Oxidized GaN(0001) surfaces studied by scanning tunneling microscopy and spectroscopy and by first-principles theory
Oxidized Ga-polar GaN surfaces have been studied both experimentally and theoretically. For in-situ oxidization at 550°C using molecular oxygen, Auger electron spectroscopy indicates a saturation oxygen coverage of 2.1 + / - 0.5 monolayers. For these surfaces scanning tunneling microscopy reveals two surface phases, one with 3rt(3)x3rt(3)-R30° periodicity and the other with disordered 2x periodicity. Scanning tunneling spectroscopy revealed a surface band gap with size close to that of GaN, indicating that any states of the oxide lie predominantly outside of the GaN gap. From first-principles total energy calculations of surface formation energies two models of energetically favorable surfaces structures are developed, with oxygen coverages of 1.25 and 2 monolayers, respectively. Both structures satisfy electron counting and have surface band gaps close in size to that of GaN, in agreement with experiment.