Urbach's tails in the absorption spectra of CuInTe2 single crystals with various deviations from stoichiometry

The optical absorption coefficient a near the band edge is found to vary exponentially with photon energy in several samples of CuInTe2 single crystals. This behavior, where the extrapolation of In alpha vs h upsilon at different temperatures converges to a single point, is consistent with Urbach's rule. The temperature dependence of Urbach energy KT/sigma, calculated from the steepness parameter sigma, agrees with the theoretical model that takes into account the contribution of both electron-phonon interaction and structural disorder. It is established that the observed phonon energy higher than the average value calculated from the reported optical modes is due to structural disorder which is caused by the compositional deviation from the ideal stoichiometry. (C) 1998 American Institute of Physics.