TEM investigation of grain boundaries in YBa₂Cu₃O₇ thin films grown on SrTiO₃ bicrystal substrates

Thin films of YBa₂Cu₃O₇ were deposited by high-oxygen-pressure dc-sputtering on [001]-tilt bicrystals of SrTiO₃ with rotation angles of 10°, 24°, 36° and 45°. The substrates and the films were investigated by transmission electron microscopy. The lateral roughness of the substrate grain boundaries was found to be about 3 nm, while the film grain boundaries exhibited a roughness of 20 nm to 200 nm. The film boundaries were dominantly asymmetrical and faceted so that the grain boundary plane corresponds to a low-index plane of one of the two adjacent grains. The majority of these low-index planes were (100) or (110) planes. Symmetrical segments were only observed for the 24° grain boundary. For the 45° boundary only (100)/(110) facets were found. The atomic structure of a (100)/(110) facet was investigated in detail by comparing the experimental high-resolution electron micrographs with calculated images. The atomic order at the grain boundary was found to be perturbed within one or two interatomic spacings from the interface.