Ultraviolet light-emitting diodes (UV-LEDs) are attracting much attention for their wide variety of potential applications, such as sterilization, water or air purification, medicine and biochemistry, and so on. In this paper, we demonstrated recent advances in AlGaN-based UV-LEDs achieved by the development of growth techniques for high-quality A1N and AlGaN semiconductor crystals. Low threading-dislocation density (TDD) A1N crystals on sapphire substrates were achieved by introducing novel 'ammonia pulse-flow growth' technique. Significant increases in internal quantum efficiency (IQE) have been achieved for emissions in UVC range from AlGaN and quaternary InAlGaN quantum wells (QWs) by fabricating them on a low TDD AlN buffer layers. The electron injection efficiency (EIE) of the LEDs was also significantly increased by introducing a multi-quantum barrier (MQB) for an electron-blocking layer (EBL). We also increased light-extraction efficiency (LEE) of the UV-lEDs by introducing a transparent p-AlGaN contact layer and highly reflective electrode. Using these techniques, we demonstrated UVA~UVC LEDs with wavelength between 222-351 nm, including the shortest wavelength for AlGaN LED. The maximum external quantum efficiency (EQE) and output power were 7% and higher than 30 mW, respectively, for a 280 nm UVC-LED. Efficiency of UVC-LED could be increased up to several tens of percent in near future by improving LEE, and such high-efficiency UVC-LED lumps will be attractive for sterilization applications as substitutions of mercury lamps.
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