Hyejung Choi

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.

Resistance switching of the nonstoichiometric zirconium oxide for nonvolatile memory applications

The resistance switching behavior and switching mechanism of nonstoichiometric zirconium oxide thin films were investigated for nonvolatile memory application. The Pt/ZrO/sub x//p/sup +/-Si sandwich structure fabricated by reactive sputtering shows two stable resistance states. By applying proper bias, resistance switching from one to another state can be obtained. The composition in ZrO/sub x/ thin films were confirmed from X-ray photoelectron spectroscope (XPS) analysis, which showed three layers such as top stoichiometric ZrO/sub 2/ layer with high resistance, transition region with medium resistance, and conducting ZrO/sub x/ bulk layer. The resistance switching can be explained by electron trapping and detrapping of excess Zr/sup +/ ions in transition layer which control the distribution of electric field inside the oxide, and, hence the current flow.

An electrically modifiable synapse array of resistive switching memory

This paper describes the resistive switching of a cross-point cell array device, with a junction area of 100 nm x 100 nm, fabricated using ultraviolet nanoimprinting. A GdO(x) and Cu-doped MoO(x) stack with platinum top and bottom electrodes served as the resistive switching layer, which shows analog memory characteristics with a resistance ratio greater than 10. To demonstrate a neural network circuit, we operated the cell array device as an electrically modifiable synapse array circuit and carried out a weighted sum operation. This demonstration of cross-point arrays, based on resistive switching memory, opens the way for feasible ultra-high density synapse circuits for future large-scale neural network systems.

Flexible and twistable non-volatile memory cell array with all-organic one diode–one resistor architecture

Flexible organic memory devices are one of the integral components for future flexible organic electronics. However, high-density all-organic memory cell arrays on malleable substrates without cross-talk have not been demonstrated because of difficulties in their fabrication and relatively poor performances to date. Here we demonstrate the first flexible all-organic 64-bit memory cell array possessing one diode-one resistor architectures. Our all-organic one diode-one resistor cell exhibits excellent rewritable switching characteristics, even during and after harsh physical stresses. The write-read-erase-read output sequence of the cells perfectly correspond to the external pulse signal regardless of substrate deformation. The one diode-one resistor cell array is clearly addressed at the specified cells and encoded letters based on the standard ASCII character code. Our study on integrated organic memory cell arrays suggests that the all-organic one diode-one resistor cell architecture is suitable for high-density flexible organic memory applications in the future.

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.

Excellent Switching Uniformity of Cu-Doped

We have investigated a Cu-doped MoO_x/GdO_x bilayer film for nonvolatile memory applications. By adopting an ultrathin GdO_x layer, we obtained excellent device characteristics such as resistance ratio of three orders of magnitude, uniform distribution of set and reset voltages, switching endurance up to 10⁴ cycles, and ten years of data retention at 85degC. By adopting bilayer films of Cu-doped MoO_x/GdO_x, a local filament was formed by a two-step process. Improved memory characteristics can be explained by the formation of nanoscale local filament in the ultrathin GdO_x layer.

\hbox{MoO}_{x}/\hbox{GdO}_{x}

This letter reports on reversible switching behavior of metal-insulator-metal type nonvolatile organic memory devices using polyfluorene-derivative (WPF-oxy-F) single layer film. The current-voltage (I-V) characteristics showed that the WPF-oxy-F single layer film has two distinguished resistance states, low resistance state and high resistance state, with four orders of on/off ratio (Ion∕Ioff∼104). From the analysis of I-V curves, area dependent I-V characteristics, and current images obtained by conducting atomic force microscopy we propose that the space charge limited current with filamentary conduction is a potential mechanism for the reversible switching behavior of WPF-Oxy-F memory devices.

Bilayer for Nonvolatile Memory Applications

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.

Reversible switching characteristics of polyfluorene-derivative single layer film for nonvolatile memory devices

A systematic study on the switching mechanism of an Al/ Pr0.7Ca0.3MnO3 (PCMO) device was performed. A polycrystalline PCMO film was deposited using a conventional sputtering method. A thin Al layer was introduced to induce a reaction with the PCMO, forming aluminum oxide (AlOx). Transmission electron microscopy analysis of the interface between Al and PCMO showed that resistive switching was governed by the formation and dissolution of AlOx. Some basic memory characteristics, such as good cycle endurance and data retention of up to 104 s at 125°C, were also obtained. It also showed excellent switching uniformity and high device yield.

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

The resistive switching characteristics of polyfluorene-derivative polymer material in a sub-micron scale via-hole device structure were investigated. The scalable via-hole sub-microstructure was fabricated using an e-beam lithographic technique. The polymer non-volatile memory devices varied in size from 40 x 40 microm(2) to 200 x 200 nm(2). From the scaling of junction size, the memory mechanism can be attributed to the space-charge-limited current with filamentary conduction. Sub-micron scale polymer memory devices showed excellent resistive switching behaviours such as a large ON/OFF ratio (I(ON)/I(OFF) approximately 10(4)), excellent device-to-device switching uniformity, good sweep endurance, and good retention times (more than 10,000 s). The successful operation of sub-micron scale memory devices of our polyfluorene-derivative polymer shows promise to fabricate high-density polymer memory devices.