In this paper, we study the diffusion of impurities from three types of foreign substrates (graphite, alumina and mullite) during thermal chemical vapour deposition (CVD) of a polycrystalline Si film. For this we use a rapid thermal CVD (RTCVD) system and characterization techniques such as secondary ion mass spectroscopy (SIMS) and deep level transient spectroscopy (DLTS). Results show that, in the case of materials like graphite, metallic contaminants can freely outdiffuse into the deposited layer and the environment. In contrast, the ceramic substrates release only a very limited amount of contaminants during the CVD process, making the need of a diffusion barrier much less severe. (C) 2000 Elsevier Science B.V. All rights reserved.