Tunability of the superconductivity of tungsten films grown by focused-ion-beam direct writing

Abstract

We have grown tungsten-containing films by focused-ion-beam FIB -induced chemical vapor deposition. The films lie close to the metal-insulator transition with an electrical conductivity which changes by less than 5% between room temperature and 7 K. The superconducting transition temperature Tc of the films can be controlled between 5.0 and 6.2 K by varying the ion-beam deposition current. The Tc can be correlated with how far the films are from the metal-insulator transition, showing a nonmonotonic dependence, which is well described by the heuristic model of Osofsky et al. (Phys. Rev. Lett. 87, 197004 (2001)). Our results suggest that FIB direct-writing of W composites might be a potential approach to fabricate mask-free superconducting devices as well as to explore the role of reduced dimensionality on superconductivity.Keywords: Direct-writing, nanoscale structures, superconductivity, tungsten, focused-ionbeam deposition IntroductionThe investigation of enhanced superconductivity has been a major activity in solid-state physics and materials science in the last few decades. There are only 27 elements which, in bulk form, are superconductors at ambient pressure. The highest critical temperature, T c , of an elemental bulk material is that of niobium (T c = 9.3 K). Thus the search for higher-T c superconductors was expanded to alloys, compounds and composites. [1][2][3][4][5][6][7][8] In an alloy, the degree of order can be very important in determining the superconducting properties. For high transition-temperature superconductors the stoichiometric composition and the structural order are of great importance in obtaining a maximal T c . By contrast, highly disordered transition metals mostly display a higher T c than their annealed ordered counterparts. Tungsten is the superconductor of choice for fabrication of transition-edge sensors (TESs) for optical and near-infared wavelengths due to the tunability of its superconducting transition temperature in the range 100 mK and its relatively weak electron-phonon coupling at such temperatures.[9] Single-crystal tungsten has a very low transition temperature: T c of alpha (bcc) W is 15 mK and that of beta (A15) W is in the range 1 K to 4 K. Amorphous tungsten films [4][5] and alloys, [6][7][8] when rendered into a disordered or granular state, have a T c which can be up to two orders of magnitude larger than that of single-crystal bcc W.Recently the use of a focused ion-or electron-beam to induce the deposition of composite materials from metal-organic precursors has emerged as an important nanofabrication technique. [10][11][12][13][14][15][16][17][18] Such techniques enable the formation of metallic

Focused-Ion-Beam Direct-Writing of Ultra-Thin Superconducting Tungsten Composite Films

“…Before the tungsten deposition, gold contact patterns were formed onto the oxidised silicon substrates by conventional photolithography-based processes [6]. For site-specific tungsten deposition, W(CO) 6 gas was injected onto the sample surface through a nozzle, creating a local high pressure in the region scanned by the ion beam.…”

“…Before the tungsten deposition, gold contact patterns were formed onto the oxidised silicon substrates by conventional photolithography-based processes [6]. For site-specific tungsten deposition, W(CO) 6 gas was injected onto the sample surface through a nozzle, creating a local high pressure in the region scanned by the ion beam. The base pressure before introducing the precursor gas was 8. pA ion-beam current scanned over a 20 m 1 m rectangular area; then 4 other thicker strips were deposited using identical deposition conditions to connect the strip of interest to the large Au pads.…”

“…By varying the ion-beam deposition current, the proximity to the MIT can be controlled within a certain range. We also investigated the controlled growth and superconductivity of "flying" three-dimensional W-containing nanostructures [6].…”

Geometry-related magnetic interference patterns in longSNSJosephson junctions

“…4 The superconducting critical temperature of the wires produced is T c ∼ 4 K, an order of magnitude higher than the bulk T c of W. 5 This could be due to the inclusion of Ga, which is itself a superconductor with T c = 1 K. The critical magnetic field of the wires is also strikingly high: at 1 K, H c = 7 T. 6 The W wires are 200 nm wide and 100 nm thick. The dependence of the superconducting properties of these wires on the deposition conditions has been investigated in detail in Li et al 7 The superconducting gap as well as the Abrikosov flux lattice have been studied by scanning tunneling microscopy experiments. 8 The investigation of proximity- induced superconductivity in metallic nanowires contacted by FIB has also been recently performed.…”

Superconductivity in the system MoxCyGazOδ prepared by focused ion beam induced deposition

“…We conclude the analysis by another interesting observation. In the R(T )-curve for sample 6 a rather distinct change of slope occurs at about 5 K, clearly far above the actual onset of the superconducting transition at 3.8 K. We note, that the superconductor W x C y Ga z O δ typically shows the onset to superconductivity at 5 K 5, [14][15][16][17] . From this observation one may speculate that the slope change in sample 6 is an indication for the formation of a non-percolative and inhomogenous superconducting phase with a critical temperature of 5 K. The question then arises whether the specific properties of the refractory metal component, W or Mo, is really decisive for the formation of the superconducting state or if the substrate of the superconduting phase is rather the carbon component, as was recently speculated for W-based FEBID structures with sulphur doping 18,19 .…”