Comparative study of tetragonal Cu2In7Se11.5 and trigonal CuIn5Se8 by spectroscopic ellipsometry

A comparative study of chalcopyrite-related Cu2In7Se11.5 and CuIn5Se8 crystals by spectroscopic ellipsometry is presented. Their complex dielectric function ɛ(ω) = ɛ1(ω) + iɛ2(ω) has been determined in the 0.8-4.7 eV photon energy range. The spectral dependence of ɛ1(ω) and ɛ2(ω) as well as the complex refractive index, the absorption coefficient and the normal-incidence reflectivity for tetragonal Cu2In7Se11.5 and trigonal CuIn5Se8 crystals have been modelled using the Adachi's model for interband transitions. The results are in a good agreement with the experimental data over the entire range of photon energies. The model parameters, including the energies corresponding to the lowest direct gap (E0) and higher critical points (E1A, E1B), have been determined using the simulated annealing algorithm. The results show that the tetragonal CuIn3Se5, the tetragonal Cu2In7Se11.5 and the trigonal CuIn5Se8 crystals belong to different phases of the Cu4In2nSe2+3n system, n = 6, n = 7 and n = 10, respectively.