Understanding of proton induced synaptic behaviors in three-terminal synapse device for neuromorphic systems.

In this study, we investigate a proton-based three-terminal (3-T) synapse device to realize linear weight-update and I-V linearity characteristics for neuromorphic systems. The conductance states of the 3-T synapse device can be controlled by modulating the proton concentration in the WOx channel. Therefore, we estimate the dynamic change of proton concentration in the channel region, which directly affects synaptic behaviors. Our findings indicate that the supply of an excess number of protons from the SiO2-H electrolyte and low proton diffusivity in the WOx channel result in asymmetric and non-linear weight-update characteristics. In addition, though the linear I-V characteristics can be obtained using non-stoichiometric WOx, we observe that significant oxygen deficiency in the channel region increases the operating current levels. Thus, based on this information, we introduce optimized conditions of each component in the 3-T synapse device and shape of the gate voltage pulses. As a result, an excellent classification accuracy is achieved using linear weight-update and I-V linearity characteristics under optimized device and pulse conditions.

Effect of cation amount in the electrolyte on characteristics of Ag/TiO2 based threshold switching devices.

In this study, we investigate the effect of cation amount in an electrolyte on Ag/TiO2 based threshold switching (TS) devices, based on field-induced nucleation theory. For this purpose, normal Ag/TiO2, annealed Ag/TiO2, and AgTe/TiO2 based TS devices are prepared, which have different cation amounts in their electrolytes during the switching process. First, we find that all of the prepared TS devices follow the field-induced nucleation theory with different nucleation barrier energy (W0), by investigating the delay-time dependency at various voltages and temperatures. Based on the investigation, we reveal that the amount of cations in the electrolyte during the switching process is the control parameter that affects the W0 values, which are found to be inversely proportional to the turn-off speed of the TS devices. This implies that the turn-off speed of the TS devices can be modulated by controlling the amount of cations in the matrix.

A C-Te-based binary OTS device exhibiting excellent performance and high thermal stability for selector application.

In this letter, we demonstrate a new binary ovonic threshold switching (OTS) selector device scalable down to ø30 nm based on C-Te. Our proposed selector device exhibits outstanding performance such as a high switching ratio (Ion/Ioff > 105), an extremely low off-current (∼1 nA), an extremely fast operating speed of <10 ns (transition time of <2 ns and delay time of <8 ns), high endurance (109), and high thermal stability (>450 °C). The observed high thermal stability is caused by the relatively small atomic size of C, compared to Te, which can effectively suppress the segregation and crystallization of Te in the OTS film. Furthermore, to confirm the functionality of the selector in a crossbar array, we evaluated a 1S-1R device by integrating our OTS device with a ReRAM (resistive random access memory) device. The 1S-1R integrated device exhibits a successful suppression of leakage current at the half-selected cell and shows an excellent read-out margin (>212 word lines) in a fast read operation.

Effect of conductance linearity and multi-level cell characteristics of TaOx-based synapse device on pattern recognition accuracy of neuromorphic system.

To improve the classification accuracy of an image data set (CIFAR-10) by using analog input voltage, synapse devices with excellent conductance linearity (CL) and multi-level cell (MLC) characteristics are required. We analyze the CL and MLC characteristics of TaOx-based filamentary resistive random access memory (RRAM) to implement the synapse device in neural network hardware. Our findings show that the number of oxygen vacancies in the filament constriction region of the RRAM directly controls the CL and MLC characteristics. By adopting a Ta electrode (instead of Ti) and the hot-forming step, we could form a dense conductive filament. As a result, a wide range of conductance levels with CL is achieved and significantly improved image classification accuracy is confirmed.

Controllable quantized conductance for multilevel data storage applications using conductive bridge random access memory.

In this paper, we investigate the quantized conduction behavior of conductive bridge random access memory (CBRAM) with varied materials and ramping rates. We report stable and reproducible quantized conductance states with integer multiples of fundamental conductance obtained by optimizing the voltage ramping rate and the Ti-diffusion barrier (DB) at the Cu/HfO2 interface. Owing to controlled diffusion of Cu ions by the Ti-DB and the optimized ramping rate, through which it was possible to control the time delay of Cu ion reduction, more than seven levels of discrete conductance states were clearly observed. Analytical modeling was performed to determine the rate-limiting step in filament growth based on an electrochemical redox reaction. Our understanding of the fundamental mechanisms of quantized conductance behaviors provide a promising future for the multi-bit CBRAM device.

Glycothermal Synthesis and Photocatalytic Properties of Highly Crystallized Anatase TiO2 Nanoparticles.

Highly crystallized anatase TiO2 nanoparticles were synthesized at a temperature as low as 120 °C through a glycothermal reaction using amorphous titanium hydrous gel as precursor and 1,4-butanediol and water as solvent. X-ray diffraction (XRD) and transmission electron microscopy (TEM) data support that the glycothermal processing method provides a simple low-temperature route for producing highly crystallized anatase TiO2 nanoparticles without pH adjustment. It is demonstrated that the shape and dispersability of TiO2 nanoparticles can be controlled by the reaction conditions, such as the reaction temperature and variation of the volume ratio of 1,4- butanediol/water (B/W). It was observed that TiO2 samples glycothermally prepared at 220 °C and the B/W ratio of 8/0 showed excellent photocatalytic behavior. The high activity is attributed to the high crystallinity and bipyramidal shape of the particles, which have fewer defects and more active {101} surfaces.

Hardware implementation of associative memory characteristics with analogue-type resistive-switching device.

We have investigated the analogue memory characteristics of an oxide-based resistive-switching device under an electrical pulse to mimic biological spike-timing-dependent plasticity synapse characteristics. As a synaptic device, a TiN/Pr0.7Ca0.3MnO3-based resistive-switching device exhibiting excellent analogue memory characteristics was used to control the synaptic weight by applying various pulse amplitudes and cycles. Furthermore, potentiation and depression characteristics with the same spikes can be achieved by applying negative and positive pulses, respectively. By adopting complementary metal-oxide-semiconductor devices as neurons and TiN/PCMO devices as synapses, we implemented neuromorphic hardware that mimics associative memory characteristics in real time for the first time. Owing to their excellent scalability, resistive-switching devices, shows promise for future high-density neuromorphic applications.

Fabrication of nano-LaCrO3 receptor by polymeric precursor method and its impedancemetric NOx sensing properties.

LaCrO3 was prepared by using the polymeric precursor method for use as a receptor material and its NOx sensing characteristics were investigated. Nano-LaCrO3 powders were synthesized at the optimum compositions of the mole ratio of [La-, Cr-source]:[EG]:[AcAc] = [a, a]:[160 a]:[8 a] with 1 wt% polyvinylpyrrolidone (PVP) using ethylene glycol (EG) as a solvent, acetyl acetone (AcAc) as a chelating agent, and PVP as a polymer additive. The thermal decomposition behavior, crystal structure, morphology, and particle sizes of nano powders were characterized by a thermal analysis (TG-DTA), X-ray diffraction (XRD), a field emission scanning electron microscopy (FE-SEM), and a particle size analyzer, respectively LaCrO3 powders were mainly orthorhombic in structure and the primary particle size was 30 nm according to the XRD results. All solid-state compact impedancemetric-type sensor devices composed of Li1.5Al0.5Ti1.5(PO4)3 (LATP) as a transducer and a perovskite-type LaCrO3 nano powder as a receptor, have been investigated for their ability to detect NOx (NO and NO2) in the range of 1-250 ppm at 400 degrees C. The sensor device showed high gas sensitivities at NO gas, but relatively low gas sensitivities for NO2 gas.

Fabrication of a pure TiO2 thin film using a self-polymeric titania nano-sol and its properties.

A pure TiO2 thin film without adding any organic binder was fabricated by using a self-polymeric titania nano-sol (14 mass%), which was prepared by the acid peptization method. The particle size distribution in the 14 mass% TiO2 sol, in which almost of particles had a size below 10.2 nm and the crystal phase confirmed by X-ray diffraction analysis was anatase. The diluted nano-sol had a capability to form a thin film at a low temperature (100-400 degrees C) on the slide glass by dipping method. The average thickness of a coating film was measured to be about 0.25-0.30 microm. A coated film had a high refractive index over 1.88 at least irrespective of the heat-treatment even at room temperature drying and showed a super-hydrophilicity (< 5 degrees) after 20 minutes under Ultra Violet light irradiation, and it sustained in the darkness during a long period over 7 days depending on the heat-treatment conditions. Atomic Force Microscopic observation shows that the morphology of a heat-treated film had a relationship with the long-term hydrophilicity in the darkness.

Nanometer-Scale Phase Transformation Determines Threshold and Memory Switching Mechanism.

Creation of nanometer-scale conductive filaments in resistive switching devices makes them appealing for advanced electrical applications. While in situ electrical probing transmission electron microscopy promotes fundamental investigations of how the conductive filament comes into existence, it does not provide proof-of-principle observations for the filament growth. Here, using advanced microscopy techniques, electrical, 3D compositional, and structural information of the switching-induced conductive filament are described. It is found that during in situ probing microscopy of a Ag/TiO2 /Pt device showing both memory- and threshold-switching characteristics, a crystalline Ag-doped TiO2 forms at vacant sites on the device surface and acts as the conductive filament. More importantly, change in filament morphology varying with applied compliance currents determines the underlying switching mechanisms that govern either memory or threshold response. When focusing more on threshold switching features, it is demonstrated that the structural disappearance of the filament arises at the end of the constricted region and leads to the spontaneous phase transformation from crystalline conductive state into an initial amorphous insulator. Use of the proposed method enables a new pathway for observing nanosized features in a variety of devices at the atomic scale in three dimensions.

Effects of High-Pressure Hydrogen Annealing (HPHA) on Reliability Characteristics of RRAM.

Reliability characteristics (retention and endurance) of RRAM are critical for its practical realization and need to be improved. In this work, we confirmed the trade-off between retention and endurance by using various top electrode thickness conditions. The trade-off between retention and endurance characteristics was mainly due to the different amount of oxygen in scavenging layer (Ta) and the amount of oxygen vacancy in switching layer (HfO2). The amount of the oxygen in scavenging layer (Ta) and the amount of the oxygen vacancy in switching layer (HfO2) will be increased with the increase of Ta thickness. Therefore, the thicker Ta cells have worse retention because the large amount of oxygen in scavenging layer (Ta) can diffuse back into switching layer (HfO2) and recombine with oxygen vacancies in the filament. However, it has longer endurance because the large amount of oxygen vacancy in switching layer (HfO2) can be a source of the filament. Hence, there exists a trade-off relation between retention and endurance under the various Ta thickness conditions. To improve both retention and endurance characteristics, we proposed a new method by using high-pressure hydrogen annealing (HPHA). The thin Ta cells have longer retention and worse endurance because it has small amount of both oxygen in scavenging layer (Ta) and oxygen vacancy in switching layer (HfO2). Therefore, to generate more oxygen vacancies in switching layer (HfO2) maintaining small amount of oxygen in scavenging layer (Ta), we treated the samples by HPHA before Ta deposition. Finally, we obtained both improved retention and endurance characteristics in HfO2 based RRAM devices after high-pressure hydrogen annealing treatment.

Structurally engineered stackable and scalable 3D titanium-oxide switching devices for high-density nanoscale memory.

A 3D high-density switching device is realized utilizing titanium oxide, which is the most optimum material, but which is not practically demonstrated yet. The 1S1R (one ReRAM with the developed switching device) exhibits memory characteristics with a significantly suppressed sneak current, which can be used to realize high-density ReRAM applications.

Direct observation of internal fluidity in a water droplet during sliding on hydrophobic surfaces.

In the current study, we used a high-speed camera system with particle image velocimetry to observe the internal fluidity of water droplets during sliding. The droplets' velocity during sliding was controlled by slipping and rolling motions. On the superhydrophobic surface, with a contact angle of 150 degrees, the droplet fell at high velocity by slipping. However, on a normal hydrophobic surface whose water contact angle was around 100 degrees, both slipping and rolling controlled the droplet's velocity during sliding. In addition, the advancing velocity might be large when the slip velocity is large and the contact area is small.