Abstract
► Developed a technique of size reduction to sub-ten nanometers by FIB milling. ► FIB milling is a top-down and site-specific approach with atomic resolution. ► No remarkable morphology and microstructure changes of W composite nanowires were observed after thinning. ► FIB thinning is an effective method towards size and quantum effects observation. Techniques for scalable fabrication of one-dimensional or quasi-one-dimensional nanowires are of great importance to observe quantum size effects and build quantum information devices. In this work, we developed a technique for size reduction of both lateral and freestanding tungsten composite nanostructures using focused-ion-beam (FIB) thinning. Different exposure times and ion-beam currents were used to control the final size and the thinning rate and accuracy of a group of nanowires, an individual nanowire and a portion of a nanowire by low-current site-specific milling. A transmission electron microscope image of a thinned superconducting tungsten composite nanowire with width reduced from 80 nm to 50 nm shows uniform shrinking along the length of the wire and high resolution image shows no obvious changes of the morphology after thinning. The variation of the superconducting critical current density upon thinning is insignificant; it is 1.7 × 10 5 and 1.4 × 10 5 A/cm 2 at 4.26 K for the as-deposited and wire with width reduced to 50 nm, respectively. These results suggest that FIB-milling is a potential approach for controllable size reduction enabling the observation of size- and quantum effects.
Highlights
It has been demonstrated that when the diameter of a superconducting nanowire is reduced smaller than the Ginsburg–Landau phase coherence length and the magnetic penetration depth, below or near the superconducting transition temperature, the resistance of a 1D wire is no longer zero [1]
Our results suggest that FIB-thinning is a potential approach for controllable size reduction with high resolution towards the observation of size- and quantum effects
The transition width of the 50 nm wide nanowire is slightly wider than that of the as-deposited one (80 nm). This observation is consistent with the previous report [14] that for FIB-grown tungsten wires the width of the transition increases for wires of diameter below 60 nm.From the I–V curves, it can be derived that the critical current density upon thinning is insignificant, being 1.7 Â 105 and 1.4 Â 105 A/cm2 at 4.26 K for the as-deposited and wire with width reduced to 50 nm, respectively
Summary
It has been demonstrated that when the diameter of a superconducting nanowire is reduced smaller than the Ginsburg–Landau phase coherence length and the magnetic penetration depth, below or near the superconducting transition temperature, the resistance of a 1D wire is no longer zero [1]. In addition it has been reported that vertical tungsten nanowires grown by FIB-induced deposition are superconducting with much enhanced superconductivity [12,13,14], and could be used to form 3D nano-SQUIDs that potentially are able to detect the magnetic field both parallel and perpendicular to the substrate surface [2]. In detail, freestanding tungsten nanowires were grown on SiO2/Si by FIB-CVD using a 1 pA ion-beam current; nanowires were felled by FIB milling followed by low current FIB thinning to reduce the size locally. A 200 pA ion beam current was used to irradiate vertically grown nanowires by reduced-raster scanning to form electrical contacts as well as to reduce the size of the freestanding wires simultaneously. Our results suggest that FIB-thinning is a potential approach for controllable size reduction with high resolution towards the observation of size- and quantum effects
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