Self-assembled quantum dots: A theoretical study of the exciton in single and vertically coupled type-II quantum dots.

摘要

In recent years, quantum dots (QD's) have attracted considerable interest because they can be regarded as ideal model systems for quasi-zero-dimensional systems. They are therefore often called artificial atoms. Two such artificial atoms can be placed above each other, resulting in vertically coupled quantum dots or artificial molecules. The possibility to discover new physics in these zero-dimensional systems together with possible applications in opto-electronics has led to many experimental and theoretical results. The so-called self-assembled quantum dots, as studied in my thesis, form spontaneously when growing two semiconductor materials with a substantially different lattice parameter on top of each other. In such self-assembled quantum dots, it is possible to create an exciton (i.e. a bound electron-hole pair), which will be trapped due to the different band structure of the two materials. One can distinguish between two types of dots: type-I, where both the electron and the hole are confined inside the dot, and type-II, where one of the particles is located in the barrier material.; In my thesis, I studied theoretically the behaviour of one exciton in a type-II quantum dot, where the electron is sitting inside the dot and the hole is located in the barrier under the influence of an externally applied magnetic field. The quantum dot was modeled by a disk of finite radius R and thickness d.; In the first part of the thesis, the exciton properties were examined in the absence of strain effects. For the study of a single disk, we found a different behaviour for a disk-like system (d 2R) and for a pillar-like system (d 2 R), where the latter exhibits angular momentum transitions for the hole with increasing magnetic field. These transitions follow from the fact that the hole is located at the radial boundary of the disk for a pillar-like system, and is pushed against the barrier when the magnetic field is applied. When studying a system of vertically coupled disk, we found that this system exhibits a similar behaviour as the single pillar-like system.; The second part of the thesis is dedicated to the study of the exciton in quantum disk in the presence of strain fields. These strain fields are important, as the cause the formation of the self-assembled dots. They also have an appreciable influence on the band structure and it is therefore necessary to take them into account. The strain was calculated using the isotropic elasticity model. (Abstract shortened by UMI.)