Optical functions of semiconductors beyond density-functional theory and random-phase approximation

The linear optical response of semiconductors has been studied beyond the density-functional theory with ab initio pseudopotentials and the random-phase approximation. Effects of the macroscopic local fields and the microscopic exchange-correlation interaction are included in the description of the optical spectra. Quasiparticle corrections to the single-particle energies have been added in the polarization function. Numerical calculations are performed for the group-IV materials Si, SiC, and diamond as model substances. In the static limit and in the low-frequency region, corrections due to the local fields reduce the dielectric function, whereas inclusion of the exchange-correlation interaction enhance the oscillator strengths. In the high-energy region these effect have a more complex character, and the signs of the relevant corrections change in dependence on the photon energy. The effects considered strongly modify the plasmon resonance in the energy-loss function. The results obtained are discussed in comparison with theoretical and experimental data available.