Identification of optical transitions in cubic and hexagonal GaN by spatially resolved cathodoluminescence

Abstract

The hexagonal and cubic phases of GaN are characterized by spatially resolved cathodoluminescence (CL) spectra from micrometer-size single crystals with either hexagonal or cubic habits grown by plasma-assisted molecular-beam epitaxy. At 5 K, distinct narrow excitonic lines are found 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 determine the donor-band and acceptor-band transition energy for this phase. In addition, phonon replicas of the exciton line and of the donor-acceptor pair transition are observed at 3.185 and 3.064 eV, respectively.