The kinetics of the growth of epitaxial and polycrystalline undoped and heavily p-type-doped Si and Si1-xGex layers have been studied over the pressure range from 4 x 10(-4) to 1.5 Torr at 625-degrees-C in an ultrahigh vacuum chemical vapour deposition reactor by growing layers for various times and measuring the resulting thicknesses. The pressure influence on the epitaxial as well as on the polycrystalline Si growth rate is discussed. We observed that nucleation of Si on Si as well as on SiO2 is retarded. A model is proposed which explains this incubation time for the nucleation of Si on Si and the influence of the pressure on it. The addition of GeH4 to the SiH4 growth environment is found to retard the nucleation on SiO2 even longer. On the other hand, a pressure increase and the addition of B2H6 accelerate nucleation. Epitaxial layers can be grown selectively on Si by limiting the growth period to below the incubation time of polycrystalline material. Maximum selective thicknesses for various types and combinations of layers have been determined.