Browsing by Subject "2D Semiconductor"

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    Continuous and Reversible Modulation of Interfacial Electrochemical Phenomena on Back-Gated Two Dimensional Semiconductors
    (2017-08) Kim, Chang-Hyun
    The continuous increases in global population and energy consumption have raised major concerns about the security of our energy future. Limited amount of fossil fuels and increasing concerns over their combustion product, carbon dioxide, on climate change make renewable energy sources, such as sun light and wind, attractive options. Accordingly, energy conversion and storage devices based on (photo)electrochemical processes (e.g., batteries, fuel cells, water splitting, solar-to-fuel conversion, etc.) have received great attention as promising solutions to overcome the challenges in the intermittent renewable energy sources. To develop highly efficient and economically viable energy devices, fundamental understanding of electrochemical phenomena occurring at the electrode/electrolyte interfaces is essential. In this dissertation, we introduce a new approach, inspired by the operating mechanism of field-effect transistors (FETs), to modify and study such electrochemical interfaces. The devices studied in this dissertation project have a “gate-insulator-electrode” stack structure, which is essentially similar to that of a FET but the (typically) thick semiconductor layer in regular FETs is replaced with an ultrathin or two-dimensional (2D) active electrode for electrochemical processes in our devices. In such a device, due to the extreme thinness of the active electrode, electronic properties at the electrode surface can be dramatically altered by extra charge carriers induced with a voltage bias to the gate. This, in turn, makes thermodynamics and kinetics of electrochemical processes at the electrode/electrolyte interface be determined by the gate bias as well as by the electrode potential (with respect to the solution or reference electrode potential). In this project, three interfacial electrochemical phenomena are of our main interest: (1) electric double layer charging, (2) electron transfer across interface, and (3) surface binding of reaction species on electrode surface. First, responses of electric double layer structure to the gate bias is investigated using graphene devices, and a method to experimentally separate the band filling potential and the double layer charging potential has been developed. Second, continuous and reversible modulation of outer-sphere electron transfer kinetics by a gate bias was demonstrated on ZnO devices for the first time using cyclic voltammetry. Third, quantitative analysis of the electron transfer kinetics has been conducted using microchannel flow cells, in which continuous supply of fresh electrolyte through the microfluidic channel generates time-invariant diffusion layers near the active electrode surface, allowing electrochemical measurements in steady states. To collectively explain our observations, a simple but very useful physical model is proposed; the model indicates that the observed changes in the interfacial electrochemical phenomena essentially result from the gate-induced band alignment shift at the electrode/electrolyte interfaces. Lastly, based upon the results and the insight gained from the previous experiments, possibilities and challenges in field-effect control of surface binding energies in electrocatalytic systems are explored, and rational strategies to overcome the difficulties are proposed.
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    Exploration of Carrier Transport and Novel Devices in Emerging Semiconductors
    (2022-08) Golani, Prafful
    With scaling and performance of silicon-based transistors reaching their fundamental limits, a cross-disciplinary effort has gone into identification of novel material systems and device architectures that can outperform conventional solutions. Two systems that have shown good promise are van der Waals (vdW) semiconductors and semiconducting perovskite oxides. vdW semiconductors have already been used to demonstrate conventional MOSFETs and TFETs because of their atomically smooth surfaces and extremely thin body thicknesses which result in enhanced electrostatic gate control and improved scalability. On the other hand, semiconducting perovskite oxides have a large bandgap, low carrier effective masses and ability to form unique heterostructures making them interesting candidates for high-power high-frequency applications. The purpose of this thesis is to explore the electrical and material characterization results of electronic devices fabricated from Black Arsenic (vdW semiconductor) and SrSnO3 (perovskite oxide), by diving into their fundamental carrier transport studies. Exfoliated flakes of black arsenic (BAs) were used to fabricate MOSFETs which demonstrated ambipolar transport. The fabricated devices showed layer-dependent transport with high on/off ratios, high mobility and low off-current. Low temperature characterization revealed presence of low Schottky barrier height at the Ni/BAs interface while electron (hole) mobility vs temperature plot showed mobility was phonon limited. To show practical applications, ambipolarity of the devices was used to demonstrate an inverter and a frequency doubler as well. Ni/BAs interface was further explored, which revealed formation of an in-plane metallic contact to the semiconducting channel. Based upon this observation a self-aligned FET with lowered contact resistance is also proposed. Doped SrSnO3 had already been used to demonstrate MESFETs and RF FETs. However, SrSnO3 has low thermal conductivity which can result in degraded performance due to self-heating. An all-electrical method based on pulsed I-V characterization was performed to determine the thermal resistance and quantify the rise in channel temperature of two-terminal devices under electrical bias. TCAD simulations were performed to show that the rise in channel temperature was in close agreement with the experimental values. To further explore the carrier transport, electrical breakdown in undoped films was studied and contact optimization to doped SrSnO3 was also performed.