First principle studies of oxygen absorption on GaN(0001) surface with steps

We performed systematical first principle calculations on the atomic and electronic structures of GaN(0001) surface with steps, and investigated the O₂ adsorption behavior with the terraces and edges. Energetically favorite step configuration was found, Ga atoms on the terrace relaxed up and down alternately after structural optimization. The upwardly shifted Ga atoms and step edges caused the surface states, in which the isolated surface states are from step edges. 17 possible adsorption sites of O₂ on the stepped surface were considered. For the situation with the lowest adsorption energy, O₂ oxidized the Ga–Ga dimer at the Ga-terminated edge, forming Ga–O bonds and passivating the surface states originated from the Ga–Ga dimer. O₂ adsorbed on the terraces interact weakly with the surface, and retained molecular form geometrically and electronically, leaving the localized surface states. These theoretical understandings may provide useful guidance for designing and controlling the surfaces and interfaces of the GaN devices.