## 3 Optical Properties
### Lateral Carrier Diffusion in QWDs  
Carrier diffusion plays an important role in understanding the performance and optimization of parameters of optoelectronic devices, particularly lasers and light-emitting diodes. The diffusion length sets the ultimate limit on the active device diameter. For this reason, a great effort has been  
Figure 7: Evolution of the PL spectra of the QWDs with the increase in the nominal thickness of the deposited InGaAs from 7 ML to 10 ML, taken at 100 K.  
dedicated to the development of fabrication technology for nanostructures with a smaller diffusion length [38; 39; 40]. The diffusion process is drastically influenced by the dimensionality of the active region: while 3D carrier diffusion is possible in a bulk unconfined active region, diffusion is restricted in a plane for a 2D quantum well, and to a single direction for a quantum wire. Ultimately, carrier localization in all three spatial directions in an ideal 0D QDs [41] affects different device characteristics, including the weak sensitivity to dislocations and etched sidewalls. Suppressed carrier diffusion in QDs has been indirectly proved in various experiments, such as improved scaling characteristics of ridge narrow stripe lasers [39], increased tolerance to radiation-induced damage [42; 43], and improved performance of ultrasmall devices [44; 45; 46]. Recently, room-temperature lasing has been realized in QD based microdisks as small as 1 \(\upmu\)m [47]. As it follows from the above consideration, QWDs are an intermediate case between the 2D and 0D situations. Carriers can be localized in the minima of the potential profile of the QWD layer due to inhomogeneities in its thickness and composition, even in case the potential profile is not purely zero-dimensional. Thus, the ability of charge carriers to diffusion is suppressed. As the temperature or the injection level is increased, more and more carriers are thermally evaporated into higher energy levels and continuum states. Even in unconfined energy states within the band, the carrier mobility and, therefore, the diffusion length is strongly reduced by scattering due to spatial inhomogeneities, as evidenced, e.g., in the AlGaN and InGaAs material systems [48; 49]. Carrier diffusion in different types of quantum sized nanostructures can be studied by using temperature-dependent PL from mesa arrays of different diameters [50]. The smaller the mesa size, the easier for the carriers to reach the mesa sidewalls and to recombine non-radiatively. The drop of the mesa luminescence intensities with a decrease in mesa diameter will reflect the enhancement of the role of non-radiative recombination at sidewalls. Figure 8 compares integrated PL intensity measured at 77 K (a) and 290 K (b) for mesas of various diameters. The mesas arrays were formed by electron beam lithography and etching from three types of In(Ga)As/Ga(Al)As heterostructures having different active regions: InGaAs/GaAs 2D quantum wells, InGaAs/GaAs QWDs and InAs/InGaAs/GaAs 0D QDs [51]. Note that the latter QDs emit near 1.3 \(\upmu\)m and; therefore, provide strong confinement of electrons and holes. The intensities for each type of structure are normalized to their values measured for the largest (10 \(\upmu\)m in size) mesas either at 77 K (a) or at 300 K (b). At 77 K, the intensity of the mesa array with QDs is independent of the mesa diameter. This is due to the fact that the carries are localized inside the QDs and do not contribute to lateral diffusion, whereas carrier concentration inside the wetting layer and GaAs matrix is negligible [50]. Figure 8: PL intensity mesas with QDs, QWDs, and QWs as a function of mesa diameter measured at 77 K (**a**) and 290 K (**b**). Insets: microscopic images of mesa arrays taken with different magnification. Carrier diffusion and correspondingly surface non-radiative recombination is activated with temperature increase.