Godeuni Choi

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, 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.

for High-Density Bipolar ReRAM Applications

Hydrogenation of thin-film indium-gallium-zincoxide (IGZO) is carried out by applying a high-pressure hydrogen annealing (HPHA) process under 5-atm pressure at different temperatures of 260°C, 270°C, and 280°C. The HPHA effectively increases the carrier concentration and the Hall mobility up to ~ 10¹⁹ cm^-3 and ~ 6.4 cm²/Vs, respectively. The HPHA-IGZO films exhibit smoother surfaces as compared with the as-grown films. The HPHA performs at a temperature of 260 ^°C that greatly enhances the electrical characteristics of IGZO TFTs, leading to a saturation field effect mobility of 7.4 cm²/Vs, a subthreshold slope of 0.37 V/decade, a threshold voltage of 2.2 V, and an ION/IOFF of 2.0×10⁶.

Ultrathin (l10nm) Nb 2 O 5 /NbO 2 hybrid memory with both memory and selector characteristics for high density 3D vertically stackable RRAM applications

For the first time, we have investigated the resistive switching characteristics in extreme size (sub-5nm) device. Less than 5nm effective electrode radius is confirmed by conductive-AFM and FIB-TEM analysis. The conductive filament source (Cu ions) is limited by applying novel fabrication technology. Due to the limited Cu source, we observe the quantized formation of conductive path, which results in the distinguishable conductance states and shows the feasibility of multi-bit operation. By controlling the motion of Cu ions precisely, ideal selection behavior for xpoint memory application was achieved.

Hydrogenated IGZO Thin-Film Transistors Using High-Pressure Hydrogen Annealing

We demonstrate a varistor-type bidirectional switch (VBS) with excellent selection property for future 3D bipolar resistive memory array. A highly non-linear VBS showed superior performances including high current density (>;3×10⁷A/cm²) and high selectivity (~10⁴). The non-linear I-V characteristics can be explained by varistor-type multi-layer tunnel barriers, which were formed by Ta incorporation into thin TiO₂. Furthermore, the 1S1R device showed excellent suppression of leakage current (>;10⁴ reduction) at 1/2V_READ, which is promising for ultra-high density resistive memory applications.

Quantized conductive filament formed by limited Cu source in sub-5nm era

For the high-density memory application of resistive random access memory (ReRAM), we study the complementary resistive switch (CRS) behavior of a HfOx-based ReRAM with a TiOx-based ReRAM. To control the operation voltages of the CRS device, we used ReRAMs having asymmetric set and reset voltages. Consequently, we achieved a wider voltage window for the read process, high switch speed, high reliability, and more than ten times readout margin from the heterodevice CRS.

Varistor-type bidirectional switch (J MAX >10 7 A/cm 2 , selectivity∼10 4 ) for 3D bipolar resistive memory arrays

We proposed for the first time thermally-assisted resistive switching random access memory (ReRAM) device to overcome the voltage-time dilemma [1] using Ti/Pr<inf>0.7</inf>Ca<inf>0.3</inf>MnO<inf>3</inf> (Ti/PCMO) with Ge<inf>2</inf>Sb<inf>2</inf>Te<inf>5</inf> (GST) thermoelectric heater as well as thermal barrier (Fig. 1). “Thermoelectric heating effect” from GST/Ti junction [2] and “thermal barrier effect” from the heat confinement by GST and PCMO thermal insulators [3,4] successfully improved switching speed while “large effective Schottky barrier (SB) height (Ф<inf>eff</inf>)” provided excellent robustness to high-temperature retention and read/set/reset disturbance characteristics (Fig. 2).

Operation Voltage Control in Complementary Resistive Switches Using Heterodevice

We report a TiOx-based resistance-switching device that exhibits non-linearity in the low resistance state (LRS) under high operating current conditions for cross-point array applications. The transition of the conduction behavior in the LRS from linear to non-linear type was observed in the TiOx/TiOy bilayered structure by controlling programming currents. Our results suggest that the non-linear conduction behavior is activated in a thermally formed suboxide region due to lots of heat during the switching. By using achieved non-linearity of device, a one-resistor memory cell can be used for the suppression of sneak-path currents without the need for additional selector device.