For quite some time we have been working on techniques to combine ultra-clean carbon nanotubes and their regular electronic spectrum with superconducting material systems. One of our objectives is to perform high-frequency measurements on carbon nanotube nano-electromechanical systems at millikelvin temperatures. With this in mind we have established the fabrication and characterization of compatible superconducting coplanar resonators in our research group. A serious challenge here was that the high-temperature process of carbon nanotube growth destroys most metal films, or if not, at least lowers the critical temperature Tc of superconductors so much that they are not useful anymore.
In the present manuscript, we demonstrate deposition of a molybdenum-rhenium alloy of variable composition by simultaneous sputtering from two sources. We characterize the resulting thin films using x-ray photoelectron spectroscopy, and analyze the saturation of the surface layers with carbon during the nanotube growth process. Low-temperature dc measurements show that specifically an alloy of composition Mo20Re80 remains very stable during this process, with large critical currents and critical temperatures even rising to up to Tc~8K. We use this alloy to fabricate coplanar resonator structures and demonstrate even after a nanotube growth high temperature process resonant behaviour at Gigahertz frequencies with quality factors up to Q~5000. Observation of the temperature dependent behaviour shows that our devices are well described by Mattis-Bardeen theory, in combination with dissipation by two-level systems in the dielectric substrate.
"Co-sputtered MoRe thin films for carbon nanotube growth-compatible superconducting coplanar resonators"
K. J. G. Götz, S. Blien, P. L. Stiller, O. Vavra, T. Mayer, T. Huber, T. N. G. Meier, M. Kronseder, Ch. Strunk, and A. K. Hüttel
accepted for publication in Nanotechnology; arXiv:1510.00278 (PDF)
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