# 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\({}^{2}\)
### Use of High Miscut Sapphire  
During our optimization for the miscut angle of sapphire oriented along the m-axis, the improved-AlN crystallinity was observed when changing the miscut angle from 0.3\({}^{\circ}\) to 1.5\({}^{\circ}\). Similar phenomenon was observed for the CHT growth of AlN. However, based on the conventional understanding that a rough surface with macrosteps, of which height gap between flat surfaces is greater than a unit cell, is unsuitable to grow LED layers on it, Mg was added to AlN with an aim of making AlN template smoother in the case of using sapphire with 0.5\({}^{\circ}\) miscut [109]. The macrostep in this paper might include stairway structure of bunched m(10-10)-, r(1-102)- and narrow c(0001)-surfaces. In the past, using molecular beam epitaxy at the temperature of 700 \({}^{\circ}\)C, an improved crystallinity of AlN was observed in the case of using sapphire with higher miscut of 1.0\({}^{\circ}\) in addition to the exhibit of multi-atomic layer macrosteps, and the maximum miscut angle to obtain stepflow surface was reported to be 0.5\({}^{\circ}\)[111]. Using molecular beam epitaxy, the improved (0002) and (10-12) XRCs of GaN were also observed in the case of using sapphire with higher miscut of 1.0\({}^{\circ}\) and 2.0\({}^{\circ}\) with the exhibit of bunched steps. The maximum miscut angle of 0.5\({}^{\circ}\) to exhibit the stepflow surface [112] was reported. The groups using EM reported the smooth surfaces of AlN with stepflow [71,113] to make LEDs. These reports indicate that the macrosteps were not expected to be useful to fabricate LEDs. Despite this general understanding that excess miscut angle cannot be suitable to fabricate LEDs due to macrosteps, not to miss the optimal angle, the investigation on a wide range of miscut angle between 0.1\({}^{\circ}\) and 3.0\({}^{\circ}\) was carried out. The result is shown in Figure 3. The maximum miscut angle for which a stepflow surface could be obtained was limited to below 0.3\({}^{\circ}\). This agrees well with the results of other groups. The highest crystal quality was observed at a miscut angle of 1.5\({}^{\circ}\). It was unexpectedly found that AlN with dense macrosteps is suitable for the fabrication of DUV-LEDs, contradicting the common understanding of QWs. In fact, the miscut angle is usually set to between 0.15\({}^{\circ}\) and 0.3\({}^{\circ}\) to produce InGaN LEDs with stepflow surfaces. There was indeed no evidence to indicate that AlN with macrosteps is suitable for fabrication of DUV-LEDs. Figure 3: (**a**) Relationship between FWHM of XRC (10-10) for AlN and AlGaN on AlN templates and miscut angle of sapphire obtained before optimizing AlN growth recipe. (**b**) At that time, the (10-10) XRC of AlGaN experimentally suggested a relationship with the EL output. The relationship between the EL output and FWHM of (0002) XRC was unclear [38] at that time. The n-AlGaN cladding layers showed slightly higher crystallinity than the AlN layers. (**(a)** is reused from Proc. SPIE 9926, 99260C (2016); and (**b**) is permitted to be reused from Physica Stat. Solidi A 2011, 208, 1594-1596 \(\copy\) Wiley & Sons, Inc.). Figure 3a shows the average FWHM of the (10-10) XRC of AlN templates grown on sapphire with miscut angles between 0.1\({}^{\circ}\) and 3.0\({}^{\circ}\). The FWHMs of (10-10) XRCs for n-AlGaN on AlN on sapphire with various miscut angles are also shown. The EL outputs of DUV-LEDs for different miscut angles reported in our articles [13; 37; 38] are shown in Figure 4. Figure 5 shows AFM images of our recently grown AlN templates [13] on sapphires with various miscut angles using the same growth recipe. After recipe optimization along with the increased EL, steps had unexpectedly appeared for all sapphire substrates with various miscut angles. Fortunately, AlN templates with macrosteps have been proven to be useful for producing AlGaN-based DUV-LEDs with superior performance to those grown on flat AlN.