Spectroscopic characterization of local valence change processes in resistively switching complex oxides

An increasingly interconnected world creates a high demand for high-density and low-cost data storage. Redox-based memristive devices, which allow switching between high and low electrical resistances through the application of voltages, are highly attractive candidates for next-generation non-volatile memory. But their control and rational design is complicated by poorly understood switching and failure mechanisms.The complex nanoscale redox processes that are suspected to drive so-called resistive switching in these devices remain inadequately characterized. Especially, quantitative information about these processes, which is essential for further advances in the educated design, has been experimentally inaccessible so far.Therefore, spectroscopic tools with high spatial resolution are employed in this work to elucidate both switching and failure mechanism of memristive devices based on the model material SrTiO3. After thorough electrical characterization, two alternative photoelectron emission microscopy approaches are used. As photoemission is a surface sensitive process, the top electrodes of the devices are removed before investigation in the first approach. In the second step, thin graphene electrodes are employed,enabling in operando characterization. In combination with cross sectional, in operando transmission electron microscopy and spectroscopy, a clear evidence of a reversible, localized redox reaction is identified. In the low resistance state, a nanoscale filament in the SrTiO3 is oxygen-deficient, while it is nearly stoichiometric in the high resistance state, resulting in a valence change between Ti3+ and Ti4+. The carrier concentration modulation resulting from this valence change is quantified through comparison with calibration spectra. A carrier concentration change by a factor of two causes two orders of magnitude change in device resistance through a modulation of the effective Schottky barrier at the electrode/oxide interface. The microscopic origin of the polarity of the resistance hysteresis in these devices has long been debated, as it cannot be explained by the typically involved purely internal redistribution of oxygen vacancies. The spectroscopic results of this work reveal that instead, oxygen evolution and reincorporation reactions at the electrode/oxide interface are responsible for the valence change in the SrTiO3. Regarding the failure mechanism, it is found that fast reoxidation frequently results in retention failure in SrTiO3 devices, which can be inhibited by incidental, local phase separations. Mimicking this phase separation by intentionally introducing retention-stabilization-layers with slow oxygen transport is therefore derived as a design rule for retention-failure-resistant devices