TEM was used to observe lattice defects in Si0.75Ge0.25 islands and in the film subsequently grown by conventional LPCVD at 680 degrees C on (100)-oriented and 0.5 degrees-misoriented Si substrates without any buffer layers. Large isolated islands grew on the oriented substrate. The lattice-mismatched islands, which contained the defects originated from the surface contaminants, released their strain energy by generating lattice-misfit dislocations and thus the thick film was relaxed. Reducing the growth temperature decreased the size of the islands, which enabled a strained film to be grown. Many coalesced islands grew on the misoriented substrate at 680 degrees C. No misfit dislocations were observed in the thick film. This was thought to be due to the increase in the nucleation sites depending on the step density of the substrate surface. In contrast to the undoped alloy, the highly B-doped alloy grew coherently on the oriented substrate at 680 degrees C. The high B-doping seems to reduce lattice strain so that the alloy growth initiates in a two-dimensional growth mode.