Nanoelectronics based on epitaxial oxides
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Crystalline oxide materials and heterostructures have been under extensive investigation owing to the richness of the physical, chemical, and electrical properties they exhibit, including ferromagnetism, ferroelectricity, ferrotoroidicity, superconductivity, metal-insulator transition, multiferroics, and 2-dimensional electron liquids. In recent years, the advancement of thin film growth techniques such as molecular beam epitaxy and atomic layer deposition has made possible monolithic integration of these crystalline oxide materials with mainstream semiconductor substrate materials such as Si and Ge, which opens new avenues for improving existing device performance and provides many opportunities for adding various solid-state device functionalities to electronic devices that are unachievable with conventional semiconductor materials. Epitaxial oxide heterostructures with a perovskite crystal structure are emerging as outstanding candidates for realization of devices in which diverse material properties - ferromagnetism, piezoelectricity, ferroelectricity, and others - are flexibly coupled to achieve new functionality. In the first part of this dissertation, the strain-dependent ferromagnetism in LaCoO3, piezoelectric response in SrTiO3, and their strain coupling in a single-crystal oxide heterostructure grown on Si (001) are employed to enable a novel approach to modulating ferromagnetism and magnetoresistance by application of a gate voltage in a suitably fabricated device. The second part of the dissertation addresses the resistive switching behavior and physics of epitaxial single-crystal anatase TiO2 on silicon and demonstrates several unique advantages of using single-crystal metal oxide films as an active switching layer, including a high ON/OFF ratio, a great potential for device scaling, highly linear current-voltage characteristics, and room-temperature, reproducible quantization of conductance, etc. Finally, epitaxial SrHfO3-based gate stacks for Ge metal-oxide-semiconductor devices are investigated as an approach to alleviate the gate dielectric interface quality problem that has tremendously hampered the adoption of next-generation Ge-based transistors. Different methods are shown to effectively decrease the interface trap density, and the gate stacks developed in this dissertation represent the state of the art in terms of the combination of equivalent oxide thickness and gate leakage. In summary, this dissertation presents several results in the design and modeling, process integration, characterization, and analysis of device prototypes for functional and nano- electronics applications using epitaxial oxide films. These results provide a foundation for further exploration of solid-state device applications using epitaxial crystalline oxide materials.