Study of a charge transition-driven resistive switching mechanism in TiO ₂ -based random access memory via density functional theory

The nature of the conducting filament (CF) with a high concentration of oxygen vacancies (V_Os) in oxide thin film-based resistive random access memory (RRAM) remains unclear. The V_Os in the CF have been assumed to be positively charged (V_O²⁺) to explain the field-driven switching of RRAM, but V_O²⁺ clusters in high concentration encounter Coulomb repulsion, rendering the CF unstable. Therefore, this study examined the oxidation state of V_Os in the CF and their effects on the switching behavior via density functional theory calculations using a Pt/TiO₂/Ti model system. It was concluded that the V_Os in the CF are in a low oxidation state but are transformed to V_O²⁺ immediately after release from the CF. In addition, the short-range interactions between V_Os were confirmed to facilitate the rupture and rejuvenation of the CF by reducing the required activation energy. Finally, an improved switching model was proposed by considering the charge transition of V_Os, providing a plausible explanation for the reported coexistence of two opposite bipolar switching polarities: the eight-wise and the counter-eight-wise polarities.