Thin-film photonic crystal LEDs with enhanced directionality

Abstract

The use of photonic crystals for light extraction from light-emitting diodes (LEDs) gives the possibility to shape the farfield emission pattern. This is of particular interest for étendue-limited LED applications that require a more directional farfield than state- of-the-art Lambertian emitters. However, the application of a photonic crystal in a LED results in directional emission only if the photonic crystal and the distribution of guided modes in the LED are tuned correctly. In this thesis, red- and blue-emitting thin-film PhC-LEDs in the AlGaInP and InGaN material systems were modelled, designed, fabricated and characterized. The first experimental results show that light extraction with photonic crystals from AlGaInP thin-film LEDs several microns thick is neither directional nor more efficient than state-of-the-art LEDs with a rough surface structure. Directional light extraction for AlGaInP PhC-LEDs is for the first time demonstrated in much thinner devices where the photonic crystal light extraction of guided modes is combined with the resonant-cavity effect. In an attempt to approach the ideal PhC-LED, strong photonic crystal farfield shaping is demonstrated in InGaN thin-film LEDs of sub-micron thickness. Analysis of their spectral farfields unexpectedly shows that high order diffraction contributes significantly to the light extraction efficiency if the mode absorption is sufficiently low. It is also demonstrated that directional photonic crystal light extraction is possible in InGaN thin-film LEDs several microns thick. The directionality stems from the modulation of the spontaneous emission caused by the proximity of the active region to the bottom mirror. Two new concepts for enhanced light extraction and high directionality are presented: Photonic crystals with two dominating lattice constants are found to outperform conventional photonic crystal LEDs. An alternative approach is the dielectric PhC-LED - FDTD simulations show that the high extraction efficiency of LEDs with surface roughness is combined with the higher directionality of photonic crystal light extraction.