Effect of substrate vicinality on the formation of precipitates and twins in YBa₂Cu₃O₇-x thin films

Twinning and precipitates in YBa2Cu3O7-delta (YBCO) thin film grown on vicinal SrTiO3 (STO) (100) substrates are investigated. The direction of the in-plane cut of the substrate was chosen to be close to either [010] or [110]. In the former case the surface steps create an anisotropic surface morphology, the YBCO film grows with the c-axis perpendicular to the SrTiO3 (100) crystal lattice plane and the twinning in one in-plane direction can be suppressed. In the latter case the substrate surface is dominated by kinks in the steps, the YBCO film grows with the c-axis perpendicular to the surface geometrical plane and the film is twinned unidirectionally. Films deposited on substrates cut along [110] are rather smooth whereas deep holes are observed in YBCO films grown on substrates vicinally cut along [010]. It is proved by cross-sectional TEM that these holes originate on Y2O3, CuO and Y2O3-CuO precipitates. We find no direct correlation of the precipitate formation with the substrate/film microscopic features, however possible mechanisms are considered.