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

Abstract A comparative study of chalcopyrite-related Cu 2 In 7 Se 11.5 and CuIn 5 Se 8 crystals by spectroscopic ellipsometry is presented. Their complex dielectric function ɛ ( ω ) =  ɛ 1 ( ω ) +  iɛ 2 ( ω ) has been determined in the 0.8–4.7 eV photon energy range. The spectral dependence of ɛ 1 ( ω ) and ɛ 2 ( ω ) as well as the complex refractive index, the absorption coefficient and the normal-incidence reflectivity for tetragonal Cu 2 In 7 Se 11.5 and trigonal CuIn 5 Se 8 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 ( E 0 ) and higher critical points ( E 1 A , E 1 B ), have been determined using the simulated annealing algorithm. The results show that the tetragonal CuIn 3 Se 5 , the tetragonal Cu 2 In 7 Se 11.5 and the trigonal CuIn 5 Se 8 crystals belong to different phases of the Cu 4 In 2 n Se 2+3 n system, n  = 6, n  = 7 and n  = 10, respectively.