Quasi-epitaxial orientations of grains and structure of grain boundaries in YBa2Cu3O7−x thin films

Abstract

When a thin film is grown on a substrate with a large difference in lattice constants, the interface energy as a function of relative orientation shows several cusped minima. If the lattice mismatch is small, or if the growth temperature is high and the deposition rate is slow, most of the nucleated islands assume the orientation corresponding to the minimum interface energy, and a nearly single crystalline film is obtained. If the growth conditions are not so favorable, a polycrystalline film is obtained in which each of the grains may take a variety of orientations corresponding to local minima of interface energy. These relative orientations between the film and substrate determine the tilt angles at the grain boundaries.YBCO films grown on MgO at high temperatures or on SrTiO3 where the lattice mismatch is minimal are found to be nearly single crystal.