Unique behavior of F-centers in high-k Hf-based oxides

Physical properties of F-center in HfO2, have been investigated by first-principles calculations. The F-centers (doubly occupied oxygen vacancy V-o) are found to generate gap states at 1.05 eV below the bottom of HfO, conduction band and they are remarkably elevated by extracting electrons from the F-center. The negative-U behavior of F-centers in HFO2 is very different from those in the ionic rocksalt oxide such as MgO or CaO, which possesses positive-U character. The reason for this unique behavior of F-centers in HfO2 is considered to be due to highly positively ionized H& ions, which are largely relaxed outward away from singly occupied or non-occupied F-centers. This outward relaxation is the origin of the remarkable elevations of the F-center levels. Moreover, similar negative-U behavior is also observed in HfON. (c) 2005 Elsevier B.V. All rights reserved.