Effect of localized modes in the absorption spectra of CuInTe 2, CuIn 3Te 5 and CuIn 5Te 8

Abstract

The optical absorption coefficient α as a function of the incident photon energy hν at different temperatures between 10 and 300 K and the infrared spectra between 50 and 300 meV have been studied in CuInTe 2, CuIn 3Te 5 and CuIn 5Te 8 of the Cu–In–Te system. It is found from the analysis of Urbach tail, observed in these compounds just below the fundamental absorption edge, that the phonon energy involved in the electron/exciton–phonon interaction is much higher than the average of their corresponding lattice vibrational modes. However, after discarding the donor/acceptor transitions and also the replicas of phonon modes of lower energies observed in their infrared spectra, the combined average energy hν p ∗ of the remaining localized and lattice vibrational modes is found to be in good agreement with the phonon energy hν p obtained from the study of Urbach tail in these compounds. This confirms the suggestion made earlier [Phys. Rev. B 64 (2001) 195101] that in the formation of Urbach tail, in addition to the contribution of the lattice vibrational modes, other localized modes that have their origin in the structural defects caused by the deviation from ideal stoichiometry are also involved.