Vertically Stackable Ovonic Threshold Switch Oscillator Using Atomic Layer Deposited Ge0.6Se0.4 Film for High-Density Artificial Neural Networks.

Nanodevice oscillators (nano-oscillators) have received considerable attention to implement in neuromorphic computing as hardware because they can significantly improve the device integration density and energy efficiency compared to complementary metal oxide semiconductor circuit-based oscillators. This work demonstrates vertically stackable nano-oscillators using an ovonic threshold switch (OTS) for high-density neuromorphic hardware. A vertically stackable Ge0.6Se0.4 OTS-oscillator (VOTS-OSC) is fabricated with a vertical crossbar array structure by growing Ge0.6Se0.4 film conformally on a contact hole structure using atomic layer deposition. The VOTS-OSC can be vertically integrated onto peripheral circuits without causing thermal damage because the fabrication temperature is <400 °C. The fabricated device exhibits oscillation characteristics, which can serve as leaky integrate-and-fire neurons in spiking neural networks (SNNs) and coupled oscillators in oscillatory neural networks (ONNs). For practical applications, pattern recognition and vertex coloring are demonstrated with SNNs and ONNs, respectively, using semiempirical simulations. This structure increases the oscillator integration density significantly, enabling complex tasks with a large number of oscillators. Moreover, it can enhance the computational speed of neural networks due to its rapid switching speed.

2Memristor-1Capacitor Integrated Temporal Kernel for High-Dimensional Data Mapping.

Compact but precise feature-extracting ability is core to processing complex computational tasks in neuromorphic hardware. Physical reservoir computing (RC) offers a robust framework to map temporal data into a high-dimensional space using the time dynamics of a material system, such as a volatile memristor. However, conventional physical RC systems have limited dynamics for the given material properties, restricting the methods to increase their dimensionality. This study proposes an integrated temporal kernel composed of a 2-memristor and 1-capacitor (2M1C) using a W/HfO2/TiN memristor and TiN/ZrO2/Al2O3/ZrO2/TiN capacitor to achieve higher dimensionality and tunable dynamics. The kernel elements are carefully designed and fabricated into an integrated array, of which performances are evaluated under diverse conditions. By optimizing the time dynamics of the 2M1C kernel, each memristor simultaneously extracts complementary information from input signals. The MNIST benchmark digit classification task achieves a high accuracy of 94.3% with a (196×10) single-layer network. Analog input mapping ability is tested with a Mackey-Glass time series prediction, and the system records a normalized root mean square error of 0.04 with a 20×1 readout network, the smallest readout network ever used for Mackey-Glass prediction in RC. These performances demonstrate its high potential for efficient temporal data analysis.

Atomic Layer Deposition of Sb2 Te3 /GeTe Superlattice Film and Its Melt-Quenching-Free Phase-Transition Mechanism for Phase-Change Memory.

Atomic layer deposition (ALD) of Sb2 Te3 /GeTe superlattice (SL) film on planar and vertical sidewall areas containing TiN metal and SiO2 insulator is demonstrated. The peculiar chemical affinity of the ALD precursor to the substrate surface and the 2D nature of the Sb2 Te3 enable the growth of an in situ crystallized SL film with a preferred orientation. The SL film shows a reduced reset current of ≈1/7 of the randomly oriented Ge2 Sb2 Te5 alloy. The reset switching is induced by the transition from the SL to the (111)-oriented face-centered-cubic (FCC) Ge2 Sb2 Te5 alloy and subsequent melt-quenching-free amorphization. The in-plane compressive stress, induced by the SL-to-FCC structural transition, enhances the electromigration of Ge along the [111] direction of FCC structure, which enables such a significant improvement. Set operation switches the amorphous to the (111)-oriented FCC structure.

Atomic layer deposition of SnSex thin films using Sn(N(CH3)2)4 and Se(Si(CH3)3)2 with NH3 co-injection.

This study introduces the atomic layer deposition (ALD) of tin selenide thin films using Sn(N(CH3)2)4 and Se(Si(CH3)3)2 with NH3 co-injection. The co-injection of NH3 with Se(Si(CH3)3)2 is essential for film growth to convert the precursor into a more reactive form. The most critical feature of this specific ALD process is that the chemical composition (Sn/Se ratio) could be varied by changing the growth temperature, even for the given precursor injection conditions. The composition and morphology of the deposited films varied depending on the process temperature. Below 150 °C, a uniform SnSe2 thin film was deposited in an amorphous phase, maintaining the oxidation states of its precursors. Above 170 °C, the composition of the film changed to 1 : 1 stoichiometry due to the crystallization of SnSe and desorption of Se. A two-step growth sequence involving a low-temperature seed layer was devised for the high-temperature ALD of SnSe to improve surface roughness.

Time-varying data processing with nonvolatile memristor-based temporal kernel.

Recent advances in physical reservoir computing, which is a type of temporal kernel, have made it possible to perform complicated timing-related tasks using a linear classifier. However, the fixed reservoir dynamics in previous studies have limited application fields. In this study, temporal kernel computing was implemented with a physical kernel that consisted of a W/HfO2/TiN memristor, a capacitor, and a resistor, in which the kernel dynamics could be arbitrarily controlled by changing the circuit parameters. After the capability of the temporal kernel to identify the static MNIST data was proven, the system was adopted to recognize the sequential data, ultrasound (malignancy of lesions) and electrocardiogram (arrhythmia), that had a significantly different time constant (10-7 vs. 1 s). The suggested system feasibly performed the tasks by simply varying the capacitance and resistance. These functionalities demonstrate the high adaptability of the present temporal kernel compared to the previous ones.

Comparison of screw-home mechanism in the unloaded living knee subjected to active and passive movements.

BACKGROUND: The screw-home mechanism (SHM) plays an important role in the stability of the knee. Accordingly, the analysis of tibial rotation patterns can be used to elucidate the effect of SHM-related factors. OBJECTIVE: The purpose of this study was to compare the magnitude of the angle and the pattern of SHM between passive and active movements. METHODS: We studied twenty healthy males, of which the angle of knee flexion-extension and tibial longitudinal rotation (TLR) during active and passive movements were measured using the inertial measurement unit. Student's t-tests were used to compare the magnitude of TLR. The waveform similarity was quantified using a coefficient of multiple correlation (CMC). RESULTS: Significant differences were found in the TLR between the active and passive movements (p< 0.05). The knee flexion-extension waveform similarity was excellent (CMC = 0.956). However, the waveform similarity of TLR was weak (CMC = 0.629). CONCLUSION: The SHM increased abruptly during the last 20∘ of the active (extension) movement compared with passive extension. The SHM occurred mainly owing to the geometry and shape of the articular surfaces of the knee joint. In addition, muscle contraction was considered to be an important factor in the articulation movement.

Atomic Layer Deposition of GexSe1-x Thin Films for Endurable Ovonic Threshold Selectors with a Low Threshold Voltage.

An ovonic threshold switch (OTS) based on amorphous chalcogenide materials possesses several desirable characteristics, including high selectivity and fast switching speed, enabling the fabrication of one selector-one resistor (1S-1R) crossbar array (CBA) for random access memory. Among the several chalcogenide materials, GeSe offers high selectivity and a strong glass-forming ability with environment-friendly, simple binary composition. In this report, the GeSe thin films were deposited via atomic layer deposition (ALD) using Ge(N(Si(CH3)3)2)2 and ((CH3)3Si)2Se for its envisioned application in fabricating three-dimensional vertical-type phase-change memory. Highly conformal GexSe1-x films were obtained at a substrate temperature ranging from 70 to 160 °C. The unique deposition mechanism that involves Ge intermediates provided a way to modulate the composition of the Ge-Se films from 5:5 to 7:3. Low threshold voltages ranging from 1.2 to 1.4 V were observed depending on the composition. A cycling endurance of more than 106 was achieved with the Ge0.6Se0.4 composition with 104 half-bias nonlinearity. This work presents the foundations for the future development of vertical-type 1S-1R arrays when combined with the ALD technique for Ge2Sb2Te5 phase-change materials.

Electroforming-Free Bipolar Resistive Switching in GeSe Thin Films with a Ti-Containing Electrode.

Chalcogenide materials have been regarded as strong candidates for both resistor and selector elements in passive crossbar arrays owing to their dual capabilities of undergoing threshold and resistance switching. This work describes the bipolar resistive switching (BRS) of amorphous GeSe thin films, which used to show Ovonic threshold switching (OTS) behavior. The behavior of this new functionality of the material follows filament-based resistance switching when Ti and TiN are adopted as the top and bottom electrodes, respectively. The detailed analysis revealed that the high chemical affinity of Ti to Se produces a Se-deficient GexSe1-x matrix and the interfacial Ti-Se layer. Electroforming-free BRS behavior with reliable retention and cycling endurance was achieved. The performance improvement was attributed to the Ti-Se interfacial layer, which stabilizes the composition of GeSe during the electrical switching cycles by preventing further massive Se migration to the top electrode. The conduction mechanism analysis denotes that the resistance switching originates from the formation and rupture of the high-conductance semiconducting Ge-rich GexSe1-x filament. The high-resistance state follows the modified Poole-Frenkel conduction.

Matrix Mapping on Crossbar Memory Arrays with Resistive Interconnects and Its Use in In-Memory Compression of Biosignals.

Recent advances in nanoscale resistive memory devices offer promising opportunities for in-memory computing with their capability of simultaneous information storage and processing. The relationship between current and memory conductance can be utilized to perform matrix-vector multiplication for data-intensive tasks, such as training and inference in machine learning and analysis of continuous data stream. This work implements a mapping algorithm of memory conductance for matrix-vector multiplication using a realistic crossbar model with finite cell-to-cell resistance. An iterative simulation calculates the matrix-specific local junction voltages at each crosspoint, and systematically compensates the voltage drop by multiplying the memory conductance with the ratio between the applied and real junction potential. The calibration factors depend both on the location of the crosspoints and the matrix structure. This modification enabled the compression of Electrocardiographic signals, which was not possible with uncalibrated conductance. The results suggest potential utilities of the calibration scheme in the processing of data generated from mobile sensing or communication devices that requires energy/areal efficiencies.

Association between Cigarette Smoking Frequency and Health Factors among Korean Adults.

BACKGROUND: Recently, there has been a trend that cigarette smoking rate in Asian and Africa adults has increased while the age group to start smoking has decreased gradually. This study aimed to investigate the relationships between lifetime smoking and hypertension, diabetes, obesity, waist measure, fasting blood pressure and food consumption, in order to look into health status depending on smoking status in Koreans. METHODS: Totally, 1075 men and 697 women with no disease participated in this study, in which one-way ANOVA was conducted by using SPSS version 18.0 for statistical process. The level of statistical significance was 0.05. RESULTS: As a result of analysis on relationship between lifetime smoking and hypertension, obesity and diabetes, statistically significant differences were revealed.Lifetime smoking was found to be significantly associated with increased waist measure, higher level of fasting blood sugar, and more ingestion of nutrients (carbohydrate, fat, and protein). CONCLUSION: Increased amount of lifetime cigarette smoking was shown to negatively influence various health factors, which might become to be a drive to cause diseases. Therefore, method to improve health factors must be sought for via education and campaign to control an amount of cigarette smoking in Korean adults.