GdSi2 film with almost perfect interface was grown on a Si(111) substrate via phase transformation assisted by interfacial SiO2 layer. The evolution of Gd silicide and the role of an oxide layer were investigated by using in situ reflection of high-energy electron diffraction, x-ray diffraction (XRD), atomic force microscopy (AFM), x-ray photoelectron spectroscopy, and high-resolution transmission electron microscopy (HRTEM). The XRD and AFM results confirm structural transformation from the initial GdSi1.7 layer to the GdSi2 layer after the post-annealing at 900degreesC. The HRTEM image suggests that the formation of GdSi2 follows kinetic growth process, where the grain growth is dominated by the abundance: of Si at the reacting surface. The thermally decomposed interfacial oxide initiates rapid phase transformation' and finally results in almost perfect GdSi2/Si interface without any residual oxide or mixed structure. (C) 2005 American Vacuum Society.