High energy Urbach characteristic observed for gallium nitride amorphous surface oxide

We have observed an “above band-gap” Urbach like characteristic for gallium nitride films (at the high energy side of the band-edge). A combination of X-ray diffraction, secondary ion mass spectroscopy and optical transmission measurements were taken for gallium nitride samples of different thickness. From this data we demonstrate that the high energy Urbach like characteristic is related to the presence of an amorphous surface oxide. It is shown to dominate the absorption spectra of thin gallium nitride samples, for which the influence of surface oxidation is strongest.