# A Review of AlGaN-Based Deep-Ultraviolet Light-Emitting Diodes on Sapphire
## 13 Characteristic Behaviors of Uneven QWs  
### Redshift of EL Spectra with Increasing Number of Macrosteps  
The FWHMs of the XRCs for the (0002) and (10-10) \(\omega\)-scans in Figure 9 indicate a TDD higher than 10\({}^{8}\)/cm\({}^{2}\), which led us to consider the carrier localization effect for both 265 and 285 nm LEDs. The carrier localization effect for AlGaN QWs has been reported [86; 130]. In addition, it is well known that reducing the TDD increases the IQE in QWs by reducing the number of nonradiative recombination centers (NRCs) [77]. Therefore, the following experiments are considered to be affected by both the lowered TDD and the enhanced carrier localization resulting from the use of high-miscut sapphire. The redshift of the EL spectra as a function of miscut angle was observed in the case of high-miscut sapphire as shown in Figure 15. To minimize the run-to-run fluctuation, simultaneous growth using multiple wafer trays was performed several times and no exceptions to the results in Figure 15 were observed. In Figure 15a, the results for two pairs of the simultaneously grown 260 and 280 nm LEDs are shown, and in Figure 15b the results for three pairs of simultaneously grown 265, 285, and 300 nm LEDs are shown. The reasons for the spectral redshift in the case of higher-miscut sapphire are next hypothesized. The enhancement of compositional and thickness modulation by macrosteps may be among the reasons for the increased potential modulation. The potential nonuniformity will increase the IQE in MQWs due to the localized carriers. Note existence of a spectral shoulder at around  
Figure 14: Reliability tests for 265 and 285 nm LEDs. The tests were carried out at 350 mA (>100 mA/cm\({}^{2}\) in the junction area), which is 3.5 times higher than 100 mA in our previous report [13], demonstrating the robustness of the AlGaN-based uneven QWs. 295 nm in two of the spectra in Figure 15b shown by a gray dashed line. The wavelength of the shoulder is common for the 285 and 300 nm LEDs and suggests the existence of quasi-stable AlN in AlGaN. This quasi-stable AlN is suggested to be related to the high-EQE region between 280 and 300 nm in Figure 1. The peak EQE of 295 nm can also be seen in the EQE mapping of Figure 1. Thus, enhanced compositional nonuniformity should occur around this wavelength, and the carrier localization induced by the compositional nonuniformity is considered to boost the IQE. Note that this does not imply that the compositional fluctuation on a flat terrace should be neglected, because somewhat lower EQEs were reported for flat QWs for the DUV-LEDs with p-GaN [19; 21]. In addition, the enhanced mass transport near the macrosteps may cause thickness modulation because it is reasonable to consider the different growth rate between the surfaces of the step-edge and (0001) surface of AlGaN. The growth rate on the step-edge should be larger. Therefore, the enhanced carrier localization may be due to a complicated combination of mechanisms. Figure 15: (**a**,**b**) Redshift of EL spectra observed in case of using high-miscut sapphire. The spectra in (**a**) were controlled to have single peaks. (**c**,**d**) I-L characteristics of 260 and 280 nm LEDs for different miscut angles of sapphire, respectively. Graph (**b**) includes spectra with a shoulder to show the possible existence of quasi-stable AlN in AlGaN. The indices above the spectra or I-L characteristics in (**b**,**d**) denote the miscut angles of sapphire. Both (**c**,**d**) show that the EL output increases with increasing miscut angle of sapphire for the samples in (**a**). (**b**) is permitted to reuse from [13] and (**a**,**c**,**d**) are permitted to reuse from Proc. SPIE 9926, 99260C (2016) [131]). In Figure 15a, a smaller spectral redshift and less spectral broadening with the increase in the miscut angle of sapphire can be seen for the 260 nm LEDs.