Stoichiometry and structural disorder effects on the electronic structure of Ni and Pd silicides

Synchrotron-radiation ultraviolet photoemission studies of the electronic structure of both stoichiometric epitaxial silicides (Ni${\mathrm{Si}}_2$ and ${\mathrm{Pd}}_2$Si) and metal-rich phases (${\mathrm{Ni}}_x\mathrm{Si}$ and ${\mathrm{Pd}}_x\mathrm{Si}$) obtained by low-energy ion sputtering (1 keV) are performed. The data for thick (1000-2000 Å), annealed epitaxial stoichiometric silicides (Ni${\mathrm{Si}}_2$ and ${\mathrm{Pd}}_2$Si) are in good agreement with first-principles and semiempirical calculations, indicating little variation of the surface electronic structure with respect to the bulk. The spectra associated with varying average metal concentrations ($x=0.5\mathrm{} \mathrm{to} 3.5$ for nickel silicides and $x=2\mathrm{} \mathrm{to} 7.6$ for palladium silicides) are dominated by the local atomic bonding configuration: For nickel silicides, the Ni${\mathrm{Si}}_2$ structure is replaced by NiSi and ${\mathrm{Ni}}_2$Si subunits in a discrete fashion as the average metal concentration increases, while, for palladium silicides, the ${\mathrm{Pd}}_2$Si local bonding remains dominant. A large number of ${\mathrm{Ni}}_x\mathrm{Si}$ and ${\mathrm{Pd}}_x\mathrm{Si}$ mixtures could be obtained by preferential-sputtering and partial-annealing techniques. Our studies of these mixtures which have different microscopic local configurations may be useful in modeling metal-silicides and metal-silicon interfaces.