Grain Structure and Grain Boundaries in High T_c Superconducting YBa₂Cu₃O_7−x Thin Films

Abstract

We present electron microscopy studies of YBa₂Cu₃O_7−x high temperature superconducting thin films grown by reactive evaporation, reactive sputtering and laser-ablation deposition on MgO, SrTiO₃ and ZrO₂ substrates at various temperatures. Combination of this work with studies of the superconducting properties of small (1μm wide) lines patterned in these films and X-ray pole figure studies has focused attention on large angle tilt boundaries found in c-axis films. Different epitaxial arrangements, corresponding grain boundaries and the grain size distributions can be generated by different choices of substrate, substrate temperature and deposition mode. Examples of grain structure, grain size distribution and high resolution studies of high angle tilt boundaries are presented. The role of the substrate in nucleating defects and grain boundaries will be reviewed. Correlation of these studies with the superconducting properties identifies high angle boundaries as a possible source of significant weakening of the critical current density. Alternatively, misoriented grains may be associated with the weak link behavior. High resolution electron microscopy permits atom structure imaging of these boundaries and, together with microanalytical work, these observations show the boundaries to be normally clean.