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 pu
lsed laser deposition (PLD) in a nitrogen atmosphere. Material quality is i
nvestigated using low temperature photoluminescence (PL) (which yields a li
ne-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 de
rived 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 obse
rved in the low- and high-temperature regimes. We discuss these data in ter
ms of the possible mixed cubic/hexagonal phase structure of the material, a
nd examine evidence for the localisation of the electrons and holes in the
different phases. (C) 2001 Elsevier Science B.V. All rights reserved.