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 diffract ion (LEED), X-ray photoelectron spectroscopy (XPS) and high-resolution elec tron energy loss spectroscopy (HREELS). The orientation of facets induced b y annealing at elevated temperatures was determined from the extra LEED dif fraction 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. Wa ter 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 vib rations are observed initially, while the Ga-H vibration becomes prominent only after the N-H peak has saturated. A reconstructed (3 x 3) phase develo ps upon heating the sample at moderate temparatures under simultaneous Ga e xposure which again backs our orientation assessment as this (3 x 3) phase had previously been attributed to GaN(000 (1) over bar).