X‐ray reflectivity is applied to investigate the effect of a surfactant on the growth of Ge1−xSix/Si superlattices. It is demonstrated that the antimony layer deposited on the surface can effectively prevent the intermixing of silicon and germanium. The specular reflectivity curves show that the width of the interface is sufficiently reduced by the surfactant. The transverse scans show that the interface roughening exponenthfor the sample with surfactant is larger than for the sample without surfactant, and the in‐plane correlation length for the former is much larger than for the latter. This indicates that the surfactant makes less jagged and smoother interfaces and induces a different surface growth mode. ©1995 American Institute of Physics.