Analysis of extraction routes in surface textured thin-film optical emitters with transparent substrates

We propose a systematic strategy that enables analysis of the outcoupling efficiency through each extraction route for two-dimensional (2D) patterned thin-film optical emitters with transparent substrates. Full-vectorial simulations combined with the near-to-far-field transformations are applied to quantitatively analyze the extraction routes of blue-emitting InGaN/GaN light-emitting diodes (LEDs) on 2D patterned sapphire substrate, from which the main extraction routes are through the top and side surfaces of the substrate. For patterned sapphire substrates, the ratio of top to side emission is calculated for various lattice constants (a) of the pattern; for example, the efficiencies through the top and side routes are nearly equal at a = 3000 nm. We find that the top extraction of light is dramatically improved by increasing the index contrast in patterned substrates, suggesting high-index-contrast patterned substrates containing hollow cavities. The dramatic enhancement in top emission is verified by measuring the far-field distribution of InGaN/GaN LED devices fabricated on sapphire substrate containing hollow cavities and reference patterned sapphire substrate. The simulation algorithm studied herein will provide valuable design freedom for thin-film optical emitters such as GaN-based LEDs and organic LEDs.