Role of the V/III precursor ratio on exciton dynamics in GaN MOCVD epilayers

Time-resolved photoluminescence spectroscopy is performed on strained epilayers of GaN, grown by MOVPE on Al2O3 substrates. As the V/III precursor ratio R is increased, the decay time of the donor–accepter pair (DAP) recombinations at ∼3.22 and ∼3.29 eV is changed from ∼1 ns to much more than 20 ns. Coherently with this change, the XA and XB free excitons and a donor-bound exciton DoX lying 5.7 meV below XA are subject to significant variations. For R∼8000, DAP transitions are fast and the DoX recombination dominates the spectrum at T=8 K, with decay times of ∼35–45 ps, while free excitons decay on a ∼25 ps time scale. For R∼10000, the DAP transitions are very slow and all excitonic recombinations become faster: times of up to ∼280 ps are obtained for DoX and 35 ps for the XA line, which now dominates the time-integrated spectrum. Exciton dynamics in GaN epilayers can thus be controlled by the V/III precursor ratio, which affects both the density of vacancies and extrinsic impurity incorporation.