Influence of nonstoichiometry on the Urbach's tails of absorption spectra for CuInSe2 single crystals

Optical absorption spectra of CuInSe2 single crystals were measured for the samples with -0.150 less than or equal to x less than or equal to 0.053, where x represents a degree of nonstoichiometry in the formula Cu(1-x)Tn(1+x)Se(2). The Urbach's tail was observed for all samples between 90 K and room temperature. The Urbach's energy, which represents an arbitrary intensity of exciton-phonon interaction, was almost constant for the Cu-rich samples (x<0), while it increased with increasing In composition for the In-rich ones (x>0). Such an Increase of the Urbach's energy was explained to be due to enhanced electronic distortion caused by the compositional deviation from stoichiometry in terms of simultaneous influence of electron-phonon interaction and structural disorder. (C) 1996 American Institute of Physics.