Technical Insight
Koreans slam Auger as the primary cause of LED droop
A Korean partnership has joined the controversial debate over the origin of LED droop. It claims that its sophisticated approach to curve fitting of experimental data demonstrates that Auger recombination only makes a small contribution to droop, the decline in GaN LED external quantum efficiency at higher current densities.
This team from Inha University and Hanyang University arrived at this conclusion by manipulating the standard rate equations, and reducing fit parameters from three to just one. Selecting a suitable value for the Shockley-Read-Hall (SRH) non-radiative recombination coefficient generated a value for Auger recombination (the non-radiative interaction of an electron, a hole, and a third carrier) that is substantially different from that quoted by those backing Auger as the cause of LED droop.
To slash the number of fit parameters to just one, the researchers assumed an injection efficiency of 100 percent.
In addition, they exploited the fact that the gradient of the peak of the internal quantum efficiency as a function of drive current is zero at the maximum value for efficiency.This allowed the construction of new equations that related the rates for SRH recombination, bimolecular radiative recombination, and Auger recombination to the quantum well thickness and the maximum values for current density and internal quantum efficiency.
Experimental results were obtained by taking a 460 nm LED produced by a domestic supplier, and measuring its internal quantum efficiency as a function of drive current. This supplier also provided a value for the quantum well thickness, and the researchers determined the non-radiative carrier lifetime from time-resolved photoluminescence measurements. Lead author Han-Youl Ryu says that this type of photoluminescence measurement could not be used on the LED chips. “Instead, we performed the measurement on an LED wafer with similar layer structures, and estimated the nonradiative carrier lifetime to be about 50 ns.”
The researchers then deduced an Auger recombination rate of 10-27 cm6/s, by assuming a maximum internal quantum efficiency of 79 percent, and determining the value for the SRH recombination rate from the non-radiative carrier lifetime measurements.
This value for the Auger recombination rate is at least 1000 times higher than that quoted by other groups that claim Auger is the primary cause of droop, and implies that an alternative non-radiative mechanism is needed to account for declining LED efficiencies at high drive currents.
To slash the number of fit parameters to just one, the researchers assumed an injection efficiency of 100 percent.
In addition, they exploited the fact that the gradient of the peak of the internal quantum efficiency as a function of drive current is zero at the maximum value for efficiency.This allowed the construction of new equations that related the rates for SRH recombination, bimolecular radiative recombination, and Auger recombination to the quantum well thickness and the maximum values for current density and internal quantum efficiency.
Experimental results were obtained by taking a 460 nm LED produced by a domestic supplier, and measuring its internal quantum efficiency as a function of drive current. This supplier also provided a value for the quantum well thickness, and the researchers determined the non-radiative carrier lifetime from time-resolved photoluminescence measurements. Lead author Han-Youl Ryu says that this type of photoluminescence measurement could not be used on the LED chips. “Instead, we performed the measurement on an LED wafer with similar layer structures, and estimated the nonradiative carrier lifetime to be about 50 ns.”
The researchers then deduced an Auger recombination rate of 10-27 cm6/s, by assuming a maximum internal quantum efficiency of 79 percent, and determining the value for the SRH recombination rate from the non-radiative carrier lifetime measurements.
This value for the Auger recombination rate is at least 1000 times higher than that quoted by other groups that claim Auger is the primary cause of droop, and implies that an alternative non-radiative mechanism is needed to account for declining LED efficiencies at high drive currents.
