|Titel:||Luminescence of droplet-etched GaAs quantum dots at varied temperature and in electric and magnetic fields||Sprache:||Englisch||Autor*in:||Ranasinghe Arachchige, Leonardo||Erscheinungsdatum:||2022-08-09||Tag der mündlichen Prüfung:||2022-07-18||Zusammenfassung:||
Semiconductor quantum dots (QDs) are important building blocks for a range of applications, from QD based lasers to quantum cryptography and quantum information processing, to further optoelectronic applications. QDs suitable for these purposes need to fulfil highly demanding criteria in terms of optical quality and versatility. Local droplet etching (LDE) has established as an epitaxial growth method to obtain low-density, strain-free, pure and highly uniform QDs which can meet these standards. In the LDE method, which is fully compatible with the conventional molecular beam epitaxy, Al droplets are deposited on an AlGaAs or AlAs surface and perform self-assembled drilling of nanoholes with shape and size that are tunable through the process parameters. The nanoholes can then be filled with GaAs and capped with AlGaAs to form GaAs QDs. The LDE QDs have sharp excitonic lines with linewidth down to 25 μeV, precisely controllable emission wavelength, neutral exciton fine-structure splitting below 5 μeV and a nearly perfect single photon emission, with a minimum of the second order correlation function of 0.01.
The optical properties of the LDE QDs have mostly been studied at liquid helium temperatures. However, for many QD based devices, operation in ambient condition is crucial. Hence, an analysis of the behaviour of the QD photoluminescence (PL) emission at and close to room temperature is here provided. It is shown that the observation of the optical emission from the QDs is hindered by the emission from the GaAs substrate and that a significant loss of intensity by more than four orders of magnitude is observed compared to low temperature luminescence. However, using an optimised sample with an AlGaAs barrier layer thicker than the penetration depth of the photons from a 532 nm green laser, leads to clear QD peaks at high temperatures. Furthermore, when the AlGaAs layer is thin, QD emission at room temperature can still be observed if the QD is excited with a 405 nm blue laser, since it has a reduced optical penetration depth. A model and a fit to the experimental results is presented to understand possible reasons for the intensity loss. This analysis identifies as the main causes of intensity loss the exciton dissociation in the barrier at T < 100 K and the thermal escape of bound excitons from the QD ground-state at T > 160 K.
Besides the impact of the rise in temperature, this work also covers the influence of applying external electric and magnetic fields on the LDE QDs. In particular, QDs with a cone-shell shape, obtained by drilling nanoholes with Al droplets into AlGaAs instead of AlAs, are here considered, as their peculiar shape makes them highly versatile structures for wavefunction manipulation. In fact, the application of an electric field along the QD growth direction is shown to cause a strong separation between charge carriers, which leads the probability distribution of either the electrons or the holes to take a ring shape, whilst the other charge carrier remains as a dot. The dot to ring transformation can be precisely controlled by the applied gate voltage and has various consequences. One of these, which is here studied both with computations and experiments, is the non-parabolic Stark-shift with a regime of approximately constant exciton emission energy. Furthermore, numerical simulations predict that the exciton recombination lifetime can be increased from nanoseconds up to seconds, as a consequence of the strong charge carrier separation and reduced Coulomb interaction at strong electric fields. The lifetime elongation suggests cone-shell QDs for applications as light storage in the field of quantum information technology.
The formation of quantum rings with adjustable diameter by means of an electric field makes cone-shell QDs interesting also for magnetic field dependent PL measurements. Here, predictions of the behaviour of QD PL under the influence of an external vertical magnetic field are drawn through simulations. The computational studies on the excitonic PL emission predict the possibility of observing Ahronov-Bohm (AB) states in the spectra with the application of a magnetic field of less than 1 T when applying an electric field of 12 kV/cm. However, due to selection rules, only the vanishing of the PL emission and, under certain conditions, its recurrence are expected to be experimentally observed. It is thus also shown that, through the combined effect of the electric and magnetic fields, it is possible to form a hybrid system between a quantum ring and a QD, in which, as a consequence of the varying ground-state magnetic quantum number as function of the magnetic field, the magnetic field can be used to switch on and off the luminescence.
|Enthalten in den Sammlungen:||Elektronische Dissertationen und Habilitationen|
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geprüft am 31.03.2023
geprüft am 31.03.2023