University of California researchers confirm that atomic structure defects cause LED performance degradation

Researchers at the University of California, Santa Barbara (UCSB) have confirmed specific types of defects in LED atomic structures, resulting in reduced LED performance. Researchers predict that such defect characterization may make it difficult to produce LEDs that are more efficient and emit longer.

Conceptual illustration of UCSB: lattice defects of GaN

Chris Van de Walle led the research team to carry out this work. He said that if LED materials have such defects, the technology can be used to find such defects. These techniques can be used to improve the quality of materials. Not all LEDs created are the same. In fact, it is difficult to create LEDs with the same performance and characteristics. Efficiency is the most important feature of LEDs. At the atomic level, the performance of LEDs is highly dependent on the quality of the semiconductor material.

Van de Walle said: "In LEDs, electrons are injected from one side and holes are injected from the other side." They pass through the crystal lattice of the semiconductor - a white LED material based on gallium nitride. Then, electrons and holes (electron missing) will cause the diode to illuminate. When an electron encounters a hole, it transitions to a low energy state, releasing photons.

Sometimes, although the charge carriers meet, they do not illuminate, resulting in a so-called Shockley Read Hall (SRH) recombination. According to the researchers, SRH recombination occurs when trapped charge carriers are captured in lattices that meet but do not illuminate.

Researchers have confirmed gallium vacancy complex defects in the presence of oxygen and hydrogen. Cyrus Dreyer, the first author of the study, said: "These defects were previously observed in nitride semiconductors, but until now, they have understood the harmful effects they have."

Audrius Alkauskas, co-author of Van de Walle, developed a theoretical framework for calculating the defect rate of trapping electrons and holes. Van de Walle said: "This combines the defects we have considered for many years with the new theory and achieves breakthrough research." The method they designed is suitable for distinguishing themselves from other defects and clarifying the mechanism by which SRH complexes occur.

He said: "These gallium vacancy complexes are certainly not the only harmful defects. Now that we have found a way, we are actively investigating other potential defects and assessing their impact on non-radiative recombination." The researchers detailed their findings. It will be released in April by the Applied Physics Letter [APL 108, 141101 (2016)] and in the cover of the magazine.

The study was funded by the US Department of Energy's Office of Science and Technology and the EU Marie Scarodosca Curie Action Plan.