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In situ preparation of Y1Ba2Cu4O8 thin films by the laser ablation technique
PHYSICA PhysieaC231 (1994) 335-340
ELSEVIER
In situ preparation of YiBa2Cu408 thin films by the laser ablation technique M. Badaye, Y. Kanke, K. Fukushima, T. Morishita International Superconductivity Technology Center, 10-13 Shinonome, l-chome, Koto-ku. Tokyo 135, Japan
Received 24 June 1994
Abstract Thin fdms of Y~Ba2Cu4Oahave been prepared by the laser ablation of sintered Y~Ba2Cu4Ostargets, followed by a short in situ oxygen annealing. The films obtained under optimum conditions show a high degree of phase purity compared to the films obtained by the other methods. The films' surface morphology has been studied by atomic force microscopy (AFM), their crystalline structure was examined by the X-ray diffraction (XRD) method, and their transport characteristics have been measured by the DC four-probe technique. It is shown that the fabricated films are of high enough quality to be considered for device applications. 1. Introduction The Y~Ba2Cu4Os phase is known to have a superior thermal stability compared to the 123 phase [ 1 ]. Therefore, from the application point of view the Y~BaECu4Os thin films are very important. The difficulty with this phase is that it cannot be grown directly in the low pressures usually used in thin film processes such as laser ablation and sputtering. The extrapolation of the 124 phase diagrams [ 2 ] to low pressures shows that in the l 0-3 torr region, which is usually used in laser deposition, Y~Ba2Cu4Os is not stable unless the temperature is lower than 550°C. Films deposited at such low temperatures are amorphous, and need to be annealed at high pressures to acquire crystalline structure. Several methods have been reported in the past for preparation of Y~Ba2Cu4Os thin films [3-5 ], but all of them involve ex situ annealing of the films grown at room temperature in an oxygen environment. In addition to the need for a furnace for ex situ annealing, the majority of post-annealed films have shown
the 123 phase impurity, and they are potentially less pure than in situ annealed films. In this paper we report the fabrication of 124 thin films by the laser ablation method followed by oxygen annealing in the deposition chamber. The effect of deposition and annealing conditions on the final film characteristics is fully investigated. It is found that the films made under optimum conditions are free of other phases, and the transport characteristics of these films are similar to those reported for Y1Ba2Cu4Os thin films.
2. Experimental method The thin films were deposited on SrTiO3(100) substrates by ablation of a sintered stoichiometric Y1Ba2Cu4Os target using an ArF excimer laser. The laser fluence was typically around 3 J / c m 2, and the substrate-target distance was 5 cm. In general, it was found that the film characteristics depended on both the deposition and the annealing conditions. To investigate the effect of deposition conditions, films
0921-4534/94/$07.00 © 1994 ElsevierScienceB.V. All rights reserved SSDI 0921-4534 (94) 00507-9
336
M. Badaye et al. / Physica C 231 (1994) 335-340
were deposited at substrate temperatures varying from room temperature to 750°C, in oxygen pressures varying from 100 to 500 mtorr. The deposited films were then annealed in the deposition chamber in 700 torr oxygen ambient at 750°C for one hour. The effect of annealing conditions on the film properties was investigated by annealing the samples deposited under identical conditions for varying lengths of time and temperatures. Annealing was usually carried out by a single step, in which the substrate temperature was raised immediately after deposition to the annealing temperature at a rate of 20°C/min, and after annealing for the specified time the sample was usually cooled down to room temperature at a rate of 10*C/min. The annealing pressure was 700 torr for all samples. The fabricated • m s were examined by the X-ray diffraction ( X R D ) method for crystallinity and phase purity. The superconducting characteristics of these films were studied by standard four-probe method. Atomic force microscopy (AFM) was employed to observe the surface structure of the films. The optim u m film was considered to be the one showing the highest crystallinity while having no peak from other phases, the closest superconducting characteristics to that of the bulk Y124, and possibly the smoothest surface.
3. Results
At 500 mtorr oxygen, the as-deposited films grown at substrate temperatures above 600°C were YIBa2CuaOv_x, and subsequent annealing did not convert them to Y1Ba2Cu408. Below 600°C, the asdeposited films were found to be amorphous. Of these low-temperature deposited films, only those formed in the region 400°C < Tsub< 600°C could convert to Y 124 by annealing. The films deposited below 400°C remained amorphous after annealing. A similar phenomenon (existence of a lower limit for substrate temperature for realization of the 124 phase) has also been observed in the growth of 124 films by the sputtering method [6 ]. Fig. 1 shows the X R D measurement results for different substrate temperatures representing the different regions mentioned above. The peaks at about 11.6 °, 15.7 ° and 41.8 ° are all machine artifacts which were even observed in amorphous
STO
8
6
STO
STO
I
II
V
'==
V
I i
I V
V
•
~
~
750°C
'
'
550°C
•
~ 20
30
40
50
400°C 60
70
2 O (Degrees) Fig. 1. The XRD pattern for the samples deposited at different substrate temperatures and annealed under identical conditions. ~7 shows the peaks due to the 123 phase, 0 shows the (001) peaks of the 124 phase, and • shows the spurious peaks generated by the XRD machine.
samples. The 750°C sample has only the c-axis oriented 123 phase, and is therefore undesirable. The 400°C sample has only small peaks due to CuO and minor amounts of Y124 and Y123 phases. The 550°C sample, on the other hand, shows only the c-axis 124 peaks, and this therefore seems to be the best deposition temperature at 500 mtorr. The four-probe measurements on these films showed the Y 123 characteristics for the 750°C sample, insulating for the 450°C film, and close to Y124 behavior for the 550°C sample. The oxygen pressure during laser ablation affects the deposition rate, plume size, deposited particles energy, and phase status of the deposited material. At high ambient pressures, the plume becomes small, the deposition rate reduces, and the plasma species have low energy: Owing to these restrictions we found that 500 mtorr was the upper limit for successful growth of the phase-pure Y124 films in the present set up. To find the proper operating pressure, several films were deposited in the pressure range 100-500 mtorr. The substrate temperature for all these samples was 550°C, and they were all annealed under the same conditions as mentioned above. The X R D measure-
337
M. Badaye et aL / Physica C 231 (1994) 335-340
ment results corresponding to these samples are shown in Fig. 2. As shown, only for pressures above 400 mtorr do the crystalline 124 phase peaks start to appear. The 500 mtorr deposited sample has the highest orientation and phase purity in the present work. Annealing temperature was found to be the most important parameter of the 124 film fabrication process. We found that only in a narrow temperature range ( 7 4 0 ° < T < 7 6 0 " C ) could pure 124 phase be formed in thin films which were deposited under optimum conditions (550°C and 500 mtorr). Experiments showed that the samples annealed below 730°C were amorphous, and generally insulating. It was also found that above the annealing temperature of 760°C the 123 phase appeared in the thin film. Figs. 3 and 4 compare the crystalline structure and superconducting behavior of the samples deposited under identical conditions (Tsub=550°C and Po2=500 mtorr), and annealed at different temperatures for one hour in 700 torr of oxygen. In Fig. 3, although the XRD peaks corresponding to 780 ° and 800°C have slightly higher intensity, they also show impurity peaks from 123 phase. As a result, 750°C was found
,,i .... , .... i'|
°
STO '| .... i .... ~'"¢
10
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~,r,
I ....
r ....
I'''
i ....
I ....
' ....
I''''
''
I .... i ....
V
6
V
•
V 800 °C
i
750o C
,
10
20
30
~
780°C
50
40
60
2 O (Degrees)
Fig. 3. The XRD pattern for the samples deposited under identical conditionsand annealed at different temperatures. • shows the (001) peaks of the 124 phase, and ~7 showsthe peaks due to the 123 phase.
STO ....
I
II )~500 mtorr
~
.5
r,~
1.0
..............
"
"
V
P--.400 m t o r r 0.5
0.0 ~
-
P=300 mtorr
:i// t
50
I e = , , I , t ~ i I I '
100
150
200
'
'
I ~ ~ e i
250
Temperature °K
Fig. 4. The P-T variations in the samples of Fig. 3. P = 1O0 m t o r r
10
20
30
40
50
60
28 (Degrees)
Fig. 2. The XRD pattern for the samplesdepositedat Tsub=550*(2 in different oxygenpressures, and annealed under identical conditions, x7 showsthe peaks due to the 123 phase, and • shows the (001) peaks of the 124 phase.
to be about the optimum annealing the production of 124 thin films. The AFM studies showed that in in this work there were two groups particles in the first group were
temperature for the films grown of particles; the relatively small
338
M. Badaye et al. /Physica C 231 (I 994) 335-340
( < 500 A), and the other group species were rather large (typically 0.2 ~tm) in diameter. The films had a relatively smooth matrix with a roughness of less than 100 A. The panicle density and the matrix smoothness were found to be dependent on the film fabrication conditions. The AFM examination of the films annealed at different temperatures showed that with increasing temperature, the large particle density decreased but the size of particles increased, and the matrix became rougher, Fig. 5. The other important parameter is the annealing duration. To study the effect of annealing time, samples deposited under identical conditions (Tsub= 550°C and Po: = 500 mtorr) were annealed for 30 rain, l, 1.5, and 2 h in 750°C. Figs. 6 and 7 show the XRD and resistivity measurement results made on these samples. In Fig. 6 the XRD corresponding to the 1.5 h annealed sample shows slightly higher peaks than those of the l h annealed film, but since the latter has a superior superconducting behavior (lower normal state resistivity and slightly higher To) we chose one hour as the optimum annealing time. The films' matrix smoothness was found to improve with increasing annealing time but, simultaneously, the panicle size on the average enlarged, Fig. 8. The effect of annealing cycles was investigated by annealing a sample in two steps. In the first step the temperature was raised to 650°C and kept at that level for 1 h. In the second step it was raised to 750°C and kept for one more hour. The resulting thin film showed a better crystallinity but slightly lower onset Tc and zero resistance temperatures than those of the single-step annealed sample.
4 5 IqM ?2 ~M UM
UM
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111t
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tim
4. Discussion
Previously, Y124 films were made by the laser ablation, sputtering, and thermal evaporation of separate metallic cations in the oxide and fluoride forms [3,7]. The 124 phase in these cases was formed by annealing in a flowing oxygen furnace at atmospheric pressure. In the ablation of a single stoichiometric 124 target a completely different chemistry is involved in the formation of 124 film. In this situation there is no oxidizing and catalytic agent except ambient oxygen molecules. This presumably reduces the formation rate of Y 124. On the other hand, this method is
Fig. 5. The AFM image of the samples of Fig. 3. simple, and the final film bound to have a higher chemical purity. Starting with metallic oxides or other ternary and quaternary compounds made of Y, Ba, Cu, and oxygen, many reactions can give rise to the Y124 phase. Of all these reactions only a few are pos-
M. Badaye et aL / Physica C 231 (1994) 335-340
'F"" ......... ,..,1.,
i
,,, .........
, .... I"
't .........
4
339
i
!
;h
2
0
. . . . 10
20
30
i [, ' ~ w ~ 40
50
30 mir 60
20 (Degrees) Fig. 6. T h e X R D p a t t e r n f o r the samples deposited under iden
tical conditions and annealed for different lengths of time. • show: the ( 0 0 1 ) peaks of the 124 phase.
4 ± 1 9 lq~ $ 9 NM
g 8 a
g8 NH
1.98
f
,.¢ 2
.................i
st
z
1
o
aM
% ~. 9 9
~.
99
HM
UM
III'I
1.98
UM
~ UN
i
50
100
150 Temperature
200
250
(°C)
Fig. 7. The p--T variations in the samples of Fig. 6. For of clarity, different marks are used for different curves.
reasons
sible in the growth conditions used in this work. For example, the reaction YBa2 Cu3 0
7 "1-CuO*-+YBa2 Cu4 O8
Fig. 8. The AFM images of the samples annealed for different lengths of time. can be ruled out because this process needs a very high pressure to occur [ 2 ]. We also confirmed this by observing that when the as-deposited film was 123 it could not be converted to the 124 phase by annealing. Simple combination of metallic oxides
340
M. Badaye et aL / Physica C 231 (1994) 335-340
( Y 2 0 3 + C u O + B a O ) also needs high pressure and high temperature [ 8 ], which were not applied during annealing in this work. On the other hand, we observed that the 124 could only be formed when deposition conditions, i.e., temperature and pressure, lay in the stability region of the 124 phase [ 2 ]. This result suggests that the 124 phase was formed during deposition. The annealing process only helped orienting the crystallites formed during deposition. Using this qualitative model we may explain the changes in the X R D variations with deposition conditions. A linear extrapolation of Karpinsky's phase diagrams [2 ] to the low temperature-pressure regions shows that the 124 phase should remain stable at low temperatures at a fixed pressure level of 500 mtorr. The reason for not obtaining a 124 film by depositing at T < 400°C, followed by annealing, should be the lack of energy (during deposition) needed for reactions at the substrate, leading to the formation of 124 crystallites. Clearly in 500 mtorr oxygen pressure and 750°C only the 123 phase is stable [2]. So the X R D and p - T measurements indicate only the 123 phase characteristics. When deposition pressure is less than 300 mtorr at 550°C, the 124 phase is not stable, and therefore little or no 124 phase is formed during deposition. As a result, at such low pressures the resulting film (after annealing) does not show any 124 peak in the X R D pattern (Fig. 2). The X R D and the resistivity variations with annealing conditions can also be interpreted on the basis of the suggested growth mechanism. Although we believe that the chemical reactions leading to the 124 phase do not occur during annealing, the 123 phase can easily be formed in one atmosphere oxygen pressure and high temperatures, e.g., via decomposition of the existing Y124 phase. In this work it was observed that when annealing temperature exceeded 750°C the 123 phase showed up in the thin film. The samples annealed at 780 ° and 800°C (containing 123 phase) did not show superior p - T characteristics compared to the sample annealed at 750°C (free of 123 phase). This may be due to the oxygen deficiency of the 123 phase present in these films; The effect of annealing time on X R D peak intensities, Fig. 6, is not drastic, but nevertheless noticeable. Apparently, some mechanism, such as decomposition, deteriorates the 124 phase beyond 1.5 h annealing. Clearly, for periods shorter than 1.5 h the crystallinity and superconductivity constantly improve with increasing annealing time.
The drastic changes in the particle size and density with changing annealing conditions imply that particles have a relatively lower melting point than that in the matrix, which is mainly the 124 phase. This in turn indicates that the particles' composition differs from YBCO 123, 124, and 123.5 phases. The reason is that the latter compounds have a much higher melting point than the annealing temperatures used in this work. If particles were made of any of these compositions, they could not melt or reshape under the applied annealing conditions. In summary, we have reported the production of single-phase Y124 thin films by laser ablation followed by an in situ annealing. The X R D and resistance measurements showed that the samples showing the best superconducting characteristics and phase purity were those deposited at 550°C in 500 mtorr of oxygen, and annealed at 750°C in near atmospheric oxygen pressure for a period of one hour. The film prepared under these conditions showed a Jc of about 5 × 103 A / c m 2 at 14 K in zero field. In these films the largest particle-free area was found to be about 2 X 2 p.m2.
Acknowledgement This work was partially supported by the New Energy and Industrial Technology Development Organization (NEDO) for the R&D of Industrial Science and Technology Frontier Program.
References [ 1] J. Karpinsky,S. Rusiecki, E. Kaldis, B, Bucher and E. Jilek, Physica C 160 ( 1989) 449. [2] J. Karpinsky,H. Schwer, K. Conder, E. Jilek and E. Kaldis, Appl. Supercond. 1 (1993) 333. [3] M.L. Mandich, A.M. Desantolo, R.M. Fleming, P. Marsh, S. Nakahara, S. Sunshine, J. Kwo, M. Hong, T. Booneand T.Y. Kometani, Phys. Rev. B 38 ( 1988) 5031. [4] K. Char, A.D. Kent, A. Kapitulnik, M.R. Beasleyand T.H. GebalUe,Appl. Phys. Lett. 51 (1987) 1370. [5]R. Kita, S. Kawamoto, K. Ohata, H. Izumi, R. Itti, T. Morishita and S. Tanaka, Physica C 170 (1990) 500. [6] K. Kuroda, K. Kojima, M. Tanioku, K. Yokoyamaand K. Hamanaka, Jpn. J. Appl. Phys. 29 (1990) L1439. [7] D.D. Berkley,D.H. Kim, B.R. Johnson, A.M. Goldman, M.L. Mecartney, K. Beauchampand J. Maps, Appl. Phys. Lett. 53 (1988) 708. [8] D.E. Morris, N.G. Asmar, J.H. Nickel, R.L. Sid and J.Y.T. Wei, PhysicaC 159 (1989) 287.
ELSEVIER
In situ preparation of YiBa2Cu408 thin films by the laser ablation technique M. Badaye, Y. Kanke, K. Fukushima, T. Morishita International Superconductivity Technology Center, 10-13 Shinonome, l-chome, Koto-ku. Tokyo 135, Japan
Received 24 June 1994
Abstract Thin fdms of Y~Ba2Cu4Oahave been prepared by the laser ablation of sintered Y~Ba2Cu4Ostargets, followed by a short in situ oxygen annealing. The films obtained under optimum conditions show a high degree of phase purity compared to the films obtained by the other methods. The films' surface morphology has been studied by atomic force microscopy (AFM), their crystalline structure was examined by the X-ray diffraction (XRD) method, and their transport characteristics have been measured by the DC four-probe technique. It is shown that the fabricated films are of high enough quality to be considered for device applications. 1. Introduction The Y~Ba2Cu4Os phase is known to have a superior thermal stability compared to the 123 phase [ 1 ]. Therefore, from the application point of view the Y~BaECu4Os thin films are very important. The difficulty with this phase is that it cannot be grown directly in the low pressures usually used in thin film processes such as laser ablation and sputtering. The extrapolation of the 124 phase diagrams [ 2 ] to low pressures shows that in the l 0-3 torr region, which is usually used in laser deposition, Y~Ba2Cu4Os is not stable unless the temperature is lower than 550°C. Films deposited at such low temperatures are amorphous, and need to be annealed at high pressures to acquire crystalline structure. Several methods have been reported in the past for preparation of Y~Ba2Cu4Os thin films [3-5 ], but all of them involve ex situ annealing of the films grown at room temperature in an oxygen environment. In addition to the need for a furnace for ex situ annealing, the majority of post-annealed films have shown
the 123 phase impurity, and they are potentially less pure than in situ annealed films. In this paper we report the fabrication of 124 thin films by the laser ablation method followed by oxygen annealing in the deposition chamber. The effect of deposition and annealing conditions on the final film characteristics is fully investigated. It is found that the films made under optimum conditions are free of other phases, and the transport characteristics of these films are similar to those reported for Y1Ba2Cu4Os thin films.
2. Experimental method The thin films were deposited on SrTiO3(100) substrates by ablation of a sintered stoichiometric Y1Ba2Cu4Os target using an ArF excimer laser. The laser fluence was typically around 3 J / c m 2, and the substrate-target distance was 5 cm. In general, it was found that the film characteristics depended on both the deposition and the annealing conditions. To investigate the effect of deposition conditions, films
0921-4534/94/$07.00 © 1994 ElsevierScienceB.V. All rights reserved SSDI 0921-4534 (94) 00507-9
336
M. Badaye et al. / Physica C 231 (1994) 335-340
were deposited at substrate temperatures varying from room temperature to 750°C, in oxygen pressures varying from 100 to 500 mtorr. The deposited films were then annealed in the deposition chamber in 700 torr oxygen ambient at 750°C for one hour. The effect of annealing conditions on the film properties was investigated by annealing the samples deposited under identical conditions for varying lengths of time and temperatures. Annealing was usually carried out by a single step, in which the substrate temperature was raised immediately after deposition to the annealing temperature at a rate of 20°C/min, and after annealing for the specified time the sample was usually cooled down to room temperature at a rate of 10*C/min. The annealing pressure was 700 torr for all samples. The fabricated • m s were examined by the X-ray diffraction ( X R D ) method for crystallinity and phase purity. The superconducting characteristics of these films were studied by standard four-probe method. Atomic force microscopy (AFM) was employed to observe the surface structure of the films. The optim u m film was considered to be the one showing the highest crystallinity while having no peak from other phases, the closest superconducting characteristics to that of the bulk Y124, and possibly the smoothest surface.
3. Results
At 500 mtorr oxygen, the as-deposited films grown at substrate temperatures above 600°C were YIBa2CuaOv_x, and subsequent annealing did not convert them to Y1Ba2Cu408. Below 600°C, the asdeposited films were found to be amorphous. Of these low-temperature deposited films, only those formed in the region 400°C < Tsub< 600°C could convert to Y 124 by annealing. The films deposited below 400°C remained amorphous after annealing. A similar phenomenon (existence of a lower limit for substrate temperature for realization of the 124 phase) has also been observed in the growth of 124 films by the sputtering method [6 ]. Fig. 1 shows the X R D measurement results for different substrate temperatures representing the different regions mentioned above. The peaks at about 11.6 °, 15.7 ° and 41.8 ° are all machine artifacts which were even observed in amorphous
STO
8
6
STO
STO
I
II
V
'==
V
I i
I V
V
•
~
~
750°C
'
'
550°C
•
~ 20
30
40
50
400°C 60
70
2 O (Degrees) Fig. 1. The XRD pattern for the samples deposited at different substrate temperatures and annealed under identical conditions. ~7 shows the peaks due to the 123 phase, 0 shows the (001) peaks of the 124 phase, and • shows the spurious peaks generated by the XRD machine.
samples. The 750°C sample has only the c-axis oriented 123 phase, and is therefore undesirable. The 400°C sample has only small peaks due to CuO and minor amounts of Y124 and Y123 phases. The 550°C sample, on the other hand, shows only the c-axis 124 peaks, and this therefore seems to be the best deposition temperature at 500 mtorr. The four-probe measurements on these films showed the Y 123 characteristics for the 750°C sample, insulating for the 450°C film, and close to Y124 behavior for the 550°C sample. The oxygen pressure during laser ablation affects the deposition rate, plume size, deposited particles energy, and phase status of the deposited material. At high ambient pressures, the plume becomes small, the deposition rate reduces, and the plasma species have low energy: Owing to these restrictions we found that 500 mtorr was the upper limit for successful growth of the phase-pure Y124 films in the present set up. To find the proper operating pressure, several films were deposited in the pressure range 100-500 mtorr. The substrate temperature for all these samples was 550°C, and they were all annealed under the same conditions as mentioned above. The X R D measure-
337
M. Badaye et aL / Physica C 231 (1994) 335-340
ment results corresponding to these samples are shown in Fig. 2. As shown, only for pressures above 400 mtorr do the crystalline 124 phase peaks start to appear. The 500 mtorr deposited sample has the highest orientation and phase purity in the present work. Annealing temperature was found to be the most important parameter of the 124 film fabrication process. We found that only in a narrow temperature range ( 7 4 0 ° < T < 7 6 0 " C ) could pure 124 phase be formed in thin films which were deposited under optimum conditions (550°C and 500 mtorr). Experiments showed that the samples annealed below 730°C were amorphous, and generally insulating. It was also found that above the annealing temperature of 760°C the 123 phase appeared in the thin film. Figs. 3 and 4 compare the crystalline structure and superconducting behavior of the samples deposited under identical conditions (Tsub=550°C and Po2=500 mtorr), and annealed at different temperatures for one hour in 700 torr of oxygen. In Fig. 3, although the XRD peaks corresponding to 780 ° and 800°C have slightly higher intensity, they also show impurity peaks from 123 phase. As a result, 750°C was found
,,i .... , .... i'|
°
STO '| .... i .... ~'"¢
10
]
~,r,
I ....
r ....
I'''
i ....
I ....
' ....
I''''
''
I .... i ....
V
6
V
•
V 800 °C
i
750o C
,
10
20
30
~
780°C
50
40
60
2 O (Degrees)
Fig. 3. The XRD pattern for the samples deposited under identical conditionsand annealed at different temperatures. • shows the (001) peaks of the 124 phase, and ~7 showsthe peaks due to the 123 phase.
STO ....
I
II )~500 mtorr
~
.5
r,~
1.0
..............
"
"
V
P--.400 m t o r r 0.5
0.0 ~
-
P=300 mtorr
:i// t
50
I e = , , I , t ~ i I I '
100
150
200
'
'
I ~ ~ e i
250
Temperature °K
Fig. 4. The P-T variations in the samples of Fig. 3. P = 1O0 m t o r r
10
20
30
40
50
60
28 (Degrees)
Fig. 2. The XRD pattern for the samplesdepositedat Tsub=550*(2 in different oxygenpressures, and annealed under identical conditions, x7 showsthe peaks due to the 123 phase, and • shows the (001) peaks of the 124 phase.
to be about the optimum annealing the production of 124 thin films. The AFM studies showed that in in this work there were two groups particles in the first group were
temperature for the films grown of particles; the relatively small
338
M. Badaye et al. /Physica C 231 (I 994) 335-340
( < 500 A), and the other group species were rather large (typically 0.2 ~tm) in diameter. The films had a relatively smooth matrix with a roughness of less than 100 A. The panicle density and the matrix smoothness were found to be dependent on the film fabrication conditions. The AFM examination of the films annealed at different temperatures showed that with increasing temperature, the large particle density decreased but the size of particles increased, and the matrix became rougher, Fig. 5. The other important parameter is the annealing duration. To study the effect of annealing time, samples deposited under identical conditions (Tsub= 550°C and Po: = 500 mtorr) were annealed for 30 rain, l, 1.5, and 2 h in 750°C. Figs. 6 and 7 show the XRD and resistivity measurement results made on these samples. In Fig. 6 the XRD corresponding to the 1.5 h annealed sample shows slightly higher peaks than those of the l h annealed film, but since the latter has a superior superconducting behavior (lower normal state resistivity and slightly higher To) we chose one hour as the optimum annealing time. The films' matrix smoothness was found to improve with increasing annealing time but, simultaneously, the panicle size on the average enlarged, Fig. 8. The effect of annealing cycles was investigated by annealing a sample in two steps. In the first step the temperature was raised to 650°C and kept at that level for 1 h. In the second step it was raised to 750°C and kept for one more hour. The resulting thin film showed a better crystallinity but slightly lower onset Tc and zero resistance temperatures than those of the single-step annealed sample.
4 5 IqM ?2 ~M UM
UM
1 IIM I
LIM
2. t i n
~
111t
1.77
I'iNI
:3~q lqM i:
6
tim
~
2
I.!14
~
HM
tl~
tim
4. Discussion
Previously, Y124 films were made by the laser ablation, sputtering, and thermal evaporation of separate metallic cations in the oxide and fluoride forms [3,7]. The 124 phase in these cases was formed by annealing in a flowing oxygen furnace at atmospheric pressure. In the ablation of a single stoichiometric 124 target a completely different chemistry is involved in the formation of 124 film. In this situation there is no oxidizing and catalytic agent except ambient oxygen molecules. This presumably reduces the formation rate of Y 124. On the other hand, this method is
Fig. 5. The AFM image of the samples of Fig. 3. simple, and the final film bound to have a higher chemical purity. Starting with metallic oxides or other ternary and quaternary compounds made of Y, Ba, Cu, and oxygen, many reactions can give rise to the Y124 phase. Of all these reactions only a few are pos-
M. Badaye et aL / Physica C 231 (1994) 335-340
'F"" ......... ,..,1.,
i
,,, .........
, .... I"
't .........
4
339
i
!
;h
2
0
. . . . 10
20
30
i [, ' ~ w ~ 40
50
30 mir 60
20 (Degrees) Fig. 6. T h e X R D p a t t e r n f o r the samples deposited under iden
tical conditions and annealed for different lengths of time. • show: the ( 0 0 1 ) peaks of the 124 phase.
4 ± 1 9 lq~ $ 9 NM
g 8 a
g8 NH
1.98
f
,.¢ 2
.................i
st
z
1
o
aM
% ~. 9 9
~.
99
HM
UM
III'I
1.98
UM
~ UN
i
50
100
150 Temperature
200
250
(°C)
Fig. 7. The p--T variations in the samples of Fig. 6. For of clarity, different marks are used for different curves.
reasons
sible in the growth conditions used in this work. For example, the reaction YBa2 Cu3 0
7 "1-CuO*-+YBa2 Cu4 O8
Fig. 8. The AFM images of the samples annealed for different lengths of time. can be ruled out because this process needs a very high pressure to occur [ 2 ]. We also confirmed this by observing that when the as-deposited film was 123 it could not be converted to the 124 phase by annealing. Simple combination of metallic oxides
340
M. Badaye et aL / Physica C 231 (1994) 335-340
( Y 2 0 3 + C u O + B a O ) also needs high pressure and high temperature [ 8 ], which were not applied during annealing in this work. On the other hand, we observed that the 124 could only be formed when deposition conditions, i.e., temperature and pressure, lay in the stability region of the 124 phase [ 2 ]. This result suggests that the 124 phase was formed during deposition. The annealing process only helped orienting the crystallites formed during deposition. Using this qualitative model we may explain the changes in the X R D variations with deposition conditions. A linear extrapolation of Karpinsky's phase diagrams [2 ] to the low temperature-pressure regions shows that the 124 phase should remain stable at low temperatures at a fixed pressure level of 500 mtorr. The reason for not obtaining a 124 film by depositing at T < 400°C, followed by annealing, should be the lack of energy (during deposition) needed for reactions at the substrate, leading to the formation of 124 crystallites. Clearly in 500 mtorr oxygen pressure and 750°C only the 123 phase is stable [2]. So the X R D and p - T measurements indicate only the 123 phase characteristics. When deposition pressure is less than 300 mtorr at 550°C, the 124 phase is not stable, and therefore little or no 124 phase is formed during deposition. As a result, at such low pressures the resulting film (after annealing) does not show any 124 peak in the X R D pattern (Fig. 2). The X R D and the resistivity variations with annealing conditions can also be interpreted on the basis of the suggested growth mechanism. Although we believe that the chemical reactions leading to the 124 phase do not occur during annealing, the 123 phase can easily be formed in one atmosphere oxygen pressure and high temperatures, e.g., via decomposition of the existing Y124 phase. In this work it was observed that when annealing temperature exceeded 750°C the 123 phase showed up in the thin film. The samples annealed at 780 ° and 800°C (containing 123 phase) did not show superior p - T characteristics compared to the sample annealed at 750°C (free of 123 phase). This may be due to the oxygen deficiency of the 123 phase present in these films; The effect of annealing time on X R D peak intensities, Fig. 6, is not drastic, but nevertheless noticeable. Apparently, some mechanism, such as decomposition, deteriorates the 124 phase beyond 1.5 h annealing. Clearly, for periods shorter than 1.5 h the crystallinity and superconductivity constantly improve with increasing annealing time.
The drastic changes in the particle size and density with changing annealing conditions imply that particles have a relatively lower melting point than that in the matrix, which is mainly the 124 phase. This in turn indicates that the particles' composition differs from YBCO 123, 124, and 123.5 phases. The reason is that the latter compounds have a much higher melting point than the annealing temperatures used in this work. If particles were made of any of these compositions, they could not melt or reshape under the applied annealing conditions. In summary, we have reported the production of single-phase Y124 thin films by laser ablation followed by an in situ annealing. The X R D and resistance measurements showed that the samples showing the best superconducting characteristics and phase purity were those deposited at 550°C in 500 mtorr of oxygen, and annealed at 750°C in near atmospheric oxygen pressure for a period of one hour. The film prepared under these conditions showed a Jc of about 5 × 103 A / c m 2 at 14 K in zero field. In these films the largest particle-free area was found to be about 2 X 2 p.m2.
Acknowledgement This work was partially supported by the New Energy and Industrial Technology Development Organization (NEDO) for the R&D of Industrial Science and Technology Frontier Program.
References [ 1] J. Karpinsky,S. Rusiecki, E. Kaldis, B, Bucher and E. Jilek, Physica C 160 ( 1989) 449. [2] J. Karpinsky,H. Schwer, K. Conder, E. Jilek and E. Kaldis, Appl. Supercond. 1 (1993) 333. [3] M.L. Mandich, A.M. Desantolo, R.M. Fleming, P. Marsh, S. Nakahara, S. Sunshine, J. Kwo, M. Hong, T. Booneand T.Y. Kometani, Phys. Rev. B 38 ( 1988) 5031. [4] K. Char, A.D. Kent, A. Kapitulnik, M.R. Beasleyand T.H. GebalUe,Appl. Phys. Lett. 51 (1987) 1370. [5]R. Kita, S. Kawamoto, K. Ohata, H. Izumi, R. Itti, T. Morishita and S. Tanaka, Physica C 170 (1990) 500. [6] K. Kuroda, K. Kojima, M. Tanioku, K. Yokoyamaand K. Hamanaka, Jpn. J. Appl. Phys. 29 (1990) L1439. [7] D.D. Berkley,D.H. Kim, B.R. Johnson, A.M. Goldman, M.L. Mecartney, K. Beauchampand J. Maps, Appl. Phys. Lett. 53 (1988) 708. [8] D.E. Morris, N.G. Asmar, J.H. Nickel, R.L. Sid and J.Y.T. Wei, PhysicaC 159 (1989) 287.