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Synthesis and cation stoichiometry effects of MBE-grown strained Ca₂Fe₂O₅ thin films
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Synthesis and cation stoichiometry effects of MBE-grown strained Ca₂Fe₂O₅ thin films

Yizhou Yang
Master of Science (M.S.), Drexel University
May 2018
DOI:
https://doi.org/10.17918/htj0-yj98
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Abstract

Perovskite Optoelectronic devices--Materials Thin films--Optical properties Materials Science
The physical properties of perovskite oxide (ABO₃) thin films can be tailored through cation deficiency, epitaxial strain, and anion substitution. Similar approaches should be applicable to A₂B₂O₅ brownmillerite materials that consist of alternating layers of BO₆ octahedra and BO₄ tetrahedra, however, such strategies have yet to be explored in detail in brownmillerite films. In this thesis, Ca₂Fe₂O₅ thin films are synthesized using molecular beam epitaxy, and anion substitution is explored through vapor transport fluorination. Cation off-stoichiometric effects, oxygen vacancy channel orientation, and fluorine substitution are studied through characterization methods such as X-ray diffraction, X-ray reflectivity, X-ray photoelectron spectroscopy, ellipsometry, and Rutherford backscattering spectrometry. The results suggest that Ca deficiency leads to minimal changes to the lattice parameter, while Fe deficient films exhibit c-axis expansion. Epitaxial strain is shown to control the orientation of the vacancies, which is consistent with previous reports. For fluorination, the mechanism is depended on the orientation of vacancies. When the oxygen vacancies are oriented parallel to the film/substrate interface, fluorination is dominated by fluorine substitution for oxygen, while the mechanism for perpendicularly oriented oxygen vacancies is a combination of both fluorine substitution and insertion into anion vacancy sites.

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