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 CeO 2 -buffered sapphire wafers and LaAlO 3 cylinders as well as sputter deposited onto 2 in diameter LaAlO 3 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.