AbstractWe carry out an investigation of grown-in nonradiative defects in Si and SiGe/Si heterostructures grown by molecular-beam-epitaxy (MBE). A number of such defects are observed by the optical detection of magnetic resonance (ODMR) technique, in samples with various structures and growth conditions. These defects are shown to provide efficient nonradiative shunt paths for carrier recombination, competing with and reducing radiative recombination processes. It is revealed that the dominant nonradiative defect is a low-symmetry vacancy-related complex, evident from a characteristic hyperfine structure due to 29Si ligands (with nuclear spin I=1/2 and natural abundance of 4.67 %) connected to the dangling bonds. The introduction of these defects is believed to be largely due to a low surface adatom mobility during the low temperature growth. By varying the substrate bias during the MBE growth, it is shown that the formation of these nonradiative defects can be effectively enhanced by exposure to accelerated positive ions, presumably dominated by the Si+ ions. Effects of hydrogenation on these defects are also studied.