High excitation density recombination dynamics in InGaN/GaN quantum well structures in the droop regime

We report on the low temperature time-integrated and time-resolved photoluminescence properties of an (0001) InGaN/GaN multiple quantum well structure. From excitation power dependent studies, we observe a clear correlation between the onset of efficiency droop and the broadening of the time-integrated luminescence spectra. Time-resolved spectroscopy reveals that this spectral broadening is associated with a large and rapid red-shift of the spectrum with increasing time after the excitation pulse. This dynamic shift of the spectrum also modifies the form of the luminescence transients, yet the spectrally integrated luminescence dynamics have the same non-exponential shape as those obtained under much lower excitation levels. This observation is consistent with the emission originating from the recombination of independently localised carriers, even under very high excitation levels. Nevertheless, at these high excitation levels we propose that there also is a population of weakly localised carriers, probably electrons. We suggest that these more mobile carriers are effective in screening the built-in electric field across the quantum well as well as being more susceptible to non-radiative loss. The collapse in this population, due to radiative and non-radiative recombination, restores the built-in polarisation induced field and results in the observed time dependent red-shift of the luminescence spectrum. (C) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim