Linear and non-linear optical dispersion parameters of Te<inf>81</inf>Ge<inf>15</inf>Bi<inf>4</inf> chalcogenide glass thin films for optoelectronic applications

Abstract

Chalcogenide composition of Te81Ge15Bi4 was prepared by the melt quenching technique in bulk form and by the thermal evaporation method in thin film form. The amorphicity of the investigated composition was examined by X-ray diffraction (XRD) analysis. The actual percentage of the prepared film samples was checked by energy dispersive X-ray (EDX) spectroscopy. The transmittance T(λ) of the investigated films in the thickness range from 143 nm to 721 nm was measured in the spectral range (400–2500 nm). Swanepoel's method was used to calculate the linear optical parameters (refractive index (n) and absorption index (k)) from the transmittance T(λ) data. Both n and k are found to be independent of the film thickness through the studied range. The type of allowed transitions was examined and found to be indirect allowed transitions with an optical energy band gap (Egopt) and the optical absorption edge (Ee) equal to 0.652 eV and 0.089 eV using Tauc's extrapolation procedure and Urbach tail, respectively. The optical conductivity σopt of Te81Ge15Bi4 film samples increased with increasing the incident photon energy (hν). The optical dispersion parameters (Eo and Ed), the high frequency dielectric constant ε∞, the average oscillator strength So, the lattice dielectric constant εL, the ratio between the free charged carriers concentration and its effective mass N/m* were obtained from the analysis of the refractive index results. The other parameters related to dielectric constant ε1, dielectric loss ε2, dissipation factor tanδ, relaxation time τ, volume energy loss (VELF) and surface energy loss (SELF) functions show a dependence on photon energy (hν) for all investigated films. The non-linear optical susceptibility and the non-linear index of refraction n2 were also calculated using the linear dispersion parameters.