Dependence of optical properties on the thickness of amorphous Ge30Se70 thin films

In order to calculate the optical properties of evaporated Ge30Se70 thin films and their relation to thickness, amorphous Ge30Se70 thin films of different thicknesses were deposited by thermal evaporation at a base pressure of 7.5 × 10−6 Torr at room temperature. The optical transmission and reflection spectra of all films were measured in the wavelength range of 0.2–2.5 μm. Efficient parameterization of the spectral dependence of the optical constants of amorphous Ge–Se thin films was obtained by applying a suitable dielectric function model. The O’Leary, Johnson and Lim (OJL) model, based on joint density of states functions, was used to analyze the optical spectra. The best fit was obtained by configuring the film as two layers, the top layer consisting of bulk Ge30Se70 material embedded in air medium containing different volumes of voids. Therefore, the OJL model coupled with the Bruggeman effective-medium approximation model were used to determine the optical constants of the Ge30Se70 thin films. The photon energy dependence of the dielectric function, ɛ = ɛr − iɛi of the investigated films is presented. The film thickness, absorption coefficient α, refractive index n, extinction coefficient k, static refractive index n(0) and optical band gap Eg were deduced. We found that increasing the film thickness increased the direct optical energy gap and decreased the refractive index.