Grain boundary faceting in YBa2Cu3O7-x bicrystal thin films on SrTiO3 substrates

The structure of [001] tilt boundaries in YBa2Cu3O7-x (YBCO) thin films deposited on [001] tilt SrTiO3 (STO) bicrystal substrates has been characterized by transmission electron microscopy (TEM). These boundaries are (100)/(210), (310)/(510), (410)/(310), (510)/(210), (210)/(410), and (210)/(310), with corresponding misorientation angles of 26degrees, 29degrees, 32degrees, 37degrees, 40degrees, and 44degrees. It was found that the YBCO film boundaries were meandering along the relatively straight substrate boundaries. High-resolution lattice images indicated that the microscopic meandering of the film boundary essentially consisted of many straight segments of facets at the atomic scale. On the basis of the observed facets, three competing factors controlling the formation of facets are discussed. First, the boundary plane is defined by Miller indices (hk0) in both crystals with sufficiently small h, k (i.e., h, k less than or equal to 5) and sufficiently large effective interplanar spacing (i.e., d(eff) > 0.06 nm). Second, the closure failure defined by the difference between the local misorientation from the design misorientation is small, i.e., less than 2degrees. Third, the deviation of a local facet plane is observed to be less than 30degrees from the design boundary plane. Higher values of d(eff)s are observed to give tolerance to higher deviation angles.