Formation, electronic structure, and optical properties of self-assembled quantum-dot single-photon emitters in Ga(N,As,P) nanowires

Quantum dots (QDs) formed in semiconductor nanowires (NWs) form a basis for studying interesting quantum phenomena and provide an exciting platform for various device applications, where in-depth understanding of the formation and electronic properties of such QDs is a requirement. In this work we present detailed investigations of structural, electronic, and optical properties of nitrogen-induced self-assembled QDs in novel Ga(N,As,P) NWs. We show that the three-dimensional quantum confinement of the excitons caused by short-range fluctuations in the N content leads to single-photon emission. We demonstrate that valence band character varies between the QD emitters and attribute it to a varying degree of hole localization within the QDs induced by fluctuations in the As/P ratio. These fluctuations in the chemical compositions are also believed to be at least partly responsible for the observed large variation in exciton lifetimes of the QDs, ranging between 0.5 and 10 ns. The rather long lifetimes demonstrate the resilience of the QDs to nonradiative recombination-a merit for efficient single-photon emitters. Furthermore, we show that the principal axes of the formed QDs are oriented radially, normal to the NW side facets, in contrast to the N-induced QDs in GaNAs NWs which are primarily aligned along the NW axis. Our results, therefore, underline the impacts of the host alloy on the QD formation, which could provide an additional degree of freedom for band structure engineering in nanowires.

Tailored-Potential Pyramidal Quantum Dot Heterostructures

Valentina Troncale

Semiconductor quantum dots are usually compared to artificial atoms, because their electronic structure consists of discrete energy levels as for natural atoms. These artificial systems are integrated in solid materials and can be localized with a spatial precision of the order of nanometers. Besides, they conserve their quantum properties even at quite high temperatures (∼ 10 K). These properties make quantum dots one of the most suitable systems for the realization of quantum devices and computers. However, the energy states and the optical properties of quantum dots are much more complicated than for atoms, because a quantum dot is never an isolated potential well. Instead, its electronic structure depends on the crystallin structure of the semiconductor material and on the Coulomb ion-electron and electron-electron correlations. In particular, the presence of several valence bands and their mixing, induced by quantum confinement, gives rise to novel properties which are still not completely understood and exploited in applications. To get a major advance in this field, a full deterministic control of the spatial shape of the quantum confinement is needed, combined with a deeper understanding of the connections between electronic and optical properties. This thesis work has these two main objectives. We realized and experimentally studied different quantum dot systems, in pyramidal hetero-structures grown with MOCVD techniques. These systems allowed the realization of several different geometries for the carrier confining potential, with a precision in the order of nanometers. The optical characterization has been obtained in particular by means of polarization-resolved microphotoluminescence, magneto-photoluminescence, excitation photoluminescence (PLE), and interferometry techniques. For single quantum dots, we have observed and characterized for the first time new excitonic complexes, arising from excited hole states. This allowed a full caracterizatioon of the valence band hole states in our peculiar system. By means of photon correlation measurements, we have also experimentally demonstrated that, even in presence of a large family of exciton states, these quantum dot systems can emit single photons. We have then realized much more complex quantum dot structures, double dot systems (quantum dot molecules) and a completely new system called Dot-in-Dot (DiD). This latter is composed by a small inner dot surrounded by an electrostatic potential well (which can be considered as an outer elongated dot). Such a composite system is characterized by a strong valence band mixing. This state superposition is however very sensitive to small variations of the confining potential. Therefore the degree of valence band mixing can be easily switched by the introduction of a week external field. Since the valence band mixing determines the polarization properties of the emitted light, the DiD changes the polarization properties of its emission spectrum under the action of an external field. In particular, we have experimentally demonstrated this effect for an external static magnetic field, while we have numerically predicted a very similar effect for a static electric field. In the latter case, the polarization switching is a direct consequence of the quantum confined Stark effect induced in the DiD. Hence the DiD appears to be an ideal candidate for realizing emitters of single photons with tunable and controllable polarization.

EPFL2010

Three-dimensional nanoscale study of Al segregation and quantum dot formation in GaAs/AlGaAs core-shell nanowires

Sonia Conesa Boj, Yannik Fontana, Anna Fontcuberta i Morral, Luca Francaviglia, Martin Heiss, Eleonora Russo

GaAs/Al-GaAs core-shell nanowires fabricated by molecular beam epitaxy contain quantum confining structures susceptible of producing narrow photoluminescence (PL) and single photons. The nanoscale chemical mapping of these structures is analyzed in 3D by atom probe tomography (APT). The study allows us to confirm that Al atoms tend to segregate within the AlGaAs shells towards the vertices of the hexagons defining the nanowire cross section. We also find strong alloy fluctuations remaining AlGaAs shell, leading occasionally to the formation of quantum dots (QDs). The PL emission energies predicted in the framework of a 3D effective mass model for a QD analyzed by APT and the PL spectra measured on other nanowires from the same growth batch are consistent within the experimental uncertainties. (C) 2014 AIP Publishing LLC.

American Institute of Physics2014

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

Luca Francaviglia

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.