High efficiency GaN-based LEDs: light extraction by photonic crystals

Issue		Ann. Phys. Fr. Volume 31, Number 6, 2006


Page(s)		1 - 235
DOI		http://dx.doi.org/10.1051/anphys:2007003
Published online		14 December 2007

Ann. Phys. Fr., Vol. 31, N°6 2006, pp. 1-235
DOI: 10.1051/anphys:2007003

High efficiency GaN-based LEDs: light extraction by photonic crystals

A. David^{1, 2, 3}

¹ Materials Department, University of California, Santa Barbara, CA 94301, USA.
² Laboratoire Charles Fabry de l'institut d'optique, Campus polytechnique, 91127 Palaiseau, France.
³ Laboratoire de physique de la matière condensée, École polytechnique, 91128 Palaiseau, France.

(Published online: 14 December 2007)

Abstract
Gallium nitride (GaN)-based light-emitting diodes (LEDs) are seen as promising candidates for the replacement of conventional light sources by all-solid state lighting. However, their efficiency is currently limited by the difficulty of extracting the light emitted within the LED, which is mostly trapped in the material by total internal reflection. This book explores the use of photonic crystals (PhCs) to increase light extraction in a deterministic fashion. PhCs act as bi-dimensional diffraction gratings which extract guided light from the LED. However, the peculiarities of GaN epitaxial layers make the optimization of such structures challenging. In particular, it turns out that the vertical structure of the LED has to be engineered so that all of the guided light may interact efficiently with the photonic crystal. Various implementations of such LEDs are proposed, modeled (by 3D methods), fabricated and characterized.

Résumé - Diodes GaN à haute efficacité: extraction de la lumière par cristaux photoniques.
Les diodes électro-luminescentes à base de nitrure de gallium (GaN) ont le potentiel de remplacer les sources d'éclairage conventionnel en offrant des économies d'énergie considérables. Toutefois, leur efficacité est actuellement limitée par le problème de l'extraction de la lumière émise dans la diode, dont une grande partie reste piégée dans le matériau par réflexion totale interne. Cet ouvrage explore l'emploi de cristaux photoniques (PhC) pour augmenter l'extraction de la lumière de façon déterministe. Les PhC sont ici employés comme réseau de diffraction à deux dimensions afin d'extraire la lumière guidée hors de la diode. Les caractéristiques des couches épitaxiées de GaN rendent toutefois l'optimisation de telles structures difficile. Notamment, il apparaît que la structure verticale de la diode doit être adaptée pour que toute la lumière guidée puisse interagir efficacement avec les cristaux photoniques. Diverses implémentations de telles diodes sont proposées, modélisées (calcul numérique 3D), fabriquées et caractérisées.

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