Comparative Study of H₂O and O₂Adsorption on the GaN Surface

GaN is an excellent candidate for photocatalytic, optoelectronic, and high-power devices, and the interaction between the GaN surface and ambient species, especially H2O and O2, has drawn exceptional attention. In this study, the evolution of the n-GaN(0001) surface geometric structure and the corresponding band bending (a key parameter that describes the surface electronic structure of a semiconductor) during H2O and O2 exposure is predicted from first-principles calculations and confirmed by ambient pressure X-ray photoemission spectroscopy (AP-XPS) measurements. Overall, the AP-XPS results are in good agreement with the predictions, and we discuss the possible origin of the difference in the band bending of H2O- and O2-adsorbed surfaces. In the case of the O2-exposed surface, upward band bending is observed above the effective coverage of 3/4 ML (3/8 ML of O atoms) because the Fermi level becomes pinned to the N-2p-originated surface states, which is formed through Ga-N bond scission by O atom adsorption and insertion into the slab. As for the H2O-exposed surface, the saturated band bending depends on the H2O supply rate: When the supply rate is high, half dissociation of H2O is dominant and the band bending approaches the flat-band condition due to the termination of surface Ga dangling bonds by H and OH; when the supply rate is low, the saturated band bending matches that of the O2-adsorbed surface, presumably due to the O atoms that are formed by full dissociation of H2O.