NUSOD Blog

Connecting Theory and Practice in Optoelectronics

The dilemma of modeling the efficiency droop in GaN-based light-emitting diodes

LED efficiency droopThe Journal of Computational Electronics just published a special issue on “Simulation of GaN-based Light-Emitting Diodes” (link) which presents a collection of invited papers from leading modeling groups currently working in this much discussed field. The debate focuses on the microscopic mechanism behind the  efficiency droop in GaN-LEDs. With rising current, injected electron-hole pairs disappear increasingly in non-radiative recombination processes, thereby reducing the fraction that delivers photons (internal quantum efficiency). The two leading explanations are Auger recombination and electron leakage, respectively. Very few direct measurements of either mechanism are published thus far, none of which reveals a dominating magnitude. Most publications base their claims on modeling and simulation. For the first time, this special journal issue includes a direct comparison of four different models by applying them to the same LED and the same efficiency measurement.  The difference between the four modeling approaches is mainly found in the carrier transport models. Besides the common drift-diffusion concept, authors include the Monte-Carlo method, the non-equilibrium Green’s function, a non-local transport model, and percolation transport considering random alloy fluctuations, respectively. While the normalized efficiency droop is fairly well reproduced in most cases, the conclusions are quite different. Two papers find that the measured droop is caused by Auger recombination, the others observe significant electron leakage. This ongoing dilemma represents a serious challenge to the computational electronics community. Further efforts are needed to unambiguously identify the cause of the GaN-LED efficiency droop. The future of these modern light sources may depend on it.

Update 9/1/15: Latest results are now published in Applied Physics Letters.

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