The optical properties around the absorption edge of high-quality wurtzite $c$-plane AlN layers are investigated by spectroscopic ellipsometry focusing on the anisotropy of the optical response. The spectral dependence of dielectric function shows a strong contribution of exciton-phonon coupling superimposed to the exciton continuum. Crystal field splitting and spin-orbit coupling energies are found to be ${\Delta }_{\mathrm{cf}}=-212$ meV and ${\Delta }_{\mathrm{so}}=16$ meV, respectively. These values are accessible because our data allow extraction of the transition energies of excitons with holes from all three highest valence bands. Energy positions are cross-checked by photoluminescence measurements. As the samples are grown on different substrates and exhibit varying biaxial strain determined by high resolution x-ray diffraction, we are also able to determine the deformation potentials $a-D_1=-6.9$ eV, $a-D_2=-15.2$ eV, $D_3=8.3$ eV, and $D_4=-4.15$ eV for AlN.

• Received 27 January 2011

DOI:https://doi.org/10.1103/PhysRevB.83.195202