First-order dielectric susceptibilities of tetrahedrally coordinated semiconductors

This work describes the computation of the first-order dielectric susceptibilities of tetrahedrally coordinated semiconductors, within a molecular model. It allows the discussion of several approximations which have been frequently used in the study of second-order dielectric susceptibilities and the estimation of local-field effects. First of all, it is shown that the accuracy of a bonding-antibonding model decreases with increasing ionicity. Secondly, using a method of moments and a model curve for the ${\epsilon }_2\mathrm{}\left(E\right)\mathrm{}$ spectrum, reasonable values of the low-energy threshold of ${\epsilon }_2\mathrm{}\left(E\right)\mathrm{}$ as well as of the average dielectric gap defined by Phillips are obtained. Then local-field effects are estimated. A Lorentz-Lorenz correction seems to be valid, if one includes the drastic reduction due to self-polarization effects. Finally, the separation of the contributions of bond charge and charge transfer to ${\epsilon }_1\mathrm{}\left(0\right)\mathrm{}$ is discussed.