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Is Auger definitely the cause of droop?

Researchers question claims of a definitive proof for Auger recombination as the cause of LED droop.

One of the mosthotly debated issues within the compound semiconductor community is whether Auger recombination is the primary cause of droop – the decline in LED efficiency as the current passing through the device is cranked up.

This April, researchers at the University of California, Santa Barbara, (UCSB) and the École Polytechnique, France, claimed that they had finally brought this controversial, long-running debate to an end when they reported the results of a novel experiment.

By simultaneously measuring the energy of the electrons passing through a GaN-based LED and the photons emitted by it, they argued that they had gathered undisputable evidence that Auger recombination is the cause of droop.

However, while some peers in the nitride community were impressed by the elegance of this experiment, they were not convinced that these results offered undisputable proof that Auger recombination is the origin of droop.

“We think that they had a truly brilliant idea to apply electron emission spectroscopy for elucidating Auger processes in GaN-based LEDs,” says Michele Goano from Politecnico di Torino, Turin, Italy. But he questions the interpretation of the experimental results, and is voicing his concerns in a comment submitted to Physical Review Letters, co-authored with other researchers from the Polytechnic University of Turin and Masahiko Matsubara and Enrico Bellotti from Boston University.

One of their criticisms is that the UCSB-École Polytechnique team claim that the higher energy electron peak observed at about 1.5 eV originates from a satellite valley in the conduction band. Goano and his co-workers argue that the energy of satellite band is as high as 2.5 eV.

This value is based on their calculations, which give similar results to those of other groups. The precise energy of this satellite valley does not actually matter, however, according to Goano: “The point is that, according to our calculations, the scattering rates in the satellite valleys are so high that electrons there would undergo relaxation to the bottom of the conduction band well before they reach the LED surface.” In other words, electrons in satellite valleys would not account for the experimentally observed high-energy peak.

Another difference of opinion concerns the level of carrier heating in the LED. Researchers from the UCSB-École Polytechnique partnership claim that it is negligible, while the theoretical team have calculated that it may account for the high-energy peak seen in the experiment.

“According to our present view, electrons leaked from the active region – regardless of the originating leakage mechanism – thermalize at the bottom of the conduction band and, upon reaching the narrow band-bending region below the surface, propagate ballistically through it, thanks to the high electric field there. In other words, they convert their potential energy into kinetic energy,” explains Goano.

Although he and his co-workers are critical of the claim that the experimental evidence proves that Auger is the cause of droop, they point out that the results of their calculations do not imply that Auger, and possibly Auger-induced leakage, play a negligible role in LED droop. 

“The LED community is probably coming to terms with the idea that there is not a single cause of droop,” says Goano.

In his opinion, experiments similar to the one by the UCSB-École Polytechnique team, but measuring electron energies in carefully designed test structures, could help to shed new light on this debate.

Theorists are questioning the interpretation of results obtained by researchers Jacques Peretti, Claude Weisbuch, and Lucio Martinelli who have used a spectrometer to measure the energy of electrons emitted from a GaN LEDs. Photo credit: Ecole Polytechnique, Ph. Lavialle.

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