D. S. Wuu

Defect reduction and efficiency improvement of near-ultraviolet emitters via laterally overgrown GaN on a GaN/patterned sapphire template

An approach to improve the defect density and internal quantum efficiency of near-ultraviolet emitters was proposed using a combination of epitaxial lateral overgrowth (ELOG) and patterned sapphire substrate (PSS) techniques. Especially, a complementary dot array pattern corresponding to the underlying PSS was used for the ELOG-SiO2 mask design. Based on the transmission-electron-microscopy and etch-pit-density results, the ELOG∕SiO2∕GaN∕PSS structure can reduce the defect density to a level of 105cm−2. The internal quantum efficiency of the InGaN-based ELOG-PSS light-emitting diode (LED) sample showed three times in magnitude as compared with that of the conventional GaN/sapphire one. Under a 20mA injection current, the output powers of ELOG-PSS, PSS, and conventional LED samples were measured to be 3.3, 2.9, and 2.5mW, respectively. The enhanced output power could be due to a combination of the reduction in dislocation density (by ELOG) and improved light extraction efficiency (by PSS). Unlike the previ...

GaN-based light-emitting diodes with indium tin oxide texturing window layers using natural lithography

There is a significant gap between the internal and external efficiencies of conventional GaN light-emitting diodes (LEDs). The reason for this shortfall is the narrow escape cone for light in high refractive index semiconductors. In this letter, the p-side-up GaN∕sapphire LEDs with surface textured indium tin oxide (ITO) widow layers were investigated using natural lithography with polystyrene spheres as the etching mask. Under optimum etching conditions, the surface roughness of the ITO film can reach 140 nm while the polystyrene sphere on the textured ITO surface is maintained at about 250–300 nm in diameter. The output power of the ITO∕GaN LED with and without surface texturing is 10.9, and 8.5 mW at 20 mA, respectively. The LEDs fabricated using the surface-textured ITO produced an output power that exceeded that of the planar-surface LED by about 28% at 20 mA.

Fabrication of Pyramidal Patterned Sapphire Substrates for High-Efficiency InGaN-Based Light Emitting Diodes

In this study, a wet-etched pyramidal patterned sapphire substrate (PSS) was used to fabricate the near-ultraviolet InGaN-based light-emitting diodes (LEDs). The pyramidal PSS was etched using a 3H 2 SO 4 :1H 3 PO 4 mixture solution and the activation energy of this reaction is determined to be 28.2 kcal/mol. Three symmetric sidewall facets of the etched pyramidal hole were {112k} on the (0001) sapphire. It was found that the GaN epi layer grew laterally from the top of the pyramid pit and overhung the cavity. An evident reduction in dislocation density of the GaN-on-PSS sample can be confirmed by the etch-pit-density, double-crystal X-ray, and micro photoluminescence measurement results. Under a 20 mA forward injection current, the output power of the conventional and pyramidal PSS LEDs (in epoxy lamp form, λ D = 400 nm) were 7.45 and 9.35 mW, respectively. A 25% enhancement in output power was achieved in the pyramidal PSS LED as compared with that of the conventional LED sample. The enhanced output power is not only due to the improvement of the internal quantum efficiency upon decreasing the dislocation density, but also due to the enhancement of the extraction efficiency using a pyramidal PSS. From light-tracing calculation, the pyramidal reflector arrays can offer more probability of escaping photons from the GaN/sapphire interface, resulting in an increase in light extracting efficiency.

High-power GaN-mirror-Cu light-emitting diodes for vertical current injection using laser liftoff and electroplating techniques

A large-area (1 /spl times/ 1 mm) vertical conductive GaN-mirror-Cu light-emitting diode (LED) fabricated using the laser liftoff and electroplating techniques is demonstrated. Selective p-GaN top area was first electroplated by the thick copper film, and then an excimer laser was employed to separate the GaN thin film from the sapphire substrate. The luminance intensity of the vertical conductive p-side-down GaN-mirror-Cu LED presented about 2.7 times in magnitude as compared with that of the original GaN-sapphire LED (at 20 mA). The light output power for the GaN-mirror-Cu LED was about twofold stronger (at 500 mA). A more stable peak wavelength shift under high current injection was also observed.

AlGaInP light-emitting diodes with mirror substrates fabricated by wafer bonding

An AlGaInP light-emitting diode (LED) with a Au/AuBe/SiO2/Si mirror substrate has been fabricated using wafer bonding. The bonded mirror-substrate LED is capable of emitting luminous intensity of 90 and 205 mcd under 20 and 50 mA injection, respectively. The emission wavelength was found to be independent of the injection current. This feature is attributed to the Si substrate providing a good heat sink.

Effects of Growth Conditions on Structural Properties of ZnO Nanostructures on Sapphire Substrate by Metal–Organic Chemical Vapor Deposition

ZnO was grown on sapphire substrate by metal–organic chemical vapor deposition using the diethylzinc (DEZn) and oxygen (O2) as source chemicals at 500 °C. Influences of the chamber pressure and O2/DEZn ratio on the ZnO structural properties were discussed. It was found that the chamber pressure has significant effects on the morphology of ZnO and could result in various structures of ZnO including pyramid-like, worm-like, and columnar grain. When the chamber pressure was kept at 10 Torr, the lowest full width at half-maximum of ZnO (002) of 175 arc second can be obtained. On the other hand, by lowering the DEZn flow rate, the crystal quality of ZnO can be improved. Under high DEZn flow rate, the ZnO nanowall-network structures were found to grow vertically on the sapphire substrate without using any metal catalysts. It suggests that higher DEZn flow rate promotes three-dimensional growth mode resulting in increased surface roughness. Therefore, some tip on the ZnO surface could act as nucleation site. In this work, the growth process of our ZnO nanowall networks is said to follow the self-catalyzed growth mechanism under high-DEZn flow rate.

Formation process and material properties of reactive sputtered IrO2 thin films

Abstract IrO 2 thin films were deposited by reactive sputtering in various O 2 /(O 2 +Ar) mixing ratios (OMR). The systematic study of the OMR effect on the properties of IrO 2 thin films has been reported. It was found that the formation of IrO 2 could be classified into two classes, depending on the O 2 flow ratio. At low OMR (10–30%), the Ir target and Si substrate were not oxidized and a high deposition rate and high crystallinity IrO 2 could be obtained. On the other hand, at high OMR (>30%), the target and Si substrate were oxidized. It resulted in a lower deposition rate of IrO 2 and yielded poor structural properties. Moreover, the high OMR provided O atoms, incorporated into the IrO 2 thin film. This point could be confirmed by X-ray photoelectron spectroscopy. The excess O defects would also make the resistivity of IrO 2 increase as the samples were prepared at high OMR. The effect of substrate temperature on the resistivity was also discussed. It was found that the resistivity of the IrO 2 films decreased with an increase of the substrate temperature and a minimum resistivity of 70 μΩcm was obtained as films deposited at 600°C using 10% OMR.

Improved thermal management of GaN/sapphire light-emitting diodes embedded in reflective heat spreaders

Using maskless lithography and electroforming techniques, we have demonstrated an enhanced performance of GaN/sapphire light-emitting diode (LED) embedded in a reflective copper heat spreader. The chip size and dominant wavelength of the blue emitter used in this research is 1×1 mm2 and 455 nm, respectively. The cup-shaped LED heat sink is electroformed on sapphire directly using the spin-coated photoresist coated with the Au/Cr/Ag mirror as a mold and dicing into the embedded LED with a Cu base dimension of 3×3 mm2, which effectively enhances the heat dissipation down to the metal frame and reaps the light flux generated from the side emission. With the aid of a reflective heat spreader, the encapsulated LED sample driven at 1 A yields the light output power of 700 mW and around 2.7-times increase in the wall-plug efficiency compared to that of the conventional GaN/sapphire LED. Infrared thermal images confirm the GaN/sapphire LED with more efficient heat extraction and better temperature uniformity. The...

Effects of rapid thermal process on structural and electrical characteristics of Y2O3 thin films by r.f.-magnetron sputtering

Abstract Y2O3 thin films have been deposited on (100) Si substrates by r.f.-magnetron sputtering and subsequently submitted to rapid thermal processing (RTP). X-ray examinations show that the sputtered Y2O3 was dominated by the (111) cubic structure. With increasing RTP temperature (>700°C), the crystallinity of films was improved, especially for the intensity of (400) diffraction peak. The as-deposited films show good dielectric properties in terms of a relative dielectric constant of 16.67 and leakage current density of 6×10−7 A cm−2 (at 1.8 MV cm−1). After the RTP treatment, both the dielectric constant and leakage current of Y2O3 were found to decrease. A typical dielectric constant decreased to 14.77 and its leakage current density lowered to 3×10−8 A cm−2 (at 1.8 MV cm−1) for the film annealed at 850°C. The observed behavior of dielectric constant may be due to the intermediate oxide formation between Y2O3 and Si. Capacitance-voltage characteristics confirm that the reduction of leakage current at high electric field comes from the improvement of interface states.

AlGaInP/AuBe/glass light-emitting diodes fabricated by wafer bonding technology

An AlGaInP/AuBe/glass light-emitting diode (LED) was fabricated by a wafer bonding technique. The AlGaInP LED was grown on a temporary GaAs substrate by metalorganic vapor phase epitaxy. By bonding the AuBe/glass substrate on top of epitaxial layers, the temporary GaAs substrate was removed. The luminance of this wafer-bonded device is about 3050 cd/m2 (600 nm wavelength) at an operating current of 20 mA. It is about three times brighter than a conventional device with an absorbing GaAs substrate. This could be due to the fact that the AuBe/glass substrate serves as a reflective mirror, improving the light extraction efficiency.