Investigation of Au droplet formation and growth of SixGe1−x nanowires by molecular beam epitaxy

Abstract

Silicon (Si) and silicon–germanium (Si_xGe_1−x) nanowires (NWs) were grown out-of-plane by molecular beam epitaxy using gold (Au) as a solvent. For this purpose, a thin film of gold was deposited onto a heated substrate, resulting in the formation of droplets with a characteristic size distribution dependent on temperature. The amount, yield and NW diameter were estimated by the individual size of Au droplets and their number per area. Due to increased surface diffusion velocity with raising temperature, the droplet diameter distribution shifted from a positive skewed distribution with a high fraction of smaller droplets to a negative skewed distribution with a high fraction of larger droplets. Therefore, the distribution function for Ostwald ripening can only be applied at a temperature above 550 °C. Below 550 °C, the gamma distribution function correlates with the droplet diameter distributions. The temperature dependency of the most probable droplet diameter and the number of Au droplets per area has been determined in the inspected temperature range and can be applied for the MBE growth of Si and Si_xGe_1−x NWs. During the Si_xGe_1−x NW formation, the Au surface diffusion velocity increased, due to the epitaxial layer formation on the substrate. Therefore, the most frequently observed diameter of Si and Si_xGe_1−x NWs shifted compared to the previously formed Au droplet diameter with the highest probability, while the number of Au droplets per area decreased by a constant factor due to an increase of the contact angle. The specific diameter range for successful NW formation is reduced by incorporation of Ge into Si_xGe_1−x NWs.