Opposite Behavior of Multilayer Graphene/ Indium-Tin-Oxide p-Electrode for Gallium Nitride Based-Light Emitting Diodes Depending on Thickness of Indium-Tin-Oxide Layer.

In order to improve EQE, we have investigated on the role of multilayer graphene (MLG) on the electrical and optical properties of GaN based light-emitting diodes (LEDs) with ultrathin ITO (5 nm or 10 nm)/p-GaN contacts. The MLG was transferred on the ITO/p-GaN to decrease sheet resistance of thin ITO p-electrode and improve the current spreading of LEDs. The LEDs with the ITO 5 nm and MLG/ITO 5 nm structures showed 3.25 and 3.06 V at 20 mA, and 11.69 and 13.02 mW/sr at 400 mA, respectively. After forming MLG on ITO 5 nm, the electro-optical properties were enhanced. Furthermore, the GaN based-LEDs applied to the ITO 10 nm, and MLG/ITO (10 nm) structures showed 2.95 and 3.06 V at 20 mA, and 20.28 and 16.74 mW/sr at 400 mA, respectively. The sheet resistance of the MLG transferred to ITO 5 nm was decreased approximately four fold compared to ITO 5 nm. On the other hand, the ITO 10 nm and MLG/ITO 10 nm showed a similar sheet resistance; the transmittance of the LEDs with ITO 10 nm decreased to 16% due to MLG formation on ITO. This suggests that the relationship between the sheet resistance and transmittance according to the ITO film thickness affected the electro-optical properties of the LEDs with a transparent p-electrode with the MLG/ITO dual structure.

Monolithic Inorganic ZnO/GaN Semiconductors Heterojunction White Light-Emitting Diodes.

Monolithic light-emitting diodes (LEDs) that can generate white color at the one-chip level without the wavelength conversion through packaged phosphors or chip integration for photon recycling are of particular importance to produce compact, cost-competitive, and smart lighting sources. In this study, monolithic white LEDs were developed based on ZnO/GaN semiconductor heterojunctions. The electroluminescence (EL) wavelength of the ZnO/GaN heterojunction could be tuned by a post-thermal annealing process, causing the generation of an interfacial Ga2O3 layer. Ultraviolet, violet-bluish, and greenish-yellow broad bands were observed from n-ZnO/p-GaN without an interfacial layer, whereas a strong greenish-yellow band emission was the only one observed from that with an interfacial layer. By controlled integration of ZnO/GaN heterojunctions with different postannealing conditions, monolithic white LED was demonstrated with color coordinates in the range (0.3534, 0.3710)-(0.4197, 0.4080) and color temperatures of 4778-3349 K in the Commission Internationale de l'Eclairage 1931 chromaticity diagram. Furthermore, the monolithic white LED produced approximately 2.1 times higher optical output power than a conventional ZnO/GaN heterojunction due to the carrier confinement effect at the Ga2O3/n-ZnO interface.

High-Output Lead-Free Flexible Piezoelectric Generator Using Single-Crystalline GaN Thin Film.

Piezoelectric generators (PEGs) are a promising power source for future self-powered electronics by converting ubiquitous ambient mechanical energy into electricity. However, most of the high-output PEGs are made from lead zirconate titanate, in which the hazardous lead could be a potential risk to both humans and environment, limiting their real applications. III-Nitride (III-N) can be a potential candidate to make stable, safe, and efficient PEGs due to its high chemical stability and piezoelectricity. Also, PEGs are preferred to be flexible rather than rigid, to better harvest the low-magnitude mechanical energy. Herein, a high-output, lead-free, and flexible PEG (F-PEG) is made from GaN thin film by transferring a single-crystalline epitaxial layer from silicon substrate to a flexible substrate. The output voltage, current density, and power density can reach 28 V, 1 µA·cm-2, and 6 µW·cm-2, respectively, by bending the F-PEG. The generated electric power by human finger bending is high enough to light commercial visible light-emitting diodes and charge commercial capacitors. The output performance is maintained higher than 95% of its original value after 10 000-cycle test. This highly stable, high-output, and lead-free GaN thin-film F-PEG has the great potential for future self-powered electronic devices and systems.

Impact of Plasma Electron Flux on Plasma Damage-Free Sputtering of Ultrathin Tin-Doped Indium Oxide Contact Layer on p-GaN for InGaN/GaN Light-Emitting Diodes.

The origin of plasma-induced damage on a p -type wide-bandgap layer during the sputtering of tin-doped indium oxide (ITO) contact layers by using radiofrequency-superimposed direct current (DC) sputtering and its effects on the forward voltage and light output power (LOP) of light-emitting diodes (LEDs) with sputtered ITO transparent conductive electrodes (TCE) is systematically studied. Changing the DC power voltage from negative to positive bias reduces the forward voltages and enhances the LOP of the LEDs. The positive DC power drastically decreases the electron flux in the plasma obtained by plasma diagnostics using a cutoff probe and a Langmuir probe, suggesting that the repulsion of plasma electrons from the p -GaN surface can reduce plasma-induced damage to the p -GaN. Furthermore, electron-beam irradiation on p -GaN prior to ITO deposition significantly increases the forward voltages, showing that the plasma electrons play an important role in plasma-induced damage to the p -GaN. The plasma electrons can increase the effective barrier height at the ITO/deep-level defect (DLD) band of p -GaN by compensating DLDs, resulting in the deterioration of the forward voltage and LOP. Finally, the plasma damage-free sputtered-ITO TCE enhances the LOP of the LEDs by 20% with a low forward voltage of 2.9 V at 20 mA compared to LEDs with conventional e-beam-evaporated ITO TCE.

Vertical-Injection AlGaInP LEDs with n-AlGaInP Nanopillars Fabricated by Self-Assembled ITO-Based Nanodots.

The light output power of AlGaInP-based vertical-injection light-emitting diodes (VI-LEDs) can be enhanced significantly using n-AlGaInP nanopillars. n-AlGaInP nanopillars, ~200 nm in diameter, were produced using SiO2 nanopillars as an etching mask, which were fabricated from self-assembled tin-doped indium oxide (ITO)-based nanodots formed by the wet etching of as-deposited ITO films. The AlGaInP-based VI-LEDs with the n-AlGaInP nanopillars provided 25 % light output power enhancement compared to VI-LEDs with a surface-roughened n-AlGaInP because of the reduced total internal reflection by the nanopillars at the n-AlGaInP/air interface with a large refractive index difference of 1.9.

Electron beam irradiated silver nanowires for a highly transparent heater.

Transparent heaters have attracted increasing attention for their usefulness in vehicle windows, outdoor displays, and periscopes. We present high performance transparent heaters based on Ag nanowires with electron beam irradiation. We obtained an Ag-nanowire thin film with 48 ohm/sq of sheet resistance and 88.8% (substrate included) transmittance at 550 nm after electron beam irradiation for 120 sec. We demonstrate that the electron beam creates nano-soldering at the junctions of the Ag nanowires, which produces lower sheet resistance and improved adhesion of the Ag nanowires. We fabricated a transparent heater with Ag nanowires after electron beam irradiation, and obtained a temperature of 51 °C within 1 min at an applied voltage of 7 V. The presented technique will be useful in a wide range of applications for transparent heaters.

Anti-arthritic effects of Ephedra sinica STAPF herb-acupuncture: inhibition of lipopolysaccharide-induced inflammation and adjuvant-induced polyarthritis.

Anti-inflammatory and anti-arthritic effects of water distillates of Ephedra sinica STAPF (ES), in herb-acupuncture, on the inflammatory responses of arthritis was investigated using phorbol 12-myristate 13-acetate (PMA)/lipopolysaccharide (LPS)-induced human macrophage and adjuvant-induced arthritic rat. The luciferase reporter vectors driven by the tumor necrosis factor (TNF)-alpha and cyclooxygenase-2 promoters were transiently transfected into U937 cells, which were then differentiated and stimulated by PMA and LPS, respectively, to develop an in vitro anti-inflammation assay system. The luciferase activities, observed in the activated U937 cells, were significantly inhibited by ES herb-acupuncture, compared to those of PD98509 and berberine. To evaluate ES herb-acupuncture as a novel anti-arthritic therapy, a polyarthritic rat model was developed using heat-killed Mycobacterium tuberculosis, and 50 mul of ES distillate was subcutaneously injected into the ST36 acupoint on each knee joint. While the articular indexes of arthritic rats were evidently decreased by ES herb-acupuncture, their body weights did not regain their initial levels. This may be due to the accelerating effects of ES on weight-loss and fat consumption. The mRNA expressions of TNF-alpha and interleukin (IL)-6 genes, which were closely stimulated in the arthritic rat joints, were found to be restored to the normal levels through the ES treatment. In the case of IL-1beta, the recovery was not significant but substantial. The anti-arthritic effect of ES herb-acupuncture was not found in the ES-treated/non-acupoint group. In conclusion, the ES herb-acupuncture into the ST36 acupoint was found to be effective in alleviating the inflammatory response and thus arthritic symptoms in adjuvant-induced arthritic rats.