EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

The use of superconducting thin films on copper are proven technology for application in superconducting radio frequency (SRF) cavities, with Nb-coated cavities already in use at both the LEP and LHC at CERN. NbN is a candidate material for multilayer (ML), superconductor-insulator-superconductor (SIS) film structures which are reported to improve superconducting properties further. NbN offers several advantages over Nb, such as a higher superconducting transition temperature and higher critical field. In this respect, single layer NbN thin films and Nb/AlN/NbN ML SIS films, have been deposited onto Cu substrates by DC magnetron sputtering. To understand and optimize the thin-film microstructure and the structure-property-relationship, cross sections of the films were investigated by transmission electron microscopy (TEM).

The cross sections are obtained by conventional preparation of TEM cross-sectional samples in conjunction with optimized, single-sector Ar-ion milling [1]. The TEM analysis of single layer NbN thin film samples includes the detailed characterization of the obtained microstructure of the NbN film as well as the NbN/Cu. For ML SIS films, the microstructure, morphological evolution and epitaxial relationship between layers are observed.

For high critical temperature NbN thin films, BF-TEM images revealed a well-ordered columnar morphology with a high degree of crystallographic texture as proven by electron diffraction. High entry field NbN thin films featured a nanocrystalline structure [2]. This shows that the superconducting properties of the NbN thin films are highly dependent on its crystallinity and morphology.

Acknowledgements:

The EASITrain project has received funding from the European Union's Horizon 2020 research and innovation programme under grant No. 764879.

References:

[1] L. Dieterle, B. Butz, E. Müller, Optimized Ar+-ion milling procedure for TEM cross-section sample preparation, Ultramicroscopy 111 (2011) 1636-1644

[2] S. Leith, M. Vogel, J. Fan, Superconducting NbN thin films for use in superconducting radio frequency cavities, Supercond. Sci. Technol. 34 (2021) 025006 (12pp)