YBa₂Cu₃O_7-δ films on SrTiO₃ are characterized by a network of anti-phase boundaries (APB's), protruding from the interface to the film surface. We propose that the island morphology of these films is to a large extent determined by this network. At APB-outcrops deep trenches form which separate the growth islands. At the cross-section of three APB's deep holes are formed. It is found that the growth island size can be increased by increasing the substrate temperature or the oxygen pressure during growth.

Epitaxial YBa2Cu3O7-δ films nucleate in c-axis oriented single-crystalline islands. The surface of the single-crystalline SrTiO3 substrates exhibit steps of one third of the YBa2Cu3O7-δ c-axis. These steps generate a mismatch in the island boundaries between the CuO2 superconducting blocks. We show that these defect regions are strong candidates for being the pinning centers responsible for the large critical currents observed in Laser Ablated and Sputtered thin films.