On Nb2O5growth and tunneling through Nb2O5

Nb-Nb2O5interfaces are not only well known for deteriorated superconductivity but also for contradictory results on barrier height sampled by tunneling electrons. According to the Cabrera-Mott theory tunneling electrons define the oxide growth and thus oxidation and tunneling are linked. The oxidation of Nb is dominated by the growth of Nb2O5microcristallites and by oxygen diffusion. Both effects heavily strain the Nb-Nb2O5interface creating defects in Nb into which O is injected. This causes defects in Nb2O5as counterpart. These defect system weaken locally the superconductivity of Nb and the insulating properties of Nb2O5yielding so a serrated and eroded transition superconductor - insulator. In Nb3Sn-, NbC-, NbN-, ... oxidation the defect creation in the metal is reduced yielding so thinner and less defective Nb2O5than in Nb oxidation. Tunnel measurements allow quantitatively the following classification: -NbO6octahedra blocks (≥ 1 nm) with eV, with -channels with eV in between and with -localized electron states /cm³eV as extended and localized defects. For Nb oxidation the channels are linked to NbOxlumps ( K) at the Nb surface. These defects yield the tunnel channels and the tunnel anomalies encountered in Nb2O5dominating below 50 meV the tunnel current.