Abstract
Superconducting BiSrCaCuO ultra thin films with a thickness of 30 nm were grown epitaxially on (100)MgO substrates by using modified single-target sputtering and by optimizing the heat-treatment. The value of the superconducting transition temperature Tc,0 obtained was as high as 102 K, which is the highest level at this thickness of the Bi system. The target used in this study was modified to a concave-surface shape by heaping up the oxide powders in the sputter-erosion areas so that sputtered particles are continuously directed toward the central part of the substrate holder. The optimum heat-treatment consisted of the multistep annealing at 1148 K for 2 min, down to 1144 K and to 1123 K at the elaborately controlled rates of cooling, respectively, followed by final cooling to room temperature. Particularly, the short term annealing was necessary to suppress undesiable compositional changes due to the evaporation of constituent atoms and the reaction between thin films and substrates.
