Microstructure and microwave surface resistance of typical YBaCuO thin films on sapphire and LaAlO₃

Epitaxial c-oriented YBCO films laser deposited onto 3 in diameter CeO₂-buffered sapphire wafers and LaAlO₃ cylinders as well as sputter deposited onto 2 in diameter LaAlO₃ wafers were characterized by integral and spatially resolved measurements of the critical current density j_c and the microwave surface resistance R_s, by microstructure investigations using optical and electron microscopy and by x-ray diffraction. Epitaxial misorientations of in-plane-rotated as well as of a-axis-oriented grains were found in amounts up to 10%. The in-plane rotation seriously degraded R_s while the a orientation mainly lowered j_c. Moreover, a degradation of R_s and of the microwave power handling could be clearly correlated with the density of microcracks occasionally found in YBCO films on sapphire. Inhomogeneities like a-axis-oriented grains were observed to `disperse' microcracks, probably in favour of the electrical properties. The impact of further microstructure imperfections on R_s, in particular of the typical twin lamellae and their domains, is discussed in view of findings from transmission electron microscopy.