Euijun Cha

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

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.

Improved switching uniformity in resistive random access memory containing metal-doped electrolyte due to thermally agglomerated metallic filaments

We demonstrate improved switching uniformity in resistive random-access memory (RRAM) containing metal-doped electrolyte due to thermally agglomerated metallic filaments. Rapid thermal annealing (RTA) produced copper-doped carbon (CuC) devices that exhibited better switching parameters, such as on/off resistance and set/reset voltage, than a control device. High-resolution transmission electron microscopy, electron dispersive spectroscopy, and conductive atomic force microscopy revealed that Cu atoms were agglomerated during the RTA process and formed a Cu filament in the CuC film. Consequently, the forming process can be eliminated, which is desirable for practical RRAM applications.

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.

Effect of interfacial oxide layer on the switching uniformity of Ge2Sb2Te5-based resistive change memory devices

We report the effect of the interfacial oxide layer on switching uniformity in Ge2Sb2Te5 (GST)-based resistive switching memory devices. An interfacial oxide layer acting as an internal resistor was fabricated by the simple thermal oxidation process at low temperature and confirmed by x-ray photoelectron spectroscopy analysis. TiN/oxidized GST/GST/Pt devices showed extremely uniform resistance states owing to intentionally controlled current flow induced by the interfacial oxide layer, despite the filaments being randomly formed. Furthermore, the devices showed good memory performance, e.g., a large on/off resistance ratio (over four orders of magnitude) and reliable data retention (up to 104 s at 85 °C).

Dependence of reactive metal layer on resistive switching in a bi-layer structure Ta/HfOx filament type resistive random access memory

The dependence of reactive metal layer on resistive switching characteristics is investigated in a bi-layer structural Ta/HfOx filament type resistive random access memory (ReRAM). By increasing the oxygen absorption rate of the reactive metal layer, formation of an induced resistive switching region that led to significant changes in the resistive switching characteristics of the ReRAM was observed. Electrical and physical analyses showed that the induced TaOx-resistive switching region can result in self-compliance behavior, uniform resistive switching, and a gradual set process, which can be utilized for low power and analog operations.

Tunnel barrier engineering of titanium oxide for high non-linearity of selector-less resistive random access memory

In this study, the effect of the oxygen profile and thickness of multiple-layers TiOx on tunnel barrier characteristics was investigated to achieve high non-linearity in low-resistance state current (ILRS). To form the tunnel barrier in multiple-layer of TiOx, tunnel barrier engineering in terms of the thickness and oxygen profile was attempted using deposition and thermal oxidation times. It modified the defect distribution of the tunnel barrier for effective suppression of ILRS at off-state (½VRead). By inserting modified tunnel barrier in resistive random access memory, a high non-linear ILRS was exhibited with a significantly lowered ILRS for ½VRead.

Thermally activated non-linearity of device in resistance-switching memory for cross-point array applications

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.

Multilayer-oxide-based bidirectional cell selector device for cross-point resistive memory applications

In this study, we propose a multilayer structure as an insulating oxide/conducting oxide/insulating oxide for a high-performance cell selector device. To achieve a desirable selector device for cross-point memory applications, the electrical characteristics of the selector device with a multilayer oxide have been systemically investigated by using various approaches such as interface engineering and by considering factors such as material dependence. Through the introduction of a multilayer oxide, a field-sensitive device structure that exhibits a highly nonlinear I-V curve is formed. Therefore, both high current density (JMAX > 107 A/cm2) and better off-current (IOFF < 100 nA) can be achieved.

Bidirectional selection device characteristics of ultra-thin (&#60;3nm) TiO 2 layer for 3D vertically stackable ReRAM application

We propose the feasibility of bidirectional selection device characteristics in ultrathin (<;3nm) TiO₂ layer. We utilized the localized conducting path as virtual electrode to investigate device property at extremely scaled area. By using electrical method such as “forming” and “reset” processes in oxide, virtual electrode/sub-3nm-thick TiO₂/virtual electrode structure was achieved. The measured current-voltage characteristics of fabricated device exhibited uniform bidirectional selection behavior with a high selectivity (~10⁵) and showed the feasibility of high current density (>;10⁶A/cm²).

Comprehensive analysis of electro thermally driven nanoscale insulator–metal transition SmNiO3-based selector for cross-point memory array

To implement a cross-point memory array successfully, it is highly required to develop nonlinear selector devices. Insulator–metal transition (IMT) devices are promising candidates for selector applications. Although IMT characteristics of SmNiO3 with a high transition temperature of 130 °C have already been reported, the film deposition conditions following high-pressure oxygen annealing at high temperatures are not practical for high-density memory applications. In this report, we propose a simple electrical method to form a localized IMT SmNiO3 region in a sputter-deposited SmNiOx film. The nanoscale IMT device formed by the electro thermal effect shows promising selector characteristics such as switching uniformity, switching endurance (>105 cycles), and high temperature stability. The feasibility of good selector characteristics is also investigated by serially connecting the selector device (SmNiO3 IMT) to a Ta/Ta2O5/Pt resistive random access memory (ReRAM) device.