The importance of singly charged oxygen vacancies for electrical conduction in monoclinic HfO2

Abstract

The point-defect structure of monoclinic HfO2 (m-HfO2) was studied by means of equilibrium electrical conductance measurements as a function of temperature 1050≤T/K≤1200 and oxygen partial pressure −20≤log(pO2/bar)≤−2. The total conductivity σ displayed similar behavior at each temperature examined. In oxidizing conditions (pO2≥10−7bar), the total conductivity increased with increasing oxygen partial pressure and was assigned to hole conduction. Around 10−10 bar, a region of almost constant conductivity was found; this is ascribed to ionic conduction by means of doubly charged oxygen vacancies. In reducing conditions (pO2≤10−16bar), the total conductivity surprisingly decreased with decreasing oxygen partial pressure. Defect-chemical modeling indicates that this behavior is consistent with the conversion of mobile doubly charged oxygen vacancies into less mobile singly charged vacancies by electron trapping. Point-defect concentrations at the oxygen partial pressures relevant to resistive switching devices are predicted and discussed.