Lattice Vibration Spectra of Aluminum Nitride

The reflectivity and transmission spectra of aluminum nitride have been measured using unpolarized radiation in the wavelength region 2 to 30 {μ}. The dielectric parameters and the zone-center (k0) opticalmode energies [transverse (TO) and longitudinal (LO)] have been calculated from the reflectivity measurements using dispersion theory. The values obtained are TO=82.7 meV (666.7 cm^-1), LO=113.6 meV (916.3 cm^-1), ε_∞=4.84, and ε₀=9.14. From these data the value calculated for the Szigeti effective charge is q*=1.2. Polarization measurements have revealed a sharp minimum in the reststrahlen band for E parallel to the c axis at {~}111.0 meV. Otherwise the reflectivity curves appear identical for both polarizations. Thirteen absorption bands have been observed on the high-energy side of the reflectivity band. A tentative interpretation is proposed in which these bands are considered to be due to combinations of six representative phonon branches at or near the Brillouin-zone boundary (kkₘₐₓ). The phonon energies resulting from this interpretation are LO=91.4, TO₁=82.5, TO₂=78.1, LA=62.9, TA₁=55.3, and TA₂=50.9 meV.