Ultralow voltage resistive switching characteristics of HfOx/NiOx stacks

Abstract

HfOx/NiOx stacks composed of polycrystalline HfOx and ⟨111⟩/⟨100⟩-orientation preferred NiOx films were prepared by magnetron sputtering. Triangular pyramid-shaped NiOx⟨111⟩ grains assemble into the dense but rough NiOx(111) film, while square columnar NiOx⟨100⟩ grains make up the smooth NiOx(100) film. Compared with the NiOx films, HfOx/NiOx stacks exhibit significantly improved bipolar resistive switching (RS) characteristics. NiOx(100)/HfOx stacks show a ratio of the high resistance to the low resistance (RH/RL) varying between 7 × 102 and 9 × 103, and the optimized HfOx/NiOx(111) stack exhibits a high RH/RL ratio of ∼2 × 105 at a ultralow switching voltage of <±0.2 V. Both current–voltage curves in the high and low resistance states follow the relationship of ohmic conduction. It is believed that the HfOx film works as a buffer matrix to depress the leakage current and block the diffusion of Ag ions, while Ag ions diffused into the HfOx matrix act as oxygen reservoirs to store or release oxygen ions, which promotes the formation and rupture of oxygen-vacancy conductive filaments in the NiOx film, resulting in ultralow voltage RS behaviors.