Studies of efficiency droop in GaN based LEDs



EL measurements have been made on a number of commercial LEDs as a function of drive current density “J” and temperature “T”. For one type of LED, (sample A), the magnitude of the peak in quantum efficiency (QE) is almost independent of temperature and the QE as a function of J can be successfully modelled in terms of the well-known J=An+Bn2+Cn3 expression using the same relative magnitudes of B and C at different temperatures, with A alone increasing with increasing temperature. In another type, the peak in QE occurs at values of J that decrease with decreasing temperature but in this sample there is evidence of quantum well inhomogeneity. The weak temperature dependence of the C coefficient in sample A is strong evidence against thermally activated current leakage, mid-gap trapping centres or intra-band Auger recombination being dominant mechanisms for droop. The EL spectra show strong phonon replicas at low temperatures, but a lineshape analysis allows the main emission linewidth to be derived as a function of J and T. A model based on inhomogeneous broadening provides a good explanation for the results, and provides a link between the value of B and the effective 2D density of states. The results suggest that the most likely mechanism for droop in these devices is an indirect Auger effect mediated by phonons or localised states (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)