We have developed a two steps chemical vapor deposition (CVD) process which permits to dissociate the nucleation and the growth of silicon nano-crystals. In the first step, silicon ''clusters'' of a diameter below 1nm are nucleated by exposure of the SiO 2 substrate to SiH 4 . In the second step, these clusters are grown selectively using SiH 2 Cl 2 as silicon precursor. TEM analysis shows that the silicon quantum dots (Si-QDs) obtained with this process are mono-crystalline. The main advantage of this two steps process is that the size dispersion is sharpened compared to a standard one step process because of the dissociation of the nucleation and the growth of the Si-QDs.