Nonlinear Shift of the Raman A₁ Mode in Ga-Incorporated CuInSe₂ Thin Films

Composition dependence of quaternary CuIn_1-xGa_xSe₂ films on Ga content has been systematically investigated by Raman scattering. The dominant A₁ mode shifts from 174cm^-1 for CuInSe₂ to 185cm^-1 for CuGaSe₂ in an approximately polynomial curve other than a linear curve, indicating existence of asymmetric distribution of Ga and In on a microscopic scale in films. With Ga content x>0.3, the significantly broadening and intensity decrease of A₁ modes suggest the degradation of crystalline quality of chalcopyrite phase. Additionally, the quenching of additional Raman band at 183cm^-1 for the Ga-rich films reveals that CuAu-ordered phase can coexist in nominal chalcopyrite CuInSe₂ films but not in CuGaSe₂, due to Ga inhibition effect.