Reflection measurements in the near-band-edge region of bulk AlN crystals have been performed as a function of temperature. The optical reflectance spectra of an $a$-face AlN sample show a feature at about 6.029 eV, which we assign to the free exciton $A$. The observation of the free exciton $A$ first excited state yields the estimated values of the direct band gap and the limit of the electron effective mass. Optical reflectance measurements performed on AlN samples with two different crystallographic orientations allow the observation of transitions associated with the optical selection rules related to the so-called $A,B$, and $C$ excitons. It was not possible to fully resolve the $B$- and $C$-excitonic transitions, which were observed at about 6.243 and 6.268 eV, respectively. Using the measured exciton energies and a quasicubic model developed for the wurzite crystal structure we estimate the spin-orbit splitting $\delta =36\mathrm{meV}$ and the crystal-field splitting $\Delta =-225\mathrm{meV}$.

• Received 27 September 2004

DOI:https://doi.org/10.1103/PhysRevB.71.041201