Microstructure of YBa2Cu3O7−x thin films grown on single-crystal SrTiO3

YBa2Cu3O7−x thin films on (001) and (014) SrTiO3 were prepared by coevaporation of Y, BaF2, and Cu, and annealing in oxygen at 850 °C. Cross-sectional samples prepared by cutting films and substrates along the SrTiO3 (100) plane were studied with transmission electron microscopy. The growth of YBa2Cu3O7−x in as-deposited films is extremely anisotropic with the [001] being the slow direction and the (001) plane being the fast growth plane. When the (001) planes of YBa2Cu3O7−x grains are parallel to the film plane, as on (001) SrTiO3, there is a dominance of this orientation over the whole film. Three types of YBa2Cu3O7−x grains, 90° misoriented from each other, are observed to grow on both (001) and (014) surfaces of single-crystal SrTiO3 substrates with their c directions aligned with one of the three 〈100〉 directions of cubic SrTiO3. This ambiguity creates a threefold degenerate epitaxy of YBa2Cu3O7−x on SrTiO3, which results in 90° grain boundaries in the final YBa2Cu3O7−x thin films. These boundaries are free of any second phase and decorations. While some extraneous phases were observed in these films, they were located away from the crystallographically special, 90° boundaries. Therefore, the presence of extraneous phases elsewhere in the films was not damaging to the superconducting transport.