# 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
To increase understanding of the background to achieving the current AlGaN-based LEDs with high performance, optimizing AlN growth recipes during the preparation before production is discussed in the following. The MOCVD performed in the experiment used a tray of multiple wafers. The effect of the miscut angles was investigated for two years, where we expected that AlN grown on 0.2\({}^{\circ}\)- and 0.3\({}^{\circ}\)-miscut sapphire would have a stepflow surface and that AlN on 1.0\({}^{\circ}\)-miscut sapphire would have dense macrosteps. The comparison was based on XRC measurements and EL output measurements in reliability tests. During the optimization procedure, the AlN growth recipe was modified on the basis of this routine analysis. The appearance of macrosteps on AlN on sapphire with miscut angles less than 0.3\({}^{\circ}\) was overlooked simply because there was no increase in the number of device faults. Before starting the production, AlN surfaces on sapphire with various miscut angles were observed by AFM for confirmation. It was indeed surprising to see that the entire AlN surface exhibited was rough. Including the case of AlN grown on 1.0\({}^{\circ}\)-miscut sapphire, which was expected to have dense macrosteps, the number of LED wafers subjected to the reliability tests was about 1000 and the number of LED dies was about 10,000. Statistically, the rough surface on AlN cannot be a reason for device faults. During this preparation before the mass production of DUV-LEDs, different reasons for device faults were revealed and measures were taken. Due to this background, the results using sapphire with a miscut of less than 0.3\({}^{\circ}\) reported in this article might include those of devices both smooth and rough surfaces. Figure 4: Our reported EQEs [13; 37; 38] for various miscut angles of the sapphire substrate. The EL output tends to increase in the case of using high-miscut sapphire. (Permitted to be reused from Jpn. J. Appl. Phys. [13]. © Japan Society Applied Physics). The use of high miscut sapphire has the additional advantages of lowering the growth temperature and enabling the use of a thinner AlN layer to obtain AlN with a TDD on the order of 10\({}^{8}\)/cm\({}^{2}\). When the growth temperature was set to lower than 1300 \({}^{\circ}\)C [13,38], the growth rate of AlN was reduced to around 1 \(\upmu\)m/h. However, the use of 1.0\({}^{\circ}\)-miscut sapphire enables the growth of a 2-3-\(\upmu\)m-thick AlN layer with a TDD on 5 \(\times\) 10\({}^{8}\)/cm\({}^{2}\) on sapphire. The growth time for the AlN layer did not exceed about 3 h, meaning that it is practically applicable to mass-produce DUV-LEDs. Despite the decreased growth rate, the lower growth temperature solved the problem of a short lifetime for the consumables in the growth chamber. The lifetime of the consumables is in a trade-off relationship to the growth rate and the lower growth temperature increases the interval between growth chamber maintenance. From an industrial viewpoint, increasing the yield of the dies in a wafer as well as the yield of wafers due to reduced run-to-run fluctuation is also important and the reduction of heat transfer due to infrared radiation contributes to reduce the run-to-run fluctuation. To conclude this Section, the use of high-miscut sapphire is considered to be effective in the current situation.