An investigation of Si2H6 and H-2 for rapid thermal chemical vapor deposition (RTCVD) of silicon on SiO2 has been performed at temperatures ranging from 590 to 900 degrees C and pressures ranging from 0.1 to 1.5 Torr. Deposition at 590 degrees C yields amorphous silicon films with the corresponding ultramooth surface with a deposition rate of 68 nm/min. Cross-sectional transmission electron microscopy of a sample deposited at 625 degrees C and 1 Torr reveals a bilayer structure which is amorphous at the growth surface and crystallized at the oxide interface. Higher temperatures yield polycrystalline films where the surface roughness depends strongly on both deposition pressure and temperature. Silane-based amorphous silicon deposition in conventional systems yields the expected ultrasmooth surfaces, but at greatly reduced deposition rates unsuitable for single-wafer processing. However, disilane, over the process window considered here, yields growth rates high enough to be appropriate for single-wafer manufacturing, thus providing a viable means for deposition of very smooth silicon films on SiO2 in a single-wafer environment.