Defect luminescence of GaN grown by pulsed laser deposition

GaN films with similar to0.4 mum thickness, without any intentional buffer layer, have been successfully grown on a sapphire (0 0 0 1) substrate by pulsed laser deposition (PLD) in a nitrogen atmosphere. Material quality is investigated using low temperature photoluminescence (PL) (which yields a line-width of 15 meV for the donor-bound exciton (D-X) transitions) and X-ray diffraction (XRD) measurements. Activation energies of 26.5 and 230 meV derived from the temperature dependence PL studies of the donor-acceptor (D-A) transitions are consistent with previous studies. In addition, two values of activation energy (i.e. 13.5 and 25 meV) for the 3.41 eV peak were observed in the low- and high-temperature regimes. We discuss these data in terms of the possible mixed cubic/hexagonal phase structure of the material, and examine evidence for the localisation of the electrons and holes in the different phases. (C) 2001 Elsevier Science B.V. All rights reserved.