Anisotropic absorption and emission of bulk (1(1)over-bar00) AlN

The intrinsic anisotropic optical properties of wurtzite AlN are investigated in absorption and emission. Full access to the anisotropy of the optical response of the hexagonal material is obtained by investigating the (1 (1) over bar 00) plane of a high-quality bulk crystal allowing electric field E polarization perpendicular (E perpendicular to c) and parallel (E parallel to c) to the optical axis c. Spectroscopic ellipsometry yields the ordinary (epsilon(perpendicular to)) and extraordinary (epsilon(parallel to)) dielectric functions (DFs) from 0.58 up to 20 eV. The comparison of the experimental data with recently calculated DFs demonstrates that Coulomb interaction has a strong impact not only on the spectral dependence around the fundamental absorption edge but also on the high-energy features usually discussed in terms of van Hove singularities. The fits of the second-order derivatives of epsilon(parallel to) and epsilon(perpendicular to) provide the transition energies for the main features in this range. The DFs close to the fundamental absorption edge, dominated by free excitons, exciton-phonon complexes, and the exciton continuum, are independently confirmed by reflectivity and synchrotron-based photoluminescence excitation studies. Values for the band gaps, the crystal field (Delta(cf) = -221 +/- 2 meV), and spin-orbit splittings (Delta(so) = 13 +/- 2 meV) are obtained. Furthermore, we obtain accurate values for the static dielectric constants of epsilon(S perpendicular to) = 7.65 and epsilon(S parallel to) = 9.21, entering, e. g., the calculations of exciton binding energies. Photoluminescence is used to investigate the emission properties of the same sample.