“UV treatment utilizing LEDs would be more cost-effective, energy efficient and longer lasting,” says Dr. Ramón Collazo, an assistant professor of materials science and engineering at NC State and lead author of a paper describing the research. “Our work would also allow for the development of robust and portable water-treatment technologies for use in developing countries.”
LEDs utilize aluminum nitride (AlN) as a semiconductor, because the material can handle a lot of power and create light in a wide spectrum of colors, particularly in the UV range. However, technologies that use AlN LEDs to create UV light have been severely limited because the substrates that served as the foundation for these semiconductors absorbed wavelengths of UV light that are crucial to applications in sterilization and water treatment technologies.
A team of researchers from North Carolina and Japan has developed a solution to the problem. Using computer simulation, they determined that trace carbon atoms in the crystalline structure of the AlN substrate were responsible for absorbing most of the relevant UV light. By eliminating the carbon in the substrate, the team was able to significantly improve the amount of UV light that can pass through the substrate at the desired wavelengths.
“Once we identified the problem, it was relatively easy and inexpensive to address,” says Dr. Zlatko Sitar, Kobe Steel Distinguished Professor of Materials Science and Engineering at NC State and co-author of the paper.
Commercial technologies incorporating this research are currently being developed by HexaTech Inc., a spin-off company from NC State.
“This is a problem that’s been around for more than 30 years, and we were able to solve it by integrating advanced computation, materials synthesis and characterization,” says Dr. Doug Irving, assistant professor of materials science and engineering at NC State and co-author of the paper. “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, “On the origin of the 265 nm absorption band in AlN bulk crystals,” is published online in Applied Physics Letters. Co-authors include Benjamin Gaddy, Zachary Bryan, Ronny Kirste and Marc Hoffman from NC State, as well as researchers from HexaTech Inc., Tokyo University of Agriculture and Technology, and the Tokuyama Corporation. The research was supported with funding from the U.S. Department of Defense.
Note to Editors: The study abstract follows.
“On the origin of the 265 nm absorption band in AlN bulk crystals”
Authors: Ramon Collazo, Benjamin E. Gaddy, Zachary Bryan, Ronny Kirste, Marc Hoffman, Douglas L. Irving and Zlatko Sitar, North Carolina State University; Jinqiao Xie, Rafael Dalmau and Baxter Moody, HexaTech, Inc.; Yoshinao Kumagai and Akinori Koukitu, Tokyo University of Agriculture and Technology; Toru Nagashima, Yuki Kubota and Toru Kinoshita, Tokuyama Corporation
Published: Online May 2012, Applied Physics Letters
Abstract: Single crystal AlN provides a native substrate for Al-rich AlGaN that is needed for the development of efficient deep UV LEDs and laser diodes. An absorption band centered around 4.7 eV (~265 nm) with an absorption coefficient above 1000 cm-1 is observed in these substrates. Based on DFT calculations, substitutional carbon on the nitrogen site introduces absorption at this energy. A series of single crystalline wafers grown by PVT and homoepitaxially by HVPE were used to demonstrate that this absorption band linearly increased with carbon, strongly supporting the model that CN- is the predominant state for carbon in AlN.