We report on an InGaN-based light-emitting diode ͑LED͒ with a top p-GaN surface microroughened using the metal clusters as a wet etching mask. The light-output power for a LED chip with microroughening was increased compared to that for a LED chip without one. This indicates that the scattering of photons emitted in the active layer was much enhanced at the microroughened top p-GaN surface of a LED due to the angular randomization of photons inside the LED structure, resulting in an increase in the probability of escaping from the LED structure. By employing the top surface microroughened in a LED structure, the power conversion efficiency was increased by 62%.

To improve the extraction efficiency of InGaN/GaN multiple quantum well light-emitting diodes ͑LEDs͒, nanosize cavities were fabricated on a top p-GaN surface by inductively coupled plasma etching utilizing self-assembled platinum clusters as an etch mask. The relative output power was increased up to 88% compared to that of the LED without nanosize cavities. This result could be attributed to an enhancement in the escape of light due to the angular randomization by the nanosize cavities and to the reduced contact resistance due to the increased contact area between the transparent metal layers and the p-GaN.GaN-based optical devices, especially light-emitting diodes ͑LEDs͒, have considerable potential for use in many applications, including automobile brake lights, full color display panels, and solid state lighting sources. 1-3 In applications for lighting sources, GaN-based LEDs have several advantages, including higher reliability and high brightness. For these reasons, LEDs are increasingly replacing conventional lighting sources and are finding many new applications. In spite of these advantages, the total light-output continues to be low, and modification of device design is needed to increase the light-output efficiency of LEDs. 4 In general, for conventional planar LEDs, the internal quantum efficiencies are close to 100%. 5 However, the external quantum efficiency is only 3-30 % for most commercial LEDs. 6 The origin of this large discrepancy is due to the fact that the efficiency of light extraction of most conventional LEDs is limited by the total internal reflection of the light generated in the active region of the LED, which occurs at the semiconductor-air interface. This is due to the large difference in the refractive index between the semiconductor and air. For GaN-based LEDs, the refractive indexes of GaN ͑n GaN ͒ and air ͑n air ͒ are 2.5 and 1, respectively. 7 In this case, the critical angle ͓ c = sin −1 ͑n air /n GaN ͔͒ for the light generated in the active region to escape is about 23°. Because light emission from the active region of an LED is directionally isotropic and can escape from the chip only if the angle of incidence to the wall is less than the critical angle, a small fraction of light generated in the active region of the LED is able to escape to the surrounding air. Therefore, for a simple planar GaN-based LED, the external quantum efficiencies are limited to a few percent due to the high refractive index of GaN as well as absorption in the metal pad for current injection.A number of approaches have been proposed to create more highly efficient LEDs. These include flip-chip packaging for high power light emitters, 4 substrate separation to minimize light absorption by the substrate, 8 transparent layer design, 9 shaping chip design, 10 and a distributed Bragg reflector layer. 11 These findings indicate that the performance of this type of LED can be significantly enhanced compared to that of conventional LEDs. In this paper, we report on roughening of the p-GaN surface using se...

Two different InGaN/GaN multiple-quantum well ͑MQW͒ microdisk light emitting diodes ͑-LEDs͒ with different In compositions in the MQW were fabricated. The optical output power was greatly increased with a reduction of LED size. This can be attributed to the enhanced current density and internal quantum efficiency in-LEDs. The peak shift and the enhancement of output power were larger in-LED with a higher In composition in the MQW. These can be explained by a reduced piezoelectric field due to a partial strain relief and also more efficient carrier confinement due to a higher In composition in the MQWs.

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