We present a comparative study of nucleation and growth of Si quantum dots on SiO2, SiOxNy and Si3N4 substrates using silane low pressure chemical vapor deposition (LPCVD) at low temperature (570-610 degrees C). The samples are investigated by atomic force micoscopy (AFM), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM) and spectroscopic ellipsomehy (SE). We show that the chemical nature of the surface, precisely, the presence of SiO bonds, decreases the Si quantum dot density. By optimising the deposition parameters, a Si dot density of 10(12) cm(-2) can be obtained below 600 degrees C on a pure Si3N4 surface. The influence of hydrogen, provided by silane decomposition, on the Si nucleation mechanism will be discussed.