The effect of silicon substrate orientation on the formation of Gd-silicide phases

The formation and epitaxy of Gd-silicide compounds were investigated in the solid phase reaction of Gd thin film on (111) and (100) oriented Si substrate by X-ray diffraction, Rutherford backscattering and transmission electron microscopy. It was recognised that the effect of substrate orientation on the phase formation became dominant for Gd films thinner than 30 nm. To determine the role of the substrate orientation on the phase formation, 20 nm Gd films were annealed in-situ under the same conditions for (111) and (100) pairs. At low temperature (320°C for 5 min) the first phase (hexagonal GdSi2−x) was formed epitaxially on Si(111), while on Si(100) an amorphous alloy formed. At higher temperatures — 350, 430, 500, 550 and 650°C, and 5 min annealing — epitaxial hexagonal GdSi2−x was found on Si(111), that could not transform into the second phase (orthorhombic GdSi2), in contrast to the case, where films were thicker than 30 nm. Meanwhile on Si(100) epitaxial orthorhombic GdSi2 was found under the same annealing conditions. It was demonstrated, that under a certain thickness (30 nm) the formed phase was determined by the substrate orientation instead of the usual diffusion and reaction processes. Our results show the possibility of amorphous phase formation in GdSi reaction, which is reasonable because of the high mobility difference of the diffusing species.