Photoluminescence, cathodoluminescence, and reflectance study of AlN layers and AlN single crystals

Strained AlN layers grown by MOVPE on sapphire and nominally unstrained AlN single crystals were studied employing photoluminescence, cathodoluminescence, and reflectance spectroscopy in the near-band edge range. The data allow one to determine fundamental optical parameters such as the band edge energy with its crystal field splitting and strain dependence which are still under discussion. Reflection measurements performed on crystal facets with different orientations and subjected to varying selection rules serve to assign the observed transitions to valence band states with specific symmetries.Near-band edge excitonic luminescence at around 6 eV was recorded as a function of temperature (8…300K). Fits to the data using standard models from the literature yield the temperature dependence Eg(T)Eg(T) of the band gap.