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We investigated the electronic aspect of oxygen vacancy in hafnium oxide film, using hard and soft x-ray absorption spectroscopies (XAS). We focused on the change in the physical and chemical properties of the oxide in the presence of the oxygen vacancy. A 2nm-thick hafnium oxide film was grown on a clean Si(100) surface by means of pulsed laser deposition (PLD) technique. The oxygen deficiency of the oxide was about 10% (i.e. HfO~1.8). By analyzing the modulation in the absorption spectra above the Hf L3-edge energy(h> 9.5keV), the local characteristics around the Hf atom, such as the bond length or the number of nearest neighbors are scrutinized; extended x-ray absorption fine structures (EXAFS). In the presence of oxygen vacancy, the average Hf-O bond length is found to reduce more than -0.1 Å as compared with that of the stoichiometric hafnia. This reduction in bond length is interpreted as a relaxation due to the formation of the oxygen vacancy; the loss of attractive Madelung interaction between Hf and O atoms causes a migration of the nearest Hf atoms toward the remaining oxygen atoms, so that the average Hf-O bond length will reduce. Thus the effect of the oxygen vacancy in this system is strong enough to influence the electronic structure of the system. In practice, the unoccupied electronic structure of the oxygen-deficient oxide, deduced from the O K-edge (hν~530eV) XAS spectra, shows a band tail state beneath the genuine Hf 5d conduction band. This band tail might be responsible for lifting of the fermi-level as well as a small occupation in Hf d shell. However, it does not involve any long-range spin correlation in the oxide; this is confirmed by the null signal in the Hf N3-edge (hν~380eV) magnetic circular dichroism (MCD) measurement. This study concerns the role of the oxygen vacancy rather than the detailed band structure of the oxide itself, so this spectroscopic approach could be extended to other large bandgap oxides. |