Origin of attendant phenomena of bipolar resistive switching and negative differential resistance in SrTiO3:Nb/ZnO heterojunctions

Epitaxial ZnO thin films were grown on SrTiO3:Nb (NSTO) substrates by pulsed laser deposition. The NSTO/ZnO heterojunctions exhibit a typical rectification characteristic under a small voltage, while two attendant behaviors of bipolar resistive switching and negative differential resistance appear under a large voltage. The NSTO/ZnO heterojunctions show extremely weak resistance switching hysteresis without applying a forward bias. However, when the forward bias increases to some extent, the hysteresis becomes more and more prominent and negative differential resistance gradually appears. Furthermore, the high resistance state is obtained when sweeping from negative to positive voltage bias, and vice versa. We propose a model for these behaviors at NSTO/ZnO interface, in which the space charge region in ZnO is wide in high resistance state when the interface state is unoccupied, while the space charge region becomes narrower in low resistance state due to Fermi pinning when the interface state is completely occupied, and the low resistance state is remained until electrons are detrapped from the interface state.