Study of a charge transition-driven resistive switching mechanism in TiO2-based random access memory via density functional theory.

The nature of the conducting filament (CF) with a high concentration of oxygen vacancies (VOs) in oxide thin film-based resistive random access memory (RRAM) remains unclear. The VOs in the CF have been assumed to be positively charged (VO2+) to explain the field-driven switching of RRAM, but VO2+ clusters in high concentration encounter Coulomb repulsion, rendering the CF unstable. Therefore, this study examined the oxidation state of VOs 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 VOs in the CF are in a low oxidation state but are transformed to VO2+ immediately after release from the CF. In addition, the short-range interactions between VOs 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 VOs, providing a plausible explanation for the reported coexistence of two opposite bipolar switching polarities: the eight-wise and the counter-eight-wise polarities.

Reversible transition between the polar and antipolar phases and its implications for wake-up and fatigue in HfO2-based ferroelectric thin film.

Atomic-resolution Cs-corrected scanning transmission electron microscopy revealed local shifting of two oxygen positions (OI and OII) within the unit cells of a ferroelectric (Hf0.5Zr0.5)O2 thin film. A reversible transition between the polar Pbc21 and antipolar Pbca phases, where the crystal structures of the 180° domain wall of the Pbc21 phase and the unit cell structure of the Pbca phase were identical, was induced by applying appropriate cycling voltages. The critical field strength that determined whether the film would be woken up or fatigued was ~0.8 MV/cm, above or below which wake-up or fatigue was observed, respectively. Repeated cycling with sufficiently high voltages led to development of the interfacial nonpolar P42/nmc phase, which induced fatigue through the depolarizing field effect. The fatigued film could be rejuvenated by applying a slightly higher voltage, indicating that these transitions were reversible. These mechanisms are radically different from those of conventional ferroelectrics.