Abstract The epitaxial growth conditions and the microstructure of (111) and (100) single-crystal films of silicon and germanium were studied using in situ reflection high energy electron diffraction and using transmission electron diffraction and transmission electron microscopy. The films were grown epitaxially by evaporation in an ultrahigh vacuum system onto a CaF 2 /epi-Si/Si substrate, where epi-Si is epitaxial silicon, at 450–710°C for Si(111), 450–650°C for Si(100) and 400–550°C for Ge(111) and Ge(100). The epi-Si, 2000 A thick, was deposited at 580–750°C onto an Si(111) or Si(100) wafer heat treated at 720–750°C in order to overcome the problem of the initial amorphous silicon oxide. The (111)-oriented films contained double-positioning domains, i.e. (111) twins, up to 2 μm in size. Each of the domains contained triangular islands of the complementary domain up to 1000 A in size. Planar and line defects were concentrated at the (111) twin boundaries. On lowering the growth temperature for Si(111) from 710 to 680°C the defect concentration increased by more than an order of magnitude. Hexagonal-phase particles of up to 2000 A in size with a density of 10 6 cm -2 were also present in the (111)-oriented films.