Chemical termination of the CsCl-structure FeSi/Si(111) film surface and its multilayer relaxation

Metallic FeSi films, epitaxially stabilized on Si(111) in CsCl structure, are investigated experimentally by quantitative low-energy electron diffraction (LEED) and theoretically by total energy calculations using density functional theory (DFT). Both methods show clearly that the surface is Si terminated. Additionally, LEED and DFT agree in retrieving an unusual multilayer relaxation of +6%, $-16\%,$ and +14% from the top layer into the bulk for the first three layer spacings. This relaxation pattern is explained by an enhanced covalent bonding between the subsurface iron and silicon layers.