Cation Interdiffusion at YBCO/MgO Interfaces
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CATION INTERDIFFUSION AT YBCO/MgO INTERFACES.
M. Lanham, J. Mayer, S.J. Golden, F.F. Lange, and M. Ruihle* Materials Department, University of California, Santa Barbara, CA. 93106 *Max-Planck-Institut ffir Metallforschung, Stuttgart, West Germany.
ABSTRACT Resistivity measurements of YBCO thin films on MgO substrates show that the transition to the superconducting state depends on film thickness. TEM investigations of the films revealed interdiffusion of Cu and Mg. The concentrations of the cations at the interface were quantitatively measured. These data show that their diffusion rates are much different (MgODCu/YBCODMg=30); this indicates that the diffusion mechanism is complicated by the layered structure of the perovskite-like films. No impurity segregation was detected at the interface. Diffraction study of several YBCO grains confirms that the film c-axis is oriented perpendicular to the substrate, but that the a/b plane has no fixed orientation, indicating that no epitaxial relationship exists. INTRODUCTION Precursors deposited on ceramic substrates require heat treatment to form the superconductor films. Undesirable interactions between the film and the substrate often occur. Yet, there has been little detailed investigation of the effects even for the most commonly used substrate materials, such as MgO. We have found that the transition to the superconducting state of YBCO films on MgO substrates has a marked dependence on film thickness. Here we report on a detailed TEM investigation, which revealed that cation interdiffusion is a major contributor to film degradation. EXPERIMENTAL YBCO films were prepared by spray-pyrolysis of nitrate precursors onto singlecrystal MgO (100) substrates. The thickness was varied by changing the spraying time. The films were heat treated at 950 0 C for 30 minutes. Silver paint contacts and a current of 10A were used for transport measurements. TEM cross-section samples were prepared by first sandwiching two pieces of film/substrate together. The sandwiches were epoxied into special ceramic holders, sliced, dimpled and ion milled. The experiments were carried out in a 400 kV STEM (JEOL 4000FX) equiped with a high-angle take off X-ray detector. The cation concentration gradients across the interface were measured by acquiring X-ray spectra (EDS) from sample areas with constant thickness. This was achieved by stepping a small probe (diameter 10 nm) parallel to the edge of the wedge produced by ion-milling. To verify that the measured cation concentrations were not due to a surface contamination sputtered on while ion-milling, the concentration measurements were repeated parallel to the interface, i.e., for increasing thickness.
Mat. Res. Soc. Symp. Proc. Vol. 169. ©1990 Materials Research Society
1174
RESULTS The resistivity measurements show (figure 1) that the transition to the superconducting state is sharper for thicker films [1).
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