Memristive Switches with Two Switching Polarities in a Forming Free Device Structure
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Memristive Switches with Two Switching Polarities in a Forming Free Device Structure Rainer Bruchhaus1, Christoph R. Hermes1 and Rainer Waser1,2 1
Peter Grünberg Institut, Forschungszentrum Jülich, 52425 Jülich, Germany and JARA –
Fundamentals for Future Information Technology, Forschungszentrum Jülich, 52425 Jülich, Germany 2
Institut für Werkstoffe der Elektrotechnik II, RWTH Aachen, 52074 Aachen, Germany
ABSTRACT In this study an electroforming free device structure based on 25nm thin TiO2 thin films is presented. The TiO2 films are deposited on CMOS compatible W plugs. The use of 5nm thick interlayers of Ti and W between the TiO2 and the Pt electrode turn out to be the key step to achieve the forming free performance. In these Pt/Ti/TiO2/W or Pt/W/TiO2/W samples the switching polarity can be repeatedly changed from “eightwise” to “counter-eightwise” in one device by a proper adjustment of the I-V measurement conditions. The most simple explanation for this observation is that the switching interface can be flipped back and forth from the bottom to the top electrode. INTRODUCTION Under the term “resistive switching” numerous approaches to use voltage induced changes of the resistance of simple two terminal metal-isolator-metal (MIM) structures for future high density data storage are subsumed. Recently, the field was reviewed and for the material systems based on nanoionic transport and redox reactions at least three different working principles were identified [1]. MIM structures based on TiO2 with noble metal electrodes like Pt belong to the valence change mechanism (VCM). Under VCM the elemental steps of the redox reaction are anion transport and subsequent valence changes in the cation sublattice to form conducting mixed valence states or conducting phases. For TiO2 with Pt electrodes usually the MIM structure needs an electroforming step which transforms the device from an initial high resistive state into a switchable state [2]. The subsequent bipolar switching is dominated by the metal/oxide interface and involves local changes to the electronic barrier due to the drift of oxygen vacancies in the applied electric field [3]. Different switching polarities have been observed depending on which metal/oxide interface played the key role during switching [3]. Thus interface engineering by applying different electrode materials is a fruitful approach to deepen the understanding of the switching process and the role of the interfaces. In this paper a systematic study on resistive switching of devices with W bottom electrode and three different top electrode materials is presented.
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EXPERIMENT The CMOS compatible metal W prepared as plugs embedded into a Si3N4-layer was used as bottom contact electrode material. The plugs had different diameters ranging from 60nm to 700nm. Prior to the TiO2 deposition by reactive sputtering (300W, 46sccm Ar, 17sccm O2) the W-plugs were cleaned by a short etching step for 30s in an Ar plasma. Film thickness of the TiO2 in this study is 25nm. A set of samples with three dif
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