Anisotropy and dispersion of coupled plasmon-LO-phonon modes in Sb2Te3

Coupled plasmon-LO-phonon modes have been studied in Sb2Te3 (bulk samples and epitaxial layers) by Raman scattering and IR-reflectivity. The high frequency plasmonlike mode (Ω3: 300–2,000 cm−1) is observed, in contrast to the other two phonon-like plasmon-LO-phonon modes. The latter are expected to have negligible scattering intensities, since the IR-phonons in Sb2Te3 are not Raman-active. The Ω3-mode shows up with different frequencies since the effective masses, i.e. the plasmon is anisotropic. Dispersion measurements and their comparison with the results of a theoretical description (hydrodynamical theory) gives good agreement. From this values for the anisotropic effective masses are obtained.