Critical Current Density and Microstructure of Yba 2 Cu 3 O 7-X Films as a Function of Film Thickness
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CRITICAL CURRENT DENSITY AND MICROSTRUCTURE OF YBa 2 Cu 3 O7 x FILMS AS A FUNCTION OF FILM THICKNESS A. MOGRO-CAMPERO, L.G. TURNER, E.L. HALL, AND N. LEWIS CE Research and Development Center, Schenectady, NY 12301
ABSTRACT Thin films of nominal composition YBa 2 Cu 3 O7 _x (YBCO) were produced on (100) SrTiO3 substrates by coevaporation and furnace annealing. Film thicknesses in the range of 0.2 to 2.4,pm were analyzed. Microstructural investigations by cross sectional transmission electron microscopy (TEM) reveal a continuous layer of about 0.4 um thickness adjacent to the substrate with c-axis normal to the substrate plane. In thicker films the remaining top portion has the c-axis in the film plane. The critical current density (Jc) at 77 K decreases with increasing thickness in the thickness range exceeding 0.4 pm, qualitatively consistent with the microstructural observations, but quantitatively inconsistent with a simple model based on the microstructural data.
INTRODUCTION Studies of the dependence of the critical current density (Jc) on film thickness for films of high temperature superconductors may contribute to an understanding of the factors which determine J Few of these studies have C'5 -2 at been reported. Akoh et al. [1] found a constant value of about 10 A cm2 0 7 4.2 K for film thicknesses in the range 0.2 to . um. Luborsky et al. [2] 2 reported Jc values somewhat under 106 A cmat 77 K for YBCO films sputtered onto SrTiO3 substrates and postannealed. Jc decreased by about an order of magnitude in the range of 0.25 to 2 pm. EXPERIMENTAL PROCEDURE Coevaporation of Y, BaF 2 , and Cu was performed as described elsewhere [3] onto (100) SrTiO3 substrates at ambient temperature. All samples were annealed in the same furnace at 8500C with oxygen and water vapor for 3.5 h; ramp-up and ramp-down in oxygen only. Critical current measurements were performed on bridges 400 pm long and 20 )m wide patterned by photolithography and a dilute nitric acid etch, using a 1ijV criterion.
RESULTS AND DISCUSSION Cross sectional images of samples of the six thicknesses employed in this study are shown in Fig. 1 with the measured film thicknesses indicated. Using thickness determinations from these measurements and a scaling based on deposition parameters, one arrives at the following set of thicknesses to be used in this study: 0.2, 0.4, 0.6, 0.8, 1.6, and 2.4 pim. These values are indicated by arrows in Fig. 2. The layer of each film nearest the interface was determined by electron diffraction to be epitaxial with the substrate and to have the YBCO c-axis perpendicular to the substrate. For film thicknesses greater than 0.4 jm, the top portion of the film is also epitaxial with the substrate, but with the c-axis pointing in one of the two mutually perpendicular directions in the film plane parallel to the substrate cube axes. Thus all films are epitaxial with the substrate, with
Mat. Res. Soc. Symp. Proc. Vol. 169. 'c1990 Materials Research Society
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