Blank, David H.A.

Brinkman, Alexander

Hilgenkamp, Johannes W.M.

Mijatovic, D.

Oomen, I.

Rijnders, Augustinus J.H.M.

Rogalla, Horst

English

Josephson junctions have been realized in which two superconducting magnesium-diboride (MgB2) layers are separated by a thin MgO barrier layer, using the ramp-type configuration. Their current–voltage characteristics follow the behavior described by the resistively shunted junction model, with an excess current of about 30% of the critical current Ic. A suppression of 70% of Ic was achieved in applied magnetic fields. Shapiro steps were observed by irradiating the junctions with 10.0 GHz microwaves, and the dependence of the step height on applied rf current is well described by a current–source model. Reference samples prepared without the MgO layer showed strong-link behavior with large Ic values

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APPLIED PHYSICS LETTERS VOLUME 80, NUMBER 12 25 MARCH 2002Magnesium-diboride ramp-type Josephson junctionsDragana Mijatovic, Alexander Brinkman, Ingrid Oomen, Guus Rijnders,Hans Hilgenkamp,a) Horst Rogalla, and Dave H. A. BlankLow Temperature Division, Department of Applied Physics, and MESA1 Research Institute,University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands~Received 2 January 2002; accepted for publication 21 January 2002!Josephson junctions have been realized in which two superconducting magnesium-diboride (MgB2)layers are separated by a thin MgO barrier layer, using the ramp-type configuration. Their current–voltage characteristics follow the behavior described by the resistively shunted junction model, withan excess current of about 30% of the critical currentI c . A suppression of 70% ofI c was achievedin applied magnetic fields. Shapiro steps were observed by irradiating the junctions with 10.0 GHzmicrowaves, and the dependence of the step height on applied rf current is well described by acurrent–source model. Reference samples prepared without the MgO layer showed strong-linkbehavior with largeI c values. © 2002 American Institute of Physics.@DOI: 10.1063/1.1462869#mdeaireeBsmaothupiyp-tio1aArsurfmg-ithisithd.erinthenhethe-pre-stedof-The remarkably high superconducting transition teperatureTc of 39 K that was discovered1 for the noncuprateintermetallic compound MgB2, the correspondingly largeenergy gap2,3 as well as the large carrier density,4 make thismaterial attractive for use in superconducting electronicvices, possibly cooled by cryocoolers. To this end, the avability of high-quality thin films and Josephson junctions aessential, which triggered an intensive research in thareas.5–20The realization of various Josephson devices in Mg2has been reported, based on, e.g., thin film nanobridge16localized ion damage,17 point contacts,18 and thin film het-erostructures with one MgB2 electrode.19,20To optimally ex-ploit the beneficial properties of MgB2 and to be able to tunethe junction characteristics by choosing the barrier paraeters, the next challenging step is to realize and investigMgB2/artificial-barrier/MgB2 thin film Josephson junctions.In this letter the fabrication and characteristicsmultilayer Josephson junctions with MgB2 electrodes and adielectric barrier is presented. The proper operation ofjunctions is evidenced by a clear modulation of the supercrent by applied magnetic fields and the occurrence of Shasteps under microwave irradiation.The junctions were prepared according to the ramp-tconfiguration21 ~Fig. 1!. First, a bilayer of MgB2 ~200 nm!and MgO ~100 nm! was depositedin situ by pulsed laserdeposition~PLD! on a MgO substrate. The MgB2 film wasprepared from a Mg-enriched MgB2 target, using the procedure described in Refs. 10 and 15. In short, the depositook place at 200 °C in 0.17 mbar Ar pressure for 6 min atHz, using a laser density of 4 J/cm2. Subsequently, the MgB2layer was annealed at 600 °C for 5 min in a Mg plasmgenerated by ablation from a Mg target in 0.22 mbarAfter cooling down to 200 °C, the MgO insulation layer waformed by ablating from a Mg target in an oxygen pressof 0.5 mbar for 50 s at 10 Hz, using an energy density oJ/cm2. Subsequently, the film was cooled down to room teperature keeping the pressure in the chamber unchana!Electronic mail: h.hilgenKamp@tn.utwente.nl2140003-6951/2002/80(12)/2141/3/$19.00Downloaded 13 Mar 2008 to 130.89.112.51. Redistribution subject to AIP--l-se,-tefer-roen0,.e4-ed.Then, a beveled edge~ramp! was defined by photolithography and argon ion beam milling, under an angle of 45°. Wa beam voltage of 500 V the etching rate of the bilayerabout 8 nm/min. Due to the difference in etching rate wthe photoresist a slope of the ramp of about 20° is obtaine22After removing the photoresist, a 12 nm MgO barrier laywas deposited by ablating Mg for 6 s at 10 Hzwith annergy density of 4 J/cm2, in 0.5 mbar O2 at 200 °C. Subse-quently, a 200 nm MgB2 counterelectrode was depositedthe same way as the base electrode. The sample waspatterned by photolithography and ion milling to define tjunctions, with an overlap of the counterelectrode of 3mm.After sample preparation, the transition temperatures ofMgB2 layers were typically 23 K. This reduced critical temperature as compared to the bulk value is discussed invious articles and is attributed to the small MgB2 grain sizein thesein situ fabricated films and to impurities, such aMgO inclusions.10,15Figure 1 displays the current–voltage (I –V) character-istic of a 7mm wide junction atT54 K. This characteristicfollows the behavior expected from the resistively shunjunction ~RSJ! model, with an additional excess currentabout 30% of the critical currentI c . At this temperature theI c value is 130mA, which implies that the junction width isFIG. 1. Schematic of a MgB2 /MgO/MgB2 ramp-type junction, and a measured current–voltage characteristic of a 7mm wide junction at 4 K.1 © 2002 American Institute of Physics license or copyright; see http://apl.aip.org/apl/copyright.jspnntinatri-thnafoo-y-nrItthandtheortthetheed byr.nt-ectFur-ts air-lowjunc-eral-algsJ-ph-Jo-theirra-fis aereds vs2142 Appl. Phys. Lett., Vol. 80, No. 12, 25 March 2002 Mijatovic et al.much smaller than the Josephson penetration depth. Themal state resistanceRn is almost temperature independeand has a value of about 1V. The I cRn product is 130mV.Systematic studies of the temperature dependence ofI c andI cRn are in progress and will be reported elsewhere.The application of a magnetic fieldH perpendicular tothe current direction and parallel to the substrate resultedmodulation of the critical current~Fig. 2!. A suppression ofthe critical current by up to 70% was observed. TheI c(H)dependence differs from the Fraunhofer dependence thexpected for a small junction with a uniform current distbution, with the large amplitude of the side peaks andincomplete suppression of the critical current being sigtures of a nonhomogeneity of the barrier.A complete suppression of the supercurrent and themation of Shapiro steps at multiples ofV520.7mV wereobserved by irradiating the junction with 10.0 GHz micrwave fields atT54.2 K ~Fig. 3!. The voltages at which thecurrent steps appear are as expected from the frequencthe applied radiationn r : Vn5(h/2e)n rn, with 2e/h5483.6 MHz/mV. The modulation of the height of the Shapiro steps as a function of applied rf current is presentedFig. 4. To describe the junction under microwave irradiatiothe RSJ model is extended with a rf current–source tewith a large source impedance.23 With this current–sourcemodel a good fit to the experimental data was obtained.noted that the fitting parameters were the same for allShapiro steps. The parameterV, describing the ratio of theFIG. 2. Modulation of the critical current by a magnetic field applied ppendicular to the current direction and parallel to the substrate, measur3.4 K.FIG. 3. Current–voltage characteristics of a 7mm wide junction with andwithout 10.0 GHz microwave irradiation atT54.2 K.Downloaded 13 Mar 2008 to 130.89.112.51. Redistribution subject to AIPor-aise-r-ofin,misefractions of rf current passing through the resistive shuntthe Josephson element was found to be 0.35.To determine the effects of the interfaces betweenMgO barrier and the electrodes on the junction transpproperties, reference samples were prepared followingsame fabrication procedure, except for the deposition ofbarrier layer. It has been shown, e.g., in high-Tc Josephsonjunction technology24 that a barrier is formed at the interfacbetween the electrodes, due to structural damage invokeion milling. In contrast, for our MgB2 ramp-type contactsthis ion-milling procedure did not lead to weak link behavioCritical currents up to 31 mA were obtained for 5mm widecontacts atT54.2 K, which corresponds to a critical curredensity of 33106 A/cm2. This implies that the interface region has good superconducting properties and will not affthe transport characteristics of the Josephson junctions.thermore, it indicates that the ramp-type contact presenuseful configuration for via contacts in future multilayer ccuits.The nonhysteretic character ofI –V curves of the junc-tions results from the low normal state resistance and thecapacitance. The capacitance can be estimated from thetion geometry, yieldingC540 fF for the junction displayedin Fig. 1. This leads to a value for the Stewart–McCumbparameterbc of the order of 1022, which is in the nonhys-teretic regime. TheI cRn product of 130mV at T54 K isconsiderably below the expectations following from the vues of the energy gap in the MgB2.3 This is attributed pri-marily to barrier inhomogeneity and to the reduced critictemperatures of thesein situ grown MgB2 films. It is antici-pated that higherI cRn products are attainable by improvinfilm smoothness and enhancing theTc.In conclusion, all-MgB2 ramp-type Josephson junctionwith a 12 nm MgO barrier layer were realized, showing RSlike current–voltage characteristics. Both, dc and ac Joseson effects were demonstrated by the modulation of thesephson current by applied magnetic fields andappearance of Shapiro steps under applied microwavediation. The establishment of an all-MgB2 Josephson junc-tion technology, employing an artificial barrier layer owhich the composition and thickness can be selected,-atFIG. 4. Amplitude of the supercurrent and the first three Shapiro stepapplied rf current~10.0 GHz! at T54.2 K. The solid line shows a fit usingthe current–source model. license or copyright; see http://apl.aip.org/apl/copyright.jsppldonestsus,s,,.iu,I.Hcey,X.NMd, S.,er-ndC.Cd.e,rk,do,21-2143Appl. Phys. Lett., Vol. 80, No. 12, 25 March 2002 Mijatovic et al.further step towards electronic devices based on this suconductor.The authors thank Frank Roesthuis, Henk-Jan Smiand Gerrit Gerritsma for their assistance and discussiThis work was supported by the Dutch Foundation for Rsearch Matter~FOM! and the Royal Dutch Academy of Artand Sciences and is part of the MESA1 Strategic Orientationon Materials Science of Interfaces.1J. Nagamatsu, N. Nakava, T. Muranaka, Y. Zenitani, and J. AkimiNature~London! 410, 63 ~2001!.2G. Kareptrov, M. Iavarone, W. K. Kwok, G. W. Crabtree, and D. G. HinkPhys. Rev. Lett.86, 4374~2001!.3A. Brinkman, A. A. Golubov, H. Rogalla, O. V. Dolgov, and J. Kortucond-mat/0111115~2001!.4W. N. Kang, C. U. Jung, K. H. P. Kim, M.-S. Park, S. Y. Lee, H.-J. KimE.-M. Choi, K. H. Kim, M.-S. 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Magnesium-diboride ramp-type Josephson junctions

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Magnesium-diboride ramp-type Josephson junctions

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