Boron incorporation in Si during Si epitaxy was studied in an ultrahigh-vacuum rapid thermal chemical vapor deposition (UHV-RTCVD) reactor. The films were deposited using Si2H6 and B2H6 diluted in H-2 as the reactive gases over a doping range from 1 X 10(16) to 1 X 10(19) cm(-3). The experiments were carried out in a temperature range of 650-800 degrees C and a total pressure of 80 mTorr. The experimental results revealed a minimal effect of B2H6 on the Si growth rate. Boron concentration in Si was found to be proportional to the B2H6 flow rate and independent of the deposition temperature. This was found to be true even though the growth rate changed by a factor of five in the temperature range investigated. This is suggestive of a thermodynamic equilibrium between B in the gas phase and B on the growing Si surface. The results are consistent with the claim that B2H6 dissociates as BH3 in the gas phase which chemisorbs on the growing Si surface. In this paper, we demonstrate that by UHV-RTCVD, retrograde doping profiles with sharp doping transitions and doped multilayers with a precise control over film thickness can be obtained. This is attributed to the cold-walled nature of the growth environment which eliminates the chamber memory effect typically observed in hot-wall reactors.