The efficiency of LED devices is limited by carbon point defects in the aluminum nitride transparency layer, which absorb useful wavelengths of light (around 4.7 eV). North Carolina State University researchers used computational methods and detailed experiments to make the discovery. Credit: Doug Irving; NC State.
The Materials Genome Initiative is shining the spotlight on modeling, simulation and computational methods for materials science, mostly for materials development.
Computational methods can be used to solve old problems, too, as a recent paper published by a group out of North Carolina State University demonstrates.
The 30-year old problem relates to absorption of ultraviolet light by single crystal aluminum nitride, a semiconductor that is used as a substrate for AlGaN-based UV emitting LEDs and laser diodes. Because these devices emit UV, they could be used to kill pathogens, for example, in drinking water or on surgical instruments. In a university press release, lead author Ramón Callazo says, “UV treatment utilizing LEDs would be more cost-effective, energy efficient and longer lasting.” Callazo is an assistant professor of MSE at NC State.
However, taking advantage of the UV-emitting property has been difficult because the substrates themselves absorb a fair amount of the UV.
Using density functional theory calculations, a research team at NC State determined that trace amounts of carbon were the culprit and that, according to the abstract, “substitutional carbon on the nitrogen site introduces absorption at this energy.” The team followed up by characterizing a series of single crystal wafers and showed that the absorption band increased linearly with increased carbon.
In a press release, coauthor Zlatko Sitar, professor of MSE at NC State, says,” Once we identified the problem, it was relatively easy and inexpensive to address.” HexaTech Inc., a university spin-off company that specializes in growing single crystal aluminum nitride, is already working on incorporating the research into its technology.
Of the 30-year old problem, Doug Irving, also a coauthor and department assistant professor, says in the press release, “we were able to solve it by integrating advanced computation, material synthesis and characterization. I think we’ll see more work in this vein as the Materials Genome Initiative moves forward, and that this approach will accelerate the development of new materials and related technologies.”
The paper is, “On the origin of the 265 nm absorption band in AlN bulk crystals,” is published online in Applied Physics Letters, http://dx.doi.org/10.1063/1.4717623.