Optically functional surface composed of patterned graded-refractive-index coatings to enhance light-extraction of GaInN light-emitting diodes

Abstract

Graded-refractive-index (GRIN) coatings, composed of multiple dielectric layers of TiO2 and SiO2 are sputter-deposited on the nitrogen-face of thin-film GaInN/GaN light emitting diodes (LEDs). The thickness and refractive index of each layer in the GRIN stack is designed to minimize light trapping inside the LED caused by total internal reflection at the semiconductor-air interface. Patterning the GRIN stack forms an optically functional surface, which converts trapped modes of light into desirable extracted modes that have preferential directions. Inductively coupled-plasma reactive-ion-etching is used to fabricate various patterns, including arrays of cylindrical pillars and diamond-shaped pillars on the GRIN coatings. In comparison to an uncoated planar reference device, the light-output power is enhanced by 131% and 104% for an array of GRIN diamond-shaped pillars and an array of GRIN cylindrical pillars, respectively. This enhancement in light-output power is comparable to N-face roughened LEDs, which show an enhancement of 124%. In addition, the peak emission intensity of the GRIN LEDs with an array of GRIN pillars is between 25 degrees and 55 degrees off-surface-normal. (C) 2011 American Institute of Physics. [doi:10.1063/1.3632072]