The nucleation mechanisms for SiC thin films on Si were investigated by interrupting the growth at very brief times (similar to 1 to 10 s) using rapid thermal chemical vapor deposition in conjunction with hydrocarbon carbonization. The resulting SiC nuclei and films on Si have been studied by scanning electron microscopy and atomic force microscopy. The hydrocarbon partial pressure in the gas stream was found to determine the nucleation mode. Low precursor concentration results in initial three-dimensional(island) growth which enables the study of trench and void formation. Voids were observed to initiate when two neighboring nuclei come in contact. Trenches in the Si substrate surround each isolated nucleus, with the trench depth increasing with the diameter of the island. SiC films grown for a nominal reaction time of 1 s indicate that increasing the propane concentration results in decreases in SiC grain size and surface roughness and an increase in the nuclei density. A model is proposed for the nucleation process of SiC growth on Si by carbonization consisting of the following key steps : (i) the initial nucleation density is determined by the precursor concentration; (ii) lateral and vertical growth of individual nuclei proceeds by consumption of Si atoms around their periphery, forming trenches in the substrate; (iii) Si Voids are formed in the Si substrate near the SiC/Si interface when nuclei grow large enough to come in contact and, thus, restrict the supply of Si atoms.