In practice, this means that, on the one hand, the size of a QD should be large enough to provide sufficient localization energy of electrons and holes at the ground state. On the other hand, the QD size should not be too large to prevent the formation of a number of closely spaced electron and hole energy levels. For the device application, these conditions must be satisfied at room temperature, which implies that confinement energy of electrons and holes with respect to the wetting layer/matrix (and, ideally, first excited state) should exceed at least 3 \(k_{\text{B}}T\) (where \(k_{\text{B}}\)--Boltzmann constant). In addition, homogenous and \begin{table}
\begin{tabular}{c c c} \hline
**Properties** & **InGaAs/GaAs QWs** & **In(Ga)As/GaAs QDs** \\ \hline Carrier localization & 2D & 0D \\ \hline Maximal emission wavelength & 1100 nm & 1350 nm \\ \hline Optical gain/absorption per layer & \(>\)20 cm\({}^{-1}\) & 3–8 cm\({}^{-1}\) \\ \hline Carrier lateral diffusion length & Several microns & \(\sim\)100 nm \\ \hline Number of stacked layers & 3 (for \(\lambda\sim\) 1 \(\upmu\)m) & 15–20 (for \(\lambda\) 1.1–1.3 \(\upmu\)m) \\ (without strain compensation) & & \\ \hline Carrier capture time & \(\sim\)ps or less & few ps \\ \hline \end{tabular}
\end{table} 
Table 1: Comparison of the characteristics of quantum wells and quantum dots. inhomogeneous broadening of QD array also deteriorates its properties as compared to the case of an ensemble of \"ideal 3D emitters\". Given the above considerations, fabrication and studies of nanostructures, which properties are intermediate between QWs and QDs, are of special fundamental interest and beneficial for certain practical applications. Such nanostructures should combine useful properties of both QWs and QDs and, to some extent, be free of their disadvantages. Starting from the QW side, such InGaAs nanostructure can represent an InGaAs QW comprising In-rich regions and local thickenings, which are capable of localizing electrons and holes. Starting from the QD side, it can be implemented as a dense array of shallow In\({}_{\mathrm{x}}\)Ga\({}_{1-\mathrm{x}}\)As QDs inside the thin In\({}_{\mathrm{y}}\)Ga\({}_{1-\mathrm{y}}\)As layer (\(x>y\)). Two-dimensional QWs and 0D QDs are usually formed when the In content in the depositing InGaAs material is either low (\(x<0.3\)) or high (\(x>0.5\)), respectively. The growth and investigation of structural and optical properties of In\({}_{\mathrm{x}}\)Ga\({}_{1-\mathrm{x}}\)As layers with moderate In concentrations (\(0.3<x<0.5\)) is expected to be useful for the formation of the aforementioned nanostructures of intermediate 2D-0D dimensionality. A number of research groups have investigated nanostructures that can be considered as intermediate cases between QWs and QDs. These structures were referred to as modulated quantum wells [16; 17], as quantum wires [18; 19; 20; 21; 22; 23; 24], as \"Wires-on-Well' (WoW) [25; 26], and as 'Well-Island' [27]. One approach is to use a dense array of small-sized QDs [28; 29; 21]. For instance, in references [28; 29], the structures were grown by submonolayer migration-enhanced epitaxy on vicinal substrates with the amount of deposited InAs close to the critical value of 1.8 monolayers (ML). A blue shift and narrowing of the photoluminescence (PL) band with increasing misorientation angle was observed. The fact that QDs become smaller and more uniform in size was attributed to lateral confinement of QDs on terraces formed due to step bunching effect. In reference [21], the alignment of InAs QDs on very regularly formed multiatomic GaAs steps, produced by step bunching on vicinal (100) substrates, was studied. This approach was used for growing regular arrays of quantum wire (QWR) structures formed along the step edges. Another approach to form nanostructures of mixed dimensionality is to apply MOCVD or MBE growth of InGaAs with low In composition [16; 25; 30]. Partial strain release results in the appearance of thickness and compositional modulations.