Beam-shaping properties of InGaN thin-film micro-cavity light-emitting diodes with photonic crystals

Photonic crystals (PhCs) are known to diffract guided modes in a light-emitting diode into the light extraction cone according to Bragg's law. The extraction angle of a single mode is determined by the phase match between the guided mode and the reciprocal lattice vector of the PhC. Hence, light extraction by PhCs enables strong beam-shaping if the number of guided modes can be kept to a minimum. InGaN thin-film micro-cavity light-emitting diodes (MCLEDs) with photonic crystals (PhCs) emitting at 455 nm have been fabricated. The GaN layer thickness of the processed MCLEDs with a reflective metallic p-contact was 850 nm. One and two-dimensional PhCs were etched 400 nm into the n-GaN to diffract the guided light into air. The farfield radiation pattern was strongly modified depending on the lattice type and lattice constant of the PhC. Two-six-and twelve-fold symmetry was observed in the azimuthal plane from 1D lines, hexagonal lattices and Archimedean A7 lattices, respectively. The emission normal to the LED surface was enhanced by up to 330% compared with the unstructured MCLEDs. The external quantum efficiency was enhanced by 80% for extraction to air. The flux from PhC-MCLEDs in a radial lens was 15.7 mW at 20 mA and 36% external quantum efficiency was measured at 3 mA. High order diffraction was found to contribute significantly to the enhancements in efficiency and directionality. The experimental results are compared with FDTD simulations.