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-degrees-C, ion mixing resulted in an amorphous alloy of Y and Si. For temperatures of 125-190-degrees-C, we observed formation of the YSi phase. YSi is not formed during thermal anneals of bilayers. Ion mixing at higher temperatures (greater-than-or-equal-to 205-degrees-C) results in the formation of the stable YSi1.7 phase. Such sequential silicide formation has not been observed for comparable rare-earth silicides. The minimum temperatures for ion-induced YSi1.7 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.