Coexistence of unipolar and bipolar resistive switching in Pt/NiO/Pt

We demonstrate the coexistence of unipolar and bipolar resistive switching in NiO thin film fabricated by reactive sputtering. It was shown that the required voltages for switching operations in the bipolar switching mode are smaller than the voltages in the unipolar mode, while the ON/OFF resistance levels are similar for the two modes. The NiO resistive switching device also exhibited transitions between the two switching modes, with the subsequent I-V characteristics unaffected by the switching history, which clearly indicates that the two switching mechanisms are directly related. The switching versatility in NiO is expected to facilitate improved device performance, for example, by increasing the margin of programming voltages. Based on the investigation of device characteristics, we discuss the switching processes for the two switching modes and their relationship.