Epitaxial Growth of Sputtered Ultra-thin NbN Layers and Junctions on Sapphire

J-C. Villegier, S. Bouat, P. Cavalier, R. Setzu, R. Espiau de Lamaëstre,
C. Jorel, Ph. Odier, B. Guillet, L. Mechin,  M. P. Chauvat, P. Ruterana

Abstract—High crystalline quality of ultra-thin NbN layers and of NbN-MgO-NbN tri-layers, epitaxially grown by DC-magnetron sputtering in the superconducting B1-cubic phase has been achieved in a reproducible way on three different orientations of sapphire substrates i.e. R-, A- and M-planes. Significant improvements such as higher T_c, higher J_c and lower resistivity have been obtained by growing untwined (110) oriented NbN layers on M-plane orientation of sapphire.  Uniform, low roughness, 3-5 nm thick films with T_c above 12 K and J_c above 5 MA/cm² at 4.2K were obtained. Characterizations by TEM, AFM and X-Ray diffraction evidence that growth of untwined NbN on M-plane lead to a better epitaxy in comparison with twinned films observed on other sapphire orientations. We observe that the reduction of the substrate temperature from 600°C to 300°C during the deposition of NbN or NbN-MgO-NbN layers thicker than 20 nm prevents the nucleation of the competing HCP NbN phase. Moreover, 1.5 nm thick AlN or MgO over-layers sputtered in-situ prevent ultra-thin NbN films degradation through aging. The formation of Nb₂N_yO_5-x (~2.2 nm) at the unprotected NbN surface and of interfacial NbO (~0.7 nm) native oxides has been observed by XPS. It is forecasted that such improvements in ultra-thin NbN films deposited uniformly on 3 and 4 inch sapphire wafers is a key in the future development of superconducting single photon detectors, THz HEB mixers and also in low noise quantum analogical and digital Josephson devices.

Index Terms— Josephson epitaxial junctions, Niobium nitride compounds, Superconducting filaments and wires, Superconducting epitaxial layers, Thin films.

Manuscript received 26 August 2008.
This work was supported in part by the Grant EC Sinphonia NMP4-CT-2005-16433 and by the Grant ’HyperSCAN’ANR T-COM-06-023.
S. Bouat, P. Cavalier, R. Setzu and J-C. Villegier are with the CEA, Institute of Nanosciences and Cryogenics, SPSMS, CEA-Grenoble, 38054 Grenoble-Cedex-9, France (corresponding author J-C Villegier, phone: 33+438783587; fax: 33+438785096; e-mail: jean-claude.villegier@cea.fr).
R. Espiau de Lamaëstre is with CEA-Leti-MINATEC, 38054 Grenoble, Fr.
C. Jorel is with Leti-MINATEC/CNRS-LTM, 38054 Grenoble-Cdx-9, Fr.  
Ph. Odier is with the CNRS, Institut Néel, 38042 Grenoble-Cedex, Fr.
B. Guillet, L. Mechin are with GREYC(UMR 6072) – CNRS – ENSICAEN – Univ. Caen Basse-Normandie; M.P. Chauvat and P. Ruterana with CIMAP(UMR 6252) – CNRS – CEA - ENSICAEN, 6 bd Marechal Juin, 14050 Caen-Cedex, Fr.

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