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Quantum confinement effects in silicon nanowires (SiNWs) are expected when their diameter is less than the size of the free exciton (with a Bohr radius ∼5 nm) in bulk silicon. However, their synthesis represents a considerable technological challenge. The vapor–liquid–solid (VLS) mechanism, mediated by metallic nanoclusters brought to the eutectic liquid state, is most widely used for its simplicity and control on the SiNWs size, shape, orientation, density, and surface smoothness. VLS growth is often performed within high-vacuum physical vapor deposition systems, where the eutectic composition and the pressure conditions define the minimum diameter of the final nanowire usually around 100 nm. In this article, we present and discuss the SiNWs’ growth by the VLS method in a plasma-based chemical vapor deposition system, working in the mTorr pressure range. The purpose is to demonstrate that it is possible to obtain nanostructures with sizes well beyond the observed limit by modulating the deposition parameters, like chamber pressure and plasma power, to find the proper thermodynamic conditions for nucleation. The formation of SiNWs with sub-5 nm diameter is demonstrated.

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The formation of Si quantum dots on SiO2 by chemical vapor deposition of SiH4 has been investigated in the range from the submonolayer to the complete coverage with Si. In order to investigate the very early stages of the nucleation process of Si on SiO2, the energy filtered transmission electron microscopy has been chosen as the main characterization technique, because of the high spatial resolution typical of the transmission electron microscopy analysis, coupled to the compositional information obtained by the electron energy loss spectroscopy. The plan view configuration has been used to measure the dot size distributions down to dimensions of about 1 nm, and in cross section to evaluate the dot wetting angle. For all the several experimental conditions, a wetting angle distribution has been obtained and has shown to be centered at about 90°. Data on the dot size distributions are shown and discussed in the framework of a continuous nucleation model, which has been implemented to take into account the dot coalescence process contribution. Through the comparison with experimental data the relevant thermodynamic parameters of the process have been evaluated: the critical radius, the free energy barrier for nucleation, and the concentration of nucleation sites.

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