Crystallization of SiO by HE⁺ bombardment

Amorphous silicon monoxide (∼100 nm thick), prepared by evaporation on rock salt, was bombarded with 80 keV He+ ions. Examination by transmission electron microscopy revealed that crystalline regions up to 15 nm diameter were produced throughout the amorphous matrix when the ion flux was greater than 2.0 × 10l8 ions/m2-sec in which the sample temperature was 850 ± C due to beam heating. Heating unirradiated samples to this temperature did not cause crystallization. The crystalline precipitates were identified as silicon using electron diffraction. The Si precipitates are thought to be initially present in unirradiated SiO and grow to an observable size during bombardment. The crystallization process followed nucleation and growth kinetics, and was found to saturate after .8% of the SiO had transformed to crystalline Si at a dose of 1.5 × 1022 ions/m2. Silicon monoxide fits into a group with other amorphous nonmetallic solids that have a crystallization to melting temperature ratio (Tc/Tm) greater than 0.30 and partially crystallize under ion bombardment. These materials form crystalline precipitates by an extended diffusion of atoms, and in compounds the stochiometry of the precipitate is typically lower than that of the initial amorphous matrix due to preferential sputtering effects on one of the species. On the other hand amorphization of crystalline nonmetals withTc/Tm>0.30 and crystallization of amorphous nonmetals withTc/Tm < 0.30 are not diffusional processes but rather transform in a localized region surrounding the individual ion paths.