SPEAKER: Prof. Ramón Collazo, NC State

“Growth and Point Defect Control of AlGaN for UV Laser Diodes”

Abstract  The AlGaN material system offers unique opportunities to develop next-generation UV lasers with emissions ranging from 210-350 nm. Such devices find direct and immediate uses in health care, bio-defense, and other commercial and defense applications. However, despite many efforts, only a few examples of electrically injected laser diodes with emission wavelength <350 nm have been demonstrated. Among others, challenges for these devices include low doping and low carrier injection efficiency, absorbing layers and defects, and non-ohmic contacts. Here, we present recent advances in the growth and fabrication of UV laser diodes on single-crystal AlN and GaN substrates focusing on necessary point defect control and relaxation schemes during growth by MOCVD. Point defect incorporation in Al/GaN is dependent on the defect formation energy and hence on associated chemical potentials and the Fermi level. We demonstrate a systematic point defect control by employing the defect formation energy as a tool by (a) chemical potential control and (b) Fermi level control. In addition, AlGaN growth on AlN is under compression but on GaN is under tension. We demonstrate specific relaxation schemes to avoid AlGaN cracking for the growth of high Ga-content AlGaN on GaN substrates. In terms of UV laser fabrication, all steps needed to achieve electrically injected UV lasing will be described. First, it is shown that the MOCVD growth on AlN substrates results in high-quality AlGaN layer with low defect concentration and excellent doping capabilities after implementation of the point defect control schemes. Next, the design of the active region (MQW) is discussed and a low threshold optically pumped lasing is demonstrated. Considering simulation results, the design and growth of a complete UV laser diode are shown and fabrication challenges are analyzed. Finally, we present electrical data and electroluminescence spectra from fabricated diodes and discuss the challenges that need to be addressed to realize electrically injected UV laser diodes.

Biography Dr. Ramón Collazo is an Associate Professor at NC State University Department of Materials Science and Engineering (MSE), researching the growth and characterization of wide bandgap semiconductor thin films for optoelectronics and power applications. He co-directs the Wide Bandgaps Laboratory at NC State. He received his B.S. in Physics from the University of Puerto Rico, San Juan, and his Ph.D. in MSE at NC State in 2002. His current research involves the development of the growth and characterization technologies for III-nitrides, especially the Al-rich AlGaN alloy system for UV optoelectronics. This includes the development of UV light-emitting diodes, laser diodes, and related optoelectronics. He was awarded the NSF CAREER award supporting his work on point defects in these wide bandgap semiconductors. He has published over 200 research articles in refereed journals, holding 11 patents, five exclusively licensed disclosures, and numerous presentations at national and international conferences.