Abstract
We report microstructures of superconducting YBa 2 Cu 3 O 7-x thin films epitaxially grown on a SrTiO 3 (100) substrate. Transmission electron microscopy studies reveal epitaxial, highly ordered grains ∼5000 Å in size for both types of films with either the 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 c observed in thin-film samples.