Transport Properties of NbN Thin Films Patterned With a Focused Helium Ion Beam

Abstract

Niobium nitride (NbN) is an important material for superconducting electronics because of its relatively high transition temperature in comparison to other conventional superconductors. Recent advances in the use of gas field focused ion beams for material modification motivate directly written NbN electronics. In this work, we study the electrical transport properties of ultra-thin film NbN microbridges irradiated with a focused helium ion beam. Twenty 4- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula> m wide strips were structured into an NbN thin film and irradiated with a helium ion microscope with ion fluences ranging from <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$1 \times 10^{18}$</tex-math></inline-formula> He <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^+/$</tex-math></inline-formula> cm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{2}$</tex-math></inline-formula> to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$3 \times 10^{18}$</tex-math></inline-formula> He <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^+/$</tex-math></inline-formula> cm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{2}$</tex-math></inline-formula> . We report the temperature and magnetic field dependence of the transport properties. At a higher dose of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$1 \times 10^{20}$</tex-math></inline-formula> He <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^+$</tex-math></inline-formula> , the irradiation reduces the critical temperature of a narrow region resulting in a planar superconductor-reduced <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$T_{c}$</tex-math></inline-formula> superconductor-superconductor (SS'S) Josephson junctions. This establishes that NbN can be modified in this manner for nanoelectronics, opening up possibilities for superconducting logic circuits and other higher-speed and high-temperature applications.