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 (${\mathrm{ML}}^{\mathrm{ps}}$) on clean Nb(110), but then undergoes a structural transition to Bi(111) below about 8 ${\mathrm{ML}}^{\mathrm{ps}}$. Comparison with two oxygen-reconstructed Nb(110) surfaces, the $\mathrm{Nb}{\mathrm{O}}_x$ 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 $\mathrm{Nb}{\mathrm{O}}_x$ phase I, our results indicate that Bi(110) remains stable on $\mathrm{Nb}{\mathrm{O}}_x$ 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.

中文翻译：

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清洁和氧还原的Nb（110）上超薄Bi膜的比较生长研究

我们提出了根据Bi覆盖率和Nb表面质量在超导Nb（110）衬底上Bi膜的生长的详细研究。我们发现Bi以（110）方向生长，覆盖率低，相当于大约五个伪晶单层（${\mathrm{ML}}^{\mathrm{ps}}$）在干净的Nb（110）上，但随后在低于约8的温度下发生结构转变为Bi（111） ${\mathrm{ML}}^{\mathrm{ps}}$。与两个氧气重建的Nb（110）表面比较，$铌{\mathrm{Ø}}_X$阶段I和阶段II揭示了发生（110）到（111）转变的膜厚取决于表面质量。而在较低的覆盖范围内观察到$铌{\mathrm{Ø}}_X$ 第一阶段，我们的结果表明Bi（110）在 $铌{\mathrm{Ø}}_X$第二阶段直到此处研究的最大膜厚度，即18 ML。Bi薄膜的质量和光滑度在很大程度上取决于Nb衬底的清洁度，显示出在无氧的清洁Nb（110）表面上生长的最平坦且无缺陷的Bi薄膜。