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 Δcf=−212 meV and Δso=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−D1=−6.9 eV, a−D2=−15.2 eV, D3=8.3 eV, and D4=−4.15 eV for AlN.
