Seunghyup Lee

Resistive switching characteristics of ZnO thin film grown on stainless steel for flexible nonvolatile memory devices

This paper reports a resistive switching device of Au/ZnO/stainless steel (SS) and its applicability as a flexible resistive random access memory (ReRAM). The Au/ZnO/SS structure was fabricated by radio frequency sputtering deposition of a ZnO thin film on the SS substrate. The fabricated device showed stable unipolar and bipolar resistive switching behaviors with reliable switching responses over 100 cycles. The device performance was not degraded upon bending, which indicates high potential for flexible ReRAM applications.

Resistance Switching Behaviors of Hafnium Oxide Films Grown by MOCVD for Nonvolatile Memory Applications

Resistive switching characteristics of hafnium oxide were studied for possible nonvolatile memory device applications. The HfO 2 films were grown by metallorganic chemical vapor deposition (MOCVD) at 400°C using tetrakis(diethylamido)hafnium [Hf(N(C 2 H 5 ) 2 ) 4 ] as a precursor and oxygen gas as an oxidizing agent. The film was polycrystalline and had gradational compositions of Hf and O atoms. Current-voltage characteristics of the films were investigated with 1 mA compliance. A reproducible resistance switching behavior was observed with high resistance ratio of about 10 4 -10 9 , which is higher than other comparable materials, such as TiO 2 and ZrO 2 . SET and RESET voltages were measured about 0.8 and 1.5 V, respectively, indicating that the device can be operated below 2 V. The devices were operated in ohmic conduction mechanism. During the forming process, characteristics of Schottky emission were observed, which indicates that conduction mechanisms between SET and forming processes are different. The bipolar resistance switching behavior was also observed as well as unipolar resistance switching behavior.

Coexistence of unipolar and bipolar resistive switching characteristics in ZnO thin films

This paper reports the coexistence of unipolar and bipolar resistive switching (BRS) in an Ag/ZnO/Pt structure fabricated by sputtering deposition at room temperature. The structure shows reproducible and stable unipolar resistive switching after electroforming with high compliance current (Ic)=40 mA, regardless of the applied voltage polarity. With low Ic=5 mA at electroforming, BRS was also observed; this phenomenon depended on the voltage polarity. Both states were stable and reproducible over 100 cycles. The switching mode was changed with adjusting Ic but the transition was irreversible. Based on the results, switching mechanism based on filament theory is proposed to explain both resistive switching.

A Light Incident Angle Switchable ZnO Nanorod Memristor: Reversible Switching Behavior Between Two Non‐Volatile Memory Devices

A light incident angle selectivity of a memory device is demonstrated. As a model system, the ZnO resistive switching device has been selected. Electrical signal is reversibly switched between memristor and resistor behaviors by modulating the light incident angle on the device. Moreover, a liquid passivation layer is introduced to achieve stable and reversible exchange between the memristor and WORM behaviors.

Overcoming The Water Vulnerability Of Electronic Devices: A Highly Water‐Resistant ZnO Nanodevice With Multifunctionality

In general, electronic devices are highly vulnerable to water exposure, which causes serious operational failure of the devices and potentially dangerous electrical shock to the user. Even though current electronics are well sealed and the integrated devices inside are packaged with polymer passivation, the penetration of a single water droplet can be fatal to electrical circuits. To prevent water from causing device failure, and even more serious electric shock, water resistance would be an ideal and useful feature for the device. One way to produce a waterproof electronic device is to incorporate nonwetting, superhydrophobic components into the electronic device. The applications of superhydrophobicity have attracted much attention in various fi elds, including optical devices, [ 1–5 ] catalysis, [ 6 ]

Oxidation-induced traps near SiO2/SiGe interface

Using an Al/SiO2(wet)/Si0.9Ge0.1/n–Si/Al capacitor structure, effects of oxidation on bulk trap and interface states near the SiO2/SiGe interface are investigated. Two peaks at the energy levels of 0.23 eV (D1) and 0.40 eV (D2) below the conduction band edge are observed with the capacitance deep level transient spectroscopy (DLTS) method. The DLTS measurement results show a characteristic feature of interface states for the D1 peak. The interface state distribution obtained by the capacitance–voltage method also has a high density (6.9×1012/cm2 eV) peak at an energy level of 0.23 eV below the conduction band edge. The Si–O– dangling bonds are thought to be the source of the D1 peak. The annealing behaviors of the D2 peak show that D2 is a divacancy related bulk trap. The capture cross section and the trap density for the bulk trap D2 are 2.06×10−15/cm2 and 1.8×1014/cm3, respectively. The density of D2 is significantly reduced after low temperature postmetallization annealing.

Resistive Switching WOx‐Au Core‐Shell Nanowires with Unexpected Nonwetting Stability Even when Submerged Under Water

The resistive switching (RS) characteristics of a tungsten oxide (WO(x) )-Au core-shell nanowire device array is demonstrated for the first time. In addition to the stable bipolar RS characteristics, the nanowire structure of our RS devices provides superhydrophobic properties. The superhydrophobic RS nanowires repelled water that was poured over, such that the device was protected from failure by water contact-driven leakage currents. Moreover, surprisingly, the devices still work even with when the device is submerged underwater.

In situ ultraviolet photoemission spectroscopy measurement of the pentacene-RuO2/Ti contact energy structure

In situ ultraviolet photoemission spectroscopy was used during the pentacene layer growth on ruthenium and ruthenium oxide films to measure the energy barrier in the metal-semiconductor contact. The measurement showed that ruthenium oxide film formed lower hole-injection barrier with pentacene than ruthenium or gold metal film due to its high work function of 4.92 eV and low resistivity of ∼350 μΩ cm. Pentacene thin film transistor with ruthenium oxide electrode showed higher mobility of 0.435 cm2/V s and on-off ratio of 106 than ruthenium (μ: 0.205 cm2/V s, on/off ratio: 106) or gold electrode (μ: 0.338 cm2/V s, on/off ratio: 106) of the same structure.

Atomic layer deposition of hafnium silicate film for high mobility pentacene thin film transistor applications

The performance of pentacene thin film transistors (TFTs) was improved using a hafnium silicate (HfxSi1−xO2) thin film as a high-k dielectric layer. For growth of the HfxSi1−xO2 thin films, an atomic layer chemical vapor deposition (ALCVD) process was optimized using silicon alkoxide and hafnium amido as precursors. The self-limiting surface reactions of each precursor were observed, indicating the ALCVD growth characteristics. The film thickness linearly increased depending on the number of process cycles, with a remarkably high growth rate of 2.3 A per cycle. The chemical binding states, thermal stability and electrical characteristics of the films grown were investigated using XPS, XRD and capacitance–voltage and leakage current–voltage analysis. The pentacene TFTs fabricated with the ALCVD-grown Hf0.67Si0.33O2 dielectric layer were characterized and the results were compared to the pentacene TFTs using Al2O3 and SiO2 film as dielectric layers. The pentacene/Hf0.67Si0.33O2 TFT showed a three-fold and five-fold higher mobility than a pentacene/Al2O3 TFT and a pentacene/SiO2 TFT, respectively. With additional treatments to enhance the characteristics of the OTFT, pentacene/HfxSi1−xO2 TFTs have great potential as high mobility devices with low operational voltage.

Impact of transparent electrode on photoresponse of ZnO-based phototransistor

ZnO-based photo-thin film transistors with enhanced photoresponse were developed using transparent conductive oxide contacts. Changing the electrode from opaque Mo to transparent In-Zn-O increases the photocurrent by five orders of magnitude. By changing the opacity of each source and drain electrode, we could observe how the photoresponse is affected. We deduce that the photocurrent generation mechanism is based on an energy band change due to the photon irradiation. More importantly, we reveal that the photocurrent is determined by the energy barrier of injected electrons at the interface between the source electrode and the active layer.