Effective electron mass and phonon modes inn-type hexagonal InN

Infrared spectroscopic ellipsometry and micro-Raman scattering are used to study vibrational and electronic properties of high-quality hexagonal InN. The $0.22-\mu \mathrm{m}$-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 $477\hspace{0.5em}{\mathrm{cm}}^{-1}$ in the ellipsometry spectra. Despite the high electron concentration in the film, a strong Raman mode occurs in the spectral range of the unscreened $A_1\left(\mathrm{LO}\right)$ 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 $590\hspace{0.5em}{\mathrm{cm}}^{-1}$ constitutes a lower limit for the unscreened $A_1\left(\mathrm{LO}\right)$ phonon frequency.