Effect of localized modes in the optical absorption spectra of CuGaSe2 and CuGa3Se5

The optical absorption coefficient α as a function of the incident photon energy hν at different temperatures between 50 and 300 K and the infrared spectra between 80 and 375 meV have been studied in the ternary compound CuGaSe2 and the ordered defect compound CuGa3Se5. It is found that the phonon energy hνp involved in the formation of Urbach tail is much higher than the highest lattice vibrational modes in these compounds. A combined analysis of the Raman modes and infrared spectra shows, after discarding the possible donor/acceptor transitions, replicas of phonon modes at higher energies in both Raman and infrared spectra, and the peaks originating from the spectrophotometer, that the average phonon energy hνp∗ of the remaining lattice vibrational and localized modes are in good agreement with the phonon energy hνp obtained from the study of Urbach tail. This confirms that in the formation of Urbach tail, in addition to the contribution of lattice vibrational modes, other localized modes caused by structural defects and enhanced electronic distortion, originating from ordered defects, are also involved. It is suggested that this type of study can be a useful tool in determining the quality of the samples and the ingots, important in device applications.