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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 10(5) cm(-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 20 mA injection current, the output powers of ELOG-PSS, PSS, and conventional LED samples were measured to be 3.3, 2.9, and 2.5 mW, 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 previous double ELOG approaches, the presented ELOG-PSS structure needs only one regrowth process and will have high potential in future high-quality ultraviolet emitters, even blue/green laser diode applications. (c) 2006 American Institute of Physics

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. (c) 2005 American Institute of Physics

Summary This paper investigates the finite‐time stabilization of a class of switched stochastic nonlinear systems under arbitrary switching, where each subsystem has a chained integrator with the power r (0 < r < 1). By using the technique of adding a power integrator, a continuous state‐feedback controller is constructed, and it is proved that the solution of the closed‐loop system is finite‐time stable in probability. Two simulation examples are provided to show the effectiveness of the proposed method. Copyright © 2015 John Wiley & Sons, Ltd.

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 3normalH2SnormalO4:1normalH3PnormalO4 mixture solution and the activation energy of this reaction is determined to be 28.2kcal∕mol . Three symmetric sidewall facets of the etched pyramidal hole were {112false¯kfalse¯} 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 20mA forward injection current, the output power of the conventional and pyramidal PSS LEDs (in epoxy lamp form, λnormalD=400nm ) were 7.45 and 9.35mW , 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.

Articles you may be interested inMorphology and microstructure evolution of Al x Ga 1 − x N epilayers grown on GaN/sapphire templates with AlN interlayers observed by transmission electron microscopy Morphology and microstructure of dislocation etch pits in GaN epilayers etched by molten KOH have been investigated by atomic force microscopy, scanning electron microscopy, and transmission electron microscopy ͑TEM͒. Three types of etch pits ͑␣, ␤, and ␥͒ are observed. The ␣ type etch pit shows an inversed trapezoidal shape, the ␤ one has a triangular shape, and the ␥ type one has a combination of triangular and trapezoidal shapes. TEM observation shows that ␣, ␤, and ␥ types etch pits originate from screw, edge, and mixed-type threading dislocations ͑TDs͒, respectively. For the screw-type TD, it is easily etched along the steps that the dislocation terminates, and consequently, a small Ga-polar plane is formed to prevent further vertical etching. For the edge-type TD, it is easily etched along the dislocation line. Since the mixed-type TDs have both screw and edge components, the ␥ type etch pit has a combination of ␣ and ␤ type shapes. It is also found that the chemical stabilization of Ga-polar surface plays an important role in the formation of various types of dislocation etch pits.

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