Charge Transition of Oxygen Vacancies during Resistive Switching in Oxide-based Memristors

Resistive switching (RS) memristors find applications in non-volatile memory and neuromorphic computing. RS in oxide memristors originates from the redistribution of oxygen vacancies (V$_{\mathrm{O}}$s) to form conducting filaments of aggregated V$_{\mathrm{O}}$s. Since the ion migration is facilitated by an applied electric field, V$_{\mathrm{O}}$s in RS are considered positively charged (V$_{\mathrm{O}}^{\mathrm{2+}})$. We performed density function theory calculations to study the interactions between neutral and charged V$_{\mathrm{O}}$s in amorphous Ta$_{\mathrm{2}}$O$_{\mathrm{5}}$. The cohesive energy between charged V$_{\mathrm{O}}$s is strongly repulsive at short range, contradicting the experimentally observed high V$_{\mathrm{O}}$ concentration in filaments. On the other hand, neutral V$_{\mathrm{O}}$s exhibit a short-range attraction that facilitates aggregation, but their charge neutrality precludes interactions with an electric field. We propose a series of charge-transition processes at work during RS that enable V$_{\mathrm{O}}$ drift and aggregation. We experimentally support the proposed model with electrical measurements under visible-light illumination that induce charge transitions of V$_{\mathrm{O}}$s and enhance the programming and erasing processes. Our results provide microscopic understanding of the RS mechanism and the effect of light on ions/defects migration.