Relationship between Ta Oxidation State and Its Local Atomic Coordination Symmetry in a Wide Range of Oxygen Nonstoichiometry Extent of TaOx

Evolution of electronic and atomic structure of amorphous Ta2O5 during sputtering by Ar+ ions was investigated by means of X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. XPS Ta 4f spectra were analyzed through spectral decomposition by paying special attention to inelastic scattered electron background subtraction from the Ta 4f spectra. The decomposition revealed the formation of Ta4+, Ta3+, Ta2+, Ta+, and Ta-0 chemical states during the sputtering. The dynamics of the Ta chemical states evolution was analyzed and referred to the modification of the local atomic structure revealed by the NEXAFS O K-edge absorption spectra analysis. The transformation of the O K-edge spectra during the sputtering suggests the preservation of a significant part of octahedrons (the structural units of stoichiometric amorphous Ta2O5), which low-symmetry transformed from the octahedra during the sputtering. Possible mechanisms of the octahedra transformation were discussed based on the O K-edge spectra and orbital correlation diagrams. It is noteworthy that the experiment showed a possibility of complete Ta metallization by Ar+ ion bombardment of amorphous Ta2O5 on the Si substrate.