Ultrathin epitaxial NbN superconducting films with high upper critical field grown at low temperature. Materials Research Letters, 9:8, 336-342. (https://doi.org/10.1080/21663831.2021.1919934)
 
Background
Niobium nitride (NbN) superconducting films are widely used in a variety of electronic devices due to their relatively high superconducting transition temperature (Tc) and excellent critical current density-magnetic field (Jc-H) characteristics, such as superconducting Nanowire single photon detector (SNSPD) and superconducting Josephson junction. Studies have shown that the superconductivity of niobium nitride films is not only related to the thickness of the film itself, but also closely related to the deposition method and deposition temperature used in the film deposition process. For some specific applications, such as superconducting quantum interferometers and fast single-flux quantum logic circuits, not only the thickness of the niobium nitride film has high requirements (3-5 nanometers), but also epitaxial growth technology is required. Therefore, in order to optimize the performance of the niobium nitride film in specific applications, it is important to study the relationship between its growth-structure-property.
 
Results presentation
In this article, the team of Mingwei Zhu from the American Applied Materials Corporation and the team of Professor Quanxi Jia of the University of Buffalo have cooperated, using magnetron reactive sputtering technology to successfully prepare high-quality ultra-thin (5) at a lower temperature (400 degrees Celsius). -50 nm) epitaxially grown niobium nitride superconducting film. It is important that high-quality superconducting films on wafer scale sizes can be prepared using Applied Materials' Endura® 300-mm Impulse™ PVD physical vapor deposition equipment. This paper also discusses the close relationship between its superconducting transition temperature, upper critical magnetic field strength, irreversible magnetic field strength, superconducting coherence length and other physical properties and film thickness. The results show that the superconducting transition temperature of the epitaxial niobium nitride film with (111) crystal orientation gradually decreases with the film thickness, and will completely disappear when the film thickness is 1.4 nm. Even at an ultra-thin thickness of 5 nanometers, it still has a superconducting transition temperature of 11.2K and an upper critical magnetic field of 36±2T.
Related important results were published on Materials Research Letters with the title "Ultrathin epitaxial NbN superconducting films with high upper critical field grown at low temperature".
Graphic guide
Figure 1 X-ray diffraction spectra of niobium nitride films with different thicknesses. The inset in the upper right corner shows the relationship between the niobium nitride lattice constant and the half-height width of the (111) diffraction peak and its thickness.
 
Figure 2 STEM analysis and EDS spectrum of a niobium nitride film with a thickness of 5 nm.
 
Figure 3 The relationship between superconducting transition temperature and transition width and film thickness.
 
Figure. 4 Superconducting properties of niobium nitride thin films related to magnetic field. (A) The relationship between the superconducting transition width and the magnetic field at different thicknesses. (B) Resistivity-temperature characteristic curves of niobium nitride films with a thickness of 5 nm under different magnetic field strengths (0-7T). (C) and (d): The temperature dependence of the upper critical magnetic field and irreversible magnetic field strength of 5nm and 50nm films.

 
Summary
This work has realized the epitaxial growth of niobium nitride films with a wafer-scale size under relatively low temperature conditions, and deeply discussed the close relationship between the superconducting properties of niobium nitride superconducting films and the film thickness. It proved the practical feasibility of using industrial-scale physical vapor deposition technology to produce high-performance epitaxial superconducting thin films for quantum devices.