Synthesis of thin niobium films on silicon and study of their superconducting properties in the dimensional crossover region

Niobium films of 4–100 nm thickness were synthesized on a silicon substrate under ultrahigh vacuum conditions. Measurements of electrical resistance showed a high temperature of the superconducting transition $T_c$, in the range of 4.7–9.1 K, and extremely small transition widths $\Delta T_c$ in the range of 260–11 mK. The dependences of $T_c$ and $\Delta T_c$ on the magnetic field were studied, and superconducting coherence lengths and mean free paths of the conduction electrons were determined for different thicknesses of the synthesized films. A specific effect of the magnetic field on $\Delta$Tc was found, which reveals a transition from three-dimensional to two-dimensional superconductivity at thicknesses below 10 nm. The dependences of $T_c$ and $\Delta T_c$ on the films thickness and the magnitude of the magnetic field are discussed in the framework of existing concepts of superconductivity in thin films of superconducting metals.