# A Review of AlGaN-Based Deep-Ultraviolet Light-Emitting Diodes on Sapphire
## 4 Attempted Improvements of AlN on Sapphire to Obtain TDD Less Than 10\({}^{9}\)/cm\({}^{2}\)  
### Epitaxial Lateral Overgrowth (ELO) and Patterned Sapphire Substrate (PSS)  
GaN improvement techniques are briefly introduced as a contrast to those for AlN. The low-temperature (LT) buffer layer technique [91; 92] has enabled a GaN film with a TDD of less than \(10^{9}\)/cm\({}^{2}\) to be produced on sapphire. The additional technique to improving GaN on LT buffer layer on sapphire is well known as ELO, and a GaN with a TDD of lower than \(10^{7}\)/cm\({}^{2}\) successfully led to demonstrate laser diodes (LDs) [68; 92]. The variations of crystal improvement techniques has been attempted for GaN on LT buffer, including pendeoepitaxy [93] and facet-controlled ELO [94], which involve the masking of dislocations and utilizing the mass transfer of GaN. In addition, the use of a PSS gives a TDD on the order of \(10^{8}\)/cm\({}^{2}\)[95; 96]; PSS technique utilizes the decomposition of irregularly oriented GaN seed crystals on sapphire. GaN substrates with TDD of lower than \(10^{6}\)/cm\({}^{2}\) are currently used for the production of LDs, and ELO-GaN has been replaced by GaN substrates [97; 98] to produce LDs. Despite the smaller improvement of GaN for PSSs, only the PSS technique is used to produce commercialized InGaN-based LEDs (<365 nm) on sapphire owing to its moderately good crystallinity on sapphire and the simple growth procedure without regrowth and masking of dislocations.  
It is reasonable to consider that AlGaN with moderate crystal quality is desirable for fabrication of AlGaN LEDs. AlN and GaN have a wurtzite structure and similar techniques are expected to be effective for improving AlN. Similar to GaN, improvements of AlN and AlGaN using ELO and PSS have been attempted because TDD on the order of \(10^{9}\)/cm\({}^{2}\) is too high to obtain high IQEs [69; 99]. With this background, the ELO of AlN on a PSS using trenched sapphire has been investigated [33; 100; 101]. The AlN crystallinity was reported to be improved using PSSs. DUV-LEDs were fabricated using ELO on PSS [27; 39], but significant increases in EQEs have not been reported. Based on our experiment [39], although AlN improvement in the TDD of AlN (<\(10^{8}\)/cm\({}^{2}\)) above the groove of sapphire was achieved, reducing the miss-coalescence of the entire wafer was difficult. The lack of mass transfer for AlN is considered to cause the difficulty. To overcome the lack of mass transfer of AlN, a nanoimprint (NIP) technique has been applied to a Si substrate, in which submicron circles were patterned [102; 103]. Coalescence was observed, as were \(\upmu\)m-size PSSs. However, the crystallinity was not improved significantly. All the experiments on AlN-ELO incorporated PSS with flat tops to obtain AlN seed crystals, and the misoriented small AlN grains around the grooves were observed. The misoriented AlN seed crystals are considered to remain due to the insufficient decomposition of the irregularly oriented seed AlN crystals. In addition, there are reports that the polarity of AlN is difficult to align [104; 105]. AlN is expected to behave similarly to GaN [106; 107; 108] if the growth temperature of AlN is increased much higher than 1300 \({}^{\circ}\)C. Currently, the gap between AlN and GaN is difficult to close, and ELO and PSS techniques are unlikely to be used in practice. The coalescence faults resulted in low yield.