Microwave surface resistance of YBCO thin films on cerium oxide buffer

The effects of the crystallinity and surface roughness of CeOz buffer on It-plane A1}O3 substrate on the microwave surface resistance (RJ of YBBjCUjO YBCO) thin films are discussed. We estimated Rf from the transmission characteristics of the microstrip line resonator at 25 K and 6.7 GHz. X-ray diffraction (XRD) of 6-26 and -scan showed that CeO, was completely (OOl)-oriented and in-plane aligned crystal Four CeO: samples with different thicknesses were prepared nsing identical conditions except for the deposition time. The dependence of Rf on CeO, thickness was measured in the range from 10 nm to 200 nm. The value of Rt was minimum at CeO2 thickness of 100 nm. The dependence of A( vs CeO2 thickness was similar to that of the amount of a-axis domains against the thickness. The crystallinity of thin CeOj was poor because the lattice was strongly strained by A12O3. This affected the quality of the upper YBCO layer. In contrast, thick CeO2 had excellent crystallinity. However, for the thickness of more than 100 nm a drastic change in surface morphology was observed by atomic force microscopy (AFM). A number of projections appeared on the CeO,surface. These projections act as nucleation centers for the a-axis domains.