Effects of post-annealing temperature of CeO$_2$ buffer layers on the surface morphology, structures and microwave properties of YBa$_2$Cu$_3$O$_{7-{\delta}}$ films on sapphire

Effects of the post-annealing temperature of CeO$_2$ buffer layers on the properties of YBCO films on CeO$_2$-buffered sapphire were investigated. 45 nm-thick CeO$_2$ buffer layer was prepared in-situ on r-cut sapphire using an on-axis rf magnetron sputtering method, which was later post-annealed at temperatures between 950$^{\circ}$C and 1100$^{\circ}$C in an oxygen-flowing environment. YBCO films were prepared on CeO$_2$-buffered sapphire (CbS), for which the surface morphology, crystal structures and electrical properties of the YBCO films were studied. YBCO films on post-annealed CbS appeared to have better properties than those on as-grown CbS with regard to the morphological, structural and electrical properties when the YBCO films were prepared on CeO$_2$ buffer layer post-annealed at temperatures of 1000 - 1050$^{\circ}$C. A TE$_{011}$ mode rutileloaded cylindrical cavity resonators was fabricated with the YBCO films placed as the endplates, for which the unloaded Q of the resonator was measured. It turned out that the resonator with the endplates prepared from the YBCO films on postannealed CbS at 1000 $^{\circ}$C showed the highest unloaded Q with the value more than 8 ${\times}$ 10$^5$ at 30 K and 8.6 CHz, revealing that the YBCO films on post-annealed CbS at 1000$^{\circ}$C the temperature could be the lowest among the YBCO films on post-annealed CbS.