Löbl, Matthias Christian. Excitons in quantum dots and design of their environment. 2019, Doctoral Thesis, University of Basel, Faculty of Science.

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Abstract
Selfassembled semiconductor quantum dots confine single carriers on the nanometerscale. 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 electronholes), they fill the states according to Hund's rules. The recombination of a single exciton (a bound electronhole 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 twolevel 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. Tunnelcoupling the quantum dot to a Fermireservoir 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, closeby surfaces cause significant charge noise. To reduce the charge noise, a new type of ultrathin 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 spinphysics 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, ultralow 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 socalled 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 twodimensional 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 wettinglayer 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 selfassembled 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 postgrowth 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 socalled 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 wellisolated quantum dot, there are higher shells of the quantum dot itself which can lead to effects beyond a twolevel 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 redshifted 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 singleparticle 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.
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. Tunnelcoupling the quantum dot to a Fermireservoir 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, closeby surfaces cause significant charge noise. To reduce the charge noise, a new type of ultrathin 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 spinphysics 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, ultralow 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 socalled 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 twodimensional 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 wettinglayer 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 selfassembled 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 postgrowth 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 socalled 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 wellisolated quantum dot, there are higher shells of the quantum dot itself which can lead to effects beyond a twolevel 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 redshifted 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 singleparticle 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.
Advisors:  Warburton, Richard and Finley, Jonathan J. and Poggio, Martino 

Faculties and Departments:  05 Faculty of Science > Departement Physik > Physik > Experimental Physics (Warburton) 
UniBasel Contributors:  Löbl, Matthias 
Item Type:  Thesis 
Thesis Subtype:  Doctoral Thesis 
Thesis no:  13645 
Thesis status:  Complete 
Number of Pages:  1 OnlineRessource (136 Seiten) 
Language:  English 
Identification Number: 

edoc DOI:  
Last Modified:  01 Dec 2020 02:30 
Deposited On:  31 Aug 2020 13:05 
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