Grain-boundary structure of thin films of YBa₂Cu₃O₇ and Bi₂Sr₂CaCu₂O₈ on bicrystalline substrates

Thin films of YBa₂Cu₃O₇ and Bi₂Sr₂CaCu₂O₈ were deposited by high-pressure oxygen sputtering and laser ablation, respectively, on bicrystalline substrates of SrTiO₃ with angles of 10,24,36 and 45 degrees. The microstructure of the grain boundary in the substrate and in the film was characterized by transmission electron microscopy. While the grain boundary in the substrate was found to be almost rectilinear with a maximum roughness of about 3 nm, the grain boundaries in the superconducting films exhibited a roughness of up to 200 nm for YBa₂Cu₃O₇ and up to 1 μm for Bi₂Sr₂CaCu₂O₈. The grain boundaries consist of facets, which have a low index habit plane in common with one of the adjacent grains. These facets can be found on a length scale extending from several nanometers up to one micrometer. It is concluded that the exact lateral position of the film grain boundary is not only determined by the substrate but also by the coalescence of growth islands during the film-deposition process. The microstructural results are discussed with respect to their impact on the electrical properties.