Highly oriented thin films of cubic zirconia on sapphire through grain growth seeding
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A two-step process has been developed to form highly oriented thin films in material systems with dissimilar crystal structures and interatomic spacings. This processing method utilizes current polycrystalline thin film deposition techniques. In this method, a polycrystalline thin film is first deposited and heat treated to promote its breakup into isolated grains. The breakup process favors those grains that have a low substrate interfacial energy and so produces a film of highly oriented but isolated grains. In the second process step, another polycrystalline thin film is deposited. The remnant grains act as seeds for the growth of a highly oriented thin film. The process is demonstrated through the growth of highly (100) oriented thin films of cubic ZrO 2 (25 mol % Y 2 O 3 ) on (0001) A12O3 single crystal substrates, a material system in which film and substrate have dissimilar structures and interatomic spacings. Implications for the growth of epitaxial films using this method are discussed.
I. INTRODUCTION An epitaxial thin film has a single orientation that is determined by its substrate crystal structure. Such films are generally produced through the controlled condensation of a vapor1 (e.g., MBE, MOCVD, etc.) or solidification of a liquid2 (LPE) upon a single crystal substrate that has similar crystal structure and lattice parameters. In practice, growth on a substrate of dissimilar structure generally produces a polycrystalline thin film, presumably because of the lack of a suitable atomic pattern as the first few atomic layers are deposited. As will be shown, the need for a substrate of similar crystal structure and lattice parameters need not necessarily be a requirement for forming a single crystal thin film. In principle, an epitaxial thin film can be grown by exploiting the special orientational relations of low interfacial energy that may exist between dissimilar structures. In this paper, we describe a simple two-step process that utilizes this principle, and use it to produce highly oriented thin films of zirconia on sapphire, a system in which the film and substrate materials have dissimilar crystal structures. II. PROCESS DESCRIPTION Figure 1 schematically illustrates the two processing steps. In the first step, a polycrystalline thin film is deposited upon a single crystal substrate. This thin film is heat treated to promote the breakup of the thin film into single crystal islands. As will be shown, this process favors those grains that have a special low energy orientation with the substrate, and so produces a film of highly oriented but isolated grains. In the second step, the substrate containing the isolated, oriented grains is J. Mater. Res., Vol. 6, No. 11, Nov 1991 http://journals.cambridge.org
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recoated with a second film of the same material. The remnant grains now act as seeds for the growth of a continuous thin film of high orientation. Previous work suggests that the breakup of thin films may indeed be an effective tool for the formation of epitaxial seeds.
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