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 ($k\simeq 0$) 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 ${\mathrm{cm}}^{-1\mathrm{}}$), LO=113.6 meV (916.3 ${\mathrm{cm}}^{-1\mathrm{}}$), ${\epsilon }_{\infty }=4.84\mathrm{}$, and ${\epsilon }_0=9.14\mathrm{}$. From these data the value calculated for the Szigeti effective charge is $q^{*}=1.2\mathrm{}$. 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 ($\mathbf{k}\simeq {\mathbf{k}}_{max}$). The phonon energies resulting from this interpretation are LO=91.4, T${\mathrm{O}}_1$=82.5, T${\mathrm{O}}_2$=78.1, LA=62.9, T${\mathrm{A}}_1$=55.3, and T${\mathrm{A}}_2$=50.9 meV.