Deposition and analysis of silicon clusters generated by laser-induced gas phase reaction

Abstract

Silicon clusters have been generated by CO2-laser-induced decomposition of SiH4 in a flow reactor. By introducing a conical nozzle into the reaction zone, they are extracted into a molecular beam apparatus and analyzed with a time-of-flight mass spectrometer. The mass spectra show that the cluster source emits, besides small clusters, also nanosized species containing around 103 atoms. These clusters were deposited on silicon and sapphire targets at room temperature. The deposited films have been analyzed with a Raman spectrometer and with a field emission scanning electron microscope (FE-SEM). The Raman spectra reveal a broad amorphouslike band and a relatively sharp peak at 518.1 cm−1. Interpretation of the sharp Raman feature, based on the phonon confinement model, suggests the presence of silicon nanocrystallites in the deposited films with a particle size of about 3–3.6 nm in diameter. The FE-SEM micrographs show an agglomerate of spherical particles of 3–12 nm in diameter, with a pronounced maximum in the size distribution at around 3.5 nm. The various methods of characterization allow us to conclude that the size of the nanoclusters is largely preserved if they are deposited on the substrate. Therefore, the technique presented here might be an efficient means to produce silicon quantum dots of about 3 nm in diameter.