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

We have fabricated low threading dislocation density (TDD) AlN templates on sapphire substrates for application to deep‐ultraviolet (DUV) light‐emitting diodes (LEDs) by employing an epitaxial lateral overgrowth (ELO) process. An AlN stripe structure with a width of 5 μm and a spacing of 3 μm was formed by using inductive coupled plasma (ICP) etching. The AlN stripes were completely coalesced and embedded to create a flat surface after the growth of an approximately 15 μm‐thick ELO‐AlN layer, grown by low‐pressure metal‐organic chemical vapor deposition (LP‐MOCVD). We systematically investigated the appropriate V/III ratio and growth temperature for the ELO‐AlN. The edge‐type TDD of the AlN was reduced from 2×109 cm–2 (AlN stripe region) to 3×108 cm–2 (in the wing region of the ELO‐AlN), as estimated by cross‐sectional transmission electron microscope (TEM) imaging. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)