Jiyong Woo

RRAM-based synapse for neuromorphic system with pattern recognition function

Feasibility of a high speed pattern recognition system using 1k-bit cross-point synaptic RRAM array and CMOS-based neuron chip has been experimentally demonstrated. Learning capability of a neuromorphic system comprising RRAM synapses and CMOS neurons has been confirmed experimentally, for the first time.

Threshold Selector With High Selectivity and Steep Slope for Cross-Point Memory Array

In this letter, we demonstrated a new type of threshold selector with excellent electrical characteristics for cross-point memory array. The proposed Ag/TiO2-based threshold selector device showed high selectivity (~107) and steep slope (<;5 mV/decade). The observed threshold switching in programmable metallization cell device occurred due to the spontaneous rupturing of silver (Ag) filament. The Ag ionization to minimize the steric repulsion between Ag and surrounding TiO2 electrolyte was the main origin of the spontaneous rupture.

Nanoscale (∼10nm) 3D vertical ReRAM and NbO 2 threshold selector with TiN electrode

The scaling and 3-D integration issues of NbO₂ with threshold switching characteristics were investigated for ReRAM selector device. To avoid the process problems of Pt electrode, we tested ReRAM and selector devices with conventional electrodes (TiN and W). By adopting 10nm-thick TiN bottom electrode with low thermal conductivity, we could significantly reduce the threshold current for insulator-metal transition (I-M-T) due to the heat confinement effect. We have evaluated for the first time both 1S1R (NbO₂/TaO_x) and hybrid (NbO₂/Nb₂O₅) devices. We have confirmed the feasibility of high density vertical memory device by adopting NbO₂ I-M-T selector device.

Improved Synaptic Behavior Under Identical Pulses Using AlO x /HfO 2 Bilayer RRAM Array for Neuromorphic Systems

We analyze the response of identical pulses on a filamentary resistive memory (RRAM) to implement the synapse function in neuromorphic systems. Our findings show that the multilevel states of conductance are achieved by varying the measurement conditions related to the formation and rupture of a conductive filament. Furthermore, abrupt set switching behavior in the RRAM leads to an unchanged conductance state, leading to degradation in the accuracy of pattern recognition. Thus, we demonstrate a linear potentiation (or depression) behavior of conductance under identical pulses using the effect of barrier layer on the switching, which was realized by fabricating an RRAM on top of an Al electrode. As a result, when the range of the conductance is symmetrically controlled at both polarities, a significantly improved accuracy is achieved for pattern recognition using a neural network with a multilayer perceptron.

Demonstration of Low Power 3-bit Multilevel Cell Characteristics in a TaO x -Based RRAM by Stack Engineering

Multilevel cell (MLC) storage technology is attractive in achieving ultrahigh density memory with low cost. In this letter, we have demonstrated 3-bit per cell storage characteristics in a TaOx-based RRAM. By analyzing the key requirements for MLC operation mainly the switching uniformity and stability of resistance levels, an engineered stack based on thermodynamics in top electrode/(vacancy reservoir/defect control layer)/switching layer/bottom electrode structure was designed. In the optimized stack with ~10-nm Ta layer incorporated at W/TaOx interface, seven low resistance state levels with same high resistance state were obtained by controlling the switching current down from 30 μA enabling low power 3-bit storage in contrast to the control device which shows 2-bit MLC with resistance saturation. The improved switching and MLC behavior is attributed to the minimized stochastic nature of set/reset operations due to filament confinement by favorable electric field generation and formation of thin but highly conductive filament which is confirmed electrically.

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.

Defect engineering: reduction effect of hydrogen atom impurities in HfO2-based resistive-switching memory devices

In this study, we propose a new and effective methodology for improving the resistive-switching performance of memory devices by high-pressure hydrogen annealing under ambient conditions. The reduction effect results in the uniform creation of oxygen vacancies that in turn enable forming-free operation and afford uniform switching characteristics. In addition, H(+) and mobile hydroxyl (OH(-)) ions are generated, and these induce fast switching operation due to the higher mobility compared to oxygen ions. Defect engineering, specifically, the introduction of hydrogen atom impurities, improves the device performance for metal-oxide-based resistive-switching random access memory devices.

TiO x -Based RRAM Synapse With 64-Levels of Conductance and Symmetric Conductance Change by Adopting a Hybrid Pulse Scheme for Neuromorphic Computing

We propose TiOx-based resistive switching device for neuromorphic synapse applications. This device is capable of 64-levels conductance states because of their optimized interface between the metal electrode and the TiOx film. To compensate the change in switching power with increasing pulse number, we propose the use of fixed voltage and current pulses in potentiation and depression conditions, respectively. By adopting a hybrid pulse scheme, the symmetry of conductance change under both potentiation and depression conditions is shown to be significantly improved. Both the improved conductance levels and the symmetry of conductance change are directly related with enhanced pattern recognition accuracy, which is confirmed by a neural network simulation.

Vertically Stacked ReRAM Composed of a Bidirectional Selector and CB-RAM for Cross-Point Array Applications

In this letter, we discuss our technique for fabricating a vertically stacked ReRAM device composed of one selector and one resistor (1S-1R). We demonstrate that the nanoscale via-hole structure and 1-kb array architecture of selector device exhibit higher current density ( ~ 107 A/cm2) and reliability, and we introduce bipolar resistive switching element-a conductive-bridge RAM that can be stacked on top of the selector device. The resulting integrated 1S-1R device performs robust bipolar switching operations and significantly reduces the leakage current in cross-point applications.

Bidirectional threshold switching in engineered multilayer (Cu2O/Ag:Cu2O/Cu2O) stack for cross-point selector application

In this study, we achieved bidirectional threshold switching (TS) for selector applications in a Ag-Cu2O-based programmable-metallization-cell device by engineering the stack wherein Ag was intentionally incorporated in the oxide (Cu2O) layer by a simple approach comprising co-sputtering and subsequent optimized annealing. The distribution of the Ag was directly confirmed by transmission electron microscopy and energy dispersive spectroscopy line profiling. The observed TS occurred because of the spontaneous self-rupturing of the unstable Ag filament that formed in the oxide layer.