Characterization of three-dimensional grain boundary topography in a YBa2Cu3O7-d thin film bicrystal grown on a SrTiO3 substrate

The topography and crystallography of YBa2Cu3O7-d (YBCO) bicrystal films gr own epitaxially on oriented SrTiO3 (STO) bicrystals have been characterized by scanning and transmission electron microscopies (SEM and TEM) and atomi c force microscopy (AFM). The YBCO films were formed by laser ablation on m elt-grown Sigma 13 STO bicrystals with a misorientation of 24 degrees aroun d the [001] tilt axis. In agreement with previous reports, TEM analysis rev ealed that the grain boundary in the film did not always follow the planar substrate grain boundary faithfully, but undulated about the average bounda ry plane. High resolution electron microscopy observations of the apparentl y complex undulating boundary structures could be explained as a result of an overlap between different orientation variants of the orthorhombic YBCO film. Cross correlation between SEM, AFM, and TEM imaging gave a clear evid ence that an island growth mechanism is responsible for the observed grain boundary structure and morphology for which a schematic model is presented. It is seen that meandering of the YBCO grain boundary (GB) is necessarily coupled to a wide range of inclination of the GB plane in the z direction. The implications of this interfacial structure for the behavior of GB based Josephson junctions are discussed and compared to models proposed in the l iterature. It is also seen that inclination of the GB may be responsible fo r the poor correlation usually found in the literature between calculations and experimental curves of current density J(c) versus the GB angle since the most elaborate models proposed up to now take into account only pure ti lt GB plane facets, that is to say facets in the zone of the tilt axis. Mor eover, such a GB structure may affect the interpretation of recent phase se nsitive experiments done on bicrystal or tricrystal high T-c superconductor s to determine the symmetry of the order parameter. (C) 1998 American Insti tute of Physics. [S0021-8979(98)01921-5].