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 jc and the microwave surface resistance Rs, 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 Rs while the a orientation mainly lowered jc. Moreover, a degradation of Rs 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 Rs, in particular of the typical twin lamellae and their domains, is discussed in view of findings from transmission electron microscopy.