Growth and Characterization of Epitaxial NbN Superconducting Films on SiC Substrate.
Hsiao Wen Chang1*, Hsun Hsieh1,2, Chun Lun Wang1, Yen Ru Huang1, Tse Jun Chen1, Ming Jye Wang1,3
1Institute of Astronomy and Astrophysics, Academia Sinica, Taipei, Taiwan
2Department of Physics, National Taiwan University, Taipei, Taiwan
3Institute of Physics, Academia Sinica, Taipei, Taiwan
* Presenter:Hsiao Wen Chang
The Nb-N system is interesting because of the superconducting behavior of the δ-NbN phase are promising superconducting components for use in high-frequency and high-speed devices because of their prominent superconductivity [1]. The new discovery of superconductivity in polycrystalline hexagonal ε-NbN has been discovered by Zou et al. [2], recently. The transition temperature (TC) is below ∼11.6 K, and exhibits higher hardness, bulk and shear moduli compared to those for cubic δ-NbN. Wang et. al. [3] showed that the cubic δ-NbN was metastable and the hexagonal-structured NbN (e.g. WC-type structure) was more stable than the cubic counterpart.
We have realized the epitaxial growth of NbN films on (100)-oriented 3C-SiC/Si substrates and (0001)-orientated 4H-SiC substrate by dc reactive magnetron sputtering at 760oC with a deposition rate about 0.05 nm/sec. High-resolution transmission electron microscope images confirm the excellent epitaxy of these films. The (TC) of a 21.5 nm-NbN film deposited on 4H-SiC substrate is 12.4 K. The diffraction pattern reveals the film exhibits a hexagonal structure. It's also possible to evaluate both c- and a-axis parameters of the NbN film deposited on 4H-SiC substrate that are 0.4998 nm and 0.3058 nm, respectively. These values of the lattice constants are close to the reference value of Nb2N. Moreover, the deposited δ-NbN ultrathin films show excellent superconducting and normal state properties compared with the reported results. Even with a thickness of 1.3 nm (~3 unit-cells), the δ-NbN film deposited on 3C-SiC substrate shows a superconducting transition above 8 K. Our ultrathin δ-NbN films demonstrate a long Ginzburg-Landau superconducting coherent length (ξGL (0)>5 nm) with critical current density about 2.2 MA/cm2. The TC and the normal state resistivity Rn of film degrade negligibly after being stored in an ambient environment for more than 100 days, indicating good stability for device application [4].

[1] Shurakov A, Lobanov Y and Goltsman G, Superconducting hot-electron bolometer: from the discovery of hot-electron phenomena to practical applications, Supercond. Sci. Technol. 29 023001 (2016).
[2] Zou Y. et al., Discovery of Superconductivity in Hard Hexagonal ε-NbN, Sci. Rep. 6, 22330 (2016).
[3] Wang, Z. H. et al., Pressure-induced structural transition and thermodynamic properties of NbN and effect of metallic bonding on its hardness. Europhys. Lett. 92, 56002 (2010).
[4] Chang H W, Wang C L, Huang Y R, Chen T J and Wang M J 2017 Growth and characterization of few unit-cell NbN superconducting films on 3C-SiC/Si substrate Supercond. Sci. Technol. 30 115010.

Keywords: magnetron sputtering, epitaxial growth, superconducting film, δ phase-NbN, hexagonal phase-NbN