Large-area YBCO films for microwave applications

This paper presents techniques for the in- situ deposition of epitaxial YBCO films on two-inch diameter wafers of LaAlO3(001) or α-Al2O3 (1 bar 102) with a Sr-doped La2CuO4(001) (LSCO) buffer layer. The inductively-measured transition temperature varied within the values of 91.1 ± 0.5K across the surface of the LaAlO3 wafer, and 90.7 ± 0.4K across the buffered sapphire wafer. The epitaxial LSCO buffer layer acted not only as a barrier to diffusion of Al into the YBCO films, but improved the YBCO (005) x-ray rocking curve widths from 4 for films grown on bare sapphire to 1.2 for films on the LSCO-buffered sapphire. The typical buffer-layer thickness was 40 nm although layers as thin as 4 nm appeared to be equally effective. The transport critical current density was greater than 106 A/cm2 at 77 K for films on buffered sapphire. At 8.8 GHz and 4.2K, the rf surface resistance was lower than that of gold, indicating that the films on sapphire will be useful in large-area UHF applications