Buried Si1−xGexlayers grown on Si at elevated temperatures of 700 to 800 °C generally exhibit x‐ray rocking curves which are significantly broader than those predicted for perfect crystals, suggesting that the layers are strain‐relieved. However, calculations using these rocking curves show the materials to be either nearly‐ or fully‐strained. The source of x‐ray broadening accompanied by high strain is found to be an abrupt, thermally‐induced fragmentation of the layer into small, slightly misoriented regions during the cool‐down, such that the as‐grown strain remains unchanged. The fragments are typically rectangles a few micrometers wide, with well‐defined boundaries along [110]‐type directions.