Surface reconstruction and their transition have been examined for MBE-grown GaN epitaxial layers. Several types of reconstruction were observed depending on the growth condition, crystal structure etc., and the reconstruction transitions were found to be related to effective III/V stoichiometry on the growing surface. It is shown that the surface reconstruction phase diagram and its phase transition line are useful for the optimization of MBE growth of GaN. Little amount of As4 residual pressure was found to affect the structure of GaN growing surfaces. These As surfactant effects are discussed and the growth of cubic GaN under small amount of As4 pressure is reported. The correspondence between the lattice polarity of hexagonal GaN epilayers and the surface reconstruction was confirmed by CAICISS technique. The quality of the epilayers which are grown so that the Ga-polarity is achieved is much improved.
