SPECTRAL DEPENDENCE OF THE ABSORPTION-COEFFICIENT OF THIN-FILMS OF NONSTOICHIOMETRIC CU2-XTE

A transmission and reflection spectra of polycrystalline thin films (from about 500 to about 3000 angstrom) of Cu2-xTe compounds, where 0 < x less-than-or-equal-to 0.25 with steps in x of 0.05, have been studied at room temperature in the wavelength region from 0.295 to 2.5-mu-m. A new technique is described to determine either the composition or the thickness of the thin film of copper telluride. In this method reflectance-transmittance data for the Cu2-xTe film are needed in the small wavelength region 0.5-1.0-mu-m. The position and the value of the reflectance minima and the transmittance maxima uniquely determine the film composition and its thickness respectively. The absorption coefficient-alpha of the different compositions of Cu2-xTe have been determined for at least three different thicknesses. For all the compositions, direct and indirect transitions exist. The value of the energy gap corresponding to the direct transitions strongly depends on the value of x, increasing as x increases. On the contrary, the indirect energy gap corresponding to the indirect transitions slightly decrease as x increases. The frequency dependence of the optical constants n, k, epsilon' = n2 - k2, epsilon" = 2nk was also given. It was found that the value of the high frequency optical constants, as estimated from extrapolation of the curves, did not depend on the value of x The theoretical calculations of the direct energy gaps of all compositions using the simple relation E(g)(x,T) = 1.233 + 3.374x-3.1 x 10(-4)T(1-2x) agree fairly well with the experimental values.