Microstructure and microwave surface resistance of YBaCuO thin films

Epitaxial YBaCuO thin films on 73 mm diam. sapphire and 50 mm LaAlO/sub 3/ wafers prepared for microwave applications were characterized by optical and electron microscopy as well as by X-ray diffraction in order to guide optimizing the film properties. The surface resistance R/sub s/, measured at 8.5, 19 or 145 GHz, partially as a function of microwave field amplitude B/sub s/, was taken as the key parameter. Typical results scaled quadratically in frequency to 0.25-0.75 m/spl Omega/ at 10 GHz, 77 K, and low field levels. In case of sapphire substrates, a challenge for applications is microcracking of the films. However, cracks are less deleterious if "dispersed" by other heterogeneities such as a-oriented grains. Narrow microcracks should allow for tunnel currents and flux pinning. In case of LaAlO/sub 3/, films sensitive to high microwave power exhibited some in-plane rotational misorientation and a-oriented grains as well as inhomogeneous layering of these grains and of additional Cu-O planes within the film thickness, with possible influence on R/sub s/.