Femtosecond exciton dynamics and the mott transition in GaN under resonantexcitation

We present resonant femtosecond pump-probe reflectance measurements of exci tons in high quality wurtzite GaN epilayers for a range of lattice temperat ures and various pump intensities. From the density dependence of the excit onic bleaching we find that the Mott density is n(Mott) less than or equal to 2.2 x 10(19) cm(-3). At 4 K we find that the exciton dynamics is dominat ed by trapping at defects via acoustic phonon emission on a timescale of si milar to 16 ps. At temperatures above 60 K we observe a much longer relaxat ion component of similar to 350 to 400 ps, which we ascribe to the radiativ e recombination of free excitons.