Formation process of beta-FeSi from amorphous Fe-Si synthesized by ion implantation: Fe concentration dependence

Formation processes of beta-FeSi(2) from amorphous Fe-Si layers have been investigated using transmission electron microscopy (TEM). Si(111) substrates were irradiated with 120 keV Fe ions at -150 degrees C to fluences of 1.0 x 10(17) and 4.0 x 10(17) cm(-2). An amorphous Fe-Si layer embedded in an amorphous Si was formed in the low-fluence sample, whereas an amorphous Fe-Si surface layer on an amorphous Si was obtained in the high-fluence one. The amorphous Fe-Si layers were crystallized to beta-FeSi(2) after thermal annealing at 800 degrees C for 2 h. Cross-sectional and plan-view TEM observations revealed that, prior to the formation of beta-FeSi(2), the amorphous Fe-Si layers crystallized to alpha-FeSi(2) in the low-fluence sample and to epsilon-FeSi in the high-fluence one. The absence of metastable gamma-FeSi(2) which is considered as a precursor of epitaxially grown beta-FeSi(2) on Si was attributed to the instability of gamma-phase in an amorphous matrix.