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

The point-defect structure of monoclinic HfO 2 (m-HfO 2) was studied by means of equilibrium electrical conductance measurements as a function of temperature 1050 ≤ T / K ≤ 1200 and oxygen partial pressure - 20 ≤ log (p O 2 / bar) ≤ - 2. The total conductivity σ displayed similar behavior at each temperature examined. In oxidizing conditions (p O 2 ≥ 10 - 7 bar), 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 (p O 2 ≤ 10 - 16 bar), 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.