The first- and second-order Raman scattering and IR reflection have been studied for hexagonal InN layers grown on (0001) and (11̄02) sapphire substrates. All six Raman-active optical phonons were observed and assigned: E2(low) at 87 cm−1, E2(high) at 488 cm−1, A1(TO) at 447 cm−1, E1(TO) at 476 cm−1, A1(LO) at 586 cm−1, and E1(LO) at 593 cm−1. The ratio between the InN static dielectric constants for the ordinary and extraordinary directions was found to be ε⊥0/ε∥0=0.91. The phonon dispersion curves, phonon density-of-state function, and lattice specific heat were calculated. The Debye temperature at 0 K for hexagonal InN was estimated to be 370 K.
The carrier concentration dependence of the interaction between free carriers and longitudinal optical ͑LO͒ phonons of InN is studied by Raman scattering and Fourier transform infrared measurements. InN is grown on a sapphire ͑0001͒ surface by plasma-assisted molecular beam epitaxy. The carrier concentration is varied from 1.8ϫ10 18 to 1.5ϫ10 19 cm Ϫ3 by Si doping. The infrared reflection spectra, to which the vibration in the a-b plane contributes, reveal a linear coupling between the E 1 (LO) phonon and the plasma oscillation of the free carriers. From the plasma frequency the electron effective mass is estimated to be m eЌ * ϭ0.085m 0 for the intrinsic InN. The Raman spectra, to which the vibration along the c axis contributes, reveal that the A 1 (LO) phonon and free carriers couple nonlinearly, where a Fano interference between the zone-center LO phonon and the quasicontinuum electronic state along the c axis is prominent. With these results, the anisotropic electronic structure of InN is discussed.
The temperature dependence of the resistivity of InN was investigated as a function of carrier density. The carrier density was changed from n e = 1.8ϫ 10 18 cm −3 to 1.5ϫ 10 19 cm −3 by Si doping. The InN investigated showed metallic conduction above 20 K. At lower temperatures there was a resistivity anomaly originating from carrier localization in the a-b plane, which was confirmed by the magnetoresistance at 0.5 K. The Shubnikov-de Haas oscillation showed that InN had a spherical Fermi surface and its radius increased according to the increase of n e when n e Ͻ 5 ϫ 10 18 cm −3. In addition, an oscillation corresponding to the constant carrier density of 4.5ϫ 10 12 cm −2 was observed in the field applied perpendicular to the a-b plane. This oscillation showed an anomalous angle dependence on the magnetic field. Taking into account this density, we determined the critical carrier density of the Mott transition to be 2 ϫ 10 17 cm −3. Anisotropy of localization was observed within the a-b plane, which indicates that the distribution of the electrons was not uniform in the a-b plane. The n e dependence of the magnetoresistance revealed an electronic structure change around 5 ϫ 10 18 cm −3. From these results, an electronic structure at the fundamental absorption edge of InN grown on sapphire ͑0001͒ was presented.
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