GaN exists in both wurtzite and zinc-blende phases and the growths of the two on its ͑0001͒ or ͑111͒ surfaces are achieved by choosing proper deposition conditions of molecular-beam epitaxy ͑MBE͒. At low substrate temperatures but high gallium fluxes, metastable zinc-blende GaN films are obtained, whereas at high temperatures and/or using high nitrogen fluxes, equilibrium wurtzite phase GaN epilayers resulted. This dependence of crystal structure on substrate temperature and source flux is not

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... ffected by deposition rate. Rather, the initial stage nucleation kinetics plays a primary role in determining the crystallographic structures of epitaxial GaN by MBE.