|dc.description.abstract||The crystallization of complex-oxide materials through a transformation from the amorphous to crystalline forms presents a range of new opportunities to synthesize new materials, and simultaneously poses important scientific challenges. New crystallization method complements more conventional vapor-phase epitaxy techniques for epitaxial complex-oxide thin film growth that involve long-range surface diffusion on 2D planar crystal surfaces. The vapor-phase techniques are not readily adaptable to creating nanoscale epitaxial complex-oxide crystals. The alternative synthesis method described in this thesis is solid-phase crystallization, which is the crystallization of amorphous oxides, often in the form of thin films, by post-deposition heating. The creation of epitaxial complex-oxide nanostructures can facilitate their integration in 3D electronic, optoelectronic and ionic devices.
Epitaxial complex-oxide crystals in intricate geometries can be created by solid-phase crystallization employing patterned substrates with a distribution of isolated crystalline seeds. This method requires the study of distinct crystal growth and nucleation kinetics on epitaxial and non-epitaxial surfaces. Nanoscale seeded crystallization can be achieved by understanding the relative rates of nucleation and lateral crystal growth processes, and the role of seeds in determining the overall orientation of the resulting crystals. Epitaxial complex-oxide thin films in intricate geometries with an expanded range of compositions can be created by combining the use of atomic layer deposition (ALD) and solid-phase crystallization, with the development of new ALD procedures to deposit amorphous oxide films and the study of the subsequent crystallization processes to select the crystalline structures of the crystallized film. ALD itself allows for the conformal deposition of thin films over non-planar surfaces.
Solid-phase crystallization can also be used to deposit epitaxial complex-oxide thin films with a wider range of compositions, including those that cannot be deposited from the vapor phase at high temperatures. Such oxides include the oxides that have complex compositions and volatile components. The different kinetic constraints of solid-phase crystallization allow the epitaxial growth of those oxide thin films because of the slow diffusion in the solid state at relatively low crystallization temperatures.
This thesis describes the discovery that, at low crystallization temperatures, epitaxial crystal growth of the model perovskite SrTiO3 on single-crystal SrTiO3 propagates over long distances without nucleation of SrTiO3 on Si with a native oxide. Two kinds of isolated nanoscale seed crystals are employed to study the seeded lateral crystallization of SrTiO3, yielding highly similar results. Micron-scale crystalline regions form surrounding the seeds before encountering separately nucleated crystals away from the seeds. Seed crystals play an important role in determining the orientations of the resulting crystals. New chemical precursors and ALD procedures were developed to grow amorphous PrAlO3 films. An epitaxial γ-Al2O3 layer formed at the interface between the PrAlO3 film and (001) SrTiO3 substrate during the deposition. Epitaxial PrAlO3 films were achieved on (001) γ-Al2O3/SrTiO3 by solid-phase epitaxy. The study of SrTiO3 and PrAlO3 is also applicable to a series of chemically and structurally similar functional ABO3 compounds.
The concepts of solid-phase crystallization also apply to oxides with multiple metal ions and more complex crystal structure. The kinetic processes occurring during the crystallization of ScAlMgO4, on (0001) sapphire substrates are quite different at two different temperatures. Epitaxial ScAlMgO4 crystals grow through the film thickness at a crystallization temperature of 950 ºC. Solid-state reaction and evaporation of the component Sc prohibits the formation of large ScAlMgO4 crystals at a crystallization temperature of 1400 ºC. Low-temperature crystallization can be used to create epitaxial oxide thin films with complex compositions and volatile components.||en_US