Permittivity of Ge, Te and Se thin films in the 200-1500 nm spectral range. Predicting the segregation effects in silver

Optical properties of well-known bulk materials can be significantly modified by decreasing dimensions to nm-size. Using Molecular Beam Epitaxy (MBE) and e-beam Physical Vapour Deposition (PVD) we have fabricated 20-30 nm-thick amorphous Ge, Te and Se films. The permittivities of investigated layers have been extracted from measurements of the Ψ and Δ ellipsometric azimuths. We found that for all of the investigated films, the intensity of all bands in the permittivity spectrum is smaller than for bulk materials or thick (> 100 nm) films. Using the acquired optical constants along with the permittivity of a 20 nm-thick silver film, we have applied the Maxwell-Garnett equation to predict the permittivities of a silver film with Ge, Se or Te segregated in its structure. Implementing the parameters of 20 nm-thick Ge results in an 81 nm redshift of the segregation-induced band with respect to the experimental value, while implementing the parameters of 2 nm-thick Ge film results in a 95 nm blueshift of this band.