Microstructure and growth morphology as related to electro-optical properties of heteroepitaxial wurtzite GaN on sapphire (0001) substrates

We investigate the microstructural development during growth and the related electro-optical properties of gallium nitride (GaN) films deposited on (0001) sapphire substrates by gas source molecular beam epitaxy. We use transmission electron microscopy, atomic force microscopy, scanning tunnelling microscopy, spectrally resolved cathodoluminescence (CL) mapping and photoluminescence (PL). We report on specimens exhibition UV luminescence (band-to-band transition at 358 nm/3.46 eV) in CL/PL and an additional strong yellow luminescence (Gaussian shaped CL peaks at around 528 nm/2.35 eV). The specimens show a surface topology with facetted hexagonal islands with a width of 1-2 mu m at a height of 50 nm. A correlation with spectrally resolved CL images shows: the yellow luminescence is homogeneously distributed over the whole of the specimens as are the pure screw dislocations with (b) over right arrow = [0001], while the UV luminescence is confined to troughs between adjacent hexagonal islands where edge type dislocations with (b) over right arrow = 1/3[<(2)over bar 110>]) or (b) over right arrow = 1/3[<2(11)over bar 3>]) form small angle boundaries. These dislocations do favour or at least do nut impair UV luminescence. Formation mechanisms for the different defect types and possibilities for their reduction are briefly considered. (C) 1997 Elsevier Science S.A.