- Email: [email protected]
Y1Ba2Cu3O7−x interface by ion beam mixing
Nuclear Instruments North-Holland
and Methods
in Physics Research
B59/60
1439
(1991) 1439-1442
Significant improvement in the electrical properties of Ag/Y,Ba ,Cu 307 _ x interface by ion beam mixing R. Viswanathan, Department
R.R. Rao, S.N. Yedave
of Physics, Unruersity
of Poona,
and S.B. Ogale
Pune 411 007, India
Ion beam and thermally induced interface reactions between high-T, superconducting thin film of Y,BazCusO,_, (YBaCuO) and metal overlayer of Ag are studied with a view to improve the electrical properties of the interface. The interface reaction is induced by 100 keV Art ion beam with a dose value ranging from 5 X lOI to 2 X lOI ions/cm2. The electrical properties of the Ag/YBaCuO film interface. specifically contact resistance, have been investigated by the “three-probe vs four-probe” configuration.
1. Introduction The applications of high-T, superconducting thin films in devices most often requires the incorporation of metal/superconductor junctions. In this context, the electrical quality of the junction becomes extremely important [l-3]. If a patterned metallization is thermally annealed in vacuum at sufficiently high temperature for interface reaction to occur, then the superconductor is depleted of oxygen. If annealing is done in oxygen atmosphere. many types of metallization can degrade [4]. Hence low-temperature processing such as ion beam mixing [5] is desirable for the improvement of the contact properties. We show in this article that the ion beam mixing followed by low-temperature annealing improves the electrical quality of the Ag/YBaCuO interface.
The electrical measurements were carried out by employing three-probe [8] and conventional four-probe configurations to obtain the contact resistance (R,) and R-T data respectively after each stage of processing. The films were also characterized by low-angle X-ray diffraction (XRD) measurements (Rotaflex RU2OO B) using Cu Ka radiation employing Seeman-Bohlin geometry. Since the X-ray penetration depth in a sample depends upon the angle of incidence, a grazing angle of 0.6 o was chosen to probe the Ag/Y BaCuO interface.
3. Results and discussion The R-T (four probe) plot of YBaCuO film for different # values is shown in fig. 1. The as-deposited
2. Experimental Thin films of YBaCuO were deposited by the laser beam ablation process on single-crystal ZrO, substrates [6]. Four silver contact pads (thickness of 400 A) were deposited on YBaCuO films by the thermal evaporation process. The silver contact pad areas were irradiated through a shadow mask (avoiding any exposure to the superconducting area of the film between the contact pads) with Ar+. to enable the mixing at the Ag/YBaCuO interface. To ensure that the projected range of ions falls at the Ag/YBaCuO interface, an Ar+ ion beam energy value of 100 keV has been chosen in the process. The ion current density was maintained at - 0.5 kA/cm’. Since the amorphization threshold [7] in case of YBaCuO films for Art is = 10” ions/cm*, the mixing was carried out with ion dose (+) values ranging from lOi to lOi ions/cm*. 0168-583X/91/$03.50
0 1991 - Elsevier Science Publishers
-
Temperature
(Kl
Fig. 1. Four-probe R-T plot of YBaCuO film on ZrO, (001); (a) as it is with Ag overlayer; after ion beam mixing with dose values (b) 3.0 X lOI and (c) 2.0X 10’s ions/cm2: subsequently annealed at 200° C for (d) 15 min. (e) 1 h.
B.V. (North-Holland)
XI. HIGH-T
SUPERCONDUCTORS
1440
R. Vmtunothan
et al. / Improwment
film shows the transition at 92 K with transition width of about 1 K (see fig. la). Upon ion beam treatment. the slope of the linear region from room temperature to just above the onset temperature was found to increase with the increase in + values. When an energetic ion enters a solid, it causes a series of branching cascades of atomic collisions pushing silver atoms across the interface into the YBaCuO matrix with simultaneous outmigration of atomic species from the YBaCuO matrix. The extent of these processes would indeed depend upon the + value. The increase in + generates a higher number of scattering centers and thus leads to a little but finite increase in room-temperature resistance ( R,) values. It is observed that for dose values less than 2 X lOI ions/cm2, there is no variation in the transition temperature of the superconducting film (fig. lb). For +=2x10 I5 ions/cm’, however. there is a substantial change in the shape of the R-T curve. As shown in fig. Ic. the transition temperature (K) is reduced from 92 to 85 K. and the sample shows as well semiconducting behavior over the temperature region of 200-300 K. This exhibits the onset of crystalline to amorphous transition of YBaCuO film just below the contact pads possibly by collisional disordering [9,10]. The results of isothermal annealing of the irradiated samples are shown in figs. Id and le. It is found that annealing in oxygen atmosphere at 200” C for 15 min helps to regain its metallic nature but still with larger transition width. Further annealing, however, improves the quality of transition, but R, is still greater than that of the original sample. These results indicate that upon annealing, there could be radiation-enhanced diffusion wherein Ag atoms from the interior of YBaCuO crystallite can be driven to grain boundaries. leading subsequently to the restoration of superconducting property. Some structural disorder in the interface region of respective layers still exists leading to increase in R, value. The R-T (three probe) plot of the YBaCuO film with Ag overlayer for various values is shown in fig. 2. The variation of three-probe resistance (R(3P)) with temperature shows a similar behavior as in the case of the four-probe configuration (R(4P)) (viz. fig. 1) but with a higher resistance above the onset temperature. The contact resistance (R,) measured in this context. is the algebraic addition of metal resistance. the Ag/YBaCuO interfacial resistance and the contribution due to the intergranular weak links just below the contact pads. The residual resistance at 77 K. essentially R, value. is observed to vary significantly with the + values. It is clearly seen that the implantation with + = 1.75 x 10 I4 ions/cm’ reduces the R, value by 39%. With the increase in + value to 3 x lOI ions/cm’. the R, is found to decrease further by 17%‘. To anneal the defects created by implantation process, the films were subsequently subjected to thermal treatment at 200°C
in electrical propertres
of AR/
Y, Ba,Cu,O,
_,
10
-
I
a-
6-
L I
;
2 -z -6 p
I
,
I
,
I
,
I
,
I
,
,
,
,
O:.
0 lb- 1 ‘;i m g 12-
,/ * 0’
_-
___----__
--.
--__
--(a
: (b;
0
0
j
’ 3’ ” LO 80 ~
120
1
”
160
Temperature
”
200
”
2LO
’
280
(K) v
Fig. 2. Three-probe R-T plot of YBaCuO film on ZrO, (001): I(a) wth Ag overlayer; after ion-beam mixing with dose values I(b) 1.75 X 1014. I(c) 3.0~ 1014, I(d) 3.0~ lOI ions/cm2 and
annealed at 200 o C/15 min in oxygen ambient. II(a) 2.0 X lo”, and subsequent annealing in oxygen ambient at 200° C for II(b) 15 min. II(c) 1 h.
in oxygen ambient for the duration of 15 min (see fig. 2. curve Id). Interestingly, this process reduced R, further by about 24%. In the context of metallization, it has already been shown that [ll] the diffusion of contact metal in the sample can contribute in lowering the R, value. However, the implantation at + = 2 X lOI ions/cm* (see fig. 2, curve Ila) increased R, enormously by 85%, which replicates R(4P) signature in fig. lc. The samples, subjected to dose value - 2 X 10” ions/cm*. were subsequently annealed in oxygen ambient. As seen in fig. 2, curves Ilb and 11~. the R, value dropped by 75%. However, it is observed that the electrical quality of the interface has not improved compared to that treated at lower + = 3 x lOI ions/cm*. These above discussed features are brought out in fig. 3. At sufficiently low $ values Ag incorporation is limited so that it only affects R,. At higher dose values the extent of alloying increases, leading to the degradation of the quality of transition. The XRD results shown in fig. 4 supports this proposition. The bulk XRD of the as-deposited YBaCuO film (fig. 4a) is highly c-axis oriented. The X-ray spectrum corresponding to YBaCuO film with Ag overlayer (fig. 4b) shows various contribu-
R. Viswanathan
0'
I
Fig. 3. Variation
I
of contact
et al. / Improoement
I
I
resistance
1
I
with ion dose values.
(Cl)
in electrical properties
ofAg/
1441
y/ Ba,Cu,O,-
tions of YBaCuO and silver. Upon ion beam mixing at 9 = 3 x lOi ions/cm2. there is considerable reduction in the intensity of various peak contributions (fig. 4c). probably by defect-assisted diffusion across the interface. Upon annealing, it was observed that there is no significant change in the basic character of the spectrum, though the interface densifies to yield slightly higher intensities of some peak contributions (result not shown). These results show that the significant improvement in the electrical property is primarily an interface-related effect with minimal bulk mixing. In order to confirm the role played by the ion beam mixing in the reduction of R,. the samples were annealed in vacuum as well as in 0, partial pressure at 200°C for 15 min (results not shown). In the case of vacuum annealing, the depletion of oxygen even at such low temperature is clearly observed by an increase in R, as well as degrading the quality of transition. On the other hand, in the case of the samples annealed in oxygen atmosphere, no change in R, value was observed. We believe that ion beam mixing, being a forced process, may incorporate silver atoms into YBaCuO matrix, rather than a mere distribution on the surface and along the grain boundaries as reported by several authors [12]. This may enhance the electrical quality of the interface.
4. Conclusion
tb)
In conclusion, it has been brought out that the low-temperature processing of ion-beam mixing can serve as an important method in improving the electrical quality of Ag/YBaCuO interface.
Acknowledgement The authors gratefully acknowledge the financial support from the Council of Scientific and Industrial Research (CSIR), Program Management Board of DST on superconductivity and University Grants Commision (UGC). India.
References
~
28
(Dqree)
-
Fig. 4. X-ray diffraction patterns of (a) YBaCuO film, (b) YBaCuO film with Ag overlayer, (c) YBaCuO film with Ag overlayer and implanted with a dose value of 3 x lOI ions/cm’.
[l] Y. Tzeng. A. Holt and R. Ely, Appl. Phys. Lett. 52 (1988) 155. [2] J.W. Ekin. A.J. Panson and B.A. Blankenship. Appl. Phys. Lett. 52 (1988) 331. [3] Q.X. Jia and W.A. Anderson. J. Phys. D22 (1989) 1565. [4] J.W. Ekin. T.M. Larson. N.F. Bergren. A.J. Nelson, A.B. Swartzlander, L.L. Kazmerski, A.J. Panson and B.A. Blankenship, Appl. Phys. Lett. 52 (1988) 1819. XI. HIGH-T
SUPERCONDUCTORS
1442
R. Viswnnnthan
et al. / Improvement
[5] SW. Chiang, T.P. Chow, R.F. Reihl and K.L. Wang, J. Appl. Phys. 52 (1981) 4027. [6] S.T. Bendre, V.N. Koinkar, R.D. Vispute, R. Viswanathan, A.M. Dhote. S.M. Chaudhari, S.M. Kanetkar and S.B. Ogale, Solid State Commun. 73 (1990) 345. [7] A.E. White et al., Phys. Rev. B37 (1988) 3755. [8] Y. Suzuki, T. Kusaka. A. Aoki, T. Aoyama, T. Yotsuya and S. Ogawa, Jpn. J. Appl. Phys. 28 (1989) 2463. [9] G.J. Clark, F.K. LeGoues, A.D. Marwick, R.B. Laibowitz and R. Koch, Appl. Phys. Lett. 51 (1987) 1462.
in
electricalproperties of Ag/ Y, Ba,Cu@_
x
1101 0. Meyer, B. Egner, J. Geerk, Weschenfelder. X.X. Xi and Meth. B37/38 (1989) 917. Pll K. Mizusmma, M. Sagoi. T. Phys. Lett. 52 (1988) 1101. 1121 A.G. Schrott, K.N. Tu, N.C. C.C. Tsuei, Appl. Phys. Lett.
R. Gerber, G. Linker, F. G.C. Xiong. Nucl. Instr. and Miura
and J. Yosmda,
Appt
Yeh, G. Singco, A. Levi and 55 (1989) 2126.
and Methods
in Physics Research
B59/60
1439
(1991) 1439-1442
Significant improvement in the electrical properties of Ag/Y,Ba ,Cu 307 _ x interface by ion beam mixing R. Viswanathan, Department
R.R. Rao, S.N. Yedave
of Physics, Unruersity
of Poona,
and S.B. Ogale
Pune 411 007, India
Ion beam and thermally induced interface reactions between high-T, superconducting thin film of Y,BazCusO,_, (YBaCuO) and metal overlayer of Ag are studied with a view to improve the electrical properties of the interface. The interface reaction is induced by 100 keV Art ion beam with a dose value ranging from 5 X lOI to 2 X lOI ions/cm2. The electrical properties of the Ag/YBaCuO film interface. specifically contact resistance, have been investigated by the “three-probe vs four-probe” configuration.
1. Introduction The applications of high-T, superconducting thin films in devices most often requires the incorporation of metal/superconductor junctions. In this context, the electrical quality of the junction becomes extremely important [l-3]. If a patterned metallization is thermally annealed in vacuum at sufficiently high temperature for interface reaction to occur, then the superconductor is depleted of oxygen. If annealing is done in oxygen atmosphere. many types of metallization can degrade [4]. Hence low-temperature processing such as ion beam mixing [5] is desirable for the improvement of the contact properties. We show in this article that the ion beam mixing followed by low-temperature annealing improves the electrical quality of the Ag/YBaCuO interface.
The electrical measurements were carried out by employing three-probe [8] and conventional four-probe configurations to obtain the contact resistance (R,) and R-T data respectively after each stage of processing. The films were also characterized by low-angle X-ray diffraction (XRD) measurements (Rotaflex RU2OO B) using Cu Ka radiation employing Seeman-Bohlin geometry. Since the X-ray penetration depth in a sample depends upon the angle of incidence, a grazing angle of 0.6 o was chosen to probe the Ag/Y BaCuO interface.
3. Results and discussion The R-T (four probe) plot of YBaCuO film for different # values is shown in fig. 1. The as-deposited
2. Experimental Thin films of YBaCuO were deposited by the laser beam ablation process on single-crystal ZrO, substrates [6]. Four silver contact pads (thickness of 400 A) were deposited on YBaCuO films by the thermal evaporation process. The silver contact pad areas were irradiated through a shadow mask (avoiding any exposure to the superconducting area of the film between the contact pads) with Ar+. to enable the mixing at the Ag/YBaCuO interface. To ensure that the projected range of ions falls at the Ag/YBaCuO interface, an Ar+ ion beam energy value of 100 keV has been chosen in the process. The ion current density was maintained at - 0.5 kA/cm’. Since the amorphization threshold [7] in case of YBaCuO films for Art is = 10” ions/cm*, the mixing was carried out with ion dose (+) values ranging from lOi to lOi ions/cm*. 0168-583X/91/$03.50
0 1991 - Elsevier Science Publishers
-
Temperature
(Kl
Fig. 1. Four-probe R-T plot of YBaCuO film on ZrO, (001); (a) as it is with Ag overlayer; after ion beam mixing with dose values (b) 3.0 X lOI and (c) 2.0X 10’s ions/cm2: subsequently annealed at 200° C for (d) 15 min. (e) 1 h.
B.V. (North-Holland)
XI. HIGH-T
SUPERCONDUCTORS
1440
R. Vmtunothan
et al. / Improwment
film shows the transition at 92 K with transition width of about 1 K (see fig. la). Upon ion beam treatment. the slope of the linear region from room temperature to just above the onset temperature was found to increase with the increase in + values. When an energetic ion enters a solid, it causes a series of branching cascades of atomic collisions pushing silver atoms across the interface into the YBaCuO matrix with simultaneous outmigration of atomic species from the YBaCuO matrix. The extent of these processes would indeed depend upon the + value. The increase in + generates a higher number of scattering centers and thus leads to a little but finite increase in room-temperature resistance ( R,) values. It is observed that for dose values less than 2 X lOI ions/cm2, there is no variation in the transition temperature of the superconducting film (fig. lb). For +=2x10 I5 ions/cm’, however. there is a substantial change in the shape of the R-T curve. As shown in fig. Ic. the transition temperature (K) is reduced from 92 to 85 K. and the sample shows as well semiconducting behavior over the temperature region of 200-300 K. This exhibits the onset of crystalline to amorphous transition of YBaCuO film just below the contact pads possibly by collisional disordering [9,10]. The results of isothermal annealing of the irradiated samples are shown in figs. Id and le. It is found that annealing in oxygen atmosphere at 200” C for 15 min helps to regain its metallic nature but still with larger transition width. Further annealing, however, improves the quality of transition, but R, is still greater than that of the original sample. These results indicate that upon annealing, there could be radiation-enhanced diffusion wherein Ag atoms from the interior of YBaCuO crystallite can be driven to grain boundaries. leading subsequently to the restoration of superconducting property. Some structural disorder in the interface region of respective layers still exists leading to increase in R, value. The R-T (three probe) plot of the YBaCuO film with Ag overlayer for various values is shown in fig. 2. The variation of three-probe resistance (R(3P)) with temperature shows a similar behavior as in the case of the four-probe configuration (R(4P)) (viz. fig. 1) but with a higher resistance above the onset temperature. The contact resistance (R,) measured in this context. is the algebraic addition of metal resistance. the Ag/YBaCuO interfacial resistance and the contribution due to the intergranular weak links just below the contact pads. The residual resistance at 77 K. essentially R, value. is observed to vary significantly with the + values. It is clearly seen that the implantation with + = 1.75 x 10 I4 ions/cm’ reduces the R, value by 39%. With the increase in + value to 3 x lOI ions/cm’. the R, is found to decrease further by 17%‘. To anneal the defects created by implantation process, the films were subsequently subjected to thermal treatment at 200°C
in electrical propertres
of AR/
Y, Ba,Cu,O,
_,
10
-
I
a-
6-
L I
;
2 -z -6 p
I
,
I
,
I
,
I
,
I
,
,
,
,
O:.
0 lb- 1 ‘;i m g 12-
,/ * 0’
_-
___----__
--.
--__
--(a
: (b;
0
0
j
’ 3’ ” LO 80 ~
120
1
”
160
Temperature
”
200
”
2LO
’
280
(K) v
Fig. 2. Three-probe R-T plot of YBaCuO film on ZrO, (001): I(a) wth Ag overlayer; after ion-beam mixing with dose values I(b) 1.75 X 1014. I(c) 3.0~ 1014, I(d) 3.0~ lOI ions/cm2 and
annealed at 200 o C/15 min in oxygen ambient. II(a) 2.0 X lo”, and subsequent annealing in oxygen ambient at 200° C for II(b) 15 min. II(c) 1 h.
in oxygen ambient for the duration of 15 min (see fig. 2. curve Id). Interestingly, this process reduced R, further by about 24%. In the context of metallization, it has already been shown that [ll] the diffusion of contact metal in the sample can contribute in lowering the R, value. However, the implantation at + = 2 X lOI ions/cm* (see fig. 2, curve Ila) increased R, enormously by 85%, which replicates R(4P) signature in fig. lc. The samples, subjected to dose value - 2 X 10” ions/cm*. were subsequently annealed in oxygen ambient. As seen in fig. 2, curves Ilb and 11~. the R, value dropped by 75%. However, it is observed that the electrical quality of the interface has not improved compared to that treated at lower + = 3 x lOI ions/cm*. These above discussed features are brought out in fig. 3. At sufficiently low $ values Ag incorporation is limited so that it only affects R,. At higher dose values the extent of alloying increases, leading to the degradation of the quality of transition. The XRD results shown in fig. 4 supports this proposition. The bulk XRD of the as-deposited YBaCuO film (fig. 4a) is highly c-axis oriented. The X-ray spectrum corresponding to YBaCuO film with Ag overlayer (fig. 4b) shows various contribu-
R. Viswanathan
0'
I
Fig. 3. Variation
I
of contact
et al. / Improoement
I
I
resistance
1
I
with ion dose values.
(Cl)
in electrical properties
ofAg/
1441
y/ Ba,Cu,O,-
tions of YBaCuO and silver. Upon ion beam mixing at 9 = 3 x lOi ions/cm2. there is considerable reduction in the intensity of various peak contributions (fig. 4c). probably by defect-assisted diffusion across the interface. Upon annealing, it was observed that there is no significant change in the basic character of the spectrum, though the interface densifies to yield slightly higher intensities of some peak contributions (result not shown). These results show that the significant improvement in the electrical property is primarily an interface-related effect with minimal bulk mixing. In order to confirm the role played by the ion beam mixing in the reduction of R,. the samples were annealed in vacuum as well as in 0, partial pressure at 200°C for 15 min (results not shown). In the case of vacuum annealing, the depletion of oxygen even at such low temperature is clearly observed by an increase in R, as well as degrading the quality of transition. On the other hand, in the case of the samples annealed in oxygen atmosphere, no change in R, value was observed. We believe that ion beam mixing, being a forced process, may incorporate silver atoms into YBaCuO matrix, rather than a mere distribution on the surface and along the grain boundaries as reported by several authors [12]. This may enhance the electrical quality of the interface.
4. Conclusion
tb)
In conclusion, it has been brought out that the low-temperature processing of ion-beam mixing can serve as an important method in improving the electrical quality of Ag/YBaCuO interface.
Acknowledgement The authors gratefully acknowledge the financial support from the Council of Scientific and Industrial Research (CSIR), Program Management Board of DST on superconductivity and University Grants Commision (UGC). India.
References
~
28
(Dqree)
-
Fig. 4. X-ray diffraction patterns of (a) YBaCuO film, (b) YBaCuO film with Ag overlayer, (c) YBaCuO film with Ag overlayer and implanted with a dose value of 3 x lOI ions/cm’.
[l] Y. Tzeng. A. Holt and R. Ely, Appl. Phys. Lett. 52 (1988) 155. [2] J.W. Ekin. A.J. Panson and B.A. Blankenship. Appl. Phys. Lett. 52 (1988) 331. [3] Q.X. Jia and W.A. Anderson. J. Phys. D22 (1989) 1565. [4] J.W. Ekin. T.M. Larson. N.F. Bergren. A.J. Nelson, A.B. Swartzlander, L.L. Kazmerski, A.J. Panson and B.A. Blankenship, Appl. Phys. Lett. 52 (1988) 1819. XI. HIGH-T
SUPERCONDUCTORS
1442
R. Viswnnnthan
et al. / Improvement
[5] SW. Chiang, T.P. Chow, R.F. Reihl and K.L. Wang, J. Appl. Phys. 52 (1981) 4027. [6] S.T. Bendre, V.N. Koinkar, R.D. Vispute, R. Viswanathan, A.M. Dhote. S.M. Chaudhari, S.M. Kanetkar and S.B. Ogale, Solid State Commun. 73 (1990) 345. [7] A.E. White et al., Phys. Rev. B37 (1988) 3755. [8] Y. Suzuki, T. Kusaka. A. Aoki, T. Aoyama, T. Yotsuya and S. Ogawa, Jpn. J. Appl. Phys. 28 (1989) 2463. [9] G.J. Clark, F.K. LeGoues, A.D. Marwick, R.B. Laibowitz and R. Koch, Appl. Phys. Lett. 51 (1987) 1462.
in
electricalproperties of Ag/ Y, Ba,Cu@_
x
1101 0. Meyer, B. Egner, J. Geerk, Weschenfelder. X.X. Xi and Meth. B37/38 (1989) 917. Pll K. Mizusmma, M. Sagoi. T. Phys. Lett. 52 (1988) 1101. 1121 A.G. Schrott, K.N. Tu, N.C. C.C. Tsuei, Appl. Phys. Lett.
R. Gerber, G. Linker, F. G.C. Xiong. Nucl. Instr. and Miura
and J. Yosmda,
Appt
Yeh, G. Singco, A. Levi and 55 (1989) 2126.