On the natural Nb 2O 5 growth on Nb at room temperature

Abstract

Abstract Photoelectrons allow accurate measurements of the oxide growth on Nb below 400 K with respect to the stoichiometry of the different oxides, the growth rates and the adsorbates. Whereas at 300 K the oxidation of Nb in H2O saturates at 6 nm Nb2O5 in some minutes, the oxidation in dry O2 is slowly reaching 6 nm Nb2O5 only after some days. The slowness of this growth results in the separation of the following steps in the oxide growth on Nb: (a) Rapid growth of about 0.5 nm interface oxide NbOx (x ≈1). (b) Slow, logarithmic growth of Nb2O5 (∫ ln(tpo2 + 1)), which becomes more rapid with O2 pressure, (c) Depending on the oxidation conditions the — logarithmic — growth becomes more rapid above 2 nm Nb2O5. (d) Above 6 nm the rapid Nb2O5 growth is levelling off and changing to the slow, thermally activated diffusion. This Nb2O5 growth can be explained by an O2 pressure dependent Cabrera-Mott process up to 6 nm. The saturation thickness of 6 nm is given by the diminishing electron tunnel probability through the Nb2O5 layer.