LEED STRUCTURE DETERMINATION OF THE (100)-SURFACE OF A COSI2 CRYSTAL AND A COSI2 FILM GROWN EPITAXIALLY ON SI(100)

The surface structures of CoSi2(100) for a bulk crystal and a 12 Angst rom thin film grown epitaxially on Si(100) are compared by means of qu antitative low energy electron diffraction (LEED). Similar LEED patter ns are observed for both systems corresponding to a c(2 x 2) periodici ty with respect to the Si(100) surface. Also, intensity versus energy curves are very similar for the film and bulk samples. Consequently, w e derive practically the same best-fit model when fitting the experime ntal data with spectra obtained from full dynamical LEED calculations: the CoSi2 surface is arranged in CaF2 structure, terminated by one fu ll Si layer with an additional c(2 x 2) Si overlayer of half monolayer coverage. The top silicon adatoms reside in four-fold symmetric hollo w sites. The unfavourable energetics of this rather peculiar bonding g eometry are obviously overcompensated by the energy gain from the satu ration of dangling bonds on the surface. The geometry found is consist ent with the chemical passivity of the surface.