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

• Published in: IEEE transactions on applied superconductivity 2009-06, Vol.19 (3), p.3375-3378
• Main Authors: Villegier, J.-C., Bouat, S., Cavalier, P., Setzu, R., Espiau de Lamaestre, R., Jorel, C., Odier, P., Guillet, B., Mechin, L., Chauvat, M.P., Ruterana, P.
• Format: Article
• Language: English
• Subjects: Applied sciences; Circuit properties; Conductivity; Crystallization; Deposition; Electric, optical and optoelectronic circuits; Electrical engineering. Electrical power engineering; Electromagnets; Electronic circuits; Electronics; Engineering Sciences; Epitaxial growth; Exact sciences and technology; Josephson epitaxial junctions; Josephson junctions; Micro and nanotechnologies; Microelectronics; Microwave circuits, microwave integrated circuits, microwave transmission lines, submillimeter wave circuits; niobium nitride compounds; Optoelectronic devices; Orientation; Roughness; Sapphire; Semiconductor electronics. Microelectronics. Optoelectronics. Solid state devices; Signal convertors; Sputtering; Substrates; Superconducting device noise; Superconducting epitaxial layers; superconducting filaments and wires; Superconducting films; Superconductivity; Thick films; thin films; Various equipment and components; X-ray photoelectron spectroscopy
• ISSN: 1051-8223; 1558-2515
• DOI: 10.1109/TASC.2009.2019243

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 Tc, higher Jc and lower resistivity have been obtained by growing un-twined (110) oriented NbN layers on M-plane orientation of sapphire. Uniform, low roughness, 3-5 nm thick films with Tc above 12 K and Jc above 5 MA/cm 2 at 4.2 K were obtained. Characterizations by TEM, AFM and X-Ray diffraction evidence that growth of un-twined 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 600degC to 300degC 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 Nb 2 N y O 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.