Defect state features were detected in second derivative 0K edge spectra for thin films of nano-crystalline TiO2 and HfO2. Based on soft X-ray photo-electron band edge spectra, and the occurrence of occupied band edge 4f states in Gd(Sc,Ti)O3, complementary spectroscopic features were confirmed in the pre-edge (<530eV) and vacuum continuum (>545eV) regimes of 0K edge spectra. Qualitatively similar spectral features were obtained for thin films of HfO2 and TiO2, and these were assigned to defect states associated with vacancies. The two electrons/removed O-atom were not distributed uniformly over the TM atoms defining the vacancy geometry, but instead were localized in equivalent d-states: a d2 state for a Ti monovacancy and a d4 state for a Hf divacancy. This new model for electronic structure provides an unambiguous way to differentiate between monovacancy and divacancy arrangements, as well as immobile (or fixed) and mobile vacancies.

Spectroscopic Differentiation between O-Atom Vacancy and Divacancy Defects, Respectively, in TiO2 and HfO2 by X-Ray Absorption Spectroscopy. Lucovsky, G., Chung, K.B., Kim, J.W., Norlund, D.: Microelectronic Engineering, 2009, 86[7-9], 1676-9