The nanopatterned sapphire substrates (NPSSs) with aspect ratio that varied from 2.00 to 2.50 were fabricated by nanoimprint lithography. We could improve the epitaxial film quality and enhance the light extraction efficiency by NPSS technique. In this work, the aspect ratio effects on the performances of GaN-based light-emitting diodes (LEDs) with NPSS were investigated. The light output enhancement of GaN-based LEDs with NPSS was increased from 11% to 27% as the aspect ratio of the NPSS increases from 2.00 to 2.50. Owing to the same improvement of crystalline quality by using various aspect ratios of NPSS, these results indicated that the aspect ratio of the NPSS is strongly related to the light extraction efficiency.
GaN-based light-emitting diodes (LEDs) grown on photonic crystal-patterned sapphire substrates (PCPSS) have been demonstrated. PCPSS was fabricated by nanosphere lithography, and the photonic crystal structure was the hexagonal-lattice pattern. The forward voltages of PCPSS and patterned sapphire substrates (PSS) LEDs were smaller than that of conventional sapphire substrates (CSS) LED, and it infers the epitaxial film quality of PCPSS and PSS LEDs has been slightly improved. The luminance intensity of PCPSS LED was 1.63 and 1.51 times higher than those of CSS and PSS LED at 20 mA injection current. The enhancement in the luminance intensity of PCPSS LED is attributed to the photonic crystal structure.
Device performances were investigated for InGaN/sapphire light-emitting diodes ͑LEDs͒ with advanced heat dissipation design from chip to package. By directly contacting a copper heat spreader with sapphire, the maximum junction temperature of the LED chip was reduced from 62.9°C of a conventional LED to 48.3°C at an injection current of 350 mA. Further temperature reduction to 37.3°C could be achieved by packaging the copper-surrounded LED chip on the heat sink coated with a diamond-like layer which acts as the second heat spreader. The reduced junction temperature was attributed to good heat dissipation from both the copper and the diamond-like layer due to their low thermal resistance. The copper heat spreader not only extracts heat efficiently, but also enhances the light extraction of the LED, as the copper was designed with a proper geometry such as cup-shaped profile. The improved LED performance suggests that the proposed thermal management from chip to package is an efficient alternative for high power applications. High power GaN-based light-emitting diodes ͑LEDs͒ are essential for next-generation lighting applications. To achieve higher light output performance, it is necessary to drive a GaN-based LED to a high current density level. However, under such a high current injection, the LED typically exhibits the performance deterioration of power saturation.
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