Disordered structure of cubic iron silicide films on Si(111)

X-ray diffraction has been used to analyze a series of thin-film samples of Fe{sub {ital x}}Si as a function of thickness and the composition variable {ital x}. Unannealed samples are found to be composed entirely of a CsCl-type structure not present in the bulk phase diagram. A careful crystallographic analysis is used to show that the variable composition is accommodated by vacancies on the Fe sites. The unit cells of the film are nearly lattice matched to the substrate, which results in a rhombohedral distortion; this lateral strain is compressive for samples near the FeSi composition and tensile for samples near the Fe{sub 0.5}Si composition. Films annealed to 630 {degree}C are also lattice matched to the substrate but composed of the {alpha}-FeSi{sub 2} structure.