First nucleation steps of vanadium oxide thin films studied by XPS inelastic peak shape analysis

The initial states of deposition of vanadium oxide thin films have been studied by analysis of the peak shape (both inelastic background and elastic contributions) of X-ray photoemission spectra (XPS) after successive deposition experiments. This study has permitted to assess the type of nucleation and growth mechanisms of the films. The experiments have been carried out in situ in the preparation chamber of a XPS spectrometer. Thin films of vanadium oxide have been prepared on Al2O3 and TiO2 by means of thermal evaporation, ion beam assisted deposition and plasma enhanced chemical vapour deposition. The thin films prepared by the first two procedures consisted of V2O4, while those prepared by the latter had a V2O5 stoichiometry. The analysis of the inelastic background of the photoemission spectra has shown that the films prepared by thermal evaporation on Al2O3 are formed by big particles that only cover completely the surface of the substrate when their height reaches 16 nm. By contrast, the thin films prepared with assistance of ions on Al2O3 or with plasma on TiO2 consist of smaller particles that succeed in covering the substrate surface already for a height of approximately 4 nm. Thin films prepared by plasma-assisted deposition on Al2O3 depict an intermediate situation where the substrate is completely covered when the particles have a height of approximately 6 nm. The type of substrates, differences in the deposition procedure or the activation of the adatoms by ion bombardment are some of the factors that are accounted for by to explain the different observed behaviours.