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J Chem Phys. 2008 Nov 14;129(18):184701. doi: 10.1063/1.3009223.

Influence of the implantation profiles of Si(+) on the dielectric function and optical transitions in silicon nanocrystals.

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Universite Paul Verlaine-Metz, Labo: LPMD, 1 Bd Arago, 57078 Metz, France.


We report optical characterization of silicon nanocrystals embedded in silica thin films by spectroscopic ellipsometry (SE). Silicon nanocrystals (nc-Si) are produced by single energy ion implantation and multienergy Si(+) ion implantation into 250 nm of thermal oxide (SiO(2)) layers on silicon substrate. After thermal annealing, the obtained nc-Si have a Gaussian and uniform profiles for single and multienergy implantation, respectively. SE measurements are performed at room temperature at spectral range from 0.6 to 6.5 eV using the photoelastic modulated spectroscopic ellipsometer. Physical models based on the Maxwell-Garnet approximation combined with Forouhi-Bloomer dispersion formulas and wavelength by wavelength inversion are developed to extract the optical parameters of the layers. The complex dielectric function epsilon(E)=epsilon(r)(E)-iepsilon(i)(E) of nc-Si is determined and analyzed. The obtained epsilon(E) spectra of both uniform and Gaussian profiles are given and compared with those of bulk Si. The nc-Si exhibit a significant reduction of the dielectric function in comparison with bulk Si. We have determined the optical transitions E(1) and E(2) corresponding to Van Hove singularities in the joint density of states. A reduction of the amplitude of E(1) peak with a very weak shift of its energy position is observed. The transition E(2) is characterized by a rather broad peak; the amplitude of this peak is more important than that of E(1). The extended Forouhi-Bloomer model to semiconductor is also used to determine the dielectric functions of nc-Si and optical transitions. In epsilon(i)(E) spectra of nc-Si we have observed that not only the optical transition E(1) peak reduced but it tends to disappear and to form with E(2) only a single broad peak centered at around 4.3 eV. The influence of the distribution profile on the sample's structural and optical characteristics is also investigated. Defects caused by implantation are identified by analyzing the dielectric function behavior. For more reliability, photoluminescence analysis are used to obtain direct optical responses of nc-Si.

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