Stable superconducting niobium ultrathin films
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Stable superconducting niobium ultrathin films Cécile Delacour, Luc Ortega, Bernard Pannetier and Vincent Bouchiat Institut Néel, CNRS-Université Joseph Fourier-Grenoble INP, BP 166, F-38042 Grenoble, France. ABSTRACT We report on a combined structural and electronic analysis of niobium ultrathin films (from 2.5 to 10 nm) epitaxially grown in ultra-high vacuum on atomically flat sapphire wafers. We demonstrate a structural transition in the early stages of Nb growth, which coincides with the onset of a superconducting-metallic transition (SMT). The SMT takes place on a very narrow thickness range (1 ML). The thinnest superconducting sample (3 nm/ 9ML) has an offset critical temperature above 4.2K and allows to be processed by standard nanofabrication techniques to generate air and time stable superconducting nanostructures. INTRODUCTION Superconductivity has been recently shown to survive down to extremely confined nanostructures such as metal monolayers [1] or clusters made of few organic molecules [2]. While these structures are extremely interesting to probe the ultimate limits of superconductivity, their studies are limited to in-situ measurements. Preserving a superconducting state in ultrathin films that can be processed by state-of-the-art nanofabrication techniques and withstand multiple cooling cycles remains technologically challenging and is timely for quantum devices applications. Having the highest critical temperature among elemental superconductors, niobium is an biquitous material for superconducting thin films and its performance is known to increase when epitaxy conditions can be reached. Hetero-epitaxy of niobium is best performed on sapphire substrates since both their lattice parameters and thermal dilatation coefficients match rather well [3]. Most combined structural and transport studies have involved A-plane (1000) oriented sapphire as the growing substrate. However the high step-edge roughness [4] and interfacial stress found for that crystal orientation is likely to be a probable source of rapid aging. Only few studies [5] have described Nb growth on R-plane oriented (1-102) sapphire. However, this orientation provides an ideal substrate to promote epitaxy of Nb with (001) orientation. We present here a combined structural and electronic transport analysis of the early growth stages of niobium films on that orientation with the proof of concept of the interest of these films for quantum device fabrication. EXPERIMENT DETAILS The niobium films are grown in ultra-high vacuum using a specific recipe: R-Plane oriented sapphire substrates that are cleaned using water-based surfactants in ultrasonics bath. The surface is then prepared by annealing during 1 hour in air at 1100°C. Wafers are transferred in the deposition chamber (base pressure ~10-10 torr). Before niobium deposition, in-situ cleaning of the surface is performed during several hours using Argon milling. Then, the substrate is heated at 660°C and Niobium is deposited using electron gun at a rate of 0.02 nm/s. The vacuum is kept at
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