Shape evolution of SiGe islands fabricated by ultra-high vacuum chemical vapor deposition (UHV/CVD) using different SiH4 flow was investigated by atomic force microscopy. It is found that the threshold size at which SiGe islands change from pyramids to domes (44, 50 and 65 nm) increases with Si content in islands (0.032, 0.09 and 0.20). With Si content being considered, an improved model was established to explain the dependence of threshold size on Si content. According to the model, the threshold size as well as the shape and size distribution of self-assembled SiGe islands could be effectively controlled by adjusting Si content in islands (here using different SiH4 flow during growth). Based on our model, threshold size of self-assembled SiGe islands can be calculated from Raman scattering spectra. (c) 2006 Elsevier B.V. All rights reserved.