Cross-sectional transmission electron microscopy and Raman spectra are used to investigate the effect of monolayer Sb as a surfactant on the inner diffusion of epilayer Ge atoms into Si substrate. Without Sb, inner diffusion of the epilayer Ge atoms into the Si substrate occurs, resulting in a severe intermixing of atoms at the Ge-Si interface. With the presence of Sb as a surfactant, the inner diffusion of epitaxial Ge atoms into the Si substrate is greatly suppressed. This result is explained in terms of the strain relief in the Si substrate by the Sb surfactant.