The initial growth mode of Si on Ge(100) was studied using SiH4 CVD under a reduced-pressure N-2 ambient at 500 and 575 degrees C. We show that, using the appropriate conditions, 3D growth can be avoided and growth occurs in a layer-by-layer mode. To the authors' knowledge, this had up to now not been reported in literature. The critical thickness for relaxation of the Si film was found to be below 2 nm. Relaxation occurs through the formation of misfit dislocations which preserve the 2D character of the Si film. No convincing evidence for Ge up-diffusion during growth at 500 degrees C was found. The epitaxial growth of Ge on Ge(100) from the pyrolysis of GeH4 is also studied under H-2 ambient in both the kinetic and mass-flow controlled regime. The outstanding features are discussed and some analysis is given. (c) 2005 Elsevier B.V. All rights reserved.