ISBN 978-3-8439-2788-8

978-3-8439-2788-8, Reihe Physik

Andreas Graf
Simulation of GaAs/AlGaAs quantum dots and molecules

130 Seiten, Dissertation Universität Hamburg (2016), Softcover, A5

Zusammenfassung / Abstract

The main focus of this thesis is the research on energetic properties of strain-free GaAs quantum dots and molecules, embedded in AlGaAs barrier material. The samples are fabricated by local droplet etching in a molecular beam epitaxy system. Metal droplets are utilized to drill self-assembled nanoholes into AlGaAs surface, which are filled with GaAs up to an accurately controlled level, resulting in quantum dots with a defined size. As a consequence, the exciton recombination energy can be controllably varied over a range of over 130 meV by the dot size. Furthermore, repeated filling processes in sufficiently deep nanoholes allow the fabrication of vertically aligned quantum-dot pairs. Depending on the distance between the dots, the quantum-dot pair can be regarded as a quantum dot molecule.

Quantum dots and molecules are studied by photoluminescence measurements in an optical continuous-flow cryostat at 6.4 K. For this, micro- and macro-photoluminescence setups have been modified to fit our experimental requirements. The electron-hole recombinations from the quantum-dot ground states, the so called s shells, were analyzed. In order to assign the recombination energies of the observed transitions to exciton or biexciton states, excitation power and polarization dependent measurements were performed. Effects of vertical electric field are studied for both, single quantum dots as well as molecules.

Due to the utilized fabrication method, indirect quantum-dot-shape determination via atomic force microscopy of unfilled nanoholes is possible. This enables a detailed modeling of the studied nanostructures. Calculations based on kp theory and configuration-interaction scheme were performed on realistically modeled quantum dots and molecules in order to investigate the general behavior of the excitonic states as well as the influence of the dot shape and size. Electric field effects, such as single- and few-particle Stark effect as well as coupling effects in molecules, such as hybridization and its suppression, are realistically expressed by the calculation.

Three distinct types of quantum dots related to different schemes of the nanohole fabrication method are studied. The types differ in hole density, depth, acutance, and symmetry. These properties are transferred to the studied heterostructures.