The surface hydrooxidation of epitaxial LaNiO3-x thin films
Mickevicius, Sigitas1; Bondarenka, Vladimiras1; Grebinskij, Sergej1; Lisauskas, Vaclovas1; Sliuziene, Kristina1; Vengalis, Bonifacijus1; Orlowski, Bronislaw2; Osinniy, Wiktor2; Drube, Wolfgang3
1Lithuania;
2Poland;
3Germany

The surface segregation of elements and possible hydrooxidation of epitaxial LaNiO3-x thin films were studied by means of the volume sensitivity of Tunable High-Energy X-ray photoelectron spectroscopy at the X-ray wiggler beam line BW2 of HASYLAB (Hamburg). Epitaxial LaNiO3-x films deposited onto (100)-plane oriented NdGdO3 substrate was obtained by using a reactive DC magnetron sputtering technique. Films demonstrate the excellent in-plane orientation and the surface La/Ni ratio close to the bulk stoichiometric value. Both core and satellite peaks were used for the identification of the chemical state of elements and further quantitative analysis. The films were examined at different angles to distinguish between the chemical state of lanthanum and nickel at the surface and slightly deeper into the material. The evident difference were observed between spectra measured at normal (Θ = 0°) and grazing (Θ = 89.3°) take-off angles, indicating that the significant alteration of the film chemical composition take place within thin (scanning depth ~ 3 nm) surface layer.
It was shown, that the hydroxyl-containing phase, located near the film surface may be attributed to the lanthanum and nickel hydroxide species. For the all spectral lines in question hydroxide/oxide peaks intensity ratio R strongly affected by the geometry of the experiment, thus directly indicating that the thickness of the hydroxide enriched layer is of about few nanometers.
The more quantitative analyses were carried out assuming the exponential and/or step spatial distributions of hydroxide species concentration within the bulk of the film. The thickness of this hydroxide enriched layer, estimated from the oxide and hydroxide peak intensities and taking into account that the lattice constant for LaNiO3 ~ 0.35 nm is of about 2 nm.
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