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YBCO Josephson junctions on bicrystalline NdGaO3 substrates
Physica C 241 ( 1995) 316-318
ELSEVIER
YBCO Josephson junctions on bicrystalline NdGaO3 substrates R. Unger a, T.A. Scherer a,., W. Jutzi a, Z.G. Ivanov b, E.A. Stepantsov c " University ofKarlsruhe, lnstitutJ~r Elektrotechnische Grundlagen derlnformatik, Hertzstrasse 16, 1)-76187 Karlsruhe, Germany b Dept. of Physics, Chalmers University of Technology, G6teborg, Sweden ¢Academy of Sciences, Moscow, Russian Federation
Received 3 October 1994; revised manuscript received 7 November 1994
Abstract
YBCO Josephson junctions on bicrystalline NdGaO3 substrates with a misorientation angle O= 22 ° of the grain boundary have been fabricated with a current density Ofjm~ = 18 kA/cm 2 at T= 77 K. The dimension of the microbridge across the grain boundary was w= 4.8 ~tm with a thickness t = 290 nm. A rather uniform current-density distribution along the grain boundary is deduced with a nearly Fraunhofer behavior of the magnetic-field dependence of the maximum critical current. An IcRN product of 70 ~tV at T=77 K has been achieved.
1. Introduction
For the fabrication of cryoelectronic circuits it is necessary to realize Josephson junctions with a small standard deviation cr(Ij) of the critical current. The present standard deviation of grain boundary junctions on bicrystalline substrates is smaller than for step-edge junctions [ 1 ]. Many step-edge junctions on untwinned NdGaO3 substrates have been used for microwave applications [ 2 ]. Bicrystal junctions have been fabricated on different types o f substrates as SrTiO3, MgO, Y-ZrO2 and (110) oriented NdGaO3 [3,5-8]. Grain-boundary junctions (GB's) of untwinned YBCO on (001 ) NdGaO3 bicrystals should have interesting properties.
ented NdGaO3 single crystals. The misorientation was characterized by (001 ) rotation of the crystal lattices of both crystals on I 1 ° in opposite directions with their (001) planes being parallel to each other as shown schematically in Fig. I. A misorientation angle of the substrates O = 22 ° has been chosen to get low-energy YBCO grain boundaries. Crystal composites with an artificially made plane grain boundary were obtained as the result of intergrowing. These composites were cut into thin ( 1 m m ) slices perpendicular to the grain boundary. In a last step the slices
1001]
[00~]
2. Fabrication
Stepantsov manufactured the bicrystalline substrates by solid-phase intergrowing [ 9 ] of misori* Corresponding author.
Fig. 1. Crystallographicorientations of a NdGaO3 bicrystal substrate with a misorientation angle O= 22 °. The Miller index notation corresponds to the orthorhombic lattice of NdGaOa.
0921-4534/95/$09.50 © 1995 Elsevier Science B.V. All fights reserved SSDI0921-4534(94)02361-1
R. Ungeret al. /PhysicaC241(1995)316-318
317
were polished in order to achieve good surface characteristics for epitaxial film growth. C-axis oriented YBCO thin films have been deposited on the bicrystalline substrates by an off-axis sputtering technique with process parameters as reported in Refs. [10] and I l l ] . In order to avoid strain in the substrate and the YBCO film during the heating process a radiation heater with pyrometric control of the surface temperature has been used instead of a glueing technique with silver paint. The YBCO strip lines have been etched with an Ar ion beam on a cooled stage at liquid-nitrogen temperature. Thus, tilt GB junctions have been fabricated on (001) oriented NdGaO3 bicrystals with a junction width w=4.8 ~tm and a film thickness t= 290 nm.
to the film. Fig. 3 shows approximately a Isin x/xl behavior corresponding to a rather uniform currentdensity distribution along the junction. The ratio of the magnetic inductions B1 and Bo at both minima of the current/j yields B~/Bo=2.57 and has only a minor deviation from the expected value B~/Bo= 2. The magnetic induction Bo at the first minimum oflj can be estimated [ 2,13 ]:
3. Measurements
(Bo>= ~(Bo+ ~)=151~tT.
The measured current-voltage characteristics at T = 77 K for the mentioned contact is demonstrated in Fig. 2 for a swept magnetic field. The normal resistance is extrapolated RN~0.33 ~. The measured maximum Josephson current I~=210 ~tA yields a product I~.RN= 70 ~tV at T = 77 K. The excess current is negligibly small. The uniformity of the current distribution along the junction has been examined by measuring the dependence of the maximum Josephson current /j as a function of the magnetic induction B perpendicular
The deviation of calculated and measured values is small:
2q'o
Bo~ w2 , where Cbo=h/2eand w are the flux quantum and the width of the Josephson junction, respectively. For w= 4.8 p.m, measured with an optical microscope, the calculated induction Bo,¢~l= 180 ~tT is in a reasonable agreement with the mean value < Bo > determined by
1
AB= BoxaJ- ( Bo > ~ 16%. Bo,cal The Josephson current density of YBCO GB junctions on different bicrystalline substrates (e.g.: YZrO2, MgO, SrTiO3 and NdGaO3) as a function of the misorientation angle O can be approximated by an exponential law [ 6,7,12 ]:
Jmax=k~ exp( -k20);
0 < 4 5 ° and T = c o n s t .
A half logarithmic plot of
I
Jm~x/k~ versus the mis-
600 200
~oo ~A 200
I00
o J', m L ~
-200
o
-~oo -I00
.~,
~
.~
-600
40 6 s6 ,do ,s'o 260
-200
/ i
i
100
200
i
300
400
500
U/pV.-~.,,-
Fig. 2. Current-voltage characteristics at T=77 K of a w=4.8 vtmwideand t = 290 nm thickgrain-boundaryjunction on a (001) NdGaO~bicrystal with a misorientation angle O= 22 °.
B/~tT
..-
Fig. 3. Josephson current lj as function of the external magnetic induction B.
318
R. Unger et aL / Physica C 241 (1995) 316-318
orientation angle O yields nearly the same straight line for all the substrates mentioned in Table 1. The factor k~ represents the critical-current density of a thin YBCO film without grain boundary (O= 0 ° ). kl is a function of temperature and depends strongly on the quality of the deposition process. The factor k2, i.e. the slope of the straight line, only slightly varies. The Josephson current density versus misorientation angle O for Y - Z r O 2 at T = 7 7 K is shown in Fig. 4. The two circles corresponding to NdGaO3 bicrystals at O = 0 ° and 22 ° are very close to the straight line for Y - Z r O 2. At 0 = 2 2 ° and on (001) NdGaO3 substrates the Josephson current density isjm~x= 18 kA/ cm 2. Additional measurements at other misorientation angles than 0 ° and 22 ° are needed to determine the deviations from the straight line in Fig. 4. Unfortunately, measurements at other misorientation angles are not yet available. Table 1 Parameters kl and k2 for the dependence of the Josephson current density on the misorientation angle O for MgO, SrTiO3 and Y-ZrO2 Substrate
MgO SrTiO3 Y-ZrO2
Temperature T (K)
kl
k2
(MA/cm 2)
(°)
References
4.2 4.2 77
8.62 5.37 2.98
0.1817 0.1691 0.2192
[6] [12] [7]
107
1" ~ax A/cm 2
108, 105 10¢ 103 102 10
20
30
40
50
Misorientation angle ® / °
Fig. 4. Josephson current density at T=77 K of YBCO grain boundary junctions on Y-ZrO2 [7] marked with (Q) and NdGaO3 bicrystals marked with ( O ) [ 10,14-16 ] as function of the misorientation angle O.
4. Conclusion Bicrystalline NdGaO3 substrates with small misorientation angles have been fabricated. After off-axis sputtering of thin YBCO film stripes, comprising a single planar grain-boundary junction, have been milled with an Ar ion beam. A nearly uniform distribution of the Josephson current density has been found. At T = 7 7 K a current density jmax= 18 kA/ cm 2 of a junction on a NdGaO3 bicrystalline substrate with a misorientation angle O= 22 ° has been achieved. The current density seems to depend mainly on the misorientation angle and less on the kind of substrate. References [ 1] R. Gross, in: Grain Boundary Josephson Junctions in the High-Temperature Superconductors, Interface in Supercond. Systems, Eds. S.L. Shiade and D. Rudnan (Springer, New York, 1992). [2]A. Vogt, H. Matz, R. Dolata, R. Herwig, M. Neuhaus, P. Marienhoff, T. Scherer, W. Jutzi, U. Fath, G. Hrfer and H. Kratz, IV. Trilateral German-Russian-Ukrainian Seminar on HTSC, Dubna, Russia (1993). [3] P. Chaudhari, J. Mannhart, D. Dimos, C.C. Tsuei, J. Chi, M.M. Oprysko and M. Scheuermann, Phys. Rev. Len. 60 (1988) 1653. [4] D. Dimos, P. Chaudhari, J. Mannhart and F.K. LeGoues, Phys. Rev. Lett. 61 (1988) 219. [ 5 ] D. Dimos, P. Chaudhari and J. Mannhart, Phys. Rev. B 41 (1990) 4038. [6] H.B. Lu, T.W. Huang, J.J. Wang, J. Lin, S.L. Tu, S.J. Yang and S.E. Hsu, IEEE Trans. Appl. Supercond. 3 ( 1993 ) 2325. [7] Z.G. Ivanov, P.A. Nilsson, D. Winkler, J.A. Alasco, T. Claeson, E.A. Stepantsov and A.Y. Tzalenchuk, Appl. Phys. Lett. 59 ( 1991 ) 3030. [8] P.G. Quincey, Appl. Phys. Lett. 64 (1994) 517. [9] E.A. Stepantsov, A.S. USSR No 1116100 C1. C30b33\00 ( 1982 ), Bull. Izobreteniy 36 (1984) 77. [ 10] T. Scherer, R. Herwig, P. Marienhoff, M. Neuhaus, A. Vogt and W. Jutzi, Cryogenics 31 ( 1991 ) 975. [ 11 ] T. Scherer, P. Marienhoff, R. Herwig, M. Neuhaus and W. Jutzi, Physica C 197 (1992) 79. [ 12 ] R. Gross and B. Mayer, Physica C 180 ( 1991 ) 235. [l 3] R.G. Humphreys, J.S. Satchell, J.A. Edwards, N.G. Chew, S.W. Goodyear, M.N. Keene and S.F. Morgan, IEEE Trans. Appl. Supercond. 3 (1993) 2026. [ 14] K.H. Young, G.V. Negrete and J.Z. Sun, Jpn. J. Appl. Phys. 30 (1991) L1355. [ 15 ] K.H. Young, G.V. Negrete, M.M. Eddy and E.J. Smith, Jpn. J. Appl. Phys. 30 (1991) L1359. [ 16] Yu. Boikov, G. Brorsson, T. Claeson and Z.G. Ivanov, Appl. Phys. Lett. 59 ( 1991 ) 2606.
ELSEVIER
YBCO Josephson junctions on bicrystalline NdGaO3 substrates R. Unger a, T.A. Scherer a,., W. Jutzi a, Z.G. Ivanov b, E.A. Stepantsov c " University ofKarlsruhe, lnstitutJ~r Elektrotechnische Grundlagen derlnformatik, Hertzstrasse 16, 1)-76187 Karlsruhe, Germany b Dept. of Physics, Chalmers University of Technology, G6teborg, Sweden ¢Academy of Sciences, Moscow, Russian Federation
Received 3 October 1994; revised manuscript received 7 November 1994
Abstract
YBCO Josephson junctions on bicrystalline NdGaO3 substrates with a misorientation angle O= 22 ° of the grain boundary have been fabricated with a current density Ofjm~ = 18 kA/cm 2 at T= 77 K. The dimension of the microbridge across the grain boundary was w= 4.8 ~tm with a thickness t = 290 nm. A rather uniform current-density distribution along the grain boundary is deduced with a nearly Fraunhofer behavior of the magnetic-field dependence of the maximum critical current. An IcRN product of 70 ~tV at T=77 K has been achieved.
1. Introduction
For the fabrication of cryoelectronic circuits it is necessary to realize Josephson junctions with a small standard deviation cr(Ij) of the critical current. The present standard deviation of grain boundary junctions on bicrystalline substrates is smaller than for step-edge junctions [ 1 ]. Many step-edge junctions on untwinned NdGaO3 substrates have been used for microwave applications [ 2 ]. Bicrystal junctions have been fabricated on different types o f substrates as SrTiO3, MgO, Y-ZrO2 and (110) oriented NdGaO3 [3,5-8]. Grain-boundary junctions (GB's) of untwinned YBCO on (001 ) NdGaO3 bicrystals should have interesting properties.
ented NdGaO3 single crystals. The misorientation was characterized by (001 ) rotation of the crystal lattices of both crystals on I 1 ° in opposite directions with their (001) planes being parallel to each other as shown schematically in Fig. I. A misorientation angle of the substrates O = 22 ° has been chosen to get low-energy YBCO grain boundaries. Crystal composites with an artificially made plane grain boundary were obtained as the result of intergrowing. These composites were cut into thin ( 1 m m ) slices perpendicular to the grain boundary. In a last step the slices
1001]
[00~]
2. Fabrication
Stepantsov manufactured the bicrystalline substrates by solid-phase intergrowing [ 9 ] of misori* Corresponding author.
Fig. 1. Crystallographicorientations of a NdGaO3 bicrystal substrate with a misorientation angle O= 22 °. The Miller index notation corresponds to the orthorhombic lattice of NdGaOa.
0921-4534/95/$09.50 © 1995 Elsevier Science B.V. All fights reserved SSDI0921-4534(94)02361-1
R. Ungeret al. /PhysicaC241(1995)316-318
317
were polished in order to achieve good surface characteristics for epitaxial film growth. C-axis oriented YBCO thin films have been deposited on the bicrystalline substrates by an off-axis sputtering technique with process parameters as reported in Refs. [10] and I l l ] . In order to avoid strain in the substrate and the YBCO film during the heating process a radiation heater with pyrometric control of the surface temperature has been used instead of a glueing technique with silver paint. The YBCO strip lines have been etched with an Ar ion beam on a cooled stage at liquid-nitrogen temperature. Thus, tilt GB junctions have been fabricated on (001) oriented NdGaO3 bicrystals with a junction width w=4.8 ~tm and a film thickness t= 290 nm.
to the film. Fig. 3 shows approximately a Isin x/xl behavior corresponding to a rather uniform currentdensity distribution along the junction. The ratio of the magnetic inductions B1 and Bo at both minima of the current/j yields B~/Bo=2.57 and has only a minor deviation from the expected value B~/Bo= 2. The magnetic induction Bo at the first minimum oflj can be estimated [ 2,13 ]:
3. Measurements
(Bo>= ~(Bo+ ~)=151~tT.
The measured current-voltage characteristics at T = 77 K for the mentioned contact is demonstrated in Fig. 2 for a swept magnetic field. The normal resistance is extrapolated RN~0.33 ~. The measured maximum Josephson current I~=210 ~tA yields a product I~.RN= 70 ~tV at T = 77 K. The excess current is negligibly small. The uniformity of the current distribution along the junction has been examined by measuring the dependence of the maximum Josephson current /j as a function of the magnetic induction B perpendicular
The deviation of calculated and measured values is small:
2q'o
Bo~ w2 , where Cbo=h/2eand w are the flux quantum and the width of the Josephson junction, respectively. For w= 4.8 p.m, measured with an optical microscope, the calculated induction Bo,¢~l= 180 ~tT is in a reasonable agreement with the mean value < Bo > determined by
1
AB= BoxaJ- ( Bo > ~ 16%. Bo,cal The Josephson current density of YBCO GB junctions on different bicrystalline substrates (e.g.: YZrO2, MgO, SrTiO3 and NdGaO3) as a function of the misorientation angle O can be approximated by an exponential law [ 6,7,12 ]:
Jmax=k~ exp( -k20);
0 < 4 5 ° and T = c o n s t .
A half logarithmic plot of
I
Jm~x/k~ versus the mis-
600 200
~oo ~A 200
I00
o J', m L ~
-200
o
-~oo -I00
.~,
~
.~
-600
40 6 s6 ,do ,s'o 260
-200
/ i
i
100
200
i
300
400
500
U/pV.-~.,,-
Fig. 2. Current-voltage characteristics at T=77 K of a w=4.8 vtmwideand t = 290 nm thickgrain-boundaryjunction on a (001) NdGaO~bicrystal with a misorientation angle O= 22 °.
B/~tT
..-
Fig. 3. Josephson current lj as function of the external magnetic induction B.
318
R. Unger et aL / Physica C 241 (1995) 316-318
orientation angle O yields nearly the same straight line for all the substrates mentioned in Table 1. The factor k~ represents the critical-current density of a thin YBCO film without grain boundary (O= 0 ° ). kl is a function of temperature and depends strongly on the quality of the deposition process. The factor k2, i.e. the slope of the straight line, only slightly varies. The Josephson current density versus misorientation angle O for Y - Z r O 2 at T = 7 7 K is shown in Fig. 4. The two circles corresponding to NdGaO3 bicrystals at O = 0 ° and 22 ° are very close to the straight line for Y - Z r O 2. At 0 = 2 2 ° and on (001) NdGaO3 substrates the Josephson current density isjm~x= 18 kA/ cm 2. Additional measurements at other misorientation angles than 0 ° and 22 ° are needed to determine the deviations from the straight line in Fig. 4. Unfortunately, measurements at other misorientation angles are not yet available. Table 1 Parameters kl and k2 for the dependence of the Josephson current density on the misorientation angle O for MgO, SrTiO3 and Y-ZrO2 Substrate
MgO SrTiO3 Y-ZrO2
Temperature T (K)
kl
k2
(MA/cm 2)
(°)
References
4.2 4.2 77
8.62 5.37 2.98
0.1817 0.1691 0.2192
[6] [12] [7]
107
1" ~ax A/cm 2
108, 105 10¢ 103 102 10
20
30
40
50
Misorientation angle ® / °
Fig. 4. Josephson current density at T=77 K of YBCO grain boundary junctions on Y-ZrO2 [7] marked with (Q) and NdGaO3 bicrystals marked with ( O ) [ 10,14-16 ] as function of the misorientation angle O.
4. Conclusion Bicrystalline NdGaO3 substrates with small misorientation angles have been fabricated. After off-axis sputtering of thin YBCO film stripes, comprising a single planar grain-boundary junction, have been milled with an Ar ion beam. A nearly uniform distribution of the Josephson current density has been found. At T = 7 7 K a current density jmax= 18 kA/ cm 2 of a junction on a NdGaO3 bicrystalline substrate with a misorientation angle O= 22 ° has been achieved. The current density seems to depend mainly on the misorientation angle and less on the kind of substrate. References [ 1] R. Gross, in: Grain Boundary Josephson Junctions in the High-Temperature Superconductors, Interface in Supercond. Systems, Eds. S.L. Shiade and D. Rudnan (Springer, New York, 1992). [2]A. Vogt, H. Matz, R. Dolata, R. Herwig, M. Neuhaus, P. Marienhoff, T. Scherer, W. Jutzi, U. Fath, G. Hrfer and H. Kratz, IV. Trilateral German-Russian-Ukrainian Seminar on HTSC, Dubna, Russia (1993). [3] P. Chaudhari, J. Mannhart, D. Dimos, C.C. Tsuei, J. Chi, M.M. Oprysko and M. Scheuermann, Phys. Rev. Len. 60 (1988) 1653. [4] D. Dimos, P. Chaudhari, J. Mannhart and F.K. LeGoues, Phys. Rev. Lett. 61 (1988) 219. [ 5 ] D. Dimos, P. Chaudhari and J. Mannhart, Phys. Rev. B 41 (1990) 4038. [6] H.B. Lu, T.W. Huang, J.J. Wang, J. Lin, S.L. Tu, S.J. Yang and S.E. Hsu, IEEE Trans. Appl. Supercond. 3 ( 1993 ) 2325. [7] Z.G. Ivanov, P.A. Nilsson, D. Winkler, J.A. Alasco, T. Claeson, E.A. Stepantsov and A.Y. Tzalenchuk, Appl. Phys. Lett. 59 ( 1991 ) 3030. [8] P.G. Quincey, Appl. Phys. Lett. 64 (1994) 517. [9] E.A. Stepantsov, A.S. USSR No 1116100 C1. C30b33\00 ( 1982 ), Bull. Izobreteniy 36 (1984) 77. [ 10] T. Scherer, R. Herwig, P. Marienhoff, M. Neuhaus, A. Vogt and W. Jutzi, Cryogenics 31 ( 1991 ) 975. [ 11 ] T. Scherer, P. Marienhoff, R. Herwig, M. Neuhaus and W. Jutzi, Physica C 197 (1992) 79. [ 12 ] R. Gross and B. Mayer, Physica C 180 ( 1991 ) 235. [l 3] R.G. Humphreys, J.S. Satchell, J.A. Edwards, N.G. Chew, S.W. Goodyear, M.N. Keene and S.F. Morgan, IEEE Trans. Appl. Supercond. 3 (1993) 2026. [ 14] K.H. Young, G.V. Negrete and J.Z. Sun, Jpn. J. Appl. Phys. 30 (1991) L1355. [ 15 ] K.H. Young, G.V. Negrete, M.M. Eddy and E.J. Smith, Jpn. J. Appl. Phys. 30 (1991) L1359. [ 16] Yu. Boikov, G. Brorsson, T. Claeson and Z.G. Ivanov, Appl. Phys. Lett. 59 ( 1991 ) 2606.