Raman Spectra of Ordered Vacancy Compounds in the Cu-In-Se System

Raman spectra have been measured on bulk crystals of ordered vacancy compound (OVC) CuIn₂Se_3.5 at room temperature. Signals belonging to the possible Raman active modes of Γ₁, Γ₃, Γ₄ and Γ₅ of D _2d symmetry are detected from the polarized Raman spectra measurements in the backscattering geometries. A broad signal peaked at 154 cm⁻1 is found to include signals of Γ₁, one of Γ₃ and a set of LO and TO modes of Γ₅ symmetries. Raman spectra from the (001) face are weaker and broader than those from the (100) and (110) faces mainly because of the less quality in the surface morphology of (001) face, which is considered to be related to the compositional fluctuation observed in OVC crystals.