Mechanisms for hetero-epitaxial nucleation of YBa₂Cu ₃O_∼6.1 at the buried precursor/SrTiO₃ interface in the postdeposition reaction process

The mechanisms have been identified for the hetero-epitaxial nucleation of YBa₂Cu₃O_∼6.1 (YBCO) at the buried interface between a precursor film and SrTiO₃ for the so-called BaF ₂ process which is a postdeposition reaction process for the synthesis of epitaxial YBCO films. It is shown that the preferential nucleation of YBCO at the interface is due to (1) the strong chemical affinity of the (Y,Ba)-oxy-fluoride, an intermediate phase, to SrTiO₃ and (2) the epitaxial alignment of its (111) planes with the (001) surface of the SrTiO ₃ which reduces the activation barrier for the formation of YBCO. In thin films (<2-3 μm) the YBCO nuclei, whose c axes are perpendicular to the SrTiO₃ surface, form directly from this aligned oxy-fluoride. In thick films (5 μm), however, this oxy-fluoride decomposes into a disordered transitory cubic phase which then orders to form YBCO nuclei with three orientational variants, one with its c axis perpendicular and two with their c axes parallel to the (001) plane of SrTiO₃.