Transmission electron microscopy and localized electron energy loss spectroscopy, in the bandgap-energy regime, were applied to this system. Although grown on a cubic substrate, the GaN film exhibited hexagonal phases having 2 different orientational relationships with respect to the substrate. One of these partially transformed into a cubic phase, via sequential faulting. The band-gap was determined in local regions of good crystallinity, and in the proximity of stacking faults, dislocations, grain boundaries, and hetero-interfaces. Extrapolation of the electron energy loss spectra gave a band-gap value of 3.4eV for the unfaulted hexagonal close-packed phase. This was consistent with the predicted value. The band-gap itself was heavily masked, in the electron energy loss spectra, by near-bandedge states around 3.2 and 3.3eV; especially in faulted grains. Local variations in the mid-gap and near-bandedge density of states in the electron energy loss spectra were found to be related to microstructural defects in the GaN film. Photoluminescence measurements at 6K did not reveal the bound exciton at about 3.47eV, but instead suggested the presence of donor acceptor pairs at 3.27eV.

U.Bangert, A.Harvey, J.Davidson, R.Keyse, C.Dieker: Journal of Applied Physics, 1998, 83[12], 7726-9