Comparative study of photoluminescences for the Ga-/N-faces of a free-standing GaN bulk fabricated by using hydride vapor-phase epitaxy and self-separation technique

The authors report on a comparative study of photoluminescences for the Ga-/N-faces of a freestanding GaN bulk, fabricated by using hydride vapor-phase epitaxy and self-separation technique. It is found that the Ga-face and the N-face of the same bulk have apparently different optical properties: i) the Ga-face has well-defined excitionic emission lines while the N-face has just broad emission peaks at similar energy levels and ii) the Ga-face is characterized by a transition crossover between DoX (neutral donor-bound exciton) and FXA (A free exciton) in changing temperature while the N-face is characterized by changes in the dominant emission mechanism in the order of DoX, 3.43-eV emission, DoAo (neutral donor-acceptor) pair and eAo (neutral electron-acceptor), and FXA with increasing temperature from 10 K to room temperature. On the other hand, the N-face is shown to have a rougher surface morphology and a larger density of edge dislocations, which is due to structural imperfections generated during the lift-off process. Also, the N-face is shown to have a higher conductivity and a larger electron concentration, which indicates that the N face contains a larger density of donor-type defects. It is suggested that the different emission mechanism of the Ga-/N-faces in the free-standing GaN bulk is ascribed to the fact that the N-face contains a larger density of electrically active radiative and nonradiative recombination centers, which are generated during the self-separation process.