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http://www.eenewseurope.com/news/better-light-extraction-deep-uv-leds/page/0/1
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November 03, 2016 // By Julien Happich
Better light extraction for deep UV LEDs
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.
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.
Although it proved beneficial, the mesa stripe geometry itself caused anisotropy in light extraction, only effective transversally, not along the stripes' length. One way to circumvent this limitation was to change the form factor of the actual devices, adopting an isotropic geometry, namely truncated cone (TC)-shaped active mesas. Because each circular MQW emitting region is encapsulated within an MgF2/Al reflector-clad truncated cone, the TM-polarized photons bounce off the inclined sidewalls, down through the sapphire substrate, regardless of their emission direction.
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Schematic illustrations for one TC structure. An MgF2 passivation layer is deposited on the whole area except p-contact regions, while an Al over-layer is deposited on the whole area in order to reflect and redirect sidewall-heading DUV photons down. The Al mirror electrically connects to the p-contact metal of each TC.
Ray tracing simulations first confirmed that such TC-shaped DUV LEDs would exhibit an isotropic emission pattern with enhanced light-output power. Then the researchers validated their simulation through experiment. They grew several DUV LED epitaxial structures on 4-inch c-plane sapphire substrates, then etched TC active mesas in various array sizes, ranging from 5×5 to 25×25 on chips measuring 1x1mm2. Across these samples, the base of the TC-shaped active mesas varied from about 150µm in diameter to about 40µm in diameter for the denser arrays.
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Bird’s-eye view scanning electron microscope images for one TC taken from a 20×20 array (scale bar is 6μm).
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Optical microscopy images of the TC LEDs with various arrays ranging from 5×5 to 25×25 (scale bar is 250μm)
The result was up to 37.1% improvement in light emission compared to stripe-type LEDs. They also found that the operating voltages of TC LEDs with low perimeter lengths are lower than that of the stripe-type LEDs, improving simultaneously both the optical and electrical properties of the LED through design optimization.
Contributing to this research were teams from Pohang University of Science & Technology (POSTECH), from the Rensselaer Polytechnic Institute, and also from Samsung Electronics' LED Business.
Visit the Pohang University of Science & Technology (POSTECH) at http://npol.postech.ac.kr
Visit the Rensselaer Polytechnic Institute (RPI) at https://rpi.edu
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