Mechanisms shifting the emission energy of self-assembled quantum dots in nanowires

This thesis is dedicated to the engineering of the emission energy of quantum dots embedded in core-shell GaAs-AlGaAs nanowires. The goal is to redshift the quantum-dot emission energy in resonance with a gas of Rb atoms. Segregation processes on the nanowire surface account for the formation of the quantum dots as Ga-rich nanoclusters that behave as optically active single-photon emitters. The study presented in this thesis shows ways to enhance the control on the composition of semiconductors alloys, in order to form homogeneous compounds or to direct segregation and diffusion phenomena at the atomic scale. The first part of this thesis elucidates the role of crystal defects, namely rotational twins in the nanowire structure, in tuning the diffusion-driven quantum-dot formation. The combination of high-resolution cathodoluminescence and transmission electron microscopy evidences that the quantum-dot occurrence increases in presence of crystal defects, while the quantum-dot emission energy decreases. Simulations of the emission energy of quantum dots crossed by crystal defects as well as the twin-driven adatom-diffusion dynamics on the nanowire surface account for the observed correlation. The control on the occurrence of crystal defects in the nanowires turns into an important parameter to master the composition and light emission of the nanowire shell. The second part of this work describes two mechanisms used to redshift the quantum-dot emission energy to match the D2 emission line of Rb. By the first mechanism, a silicon-oxide coating deposited on the nanowires applies tensile stress to the embedded quantum dots and redshifts their emission-energy distributions by more than 100 meV. The anisotropy of the strain is evinced from Raman-spectroscopy measurements and correlated with the oxide microstructure. The application range of this simple, yet effective, straining device is expanded by testing further materials and deposition techniques. The second redshifting mechanism consists in alloying the nanowire shell with Indium to form a quaternary AlGaInAs shell. This mechanism redshifts the quantum-dot emission-energy distributions by about 300 meV. The spatially-resolved compositional analysis of the nanowire cross sections reveals that Ga, Al, and In segregate in the quaternary alloy according to a facet- and polarity-driven 3-fold symmetry. Wedge-shaped In-rich segregates, not observed before, form as the result of the competitive diffusion of the group-III adatoms on the nanowire facets. Preliminary measurements on the optical coupling between quantum dots and Rb demonstrate the achievement of the target emission energy, which is the first fundamental step towards the integration of these single-photon emitters into a quantum network.