In this paper, recent advances in AlGaN-based deep-ultraviolet (DUV) light-emitting diodes (LEDs) are demonstrated. 220–350-nm-band DUV LEDs have been realized by developing crystal growth techniques for wide-bandgap AlN and AlGaN semiconductors. Significant increases in internal quantum efficiency (IQE) have been achieved for AlGaN DUV emissions by developing low-threading-dislocation-density (TDD) AlN buffer layers grown on sapphire substrates. The electron injection efficiency (EIE) of the LEDs was also significantly increased by introducing a multiquantum barrier (MQB). We also discuss light extraction efficiency (LEE), which is the most important parameter for achieving high-efficiency DUV LEDs. We succeeded in improving LEE by developing a transparent p-AlGaN contact layer. The maximum external quantum efficiency (EQE) obtained was 7% for a 279 nm DUV LED. EQE could be increased by up to several tens of percent through the improvement of LEE by utilizing transparent contact layers and photonic nanostructures in the near future.

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