FILM THICKNESS DEPENDENCE OF CRITICAL CURRENT-DENSITY AND MICROSTRUCTURE FOR EPITAXIAL YBA2CU3O7-X FILMS

Films of nominal composition YBa2Cu3O 7-x(YBCO) were made on (100) SrTiO3 substrates by coevaporation and furnace annealing. Film thickness was in the range 0.2-2.4 mu m. Chemical analysis by inductively coupled plasma emission spectroscopy reveals that the thinner films had an excess of Cu and Ba relative to Y, which led to Cu- and Ba-rich particles on the film surface. Transmission electron microscopy was used to define the epitaxial nature of the films: a continuous layer of about 0.4 mu m thickness adjacent to the substrate and with the c axis normal to the substrate plane, followed by material with the c-axis in the film plane. A simple model based on microstructural observations and the strongly anisotropic transport properties of YBCO is compared with the measurements of room-temperature resistivity and critical current density at 77 K as a function of film thickness. The critical current density is found to decrease as a function of increasing film thickness by a much larger factory than the simple model predicts.