# III-Nitride Short Period Superlattices for Deep UV Light Emitters
## 2 Growth and Structural Characterization  
Most of III-N SPSLs for deep UV light emitters were grown using both molecular beam epitaxy (MBE) [41; 42; 43] and metal organic chemical vapor deposition (MOCVD) [37; 44] methods on (0001) sapphire, Si (111), and (0001) GaN/sapphire template substrates. The detailed analysis of deep UV LED efficiency (internal and external), grown on different substrates, can be found in ref. [44]. One well-known advantage of MBE over MOCVD is the in situ monitoring of the growth process using the reflection high energy electron diffraction (RHEED) [49]. Analysis of the RHEED patterns in real time allows for controlling, at the monolayer (ML) scale, the structural properties of any substrate at the onset of epitaxy, the nucleation process and growth mode of the epitaxial layer, the growth rate of III-N compounds, and the composition of their alloys, by monitoring the period of RHEED intensity oscillations during deposition [50; 51; 52]. This statement can be illustrated by a few RHEED patterns. The evolution of RHEED patterns illustrating the onset of gas source MBE (GSMBE) with ammonia on bulk (0001) AlN substrate is shown in Figure 1. As seen in Figure 1a, there are two types of reflections indicated separately by black solid and white dashed arrows. It was shown that this complex RHEED pattern can be attributable to the presence of Al\({}_{2}\)O\({}_{3}\) surface islands [53]. The well-defined (00), (01), and (-01) reflections indicated by solid arrows can be attributed to the (1 \(\times\) 1) AlN (0001) surface reconstruction at low temperatures. The weak additional reflections, indicated by dashed arrows, arise from formation of crystalline Al\({}_{2}\)O\({}_{3}\) islands on the AlN surface. These islands cannot be removed by baking of the AlN substrate at high temperatures, up to \(\sim\)1100 \({}^{\circ}\)C [53]. However, nitridation of such a surface, by exposing it to the flux of ammonia for a few minutes at a substrate temperature of \(\sim\)800 \({}^{\circ}\)C, yields formation of a pure (1 \(\times\) 1) surface structure on AlN (0001), as shown in Figure 1b. This (1 \(\times\) 1) surface reconstruction was stable up to 900 \({}^{\circ}\)C. At this temperature, AlN, Al\({}_{0.6}\)Ga\({}_{0.4}\)N, and SPSL of AlN (3 ML)/Al\({}_{0.08}\)Ga\({}_{0.92}\)N (3 ML), with a total of 100 pairs, were successfully grown. The entire SPSL was grown in the 2D mode, and formation of a (2 \(\times\) 2) surface reconstruction is shown in Figure 1c. The surface was very flat, with the root mean square (rms) roughness of less than 1 nm, as measured by 1 \(\times\) 1 \(\upmu\)m\({}^{2}\) scans, using atomic force microscopy. Figure 2 shows the evolution of the RHEED patterns illustrating 2D\(\rightarrow\)3D\(\rightarrow\)2D growth mode transitions during ammonia GSMBE of Al\({}_{0.55}\)Ga\({}_{0.45}\)N (barrier)/Al\({}_{0.45}\)Ga\({}_{0.55}\)N (well) structure on Al\({}_{0.55}\)Ga\({}_{0.45}\)N/AlN buffer grown on (0001) sapphire substrate [54; 55]. All the barrier layers were grown in 2D growth mode with \(1\times 1\) surface reconstruction, as shown in Figure 2a, when an ammonia flux was sustained at 20 sccm. The wells grown under the same ammonia flux also demonstrated 2D growth mode. This mode was maintained at ammonia fluxes greater than 7 sccm (N-rich conditions), at a substrate temperature of 795 \({}^{\circ}\)C. With the ammonia flux reduced to 5.5 sccm, the RHEED patterns become quite spotty, as shown in Figure 2b. This behavior of the RHEED pattern is typical when the growth mode changes from 2D to 3D [56]. These growth conditions can be attributed to the \"metal (Ga, Al)-rich\" conditions, although additional experiments are required. It was shown that the barrier layer recovers when ammonia flux increased to 20 sccm and the RHEED pattern shows 2D growth mode, as shown in Figure 2c, by the time the next well is grown. Note that significant increase in the deep UV cathodoluminescence (CL) and photoluminescence (PL) emission from such grown structures was observed [57].