Light-induced narrowing of excitonic absorption lines in GaN

The fundamental absorption edge of GaN has been investigated in GaN/AlxGa1−xN heterostructures. A broad excitonic line is observed after the sample has been cooled in darkness. A metastable narrowing occurs and three excitonic absorption lines are observed after the sample has been illuminated. Measurements of this effect have been made as a function of temperature and photon energy. It seems that the broadening is induced by an electric field present around dislocations in GaN. This field is reduced by free carriers created during illumination, which results in narrowing of the excitonic lines.