Free carrier plasma edge and plasmonic excitations in heavily doped surface grated n-type Si

Infrared spectroscopic ellipsometry (SE) in the MID IR region was applied at room temperature to heavily doped n++-Si to obtain information about the free carrier plasma edge and plasmonic excitations before and after the Si surface was grated with a period ? of 294 nm to a nominal depth of 20 nm by using laser lithography. Besides, SE measurements were performed in the NIR-VIS-UV spectral range to validate the optical models for non-grated and grated Si. In the latter case a thin grated layer on silicon surface was alternatively modelled either within Bruggeman effective medium approximation (BEMA) or by treating this layer as an anisotropic thin film (ATF). A model that included SiO2 (2.1 nm), n++-Si substrate and a 24.5 nm-thick ATF for grated layer was found to be good enough to reproduce the obtained ellipsometric parameters in the accessed spectral range. Intraband op-tical transitions are shown to overwhelmingly contribute to the MID IR dielectric function (DF) below the free carrier plasma edge (FCPE) of non-grated n++-Si. The parameters obtained for conduction electrons from Drude fit to this function agree well with the relevant data reported for Si so far. FCPE of non-grated n++-Si is positioned at 1416 cm-1. For graded layer this value is preserved within determination accuracy. The life time of plasmonic excitations that appeared as a single peak with a perfect Lorentz line-shape in the dielectric energy loss function of non-grated Si and grated Si layer is about 70 fs. Shape induced optical anisotropy, along with presumably conical diffraction that identified itself through a distinct angle-dependent upturn in the reflection spectra of p-polarized light incident upon surface in the plane perpendicular to ? is behind the whole set of ellipsometric data obtained for n++-Si with subwavelength grating on the surface.