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Superconducting proximity effect in Y1Ba2Cu3O7−δ - Ag-Pb trilayer structures
Solid State Communications,Vol. 71, No. 7, pp. 575-577. 1989. Printed in Great Britain.
SUPERCONDUCTING
0038-1098/89$3.00+.00 Maxwell Pergamon Macmillan plc
PROXIMITY EFFECT IN YlBa2Cu307,- Ag-Pb TRILAYER STRUCTURES
M.A.M. Gijs, D.Scholten, Th. van Rooy and R. IJsselsteijn Philips Research Laboratories,
5600 JA Eindhoven,
The Netherlands
( Received by S. Amelinckx - June lst, 1989 ) We have studied the superconducting critical current I, for YrBazCu307_, (123) Ag - Pb proximity structures as a function of temperature and magnetic field. The temperature dependence of I, is in agreement with the theory of de Gennes; the magnitude of the critical current density is as large as 1.7 x IO5 A/cm2 at 4.0 K which indicates strong proximity coupling. The decrease of I, with magnetic field is explained by the reduction of the normal metal (Ag) coherence length by the field.
The fabrication of Josephson and tunnel junctions of the oxide superconductors has gained a lot of inThe possibility of using these junctions in terest. superconducting electronics at liquid nitrogen temperature indeed seems very promising. However it is extremely difficult to fabricate a good working device [ 1,2] . The main reason for this problem is the very small coherence length of the oxide superconductors, which is typically of the order of 1 nm [3] . Hence to fabricate a good junction it is necessary to have a defectless and sharp superconductor-insulator interface on an atomic scale. A solution for this problem may be the use of a noble metal buffer layer between superconductor and insulator. Indeed a noble metal (Ag or Au) does not affect the superconducting oxide and has a large coherence length (about 100 nm at 4.2 K), so that interface defects are much less important. Therefore, if the noble metal layer is made thin enough, a supercurrent due to the proximity effect [4] can exist and in this way a Josephson junction can be properly realized. This method has already been proved to be fruitful in the realization of Recently also 123-Ag-A1203-Pb junctions [SJ . 123-Au-Nb proximity structures with a critical current density of 19 A/cm2 at 4.2 K have been realized. [6] In this paper we report on the fabrication of 123-Ag-Pb proximity junctions with a critical current density of 1.7 x 10” A/cm2 at 4.0 K. This large value indicates a very strong proximity coupling between the 123 and the Pb. The temperature dependence of I, is in agreement with the theory of de Gennes [4] . The magnetic field dependence of I, indicates a strong shielding effect of the superconducting electrodes on the junction. From the comparison with theory we find that the magnetic field reduces the normal metal coherence length of the Ag, which gives rise to a decrease of I, . A schematic of the junction is shown in Figure 1. Firstly a 1 mm wide and 210 nm thick 123 strip is sputtered on a (100) SrTiO3 substrate with an Ar pressure of 3 x 10-J Torr. Superconductivity is obtained after annealing in flowing O2 at 850 “C for l/2
SrTiO,
a
b
Figure 1: Top view (a) and side view (b) of the proximity junction geometry.
hour [7]. Then a 40 nm thick Ag layer is sputtered over the whole substrate. Standard lithography and Ar ion bombardment is used to define square Ag areas of 6 x 6 pm2 and 8 x 8 pm2 . Then the sample is heated up to 450 “C in flowing 02 during l/2 hour in order to obtain a good superconducting contact between 123 and Ag [8] . Finally a 200 nm thick Pb strip is evaporated at an angle of 90” with respect to the 123 strip. The crossed strip junction geometry permits a proper measurement of the junction interface. The electric contact of the Pb on the Ag is very good; on the other hand when Pb is directly in contact with 123 it forms an insulating layer with very high resistance, due to the extraction of oxygen out of the 123 by the Pb. Hence we realize an effective junction area equal to the Ag surface. After contacting the sample with In, it was mounted in a continuous flow cryostat. An electromagnet generated the magnetic field. In figure 2 we present the current-voltage characteristics of a proximity sandwich with a junction surface area of 36 pm2 at three different temperatures. At T=4.0 K the supercurrent is 62 mA which correcritical current density of sponds to a 1.7 x 10” A/cm2 . This high value is only possible when the proximity coupling is very strong, as is the case when the normal metal layer is very thin and when no spurious insulating layers are present at the 575
576
t
Y1Ba2Cu30,_6 - Ag .I00
3 -
Vol.
STRUCTURES
71,
No.
T (K)
1.
I
Pb TRILAYER
4.0
,060 ,020 -.020
I -1 .oo
I
I -.500
I ,000
,500 u(10-4v)
, 1 .oo
4.60
5.80
7.06
w
T(K) -
Figure 2: Current-voltage characteristics of a proximity junction at three different temperatures.
Figure 3: Temperature dependence of the critical current of a proximity iunction. The full curve is calculated using expres$on (1).
interfaces. With increasing temperature the supcrcurrent quickly drops and at 7.2 K (the critical temperature of Pb) it has disappeared completely. The temperature dependence of the critical current as determined from the I-V characteristics is shown in Figure 3. The experimental points are represented by the symbols. The full curve represents the proximityeffect theory of de Genncs, based on the linearized Gorkov equation [4] , which states that the critical current is given by
I’=* -&;T)
sinhC2;rN(T),
h,(Tk%23(T)
(1)
where A is a geometry dependent prefaktor, t is the normal metal layer thickness and A&T) and Aj23(T) are the induced pair potentials at the Pb/Ag resp. the 123/Ag boundery. lN(T) is the normal metal coherence length and is given by tN(T)=(hD/2nkoT)“2 in the case of a disordered metal. The diffusion constant D EEv&/3 , with vr; the Fermi velocity and 1, the elastic scattering length. For Ag we find with vF= I .39 x 106 m/s and with an estimate of I,=20 nm , that D=9.3 x 10-3m2/s . The temperature dependence of the pair potential Aph is, near the critical temperature of the Pb, proportional to (I - T/Tc,& , while Aj2) , due to the high critical temperature of the 123, essentially is constant in the temperature range studied. From the comparison of our experiment with expression (I), and using the above definition of the normal metal coherence length (c&=4.2 K) =.52 nm) , we obtain as best lit a Ag thickness t= 100 nm, which is of the order of the nominal Ag thickness of 40 nm. We also studied the magnetic field dependence of the critical current for fields directed along the junction plane. In Figure 4 we show our data measured at 4.0 K. We observe that up to B==400 Gauss, the critical current I, is nearly independent of field, while above 400 Gauss there is a strong dccreasc of I, , This behaviour can be explained by the shielding effects of the superconducting electrodes of the proximity sandwich. At 4.0 K the critical field of the thick Pb film, which is a type I superconductor, is about 600 Gauss [9] , while the lower critical field B,, of the 123 film is 400 - 500 Gauss [3] . At this field flux lines cntcr lhe proximity sandwich and the critical current starts to decrease, as is clear from figure 4. The full
100
300
500 B (Gauss)
700 -
Figure 4: Magnetic field dependence at 4.0 K of the critical current of a proximity junction. The full curve is calculated using expression (I) and (2). curve in Figure 4 is calculated using equation (1) without changing fitting parameters, but taking into account the magnetic field dcpendcnce of the normal metal coherence length. According to the work of Hsiang and Finncmore [IO] this dependence is given by tn=cn=u(l
+ B/B,)-“’
where B, is a characteristic field for pair breaking in the normal metal. We replaced the field B in equation (2) by (B - 400 G) to take into account the shielding effect and find that BP=63 Gauss . At tcmpcratures above 4.0 K I, is nearly constant with field until the B,, field of the 123 or the critical field of the Pb is reached, at which I, quickly drops. In conclusion we have fabricated 12%Ag-Pb proximity structures with a critical current density of 1.7 x 10’ A/cm2 at 4.0 K. The tcmpcrature dependence of 1, well agrees with the theory of de Gennes. The magnetic field dcpcndencc indicates a decrcasc of the normal metal coherence length by the field. Acknowledgements. We would like to thank P.F. Bongcrs, B. Dam, J.B. Gicsbcrs, M.F.H. Schuurmans and C.H.M. Witmcr for stimulating discussions.
7
Vol. 71, NO. 7
YlBa2Cu307_6- kit- Pb TRILAYER STRUCTURES
References 1. M.A.M. Gijs, J.W.C. de Vries and G.M. Stollman, Phys. Rev. B 31 , 9837 (1988) 2. J. Takada, T. Terashima, Y. Bando, H. Mazaki, K. Iijima, K. Yamamoto and K. Hirata, Appl. Phys. Lett. 33 , 2689 (1988) 3. W.K. Gallagher, T.K. Worthington, T.R. Dinger, F. Holtzberg, D.L. Kaiser and R.L. Sandstrom, Physica 14&B , 228 (1987) 4. P.G. de Gennes, Rev. Mod. Phys. 36 , 225 (1964) 5. J. Yoshida, K. Mizushima, M. Sagoi, Y. Terashima and T. Miura, Ext. Abstr. of the 20th Conf. on Solid State Dev. and Mat., Tokyo (1988)
6. H. Akoh, F. Shinoki, M. Takahashi and S. Takada, Jap. J. of Appl. Phys. 21, L 519 (1988) 7. J.W.C. de Vries, B. Dam, M.G.J. Heijrnan, G.M. Stollman, M.A.M. Gijs, C.W. Hagen and R.P. Griessen, Appl. Phys. Lett. 2, 1904 (1988) 8. M.A.M. Gijs, B. Dam, M.G.J. Heijman and Th.A.W van Elswijk, J. Less Common Met., to be published (1989) 9. C. Kittel, Quantum Theory of Solids (John Wiley and Sons, New York, 1966) 10. T.Y. Hsiang and D.K. Finnemore, Phys. Rev. B22 , 154 (1980)
577
SUPERCONDUCTING
0038-1098/89$3.00+.00 Maxwell Pergamon Macmillan plc
PROXIMITY EFFECT IN YlBa2Cu307,- Ag-Pb TRILAYER STRUCTURES
M.A.M. Gijs, D.Scholten, Th. van Rooy and R. IJsselsteijn Philips Research Laboratories,
5600 JA Eindhoven,
The Netherlands
( Received by S. Amelinckx - June lst, 1989 ) We have studied the superconducting critical current I, for YrBazCu307_, (123) Ag - Pb proximity structures as a function of temperature and magnetic field. The temperature dependence of I, is in agreement with the theory of de Gennes; the magnitude of the critical current density is as large as 1.7 x IO5 A/cm2 at 4.0 K which indicates strong proximity coupling. The decrease of I, with magnetic field is explained by the reduction of the normal metal (Ag) coherence length by the field.
The fabrication of Josephson and tunnel junctions of the oxide superconductors has gained a lot of inThe possibility of using these junctions in terest. superconducting electronics at liquid nitrogen temperature indeed seems very promising. However it is extremely difficult to fabricate a good working device [ 1,2] . The main reason for this problem is the very small coherence length of the oxide superconductors, which is typically of the order of 1 nm [3] . Hence to fabricate a good junction it is necessary to have a defectless and sharp superconductor-insulator interface on an atomic scale. A solution for this problem may be the use of a noble metal buffer layer between superconductor and insulator. Indeed a noble metal (Ag or Au) does not affect the superconducting oxide and has a large coherence length (about 100 nm at 4.2 K), so that interface defects are much less important. Therefore, if the noble metal layer is made thin enough, a supercurrent due to the proximity effect [4] can exist and in this way a Josephson junction can be properly realized. This method has already been proved to be fruitful in the realization of Recently also 123-Ag-A1203-Pb junctions [SJ . 123-Au-Nb proximity structures with a critical current density of 19 A/cm2 at 4.2 K have been realized. [6] In this paper we report on the fabrication of 123-Ag-Pb proximity junctions with a critical current density of 1.7 x 10” A/cm2 at 4.0 K. This large value indicates a very strong proximity coupling between the 123 and the Pb. The temperature dependence of I, is in agreement with the theory of de Gennes [4] . The magnetic field dependence of I, indicates a strong shielding effect of the superconducting electrodes on the junction. From the comparison with theory we find that the magnetic field reduces the normal metal coherence length of the Ag, which gives rise to a decrease of I, . A schematic of the junction is shown in Figure 1. Firstly a 1 mm wide and 210 nm thick 123 strip is sputtered on a (100) SrTiO3 substrate with an Ar pressure of 3 x 10-J Torr. Superconductivity is obtained after annealing in flowing O2 at 850 “C for l/2
SrTiO,
a
b
Figure 1: Top view (a) and side view (b) of the proximity junction geometry.
hour [7]. Then a 40 nm thick Ag layer is sputtered over the whole substrate. Standard lithography and Ar ion bombardment is used to define square Ag areas of 6 x 6 pm2 and 8 x 8 pm2 . Then the sample is heated up to 450 “C in flowing 02 during l/2 hour in order to obtain a good superconducting contact between 123 and Ag [8] . Finally a 200 nm thick Pb strip is evaporated at an angle of 90” with respect to the 123 strip. The crossed strip junction geometry permits a proper measurement of the junction interface. The electric contact of the Pb on the Ag is very good; on the other hand when Pb is directly in contact with 123 it forms an insulating layer with very high resistance, due to the extraction of oxygen out of the 123 by the Pb. Hence we realize an effective junction area equal to the Ag surface. After contacting the sample with In, it was mounted in a continuous flow cryostat. An electromagnet generated the magnetic field. In figure 2 we present the current-voltage characteristics of a proximity sandwich with a junction surface area of 36 pm2 at three different temperatures. At T=4.0 K the supercurrent is 62 mA which correcritical current density of sponds to a 1.7 x 10” A/cm2 . This high value is only possible when the proximity coupling is very strong, as is the case when the normal metal layer is very thin and when no spurious insulating layers are present at the 575
576
t
Y1Ba2Cu30,_6 - Ag .I00
3 -
Vol.
STRUCTURES
71,
No.
T (K)
1.
I
Pb TRILAYER
4.0
,060 ,020 -.020
I -1 .oo
I
I -.500
I ,000
,500 u(10-4v)
, 1 .oo
4.60
5.80
7.06
w
T(K) -
Figure 2: Current-voltage characteristics of a proximity junction at three different temperatures.
Figure 3: Temperature dependence of the critical current of a proximity iunction. The full curve is calculated using expres$on (1).
interfaces. With increasing temperature the supcrcurrent quickly drops and at 7.2 K (the critical temperature of Pb) it has disappeared completely. The temperature dependence of the critical current as determined from the I-V characteristics is shown in Figure 3. The experimental points are represented by the symbols. The full curve represents the proximityeffect theory of de Genncs, based on the linearized Gorkov equation [4] , which states that the critical current is given by
I’=* -&;T)
sinhC2;rN(T),
h,(Tk%23(T)
(1)
where A is a geometry dependent prefaktor, t is the normal metal layer thickness and A&T) and Aj23(T) are the induced pair potentials at the Pb/Ag resp. the 123/Ag boundery. lN(T) is the normal metal coherence length and is given by tN(T)=(hD/2nkoT)“2 in the case of a disordered metal. The diffusion constant D EEv&/3 , with vr; the Fermi velocity and 1, the elastic scattering length. For Ag we find with vF= I .39 x 106 m/s and with an estimate of I,=20 nm , that D=9.3 x 10-3m2/s . The temperature dependence of the pair potential Aph is, near the critical temperature of the Pb, proportional to (I - T/Tc,& , while Aj2) , due to the high critical temperature of the 123, essentially is constant in the temperature range studied. From the comparison of our experiment with expression (I), and using the above definition of the normal metal coherence length (c&=4.2 K) =.52 nm) , we obtain as best lit a Ag thickness t= 100 nm, which is of the order of the nominal Ag thickness of 40 nm. We also studied the magnetic field dependence of the critical current for fields directed along the junction plane. In Figure 4 we show our data measured at 4.0 K. We observe that up to B==400 Gauss, the critical current I, is nearly independent of field, while above 400 Gauss there is a strong dccreasc of I, , This behaviour can be explained by the shielding effects of the superconducting electrodes of the proximity sandwich. At 4.0 K the critical field of the thick Pb film, which is a type I superconductor, is about 600 Gauss [9] , while the lower critical field B,, of the 123 film is 400 - 500 Gauss [3] . At this field flux lines cntcr lhe proximity sandwich and the critical current starts to decrease, as is clear from figure 4. The full
100
300
500 B (Gauss)
700 -
Figure 4: Magnetic field dependence at 4.0 K of the critical current of a proximity junction. The full curve is calculated using expression (I) and (2). curve in Figure 4 is calculated using equation (1) without changing fitting parameters, but taking into account the magnetic field dcpendcnce of the normal metal coherence length. According to the work of Hsiang and Finncmore [IO] this dependence is given by tn=cn=u(l
+ B/B,)-“’
where B, is a characteristic field for pair breaking in the normal metal. We replaced the field B in equation (2) by (B - 400 G) to take into account the shielding effect and find that BP=63 Gauss . At tcmpcratures above 4.0 K I, is nearly constant with field until the B,, field of the 123 or the critical field of the Pb is reached, at which I, quickly drops. In conclusion we have fabricated 12%Ag-Pb proximity structures with a critical current density of 1.7 x 10’ A/cm2 at 4.0 K. The tcmpcrature dependence of 1, well agrees with the theory of de Gennes. The magnetic field dcpcndencc indicates a decrcasc of the normal metal coherence length by the field. Acknowledgements. We would like to thank P.F. Bongcrs, B. Dam, J.B. Gicsbcrs, M.F.H. Schuurmans and C.H.M. Witmcr for stimulating discussions.
7
Vol. 71, NO. 7
YlBa2Cu307_6- kit- Pb TRILAYER STRUCTURES
References 1. M.A.M. Gijs, J.W.C. de Vries and G.M. Stollman, Phys. Rev. B 31 , 9837 (1988) 2. J. Takada, T. Terashima, Y. Bando, H. Mazaki, K. Iijima, K. Yamamoto and K. Hirata, Appl. Phys. Lett. 33 , 2689 (1988) 3. W.K. Gallagher, T.K. Worthington, T.R. Dinger, F. Holtzberg, D.L. Kaiser and R.L. Sandstrom, Physica 14&B , 228 (1987) 4. P.G. de Gennes, Rev. Mod. Phys. 36 , 225 (1964) 5. J. Yoshida, K. Mizushima, M. Sagoi, Y. Terashima and T. Miura, Ext. Abstr. of the 20th Conf. on Solid State Dev. and Mat., Tokyo (1988)
6. H. Akoh, F. Shinoki, M. Takahashi and S. Takada, Jap. J. of Appl. Phys. 21, L 519 (1988) 7. J.W.C. de Vries, B. Dam, M.G.J. Heijrnan, G.M. Stollman, M.A.M. Gijs, C.W. Hagen and R.P. Griessen, Appl. Phys. Lett. 2, 1904 (1988) 8. M.A.M. Gijs, B. Dam, M.G.J. Heijman and Th.A.W van Elswijk, J. Less Common Met., to be published (1989) 9. C. Kittel, Quantum Theory of Solids (John Wiley and Sons, New York, 1966) 10. T.Y. Hsiang and D.K. Finnemore, Phys. Rev. B22 , 154 (1980)
577