Significant improvements in the surface smoothness of YBa₂Cu₃O_7-δ films on high-temperature annealed CeO₂-buffered r-cut sapphire

YBa₂Cu₃O_7-δ (YBCO) films were grown on CeO₂-buffered r-cut sapphire (CbS) substrates using an off-axis dc magnetron sputtering method. Both as-grown and post-annealed CbS substrates were used for this purpose, with the post-annealed CbS prepared by annealing the as-grown CbS at 1000 °C for 1 h. The YBCO films on post-annealed CbS showed significantly improved surface morphology and structures than those on the as-grown CbS, with a peak-to-valley roughness of 3.2 nm and a full-width at half-maximum of (005) YBCO rocking curve of 0.47° for a 300 nm thick YBCO. The surface resistance of the 300 nm thick YBCO film on post-annealed CbS appeared as small as about 110 µΩ at 60 K and about 230 µΩ at 77 K at about 8.6 GHz.