- Email: [email protected]
Fabrication and measurement of high Tc weak links
PHYSICA
Physica B 194-196 (1994) 1703-1704 North-Holland
Fabrication and m e a s u r e m e n t of high T c weak links S. T. Herbert, S. E. Hebboul and J. C. Garland Department of Physics, Ohio State University, Columbus, Ohio 43210, USA* We have fabricated Y1Ba2Cu3OT.~ (YBCO) superconducting weak links by depositing a perpendicular stripe of YBCO film on top of a 0.25/am-l.5pm wide line of normal metal (e.g. Ag) previously deposited onto a SrTiO 3 substrate. The c-axis orientation of the YBCO film is interrupted over the metal line, creating a disordered grain boundary junction. Measurements of the resistive transition, critical current and currentvoltage (IV) characteristics of these structures show definite weak link behavior: the critical current is significantly reduced from that of samples with no metal line present and the IV curves show qualitative agreement with the Ambegaokar-Halperin (AH) model of Josephson weak links with thermal noise. Since the advent of high T c thin films, efforts have been made at developing Josephson devices utilizing high T c superconductors. Early studies of YBCO films found that grain-boundary critical currents were significantly reduced from intra-grain critical current values, and that this reduction was dependent upon the misorientation angle in the basal plane between the two grains [1]. This finding lead to the development of single grainboundary weak links using bicrystalline substrates [2] and bi-epitaxial deposition [3]. More recent efforts have involved step edge techniques to create S-N-S junctions [4,5]. Here we present a very simple method of fabricating disordered grain boundary weak links which are well-suited to array applications. Our method involves depositing lines of Ag metal onto a (100) SrTiO 3 substrate to act as a passivation layer. A subsequently deposited YBCO film is thus prevented from ordering its c-axis perpendicular to the substrate, causing an area of random grain growth which leads to weak-link coupling in the YBCO region over the metal line. A standard e-beam lithography and liftoff technique is used to]pattern onto a single substrate eight Ag lines, 350A thick, 25/Jm long, and with widths ranging from 0.25/~m to 1.5/~m. The lithography was done using a Cambridge 10.5 EBMF e-beam writer at the National Nanofabrication Facility. Next, a 1000~ thick film of YBCO was deposited over the entire substrate by co-evaporation of Y, BaF and Cu, and annealed
in wet 0 2 at 800°C for 90 minutes, followed by an oxygenation anneal at 500°C for 80 minutes. The YBCO overlay was then patterned using contact lithography and a wet etch in dilute HNO 3 to form a 5 /~m wide strip over each Ag line along with contact pads for transport measurements (see Fig. 1). Finally, -0.3/~m of Ag was evaporated onto the contact pads to reduce contact resistance. Each weak link sample consisted of a "junction" (Ag line covered by YBCO strip) and a control sample (YBCO strip with no Ag line) side by side. Transport measurements were performed using a standard four-terminal a.c. lock-in amplifier technique with a square wave excitation signal of 13Hz. The data were taken in a shielded cryostat which reduced the ambient field to < 0.01G and which regulated the temperature to -+15mK. In Fig. 2 we show the resistive transition for a junction with a 1.5/~m wide Ag line and its accompanying control sample. The junction
~
junction
Fig. 1 Schematic of weak link sample showing the junction and control sample geometry. YBCO stripe is 5/~m wide.
* Supported in part by D.O.E. (MISCON),grant # DE-FG02-90ER45427. 0921-4526/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved S S D I 0921-4526(93)1433-M
ntrol
1704
160
50O
8
••
•
7 120
J 250
~s
~
Junction
A Control
sample sample
0 80
:
. . . . . . . . . . .
-<
2
/!
2~
~°24
~00
• • •
I
0 80
v.,~n,-1, 88
84
,
30
, 9~
sample exhibits a higher resistivity, both at 300°K (not shown) and at the transition (-90°K). The junction also shows a pronounced resistive tail beginning at about 89°K and extending down to 82°K, whereas the control shows a much sharper transition with only a little rounding near p = 0. Figure 3 shows the IV curves of the junction sample for temperatures ranging from 90°K (upper left) to 30°K (lower right). The IV curves display behavior ranging from ohmic at high temperatures to a strong critical current at low temperatures. At intermediate temperatures, the IV curves are ohmic for small currents but show an increase in slope with increasing current, followed by a return to ohmic behavior at the highest currents. Both the slope of the non-ohmic portion of the IV
10 -I
]0
60
"70
80
90
Fig. 4 Critical current density vs. T(K) for the junction and control samples.
Fig. 2 Resistivity vs. T(K) for junction (•) and control (~x) samples. Inset shows the full transition for the junction sample only.
a
105
50
T (K)
(~<)
10
40
~
curves, and the current value at which the nonohmic portion appears, tends to increase with decreasing temperature. Also in Fig. 3 we show the IV curves predicted by the Ambegaokar-Halperin model for a Josephson weak link with thermal fluctuations (solid lines)[6]. These curves showgood qualitative agreement with our data, although the critical current values used in the calculations are a factor of 30 less than actual current values of the data. This disparity suggests that our random grainboundary junctions likely consist of many weak links in parallel/series combinations, with a composite critical current larger than that of a single junction. In Fig. 4 we plot the critical current density vs. temperature for both the junction and control samples. Here, we define the critical current for the IV curves having no "ohmic tail" to be the intersection of the extrapolated curves with the current axis. The data show a three to four-fold reduction in the critical current of the junction over the control sample. This result, as well as the shape of the IV curves, is the same for all values of the Ag linewidth studied and suggests that the technique is a useful way to fabricate weak-link junctions in complex array geometries.
B
REFERENCES
107 10 -8 lO
9
10
10 10
6
108
104
10 a
10
a
~(A) Fig. 3 IV curves for the junction sample for temperatures from 30K to 90K. Solid lines show calculated curves from the model of AH.
1. Dimos, et al., Phys. Rev. Lett. 61, 219 (1988). 2. J. Mannhart, et al., Phys. Rev. Lett. 61, 2476 (1988). 3. K. Char, et al., Appl. Phys. Lett. 59, 733 (1991). 4. M.S. DiIorio, et al., Appl. Phys. Lett. 58, 2252 (1991). 5. R.Ono, et al., Appl. Phys. Lett. 59, 1126 (1991). 6. V. Ambcgaokar and B.I. Halperin, Phys. Rev. Lett. 22, 1364 (1969).
Physica B 194-196 (1994) 1703-1704 North-Holland
Fabrication and m e a s u r e m e n t of high T c weak links S. T. Herbert, S. E. Hebboul and J. C. Garland Department of Physics, Ohio State University, Columbus, Ohio 43210, USA* We have fabricated Y1Ba2Cu3OT.~ (YBCO) superconducting weak links by depositing a perpendicular stripe of YBCO film on top of a 0.25/am-l.5pm wide line of normal metal (e.g. Ag) previously deposited onto a SrTiO 3 substrate. The c-axis orientation of the YBCO film is interrupted over the metal line, creating a disordered grain boundary junction. Measurements of the resistive transition, critical current and currentvoltage (IV) characteristics of these structures show definite weak link behavior: the critical current is significantly reduced from that of samples with no metal line present and the IV curves show qualitative agreement with the Ambegaokar-Halperin (AH) model of Josephson weak links with thermal noise. Since the advent of high T c thin films, efforts have been made at developing Josephson devices utilizing high T c superconductors. Early studies of YBCO films found that grain-boundary critical currents were significantly reduced from intra-grain critical current values, and that this reduction was dependent upon the misorientation angle in the basal plane between the two grains [1]. This finding lead to the development of single grainboundary weak links using bicrystalline substrates [2] and bi-epitaxial deposition [3]. More recent efforts have involved step edge techniques to create S-N-S junctions [4,5]. Here we present a very simple method of fabricating disordered grain boundary weak links which are well-suited to array applications. Our method involves depositing lines of Ag metal onto a (100) SrTiO 3 substrate to act as a passivation layer. A subsequently deposited YBCO film is thus prevented from ordering its c-axis perpendicular to the substrate, causing an area of random grain growth which leads to weak-link coupling in the YBCO region over the metal line. A standard e-beam lithography and liftoff technique is used to]pattern onto a single substrate eight Ag lines, 350A thick, 25/Jm long, and with widths ranging from 0.25/~m to 1.5/~m. The lithography was done using a Cambridge 10.5 EBMF e-beam writer at the National Nanofabrication Facility. Next, a 1000~ thick film of YBCO was deposited over the entire substrate by co-evaporation of Y, BaF and Cu, and annealed
in wet 0 2 at 800°C for 90 minutes, followed by an oxygenation anneal at 500°C for 80 minutes. The YBCO overlay was then patterned using contact lithography and a wet etch in dilute HNO 3 to form a 5 /~m wide strip over each Ag line along with contact pads for transport measurements (see Fig. 1). Finally, -0.3/~m of Ag was evaporated onto the contact pads to reduce contact resistance. Each weak link sample consisted of a "junction" (Ag line covered by YBCO strip) and a control sample (YBCO strip with no Ag line) side by side. Transport measurements were performed using a standard four-terminal a.c. lock-in amplifier technique with a square wave excitation signal of 13Hz. The data were taken in a shielded cryostat which reduced the ambient field to < 0.01G and which regulated the temperature to -+15mK. In Fig. 2 we show the resistive transition for a junction with a 1.5/~m wide Ag line and its accompanying control sample. The junction
~
junction
Fig. 1 Schematic of weak link sample showing the junction and control sample geometry. YBCO stripe is 5/~m wide.
* Supported in part by D.O.E. (MISCON),grant # DE-FG02-90ER45427. 0921-4526/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved S S D I 0921-4526(93)1433-M
ntrol
1704
160
50O
8
••
•
7 120
J 250
~s
~
Junction
A Control
sample sample
0 80
:
. . . . . . . . . . .
-<
2
/!
2~
~°24
~00
• • •
I
0 80
v.,~n,-1, 88
84
,
30
, 9~
sample exhibits a higher resistivity, both at 300°K (not shown) and at the transition (-90°K). The junction also shows a pronounced resistive tail beginning at about 89°K and extending down to 82°K, whereas the control shows a much sharper transition with only a little rounding near p = 0. Figure 3 shows the IV curves of the junction sample for temperatures ranging from 90°K (upper left) to 30°K (lower right). The IV curves display behavior ranging from ohmic at high temperatures to a strong critical current at low temperatures. At intermediate temperatures, the IV curves are ohmic for small currents but show an increase in slope with increasing current, followed by a return to ohmic behavior at the highest currents. Both the slope of the non-ohmic portion of the IV
10 -I
]0
60
"70
80
90
Fig. 4 Critical current density vs. T(K) for the junction and control samples.
Fig. 2 Resistivity vs. T(K) for junction (•) and control (~x) samples. Inset shows the full transition for the junction sample only.
a
105
50
T (K)
(~<)
10
40
~
curves, and the current value at which the nonohmic portion appears, tends to increase with decreasing temperature. Also in Fig. 3 we show the IV curves predicted by the Ambegaokar-Halperin model for a Josephson weak link with thermal fluctuations (solid lines)[6]. These curves showgood qualitative agreement with our data, although the critical current values used in the calculations are a factor of 30 less than actual current values of the data. This disparity suggests that our random grainboundary junctions likely consist of many weak links in parallel/series combinations, with a composite critical current larger than that of a single junction. In Fig. 4 we plot the critical current density vs. temperature for both the junction and control samples. Here, we define the critical current for the IV curves having no "ohmic tail" to be the intersection of the extrapolated curves with the current axis. The data show a three to four-fold reduction in the critical current of the junction over the control sample. This result, as well as the shape of the IV curves, is the same for all values of the Ag linewidth studied and suggests that the technique is a useful way to fabricate weak-link junctions in complex array geometries.
B
REFERENCES
107 10 -8 lO
9
10
10 10
6
108
104
10 a
10
a
~(A) Fig. 3 IV curves for the junction sample for temperatures from 30K to 90K. Solid lines show calculated curves from the model of AH.
1. Dimos, et al., Phys. Rev. Lett. 61, 219 (1988). 2. J. Mannhart, et al., Phys. Rev. Lett. 61, 2476 (1988). 3. K. Char, et al., Appl. Phys. Lett. 59, 733 (1991). 4. M.S. DiIorio, et al., Appl. Phys. Lett. 58, 2252 (1991). 5. R.Ono, et al., Appl. Phys. Lett. 59, 1126 (1991). 6. V. Ambcgaokar and B.I. Halperin, Phys. Rev. Lett. 22, 1364 (1969).