Thermal effects and instability in unipolar resistive switching devices

Metal oxide based resistive switching devices have demonstrated promising characteristics for next-generation nonvolatile memory applications. These devices can be electrically switched between a high-resistance state (HRS, or OFF-state) and a low-resistance state (LRS, or ON-state). The switching from HRS to LRS is called SET and that from LRS to HRS is RESET. Resistive switching may be bipolar (i.e., SET and RESET in opposite bias directions) or unipolar (i.e., SET and RESET in the same bias direction). Unipolar resistive switching devices are more compatible with two-terminal selection devices (e.g., diodes) for 3D stackable crossbar arrays. Although the resistive switching mechanisms are not yet clearly understood, it is generally believed that the switching is caused by mixed ionic/electronic effects involving some defects (e.g., mobile ions, charge traps, oxygen vacancies, etc). The RESET in bipolar resistive switching can be explained by the reversal of the physical process that causes SET. For example, the reverse migration of ions/vacancies during RESET may annihilate the conductive channels formed during SET. Since no such reversal exists in unipolar resistive switching, the RESET process in unipolar switching is often explained by thermal effects. The similarity between SET and RESET operation conditions in unipolar switching devices may cause competition between SET and RESET processes, which reflects as instability in the switching characteristics. This paper presents some experimental evidences supporting the hypothesis of power-induced thermal nature of RESET and analyzes the instability associated with the SET-RESET competition in unipolar resistive switching, using data measured on Cu₂O-based resistive switching devices.