Surface Structure of GaN(0001) in the Chemical Vapor Deposition Environment

We report the first observation of the atomic-scale structure of the GaN(0001) surface in the metal-organic chemical vapor deposition environment. Measurements were performed using in situ grazing-incidence x-ray scattering. We determined the surface equilibrium phase diagram as a function of temperature and ammonia partial pressure, which contains two phases with 1×1 and (√3×2√3)R30^∘ symmetries. The (√3×2√3)R30^∘ phase is found to have a novel ``missing row'' structure with 1/3 of the surface Ga atoms absent.