Optical properties of Ge and Si nanocrystallites fromab initiocalculations. I. Embedded nanocrystallites

We present parameter-free calculations of the frequency-dependent dielectric function in order to understand the optical properties of Ge and Si nanoparticles embedded in a crystalline matrix. The calculations are based upon the independent-particle approximation and a pseudopotential-plane-wave method. The nanoparticles are modeled by clusters of up to 239 atoms embedded in cubic SiC as a host material with a wide energy gap. The dependence of the resulting optical spectra on the nanocrystallite size, the crystallite-host interface, and the crystallite-crystallite interaction is studied.