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 analogue of a Rayleigh mode, is predicted; Raman spectroscopy should be able to detect it. *Laboratory of Atomic and Solid State Physics and Materials Science Center, Cornell University, Ithaca, NY 14853 **Department of Physics, University of Pennsylvania, Philadelphia, PA 19104