Abstract— Bipolar switching is reported for the first time using solution deposited amorphous zinc-tin-oxide (ZTO). The impact of compliance current (CC) on the SET voltage, the magnitude of the low and high resistance states, and the switching ratio is investigated for Al/ZTO/Ir resistive random access memory (RRAM) devices. I. INTRODUCTION ESISTIVE random access memories (RRAM) (also known as memristors) have attracted attention for next- generation non-volatile memory devices and are being considered as a potential replacement for flash due to their simple structure, rapid program/erase speed, and low power consumption [1]. A resistive switching memory cell can be electrically programmed between at least two resistive states, where the cell can be switched from the high resistance state (HRS) to low resistance state (LRS) in the SET process and switched from LRS to HRS in the RESET process. Both unipolar and bipolar switching has been observed for RRAM devices. In the case of unipolar switching the polarities of the external voltage for the reset and set processes are identical, while for bipolar switching the polarities of the external voltage for the reset and set processes are opposite. Unipolar switching may be considered to be less reliable due to issues related to the high operating and compliance currents that are required for the forming process. It has been demonstrated that a large assortment of materials can exhibit resistive switching including TiO2 [1], NiO [2], ZrO 2 [3], ZnO [4], Pr 0.7 Ca 0.3 MnO 3 [5], and many others [6]. There are a number of physical phenomena that can drive resistive switching however detailed understandings of the switching mechanisms are not well understood. One general model that has been adopted to Manuscript received June 30, 2011. This work was partially supported by the Office of Naval Research (ONR) Contract Number 200CAR262, the Oregon Nanoscience and Microtechnologies Institute (ONAMI), and the National Science Foundation through DMR-0805372. Bipolar Resistive Switching in Zinc-Tin-Oxide Resistive Random Access Memory. S. Murali and J. F. Conley, Jr. are with the School of Electrical Engineering and Computer Science, Oregon State University, Corvallis, OR 97331 USA. (Phone: 541-224-2312; e-mail:muralis@onid.orst.edu, jconley@eecs.oregonstate.edu). J. S. Rajachidambaram, S-Y. Han, C.-H. Chang, and G. S. Herman are with the School of Chemical, Biological and Environmental Engineering, Oregon State University, Corvallis, OR 97331 USA. (Email: rajachij@onid.orst.edu, hanse@engr.orst.edu, chih- hung.chang@oregonstate.edu, greg.herman@oregonstate.edu) explain resistive switching in insulating oxide films is the formation and rupture of conductive filaments [7]. The control of filament growth can be achieved by setting the compliance current (CC) where optimization has been shown to assist in obtaining stable and reliable resistive switching behavior [8]. There has been recent interest in evaluating new materials that have dual functionality as the active switching material as well as the active semiconductor for thin film transistors (TFT). An early example of this is the amorphous oxide semiconductor indium-gallium-zinc-oxide (IGZO) which has attracted considerable interest for TFT applications [9], and more recently for RRAM [10]. In this study we present results for an alternative amorphous oxide semiconductor zinc-tin-oxide (ZTO) for RRAM applications. It has been demonstrated that ZTO performs well as an active semiconductor for TFTs where ZTO has high transparency, good electron mobility, and low processing temperatures [11]. An advantage of ZTO, compared to IGZO, is that it avoids the use of In and Ga, which are increasingly expensive elements. We have observed bipolar switching for solution deposited ZTO when the compliance current was slowly increased. The impact of compliance current on the magnitude of the on and off resistance states, the SET voltage, and the switching ratio were studied. II. EXPERIMENTAL DETAILS A. Materials Preparation & Device Fabrication We have fabricated RRAM devices using standard metal-insulator-metal (MIM) structures, where ZTO is the active switching layer located between the two electrodes and Ir and Al are the bottom and top contacts, respectively. For this work, a blanket Ir film was deposited on a Si substrate using electron beam evaporation and a thin Ti adhesion layer was incorporated at the Ir/Si interface. Atomic force microscopy (AFM) was used to estimate the root-mean-square (RMS) roughness. For the Ir bottom electrode the RMS roughness was 11 +/- 1 nm with a peak roughness of 120 +/- 10 nm. Zinc chloride (ZnCl 2 ) and tin chloride (SnCl 2 ) metal precursors were dissolved in a mixture of acetonitrile and ethyleneglycol with a 1:1 volume ratio. This ZTO precursor solution was spin coated onto the Ir metal surface to form a uniform continuous film. Prior studies have indicated that the aprotic solvent is volatile and does not dissociate the metal-halide precursor [11]. During spin-coating the metal Bipolar Resistive Switching of Zinc-Tin-Oxide Resistive Random Access Memory Santosh Murali 1 , Jaana Saranya Rajachidambaram 2 , Seung-Yeol Han 2 , Chih-Hung Chang 2 , Gregory S. Herman 2 , and John F. Conley, Jr. 1 , Senior Member, IEEE R 2011 11th IEEE International Conference on Nanotechnology Portland Marriott August 15-18, 2011, Portland, Oregon, USA U.S. Government work not protected by U.S. copyright 740