Electron beam irradiation of Y1Ba2Cu3O72 x grain boundary Josephson junctions F. Tafuria) Dipartimento di Ingegneria, Seconda Universita` di Napoli, Aversa (CE) (Italy) and INFM-Dipartimento di Scienze Fisiche-Universita` di Napoli ‘‘Federico II’’, Napoli (Italy)
S. Shokhor, B. Nadgorny,b) and M. Gurvitch Department of Physics, State University of New York at Stony Brook, Stony Brook, New York 11794
F. Lombardi and A. Di Chiara INFM-Dipartimento di Scienze Fisiche-Universita` di Napoli ‘‘Federico II’’, Napoli (Italy)
~Received 10 March 1997; accepted for publication 5 May 1997! The properties of the Y1Ba2Cu3O72x biepitaxial Josephson junctions were reproducibly modified by a focused electron beam irradiation of the interface region. The junctions were fabricated by depositing Y1Ba2Cu3O72x thin film by cylindrical magnetron sputtering technique on the ~110! SrTiO3 substrate, partially covered by a pregrown MgO seed layer. The junction parameters can be adjusted controllably by applying an appropriate dose. Electron irradiation decreased the critical current of the junctions I C and increased the normal state resistance times area to values of the order of 1( m V cm2). Some other effects, such as the disappearance of the excess current, were also observed. The original properties of the junctions could be partly restored by isothermal annealing. We also speculate that some aspects of the nature of the grain boundary barriers can be better understood from the study of the properties of irradiated junctions. © 1997 American Institute of Physics. @S0003-6951~97!03127-6# Josephson junctions, the backbone of superconducting electronics, can be also used as a powerful tool to explore the fundamental properties of superconductors.1 In the last few years, significant progress has been made in the development of high-T C Josephson junctions ~HTS-JJ!,2,3 superconducting quantum interferometer devices, and digital devices.4 Nevertheless, the problems of reproducibility and uniformity of the existing technologies still impose severe restrictions on practical applications,2 especially in the area of digital multijunctions HTS circuits. The peculiar properties of HTS, such as short coherence length, symmetry of the order parameter, and the actual structure of the junction interfaces,5–7 only add to the puzzling and often contradictory phenomenology. This justifies the efforts presently devoted to understanding the nature of the transport mechanisms across the junction interface in order to improve the Josephson junction technology.2 In this letter, we present a study of the properties of biepitaxial YBa2Cu3O7 ~YBCO! Josephson junctions8 modified by electron beam irradiation.9 The aim of this work is to controllably change the properties of HTS Josephson junctions and to characterize their barrier, by considering the junction response to irradiation. This is achieved by disordering oxygen in the region of the junction interface by an electron beam9,10 and by studying the induced change in the current–voltage characteristics. This novel technique makes it possible to adjust controllably the properties of a single junction, which may be a part of a more complex multijunction circuit. The procedure can be used in circuit design and testing to modify the parameters of one or a few selected junctions ~one or several times sequentially!, in order to achieve the desired device performance. a!
Electronic mail:
[email protected] b! Present address: Naval Research Laboratory, Washington DC 20375.
In order to fabricate the biepitaxial junctions, a ~110! oriented MgO film was deposited on a ~110! SrTiO3 substrate. It served as a seed layer to modify the crystal orientation of YBCO on SrTiO3. In the junctions studied in this work, YBCO grows predominantly ~103! oriented on the SrTiO3 substrate and ~001! oriented on the MgO layer, respectively8 ~see the inset of Fig. 1!. These junctions are characterized by high I C R N values, low critical current densities (J C ), high normal state resistivities ( r N ), and exhibit a Josephson behavior over the entire working temperature range.8 Details of the fabrication procedure for biepitaxial junctions and of the electron beam technique were described elsewhere;8,9 here we only briefly outline the major processing steps. A 20-nm-thick MgO film was deposited by magnetron sputtering from a stoichiometric oxide target on ~110! SrTiO3 at a temperature of 600 °C. A 120-nm-thick YBCO
FIG. 1. The typical I vs V curves of the junction biep#20 before irradiation are shown as a function of temperature. The inset shows a sketch of the biepitaxial Josephson junction ~BJ! irradiated by an electron beam.
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TABLE I. Typical junction parameters before ~BI! and after ~AI! irradiation and after a variable annealing time ~A time!, respectively. Data for biep#20 and biep#36 correspond to T54.2 and T58 K, respectively. The width of the junctions was 5 mm. The critical current densities of ~103!-oriented YBCO microbridges#3 correspond to T550 K. T C ~K!
Sample
FIG. 2. The typical I vs V curves of the junction biep#20 after irradiation are shown as a function of temperature. In the inset, I vs V curves ~measured at T58 K! are reported before irradiation ~BI! ~triangles down!, after irradiation ~AI! ~circles!, after an annealing for one day at room temperature ~A24h! ~triangle up! respectively.
film was deposited by inverted cylindrical magnetron sputtering technique typically at a temperature of 780 °C. Prior to the YBCO deposition, the MgO was patterned by a standard Nb masking technique, which uses CF4 reactive ion etching and Ar1 ion milling.8 A diluted H3PO4 or HNO3 wet etch was finally used to define the YBCO microbridge. To modify the junction, we used an electron beam of the scanning electron microscope ~SEM! Philips CM-12 with an energy of 120 keV. To irradiate the biepitaxial junctions and the microbridges, we used the same doses and spot sizes usually employed to decrease the T C of a c-axis oriented YBCO microbridge by 30–40 K.11,9 In order to study the time variation of the junction parameters, the junctions were measured after a series of isothermal annealings in the He atmosphere of the cryostat.9 The typical I–V curves as a function of temperature before and after irradiation are shown in Figs. 1 and 2, respectively. The critical current of this junction ~biep#20! decreases by a factor of '20 at T58 K, while the maximum working temperature of the junction T C decreases from 70 to about 40 K. A significant increase of R N was also observed ~from 70 to 220 V!. It is interesting to note that the excess current observed in the original junctions ~see Fig. 1! disappears after the electron irradiation, so that the I–V characteristics of the irradiated junction can be properly described by the resistively shunted junction ~RSJ! model. The properties of the original junctions can be partly restored by isothermal annealing ~see also inset in Fig. 2!. In the course of these annealings, I C and T C increase, while no significant change of R N was observed. The summary of the junction parameters for two typical samples is given in Table I. The original I C R N value decreases after electron irradiation, but then increases controllably as a function of annealing time. Since the region damaged by the electron beam is wider than the nominal spot size,9,10 the effect of the irradiation extends to the areas of the film close to the interface. Therefore, in order to draw meaningful conclusions from the modification of the junction properties, it is necessary to know the effects of irradiation on ~103!-oriented YBCO microbridges and to recall some established9,10 results on ~001! YBCO films. In contrast to the irradiation of the
biep#20 biep#20 biep#20 biep#20 biep#36 biep#36 biep#36 biep#36 biep#36 micr#3 micr#3 micr#3
BI AI A20m A24h BI AI A12m A1h A34h BI AI A10m
68 40 44 50 73 45 50 57 60 87 72 78
I C R N ~mV! 2.8 0.44 0.77 1.54 0.8 0.36 0.37 0.38 0.41
J C (A/cm2) 3
3.5310 2 3102 3.53102 7 3102 5 3103 7 3102 9 3102 1.23103 1.33103 4 3105 8 3104 1.43105
R N ~V! 80 220 220 220 20 65 52 40 40
c-axis films, the electron beam in the microbridges made out of ~103! YBCO film was scanned almost perpendicular to the c axis of the film, as shown in the inset in Fig. 3. The T C of the ~103!-oriented microbridges typically decreases from 87 down to 72–78 K. The critical current density J C dependence on the temperature is shown in Fig. 3 before irradiation, after irradiation, and after a 10 min annealing at 330 K, respectively. The change in the T C and J C of ~103! YBCO films after irradiation is on average smaller than the one found for the c-axis defined microbridges. These results are not surprising since a highly anisotropic material, such as YBCO, should have a strong dependence of the scattering cross section on the irradiation angle. We believe that the substantial increase of R N can be explained in terms of modifications of the properties of the barrier after the electron beam irradiation. In order to understand this, it is necessary to compare variations of the specific conductance s N with the data on the c-axis films. The electron irradiation of c-axis YBCO films causes a maximum variation of s N of the order of 103 ( m V cm2) 21 . The actual
FIG. 3. The temperature dependence of the critical current J C is shown for a ~103! YBCO film, before irradiation ~BI!, after irradiation ~AI! and after an annealing of 10 min at room temperature, respectively. In correspondence to this change of J C , T C decreased from 87 to 72 K. In the inset a sketch of the microbridge is shown.
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FIG. 4. The critical current I C of the samples biep#36 is shown as a function of the temperature before irradiation ~filled squares!, after irradiation ~filled circles! and after an annealing of 34 h ~open squares!, respectively. As a comparison theoretical curves ~Ref. 15! are reported for values of the ratio L/ j N* '3 ~crosses! and 5 ~diamonds!. The gap value of the superconducting electrodes to which theoretical curves are usually normalized is '0.8 meV. In the inset ~a!, the Josephson current, normalized to the corresponding value at T50.2 T C , is reported vs the reduced temperature T/T C . In the inset ~b!, a possible profile of D(r) before and after irradiation is shown: g B1,2 is proportional to the specific resistance of the S/N boundary ~Ref. 17!.
variation of s N of the irradiated junctions is much smaller, of the order of 1( m V cm2) 21 . This effect is probably due to the oxygen exchange between the barrier and the adjacent regions. Other features in I–V curves which are affected by the electron irradiation are the Fiske steps12,8 observed at 200– 300 mV. The shift of their position ~induced by electron irradiation! ~of the order of 50 mV! was observed in I–V characteristics of biep#36. This would be directly related to changes in the barrier structure.13 The disappearance of the excess current I exc after irradiation, corresponding to the J C decrease, must also be connected to the nature of the barrier and the adjacent regions. According to Clarke,14 the I exc could be directly related to ac supercurrents with a nonzero time average. For high currents ~in our case 40–50 mA!, the junction is long, when we decrease the critical current to 2–5 mA by irradiating the junction, it becomes short and the I exc disappears. A different explanation of the I exc can be given in terms of the proximity effect in the barrier region and nonequilibrium state of quasiparticles in the superconducting electrode.15,16 In this scenario, the irradiation would reduce the effects due to proximity. In Fig. 4, the critical current I C of junction biep#36 is shown as a function of temperature before irradiation, after irradiation, and after an annealing for 34 h, respectively. This dependence is typical of the ~SNS! structures.15 The ratio L/ j N* '3 between the microbridge length L and the coherence length j N* at T'T C fits the experimental data before irradiation reasonably well at higher temperatures. The behavior of the Josephson current ~normalized to the corresponding values at T50.2 T C ! plotted versus the reduced temperature t5T/T C is qualitatively the same before and after irradiation and after annealing ~see inset a in Fig. 4!. These results reveal that an increase of L/ j N* is not the only
effect. An additional increase of the boundary resistance may also occur.17 Electron irradiation can significantly increase impurity scattering and possibly reduce carrier concentration in the barrier.10 The oxygen redistribution would change the effective length of the region that separates the two HTS superconducting banks as well as the order parameter profile D(r), as for instance shown in the inset ~b! of Fig. 4. In summary, we have shown that the properties of a HTS Josephson junction can be modified by electron beam irradiation of the grain boundary. The change in the microbridge properties seems to be characteristic of the junction configuration and dominated by the modifications induced by irradiating the barrier. Although we have only studied the effect in a particular type of junction, the ease of oxygen desorption from the grain boundary indicates that the electron beam technique may be applied to modify other types of grain boundary and possibly step-edge junctions. This technology allows controllable modification of the parameters of a single junction, which may be used in testing and optimizing characteristics of HTS Josephson devices. The authors would like to thank Professor Antonio Barone for valuable comments. One of the authors ~F.T.! was supported by a grant of Universita` degli Studi di Napoli ‘‘Federico II.’’ This work has been partially supported by INFM.
1
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