Temperature dependence of excitonic transitions in a-plane AlN epitaxial layers

The excitonic optical properties of a-plane AlN epitaxial layers have been studied by means of temperature-dependent photoluminescence (PL) and optical reflectance (OR) spectroscopy. An exciton resonance and free-exciton luminescence were clearly observed up to room temperature in the OR and PL measurements, respectively. Analysis of the low temperature OR spectrum enabled us to obtain a splitting energy of 7.3 meV between longitudinal and transverse-exciton resonances for AlN, which was approximately one order of magnitude larger than that for GaN. An emission from the upper branch of the excitonic polariton was also observed at temperatures above 100 K, reflecting thermal repopulation of excitonic polaritons from the lower to the upper branch. In addition, the temperature dependence of the transverse-exciton resonance could be well described using an empirical equation based on Bose-Einstein statistics, in which the Einstein characteristic temperature was estimated to be 455 K.