Electronic structure of stoichiometric and Ar+-bombarded ZrO2 determined by resonant photoemission.

Abstract

The electronic properties of thermally grown ${\mathrm{ZrO}}_{2}$ thin films before and after ${\mathrm{Ar}}^{+}$ bombardment have been studied with resonant photoemission spectroscopy using synchrotron radiation. For stoichiometric ${\mathrm{ZrO}}_{2}$ thin films the experimental valence-band spectra are in good agreement with the calculated density of states for bulk ${\mathrm{ZrO}}_{2}$. For both stoichiometric and ${\mathrm{Ar}}^{+}$-bombarded ${\mathrm{ZrO}}_{2}$ thin films, resonant photoemission from the valence band was observed when the photon energy was swept through the Zr 4p\ensuremath{\rightarrow}4d transition energy. The resonant profile was found to exhibit a maximum at h\ensuremath{\nu}=39 eV, followed by a second well-resolved broad maximum around 50 eV. The feature at 39 eV is consistent with resonant enhancement of the Zr 4d states and has been used to identify those regions of the valence band with an important Zr 4d admixture. The results are in good agreement with the calculated Zr 4d partial density of states. The intensity increase observed at h\ensuremath{\nu}\ensuremath{\sim}45--50 eV is found to be associated with the nonbonding region of the valence band, although a proper interpretation is needed. In addition, it was found that ${\mathrm{Ar}}^{+}$ bombardment induces electronic states in the band-gap region and changes in the O 2p valence band. Three distinct emission bands were identified in the band gap as a function of the ${\mathrm{Ar}}^{+}$ dose. They are associated with the formation of oxygen vacancies and mixed oxidation states due to preferential sputtering of the oxygen atoms. Resonant photoemission of these ${\mathrm{Ar}}^{+}$-bombarded films demonstrates both the cationic character of the band-gap states and the increase of the cationic contribution to the O 2p valence band.