Silicon is the most important material for microelectronics, but its usefulness is limited in photonic or optoelectronic applications. In nature, silicon as well as germanium and its alloys crystalize in a cubic crystal structure and possess an indirect bandgap which suppresses the efficient emission of photons.
Interestingly, Si$_{1-x}$Ge$_{x}$ alloys with x > 0.6 which crystalize in the metastable hexagonal crystal structure have a direct bandgap. TU Eindhoven succeeded in growing wurtzite GaAs core nanowires (NWs) with hexagonal Si$_{1-x}$Ge$_{x}$ shells, and they subsequently measured their optical properties, showing excellent light emission capability.
Ohmic contacts, especially low resistance ohmic contacts, are important for further device fabrication. In this work, we investigate the electronic contact formation on doped and undoped hexagonal Si$_{0.2}$Ge$_{0.8}$ NWs. Metals with different work functions are used to contact the NWs, and the influence of post-annealing are investigated. By using the transfer line method (TLM) on ohmic contacts, we determine the sheet resistance as well as the contact resistivity.
Existing Schottky barriers can be reduced by increasing the charge carrier concentration. This can be achieved by the implantation of dopants (gallium for p-doping and arsenic for n-doping). However, implantation doping damages the crystal structure and leads to an amorphous surface layer. Using transmission electron microscope (TEM), we study the recovery of the hexagonal SiGe crystal structure after high temperature annealing.

Two-point probe measurements of undoped Si$_{0.2}$Ge$_{0.8}$ nanowires persistently showed Schottky barriers. The barrier height does not change significantly using different metals (Ti, Ni and Al) for the contacts. Therefore, it is very likely that Fermi level pinning occurs at the metal/NW interface, as expected for cubic germanium. However, after contact annealing, a linear I-V response is obtained indicating the formation of ohmic contacts.
For the implanted NWs, we show that recrystallization into the metastable hexagonal crystal phase is indeed possible, which is an important step towards Hex SiGe optoelectronic devices.