Reactivity and morphology of (10(12)over-bar)-faceted and (3x3)-reconstructed GaN(000(1)over-bar) epilayers grown on sapphire(0001)

We have investigated structural and chemical properties of heteroepitaxial films of GaN on Al2O3(0001) using low energy electron diffraction (LEED), x-ray photoelectron spectroscopy (XPS) and high resolution electron energy loss spectroscopy (HREELS). Depending on the preparation conditions, two faceted surfaces with different morphologies and a flat, (3 x 3)reconstructed surface were obtained. For one of the faceted surfaces a (10<(12)over bar>) orientation of the facet planes could be determined. For a sample grown by metal-organic chemical vapour deposition (MOCVD) N-termination and (000 (1) over bar) polarity could be established by a combination of HREELS and angle-resolved XPS. For a different sample grown by molecular beam epitaxy (MBE) the same (000 (1) over bar) polarity could be identified, this time concluded from the observation of the (3 x 3) reconstruction in LEED, which moreover is shown to be Ga rich by the present XPS measurements. Furthermore, we find the (000 (1) over bar) surface to be highly reactive, causing water from the residual gas to adsorb dissociatively. Finally exposure to atomic hydrogen leads to the observation in HREELS of N-H and Ga-H stretching vibrations at 403 meV and 227-247 meV, respectively, as expected for a faceted surface. Based on these observations, a structural model for the facets is proposed.