Quantized Conductance and Neuromorphic Behavior of a Gapless-Type Ag-Ta 2 O 5 Atomic Switch
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Quantized Conductance and Neuromorphic Behavior of a Gapless-Type Ag-Ta2O5 Atomic Switch Tohru Tsuruoka1,2, Tsuyoshi Hasegawa1,2, Kazuya Terabe1, and Masakazu Aono1 1 International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), Tsukuba, 305-0044, Japan. 2 Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Agency (JST), Chiyoda-ku, 102-0075, Japan. ABSTRACT We investigated quantization behavior in conductance of an Ag/Ta2O5/Pt gapless-type atomic switch. Stepwise increases and decreases in the conductance were observed when small positive and negative bias voltages were applied to the Ag electrode, respectively, where each step corresponds to the conductance of a single atomic point contact. The conductance level could also be controlled by applying voltage pulses with varied amplitudes. Furthermore, when the interval time of consecutive input pulses was turned, we also observed long-term potentiation behavior similar to that of biological synapses. These results indicate that the oxide-based, gapless-type atomic switch has potential for use as a building block of neural computing systems. INTRODUCTION Currently, there is great technological interest in resistive switching devices based on the formation and dissolution of a metal filament in a thin oxide layer for volatile and nonvolatile memory applications [1,2]. The basic structure of the devices consists of a simple metal/insulator/metal (MIM) cell, in which an ion conductive layer is sandwiched between electrochemically active (usually, Cu or Ag) and inert metal electrodes (such as Pt). Because its operation mechanism is essentially identical to that of a “gap-type atomic switch”, whose resistance across a nanometer gap is controlled by the formation and annihilation of a metal bridge [3], we call this MIM structured cell a “gapless-type atomic switch” [4]. In addition to the simple structure, oxide-based atomic switches have many unique features, such as low ON resistances, high ON/OFF resistance ratios, good scalability, and low-power consumption. In addition to usual bi-resistive switching behavior (SET from the OFF state to the ON state at positive bias and RESET from the ON state to the OFF state at negative bias), the unique characteristics of the gap-type atomic switch are conductance quantization and neuromophic behavior. The conductance of the gap-type atomic switch changed in a stepwise fashion under voltage bias, which was attributed to the formation of an atomic point contact in the nanometer gap [3]. In addition, it was recently demonstrated that atomic switch could show short-term memory behavior for lower repetition rates of input voltage pulses and long-term memory behavior for higher repetition rates, mimicking the long-term potentiation (LTM) of biological synapses [5]. For practical applications, it is important to realize these behaviors in oxide-based, gapless-type atomic switches, because of their high compatibility with the fabrication processes o
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