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Within this work, the growth of out-of-plane Si and SixGe1-x and in-plane Ge nanowire have been investigated. For this purpose, nanowires have been grown ccording to the vapor-liquid-solid mechanism by means of molecular beam epitaxy with Au as metallic solvent on Si(111) and nano-structured Si(001)/SiO2 substrates. Each stage from surface preparation to final nanowire growth has been examined in order to determine relevant parameters which influence the nucleation process nanowire elongation. Particular attention has been given to describe the variation of Au droplet size distribution on Si(111) to evaluate the subsequent nanowire growth and to obtain optimal growth conditions for an site-selective nucleation on the nano-structured substrates.
Due to increased surface diffusion velocity with raising temperature, the droplet diameter distribution shifts from a positive skewed distribution with a high fraction of smaller droplets to a negative skewed distribution with a high fraction of larger droplets. The temperature dependency of the most probable droplet diameter and the number of Au droplets per area has been determined, which can be applied to predict the resulting nanowire diameters.
The out-of-plane Si and SixGe1-x on Si(111) predominantly grow along the surface normal, exhibiting the characteristic sawtooth-like sidewall faceting. During the nanowire formation, the Au surface diffusion velocity increased and causes an increase of the most frequently observed diameter. Furthermore, the total number of droplets/nanowires decreases by a constant factor due to an increase of the contact angle during Si/Ge deposition. The specific diameter range for an possible nanowire formation is reduced by the incorporation of Ge into SixGe1-x nanowires.
To obtain a regular and uniform nanowire growth, a nano-structured substrate consisting of Si(001) terminated pillars surrounded by a SiO2 matrix has been utilized. The initial growth of Ge nanowire starting from Si-Au droplets with SixGe1-x nucleation from ternary alloy is discussed from a thermodynamic point of view and a model based on the Si-Ge-Au ternary phase diagram has been developed to predict the SixGe1-x concentration gradient in the nanowire base. The fully relaxed in-plane Ge nanowires occur within one of the four distinct in-plane ⟨110⟩ directions and nanowires are mainly bounded by two 55° inclined {111} facets and a less pronounced planar (001) top facet.
High-resolution scanning X-ray diffraction microscopy reveals a slightly tilted growth of individual nanowires with respect to each other, causing an abrupt change in the orientation at junction points of interconnected nanowires.