CEMS study of iron disilicide fomation by ⁵⁷Fe ion implantation into silicon

Doses from 2 x 10¹⁵cm^-2 to 2 x 10¹⁷cm^-2 of ⁵⁷Fe ions were implanted with 40 keV into n-type Si(111) at 350°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 Mössbauer spectroscopy. Depending on the implanted dose two different regions of phase formation are found. For doses £ 1 x 10¹⁶cm^-2 only the metastable g-FeSi₂ is present whereas at values ³ 2 x 10¹⁶cm^-2 a mixture of a- and b-FeSi₂ is formed. The phase composition in this second region is dose dependent and with increasing iron content an enlarged fraction of the b-phase is found. At the highest dose of 2 x 10¹⁷cm^-2 mostly b-FeSi₂ and no a-FeSi₂ is present. But additionally to the b-FeSi₂ a second component occurs in the Mössbauer 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°C and 1150°C lead to the formation of the pure b- and a-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°C precipitates of the a-FeSi₂ grow in the Si matrix.