Identity of the Conducting Nano-Filaments in TiO2 and Resistance Switching Mechanism of TiO2/NiO Layer

The resistance of TiO2 thin film can be switched reversibly by a voltage pulse, which can be exploited for applications in the next-generation non-volatile memory.[1] It has been argued that the current in the high-conductivity state flows through localized filaments in several oxide materials, but these filaments have been characterized only indirectly, limiting our understanding of the switching mechanism. This study observed and confirmed the conductive nano-filaments directly by highresolution transmission electron microscopy (HRTEM) in a Pt/TiO2/Pt system.[2] The HRTEM images identified the Magneli structure filaments (mostly Ti4O7 phase) of both set and reset states (Fig. 1). The local electrical properties were confirmed by in-situ current-voltage measurements in HRTEM. Electrical conductivity measurement in pad structure sample at low temperatures also confirmed that the overall resistance switching was induced by the formation and disruption of Magneli. The in-situ HRTEM observation in the structural change of filament during set and reset switching were confirmed. These results suggest that the vacancy ordering at the nanometer scale is important for the resistance-switching mechanism. Although filamentary switching is an obvious and useful mechanism for the resistance switching, high power and long time are necessary to achieve the off switching.[3] Therefore, n-type TiO2 and p-type NiO, which are the anode and cathode localized switching materials[4,5], respectively, were stacked, and the resistance switching properties were examined. Interestingly, the location where the filamentary switching occurs was arbitrarily controlled from the interface with metal electrode to the oxide/oxide interface by the proper voltage pulse. When the location of the switching region was within the oxide materials (Fig. 2), the heat loss through the electrode can be minimized so that the off switching requires a much smaller power and shorter time. Especially, the switching time was decreased by ~ 1,000 times compared to the single layer material.(Fig. 3) This study will also provide an in-depth understanding on the filamentary switching mechanisms in various oxide materials.