Impact of carrier diffusion on the internal quantum efficiency of InGaN quantum well structures

Internal quantum efficiency (IQE) is studied in a large set of polar and non-polar InGaN/GaN quantum well structures, 57 samples in total. In the search for universal factors limiting IQE, the structures having very different parameters and grown by different researchers within the last decade have been included into the study. Time-resolved light-induced transient gratings and differential transmission techniques were used to measure the carrier lifetime and the diffusion coefficient, together with calibration of the absolute IQE value using an integrating sphere. Comparison of the peak IQE values calculated from the ABC model with the measured ones confirms the existence of an additional nonradiative process in both polar and nonpolar structures, whose rate increases with carrier density and correlates with the Shockley-Read-Hall recombination rate. These facts are consistent with trap-assisted Auger recombination, which is shown to be responsible for the efficiency droop in the low quality structures with peak IQE < 10%, while intrinsic Auger recombination is clearly observed in those with IQE > 20%. Trap-related recombination constants are shown to play the dominant role in determining the decrease of IQE with the sample thickness. The correlation between the nonradiative trap-related recombination rates and the carrier diffusion coefficient is demonstrated, proving that carrier transport towards the sample regions with higher defect density facilitates the nonradiative losses in InGaN quantum well structures.