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STRATEGIES TO REALIZE NEVER-BEFORE-SYNTHESIZED EPITAXIAL FILMS

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Abstract

Despite practical difficulty, epitaxial growth of phases with increasingly complex crystal structures has enabled advances in both condensed matter physics and in commercial applications. This progress has been enabled by creativity and systematic investigation to improve synthesis science. The hard work of my predecessors has exposed previously inaccessible material properties and brought materials previously reserved for university research to industrial production lines. In this thesis, I present my contributions to synthesis science by developing new strategies and techniques to realize first-of-their-kind oxide thin films using molecular-beam epitaxy.The first half of my thesis focuses on the synthesis science of Ruddlesden-Popper titanates with formula (ATiO3)nAO. I present my work to precisely control the interlayer distance in these layered Ruddlesden-Popper titanates (as well as other layered thin films) by developing an ex situ x-ray diffraction technique to quantitatively compute source fluxes and correct Ruddlsden-Popper synthesis recipes. The power of this technique is demonstrated by synthesis of a (ATiO3)20AO with 60% barium on the A-site. Subsequently, I elaborate on the novel in situ reflection high energy electron diffraction technique that I use to calibrate and guide growth of (ATiO3)20AO films and systematically study the influence of substrate temperature, epitaxial strain, and barium content on the crystal quality of the thin films. This thorough synthetic investigation provides a detailed roadmap for the future of research on these previously inaccessible phases. In the second half of my thesis, I focus on one of the greatest problems plaguing oxide electronics: the absence of a practical p-type conducting oxide. Through my work, I demonstrate the first epitaxial growth of a promising new p-type oxide candidate, Ta2SnO6. While I successfully synthesized the first ever epitaxial thin film of Ta2SnO6, I ultimately deduce that the generation of holes is compensated by the spontaneous formation of oxygen vacancies at MBE-compatible growth conditions. Although our doping attempts were unsuccessful, computations indicate that hole doping is possible within a range of synthetic conditions, encouraging further study by alternative techniques. I perform a thorough structural analysis of the first-ever epitaxial Ta2SnO6 films and enable the first measurement of the 2.4 eV bandgap of the candidate p-type oxide. This work on epitaxy of Ta2SnO6 provided the insight that enabled the first ever epitaxial synthesis of (110)-oriented SnO, an immense synthetic achievement as it is the first ever epitaxial synthesis of any 2D van der Waals crystal with the layers vertically aligned.

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163 pages

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Date Issued

2022-08

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Keywords

Complex Oxides; Dielectrics; Epitaxial Synthesis; Ferroelctrics; Oxide Electronics; p-type Oxides

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Committee Chair

Schlom, Darrell

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Fennie, Craig J.
Kourkoutis, Lena F.

Degree Discipline

Materials Science and Engineering

Degree Name

Ph. D., Materials Science and Engineering

Degree Level

Doctor of Philosophy

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Government Document

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dissertation or thesis

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