The kinetics of the nucleation and growth of Si films on amorphous SiO2‐covered Si using rapid thermal chemical vapor deposition from SiH4and Si2H6(5% in He) were compared at temperatures between 600 and 800 °C and reactant gas pressures between 1 and 25 mTorr. Quantitative assessment of the nucleation parameters and the structures of the deposited Si films have been determined usinginsitureal time single wavelength and spectroscopic ellipsometry. In addition to ellipsometry, atomic force microscopy, scanning electron microscopy, and cross‐sectional transmission electron microscopy were usedexsituto observe the nucleation stage and the microstructures of the films. This study compares the initial growth parameters for SiH4: nuclei density (6×108cm−2), nuclei size (94 nm), incubation time (4.2 min), and degree of selectivity (42 nm) with those for Si2H6: 1.3×1010cm−2, 31 nm, 0.4 min, and 10 nm, respectively. The incubation times for SiH4and Si2H6are different, as is the degree of selectivity, but they show similar activation energies of about 1 eV in the 600–800 °C range. The Si film quality in terms of surface roughness and grain structure was better for the Si film derived from Si2H6than from SiH4.