Ion-induced formation of stable and metastable phases in the Y-Si system

Bilayers of yttrium and amorphous silicon have been irradiated with 60 keV inert ions. Between liquid-nitrogen temperature and 100°C, ion mixing resulted in an amorphous alloy of Y and Si. For temperatures of 125-190°C, we observed formation of the YSi phase. YSi is not formed during thermal anneals of bilayers. Ion mixing at higher temperatures (≥205°C) results in the formation of the stable YSi_1.7 phase. Such sequential silicide formation has not been observed for comparable rare-earth silicides. The minimum temperatures for ion-induced YSi_1.7 formation agrees with the prediction by a simple model which correlates vacancy mobility to phase transformation. The YSi formation temperature is associated with the onset of radiation-enhanced diffusion. This temperature does not correlate well with the prediction of the model, but agrees with a scaling based on the average cohesive energy.