Study of the physical properties of quaternary Ge–As–Te–Pb thin films for technology applications

Abstract

As the chalcogenide thin films have broad technological applications due to the ability of tuning their properties through composition change, lead addition has been used to tune the energy gap, refractive index, and non-linear optical parameters of Ge₂₅As₁₀Te_65−xPb_x thin films. The optical constants of Ge–As–Te thin films have been studied when introducing different contents of Pb. Tauc’s rule of the allowed indirect transitions was successfully used to describe the optical transitions of the Ge₂₅As₁₀Te_65−xPb_x (0 ≤ x ≤ 10 at. %) thin films. It was found that when increasing Pb content the optical band gap ($E_{\text{g}}^{\text{opt}}$) decreases whereas the index of refraction increases. The obtained behavior of $E_{\text{g}}^{\text{opt}}$ was interpreted in terms of Mott and Davis model. The energy reliance of the index of refraction (n) shows a normal dispersion that may be explained using the single oscillator model. Applying such model allowed to estimate the static index of refraction (n_o) and the energies of both single oscillator (E_o) and dispersion (E_d). In addition, the non-linear optical parameters such as the third-order susceptibility (χ⁽³⁾) and nonlinear index of refraction ($n_{2}^{*}$) have been deduced from E_o, E_d and n_o values. Both χ⁽³⁾ and $n_{2}^{*}$ increase with increasing Pb content.