Silicon nucleation on silicon dioxide and selective silicon epitaxial growth (SEG) were studied in an ultrahigh vacuum rapid thermal chemical vapor deposition (UHV-RTCVD) reactor using 10% Si2H6 diluted in H-2. Silicon was deposited on SiO2 patterned Si (100) substrates over a pressure range of 10-100 mTorr and a temperature range of 650 and 850-degrees-C. Under these conditions, the growth rate ranged from 50 to 330 nm/minute, demonstrating compatibility with single wafer manufacturing throughput requirements. A pressure dependence in the activation energy in the surface reaction limited regime was observed and attributed to a variation in the steady-state hydrogen coverage on the growing surface. The incubation time for loss of selectivity via Si nucleation on SiO2 was found to increase at lower pressure and remained constant over the experimental temperature range. However, the incubation thickness defined as the film thickness that can be deposited before loss of selectivity occurs was found to increase both at low pressures and high temperatures. We show that a 100 nm thick epitaxial film can be grown selectivity with no Cl addition at 750-degrees-C/10 mTorr.