Optical and phonon excitations of modified Pandey chains at the Si(111)-2×1 surface

Abstract

The modified Pandey $\ensuremath{\pi}$-bonded chain structure for the (2$\ifmmode\times\else\texttimes\fi{}$1) reconstruction of the Si(111) surface has two nearly degenerate structures in which one or the other of two atoms in the $\ensuremath{\pi}$ chain is the outermost atom at the surface. Recent experimental data suggest that both structures can be found on a single Si(111) surface. Surface structures, reflectance anisotropy spectra, and the phonon contribution to the surface conductivity are calculated using a hybrid density functional theory (DFT) method. The modified Pandey chain with a positive tilt of the atoms in the $\ensuremath{\pi}$ chain has the lowest energy. A phonon around 50 meV polarized parallel to the $\ensuremath{\pi}$ chains has anomalously large, anisotropic Born effective charges and dominates the conductivity. The hybrid DFT approach is suitable for calculating excitations in complex surface and interface structures with large unit cells.