Conductive Atomic Force Microscopy Investigation of Switching Thresholds in Titanium Dioxide Thin Films

Titanium dioxide thin films have attracted increasing attention due to their potential in next-generation memory devices. Of particular interest are applications in resistive random access memory (RRAM) devices, where such thin films are used as active layers in metal-insulator-metal (MIM) configurations. When these devices receive a bias above a certain threshold voltage, they exhibit resistive switching (RS), that is, the resistance of the oxide thin film can be tuned between a high resistive state (HRS) and a low resistive state (LRS). In the context of this work, we have used conductive atomic force microscopy (C-AFM) to identify the resistive switching thresholds of titanium dioxide thin films deposited on Si/SiO2/Ti/Pt stacks to be used in memory devices. By performing a set of reading/writing voltage scans over pristine areas of the thin films, we have identified the critical thresholds, which define a reversible operation (soft-breakdown, SB) via localized changes in electrical resistance across the film and an irreversible operation (hard-breakdown, HB) that includes both changes in local electrical resistance and thin film topography. We have also assessed the transition from SB to HB when thin films are stimulated repeatedly with potentials below the identified onsets of HB, validating a history dependent behavior. This study is therefore aimed at presenting new insights in RRAM device programmability, reliability, and eventually failure mechanisms.