Structural, vibrational, and optical properties of silicon cluster assembled films

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Abstract
Distributions of neutral Si-clusters centered around mean sizes of 50 and 200 atoms have been produced using a laser vaporization source and deposited on various substrates at room temperature in ultrahigh vacuum. The Si-cluster assembled films obtained, resulting from the random stacking of incident free nanosize clusters, were subsequently coated by appropriate protective layers before removing in air to perform ex situ infrared, visible, Raman, and photoluminescence spectrometry measurements, as well as transmission electron microscopy observations. The main characteristics of the cluster films are comparable to those observed for amorphous hydrogenated silicon and quite different to those of conventional nanoporous structures or clusters larger than 2–3 nm. The observed intense photoluminescence signal and band gap suggest the presence of a low number of dangling bonds probably due to surface reconstruction effects, connections between adjacent clusters, and oxygen contamination. As for the oxygen contamination, infrared and x-ray photoemission spectrometry measurements agree with the assumption of oxygen atoms trapped at the cluster surface. Finally, all the results on the vibrational and optical properties tend to confirm the failure of the classical confinement model in a diamond lattice to explain the behavior of such nanostructured materials with grain size typically in the nanometer range. The presence of five-membered rings characteristic of the Si-cluster structures in this size range with the subsequent rehybridization effects, as well as the connection process between adjacent clusters seem to be a track which is discussed for a better interpretation of the results.