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The defence will be in English.
Abstract: The primary goal of this PhD is to establish comprehensive control over core-shell InAs-ZnTe nanowires growth for their subsequent utilization as tunnel barriers in semiconductorsuperconductor nanowires devices designed for topological superconductivity research. We choose InAs as the nanowire core because of its superior electronic properties such as a high electron mobility and a large spin-orbit coupling. For the shell, we chose a semiconductor material with a higher bandgap that would form a barrier for electrons and passivate the core surface. Moreover, it is preferable for the core and shell materials to be lattice-matched to favor epitaxy and reduce the formation of strain related defects. Additionally, a high structural quality is crucial, and it is desirable to avoid the formation of stacking faults within the hexagonal structure of the InAs core.
The optimization of InAs nanowire cores necessitates a precise control of their length and diameter. Yet, it is important to understand the role that each element, In and As, play on the dynamics of the nanowire growth. Previous studies focused on the role of the indium current solely in Au-assisted Vapour-Liquid-Solid (VLS) nanowire growth, whereas self-catalyzed nanowires growth models take into account the arsenic current. Based on these foundational works, we proposed a dual-adatom model to describe the growth dynamics of InAs nanowires on InAs (111)B substrates. The model considers the features of both species, where the impact of each depends on the growth parameters and nanowire dimensions. In parallel, we grew by molecular beam epitaxy several series of nanowires and analyzed them by scanning electron microscopy (SEM). The length-radius dependence obtained in different growth regimes from As to In limited conditions were used to test and validate our model.
The objective of ZnTe shell growth is to achieve the controllable formation of an epitaxial shell with a homogeneous thickness of a few monolayers to a few tens of nanometers. To analyze the structures of such small dimensions, we employed energy-dispersive X-ray spectroscopy (EDS) and high resolution transmission electron microscopy (TEM) analyses, which provided the necessary high precision for measuring the shell thickness. High resolution imaging showed perfect epitaxy between ZnTe and InAs, as well as the absence of crystalline defects originating from the shell. Cross-section TEM images revealed that shell growth was non-uniform across different facets of the core surface: the growth rate along the a-facets is faster than along the m-facets. To understand this behavior and subsequently achieve homogeneous shell growth, we applied the Kinetic Wulff Construction to our system. We found controlling facet formation during InAs growth is mandatory to obtain shells of constant thickness around the whole core. As a result, the ZnTe shells developed during this thesis showed controllable thicknesses ranging between 2-3 nm and 30 nm along the m-facets. Finally, we propose optimized growth parameters for InAs-ZnTe core-shell nanowires with varying shell thicknesses, for subsequent investigation of the passivation of InAs nanowire surfaces and tuning the proximity effect between superconductor and semiconductor.
