Effect of valence electrons on the core level x-ray photoelectron spectra of niobium oxide thin films prepared by molecular beam epitaxy

X-ray photoelectron spectroscopy (XPS) is a versatile tool for identifying the chemical and electronic state of an element. However, the presence of various initial- and final-state effects makes the interpretation of XPS spectra tricky and erroneous. Metal oxides show multipeak XPS spectra when the charge carriers are coupled with the core hole of the photoion after photoemission. The extra peaks are often misconstrued to originate from other oxidation states of the cation. With a systematic approach to the analysis of Nb 3d XPS spectra of partially oxidized NbOx thin films prepared by molecular beam epitaxy, we found that the valence band electrons close to the Fermi level contribute to a satellite peak in Nb-oxides. Insulating Nb-oxide containing Nb5+ (d0) with no or fewer electrons near the Fermi level has no such peaks; on the other hand, Nb5+ containing thin films exhibit this satellite peak when sufficient surface charge carriers contribute to the core hole-electron coupling.