Microstructure and Critical Currents of YBa₂Cu₃O_7-x Thin Films on SrTiO₃ and a New Substrate: KTaO₃

Abstract

Superconducting thin films of high-T_C materials are of interest for use in fundamental research as well as for a variety of device applications. Epitaxial thin films of high-T_C superconductors have previously been produced using substrates such as SrTiO₃, cubic ZrO₂, MgO, and Al₂O₃ with the best results being reported for (100) surfaces of strontium titanate. In the present work, the properties of epitaxial films of YBa₂Cu₃O_7-x (Y123) formed by coevaporation of Y, Cu, and BaF₂ onto (100) surfaces of SrTiO₃ are presented and compared with those found for identical films on a new substrate material: single crystal KTaO₃ or potassium tantalate. This material, like SrTiO₃, is also a cubic perovskite with a suitable lattice constant for promoting epitaxial growth. In Sections I and II of the present work, the details of the film preparation are outlined, along with the properties of the resulting high-T_a films formed on (100) and (110) surfaces of SrTiO₃. These results are similar to the findings by others and a critical dependency on the film microstructure and orientation is found. In Section III, the results obtained for the new substrate material KTaO₃ are presented. It is shown that the properties of Y123 films epitaxially grown on the (100) surface of KTaO₃ single crystals compare favorably with those of films formed on SrTiO₃. In Section IV more detailed comparisons of the microstructural properties of epitaxial films on the two substrate types are made. The results show that KTaO₃ represents a promising new substrate material for the epitaxial growth of high-T_C Y123 films.

Research sponsored by the Division of Materials Sciences, U.S. Department of Energy under contract no. DE-ACO5-84OR21400 with Martin Marietta Energy Systems, Inc.