In this letter, we present experimental evidence on desorption of O and C from a Si surface resulting in impurity levels below the detection levels of secondary ion mass spectroscopy. We then propose a surface preperation method for silicon epitaxy that consists of anexsituclean and aninsitulow thermal budget prebake in an ultrahigh vacuum rapid thermal chemical vapor deposition (UHV‐RTCVD) reactor. Theexsituclean consists of a standard RCA clean followed by a dilute HF dip and rinse in de‐ionized water. Theinsituclean is either carried out in vacuum or in a low partial pressure of 10% Si2H6in H2. The experiments were conducted in an UHV‐RTCVD reactor equipped with oil‐free vacuum pumps. We propose that the responsible mechanism is desorption of oxygen and hydrocarbons from the Si surface due to the low partial pressures of these contaminants in the growth chamber. If Si2H6is used during the prebake, a sufficiently low growth rate is required in order to provide sufficient time for desorption and avoid Si overgrowth on the O and C sites. ©1995 American Institute of Physics.