Abstract

We investigated the resistive switching behavior of WO x films. WO x was obtained from the thermal oxidation of W thin layers. The parameters under investigation were the influence of the temperature (450-500°C) and time (30-220 s) used to obtain the WO x on the resistive switching characteristics of Si\W\WO x\Metal-electrode ReRAM cells. The metal top electrodes (TE) tested were Pt, Ni, Cu and Au. The elemental composition and microstructure of the samples were characterized by means of elastic recoil detection analysis (ERD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray reflectivity (XRR). Electrical measurement of the WO x-based memory elements revealed bipolar and unipolar switching and this depended upon the oxidation conditions and TE selected. Indeed, switching events were observed in WO x samples obtained either at 450°C or 500°C in time windows of 180-200 s and 30-60 s, respectively. Pt and Au TE promoted bipolar switching while unipolar behavior was observed with Ni TE only; no switching events were observed with Cu TE. Good switching characteristics seems not related to the overall thickness, crystallinity and composition of the oxide, but on the W 6+/W 5+ ratio present on the WO x surface, surface in contact with the TE material. Interestingly, W 6+/W 5+ ratio can be tuned through the oxidation conditions, showing a path for optimizing the properties of the WO x-based ReRAM cells. © 2011 Materials Research Society.