Redox-based resistive switching random access memory (ReRAM) has the potential to fulfill the demands of future Green IT solutions for non-volatile memory circuits with high-speed data access, high reliability, and low power consumption. A single valence-change-mechanism (VCM)-type switching cell is typically built from a transition metal oxide layer sandwiched between metal electrodes of different oxidation enthalpy. It is accepted in the scientific community that oxygen exchange and drift/diffusion processes play a major role in the resistive switching process. Recently, the importance of oxygen exchange reactions has been demonstrated for Ta₂O₅ based devices,[1] and several studies reported on the coexistence of switching events of opposite polarity in cells built from heteroepitaxial SrTiO₃ films.[2,3] Among various explanations for such diverse switching behaviors, the inhomogeneous dopant distribution was suggested as the main source.[4] For technically relevant devices based on amorphous metal oxide layers, a comparable phenomenon but with partly volatile resistance states was reported for symmetric Pt/TiO₂/Pt cells.[5]

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 [6] 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.

[1] W. Kim, S. Menzel, D. J. Wouters, Y. Guo, J. Robertson, B. Rösgen, R. Waser, V. Rana, Nanoscale 8, 17774-17781 (2016)

[2] R. Muenstermann, T. Menke, R. Dittmann, R. Waser, Adv. Mater. 22, 4819-4822 (2010)

[3] M. Kubicek, R. Schmitt, F. Messerschmitt, J. L. M. Rupp, ACS Nano 9, 10737-10748 (2015)

[4] J. S. Lee, S. B. Lee, B. Kahng, T. W. Noh, Appl. Phys. Lett. 102, 253503/1-4 (2013)

[5] F. Miao, J J. Yang, J. Borghetti, G. Medeiros-Ribeiro, R. S. Williams, Nanotechnology 22, 254007 (2011)

[6] Cooper, C. Baeumer, N. Bernier, A. Marchewka, C. La Torre, R. E. Dunin-Borkowski, S. Menzel, R. Waser, R. Dittmann, Adv. Mater., 1700212 (2017)