Optical transitions in cubic GaN investigated by spatially resolved cathodoluminescence

Spatially resolved cathodoluminescence (CL) spectroscopy in connection with scanning electron microscopy performed on cubic (c) GaN between 5 and 300 K reveals that at low temperatures the CL spectra of c-GaN single crystals consist of four well-separated lines. The two lines highest in energy were previously identified as excitonic and donor-acceptor transitions, respectively. Here, we show that the lines lowest in energy are due to an additional free-to-bound transition, involving an impurity different from those related to the donor-acceptor transition, and its phonon replica. The CL spectra of c-GaN layers, while being rather broad, are composed of these four lines. Moreover, at 300 K the spectra of the layers and of the crystals are both dominated by the excitonic transition and closely resemble each other.