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On the role of Ag in enhancement of Jc in YBa2Cu3O7−δ thin films
PHYSICA ELSEVIER
PhysicaC248 (1995) 276-280
On the role of Ag in enhancement of Jc in Y B a 2 f u 3 0 7 _ thin films
8
D.S. Misra a,*, Binto John a, R. Pinto b, L.S. Mombasawala c, S.B. Palmer d a Department of Physics, Indian Institute of Technology, Bombay 400 076, India b Tata Institute of Fundamentul Research, Homi Bhaba Road, Bombay 400 005, India c R.S.LC, Indian Institute of Technology, Bombay 400 076, India d Department of Physics, University of Warwick, Coventry, CV4 7AL, UK
Received 17 January 1995; revised manuscriptreceived2 May 1995
Abstract The role of Ag in the enhancement of Jc in YBa2Cu307_ 8 (YBCO) thin films is investigated. Studies show that the concentration of Ag in laser-ablated films made from YBCO targets containing 1-20 wt.% Ag, is less than 0.1%. J~ has been found to improve with Ag doping by a factor of 8 in the films which were prepared from targets containing 5 wt.% Ag. A theoretical model based on the reduction in the resistive grain-boundary size is proposed to explain the observed enhancement of Jc.
1. Introduction It has been observed that silver doping in "YBa2Cu307_ 8 (YBCO) results in improved superconducting properties (J¢) in bulk as well as thin films [1-9]. Scanning electron microscopy (SEM) has clearly shown that in the case of bulk YBCO, Ag is incorporated in the grain boundaries (GB's) and results in improvement in GB conduction. However, the enhancement of the critical current density (J¢ at 77 K) in thin films is much more pronounced than for bulk material. It has been proposed that Ag may play a similar role in both the thin film and bulk material although the concentration of Ag in the films has not been ascertained reliably. It is therefore premature to presume that the mechanism of J~ enhancement is the same in bulk and thin films. The * Corresponding author.
temperature (T) dependence of Jc is an important measurement that provides indirect information about the factors which limit J~ in these films. Generally Jc varies with T as (1 - T / T ~ ) ' , with 1.5 _< n _< 3.5 [10-12]. The index n is found to vary with the thickness of the film and may be related to the size and width of the grain boundaries [12]. As discussed by Deutscher and Muller, [13] the index n assumes a value equal to 2 for YBCO which has a small coherence length. In the present study, we have measured the temperature dependence of J~ in films grown from targets containing varying weight percentages of Ag. We find that the index n is sensitive to the percentage of Ag contained in the target and is found to assume a value equal to 2 for the films with highest J~. We have also estimated the concentration of Ag in these films and found it to be much less than the concentration in the targets. The results imply that
0921-4534/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0921-4534(95)00257-X
277
D.S. Misra et al. / Physica C 248 (1995) 276-280 Table 1 Various parameters for YBCO films deposited from targets containing varying percentages of Ag No.
1 2 3 4
% of Ag in target
Tco (K)
0.0 2.0 5.0 7.0
88.0 88.5 89.5 89.5
Av. grain size (ixm)
Jc (A/cm2) at 77 K, B = 0
J~(77 K)/Jc(77 K) Observed
Calculated
0.33 0.55 0.85 -
0.9 1.2 8.3 4.7
1.0
1.0
1.3 8.8 5.2
1.8 9.4
the improvement in properties results from the improved connectivity between grains rather than the incorporation of Ag into the grain boundaries. Our calculations based on this assumption appear to be in agreement with the experimental results.
2. Experimental YBCO targets containing 1-20 wt.% Ag were prepared by mixing Ag20 powder (99.9%) with presintered YBCO powder. Thin films were deposited onto (100) SrTiO 3 and LaAIO 3 substrates using pulsed-laser ablation with a 248 nm KrF excimer laser generating a fluence of 3 J / c m 2 on the target. The substrate temperature (Ts) was maintained constant at 720°C and the 0 2 pressure at 200 mTorr. All the films were deposited with a thickness of 2000 ,~. Silver-contact pads evaporated onto the films allowed four-point probe resistivity measurements to be made. All the films showed a transition temperature for zero resistance, Tc, with values of 89 K and a transition width of 1 to 1.5 K. The films were patterned into 10 micron wide micro bridges using a photolithography technique, to allow transport measurements of Jc to be performed where a criterion of 1 txN/mm was used. All measurements were carried out on a cold-head which could be cooled to 10 K. Structural studies were carried out using X-ray diffraction (XRD). SEM and STM studies were conducted in order to understand the surface morphology. Finally the films were subjected to inductively coupled plasma atomic emission spectroscopy (ICPAES) to estimate the silver content. The films (weighing about 270 Ixg) were dissolved in dilute HNO 3 to facilitate the analysis which has a resolution of 100 ppb and an accuracy of 1%.
X X X ×
106 106 106 106
3. Results and discussion One of the important aspects of YBCO thin films is that Jc depends critically on the microstructure of the films. In other words, even though the films are near epitaxial with a high degree of c-axis orientation normal to the substrate plane, the in-plane mosaicity or the distribution of low-angle grain boundaries in the a - b plane, determines the critical current densities of the films [7]. Fig. 1 shows the XRD spectra obtained for undoped and 5 wt.% Ag doped films deposited o n L a A 1 0 3. The spectra indicate a high degree of c-axis orientation with (001) reflections. The XRD spectra obtained for 3, 7 and 10 wt.% Ag doped films were similar to those in Fig. 1. Although the undoped films showed a slightly low Jc = 0.9 × 106 A / c m 2 at 77 K (zero field), 5 wt.% Ag doped films showed a high Jc of 8.3 X 10 6 A / c m 2 at 77 K. Combined evidence of the high INTENSITY (A.U.) .001
o"
,..-002
0O3 r-
rrl G') ~D rn Cn v
LOAi0 3
~-004
005 006 r
',.
-~
LoALO3 1
l'- 007 Fig. 1. XRD spectra for (a) undoped and (b) 5 wt,% Ag doped films.
D.S. Misra et al. / Physica C 248 (1995) 276-280
278 1.6
~
J o A
A
800
i
T
,
I
80
I
D 10% 41.3"/.
5% 7%
1.2 4
...o 40C
._u 0 . 8
40
0.~
0 0.8/.
0 0.65
L 0.88
0.92
0.96
,
I0 0.75
0.85 t
t
Fig. 2. Variation of normalized critical current (Jc) with reduced temperature (t) of YBCO films prepared from targets containing 5 and 7% Ag.
Fig. 3. Variation of normalized critical current (Jc) with reduced temperature (t) of YBCO films prepared from targets containing 3 and 10% Ag.
degree of c-axis orientation and Jc values of the order of 106 A / c m 2 at 77 K, indicate the absence of large-angle boundaries in our films. Zero-field Jc (77 K) values of the other films are given in Table 1. Fig. 2 shows the plot of the normalized critical current density j~ ( J ~ ( T ) / J ~ ( 7 7 K)) as a function of reduced temperature t ( T / T ~ ) for the films deposited from targets containing 5 and 7% Ag. Similar results for the films deposited from targets containing 3 and 10% Ag are plotted in Fig. 3. We find that all the samples follow a quadratic polynomial of the form
ment of the grains has occurred in all these samples. A few films were chemically etched using 5% acetic acid to facilitate the observation of the grains and grain boundaries using SEM and STM. Tunnelling microscopy was carded out at different places across the grains. An STM micrograph (line scan) of a chemically etched YBCO film prepared from a target
J0(1 - a t + f i t 2 ) ,
(1)
and that irrespective of the quality of the films, for a given percentage of Ag, the coefficients a and /3 remain the same. The variation of coefficients a and /3 with the percentage of Ag in the target is shown in Fig. 4. As can be noted, both a and /3 have a minimum value of 2 and 1, respectively, for films prepared from targets containing 5% Ag. We can therefore write Eq. (1) as J¢ = J 0 ( 1 - t) 2 for the above, in exact agreement with the theoretical predictions [13]. This behavior is characteristic of SNS junctions. It is also worth noting that films made from targets doped with 5% Ag have also been found to exhibit the best values of J~ = 8.3 × 10 6 A / c m 2 at 77 K. Measurements show that enlarge-
I
I
I
i
I
i
2.00 -
-p 0
~
,/ 1.2o
0Zg
J
I 8.oo Ag PERCENTAGE IN TARGET
~.oo
L
12.oo
Fig. 4. Variation of coefficients a and fl with percentage of Ag in target.
D.S. Misra et al. /Physica C 248 (1995) 276-280
279
535A °
g 3 1 7A ° I--4
2 99A ° ONM
5001~d
1000blVI
HORIZONTAL SCAN Fig. 5. STM micrograph (line scan) of a YBCO film. Arrows indicate fall of tunneling current at grain boundaries.
containing 7% Ag is shown in Fig. 5. We observe a sharp and monotonic decrease in the tunnelling current near the grain boundaries (marked by arrows in Fig. 5), casting doubt on the assumption that the silver may be present in the grain boundaries in doped YBCO films. The results of ICPAES revealed that the percentage of Ag in all the films is less than 0.1% while the percentage of Ag in the various targets was confirmed to have their expected values. The results of the experiments indicate that the enhancement in Jc may not be due to the presence of silver in the samples. Reports of silver doping in bulk YBCO showed that a maximum J~ is observed for samples containing 5-7% Ag. It is interesting to note that in thin films, J~ attains its maximum value when the targets contained 5-7% Ag, though the actual percentage of Ag in films made from these targets were found to be much less. It has therefore been proposed that silver may help in crystallization, enabling larger crystals to be formed (grain enlargement) by providing greater mobility to adatoms on the substrate. Moreover, Ag also helps in better incorporation of 0 2 during the growth process itself [141. As the pinning potential of Ag doped films does not vary appreciably from that of undoped samples, it may be assumed that the boundary junction (B J) itself is the major limiting factor on J~ [7]. Enlarge-
ment of grains thus results in a reduction in the size of the grain boundary, providing the mechanism for enhanced properties. In Fig. 6 the resistive grain boundary is pictorially represented in the case of both undoped and doped samples. In the doped case, the size of the resistive grain boundary is shown to be reduced. The dimensions of the grains are denoted by a and ka in the respective cases and the dimensions of the boundary by b and b'. We can write for the general case
Jcozl/(r)", }
(2) 1 uhlt
I-..-+h- I
(o}
F"*--t--~
}
[b)
1 unlt
l
Fig. 6. Pictorial representation of grains and resistive grain boundaries (shaded portion) in films (a) undoped, (b) doped with Ag.
280
D.S. Misra et al. / Physica C 248 (1995) 276-280
where r denotes the grain-boundary resistance and n is a suitable index. Gross et al. [15] have reported that for YBCO samples, the index n assumes a value equal to 1.5. Assuming that resistance is proportional to the dimension of the boundary b, Eq. (2) can be written as
J c a l / ( b ) 1"5.
(3)
If N be the number of grains in unit length, we can write the following relation connecting a and b N ( a + b) = 1.
(4a)
In the doped case a similar relation is N(ka + b') = 1.
(4b)
For the ratio of critical currents in the two cases we have using the above, the following relation:
J~/Jc = (b/b') 1"5.
(5a)
Using the relations above, we obtain
Jc/Je = [( 1 - N a ) / ( 1 - Nka)] 1.5
(5b)
Any effects of grain growth in the two-dimensional plane (direction normal to current flow) may be considered negligible as the width of the microbridge is kept constant at 10 Ixm in all the films studied. Table 1 summarizes the results of the calculations based on Eq. (5b) in which we have chosen the unit length as 1 I~m which implies N = 1. As can be seen from the table, the agreement between calculated and observed values for enhancement in J¢ is fairly good. The model is not applicable in the case of films deposited from targets containing more than 7 wt.% Ag because beyond this concentration it is likely that the Ag becomes incorporated into the lattice itself and destroys the superconducting properties. For the same reason, J¢ attains its maximum value in films deposited from targets containing 5 wt.% Ag, and decreases for higher percentages, though the grain sizes in these films remain nearly the same as that of the films exhibiting the highest J~. The reason for the extremely low concentration of Ag in laser-ablated films is probably due to the re-evaporation of Ag which takes place at the substrate, while the film is being grown.
4. Conclusion The role of Ag in the enhancement of J¢ has been studied using both direct and indirect evidence. Jc versus T measurements show that increase in J¢ may be produced by a reduction in the size of the grain boundaries. An estimation of the percentage of Ag in laser-ablated films using ICPAES showed that it is less than 0.1%. We propose that the enhancement in J~ is due to reduction in the resistive grain-boundary size which results due to enlarged grains. We have presented a calculation based on the above model which predicts the observed enhancement in J~. References [1] P.N. Peters, R.C. Sisk, E.W. Urban, C.Y. Huang and M.K. Wu, Appl. Phys. Lett. 52 (1988) 2066. [2] Y. Matsumoto, J. Horabo, Y. Yamaguchi, M. Nishida and A. Chiba, Appl. Phys. Lett. 56 (1990) 1585. [3] B. Dwir, M. A.ffronte and D. Pavuna, Appl. Phys. Lett. 55 (1989) 399. [4] T.B. Lindemer, F.A. Washburn and C.S. MacDougall, Physica C 196 (1992) 390. [5] D. Lee, X. Chand and K. Salama, Jpn. J. Appl. Phys. 31
(1992) 2411. [6] A. Oota, T. Horio, K. Ohba and K. Iwasaki, J. Appl. Phys. 71 (1992) 5997. [7] D. Kumar, M. Sharon, R. Pinto, P.R. Apte, S.P. Pai, S.C. Purandare, L.C. Gupta and R. Vijayaraghavan, Appl. Phys. Lett. 62 (1993) 3522. [8] R. Pinto, N. Goyal, S.P. Pal, P.R. Apte, L.C. Gupta and R. Vijayaraghavan, J. Appl. Phys. 73 (1993) 5105. [9] T. Manabe, W. Kondo, S. Mizuta and T. Kuamgai, Appl. Phys. Lett. 60 (1992) 3301. [10] S.B. Ogale, D. Dijkkamp, T. Venkatesan, X.D. Wu and A. Inam, Phys. Rev. B 36 (1987) 7210. [11] C.W. Yuan, B.R. Zhao, Y.Z. Zhang, Y.Y. Zhao, Y. Lu, H.S. Wang, Y.H. Shi, J. Gao and L. Li, J. Appl. Phys. 64 (1988) 4091. [12] L.H. Allen, P.R. Broussard, J.H. Claassen and S.A. Wolf, Appl. Phys. Lett. 53 (1988) 1338. [13] G. Deutscher and K.A. Muller, Phys. Rev. Lett. 59 (1987) 1745. [14] D. Kumar, M. Sharon, P.R. Apte, R. Pinto, S.P. Pal, S.C. Purandare C.P. D'souza, L.C. Gupta and R. Vijayaraghavan, J. Appl. Phys. 76 (1994). [15] R. Gross, P. Chaudhari, M. Kawasaki and A. Gupta, Phys. Rev. B 42 (1990) 10735.
PhysicaC248 (1995) 276-280
On the role of Ag in enhancement of Jc in Y B a 2 f u 3 0 7 _ thin films
8
D.S. Misra a,*, Binto John a, R. Pinto b, L.S. Mombasawala c, S.B. Palmer d a Department of Physics, Indian Institute of Technology, Bombay 400 076, India b Tata Institute of Fundamentul Research, Homi Bhaba Road, Bombay 400 005, India c R.S.LC, Indian Institute of Technology, Bombay 400 076, India d Department of Physics, University of Warwick, Coventry, CV4 7AL, UK
Received 17 January 1995; revised manuscriptreceived2 May 1995
Abstract The role of Ag in the enhancement of Jc in YBa2Cu307_ 8 (YBCO) thin films is investigated. Studies show that the concentration of Ag in laser-ablated films made from YBCO targets containing 1-20 wt.% Ag, is less than 0.1%. J~ has been found to improve with Ag doping by a factor of 8 in the films which were prepared from targets containing 5 wt.% Ag. A theoretical model based on the reduction in the resistive grain-boundary size is proposed to explain the observed enhancement of Jc.
1. Introduction It has been observed that silver doping in "YBa2Cu307_ 8 (YBCO) results in improved superconducting properties (J¢) in bulk as well as thin films [1-9]. Scanning electron microscopy (SEM) has clearly shown that in the case of bulk YBCO, Ag is incorporated in the grain boundaries (GB's) and results in improvement in GB conduction. However, the enhancement of the critical current density (J¢ at 77 K) in thin films is much more pronounced than for bulk material. It has been proposed that Ag may play a similar role in both the thin film and bulk material although the concentration of Ag in the films has not been ascertained reliably. It is therefore premature to presume that the mechanism of J~ enhancement is the same in bulk and thin films. The * Corresponding author.
temperature (T) dependence of Jc is an important measurement that provides indirect information about the factors which limit J~ in these films. Generally Jc varies with T as (1 - T / T ~ ) ' , with 1.5 _< n _< 3.5 [10-12]. The index n is found to vary with the thickness of the film and may be related to the size and width of the grain boundaries [12]. As discussed by Deutscher and Muller, [13] the index n assumes a value equal to 2 for YBCO which has a small coherence length. In the present study, we have measured the temperature dependence of J~ in films grown from targets containing varying weight percentages of Ag. We find that the index n is sensitive to the percentage of Ag contained in the target and is found to assume a value equal to 2 for the films with highest J~. We have also estimated the concentration of Ag in these films and found it to be much less than the concentration in the targets. The results imply that
0921-4534/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0921-4534(95)00257-X
277
D.S. Misra et al. / Physica C 248 (1995) 276-280 Table 1 Various parameters for YBCO films deposited from targets containing varying percentages of Ag No.
1 2 3 4
% of Ag in target
Tco (K)
0.0 2.0 5.0 7.0
88.0 88.5 89.5 89.5
Av. grain size (ixm)
Jc (A/cm2) at 77 K, B = 0
J~(77 K)/Jc(77 K) Observed
Calculated
0.33 0.55 0.85 -
0.9 1.2 8.3 4.7
1.0
1.0
1.3 8.8 5.2
1.8 9.4
the improvement in properties results from the improved connectivity between grains rather than the incorporation of Ag into the grain boundaries. Our calculations based on this assumption appear to be in agreement with the experimental results.
2. Experimental YBCO targets containing 1-20 wt.% Ag were prepared by mixing Ag20 powder (99.9%) with presintered YBCO powder. Thin films were deposited onto (100) SrTiO 3 and LaAIO 3 substrates using pulsed-laser ablation with a 248 nm KrF excimer laser generating a fluence of 3 J / c m 2 on the target. The substrate temperature (Ts) was maintained constant at 720°C and the 0 2 pressure at 200 mTorr. All the films were deposited with a thickness of 2000 ,~. Silver-contact pads evaporated onto the films allowed four-point probe resistivity measurements to be made. All the films showed a transition temperature for zero resistance, Tc, with values of 89 K and a transition width of 1 to 1.5 K. The films were patterned into 10 micron wide micro bridges using a photolithography technique, to allow transport measurements of Jc to be performed where a criterion of 1 txN/mm was used. All measurements were carried out on a cold-head which could be cooled to 10 K. Structural studies were carried out using X-ray diffraction (XRD). SEM and STM studies were conducted in order to understand the surface morphology. Finally the films were subjected to inductively coupled plasma atomic emission spectroscopy (ICPAES) to estimate the silver content. The films (weighing about 270 Ixg) were dissolved in dilute HNO 3 to facilitate the analysis which has a resolution of 100 ppb and an accuracy of 1%.
X X X ×
106 106 106 106
3. Results and discussion One of the important aspects of YBCO thin films is that Jc depends critically on the microstructure of the films. In other words, even though the films are near epitaxial with a high degree of c-axis orientation normal to the substrate plane, the in-plane mosaicity or the distribution of low-angle grain boundaries in the a - b plane, determines the critical current densities of the films [7]. Fig. 1 shows the XRD spectra obtained for undoped and 5 wt.% Ag doped films deposited o n L a A 1 0 3. The spectra indicate a high degree of c-axis orientation with (001) reflections. The XRD spectra obtained for 3, 7 and 10 wt.% Ag doped films were similar to those in Fig. 1. Although the undoped films showed a slightly low Jc = 0.9 × 106 A / c m 2 at 77 K (zero field), 5 wt.% Ag doped films showed a high Jc of 8.3 X 10 6 A / c m 2 at 77 K. Combined evidence of the high INTENSITY (A.U.) .001
o"
,..-002
0O3 r-
rrl G') ~D rn Cn v
LOAi0 3
~-004
005 006 r
',.
-~
LoALO3 1
l'- 007 Fig. 1. XRD spectra for (a) undoped and (b) 5 wt,% Ag doped films.
D.S. Misra et al. / Physica C 248 (1995) 276-280
278 1.6
~
J o A
A
800
i
T
,
I
80
I
D 10% 41.3"/.
5% 7%
1.2 4
...o 40C
._u 0 . 8
40
0.~
0 0.8/.
0 0.65
L 0.88
0.92
0.96
,
I0 0.75
0.85 t
t
Fig. 2. Variation of normalized critical current (Jc) with reduced temperature (t) of YBCO films prepared from targets containing 5 and 7% Ag.
Fig. 3. Variation of normalized critical current (Jc) with reduced temperature (t) of YBCO films prepared from targets containing 3 and 10% Ag.
degree of c-axis orientation and Jc values of the order of 106 A / c m 2 at 77 K, indicate the absence of large-angle boundaries in our films. Zero-field Jc (77 K) values of the other films are given in Table 1. Fig. 2 shows the plot of the normalized critical current density j~ ( J ~ ( T ) / J ~ ( 7 7 K)) as a function of reduced temperature t ( T / T ~ ) for the films deposited from targets containing 5 and 7% Ag. Similar results for the films deposited from targets containing 3 and 10% Ag are plotted in Fig. 3. We find that all the samples follow a quadratic polynomial of the form
ment of the grains has occurred in all these samples. A few films were chemically etched using 5% acetic acid to facilitate the observation of the grains and grain boundaries using SEM and STM. Tunnelling microscopy was carded out at different places across the grains. An STM micrograph (line scan) of a chemically etched YBCO film prepared from a target
J0(1 - a t + f i t 2 ) ,
(1)
and that irrespective of the quality of the films, for a given percentage of Ag, the coefficients a and /3 remain the same. The variation of coefficients a and /3 with the percentage of Ag in the target is shown in Fig. 4. As can be noted, both a and /3 have a minimum value of 2 and 1, respectively, for films prepared from targets containing 5% Ag. We can therefore write Eq. (1) as J¢ = J 0 ( 1 - t) 2 for the above, in exact agreement with the theoretical predictions [13]. This behavior is characteristic of SNS junctions. It is also worth noting that films made from targets doped with 5% Ag have also been found to exhibit the best values of J~ = 8.3 × 10 6 A / c m 2 at 77 K. Measurements show that enlarge-
I
I
I
i
I
i
2.00 -
-p 0
~
,/ 1.2o
0Zg
J
I 8.oo Ag PERCENTAGE IN TARGET
~.oo
L
12.oo
Fig. 4. Variation of coefficients a and fl with percentage of Ag in target.
D.S. Misra et al. /Physica C 248 (1995) 276-280
279
535A °
g 3 1 7A ° I--4
2 99A ° ONM
5001~d
1000blVI
HORIZONTAL SCAN Fig. 5. STM micrograph (line scan) of a YBCO film. Arrows indicate fall of tunneling current at grain boundaries.
containing 7% Ag is shown in Fig. 5. We observe a sharp and monotonic decrease in the tunnelling current near the grain boundaries (marked by arrows in Fig. 5), casting doubt on the assumption that the silver may be present in the grain boundaries in doped YBCO films. The results of ICPAES revealed that the percentage of Ag in all the films is less than 0.1% while the percentage of Ag in the various targets was confirmed to have their expected values. The results of the experiments indicate that the enhancement in Jc may not be due to the presence of silver in the samples. Reports of silver doping in bulk YBCO showed that a maximum J~ is observed for samples containing 5-7% Ag. It is interesting to note that in thin films, J~ attains its maximum value when the targets contained 5-7% Ag, though the actual percentage of Ag in films made from these targets were found to be much less. It has therefore been proposed that silver may help in crystallization, enabling larger crystals to be formed (grain enlargement) by providing greater mobility to adatoms on the substrate. Moreover, Ag also helps in better incorporation of 0 2 during the growth process itself [141. As the pinning potential of Ag doped films does not vary appreciably from that of undoped samples, it may be assumed that the boundary junction (B J) itself is the major limiting factor on J~ [7]. Enlarge-
ment of grains thus results in a reduction in the size of the grain boundary, providing the mechanism for enhanced properties. In Fig. 6 the resistive grain boundary is pictorially represented in the case of both undoped and doped samples. In the doped case, the size of the resistive grain boundary is shown to be reduced. The dimensions of the grains are denoted by a and ka in the respective cases and the dimensions of the boundary by b and b'. We can write for the general case
Jcozl/(r)", }
(2) 1 uhlt
I-..-+h- I
(o}
F"*--t--~
}
[b)
1 unlt
l
Fig. 6. Pictorial representation of grains and resistive grain boundaries (shaded portion) in films (a) undoped, (b) doped with Ag.
280
D.S. Misra et al. / Physica C 248 (1995) 276-280
where r denotes the grain-boundary resistance and n is a suitable index. Gross et al. [15] have reported that for YBCO samples, the index n assumes a value equal to 1.5. Assuming that resistance is proportional to the dimension of the boundary b, Eq. (2) can be written as
J c a l / ( b ) 1"5.
(3)
If N be the number of grains in unit length, we can write the following relation connecting a and b N ( a + b) = 1.
(4a)
In the doped case a similar relation is N(ka + b') = 1.
(4b)
For the ratio of critical currents in the two cases we have using the above, the following relation:
J~/Jc = (b/b') 1"5.
(5a)
Using the relations above, we obtain
Jc/Je = [( 1 - N a ) / ( 1 - Nka)] 1.5
(5b)
Any effects of grain growth in the two-dimensional plane (direction normal to current flow) may be considered negligible as the width of the microbridge is kept constant at 10 Ixm in all the films studied. Table 1 summarizes the results of the calculations based on Eq. (5b) in which we have chosen the unit length as 1 I~m which implies N = 1. As can be seen from the table, the agreement between calculated and observed values for enhancement in J¢ is fairly good. The model is not applicable in the case of films deposited from targets containing more than 7 wt.% Ag because beyond this concentration it is likely that the Ag becomes incorporated into the lattice itself and destroys the superconducting properties. For the same reason, J¢ attains its maximum value in films deposited from targets containing 5 wt.% Ag, and decreases for higher percentages, though the grain sizes in these films remain nearly the same as that of the films exhibiting the highest J~. The reason for the extremely low concentration of Ag in laser-ablated films is probably due to the re-evaporation of Ag which takes place at the substrate, while the film is being grown.
4. Conclusion The role of Ag in the enhancement of J¢ has been studied using both direct and indirect evidence. Jc versus T measurements show that increase in J¢ may be produced by a reduction in the size of the grain boundaries. An estimation of the percentage of Ag in laser-ablated films using ICPAES showed that it is less than 0.1%. We propose that the enhancement in J~ is due to reduction in the resistive grain-boundary size which results due to enlarged grains. We have presented a calculation based on the above model which predicts the observed enhancement in J~. References [1] P.N. Peters, R.C. Sisk, E.W. Urban, C.Y. Huang and M.K. Wu, Appl. Phys. Lett. 52 (1988) 2066. [2] Y. Matsumoto, J. Horabo, Y. Yamaguchi, M. Nishida and A. Chiba, Appl. Phys. Lett. 56 (1990) 1585. [3] B. Dwir, M. A.ffronte and D. Pavuna, Appl. Phys. Lett. 55 (1989) 399. [4] T.B. Lindemer, F.A. Washburn and C.S. MacDougall, Physica C 196 (1992) 390. [5] D. Lee, X. Chand and K. Salama, Jpn. J. Appl. Phys. 31
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