Nature of textured growth of laser-deposited YBaCuO thin films on (100) MgO

We have studied, using X-ray and transmission electron microscopy techniques, the texturing of YBa₂Cu₃O₇ superconducting thin films with c axis perpendicular to the lattice-mismatched (100) magnesium oxide substrate. The results were compared with epitaxial growth on the (100) SrTiO₃ where the c axis of the film is either perpendicular or parallel to the substrate. Texturing with c axis perpendicular to the substrate occurs as a result of preferential grain growth in the "a" and "b" directions, whereas epitaxial growth involves lattice matching with the underlying substrate. Thin films of YBa₂Cu₃O_7-x were deposited using a pulsed-laser evaporation technique and were further annealed in oxygen to recover the superconducting properties, and to study the nature of textured growth. The grain growth in films was investigated as a function of annealing treatments. The high-temperature annealed films exhibited large textured grains (about 5-10 μm) with c axis perpendicular to the substrate, but the (001) planes were found to be rotated randomly in the plane parallel to the substrate. The low-temperature annealed films showed small grains (≈100 nm) with no preferred texturing. From the microstructural variations between the high- and low-temperature annealed films, the process of grain growth in high-T_c superconducting films was analysed. We propose a model based upon higher mobility of the a-b grain boundaries to explain the texturing of thin films with c axis perpendicular to the substrate.