The effects of Ge overlayers on the reaction of one monolayer of Ni on Si(001) substrate were investigated. The samples were then ex situ annealed at 400 to 800 degrees C. The reaction process began with dissolution of the Ni atoms into the amorphous Ge layer. Then, solid-phase epitaxy of the amorphous layer was observed. The Ni atoms were expelled from the crystallized region and redistributed in the amorphous region. Until the concentration of Ni in the amorphous Ge layer exceeded the solubility limit, the Ni atoms diffused through the crystallized Ge layer and reacted with the substrate to form NiSi2. The NiSi2 phase formation was kinetically controlled by the rate of solid-phase epitaxy of the Ge overlayers for the reaction system. (c) 2008 The Electrochemical Society.