Electrical Characterization of Hex-SiGe Nanowires

The semiconductor industry has required a significant increase in energy efficiency and computational performance of the electronic devices for decades. This results in the search for low-power, high-frequency devices. To meet these demands it was necessary to replace the electrical signals with photonic signals. One of the possible candidates are group III-V semiconductor materials, which can efficiently emit light, but they are not CMOS compatible, so their integration with Si chips is very expensive. Ge-rich SiGe alloys grown in the hexagonal crystal structure showed a direct bandgap nature. SiGe compounds are epitaxially grown in the hexagonal phase by MOVPE on thin hexagonal GaAs nanowires, that act as templates. The result is a hex-GaAs core surrounded by a hex-SiGe shell in a GaAs/SiGe core/shell nanowire configuration. While it is difficult to engineer the growth of this novel material, it has been proven to efficiently emit light and it is CMOS compatible as well. Thus, it could revolutionize the optoelectronics industry, replacing group III-V semiconductor materials. Despite the promising experimental results obtained regarding this novel material, many of its electrical, mechanical and optical properties still need to be investigated. The purpose of this thesis is the electrical characterization of hexagonal Ge-rich SiGe alloys carried out in the well-equipped BRNC cleanroom facility of IBM Research Europe. Previous attempts highlighted some issues in its development, including the difficulty in obtaining ohmic contacts at the metal-semiconductor interface. For this reason, one of the main objectives of this project is the investigation of the current-voltage characteristics performed on the material to analyze the occurrence of Schottky potential barriers. From this analysis arises the need to highly dope the metal-semiconductor contact areas by solid-state diffusion to overcome the potential barriers due to the tunnel effect in order to obtain ohmic contacts. In addition, from resistivity measurements it is possible to estimate the doping concentration in the material to verify the effectiveness of the process. Furthermore, some annealing tests highlighted the interference by arsenic impurities coming from the GaAs core in the nanowires during solid-state diffusion doping. This required etching the GaAs core from the nanowires in order to analyze the results of the process on the SiGe shells without additional interference.