# A Review of AlGaN-Based Deep-Ultraviolet Light-Emitting Diodes on Sapphire
## 1 Introduction  
### General Overview  
AlGaN ternary alloy has a direct bandgap tunable between 3.43 eV and 6.11 eV and is suitable for the fabrication of optical devices with a wavelength range between about 200 nm and 365 nm. In this wavelength range, applications include sterilization [1; 2; 3; 4], ultraviolet (UV) curing and printing [5; 6], phototherapy and medical applications [7; 8], deodorization by the use of a photocatalyst [9; 10], and sensing applications for materials such as urea [11; 12]. To date, these applications mainly involve the use of mercury-based lamps. The Minamata Convention on Mercury took effect on 16 August 2017, and AlGaN-based light-emitting diodes (LEDs) are expected to be used as an alternative to mercury lamps. The authors' group has reported AlGaN-based deep-ultraviolet (DUV) LED bare dies with external quantum efficiencies (EQEs) of 3.5% at 265 nm, over 4.5% at 270 nm, and over 6% between 280 nm and 300 nm without encapsulation and with a p-GaN contact layer [13]. These EQEs correspond to high wall-plug-efficiencies (WPEs) due to the low turn-on voltage (V\({}_{t}\)) of 5-6 V at 100 mA (31 A/cm\({}^{2}\) at the junction area on 1 \(\times\) 1 mm\({}^{2}\) dies). The recorded WPEs of bare dies at 100 mA were 2.7%, 3.1%, 4.4%, and 4.3% at 266 nm, 271 nm, 281 nm, and 298 nm, respectively, and the peak WPEs ofthe 281 and 298 nm LEDs exceeded 5%. When similar DUV-LEDs in Ref. [13] were mounted on the commercialized packages, a lifetime of more than 10,000 h was achieved. If encapsulation is assumed to be used, the WPEs are expected to be improved 1.5-fold [14,15] and the WPEs are predicted to be 4.1%, 4.7%, 6.6%, and 6.5%, respectively, for 100 mA operation. These values are significantly greater than those of 2-3% for incandescent lamps. The WPEs of high- or ultrahigh-pressure mercury lamps are in the range of 10-15% over their entire spectra. Since UV applications often do not utilize all the spectral light of UV bulbs, the utilization factor should be considered; thus, these performances indicate that conventional mercury lamps can be replaced with such LEDs. The lifetimes of high- and ultrahigh-pressure mercury lamps are normally in the range of 100-2000 h, clearly illustrating the advantages of DUV-LEDs with lifetimes of over 10,000 h.  
The trend of the EQE dependence on the wavelength is shown in Figure 1. Note that all the EQEs in Figure 1 were achieved for simple devices fabricated with a p-GaN contact layer opaque to DUV (\(\uplambda<300\) nm) and without any reflective electrodes. DUV-LEDs with wavelength between 280 nm and 300 nm are considered to be suitable for obtaining higher EQEs and WPEs. The collection of published EQE values without distinguishing the incorporated features such as reflective electrodes [16] shows a similar trend to that in Figure 1. Figure 2 illustrates the LEDs demonstrated by the authors' group. These samples also have a thick p-GaN contact layer. The characteristic strong spectral peaks of mercury are located at 365 nm (i-line) and 253.7 nm (sterilization line). One of the advantages of AlGaN-based LEDs is their brightness in the region where bright lines from mercury lamps are lacking. In addition, the tunable wavelength of AlGaN-based LEDs is considered to be highly suitable for sensing applications, which are the most accessible applications for DUV-LEDs in their early stage of development. However, the WPE of low-pressure mercury lamps exceeds 25%. In addition, about 90% of the emitted light from low-pressure mercury lamps is sterilization line. Sterilization is currently considered to be a difficult application for DUV-LEDs when considering only WPE and the production cost. For example, a concept of intermittent lighting  
Figure 1: EQE mapping of AlGaN-based LED dies with p-GaN contact layer. The data are mainly for LED dies on planar sapphire from our group (colored). EQEs of LED dies from elsewhere (uncolored) using AlN substrates [17,18] and sapphire substrates [19,20, 21] with a p-GaN contact layer are also shown.