A comparative study on the dielectric functions of isolated Si nanocrystals and densely-stacked Si nanocrystal layer embedded in SiO2 synthesized with Si ion implantation

Both isolated Si nanocrystals (nc-Si) dispersedly distributed in a SiO2 matrix and densely stacked nc-Si layers embedded in SiO2 have been synthesized with the ion implantation technique followed by high temperature annealing. The dielectric functions of the isolated ne-Si and densely-stacked nc-Si layer embedded in SiO2 have been determined with spectroscopic ellipsometry (SE) in the photon energy range of 1.1-5 eV. The dielectric functions of these two different Si nanostructures were successfully extracted from the SE fitting based on a multi-layer fitting model that takes into account the distribution of nc-Si in SiO2 and a five phase model (i.e., air/SiO2 layer/densely-stacked nc-Si layer/SiO2 layer/Si), respectively. The dielectric spectra of isolated nc-Si distributed in SiO2 present a two-peak structure, while the dielectric spectra of densely-stacked nc-Si layer show a single broad peak, being similar to that of amorphous Si. The dielectric functions of these two Si nanostructures both show significant suppressions as compared with bulk crystalline Si. However, it has been observed that the densely stacked nc-Si layer exhibits a more significant suppression in the dielectric spectra than the isolated nc-Si dispersedly embedded in SiO2. This is probably related to the two factors: (i) the nc-Si size (similar to 3 nm) of the densely stacked nc-Si layer is smaller than that (similar to 4.5 nm) of the isolated nc-Si embedded in SiO2 matrix, and (ii) the densely stacked ne-Si layer has an amorphous phase.