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Khadka, SudikshaGrowth Techniques and Optoelectronic Study of 2D Semiconductor Based Devices
Doctor of Philosophy (PhD), Ohio University, 2018, Physics and Astronomy (Arts and Sciences)
Since Graphene was discovered in 2004, two-dimensional (2D) materials have established themselves as one of the most promising materials for next generation electronics and optoelectronics. Despite graphene’s exceptional electron mobility, its absence of a naturally occurring band gap restricts its applicability in optical and digital electronics. While on the other hand, MoS2 is an indirect semiconductor in its bulk form which transitions into a direct band gap material in the monolayer limit. It is reported to have a high current on/off ratio of 108, and room temperature mobility of few hundreds of cm2/Vs. Stable individual layers, without any dangling bonds and a breaking strength comparable to that of steel, support the expectation that this material will compliment graphene in future devices. However, scalable fabrication of 2D materials-based devices with consistent characteristics remains a significant impediment in the field. This dissertation establishes a simple, deterministic and scalable method of 2D material based device fabrication which is compatible with silicon processing. It is based on concurrent growth and formation of electrical contact between bulk metal features and the mono-to-few layer semiconducting channel grown in between. In addition to producing high-quality material, it opens a completely new field of hetero-structured devices with as grown electrical contacts. Detailed optical and electrical characteristics of MoS2 ¬based photodetectors grown using this method show high responsivity (~1 A/W) even at a low drain-source voltage (VDS) of 1.5 V and a maximum responsivity of up to 15 A/W when VDS = 4 V with an applied gate voltage of 8 V. The response time of these devices is found to be on the order of 1 µs, an order of magnitude faster than previous reports based on devices fabricated using conventional method. Preliminary results on calculation of Schottky barrier height (SBH) and related future works are presented.

Committee:

Eric Stinaff, Associate Professsor (Advisor); Martin Kordesch, Professor (Committee Member); Nancy Sandler, Professor (Committee Member); Wojciech Jadwisienczak, Associate Professor (Committee Member)

Subjects:

Condensed Matter Physics; Optics; Physics

Keywords:

2D Semiconductor; concurrent growth of 2D semiconductors based devices; naturally grown electrical contacts; MoS2 and WS2 based as-grown photo detectors

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.

Committee:

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

Subjects:

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

Keywords:

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