# A Review of AlGaN-Based Deep-Ultraviolet Light-Emitting Diodes on Sapphire
## 15 Prospects for EQE and WPE with Advanced Packaging  
For the devices with a p-GaN contact layer and no reflective p-contact, improving the LE from the sapphire side is important. Using thick sapphire without thinning, the increase in the surface area of sapphire can increase the extraction of EL propagating to the sapphire. However, the shape of the sapphire opposite the device layers of dies is a rectangular parallelepiped, resulting in a significant amount of EL being wasted. EL from the MQWs is reflected at the sapphire-air boundary. Thus, a part of the EL returns to the device layers and is absorbed in the p-GaN or not reflected from the n-electrode area. The shape effect has not been sufficiently utilized to increase LE. A material with  
Figure 18: The reported IQEs as a function of TDD are mapped. The data for the flat AlGaN MQWs determined under nonsaturated conditions are plotted with \(\blacklozenge\)[61], \(\blacklozenge\)[137], \(\blacklozenge\)[24; 25], \(\blacksquare\)[90] and \(\blacklozenge\)[136] solid symbols. The solid blue plots are the measured IQEs considering excitation conditions for the flat MQWs with wavelengths 230 nm (\(\blacklozenge\)), 250 nm (\(\blacksquare\)), 300 nm (\(\blacklozenge\)), and 350 nm (\(\blacktriangle\)), and the blue curve show the relationship between TDD and IQE [77]. AlN indicates the use of the bulk AlN substrate, which is gathered at TDD of 10\({}^{6}\)/cm\({}^{2}\). The violet symbols denote the uneven MQWs on macrosteps for our samples from Table 2. The gray circles (\(\bullet\)) indicate the IQE boosted by Si-doped MQW [90] and the rest is considered to be data for undoped MQWs (colored symbols). AlN\({}^{\ddagger}\) indicates the uneven QWs. a spherical shape is considered to be effective for improving our rectangular parallelepiped dies. However, the shaping of the thick sapphire is difficult due to its hardness. In addition, the difference between the refractive indices of sapphire (1.8) and air is large. The use of a material with a refractive index between those of air and sapphire is also desired. To overcome these problems, resin encapsulation using silicone [54], fluorine [13,14], and unknown type of resins [17,19] has been reported. Our intensive studies on the use the fluoropolymer to encapsulate DUV-LED dies clarified a molecular structure with robustness against DUV [13,14,139]. The LEE and WPE were improved 1.5-fold in the case of hemispherical encapsulation using the fluoropolymer. In addition to the encapsulation, the minor practical improvements of: (i) using an Al reflector around the die; (ii) forming an Al coating on the submount; and (iii) forming an Al coating on the insulation film on the mesa sidewall are thought to be effective for improving the net WPE without reducing the lifetime due to the use of p-AlGaN and a reflective p-contact. In addition, (iv) the formation of a moth-eye (PhC) structure on the sapphire surface can also increase the net EQE and WPE without reducing the LED lifetime. Although the flexible shaping of sapphire is difficult, (v) charfering the sapphire of the die is considered possible and may be an effective measure in addition to encapsulation using a fluoropolymer. Based on our limited experimental attempts, an additional 1.2-fold improvement is conservatively expected by adapting (i)-(v). In Figure 13, the 1.5-fold WPEs at 100 mA are estimated to be 4.1%, 4.6%, 6.6%, and 6.5% at 266, 271, 283, and 298 nm, respectively, from the WPEs corresponding to red solid circles in Figure 1. In addition, when additional improvement of 1.2-fold is assumed, WPEs of 4.9%, 5.5%, 7.9% and 7.8% at 100 mA are expected. A single bare die in the market fabricated by our group demonstrated about 80 mW at 285 nm on AlN-sheet submount at 350 mA without using an encapsulation nor the techniques of (i)-(v). Only with the use of encapsulation to the current product, more than 100 mW per die is considered to be sufficiently feasible by increasing LEE.