First-principles calculation of optical properties:: Application to embedded Ge and Si dots

We calculate both the imaginary and real part of the dielectric function for Ge and Si dots embedded in cubic SiC and other wide-ap semiconductors simulated by a hydrogen coverage. Nanocrystals of up to 239 atoms are studied using supercells of normally 512 atoms. Ab-initio electronic structure calculations are performed in the independent-particle approximation and using a pseudopotential-plane-wave code. Absorption spectra are studied in dependence on the dot size. The results are compared with recent measurements, e.g. for Ge nanocrystals in sapphire.