Optical phonons of aluminum nitride

Oriented polycrystalline films of AlN on Si(111) substrates and amorphous AlN films on sapphire substrates have been prepared by sputter deposition of aluminum in a nitrogen plasma. Phonons at 303, 426, 514, 614, 663, and 831 cm−1 (±3%) have been observed in the Raman spectra of the crystalline films. The 303-cm−1 phonon shifts to 288 cm−1 and the 663-cm−1 phonon to 650 cm−1 in the amorphous sample. By means of infrared absorption measurements with the electric field perpendicular to the c axis we have tentatively identified E1(LO) and E1(TO) modes near 800 and 610 cm−1, respectively. A tentative assignment of the other modes is made from partial data from orientation dependent scattering and results for other wurtzite structure compounds. A comparison of the AlN phonon frequencies with those of other known wurtzite structure compounds shows that Martin’s scaling equation is satisfied.