Control of thin NbN film superconducting properties by ScN buffer layer

This work investigates the effect of a scandium nitride buffer layer on the superconducting properties of niobium nitride thin films. The use of a ScN buffer layer significantly improves the characteristics of 29 nm thick NbN films: the critical temperature Tc increases from 9 K to 12.5 K, while the resistivity at 20 K decreases from 330 mkOhmcm to 210 mkOhmcm compared to films without a buffer layer. These enhancements are attributed to the better lattice matching between NbN and ScN, which results in a higher quality crystal lattice of the NbN film, as confirmed by transmission electron microscopy and X-ray diffraction data.

💡 Research Summary

This paper investigates the influence of a scandium nitride (ScN) buffer layer on the superconducting performance of niobium nitride (NbN) thin films deposited on silicon substrates with a thermally grown SiO₂ layer. A series of multilayer structures (Si/SiO₂/ScN/NbN) were fabricated by reactive magnetron sputtering at room temperature. The ScN thickness was varied from 3.6 nm to 38 nm, while the NbN thickness was kept constant at 29 nm. Reference NbN films without a buffer layer were also prepared under identical conditions for comparison.

Key material properties of ScN make it an attractive buffer: it is chemically and thermally stable, non‑hygroscopic, dielectric, and possesses a cubic NaCl‑type lattice with a lattice constant of 0.450 nm, which is very close to NbN’s 0.439 nm. This near‑perfect lattice match is expected to reduce strain and promote epitaxial-like growth of NbN.

Structural characterization was performed using X‑ray reflectometry (XRR), X‑ray diffraction (XRD), and cross‑sectional transmission electron microscopy (TEM). XRR confirmed that the NbN layer thickness remained precisely 29 nm across all samples and revealed a thin surface NbₓOᵧ layer formed by ambient oxidation. TEM images showed that for ScN layers thicker than ~10 nm a columnar grain structure develops in both ScN and NbN, with an average column width of ~5 nm. NbN grains appear to inherit the crystallographic orientation of the underlying ScN, and the NbN lattice parameter shifts toward that of ScN as the buffer becomes thicker. XRD peak narrowing and position shifts corroborate this lattice relaxation.

Electrical transport was measured from 2.6 K to 20 K using patterned microbridges (≈5 µm wide, 628 µm long). The critical temperature (Tc) was defined at the maximum of dR/dT. The reference NbN film (no buffer) exhibited Tc ≈ 9 K and a resistivity at 20 K (ρ20K) of 330 µΩ·cm, with a residual‑resistance ratio (RRR) of ~0.77. Introducing a 3.6 nm ScN layer raised Tc to 9.9 K and lowered ρ20K to 265 µΩ·cm. As the ScN thickness increased to 30 nm, Tc reached 12.5–12.9 K and ρ20K dropped to 210 µΩ·cm, representing a ~30 % reduction in resistivity and a ~40 % increase in Tc. The RRR remained essentially unchanged (~0.8), indicating that the bulk scattering mechanisms in NbN were not adversely affected.

Critical current (Ic) versus temperature was measured in voltage‑source mode. The Ic(T) curves follow the two‑fluid model: Ic(T)=Ic(0)