Thickness dependence of the resistive switching behavior of nonvolatile memory device structures based on undoped ZnO films

The resistive switching behavior of Al/ZnO/Al layered memory device structures was investigated in connection with varying ZnO layer thickness and related changes in crystallinity and concentration of oxygen-related defects. It was observed that, with increasing thickness, the crystallinity of the ZnO layer was improved and the concentration of oxygen-related defects within the layer increased. While the device showed unipolar switching characteristics, the current–voltage hysteresis was dependent on the thickness of the ZnO layer. In particular, the set voltage gradually increased with increasing layer thickness in the high resistive state whereas the reset voltage remained almost constant in the low resistive state. The observed operation characteristics of the device structures in relation to the crystallinity and oxygen-related defect concentration of the ZnO layer suggest that extended defects such as grain boundaries and dislocations play important roles in determining device performances.