title: Nanoscale cation motion in TaO(x), HfO(x) and TiO(x) memristive systems.
abstract: A detailed understanding of the resistive switching mechanisms that operate in redox-based resistive random-access memories (ReRAM) is key to controlling these memristive devices and formulating appropriate design rules. Based on distinct fundamental switching mechanisms, two types of ReRAM have emerged: electrochemical metallization memories, in which the mobile species is thought to be metal cations, and valence change memories, in which the mobile species is thought to be oxygen anions (or positively charged oxygen vacancies). Here we show, using scanning tunnelling microscopy and supported by potentiodynamic current-voltage measurements, that in three typical valence change memory materials (TaO(x), HfO(x) and TiO(x)) the host metal cations are mobile in films of 2 nm thickness. The cations can form metallic filaments and participate in the resistive switching process, illustrating that there is a bridge between the electrochemical metallization mechanism and the valence change mechanism. Reset/Set operations are, we suggest, driven by oxidation (passivation) and reduction reactions. For the Ta/Ta2O5 system, a rutile-type TaO2 film is believed to mediate switching, and we show that devices can be switched from a valence change mode to an electrochemical metallization mode by introducing an intermediate layer of amorphous carbon.
authors.affiliation: Peter Gruenberg Institute, Research Centre Juelich, Juelich 52425, Germany.
authors.forename: Anja
authors.identifier: http://orcid.org/0000-0002-2880-1825
authors.initials: A
authors.lastname: Wedig
authors.affiliation: Peter Gruenberg Institute, Research Centre Juelich, Juelich 52425, Germany.
authors.forename: Michael
authors.initials: M
authors.lastname: Luebben
authors.affiliation: Department of Physics, Chonbuk National University, Jeonju 561-756, Korea.
authors.forename: Deok-Yong
authors.identifier: http://orcid.org/0000-0001-5789-8286
authors.initials: DY
authors.lastname: Cho
authors.affiliation: Peter Gruenberg Institute, Research Centre Juelich, Juelich 52425, Germany.
authors.forename: Marco
authors.initials: M
authors.lastname: Moors
authors.affiliation: Peter Gruenberg Institute, Research Centre Juelich, Juelich 52425, Germany.
authors.forename: Katharina
authors.initials: K
authors.lastname: Skaja
authors.affiliation: Peter Gruenberg Institute, Research Centre Juelich, Juelich 52425, Germany.
authors.forename: Vikas
authors.initials: V
authors.lastname: Rana
authors.affiliation: International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan.
authors.forename: Tsuyoshi
authors.initials: T
authors.lastname: Hasegawa
authors.affiliation: Department of Nuclear Science and Engineering, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA.|Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA.
authors.forename: Kiran K
authors.initials: KK
authors.lastname: Adepalli
authors.affiliation: Department of Nuclear Science and Engineering, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA.|Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA.
authors.forename: Bilge
authors.initials: B
authors.lastname: Yildiz
authors.affiliation: Peter Gruenberg Institute, Research Centre Juelich, Juelich 52425, Germany.|Institute for Materials in Electrical Engineering II, RWTH Aachen University, Sommerfeldstrasse 24, Aachen 52074, Germany.
authors.forename: Rainer
authors.initials: R
authors.lastname: Waser
authors.affiliation: Peter Gruenberg Institute, Research Centre Juelich, Juelich 52425, Germany.|Institute for Materials in Electrical Engineering II, RWTH Aachen University, Sommerfeldstrasse 24, Aachen 52074, Germany.
authors.forename: Ilia
authors.identifier: http://orcid.org/0000-0002-0728-7214
authors.initials: I
authors.lastname: Valov