A commercially available 300 mm single-wafer UHV-CVD reactor was used to grow strained pseudomorphic Si0.79Ge0.21 layers beyond the critical thickness on Si substrates. A unique in situ method of introducing controlled amounts of oxygen at the growth interface, combined with a very sensitive defect etching technique, was used to study the crystal defects present in the strained SiGe layers immediately after growth and after thermal annealing over large areas (many square centimeters). The etching results showed that the density of growth-related defects (stacking faults) originating from residual O at the epitaxy/substrate interface increases exponentially when the interfacial O concentration exceeds a level of about 1013 O/cm 2 as measured by secondary ion mass spectroscopy. By defect etching the annealed samples we demonstrate that all the strain-relieving defects (dislocations) we observed originate from these growth defects. Therefore, we confirm previous claims that the strain metastability of low-misfit SiGe layers is controlled mainly by the initial concentration of growth defects. These findings have important implications regarding historical studies of metastability in the strained SiGe/Si system as well as contemporary efforts to grow highly metastable strained layers for IC applications. © 2010 Elsevier B.V. All rights reserved.