IDENTIFICATION OF OPTICAL-TRANSITIONS IN CUBIC AND HEXAGONAL GAN BY SPATIALLY-RESOLVED CATHODOLUMINESCENCE

The hexagonal and cubic phases of GaN are characterized by spatially r esolved cathodoluminescence (CL) spectra from micrometer-size single c rystals with either hexagonal or cubic habits grown by plasma-assisted molecular-beam epitaxy. At 5 K, distinct narrow excitonic lines are f ound at 3.472 and 3.272 eV for the hexagonal and cubic phase, yielding energy gaps of 3.500 and 3.300 eV, respectively. Detailed temperature - and intensity-dependent CL measurements on cubic GaN crystals enable us to clearly identify the exciton (free: 3.272 eV, bound: 3.263 eV) and the donor-acceptor pair (3.150 eV) transition. Moreover, we determ ine the donor-band and acceptor-band transition energy for this phase. In addition, phonon replicas of the exciton line and of the donor-acc eptor pair transition are observed at 3.185 and 3.064 eV, respectively .