SURFACE SEGREGATION OF SILICON IN PLATINUM(111)

We present a study of an ultrathin surface layer of platinum silicide formed on a Pt(lll) crystal as a result of surface segregation of Si a nd Ca trace impurities. The structure and composition of this gated wi th low energy He+ ion scattering, (LEIS), x-ray photoelectron spectros copy (XPS), low energy electron diffraction (LEED), and scanning tunne ling microscopy (STM). Si surface segregation onto Pt(lll) is thermall y activated; annealing at temperatures between 750 and 1100 K results in the formation of a surface silicide. The highest Si coverage that c an be reached is 0.4 monolayers; the Ca coverage at saturation is belo w 0.02 monolayers. The enthalpy of Si segregation is found to be -(105 +/-30) kJ/mole. A well-ordered (root 19 X root 19)R23.41 degrees or /( 3)(-2) (2)(5)/ structure is observed by LEEDS and STM; the amount of s urface area covered with this structure is proportional to the Si cove rage measured with LEIS. At low annealing temperatures up to 800 K, tw o domains coexist with/(3)(-2) (2)(5)/ and /(2)(-3) (3)(5)/ orientatio ns, but only the first one is stable at Si saturation coverage. No lar ge relaxations of substrate interatomic distances are detected upon fo rmation of the overlayer. (C) 1996 American Vacuum Society.