Abstract
This study examined systematically the mechanism of light interaction in the sapphire/MgF2/Al triple-layer omnidirectional reflectors (ODR) and its effects on the light output power in near ultraviolet light emitting diodes (NUV-LEDs) with the ODR. The light output power of NUV-LEDs with the triple-layer ODR structure increased with decreasing surface roughness of the sapphire backside in the ODR. Theoretical modeling of the roughened surface suggests that the dependence of the reflectance of the triple-layer ODR structure on the surface roughness can be attributed mainly to light absorption by the Al nano-structures and the trapping of scattered light in the MgF2 layer. Furthermore, the ray tracing simulation based upon the theoretical modeling showed good agreement with the measured reflectance of the ODR structure in diffuse mode.
Highlights
This study examined systematically the mechanism of light interaction in the sapphire/MgF2/Al triple-layer omnidirectional reflectors (ODR) and its effects on the light output power in near ultraviolet light emitting diodes (NUV-LEDs) with the ODR
AlGaN-based UV-LEDs normally suffer from very low external quantum efficiency (EQE) due to the poor light extraction efficiency caused by the epitaxial absorption of UV light
The AlGaN/InGaN MQW NUV-LED chips were fabricated on c-plane sapphire substrates, and the sapphire substrate was thinned to 110 mm by backside grinding and lapping using a range of slurries with different diamond sizes of 1, 3 and 5 mm, respectively, to control the surface roughness of the sapphire backside
Summary
This study examined systematically the mechanism of light interaction in the sapphire/MgF2/Al triple-layer omnidirectional reflectors (ODR) and its effects on the light output power in near ultraviolet light emitting diodes (NUV-LEDs) with the ODR. The light output power of NUV-LEDs with the triple-layer ODR structure increased with decreasing surface roughness of the sapphire backside in the ODR. Reduced indium compositional fluctuations in InGaN quantum wells with a small indium content weakens the carrier localization in localized energy states, which in turn increases the possibility of carrier trapping at the nonradiative recombination centers by the increase in in-plane carrier diffusion To overcome these obstacles in AlGaN-based UV-LEDs, noble device structures, such as chip-shaped LED design[6], photonic crystal[7], distributed Bragg reflector (DBR)[8], omnidirectional reflector (ODR)[9], surface texturing reducing internal light reflection, and surface plasmon[10,11], have been proposed. The surface roughness of the sapphire backside critically affected the light extraction efficiency of the ODR scheme and should be minimized to enhance light extraction effectively
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