## 3 Optical Properties  
### The Impact of Indium Composition and Average Layer Thickness on Photoluminescence Spectra  
The existence of two types of quantum-sized objects with different sizes and correspondingly with different confinement energy (QWDs and QDs) results in a characteristic modification of photoluminescence (PL) spectra with temperature increase. At low-temperature thermal escape of carriers from both types of localized states (QWDs and QDs) to the matrix is suppressed (Figure 5) and random (or non-equilibrium) population of the joint QWD/QD array is realized. The PL spectrum was taken at 20 K and moderate excitation power density is composed of ground-state emission from both QWDs and QDs and reflects their size distribution and density. Temperature raise opens the possibility of carrier transport between localizing objects resulting in a redistribution of intensities of the corresponding peaks in the PL spectrum. In the middle-temperature range (\(-\)80-150 K), it can be expected that carrier transport is controlled heavily by the activation energy of confined states (i.e., the energy gap between confined state and matrix) because carrier escape probability is increasing with decreasing activation energy of the confined state. This results in one-way carrier transport from QWDs with smaller confinement energy to QDs providing stronger confinement. Further increase in the temperature (180-300 K) leads to blurring of differences in the probability of carrier escape from QWD and QDs resulting in two-way carrier transport, or in other words, establishing of equilibrium population in joint QWD/QD ensemble according to Fermi-Dirac statistics. The PL spectra of the samples with QWDs formed by the deposition of In\({}_{\mathrm{x}}\)Ga\({}_{1-\mathrm{x}}\)As with \(x=0.3\), 0.4, and 0.5 are in good agreement with the results of TEM studies. The structure with QWDs formed by the deposition of 16 monolayers (ML) of In\({}_{0.3}\)Ga\({}_{0.7}\)As demonstrates a narrow single PL peak (Figure 6a), pointing out that there are no other quantum-sized objects apart of QWDs. The peak is gradually red-shifted and broadened with temperature increases (Figure 6a-c). Figure 5: _Cont._PL spectra of QWDs formed by the deposition In\({}_{0.4}\)Ga\({}_{0.6}\)As show quite different behavior. At 20 K, the main PL peak is accompanied by a shoulder on the long-wavelength side pointing to the presence of two kinds of quantum-sized objects in the active area separated by a potential barrier. On the base of TEM studies, we attribute the shorter- and longer-wavelength peaks to emission from QWDs and QDs, respectively. The ratio of QWD-to-QD surface densities is estimated as \(\sim\)10:1, according to the relative peak intensities in PL at 20 K. Temperature rising up to 100 K leads to gradual PL intensity redistribution from the QWD peak to the QD one, which is accompanied by a dramatic change in the PL spectrum shape (Figure 6b). However, further temperature increase results in reverse evolution of the spectrum--QD shoulder has nearly vanished, and only the main peak, which corresponds to QWDs, remains in the spectrum. One can conclude that the samples formed by the deposition of InGaAs with 40% indium concentration comprises QWDs as major quantum-sized objects, and there is an only small fraction of QDs, which are characterized by deeper localization energies. The PL signal from QDs state is noticeable only under the one-way carrier transport conditions in the temperature range 80-150 K.  
Figure 5: Schematic representation of temperature-dependent evolution of QWD PL spectra: low (**a**), moderate (**b**), and elevated (**c**) temperatures. Figure 6: PL spectra of InGaAs QWDs with 30%, 40%, and 50% In composition measured in the conditions of random carrier population—20 K (**a**), one-way carrier transport—100 K (**b**) and equilibrium (Fermi) carrier distribution—300 K (**c**) in joint QWD/QD array.