Pairing of cation vacancies and gap-state creation in TiO2 and HfO2

Based on the first-principles calculations, the authors study defect-defect interactions between cation vacancies in rutile TiO2 and monoclinic HfO2. It is found that vacancies are greatly stabilized at small separations because of a large reconstruction of nearby oxygen atoms that have two broken bonds. As a result, O-O bonds resembling O-2 or O-3 molecules are formed near the divacancy site. The defect levels originated from antibonding states of Op orbitals are identified within the energy gap, which can affect leakage currents and the density of trapped charges of oxides substantially. (c) 2007 American Institute of Physics.