DEPARTMENT OF PHYSICS DISSERTATION DEFENSE: Kevin Crust

Public zoom link:   https://stanford.zoom.us/j/95173484454?pwd=MEuoYqI8eaJLsb516MkaapsVhGWX…

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Title: 

Development and Characterization of Complex Oxide Dielectrics for Functional Heterostructures

Abstract: 

Complex oxide materials host a wide range of functional properties that are closely linked to their underlying structural phases. Precise control over the competition between these phases provides a direct pathway for tuning material functionality. Such control is increasingly important as modern microelectronics seek alternative materials platforms to improve efficiency and expand capabilities beyond the limits of conventional semiconductors. Epitaxial thin films offer powerful methods to engineer these properties: strain, reduced dimensionality, and lattice symmetry can alter the balance between nearly degenerate ground states while preserving crystalline quality, thereby stabilizing emergent phases not present in bulk crystals. Among complex oxides, ferroelectric and antiferroelectric materials are of particular interest due to switchable electrical polarizations and field-induced metastable states which can be utilized for next-generation computation and energy storage applications. In this talk, I will discuss our work on two perovskite oxides, NaNbO3 and BaTiO3, utilizing various synthesis control methods to probe fundamental phase competition while enhancing their functional performance. Freestanding NaNbO3 membranes are released and transferred from their growth substrates to isolate their intrinsic thickness-dependent scaling without the influence of epitaxial strain, providing experimental evidence for an antiferroelectric-to-ferroelectric transition governed solely by thickness. We then intentionally combine these size effects with epitaxial strain to engineer a ferroelectric – antiferroelectric phase coexistence, enabling both multistate switching and enhanced energy storage efficiency. Furthermore, breaking in-plane symmetry is used to induce anisotropic strain relaxation in BaTiO3 thin films grown on 110-oriented substrates to stabilize a non-bulk crystal structure that is promising for integration into electro-optic waveguide modulators. Together, this work demonstrates how intentional control of mechanical boundary conditions can unveil emergent ferroic phases for enhancements in functional devices.