For this study, we fabricated 100 nm x 100 nm size nano-crossbar devices built from a Pt bottom electrode, a 3- to 5-nm-thick TiO2 layer grown by thermal atomic layer deposition and a 10 nm thick Ti top electrode covered by Pt. Metal layers were grown by e-beam evaporation. Electroforming and subsequent reset at moderate voltages initiate a filamentary-type bipolar resistive switching behavior with counter-eightwise (c8w) switching direction. This means that an abrupt set event from the c8w high resistance state (HRS) to the c8w low resistance state (LRS) is observed for negative voltages applied to the Pt Schottky electrode. When the positive voltage signal applied to the Pt electrode is increased, the switching direction changes towards eightwise (8w). This means that the cell sets to conducting low resistance state (8w-LRS*) for positive voltages applied to the Pt electrode, and resets to an insulating (8w) high resistance state (HRS*). The c8w- and 8w- switching hysteresis share the c8w-HRS and the 8w-LRS* states.
In this presentation, we will characterize the different stable states using the temperature dependence of the measured current-voltage characteristics. Based on recent findings for the 8w-switching Nb:STO/STO/Pt system  we will discuss arguments for an interplay of drift/diffusion and oxygen exchange reactions that can explain the different switching events happening at the same Pt/TiO2 interface.
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