Oscillator strengths and excitation energies of Ge and Si nanocrystals from ab initio supercell calculations

We demonstrate that strained germanium nanocrystals embedded in hexagonal SiC constitute a type-I heterostructure with localization for both electrons and holes. Nonetheless, the oscillator strengths for the lowest transitions are very small. We relate these results to the pressure dependence of the oscillator strengths and radiative lifetimes in free Ge crystallites. The pressure dependence of the gap energies of the free crystallites as well as the strong reduction of the transition probabilities is interpreted in terms of recent results of the pressure dependence of the respective bulk materials.