Thermal effect on the oxides on Nb(100) studied by synchrotron-radiation x-ray photoelectron spectroscopy

Abstract

Synchrotron-radiation glancing-incidence and angle-resolved photoelectron spectroscopy (PES) is used to study the oxides grown in air on a single-crystal Nb(100) surface. Both core-level and valance-band PES are measured for various heat treatments. Glancing-incidence excitation was used to characterize the outer 2-nm Nb2O5 layer without influence from the substrate and to profile through various oxides by varying the incidence angle. Immediately adjacent to this amorphous layer is a layer of NbO2, which contributes to the density of states at 1.2eV below the Fermi level. Thereafter, there exist other oxides that interface with the metal substrate. Annealing at 430K induces a “mild” chemical reaction, which is contained essentially in the oxide layer. Annealing at 550K causes oxygen diffusion into the metal and leaves an oxide layer consisting primarily of Nb2O and a roughened surface, as evidenced by angle-resolved photoemission measurements. Oxide composition alteration caused by annealing at these moderated temperatures is believed to be interface mediated.