Epitaxial silicon films have been grown on gallium phosphide by molecular beam and solid phase epitaxy or combinations of both methods. During molecular beam epitaxial growth Ga segregates on top of the Si films, which can be considerably reduced by solid phase epitaxy but not completely suppressed. The cause of segregation is investigated using Rutherford backscattering, defect etching, and scanning electron microscopic inspection. We conclude that imperfections in the epitaxial layer act as diffusion pipes for atoms.