# A Review of AlGaN-Based Deep-Ultraviolet Light-Emitting Diodes on Sapphire
## 10 DUV-LED Layers with Uneven QWs Grown on Macrosteps  
Figure 9 shows the result of AlN grown on 430-\(\upmu\)m-thick sapphire with miscut angles of 0.2\({}^{\circ}\), 0.3\({}^{\circ}\), and 1.0\({}^{\circ}\) relative to the m-axis of (0001) sapphire. This experiment was performed in parallel with optimization of the growth recipe for AlN for two years. The growth temperature was set to be between 1220 and 1280 \({}^{\circ}\)C during the growth of AlN layer. Based on the result shown in Figure 9, the miscut angle of the sapphire substrate was set to 1.0\({}^{\circ}\) to fabricate LED wafers. The crystallinity of 2-\(\upmu\)m-thick AlN in the case of 1.0\({}^{\circ}\)-miscut sapphire was stable [13]. The FWHMs of the \(\omega\)-scans of (0002) and (10-10) XRCs were about 150 and 650 arcsec, respectively. The total of the FWHMs of the \(\omega\)-scans of (0002) and (10-10) XRCs was determined to be quite stable at about 800 \(\pm\) 15.5 arcsec because of the trade-off relationship between tilt (0002) and twist (10-10) [13]. A TDD of 5 \(\times\) 10\({}^{8}\)/cm\({}^{2}\) was estimated for our AlN in the case of using 1.0\({}^{\circ}\)-miscut sapphire. The AlN on the sapphire exhibited dense macrosteps after optimizing the AlN growth recipe, as shown in Figure 5. In the case of the 1.0\({}^{\circ}\)-miscut sapphire, the terrace width was about 300 nm and the step height was often greater than 10 nm, which is greater than the well thickness of 2-3 nm or the quantum barrier thickness of 6-7 nm. To determine the roughness of the quantum well, n-Al\({}_{x}\)Ga\({}_{1-x}\)N cladding layers were grown on AlN templates and atomic force microscope (AFM) images were taken for cladding layers with x = 0.67 and 0.61, which were prepared for 265 and 285 nm LEDs, respectively. After characterizing the n-Al\({}_{x}\)Ga\({}_{1-x}\)N, 3 QW and 20 nm-thick-electron blocking (EB) layers were regrown on the n-AlGaN templates. Figure 10 shows AFM images of the n-AlGaN templates and EB surfaces. Al-rich EB surfaces were used to determine the roughness of the final surface of the MQW because the decomposition of the Ga-rich MQW during the cooling process of MOCVD must be prevented. The large step edges on the AlN surface were divided into multiple macrosteps on the n-AlGaN and EB surfaces for both the x = 0.67 (265 nm) and 0.61 (285 nm) cases. This is considered as a footprint of mass transport during the AlGaN growth. In addition, the step edges lost their sharpness at the AlN surfaces which can be seen in Figure 5, suggesting that the macrosteps collapsed during the AlGaN growth. However, the height of the macrosteps on n-AlGaN and EB surfaces often exceeded 10 nm, which is greater than the quantum well and quantum barrier thicknesses. Since uneven QWs are thought to be unsuitable to grow QWs on it, the authors' group previously attempted to make a flat template [109]. EM groups also reported n-AlGaN with a flat surface with an RMS of less than 0.3 nm [71; 113]. We unexpectedly observed that the uneven QWs function well. Then, samples with EB surfaces were used to regrow p-GaN layers, where the thickness of each p-GaN layer was about 0.5 \(\upmu\)m. A laser microscopic image of the p-GaN surface is shown in Figure 11. Due to compressive strain given by the EB surface, the p-GaN growth  
became three-dimensional mode before forming a smoother p-GaN surface. A high miscut angle of the sapphire prevented the formation of stepflow surfaces of p-GaN. Despite the surface having an arithmetic average roughness (Ra) of 5.86 for 6 \(\times\) 6 \(\upmu\)m\({}^{2}\) area, this thick p-GaN layer is considered to be indispensable for obtaining a lifetime of over 10,000 h. Similar to the manufacturers of DUV-LEDs using sapphire, manufacturers of DUV-LEDs on AlN bulk substrates also incorporate thick p-GaN layers [30; 62]. Figure 10: AFM images of n-Al\({}_{\mathrm{x}}\)Ga\({}_{1-\mathrm{x}}\)N surface and EB surface used to determine the external forms of uneven QWs for: (**a**) x = 0.61 (265 nm LEDs); and (**b**) x = 0.67 (285 nm LEDs).