Bulk silicides and Si-metal interface reaction:Pd2Si

We report synchrotron-radiation photoemission studies of cleaved ${\mathrm{Pd}}_2$Si in which we use $4d$ Cooper minimum and $4p\to 4d$ resonance techniques to identify the bonding Si $p-\mathrm{Pd} d$ states 3.5-6.5 eV below $E_F$, the Si $s$ states at 9.3 eV, the nonbonding Pd $4d$ states centered at 2.5 eV, and hybridized Si-Pd or Pd $\mathrm{sp}$ states within 1.5 eV of $E_F$. The resonant photoemission technique was shown to yield detailed information about the orbital character of the valence states and to be applicable in principle to all $\mathrm{Si}-d$-metal systems. Comparison of bulk ${\mathrm{Pd}}_2$Si with palladium silicide thermally grown on Si(111) shows the main $4d$ emission feature at 2.8 eV shifted 0.3 eV to higher binding energy relative to bulk ${\mathrm{Pd}}_2$Si, an increased full width at half maximum (2.5 versus 1.5 eV), and substantially greater emission from Si $p$- metal $d$ bonding states. This confirms that the palladium silicide formed on Si is Si rich near the silicide-vacuum interface and shows that the electronic structure of the uppermost silicide layers differs from that of bulk ${\mathrm{Pd}}_2$Si.