## 3 Optical Properties
### Dynamic Characteristics of QWDs  
The quantum dimensionality naturally influences carrier relaxation and recombination processes in low dimensional structures. In this section, we compared results of time-resolved PL studies for 0D InAs/InGaAs/GaAs quantum dots [51], 2D InGaAs/GaAs quantum wells, and InGaAs/GaAs QWDs nanostructures of mixed (0D/2D) dimensionality.  
The PL spectra at the CW excitation are demonstrated in Figure 9a. The spectrum of the QD sample contains a dominant peak at 1270 nm due to the emission from the ground-state optical transition as well as weaker peaks at higher energies due to the emission from the first and second excited-state transitions. The spectra of QWD- and QW-samples show intense peaks of the ground-state optical transition and weaker shoulder from the higher energy states. All the samples show high optical quality: the drop in integrated PL intensity with temperature increase from 78 K to 300 K is 3.2, 1.6, and 1.3 for QDs, QWDs, and QWs, respectively.  
Figure 9b compares the temporal evolution of PL signal for the QD, QWD, and QW structures [53]. In case of QDs, the PL intensity temporal evolution can be well fitted by bi-exponential expression PL(\(t\)) = A\({}_{1}\)exp(\(\tau_{1}\)/\(t\)) + A\({}_{2}\)exp(\(\tau_{2}\)/\(t\)). The decay of PL signal is characterized by the fast component (\(\tau_{1}\) = 1.1 ns) and slow component (\(\tau_{2}\) = 7 ns). This agrees with previous estimations of PL decay time about 1 ns made for self-organized In(Ga)As QDs by several research groups [54; 55]. The slow component is attributed to carrier radiative recombination from the QD ground state. It is generally believed that at room temperature, the carrier lifetime in semiconductors is limited by non-radiative recombination [56]. The obtained large value of PL decay time can be explained by carrier recapture in  
Figure 9: Normalized PL spectra (**a**) and temporal evolution of PL signal at its spectral maximum (**b**) for quantum dots (blue), quantum well-dots (green), and quantum well (red).  
QD array [57; 58] as well as to the absence of fast non-radiative recombination channels in the sample under study. The total PL decay time at 1/e level for the QD ground state transition is estimated as 6 ns.  
QWD PL intensity temporal dependence can be fitted by a mono-exponential expression, which is ascribed to the absence of discrete high energy levels involved in carrier relaxation processed to the ground state. The PL decay time at 1/e level for ground-state transition is about 6 ns.  
The QW structure shows the slowest PL decay with a characteristic time of 20 ns. This value is comparable with carrier radiative lifetime measured for QWs in the temperature range 150-250 K [59]. At higher temperatures, PL decay time is limited by non-radiative lifetime and is decreasing rapidly. Taking into account extremely high optical quality of the structures under study, which demonstrates room-temperature PL intensity comparable to the one at 78 K (to compare, in contrast, in [59], PL intensity degradation in the temperature range 10-250 K is higher than two orders of magnitude), one can suppose that non-radiative recombination processes are still not significant even at room temperature, and PL decay time corresponds to radiative recombination time.  
Summarizing, the presented data show, that the carrier radiative recombination time is affected by structure dimensionality. The localization of charge carriers in QWDs results in the attraction of the carrier of opposite electrical charge and facilitates faster radiative recombination as compared to QWs.