two dimensional (2D) materials have the potential to revolutionize performance and tailorability of electro-optical devices fabricated entirely from molecularly thin materials. In a departure from traditional exfoliation or high temperature chemical vapor deposition approaches for 2D materials synthesis, novel plasma-based physical vapor (PVD) techniques were used to fabricate uniform films over large areas. This experimental approach allowed unique studies. For example, vapor phase growth allowed systematically variation of the sulfur vacancy concentration in MoS2 and WS2 and subsequent correlation to electronic properties. This effort leads to controlled bottom-up assembly of 2D devices on flexible and standard substrates to experimentally couple the remarkable intrinsic mechanical and electronic properties of ultrathin materials, which are particularly appealing for molecular sensing. The pursuit of an all physical vapor deposited field effect transistor (FET) is the main priority for the 2D materials community as definitive demonstration of the feasibility of physical vapor deposition as a scalable technique for consumer electronics. PVD sputtered Titanium Nitride (TiN) and Tungsten (W) were experimentally characterized as potential back gated materials, Plasma Vapor Deposited (PLD) a-BN was electrically characterized as a uniform ultra-thin low temperature dielectric, and sputtered MoS2 and WS2 were electrically characterized as a semiconductor material. Tungsten deposition methods were previously researched and mimicked for smooth and conductive back gate material depositions. TiN was parameterized and the best room temperature deposition conditions were 70V applied to the sputtering gun with 25 sccm gas flow of 90% N2 and 10% Ar for 60 minutes. The best high temperature depositions were done at 500oC, 70V applied to the sputtering gun with 25 sccm gas flow of 90% N2 and 10% Ar for 30 minutes. Dielectric a-BN electrical characterization began to occur after 6nm which equated to 100 pulses, while 200 pulses equated to 16.5nm thickness. A dielectric constant of 5.90 ± .65 is reported for a-BN for under 20nm thickness. Soft probing techniques by conductively pasted gold wires on the probe tips were required to obtain true electrical measurements of 2D materials in a stacked structure, otherwise scratching would occur and uniformity would cease to exist in the film. Chemical Vapor Deposition (CVD) and mechanical exfoliation have provided the only working TMD semiconductor 2D materials in MOSFET structure to date with lithographic electrical connections. PVD sputtering as a new synthesis method for crystalline TMD with a stoichiometric ratio is achievable over large areas. Though, reduced area depositions are required for doped Silicon and Silicon Oxide (SiO2) based FET structures to limit the chance of encountering a pinhole. With reduced area and stoichiometric enhancement control, sputtered TMD films exhibit high sensitivity to oxygen and are electrically conductive even when exposed to a field effect. Increasing the grain size of the sputtered materials is the next driving force towards a fully recognizable TMD thin film transistor.


Christopher Muratore, PhD (Committee Chair); Terrence Murray, PhD (Committee Member); Kevin Myers, DSc (Committee Member)


Aerospace Engineering; Aerospace Materials; Electrical Engineering; Engineering; Materials Science


PVD; materials; 2D materials; Nanoelectronics; TMDs; 2D Transistors; Molybdenum Disulfide; MoS2; WS2; Tungsten Disulfide