The deposition of in situ boron doped silicon films from boron trichloride BCl3 and silane SiH4 in a conventional low-pressure chemical vapour deposition reactor has been studied for high boron doping levels and two kinds of substrates (SiO2 and Si3N4)On the basis of transmission electron microscopy and X-ray photoelectron spectroscopy results, these films appear to be highly sensitive to the local electronic environment of both substrate and deposited atoms. Indeed, beyond a critical doping level, this material becomes more and more amorphous, due to the occurrence of a particular organization of boron atoms in the silicon matrix. This behaviour results in a lowering of the well-known boron enhancement effect for deposition rate and crystalline fraction. (C) 2003 Elsevier B.V, All rights reserved.