Showing papers by "Kazuya Terabe published in 2011"

Short-term plasticity and long-term potentiation mimicked in single inorganic synapses

Abstract: The electronic properties of inorganic devices such as memristors can be used to simulate neurological behaviour. In particular, ionic and electronic effects in a silver sulphide device are now shown to mimic short- and long-term synaptic functions. Memory is believed to occur in the human brain as a result of two types of synaptic plasticity: short-term plasticity (STP) and long-term potentiation (LTP; refs 1, 2, 3, 4). In neuromorphic engineering5,6, emulation of known neural behaviour has proven to be difficult to implement in software because of the highly complex interconnected nature of thought processes. Here we report the discovery of a Ag2S inorganic synapse, which emulates the synaptic functions of both STP and LTP characteristics through the use of input pulse repetition time. The structure known as an atomic switch7,8, operating at critical voltages, stores information as STP with a spontaneous decay of conductance level in response to intermittent input stimuli, whereas frequent stimulation results in a transition to LTP. The Ag2S inorganic synapse has interesting characteristics with analogies to an individual biological synapse, and achieves dynamic memorization in a single device without the need of external preprogramming. A psychological model related to the process of memorizing and forgetting is also demonstrated using the inorganic synapses. Our Ag2S element indicates a breakthrough in mimicking synaptic behaviour essential for the further creation of artificial neural systems that emulate characteristics of human memory.

A Polymer-Electrolyte-Based Atomic Switch

Abstract: Studies on a resistive switching memory based on a silver-ion-conductive solid polymer electrolyte (SPE) are reported. Simple Ag/SPE/Pt structures containing polyethylene oxide–silver perchlorate complexes exhibit bipolar resistive switching under bias voltage sweeping. The switching behavior depends strongly on the silver perchlorate concentration. From the results of thermal, transport, and electrochemical measurements, it is concluded that the observed switching originates from formation and dissolution of a silver metal fi lament inside the SPE fi lm caused by electrochemical reactions. This is the fi rst report of an electrochemical “atomic switch” realized using an organic material. The devices also show ON/OFF resistance ratios greater than 10 5 , programming speeds higher than 1 μ s, and retention times longer than 1 week. These results suggest that SPE-based electrochemical devices might be suitable for fl exible switch and memory applications.

Temperature effects on the switching kinetics of a Cu–Ta2O5-based atomic switch

Abstract: Voltage–current (I–V) measurements in a wide temperature range from 88 to 573 K demonstrated the effects of temperature on the switching behavior of a Cu/Ta2O5/Pt resistive memory cell that is referred to as a gapless-type atomic switch. After the forming process, the cells were SET from the OFF state to the ON state at a positive bias to the Cu electrode and then RESET from the ON state to the OFF state at a negative bias. In a previous study (Tsuruoka et al 2010 Nanotechnology 21 425205), it was demonstrated that the SET process corresponds to the reformation of a metal filament between the electrodes by the inhomogeneous nucleation and subsequent growth of Cu whereas the RESET process can be attributed to the Joule-heating-assisted dissolution of the metal filament. In the work described here, we observed that the voltages at which the cells are SET and RESET (SET and RESET voltages) decreased in magnitude with an increase in temperature. From calculations of the nucleation rate of Cu nuclei based on the classical nucleation theory, it was found that the observed temperature variation of the SET voltage is primarily determined by supersaturation in the vicinity of the Pt electrode, which is controlled by the application of positive bias. The supersaturation required for spontaneous growth of a Cu nucleus decreases with increasing temperature, resulting in lower SET voltages at higher temperatures. The RESET voltage is determined by the thermal stability of the metal filament formed. Moreover, using the temperature variation in cell resistances of the ON state, the growth speed of the Cu nucleus after the nucleation was found to decease with increasing temperature. These results are consistent with our switching model.

Switching kinetics of a Cu2S-based gap-type atomic switch

Abstract: The switching time of a Cu2S-based gap-type atomic switch is investigated as a function of temperature, bias voltage, and initial off-resistance. The gap-type atomic switch is realized using a scanning tunneling microscope (STM), in which the formation and annihilation of a Cu-atom bridge in the vacuum gap between the Cu2S electrode and the Pt tip of the STM are controlled by a solid-electrochemical reaction. Increasing the temperature decreases the switching time exponentially with an activation energy of about 1.38?eV. Increasing the bias voltage also shortens the switching time exponentially, exhibiting a greater exponent for the lower bias than for the higher bias. Furthermore, faster switching has been achieved by decreasing the initial off-resistance between the Cu2S electrode and STM tip. On the basis of these results, we suggest that, in addition to the chemical reaction, the electric field in the vacuum gap plays a significant role in the operation of a gap-type atomic switch. This investigation advances our understanding of the operating mechanism of an atomic switch, which is a new concept for future electronic devices.

Memristive operations demonstrated by gap-type atomic switches

Abstract: We demonstrate memristive operations using gap-type Ag2S atomic switches, in which the growth and shrinkage of an Ag protrusion are controlled by using solid-electrochemical reactions. In addition to conventional memristive operations such as those proposed and demonstrated by resistive random-access memories (ReRAMs) using metal oxide compounds, gap-type Ag2S atomic switches also show new types of memristive operations by storing information from input signals without changing their output until a sufficient number of signals are inputted. The new types of memristive operations resemble the learning process seen in neuroplasticity, where changes occur in the organization of the human brain as a result of experience.

Volatile/Nonvolatile Dual-Functional Atom Transistor

Abstract: We demonstrate a conceptually new atom transistor operation by electric-field control of the nanoionic state. The new atom transistor possesses novel characteristics, such as dual functionality of selective volatile and nonvolatile operations, very small power consumption (pW), and a high ON/OFF ratio [106 (volatile operation) to 108 (nonvolatile operation)], in addition to complementary metal oxide semiconductor (CMOS) process compatibility enabling the development of future computing systems that fully utilize highly-integrated CMOS technology. Cyclic endurance of 104 times switching was achieved with the prototype.

Theoretical investigation of kinetics of a Cu2S-based gap-type atomic switch

Abstract: Atomic switch, operating by forming and dissolving a metal-protrusion in a nanogap, shows an exponentially large bias dependence and a faster switching with increasing temperature and decreasing off-resistance These major characteristics are explained with a simple model where the electrochemical potential at the subsurface of solid-electrolyte electrode determines the precipitation rate of metal atoms and the electric-field in the nanogap strongly affects the formation of metal-protrusion Theoretically calculated switching time, based on this model, well reproduced the measured properties of a Cu2S-based atomic switch as a function of bias, temperature and off-resistance, providing a significant physical insight into the mechanism