Polarity determination of a GaN thin film on sapphire (0001) with x-ray standing waves

The x-ray standing wave technique was used to determine the polarity of a 1 μm thick GaN film grown by molecular beam epitaxy on an α-Al2O3(0001) single crystal. The standing wave was generated by x-ray diffraction from the GaN film. The Ga Kα fluorescence yield was recorded as a function of incidence angle within the range of the GaN(0002) reflection. Analysis of the data reveals that the film has grown with N polarity, i.e., the nitrogen atoms occupy the top half of the wurtzite (0001) bilayers.