Origin of dislocation-related photoluminescence bands in very thin silicon-germanium layers grown on silicon substrates

We measured the photoluminescence spectra of very thin and partially strained Si_1-xGe_x (0.2≤x ≤0.5) layers grown on silicon substrates with varying degrees of strain relaxation. We observed photoluminescence lines, so-called D lines, which arise from dislocations in the Si_1-xGe_x/Si alloys. Surprisingly, we observed no D lines originating from the Si_1-xGe_x layers. We identify the origin of the D lines 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.