# A Review of AlGaN-Based Deep-Ultraviolet Light-Emitting Diodes on Sapphire
## 2 Groups Manufacturing AlGaN-Based DUV-LEDs  
Currently, there are many groups manufacturing AlGaN-based near-ultraviolet (NUV: 300 nm \(<\)\(\lambda\) < 400 nm, about 300-350 nm for AlGaN devices) and DUV (200 nm \(<\)\(\lambda\) < 300 nm) LEDs. The groups can  
Figure 2: Demonstration samples of DUV-LEDs fabricated by UV Craftory Co., Ltd. Until 2014, the effect of the miscut angle of sapphire [13] was studied to increase EQE. In parallel, the heat dissipation was improved and the chip size was increased to obtain higher output power. A thick p-GaN layer is considered to be necessary to obtain a longer lifetime and to reduce device faults. Since 2012, resin encapsulation using fluorine polymers has been intensively investigated [14] to increase the extracted electroluminescence (EL). (Photographs are permitted to be reused from Oyobutsuri [22] (in Japanese). © Japan Society Applied Physics.). be distinguished into AlN bulk and sapphire groups according to the substrate. Mergers and alliances often occur in science ventures, and the owner of the intellectual properties and the manufacturer are often different. To our best knowledge, the major players can be grouped as shown in Table 1, which is used to clarify the trends of research and development. The groups employing AlN bulk substrates have mainly followed two approaches to fabricating AlGaN-based LEDs. The problem in growing AlN substrates by physical vapor transport (PVT) [55], which is also referred to as sublimation [56; 57], is absorption in the UV region due to carbon [58] and oxygen [59] impurities. The advantage of AlN bulk substrates is high crystallinity with a TDD lower than \(10^{4}\)/cm\({}^{2}\)[55; 56]. Normally, dislocations are considered to be related to nonradiative recombination (NRCs). Based on this superficial understanding, the advantage of using AlN substrates is expected to result in a higher internal quantum efficiency (IQE) [60; 61]. It is conventionally considered that high crystal quality contributes to increased reliability [30; 62]. However, nitride materials are robust and InGaN-based LEDs [63; 64; 65; 66] and LDs [67; 68] with high TDDs of \(10^{9}\)/cm\({}^{2}\) and \(10^{6}\)/cm\({}^{2}\), respectively, have been commercialized. According to the experimental data in Figure 1 for AlGaN-based DUV-LEDs, the EQEs of LEDs on a sapphire substrate are comparable to those on an AlN substrate with TDD of less than \(10^{6}\)/cm\({}^{2}\). The crystal quality required for AlGaN-based DUV-LEDs will be discussed and partially clarified in this paper. The groups employing sapphire substrates can be roughly categorized in two subgroups: those attempting enhanced migration (EM) [69; 70; 23; 53; 60; 71] by applying an alternating or pulsed supply of source precursors and us focusing on continuous high-temperature (CHT) growth [13; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40]. Compared with GaN, mass transportation of AlN during the growth is very small and EM and CHT growth were investigated to recover the lack of mass transport. For the EM technique, the growth mechanism is similar to atomic-layer epitaxy, and the growth technique was first called as pulsed atomic-layer epitaxy (PALE) [23]. Improvements were then carried out by the