Free-and bound-exciton luminescences of GaN epitaxial layers grown by a sublimation technique on 6H-SiC substrates were investigated using time-integrated and time-resolved photoluminescence measurements at low temperatures. Lifetimes were determined for the donor-bound exciton at 3.4722 eV and for two acceptor-bound excitons with energies of 3.4672 eV and 3.459 eV. On the basis of our results we obtain an upper limit of the free-exciton oscillator strength of 0.0046 for GaN. Luminescences between 3.29 eV and 3.37 eV are identified as due to excitons deeply bound to centers located near the substrate-epilayer interface. Free excitons are captured by these centers within 20 ps.

We report on strong excitonic luminescence in wurtzite GaN at 3.309 and 3.365 eV ͑Tϭ6 K͒. These lines lie well below the band gap and are found commonly in layers grown by different techniques and on different substrates. From detailed photoluminescence investigations we find small thermal activation energies and a very weak electron-phonon coupling. The photoluminescence behavior under hydrostatic pressure is indicative of strongly localized defects. These findings are similar to observations of excitons localized at extended defects such as dislocations in II-VI compounds.

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