Raman spectra of ordered vacancy compounds in the Cu-In-Se system

Raman spectra of ordered vacancy compounds (OVC) of CuIn2Se3.5 and CuIn3Se5 have been measured at room temperature. Similar nonpolarized Raman spectra are obtained for CuIn2Se3.5 and CuIn3Se5. However; polarized Raman spectra are not available for CuIn3Se5 because of its unfavorable crystallinity. A signal at 154 cm(-1) is assigned to be Gamma(1) mode of D-2d 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 CuInSe2 of chalcopyrite-type in the Cu-In-Se system. The lowering of Gamma(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 Gamma(3) mode are unexpectedly observed in the geometries allowing the longitudinal optical (LO) modes of Gamma(4) and Gamma(5) symmetries. The respect ive mode peaks are found to be more dispersive in frequency for CuIn2Se3.5 than for CuInSe2.