Abstract
Tungsten composite films of thickness as low as 19 nm have been deposited using a 30 keV Ga <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> focused-ion-beam with tungsten carboxyl (W(CO) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> ) as the gas precursor. Films of thickness 25 nm or more are superconducting with a transition temperature exceeding 5 K. Films in the thickness range 25 nm to 50 nm show an <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">increasing</i> T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> for a <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">decreasing</i> film thickness. This correlates well with the measured dependence of the normal state resistivity upon film thickness. We attribute this behavior to an increase in the BCS electron-phonon interaction potential resulting from a reduction in the electron mean-free-path as the film thickness is reduced. In the light of these data we discuss the applicability of FIB-deposited tungsten for devices requiring ultra-thin superconducting films, including photon detectors and phase-slip qubits.
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