Jeonghwan Song

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.

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.

Effects of RESET Current Overshoot and Resistance State on Reliability of RRAM

Current overshoot has severe effects on the reliability of resistive random access memory (RRAM). It is well known that the current overshoot during the SET process is caused by parasitic capacitance. In this letter, we observed a different type of current overshoot during the RESET process. The RESET current overshoot was confirmed to have severe effects on the endurance of RRAM. We also demonstrated the relation between the current overshoot and the intrinsic capacitive elements of each state of RRAM. Finally, an optimized pulse shape was proposed to minimize the current overshoot and was experimentally verified to significantly improve the variability and endurance in a typical RRAM device with a W/Zr/HfO2/TiN structure.

Selector-less ReRAM with an excellent non-linearity and reliability by the band-gap engineered multi-layer titanium oxide and triangular shaped AC pulse

The effect of oxygen profile control of a multi-layer TiOx on tunnel barrier characteristics has been investigated to achieve high non-linearity, endurance, and uniformity of a selector-less ReRAM. By optimizing oxygen profile of TiOx layer in the selector-less ReRAM, non-linearity and a readout margin (low ILRS at ½VRead) have been significantly improved compared with 1S1R-type devices and non-linear ReRAMs (Figs. 1-2) [2]-[5]. In addition, AC behaviors of the selector-less ReRAM have been investigated with various AC pulse shape to realize the AC operation of the selector-less ReRAM in cross-point array. Hence, significantly improved AC switching reliability of the selector-less ReRAM was obtained by adopting triangular AC pulse shapes for both set and reset mode.

Highly Reliable Resistive Switching Without an Initial Forming Operation by Defect Engineering

The effects of stack and defect engineering of metal-oxide layers on resistive switching uniformity were investigated to obtain resistive random access memory (ReRAM) with excellent switching reliability. Uniform switching, parameters, such as set voltage (Vset), reset voltage (Vreset), low-resistance state, high-resistance state, and retention characteristics, were significantly improved by stack and defect engineering. Furthermore, the initial forming operation, which is a nuisance, was removed to realize cross-point ReRAM.

Optimized Programming Scheme Enabling Linear Potentiation in Filamentary HfO 2 RRAM Synapse for Neuromorphic Systems

In this brief, we demonstrate the multilevel cell (MLC) characteristics of an HfO2-based resistive memory (RRAM) array as a synaptic element for neuromorphic systems. We utilize various programming schemes to linearly change the resistance state with either set voltage/pulse ramping or gate voltage ramping. Our results reveal that the MLC relates to the size of the conductive filament involved in the movement of oxygen vacancies with respect to applying pulses. Thus, by optimizing the pulse for a set condition, such as an identical pulse, we achieve linearly increased MLC behavior, thereby enabling a high accuracy for pattern recognition in neuromorphic systems.