A thesis for the degree of Doctor of Philosophy defended August 2018.
The PhD School of Science, Faculty of Science, Center for Quantum Devices, Niels Bohr Institute, University of Copenhagen
Prof. Peter Krogstrup
Prof. Charles M. Marcus
Semiconductor nanowire networks grown by Molecular Beam Epitaxy: Vapor-Liquid-Solid and Selective Area Growth
This thesis presents results and advancements achieved in pursuit of high-yield in-situ growth of superconductor/semiconductor hybrid nanowire based networks. Such hybrid structures were proposed as a material platform suitable for realization of topological quantum computing based on Majorana Fermions. This specific application sets various requirements on the materials, namely strong spin-orbit coupling, large Landé g-factor, superconducting pairing, tunability of chemical potential and at last multi-terminal geometry with high-quality junctions between quasi-1D nanowires. That put together sets the
overall goal of this work.
The thesis starts by investigation of early stages of vapor-liquid-solid growth of InAs nanowires. A new role of catalyst nanoparticles, that governs the NW growth on (001) substrates is uncovered. The understanding of the growth process in combination with atypical patterning of the growth substrate by gold particles, allows for formation of ’inclined’ nanocrosses and more complex multiterminal nanowire networks. The study continues with a description of two distinct vapor-liquid-solid mechanism based growth methods, that rely on in-situ kinking of growth direction of typical  InAs nanowires. The described methods are used to grow two novel types of nanocrosses. That is enabled by specific patterning of the substrate, where gold catalysts are precisely aligned to the crystal orientation of the substrate.
The analysis of the crystal structure withing the inclined nanocrosses reveals that they form well defined polytypic wurtzite/zincblende/wurtzite junctions. Resistivity measurements through the nanowire intersections show that the zincblende inclusion affects its transparency to electron transport. We demonstrate that the inclusion can be used as an intrinsic quantum dot embedded into the junction of multi-terminal nanowire structures. In addition the overal crystal structure of the whole inclined nanocross can be modified by increase in growth temperature. That results in growth of complex but periodic polytypic structures. The two presented types of kinked nanocrosses form single crystalline junctions and no additional barriers that would affect electron transport were measured.
By implementing the described growth methods, new possibilities of scaling up the growth into larger nanowire networks are introduced together with in-situ shadow masking of hybrid semiconductor/superconductor nanowire heterostructures. The presented vaporliquid- solid based growth strategies are still limited by challenging fabrication on larger 4 networks. In order to increase the scalability potential, new advances in the in-plane InAs nanowire network selective area growth are presented. It is shown that selective area growth offers full scalability, comparable to standard top-down methods. It is also demonstrated that electronic devices can be fabricated directly on the growth substrate and used in low temperature transport measurements.
The device performance is enhanced by implementation of a GaAs(Sb) buffer layer, which improves the nanowire/substrate interface quality by allowing for partial elastic relaxation of the InAs NW. The buffered nanowires show field effect mobility and spin orbit coupling comparable to typical vapor-liquid-solid structures. In addition the buffered networks show coherent transport in Aharonov-Bohmn experiments. The characterization is concluded by demonstrating the compatibility of the selected area grown nanowires and networks with in-situ growth of radial superconductor/semiconductor heterostructures.