Photonic crystal based control of directionality in GaN based LEDs

Authors: Mercier, T.M., Krishnan, C., Shaw, P.J., Lagoudakis, P.G. and Charlton, M.D.B.

Journal: Proceedings of SPIE - The International Society for Optical Engineering

Volume: 11302

eISSN: 1996-756X

ISSN: 0277-786X

DOI: 10.1117/12.2543124

Abstract:

LED surface structuring has been widely used to increase light extraction[1]. Due to the high refractive index of the thick GaN epitaxy layers, most emitted light becomes trapped and reabsorbed by the epitaxial layers. While random structuring can effectively scatter trapped light out of the LED, it gives little control over the resulting beam-shapef[2]. Photonic crystals however provide a means to simultaneously improve light extraction efficiency and control beam directionality. Furthermore, P-side up LEDs normally utilize a transparent top contact layer in order to allow top light emission whilst maintaining good electrical properties. In this paper we investigate a novel photonic crystal LED configuration with a nontransparent metal top contact layer, and cylindrical holes etched through the top contact layer and deep into the underlying epitaxy. In this novel configuration light emission is only possible from the etched holes giving rise to extreme beam steering effects. We utilize broadband spectroscopic reflectometry to experimentally investigate beam shape and optical properties from fabricated devices. We observe a range of achievable beam patterns with extreme deviations from the normal Lambertian. We investigate the effect of square and triangular photonic crystal lattices on beam directionality.

Source: Scopus