Comparative study of tetragonal Cu 2In 7Se 11.5 and trigonal CuIn 5Se 8 by spectroscopic ellipsometry

Abstract

A comparative study of chalcopyrite-related Cu 2In 7Se 11.5 and CuIn 5Se 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 2In 7Se 11.5 and trigonal CuIn 5Se 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 3Se 5, the tetragonal Cu 2In 7Se 11.5 and the trigonal CuIn 5Se 8 crystals belong to different phases of the Cu 4In 2 n Se 2+3 n system, n = 6, n = 7 and n = 10, respectively.