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 (NiSi₂ and Pd₂Si) and metal-rich phases (NiₓSi and PdₓSi) obtained by low-energy ion sputtering (1 keV) are performed. The data for thick (1000-2000 {}), annealed epitaxial stoichiometric silicides (NiSi₂ and Pd₂Si) 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 to 3.5 for nickel silicides and x=2 to 7.6 for palladium silicides) are dominated by the local atomic bonding configuration: For nickel silicides, the NiSi₂ structure is replaced by NiSi and Ni₂Si subunits in a discrete fashion as the average metal concentration increases, while, for palladium silicides, the Pd₂Si local bonding remains dominant. A large number of NiₓSi and PdₓSi 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.