Optical investigation of thermally evaporated Cu5Ge x Te95− x thin films
Cu5GexTe95−x (x = 14, 16, 18 and 20) thin films were successfully deposited using thermal evaporation technique. The optical transmittance (T) and reflectance (R) spectra in the wavelength range 250–2500 nm were used for the determination of the optical energy gap and the associated optical constants of the deposited thin films. It was found that, the absorption coefficient (α), refractive index (n) and the extinction coefficient (k) spectra vary with increasing Ge concentration. The indirect optical band gap Eg is found to decrease while the Urbach tail energy increases for different film compositions. The refractive index dispersion data were analysed using the Wemple–DiDomenico single-effective-oscillator model. As a result, the oscillator energy (Eso), dispersion energy (Ed), oscillator strength (Sso), and zero-frequency refractive index (no) were determined. The dependence of the optical constants of Cu5GexTe95−x on the photon energy (hν) at different Ge concentrations is discussed.