Yoocharn Jeon

An accurate locally active memristor model for S-type negative differential resistance in NbOx

A number of important commercial applications would benefit from the introduction of easily manufactured devices that exhibit current-controlled, or “S-type,” negative differential resistance (NDR). A leading example is emerging non-volatile memory based on crossbar array architectures. Due to the inherently linear current vs. voltage characteristics of candidate non-volatile memristor memory elements, individual memory cells in these crossbar arrays can be addressed only if a highly non-linear circuit element, termed a “selector,” is incorporated in the cell. Selectors based on a layer of niobium oxide sandwiched between two electrodes have been investigated by a number of groups because the NDR they exhibit provides a promisingly large non-linearity. We have developed a highly accurate compact dynamical model for their electrical conduction that shows that the NDR in these devices results from a thermal feedback mechanism. A series of electrothermal measurements and numerical simulations corroborate this model. These results reveal that the leakage currents can be minimized by thermally isolating the selector or by incorporating materials with larger activation energies for electron motion.

Influence of Surfactant Structure on Reverse Micelle Size and Charge for Nonpolar Electrophoretic Inks

Electrophoretic inks, which are suspensions of colorant particles that are controllably concentrated and dispersed by applied electric fields, are the leading commercial technology for high-quality reflective displays. Extending the state of the art for high-fidelity color in these displays requires improved understanding and control of the colloidal systems. In these inks, reverse micelles in nonpolar media play key roles in media and particle charging. Here we investigate the effect of surfactant structure on reverse micelle size and charging properties by synthesizing different surfactants with variations in polyamine polar head groups. Small-angle X-ray scattering (SAXS) and dynamic light scattering (DLS) were used to determine the micelle core plus shell size and micelle hydrodynamic radius, respectively. The results from SAXS agreed with DLS and showed that increasing polyamines in the surfactant head increased the micelle size. The hydrodynamic radius was also calculated on the basis of transient current measurements and agreed well with the DLS results. The transient current technique further determined that increasing polyamines increased the charge stabilization capability of the micelles and that an analogous commercial surfactant OLOA 11000 made for a lower concentration of charge-generating ions in solution. Formulating magenta inks with the various surfactants showed that the absence of amine in the surfactant head was detrimental to particle stabilization and device performance.

NbO 2 -based low power and cost effective 1S1R switching for high density cross point ReRAM Application

In this paper, 5Xnm cross point cell array for the low power ReRAM operation was developed with 1S1R cell structure. Through the optimization of both TiOx/TaOx based-1R and NbO2 based-1S stacks with TiN based-electrode, the worlds first and best bipolar switching characteristics with the lowest operation current (20~50uA) and sneak current (~1uA) level were acquired.

Thermally induced crystallization in NbO2 thin films.

Niobium dioxide can exhibit negative differential resistance (NDR) in metal-insulator-metal (MIM) devices, which has recently attracted significant interest for its potential applications as a highly non-linear selector element in emerging nonvolatile memory (NVM) and as a locally-active element in neuromorphic circuits. In order to further understand the processing of this material system, we studied the effect of thermal annealing on a 15 nm thick NbO2 thin film sandwiched inside a nanoscale MIM device and compared it with 180 nm thick blanket NbOx (x = 2 and 2.5) films deposited on a silicon dioxide surface as references. A systematic transmission electron microscope (TEM) study revealed a similar structural transition from amorphous to a distorted rutile structure in both cases, with a transition temperature of 700 °C for the NbO2 inside the MIM device and a slightly higher transition temperature of 750 °C for the reference NbO2 film. Quantitative composition analysis from electron energy loss spectroscopy (EELS) showed the stoichiometry of the nominal 15 nm NbO2 layer in the as-fabricated MIM device deviated from the target 1:2 ratio because of an interaction with the electrode materials, which was more prominent at elevated annealing temperature.

Improvement of characteristics of NbO2 selector and full integration of 4F2 2x-nm tech 1S1R ReRAM

In this paper, the authors report that 2x nm cross-point ReRAM with 1S1R structure has been successfully developed. Off-current at 1/2 Vsw of 1S1R is one of key factor for high-density ReRAM. NbO2 was chosen as a selector material and off-current and forming characteristics were improved by using stack engineering of top and bottom barriers as well as spacer materials. Finally array operation was characterized with the integration of selector and resistor materials.

Short-Term Relaxation in HfO x /CeO x Resistive Random Access Memory With Selector

This letter illustrates short-term relaxation in CeOx-based resistive random access memory (RRAM) devices. Our results suggest that the noise of the serial selector device can impact the short-term relaxation, reduce the operating window of the RRAM, and increase the read error. Our findings indicate that the application of longer initial forming pulses can mitigate the short-term relaxation issue.

Transient-current characteristics of dispersed charges in a non-polar medium

— Transient currents of reverse micelles in a non-polar solvent from voltage step stimuli were studied to investigate the electrophoretic behavior of the charges. The current showed a sharp peak directly after the voltage application and decayed afterward while it exhibited various time-dependent transients depending on the applied voltage and the charge content after the bias was removed. A one-dimensional drift-diffusion model could reproduce the behaviors for various conditions. The forward transient could be well-explained by a simple capacitor-charging model with a limited charge. It turned out that the broad peak in the reverse transient current is formed by a competition between an increasing number of charges available for drift and a decreasing electric field resulting from mixing of opposite charges and that the full development of the peak is a good indication of complete polarization of the charges. The slow initial release of charges from the electrodes is due to the electric field developed by accumulated charges that decreases as the charges are released by diffusion. The high density compaction of charges against the electrodes reduces electric-field screening by the accumulated charges and enables more accumulation, but individual charge-to-charge interaction limits the density.

Effect of annealing on structural changes and oxygen diffusion in amorphous HfO2 using classical molecular dynamics

Non-volatile memory is a promising alternative to present memory technologies. Oxygen vacancy diffusion has been widely accepted as one of the reasons for the resistive switching mechanism of transition-metal-oxide based resistive random access memory. In this study, molecular dynamics simulation is applied to investigate the diffusion coefficient and activation energy of oxygen in amorphous hafnia. Two sets of empirical potential, Charge-Optimized Many-Body (COMB) and Morse-BKS (MBKS), were considered to investigate the structural and diffusion properties at different temperatures. COMB predicts the activation energy of 0.53 eV for the temperature range of 1000–2000 K, while MBKS predicts 2.2 eV at high temperature (1600–2000 K) and 0.36 eV at low temperature (1000–1600 K). Structural changes and appearance of nano-crystalline phases with increasing temperature might affect the activation energy of oxygen diffusion predicted by MBKS, which is evident from the change in coordination number distribution an...