Anisotropy of Light Extraction Emission with High Polarization Ratio from GaN-based Photonic Crystal Light-Emitting Diodes

Abstract

In conclusion, we have experimentally and theoretically demonstrated that surface emitted anisotropic light extraction and polarized light from GaN-based LEDs. The EL images of the anisotropy light extraction distribution in the azimuthal direction were obtained with specially designed annual GaN PhC LED structures, which is dependent on the orientations of the PhC lattice and lattice constants and shows a four-fold symmetric light extraction patterns with varying numbers of petals in the plane of the waveguide. The regions corresponding to the various numbers of petals are determined for increasing lattice constant. More petals appear in the observed image with increasing lattice constant, and some of the petals may disappear. The regions for the appearance and disappearance of the petals are determined by the Bragg diffraction analysis using Ewald construction. In addition the angular dependence of the light extraction for waveguided light incidents to plane with various lattice orientations is also determined. The results show that the light extraction for the square lattices can only occur for certain crystal directions according to the lattice symmetry. Further, a P/S ratio of 5.5 (~85% polarization light) has been observed. The polarization characteristics are theoretically discussed by couple mode theory. At lower normalized frequency, PhC LED has better polarization property, and lattice orientation not only affects the extraction efficiency but also P/S ratio of radiative light. This polarization behavior suggests an efficient means to design and control the GaN blue PhC LEDs for polarized light emission.