The nucleation and growth of silicon (Si) films on silicon dioxide (SiO2) surfaces has implications for both selective Si deposition in microelectronics manufacturing and thin film transistor performance in liquid crystal displays. We report the evolution of Si nuclei on a SiO2 surface as observed by in situ single wavelength ellipsometry and the modeling of the Si films by spectroscopic ellipsometry. The Si is deposited on thermally grown SiO2 films by rapid thermal chemical vapor deposition (650 degrees C and 20 millitorr) with a disilane (5% in He) source gas. Single wavelength ellipsometry measurements are made at 3.65 eV (for temperature stability reasons) and spectroscopic measurements are made in the range of 2.5-5.0 eV. Modeling of the ellipsometric results indicate that the H-2 pretreatments passivate the Si sites on the oxide surface leading to sparse nuclei, hence larger grained and rougher films. Ion beam pretreatments shorten the incubation time and yield small grained, smooth films. In the range investigated, ion dose is more influential than ion energy on incubation time. Creation of nucleation sites by ion pretreatments appears to be from a physical, rather than chemical mechanism. High temperature pretreatments damage the surface less than room temperature treatments.