Surface layers on superconducting Y-Ba-Cu-O films studied with x-ray photoelectron spectroscopy

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Abstract
X‐ray photoelectron spectroscopy (XPS) examines only a thin surface layer (Tc (90 K) and high Jc (>106 A/cm2) Y‐Ba‐Cu‐O films on SrTiO3 (001) substrates were examined. From the relative intensities of the surface and bulk components of the Ba(3d) and Ba(4d) spectra taken at different take‐off angles and different escape depths [using Al Kα (1486.6 eV) and Mg Kα (1253.6 eV) excitations], we have determined the nonsuperconducting surface layer thickness to be 1 nm and the layer composition to be BaCuO2. The surface layer thickness for a superconducting film only 8 nm thick was also 1 nm. By detecting the substrate Ti signal through this film, and ruling out a high density pinholes, we provide evidence that the XPS data contain information from the superconducting phase. A polycrystalline pellet scraped in vacuum had a surface layer only 0.4 nm thick. Since typical photoelectron escape depths are about 2 nm, about 80% of the detected signal originates in the bulk. The surface layer contains Cu2+ and oxygen with a photoelectron binding energy of 531 eV.