Modeling and design of quantum dots under the effect of electric and stress field

Quantum dots are metalic and semiconductor nano materials having sizes between 2-50 nanometers. QDs have discrete energy levels and thus a well-defined band gap, which may be engineered by controlling their size, morphology, arrangement/density and material compositions. Stranski Krastanov (SK) growth mode leads to the formation of dislocation-free nanoislands, which are interconnected with a thin wetting layer. These unique properties make QDs promising candidates for designing novel electronic, photonic and magnetic devices, such as single electron transistors, lasers and solar cells, with improved performance and reliability. Experimental studies demonstrated that initial thickness of the film, crystallographic structure/orientation, diffusion and surface stiffness anisotropies, wetting contact angle, mismatch and residual stresses and the strength as well as direction of the applied external electric field are among the several parameters that affect the final morphology of nanoislands. Despite intense attention and extensive research on capillary-driven shape and microstructural evolution under various force fields, it continues to be a challenging theoretical problem in materials science. To be able to produce quantum dots with controlled morphology and self-assembly we need to understand their growth kinetics and morphology differentiation under various force fields in detail. This knowledge will allow us to control the morphology and thus the optoelectronic properties of produced QD nanostructures and lead us to develop novel fabrication techniques and design novel QD-based technologies. Experimental studies have shown that applied electric field has an effect on the QD formation and thus may be used to control structural and electronic properties of quantum dots together with residual and applied stresses. This technique is thought to be technologically more feasible in contrast to application of stress fields. In this thesis, formation, spontaneous evolution and stability of single quantum dots and quantum dot series in hetero-epitaxial systems under the simultaneous action of electric and strain fields has been studied for the first time. In this thesis, we numerically solved the the system of differential equations governing the evolution dynamics of QDs with realistic boundary conditions and finally carried out extensive simulations to reveal the interplay between the final equilibrium QD shapes and the material properties (crystal structure, surface energies, diffusion and surface stiffness anisotropies) and the applied external fields (electric and strain fields). Finally, based on the simulation results, we obtained the tendencies related to the equilibrium QD shapes for a given set of material properties and applied external force fields. The knowledge gained from these simulations will help us to control and design the QD nanostructures with optimum properties. Keywords: Quantum dots, Thin films, Diffusion, Anisotropic material properties, Computer simulation.