Superconducting NbN Resonator Parametric Amplifiers for Millimetre Wavelengths

Abstract We report the development of a reactive sputtering process for high $$T_\textrm{c}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>T</mml:mi> <mml:mtext>c</mml:mtext> </mml:msub> </mml:math> NbN films with high normal-state resistivity, tailored for kinetic inductance parametric amplifiers. The process includes precise control to ensure full nitridation of the target prior to deposition. Under optimised conditions, the resulting NbN thin films exhibit a critical temperature of $$10.5\,\textrm{K}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>10.5</mml:mn> <mml:mspace/> <mml:mtext>K</mml:mtext> </mml:mrow> </mml:math> and a resistivity of $$\sim 1000\,\mathrm {\mu \Omega \,cm}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mo>∼</mml:mo> <mml:mn>1000</mml:mn> <mml:mspace/> <mml:mrow> <mml:mi>μ</mml:mi> <mml:mi>Ω</mml:mi> <mml:mspace/> <mml:mi>cm</mml:mi> </mml:mrow> </mml:mrow> </mml:math> . The high $$T_\textrm{c}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>T</mml:mi> <mml:mtext>c</mml:mtext> </mml:msub> </mml:math> of the NbN thin films suggests strong potential for application over the entire millimetre-wave frequency range from 24 to $$300\,\textrm{GHz}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>300</mml:mn> <mml:mspace/> <mml:mtext>GHz</mml:mtext> </mml:mrow> </mml:math> , whereas the high resistivity suggests a reduced power requirement for the pump tone to achieve high gain. Resonator parametric amplifiers have been fabricated from these films using coplanar waveguide geometry. The devices were able to produce high gain exceeding $$20\,\textrm{dB}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>20</mml:mn> <mml:mspace/> <mml:mtext>dB</mml:mtext> </mml:mrow> </mml:math> at $$25\,\textrm{GHz}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>25</mml:mn> <mml:mspace/> <mml:mtext>GHz</mml:mtext> </mml:mrow> </mml:math> , with artefact-free, reproducible amplification profiles in good agreement with theoretical models.