CEMS study of iron disilicide formation by iron ion implantation into silicon

Doses from 2x1015cm-2 to 2x1017cm-2 of 57Fe ions were implanted with 40 keV into n-type Si(1 1 1) at 350 deg. C. The iron concentration profiles reach peak maximum concentrations from 0.3 at.% for the lowest dose up to about 34 at.%. For the highest dose a plateau-like profile is formed due to the effect of sputtering. The phase formation of the iron disilicides was studied by 57Fe conversion electron Moessbauer spectroscopy. Depending on the implanted dose two different regions of phase formation are found. For doses ≤1x1016 cm-2 only the metastable γ-FeSi2 is present whereas at values ≥2x1016 cm-2 a mixture of α- and β-FeSi2 is formed. The phase composition in this second region is dose dependent and with increasing iron content an enlarged fraction of the β-phase is found. At the highest dose of 2x1017 cm-2 mostly β-FeSi2 and no α-FeSi2 is present. But additionally to the β-FeSi2 a second component occurs in the Moessbauer spectrum which is observed for the first time. With respect to the iron concentration this subspectrum could be attributed to a new metastable state of the mono silicide. Different annealings of the as-implanted samples at 900 deg. C and 1150 deg. C lead to the formation of the pure β- and α-FeSi2, respectively. Additionally to the measurements of the concentration profiles investigations of the lateral element distributions in the layers were performed with scanning Auger electron microscopy. It can be concluded that during the annealing at 1150 deg. C precipitates of the α-FeSi2 grow in the Si matrix