Adsorption of film-forming species on NbO and NbO2 surfaces

Based on a mass-energy analysis, Nb, NbO, NbO2, NbO3, O, O-2, and O-3 film forming-species occur in an Nb-O-2-Ar sputtering plasma. H, H-2, H2O, and OH also emerge due to high-vacuum conditions. Surface diffusion pathways for fast moving species (H, O, and OH) and adsorption energy are obtained by density functional theory. H rattles, O hops in a [110] direction, and OH moves in < 100 > channels on NbO(001), while surface diffusion on NbO2(001) is not species specific. There are strongly adsorbed species (Nb, O, O-2, and O-3) and weakly adsorbed species (Nb-O clusters and H containing molecules). H2O, O-2, and O-3 dissociate, while Nb-O clusters and OH undergo nondissociative adsorption and H-2 is repelled. H incorporation is thus enabled via H and OH adsorption. Nb and O seem to promote an epitaxial growth of NbO, but not of NbO2. An NbO cluster on NbO(001) and an NbO2 cluster on NbO2(001) may sustain epitaxial growth. All other Nb-O clusters lead to a growth disruption or a nanostructure formation. In general, NbO can likely grow in a defect-free fashion, while this is not the case for NbO2 under kinetically limited growth conditions. (C) 2017 American Vacuum Society.