Electronic and atomic structure of the Cu/Si(111) quasi-5×5 overlayer

The quasi-5\ifmmode\times\else\texttimes\fi{}5 layer formed by annealing a monolayer of Cu on a Si(111) surface has a so-called quasiperiodic structure that differs significantly from both transition-metal silicides and metal-induced reconstructions. We have therefore performed detailed angle-resolved uv photoemission (ARUPS) measurements and ab initio band-structure calculations to investigate the atomic structure of the quasi-5\ifmmode\times\else\texttimes\fi{}5 layer and the unique bonding behavior it embodies. ARUPS results are dominated by two Cu 3d peaks separated by 0.7 eV. The intensity variation of these peaks with emission and incidence angles suggests an ordered planar layer, yet there is considerable inhomogeneous broadening. A Si 3p\char21{}derived surface state is also observed 1.2 eV below the Fermi level. Two atomic models are considered in light of these results: a widely cited nearly planar ${\mathrm{CuSi}}_{2}$ model with interstitial Cu atoms and a substitutional CuSi model. In electronic-structure calculations using the pseudofunction method of Kasowski et al., the CuSi model agrees much better than the ${\mathrm{CuSi}}_{2}$ model with ARUPS in the energy differences between Cu 3d states, in their energies relative to the Fermi level, and in the surface-state behavior. Computed results for the CuSi model also account for features seen in current-voltage relationships in scanning tunneling microscopy, the Cu atom height measured with x-ray standing waves, the observed nonreactivity of the quasi-5\ifmmode\times\else\texttimes\fi{}5 surface, and a vibrational mode at 8 meV detected using helium diffraction. The band-structure calculations show that bonding in the ``5\ifmmode\times\else\texttimes\fi{}5'' CuSi layer is different from that of transition-metal silicides. The formation of Si p\char21{}Cu d bonding hybrid orbitals appears to be important in making the CuSi structure stable, but the Cu 4s orbitals also play a significant role in hybridizing with Si 3p states. It is possible that the quasi-5\ifmmode\times\else\texttimes\fi{}5 layer is a two-dimensional electron phase in which domain boundaries are formed to accommodate a particular [Cu]:[Si] surface stoichiometry different from unity.