Optical properties of silicon nanocrystals embedded in Si3N4 matrix measured by spectroscopic ellipsometry and UV-Vis-NIR spectroscopy

Abstract

In this paper, we report a spectroscopic ellipsometry study of the optical properties of silicon nanocrystals (Si-ncs) embedded in silicon nitride matrix. The nanocomposite thin-films were elaborated by radiofrequency plasma enhanced chemical vapor deposition from ammonia and silane precursors, followed by high temperature annealing. Bruggeman effective medium approximation combined with the Tauc–Lorentz dispersion law was found to be an appropriate model in describing the ellipsometric data, and provided a fine determination of the dielectric functions or complex permittivity of Si-ncs embedded in silicon nitride. It is shown that the dielectric functions of Si-ncs undergo a large reduction in amplitude and broadening compared to the dielectric function of the bulk crystalline Si. Consequently to the disappearance of direct transition energy E1 and E2, the imaginary part ϵ2 of the dielectric function of Si-ncs exhibits a single line shape centered between E1 and E2. With decreasing Si-ncs size, we observe a red-shift of ϵ2 which cannot be attributed to bandgap expansion, but is better explained by electron–phonon interactions in the case of a Si3N4 matrix with high Young modulus. According to Tauc–Lorentz dispersion law, the obtained bandgap values of Si-ncs are between 1.58 eV and 1.67 eV for Si-ncs with diameters from 4.6 nm to 3.8 nm, which is in good agreement with measurements from UV-Vis-NIR spectroscopy.