A Survey of Metal Oxides and Top Electrodes for Resistive Memory Devices
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A Survey of Metal Oxides and Top Electrodes for Resistive Memory Devices S.M. Bishop1, B.D. Briggs1, K.D. Leedy2, S. Addepalli1, and N.C. Cady1 1 College of Nanoscale Science and Engineering, University at Albany (SUNY), Albany, NY 12203, U.S.A. 2 Air Force Research Laboratory, 2241 Avionics Circle, Dayton, OH, 45433, U.S.A. ABSTRACT Metal-insulator-metal (MIM) resistive switching devices are being pursued for a number of applications, including non-volatile memory and high density/low power computing. Reported resistive switching devices vary greatly in the choice of metal oxide and electrode material. Importantly, the choice of both the metal oxide and electrode material can have significant impact on device performance, their ability to switch, and the mode of switching (unipolar, bipolar, nonpolar) that results. In this study, three metal oxides (Cu2O, HfOx, and TiOx) were deposited onto copper bottom electrodes (BEs). Four different top electrode (TE) materials (Ni, Au, Al, and Pt) were then fabricated on the various metal oxides to form MIM structures. Devices were then characterized electrically to determine switching performance and behavior. Our results show that the metal TE plays a large role in determining whether or not the MIM structure will switch resistively and what mode of switching (unipolar, bipolar, or nonpolar) is observed. INTRODUCTION Metal oxide based resistive memory devices are characterized by an initial high resistance state (HRS) that can be modified to a low resistance state (LRS) by application of a characteristic threshold voltage. The mechanism of this switching behavior is being actively studied by a number of research groups and appears to vary based upon device architecture and the material(s) chosen. Such devices have been fabricated using a wide array of metal oxide insulator/metal electrode combinations. To date, few studies have rigorously compared devices fabricated from multiple electrode/insulator combinations. Those studies that have compared devices with different electrode/insulator combinations show that the choice of electrode plays a large role in the switching characteristics of the resulting MIM devices. Kim, et al. used several different metal TEs for TiOx–based devices, including Pt, Au, Ni, Al, and Ti [1]. When using Pt and Au electrodes, both bipolar and unipolar switching was observed, but Ag devices exhibited only bipolar behavior. Further, devices fabricated with Ti TEs were not able to be switched, while Ni and Al TEs resulted in unstable switching behavior. Similar trends were observed by Vallee, et al. for HfOx-based devices in which the switching behavior for Pt TEs was superior to Au and WSix(x>2)-based TEs [2]. Another study using HfOx-based devices showed that the conductivity of the low resistance state (LRS) was related to the choice of TE material, and the heat of formation for oxidation of that material [3]. Clearly, the choice of metal oxide and electrode materials for resistive memory devices is important for their resulting electrical behavior.
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