## 2 Growth and Structural Properties
In addition to QWDs, there are also objects with a larger size, which can be considered as conventional QDs. However, the surface density of such QDs is much lower compared to that of the QWDs, so they do not significantly affect the optical properties of the sample, as will be shown below. Leteral sizes of the QWDs and the QDs revealed from cross-section TEM (insets to Figure 2a-c) are in good agreement with the plan-view TEM data (Figure 2a-c). The average height of the QWDs for \(x=0.4\) is 3 nm, that means that the thickness modulation of the QWD layer is not high (about 30% of its average thickness). The QDs are much bigger in size than QWDs; their average lateral size is 30-40 nm, and the average height is about 6 nm. Further increase in the In composition up to \(x=0.5\), i.e., an increase in the lattice mismatch, leads to significant growth of the density of QDs, whereas the density of QWD decreases. From the viewpoint of their geometrical dimensions, these QDs resemble conventional QDs formed in Stranski-Krastanow growth regime. The QDs do not align along with the atomic steps. QWDs can be formed on the exact (100) oriented substrate as well, but their structural properties are strongly influenced by substrate orientation (Figure 3). In the case of the vicinal surface, the modulations of indium composition and layer thickness are much more pronounced; in other words, QWDs are better developed. Atomic steps existing on the vicinal surface facilitate island growth mode. Note that the thickness modulations often follow atomic steps (Figure 3a). For the exact oriented surface, the QWDs have smaller lateral size -10 nm as compared to QWDs grown on 6\({}^{\circ}\) misoriented substrate (20-30 nm). The large-sized QDs are formed only on the misoriented surface, cover many atomic steps, and likely appear due to coalescence of individual QWDs. The impact of surface misorientation angle was also studied in [18]. InGaAs/GaAsP multiple quantum wells (MQWs) were grown by MOVPE on vicinal GaAs (001) substrates with different miscut angles of 0\({}^{\circ}\), 2\({}^{\circ}\), and 15\({}^{\circ}\) towards [110]. Growth on substrates with 2\({}^{\circ}\) and 15\({}^{\circ}\) miscut angles resulted in the formation of quantum wires and quantum dots as revealed by atomic force microscopy and photoluminescence. Since the growth of QWDs is accompanied by a partial relaxation of elastic strain, there is no fast strain accumulation upon the stacking of QWD layers, and misfit dislocation formation is suppressed. This effect is similar to the case of stacking of the self-organized QDs, and is an advantage of QWDs as compared to QWs. Figure 4 shows TEM image of the sample with 15 vertically stacked QWD layers. The structure demonstrates high crystal quality, and no dislocations are observed. In contrast, vertical stacking of more than 3 InGaAs QWs with composition 15-20% would result in dislocation formation and drastic deterioration of optical quality. The possibility of dislocation-free stacking, a large number of QWD sheets is advantageous to obtain high optical gain/absorption in optoelectronic  
Figure 2: Plan-view (001) TEM images of the samples of the In\({}_{\text{N}}\)Ga\({}_{1-\text{N}}\)As QWD layers with nominal indium composition \(x=0.3\) (**a**), 0.4 (**b**), and 0.5 (**c**) (effective vector of diffraction \(g=(220)\)). Insets show a cross-section of images of the same samples in the [1; 2; 3; 4; 5; 6; 7; 8; 9; 10] zone.