Study of epitaxy in proximity coupled semiconductor - ferromagnetic insulator - superconductor heterostructures for majorana-based topological quantum computing

Abstract

One of the current endeavors in quantum technology consists of the development of topological quantum computers, which stand out as promising candidates because, contrary to the quantum states of ordinary individually trapped particles such as electrons, the topological properties of the quantum system ensure a higher stability when subjected to small perturbations. Among the different classes of topological states, the so-called Majorana bound states are of great interest, as signatures of their appearance have been observed recently in hybrid III-V semiconductor superconductor nanowires. A way to induce the topological properties, without the need of external magnetic fields that could modify the band structure of the nanowire, is given by the coupling to ferromagnetic insulators. Composite tri-crystals of semiconductor-ferromagnetic insulator-superconductor materials, such as InAs-EuS-Al, have been proposed to overcome the challenge of building adequate topological systems, where the absence of crystal defects and impurities, particularly at the interfaces between materials, is of extreme importance. Therefore, our work is centered around the study of the crystal quality of the nanostructures, especially focusing on the epitaxial relationship between the composing materials. Mainly by the means of High-angle annular dark-field (HAADF) imaging in Scanning Transmission Electron Microscopy (HAADF-STEM), where we analyze the crystal phases and interfaces at atomic scale and check the coupling of different materials atom by atom (column), combined with Electron Energy Loss Spectroscopy (EELS) in order to gain data on the chemical composition distribution, we study nanostructures grown by different techniques, in varying geometrical layouts and growth conditions. Namely, we have analyzed vertically grown nanowires of InAs by the Vapor-Liquid-Solid (VLS) method, with selective covering of the several facets with EuS and Al to observe possible favored crystal orientations depending on the faceting, planar samples of EuS layers grown on InAs by Molecular Beam Epitaxy (MBE), which is the preferred synthesis method due to its ultra-pure environment, and also the growth of InAs on GaAs substrate in the form of nanowires forming networks created by the method of Selective Area Growth (SAG)7-10, since the development of said networks is one of the final goals for the construction of the logic gates used for the calculations in a topological quantum computer. By applying Geometric Phase Analysis (GPA) to HAADF-STEM micrographs with atomic resolution, we are able to detect structural defects and deformations, such as twin boundaries, misfit dislocations, rotation and compression/dilatation within the crystal phases of these nanostructures.