Scalable Nanowire Networks: Growth and Functionality

With the rise of quantum computing and recent experiments into topological quantum computers come exciting new opportunities for III-V semiconductor quantum nanostructures. In this thesis, we explore the scalable fabrication of patterned arrays and branched networks of horizontally-oriented III-V nanowires with potential applications in the highly-relevant fields of topological quantum computing and infrared photodetection. We approach this challenge through the use of selective-area epitaxy applied to the technique of molecular beam epitaxy.

The first part of this thesis serves to introduce the reader to the relevant topics of quantum computing and epitaxial crystal growth in the context of molecular beam epitaxy. It continues by touching on the applied topics related to electrical transport in nanostructures and transmission electron microscopy.

The second part of this thesis begins with the main result which is the growth of In(Ga)As nanowires through the use of selective-area GaAs nanomembranes as templates on GaAs (111)B substrates. Here, we find that the deposition of InAs on a GaAs nanomembrane favours the growth of an intermixed InGaAs nanowire at the top of the GaAs. We build upon this by also demonstrating the growth of branched wires by patterning branched slits into the SiO2 mask. Electrical transport is then demonstrated by adding extrinsic dopants that are shown by atom probe tomography measurements to segregate to the top of the wire. Magnetotransport measurements on these wires show diffusive transport and weak localization with coherence lengths on the order of 100nm. In a follow-up publication, we then explore the remote-doping of InGaAs nanowires to improve their electrical properties and achieve quasi-1D transport. We start by optimization of the growth parameters to increase In content in the wires. This is followed by growing a remote-doped structure with which we can to improve the electron mean free path by roughly two orders of magnitude to 250nm. With this, magnetoconductance measurements now show transport in the weak anti-localization regime. At the same time, remote-doped test structures analysed by atom probe tomography uncover a dopant segregation effect taking place during the GaAs nanomembrane growth.

Rounding out the growth on GaAs substrates, the third part of the thesis switches the platform to GaAs (100). Here we find that the GaAs nanomembranes do not grow very much in the vertical direction and as a result, the nanowires grow very close to the substrate. We observe a significant reduction in intermixing between InAs and GaAs which allows for the growth of pure InAs wires. We then describe initial promising results on field-effect transport measurements performed on these nanowires.

The fourth and final section of this thesis addresses the integration of GaAs nanomembranes on the silicon platform, starting with Si (111). Here, it is found that different surface treatments before the start of GaAs growth allow for preferential orientation of GaAs nanomembranes. We further discuss the effects of a polynucleated growth regime on the formation of defects, including anti-phase boundaries, and suggest an approach to reduce such defects by encouraging mononucleated growth.

Finally, the thesis is concluded with some closing remarks, an outlook and appendices including everything from extra experiments to a summary of technical lab contributions and paper supplementaries.