_Applied_  
_sciences_  
**Light Emitting Devices Based on Quantum Well-Dots**  
**[PERSON] 1,*, [PERSON] 1, [PERSON] 2, [PERSON] 2, [PERSON] 3, [PERSON] 1, [PERSON] 2, [PERSON] 2, [PERSON] 2, [PERSON] 1, [PERSON] 2, [PERSON] 4 and [PERSON] 3**  
1 Nanophotonics Lab., Alferov University, 8/3 Kholpina, 194021 St Petersburg, Russia; [EMAIL_ADDRESS] (A.M.N. ); [EMAIL_ADDRESS] (E.I.M. ); [EMAIL_ADDRESS] (F.I.Z.) 2 Ioffe Institute, 26 Polytechnicheskaya, 194021 St Petersburg, Russia; [EMAIL_ADDRESS] (S.A.M. ); [PERSON]DDRESS] (N.A.K. ); [EMAIL_ADDRESS] (N.Y.G. ); [PERSON] (Y.M.S. ); [PERSON]_ADDRESS] (A.S.P. ); [PERSON]_ADDRESS] (V.N.N.) 3 Department of Physics, National Research University Higher School of Economics, 16 Soyuza Pechatnikov, 190008 St Petersburg, Russia; [PERSON]IL_ADDRESS] (N.V.K. ); [EMAIL_ADDRESS] (A.E.Z.) 4 Department of Electrical Engineering, University of Notre Dame, Notre Dame, IN 46556, USA; [PERSON]* Correspondence: [EMAIL_ADDRESS]; Tel. : +7-812-4486980
* 6 January 2020
* 7 January 2020
* 8 February 2020
* 9 February 2020
* 1046 *  
speed, etc.) as well as to develop devices based on novel principles such as single-photon light sources or quantum processing devices [9]. Both QWs and QDs possess a number of inherent advantages and disadvantages caused by their structural properties and energy band diagrams (see Table 1). In QD structures, carrier lateral transport is suppressed due to the efficient spatial localization in QDs of electrons and holes. In contrast, in QW structures, carriers can freely diffuse in the plane of QW. As a result, QD-based structures are less sensitive to epitaxy- or post-growth-related defects compared to their QW-based counterparts. However, in QD structures, optical gain/absorption is relatively low due to limited QD density and inhomogeneous broadening of the QD array. In QWs, optical gain/absorption is much higher. For GaAs-based structures, the emission wavelength of QDs can be much longer than that of QWs and covers telecommunication O-band (1.26 \(\upmu\)m-1.36 \(\upmu\)m). The formation of QDs is accompanied by efficient strain relaxation [1; 10] that enables stacking 20 and more QD layers with high crystalline quality. In contrast, deposition of several InGaAs QWs (\(>\) 3 in the optical region around and beyond 1 \(\upmu\)m, typical QW width and In compositions are 8-10 nm, and 15%-18%, respectively) results in the formation of the dislocation due to accumulation of excessive elastic strains in the semiconductor layers, and the use of complex strain compensation techniques is required [11]. Presently, high-quality InGaAs/GaAs QWs and QDs suitable for device applications are formed by molecular beam epitaxy (MBE) or metal-organic chemical vapor deposition (MOCVD). However even with the use of these up-to-date technologies, it is very challenging to fabricate a nanostructure with properties characteristic of an \"ideal\" 2D or 0D object. During the growth of GaAs/AlGaAs QWs, monolayer high fluctuations can be formed at the lower and upper interfaces. At cryogenic temperatures, local thickenings of the QW results in carrier localization in the lateral direction and plays the role of shallow QDs [12; 13]. In the case of InGaAs/GaAs QWs, there are fluctuations of the In composition that acts as localization potentials for electrons and holes [14]. Similar effects of QD-like emission from In-rich regions have also been observed in InAlAs/InP (see, for example [15]). Compositional and thickness fluctuations are much more enhanced in case of quaternary or quinary QWs, e.g., InGaAsN(Sb). QD arrays are usually represented by narrow bandgap inclusions (islands) inside a wide bandgap matrix. However, these \"geometrical\" QDs do not necessarily show properties that are expected for \"ideal\" QDs with delta-function like the density of states. To fulfill this condition, the confinement energy for electrons and holes in QDs should be high enough to prevent carrier thermal evaporation from the ground state to excited states and continuum (wetting layer and matrix).