Abstract
The optical properties of the ordered defect compound CuIn5Te8 that crystallizes in a chalcopyrite-related structure have been studied by Raman, optical absorption, and photoluminescence techniques. From the analysis of Raman spectra, lattice vibrational modes are identified and their possible symmetry assignments are suggested. Optical absorption measurements show that the band gap energy EG vary from 1.10 to 1.02eV between 10 and 300K. It is found that the variation of EG with temperature is mainly due to the contribution of optical phonons with a characteristic energy of about 17meV. Photoluminescence (PL) measurements reveal that the main PL band is due to a donor–acceptor recombination between donor and acceptor levels that have activation energies of 50 and 25meV, respectively. Phonon energy associated with Urbach's tail, which is higher than that of longitudinal optical phonons, is attributed to the effect of structural disorder due to deviation from stoichiometry or cation–cation antisite intrinsic defect. An empirical relation, also used earlier in the case of other ordered defect compounds of the 1:3:5 phase [S.M. Wasim, G. Marı́n, C. Rincón, P. Bocaranda, G. Sánchez Pérez, J. Phys. Chem. Solids 61 (2000) 669], explains very well the variation of Urbach's energy with temperature.
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