Operation Mechanism and Novel Functions of Oxide-Based Atomic Switches Tohru Tsuruoka 1 , Tsuyoshi Hasegawa 2 , Kazuya Terabe 1 , and Masakazu Aono 1 1 National Institute for Materials Science 1-1 Namiki, Tsukuba 305-0044, Japan Phone: +81-29-860-4117 E-mail: TSURUOKA.Tohru@nims.go.jp 2 Waseda University 3-4-1 Okuba, Shinjuku-ku, Tokyo 169-8555, Japan Abstract We have investigated the operation mechanism and resistive switching characteristics of Cu(Ag)/Ta 2 O 5 /Pt atomic switch-type resistance change memories. Ionic current associated with redox reactions at Cu(Ag)/Ta 2 O 5 interfaces was clearly observed, and it was found that moisture absorption in the oxide matrix contributes sig- nificantly to redox processes as well as switching behav- ior. In addition to bi-stable switching, novel functions such as conductance quantization and synaptic plasticity were demonstrated, which can be used for neuromorphic applications. 1. Introduction Among several emerging technologies for the next-generation nonvolatile memory, resistive switching memory based on metal ion transport in a thin oxide layer is one of the most attractive candidates, because of its promis- ing properties such as simple structure, excellent scalability, ease of operation, and high compatibility with the current CMOS fabrication processes [1]. The basic structure of the devices consists of metal-ion conductor-metal (MIM) cells, in which an ion transport layer is sandwiched between an electrochemically active metal electrode (usually, Cu or Ag) and an inert metal electrode (for example, Pt). Because its operation mechanism is essentially the identical to that of an ‘gap-type atomic switch’, whose resistance across a na- nometer gap between a mixed conductor electrode and an inert electrode is controlled by the formation and annihila- tion of a metal bridge under electrical bias [2], the MIM-structured cell with an active electrode can be referred to as a ‘gapless-type atomic switch’ [3]. This type of cells is also called an ‘electrochemical metallization (ECM) cell’ and a ‘conductive-bridge random access memory (CBRAM). Here, we present our recent results on the operation mechanism of atomic switches with a Ta 2 O 5 layer as a mod- el system and their novel functions. 2. Redox reactions at metal/oxide interfaces Ta 2 O 5 -based atomic switch cells are fabricated on a SiO 2 /Si substrate. First, Ti and Pt are deposited by elec- tron-beam (EB) evaporation as the adhesion and bottom electrode, respectively. Then, a Ta 2 O 5 layer with thicknesses of 8 – 15 nm is deposited by RF sputtering or EB evapora- tion. Finally, Cu or Ag is deposited as the top electrode cov- ered with a protective layer of Pt. The cell consists of a cross-point structure with junction sizes between 50 µm and 500 nm. Cu(Ag)/Ta 2 O 5 /Pt cells show bipolar resistive switching under bias voltage sweeping. They are SET from a high-resistance (OFF) state to a low-resistance (ON) state at positive bias to the Cu(Ag) electrode, and RESET from the ON state to the OFF state at negative bias. From the meas- urements of the forming time and the thermal stability of ON states, we proposed the switching mechanism [4]. The SET process corresponds to the formation of a metal fila- ment by nucleation and growth of the active metal, while the RESET process is attributed to the thermal dissolution of the metal filament due to Joule heating followed by the ion dif- fusion under concentration gradient and the applied electric field. Temperature measurements of switching behaviors can be explained by classical nucleation theory, suggesting the validity of our proposed mechanism [5]. We succeeded to observe the ionic current associated with redox reactions at Cu(Ag)/Ta 2 O 5 interfaces by means of cyclic voltammetry (CV) measurements [6]. The results clearly showed repeated oxidation and reduction reactions of Cu and Ag at the interface under voltage sweeping, as shown in Fig. 1. The concentration of generated ions and their diffusion coefficient were estimated from CV curves with different voltage sweep rates. The diffusion coefficient was much higher than that evaluated for Cu ions in thicker Ta 2 O 5 films. It was also found that the redox current is en- hanced and the forming voltage is reduced when the film density of Ta 2 O 5 is decreased, which are realized by differ- ent deposition methods. This indicates that Cu and Ag are more oxidized at the interface with a more porous Ta 2 O 5 Fig. 1 Cyclic voltammogram of a Cu/Ta 2 O 5 /Pt atomic switch B-1-01(Invited) Extended Abstracts of the 2016 International Conference on Solid State Devices and Materials, Tsukuba, 2016, pp63-64 - 63 -