Jaesik Yoon

An electrically modifiable synapse array of resistive switching memory

This paper describes the resistive switching of a cross-point cell array device, with a junction area of 100 nm x 100 nm, fabricated using ultraviolet nanoimprinting. A GdO(x) and Cu-doped MoO(x) stack with platinum top and bottom electrodes served as the resistive switching layer, which shows analog memory characteristics with a resistance ratio greater than 10. To demonstrate a neural network circuit, we operated the cell array device as an electrically modifiable synapse array circuit and carried out a weighted sum operation. This demonstration of cross-point arrays, based on resistive switching memory, opens the way for feasible ultra-high density synapse circuits for future large-scale neural network systems.

Excellent Switching Uniformity of Cu-Doped

We have investigated a Cu-doped MoO_x/GdO_x bilayer film for nonvolatile memory applications. By adopting an ultrathin GdO_x layer, we obtained excellent device characteristics such as resistance ratio of three orders of magnitude, uniform distribution of set and reset voltages, switching endurance up to 10⁴ cycles, and ten years of data retention at 85degC. By adopting bilayer films of Cu-doped MoO_x/GdO_x, a local filament was formed by a two-step process. Improved memory characteristics can be explained by the formation of nanoscale local filament in the ultrathin GdO_x layer.

\hbox{MoO}_{x}/\hbox{GdO}_{x}

A systematic study on the switching mechanism of an Al/ Pr0.7Ca0.3MnO3 (PCMO) device was performed. A polycrystalline PCMO film was deposited using a conventional sputtering method. A thin Al layer was introduced to induce a reaction with the PCMO, forming aluminum oxide (AlOx). Transmission electron microscopy analysis of the interface between Al and PCMO showed that resistive switching was governed by the formation and dissolution of AlOx. Some basic memory characteristics, such as good cycle endurance and data retention of up to 104 s at 125°C, were also obtained. It also showed excellent switching uniformity and high device yield.

Bilayer for Nonvolatile Memory Applications

We investigated the state stability of the low-resistance state (LRS) in a resistive switching memory having a Pt/Cu:MoOx/GdOx/Pt structure. Various resistance values of LRS were accurately controlled using an external load resistor connected in series with the resistive memory device. We found that the retention time decreased with an increase in the resistance of LRS. We performed accelerating tests for resistance transition from a low- to a high-resistance state under temperatures ranging from 200°C to 250°C. A predicted resistance of LRS for a ten-year retention period at 85°C was determined based on the Arrhenius law.

Resistive-Switching Characteristics of

We have investigated copper-doped carbon (CuC) as a new solid-state electrolyte material for resistive switching devices. Compared with CuS electrolytes, CuC devices demonstrate good memory characteristics such as a high resistance ratio of over two orders, higher operation voltage, and high temperature retention characteristics. Using 1000 cell array devices, we have also confirmed uniform distributions of resistance and switching voltages. Both high and low resistance states showed negligible degradation of resistance for over 104 s at 85 °C, confirming good retention characteristics.

\hbox{Al}/ \hbox{Pr}_{0.7}\hbox{Ca}_{0.3}\hbox{MnO}_{3}

The nanoscale resistance switching property of copper-carbon-mixed (Cu-C) layer was investigated for nonvolatile memory applications. The Cu-C layer of the cross-point cell array showed typical filament switching with two orders of on/off ratio, exhibiting stable resistance switching and a narrow distribution of set and reset voltages in the nanoscale junction. In addition, we investigated the area dependence of operation current. Based on these results and current-voltage dependence on temperature, we discussed a potential switching mechanism of Cu-C layer.

for Nonvolatile Memory Applications

We found that the charge loss behavior of metal-alumina-nitride-oxide-silicon-type flash memory was highly dependent on the amount of injected charge (Qinj). Beyond the critical level of Qinj, the direction of the dominant charge loss changed from pointing toward SiO2 to pointing toward Al2O3. The highly injected charges could cause the band bending of Al2O3, which reduced the tunneling distance across Al2O3 with the low conduction band offset. These results were verified by experimental results and theoretical device modeling through a comparison of the charge loss rate and the tunneling rate between a SiO2∕Si3N4∕SiO2 stack and a SiO2∕Si3N4∕Al2O3 stack.

Investigation of State Stability of Low-Resistance State in Resistive Memory

For the device application of resistive change memory, device performance as well as uniformity and understanding of the mechanism is necessary. The resistance change due to filament formation is undesirable, as it is hard to apply for practical application. In order to have a nonlocalized, uniformly distributed, nonfilamentary-type resistance change memory device, a switching mechanism based on oxide formation and deformation is beneficial. This phenomenon largely depends on the metal electrode on top of the memory oxides. In this study both the device performance, such as pulse switching speed, and the switching mechanism were understood with different metal electrode-oxide combinations. The switching speed is determined by the slowest process step, which in this case is the dissolving of oxide during the set process. A set pulse width of 20 μs and reset pulse width of 1 As were shown. Pulse switching speed was also confirmed by monitoring the delay time, comparing the output signal pulse with the input signal. The devices showed good uniformity and data retention over 10 years. Overall the combination of LaCaMnO 3 with a carefully designed metal electrode stack has great potential for future nonvolatile memory device applications.

Electrical and reliability characteristics of copper-doped carbon (CuC) based resistive switching devices for nonvolatile memory applications

We investigated the electrical properties of a Pt/Nb-doped SrTiO3 (Nb:STO) single crystal Schottky junction. The junction exhibited highly rectifying current–voltage (I–V) characteristics under a DC bias. To make a junction profile, the capacitance–voltage (C–V) curve for reverse bias was measured – the curve showed little change in Schottky barrier height. The large leakage current problem of Nb:STO oxide was overcome using an ultrathin metal–oxide–semiconductor (MOS) capacitor model. On the basis of our results and previous reports, we propose a plausible mechanism for a Pt/Nb:STO Schottky junction, where the tunneling effect is the main cause of the switching.

Nanoscale Resistive Switching of a Copper–Carbon-Mixed Layer for Nonvolatile Memory Applications

In this study, we examined the characteristics of Ni/Au (20 nm/80 nm) ohmic contacts to non-polar a-plane p-GaN as a function of annealing temperature. The current?voltage (I?V) curves were showed an upward curve with annealing when Ni/Au metals were used as ohmic metals to nonpolar p-GaN, which was similar to those of the other crystalline planes. The contact resistivity decreased from 2.36 to 6.95 ? 10?3 ? cm2. Secondary ion mass spectroscopy showed that the Ga atoms out-diffused from the GaN substrate after annealing at 400 ?C, which led to the generation of Ga vacancies. The formation of Ga and N vacancies was found to be a competing process during annealing.