We study the vibrational spectrum of AlN grown on Si͑111͒. The AlN was deposited using gas-source molecular beam epitaxy. Raman backscattering along the growth c axis and from a cleaved surface perpendicular to the wurtzite c direction allows us to determine the E 2 1 , E 2 2 , A 1 ͑TO͒, A 1 ͑LO͒, and E 1 ͑TO͒ phonon energies. For a 0.8-m-thick AlN layer under a biaxial tensile stress of 0.6 GPa, these are 249.0, 653.6, 607.3, 884.5, and 666.5 cm Ϫ1 , respectively. By combining the Raman and

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... ay diffraction studies, the Raman stress factor of AlN is found to be Ϫ6.3Ϯ1.4 cm Ϫ1 /GPa for the E 2 2 phonon. This factor depends on published values of the elastic constants of AlN, as discussed in the text. The zero-stress E 2 2 energy is determined to be 657.4 Ϯ0.2 cm Ϫ1 . Fourier-transform infrared reflectance and absorption techniques allow us to measure the E 1 ͑TO͒ and A 1 ͑LO͒ phonon energies. The film thickness ͑from 0.06 to 1.0 m͒ results in great differences in the reflectance spectra, which are well described by a model using damped Lorentzian oscillators taking into account the crystal anisotropy and the film thickness.