The dielectric function (DF) of hexagonal AlN on Si(111) is determined in the range between 1 and 9.8 eV by spectroscopic ellipsometry (SE). Due to its significant negative crystal field splitting, wurtzite AlN exhibits a pronounced dichroism. To demonstrate that SE is sensitive to both components of the DF around the absorption edge, a uniaxial model is applied, providing transition energies for the free excitonic state. In-plane tensile stress leads to a redshift of these transitions and an increased splitting. The experimental data is compared with the results of band structure calculations, showing excellent overall agreement. Additionally, two high-energy critical points in the ordinary DF at energies of approximately 7.75 and 8.85 eV were determined.
Rossbach, G., Röppischer, M., Schley, P., Gobsch, G., Werner, C., Cobet, C., Esser, N., Dadgar, A., Wieneke, M., Krost, A., & Goldhahn, R. (2010). Valence-band splitting and optical anisotropy of ALN. Physica Status Solidi (b), 247(7), 1679–1682. https://doi.org/10.1002/pssb.200983677