Multi-valued resistive switching characteristics in WO_x/AlO_y heterojunction resistive switching memories

The multi-valued resistive switching (RS) characteristics and mechanism in a WO_x/AlO_y heterojunction-based nonvolatile memory have been investigated. Although Al/WO_x/Pt and Al/AlO_y/Pt show different RS characteristics, bipolar and unipolar modes, respectively, the Al/WO_x/AlO_y/Pt structure shows both bipolar and unipolar RS characteristics, depending on the magnitude of the applied voltage. Three resistance states, which are defined as the low-resistance state (LRS), the mid-high-resistance state (mid-HRS) and the high-resistance state (HRS), are observed. A model combining two different RS mechanisms, oxygen ions involving the redox process at the interface and filamentary conduction, is proposed, and provides a simple physical concept to understand the multi-valued RS behavior. Our findings will be useful to design and optimize oxide heterojunction-based non-volatile RS memory devices.