Visualization of the conductive channel in a planar resistance switching device based on electrochromic materials

Abstract

In this work, bipolar resistance switching behavior was realized in an Au/tungsten oxide/Au planar device, and the evolution of the conductive channel during resistance switching was successfully visualized by the in situ optical image technique based on the color-conductivity dependence of tungsten oxide. We found that there are two types of conductive channel, named parabolic channel and bar-like channel, exist in the planar device. The parabolic channel formed firstly near the cathode and then extended to but could not touch the anode. By applying opposite electric-field, the bar-like channel formed from the cathode (i.e., foregoing anode) and extended to the parabolic channel. With alternating the external electric-field polarity, the bar-like channel showed an indirect connection and nonmonotonic disconnection with the parabolic channel at the region near the foregoing anode, corresponding to the high-to-low and low-to-high resistance switching processes of the planar device, respectively. The instable RS behavior was caused by the change of bar-like channel occurring position under the high external field condition. The conductive channel formation was ascribed to the sodium ion immersion from the soda-lime glass substrate into the tungsten oxide film and then migration driven by the electric field to form sodium tungsten bronze. These results will give some insight into the resistance switching property improvement and mechanism elucidation as well as a possibility to develop electric/optical-coupled switch and data storage devices.