Coexistence of unipolar and bipolar resistive switching characteristics in ZnO thin films

This paper reports the coexistence of unipolar and bipolar resistive switching (BRS) in an Ag/ZnO/Pt structure fabricated by sputtering deposition at room temperature. The structure shows reproducible and stable unipolar resistive switching after electroforming with high compliance current $(Ic)=40mA$⁠, regardless of the applied voltage polarity. With low $Ic=5mA$ at electroforming, BRS was also observed; this phenomenon depended on the voltage polarity. Both states were stable and reproducible over 100 cycles. The switching mode was changed with adjusting $Ic$ but the transition was irreversible. Based on the results, switching mechanism based on filament theory is proposed to explain both resistive switching.