In-Situ Preparation of Ytterbium-Barium - Superconducting Thin Films Using Pure Ozone Vapor Oxidation

A new process for preparing thin films of the YBa_2Cu_3O _{rm 7-x} high transition temperature superconducting oxide completely in-situ, without the need for a post-evaporation anneal has been developed. This work is a significant advancement in the effort to achieve a fully mature, high quality thin film-making process for scientific and technical applications. A pure ozone vapor, derived from the distilled liquid, is used to oxidize the co-evaporated metallic constituents during deposition to nucleate the superconducting phase in the vacuum chamber. Films exhibiting zero resistance transition temperatures at 85 K have been grown on strontium titanate substrates using a substrate temperature of 700 ^circC. Background evaporation pressures of 2 x 10^{-7}Torr are employed during film growth. Films prepared using this process contain primarily mixed a- and c-axis oriented grains which, as evidenced by Transmission Electron Microscopy, exhibit a high degree of epitaxial order with the substrate. Processing at lower substrate temperatures results in a depression of T _{rm c} consistent with the behavior observed for other in-situ techniques. It is unclear at this time whether this depression can be attributed to an oxygen deficiency or an expanded c-axis lattice parameter which is always associated with lower temperature processing. Measurements of the critical current of a prototype YBa_2Cu_3O _{rm 7-x}/Au/Pb proximity tunneling junction prepared in-situ using the ozone process do not exhibit the expected magnetic field and temperature dependence. This observation may be the result of a poorly defined tunneling geometry subject to the vagaries of edge effects or filamentary electrical shorts through the normal metal layer. Important implications for the investigation of an isotope effect in the high T_{rm c} superconductors is made possible by the development of the ozone technique. By distilling the ^{18}O gas into a "heavy ozone," thin films can be efficiently prepared from the outset in the exclusive environment of the heavy isotope. This method would overstep the fundamental criticism of incomplete oxygen exchange suffered by the bulk of recently conducted investigations conducted using polycrystalline material.