Currently, in the field of solid-state lighting, LEDs based on InGaN/GaN quantum wells offer the highest overall efficiency. However, while the present preferred solution for white light emission is phosphor-based wavelength conversion, it has been argued that a color-mixing approach could exploit more efficiently the potential of solid-state lighting. A similar approach implies the combination of different LEDs emitting at different wavelength, including in particular the green-yellow range of spectrum. Unfortunately, it is well known that in this range nitride-based LEDs show a systematic drop in efficiency, called "green gap".

Recently, a research published in Phisical Review Letters shed some light on the physical origin of the "green gap".

As Dr. Matthias Auf der Maur, the leading author of the paper says, "Our findings show that the random fluctuations of the indium concentration naturally present in any InGaN alloy material could explain a great part of the green gap in c-plane InGaN/GaN-based light emitting diodes. In fact - explains Dr Auf der Maur - these fluctuations are found to cause a decrease in the radiative recombination coefficient with increasing indium content. This is due to the localization of electrons and holes in the QW plane determined by statistical fluctuations in the InGaN alloy".

Thanks to the atomistic simulations performed through the software tiberCAD, it has been possible to quantitatively assess the effect of alloy fluctuations on the maximum LED efficiency. To this aim, optical transitions’ momentum matrix elements have been calculated using an atomistic empirical tight-binding approach.

"Through TiberCAD simulations – concludes Dr. Auf der Maur – we have shown that the wavelength dependence of radiative recombination is in good agreement with experimental findings and that simulation approaches based on homogeneous effective media approximations, typically used for device simulations, overestimate B parameter by an amount proportional to the mean indium content in the QW".


For further information see:
"Efficiency Drop in Green InGaN/GaN Light Emitting Diodes: The Role of Random Alloy Fluctuations"
Matthias Auf der Maur, Alessandro Pecchia, Gabriele Penazzi, Walter Rodrigues, and Aldo Di Carlo
Phys. Rev. Lett. 116, 027401 (2016)