New buffer layer for high-temperature superconducting ceramics on sapphire: LaBa2Cu3Oy/Ag bilayers

Sapphire (a-A1203) is a desirable substrate for the growth of superconducting thin films due to its low dielectric constant and the extensive interest on silicon on sapphire technology. Unfortunately there exists a strong chemical interaction between sapphire and REBa,CusO,, ( RE123 ) ceramics’-3 which complicates the growth of high-quality-thin-film ceramic superconductors. Several reports on the growth of RE-123 superconducting thin films on sapphire substrate with and without buffer layerslm7 have shown that the quality of these films is inferior to that of films grown on other suitable substrates, e.g., SrTiO, and MgO. Witanachchi et a1.7 reported the growth of YBazC!u30,, (Y-123) films on sapphire (iO12) using Ag buffer layers, but no structural information was reported for the Ag nor the Y-123 films. Here we report the growth of LaBa2Cu30,, (La-123)/ Ag bilayers on sapphire ( 1120) substrates and show them to be excellent buffer layers for the growth of RE-123 films on sapphire. This structure has the inherent advantages that it is well matched to other RE-123 films and that it provides an excellent diffusion barrier. Moreover, to our knowledge, this is the first time that well oriented non-ctextured 123 films are grown on sapphire. This is significantly important for electronics applications because of the short superconducting coherence length along the c-axis of 2-3 A which makes non-c-textured 123 films preferable for Josephson and tunnel devices. All fabrication processes were performed in situ using magnetron sputtering. The RE-123 stoichiometric ceramic targets were prepared from better than 99.9% pure La203, DY+s, BaCO3, and CuO. The calcined powders were pressed into the proper shape and sintered in air at 935 “C for 3 h. The sputtering gas was a mixture of 10% oxygen/ 90% argon and either at 100 or 300 mTorr of total pressure. The deposition geometry was always with the substrate surface opposing the target (on-axis geometry) at a vertical distance of 1 in. The substrate was positioned above the edge of the target to avoid resputtering effect. The actual substrate temperature was calibrated against the holder temperature using a second thermocouple in different runs.