Densely Rotational Twins in Epitaxial Superconducting δ-NbN on 4H-SiC Substrates

Hsiao-Wen Chang^1*, Vankayala Krishna Ranganayakulu^2,3,4, Syu-You Guan², Peng-Jen Chen², Min-Nan Ou², Yang-Yuan Chen², Tien-Ming Chuang², Chia-Seng Chang², Ting-Kuo Lee², Maw-Kuen Wu², Ming-Jye Wang^1,2

¹Institute of Astronomy and Astrophysics, Academia Sinica, Taipei, Taiwan
²Institute of Physics, Academia Sinica, Taipei, Taiwan
³Department of Engineering and System Science, National Tsing-Hua University, Hsinchu, Taiwan
⁴Taiwan International Graduate Program, Academia Sinica, Taipei, Taiwan

* Presenter:Hsiao-Wen Chang, email:hwchang@asiaa.sinica.edu.tw

The superconductivity of hexagonal ε-NbN was reported recently [1]. A reported Tc of 11.6 K in hexagonal ε-NbN powder prepared under 10 GPa pressure. Kou et. al. calculated the band structures of NbN polymorphs and found that the ε -NbN should be an insulator with topological properties [2]. To date, no further reports on the superconductivity of ε-NbN are published. We intentionally deposited NbN film on (0001) 4H-SiC substrate, with good lattice match, at 750°C by magnetron sputtering method to grow the hexagonal NbN superconductor. Rather than the hexagonal ε-NbN, the deposited films are identified as (111)-orientated δ-NbN phase with densely rotational (60°) twins. The HRTEM and HAADF-STEM images confirm the existence of nano-twins in the (111)-orientated δ-NbN film. Odd diffraction spots in SAED images are reconstructed through the vector analysis method of the double diffraction model [3]. Having high density of twin in (111)-orientated δ-NbN film is natural because of no preference in bonding energy with the Si-face substrate for both rotational twins.

In comparing with the (001)-orientated δ-NbN films, the (111)-orientated δ-NbN films have a higher normal state resistivity and lower Tc. Their Tc, normal state resistivity, and thickness follow the scaling law well but with a larger coefficient [4]. The STM results show excellent spatial uniformity of superconducting energy gap although the deposited (111)-orientated δ-NbN films have densely rotational twins.

[1] Y. Zou, X. Qi, C. Zhang, S. Ma, W. Zhang, Y. Li, et al., "Discovery of Superconductivity in Hard Hexagonal epsilon-NbN," Sci Rep, vol. 6, p. 22330, Feb 29 2016.
[2] K. R. Bahu and G.-Y. Guo, "Electron-phonon coupling, superconductivity, and nontrivial band topology in NbN polytypes," Phys. Rev. B, vol. 99, 104508, 2019.
[3] L. A. Tietz, C. B. Carter, and S. McKernan, "Top-bottom effects in double diffraction," Ultramicroscopy, vol. 60, pp. 241-246, 1995.
[4] Y. Tao, "Scaling Laws for Thin Films near the Superconducting-to-Insulating Transitio," Sci. Rep., vol. 6, 23863, 2016.

Keywords: Magnetron sputtering, Epitaxial growth, Nano twin, Niobium Nitride, Superconducting material