Nitride-based LEDs are efficient, nontoxic and long-lasting, but they are considered impractical for general lighting because of a drop in efficiency when operating at high power. Researchers at the University of California said they have now figured out why. Chris Van de Walle, a professor in the materials department at UCSB, and his colleagues investigated the drop in efficiency, a phenomenon known as “droop,” by performing quantum mechanical calculations. LED droop. (Image: UC Santa Barbara engineering) The researchers said the LED droop can be attributed to Auger recombination, a process that occurs in semiconductors in which three charge-carriers interact without giving off light. The researchers also discovered that indirect Auger effects, which involve a scattering mechanism, account for the discrepancy between the observed degree of droop and that predicted by other theoretical studies, which only accounted for direct Auger processes. In nitride LEDs, “These indirect processes form the dominant contribution to the Auger recombination rate,” said Emmanouil Kioupakis, a postdoctoral researcher at UCSB and lead author of the study. The other authors are Van de Walle, Patrick Rinke, now with the Fritz Haber Institute in Germany, and Kris Delaney, a project scientist at UCSB. LED droop can't be eliminated because Auger effects are intrinsic, but it could be minimized, the researchers said, by using thicker quantum wells in LEDs or growing devices along nonpolar or semipolar growth directions to keep carrier density low. “With Auger recombination now established as the culprit, we can focus on creative approaches to suppress or circumvent this loss mechanism,” Van de Walle said. For more information, visit: www.engineering.ucsb.edu