Femtosecond exciton dynamics and the mott transition in GaN under resonant excitation

We present resonant femtosecond pump-probe reflectance measurements of excitons in high quality wurtzite GaN epilayers for a range of lattice temperatures and various pump intensities. From the density dependence of the excitonic 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 dominated by trapping at defects via acoustic phonon emission on a timescale of similar to 16 ps. At temperatures above 60 K we observe a much longer relaxation component of similar to 350 to 400 ps, which we ascribe to the radiative recombination of free excitons.