Crystal growth, structural, and optical characterization of the ordered defect compound CuGa5Se8

Abstract

The crystal structural and optical properties of the ordered defect compound CuGa5Se8 grown by the vertical Bridgman–Stockbarger method have been studied by x-ray diffraction and optical absorption techniques, respectively. The analysis of the x-ray diffraction data shows that the material crystallizes in a tetragonal chalcopyrite-related structure with unit cell parameters a=5.4732(8) and c=10.931(6) Å. Optical data shows that the band gap energy EG varies from 1.917 to 1.811 eV in the temperature range between 10 and 300 K. It is found that the variation of EG with T is mainly due to the contribution of optical phonons with a characteristic phonon energy of about 19 meV. The optical absorption coefficient just below the absorption edge varies exponentially with photon energy confirming the existence of the expected Urbach’s tail. The phonon energy hνp associated with Urbach’s tail, which is found to be 54 meV, is about two times higher than the highest optical phonon mode reported for ternary semiconductors of the Cu–Ga–Se system. The origin of higher energy is attributed to the contribution of localized modes produced by structural disorders of low energy formation mainly due to deviation from ideal stoichiometry.