Optically active quantum dots in bottom-up nanowires

This thesis is dedicated to the discovery and progressive study of quantum emitters embedded in the shell of coaxial gallium arsenide/ aluminum gallium arsenide nanowires. The bottom-up core/shell nanowires were grown in a molecular beam epitaxy machine. During the shell growth, diffusion-driven phenomena lead to segregation effects. Gallium-rich regions are formed at the nanoscopic scale. The observation has been made that the reduced dimensionality of these regions provides true tridimensional confinement for the carriers. The recombination spectra of the electrons with the holes in what was coined shell quantum dots (shell-QDs) thus appear as sets of narrow, intense peaks. The formation of shell quantum dots is taking place on a large range of growth temperatures and nominal alloy fractions, giving freedom to engineer the growth process. The shell thickness plays an important role in the quantum dot density and total ensemble spectrum. In addition, the adjunction of an aluminum arsenide predeposition layer increasing the local curvature has been seen to foster the quantum dots formation. Single emitter spectroscopy reveals the few-particles electronic structure of quantum dots, with systematic signatures for the different degrees of occupation of the quantum dot. The shape anisotropy of the quantum dots leads to observable spin-spin interactions, which lift the degeneracy of the exciton level (one hole and one electron). Generally undesirable, this effect allows here to find that the orientation of the quantum dots in the nanowire is not hard-wired to the growth direction or to the nanowire long axis. This observation is confirmed by magneto-photoluminescence experiments. The energetic splitting and shift of the spin sublevels when an external magnetic field is applied also confirms the small size of the quantum dots. It is found that for GaAs in the strong confinement regime, the Landé coefficients of the electron and hole take opposite signs and are dependent on the angle at which the field is applied. These effects allow to tune the exciton composite Landé coefficient and could be used to reduce the splitting between the exciton spin sublevels or create optically degenerate coupled systems. Finally, the sub-nanosecond dynamics happening in the quantum dots are probed with time-correlated photon counting. It is shown that the carriers in the shell are quickly captured by the quantum dots. In addition, it is proposed that the electron population is reduced due to diffusion-assisted mechanisms or through electron-donor recombination.

Fontcuberta i Morral, Anna
Lausanne, EPFL
Other identifiers:
urn: urn:nbn:ch:bel-epfl-thesis6812-3

 Record created 2015-11-09, last modified 2018-12-05

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