Far-infrared spectra of indium selenide single crystals

Far-infrared spectra of different indium selenide compounds are reported. Both polarizations E⊥c and E∥c have been recorded in the layered materials InSe,${\mathrm{In}}_2$ ${\mathrm{Se}}_3$, ${\mathrm{In}}_4$ ${\mathrm{Se}}_3$, and ${\mathrm{InSe}}_{1.2}$ crystals. Analyses of the reflection spectra are made by application of the Kramers-Kronig relationship and a Lorentzian oscillator fitting. Accurate values for the frequencies and oscillator strengths of the infrared-active modes have been obtained. The temperature dependences of the frequency and lifetime of the phonon in InSe are investigated using the anharmonic model with two- and three-phonon decay function. Far-infrared spectra of doped layered materials and lithium-intercalated ${\mathrm{In}}_2$ ${\mathrm{Se}}_3$ are also presented. Vibrational and electronic properties are discussed. Free-carrier data are deduced from the optical measurements using a single-particle Drude model and compared with those obtained by Hall-effect measurements.