DIPOLE WAVES IN SEMICONDUCTORS - THE DIELECTRIC FUNCTION AND PLASMA-OSCILLATIONS OF SILICON

The relationship between bond polarisability, local fields and the die lectric function of bulk silicon is investigated using the discrete di pole model. In this model a valence electron pair (a bond) is treated as a point-like polarisable unit which responds to the local field at the bond by creating a dipole moment there. We assume an unretarded fo rm for bond-bond interactions (local fields) and find the normal modes of an infinite lattice of bonds with the diamond structure. We call t hese normal modes dipole waves and express the dielectric function for bulk silicon in terms of them. Dipole waves corresponding to optical transitions and bulk plasma oscillations are identified and it is show n that the experimental peak positions and intensities in both the die lectric function and the dielectric loss function of silicon are repro duced by this model. The form of bond-bond interaction chosen is the e lectrostatic interaction between point-like dipoles but this approxima tion breaks down at short range. Bond-bond interactions at short range and bond polarisabilities are therefore calculated from ab initio clu ster calculations. The static dielectric function is obtained as a fun ction of bond polarisability using point dipolar local fields and loca l fields corrected at short range for the finite size of a bond. Compa rison is made with the Clausius-Mossotti relationship between dielectr ic function and polarisability. (C) 1997 Elsevier Science Limited. All rights reserved.