Ellipsometric studies on cupric telluride thin films

The optical properties of cupric telluride (CuTe) thin films have been studied in the wavelength range 310-800 nm using spectroscopic ellipsometry (SE). Thin films of thickness between 30 and 150 nm were prepared by thermal evaporation at the rate of 15.6 Å/s on well cleaned glass substrates kept at 300 K under the vacuum better than 2×10⁻⁵ mbar. It has been found that the optical band gap increases with the thickness of the films. The refractive index of the films increases with the energy but the extinction coefficient first increases and then decreases gradually with energy. The analysis of the absorption coefficient determined from the extinction coefficient reveals that there is allowed direct transition with a band gap of about 1.5 eV. The increase in the band gap with the increase in the film thickness has been ascribed to defect levels in the band gap formed by defects in the films.