Planar superconductor-normal-superconductor Josephson junctions in MgB2

Abstract

Since the discovery of superconductivity in MgB 2 1 considerable progress has been made in determining the physical properties of the material, which are promising for bulk conductors 2-5 . Tunneling studies [6][7][8][9] show that the material is reasonably isotropic and has a well-developed s-wave energy gap (∆), implying that electronic devices based on MgB 2 could operate close to 30K. Although a number of groups have reported the formation of thin films by post-reaction of precursors 10-14 , heterostructure growth is likely to require considerable technological development, making single-layer device structures of most immediate interest. MgB 2 is unlike the cuprate superconductors in that grain boundaries do not form good Josephson junctions, and although a SQUID based on MgB 2 nanobridges has been fabricated 15 , the nanobridges themselves do not show junction-like properties. Here we report the successful creation of planar MgB 2 junctions by localised ion damage in thin films. The critical current (I C ) of these devices is strongly modulated by applied microwave radiation and magnetic field. The product of the critical current and normal state resistance (I C R N ) is remarkably high, implying a potential for very high frequency applications.Our film deposition technique has been described elsewhere. 12 Briefly, B films were deposited at room temperature on to (0001) sapphire substrates by electron beam evaporation and then ex-situ annealed in a Mg vapour at 850°C for 30 minutes to produce MgB 2 films with a final thickness of 100nm and a critical temperature (T C ) of 36K. We deposited a 20nm Au film onto the films by dc magnetron sputtering in order to protect the MgB 2 from contact with water during subsequent processing. Tracks and contact pads were patterned in the bilayer film using standard photolithography and broad beam Ar-ion milling. The T C of the patterned tracks was approximately 35K. In order to fabricate the junctions, the film was then transferred to a focused ion beam system (FIB) (Philips-FEI Inc. FIB 200). Chips were wirebonded to enable the resistance of the tracks to be monitored during the FIB milling process. 16 The barrier was defined by writing 50nm wide cuts across the width of tracks using a 4pA 30kV Ga ion beam. The depth of the cut was calibrated by comparing the cut time to that required to completely sever a track; since the Au mills more than order of magnitude faster than the MgB 2 it could be ignored in calibrating the cut depth. Compared to other materials used to fabricate SNS junctions using this technique, 17 MgB 2 offers the advantage of a relatively low milling rate and hence excellent depth control due to the low atomic mass of its constituents and high melting point.The devices were measured in dip probes placed in 4 He storage dewars. The probes are equipped with coils to provide a magnetic field perpendicular to the substrate plane, and with a microwave antenna. The inset to Fig. 1 shows the resistance against temperature for two tracks on the same substrate,...

High-Jc MgB2 Josephson junctions with operating temperature up to 40 K

“…There have been reports of Josephson junctions with one MgB 2 and one low-T c superconductor electrodes, 6-8 which cannot operate above the lower T c . All-MgB 2 weak-link junctions [9][10][11][12][13] are not ideal for integrated circuits. All-MgB 2 sandwich-type Josephson junctions have been reported, [14][15][16] some with near-ideal Fraunhofer patterns, however, the J c values are very low and drop quickly to zero at 20-25 K. In this letter, we report MgB 2 / MgO/ MgB 2 sandwich-type Josephson junctions with high J c up to 275 kA/ cm 2 at 4 K, which remains nonzero up to 40 K. The ultimate speed limit of Josephson junction-based rapid single flux quantum ͑RSFQ͒ circuits is proportional to the product I c R n , where I c is the critical current and R n the resistance of the junction in the normal state.…”

Planar josephson junctions fabricated with magnesium diboride films

“…1(a), (b), and (c)-(f), respectively, and that can be described as a thinned microbridge, narrowed microbridge, and a Superconductor-Normal-Superconductor or SNS structure. Results on thinned [2] and narrowed [3] microbridges made in somewhat lower-MgB films have been reported by other laboratories. We argue that the SNS structures have greater potential for high products so we explored several versions.…”

Directly coupled superconducting quantum interference device magnetometer fabricated in magnesium diboride by focused ion beam

“…Tunneling studies [2][3][4][5] indicate that MgB 2 is reasonably isotropic with an s-wave character -albeit with two gaps arising from multi-band superconductivity 6,7 -implying that superconducting devices based on MgB 2 could operate up to 30 K. There have been a number of reports on the growth of thin films either by deposition of a precursor followed by annealing in an Mg rich environment [8][9][10][11][12] or by direct deposition of MgB 2 [13][14][15] , however there have been few reports of all-MgB 2 thin film junctions and devices 16,17 . MgB 2 SQUIDs based on thin film nanobridges 17 and point contact junctions 18 have been reported, in the former case the relatively small voltage modulation limits the utility of the device, and the latter whilst showing impressive performance has a physical configuration that is unsuitable for many applications.…”

“…We have used our previously reported 16 technique for fabricating MgB 2 superconductor-normal metal superconductor (SNS) Josephson junctions using localized ion damage caused by a FIB to create a single-layer thin film SQUID with a directly coupled pick-up loop of outer dimension 2 mm on which we report in this letter.…”

“…Our thin film deposition and fabrication technique have been fully described elsewhere 10,16 . The device design used in this work was similar to that used for previous work on YBa 2 Cu 3 O 7-δ grain boundary SQUIDs 19 .…”