Surface states and Fermi-level pinning at epitaxial Pb/Si(111) surfaces

The difference in Schottky-barrier height of epitaxial Si(111) (√3 × √3 )R30°-Pb(β) and Si(111)(7×7)-Pb interfaces, supports the view that the Schottky-barrier height at these interfaces is not determined by bulk properties of the metal and the semiconductor, but that it depends on the local geometry and electronic structure at the interface. To elucidate the relation between the interface electronic structure and the Schottky-barrier height, we performed an angle-resolved photoemission study on the Si(111)(7×7)-Pb and the Si(111)(√3 × √3 )R30°-Pb(β) surfaces. The electronic structures of these two surfaces are rather similar. Two different surface-state bands were resolved. One of them is fully occupied and is situated below the valence-band maximum (VBM). This state is interpreted as a Si dangling-bond state that is hybridized with Pb 6${\mathit{p}}_{\mathit{x}}$,${\mathit{p}}_{\mathit{y}}$ orbitals. The other state pins the Fermi level at approximately 0.1 eV above the VBM. This state has no measurable dispersion and appears in regions of k space where a common gap is present in the projected band structure of Pb and Si. Therefore, this surface state may become a true interface state at thick overlayers, pinning the Fermi level of the Si(111)(√3 × √3 )R30°-Pb(β) Schottky diodes near the bottom of the energy gap.