Enhancing the light-extraction efficiency is the key issue for realizing highly efficient AlGaN-based ultraviolet light-emitting diodes (UV-LEDs). We introduced several features to improve the light extraction: a transparent AlGaN:Mg contact layer, a Rh mirror electrode, an AlN template on a patterned sapphire substrate, and encapsulation resin. The combination of the AlGaN:Mg contact layer and the Rh mirror electrode significantly improved the output power and the external quantum efficiency (EQE) of UV-LEDs. By introducing the aforementioned features, a maximum EQE of >20% at an emission wavelength of 275 nm and a 20-mA direct current was achieved.

1 Introduction Because of their wide direct transition energy range in UV, which is between 6.2 eV (AlN) and 3.4 eV (GaN), AlGaN and quaternary InAlGaN are attracting considerable attention as candidate materials for the realization of deep ultraviolet (DUV) laser diodes (LDs) or light-emitting diodes (LEDs) [1,2]. DUV LEDs and LDs with emission wavelengths in the range of 230-350 nm are expected for a lot of applications, such as, sterilization, water purification, medicine and biochemistry, light sources for high density optical recording, white light illumination, fluorescence analytical systems and related information sensing fields, air purification equipment, and zero-emission automobiles.Research into AlGaN-based UV LEDs for wavelengths shorter than 360 nm, i.e., wavelengths between 330-355 nm [3][4][5], was initiated by several research groups between 1996-1999. The development of short-wavelength UV LEDs is now becoming extremely competitive. Several groups have reported AlGaN-, InAlGaN-, or AlNbased DUV LEDs, 240-280 nm AlGaN multi-quantumwells (MQWs) LEDs [6-10], quaternary InAlGaN MQW LEDs [1,[11][12][13] and a 210 nm AlN LED [14].We started our research into AlGaN-based deep-UV LEDs in 1997. We reported first efficient DUV (230 nm) photoluminescence (PL) from AlGaN QWs [15], and 333 nm AlGaN-QW UV LED on SiC in 1999 [3]. We have