Effective electron mass and phonon modes in<i>n</i>-type hexagonal InN

Abstract

Infrared spectroscopic ellipsometry and micro-Raman scattering are used to study vibrational and electronic properties of high-quality hexagonal InN. The $0.22\ensuremath{-}\ensuremath{\mu}\mathrm{m}$-thick highly n-conductive InN film was grown on c-plane sapphire by radio-frequency molecular-beam epitaxy. Combining our results from the ellipsometry data analysis with Hall-effect measurements, the isotropically averaged effective electron mass in InN is determined as ${0.14m}_{0}.$ The resonantly excited zone center ${E}_{1}$ (TO) phonon mode is observed at $477 {\mathrm{cm}}^{\ensuremath{-}1}$ in the ellipsometry spectra. Despite the high electron concentration in the film, a strong Raman mode occurs in the spectral range of the unscreened ${A}_{1}(\mathrm{LO})$ phonon. Because an extended carrier-depleted region at the sample surface can be excluded from the ellipsometry-model analysis, we assign this mode to the lower branch of the large-wave-vector LO-phonon-plasmon coupled modes arising from nonconserving wave-vector scattering processes. The spectral position of this mode at $590 {\mathrm{cm}}^{\ensuremath{-}1}$ constitutes a lower limit for the unscreened ${A}_{1}(\mathrm{LO})$ phonon frequency.