Electronic and Structural Properties of a Twin Boundary in Si

A second-order twin boundary in Si, of the $\Sigma 9$ type, is investigated with first-principles density-functional theory and with an empirical tight-binding model. Of two proposed reconstructions studied, the one supported by recent experiment is energetically favored, with less bond stretching than the other. A conduction-band-edge interface electronic state associated with bond-angle strains is found. A phonon resonance, the interfacial analog of a Rayleigh mode, is predicted; Raman spectroscopy should be able to detect it.