Excitonic thermalization and recombination in homoepitaxial gallium nitride

We report temporally resolved photoluminescence (PL) measurements on a high -quality single crystal GaN film grown on GaN substrates by metalorganic ch emical vapour deposition (MOCVD). The PL decay times of free excitons and d onor-bound excitons have been measured at liquid helium temperature, under various excitation power densities. The excitonic temperature has been moni tored by analysis of the shape of the excitonic peak and the cooling time o f (100 +/- 20) ps has been determined. Under higher excitation densities th e recombination rates of both free and bound excitons are smaller (for the free exciton A the lifetime increases up to tau(d) = 200 ps). This observat ion can be explained by detrapping of donor bound excitons at higher excito nic temperatures.