Nitride-based UV, vertical-injection light-emitting diodes ͑VLEDs͒ with GaN-free and surface roughness structures operating at 365 nm were proposed and demonstrated by a combination of wafer bonding and laser lift-off processes. The GaN-free structure offers a promising potential for enhancing the light output of UV-VLEDs. The 3.2ϫ light output enhancement was performed by removing the GaN. With the help of adopting a roughened surface, the light-output power of the UV-VLEDs could be further enhanced by a factor of 2.3 as compared with that of UV-VLED without a roughened surface. The total enhancement of surface-roughened GaN-free UV-VLEDs was increased by a factor of 7.8 compared to that of conventional UV-LEDs at a driving current injection of 250 mA.High-brightness GaN-based blue and green light emitting diodes ͑LEDs͒ have already been extensively used in full-color displays, traffic signals, and exterior automotive lighting, etc., and show a greater potential to replace incandescent bulbs and fluorescent lamps. 1 UV-LEDs are of interest as a light source for exciting phosphors, medical equipment, air cleaners, and environmental sensors. In particular, most of the phosphors for white fluorescent lamps have a high conversion efficiency of less than 370 nm in the UV spectral region. 2 Therefore, it is important to develop high-efficiency UVLEDs that emit less than 370 nm light in order to fabricate high luminous white LEDs by coupling UV-LEDs and phosphors for solid-state lighting applications. Although the blue/green LEDs are commercially available, it is still difficult to manufacture highbrightness UV-LEDs due to poor quantum efficiency. Several works focusing on efficient current injection by introducing current blocking layers, 3 highly efficient radiative carrier recombination by introducing a high-quality bulk GaN substrate, 4 the emission enhancement of LEDs by the introduction of an AlInGaN quaternary active layer, 5 and the improvement in quality of LEDs by utilizing the AlGaN epitaxial lateral overgrowth technique 6 have attempted to improve device performance. However, light extraction efficiency is also limited due to the self-absorption effect of the GaN layer. The self-absorption of UV lights in the bulk GaN substrate, thick GaN buffer/contact layer under the active layer, or the p-type GaN contact layer result in the lower external quantum efficiency. Therefore, GaN-free structure design is a key issue for enhancing the light extraction of UV-LEDs. 7 Recently, state-of-the art vertical injection GaN-based LEDs were demonstrated to be high-potential lightemitting devices capable of achieving high brightness operation due to their excellent thermal dissipation. 8,9 In addition, the surface roughness technique seems to have great potential to provide large enhancement due to random scattering from the roughened surface. 10,11 Zhou et al. 12 reported Ͼ2.5 times light extraction gain for deep UV-LEDs by utilizing laser lift-off ͑LLO͒ and surface roughening to enhance the extraction efficiency of near-...

No abstract

The flip-chip light emitting diodes (FC-LEDs) with triple roughened surfaces were fabricated comprising top surface sapphire textured layer, interface patterned sapphire layer, and bottom naturally textured p-GaN layer. Light extraction efficiency was enhanced by such triple textured layers. The light output power of FC-LEDs was increased 60% (at 350 mA current injection) compared to that of conventional FC-LEDs by implementing the triple roughened surfaces. The enhancement efficiency can be simulated and the simulated results showed the same trend as the results of experiment.

No abstract

No abstract