Control of Defects and Subgrains in Epitaxial Perovskite Thin Films

Abstract: This work concerns the microstructure of epitaxial (001) perovskite thin films, studied by transmission electron microscopy (TEM), in particular high resolution TEM, electron energy-loss spectroscopy (EELS), and energy dispersive x-ray analysis. The perovskites exhibit interesting superconducting, dielectric, and magnetic properties. Crucial issues are the effect of substrate surface structure, interfacial lattice mismatch strain, and film processing parameters on thin film evolution and the effect of fine-scale microstructure on the local physical properties.

Dielectric SrTiO₃ films, rf magnetron sputtered on LaAlO₃ substrates, have been characterised. The films exhibited a columnar subgrain morphology, which was induced by the lattice mismatch strain. The microstructure was improved by use of a strain relieving YBa₂Cu₃O_7-x buffer layer, with an 80% reduction of the subgrain boundary area, 80% decrease of the density of misfit dislocations, and a 30% increase of the dielectric tunability.

EELS was used to measure the local dielectric properties of the sputtered SrTiO₃ films. Improved stoichiometry and absence of strain caused the absolute value of the imaginary part of the high-frequency dielectric function to approach the single crystal bulk value, with improving low-frequency dielectric constant.

YBa₂Cu₃O_7-x/SrTiO₃ multilayers on LaAlO₃ substrates have been studied. Multilayers provide the potential of changing the internal lattice mismatch strain and affecting the lattice parameters and properties of the multilayer constituents. Special attention was paid to the effect of changes in the individual layer thicknesses, on the strain situation within the multilayers.

High temperature superconductors are closely related to colossal magnetoresistance materials in terms of structure. Lattice strain, oxygen annealing, and grain boundaries have been found to affect the magnetoresistance. In La0.70Ba0.30MnO3 films, pulsed laser deposited on LaAlO₃ substrates, the present work showed that annealing reduced the lattice mismatch strain and decreased the density of subgrain boundaries. This illustrated the effect of interfacial strain on subgrain morphology. The use of a SrTiO₃ buffer layer had a superior strain relieving effect and further reduced the density of subgrain boundaries and misfit dislocations. The importance of a defect-free substrate/buffer layer surface, to avoid defects in the subsequent film, was shown.