Reproducible sputtering and properties of Y-Ba-Cu-O films of various thicknesses

Abstract

One goal of this work was to develop a reproducible method of preparing high quality Y-Ba-Cu-O superconducting films and to study their properties versus thickness. This was accomplished by rf diode sputtering from a single target. Twenty-seven depositions were made using a target containing 8.9-at. % Y, 37.3-at. % Ba, and 53.8-at. % Cu. Film thicknesses ranged from 0.09 to 2.4 μm. The film compositions obtained were 15.6±1.0-at. % Y, 35.8±1.0-at. % Ba, and 48.7±1.7-at. % Cu for the mean and standard deviation. The films were amorphous as-deposited and crystallized by annealing in O2 at 915 °C. X-ray diffraction, transmission electron microscopy, and scanning electron microscopy indicated that, on (100)SrTiO3 substrates, films with thickness less than ∼0.25 μm were epitaxially oriented with their c axis perpendicular to the substrate. Films on (110)SrTiO3 were oriented with their c axis parallel to the substrate. On (100)SrTiO3, zero resistance was achieved in 30 samples from 27 runs at 85.6±1.4 K with a transition width (10%–90%) of 1.8±1.1 K independent of thickness. Results for deposition on a variety of other substrates were more variable. Diamagnetic shielding of up to 38% was calculated from the initial slope of M vs H. This low value was attributed to the presence of second phases and a significant diffusion layer thickness, both observed by transmission electron microscopy. Critical currents, measured by transport using a four-point probe, reached 8.1×105 A/cm2 at 77.35 K for a 0.2-μm film deposited on (100)SrTiO3 . Comparable values were obtained by calculation from the M-H hysteresis loop, from which we infer that there are either continuous epitaxial sheets of c axis oriented 123 or, if there are grain boundaries, the coupling across them is strong.

Critical Current Density and Microstructure of Yba2Cu3O7-X Films as a Function of Film Thickness

“…For thicker films, the thickness of this layer remains constant at 0.4 pm thick as shown by the horizontal line in the figure. This model is consistent with the observations of Luborsky et al, except that in their case the breakpoint thickness was 0.25 pm [2]. Since Jc is highest in the direction perpendicular to the c-axis of YBCO, we assume to first order that all the current is carried in the bottom layer of the films.…”

“…It can be seen that the decrease of Jc shown by the data is faster than that predicted by the model based on the microstructural observations. It is interesting that a similar model was found to be in rough agreement with data for sputtered films which contained nonepitaxial material [2].…”

“…7 um. Luborsky et al [2] reported Jc values somewhat under 106 A cm-2 at 77 K for YBCO films sputtered onto SrTiO 3 substrates and postannealed. Jc decreased by about an order of magnitude in the range of 0.25 to 2 pm.…”

Fabrication and X-ray characterization of biaxially textured thick film EuBa2Cu3O7−δ homo-substrates

“…The YBCO coated conductors with a current density over 1 mA/cm 2 have been demonstrated [2,4,5]. However, it has been reported that the J c decreases rapidly as the thickness of an epitaxial YBCO layer increases either on a single-crystal substrate or a metallic substrate [6][7][8][9]. The reason for the J c drop, which seems to be universal regardless of the deposition technique, is not clear.…”

High-Jc GdBa2Cu3O7 thin films on SrTiO3 substrates by pulsed laser deposition

“…The reason for the J c decrease with thickness has been unclarified yet. In the literatures [5][6][7][8][9][10][11], however, several reasons have been suggested for J c decrease with thickness, including a transition from c-axis to a-axis orientation, a loss of in-plane and out-of-plane textures, a decrease in the number of pinning sites, and a change in the microstructure. From this viewpoint, it must be worthwhile to investigate J c dependency of film thickness for other LREBCO films.…”