Near-surface stoichiometry of high temperature superconducting YBaCuO thin films
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G-C. Wang and Y-F. Liew Department of Physics, Rensselaer Polytechnic Institute, Troy, New York 12180-3590 (Received 23 August 1989; accepted 21 March 1990)
The surface morphology and near-surface chemical composition of high temperature superconducting Y-Ba-Cu-O thin films have been studied by Scanning Electron Microscopy (SEM) and Auger Electron Spectroscopy (AES) depth profiling. These films were fabricated on SrTiO3 substrates by RF diode sputter deposition and subsequent furnace annealing in oxygen. The chemical composition at and near the surface of thin films was found to differ from the bulk composition. At about 500 A below the surface the Y, Ba, and Cu stoichiometry, as determined by AES, gradually approach that of the film interior. These results suggest that, for furnace annealed films, there may exist a minimum deposited Y-Ba-Cu-O film thickness in which superconductivity is possible. It was also found that the calculated copper concentration determined by AES during depth profiling is significantly lower than its actual value.
I. INTRODUCTION
Even high quality bulk ceramic YBa2Cu3O7 material contains grain boundary phases1 and near-grainboundary composition variations2'3 within grains. Such off-stoichiometric regions degrade the superconducting properties. For thin films, off-stoichiometric surfaces are also detrimental, resulting in high surface impedance4 and in a layer which can preclude the fabrication of superconducting weak links and Josephson junctions. Film surface quality may critically depend on the fabrication technique. The techniques for fabricating superconducting YBa2Cu3O7 films5 fall into two general classes: (1) those in which all fabrication steps are done within the deposition chamber and (2) those which utilize a furnace anneal, and therefore allow exposure to air before processing is completed. Exposure to air would give an opportunity for the film surface to react with carbon dioxide and/or water vapor prior to anneal. This reacted surface could then affect the nearsurface stoichiometry of the annealed superconducting film. In this paper we present our investigation of the near-surface stoichiometry of furnace annealed YBa2Cu3O;c films. Film surface quality may be determined by the fabrication technique. II. EXPERIMENTAL
Films were produced by RF diode sputtering of an Y-Ba-Cu oxide target having metal atom ratios of 1:3:4. The sputter gas consisted of 10 m Torr argon and 2 m Torr oxygen. The target was approximately 5 cm in diam, and it was located 3 cm above the substrate table. 1392
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Sputter power was 160 watts, and a negative 30 volt bias was applied to the substrate table. The temperature of the substrate where the films were deposited was estimated to be 125 ± 25 °C. The dc current from the substrate table through the plasma and to ground varied from run to run. Thus the two films whose properties are reported here were not made under identical conditions. Two films of different thickness (0.9 /Am and 2.5 jam) were deposited on SrTiO3. The
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