We measured the photoluminescence spectra of very thin and partially strainedSi1−xGex(0.2⩽x⩽0.5)layers grown on silicon substrates with varying degrees of strain relaxation. We observed photoluminescence lines, so-calledDlines, which arise from dislocations in theSi1−xGex/Sialloys. Surprisingly, we observed noDlines originating from theSi1−xGexlayers. We identify the origin of theDlines as the penetrated dislocations in the Si substrates extending from the SiGe/Si interface with the assistance of transmission electron microscopy. We discuss possible mechanisms of the dominance of luminescence from the dislocations in the Si substrates over those in the SiGe layers. ©1997 American Institute of Physics.