Temperature Dependence of the Exciton Gap in Monocrystalline CuGaS2

Single crystals of CuGaS2 have been grown by chemical vapour transport. Their near-band gap photoluminescence properties were investigated in the temperature range of 10-300 K. The variation of the exciton gap energy with temperature was studied by means of a three-parameter thermodynamic model, the Einstein model and the Pässler model. Values of the band gap at T=0 K, of a dimensionless constant related to the electron-phonon coupling, and of an effective and a cut-off phonon energy have been estimated. It has also been found that the major contribution of phonons to the shift of Eg as a function of T in CuGaS2 is mainly from optical phonons