## 2 Growth and Structural Properties  
The formation of QWDs takes place during the MOCVD deposition of a lattice-mismatched InGaAs thin film on the GaAs substrate. Structures have been grown on exact oriented (100) and vicinal (4-6\({}^{\circ}\) off) GaAs substrates using an MOCVD installation with a low pressure (100 mbar) horizontal reactor. Metal alkyls (trimethylgallium, trimethylaluminum, trimethylaluminum) and arsine were used as precursors. GaAs and AlGaAs layers were deposited at 700 \({}^{\circ}\)C, the ratio of molar flows of V to III group precursors is about 30, and 0.4 nm/s growth rate. InGaAs QWDs are formed at lowered growth temperatures 500-550 \({}^{\circ}\)C, the ratio of molar flows of V to III group precursors is about 30 and 0.2 nm/s growth rate. Compositional and thickness modulations appear due to surface migration of In atoms in the lateral strain fields. In other words, the appearance of In-rich islands is energetically favorable because of partial strain relaxation. On the one hand, elastic strain should be strong enough to cause such modulations. On the other hand, it should not be too high in order to avoid the transition to the Stranski-Krastanow growth mode and formation of conventional self-organized QDs. The strain energy depends both on the thickness and composition of the lattice-mismatched epitaxial material (i.e., the In content), and both parameters should be optimized. The most direct method to study the structural properties of InGaAs nanostructures is transmission electron microscopy (TEM). The combination of cross-section and plan-view TEM images allows one to determine the size, shape, and density of in-rich islands. Let us consider the growth of In\({}_{\mathrm{x}}\)Ga\({}_{\mathrm{1-x}}\)As layer of different In concentrations (\(x\)) on GaAs substrate. Deposition of In\({}_{\mathrm{0.2}}\)Ga\({}_{\mathrm{0.8}}\)As results in the formation of a planar uniform layer. If the indium concentration exceeds 60%, the growth occurs in Stranski-Krastanow mode via the formation of the wetting layer on the top of which large-sized pyramid-shaped islands are formed. In both cases of low and high In contents, QWDs are not observed in TEM images. The window of In composition to grow QWDs is from 30 to 50%. Figure 2 illustrates the impact of indium composition on structural properties of InGaAs layers formed on 6\({}^{\circ}\) misoriented GaAs (100) substrates. The strain fields caused by variation of indium composition are visualized as black-and-white contrast. Plan-view TEM images reveal the formation of islands with a higher In composition as compared to the In concentration in the surrounding InGaAs layer (residual QW). It is these islands that we refer to as quantum well-dots. The QWDs are aligned along the [1; 2; 3; 4; 5; 6; 7; 8; 9; 10] direction, have a round or oval shape and lateral size (in case of In\({}_{\mathrm{0.3}}\)Ga\({}_{\mathrm{0.7}}\)As) of 10-20 nm. The islands tend to form nanowire-shaped clusters along the [1; 2; 3; 4; 5; 6; 7; 8; 9; 10] direction, which corresponds to the direction of atomic steps [33; 34]. This tendency is weakening with increasing the indium composition. The nanowire-like objects show periodicity in the [110] direction with a period of 20-40 nm. Note that the length of monolayer steps for 6\({}^{\circ}\) misoriented GaAs (100) surface is about 3 nm. The larger value of  
Figure 1: Schematic representation of InGaAs structures of different dimensionalities—self-organized Stranski–Krastanow quantum dots, QDs (a); quantum well-dots, QWDs (b); and quantum wells, QW (c). the found periodicity is due to the \"step-bunching\" effect [35] that occurs during the MOVPE growth on vicinal substrates [16]. QWD layer formed by deposition of In\({}_{0.4}\)Ga\({}_{0.6}\)As represents the larger lateral size of the islands--20-30 nm (Figure 2b). The tendency for the QWDs to merge into wire-like clusters along atomic steps is less pronounced as compared to the case of In\({}_{0.3}\)Ga\({}_{0.7}\)As.