research review Don’t blame Auger for LED droop
Scientists say they have shown that neither direct nor indirect Auger recombination are the primary cause of droop in indium gallium nitride quantum wells.
RESEARCHERS at the Technical University of Braunschweig, Germany, believe that they have pinned down the Auger recombination coefficient C in InGaN quantum well structures. Their value of C, which is (1.8±0.2) x 1031
cm6
is lower than previous experimental estimates. The researchers believe that this quantitatively measured Auger
/s,
coefficient is likely due to indirect (phonon-or alloy-assisted) Auger processes rather than direct Auger processes. From the small magnitude of the Auger coefficient (in relation to the radiative coefficient), the team, led by Andreas Hangleiter, says they can conclude that neither direct nor indirect Auger recombination is the primary cause
Typical experimental internal quantum efficiency of a green LED compared to calculation based on the new Auger results showing that such a small Auger coefficient can not explain the droop
of droop in InGaN LEDs. Even though Auger recombination does qualitatively produce a droop-like behaviour, it does so only at much higher current densities than those observed in LEDs.
Understanding the cause of droop is important because it limits the current density applicable to LEDs and thus the amount of light that can be generated with a single LED chip. Given the small Auger coefficient found recombination is a minor contribution to droop. Other mechanisms like carrier overflow or defect-related non- radiative recombination need appraisal. Auger recombination also has an impact on laser diodes. It may contribute to the threshold current density and may lead to different design rules for laser diodes.
The results obtained by the team at Braunschweig were based on optical gain spectra of InGaN-based laser structures. Optical gain spectra provide the unique opportunity to determine carrier density. From the dependence of the non-radiative rate on carrier density, the Auger coefficient and the defect recombination coefficient can then be determined. The researchers claim that this is in contrast to other attempts to investigate, where neither the non-radiative rate nor the carrier density are known, but are the result of a fitting procedure based on a very much simplified model.
M. Brendel et al. Appl. Phys. Lett. 99 031106 (2011)
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www.compoundsemiconductor.net August / September 2011
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