Abstract

There is a growing demand for physical reservoirs that operate with low power consumption and low computational cost. We have conducted research on the basic properties of Ag₂S reservoirs, which are a type of physical reservoir. However, little research has been conducted on their applications. In this study, as a first step toward the practical application of Ag₂S reservoirs, we implemented two types of rock-paper-scissors judgment systems using Ag₂S reservoirs. In these experiments, we were able to demonstrate fast learning in the reservoir by comparing the results with methods using a single-layer perceptron and a convolutional neural network. In addition, we could obtain a maximum accuracy rate of about 98%.

Abstract

The rapid growth in demand for edge artificial intelligence increases importance of physical reservoirs that work at low computational cost with low power consumption. A Ag₂S island network also works as a physical reservoir, in which various physicochemical phenomena contribute to a reservoir operation. In this study, we investigated its frequency dependence and found that diffusion of Ag⁺ cations in a Ag₂S island, which has a relaxation time of about 100 μs, plays a major role when performance is improved. Modified National Institute of Standards and Technology (MNIST) classification task using an input pulse width of 100 μs resulted in the accuracy of 91%. Iterative operations up to 10 million cycles revealed a small enough standard deviation of output, suggesting a potential for practical use of a Ag₂S island network as a reservoir.

Abstract

In today’s advanced information society, hardware-based neuromorphic systems attract much attention for achieving more efficient information processing. Hardware-based neuromorphic systems need devices that change their resistance in an analog manner like biological synapses. A molecular-gap atomic switch exhibits analog resistance change over a wider range compared to other non-volatile memory devices. However, several issues remain with the device, such as in cyclic endurance and retention. In this study, we fabricated a molecular-gap atomic switch with a reduced switching area. We expected that the reduction would limit the number of Ag⁺ cations that contribute to a switching phenomenon and solve the remaining issues. The fabricated devices endured 1000 switching cycles and exhibited stable analog resistance change. Deep learning was successfully demonstrated using 293 fabricated devices as synapses, which resulted in the accuracy of 93.65% in 26th epoch in a 5 × 5 pixel image classification task.

Abstract

A molecular-gap atomic switch is one of the emerging devices that works as a synaptic device. It shows good enough performance such as analog resistance change over five orders of magnitude. However, low yield in device fabrication due to short-circuit of as-fabricated devices has been a big issue. In this study, we Investigated the causes of the low yield and found several possible leakage current paths in unexpected routes. A new device structure and fabrication processes that eliminate the potential leakage paths were proposed. Operating characteristics were evaluated at each step in the improvement, and finally yield in the device fabrication was improved from 10% to 80%.

Abstract

A physical reservoir that accepts direct light irradiation as input was developed using a Ag₂S island network. Short-term memory and nonlinearity required for reservoirs are achieved by the diffusion of Ag⁺ cations in each Ag₂S island and the growth of Ag filaments between Ag₂S islands. We found that direct light irradiation to Ag₂S islands changes local conductivity in a reservoir, which enhances the performance in short-term memory and nonlinearity of the reservoir. Using the effect, we performed a pattern classification of light that was irradiated to a Ag₂S island network reservoir through a rectangular slit, which resulted in the accuracy of over 95%.

Abstract

Quantum effects in novel functional materials and new device concepts represent a potential breakthrough for the development of new information processing technologies based on quantum phenomena. Among the emerging technologies, memristive elements that exhibit resistive switching, which relies on the electrochemical formation/rupture of conductive nanofilaments, exhibit quantum conductance effects at room temperature. Despite the underlying resistive switching mechanism having been exploited for the realization of next‐generation memories and neuromorphic computing architectures, the potentialities of quantum effects in memristive devices are still rather unexplored. Here, a comprehensive review on memristive quantum devices, where quantum conductance effects can be observed by coupling ionics with electronics, is presented. Fundamental electrochemical and physicochemical phenomena underlying device functionalities are introduced, together with fundamentals of electronic ballistic conduction transport in nanofilaments. Quantum conductance effects including quantum mode splitting, stability, and random telegraph noise are analyzed, reporting experimental techniques and challenges of nanoscale metrology for the characterization of memristive phenomena. Finally, potential applications and future perspectives are envisioned, discussing how memristive devices with controllable atomic‐sized conductive filaments can represent not only suitable platforms for the investigation of quantum phenomena but also promising building blocks for the realization of integrated quantum systems working in air at room temperature.

A reservoir that is more sensitive to lower frequencies is developed by a Ag₂S-island network, where Ag filament growth/shrinkage achieves non-linear transformation of input signals. Six logic operations are achieved with accuracy higher than 99%.

Atomically sharp tips are a requirement for scanning-probe microscopy, such as scanning tunneling microscopy (STM) and atomic force microscopy (AFM). Compared with STM, AFM imaging is more sensitive to the sharpness of tip apices because long-range forces act as a background signal on the high-resolution AFM images originating from short-range forces. Here we report the investigation of in situ reproducible sharp tips for AFM. We make an Ag2S crystal, a mixed ionic and electronic conductor, on a conventional Si cantilever, and controllably grow and shrink the Ag nanoprotrusion by changing the polarity of the bias voltage between the tip and the sample. We are able to reduce the contribution of long-range forces by growing a Ag nanoprotrusion on the Ag2S tip, and obtain atomic-resolution AFM images. We also confirm that the Ag2S tip with a Ag nanoprotrusion, the end of which presumably terminates in Si atoms, is capable of simultaneous AFM and STM measurements.

The rate limiting process in first resistive switching, called the "forming process", is determined by measuring the switching time of an as-fabricated Cu/Ta2O5/Pt atomic switch as a function of the ambient temperature and the Ta2O5 thickness. The temperature dependence is well fitted by the Arrhenius equation, suggesting that a certain activation process dominates the switching phenomenon. The switching time increases linearly as the Ta2O5 thickness increases. The results herein clearly suggest that the rate limiting process is the drift of Cu cations in the Ta2O5 layer. We determine that the activation energy is 0.4 eV.

Controlling movements of electrons and holes is the key task in developing today's highly sophisticated information society. As transistors reach their physical limits, the semiconductor industry is seeking the next alternative to sustain its economy and to unfold a new era of human civilization. In this context, a completely new information token, i.e., ions instead of electrons, is promising. The current trend in solid-state nanoionics for applications in energy storage, sensing, and brain-type information processing, requires the ability to control the properties of matter at the ultimate atomic scale. Here, a conceptually novel nanoarchitectonic strategy is proposed for controlling the number of dopant atoms in a solid electrolyte to obtain discrete electrical properties. Using α-Ag2+ δS nanodots with a finite number of nonstoichiometry excess dopants as a model system, a theory matched with experiments is presented that reveals the role of physical parameters, namely, the separation between electrochemical energy levels and the cohesive energy, underlying atomic-scale manipulation of dopants in nanodots. This strategy can be applied to different nanoscale materials as their properties strongly depend on the number of doping atoms/ions, and has the potential to create a new paradigm based on controlled single atom/ion transfer.

Atomic switches are nanoionic devices that are operated by controlling redox reactions and the local migration of metal ions in solids. The essential mechanism is the growth and shrinkage of a metal filament formed between two electrodes, resulting in repeatable resistive switching between high-resistance and low-resistance states, which can be used for next-generation nonvolatile memories. This review focuses on the operating mechanism and resistive switching characteristics of two- and three-terminal atomic switches using a thin metal oxide layer as an ion-conducting matrix. First, we describe the operating mechanism of a two-terminal atomic switch based on nucleation theory and present the results of temperature dependence and switching speeds to determine the validity of our switching model. Then, we discuss the effects that moisture absorption in the oxide matrix has on the fundamental processes and switching behavior in order to elucidate the importance of the porosity of the oxide matrix. Finally, we demonstrate a three-terminal atomic switch and describe the impact of the anode material or metal-ion species. These findings will contribute to the development of next-generation logic circuits with low-voltage operation and low-power consumption.

The Ag2S gap-type atomic switch based "tug of war" device is a promising element for building a new type of CMOS free neuromorphic computerhardware. Since Ag+ cations are reduced during operation of the device, it was thought that the gap-material should be a n-type polymer. In this study, we revealed that the polymer bithiophene-oligoethyleneoxide (BTOE) doped poly(ethylene oxide) (PEO), which was used as gap-material in the first demonstration of the "tug of war", is a p-type polymer. For this we used impedance spectroscopy and transistor measurements. We elaborate on how the electrochemical processes in the "tug of war" devices could be explained in the case of p-type conductive gap-materials.

The morphology and composition of tantalum oxide (Ta2O5) thin films prepared by electron-beam (EB) evaporation and radio-frequency sputtering (SP) were investigated by grazing incidence X-ray diffraction (GIXRD), X-ray reflectometry (XRR), atomic force microscopy, Fourier transformed infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). GIXRD revealed an amorphous nature for both films, and XRR showed that the density of the Ta2O5-EB films was lower than that of the Ta2O5-SP films; both films have lower density than the bulk value. A larger amount of molecular water and peroxo species were detected for the Ta2O5-EB films by FTIR performed in ambient atmosphere. XPS analyses performed in vacuum confirmed the presence of hydroxyl groups, but no trace of chemisorbed molecular water was detected. In addition, a higher oxygen nonstoichiometry (higher O/Ta ratio) was found for the EB films. From these results, we conclude that the oxygen nonstoichiometry of the EB film accounted for its lower density and higher amount of absorbed molecular water. The results also suggest the importance of understanding the dependence of the structural and chemical properties of thin amorphous oxide films on the deposition process. (C) 2016 Published by Elsevier B.V.

Redox reactions at the Cu/Ta2O5 interface and subsequent Cu ion transport in a Ta2O5 film have been investigated by means of cyclic voltammetry (CV) measurements. Under positive bias to the Cu electrode, Cu is preferentially oxidized to Cu2+ and then to Cu+. Subsequent negative bias causes a reduction of the oxidized Cu ions at the interface. It was found that CV curves change drastically with varied relative humidity levels from 5 to 85%. At higher humidity levels, the ion concentrations and diffusion coefficients, estimated from the CV curves, suggest increased redox reaction rates and a significant contribution of proton conduction to the ionic transport. The results indicate that the redox reactions of moisture are rate-limiting and highlight the importance of water uptake by the matrix oxide film in understanding and controlling the resistive switching behavior of oxide-based atomic switches. (C) 2016 The Japan Society of Applied Physics

The resistive switching behavior of Cu/Ta2O5/Pt atomic switches, in which the Ta2O5 film was deposited by electron-beam (EB) evaporation and radio-frequency sputtering (SP), was investigated under different relative humidity (RH) levels. Fourier-transformed infrared spectroscopy and X-ray photoelectron spectroscopy measurements revealed that both films possess the oxygen-rich composition and higher water absorption capability of EB films. The Cu/Ta2O5-SP/Pt cell showed a stable, nonvolatile switching behavior in the observed RH range, whereas the Cu/Ta2O5-EB/Pt cell exhibited a similar behavior up to 50% RH, but altered from nonvolatile to volatile switching at higher RH levels. The observed volatile switching behavior of the Cu/Ta2O5-EB/Pt cell can be explained by increased ion migration, assisted by absorbed water and/or proton conduction in hydrated environments. The results indicate that the water uptake ability of the matrix film plays a crucial role in determining the resistive switching behavior of oxide-based atomic switches. (C) 2016 The Japan Society of Applied Physics

The resistive switching behavior of Cu/Ta<inf>2</inf>O<inf>5</inf>/Pt atomic switches, in which the Ta<inf>2</inf>O<inf>5</inf>film was deposited by electron-beam (EB) evaporation and radio-frequency sputtering (SP), was investigated under different relative humidity (RH) levels. Fourier-transformed infrared spectroscopy and X-ray photoelectron spectroscopy measurements revealed that both films possess the oxygen-rich composition and higher water absorption capability of EB films. The Cu/Ta<inf>2</inf>O<inf>5</inf>-SP/Pt cell showed a stable, nonvolatile switching behavior in the observed RH range, whereas the Cu/Ta<inf>2</inf>O<inf>5</inf>-EB/Pt cell exhibited a similar behavior up to 50% RH, but altered from nonvolatile to volatile switching at higher RH levels. The observed volatile switching behavior of the Cu/Ta<inf>2</inf>O<inf>5</inf>-EB/Pt cell can be explained by increased ion migration, assisted by absorbed water and/or proton conduction in hydrated environments. The results indicate that the water uptake ability of the matrix film plays a crucial role in determining the resistive switching behavior of oxide-based atomic switches.

The last century was in a sense a century of solid electronic devices. In 1947,the first transistor was invented by Bardeen, Brattain, and Shockley,and thanks to ilby&#039;s idea of integrating such transistors in 1958,integrated circuit (IC) technology experienced rapid development with the aid of microfabrication technology, leading to today&#039;s revolutionary information and communication technology. In IC technology, as typified by metal-oxide-semiconductor (MOS) transistor technology, the movementof electrons (or holes) is controlled by electric field, and the atoms (or ions) constructing the stage in which the electrons (or holes) move are stationary except for thermal vibration.

We propose a simple model for an atomic switch-based decision maker (ASDM), and show that, as long as its total number of metal atoms is conserved when coupled with suitable operations, an atomic switch system provides a sophisticated "decision-making" capability that is known to be one of the most important intellectual abilities in human beings. We considered a popular decision-making problem studied in the context of reinforcement learning, the multi-armed bandit problem (MAB); the problem of finding, as accurately and quickly as possible, the most profitable option from a set of options that gives stochastic rewards. These decisions are made as dictated by each volume of precipitated metal atoms, which is moved in a manner similar to the fluctuations of a rigid body in a tug-of-war game. The "tug-of-war (TOW) dynamics" of the ASDM exhibits higher efficiency than conventional reinforcement-learning algorithms. We show analytical calculations that validate the statistical reasons for the ASDM to produce such high performance, despite its simplicity. Efficient MAB solvers are useful for many practical applications, because MAB abstracts a variety of decision-making problems in real-world situations where an efficient trial-and-error is required. The proposed scheme will open up a new direction in physics-based analog-computing paradigms, which will include such things as "intelligent nanodevices" based on self-judgment.

For a computing process such as making a decision, a software controlled chip of several transistors is necessary. Inspired by how a single cell amoeba decides its movements, the theoretical 'tug of war' computing model was proposed but not yet implemented in an analogue device suitable for integrated circuits. Based on this model, we now developed a new electronic element for decision making processes, which will have no need for prior programming. The devices are based on the growth and shrinkage of Ag filaments in alpha-Ag2+delta S gap-type atomic switches. Here we present the adapted device design and the new materials. We demonstrate the basic 'tug of war' operation by IV-measurements and Scanning Electron Microscopy (SEM) observation. These devices could be the base for a CMOS-free new computer architecture.

Cu and Ag redox reactions at the interfaces with Ta2O5 and the impact of Ta2O5 film density on the forming process of Cu,Ag/Ta2O5/Pt atomic switch structures are investigated. Cyclic voltammetry measurements revealed that under positive bias to the Cu (Ag) electrode, Cu is preferentially oxidized to Cu2+, while Ag is oxidized to Ag+ ions. Subsequent negative bias causes a reduction of oxidized Cu (Ag) ions at the interfaces. The diffusion coefficient of the Cu and Ag ions in the Ta2O5 film is estimated from the results from different bias voltage sweep rates. It is also found that the redox current is enhanced and the forming voltage of the Cu/Ta2O5/Pt cell is reduced when the density of the Ta2O5 film is decreased. This result indicates the importance of the structural properties of the matrix oxide film in understanding and controlling resistive switching behavior.

Nonvolatile three-terminal operation, with a very small range of bias sweeping (-80 to 250 mV), a high on/off ratio of up to six orders of magnitude, and a very small gate leakage current (&lt; 1 pA), is demonstrated using an Ag (gate)/Ta2O5 (ionic transfer layer)/Pt (source), Pt (drain) three-terminal atomic switch structure.

Current-voltage measurements demonstrated the effects of temperature on the resistive switching behaviour of a gapless-type atomic switch based on a silver-ion-conductive solid polymer electrolyte (SPE) consisting of a mixture of polyethylene oxide (PEO) and AgClO&lt
inf&gt
4&lt
/inf&gt
. The operation voltages decreased in magnitude with increased ambient temperature. The reduction of the operation voltages can be explained by the increased conductivity of silver ions in the amorphous PEO-salt complex region of the SPE film. This situation is completely different from a cell with a pure PEO film, in which the increased crystallinity of the PEO film may hinder the ionic conductivity, although similar switching behaviour was observed. It was also found that cells based on SPE and PEO show different behaviours under air and vacuum conditions. This is probably associated with different water uptakes from the ambient surroundings by the SPE and PEO films. The results suggest the importance of the crystallinity and water uptake ability of the matrix polymer film on the resistive switching characteristics.

ReRAMs using oxygen vacancy drift in their resistive switching are promising candidates as next generation memory devices. One remaining issue is degradation of the on/off ratio down to 102 or less with an increased number of switching cycles. Such degradation is caused by a local hard breakdown in a set process due to a very high electric field formed just before the completion of a conductive filament formation. We found that introducing an ultra-thin SiO2 layer prevents the hard breakdown by dynamical moderation of the electric field formed in the TaOx matrix, resulting in repeated switching while retaining a higher on/off ratio of about 105. (C) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

We have investigated the switching mechanisms of Cu(Ag)/Ta2O5/Pt atomic switch cells as a model system. The observed fast switching speeds indicate that oxide-based atomic switches hold potential for fast-switching memory applications.

An atomic switch is a nanoionic device that controls the diffusion of metal ions and their reduction/oxidation processes in the switching operation to form/annihilate a metal atomic bridge. Various types of atomic switches, such as two-terminal and three-terminal atomic switches, have been developed. The novel characteristics, such as their small size, low power consumption, low ON-resistance, and non-volatility, will be useful in improving present-day computers. Novel functions have also been developed in atomic switches, such as learning abilities, photo-sensing abilities, memristive operations, and synaptic functions
these functions may contribute in the development of new types of neural computing systems.

A silver sulfide (Ag2S) island as an ionic conductor in resistive switching memories was formed and a protrusion of silver from the Ag2S formed by an electrochemical reaction was observed using a scanning tunneling microscope. (C) 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Volatile and nonvolatile switching are selectively operated using a two-terminal gap-type atomic switch by choosing a sweeping bias range. The amount of Ag atoms those precipitate from an Ag2S electrode in the turning-on process is a function of a bias. Using a bias larger than the threshold bias, enough number of Ag atoms precipitate to make a bridge to the counter electrode, resulting in the nonvolatile operation. A bias smaller than the threshold bias does not make a bridge, and Ag atoms return to an Ag2S electrode while decreasing a bias to zero, resulting in the volatile operation. The selective operations are enabled by controlling electrochemical potentials. © 2014 IEEE.

We demonstrate that the resistive switching memory, called an atomic switch, emulates the synaptic plasticity underlying short-term and long-term memory formations in the human brain. The change in conductance of the atomic switch is considered analogous to the change in strength of a biological synapse that varies according to stimulating input pulses. The atomic switch also exhibits conventional memristive behavior in which the output depends on the history of input signal. These observations indicate that the atomic switch has potential for use as an essential building block for neural computing systems.

Volatile and nonvolatile switching are selectively operated using a two-terminal gap-type atomic switch by choosing a sweeping bias range. The amount of Ag atoms those precipitate from an Ag2S electrode in the turning-on process is a function of a bias. Using a bias larger than the threshold bias, enough number of Ag atoms precipitate to make a bridge to the counter electrode, resulting in the nonvolatile operation. A bias smaller than the threshold bias does not make a bridge, and Ag atoms return to an Ag2S electrode while decreasing a bias to zero, resulting in the volatile operation. The selective operations are enabled by controlling electrochemical potentials.

A redox-based three-terminal device was fabricated using Ta2O5 as the ionic transfer material, and its operation was investigated. We found that application of a negative polarity gate bias, which increases oxygen anions in the channel regions, can make a conductive path between a source electrode and a drain electrode. The insulating state of the pristine device is turned on to a semiconductor state by the application of a negative polarity gate bias. Since turning off to the insulating state could not be achieved, the switching process resembles the soft breakdown of the first turning-on process of oxygen vacancy controlled resistive random access memories, although the polarity of the bias is opposite to that used in the first turning-on process. Further application of a gate bias causes a transition from the semiconductor state to a metal state. Accordingly, there are two types of on-state. It is possible to switch between the semiconductor and metal states.

We conducted light irradiation experiments in air to clarify influence of atmosphere on the operation of a photo-assisted atomic switch. In air, Pt-Ag2S/Ag nanogap electrodes with a PTCDI thin layer in their nanogaps showed current fluctuations with an applied bias of from 1.5 V to 6 V regardless of the bias polarity and with or without light irradiation. This is in contrast to the fact that only two things cause an increase in current that result in the formation of a silver bridge and switching behavior under vacuum, namely, light irradiation and the application of positive bias to the Ag2S/Ag electrode [1]. In addition, photocurrent caused by irradiating a PTCDI thin layer was found to be sensitive to air and to N-2. These results indicate that moisture or other gas molecules in air and in N-2 have an influence on the photo-assisted atomic switching behavior.

A photo-assisted atomic switch, which has a photoconductive molecular layer in a gap of about 20 nm between an Ag2S electrode and a Pt electrode, is set to a conventional gap-type atomic switch operation mode by light irradiation with the application of a small bias that precipitates Ag atoms from an Ag2S electrode. After this initialization, the switch operates only with application of a bias. In this study, we also found that after the set-operation a photo-assisted initialized atomic switch shows different switching modes depending on the bias range, i.e., volatile switching when the applied bias is smaller than the threshold bias, and nonvolatile switching when the applied bias is larger than the threshold bias. These characteristics can be useful in reconfiguring a circuit such as in neural computing systems.

A compact neuromorphic nanodevice with inherent learning and memory properties emulating those of biological synapses is the key to developing artificial neural networks rivaling their biological counterparts. Experimental results showed that memorization with a wide time scale from volatile to permanent can be achieved in a WO3-x-based nanoionics device and can be precisely and cumulatively controlled by adjusting the device's resistance state and input pulse parameters such as the amplitude, interval, and number. This control is analogous to biological synaptic plasticity including short-term plasticity, long-term potentiation, transition from short-term memory to long-term memory, forgetting processes for short-and long-term memory, learning speed, and learning history. A compact WO3-x-based nanoionics device with a simple stacked layer structure should thus be a promising candidate for use as an inorganic synapse in artificial neural networks due to its striking resemblance to the biological synapse.

Resistive switching memories (ReRAMs) are the major candidates for replacing the state-of-the-art memory technology in future nanoelectronics. These nonvolatile memory cells are based on nanoionic redox processes and offer prospects for high scalability, ultrafast write and read access, and low power consumption. The interfacial electrochemical reactions of oxidation and reduction of ions necessarily needed for resistive switching result inevitably in nonequilibrium states, which play a fundamental role in the processes involved during device operation. We report on nonequilibrium states in SiO2-based ReRAMs being induced during the resistance transition. It is demonstrated that the formation of metallic cations proceeds in parallel to reduction of moisture, supplied by the ambient. The latter results in the formation of an electromotive force in the range of up to 600 mV. The outcome of the study highlights the hitherto overlooked necessity of a counter charge/reaction to keep the charge electroneutrality in cation-transporting thin films, making it hard to analyze and compare experimental results under different ambient conditions such as water partial pressure. Together with the dependence of the electromotive force on the ambient, these results contribute to the microscopic understanding of the resistive switching phenomena in cation-based ReRAMs.

Nonvolatile three-terminal operation is demonstrated using a Pt/Ta 2O5-x/Pt, Pt structure, by controlling oxygen vacancy drift to make/annihilate a conductive channel between a source and a drain. The as-fabricated device is in an off-state. Application of a positive gate bias moves oxygen vacancies in a Ta2O5-x layer towards a channel region, making the channel region conductive. The conductive channel remains even after unloading the gate bias. Application of a negative gate bias, which moves the oxygen vacancies back towards the gate electrode, is required to turn off the device. The device shows a high ON/OFF ratio of up to 10 6. © 2013 AIP Publishing LLC.

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.

The speed of the SET operation of a Cu/Ta2O5/Pt atomic switch from a high-resistance state to a low-resistance state was measured by transient current measurements under the application of a short voltage pulse. The SET time decreased exponentially with increasing pulse amplitude, reaching as low as 1 ns using moderate pulse voltages. This observation shows that oxide-based atomic switches hold potential for fast-switching memory applications. From a comparison with atomistic nucleation theory, Cu nucleation on the Pt electrode was found to be the likely rate-limiting process determining the SET time. Copyright 2013 Author(s). This article is distributed under a Creative Commons Attribution 3.0 Unported License. [http://dx.doi.org/10.1063/1.4795140]

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. Copyright © Materials Research Society 2013.

Quantized conductance was observed in a cation-migration-based resistive switching memory cell with a simple metal-insulator-metal (MIM) structure using a thin Ta2O5 layer. The observed conductance changes are attributed to the formation and dissolution of a metal filament with an atomic point contact of different integer multiples in the Ta2O5 layer. The results demonstrate that atomic point contacts can be realized in an oxide-based MIM structure that functions as a nanogap-based atomic switch (Terabe et al 2005 Nature 433 47). By applying consecutive voltage pulses at periodic intervals of different times, we also observed an effect analogous to the long-term potentiation of biological synapses, which shows that the oxide-based atomic switch has potential for use as an essential building block of neural computing systems.

A potential route to extend Moore's law beyond the physical limits of existing materials and device architectures is to achieve nanotechnology breakthroughs in materials and device concepts. Here, we discuss an on-demand WO3-x-based nanoionic device where electrical and neuromorphic multifunctions are realized through externally induced local migration of oxygen ions. The device is found to possess a wide range of time scales of memorization, resistance switching, and rectification varying from volatile to permanent in a single device, and these can furthermore be realizable in both two- or three-terminal systems. The gradually changing volatile and nonvolatile resistance states are experimentally demonstrated to mimic the human brain's forgetting process for short-term memory and long-term memory. We propose this nanoionic device with its on-demand electrical and neuromorphic multifunction has a unique paradigm shifting potential for the fabrication of configurable circuits, analog: memories, digital-neural fused networks, and more in one device architecture.

It is demonstrated that a Cu2S gap-type atomic switch, referred to as a Cu2S inorganic synapse, emulates the synaptic plasticity underlying the sensory, short-term, and long-term memory formations in the human brain. The change in conductance of the Cu2S inorganic synapse is considered analogous to the change in strength of a biological synaptic connection known as the synaptic plasticity. The plasticity of the Cu2S inorganic synapse is controlled depending on the interval, amplitude, and width of an input voltage pulse stimulation. Interestingly, the plasticity is influenced by the presence of air or moisture. Time-dependent scanning tunneling microscopy images of the Cu-protrusions grown in air and in vacuum provide clear evidence of the influence of air on their stability. Furthermore, the plasticity depends on temperature, such that a long-term memory is achieved much faster at elevated temperatures with shorter or fewer number of input pulses, indicating a close analogy with a biological synapse where elevated temperature increases the degree of synaptic transmission. The ability to control the plasticity of the Cu2S inorganic synapse justifies its potential as an advanced synthetic synapse with air/temperature sensibility for the development of artificial neural networks.

Resistive switching memory cells were fabricated on a plastic substrate via inkjet printing (IJP) of a solid polymer electrolyte (SPE). Using the high contrast between the surface energy of a metal electrode and the substrate, a thin SPE film could be deposited over the electrode by IJP. The fabricated Ag/SPE/Pt cells showed bipolar resistive switching behavior under electrical bias in vacuum and in air, which is attributed to the formation and dissolution of a metal filament between the electrodes. From the standpoint of the switching mechanism, our cell can be referred to as a 'gapless-type atomic switch'. The cells also exhibited stable switching behavior under substrate bending. This device fabrication technique has great potential for flexible switch/memory applications. Copyright 2012 Author(s). This article is distributed under a Creative Commons Attribution 3.0 Unported License. [http://dx.doi.org/10.1063/1.4727742]

Removal of a sulfur atom from the topmost layer of a MoS2 surface forms electronic states in the band-gap of an inherently semiconducting material. Scanning tunneling spectroscopy measured at sulfur vacancies, which were made by sulfur atom removal using the high electrical field of a scanning tunneling microscope, shows stepwise increases in the current in a band-gap region, corresponding to the formation of electronic states. The periphery of sulfur vacancies also show linear current-voltage (I/V) characteristics, suggesting that electronic states in the periphery are modified due to the removal of sulfur atoms. (C) 2012 The Japan Society of Applied Physics

Bipolar resistance switching (BRS) behavior and the effects of atmosphere (air, vacuum, O-2 gas, or N-2 gas) on BRS behavior occurred in the top and bottom interfaces in the M-(top electrode)/WO3-x/Pt-(bottom electrode) (M = Pt, Au) devices were investigated. Stable BRS only can be obtained in the interface with Pt electrode. And, the top Pt/WO3-x interface exhibited stable BRS only in an oxygen-rich atmosphere (air and O-2 gas). In contrast, the bottom WO3-x/Pt interface showed stable BRS under any atmosphere. Based on the x-ray photoelectron spectroscopy measurement on Pt, Au/WO3-x interfaces, it is identified that the oxygen migration process during resistance switching mainly occurs between the Pt/WO3-x interface and Pt electrode. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4726084]

Electrochemical equilibrium and the transfer of mass and charge through interfaces at the atomic scale are of fundamental importance for the microscopic understanding of elementary physicochemical processes. Approaching atomic dimensions, phase instabilities and instrumentation limits restrict the resolution. Here we show an ultimate lateral, mass and charge resolution during electrochemical Ag phase formation at the surface of RbAg4I5 superionic conductor thin films. We found that a small amount of electron donors in the solid electrolyte enables scanning tunnelling microscope measurements and atomically resolved imaging. We demonstrate that Ag critical nucleus formation is rate limiting. The Gibbs energy of this process takes discrete values and the number of atoms of the critical nucleus remains constant over a large range of applied potentials. Our approach is crucial to elucidate the mechanism of atomic switches and highlights the possibility of extending this method to a variety of other electrochemical systems.

In the research of advanced materials based on nanoscience and nanotechnology, it is often desirable to measure nanoscale local electrical conductivity at a designated position of a given sample. For this purpose, multiple-probe scanning probe microscopes (MP-SPMs), in which two, three or four scanning tunneling microscope (STM) or atomic force microscope (AFM) probes are operated independently, have been developed. Each probe in an MP-SPM is used not only for observing high-resolution STM or AFM images but also for forming an electrical contact enabling nanoscale local electrical conductivity measurement. The world's first double-probe STM (DP-STM) developed by the authors, which was subsequently modified to a triple-probe STM (TP-STM), has been used to measure the conductivities of one-dimensional metal nanowires and carbon nanotubes and also two-dimensional molecular films. A quadruple-probe STM (QP-STM) has also been developed and used to measure the conductivity of two-dimensional molecular films without the ambiguity of contact resistance between the probe and sample. Moreover, a quadruple-probe AFM (QP-AFM) with four conductive tuning-fork-type self-detection force sensing probes has been developed to measure the conductivity of a nanostructure on an insulating substrate. A general-purpose computer software to control four probes at the same time has also been developed and used in the operation of the QP-AFM. These developments and applications of MP-SPMs are reviewed in this paper.

The effects of temperature and moisture on the resistive switching characteristics of oxide-based atomic switches were investigated to reveal their switching mechanism. The observed temperature variations of the SET voltages can be qualitatively explained by the classical nucleation theory. The moisture absorption in oxides results in the formation of a hydrogen-bond network at grain boundaries, and metal ions are likely to migrate along the grain boundaries. Depending on the strength of hydrogen bonds in oxides, the atomic switches exhibit a different switching behavior to ambient conditions. © 2012 Materials Research Society.

Gapless-type atomic switches were fabricated on a flexible plastic substrate by printing 'solid polymer electrolyte' (SPE) layers using suitable ink and drop-on-demand ink-jet technique. High surface energy difference between Pt microelectrode patterned on the plastic substrate and the substrate itself, led to the successful printing of electrolytic solution on a bottom Pt electrodes. Bipolar resistive switching behavior was observed in Ag/SPE/Pt cross-point structures under electrical bias. The switching between ON and OFF states is attributed to the formation and dissolution of a metal filament between the electrodes. The cells also exhibited stable switching behavior under mechanical stress as performed by substrate bending. Switching characteristics measured under mechanical stress and without stress are matching well. The results demonstrate that the SPE-printed atomic switch has great potential for flexible switch/memory applications. © 2012 Materials Research Society.

Resistive switching memories based on the formation and dissolution of a metal filament in a simple metal/oxide/metal structure are attractive because of their potential high scalability, low-power consumption, and ease of operation. From the standpoint of the operation mechanism, these types of memory devices are referred to as gapless-type atomic switches or electrochemical metallization cells. It is well known that oxide materials can absorb moisture from the ambient air, which causes shifts in the characteristics of metal-oxide-semiconductor devices. However, the role of ambient moisture on the operation of oxide-based atomic switches has not yet been clarified. In this work, currentvoltage measurements were performed as a function of ambient water vapor pressure and temperature to reveal the effect of moisture on the switching behavior of Cu/oxide/Pt atomic switches using different oxide materials. The main findings are: i) the ionization of Cu at the anode interface is likely to be attributed to chemical oxidation via residual water in the oxide layer, ii) Cu ions migrate along grain boundaries in the oxide layer, where a hydrogen-bond network might be formed by moisture absorption, and iii) the stability of residual water has an impact on the ionization and migration processes and plays a major role in determining the operation voltages. These findings will be important in the microscopic understanding of the switching behavior of oxide-based atomic switches and electrochemical metallization cells.

An atomic switch is a nanoionic device that controls the diffusion of metal ions/atoms and their reduction/oxidation processes in the switching operation to form/annihilate a conductive path. Since metal atoms can provide a highly conductive channel even if their cluster size is in the nanometer scale, atomic switches may enable downscaling to smaller than the 11 nm technology node, which is a great challenge for semiconductor devices. Atomic switches also possess novel characteristics, such as high on/off ratios, very low power consumption and non-volatility. The unique operating mechanisms of these devices have enabled the development of various types of atomic switch, such as gap-type and gapless-type two-terminal atomic switches and three-terminal atomic switches. Novel functions, such as selective volatile/nonvolatile, synaptic, memristive, and photo-assisted operations have been demonstrated. Such atomic switch characteristics can not only improve the performance of present-day electronic systems, but also enable development of new types of electronic systems, such as beyond von- Neumann computers.

Memorization caused by the change in conductance in a Ag2S gap-type atomic switch was investigated as a function of the amplitude and width of input voltage pulses (V-in). The conductance changed little for the first few Vin, but the information of the input was stored as a redistribution of Ag-ions in the Ag2S, indicating the formation of sensory memory. After a certain number of Vin, the conductance increased abruptly followed by a gradual decrease, indicating the formation of short-term memory (STM). We found that the probability of STM formation depends strongly on the amplitude and width of Vin, which resembles the learning behavior of the human brain. (C) 2011 American Institute of Physics. [doi:10.1063/1.3662390]

We studied metallic Ag formation inside and on the surface of Ag2S thin films, induced by the electric field created with a scanning tunnel microscope (STM) tip. Two clear regimes were observed: cluster formation on the surface at low bias voltages, and full conductance switching at higher bias voltages (V &gt; 70 mV). The bias voltage at which this transition is observed is in agreement with the known threshold voltage for conductance switching at room temperature. We propose a model for the cluster formation at low bias voltage. Scaling of the measured data with the proposed model indicates that the process takes place near steady state, but depends on the STM tip geometry. The growth of the clusters is confirmed by tip retraction measurements and topography scans. This study provides improved understanding of the physical mechanisms that drive conductance switching in solid electrolyte memristive devices.

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 1-4). In neuromorphic engineering(5,6), 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 switch(7,8), 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.

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. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3597154]

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.

Key to single-molecule electronics is connecting functional molecules to each other using conductive nanowires. This involves two issues: how to create conductive nanowires at designated positions, and how to ensure chemical bonding between the nanowires and functional molecules. Here, we present a novel method that solves both issues. Relevant functional molecules are placed on a self-assembled monolayer of diacetylene compound. A probe tip of a scanning tunneling microscope is then positioned on the molecular row of the diacetylene compound to which the functional molecule is adsorbed, and a conductive polydiacetylene nanowire is fabricated by initiating chain polymerization by stimulation with the tip. Since the front edge of chain polymerization necessarily has a reactive chemical species, the created polymer nanowire forms chemical bonding with an encountered molecular element. We name this spontaneous reaction "chemical soldering". First-principles theoretical calculations are used to investigate the structures and electronic properties of the connection. We demonstrate that two conductive polymer nanowires are connected to a single phthalocyanine molecule. A resonant tunneling diode formed by this method is discussed.

We propose a three-terminal nanometer metal switch that utilizes a solid electrolyte where a nanoscale metal filament is stretched and retracted. Its operating principle is based on electrochemical reaction and ion migration in the electrolyte. The fabricated device is composed of a solid electrolyte layer (Cu(2)S), a gate (Cu), a source (Cu), and a drain (Pt). After the Cu filament is formed between the source and the drain by applying the drain voltage, repeatable on/off switching in the drain current is obtained by controlling the gate voltage. The on/off current ratio can be as high as 10(5), and the programmable cycle is around 50. Each state can be kept for up to 40 days. Since the gate is separated from the current path, the switching current can be reduced to 10 mu A, which is two orders of magnitude smaller than that of a two-terminal switch. In this paper, we present the operating principle and electrical characteristics of the three-terminal switches, and discuss how suitable they are for reconfigurable circuits. (C) 2011 Wiley Periodicals, Inc. Electron Comm Jpn, 94(4): 55-61, 2011; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ecj.10214

Spontaneous chain polymerization of molecules initiated by a scanning tunneling microscope tip is studied with a focus on its rate-determining factors. Such chain polymerization that happens in self-assembled monolayers (SAM) of diacetylene compound molecules, which results in a pi-conjugated linear polydiacetylene nanowire, varies in its rate P depending on domains in the SAM and substrate materials. While the arrangement of diacetylene molecules is identical in every domain on a graphite substrate, it varies in different domains on a MoS(2) substrate. This structural variation enables us to investigate how P is affected by molecular geometry. An important determining factor of P is the distance between two carbon atoms which are to be bound by polymerization reaction, R; as R decreases by 0.1 nm, P increases similar to 2 times. P for a MoS(2) substrate Is similar to 4 times higher (with the same value of R) than that for a graphite substrate because of higher mobility of molecules. The exciting correlation of the chain polymerization rate to the geometrical structure of the diacetylene molecules brings a deeper understanding of the mechanism of chain polymerization kinetics. In addition, the fabrication of one-dimensional conjugated polymer nanowires on a semiconducting MoS(2) substrate. as demonstrated here may be of immense Importance in the realization of future molecular devices.

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.

Atomic switches are nanoionic devices that control the diffusion of metal cations and their reduction/oxidation processes in the switching operation to form/annihilate a metal atomic bridge, which is a conductive path between two electrodes in the on-state. In contrast to conventional semiconductor devices, atomic switches can provide a highly conductive channel even if their size is of nanometer order. In addition to their small size and low on-resistance, their nonvolatility has enabled the development of new types of programmable devices, which may achieve all the required functions on a single chip. Three-terminal atomic switches have also been developed, in which the formation and annihilation of a metal atomic bridge between a source electrode and a drain electrode are controlled by a third (gate) electrode. Three-terminal atomic switches are expected to enhance the development of new types of logic circuits, such as nonvolatile logic. The recent development of atomic switches that use a metal oxide as the ionic conductive material has enabled the integration of atomic switches with complementary metal-oxide-semiconductor (CMOS) devices, which will facilitate the commercialization of atomic switches. The novel characteristics of atomic switches, such as their learning and photosensing abilities, are also introduced in the latter part of this review.

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. (C) 2011 The Japan Society of Applied Physics

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 filament inside the SPE film caused by electrochemical reactions. This is the first 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 mu s, and retention times longer than 1 week. These results suggest that SPE-based electrochemical devices might be suitable for flexible switch and memory applications.

It is of great interest and importance to develop new nanofabrication processes to fabricate sub-20 nm structures with sub-2 nm resolution for next-generation nanoelectronic devices. A combination of electron beam lithography (EBL) and a molecular ruler is one of the promising methods to make these fine structures. Here we successfully develop a hybrid method to fabricate sub-20 nm nanogap devices at the desired positions with a complex structure by developing a post-EBL process, which enabled us to avoid damaging the molecular ruler with the high-energy electron beam, and to fully utilize the EBL resolution. It was found that slight etching of the Ti adhesion layer of the parent metal (Pt) by ACT935J solution assisted the removal of molecular rulers, resulting in improved enhancement in the product yield (over 70%) of nanogap devices.

An atomic switch is a nanoionic device that controls the diffusion of metal ions and their reduction/oxidation processes in the switching operation to form/annihilate a metal atomic bridge, which is a conductive path between two electrodes in an ON-state. Since metal atoms can provide a highly conductive channel even if their size is in the nanometer scale, atomic switches may enable downscaling to smaller than the 11-nm technology node. Two-terminal atomic switches have the potential for use in memories and programmable switches. Three-terminal atomic switches, where the formation/annihilation of a metal atomic bridge between a source electrode and a drain electrode are controlled by a third (gate) electrode, work as nonvolatile transistors. Recent development of two-terminal atomic switches that use a metal oxide as the ionic conductive material, in which a metal atomic bridge is formed, has enabled the integration of atomic switches with complementary metal-oxide-semiconductor (CMOS) devices. Also introduced are the novel characteristics of atomic switches, such as their small size, low power consumption, low ON-resistance, nonvolatility, and learning abilities.

We report detailed current-voltage and current-time measurements to reveal the forming and switching behaviors of Cu/Ta(2)O(5)/Pt nonvolatile resistive memory devices. The devices can be initially SET (from the OFF state to the ON state) when a low positive bias voltage is applied to the Cu electrode. This first SET operation corresponds to the first formation of a metal filament by inhomogeneous nucleation and subsequent growth of Cu on the Pt electrode, based on the migration of Cu ions in the stable Ta(2)O(5) matrix. After the forming, the device exhibits bipolar switching behavior (SET at positive bias and RESET (from the ON state to the OFF state) at negative bias) with increasing the ON resistance from a few hundred Omega to a few k Omega. From the measurements of the temperature stability of the ON states, we concluded that the RESET process consists of the Joule-heating-assisted oxidation of Cu atoms at the thinnest part of the metal filament followed by diffusion and drift of the Cu ions under their own concentration gradient and the applied electric field, disconnecting the metal filament. With ON resistances of the order of a few k Omega, the SET and RESET operations are repeated by the inhomogeneous nucleation and the Joule-heating-assisted dissolution of a small filament on a remaining filament. This switching model is applicable to the operation of cation-migration-based resistive memories using other oxide materials.

Diffusivity of Cu in amorphous (a-) Ta2O5 is increased by low temperature annealing above 350 degrees C but the increase is suppressed by adding SiO2 to Ta2O5. To clarify the reasons, we investigated the structural difference between a-Ta2O5 and a-SiO2-Ta2O5. The results show that the low temperature annealing does not cause polycrystallization of Ta2O5 but purges weakly bonded oxygen atoms from the bulk and decreases the film density. Adding SiO2 to Ta2O5 is shown to increase the coordination number of Ta-O, which results in the improved thermal stability. (C) 2010 American Institute of Physics. [doi:10.1063/1.3488830]

Sulfur vacancies formed on a MoS2 surface have been predicted to have electronic states at the Fermi level, and to work as conductive atomic scale structures. We made sulfur vacancies on a MoS2 surface by removing sulfur atoms using scanning tunneling microscopy (STM) induced field evaporation, and measured the current voltage (I/V) characteristics of the vacancies. The I/V curve measured at the vacancies showed a linear increase at a zero bias region, indicating the existence of electronic states at the Fermi level. On the other hand, the I/V curve measured at a clean surface showed a gap of about 1 eV around the Fermi level, as was expected from the theoretical calculation. We also successfully carried out manipulation of Au nanoislands, which will be used as nanopads to be connected to a sulfur vacancy chain. (C) 2010 The Japan Society of Applied Physics

Learning abilities are demonstrated using a single solid-state atomic switch, wherein the formation and dissolution of a metal filament are controlled depending on the history of prior switching events. The strength of the memorization level gradually increases when the number of input signals is increased. Once the filament forms a bridge, electrons flow in a ballistic mode and long-term memorization is achieved (see figure).

The switching time of a Ag2S atomic switch, in which formation and annihilation of a Ag atomic bridge is controlled by a solid-electrochemical reaction in a nanogap between two electrodes, is investigated as a function of bias voltage and temperature. Increasing the bias voltage decreases the switching time exponentially, with a greated exponent for the lower range of bias than that for the higher range. Furthermore, the switching time shortens exponentially with raising temperature, following the Arrhenius relation with activation energy values of 0.58 and 1.32 eV for lower and higher bias ranges, respectively. These results indicate that there are two main processes which govern the rate of switching first the electrochemical reduction Ag++e(-)-&gt; Ag and, second, the diffusion of Ag+ ions. This investigation advances the fundamental understanding of the switching mechanism of the atomic switch which is essential for its successful device application.

We have developed a method using a scanning tunneling microscope (STM) probe tip to control the chain polymerization of diacetylene compounds in a self-ordered layer, thereby creating conjugated polydiacetylene nanowires. When a small amount of phthalocyanine was deposited on a molecular layer of diacetylene compound, we found adsorbed and stabilized phthalocyanine single molecules. The initiation of chain polymerization on the diacetylene molecular row to which the single phthalocyanine molecule was adsorbed was also demonstrated. ©2010 IEEE.

本稿では、固体電気化学反応を利用した金属ナノワイヤーの形成制御手法と、そのデバイス応用の現状について解説する。本手法では、イオン伝導体中における金属イオンの拡散と、その表面ないし界面における酸化・還元反応を局所的に制御することで金属ナノワイヤーが形成される。

Metal nanowire formation and annihilation are controlled using a solid electrochemical reaction simply by applying a bias voltage to an ionic conductor, such as Ag₂S and Cu₂S. This controlled formation and annihilation of a metal nanowire can be achieved both on the surface of the ionic conductor and inside of the ionic conductor, by placing a counter electrode with a gap and without a gap between the ionic conductor, respectively. A new type of a nanodevice called "Atomic Switch" has been developed using the controlled formation and annihilation of a nanowire. It shows novel characteristics, such as easy operation, simple structure, low power consumption. Recent development of an atomic switch using a metal oxide enables to integrate it with CMOS devices.<br>

For stability against the thermal budget of the CMOS BEOL process, we developed a new solid-electrolyte switch that uses a SiO2-Ta2O5 composite as the electrolyte. This switch has high thermal stability because thermal diffusion of Cu+ ions is Suppressed in the composite. Moreover, its switching characteristics after thermal annealing are similar to those of a Ta2O5 switch without annealing. The switch with the SiO2-Ta2O5 composite electrolyte has good ON-state durability against DC current stress; its durability is comparable to that of a single via in interconnects. The switch can be implemented in the local interconnection layers of LSIs.

The construction of an organic-electronic resistive switch based on polymer electrolytes is the basis to study the interfacial and bulk transport as well as the interaction between ions and electrons/holes at the nanoscale level. Moreover, it could also be potentially applied in novel nanoelectrochemical devices for sensors, fuel cells and batteries, and therefore has attracted much attention in recent years. In this work, we fabricated resistive switching devices with silver-ion-conductive polymer electrolytes. The devices showed bipolar switching behaviors in the current-voltage characteristics for different silver ion concentrations ranging from 1 to 4 wt%. A high resistance up to 1 G Omega in the OFF state and a low resistance with less than tens of k Omega in the ON state can be achieved. We believe that the observed switching results from formation and annihilation of Ag metal filaments inside the polymer film by solid electrochemical reaction. Sequential operations, such as write-read-erase-read, were also demonstrated.

A novel solid-electrolyte nonvolatile switch that we previously developed for programmable large-scale-integration circuits turns on or off when a conducting Cu bridge is formed or dissolved in the solid electrolyte. Cu+ ion migration and an electrochemical reaction are involved in the switching process. For logic applications, we need to adjust its turn-on voltage (V-ON), which was too small to maintain the conductance state during logic operations. In this paper, we clarified that V-ON is mainly affected by the rate of Cu+ ion migration in the solid electrolyte. Considering the relationship between the migration rate and V-ON, we replaced the former electrolyte, Cu2-alpha S, with Ta2O5, which enabled us to appropriately adjust V-ON with a smaller Cu+ ion diffusion coefficient.

We have investigated solid electrolyte switches that utilize electrochemical reactions (deposition and dissolution) of metallic ions. The switch turns off or on when a metallic bridge electrochemically forms or dissolves in the solid electrolyte. Each state is nonvolatile and the switching is repeatable up to 105 cycles. The promising application is a programmable switch in a field programmable logic because of its small size (&lt; 30 nm) and low ON resistance (&lt; 100 92). This paper discusses the electrical characteristics, operation principle, and applications of the solid electrolyte switch. (c) 2008 Wiley Periodicals, Inc.

We examined the structural properties of copper sulfide films as a function of the sulfurization time of 70-nm-thick Cu films. Copper sulfide films with various phases such as mixed metallic Cu-chalcocite, chalcocite, roxbyite, and covellite phases were formed with increasing sulfurization time. To evaluate the structural stability of various films, all the films were exposed to the ambient atmosphere for the same amount of time. Although the phase structure and stoichiometry of the films were maintained at a greater depth, the near-surface region of the films was oxidized and covered with overlayers of oxide, hydroxide, and/or sulfate species due to the exposure and reaction with the ambient atmosphere. The oxygen uptake and its reactivity with the copper sulfide film surfaces were enhanced with increasing sulfur content of the films. In addition, the type of divalent state of copper formed on the film surfaces depended on the phase structure, composition, and stoichiometry of the films.

A thin amorphous TiO2 layer was prepared using anodic oxidation of Ti. Resistance switching of amorphous TiO2 was investigated by sweeping in a continuous voltage scan and pulse bias voltage. The bipolar switching was observed without the need of a forming process. Switching of the Ti/amorphous-TiO2/Pt based structure showed good endurance. The switching mechanism and current-voltage characteristics were investigated from combinatorial analysis of the conducting filaments' evolution and the space-charge limited current conduction. The oxygen-vacancies migration involved redox processes, which is responsible for the recovery and rupture of sub-TiOx based conducting filaments inside the amorphous TiO2 layer. (C) 2008 The Japan Society of Applied Physics.

The luminescence and waveguide properties of single In(2)O(3) nanofibers were investigated by means of scanning near-field optical microscopy. The nanofibers showed spatially uniform emission intensity in photoluminescence (PL) images. This emission is associated with a PL emission band centered at similar to 630 nm. We found that this PL light is propagated through the nanofiber over a distance of several hundreds of micrometers. By measuring the PL intensity for different propagating distances, the optical loss of single nanofibers was evaluated. Waveguiding behavior for incident light with different wavelengths in the visible region was also observed. These results indicate that In2O3 nanofibers can be used as good waveguide elements for integrated photonic applications.

We examined the structural and electrical properties of copper sulfide films as a function of the sulfurization time of 70-nm-thick Cu films. Copper sulfide films with various phases such as mixed metallic Cu-chalcocite, chalcocite, roxbyite, and mixed roxbyite-covellite phases were formed with increasing sulfurization time. The Cu/S atomic percentage ratio of the films decreased with increasing sulfurization time, and films with various compositions such as Cu-rich and stoichiometric copper sulfide with underlying unreacted Cu as well as pure stoichiometric and S-rich copper sulfide were obtained. The surface morphology and the electrical resistivity of the films depended on the chemical phase and composition of the films. The resistivity decreased with increasing Cu deficiency in the films. Distinct switching of the resistance from high to low-state, and vice versa, with the reversal of the bias polarity of the film was observed only for the mixed metallic Cu-chalcocite phased film with underlying Cu. However, the chalcocite film with underlying Cu exhibited a semiconducting behavior. This indicated that excess Cu within the chalcocite film is required for the observation of the switching behavior of the resistance. (c) 2008 American Institute of Physics.

The emission properties of ZnS nanobelts synthesized through thermal evaporation were investigated by means of scanning near-field optical microscopy. The photoluminescence (PL) images of single nanobelts exhibited a bright line along their length. The local light emission spectra measured over the bright lines showed a green emission peak around 535 nm, which was in good agreement with a PL peak obtained for an ensemble of the nanobelts. From careful scanning-electron-microscopy observations of identical nanobelts, we found that the observed green emission is related to line or planar defects of the ZnS nanobelts.

We propose a three-terminal nanometer metal switch in this paper that utilizes a solid electrolyte where the nanoscale metal filament is stretched and retracted. Its operating principle is based on the electrochemical reaction and ion-migration in the electrolyte. The fabricated device is comprised of a solid electrolyte layer (Cu2S), a gate (Cu), a source (Cu) and a drain (Pt). After the Cu-filament is formed between the source and the drain by applying the drain voltage, repeatable ON/OFF switching in the drain current is obtained by controlling the gate voltage. The ON/OFF current ratio can be as high as 105, and the programmable cycle is around 50. Each state can be kept for up to 40 days. Since the gate is separated from the current path, the current for the switching can be reduced down to 10 μA, which is two orders of magnitude smaller than that in two-terminal switches. In this paper, we show the operating principle and electrical characteristics of the three-terminal switches, and discuss how suitable they are for reconfigurable circuits. © 2008 The Institute of Electrical Engineers of Japan.

The structure of a single polydiacetylene compound on a graphite substrate was investigated using atomic force microscopy (AFM). The linear conjugated polydiacetylenes were obtained through chain polymerization of a monomolecular layer of diacetylene compound on a graphite substrate under ultraviolet light irradiation. AFM observations revealed that the polydiacetylenes were imaged higher than the unpolymerized monomer rows. This result supports the 'lifted-up' conformation model, in which the polydiacetylene backbone is geometrically raised. To investigate why the polymer backbone is lifted, we also carried out first-principles density-functional calculations in the local density approximation. These calculations suggested that the steric hindrance between the alkyl side-chains of the monomers and the oligomer caused the lifted-up conformation.

Ag/Cu-based chalcogenide ionic conductors are candidates for use in applications in resistance- switching and nonvolatile memory devices. We report the investigation of the electrical properties of individual Ag2S/Ag heteronanowires (HNWs) by atomic force microscopy (AFM) using a nanoscale-tip electrode. Hysteretic current-voltage (IV) curves and the polarity-dependent resistance- switching phenomenon in an individual Ag2S/Ag HNW were observed. A local impedance spectroscopy measurement of Ag2S/Ag HNWs was performed to reveal the interface- related electrical characteristics. The DC-bias-dependent impedance spectra suggested the occurrence of charge and mass transfer at the interface of the electrode/mixed conductor. It is proposed that reversible resistance switching originates from the creation and rupture of filament- like conducting pathways inside the Ag2S/Ag HNW.

Silver-iodide (AgI)-based superionic conductors are attracting widespread interest for their potential applications in electrochemical devices such as sensors and batteries. A new kind of nanocomposite with highly ordered AgI nanowires embedded in an anodic-aluminum-oxide (AAO) membrane was fabricated by low-temperature step-electrochemical growth. Structural evolution, phase transition, and ionic conductivity were investigated by x-ray diffraction, differential scanning calorimetry, and impedance measurements. The phase transition from beta/gamma-AgI phase to alpha-AgI phase occurred at temperature of 168 degrees C, that is, higher than that of reported bulk AgI (147 degrees C); abnormally, the alpha to beta/gamma phase-transition temperature on cooling was also depressed as large hysteresis formed. The high-temperature phase, namely, alpha-AgI, remained at temperatures as low as 80 degrees C. The initial highly oriented-growth AgI nanowire disappeared after undergoing heating and cooling processes and a mixture of polycrystalline beta/gamma-AgI and amorphouslike interface phases formed. The cooled AgI-AAO composite displayed ionic conductivity in the order of 10(-2) S cm(-1) at room temperature. This array-structured nanocomposite of AgI-AAO may be further developed for usage as a new type of battery, i.e., "nanobatteries" and "nanosensors" with individual AgI nanowires as basic elements.

We investigated single electron tunneling (SET) behavior of dodecanethiol-coated An nanoparticles of two different sizes (average sizes are 5 nm and 2 nm) using nanogap electrodes, which have a well-defined gap size, at various temperatures. The Coulomb staircases and the Coulomb gap near-zero bias voltage caused by the suppression of the tunneling electrons due to the Coulomb blockade effect were observed in the current-voltage (I-V) curves of both sizes of nanoparticles at a low temperature (10 K). At room temperature, the Coulomb gap was observed only in the I- V curve of the smaller nanoparticles. This result indicates that the charging energy of the smaller nanoparticles is enough to overcome the thermal energy at room temperature. This suggests that it is possible to operate the SET devices at room temperature using the smaller nanoparticles as a Coulomb island. (C) 2007 Elsevier B.V. All rights reserved.

The development of nanometer-scale devices operating under a new principle that could overcome the limitations of current semiconductor devices has attracted interest in recent years. We propose that nanoionic devices that operate by controlling the local transport of ions are promising in this regard. It is possible to control the local transport of ions using the solid electrochemical properties of ionic and electronic mixed conductors. As an example of this concept, here, we report a method of controlling the transport of silver ions of the mixed-conductor silver sulfide (Ag2S) crystal and basic research on nanoionic devices based on this mixed conductor. These devices show unique functions such as atom deposition, resistance switching, and quantum point contact switching. The switches operate through the formation and dissolution of an atomic bridge between the electrodes, and the behavior is realized by control of the local solid-state electrochemical reaction. Potential nanoionic devices utilizing the unique functions and characters that do not exist in conventional semiconductor devices are discussed. (c) 2007 NIMS and Elsevier Ltd. All rights reserved.

The authors examined the electronic transport of a solid electrolyte resistive switch. Using element analysis and the temperature dependence of its electronic transport, they deduced that the conductive path is composed of Cu metal precipitated in the solid electrolyte film by an electrochemical reaction. Furthermore, they observed Coulomb blockade phenomena at 4 K when the switch was in the off state. Their observations and experimental results suggest that the metallic conductive path consists of metallic islands separated by tunneling barriers and that switching between the on and off states originates from modulation in the tunneling barriers.

The construction of an electronic-conductor/ionic-conductor heterojunction in a well-defined nanostructure is the basis of studying interfacial and bulk transport and the reactions of ions and electrons at the nanoscale level. An ionic-conductor/metal (AgI/Ag) beterostructured nanowire array is easily fabricated by a template-confined, step-electrochemical technique. The structural and morphological evolution of the AgI/Ag heterostructure before and after its release from the anodic aluminum oxide (AAO) membrane is characterized by scanning electron microscopy, X-ray diffraction, and optical spectroscopy. The structural disordering of released AgI is suggested by the appearance of a broad photoluminescence emission band at longer wavelengths and a short-range-order-like Raman peak. The ionic conductivity of the AgI nanowire embedded inside the insulating AAO membrane is measured as being on the order of 10(-3) S cm(-1), which is an enhancement by two to three orders of magnitude compared with that of bulk polycrystalline AgI at room temperature. This electrochemical method could be useful in fabricating other pure and mixed ionic conductors in heterojunction nanostructures.

I-V characteristics of single electron tunneling from a symmetric and an asymmetric double-barrier tunneling junction (DBTJ) were examined. A single Au nanoparticle was trapped in nanogap whose size was precisely controlled using a combination of electron beam lithography and molecular ruler technique. Though the symmetric junction showed a monotonic rise with a bias beyond the Coulomb gap voltage, the asymmetric junction showed Coulomb staircases. The capacitance of the junction estimated from the fitting curves using the Coulomb conventional theory was consistent with the capacitance calculated from the observed structure. The authors quantitatively found the correlation between the electrical and structural properties of DBTJ. (C) 2007 American Institute of Physics.

Chain polymerizations of diacetylene compound multilayer films on graphite substrates were examined with a scanning tunneling microscope (STM) at the liquid/solid interface of the phenyloctane solution. The first layer grew very quickly into many small domains. This was followed by the slow formation of the piled up layers into much larger domains. Chain polymerization on the topmost surface layer could be initiated by applying a pulsed voltage between the STM tip and the substrate, usually producing a long polymer of submicrometer length. In contrast, polymerizations on the underlying layer were never observed. This can be explained by a conformation model in which the polymer backbone is lifted up.

We have investigated a NanoBridge technology for a high performance LSI that is reconfigurable. The NanoBridge comprises a solid electrolyte sandwiched between two metals. The switch has two conducting states (ON/OFF) that persist in the absence of a power supply. The distinctive advantages of this switch are its small size (&lt; 30nm) and low ON resistance (&lt; 100 Omega). This paper proposes a three-terminal NanoBridge, for which the control gate is separate from the current path. This innovative switch resolves the issues arising from large current during switching that occurs in a two-terminal NanoBridge.

We developed an atomic switch consisting of an ionic and electronic mixed conductor electrode and a counter metal electrode, having a space of about 1 nm between them. Formation and annihilation of a conductive atomic bridge is controlled using a solid electrochemical reaction, which is caused by applying a certain bias voltage between the electrodes. In this study, we measured the switching time of atomic switches made of silver sulfide and copper sulfide. The switching times were different, and this difference can be attributed to the different activation energies and chemical potentials of the materials.

We present a novel solid-electrolyte switch ("NanoBridge") promising for application to field programmable gate array (FPGA). We replace a former solid electrolyte of Cu2S with Ta2O5, which has a Siprocess compatibility,. As a result; we successfully control the turn-on voltage to adapt to CMOS operation. The Ta2O5-NanoBridge exhibits a high reliability of cycling endurance (&gt; 10(4)) and a stability against EM (&gt; 10years at 2.6mA at RT). Furthermore, we demonstrate that the conducting path of the switch is a Cu precipitate with 30nm in diameter, which possibly enables to scale down the switch.

We have developed a solid-electrolyte nonvolatile switch (here we refer as NanoBridge) with a low ON resistance and its small size. When we use a NanoBridge to switch elements in a programmable logic device, the chip size (or die cost) can be reduced and performance (speed and power consumption) can be enhanced. Developing this application required solving a couple of problems. First, the switching voltage of the NanoBridge (similar to 0.3 V) needed to be larger than the operating voltage of the logic circuit (&gt; 1 V). Second, the programming current (&gt; 1 mA) needed to be suppressed to avoid large power consumption. We demonstrate how the Nanobridge enhances the switching voltage and reduces the programming current.

Metal chalcogenide-based mixed ionic-electronic conductors such as Ag2S and Cu2S can be specifically architected for application in nanoelectronic devices. We present a template-confined synthesis of metal chalcogenide (e.g., Ag2S, Cu2S) nanowires for mixed conductor-based nanoelectronics. First, the metal nanowire array was electroplated into pores of a porous alumina membrane. Anodic polarization was then used to transform the metal into the metal sulfide in aqueous hydrosulfide (HS-) solutions. The as-synthesized mixed conductors' hetero-nanowire array was characterized by X-ray diffraction and electron microscopy. Electronic transport measurements show non-linear and reproducible electrical switching characteristics. The high and low resistance states can be reversibly changed by altering the polarity of the applied voltage between the bottom and top electrodes. The electrical-switching behavior is attributed to electric-field-induced accumulation and dissolution of metallic conducting pathways inside the mixed conductors' nanowires. (c) 2006 Elsevier B.V. All rights reserved.

We fabricated Ag nanowires (AgNWs) of 4H hexagonal structure with a diameter of ca. 20nm using dc clectrodeposition inside an anodic alumina oxide (AAO) membrane at low current density. 4H-AgNWs were formed in scale inside an AAO membrane with a pore diameter of ca. 20 nm. We propose that a. small-pore-size AAO membrane and appropriate growth conditions favor the growth of 4H-Ag. Transmission electron microscopy, (TEM) revealed the coexistence of single-crystalline 4H-AgNWs and fcc-AgNWs. The 4H-AgNWs tended to transform into fcc Ag particles under intense electron beam (EB) irradiation, which may be attributed to the knock-on displacement of Ag atoms by electrons generating defects.

We have developed and tested a new method of fabricating nanogaps using a combination of self-assembled molecular and electron beam lithographic techniques. The method enables us to control the gap size with an accuracy of approximately 2 nm and designate the positions where the nanogaps should be formed with high-resolution patterning by using electron beam lithography. We have demonstrated the utility of the fabricated nanogaps by measuring a single electron tunneling phenomenon through dodecanethiol-coated Au nanoparticles placed in the fabricated nanogap.

We examined the structural and electrical properties of silver sulfide films as a function of the sulfurization time of 70-nm-thick Ag films. Variations in the sulfurization time caused variations in the Ag/S atomic percentage ratio of the silver sulfide films, and as-grown films with various compositions, such as S-rich (Ag/S=1.59), stoichiometric (Ag/S=2), and Ag-rich (Ag/S=2.16) films were formed. Amongst the various as-grown films, Ag ions existed in the most polarizable environment in the Ag-rich films. All the films existed in the acanthite alpha-phase, and the sulfurization conditions did not cause any drastic change in the preferred orientation of this phase. The resistivity of these films strongly depended on the Ag/S ratio. While the resistivity of stoichiometric or S-rich films was about 10(7)-10(8) Omega cm, excess Ag of the Ag-rich film caused a decrease in the resistivity by four orders of magnitude. The Ag/S ratio also played a significant role in our observation of the change in resistance within the films from high- to low-resistance state and vice versa with the reversal of the bias polarity of the film. Distinct switching of the resistance was observed only for the Ag-rich film. We also examined the effects of post-deposition annealing (PDA) of various films at 190 degrees C. PDA caused the formation of Ag-rich films (Ag/S=2.12-2.17) in all cases, and Ag ions existed in a more polarizable environment in all the films as compared with stoichiometric film. All the annealed films contained mixed acanthite alpha-phase and argentite beta-phase. Furthermore, all the films had low resistivities of about 0.01-0.02 Omega cm, which indicated that the coexisting metallic argentite beta-phase of the films significantly improved the conductivity of the films as compared to the as-grown film with similar Ag/S ratio. Clear switching behavior of the resistance could be observed within all the annealed films, thereby indicating that excess Ag in the silver sulfide films is a requirement for observation of such a phenomenon. (c) 2006 American Institute of Physics.

We measured the switching time of an atomic switch that is operated by controlling the formation and annihilation of an atomic bridge in a nanogap between two electrodes using solid electrochemical reaction. The switching time becomes exponentially shorter with increasing the switching bias voltage. This exponential relation indicates that the switching time is determined by the solid electrochemical reaction, which is supported by theoretical estimation using a simple model. These results suggest the possibility that the atomic switch can be operated as fast as semiconductor devices currently used.

A solid electrolyte switch turns on or off when a metallic bridge is formed or dissolved respectively in the solid electrolyte (here we use Cu2-alpha S). For logic applications, the switching voltage (&lt; 0.3 V) should be larger than the operating voltage of the logic circuit (about I V). We reveal that the switching voltage is mainly affected by Cu+ ionic transport in Cu2-alpha S and that a solid electrolyte with an ion diffusion coefficient smaller than that of Cu2-alpha S by several tens of orders of magnitude makes it possible to increase the switching voltage to I V.

固体電解質ナノスイッチは、固体電解質中での金属原子移動に基づく微細金属ブリッジのスイッチング現象を利用したスイッチであり、応用範囲の広いプログラマブルロジックを実現できる。本報告では、電流経路と制御電極を分離した3端子素子の提案と原理実証について述べる。本素子により、プログラマブルロジックに応用する際の課題であるスイッチング時の電流低減と、ブリッジを太らせることによる高EM耐性を確保できると期待される。

We propose a three-terminal solid-electrolyte nanometer switch, where the control gate is separated from the current path. This novel switch resolves the issues arising from large current during switching (&gt; 1mA) in a two-terminal solid-electrolyte switch. We demonstrate that the drain current reversibly switches when a metallic bridge electrochemically forms or dissolves between the source and drain by applying gate voltage. The ON resistance is 200-300 Omega and the ON/OFF current ratio is as high as 10(5). Each state is nonvolatile.

A large variety of nanometre-scale devices have been investigated in recent years(1-7) that could overcome the physical and economic limitations of current semiconductor devices(8). To be of technological interest, the energy consumption and fabrication cost of these 'nanodevices' need to be low. Here we report a new type of nanodevice, a quantized conductance atomic switch (QCAS), which satisfies these requirements. The QCAS works by controlling the formation and annihilation of an atomic bridge at the crossing point between two electrodes. The wires are spaced approximately 1 nm apart, and one of the two is a solid electrolyte wire from which the atomic bridges are formed. We demonstrate that such a QCAS can switch between 'on' and 'off ' states at room temperature and in air at a frequency of 1 MHz and at a small operating voltage (600 mV). Basic logic circuits are also easily fabricated by crossing solid electrolyte wires with metal electrodes.

In this paper, a reconfigurable LSI employing a nonvolatile nanometer-scale switch, NanoBridge, is proposed, and its basic operations are demonstrated. The switch, composed of solid electrolyte copper sulfide, has a &lt;30-nm contact diameter and &lt;100-Ohm on-resistance. Because of its small size, it can be used to create extremely dense field-programmable logic arrays. A 4 x 4 crossbar switch and a 2-input look-up-table circuit are fabricated with 0.18-mum CMOS technology, and operational tests with them have confirmed the switch's potential for use in programmable logic arrays. A 1-kb nonvolatile memory is also presented, and its potential for use as a low-voltage memory device is demonstrated.

固体電解質は金属イオンがその中を自由に動くことで知られている。本固体電解質を2種類の金属で挟んだ2端子素子はスイッチとして機能し、ある値以上の電圧を印加することで、電子伝導度が数桁にわたり変化する。各伝導状態はほぼ不揮発的に保たれるため、メモリ等のエレクトロニクス応用が期待される。本報告では、硫化銅を用いた固体電解質ナノスイッチの詳細な電気特性評価を行うと共に、そのスイッチング機構を明らかにする。さらに、本固体電解質ナノスイッチを用いた1Kbit不揮発性メモリのテストチップについて述べる。

We describe a nanometer-scale switch that uses a copper sulfide film and demonstrate its performance. The switch consists of a copper sulfide film, which is a chalcogenide semiconductor, sandwiched between copper and metal electrodes. Applying a positive or negative voltage to the metal electrode can repeatedly switch its conductance in under 100 mus. Each state can persist without a power supply for months, demonstrating the feasibility of nonvolatile memory with its nanometer scale. While biasing voltages, copper ions can migrate in copper sulfide film and can play an important role in switching. (C) 2003 American Institute of Physics.

In this study, we investigated the limitations of optical reading of very fine pits using conventional optics, immersion lens optics and near-field optics. Using very fine pits &lt; 100 nm in size formed by electron beam writing, a reflection-type depolarization scanning near-field optical microscope (SNOM) read very fine pits &lt; 50 nm in size. On the other hand, far-field optics read fine pits only &gt; 160 nm in size using a lens with a numerical aperture (NA) of 0.95 (17 Gb/in(2)). In addition, an oil immersion lens optics with a NA of 1.4 read fine pits only 100 nm in size (45 Gb/in(2)). Advanced near-field optics is a promising tool for achieving ultrahigh-density optical reading of trillion bits/in(2).

We have developed a nanostructuring method using the solid electrochemical reaction induced by a scanning tunneling microscope (STM). This method has some distinctive features that have not previously been obtained by conventional nanostructuring STM methods. The formation and disappearance of the nanostructure are reversible, and the rates can be controlled using STM. These features are realized via a local oxidation/reduction reaction of mobile metal ions in an ionic/electronic mixed conductor. In this study, a crystal of silver sulfide (Ag2S), a mixed conductor, was used as the material for the STM tip. A nanoscale Ag cluster was formed at the apex of the Ag2S tip when a negative bias voltage was applied to the sample. The Ag ions in the Ag2S tip are reduced to Ag atoms by the tunneling electrons from the sample, and the Ag cluster is formed by the precipitation of the Ag atoms at the apex of the tip. The Ag cluster shrank gradually and disappeared when the polarity of the sample bias voltage was switched to positive. Ag atoms in the Ag cluster are oxidized to Ag ions, and the Ag ions redissolve into the Ag2S tip. The formation and disappearance rates of the cluster were controlled by regulating the tunneling current. (C) 2002 American Institute of Physics.

Silver sulfide (Ag2S) which has Ag-ionic/electronic mixed conductivity is used for fabricating a tip used for a scanning tunneling microscope. The mixed conductor tip is capable of nanostructuring by depositing Ag atoms continuously on a sample as well as imaging the surface structure. To obtain the surface image, a nanoscale Ag protrusion is formed at an apex of the tip using a local solid electrochemical reaction, working as "a mini-tip." We fabricate a nanoscale line structure on the sample by scanning the Ag2S tip with the protrusion under appropriate bias voltages and tunneling currents. The structuring is thought to be made up of two layers of Ag atoms deposited from the protrusion. (C) 2002 American Institute of Physics.

The current status and future trend of analytical instruments are discussed. Analytical instruments for failure analyses in sub-1/4 micron dimensions or less, require high spatial resolution and sensitivity at atomic levels. Using new analytical instruments, such as the Nano-prober for electrical characteristics inspection in actual circuits, TEM-EELS for chemical bond analysis of nanometer area and GDS for precise composition analysis, it was found that a SiO2 or TiOx film formed by water from titanic acid (TiOxH2O) produced with titan, water and chlorine, was a cause of high resistivity for a contact (CVD-W/CVD-TiN/Ti/Si) in sub-1/4 micron devices.

Intra-row diffusion of Au atoms in the Si(111)-(5x2)Au reconstructed structure has been observed by using high-temperature scanning tunneling microscopy. Loosely bonded Au atoms. which are arranged with 2(n + I)a spacings at room temperature, diffuse in the twofold direction at higher temperatures among adsorbed sites arranged at 2a intervals. These diffusing Au atoms seem to be thr reason there is a nonintegral number of Au atoms in the (5x2) unit cell.

Si-island growth on a Si(111)-7 x 7 surface was studied at 350 degrees C in situ using high-temperature scanning tunneling microscopy. At the beginning of growth, deposited Si atoms formed small amorphous clusters within each triangular subunit of the 7 x 7 structure. The amorphous clusters grew, and crystallized islands also appeared as the quantity of deposited Si atoms increased. At the domain boundaries of the 7 x 7 structure, islands tended to begin forming on the unfaulted half of the 7 x 7 structure. These phenomena indicate that cancellation of the stacking fault on the substrate surface dominates island growth.

We report on the nanometer-scale modification of a MoS2 surface by scanning tunneling microscopy (STM) with an electric field lower than that required for field evaporation by STM. It is known that a Pt-Ir STM tip dissolves H-2 gas into atomic hydrogen which is chemically active. We applied this phenomenon to STM modification to lower the electric field necessary for atom detachment. A Pt-Ir tip was used to dissolve the H-2 gas on the MoS2 surface. The gas-solid reaction enhanced the evaporation of the top-layer sulfur atoms, which were removed at a low electric field of about 2.4 V nm(-1). The present study shows that we can control STM modification well with the same feedback loop as that used for STM observation.

Phase boundaries between the 7x7 and the 5x2 Au structures on a Si(111) surface, which were located along the two-fold direction of the 5x2 structure, were dynamically studied using high-temperature scanning tunneling microscopy (STM) during Au deposition at 500 degrees C. Two types of stable phase boundaries were observed depending on the growth direction of the 5x2 domain. The stable phase boundaries had adatoms of the unfaulted half at the edge of the 7x7 side. The formation of an unstable phase boundary was quickly followed by the growth of new rows of the 5x2 structure next to the boundary which formed a stable phase boundary. These results suggest that the stacking fault of the 7x7 structure at a phase boundary is unstable.

Si(111)-5 x 2 Au reconstruction was studied dynamically at 500 degrees C by high-temperature scanning tunneling microscopy (STM), Transitions of the 5 x 2 domains during growth were observed as invasive erosions of one domain by another. Whenever this occurred, the domain with its 2-fold direction crossing the domain boundary eroded away. This phenomenon suggests that growth in the 2-fold direction has a stronger driving force. Comparison with STM images taken at room temperature shows that the gold atoms seem to diffuse in the 2-fold direction at higher temperatures. The driving force is believed to derive from repulsion between the gold atoms.

We observed a change in growth mode of Cu, while dynamically observing Cu film growth on a Si(lll)-7 x 7 surface during Cu deposition at room temperature by scanning tunneling microscopy (STM). Initially, Cu atoms were adsorbed mostly on the faulted halves of the 7 x 7 structure. Then, up to about 2 ML coverage, small Cu islands appeared. As the coverage increased from 2 to 3 ML, the growth mode changed into quasi-layer-by-layer growth. With further deposition, 3-D islands having hexagonal terraces grew.

We present nanostructure fabrication techniques using field evaporation and local heating in scanning probe microscopes, especially the scanning tunneling microscope (STM), atomic force microscope (AFM), and the scanning near-field optical microscope (SNOM). The detachment of sulfur atoms from the surface of cleaved MoS2 and atomic scale fabrication were demonstrated with a field evaporation in STM. Field evaporation in AFM forms nanometer-sized gold dots on a SiO2/Si substrate. Local heating with SNOM changes a phase of the GeSbTe recording film from amorphous to crystalline, and forms high reflectivity domains 60 nm in diameter. Moreover, we discuss these applications to a semiconductor process and data storage. (C) 1995 American Vacuum Society.

The initial stage of oxygen adsorption onto a Si(111)-7×7 surface has been studied by scanning tunneling microscopy. The substrate was exposed to oxygen under a partial pressure of 1× 10-9 Torr at room temperature during the observation. Dark features, which have been reported as a main channel for oxidation, appeared exclusively in the faulted halves of the DAS structure though bright features appeared in both halves. This difference can be explained by the adsorption site of oxygen. That is, the greater energy difference of the dangling-bonds of adatoms between both halves than that of the back-bonds causes the exclusive appearance of the dark feature, which has an oxygen atom on on-top site in addition to the other in the back-bond. © 1994.

The initial stage of oxygen adsorption onto a Si(111)-7 x 7 surface has been studied at room temperature by scanning tunneling microscopy. The surface was exposed to oxygen under a partial pressure of 1 x 10(-9) Torr. Initial exposure led to bright and dark features at adatom sites. This dark feature, which was previously reported as a main channel for oxidation, only appeared in the faulted half of the dimer adatom stacking-fault (DAS) structure. The images suggest that the adsorption onto the un-faulted half starts after the faulted half are occupied with oxygen atoms. This clearly indicates that oxygen adsorption occurs at adatom sites with higher energy states.

The dynamic process of Si crystal growth on a Si(111)7X7 surface was studied in situ using high-temperature scanning tunneling microscopy. Si was evaporated onto a Si(111)7X7 surface, kept at 350-degrees-C, and the crystal growth was observed. Both step-flow growth and island growth were observed. In the step-flow growth, the [112BAR] steps became jagged with [112BAR] steps. At the [112BAR] steps, new adatoms appeared in rows along the step edges. In the island growth, the multilayer growth was observed. A rearrangement of adatoms in the first layer was observed when the second layer was formed on the first layer.

An ultrahigh vacuum atomic force microscope (UHV-AFM) with tunneling current detection has been developed. This microscope uses a new type of pantograph inchworm system. The features of the inchworm system are (i) operation of the clamp in normal clamping mode, (ii) an enlargement of the piezo device stroke for clamper stroke, and (iii) compact system for easy use. Our UHV-AFM has (i) six inchworm movements based on the new mechanism, and (ii) a sharp AFM probe whose tip is machined by a focused ion beam fabrication technique. UHV pressure experiments demonstrate that this system provides a contamination-free surface and can observe atomic resolution AFM images of MoS2 and silicon carbide.

The dynamic process of Si crystal growth on a Si(111)7 X 7 surface was studied in situ using high-temperature scanning tunneling microscopy. Si was evaporated onto a Si(111)7 X 7 surface, kept at 350-degrees-C, and the crystal growth was observed. The step-flow growth was observed as the appearance of new adatoms at the step edge. The [112BAR] steps became jagged with [112BAR] steps. At the [112BAR] steps, new adatoms appeared in rows along the step edges.

Gold adsorption onto a Si(111)7 x 7 surface is studied in situ by Scanning Tunneling Microscopy (STM). Gold is deposited onto the Si(111)7 x 7 surface, kept at 500-degrees-C during the STM observation. The 5 x 2 structure grows at a step edge and expands towards both the higher and lower terraces. Si islands are also formed during the gold deposition. These phenomena are explained by the production and migration of excess Si atoms during the phase transition from the 7 x 7 to the 5 x 2 structure.

Scanning tunneling microscopy is used to modify the surface structure of molybdenum disulfide (MoS2). A natural MoS2 Crystal often has stable point defects. This study shows that surface sulfur atoms are excised by field evaporation and these artificial defects can be used to form characters at room temperature.

Natural oxide growth on silicon facets is observed through an atomic force microscope (AFM) with current measurement. The sample is prepared by means of cleaning and heating a silicon (111) surface with direct electric heating in an ultrahigh vacuum, which creates various facets formed by step bunching. The silicon facets and steps can be observed with the AFM in air. The silicon surface structure and the current distribution can simultaneously be obtained. The results clarify that natural oxide growth on a few special high-index-orientation silicon facets is smaller than that on the other silicon facets ({111}, {110}, {100}, etc.).

The initial stage of Au adsorption onto a Si(111) surface (&lt; 0.2 ML) is studied by scanning tunneling microscopy (STM). Au is deposited at room temperature, and then the sample is annealed at 700-degrees-C. At the very early stage Au adsorption, a Au-adsorbed 5 X structure is found not to break a 7 X 7 structure. By increasing the amount of adsorbed Au, the Si substrate itself shows a 5 X structure. Rows of the Au-adsorbed 5 X structure are observed to have grown from the lower side of steps.

STMの探針を用いて行う表面修飾の方法を概観し,特に原子1個単位で操作できる手法について注目する。そのなかで二硫化モリブデン(MoS₂)表面での電界蒸発による試みについて解説する。 MoS₂表面に対してSTM像観察時よりも探針を近づけてパルス状強電界を印加して試料表面のS原子を引き出す。S原子の抜けた表面は,人工的欠陥を形成し,安定に存在する。この操作の連続により原子溝で作られた文字を形成した。

The origin and principle of scanning tunneling microscopy in reviewed. A prototype STM with a three dimensional stage which is able to select sample area and to approach the sample automatically is described, and typical images obtained by the STM are shown. Surface topograph of cleaved MoS₂ (0001) shows neat arrangement of sulfur atoms with 0.316 nm lattice constant. Some defects due to vacancies and a dislocation are also distinguished. It is considered that the imaging principle of STM depends on the probe tip and the atomic arrangement of samples. Also 7 × 7 reconstructed structure of Si (111) is shown.