Composition dependence of the Raman A1 mode and additional mode in tetragonal Cu–In–Se thin films

Composition dependence of Cu–In–Se films with nominal 60 nm thickness grown on Na-free glass substrates has been systematically investigated by Raman scattering and x-ray diffraction spectra. The most intense A1 mode shift from 175 cm−1 for Cu1.5InSe2 to 151 cm−1 for CuIn5Se8 indicates weakening of the bond-stretching forces with decrease in Cu content. According to the evolution of the phonon frequencies in films, we found that the Raman bands around 160 and 174 cm−1 observed in CuIn5Se8 are probably due to the other ordered structure phase which is very similar to chalcopyrite-related CuIn5Se8. Such coexistence mechanism of both structure phases should be associated with the influence of preferred (112) orientation in the films. Comparing our experimental data with the reported data of the related compounds, we explain the composition effect on phonon frequencies shift from Cu-rich to Cu-poor transition in this paper.