Temperature-dependence of resistive switching behaviors and tunneling mechanism of polycrystalline NiO<sup><italic>x</italic></sup> films

Abstract

Resistive switching random access memory (ReRAM) based on resistive switching of oxide films is one of potential candidates for next generation of nonvolatile memory due to its simple structure, high writing/read speed and low power consumption. NiO x thin film is one of oxide materials early observed resistive switching behaviors and the most promising materials for the application of commercial oxide resistance memory. However, NiO x thin films present diverse resistive switching behaviors which are heavily dependent on fabrication technologies, methods and conditions. From 1960s, researchers have focused on optimizations of device structure and improvements on switching performance parameters of NiO x -based ReRAMs at room temperature. On the contrary, there is few discussions on temperature-dependence of resistive switching behaviors of NiO x thin films. In this paper, temperature-dependence of resistive switching behaviors and tunneling mechanism of polycrystalline NiO x films have been discussed. 120 nm-thick NiO x films have been deposited at 280°C on Pt-covered Si(100) substrates in 0.5 Pa Ar+O2 mixed ambient by using radio frequency magnetron sputtering with a working power of 60 W. Scanning electron microscope images reveal the as-deposited film exhibits a dense and smooth surface morphology. X-ray diffraction patterns and X-ray photoelectron spectra reveal it is polycrystalline and oxygen-riched NiO x . After being covered by a 150 nm-thick Pt top electrode with a diameter of 0.2 mm, a Ag/NiO x /Pt memory cell is obtained. Its current-voltage ( I - V ) curves exhibit threshold-type resistive switching characteristics: under the positive bias ranging from 2.1 to 2.4 V and the negative bias from - 2 to - 2.2 V, one can see stable and reversible switching between the high and low resistance state. With the increase of measurement temperature, the I - V loop within the negative bias range vanishes at 140°C, while it maintains up to 270°C within the positive bias range. Power-law fitting lines for the I - V curves of memory cell located on the high/low resistance state, which is measured at room temperature (RT), demonstrate the tunneling current is dominated by defect-assistant space-charge-limited current. On the other hand, Arrhenius form (ln I vs.1/ T ) plots for the I - V curve under a positively biased condition measured at the different temperatures ranging from RT to 140°C are linear. It reveals that the variation of leakage current with measurement temperature can be attributed to Schottky tunneling mechanism. Driven by positive electric field force, Ag ions drift from Ag top electrodes into the NiO x film, then are reduced to Ag atoms near Pt bottom electrodes. After applying with periodic electric field, the forming and dissolution of conductive filaments composed by Ag atoms result in the threshold-type resistive switching behaviors of Ag/NiO x /Pt memory cells.