UCSB Researchers Identify Atomic Defects that Reduce LED Efficiency

Researchers at UCSB have identified a specific type of defect in the atomic structure of an LED that results in less efficient performance. The researchers anticipate that the characterization such point defects could lead to the fabrication of more efficient and longer lasting LEDs.

UCSB conceptual illustration of crystal-lattice-defects in GaN

Chris Van de Walle, whose research team carried out the work says that techniques are available to find if such defects are present in the LED materials, and these techniques can be used to improve the material’s quality. Not all LEDs are created equal. In fact, it is difficult for manufacturers to fabricate LEDs with exactly the same performance and characteristics. LED efficiency is probably the most important trait of an LED that leads to savings and return on investment. The performance of LEDs and their is heavily reliant on the semiconductor material quality at the atomic level.

“In an LED, electrons are injected from one side, holes from the other,” stated Van de Walle. They travel across the crystal lattice of the semiconductor — in the case of white LEDs gallium-nitride-based material. Then, the electrons and holes (the absence of electrons) meet causing the diode to emit light. When an electron meets a hole, it transitions to a lower state of energy, releasing a photon.

Sometimes, however, the charge carriers meet, but do not emit light, resulting in the so-called Shockley-Read-Hall (SRH) recombination. According to the researchers, this SRH recombination happens when the charge carriers are captured at defects in the lattice where they combine without emitting light.

The researchers have identified defects involving complexes of gallium vacancies in the presence of oxygen and hydrogen.
“These defects had been previously observed in nitride semiconductors, but until now, their detrimental effects were not understood,” explained lead author Cyrus Dreyer, who did many of the calculations.

Van de Walle credits co-author Audrius Alkauskas with the development of a theoretical framework needed to calculate the rate that defects capture electrons and holes. “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.
According to Van De Walle, the method they devised could lend itself to identifying other defects and mechanisms by which SRH recombination occurs.

“These gallium vacancy complexes are surely not the only defects that are detrimental,” he said. “Now that we have the methodology in place, we are actively investigating other potential defects to assess their impact on nonradiative recombination.
The researchers detailed their findings in the April 4 issue of Applied Physics Letters [APL 108, 141101 (2016)], with an accompanying figure on the cover of the journal.

The work was funded by U. S. Department of Energy Office of Science, and by Marie Sklodowska-Curie Action of the European Union.

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