Structural properties of Si0.7Ge0.3alloy films grown on Si(001) substrate by molecular beam epitaxy have been characterized by means of several available techniques. Different types of buffer layers were predeposited on a clean Si(001) substrate in order to relax the lattice mismatch between the topmost Si0.7Ge0.3alloy layer and Si substrate. The effect of buffer layers on the structural quality of the overgrown Si0.7Ge0.3was investigated by x-ray diffraction, x-ray photoemission spectroscopy, photoluminescence, and cross-sectional transmission electron microscope. It is confirmed that the threading dislocation density in the alloy layer drastically decreases by using buffer layers. The samples with step buffer layers relax the strain by introducing the dislocations at the interfaces, part of which goes through the alloy layer. On the other hand, the samples with superlattice buffer layers relax the strain by introducing the dislocations in the buffer layers which terminate at the interface of the superlattice buffer layer and the topmost alloy layer. The residual strain in the alloy layers on buffer layers grown at 550 °C is relaxed upon the annealing at 700 °C, or the growth at 700 °C. ©1997 American Institute of Physics.