Thermal degradation is examined in a development stage in which local damage in the crystal lattice already exists, although the known surface deformations are not yet visible. The existence of these crystal defects is shown by means of luminescence microscopy. Depth profiles reveal that the damage is not restricted to the surface. On ‘‘dark disks,’’ e.g., a spherical spreading of the degradation by some 10 μm into the crystal volume is observed. Correlation with the position of dislocation etch pits shows that all dislocations are ‘‘decorated’’ with thermally induced defects. ‘‘Dark disks,’’ however, represent no advanced stage of degradation after having been ‘‘decorated,’’ but develop separately from a different defect type, visible as ‘‘S’’ or ‘‘flat’’ etch pits. This type of defect has not yet been taken into consideration with regard to crystal quality criteria. We suspect that thermal degradation developing from this defect type is the missing link between substrate quality and certain problems in processing and device failure.