Excitons in quantum dots and design of their environment

Abstract

Self-assembled semiconductor quantum dots confine single carriers on the nanometer-scale. For the confined carriers, quantum mechanics only allows states with discrete energies. Due to the Pauli exclusion principle, two carriers of identical spin cannot occupy the same energy level. When the quantum dot hosts more carriers (electrons or electron-holes), they fill the states according to Hund's rules. The recombination of a single exciton (a bound electron-hole pair) confined to the quantum dot gives rise to the emission of a single photon. For these reasons, quantum dots are often regarded as artificial atoms or even two-level systems. However, the environment of a quantum dot has a strong effect on it. The properties of a quantum dot can significantly deviate from that of an atom when it couples to continuum states in the surrounding semiconductor material; charge noise can strongly broaden the absorption of the quantum dot beyond its natural linewidth. On the other hand, designing the environment of a quantum dot enables to control its properties. Tunnel-coupling the quantum dot to a Fermi-reservoir or integrating it into cavities and waveguides are important examples. The first part of this thesis investigates a situation in which the environment of the quantum dot is especially problematic: when the quantum dot is integrated into a nanostructured device, close-by surfaces cause significant charge noise. To reduce the charge noise, a new type of ultra-thin diode structure is developed as a host for the quantum dots. The design of the diode is challenging as it must fulfill several requirements to enable spin-physics and quantum optics on single quantum dots in nanostructures. For quantum dots embedded in the final diode structure, we simultaneously achieve full electrical control of their charge state, ultra-low charge noise, and excellent spin properties. Even when the quantum dots have a large distance to surfaces, coupling to interfaces within the semiconductor heterostructure can be a problematic source of noise and decoherence. For InGaAs quantum dots, the so-called wetting layer is an interface that forms during the growth of the quantum dots and is located in their direct spatial proximity. The continuum states of the two-dimensional wetting layer are energetically close to the $p$- and $d$-shells of the quantum dots. Problematic coupling between quantum dot and wetting layer states takes place for charged excitons. The second part of this work shows that a slight modification to the growth process of the quantum dots removes wetting layer states for electrons. The wetting-layer free quantum dots can contain more electrons than conventional InGaAs quantum dots and the linewidths of highly charged excitons significantly improve. Importantly, these quantum dots retain other excellent properties of conventional InGaAs quantum dots: control of charge and spin state, and narrow linewidths in resonance fluorescence. Also for different types of self-assembled semiconductor quantum dots, the growth has a significant influence on the optical properties of confined excitons. In the third part of this thesis, it is investigated how nucleation processes during the growth are connected to the optical properties of GaAs quantum dots in AlGaAs. Remarkably, this connection can be studied post-growth by spatially resolved optical spectroscopy. The main experimental observation is the presence of strong correlations between the optical properties of a quantum dot and its proximity to neighboring quantum dots. In particular, the emission energy and the diamagnetic shift of the quantum dot emission are strongly correlated with the area of the so-called Voronoi cell surrounding the quantum dot. The observations can be explained with the capture zone model from nucleation theory, which shows that the optical quantum dot properties reveal information about the material diffusion during the semiconductor growth. As explained before, the surrounding semiconductor environment can have a strong effect on the properties of quantum dots. However, even for a well-isolated quantum dot, there are higher shells of the quantum dot itself which can lead to effects beyond a two-level system. In the final part of this thesis, a radiative Auger process is investigated. The radiative Auger effect is directly connected to higher shells of the quantum dot and appears in its emission spectrum. It arises when resonantly exciting the singly charged exciton (trion). When one electron recombines radiatively with the hole, the other one can be promoted into a higher shell. The radiative Auger emission is red-shifted by the energy that is transferred to the second electron. The corresponding emission lines show a strong magnetic field dispersion which is characteristic for higher shells. The radiative Auger effect is observed on both types of quantum dots investigated before. Radiative Auger offers powerful applications: the single-particle spectrum of the quantum dot can be easily deduced from the corresponding emission energies; carrier dynamics inside the quantum dot can be studied with a high temporal resolution by performing quantum optics measurements on the radiative Auger photons.