# A Review of AlGaN-Based Deep-Ultraviolet Light-Emitting Diodes on Sapphire
## 6 Factors Inhibited Light Extraction at the Interfaces with Different Refractive Indices  
Currently, the use of p- or n-reflective electrodes does not practically improve DUV-LEDs. Since the waste EL comes mainly from the absorption in the p-GaN layer, the reduction of the reflection loss at the boundary between sapphire and air is recognized as a priority task to extract more light from the bare dies. Based on the concept without increasing both V\({}_{\text{f}}\) and WPE, the control of the surfaces of sapphire or the AlN substrate opposite the device layer has been studied [18; 37; 62] to increase LEE. AlN substrates are thinned to reduce DUV absorption at AlN. By roughening the thin AlN surface, an enhancement of LEE greater than 1.5-fold was achieved [18; 62]. A metheye (photonic crystal: PhC) structure [124] was fabricated on sapphire substrate [37]. As a result, the EL output obtained by wafer-based measurement without isolating into dies was improved 1.5-fold, as shown in Figure 7.  
Since the Mohs hardness of sapphire is 9 (Vickers hardness of 2300), thinning the sapphire before scribing the LED wafer is generally used to mass-produce commoditized wire-bonded InGaN-based devices with a p-transparent electrode. For the flip-chip mounted devices, heat dissipation is mainly via the p-contact and the thickness of sapphire layer is not a serious disadvantage. However, the thinning process is normally applied before scribing sapphire due to its hardness. The use of thick sapphire  
Figure 7: Relative increases in EL output power measured by wafer proper without dicing the LED wafers shown as a histogram. The inset is a scanning micro scope (SEM) image of the moth-eye (PhC) structure fabricated on sapphire. After isolating into dies from the LED wafer with maintaining the sapphire substrate thickness at 430 \(\upmu\)m, the improvement in the EL output was reduced to about 5% because the contribution of sapphire sidewalls in increasing LEE. (Graph is permitted to be reproduced from Appl. Phys. Express [37]. © Japan Society Applied Physics).  
became possible using the stealth-dicing-technique invented by [PERSON]. When LED wafers were isolated without thinning 430-\(\upmu\)m-thick sapphire, the improvement ratio of LEE resulting from using the moth-eye structure was decreased to around 5% because of the contribution of thick sapphire sidewalls for increasing in LEE. The moth-eye (PhC) structure is still substantially effective because there is no increase in V\({}_{\text{f}}\), no reduction of the lifetime, and disadvantages except for the cost. Currently, thick sapphire is considered to be a useful material with a refractive index of 1.8, between those of air and AlN (refractive index 2.1) [125]. Because of the improvement of only 5% by motheye (PhC) structure on the sapphire surface opposite to the device layers in the case of using thick sapphire, the encapsulation was focused on as a priority [14; 15].  
Due to the difference in the refractive indices between sapphire and AlN, the reflection between the sapphire and AlN layer cannot be negligible. The use of \(\upmu\)m-size PSS [39] was studied, keeping in mind the future application of PhC instead of the \(\upmu\)m-sized PPS. The use of PSS was expected to reduce this reflection loss and improve the quality of AlN crystal. For AlN to coalesce on PSS, the preferable miscut angle of sapphire was determined to be lower than 0.3\({}^{\circ}\)[126]. The use of PSS conflicted with the use high miscut sapphire. Consequently, the present device layers of DUV-LEDs were grown on planar sapphire with 1.0\({}^{\circ}\) miscut.