# A Review of AlGaN-Based Deep-Ultraviolet Light-Emitting Diodes on Sapphire
## 14 IQEs of AlGaN-Based Uneven QWs
Thus, our IQEs (\(\blacktriangle\) and \(\blacktriangledown\)) were calculated to be conservative estimates from I-L characteristics by assuming a fixed LEE of 10% and high CIEs ranging from 70% to 100%, as shown in Table 2. Our estimates of the IQEs of the uneven MQWs are compared with solid symbols (\(\blacktriangleright\), \(\blacksquare\), \(\blacksquare\), \(\blacktriangleright\), \(\blacksquare\), \(\blacktriangleright\), \(\blacktriangleright\)) of the MQWs, which are considered to be flat. In the 265-290 nm range, the uneven QWs show equal or higher values than the reported ones measured with flat MQWs on AlN grown on a sapphire substrate (\(\blacksquare\), \(\blacktriangleright\)) and on a bulk AlN substrate (\(\blacktriangleright\)). In the 255-265 nm region, data for AlGaN wells grown on the bulk AlN spread over a wide range of IQEs from 15 to 80% (\(\blacktriangleright\) and \(\blacktriangleright\)). In the 230-245 nm region, IQEs on the macrosteps were reported for the samples grown on the bulk AlN substrates (\(\blacktriangleright\)) as well as on the sapphire substrates (\(\blacksquare\)). Also, flat MQW without Si doping grown on a sapphire substrate is indicated by (\(\blacksquare\)). Figure 18 shows a map of the relationship between TDD and IQEs, eliminating the IQE data measured under unclear excitation conditions. Our IQE ranges are shown by violet triangles. The upper solid triangles (\(\blacktriangle\)) are conservative estimations for our DUV-LEDs using LEE/CIE of 10%/100%. The IQEs for undoped and Si doped MQWs are shown by \(\blacksquare\) and \(\blacktriangleright\), respectively [90]. In Figure 18, IQEs determined considering excitation conditions are added (\(\square\blacksquare\)\(\blacktriangleright\)\(\blacktriangle\)[77] with blue line) as the  
Figure 17: The reported IQE dependence on the wavelength is collected and mapped. This graph includes the calculations and IQEs under unclear excitation conditions, as shown by hollow symbols. The solid symbols \(\blacktriangleright\)[61], \(\blacktriangleright\)[137], \(\blacktriangleright\)[24], \(\blacksquare\)[25], \(\blacksquare\)[90], and \(\blacktriangleright\blacksquare\)[136] denote the nonsaturated excitations for flat MQWs. Our estimated ranges for the uneven QWs are shown by (\(\blacktriangle\) and \(\blacktriangledown\)). AlN\({}^{\ddagger}\) indicates the uneven MQW grown on bulk AlN with a miscut of 3.0\({}^{\circ}\) shown by \(\blacktriangleright\), and \(\blacksquare\) refers to the flat MQW grown on sapphire [136]. EM denotes the enhanced migration method used for AlN growth on sapphires. MBE indicates samples prepared by a molecular beam epitaxy method. references of flat MQWs. For the AlGaN QWs, the aluminum vacancy (V\({}_{\rm AI}\)) is demonstrated to act as NRC. Si-doped MQWs show higher IQEs than that of the undoped MQW because of decreasing V\({}_{\rm AI}\)[90]. The results of MQWs grown on the bulk AlN substrates are plotted at a TDD of 10\({}^{6}\) /cm\({}^{2}\) because of unknown TDDs at the MQWs. The IQEs for MQWs on the bulk AlN substrates range from 10 to 80% (\(\blacklozenge\), \(\blacklozenge\), \(\blacklozenge\)). The EL wavelength for the samples (\(\blacklozenge\), \(\blacklozenge\)) are located between 255 and 265 nm and the AlN content in AlGaN QWs are higher than those of our samples. Thus, V\({}_{\rm AI}\) is considered be among the reasons for the IQE difference [60; 137]. The IQE estimates of the uneven MQWs (\(\blacktriangle\) and \(\blacktriangledown\)) are above the IQE-TDD trend curve (blue) similarly to Si-doped MQWs (\(\blacklozenge\)) compared with undoped MQW (\(\blacksquare\)), suggesting the boosted IQEs due to macrosteps. It is possible that the macrosteps cause the mass transport of Ga and GaN components in AlGaN [138] during the growth of the MQWs. It led us to consider the possibility that the number of V\({}_{\rm AI}\) has been reduced by the mass transport adjacent to macrosteps in addition to the enhanced carrier localization due to potential modulation.