Oxidation of epitaxial Nb(110) films on a-plane sapphire substrates: an X-ray study

We studied the oxidation of epitaxial Nb(110) films on a-plane sapphire substrates at elevated temperatures with X-ray scattering techniques. Comparing atmospheric versus dry oxidation, we observe a different behaviour resulting in the formation of chemically and structurally different oxides. Under atmospheric conditions we obtain an unlimited growth of an amorphous Nb2O5 layer, while dry oxidation at 340 C and 10-3 mbar results in the growth of a crystalline and passivating NbO(111) layer with a final thickness of about 50 A. In addition atmospherically oxidized samples show the formation of an oxygen lattice gas in the remaining Nb(110) film, which is not observed parallel to the growth of NbO during dry oxidation. The use of thin film systems together with X-ray diffraction methods provides a powerful combination to investigate the oxidation process under different conditions down to the atomic scale and thus complements oxidation studies performed in the past. (orig.)