Photonic crystal light emitting diode

Abstract

This master's thesis describe electromagnetic simulations of a gallium antimonide (GaSb) light emitting diode, LED. A problem for such devices is that most of the generated light is reflected from the surface due to total internal reflection, and is therefore prevented from coupling out of the semiconductor material. Etching out a 2D photonic crystal grating on the LED surface would put aside the absolute rule of total internal reflection, and could therefore be used to increase the total transmission. The simulation method which was developed was supposed to find geometry parameters for the photonic crystal to optimize the light extraction. A set of plane waves were therefore simulated using FDTD to build an equivalent to the Fresnel equations for the photonic crystal surface. From that the total transmittance and radiation patterns for the simulated geometries were calculated. The results indicated an increase in the transmission properties of up to 70% using a square grating of holes where the holes have a radius of 0.5µm, the hole depth is 0.4µm, and the grating constant is 1µm. A hexagonal grating of holes and a square grating of isotropically etched holes were also simulated, and featured improvements on the same scale, but with different dimensions for the holes. The simulations were computationally very demanding, and the simulation structure therefore had to be highly trimmed to limit the calculation time to reasonable values. This might have reduced the accuracy of the results. Especially the optimum grating constant, and the value of the optimum improvement itself is believed to be somewhat inaccurate.