Optical anisotropy of AlN epilayer on sapphire substrate investigated by variable-angle spectroscopic ellipsometry

A wurtzite AlN epilayer grown on (0 0 0 1) sapphire substrate was characterized by variable-angle spectroscopic ellipsometry. The asymmetries of the ellipsometric spectra caused by optical anisotropy were observed below and above the Brewster angle. Tanguy’s dispersion model is employed in order to determine the extraordinary and ordinary refractive indices and extinction coefficients in the spectral range of 1.5–6.5 eV. In addition, the birefringence and dichroism were derived exhibiting near the band gap a maximum and a minimum, respectively. The anisotropy is attributed to the valence band ordering at the center of the Brillouin zone, in particular to the large negative crystal-field splitting, and the corresponding optical selection rules.