# A Review of AlGaN-Based Deep-Ultraviolet Light-Emitting Diodes on Sapphire
## 13 Characteristic Behaviors of Uneven QWs
### EL Spectral Broadening with Increase in EL Output for the Same TDD  
Higher output power is obtained for broader EL spectra when the miscut angle of sapphire is varied. However, the above results cannot eliminate the effect of the increase in EQE caused by the reduction of the TDD, because the TDD is improved by the use of high-miscut sapphire. Although the spectral redshift and the increase in EL output were accompanied with an increase in the number of macrosteps, increase in the EL output for the broader EL spectra was not clear for the sapphire with the same miscut angle. The authors expect that a simple mapping of the EL spectral width to the EL output power for the same TDD will reveal the positive relationship between the EL output and spectral width. Due to the large run-to-run fluctuation for AlGaN and AlN growth, it was difficult to ensure the relationship between EL output and spectral width during the improvement of the growth recipes. Countermeasures to the run-to-run fluctuation problem were intensively investigated before mass production. Next, a positive relationship between the EL output and spectral width was observed when the miscut angle was fixed to 1.0\({}^{\circ}\) in the routine growth of DUV-LEDs. Although it is difficult to separate the effects of potential fluctuation on the terrace and potential modulation adjacent to the macrosteps, the effect of carrier localization related to EL broadening in our experiments can be extracted.  
LED wafers grown on n-AlGaN templates with similar XRCs were collected to minimize the effect of the TDD. In this procedure, the balance between the FWHMs of the-scans of the (0001) and (1010) XRCs was considered in the analysis of 285 nm LED wafers, where the detailed procedure is described in Ref. [13]. The EL outputs determined by a wafer proper were collected for both 265 and 285 nm LED wafers. The n-AlGaN templates have a TDD of about 5 \(\times\) 10\({}^{8}\)/cm\({}^{2}\). The wafer proper measurement at the wafer center was strongly correlated with the EL output after fabricating LED dies at the wafer center, where the XRC measurements were performed. The linearity of the EL output measurements obtained with the wafer proper and the integrating sphere after isolation into dies was confirmed to be within 20%. To eliminate the wavelength dependence shown in Figure 1, the LED wafers were sorted into groups according to the wavelength of the peak EL output. Then, positive correlations between the spectral width and EL output power were observed for 265 and 285 nm LEDs, as shown in Figure 16a, b. These figures provide further evidence that the higher EL output is related to enhanced carrier localization due to increased potential nonuniformity. Using templates with similar TDDs and surfaces with macrosteps grown on 1.0\({}^{\circ}\)-miscut sapphire, an increase in the EL output for a broader spectral width was also observed. The results in Figure 16 cannot definitively conclude the enhancement of carrier localization by potential modulation adjacent to the macrosteps.  
However, this is considered to be in agreement with the hypothesis of the enhancement of EL output by carrier localization. Detailed microscopic analyses are in progress. Cathode-luminescence (CL) spectral peaks besides the macrosteps were confirmed to be redshifted, increasing the plausibility of the hypothesis that enhanced carrier localization is related to macrosteps.