Superconductivity Dependence on Epitaxial NbN Film Thickness

• Published in: IEEE transactions on applied superconductivity Vol. 29; no. 5; pp. 1 - 5
• Main Authors: Zhang, Qiyu, Wang, Huiwu, Tang, Xin, Peng, Wei, Wang, Zhen
• Format: Journal Article
• Language: English
• Published: New York IEEE 01.08.2019; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Carrier density; Charge carrier density; Coherence length; Conductivity; critical temperature; Crystal structure; Dependence; Electrical resistivity; Epitaxial growth; Film thickness; Hall effect; Magnetron sputtering; NbN film; Niobium compounds; Niobium nitride; Residual resistivity; Resistivity ratio; Single crystals; Sputtering; Substrates; Superconductivity; Temperature measurement
• ISSN: 1051-8223; 1558-2515
• DOI: 10.1109/TASC.2019.2902032

A series of NbN films (2.2-200 nm thick) were epitaxially grown on (100) oriented single crystal MgO substrates using reactive dc magnetron sputtering. As the film thickness decreased, the superconducting critical temperature (TC) and the residual resistivity ratio both decreased, the normal state resistivity increased, while the zero temperature coherence length remained basically unchanged. In addition, a negative linear relationship existed between the TC and the normal state resistivity at 20 K. Hall Effect measurements showed that the carrier density of the 200-nm-thick NbN film that exhibited the highest TC (16.63 K) decreased from 1.12 × 10 23 to 5.56 × 10 22 e/cm -3 in going to the 2.2-nm-thick NbN film that exhibited the lowest TC (9.28 K). By fitting the data using McMillan theory for strong coupling superconductors, the TC for the NbN films was determined directly from the thickness-dependent carrier density.