MICROSTRUCTURE AND DEFECTS IN ALPHA-AXIS ORIENTED YBA2CU3O7-X THIN-FILMS

MOCVD-grown, a-axis oriented YBa2Cu3O7-x thin films have been studied by transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM). The majority of the thin film material is oriented with the a-axis perpendicular to the substrate surface, while about 20% of the material has the c-axis perpendicular to the substrate. The c-axis oriented grains are disconnected from each other, and are separated by a-axis oriented grains. There are two types of a-axis oriented grains, differing by a 90-degrees rotation of the c-axis. HREM images in conjunction with image simulation analysis are used to derive the atomic structures of the 90-degrees rotation grain boundaries and of defect structures in the film. The majority of the grain boundaries is asymmetrical, bounded by (010) (001) lattice planes. However extended steps, whose segments are mostly parallel to the (013) (013) planes, are also present, thus forming short facets of symmetric grain boundaries. The latter are believed to be important in forming weak-link-free connections between the grains. The misfit at (010) (001) grain boundaries is accommodated by the formation of stacking faults, which were identified to be 124-type defects. The grain boundary structures are discussed in terms of correlations with the electric transport properties across the boundaries.