Electronic properties and bonding sites for chlorine chemisorption on Si(111)-(7×7)

The adsorption of chlorine on Si(111)-(7×7) has been studied with various photoemission techniques with the use of synchrotron radiation. At room temperature, bonding of one, two, and three chlorine atoms to a Si surface atom was observed with Si 2p core-level shifts of 930, 1830, and 2730 meV to higher binding energy, respectively. After annealing to 400 °C only monochloride species remained. For the annealed (400 °C) ordered Cl/Si(111)-(7×7) system four chlorine-induced valence states could be distinguished with polarization-dependent angle-resolved photoelectron spectroscopy, i.e., two nondispersive bonding Cl-Si states: σ ($E_B$=7.21 eV below the top of the valence band), s ($E_B$=8.7 eV), and two dispersing Cl-Cl π states of different symmetry ${\pi }^{+}$ ${\pi }^{\mathrm{-}}$). The dispersion of the ${\pi }^{+}$ (${\pi }^{\mathrm{-}}$) band is Δ$E^{+}$=1000 meV (Δ$E^{\mathrm{-}}$=320 meV) along Γ¯ M¯ and Δ$E^{+}$=900 meV (Δ$E^{\mathrm{-}}$=450 meV) along Γ¯ K¯. The photoemission angular distribution of the σ and s states shows maximum intensity in the gaps of the projected Si band structure and attenuated emission where the chlorine bands overlap with Si bands. The attenuation is explained by hybridization with the substrate bands.