Surface layer of SrRuO₃ epitaxial thin films under oxidizing and reducing conditions

Imperfect stoichiometry and heterogeneity of a surface layer of SrRuO₃ epitaxial thin films, grown on SrTiO₃ substrates, are presented with the help of various methods. Rutherford backscattering spectroscopy, x-ray photoemission spectroscopy (XPS), and time of flight secondary ion mass spectrometry are used to obtain information about the stoichiometry and uniformity of the SrRuO₃ structure. The temperature of chemical decomposition is first determined for polycrystalline samples under different conditions using thermogravimetry analysis. Then the determined values are used for thin film annealings in high and low oxygen pressure ambients, namely, air, vacuum, and hydrogen. The surface deterioration of the thin film together with changes in its electronic structure is investigated. O1s and Sr3d core lines measured by XPS for as-made samples obviously consist of multiple components indicating different chemical surroundings of atoms. Thanks to different incident beam angle measurements it is possible to distinguish between interior and surface components. Valence band spectra of the interior of the film are consistent with theoretical calculations. After annealing, the ratio of the different components changes drastically. Stoichiometry near the surface changes, mostly due to ruthenium loss (RuO_X) or a segregation process. The width and position of the Ru3p line for as-made samples suggest a mixed oxidation state from metallic to fully oxidized. Long annealing in hydrogen or vacuum ambient leads to a complete reduction of ruthenium to the metallic state. Local conductivity atomic force m- icroscopy scans reveal the presence of nonconductive adsorbates incorporated in the surface region of the film. Charge transport in these measurements shows a tunneling character. Scanning tunneling microscopy scans show some loose and mobile adsorbates on the surface, likely containing hydroxyls. These results suggest that an adequate description of a SrRuO₃ thin film should take into account imperfections and high reactivity of its surface region.