A GaN-free LED Structure for High UV-light Extraction
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A GaN-free LED Structure for High UV-light Extraction Toshio Nishida, Tomoyuki Ban,1 and Naoki Kobayashi NTT Basic Research Laboratories, NTT Corporation, Atsugi, Kanagawa, 243-0198, Japan 1 NEL Technosupport Atsugi, Kanagawa, 243-0198, Japan ABSTRACT We improved the extraction of ultraviolet light from AlGaN-based light emitting diodes (LEDs) at the wavelength of about 350 nm, by introducing a transparent structure that is free from binary GaN. We used an AlN-template layer on a sapphire substrate as starting medium of the metal organic vapor phase epitaxial growth. The buffer layer is an Al0.2Ga00.8N alloy. We also used a short period alloy superlattice as transparent and conductive p-type cladding and p-type contact layers. The resultant epitaxial structure is confirmed to be transparent with the transmittance of about 90% within the wavelength range of 340 – 400 nm. The crystal quality of the Al0.2Ga00.8N buffer layer was estimated by the transmission electron microscope (TEM) observation. The dislocation density of AlGaN buffer layer was highly reduced on the AlN template layer, both of which have a low density of screw and mixed-type dislocations, of the order of 10-8cm-2. We also found that light extraction is improved by a factor of 6 by introducing the transparent LED structure and a p-contact mirror. The resultant LED shows high performance under RT-CW operation, where 1 mW output power at 348 nm with 20 mA injection corresponds to the external quantum efficiency of 1.4 %. The maximum power was 7 mW at 220 mA. The emission spectrum is highly monochromatic with the UV-to-visible intensity ratio of about 1000. We also demonstrate the application of this transparent UV-LED to white light source in a bottom-emission geometry. INTRODUCTION Nitride semiconductors with sufficient Al content, such as AlGaN, provide short band-gap wavelengths of 200 - 360 nm in the ultraviolet (UV) range. Light sources of such UV range are promising as efficient solid-state excitation sources in wavelength conversion technologies for lighting equipment and displays, where ultraviolet lights are converted to visible light by using fluorescence dyes. They are also promising in the application field of spectro-chemical analyses and photo-catalytic chemical reactions. Therefore, clarifying their
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potential is a very important issue. We have already demonstrated a highly efficient AlGaN active layer[1,2] grown on a bulk GaN substrate[3] at the emission wavelength of 353 nm. However, the extraction efficiency of the device was as low as 1 %. The drawback of this system is the light absorption by binary GaN layers. GaN layers are conventionally used as buffer layers for high-quality crystal growth and as p-type layers for conductive cladding and metal contact. To improve extraction of UV light shorter than GaN band-gap wavelength of 363 nm, transparent buffer layers including transparent substrates and transparent p-type layers, are indispensable. In this study, we fabricated transparent LED structures that are free from
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