(Invited) Impact of Stoichiometry and Interface Configuration on the Time Stability and the Speed-Limiting Step in Memristive SrTiO3 Cells

Abstract

Resistive switching oxides are promising candidates for future non-volatile memory and for functional units of neuromorphic computing. Epitaxial SrTiO3 thin films can be regarded as model system for resistive switching thin films due to their well-known defect chemistry and the absence of grain boundaries. We have recently shown that so-called eightwise switching in SrTiO3 can be attributed to the release and reincorporation oxygen at the interface to a Pt electrode [1,2]. In the light of this knowledge, we studied in detail the impact of the Sr/Ti stoichiometry on the filament formation process, the filament stability [3] and the switching kinetics. We observed that Sr rich thin films exhibit an improved retention of the low resistive state (LRS) at low current compliance values with respect to the stoichiometric thin films. In order to identify the underlying processes, we investigated the filament formation process of SrTiO3 thin films with different stoichiometry by photoelectron emission microscopy. Stoichiometric thin films form a stable LRS state as soon as the current compliance is sufficient to trigger SrO segregation at the surface [4]. We attribute this to the impeded reoxidation of the oxygen deficient filament by the presence of the SrO at the surface. We could mimic this effect by intentionally depositing additional SrO, or other oxygen blocking layers such as Al2O3 on the surface of stoichiometric thin films and were thereby able to improve the memory window as well as the LRS retention. We furthermore investigated the switching kinetics of the SET process for different Sr/Ti stoichiometries and interface layers. We compared these results to analytical models for the switching kinetics based on ion movement, modeled by the Mott-Gurney-Law, and an oxygen exchange reaction at the interface, modeled by the Butler-Volmer-Equation, as the rate-determining steps in combination with simulated temperature and electrical potential data from finite element simulations, as shown in Figures 1 ((a) stoichiometric, (b) Sr rich). For stoichiometric SrTiO3 cells, oxygen transfer at the oxide-metal interface is determined as the rate-determining step. In contrast, for Sr-rich STO cells we propose oxygen diffusion within the SrO layer at the metal-STO interface as rate-limiting. We will discuss possible reasons for the different rate-determining steps, such as different vacancy concentrations and exchange probabilities.