Epitaxial YBCO Films on MgO, SrTiO₃, Si, and GaAs by Thermal Coevaporation

Abstract

The technique of thermal coevaporation for the preparation of thin YBa₂Cu₃O₇ films yields excellent superconducting properties and, at the same time, very smooth surfaces. Scanning tunnelling microscopy (STM) reveals tiny growth spirals with extremely high density on MgO, and layer-by-layer growth on SrTiO₃. On SrTiO₃ we also observe very pronounced RHEED-oscillations. The relatively low substrate temperatures of 600 to 650°C for optimum epitaxial growth allow film preparation also on semiconductor substrates. On silicon with buffer layers of yttria stabilized zirconia (YSZ), we have grown epitaxial films with T_c = 87..89K, j_c(77K) = 2.5·10⁶A/cm², ρ(100K) = 40μΩ, and a high degree of smoothness. Recently we have also succeeded to grow epitaxial films on GaAs with buffer layers of MgO, with T_c = 86.8K, j_c(77K) = 7·10⁵A/cm², and ρ(100K) = 100μΩ cm. These values are markedly better than those obtained with pulsed laser ablation by Fork, Nashimoto, and Geballe. Integrated superconducting and semiconducting devices have now come in sight.