Annick Bay, Nicolas Andre, Michael Sarrazin, Ali Belarouci, Vincent Aimez, Laurent A. Francis, Jean Pol Vigneron
Actual light emission diodes (LED) have most often good internal efficiencies but poor external efficiencies due to total internal reflection at the air interface. In this paper the design, fabrication and characterization of a bioinspired overlayer deposited on a GaN LED is investigated. The purpose of this overlayer is to improve light extraction into air, after the photons have been generated in the diode's high refractive-index active material. The layer design is inspired by the microstructure found in the firefly Photuris sp., described by Bay et al. : a surface with an asymmetrical triangular profile (a "factory-roof" shape), developed on the scale of a few micrometers, thus somewhat larger than usually suggested in the related literature. The profile of the overlayer corrugated surface of the coating film was copied from the natural model. Yet, the actual dimensions and material composition have been optimized to take into account the high refractive index of the GaN diode stack. The optimization procedure indicated a protrusion height of 5 micrometer, created on a photoresist film, spin-coated on the GaN-based diode. This two-dimensional pattern contrasts other designs by its unusual profile, its larger dimensions and the fact that it could be tailored to an existing diode design rather than requiring a complete redesign of the diode geometry. The gain of light extraction reaches values up to 55% in comparison to a plane surface.
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http://arxiv.org/abs/1209.4767
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