Gold-free Growth of InAs Nanowires

Abstract

Semiconductor nanowires are interesting building blocks for a variety of electronic and optoelectronic applications, and they provide an excellent platformto probe fundamental physical effects. For the realization of nanowire based devices, a deep understanding of the growth mechanism and the nanowire properties is required. In this thesis we investigate gold-free growth of InAs(Sb) nanowires and their properties. Nanowires are grown by molecular beam epitaxy on GaAs(111)B substrates. In the first part of this thesis we demonstrate the growth of InAs and InAs1ixSbx nanowires and show that polytypism can be suppressed by the incorporation of antimony. The electric properties of InAs(Sb) nanowires are studied by electrical measurements and by Raman spectroscopy, and a higher electron mobility is found for defect-free InAs0.65Sb0.35 nanowires compared to InAs nanowires. We also investigate surface passivation using aluminiumoxide. The oxide layer not only serves as passivation layer but it can also be used as gate-dielectric for top-gated field-effect devices. The second part of this thesis is dedicated to the nanowire growth direction and orientation with respect to the substrate. We analyze the existence of tilted nanowires on (111)B substrates, and demonstrate that in most cases they are a result of 3D twinning at the early stages of growth. In addition, also a few unconventional crystalline directions are observed. The ratio of tilted nanowires can be tuned by the growth conditions and substrate preparation. This allows to achieve either all vertical nanowires or a high density of tilted nanowires, whichever is desired for a certain application. Our results also shed light upon the growth mechanism of InAs nanowires, since 3D twinning is associated with the presence of a droplet. Being able to control the formation of tilted nanowires is important, but for certain applications it is also desired to modify the growth direction during growth. For example topological qubits based onMajorana Fermions require junctions and networks. In the third part of this thesis we show a new approach to change growth direction. For this, InAs nanowires are annealed in vacuum in order to create indium droplets. The droplets first formon the top facet of the nanowires and then slide down onto the nanowire side facets. These droplets can act as catalyst-particle, and re-initiation of growth results in L-shaped nanostructures. Merging of these nanostructures constitutes a new approach for the formation of nanowire networks.