Oral Presentation

Fabrication and Superconducting properties of ultra-thin NbN film

Author(s): H. W. Chang[1], H. Hsieh[1,2], C. L. Wang[1], Y. R. Huang[1], T. J. Chen[1], and M. J. Wang[1,3] ([1]Institute of Astronomy and Astrophysics, Academia Sinica [2]Department of Physics, National Taiwan University [3]Institute of Physics, Academia Sinica)

Presenter: Hsiao Wen Chang (Institute of Astronomy and Astrophysics, Academia Sinica)

The superconducting NbN ultrathin films are superior to other materials for the hot-electron bolometer (HEB) mixer application in many respects, such as having a high superconducting transition temperature (TC), a short intrinsic electron–phonon interaction time, and a high superconducting critical current density. These properties make NbN-based HEB mixers more convincing for practical applications [1].
We have realized the epitaxial growth of ultrathin δ-NbN films on (100)-oriented 3C-SiC/Si substrates by dc reactive magnetron sputtering at 760oC with a deposition rate of 0.054 nm/sec. High-resolution transmission electron microscope images confirm the excellent epitaxy of these films. Even with a thickness of 1.3 nm (~3 unit-cells), the δ-NbN film 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, and good stability in ambient environment [2].
Furthermore, we developed a technique for protecting ultra-thin NbN film during the HEB mixer fabrication by using AlN as a protection layer. A 20-nm-thick AlN over-layer was sputtered under the low growth temperature (~20oC) prevents ultra-thin NbN films degradation through HEB fabrication. The deposition rate of AlN films was about 1.6 nm/min. In the HEB fabrication tests the degradation is monitored by measuring the DC resistance of the devices during the test. The results show that using AlN cover-layer protection increases the device lifetime significantly, which is a promising for development of high quality superconducting THz HEB mixers.

[1] Shurakov A, Lobanov Y and Goltsman G 2016 Superconducting hot-electron bolometer: from the discovery of hot-electron phenomena to practical applications Supercond. Sci. Technol. 29 023001
[2] 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