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The effect of Ag or Au overlayers on underlying YBa2Cu3O7 thin films
PHYSICA
Physica C 220 (1994) 329-331 North-Holland
The effect of Ag or Au overlayers on underlying YBa2Cu307 thin films A. Mogro-Campero and L.G. Turner GE Research and Development Center, Schenectady. NY 12301, USA
Received 12 October 1993 Revised manuscript received 8 November 1993
Although Agand Au ale commonly used to provide low-resistancecontacts to YBa2Cu307 (YBCO), their effect on the electrical properties of the underlying YBCO has been largely neglected. Epitaxial YBCO thin films on LaAIO3substrates were used in this study. Thin (50 nm) and thick ( 1 I~m) layers of Ag or Au were deposited as overlayers, and in some cases annealed in oxygen at 550-650"C. Compared to samples with no metal overlayers, for both Ag and Au the normal-state parameters changed (resistivity, its temperature coefficient, and the transition width), whereas the transition temperature and critical current density remained unaltered. These results are encouraging for the use of these metals as contacts and/or conducting overlayers on YBCO.
1. Introduction A large number of studies has shown that low-resistance contacts to YBa2Cu307 ( Y B C O ) can be achieved by providing for intimate contact between the Au or Ag and YBCO. This can be achieved in practice by either the in situ deposition of YBCO and the metal contact or by a post-annealing treatment in oxygen at about 600°C. This work looks into the effect o f such Ag or Au overlayers on other properties o f the Y B C O / m e t a l bilayer. This aspect has received little attention in the literature. For Ag, the possible degradation of superconducting properties is not settled. The behavior o f the critical current density (Jc) as a function of temperature and applied magnetic field was studied for YBCO films with and without Ag overlayers [ 1 ]; the authors found that for the metal thickness ( 1 0 - 2 0 0 n m ) and annealing treatment (500°C in oxygen) used, no degradation of J~ was observed. A subsequent study [2] found that 50 nm o f Ag deposited in situ on a YBCO film produced a decrease in both the transition temperature (To), and Jc. The effect o f a 70 nm thick Ag overlayer on YBCO was investigated by an electrooptic method [ 3 ]; the authors reported degradation o f superconducting propenies beneath the Ag layer. We have encountered no
reports on the effect of Au overlayers on the properties o f the underlying YBCO.
2. Experimental procedures Thin films of YBCO were deposited onto (100) LaA103 substrates by coevaporation of Y, BaF2, and Cu, and post-annealed at 750°C in a low partial pressure of oxygen, as described in more detail elsewhere [4 ]. Electrical parameters were measured by a fourpoint probe method across 400 g m long and 25 Ilm wide bridges formed by conventional photolithography and ion milling. Thin (30 n m ) and thick (1 p.m) overlayers o f Ag or Au were evaporated on some of the samples. Some of these were annealed after metal deposition (at 550°C for Ag and 650°C for Au, in flowing oxygen for 0.5 h).
3. Results and discussion Film thickness and electrical parameters for the samples with no metal overlayers (control samples) are listed in table I. The resistivity at 295 K is lower for the thinner samples, and Jc(77 K) is higher, consistent with a previous study o f the parameter vail-
0921-4534/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved. SSDI 0921-4534(93)E0974-6
330
A. Mogro- Campero. L.G. Turner / Effect of A g or.4 u overlayer.~
Table I Parameters of YBCO control samples (no metal ovcrlayers) Film thickness (lam) Resistivity at 295 K (laf~cm ) R( 100 K)/R(295 K) Transition width ( K ) Transition temperature (K) Critical current density at 77K (MAcm -2)
0.3 390 0.30 I. 5 88 2.7
0.6 500 0.30 1.0 89 I.q
ation as a function o f film thickness [ 5 ]. As can bc seen in table 1, Jc(77 K ) > 1 M A c m -2 for all sampies, indicating the high quality o f the YBCO films used in this study. No further reference is m a d e to the YBCO film thickness. P a r a m e t e r values for samples with metal overlayers are n o r m a l i z e d to control values. In the figures, data for Au are shown as open circles, and d a t a for Ag as solid circles. The metal thickness and heat treatment are as indicated (details o f the annealing t r e a t m e n t were specified in section 2 a b o v e ) . Figure 1 shows n o r m a l i z e d values o f the resistance at 295 K. In all cases the resistance is lower with metal overlayers. This is because at this t e m p e r a t u r e the resistivity o f Ag and Au is 200 to 300 times lower than that o f YBCO. In fact, all resistances with the exception o f thin annealed Ag are close to h a n d b o o k resistivity values for Ag and Au. The higher resistance values ( a p p r o a c h i n g those o f Y B C O ) for thc thin annealed Ag samples probably indicate significant interdiffusion o f the thin Ag layer into the underlying YBCO film. The t e m p e r a t u r e coefficient o f resistance was investigated by using the resistance ratio R~oo K/R29~ K" N o r m a l i z e d values o f this ratio are shown in fig. 2. The t e m p e r a t u r e coefficients o f resistance for bulk Au and Ag are very close to that for YBCO, so that for the thicker films, the n o r m a l i z e d values are close to I. F o r the 50 nm thick metal films, a higher value o f the n o r m a l i z e d resistance ratio is observed, qualitatively consistent with the fact that at 100 K the electron mean free path in the metal is larger than the film thickness. Thus, the normal-state p a r a m e t e r s measured are different for samples with metal layers c o m p a r e d to bare YBCO samples. In particular, the resistance is substantially lower (fig. 1 ), and the,slope in the linear resistance region is shallower for samples with thin metal overlayers (fig. 2).
• f.,: .
,i
Y
1()
t~
C
10:' nO 2"
.q
,
!~C t~." t;NANNEAI
ED
(:0 n m ANNEALFf]
META~ THIC'KNFS%
Ili-A]
" ,."1 A N N t - A t E~) TP.EATME,~.;]
Fig. 1. Resistance at 295 K normalized to values for the control samples (table 1) for YBCO thin films with Au (open circles) or Ag (solid circles ) overlaycrs of the thickness shown. Annealed or unannealed refers to the heat treatment after metal deposition. Ag annealing was at 550°C and for Au at 650°C, in each case for 0.5 h in flowing oxygen. The normalized transition width is shown in fig. 3. F o r samples with metal overlayers the transition width is about half its value for the bare Y B C O control samples. At temperatures above the zero-resistance transition temperature o f Y B C O the resistances o f the YBCO and Ag films are c o m b i n e d as two resistors in parallel. Over most o f this temperature regime the total resistance is d o m i n a t e d by that o f the metal overlayer, since its resistance is much lower. This results in a narrowing o f the transition width by a lowering of the transition onset temperature. Thus, the narrower transition width is due to lower onset temperatures due to the lower resistance values. The zero resistance transition temperatures ( 8 8 89 K ) remained unaltered within experimental error ( _+0.5'-K). The proximity effect has i m p o r t a n t implications for tunneling and device work [ 6 ]. How-
A. Mogro-Campero, L.G. Turner/Effect of A g or Au overlayers
I
I
331
K for a superconductor/normal metal configuration of the type used here [2]. The critical current density at 77 K is also unaltered, as shown in fig. 2.
I
2.0 A
% 8
4. Summary and conclusion
rr
_o tr LU (.) Z •
I.LI nO N_
o
z
I
I
50 nm UNANNEALED
50 n m ANNEALED
I 1 jam ANNEALED
METAL THICKNESS, HEAT TREATMENT
Fig. 2. Resistance ratio RIOOK/R295Knormalized to values for the control samples (table 1 ) for YBCO thin films with Au or Ag oveflayers as described in the caption to fig. 1. 15
I
I
Epitaxial thin films of YBCO with Jc(77 K) > 1 M A c m - 2 w e r e used to study normal-state and superconducting-state electrical properties of samples with and without Ag or Au overlayers. Both thin and thick metal overlayers, sometimes annealed at 550650°C in flowing oxygen were used. Compared to bare YBCO film properties, for YBCO with Ag or Au overlayers ( 1 ) the superconducting parameters (To(0), Jc(77 K) ) are unaltered, (2) the transition width decreases by a factor of two and (3) in the linear resistance range (100-300 K) the resistance is substantially lower and the slope is shallower for thin (50 nm) overlayers. These results support the use of Ag or Au on YBCO as contact metals and/or conducting layers such as used for current shunting in the normal state.
Acknowledgements We thank J. McMullen for photolithography work, P. Schaeffer for wafer sawing, and J. Bray for encouragement and support.
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3 05o u.i N
References
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0 z
| 05--
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N
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I
I
I
50 nm UNANNEALED
50 nm ANNEALED
1 ~m ANNEALED
METAL THICKNESS. HEAT T R E A T M E N T
Fig. 3. Transition width and critical current density at 77 K normalized to values for the control samples (table 1 ) for YBCO thin films with Ag or Au overlayers as described in the caption to fig. 1.
ever, transition temperature shifts due to the proximity effect have been estimated as being about 10 -2
[ 1 ] R.H. Ono, J.A. Beall, T.E. Harvey, C.D. Reintsema, M. Johansson, M.W. Cromar, L.F. Goodrich, J. Moreland, A. Roshko and T.C. Stauffer, IEEE Trans. Magn. 27 (1991) 1471. [2] E. Polturak, G. Koren, D. Cohen, D. Cohen and I. Snapiro, Physica C 197 (1992) 1. [3] A.I. Belyaeva, B.1. Khamdamov and V.P. Yur'ev, Soy. Tech. Phys. Lett. 17 ( 1991 ) 220. [4] A. Mogro-Campero, L.G. Turner, A.M. Kadin and D.S. Mallory, Appl. Phys. Lett. 60 (1992) 3310. [5] A. Mogro-Campero and L.G. Turner, J. Supercond. 6 (1993) 37. [6l M. Lee, D. Lew, C.B. Eom, T.H. Geballe and M.R. Beasley, Appl. Phys. Lett. 57 (1990) 1152.
Physica C 220 (1994) 329-331 North-Holland
The effect of Ag or Au overlayers on underlying YBa2Cu307 thin films A. Mogro-Campero and L.G. Turner GE Research and Development Center, Schenectady. NY 12301, USA
Received 12 October 1993 Revised manuscript received 8 November 1993
Although Agand Au ale commonly used to provide low-resistancecontacts to YBa2Cu307 (YBCO), their effect on the electrical properties of the underlying YBCO has been largely neglected. Epitaxial YBCO thin films on LaAIO3substrates were used in this study. Thin (50 nm) and thick ( 1 I~m) layers of Ag or Au were deposited as overlayers, and in some cases annealed in oxygen at 550-650"C. Compared to samples with no metal overlayers, for both Ag and Au the normal-state parameters changed (resistivity, its temperature coefficient, and the transition width), whereas the transition temperature and critical current density remained unaltered. These results are encouraging for the use of these metals as contacts and/or conducting overlayers on YBCO.
1. Introduction A large number of studies has shown that low-resistance contacts to YBa2Cu307 ( Y B C O ) can be achieved by providing for intimate contact between the Au or Ag and YBCO. This can be achieved in practice by either the in situ deposition of YBCO and the metal contact or by a post-annealing treatment in oxygen at about 600°C. This work looks into the effect o f such Ag or Au overlayers on other properties o f the Y B C O / m e t a l bilayer. This aspect has received little attention in the literature. For Ag, the possible degradation of superconducting properties is not settled. The behavior o f the critical current density (Jc) as a function of temperature and applied magnetic field was studied for YBCO films with and without Ag overlayers [ 1 ]; the authors found that for the metal thickness ( 1 0 - 2 0 0 n m ) and annealing treatment (500°C in oxygen) used, no degradation of J~ was observed. A subsequent study [2] found that 50 nm o f Ag deposited in situ on a YBCO film produced a decrease in both the transition temperature (To), and Jc. The effect o f a 70 nm thick Ag overlayer on YBCO was investigated by an electrooptic method [ 3 ]; the authors reported degradation o f superconducting propenies beneath the Ag layer. We have encountered no
reports on the effect of Au overlayers on the properties o f the underlying YBCO.
2. Experimental procedures Thin films of YBCO were deposited onto (100) LaA103 substrates by coevaporation of Y, BaF2, and Cu, and post-annealed at 750°C in a low partial pressure of oxygen, as described in more detail elsewhere [4 ]. Electrical parameters were measured by a fourpoint probe method across 400 g m long and 25 Ilm wide bridges formed by conventional photolithography and ion milling. Thin (30 n m ) and thick (1 p.m) overlayers o f Ag or Au were evaporated on some of the samples. Some of these were annealed after metal deposition (at 550°C for Ag and 650°C for Au, in flowing oxygen for 0.5 h).
3. Results and discussion Film thickness and electrical parameters for the samples with no metal overlayers (control samples) are listed in table I. The resistivity at 295 K is lower for the thinner samples, and Jc(77 K) is higher, consistent with a previous study o f the parameter vail-
0921-4534/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved. SSDI 0921-4534(93)E0974-6
330
A. Mogro- Campero. L.G. Turner / Effect of A g or.4 u overlayer.~
Table I Parameters of YBCO control samples (no metal ovcrlayers) Film thickness (lam) Resistivity at 295 K (laf~cm ) R( 100 K)/R(295 K) Transition width ( K ) Transition temperature (K) Critical current density at 77K (MAcm -2)
0.3 390 0.30 I. 5 88 2.7
0.6 500 0.30 1.0 89 I.q
ation as a function o f film thickness [ 5 ]. As can bc seen in table 1, Jc(77 K ) > 1 M A c m -2 for all sampies, indicating the high quality o f the YBCO films used in this study. No further reference is m a d e to the YBCO film thickness. P a r a m e t e r values for samples with metal overlayers are n o r m a l i z e d to control values. In the figures, data for Au are shown as open circles, and d a t a for Ag as solid circles. The metal thickness and heat treatment are as indicated (details o f the annealing t r e a t m e n t were specified in section 2 a b o v e ) . Figure 1 shows n o r m a l i z e d values o f the resistance at 295 K. In all cases the resistance is lower with metal overlayers. This is because at this t e m p e r a t u r e the resistivity o f Ag and Au is 200 to 300 times lower than that o f YBCO. In fact, all resistances with the exception o f thin annealed Ag are close to h a n d b o o k resistivity values for Ag and Au. The higher resistance values ( a p p r o a c h i n g those o f Y B C O ) for thc thin annealed Ag samples probably indicate significant interdiffusion o f the thin Ag layer into the underlying YBCO film. The t e m p e r a t u r e coefficient o f resistance was investigated by using the resistance ratio R~oo K/R29~ K" N o r m a l i z e d values o f this ratio are shown in fig. 2. The t e m p e r a t u r e coefficients o f resistance for bulk Au and Ag are very close to that for YBCO, so that for the thicker films, the n o r m a l i z e d values are close to I. F o r the 50 nm thick metal films, a higher value o f the n o r m a l i z e d resistance ratio is observed, qualitatively consistent with the fact that at 100 K the electron mean free path in the metal is larger than the film thickness. Thus, the normal-state p a r a m e t e r s measured are different for samples with metal layers c o m p a r e d to bare YBCO samples. In particular, the resistance is substantially lower (fig. 1 ), and the,slope in the linear resistance region is shallower for samples with thin metal overlayers (fig. 2).
• f.,: .
,i
Y
1()
t~
C
10:' nO 2"
.q
,
!~C t~." t;NANNEAI
ED
(:0 n m ANNEALFf]
META~ THIC'KNFS%
Ili-A]
" ,."1 A N N t - A t E~) TP.EATME,~.;]
Fig. 1. Resistance at 295 K normalized to values for the control samples (table 1) for YBCO thin films with Au (open circles) or Ag (solid circles ) overlaycrs of the thickness shown. Annealed or unannealed refers to the heat treatment after metal deposition. Ag annealing was at 550°C and for Au at 650°C, in each case for 0.5 h in flowing oxygen. The normalized transition width is shown in fig. 3. F o r samples with metal overlayers the transition width is about half its value for the bare Y B C O control samples. At temperatures above the zero-resistance transition temperature o f Y B C O the resistances o f the YBCO and Ag films are c o m b i n e d as two resistors in parallel. Over most o f this temperature regime the total resistance is d o m i n a t e d by that o f the metal overlayer, since its resistance is much lower. This results in a narrowing o f the transition width by a lowering of the transition onset temperature. Thus, the narrower transition width is due to lower onset temperatures due to the lower resistance values. The zero resistance transition temperatures ( 8 8 89 K ) remained unaltered within experimental error ( _+0.5'-K). The proximity effect has i m p o r t a n t implications for tunneling and device work [ 6 ]. How-
A. Mogro-Campero, L.G. Turner/Effect of A g or Au overlayers
I
I
331
K for a superconductor/normal metal configuration of the type used here [2]. The critical current density at 77 K is also unaltered, as shown in fig. 2.
I
2.0 A
% 8
4. Summary and conclusion
rr
_o tr LU (.) Z •
I.LI nO N_
o
z
I
I
50 nm UNANNEALED
50 n m ANNEALED
I 1 jam ANNEALED
METAL THICKNESS, HEAT TREATMENT
Fig. 2. Resistance ratio RIOOK/R295Knormalized to values for the control samples (table 1 ) for YBCO thin films with Au or Ag oveflayers as described in the caption to fig. 1. 15
I
I
Epitaxial thin films of YBCO with Jc(77 K) > 1 M A c m - 2 w e r e used to study normal-state and superconducting-state electrical properties of samples with and without Ag or Au overlayers. Both thin and thick metal overlayers, sometimes annealed at 550650°C in flowing oxygen were used. Compared to bare YBCO film properties, for YBCO with Ag or Au overlayers ( 1 ) the superconducting parameters (To(0), Jc(77 K) ) are unaltered, (2) the transition width decreases by a factor of two and (3) in the linear resistance range (100-300 K) the resistance is substantially lower and the slope is shallower for thin (50 nm) overlayers. These results support the use of Ag or Au on YBCO as contact metals and/or conducting layers such as used for current shunting in the normal state.
Acknowledgements We thank J. McMullen for photolithography work, P. Schaeffer for wafer sawing, and J. Bray for encouragement and support.
p,., 1 . 0 v
3 05o u.i N
References
10--
0 z
| 05--
0i
N
OO
O
I
I
I
50 nm UNANNEALED
50 nm ANNEALED
1 ~m ANNEALED
METAL THICKNESS. HEAT T R E A T M E N T
Fig. 3. Transition width and critical current density at 77 K normalized to values for the control samples (table 1 ) for YBCO thin films with Ag or Au overlayers as described in the caption to fig. 1.
ever, transition temperature shifts due to the proximity effect have been estimated as being about 10 -2
[ 1 ] R.H. Ono, J.A. Beall, T.E. Harvey, C.D. Reintsema, M. Johansson, M.W. Cromar, L.F. Goodrich, J. Moreland, A. Roshko and T.C. Stauffer, IEEE Trans. Magn. 27 (1991) 1471. [2] E. Polturak, G. Koren, D. Cohen, D. Cohen and I. Snapiro, Physica C 197 (1992) 1. [3] A.I. Belyaeva, B.1. Khamdamov and V.P. Yur'ev, Soy. Tech. Phys. Lett. 17 ( 1991 ) 220. [4] A. Mogro-Campero, L.G. Turner, A.M. Kadin and D.S. Mallory, Appl. Phys. Lett. 60 (1992) 3310. [5] A. Mogro-Campero and L.G. Turner, J. Supercond. 6 (1993) 37. [6l M. Lee, D. Lew, C.B. Eom, T.H. Geballe and M.R. Beasley, Appl. Phys. Lett. 57 (1990) 1152.