In the move to use LEDs not just for indicator lamps and digital displays but to illuminate buildings and neighborhoods, gallium nitride–based semiconductors have taken center stage. They are among the few materials capable of emitting brightly at green, blue, and shorter wavelengths; as such, they are key ingredients for generating white light with LEDs.

The main allure of LEDs, and of GaN-based LEDs in particular, is their efficiency. (See the article in Physics Today, December 2001, page 42.) In theory, the energy cost of injecting an electron–hole pair into an LED is repaid nearly in full when it recombines by emitting a photon. Inevitably, though, some electrons and holes will recombine nonradiatively or not at all. In 2007 researchers at Phillips Lumileds Lighting in San Jose, California, reported experiments1 showing that the probability of such nonradiative events increases as the cube of the injected current density...

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