Adsorption of film-forming species on NbO and NbO2 surfaces

Abstract

Based on a mass-energy analysis, Nb, NbO, NbO2, NbO3, O, O2, and O3 film forming-species occur in an Nb-O2-Ar sputtering plasma. H, H2, 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, O2, and O3) and weakly adsorbed species (Nb-O clusters and H containing molecules). H2O, O2, and O3 dissociate, while Nb-O clusters and OH undergo nondissociative adsorption and H2 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.