Optical characterization of the annealing effect on Ge5Te20Se75 thin films by variable angle of-incidence spectroscopic ellipsometry

Ternary Ge5Te20Se75 thin films deposited by thermal evaporation were annealed at different temperatures below the crystallization temperature for 15 min under vacuum. The annealing effect on the optical properties of the Ge5Te20Se75 thin films was studied by means of variable angle spectroscopic ellipsometry (VASE) in the spectral range from 245 to 1700 nm. The measured VASE spectra were examined using individual Tauc–Lorentz (TL) and Cody–Lorentz (CL) dispersion models, and linear combinations of the TL or CL model with multiple Gaussian functions (G). It is demonstrated that the individual TL and CL models are not appropriate for the optical characterization of the Ge5Te20Se75 thin films. Moreover, it is shown that both the CL + 3G and TL + 3G models are satisfactory ones for the optical characterization of these films. It is found that the optical energy gap (Eg) obtained by Tauc method decreases from 1.2 to 0.86 eV with increasing the annealing temperature from room temperature to 150 °C. This result can be interpreted by the chemical bond arrangement approach. On the other hand, the maximum value of the refractive index (n) is shifted toward the long wavelength by increasing the annealing temperature.