Luminescence polarization of silicon nanocrystals - art. no. 205301

We consider theoretically the polarization of the luminescence of Si nanocr ystals that ru-ises from the anisotropy of the optical moments. No-phonon a nd phonon-assisted optical transitions are calculated in tight binding and interpreted using effective-mass theory. In contrast to direct-gap semicond uctors, we show that simple selection rules cannot be established in Si nan ocrystals because the degree of linear polarization presents large oscillat ions with respect to the size of the clusters. This effect is due to the in direct nature of the Si band gap that leads to a dependence of the optical matrix elements on the oscillatory overlaps between electron and hole state s in momentum space. However, in a statistical ensemble of crystallites elo ngated in a given direction and with size larger than 2-3 nm, we obtain tha t the light is in average polarized along this direction, in agreement with the experiments.