Excitons and optical spectrum of the Si(111)-(2 x 1) surface

We investigate: excitons at the Si(111)-(2 x 1) surface and their optical s pectrum from first principles. This is done by solving the Bethe-Salpeter e quation for the two-particle Green's function, including the electron-hole interaction. The optical spectrum of the surface is dominated by a surface exciton formed from the pi-bonded surface states. The excitonic binding ene rgy is more than 1 order of magnitude larger than in hulk Si. The two-parti cle wave function of the exciton slate is strongly localized at the surface and exhibits distinct anisotropy due to the surface reconstruction.