MICROSTRUCTURE AND GROWTH-MORPHOLOGY AS RELATED TO ELECTROOPTICAL PROPERTIES OF HETEROEPITAXIAL WURTZITE GAN ON SAPPHIRE-(0001) SUBSTRATES

We investigate the microstructural development during growth and the r elated electro-optical properties of gallium nitride (GaN) films depos ited on (0001) sapphire substrates by gas source molecular beam epitax y. We use transmission electron microscopy, atomic force microscopy, s canning tunnelling microscopy, spectrally resolved cathodoluminescence (CL) mapping and photoluminescence (PL). We report on specimens exhib ition UV luminescence (band-to-band transition at 358 nm/3.46 eV) in C L/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 arro w = [0001], while the UV luminescence is confined to troughs between a djacent hexagonal islands where edge type dislocations with (b) over r ight 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 fa vour or at least do nut impair UV luminescence. Formation mechanisms f or the different defect types and possibilities for their reduction ar e briefly considered. (C) 1997 Elsevier Science S.A.