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.

Facet-driven formation of axial and radial In(Ga)As clusters in GaAs nanowires

Akshay Balgarkashi, Didem Dede, Anna Fontcuberta i Morral, Martin George Friedl, Lucas Güniat, Wonjong Kim, Jean-Baptiste Leran, Santhanu Panikar Ramanandan, Nicolas Tappy

Embedding quantum dots in nanowires (NWs) constitutes one promising building block for quantum photonic technologies. Earlier attempts to grow InAs quantum dots on GaAs nanowires were based on the Stranski-Krastanov growth mechanism. Here, we propose a novel strain-driven mechanism to form 3D In-rich clusters on the NW sidewalls and also on the NW top facets. The focus is on ternary InGaAs nanowire quantum dots which are particularly attractive for producing single photons at telecommunication wavelengths. In(Ga)As clusters were realized on the inclined top facets and also on the {11-2} corner facets of GaAs NW arrays by depositing InAs at a high growth temperature (630 degrees C). High-angle annular dark-field scanning transmission electron microscopy combined with energy-dispersive x-ray spectroscopy confirms that the observed 3D clusters are indeed In-rich. The optical functionality of the as-grown samples was verified using optical technique of cathodoluminescence. Emission maps close to the NW tip shows the presence of optically active emission centers along the NW sidewalls. Our work illustrates how facets can be used to engineer the growth of localized emitters in semiconducting NWs.

IOP PUBLISHING LTD2020

Synthesis and properties of heterostructures in nanowires and 2-D materials

Akshay Balgarkashi

Modern solid-state devices were made possible by the discovery of semiconductor heterostructures. Heterostructures offer the ability to fabricate low-dimensional nanostructures such as quantum dots which can restrain carriers in all three-directions. Quantum dots as single photon sources can be used as building blocks in quantum photonics and information processing applications. In this thesis, we explore heterostructures in nanowires and 2-D materials towards fabrication of quantum dots. In the first part, we show how facet-driven nanostructure self-assembly can be used to grow axial and radial QDs in lattice-mismatched NW heterostructures. We investigate the growth of InAs/GaAs nanowire heterostructure arrays on Si substrates using the Ga-catalyzed method. We show how the nanowire tip morphology affects the formation of axial In(Ga)As clusters. The occurrence of In-rich clusters on the nanowires is elucidated by electron microscopy. We also show how the In(Ga)As cluster formation on the nanowire sidewalls is driven by the convex nature of the {11-2} corner facets. Cathodoluminescence and photoluminescence maps close to the NW tip evidence the presence of optically active emission centers along the NW sidewalls. The second part is dedicated to heterostructures using monolayer 2-D transition-metal dichalcogenides. Firstly, we study the heterojunction between monolayer MoS2 and gold. We provide a detailed understanding of the microscopic origins behind Raman mode splitting in monolayer MoS2 exfoliated on gold substrates. We show that splitting in the Raman modes is a convolution of two effects: structural perturbations in MoS2 due to the Au-S interaction and breaking of symmetry rules by the metallic plasmons. We confirm the proposed hypothesis by using Al as the metal which does not show any peak splitting. In the last part, we successfully combine monolayer MoS2 with a GaAs NW array as a first step towards integrating monolayer 2-D materials with III-V nanowire arrays. Using detailed structural investigation, we show three different configurations of the MoS2 monolayer on NWs. Through confocal Raman and photoluminescence mapping, we show variations in the properties of the monolayer due to nanowires. We show how the vibrational properties are mainly affected by strain and charge transfer due to the nanowires. A blueshift in the luminescence attributing to an enhanced dielectric screening due to GaAs nanowires is observed. We show how III-V nanowires can be used to engineer the properties and enhance light extraction from monolayer 2-D materials.

EPFL2022

Growth and characterization of single quantum dot devices for fiber-based quantum communications

Blandine Alloing

Single photon sources have recently attracted significant interest for their potential role in the future applications of quantum cryptography, and in the longer term, quantum computing. Among the different types of single photon emitters, semiconductor quantum dots (QDs) are good candidates as they combine discrete electronic transitions with the possibility of electrical excitation. Self-assembled InAs QDs on GaAs substrate are by far the most studied system as they can easily be grown into a microcavity structure. Embedding these into a microcavity is important as it increases the extraction efficiency, directionality and speeds up the photon emission. Until now, most studies have concentrated on QDs emitting in the λ

EPFL2006

Synthesis and properties of heterostructures in nanowires and 2-D materials

Akshay Balgarkashi

EPFL2022