DIRECT AND INDIRECT TRANSITIONS IN COPPER TELLURIDE THIN-FILMS

Transmission and reflection at nearly normal incidence of vacuum-deposited thin films of different thicknesses ranging from 100 to 3000 angstrom maintained at room temperature on glass or quartz substrates were measured in the wavelength range from 0.295 to 2.5-mu-m. The absorption coefficients-alpha were computed for different thicknesses. The analysis of the absorption coefficient data revealed the existence of two optical transition mechanisms: an allowed direct transition with E(gd) = 1.18 eV and an indirect transition with E(gi) = 0.82eV. The frequency dependences of the optical constants n, k, epsilon' and epsilon" are also given. The high frequency dielectric constants epsilon' and epsilon" as estimated from extrapolation of the curves to the high frequency range are 17 and 9 respectively.