Seungjae Jung

Analog memory and spike-timing-dependent plasticity characteristics of a nanoscale titanium oxide bilayer resistive switching device.

We demonstrated analog memory, synaptic plasticity, and a spike-timing-dependent plasticity (STDP) function with a nanoscale titanium oxide bilayer resistive switching device with a simple fabrication process and good yield uniformity. We confirmed the multilevel conductance and analog memory characteristics as well as the uniformity and separated states for the accuracy of conductance change. Finally, STDP and a biological triple model were analyzed to demonstrate the potential of titanium oxide bilayer resistive switching device as synapses in neuromorphic devices. By developing a simple resistive switching device that can emulate a synaptic function, the unique characteristics of synapses in the brain, e.g. combined memory and computing in one synapse and adaptation to the outside environment, were successfully demonstrated in a solid state device.

Excellent Selector Characteristics of Nanoscale

We herein present a nanoscale vanadium oxide (VO₂) device with excellent selector characteristics such as a high on/off ratio (>; 50), fast switching speed (<; 20 ns), and high current density (>; 10⁶ A/cm2). Owing to extrinsic defects, a large-area device with a 20-nm-thick VO₂ layer underwent an electrical short. In contrast, after scaling the device active area (<; 5 × 10⁴ nm²), excellent switching uniformity was obtained. This can be explained by the reduced defects and the metal-insulator transition of the whole nanoscale VO₂. By integrating a bipolar resistive random access memory device with the VO₂ selection device, a significantly improved readout margin was obtained. The VO₂ selection device shows good potential for cross-point bipolar resistive memory applications.

\hbox{VO}_{2}

We report a simple metal-insulator-metal (MIM)-type selection device that can alleviate the sneak current path in cross-point arrays. By connecting a nanometer-scale Pt/TiO2/TiN selection device to a Pt/TiO2−x/TiO2/W resistive random access memory (RRAM), we could significantly reduce read disturbance from unselected memory cells. This selection device could be easily integrated into an RRAM device, in which it suppressed the sneak current and significantly improved the readout margin compared to that obtained for an RRAM not using a selection device. The introduction of this MIM device can fulfill the requirement for an appropriate selection device for bipolar-type RRAM cross-point applications.

for High-Density Bipolar ReRAM Applications

We have investigated the bilayer structure of binary oxides such as HfOx and ZrOx for applications to resistance memory. The ZrOx/HfOx bilayer structure shows a lower reset current and operating voltage than an HfOx monolayer under dc sweep voltage. Furthermore, the bilayer structure exhibits a tight distribution of switching parameters, good switching endurance up to 105 cycles, and good data retention at 85 °C. The resistive switching mechanism of memory devices incorporating the ZrOx/HfOx bilayer structure can be attributed to the control of multiple conducting filaments through the occurrence of redox reactions at the tip of the localized filament.

TiO2-based metal-insulator-metal selection device for bipolar resistive random access memory cross-point application

We report excellent switching uniformity and reliability of RRAM device with ZrOx/HfOx bi-layer films. Precise control of the oxygen vacancy concentration in HfO2 layer was achieved by depositing thin Zr metal (2–15nm) layer. Scaling down active device area (ϕ=50 nm) and film thickness (&#60;2–5 nm) can significantly minimize the extrinsic defects-related non-uniform switching which was normally observed in large area (ϕ >um) device, with higher active layer thickness (>10 nm). Using back-to-back connection of two RRAM devices, we confirmed feasibility of a diode-free cross-point array with a wide readout margin and stable data reading. Considering excellent electrical and reliability characteristics of diode-free RRAM device, shows a great promise for future high density cross-point memory devices

Effect of ZrOx/HfOx bilayer structure on switching uniformity and reliability in nonvolatile memory applications

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.

Diode-less nano-scale ZrO x /HfO x RRAM device with excellent switching uniformity and reliability for high-density cross-point memory applications

We demonstrated multibit operation using a 250-nm Ir/TiOx/ TiN resistive random access memory by Schottky barrier height engineering. A Schottky barrier was formed by the interface between a high-work-function Ir top electrode and n-type TiOx. The conducting path, which was composed of oxygen vacancies, was generated in a low-resistance state, whereas a Schottky barrier was reproduced in a high-resistance state (HRS) due to the high concentration of oxygen by the electric field. By changing the reset operation voltage, we successfully engineered the Schottky barrier height, resulting in the modulation of the HRS current and demonstrating the feasibility of multibit applications.

Excellent Switching Uniformity of Cu-Doped

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.

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

We report on a simple and effective ac and dc dielectrophoresis (DEP) method that can be used to align and manipulate semiconductor gallium nitride (GaN) nanowires (NWs) with variations in the type of electrical fields as well as variations of frequency. We observed that the ability of the alignment and the formation of the assembling nanowires (single or a bundle configuration) strongly depend on the magnitude of both the ac and dc electric fields. The yield results indicate that the GaN NWs, using ac DEP, are better aligned with a higher yield rate of approximately 80% over the entire array in the chip than by using dc DEP. In addition, we first demonstrated the simple hybrid p-n junction structures assembled by n-type GaN nanowires together with a p-type silicon substrate (n-GaN NW/p-Si substrate) using dielectrophoresis. From the transport measurements, the p-n junction structures show well-defined current rectifying behaviour with a low reverse leakage current of approximately 3 x 10(-4) A at -25 V. We believe that our unique p-n junction structures can be useful for electronic and optoelectronic nanodevices such as rectifiers and UV nano-LEDs.

Bilayer for Nonvolatile Memory Applications

We report, for the first time, the novel concept of ultrathin (~10nm) W/NbO_x/Pt device with both threshold switching (TS) and memory switching (MS) characteristics. Excellent TS characteristics of NbO₂, such as high temperature stability (~160°C), fast switching speed (~22ns), good switching uniformity, and extreme scalability of device area (φ~10nm)/thickness (~10nm) were obtained. By oxidizing NbO₂, we can form ultrathin Nb₂O₅/NbO₂ stack layer for hybrid memory devices with both TS and MS. Without additional selector device, 1Kb cross-point hybrid memory device without SET/RESET disturbance up to 10⁶ cycles was demonstrated.