THE MICROSTRUCTURE OF EPITAXIAL YBA2CU3O7 FILMS ON STEEP STEPS IN LAALO3 SUBSTRATES

The microstructure of YBa2Cu3O7 grown on steep steps in (001) pseudocubic LaAlO3 substrates was studied by high-resolution electron microscopy of cross-sectional and plan-view samples. Steps with angles of about 80-degrees were obtained by ion milling. On the substrate plane, the films grew with the c-axis parallel to [001] while on the flank of a step the c-axis was parallel to the [100] direction of the substrate. As a result, two [010] tilt axis grain boundaries were formed at which the YBa2Cu3O7 lattice changed orientation by approximately 90-degrees. In the upper grain boundary, a [100] tilt axis and, on the average, a (0 1 3) habit plane alternated with a [010] tilt axis and a (1 0 3) habit plane. This alternating structure was caused by twinning in the orthorhombic film structure. The lower grain boundaries were found to be rather irregular and consisted of a chain of (0 1 3) (0 1 3) and (0 1 0) (0 0 1) type segments exhibiting a tendency to tilt the whole habit plane toward the a-b plane of the flank film. Dislocations, stacking faults and misfit strains were also observed in or close to the boundaries. Grain boundary modeling indicated a good agreement with the experimental image and permitted us to determine the atomic plane of a boundary.