We present a complete study of Si nanocrystals growth by Chemical Vapor Deposition. Si NCs are grown using SiH4 as precursor, on thermal SiO2, deposited Si3N4 and Al2O3. We have studied the influence of the experimental parameters on Si-NCs formation. On SiO2 and Al2O3, we have identified OH groups as nucleation sites'. Hence, by controlling the OH density on the SiO2 surface, we can monitor the Si-QDs density between 10(10) and 1.5 x 10(12)/cm(2). To control the Si-QDs size, we have developed an original two steps process which separates the nucleation and the growth of Si-QDs. In the first step, the density is fixed by exposing the treated SiO2 surface to SiH4 precursor. In the second step, the Si-QDs growth is obtained only on previously formed Si nuclei by using a selective precursor, namely SiH2Cl2. (C) 2004 Elsevier Ltd. All rights reserved.