Research Review: Modeling questions Auger’s contribution to droop
Their effort involved fabricating a range of LEDs with varying numbers of quantum wells, device areas and quantum efficiencies; measuring external quantum efficiency at a range of currents; and fitting the data with the well-known “ABC” equation for carrier recombination. This equation describes non-radiative recombination at defects by a term that is proportional to the carrier concentration, and uses quadratic and cubic variants to cater for radiative and Auger recombination, respectively.
“It is impossible for us to just use the ABC model and get a good fit,” explains team member Fred Schubert from RPI, who says that the team’s experiment indicates that there are contributions from second order, third order and fourth order terms caused by carrier leakage. “Some of the samples have significant fourth order contributions.”
Inserting additional terms in the carrier recombination model has enabled the USKorean partnership to fit the experimental data far better. To realize a good fit at all drive currents, it began by matching the ABC model to the data at low current densities. “This part of the curve is not in question,” says Schubert. “Everybody agrees that there is Shockley-Reed-Hall recombination and radiative recombination.”
Curve fitting was then extended to higher currents, where droop plays a significant role. Here they found that at current densities of 111 A/cm2 the higher-than-third order terms contribute 13 percent or more to the total recombination rate.
Using their model, the team extracted an coefficient for third order processes of 8 x 10-29 cm6 s-1 for the LEDs, which is comparable to values obtained by other experimentalists, but far higher than those determined by first-principle theoretical calculations for Auger recombination.
The researchers point out that this striking difference could be due to one component of carrier leakage that, like Auger recombination, is proportional to the cube of the carrier density.
Q. Dai et al. Appl. Phys Lett. 97 133507 (2010)
