Size-Independent Unipolar and Bipolar Resistive Switching Behaviors in ZnO Nanowires

In this work, we report on the mixed bipolar and unipolar resistive switching behavior in ZnO nanowires observed through conductive atomic force microscopy measurements on an individual nanowire. Both bipolar and unipolar resistive switching in ZnO nanowires are size-independent, suggesting that the switching is due to conductive filaments with dimensions much smaller than the cross-section of the nanowire. During bipolar resistive switching, both the low resistance and the high resistance states exhibit ohmic conduction at low voltage, consistent with the filament-based model. During unipolar switching, the low-resistance state is a mix of defect-free ohmic conduction and defect-dominated space-charge-limited conduction, which may not be observed in larger devices. Defects also influence transport in the high resistance state, which exhibits trap-controlled space-charge-limited conduction. Our results clearly demonstrate the key role of defects on the resistive switching behavior of ZnO-nanowires, an important consideration for optimizing the material for nonvolatile memory applications.