Growth of Si and SiGe layers using molecular beam epitaxy was carried out with the substrate at a floating, positive or negative bias, in order to investigate effects of ion bombardment on the crystalline quality of grown materials. Although ion energies (100–1500 eV) and ion/atom flux ratios (∼0.005) used in the experiments were quite low, significant lattice distortion along the growth direction (&Dgr;a⊥/asup to ∼300 ppm) was observed by high resolution x‐ray diffraction from the Si layers grown at 420 °C. At the same time, a broadband transition was observed in photoluminescence measurements from both Si and SiGe layers. Based on results of the annihilation behavior during postgrowth treatments using thermal annealing and hydrogenation, we attribute these effects to the ion bombardment induced formation and injection of different types of pointlike defects and defect clusters, which degrade the optical and electrical properties of grown layers. ©1996 American Institute of Physics.