We present evidence for quantum confinement effects on optical transitions in ensembles of Si nanocrystals (NCs) in a ${\text{SiO}}_2$ 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 $E_G$, and the critical points $E_1$ and $E_2$, 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 $d>4\text{nm}$ 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.

• Received 31 March 2010

DOI:https://doi.org/10.1103/PhysRevB.82.045302