Si(111)-Pt interface at room temperature: A synchrotron radiation photoemission study

We present extensive results on synchrotron-radiation angle-integrated photoemission from Si(111) surfaces onto which increasing amounts of Pt (coverages $\Theta$ from 0.07 to 40 monolayers) were deposited. Both core lines (Si $2p$ and Pt $4f$) and valence-band states have been measured. In the latter case we present results taken at a photon energy of $hv=80\mathrm{}$ eV where the Pt $5d$ contribution is dominant and at $hv=130\mathrm{}$ eV where the Cooper minimum effect reduces the Pt $5d$ photoemission considerably so that information on the Si contribution to the valence states can be revealed. We show that submonolayer coverages ($\Theta \ge 0.07$) of Pt disrupt the surface sufficiently to introduce considerable changes in the photoemission spectra with respect to that of clean Si(111). An interface with a photoemission spectrum resembling that of a silicide has developed at about 2-10 monolayers. At increasing $\Theta$ the region explored with photoemission shows enrichment of the metal, but the situation at $\Theta =40\mathrm{}$ is still very far from that of the pure Pt metal, thus indicating a very strong chemical interaction at room temperature on the depth scale of tens of monolayers. Opposite chemical shifts of the Si and Pt core lines are seen (Pt towards lower and Si towards higher binding energies with increasing $\Theta$) and, moreover, the shape of the Si $2p$ core lines is modified towards that typical of a metallic phase. All these results are discussed in terms of the nature of the chemical bond between Si and Pt.