Thesis presented November 07, 2019

Abstract:
In this work we investigated the valence band ground state properties of nanowire quantum dots based on the II-VI materials. The main objective was to prove experimentally the stabilization of a light hole ground state in the nanowire quantum dot and understand which parameters influence the purity of the valence band ground state. The two main factors which determine the switching between heavy and light hole and their mixing is confinement and mismatch induced strain. These parameters can be tuned by modifying the length to diameter aspect ratio of the quantum dot and by choosing properly the material which surrounds it in order to maintain confinement of the hole inside the quantum dot.
The effect of strain and confinement was studied extensively by $vec{k}cdot vec{p}$ theory on nanowire quantum dots similar to those we studied with optical measurements. More specifically we investigated the hole ground state properties of both compressive CdTe quantum dots in ZnTe nanowire and tensile ZnTe quantum dots surrounded by ZnMgTe. Strain was tuned by modifying the aspect ratio of the quantum dot and by depositing an external ZnMgTe shell to the ZnTe core. The effect of confinement was investigated by changing the valence band offset between the core and the dot and switching from a strong type I to a strong type II. Additionally, for the CdTe quantum dots we carried out calculations also under the presence of an exchange field, in order to study the spin properties of the ground state through the giant Zeeman shift. These calculations revealed a strong renormalization of the light hole Land'e factor due to a combined effect of elastic strain and spin-orbit coupling.
The nanowires were grown by molecular beam epitaxy in our group and the electronic properties of the quantum dots inserted in them, were studied by low temperature micro-photoluminescence spectroscopy. The study of the excitonic properties (identification of confined excitons, cathodoluminescence, autocorrelation) and the degree of polarization, allowed us to identify without ambiguity the presence of light holes in the valence band ground state, in agreement to what is expected from theoretical predictions.
In order to investigate the spin properties of a light hole ground state, we carried out measurements on magnetic quantum dots containing Mn atoms (concentration in the order of 10%). These quantum dots were characterized by magneto-optical spectroscopy under strong magnetic fields, up to 11 T. This study was carried out for different magnetic field configurations, using both a uniaxial and a vectorial magnet (magnetic fields applied parallel and perpendicular to the nanowire axis, rotating magnetic fields). The presence of a light hole ground state was confirmed through a quantitative study of the excitonic giant Zeeman shift. Light hole presence was manifested through the formation of an exciton magnetic polaron characterized by anisotropic magnetic properties, which were observed for the first time. The experimental data were fitted in very good agreement with a quantitative model which was developed, using the results obtained from numerical calculations.

Keywords:
semiconductors, nanostructures, quantum dots, nanowires, optical spectroscopy, spin properties

On-line thesis.