Light-extraction mechanisms in high-efficiency surface-textured light-emitting diodes

• Published in: IEEE journal of selected topics in quantum electronics Vol. 8; no. 2; pp. 248 - 255
• Main Authors: Windisch, R., Rooman, C., Dutta, B., Knobloch, A., Borghs, G., Dohler, G.H., Heremans, P.
• Format: Journal Article
• Language: English
• Published: New York IEEE 01.03.2002; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Absorption; Algorithms; Computer simulation; Efficiency; Extraction; Light emitting diodes; Light scattering; Mathematical models; Mirrors; Monte Carlo methods; Monte Carlo simulation; Particle scattering; Photons; Quantum efficiency; Scattering parameters; Studies; Surface fitting; Surface texture; Temperature; Transistors
• ISSN: 1077-260X; 1558-4542
• DOI: 10.1109/2944.999177

We present a detailed quantitative analysis of the light extraction and loss mechanisms in high-efficiency GaAs-AlGaAs surface-textured thin-film light-emitting diodes (LEDs). The analysis is based on a Monte Carlo simulation. Most input parameters, including scattering of photons at the textured surface, sub-bandgap absorption, and absorption at the metal mirror are obtained from experiments or from literature. The simulation also takes into account the effect of photon recycling and the realistic geometry of the diodes. The only remaining fitting parameter is the internal quantum efficiency, which is deduced to be about 80% at room temperature for the experimentally realized 850-nm LEDs with an external quantum efficiency of 44%. The analysis shows further that the most important loss mechanism is reabsorption in the active layer, and in particular in those parts of the active layer that are not electrically pumped. This conclusion is also valid for other types of high-efficiency LEDs. We could furthermore verify the validity of the Monte-Carlo simulation results by conducting experiments at low temperatures, where nonradiative recombination processes are reduced, resulting in the internal quantum efficiency approaching unity. The measured external quantum efficiency at 90 K is 68%, which is close to the theoretically predicted efficiency for a perfect active layer. The results demonstrate that the light extraction from surface-textured LEDs is fully understood and can be quantitatively modeled by a simple raytracing algorithm.