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>4 nm 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.