Dynamics of excitonic recombination and interactions in homoepitaxial GaN

The dynamics of free and bound excitons and their interactions have been analyzed from the results of time-resolved photoluminescence (PL) in GaN layers fabricated by metalorganic chemical vapor deposition on GaN single-crystal substrates. The PL transients of an acceptor-bound exciton (A(0)X) a donor-bound exciton ((DX)-X-0), and free excitons A and B (FXA and FXB) at 3.467 eV, 3.472 eV, 3.478 eV, and 3.483 eV, respectively, have been measured. At low excitation density (100 W/cm(2)), the PL decay times in GaN range from 30 ps for FXB up to 1000 ps for A(0)X. The dependence of time constants versus excitation and temperature has been analyzed. Below 50 K, the most important decay process of the FX is trapping on donors. At higher temperatures, delocalization of excitons is observed, and free-exciton emission becomes relatively strong. The delocalization of excitons influences the decay process. Consequently, decay times initially increase, then decrease with temperature. They are longest at about T=70 K. The time constants of the FXA and (DX)-X-0 are of the order of a few hundred ps. The exciton temperature T-X has been determined from the slope of the blue wing of the FX peak. Initially, the excitons are hot (due to high photon energy); then they cool due to interactions with phonons on a time scale of about 80 ps. In the same time range, a (DX)-X-0-FX interaction leading to a broadening of the (DX)-X-0 peak is observed.