The microscopic dielectric function in silicon and diamond

Abstract

The microscopic dielectric function epsilon (q) of diamond and silicon was calculated using a pseudopotential scheme. Sufficient plane waves were included to obtain energy convergence to within 0.1 eV. Four valence, seven conduction bands, and 11520 points were used in the sampling of the Brillouin zone. Good agreement with experiment was obtained for the static limit epsilon (0). The effect of the higher conduction bands was investigated and it was shown that it cannot be neglected. The calculated values of epsilon (q) for q>or=q (Fermi) are smaller than those predicted from the simple Penn model. The covalent 222 contribution to the valence charge density in Si was calculated as 0.219 electrons/atom, in agreement with the experimental value 0.22.