Lattice Vibration Spectra of Aluminum Nitride

Abstract

The reflectivity and transmission spectra of aluminum nitride have been measured using unpolarized radiation in the wavelength region 2 to 30 \ensuremath{\mu}. The dielectric parameters and the zone-center ($k\ensuremath{\simeq}0$) 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 ${\mathrm{cm}}^{\ensuremath{-}1}$), LO=113.6 meV (916.3 ${\mathrm{cm}}^{\ensuremath{-}1}$), ${\ensuremath{\epsilon}}_{\ensuremath{\infty}}=4.84$, and ${\ensuremath{\epsilon}}_{0}=9.14$. 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 \ensuremath{\sim}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 ($\mathbf{k}\ensuremath{\simeq}{\mathbf{k}}_{max}$). 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.