Tight-Binding based Investigation of Semiconductor Quantum Dots and Molybdenum Disulfide Nanobubbles : From Atomic Structure to Optical Spectra

Abstract

The present work achieves an understanding of the interplay between morphology, electronic, and optical properties of semiconductor nanostructures for single-photon emission. The latter is important for various quantum information applications, including future quantum communication devices. In the first part of this thesis, III-V semiconductor quantum dots are investigated. These systems have emerged as promising candidates for deterministic single-photon sources due to high repetition rates, integrability into electrical devices, and tunable Emission energy. In particular, we analyze two different quantum dot systems, which are capable of single-photon emission in the technologically important telecom C-band. The electronic single-particle energies and wave functions are calculated using a nearest neighbour sp3s empirical tight-binding (TB) model. Strain arising from lattice mismatch of the constituent materials is calculated atomistically by employing a valence-force field method. The theoretical model is based on realistic quantum dot geometries and concentration profiles, which are obtained from transmission electron microscopy. When using the measured results for a representative quantum dot geometry as well as experimentally reconstructed alloy concentrations, a combination of strain-field and TB calculations is able to reproduce the quantum dot emission wavelength in agreement with the experimentally determined photoluminescence spectrum. The inhomogeneous broadening of the latter can be related to calculated variations of the emission wavelength for the experimentally deduced In-concentration fluctuations and size variations. The results provide a deeper understanding of the interplay between morphology, electronic, and optical properties of semiconductor quantum dots. In the second part of this thesis, we investigate atomically thin layers of transition metal dichalcogenides (TMDCs), which have recently emerged as a new class of optically active materials for opto-electronic applications. These systems offer a relative ease of fabrication and the potential for large-scale applications. Engineering of the local confinement Situation can lead to discretized states, which opens the possibility for deterministic single-photon generation. One possible platform are molybdenum disulfide nanobubbles that develop when air is enclosed during the stacking of layers. An analysis of strain and dielectric effects in nanostructures reveals that an interplay of these physical effects leads to the formation of confined quantum-dot-like single-particle states that give rise to single-photon emission. The confinement of these states is caused by a wrinkling of the material, that supports the formation of strain-induced pockets. The results provide a deeper understanding of the interplay between strain, dielectric effects, electronic, and optical properties of TMDC nanostructures.