Comparative growth study of ultrathin Bi films on clean and oxygen-reconstructed Nb(110)

We present a detailed study of the growth of Bi films on superconducting Nb(110) substrates in dependence on the Bi coverages and the Nb surface quality. We find that Bi grows in a (110) orientation at low coverage equivalent to about five pseudomorphic monolayers (MLps) on clean Nb(110), but then undergoes a structural transition to Bi(111) below about 8 MLps. Comparison with two oxygen-reconstructed Nb(110) surfaces, the NbOx phases I and II, reveals that the film thickness at which the (110)-to-(111) transition takes place depends on the surface quality. Whereas it is observed at lower coverage for the NbOx phase I, our results indicate that Bi(110) remains stable on NbOx phase II up to the largest film thickness studied here, i.e., 18 ML. The quality and smoothness of the thin Bi films considerably depends on the cleanliness of the Nb substrate, revealing the most flat and defect-free Bi films grown on the oxygen-free clean Nb(110) surface.