Oxidation state and interfacial effects on oxygen vacancies in tantalum pentoxide

First-principles density-functional theory calculations are used to study the atomistic structure, structural energetics, and electron density near the O monovacancy (V-O(n); n = 0,1+, 2+) in both bulk, amorphous tantalum pentoxide (a-Ta2O5), and also at vacuum and metallic Ta interfaces. We calculate multivariate vacancy formation energies to evaluate stability as a function of oxidation state, distance from interface plane, and Fermi energy. V-O(n) of all oxidation states preferentially segregates at both Ta and vacuum interfaces, where the metallic interface exhibits global formation energy minima. In a-Ta2O5, V-O(0) is characterized by structural contraction and electron density localization, while V-O(2+) promotes structural expansion and is depleted of electron density. In contrast, interfacial V-O(0) and V-O(2+) show nearly indistinguishable ionic and electronic signatures indicative of a reduced V-O center. Interfacial V-O(2+) extracts electron density from metallic Ta, indicating that V-O(2+) is spontaneously reduced at the expense of the metal. This oxidation/reduction behavior suggests careful selection and processing of both oxide layer and metal electrodes for engineering memristor device operation. (C) 2015 AIP Publishing LLC.