Large-area YBCO films on sapphire for microwave applications

Crack-free thick YBa/sub 2/Cu/sub 3/O/sub 7-x/ films are prepared on CeO/sub 2/ buffered r-cut sapphire (2 inch in diameter) with thickness up to 700 nm, smooth surfaces ('peak-to-valley' roughness <10 nm), high critical currents (J/sub c/>2 MA/cm/sup 2/ at 77 K and 0 T) and low microwave surface resistance (R/sub s/(77 K)/spl ap/1.4 m/spl Omega/ and R/sub s/(4.2 K)/spl ap/110 /spl mu//spl Omega/ at 19 GHz), comparable to the best values reported for YBCO films on structurally better matched substrates. The improvement of the critical thickness of YBa/sub 2/Cu/sub 3/O/sub 7-x/ on structurally poorly matched sapphire from typically d/sub c//spl ap/250-300 nm to d/sub c/>700 nm is achieved by introducing defects which do not hamper the superconducting properties of the films. The modified film growth was reflected by slightly broadened rocking curves and an altered temperature dependence of R/sub s/. The thick YBCO films were able to handle high microwave power corresponding to magnetic field amplitudes (B/sub HF/) of at least 54 and 37 mT at 4.2 and 50 K (limited by the available power), and 17 mT at 77 K (often limited by quenches). The excellent high-power performance was achieved without any degradation of the samples despite frequent thermal cycling.