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

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Abstract
The chemical compositions of surface and bulk regions of pulsed laser deposited YBa2Cu3O7−x 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 concluded that there is a nonsuperconducting surface layer of 1 nm thickness with a composition of BaCuO2. This layer contains oxygen with a photoelectron binding energy of 531 eV, and Cu2+ ions. A superconducting film which was only 8 nm thick also had a layer of 1 nm thickness. By detecting the substrate Ti signal through this film, and ruling out a high density of pinholes, we provide evidence that the x‐ray photoelectron spectroscopy data contain information from the superconducting phase. A polycrystalline pellet scraped in vacuum had a similar surface layer, which was only 0.4 nm thick.