Daeseok Lee

Effect of high-pressure oxygen annealing on negative bias illumination stress-induced instability of InGaZnO thin film transistors

Negative-bias illumination stress (NBIS) of amorphous InGaZnO (IGZO) transistors can cause a large negative shift (>7.1 V) in threshold voltage, something frequently attributed to the trapping of photoinduced hole carriers. This work demonstrates that the deterioration of threshold voltage by NBIS can be strongly suppressed by high-pressure annealing under 10 atm O2 ambient. This improvement occurred through a reduction in oxygen vacancy defects in the IGZO film, indicating that a photoinduced transition from VO to VO2+ was responsible for the NBIS-induced instability.

Reproducible hysteresis and resistive switching in metal-CuxO-metal heterostructures

Materials showing reversible resistance switching between high-resistance state and low-resistance state at room temperature are attractive for today’s semiconductor technology. In this letter, the reproducible hysteresis and resistive switching characteristics of metal-CuxO-metal (M-CuxO-M) heterostructures driven by low voltages are demonstrated. The fabrication of the M-CuxO-M heterostructures is fully compatible with the standard complementary metal-oxide semiconductor process. The hysteresis and resistive switching behavior are discussed. The good retention characteristics are exhibited in the M-CuxO-M heterostructures by the accurate controlling of the preparation parameters.

Uniform resistive switching with a thin reactive metal interface layer in metal-La0.7Ca0.3MnO3-metal heterostructures

A thin samarium (Sm) metal layer was introduced to improve the resistive hysteresis and switching uniformity. Sm reacts with the La0.7Ca0.3MnO3 and forms a thin interface oxide layer, which is responsible for the switching. The switching occurs without any forming process. Compared with conventional resistive memory device based on localized filament formation, Sm∕La0.7Ca0.3MnO3 devices show area-dependent resistance which indicates uniform resistive switching. Under a positive bias, electromigration of oxygen ions (O2−) forms thicker oxide (SmOx), which dissociates under a negative bias, causes high and low resistance states, respectively. Estimated data retention of more than 10yr was observed at 85°C.

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

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 (<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

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

Improvement of reproducible hysteresis and resistive switching in metal-La0.7Ca0.3MnO3-metal heterostructures by oxygen annealing

Materials showing reversible resistance switching between high-resistance state and low-resistance state at room temperature are attractive for today’s semiconductor technology. In this letter, the improvement of reproducible hysteresis and resistive switching characteristics of metal-La0.7Ca0.3MnO3-metal (M-LCMO-M) heterostructures is demonstrated. The fabrication of the M-LCMO-M heterostructures is compatible with the standard complementary metal-oxide semiconductor process. The effect of oxygen annealing on the improvement of the hysteresis and resistive switching is discussed. The good retention characteristics are exhibited in the M-LCMO-M heterostructures by the accurate controlling of the preparation parameters.

Co-Occurrence of Threshold Switching and Memory Switching in

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.

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

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.

Cells for Crosspoint Memory Applications

We successfully fabricated a single-filament-activated resistive memory having limited conductive filament (CF) source through a novel fabrication technique. The formation of a single filament was confirmed using conductive atomic force microscopy. Compared to conventional programmable metallization cell devices having an unlimited CF source, our single-filament device showed a significantly improved switching variability by minimizing the probability of randomly formed CFs. The elimination of unlimited CF sources changed the rate-limiting parameter that determines the retention and disturbance properties and improved thermal and electrical stability.

Self-Selective Characteristics of Nanoscale

By characterizing the resistive switching state in Al/PCMO device, we propose, for the first time, the feasibility of cross-point memory without any selection device. Self-formed Schottky barrier by redox reaction at Al/PCMO interface can be used as a selection device. Using self-formed Schottky barrier, we can build simple 4F2 cross-point memory array without additional process steps.