Optical absorption spectra near the fundamental band edge in Cu₂In₄Se₇ bulk crystals

From the analysis of the optical absorption coefficient spectra of bulk crystals of Cu₂In₄Se₇ near the band edge as a function of temperature, two distinct absorption bands, one in the energy range from 0.8 to 1.0 eV and the other above 1.0 eV, were observed. The first band was identified as due to band tail–to–band transitions caused by a high concentration of native defects whereas the latter was attributed to the lowest band–to–band transition due to an allowed direct gap. The energy gap E_G was found to be 1.020 and 0.986 eV at 10 and 300 K, respectively. From the analysis of the temperature dependence of E_G in terms of the current theoretical models reported in the literature for its shift with T, the Debye temperature was estimated to be around 220 K. It was also found that the major contribution to the temperature variation of E_G in this compound is apparently due to acoustic phonons. © 2003 American Institute of Physics.