Size dependent phenomena during the formation of Gd and Fe silicide thin films

The phase formation was investigated and compared during the solid phase reaction of Gd thin film with (111) and (100) oriented Si substrate and Fe thin films with Si(111) substrate as a function of thickness and annealing by X-ray diffraction, Rutherford backscattering spectroscopy and transmission electron microscopy. For thin Gd films the phase formation was affected by the substrate orientation. The first phase was amorphous on Si(100). At higher temperatures on Si(100) epitaxial orthorhombic GdSi2 was formed and on Si(111) epitaxial hexagonal GdSi1.7 was found. For thicker gadolinium films on Si(111) a conventional diffusion-reaction process appeared. On Si(100) substrate orthorhombic GdSi2 phase was formed only. Another type of phase evolution could be experienced in case of Fe-Si solid phase reaction at constant annealing as a function of the initial iron film thickness. FeSi phase was detected in the thinner samples. Samples with Fe layer thicker than 12.5 nm contained a beta-FeSi2 phase formed by nucleation controlled mechanism. This special phase sequence was explained with the help of a model, based on the critical radius of nuclei of the new phase. The phase formation depended on the time and temperature of the annealing and even on the initial metal film thickness and substrate orientation.