Analog and bipolar resistive switching in pn junction of n-type ZnO nanowires on p-type Si substrate

Abstract

A pn junction consisting of n-type ZnO nanowires (NWs) on p-type Si substrate exhibits analog resistive switching dependent on the polarity of applied voltage before forming operation for the bipolar switching. The current-voltage curves of Ti/ZnO-NWs/ZnO-seed-layer/p+-Si substrate show diode characteristics with hysteresis in the reverse bias condition, presenting a gradually increasing and then saturated current with repeated voltage sweeps. The current is then further increased with sweeping –V and decreased during the subsequent +V sweep. This polarity-dependent analog switching remains the same during pulse measurement. The analog switching is thought to originate from gradual redistribution of oxygen vacancies, trapping and detrapping of charges in the ZnO NWs, which modulate the depletion width and space charge density. Consequently, the resistance of the pn junction is changed in an analog fashion. After the forming operation, bipolar switching is observed with a transition from high to low resistance states (SET) at +V and reverse transition (RESET) at –V, originating from the formation and rupture of filaments. These results demonstrate multiple features of the ZnO NWs based pn junction, including diode characteristics, analog-type resistive switching before forming operation, and digital-type bipolar switching after forming.