# III-Nitride Short Period Superlattices for Deep UV Light Emitters
## 2 Growth and Structural Characterization
The increase was attributed to the formation of quantum dots (QDs) within the wells. It was concluded that the greatest CL intensity and longest PL lifetime for these structures are due to formation of quantum well (QW)/QD regions in Al\({}_{x}\)Ga\({}_{1-x}\)N/Al\({}_{y}\)Ga\({}_{1-y}\)N (0.3 \(<\) x \(<\) 0.45, 0.53 \(<\) y \(\leq\) 1) QW structures [57, 58]. The approach described in [54, 55, 57, 58] was recently adjusted for a plasma-assisted MBE (PAMBE), and successfully used in an active layer of deep UV LEDs emitting at 232 nm [59]. The emission at 219 nm from PAMBE-grown 2 ML thick GaN QDs was also observed [60]. Analyzing the state-of-the art results mentioned above and discussed in literature within last two to three years, we can conclude that one of the main current trends aimed at improving internal quantum efficiency (IQE) and external quantum efficiency (EQE) of UV LEDs is the creation of a low defective and highly efficient active layer in such structures. Future research should focus on finding the optimal ratio and distribution of QDs in the active layer, as well as on the development of the growth of LED structures on relatively inexpensive templates or bulk AlN substrates. Of course, it is much more effective to design and develop such an active layer using MBE, which provides in  
Figure 1: Evolution of reflection high energy electron diffraction (RHEED) patterns for different stages at onset of gas source molecular beam epitaxy (GSMBE). (**a**) The surface of (0001) AlN substrate at low temperatures. The (00), (01), and (-01) reflections from the (1 \(\times\) 1) AlN surface and reflections from crystalline Al\({}_{2}\)O\({}_{3}\) islands are indicated by arrows; (**b**) (1 \(\times\) 1) surface reconstruction of AlN exposed to ammonia; (**c**) (2 \(\times\) 2) surface structure after deposition of about 20 pairs of short period superlattices (SPSLs). Figure 2: Evolution of RHEED patterns illustrating 2D and 3D growth modes at different (Al + Ga)/NH\({}_{3}\) flux ratios: (**a**) 2D-grown barrier at 20 sccm of ammonia; (**b**) 3D-grown well at 5.5 sccm of ammonia; (**c**) next 2D-grown barrier at 20 sccm of ammonia on a 3D-grown well. situ monitoring of the growth process. Despite the fact that the MOCVD process dominates in the industrial growth of such structures, the results obtained using the MPE should make it possible to identify the most important structural and morphological factors influencing the radiative efficiency of the active layer of the LED. These new concepts can then be transferred to the MOCVD processes. High resolution X-ray diffraction (HR-XRD) of AlN/AlGaN, AlN/GaN, and AlGaN/InGaN were carried out by many researchers [61; 62; 63; 64; 65] in order to estimate strain and dislocation density. Note, HR-XRD measurements, in conjunction with Raman measurements, allow for estimation of the residual strain in SPSLs more precisely [66; 67; 68; 69]. Usually, HR-XRD studies are carried out using a high-resolution diffractometer in double- and triple-axis alignment. A long range 20-\(\omega\) scan of the (0002) reflection is shown in Figure 3 for a typical AlN/Al\({}_{\mathrm{x}}\)Ga\({}_{1-\mathrm{x}}\)N SPSL grown on (0001) Al\({}_{0.4}\)Ga\({}_{0.6}\)N/AlN/sapphire template. Individual peaks corresponding to the AlN and Al\({}_{0.4}\)Ga\({}_{0.6}\)N buffer layers, and the 0 th, \(\pm 1\), and \(\pm 2\) satellites of the SPSL are well defined in Figure 3. The average SPSL composition, 0.68, was determined from the 20 position of the 0 th peak [61]. From the position of the 0 th and \(\pm 1\) satellite peaks, the average period of the SPSL was determined to be 2.236 nm. Using the experimentally determined period, and assuming well composition of Al\({}_{0.08}\)Ga\({}_{0.92}\)N and pure AlN barriers, the well and the barrier thicknesses are found to be 0.808 nm and 1.428 nm, respectively [61].