Tuning the growth of quantum dot multilayer arrangements: experiments and modeling

In nanotechnology, the fabrication of sophisticated devices for optoelectronics, nanophotonics or quantum computing requires the capability of producing defect-free, spatially ordered and uniformly sized Quantum Dots. In this respect, outstanding advances have been achieved combining bottom-upandtop-down methods which typically consist of an ex-situ surface manipulation of the substrates followed by self-assembly growth techniques. On the other hand,although widely flexible, these methods must face within herent problems such as limited resolution, high cost, and introduction of defects which may severely affect dots’ optical and electrical properties. Pure bottom-up methods, instead, permit higher crystalline quality and reduce the cost of processing. However, this approach requires a close tailoring of the growth in order to overcome the stochastic nature of the microscopic surface processes leading to random nucleation and size/composition fluctuations of Quantum Dots. In this thesis we studied the later al and vertical ordering of Quantum Dots in InAs/GaAs multilayer arrangements by means of Molecular Beam Epitaxy (MBE) growth technique. In the framework of the pure bottom-up methods, our approach exploits the growth of highly lattice-mismatched III-V semiconductors. In particular, we established a growth procedure for increasing the control on the growth mechanism and driving the nucleation of self-assembled InAs dots on precise locations over GaAs(001) substrates. In this work, we applied this innovative method to the growth of multilayer structures, i.e. a stack of alternate layers of dots and GaAs spacers was used for increasing control over the arrangement of the InAs islands on the final surface. We showed the formation of regularly spaced dot chains over surfaces characterized by the presence of ripples intentionally produced. Statistical analyses of the samples and Finite Element Method simulations allowed us to understand the processes involved in the in-line ordering as a function of the number of stacked layers. Moreover, we explored to what extent we are able to control the interplay between the elastic and surface curvature effects to drive the selective nucleation of single chains. Photoluminescence measurements also evidenced the potential of these structures as single photon emitters. Given the important role of the GaAs spacers in driving the ordering of the dots through the layers, we also analyzed the early stages of the formation of the GaAs cap covering the InAs dots. We examined the case of isolated dots grown on flat substrates, showing an anisotropy in the capping process of the islands. This new observed effect appeared to be a consequence of a general behavior of cations (In or Ga) under a fixed oblique As evaporation, as the one we typically use in our growth experiments. The existence of a gradient in the As pressure over the surface, indeed, triggers a mass current towards the positions where the As flux is higher. This plays a crucial role in the formation of asymmetric caps burying the dots. This topic was also discussed within a theoretical approach. We developed a two-dimensional model for describing the capping process of any III-V semiconductor under typical growth conditions. We described the time evolution of the surface profile and the concentrations of the different materials through a system of coupled equations. A numerical code, written in FORTRAN, allowed to solve this system and was applied to simulate several simple growth experiments. In particular, we tried to reproduce the experimental results found for the anisotropy in the capping process of the dots under our peculiar growth conditions. This was made possible by a preliminary study of the stress field at the surface of GaAs capping layers of variable thicknesses, burying InAs dot arrays of variable dimensions and orientations. The thesis is structured as follows. In chap. 1 a general introduction to the Quantum Dots and their device applications is reported. Chap. 2 surveys the basic aspects of our growth method. Chap.3reportsalltheexperimentalresultsofthiswork. Firstly, we discuss the multilayer growth and perform an in-depth analys is of the interplay between the elastic and morphological effects as a function of the number of layers, together with our results on these lective nucleation of dot chains. Secondly, we report thefindings of the capping mechanism of InAs islands under our particular conditions suitable to induce an asymmetry in such a process. The second part of the thesis i is mainly related to theoretical simulations and modeling and deals with the Finite Element Method simulations performed to study and fit the stress field at the surface of GaAs caps covering regular arrays of dots (chap. 4). The kinetic model developed to describe the GaAs capping process is presented in chap. 5, and the results of the numerical code are reported in chap. 6. In particular, in chap. 6 a comparison with some experimental situations is made. Finally, chap. 7 reports the summary and the conclusions of the entire work. Full details of the experimental methods and the use of the Finite Element analyses as well as the fit functions and parameters are reported in the appendices. The experimental part ofthis workwas performed in the MBElaboratory at the University of Rome Tor Vergata. The whole theoretical section is the result of an intensive collaboration with Professor Rita Magri from the University of Modena and Reggio Emilia.