Evidence of quantum confinement effects on interband optical transitions in Si nanocrystals

We present evidence for quantum confinement effects on optical transitions in ensembles of Si nanocrystals (NCs) in a SiO2 matrix by considering simultaneously the dielectric function dispersions obtained by spectroscopic ellipsometry, the absorption edge, and the photoluminescence. We find that all these quantities blueshift in a similar manner with decreasing size of the NCs for diameters d below 6 nm. The correlated behaviors of the three observed optical transitions, the optical gap EG, and the critical points E1 and E2, indicate conclusively that their measured blueshifts are associated with the same quantum confinement effect and that the "entire" band structure shifts with d. In addition, our results show that the features of the band structure of Si NCs in the d>4nm range are qualitatively similar to those of bulk Si. In particular, the indirectlike nature of the band gap and the criticality of the L and X points of the bulk Brillouin zone are retained, on the understanding that these concepts cannot be strict in nanocrystals of small dimensions.