Microstructure of Perovskite Oxides Thin Films Grown on Miscut/Small Lattice-Mismatched Substrates.

Abstract

Perovskite oxides exhibit a wide range of physical properties, such as high dielectric, piezoelectric, pyroelectric, ferroelectric/multiferroic, non-linear optical and high temperature superconducting properties. The unique combination of these properties makes possible the use of perovskite oxide thin films as core elements in the next generation devices for sensors and electric/electronic/optical circuits. Recent studies demonstrate that it is possible to tailor physical properties through controlling growth parameters during thin film deposition, for example, by choosing suitable substrates in certain oxide systems. This thesis is focused on the microstructural characterization of domain and strain engineered perovskite oxide thin films by transmission electron microscopy (TEM) and elucidation of the structure-property relationships in these thin films. The model systems studied in this thesis are ferroelectric domain engineered BiFeO3 by miscut SrTiO3 substrates, strain engineered BaTiO3 by small-lattice-mismatched substrates (GdScO3 and DyScO3) and crystallographic domain engineered PrScO3 via miscut SrTiO3 substrates. In detail, TEM results prove that the ferroelectric domain structures in multiferroic BiFeO3 thin films can be engineered by means of miscut SrTiO3 substrates. BiFeO3 thin films grown on low angle (<1°) miscut SrTiO3 substrates comprise 71° and 109° ferroelectric domains, while BiFeO3 thin films grown on 4° miscut SrTiO3 substrates comprise only 71° ferroelectric lamellae. Fully strained and partially strain-relaxed BaTiO3 thin films with coherent SrRuO3 bottom electrode can be grown on rare-earth scandate substrates (GdScO3 and DyScO3). Results corroborate that strain relaxation of the BaTiO3/SrRuO3 bilayer system is mainly determined by the lattice mismatch between each layer and the substrate, the thickness of each layer, and kinetics. Crystallographic domain structures in PrScO3 thin films can be engineered via miscut SrTiO3 substrates. PrScO3 thin films grown on high angle (≥1°) miscut SrTiO3 substrates comprise one type of crystallographic domain. In contrast, there are six types of crystallographic domains in PrScO3 thin films grown on low angle (<1°) miscut SrTiO3 substrates. These studies demonstrate that microstructure of perovskite oxide thin films can be engineered by means of miscut or small lattice-mismatched substrates. It, in turn, dramatically improves the physical properties of engineered perovskite oxide thin films.