Polarity, morphology and reactivity of epitaxial GaN films on Al2O3(0001)

The surfaces of basal plane oriented GaN films heteroexpitaxially grown on sapphire, Al2O3(0001), were investigated using low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) and high-resolution electron energy loss spectroscopy (HREELS). The orientation of facets induced by annealing at elevated temperatures was determined from the extra LEED diffraction spots emerging from the surface. Based on angle dependent XPS data the orientation of our samples could be determined to be GaN(000 (1) over bar), the nominally N terminated face. This observation is corroborated by the chemical reactivity of the surface which was investigated by HREELS. Water from the residual gas adsorbs dissociatively with the hydrogen binding exclusively to N-sites as indicated by the vibrational spectrum. Similarily, upon exposing the surface to atomic hydrogen primarily N-H stretching vibrations are observed initially, while the Ga-H vibration becomes prominent only after the N-H peak has saturated. A reconstructed (3 x 3) phase develops upon heating the sample at moderate temparatures under simultaneous Ga exposure which again backs our orientation assessment as this (3 x 3) phase had previously been attributed to GaN(000 (1) over bar).