Theoretical Technique Identifies Defects in LED Materials

The discovery and characterization of a specific type of defect in the atomic structure of an LED that results in less efficient performance could enable fabrication of even more efficient, longer-lasting light sources. Improving the internal quantum efficiency of such LEDs is a vital step for developing and proliferating optoelectronic devices.

A conceptual illustration of how defects in a crystal lattice might contribute to nonradiative recombination of electrons and holes in LEDs. Courtesy of Peter Allen/UCSB.

"In an LED, electrons are injected from one side, holes from the other," said professor and research group leader Chris Van de Walle of the University of California Santa Barbara.

As the two elements travel across the crystal lattice of the semiconductor — in this case GaN-based material — the meeting of electrons and holes (the absence of electrons) is what is responsible for the light that is emitted by the diode: As electron meets hole, it transitions to a lower state of energy, releasing a photon along the way.

Occasionally, the researchers said, the charge carriers meet and do not emit light, resulting in the so-called Shockley-Read-Hall (SRH) recombination. The charge carriers are captured at defects in the lattice where they combine, but without emitting light.

The UCSB team identified defects involving complexes of gallium vacancies with oxygen and hydrogen. These defects had been previously observed in nitride semiconductors, but until now, their detrimental effects were not understood, the researchers said.

"It was the combination of the intuition that we have developed over many years of studying point defects with these new theoretical capabilities that enabled this breakthrough," said Van de Walle, who credited co-author Audrius Alkauskas with the development of a theoretical formalism necessary to calculate the rate at which defects capture electrons and holes.

The method lends itself to future work identifying other defects and mechanisms by which SRH recombination occurs, said Van de Walle. The researchers do not believe the gallium vacancy complexes to be the only detriment defects, and will apply their methodology to investigating other potential defects to assess their impact on nonradiative recombination.

The research was published in Applied Physics Letters (doi: 10.1063/1.4942674).