Abstract
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 [0001]
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 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
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 [0001]
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 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
| Original language | English |
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| Publisher | The Niels Bohr Institute, Faculty of Science, University of Copenhagen |
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| Publication status | Published - 2018 |