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

Abstract

Raman spectra of ordered vacancy compounds (OVC) of CuIn 2 Se 3.5 and CuIn 3 Se 5 have been measured at room temperature. Similar nonpolarized Raman spectra are obtained for CuIn 2 Se 3.5 and CuIn 3 Se 5 . However, polarized Raman spectra are not available for CuIn 3 Se 5 because of its unfavorable crystallinity. A signal at 154 cm -1 is assigned to be Γ 1 mode of D 2 d symmetry, originating from the vibration of selenium atoms in the x - y ( a -axes) plane. This signal corresponds to the signal at 178 cm -1 for CuInSe 2 of chalcopyrite-type in the Cu-In-Se system. The lowering of Γ 1 mode frequency for the defect structure is attributed to looser-bound selenium atoms on account of vacancies in the cation sites. Signals assigned as Γ 3 mode are unexpectedly observed in the geometries allowing the longitudinal optical (LO) modes of Γ 4 and Γ 5 symmetries. The respective mode peaks are found to be more dispersive in frequency for CuIn 2 Se 3.5 than for CuInSe 2 .