In this paper a theoretical study of the formation of a heterojunction between a Si (001) substrate and a very thin film of the more atomically dense beta -SiC (001) is presented. The structures that describe the deposition of the first layer of carbon and then the subsequent deposition of two further layers, one of silicon and the other carbon, to form the polar SiC film are discussed. In each case the atomic structure is determined from minimizing the total energy with respect to variations in the atomic coordinates. The formation of the interface between the polar SiC film and the silicon substrate is found to give rise to a distinct dipole within the top layers of the silicon substrate. Valence electron charge is transferred from the first silicon layer to the second silicon layer.