Defect-induced structural disorder in tetragonal Cu(In1-xGax)5Se8 thin films investigated by Raman spectroscopy:: the effect of Ga addition

The dependence of the phonon frequencies of the quaternary Cu(In1-xGax)(5)Se-8 films with tetragonal structure on the Ga content have been systematically investigated by means of Raman scattering. The dominant A, mode shifts from 151 cm(-1) for CuIn5Se8 to 160 cm(-1) for CuGa5Se8 in an approximately polynomial but not linear curve, because different types of local tetrahedral cationic clusters around Se distribute in the defect crystal structures. The vibrational modes in the low-frequency region below 125 cm(-1) clearly show one-mode behaviour, whereas significant changes in the frequency range from 160 to 235 cm(-1) indicate defect-induced structural disorder and show two-mode behaviour due to Ga addition. Additionally, the quenching of the Raman band at 174 cm-1 for the Ga-rich films reveals that this mode should most probably originate from other similar localized structure phases in chalcopyrite-related CuIn5Se8 but not CuGa5Se8, due to the Ga inhibition effect. Such results should be ascribed to asymmetric distribution of Ga and In on a microscopic scale, resulting in the different properties of bonds between local clusters.