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Embedded Si and Ge nanocrystals (NCs) in wide band-gap matrices are studiedtheoretically using an atomistic pseudopotential approach. From small clustersto large NCs containing on the order of several thousand atoms are considered.Effective band-gap values as a function of NC diameter reproduce very well theavailable experimental and theoretical data. It is observed that the highestoccupied molecular orbital for both Si and Ge NCs and the lowest unoccupiedmolecular orbital for Si NCs display oscillations with respect to size amongthe different irreducible representations of the $C_{3v}$ point group to whichthese spherical NCs belong. Based on this electronic structure, first theinterband absorption is thoroughly studied which shows the importance ofsurface polarization effects that significantly reduce the absorption whenincluded. This reduction is found to increase with decreasing NC size or withincreasing permittivity mismatch between the NC core and the host matrix.Reasonable agreement is observed with the experimental absorption spectra whereavailable. The deformation of spherical NCs into prolate or oblate ellipsoidsare seen to introduce no pronounced effects for the absorption spectra. Next,intraconduction and intravalence band absorption coefficients are obtained inthe wavelength range from far-infrared to visible region. These results can bevaluable for the infrared photodetection prospects of these NC arrays. Finally,excited-state absorption at three different optical pump wavelengths, 532 nm,355 nm and 266 nm are studied for 3- and 4 nm-diameter NCs. This reveals strongabsorption windows in the case of holes and a broad spectrum in the case ofelectrons which can especially be relevant for the discussions on achievinggain in these structures.

Interband, intraband and excited-state direct photon absorption of silicon and germanium nanocrystals embedded in a wide band-gap lattice