Microstructure of YBa sub 2 Cu sub 3 O sub (7-x) superconducting thin films grown on SrTiO sub 3 (100) substrate.

We report microstructures of superconducting YBa sub 2 Cu sub 3 O sub (7-x) thin films epitaxially grown on SrTiO sub 3 (100) substrate. Transmission electron microscopy studies reveal epitaxial, highly ordered grains ~5000angstrom in size for both types of films with either a- or c-axis perpendicular to the film. Defects due to out-of-phase boundaries are observed in the former case. The high density of twin boundaries found in the latter case seems to correlate with the higher J sub c observed in thin film samples.