Thin films of semiconducting iron disilicide (β-FeSi2) with up to 8 at. % Cr addition grown on Si(001) and Si(111) substrates were studied by spectroscopic ellipsometry as well as transmission and reflection measurements at room temperature. The dielectric function was deduced in the interband spectral range. In molecular beam epitaxy (MBE) preparation part of the Fe atoms were substituted by Cr during deposition. For a low Cr amount in the doping range up to ∼0.4 at.%, Cr was found to modify epitaxial growth on Si(111) substrates with a change in dominating β-FeSi2 grain orientation. Higher amounts of Cr lead to the precipitation of CrSi2, which was detected optically and confirmed by x-ray diffraction measurements, and a deterioration of film morphology. Furthermore, β-FeSi2 thin films were implanted with Cr and subsequently annealed at various temperatures. In these samples also CrSi2 was detected. The results suggest that it is impossible to produce β-(Fe1-x,Crx)Si2 alloys (x≫0.003) by MBE or ion implantation. © 2001 American Institute of Physics.