Better light extraction for deep UV LEDs

November 03, 2016 // By Julien Happich
Improving upon prior art, an international team of researchers has open a promising path towards high efficiency deep UV LEDs (emitting at 280nm) thanks to a novel device geometry.

DUV light sources emitting in the range 250−280nm could be useful in many applications such as air and water purification or for sterilization in food processing, thanks to their ability to effectively damage or destroy the DNA or RNA of microbes including bacteria, viruses, and cancer cells.

Publishing their findings in the ACS Photonics journal in a paper titled "Arrays of Truncated Cone AlGaN Deep-Ultraviolet Light-Emitting Diodes Facilitating Efficient Outcoupling of in-Plane Emission", the researchers looked at improving the Light Extraction Efficiency (LEE) of AlGaN DUV LEDs, notoriously poor due to DUV light re-absorption. What they explain from literature is that the LEE of AlGaN DUV LEDs is particularly poor mainly due to two reasons: one is the absorption of DUV light in the p-type GaN contact layer, the second is the nature of the DUV photons that are generated, when highly transverse-magnetic (TM) polarized with a preferred in-plane emission pattern, they are more likely to be trapped and absorbed inside the device, they write.

Unlike InGaN-based visible LEDs for which micro/nanolens arrays and highly reflective mirrors can enhance light extraction, such approaches are ineffective for extracting TM-polarized anisotropic emission from AlGaN DUV LEDs. In a previous study "An Elegant Route to Overcome Fundamentally-Limited Light Extraction in AlGaN Deep-Ultraviolet Light-Emitting Diodes: Preferential Outcoupling of Strong In-Plane Emission", the researchers had demonstrated that the extraction of TM-polarized DUV photons could be enhanced in sidewall-emission-enhanced DUV LEDs by adopting internal reflectors on the inclined sidewalls of mesa stripes, the photons being reflected down through the sapphire substrate.