Composition-induced structural modifications in the quaternary CuIn_1-xGa_xSe₂ thin films:bond properties versus Ga content

In this paper the dependence of structural properties of the quaternary CuIn_1-xGa_xSe₂ films with tetragonal structure on the Ga content has been systematically investigated by Raman scattering and x-ray diffraction spectra. The shift of the dominant A₁ mode, unlike the lattice constants, does not follow the linear Vegard law with increasing Ga content x, whereas exhibits approximately polynomial change from 174 cm^-1 for CuInSe₂ to 185cm^-1 for CuGaSe₂. Such behaviour should be indicative of presence of the asymmetric distribution of Ga and In on a microscopic scale in the films, due to Ga addition. The changes in the tetragonal distortion \eta lead to a significant variation in the anion displacement parameter U, which should be responsible for the evolution of bond parameters and resultant Raman bands with x.