Disentangling the Impact of Point Defect Density and Carrier Localization-Enhanced Auger Recombination on Efficiency Droop in (In,Ga)N/GaN Quantum Wells

The internal quantum efficiency of (In,Ga)N/GaN quantumwells cansurpass 90% for blue-emitting structures at moderate drive currentdensities but decreases significantly for longer emission wavelengthsand at higher excitation rates. This latter effect is known as efficiencydroop and limits the brightness of light-emittingdiodes (LEDs) based on such quantum wells. Several mechanisms havebeen proposed to explain efficiency droop including Auger recombination,both intrinsic and defect-assisted, carrier escape, and the saturationof localized states. However, it remains unclear which of these mechanismsis most important because it has proven difficult to reconcile theoreticalcalculations of droop with measurements. Here, we first present experimentalphotoluminescence measurements extending over three orders of magnitudeof excitation for three samples grown at different temperatures thatindicate that droop behavior is not dependent on the point defectdensity in the quantum wells studied. Second, we use an atomistictight-binding electronic structure model to calculate localization-enhancedradiative and Auger rates and show that both the corresponding carrierdensity-dependent internal quantum efficiency and the carrier densitydecay dynamics are in excellent agreement with our experimental measurements.Moreover, we show that point defect density, Auger recombination,and the effect of the polarization field on recombination rates onlylimit the peak internal quantum efficiency to about 70% in the resonantlyexcited green-emitting quantum wells studied. This suggests that factorsexternal to the quantum wells, such as carrier injection efficiencyand homogeneity, contribute appreciably to the significantly lowerpeak external quantum efficiency of green LEDs.

Automated summary

The internal quantum efficiency of (In, Ga)N/GaN quantum wells can exceed 90% for blue-emitting structures, but decreases for longer wavelengths and higher excitation rates, known as efficiency droop, which limits the brightness of LEDs based on such quantum wells. Various mechanisms have been proposed to explain efficiency droop, but it is still unclear which one is the most important due to the difficulty in reconciling theoretical calculations with measurements.