https://orcid.org/0000-0003-1886-1864
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Abstract
This review addresses resistive switching devices operating according to the bipolar valence change mechanism (VCM), which has become a major trend in electronic materials and devices over the last decade due to its high potential for non-volatile memories and future neuromorphic computing. We will provide detailed insights into the status of understanding of these devices as a fundament for their use in the different fields of application. The review covers the microscopic physics of memristive states and the switching kinetics of VCM devices. It is shown that the switching of all variants of VCM cells relies on the movement of mobile donor ions, which are typically oxygen vacancies or cation interstitials. VCM cells consist of three parts: an electronically active electrode (AE), often a metal with a high work function, in front of which the switching occurs, a mixed ionic-electronic conducting (MIEC) layer consisting of a nanometer-scale metal oxide or a stack of different metal oxides, and an ohmic counter electrode (OE). After an introduction to definitions and classification, the fundamentals of solid-state physics and chemistry associated with VCM cells are described, including redox processes and the role of electrodes. The microscopic changes induced by electroforming, a process often required prior to resistive switching, are described in terms of electronic initialization and subsequent changes in chemistry, structure, and conductivity. The switching process is discussed in terms of switching polarity, geometry of the switching region, and spectroscopic detection of the valence changes. Emphasis is placed on the extreme nonlinearity of switching kinetics described by physics-based multiscale modeling, ranging from ab initio methods to kinetic Monte Carlo and finite element models to compact models that can be used in circuit simulators. The review concludes with a treatment of the highly relevant reliability issues and a description of the failure mechanisms, including mutual trade-offs.
Acknowledgements
Discussion with our colleagues and students has inspired and shaped this work. In particular, we would like to thank Dirk Wouters (RWTH), Vikas Rana (FZJ), and Victor Zhirnov (SRC Corp., USA) for providing information about the industrial state of art and about scaling aspects, Robert Spatschek (FZJ) and Martin Z Bazant (MIT, USA) for discussions about phase stability, Eike Linn (Vistec Electron Beam GmbH) for discussions about the theory of memristive devices, as well as Stanley Williams (Texas A&M Univ.), John Paul Strachan (FZJ), Thomas Mikolajick (TUD), Daniele Ielmini (Politecnico di Milano, Italy), Susanne Hoffmann-Eifert (FZJ), Ilia Valov (FZJ), Ulrich Böttger (RWTH), and Roger de Souza (RWTH) for numerous discussions on various aspects of this research area. Furthermore, we acknowledge the comprehensive collection of data by our former and present PhD students Benedikt Arndt, Christoph Bäumer, Karsten Fleck, Carsten Funck, Alexander Gutsche, Viktor Havel, Christoph Hermes, Anja Herpers, Andreas Kindsmüller, Camilla La Torre, Florian Lentz, Astrid Marchewka, Lutz Nielen, Christian Rodenbücher, Alexander Schönhals, Katharina Skaja, Moritz von Witzleben, and Stefan Wiefels. We are indebted to Thomas Pössinger for generating and postprocessing all illustrations and for crosslinking all equations, figures, and sections and to Dagmar Leisten for assistance in the illustrations. And we want to heartfully thank Maria Garcia for supporting the project in many technical ways as well as Caroline Zurhelle-Waser for the language check.
Disclosure statement
No potential conflict of interest was reported by the author(s).