Stability of N- and Ga-polarity GaN surfaces during the growth interruption studied by reflection high-energy electron diffraction

GaN films with N- and Ga-polarity were grown on sapphire (0001) substrates using different buffer layers by plasma-assisted molecular-beam epitaxy. The surface stability of each lattice-polarity film during the growth interruption was studied by reflection high-energy electron diffraction (RHEED). It was found that the surface of N-polarity film was unstable against the exposure to the nitrogen plasma flux, while that of Ga-polarity one was stable. This provides a method to clarify the lattice polarity by the in situ RHEED observation directly. A model is proposed to explain the observed phenomenon, where the origin of the phenomenon is mainly attributed to the differences in surface dynamics processes and morphologies between the two kinds of lattice-polarity films.