Defect-induced increase in the phonon energy involved in the formation of Urbach tail in Cu-ternaries

From a combined analysis of the stoichiometric composition and Urbach tail in samples of CuInSe2, CuInTe2, and CuGaTe2 of the I-III-VI2 family of chalcopyrite semiconductors, it is found that the energy hνp involved in the electron/exciton-phonon interaction is a linear function of a parameter Δz which is the sum of the deviations from ideal molecularity Δx and anion to cation ratio Δy. It gives evidence that in the copper ternaries hνp is associated to the structural defects caused by cation-cation, cation-anion, and other intrinsic disorders. The high value of hνp found in the studied samples, higher than the highest optical mode, is shown to come from the contribution of the additional phonon energy due to structural defects. This is in agreement with recently proposed models of the temperature dependence of the Urbach energy.