Optimization of the Superconducting Properties of Laser Ablated $YBa_2Cu_3O_{7-δ}$ Films on $CeO_2$-Buffered Sapphire

We use pulsed laser deposition to grow $YBa_2Cu_3O_{7-δ}$ (YBCO) superconducting films for microwave applications. The films are grown on R-cut sapphire substrates, with $CeO_2$ buffer layers, which are re-crystallized at high temperature prior to YBCO growth. Using the atomic force microscopy (AFM) and X-ray diffractometry we determine the optimal temperature for recrystallization (1000°C) and the optimal buffer layer thickness (30 nm). The properties of YBCO films of various thickness, grown on the optimized $CeO_2$ buffer layers, are studied using several methods, including AFM, magnetooptical imaging, and transport experiments. The YBCO film roughness is found to increase with the increasing film thickness, but the magnetic flux penetration in the superconducting state remains homogeneous. The superconducting parameters (the critical temperature and the critical current density) are somewhat lower than the similar parameters for YBCO films deposited on mono-crystalline substrates.