Electronic structure of stoichiometric and reducedTa2O5surfaces determined by resonant photoemission

Abstract

The electronic properties of stoichiometric and reduced ${\mathrm{Ta}}_{2}{\mathrm{O}}_{5}$ thin films have been studied with resonant photoemission using synchrotron radiation. It was found that ${\mathrm{Ar}}^{+}$ bombardment induces electronic states in the band gap region and an attenuation of the overall valence band. These changes are associated with the formation of oxygen vacancies during the reduction of the surface by ${\mathrm{Ar}}^{+}$ bombardment. For both stoichiometric and ${\mathrm{Ar}}^{+}$-bombarded ${\mathrm{Ta}}_{2}{\mathrm{O}}_{5}$ films, resonant photoemission from the valence band was observed when the photon energy was in the neighborhood of the Ta $5\stackrel{\ensuremath{\rightarrow}}{p}5d,$ $5\stackrel{\ensuremath{\rightarrow}}{p}6sp,$ and $4\stackrel{\ensuremath{\rightarrow}}{f}5d$ transition energies. The constant initial-state curves show multiple resonance maxima that are explained in terms of the Ta $5\stackrel{\ensuremath{\rightarrow}}{p}5d,$ $5\stackrel{\ensuremath{\rightarrow}}{p}6sp,$ and $4\stackrel{\ensuremath{\rightarrow}}{f}5d$ photoabsorption mechanisms, the spin-orbit splitting of the Ta $5p$ and Ta $4f$ core levels, and the splitting of the Ta $5d$ final states by crystal-field interactions. Contrarily to $3d$ and $4d$ transition metals no differences have been found in the resonance behavior of the valence-band features lying in the $\ensuremath{\sim}4--10\ensuremath{-}\mathrm{eV}$ range. This is attributed to the more extended nature of Ta $5d$ and $6sp$ orbitals with respect to $3d$ and $4d$ cationic orbitals.