In situ Raman spectroscopy of oxidation and reduction of high T superconducting materials at elevated temperatures

The superconducting transition temperature of the YBa/sub 2/CuO/sub 7-x/ system is critically dependent on the oxygen stoichiometry. Virtually all methods of preparation of thin film or bulk superconducting materials involve a final processing step at about 500 C in oxygen-containing gas which allows the oxygen stoichiometry associated with superconducting behavior to be established. The crystal structure of the superconducting phase is orthorhombic. For lower oxygen contents, a tetragonal phase is formed which does not demonstrate superconductivity at any temperature. In thin film processing, it is important to perform the anneal as rapidly as possible to add oxygen and form the orthorhombic phase while limiting diffusion of impurities from the substrate. The authors report here the use of Raman spectroscopy to determine the phases present during annealing in oxygen and argon at temperatures from 400 to 800 C. The depth probed by Raman spectroscopy is determined by the optical absorption coefficient of the sample for the exciting laser wavelength. For these highly opaque ceramics the probe depth is probably less than 1000A. This technique should be particularly useful for film superconductor processing because of its sensitivity (phase identification of 200A thick films is possible and applicability in ambient environments