# A Review of AlGaN-Based Deep-Ultraviolet Light-Emitting Diodes on Sapphire
## 18 General Conclusions and Future Outlook  
Owing to the insufficient crystallinity of the AlN template on a sapphire substrate, AlGaN-based DUV-LEDs had a low EL output due to the low EQE. The quality of the AlN template with a TDD of over \(10^{9}\)/cm\({}^{2}\) was insufficient. Since the carrier localization is expected for AlGaN-based LEDs as well as for InGaN-based LEDs, a simple method of producing an AlN template with a TDD between \(10^{8}\) and \(10^{9}\)/cm\({}^{2}\) was desired. Through studies on the use of PSS, the growth of a thick layer of AlN, and the use of high-miscut sapphire, the use of high-miscut sapphire was considered to be most suitable for the mass production of AlGaN-based DUV-LEDs to obtain an AlN template with TDD on the order of \(10^{8}\)/cm\({}^{2}\). The reasons for using high-miscut sapphire are somewhat complicated. One reason is to reduce the growth temperature to less than 1300 \({}^{\circ}\)C. In fact, there is MOCVD equipment used  
Figure 21: (**a–d**) Photographs of 285 nm irradiation modules. (**a**) Lighting; and (**b**) non-lighting 3 \(\times\) 4 array module with encapsulation, respectively. The output power is 472 mW at the injected current of 200 mA for each die. The total AlN submount area of the 12 diode array is 10.0 \(\times\) 13.2 mm\({}^{2}\). Modules without encapsulation: (**c**) photograph of 3 \(\times\) 16 array module with dies having 3.4 mm pitch; and (**d**) LED dies with denser layout of 2.5 mm pitch. The die size is 1300 \(\times\) 1300 \(\upmu\)m\({}^{2}\). ((**a,b**) are reused from Jpn. J. Appl. Phys. [15] (© Japan Society Applied Physics) and (**c,d**) are reused from Proc. SPIE 9926, 99260C (2016) [131]). to grow SiC that can be operated at 1400 \({}^{\circ}\)C. The difference between 1300 and 1400 \({}^{\circ}\)C for AlGaN growth is caused by fluctuation of radiant heat transfer mainly from the wafer tray. When growing SiC devices, the compositional difference between layers is small, such as impurity in SiC. However, during the growth of AlGaN-based LED layers, the target AlN content varies throughout the multilayer. This causes a change in the radiation rate due to the deposition inside the growth chamber, resulting in the fluctuation of the effective growth temperature even though the control temperature is fixed. Furthermore, the number of materials that can be used inside the growth chamber is limited when the growth temperature is more than 1300 \({}^{\circ}\)C. From an industrial viewpoint, stable production is necessary and a lower growth temperature will be beneficial. In the case of a lower growth temperature, the growth time for the LED wafers tends to be longer, which is not normally desirable. A suitable balance between growth time and stable production with moderately high AlN crystallinity was found to be obtained between the temperatures of 1200 and 1300 \({}^{\circ}\)C in the case of high-miscut sapphire. Furthermore, the utilization of this temperature range enabled the realization of single LED wafers with a larger effective area. Fortunately, the use of high miscut sapphire resulted in a high EQE without reducing the lifetime of LEDs. We consider that the edge of the macrostep functions as a domain for carrier localization and the detailed study on such carrier localization is in progress. However, it is suggested that the thickness and the compositional modulation on the macrosteps enhanced the carrier localization. This effect unexpectedly gave the merit of achieving LEDs with higher IQE. The device layer of DUV-LEDs is unique compared with conventional LEDs having an EL wavelength greater than 365 nm. On the AlGaN-based EB layer, p-GaN is used, even though the incorporation of AlGaN QWs by many manufacturers and p-GaN layer seriously limit LE. In addition, an AlGaN-based EB layer with a high AlN content limits the CIE for wavelengths shorter than 280 nm. These problems might not be solved in short-term research.