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

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.

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

GaAs nanowires and related prismatic heterostructures

Anna Fontcuberta i Morral

The growth of GaAs nanowires by the gallium-assisted method with molecular beam epitaxy (MBE) is presented in this review article. The structure of the grown nanowires was investigated by means of scanning and transmission electron microscopy as well as Raman spectroscopy. Their optical properties were revealed by performing photoluminescence measurements at the single nanowire level. Furthermore, by tuning the MBE conditions to planar growth, quantum heterostructures on the side facets of the nanowires were achieved. High-resolution transmission electron microscopy proved that the grown heterostructures have epitaxial precision, while photoluminescence measurements showed that they possess excellent optical quality. These quantum heterostructures constitute templates for developing novel nanowire based devices, such as a high electron mobility one-dimensional transistor or third generation solar cells.

Institute of Physics2009

Scanning Tunneling Microscopy and Luminescence of Individual Nanostructures

Theresa Lutz

A detailed investigation and characterization of the local properties of individual nanoscopic structures is of great importance for the understanding of novel physical phenomena at the nanoscale as well as for the assessment of their possible use in future applications. A unique tool for the study of such structures are scanning probe methods. These methods do not only allow to obtain atomic scale information in both topographic and spectroscopic measurements at surfaces, but can also be employed for example to stimulate local photon emission. In this thesis the growth mechanism of inorganic and organic semiconductor nanostructures and the relation between their local structure and their electronic and optical properties is investigated by scanning probe techniques. In the first part of this thesis, the growth as well as the local electronic and morphologic properties of InAs/GaAs(001) quantum dots grown by molecular beam epitaxy are described. The recorded spectra show evidence of the discretization of the density of states that can be attributed to the zero dimensional confinement of charge carriers to the quantum dots. Moreover, the shape evolution of the quantum dots during the controlled removal of material through an in situ etching gas is studied. High resolution topographic images reveal that hereby an island shape transition takes place. This shape evolution is found to be the reverse of the shape evolution that typically takes place during the growth, indicating that thermodynamic factors play an important role during the growth process and the etching process. In addition, the mechanism of the material removal from the quantum dots is investigated in detail by the analysis of the size evolution of the islands as a function of the nominal amount of etched material. The second part of this thesis is dedicated to the study of individual CdSe nanowires grown by the solution-liquid-solid approach. These nanowires consist of small alternating sections of wurtzite and zincblende lattice structures. This structural arrangement also leads to an alternating electronic structure in the form of a type II band alignment along the long axis of the nanowires. The local electronic properties of the nanowires are analyzed by scanning tunneling spectroscopy. Tunneling current induced luminescence spectra from individual nanowires are acquired by the injection of holes and electrons and their subsequent radiative recombination. In contrast to previous studies on similar systems, the coupling of tip induced plasmons to the radiation process is not required. The shape and the position of the luminescence spectra is compared to photoluminescence spectra and Raman measurements. The photon energy is found to decrease with increasing wire diameter indicating quantum confinement of the charge carriers to the wire. The bulk bandgap, that can be extrapolated from the energy vs. diameter dependence of the emission peaks, indicates that the photons are emitted from zincblende type CdSe sections. Moreover, the diameter dependence reveals that the emission stems from carriers that are confined to quantum dot like parts of the wires. The lack of light emission from the wurtzite type segments is also re ected in the appearance of non-luminescent regions in the NW as shown by spatially resolved light intensity maps. Possible mechanisms for the light excitation are discussed. The investigation of single molecules of the luminescent organic emitter tris-(2-phenylpyridine)iridium(III) (Ir(ppy)3) is the topic of the last part of this thesis. To this end, Ir(ppy)3 molecules are deposited onto metal substrates and different types of thin insulating layers. Besides their structural and electronic characterization, the optical properties of single molecules are studied by tunneling current induced luminescence. No intrinsic light emission originating from the molecules can be observed neither on metal substrates nor on thin insulating layers. This indicates that in these systems, the molecules are still not sufficiently decoupled from the underlying metallic substrate. Preliminary results of tunneling current induced luminescence measurements of Ir(ppy)3 molecules on multilayers of C60 show great promise for exciting light emission on single molecules.

EPFL2011

Scanning Tunneling Microscopy and Luminescence of Individual Nanostructures

Theresa Lutz

EPFL2011