We investigated the surface chemistry and valence band structure of as-grown thin InN films as well as their change upon exposure to oxygen and water in a combined UHV molecular beam epitaxy (MBE) and surface analysis system (base pressure < 2x10^-10 mbar). Thin InN(0001) films were grown on GaN(0001)/Al₂O₃ substrates by plasma assisted MBE and in-situ characterised by reflection high energy electron diffraction (RHEED) and photoelectron spectroscopy (XPS, UPS). In dependence of the growth parameters, we observed different surface reconstructions and corresponding electron states above the valence band maximum. The oxygen and water exposure was directly performed on the as-grown, contamination-free InN surfaces without any further preparation. Changes of the chemical surface states as well as the electronic properties upon continuous exposure to oxygen or water were detected. The surface states observed on clean InN films disappeared in both cases and two similar adsorbate-related electron states at around 5 eV and 10 eV were formed in the valence band in combination with changes in the work function and surface band bending. Furthermore, the reactivity of the different molecules was compared and revealed, that the reactivity of water is much higher than that of oxygen. Additionally, these molecules have different impact on the work function f and surface band bending. In the case of oxygen exposure, an increase in f and a reduction of the downward band bending was found. On the other hand, water decreases f and leads to a further downward shift of the bands at the surface. The details of the coverage-dependent changes will be discussed with respect to surface dipole formation, charge transfer and changes in the surface accumulation layer. Furthermore during oxidation, different chemical oxygen bonds are formed, while a direct assignment to In-O or N-O bonds is difficult due to the change in core level peak shape.

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