Thesis presented November 05, 2019

Abstract:
Semiconductor nanowires are receiving widespread interests for their novel applications in field-effect transistors, photodetectors and biosensors. The nanowire geometry provides an interesting possibility to fabricate axial heterostructures that can be easily accessed electrically by contacting the NW edges. Depending on the size, material and composition of the heterostructure, carriers can experience quantum confinement effects, allowing to fabricate quantum dots or quantum disks inside the NW. Recently, the formation of Silicide or Germanide contacts via a thermally activated solid state reaction between the metal and Si or Ge NW has drawn significant attention because of its great advantages for fabricating short channel devices from bottom up grown NWs rather than complex and high-cost photolithography top-down approaches. The advantage of this approach is that upon heating a metal enters a semiconducting NW at both ends, creating an (inter)metallic region in the NW. If the process is well controlled and stopped at the right moment, only a thin section of semiconductor is left between metallic contacts, allowing to fabricate electrically contacted quantum-dot in a wire structures in a single fabrication step.
Al/Ge NW thermal induced solid-state diffusion is a promising system since, in contrast to other metal-semiconductor combinations, no intermetallic phase is formed and a pure monocrystalline Al NW is created with a very sharp interface with the remaining Ge NW. Moreover, the combination of the intrinsically strong spin−orbit coupling in Ge and the superconducting properties of Al, make this system a promising platform to study hybrid superconductor-semiconductor devices that could be potential building blocks for superconducting quantum interference devices (SQUIDs). The challenge addressed in this PhD is to study the thermally induced exchange reaction of Al in both pure Ge as well as Si_xGe_1-x alloy NWs using in-situ observations in a transmission electron microscope (TEM), to allow better understanding and control of the mechanisms involved in the reaction.

Keywords:
thermal exchange reaction, solidification, nanowire, transmission electron microscopy, energy dispersive x-ray spectroscopy

On-line thesis.