Raman scattering from coupled plasmon-phonon modes in HgTe

Abstract

An inelastic-light-scattering experiment from the (100) face of p-type HgTe, a zero-band-gap semiconductor, is reported. The spectral features, which involve coupling of the longitudinal phonon mode with the multicomponent plasma comprised of light electrons in the Γ₈^c conduction band and heavy holes in the Γ₈^v valence band, are analyzed by calculating Im[ε⁻¹(q,ω)], where ε(q,ω) is the frequency- and wave-vector-dependent dielectric function of the medium. The sharp peak at 138 cm⁻¹ is ascribed to "forbidden" LO-phonon scattering. The partially screened allowed LO phonon appears as a broad peak around 127 cm⁻¹, which is higher than the TO-phonon frequency (~118 cm⁻¹). The hole carriers give rise to additional features in the spectrum around 160 cm⁻¹. The results of infrared-reflectivity measurements by Grynberg et al. are also discussed in light of Raman data. While the ir spectra can be analyzed using ε( ω), it is seen that a finite wave vector involved in the light-scattering experiment modifies the phonon-plasmon spectrum profoundly.