Theoretical study of O 2 adsorption on GaN surfaces

Abstract

The adsorption of oxygen on the GaN surfaces is calculated with DFT method combining a cluster model. From the total energy minimization, we found that the O 2 lying-down orientation are favored over upright orientation, and the chemisorption with the O–O axis parallel along the bridge site on the GaN (0001) surface is the most stable. The electrons transferred from the substrate to the O 2 molecule occupy the O 2 antibonding orbital, thus leading to the bond strength weakened, which can be reflected by the elongated O 2 bond length, 1.51 Å, compared to 1.21 Å of the free O 2. The energy barrier estimated for the dissociation of O 2 at the bridge site on the GaN (0001) surface is 0.7 eV. The dissociated O adatom favors the three-fold hollow (fcc) site with the Ga–O bond length 2.14 Å on the (0001) surface and 1.90 Å on the (000 1 ̄ ) surface, respectively, similarly with the bond length in bulk Ga 2O 3 ranging from 1.8 to 2.1 Å. The significantly small adsorption height (0.47 Å) of the atomic O on the (000 1 ̄ ) surface make us believe that the oxygen will be more easily incorporated on this surface, which means that the oxidation occurs on the (000 1 ̄ ) surface more easily.