Myungwoo Son

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}

Sub-10 nm Graphene Nanoribbon Arrays are fabricated over large areas by etching CVD-grown graphene. A mask is used made by the directed self-assembly of a cylindrical PS-b-PDMS block copolymer under solvent annealing guided by a removable template. The optimized solvent annealing process, surface-modified removable polymeric templates, and high Flory-Huggins interaction parameters of the block copolymer enable a highly aligned array of nanoribbons with low line edge roughness to be formed. This leads to a higher on/off ratio and stronger temperature dependence of the current for nanoribbon FETs, and a photocurrent which is 30 times larger compared to unpatterned graphene.

for High-Density Bipolar ReRAM Applications

To integrate bipolar resistive switching cells into crosspoint structures, we serially connect a threshold-switching (TS) Pt/NbO₂/Pt device with a memory-switching (MS) Pt/Nb₂O₅/ Pt device and observe the suppression of the undesired sneak current. A simpler Pt/Nb₂O₅/NbO₂/Pt bilayer oxide device was designed; it simultaneously exhibits TS and MS. The unique device characteristics in the metal/oxide/metal structure can be directly integrated into a crosspoint memory array without the diode; this can significantly reduce the fabrication complexity.

Sub-10 nm Graphene Nanoribbon Array Field-Effect Transistors Fabricated by Block Copolymer Lithography

We herein present a hybrid-type memory device in which threshold switching and bipolar resistive switching are combined. The nanoscale vanadium oxide (VOx) device simultaneously exhibited self-selective performance and memory switching by electroforming. By using W instead of Pt for the top electrode, memory performance was improved in terms of cycling, pulse endurance, and retention properties attributed to a self-formed WOx/VOx interface. Such a phenomenon in a simple metal-oxide-metal structure provides a good potential for future high-density cross-point memory devices by avoiding the sneak-path problem.

Co-Occurrence of Threshold Switching and Memory Switching in

There is significant interest in synthesizing large-area graphene films at low temperatures by chemical vapor deposition (CVD) for nanoelectronic and flexible device applications. However, to date, low-temperature CVD methods have suffered from lower surface coverage because micro-sized graphene flakes are produced. Here, we demonstrate a modified CVD technique for the production of large-area, continuous monolayer graphene films from benzene on Cu at 100–300 °C at ambient pressure. In this method, we extended the graphene growth step in the absence of residual oxidizing species by introducing pumping and purging cycles prior to growth. This led to continuous monolayer graphene films with full surface coverage and excellent quality, which were comparable to those achieved with high-temperature CVD; for example, the surface coverage, transmittance, and carrier mobilities of the graphene grown at 300 °C were 100%, 97.6%, and 1,900–2,500 cm2 V−1 s−1, respectively. In addition, the growth temperature was substantially reduced to as low as 100 °C, which is the lowest temperature reported to date for pristine graphene produced by CVD. Our modified CVD method is expected to allow the direct growth of graphene in device manufacturing processes for practical applications while keeping underlying devices intact.

\hbox{Pt}/\hbox{NbO}_{x}/\hbox{Pt}

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.

Cells for Crosspoint Memory Applications

We have investigated silicon carbide (SiC) as a new programmable metallization cell material for nonvolatile memory applications. Our Cu/SiC/Pt devices showed bipolar resistive switching; the conduction mechanisms can be explained by the formation of a Cu filament and Poole-Frenkel emission in the low-resistance and high-resistance states, respectively. In particular, our devices showed excellent state stability, e.g., nondestructive readout at various stress voltages, excellent retention characteristics at 150°C for 104 s, and stable memory operation at high ambient temperature. We attribute this state stability to the SiCs high chemical stability and the ability to act as a Cu diffusion barrier.

Self-Selective Characteristics of Nanoscale

Pristine graphene and a graphene interlayer inserted between indium tin oxide (ITO) and p-GaN have been analyzed and compared with ITO, which is a typical current spreading layer in lateral GaN LEDs. Beyond a certain current injection, the pristine graphene current spreading layer (CSL) malfunctioned due to Joule heat that originated from the high sheet resistance and low work function of the CSL. However, by combining the graphene and the ITO to improve the sheet resistance, it was found to be possible to solve the malfunctioning phenomenon. Moreover, the light output power of an LED with a graphene interlayer was stronger than that of an LED using ITO or graphene CSL. We were able to identify that the improvement originated from the enhanced current spreading by inspecting the contact and conducting the simulation.

\hbox{VO}_{x}

We have investigated the effect of a Ge2Sb2Te5 (GST) thermal barrier on the reset operations in TiN/Cu/SiC/Pt devices. When the GST film was introduced as a thermal barrier, the device showed a lower reset voltage and a lower reset current than a device without the GST layer. In particular, the reset speed of the device with the GST layer was significantly faster at room temperature compared to the device without the GST layer. We attribute the improved resistive switching to the GST thermal barrier, which induces thermally assisted electrochemical reduction of the Cu filament.

Devices for High-Density ReRAM Applications

The resistive switching behaviour of amorphous ZnO (a-ZnO) sandwiched between Ga-doped ZnO (GZO) transparent conductive oxide and Al electrode is reported. Transparent GZO films were deposited on polymer substrates as bottom electrodes using pulsed DC magnetron sputtering at 100 °C, on which a-ZnO films were deposited by RF magnetron sputtering at room temperature. The layered structure prepared in this manner was semi-transparent to visible light and its current–voltage hysteresis was representative of a bipolar resistive switching behaviour. The observation of such a resistive switching behaviour was attributed to the employment of a-ZnO as a dielectric layer and the use of Al and GZO as electrodes, which enabled the formation of Schottky barrier only at the a-ZnO/GZO interface. The conduction through the dielectric layer during the high resistance state was due to the Schottky emission as deduced from the consideration of band structures and the fitting of the current–voltage relations to the various conduction models. Switching to the low resistance state was attributed to the filament formation due to the migration of oxygen vacancies during the set process. In control experiments where crystalline ZnO was used as the dielectric layer, resistive switching behaviour was not observed.