Résumé
We report on polarization-resolved micro-photoluminescence experiments performed on a single GaN/AlN polar quantum dot (QD) grown on Si(111) substrate. We have performed a systematic study of about 50 QDs. The emission of about half of the QD excitons is strongly linearly polarized, up to 90%. Such a polarization is known to be the signature of the asymmetry of the QD shape, which lifts the degeneracy of the optically active excitons. However in the case of our GaN QDs, the polarization angles are widely distributed and do not follow the crystallographic axes, and the sharp transition lines of each QD do not appear as doublets of cross-polarized peaks. The fine structure of GaN QD excitons therefore presents strong differences with the well understood one of InAs and CdTe QDs.