Microwave surface resistance of YBCO thin films on cerium oxide buffer

The effects of the crystallinity and surface roughness of CeO/sub 2/ buffer on R-plane Al/sub 2/O/sub 3/ substrate on the microwave surface resistance (R/sub s/) of YBa/sub 2/Cu/sub 3/O/sub y/(YBCO) thin films are discussed. We estimated R/sub s/ from the transmission characteristics of the microstrip line resonator at 25 K and 6.7 GHz. X-ray diffraction (XRD) of /spl theta/-2/spl theta/ and /spl phi/-scan showed that CeO/sub 2/ was completely [001]-oriented and in-plane aligned crystal. Four CeO/sub 2/ samples with different thicknesses were prepared using identical conditions except for the deposition time. The dependence of R/sub s/ on CeO/sub 2/ thickness was measured in the range from 10 nm to 200 nm. The value of R/sub s/ was minimum at CeO/sub 2/ thickness of 100 nm. The dependence of R/sub s/ vs CeO/sub 2/ thickness was similar to that of the amount of a-axis domains against the thickness. The crystallinity of thin CeO/sub 2/ was poor because the lattice was strongly strained by Al/sub 2/O/sub 3/. This affected the quality of the upper YBCO layer. In contrast, thick CeO/sub 2/ 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/sub 2/ surface. These projections act as nucleation centers for the a-axis domains.