Authors
Shashwat Rathkanthiwar1, Maki Kushimoto2, Hiroshi Amano2,3, Yudai Shimizu4, Kazutada Ikenaga4, Mayank Bulsara4, Keitaro Ikejiri4, Leo J Schowalter1,3,5
1 Lit Thinking, Orlando, Florida, USA. 2 Graduate School of Engineering, Nagoya University, Nagoya, Japan. 3 Center for Integrated Research of Future Electronics, Institute of Materials Research and System for Sustainability, Nagoya University, Nagoya, Japan. 4 Taiyo Nippon Sanso, Innovation Unit, Yokohama, Kanagawa, Japan. 5 Department of Materials Science and Engineering, University of Central Florida, Orlando, Florida, USA
Abstract
The unique capability of far-UVC (λ<240 nm) radiation to inactivate infectious agents in indoor settings without endangering the skin and eyes of occupants has accelerated the advancement of far-UVC light sources. Given the continuing progress of achieving shorter wavelengths in LEDs based on nitride semiconductors, it is attractive to think that the well-known advantages of visible, and near UV LEDs (smaller size, lower weight, reduced operating power, optimized wavelength, and longer lifetime) could be duplicated in the far-UVC wavelength regime which would greatly advance this field. Wavelengths as short as 210 nm have been demonstrated and commercial AlGaN LEDs with wavelengths below 240 nm are currently available. However, the efficiency and lifetime of these devices at practical current densities appear to drop dramatically as the Al content is increased. A crucial observation made by several groups is that the apparent concentration of point defects increases during the epitaxial growth of high-Al content AlGaN films and it has been suggested that these defects may be playing a major role in the degradation of short wavelength UVC LEDs.
In this work, we report on the epitaxy development of far-UVC LED structures grown on single-crystal AlN substrates using a low-pressure, resistively heated, horizontal flow MOCVD reactor and intend to compare these results with other epitaxial growth techniques in an attempt to gain a greater understanding of defect incorporation. A step-flow morphology was achieved throughout the heterostructure growth. Reciprocal space mapping revealed that the entire LED structure was fully strained to the substrate indicating pseudomorphic growth. While pseudomorphic, step-flow growth has been demonstrated in vertical MOCVD designs, it was important to demonstrate that horizontal flow reactors were also capable of such high-quality growth. The Si-doped Al0.75Ga0.25N n-contact layer exhibited a sheet resistance of 390 Ω/□. Next, the optical characteristics of stand-alone, unintentionally doped AlGaN layers, multi-quantum wells, and full LED structures were characterized using temperature- and power-dependent photoluminescence. The peak emission wavelength varied less than 3 nm with temperature (300 to 4K) and about 1 nm with excitation density ranging from 10 to 5000 µJ/cm2. Finally, the study systematically investigated how growth conditions affect the IQE of far-UVC LED structures operating below 240 nm.
