Soo Gil Kim

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.

SiO2 doped Ge2Sb2Te5 thin films with high thermal efficiency for applications in phase change random access memory

This study examined the various physical, structural and electrical properties of SiO(2) doped Ge(2)Sb(2)Te(5) (SGST) films for phase change random access memory applications. Interestingly, SGST had a layered structure (LS) resulting from the inhomogeneous distribution of SiO(2) after annealing. The physical parameters able to affect the reset current of phase change memory (I(res)) were predicted from the Joule heating and heat conservation equations. When SiO(2) was doped into GST, thermal conductivity largely decreased by ∼ 55%. The influence of SiO(2)-doping on I(res) was examined using the test phase change memory cell. I(res) was reduced by ∼ 45%. An electro-thermal simulation showed that the reduced thermal conductivity contributes to the improvement of cell efficiency as well as the reduction of I(res), while the increased dynamic resistance contributes only to the latter. The formation and presence of the LS thermal conductivity in the set state test cell after repeated switching was confirmed.

NbO 2 -based low power and cost effective 1S1R switching for high density cross point ReRAM Application

In this paper, 5Xnm cross point cell array for the low power ReRAM operation was developed with 1S1R cell structure. Through the optimization of both TiOx/TaOx based-1R and NbO2 based-1S stacks with TiN based-electrode, the worlds first and best bipolar switching characteristics with the lowest operation current (20~50uA) and sneak current (~1uA) level were acquired.

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

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.

Improvement of characteristics of NbO2 selector and full integration of 4F2 2x-nm tech 1S1R ReRAM

In this paper, the authors report that 2x nm cross-point ReRAM with 1S1R structure has been successfully developed. Off-current at 1/2 Vsw of 1S1R is one of key factor for high-density ReRAM. NbO2 was chosen as a selector material and off-current and forming characteristics were improved by using stack engineering of top and bottom barriers as well as spacer materials. Finally array operation was characterized with the integration of selector and resistor materials.

Carrier capture before entering into a semiconductor quantum dot as a dominant pathway for the reduction of emission efficiency

To study the carrier reduction pathway for quantum dots (QDs), we have measured carrier lifetimes and photoluminescence spectra both at 10 K and at higher temperatures. We found that the carriers captured in QDs are robust and are not lost to nearby defects, even at elevated temperature, and that the lower emission efficiency of QDs with defects compared to the corresponding defect-free QDs is due to the capture of carriers to defects before entering into the QDs.

Full chip integration of 3-d cross-point ReRAM with leakage-compensating write driver and disturbance-aware sense amplifier

In this report, a fully integrated 3-D cross-point ReRAM is demonstrated with minimized disturbance and sneak current effect. HfOX memory cells stacked on threshold-type selector exhibit superb leakage current suppression than cells with exponential selector. Remaining leakage current is diagnosed and compensated by leakage compensating write driver. Cells are prevented from disturbance by lowering read voltage at hot temperature, which sacrifices read margin. The read margin is recovered by cell current amplifier in read circuit.

Exploring oxygen-affinity-controlled TaN electrodes for thermally advanced TaO x bipolar resistive switching

Recent advances in oxide-based resistive switching devices have made these devices very promising candidates for future nonvolatile memory applications. However, several key issues remain that affect resistive switching. One is the need for generic alternative electrodes with thermally robust resistive switching characteristics in as-grown and high-temperature annealed states. Here, we studied the electrical characteristics of Ta2O5−x oxide-based bipolar resistive frames for various TaNx bottoms. Control of the nitrogen content of the TaNx electrode is a key factor that governs variations in its oxygen affinity and structural phase. We analyzed the composition and chemical bonding states of as-grown and annealed Ta2O5−x and TaNx layers and characterized the TaNx electrode-dependent switching behavior in terms of the electrode’s oxygen affinity. Our experimental findings can aid the development of advanced resistive switching devices with thermal stability up to 400 °C.

Investigation of the retention performance of an ultra-thin HfO2 resistance switching layer in an integrated memory device

The retention behavior of a HfO2 resistive switching memory device with a diameter of 28 nm and an ultra-thin (1 nm) HfO2 layer as the switching layer was examined. Ta and TiN served as the oxygen vacancy (VO) supplying the top and inert bottom electrodes, respectively. Unlike the retention failure phenomenon reported in other thicker oxide-based resistance switching memory devices, the current of both the low and high resistance states suddenly increased at a certain time, causing retention failure. Through the retention tests of the devices in different resistance states, it was concluded that the involvement of the reset step induced the retention failure. The pristine device contained a high portion of VO-rich region and the location of the border between the VO-rich and VO-free regions played the critical role in governing the retention performance. During the reset step, this borderline moves towards the Ta electrode, but moves back to the original location during the retention period, which eventually induces the reconnection of the disconnected conducting filament (in a high resistance state) or strengthens the connected weak portion (low resistance state). The activation energy for the retention failure mechanism was 0.15 eV, which is related to the ionization of neutral VO to ionized VO.