- Email: [email protected]
Synthesis of 2212 structure Bi-Sr-Ca-Cu-O thin films superconducting in the 90 K range
PIIYSICA
Physica C 197 ( 1992 ) 64-68 North-Holland
Synthesis of 2212 structure Bi-Sr-Ca-Cu-O thin films superconducting in the 90 K range B. V e n g a l i s ~, A. F l o d s t r / S m a n d A. B r a z d e i k i s Materials Science, Department of Physics, Royal Institute of Technology, s- l O0 44 Stockholm, Sweden
Received 24 December 1991 Revised manuscript received 26 March 1992
Thin superconducting Bi-Sr-Ca-Cu-O films with dominating 2212 structure have been synthesized on MgO substrates by simultaneous evaporation of metallic Bi, Sr, Ca and Cu metals from four independent Knudsen evaporation sources and subsequent postannealing of the prepared metal alloy films in air. The thin film zero resistance temperature, To.zero,dependence on cooling rate from the annealing temperature and on composition changes was investigated. Rapid cooling of the films from 820 ° C increased Tc..... up to 94 K. Thin films with To.zeroabove 90 K were enriched in Bi and Cu with respect to the nominal 2212 composition. Electrical resistivity, X-ray diffraction and optical reflectivity measurements have been performed. Possible factors which can influence Tc variation in the films are discussed.
1. Introduction The three most stable superconducting comp o u n d s in the B i - S r - C a - C u - O system are represented by the single formula Bi2SrECa,_lCunO2,+4 with n = l, 2 a n d 3. They are known as the 2201, 2212 and 2223-phases, with n o r m a l l y d e t e r m i n e d critical zero resistance temperatures, Tc...... o f 10, 80-85 and 1 l 0 K, respectively. The large differences in Tc..... for these phases are associated with the different n u m b e r o f CuO2 layers which are stacked between equivalent subunits o f BiO a n d SrO in their crystal structures. There are great practical needs for high quality B i S r - C a - C u - O thin films with Tc exceeding LN2 temperature. However, owing to the relatively low stability o f the highest T~ phase, the corresponding 2223phase films are only p r e p a r e d with great difficulties. F u r t h e r m o r e , the T~..... values ranging in the vicinity o f 80 K for 2212-phase films seem to be too low for most applications. On the other hand, there are indications towards a T~..... increase up to 9 0 - 9 5 K [ 1 - 4 ] a n d even to 100 K [5] for 2212-phase bulk
samples if rapidly cooled from high t e m p e r a t u r e s or a n n e a l e d in reduced oxygen atmosphere. The increased Tc..... values reported in both cases were associated with lowered excess oxygen content in the superconducting material. Although recent a t t e m p t s were d e v o t e d to resolving the "oxygen p r o b l e m " in b i s m u t h oxide superconductors, the relationship between Tc and different doping levels o f the 2212phase material remains unclear. In the present work, rapid cooling a n d composition variation effects on Tc..... were examined for B i S r - C a - C u - O thin films with p r e d o m i n a n t 2212-like structure. A molecular Beam Epitaxy ( M B E ) system with four i n d e p e n d e n t Bi, Sr, Ca and Cu pure metal K n u d s e n evaporation sources has been used for the thin film fabrication. It enabled us to vary c o m p o sition o f the films in a predictable way. Electrical resistivity, X - R a y Diffraction ( X R D ) and optical reflectivity measurements in the near infrared region were p e r f o r m e d to obtain new information about the material exhibiting T¢..... in the 90 K range.
2. Experimental On leave from Semiconductor Physics Institute, Go~tauto 11, Vilnius, 2600, Lithuania
S u b m i c r o n thickness ( d = 0.20-0.30 g m ) , differ-
0921-4534/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
B. Vengalis et al. / Synthesis of 2212 BSCCO thin films
ent composition B i - S r - C a - C u - O films have been prepared on polished MgO (1 0 0) surfaces using MBE and subsequent postannealing of the metal alloy films. Evaporation of pure Bi, Sr, Ca and Cu metals was performed from four Knudsen sources under ultrahigh vacuum conditions.The typical process pressure was 5 X 10 -8 Torr. The deposition time ranged from 0.5 to 1.0 h. Different molecular fluxes were calibrated separately using a quartz crystal thickness monitor in the substrate position. Auger electron spectroscopy and electron beam-excited Xray analysis have additionally been used for composition control of the as-grown and postannealed films. Film composition in this work was reproduced within an accuracy < 5% for each element. The as-grown metal alloy films were fully oxidized in oxygen atmosphere at 350°C. To find the best high-temperature annealing conditions, the oxidized films were cut into smaller pieces which were further kept at 800-840°C for 0.5-1.5 h in air and finally cooled down to room temperature. The 1.0 h anneal at 820°C was found to be optimal with respect to superconducting properties, the surface morphology and crystal structure of the annealed films. Slow and fast film cooling with exponential temperature decrease starting from 820°C were undertaken (fig. 1 ). Slow cooling was attained by simple furnace cooling and the rapid by pulling the sample holder out of the tube furnace, allowing the sample to cool in air. To prevent evaporation of the more volatile components from the films during the high-temperature anneal and thus avoiding possible composition changes, the films were put in close contact with a polished
MgO ~'800
400
/
820 °C
50
100 Time, rain.
~=8 min "rs=180 min
150
Fig. 1. Details of the film thermal treatment.
200
65
MgO surface and the annealing performed in closed quartz crucibles filled with powdered 2212 compound. Crystal structures of the prepared films were analyzed by XRD using Cu K~ radiation. The electrical resistance measurements have been performed by a four-probe method using evaporated silver contacts and a current of less than 0.1 mA. Calibrated silicon thermoresistance and copper-constantan thermocouple were used for temperature measurements. The critical current density was determined by the 1.0 g V / c m criterion. To investigate a possible relationship between Tc..... and free carrier density, N , in the film material, the plasma frequency of free carriers, ogp, was calculated from optical reflectivity measurements. The optical reflectivity data were taken at room temperature in the wavelength range 0.5 to 4.0 gm.
3. Results B i - S r - C a - C u - O thin films with compositions close to nominal ones of the 2212- and 2223-phases were prepared. The Sr to Bi ratio for most of the films was kept constant at 1.0. For the two films with the lowest Sr+ Ca content only, these ratios were different and close to 0.7. It is convenient to plot all the compositions in the pseudoternary BiO1.5(SrO,CaO)-CuO composition diagram as shown in fig. 2. The postannealed films produced exhibited mirror-like surfaces, the typical grain size as observed in SEM ranging between 10 and 30 gin. XRD data showed that the 2212-phase dominated for all the films investigated. Single 2212-phase state films were obtained for several compositions in the vicinity of the 2212 point in the composition diagram. Small amounts of 2201-phase were found for the films with the highest Bi content. At the same time, clearly defined 2223-phase reflexes were not determined even for highest Cu content. This can be understood in view of the relatively low annealing temperature as compared to the one required for the highest Tc phase to be formed. The typical X R D spectrum for a single 2212-phase film is shown in fig. 3(a). The diffraction lines are of the (0 0/)-type, •=2, 4, 6, 8..., and clearly indicate
66
B. Vengalis et al. / Synthesis of 2212 BSCCO thin films BiO].~
O-2212 singlephase A [] -2212and impurity / \ phases / 2 2 0 1 ~ ( S r , C a/ ~ . 4 ~ n
• "1~>90 K ~ "I;,~o:(85-90K) O'I~'~°<85K
... • \
4
iii12............ ~......... ~ B i
/
(
(Sr,Ca)O
i 0.3
0.4
CuO
Cu
Fig. 2. Composition mapping diagram with respect to phase state an the zero resistance temperatures, T¢.,,~o,for Bi-Sr-Ca-Cu-O thin films rapidly cooled after 1.0 h anneal at 820°C. Sr to Bi atomic ratio for the films was close to 1.0. Only for the two films with the lowest Ca + Sr contents was this ratio 0.7.
(2.0, 2.02, 1.02, 2.08) ~,oj OOlO I0
oo2
~
a
2 MgO E/
0o4 0o6
Jl
(2.0, 1.96, 1.74, 2.78)
i
1
b
appeared in the X R D spectra. In addition, broadening o f the (0 0 l) diffraction lines was observed when the film composition was m o v e d towards the 2223 point, see e.g. X R D spectra in fig. 3(b). This observed line broadening indicates disorder in the stacking sequence of CuO2 planes when the copper content exceeds the nominal 2212 one. All the films annealed at 820 °C displayed metallic normal state electrical resistivity and sharp superconducting transitions. Rapid cooling was found to raise the superconducting transition temperatures for all of them by 5 to 10 K. Figure 4 shows the normalized resistance versus temperature data measured for slowly and rapidly cooled samples of the film with composition Bi2.oSrzo2CaLoECUzosOx. The highest zero resistance temperature, To,zero, of 94 K was measured for this film composition in the case o f rapid cooling. The critical current density, Jc(T), for this film is shown in the insert to fig. 4. C o m m o n jc(4.2 K) values for other rapidly cooled films were found to be in the range 5 to 8 × l04 A / c m 2, the critical current density decreasing with increasing cooling rate. We interpret the decrease in critical current density upon increasing cooling rate as being due to an increased tendency to the formation of microcracks with cooling rate. Composition mapping with respect to T~..... achieved for rapidly cooled films of different compositions is shown in fig. 2. It is interesting to note that relatively high T~..... values, in the range of
M 0,7
(2.0, 2.02, 1.02, 2.08)
0,6 5
10
15 20
25 30 35 20(degrees)
40
45
• 0
•
50
I'-ID
0,5
• 0
o~
0
°'-2
0 • 0
Fig. 3. XRD spectra for rapidly cooled Bi2.oSr2.o2Cal.o2Cu2.osO (a) and Bi2.0Srt.96Ca.1 7 4 C u 2 . 7 8 O y ( b ) thin films.
x
a highly oriented 2212-phase material with the c-axis normal to the film surface. Neither substantial line position changes nor line shape variations were determined for the films with different cooling rates. The lattice constant, c, for single phase films was determined to range from 30.80 to 30.85/k. By moving away from the nominal 2212 composition in the composition diagram, traces o f impurities such as the 2201-phase, (Sr, Ca)2CuO3 and CaO
ODo
~0,4 ~--~0,3 [.-, 0,2 0,1 80
oO0 • 0 • 0 • 0
8 ~6
O0 •• 0 • 0 • ~.~1
f•
o•
~4 -.&2t
zb Temperature, 4b 6bK 80
0o ,
t
,
t
,
I
,
I
,
100 120 140 160 180 200 Temperature, K
Fig. 4. Normalized resistivity vs. temperature curves for a rapidly (1) and a slowly (2) cooled Bi2.oSr2.oECat.02Cu2.osOx composition thin film. Film thickness was 0.25 p.m. Critical current density, Jc, for the rapidly cooled film is shown.
B. Vengalis et aL / Synthesis of 2212 BSCCO thin films
90 K, were obtained for films with preferential 2212 structure, although the compositions were close to the 2223 one: The reflectivity spectra, R(2), for all our single phase films displayed well-defined plasma edges similar to the reflectivity spectra obtained from bulk 2212 single crystals [ 6 ]. The R (2) curves shown in fig. 5 by full and open circles were measured for rapidly and slowly cooled films of the same composition, i.e., they were cut from the same original metal alloy films of the composition Bi2.oSr2o2Cal.o2Cu2osO~, the surface quality of the two postannealed films being almost identical. Thus, contrary to differences in To.zerovalues for the two films, there are negligible effects of the different cooling rates upon the plasma edge position and, consequently, on the plasma frequency of free carriers, top. The fit to the experimental data, displayed as a full curve in fig. 5, was calculated using a Drude model for the optical reflectivity with parameters e~ = 2.5, top = 10300 c m - ~and carrier scattering rate l / T = 1600 cm -l. In fig. 5, the additional R(2) dependence represented by squares corresponds to the other rapidly cooled film having the composition Bi2.oSrl.96CaL74Cu2.7sOy with the same 2212-1ike crystal structure but substantially higher Cu content if compared to the nominal 2212 composition. A significant plasma edge shift to shorter wavelengths is 0.6
• • - fast cooling 0.5
• ~= • • • A
o - slow cooling
°0.4 ~.
9 0.3 n'o. 2
•ml
o.~ 0
;~" I
I
(2.0, 2.02,1.o2, zos)- i, 2 (zo, 1.o6, ~.74, 2.78)- 3 i
I
,
I
,
2 3 Wavelength, 14m
Fig. 5. Optical reflectivity spectra R ( 2 ) for rapidly ( 1 ) and slowly ( 2 ) cooled films o f the Bi2.oSr2.o2CaLo2Cu2.osO= composition and rapidly cooled f i l m ( 3 ) o f the Bi2.oSr=.96CaL74CU2.TsOy composition. Full curve 4 corresponds to a D r u d e fit w i t h characteristic energy parameters, oJp= 10300 cm - 1, [ , = 1600 cm - ~ and ~o = 2.5.
67
clearly observed with increasing Cu content.
4. Discussion
The rapid cooling of the films strongly influences both the resistivity versus temperature dependence and the Tc..... values for films of different compositions, but no change in the plasma frequency, top, is deduced from the reflectance data. On the other hand, the observed shift of top to short wavelengths with increasing Cu content is not accompanied by significant change in To. Compare curves 1 and 3 in fig. 5. To understand the origin of these features it is necessary to analyze factors which can influence the measured values of resistivity (p) and the optical reflectivity (R(2)). For c-axis oriented films, both p and R (2) give information about free carrier motion parallel to the CuO2 layers. However, p, when compared to R(2), is much more sensitive to inhomogeneities, such as nonuniform doping, grain boundaries or microcracks, which can be generated during the cooling process. For polycrystalline 2212-phase material, interpretation of resistivity or Hall measurement data becomes even more complicated because of the great anisotropy (pi/pll~ l04 [8] ) of individual grains and grain boundaries. Optical reflectivity measurements performed for LaBaCuO and YBaCuO layered superconductors showed surprising stability of plasma edge spectral position with averaged doping or oxygen content change [ 9 ]. The optical reflectivity data can be interpreted assuming an almost fixed local carrier density in conducting CuO2 planes. The stabilization of free carrier density in the CuO2 planes of the layered Cu oxide superconductors has been explained by charge transfer from the so-called charge reservoir layers [ 10 ]. There are indications that BiO planes in the 2212phase act as a charge reservoir for the conducting CuO2 planes [3 ]. Thus, if dopants are introduced, it seems much easier to induce changes in superconducting properties by varying charge in the BiO layers rather than by changing the charge density in the CuO2 planes. Recently, the plasma frequency increase with n has
68
B. Vengalis et al. / Synthesis of 2212 BSCCO thin films
been found for Bi oxide superconductors hcop/x/'~oo=0.83, 1.03 a n d 1.21 eV for n = 1, 2 a n d 3, respectively [ 7 ]. If there is no strong effective mass dependence on n, it seems therefore that the carrier density corresponding to a single CuO2 layer remains nearly the same for different superconducting crystal structures. C o n t r a r y to ref. [7], the optical reflectivity m e a s u r e m e n t s in this work were perf o r m e d for films having the same preferential 2212structure. F r o m the obtained similar cop increase with Cu content. C o m p a r e curves 1 and 3 in fig. 3. It is likely that cop does c o r r e s p o n d to average CuO2 layer density in the material. It can be d e d u c e d from the c o m p o s i t i o n d i a g r a m in fig. 2 that the highest T~..... values above 90 K can be o b t a i n e d only in a relatively n a r r o w c o m p o s i t i o n region. According to the diagram, these compositions are shifted from the (2, 2, n - 1, n) composition line to the higher BiOo.15 a n d CuO content side. The present thin film data are in good agreement with similar Tc..... investigations p e r f o r m e d for rapidly cooled bulk single crystals [5]. We believe that slow a n d r a p i d cooling o f the B i S r - C a - C u - O films from annealing t e m p e r a t u r e can induce differences in their superconducting properties in two different ways. Slowly cooled material does become o v e r d o p e d by excess oxygen [1,3,4] a n d r a p i d cooling o f the films from the annealing temperature can reduce oxygen content owing to a thermally activated oxygen desorption process. In addition, we believe that enclosing the film surfaces with MgO plates, as shown in fig. 1, prevents a b s o r p t i o n o f excess oxygen during the cooling stage. Secondly, we stress the different conditions for the super ( m o d u l a t i o n ) structure formation in slowly and rapidly cooled 2212 material. The origin o f the structure is related to a m i s m a t c h in i n t e r a t o m i c distances between a t o m s in the CuO2 a n d BiO layers [ 11 ]. The a p p e a r a n c e o f strain fields in the structure can lead to a different diffusion-controlled spatial oxygen redistribution inside the charge reservoir BiO layers and, as a consequence, can lead to nonuniformity o f the superconducting properties o f the material. 5. Conclusions Cooling rate a n d c o m p o s i t i o n variations from the
n o m i n a l 2212 composition in B i - S r - C a - C u - O thin films having the 2212 structure are o f great importance for achieving Tc..... values above the 90 K range. R a p i d cooling from annealing t e m p e r a t u r e ( 8 2 0 ° C ) and slightly lower S r + C a content in the films cause Tc..... increase. Better knowledge concerning the interaction between point defects, such as excess oxygen, oxygen vacancies and substituted a t o m s in strained BiO planes, seems to be necessary to u n d e r s t a n d the origin o f unusually high Tc..... values o b t a i n e d for the thin films.
Acknowledgement This work was s u p p o r t e d by the Swedish Natural Science Research Council a n d the Swedish Research Council for the Engineering Sciences within the project for thin film synthesis and surface spectroscopy o f high-To superconductors.
References [ 1] R.G. Buckley, J.L. Tallon, I.W.M. Brown, M.R. Presland, N.E. Flower, P.W. Gilberd, M. Bowden and N.B. Milestone, Physica C 156 (1988) 629. [2] N. Knauf, J. Harnischmacher, R. Mfiller, R. Borowski, B. Roden and D. Wohlleben, Physica C 173 ( 1991 ) 414. [3] M. Nagoshi, T. Suzuki, Y. Fukuda, K. Terashima, Y. Nakanishi, M. Ogita, A. Tokiwa, Y. Syono and M. Tachiki, Phys. Rev. B 43 ( 1991 ) 10445. [4] W.A. Groen, D.M. de Leeuw and L.F. Feiner, Physica C 165 (1990) 55. [ 5 ] V.A. Murashov, S.N. Gordeev, I.S. Dubenko, V.M. lonov, A.Y. Martinkin, A.V. Titov, Y.V. Trofimov and A.M. Frolov, Supercond. Phys. Chem. Techn. 3 (1990) 963 (in Russian ). [6] D. Huang, B. Zhu, H. Ming, G. Gu, W. Cai and Y. Ruan, Appl. Phys. Lett. 58 (1991) 1098. [7] K. Hirochi, K. Setsune, S. Hayashi, K. Mizuno, T. Matsushima, Y. Ichikawa, H. Adachi and K. Wasa, Physica B 165&166 (1990) 1255. [ 8 ] S. Martin, A.T. Fiori, R.M. Fleming, L.F. Schneemeyer and J.V. Waszczak, Phys. Rev. B 41 (1990) 846. [ 9 ] S. Tajima, Electronic States of High-ToCuprates Investigated by Optical Measurements Proc. 3rd Inter. Symp. of Superconductivity, November 1990, Sendal, Japan. [ 10] R.J. Cava, A.W. Hewat, E.A. Hewat, B. Batlogg,M. Marezio, K.M. Rabe, J.J. Krajewski, W.F. Peck Jr. and UW. Rupp Jr., Physica C 165 (1990) 419. [11] H.W. Zandbergen, W.A. Groen, A. Smit and G. van Tendeloo, Physica C 168 (1990) 426.
Physica C 197 ( 1992 ) 64-68 North-Holland
Synthesis of 2212 structure Bi-Sr-Ca-Cu-O thin films superconducting in the 90 K range B. V e n g a l i s ~, A. F l o d s t r / S m a n d A. B r a z d e i k i s Materials Science, Department of Physics, Royal Institute of Technology, s- l O0 44 Stockholm, Sweden
Received 24 December 1991 Revised manuscript received 26 March 1992
Thin superconducting Bi-Sr-Ca-Cu-O films with dominating 2212 structure have been synthesized on MgO substrates by simultaneous evaporation of metallic Bi, Sr, Ca and Cu metals from four independent Knudsen evaporation sources and subsequent postannealing of the prepared metal alloy films in air. The thin film zero resistance temperature, To.zero,dependence on cooling rate from the annealing temperature and on composition changes was investigated. Rapid cooling of the films from 820 ° C increased Tc..... up to 94 K. Thin films with To.zeroabove 90 K were enriched in Bi and Cu with respect to the nominal 2212 composition. Electrical resistivity, X-ray diffraction and optical reflectivity measurements have been performed. Possible factors which can influence Tc variation in the films are discussed.
1. Introduction The three most stable superconducting comp o u n d s in the B i - S r - C a - C u - O system are represented by the single formula Bi2SrECa,_lCunO2,+4 with n = l, 2 a n d 3. They are known as the 2201, 2212 and 2223-phases, with n o r m a l l y d e t e r m i n e d critical zero resistance temperatures, Tc...... o f 10, 80-85 and 1 l 0 K, respectively. The large differences in Tc..... for these phases are associated with the different n u m b e r o f CuO2 layers which are stacked between equivalent subunits o f BiO a n d SrO in their crystal structures. There are great practical needs for high quality B i S r - C a - C u - O thin films with Tc exceeding LN2 temperature. However, owing to the relatively low stability o f the highest T~ phase, the corresponding 2223phase films are only p r e p a r e d with great difficulties. F u r t h e r m o r e , the T~..... values ranging in the vicinity o f 80 K for 2212-phase films seem to be too low for most applications. On the other hand, there are indications towards a T~..... increase up to 9 0 - 9 5 K [ 1 - 4 ] a n d even to 100 K [5] for 2212-phase bulk
samples if rapidly cooled from high t e m p e r a t u r e s or a n n e a l e d in reduced oxygen atmosphere. The increased Tc..... values reported in both cases were associated with lowered excess oxygen content in the superconducting material. Although recent a t t e m p t s were d e v o t e d to resolving the "oxygen p r o b l e m " in b i s m u t h oxide superconductors, the relationship between Tc and different doping levels o f the 2212phase material remains unclear. In the present work, rapid cooling a n d composition variation effects on Tc..... were examined for B i S r - C a - C u - O thin films with p r e d o m i n a n t 2212-like structure. A molecular Beam Epitaxy ( M B E ) system with four i n d e p e n d e n t Bi, Sr, Ca and Cu pure metal K n u d s e n evaporation sources has been used for the thin film fabrication. It enabled us to vary c o m p o sition o f the films in a predictable way. Electrical resistivity, X - R a y Diffraction ( X R D ) and optical reflectivity measurements in the near infrared region were p e r f o r m e d to obtain new information about the material exhibiting T¢..... in the 90 K range.
2. Experimental On leave from Semiconductor Physics Institute, Go~tauto 11, Vilnius, 2600, Lithuania
S u b m i c r o n thickness ( d = 0.20-0.30 g m ) , differ-
0921-4534/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
B. Vengalis et al. / Synthesis of 2212 BSCCO thin films
ent composition B i - S r - C a - C u - O films have been prepared on polished MgO (1 0 0) surfaces using MBE and subsequent postannealing of the metal alloy films. Evaporation of pure Bi, Sr, Ca and Cu metals was performed from four Knudsen sources under ultrahigh vacuum conditions.The typical process pressure was 5 X 10 -8 Torr. The deposition time ranged from 0.5 to 1.0 h. Different molecular fluxes were calibrated separately using a quartz crystal thickness monitor in the substrate position. Auger electron spectroscopy and electron beam-excited Xray analysis have additionally been used for composition control of the as-grown and postannealed films. Film composition in this work was reproduced within an accuracy < 5% for each element. The as-grown metal alloy films were fully oxidized in oxygen atmosphere at 350°C. To find the best high-temperature annealing conditions, the oxidized films were cut into smaller pieces which were further kept at 800-840°C for 0.5-1.5 h in air and finally cooled down to room temperature. The 1.0 h anneal at 820°C was found to be optimal with respect to superconducting properties, the surface morphology and crystal structure of the annealed films. Slow and fast film cooling with exponential temperature decrease starting from 820°C were undertaken (fig. 1 ). Slow cooling was attained by simple furnace cooling and the rapid by pulling the sample holder out of the tube furnace, allowing the sample to cool in air. To prevent evaporation of the more volatile components from the films during the high-temperature anneal and thus avoiding possible composition changes, the films were put in close contact with a polished
MgO ~'800
400
/
820 °C
50
100 Time, rain.
~=8 min "rs=180 min
150
Fig. 1. Details of the film thermal treatment.
200
65
MgO surface and the annealing performed in closed quartz crucibles filled with powdered 2212 compound. Crystal structures of the prepared films were analyzed by XRD using Cu K~ radiation. The electrical resistance measurements have been performed by a four-probe method using evaporated silver contacts and a current of less than 0.1 mA. Calibrated silicon thermoresistance and copper-constantan thermocouple were used for temperature measurements. The critical current density was determined by the 1.0 g V / c m criterion. To investigate a possible relationship between Tc..... and free carrier density, N , in the film material, the plasma frequency of free carriers, ogp, was calculated from optical reflectivity measurements. The optical reflectivity data were taken at room temperature in the wavelength range 0.5 to 4.0 gm.
3. Results B i - S r - C a - C u - O thin films with compositions close to nominal ones of the 2212- and 2223-phases were prepared. The Sr to Bi ratio for most of the films was kept constant at 1.0. For the two films with the lowest Sr+ Ca content only, these ratios were different and close to 0.7. It is convenient to plot all the compositions in the pseudoternary BiO1.5(SrO,CaO)-CuO composition diagram as shown in fig. 2. The postannealed films produced exhibited mirror-like surfaces, the typical grain size as observed in SEM ranging between 10 and 30 gin. XRD data showed that the 2212-phase dominated for all the films investigated. Single 2212-phase state films were obtained for several compositions in the vicinity of the 2212 point in the composition diagram. Small amounts of 2201-phase were found for the films with the highest Bi content. At the same time, clearly defined 2223-phase reflexes were not determined even for highest Cu content. This can be understood in view of the relatively low annealing temperature as compared to the one required for the highest Tc phase to be formed. The typical X R D spectrum for a single 2212-phase film is shown in fig. 3(a). The diffraction lines are of the (0 0/)-type, •=2, 4, 6, 8..., and clearly indicate
66
B. Vengalis et al. / Synthesis of 2212 BSCCO thin films BiO].~
O-2212 singlephase A [] -2212and impurity / \ phases / 2 2 0 1 ~ ( S r , C a/ ~ . 4 ~ n
• "1~>90 K ~ "I;,~o:(85-90K) O'I~'~°<85K
... • \
4
iii12............ ~......... ~ B i
/
(
(Sr,Ca)O
i 0.3
0.4
CuO
Cu
Fig. 2. Composition mapping diagram with respect to phase state an the zero resistance temperatures, T¢.,,~o,for Bi-Sr-Ca-Cu-O thin films rapidly cooled after 1.0 h anneal at 820°C. Sr to Bi atomic ratio for the films was close to 1.0. Only for the two films with the lowest Ca + Sr contents was this ratio 0.7.
(2.0, 2.02, 1.02, 2.08) ~,oj OOlO I0
oo2
~
a
2 MgO E/
0o4 0o6
Jl
(2.0, 1.96, 1.74, 2.78)
i
1
b
appeared in the X R D spectra. In addition, broadening o f the (0 0 l) diffraction lines was observed when the film composition was m o v e d towards the 2223 point, see e.g. X R D spectra in fig. 3(b). This observed line broadening indicates disorder in the stacking sequence of CuO2 planes when the copper content exceeds the nominal 2212 one. All the films annealed at 820 °C displayed metallic normal state electrical resistivity and sharp superconducting transitions. Rapid cooling was found to raise the superconducting transition temperatures for all of them by 5 to 10 K. Figure 4 shows the normalized resistance versus temperature data measured for slowly and rapidly cooled samples of the film with composition Bi2.oSrzo2CaLoECUzosOx. The highest zero resistance temperature, To,zero, of 94 K was measured for this film composition in the case o f rapid cooling. The critical current density, Jc(T), for this film is shown in the insert to fig. 4. C o m m o n jc(4.2 K) values for other rapidly cooled films were found to be in the range 5 to 8 × l04 A / c m 2, the critical current density decreasing with increasing cooling rate. We interpret the decrease in critical current density upon increasing cooling rate as being due to an increased tendency to the formation of microcracks with cooling rate. Composition mapping with respect to T~..... achieved for rapidly cooled films of different compositions is shown in fig. 2. It is interesting to note that relatively high T~..... values, in the range of
M 0,7
(2.0, 2.02, 1.02, 2.08)
0,6 5
10
15 20
25 30 35 20(degrees)
40
45
• 0
•
50
I'-ID
0,5
• 0
o~
0
°'-2
0 • 0
Fig. 3. XRD spectra for rapidly cooled Bi2.oSr2.o2Cal.o2Cu2.osO (a) and Bi2.0Srt.96Ca.1 7 4 C u 2 . 7 8 O y ( b ) thin films.
x
a highly oriented 2212-phase material with the c-axis normal to the film surface. Neither substantial line position changes nor line shape variations were determined for the films with different cooling rates. The lattice constant, c, for single phase films was determined to range from 30.80 to 30.85/k. By moving away from the nominal 2212 composition in the composition diagram, traces o f impurities such as the 2201-phase, (Sr, Ca)2CuO3 and CaO
ODo
~0,4 ~--~0,3 [.-, 0,2 0,1 80
oO0 • 0 • 0 • 0
8 ~6
O0 •• 0 • 0 • ~.~1
f•
o•
~4 -.&2t
zb Temperature, 4b 6bK 80
0o ,
t
,
t
,
I
,
I
,
100 120 140 160 180 200 Temperature, K
Fig. 4. Normalized resistivity vs. temperature curves for a rapidly (1) and a slowly (2) cooled Bi2.oSr2.oECat.02Cu2.osOx composition thin film. Film thickness was 0.25 p.m. Critical current density, Jc, for the rapidly cooled film is shown.
B. Vengalis et aL / Synthesis of 2212 BSCCO thin films
90 K, were obtained for films with preferential 2212 structure, although the compositions were close to the 2223 one: The reflectivity spectra, R(2), for all our single phase films displayed well-defined plasma edges similar to the reflectivity spectra obtained from bulk 2212 single crystals [ 6 ]. The R (2) curves shown in fig. 5 by full and open circles were measured for rapidly and slowly cooled films of the same composition, i.e., they were cut from the same original metal alloy films of the composition Bi2.oSr2o2Cal.o2Cu2osO~, the surface quality of the two postannealed films being almost identical. Thus, contrary to differences in To.zerovalues for the two films, there are negligible effects of the different cooling rates upon the plasma edge position and, consequently, on the plasma frequency of free carriers, top. The fit to the experimental data, displayed as a full curve in fig. 5, was calculated using a Drude model for the optical reflectivity with parameters e~ = 2.5, top = 10300 c m - ~and carrier scattering rate l / T = 1600 cm -l. In fig. 5, the additional R(2) dependence represented by squares corresponds to the other rapidly cooled film having the composition Bi2.oSrl.96CaL74Cu2.7sOy with the same 2212-1ike crystal structure but substantially higher Cu content if compared to the nominal 2212 composition. A significant plasma edge shift to shorter wavelengths is 0.6
• • - fast cooling 0.5
• ~= • • • A
o - slow cooling
°0.4 ~.
9 0.3 n'o. 2
•ml
o.~ 0
;~" I
I
(2.0, 2.02,1.o2, zos)- i, 2 (zo, 1.o6, ~.74, 2.78)- 3 i
I
,
I
,
2 3 Wavelength, 14m
Fig. 5. Optical reflectivity spectra R ( 2 ) for rapidly ( 1 ) and slowly ( 2 ) cooled films o f the Bi2.oSr2.o2CaLo2Cu2.osO= composition and rapidly cooled f i l m ( 3 ) o f the Bi2.oSr=.96CaL74CU2.TsOy composition. Full curve 4 corresponds to a D r u d e fit w i t h characteristic energy parameters, oJp= 10300 cm - 1, [ , = 1600 cm - ~ and ~o = 2.5.
67
clearly observed with increasing Cu content.
4. Discussion
The rapid cooling of the films strongly influences both the resistivity versus temperature dependence and the Tc..... values for films of different compositions, but no change in the plasma frequency, top, is deduced from the reflectance data. On the other hand, the observed shift of top to short wavelengths with increasing Cu content is not accompanied by significant change in To. Compare curves 1 and 3 in fig. 5. To understand the origin of these features it is necessary to analyze factors which can influence the measured values of resistivity (p) and the optical reflectivity (R(2)). For c-axis oriented films, both p and R (2) give information about free carrier motion parallel to the CuO2 layers. However, p, when compared to R(2), is much more sensitive to inhomogeneities, such as nonuniform doping, grain boundaries or microcracks, which can be generated during the cooling process. For polycrystalline 2212-phase material, interpretation of resistivity or Hall measurement data becomes even more complicated because of the great anisotropy (pi/pll~ l04 [8] ) of individual grains and grain boundaries. Optical reflectivity measurements performed for LaBaCuO and YBaCuO layered superconductors showed surprising stability of plasma edge spectral position with averaged doping or oxygen content change [ 9 ]. The optical reflectivity data can be interpreted assuming an almost fixed local carrier density in conducting CuO2 planes. The stabilization of free carrier density in the CuO2 planes of the layered Cu oxide superconductors has been explained by charge transfer from the so-called charge reservoir layers [ 10 ]. There are indications that BiO planes in the 2212phase act as a charge reservoir for the conducting CuO2 planes [3 ]. Thus, if dopants are introduced, it seems much easier to induce changes in superconducting properties by varying charge in the BiO layers rather than by changing the charge density in the CuO2 planes. Recently, the plasma frequency increase with n has
68
B. Vengalis et al. / Synthesis of 2212 BSCCO thin films
been found for Bi oxide superconductors hcop/x/'~oo=0.83, 1.03 a n d 1.21 eV for n = 1, 2 a n d 3, respectively [ 7 ]. If there is no strong effective mass dependence on n, it seems therefore that the carrier density corresponding to a single CuO2 layer remains nearly the same for different superconducting crystal structures. C o n t r a r y to ref. [7], the optical reflectivity m e a s u r e m e n t s in this work were perf o r m e d for films having the same preferential 2212structure. F r o m the obtained similar cop increase with Cu content. C o m p a r e curves 1 and 3 in fig. 3. It is likely that cop does c o r r e s p o n d to average CuO2 layer density in the material. It can be d e d u c e d from the c o m p o s i t i o n d i a g r a m in fig. 2 that the highest T~..... values above 90 K can be o b t a i n e d only in a relatively n a r r o w c o m p o s i t i o n region. According to the diagram, these compositions are shifted from the (2, 2, n - 1, n) composition line to the higher BiOo.15 a n d CuO content side. The present thin film data are in good agreement with similar Tc..... investigations p e r f o r m e d for rapidly cooled bulk single crystals [5]. We believe that slow a n d r a p i d cooling o f the B i S r - C a - C u - O films from annealing t e m p e r a t u r e can induce differences in their superconducting properties in two different ways. Slowly cooled material does become o v e r d o p e d by excess oxygen [1,3,4] a n d r a p i d cooling o f the films from the annealing temperature can reduce oxygen content owing to a thermally activated oxygen desorption process. In addition, we believe that enclosing the film surfaces with MgO plates, as shown in fig. 1, prevents a b s o r p t i o n o f excess oxygen during the cooling stage. Secondly, we stress the different conditions for the super ( m o d u l a t i o n ) structure formation in slowly and rapidly cooled 2212 material. The origin o f the structure is related to a m i s m a t c h in i n t e r a t o m i c distances between a t o m s in the CuO2 a n d BiO layers [ 11 ]. The a p p e a r a n c e o f strain fields in the structure can lead to a different diffusion-controlled spatial oxygen redistribution inside the charge reservoir BiO layers and, as a consequence, can lead to nonuniformity o f the superconducting properties o f the material. 5. Conclusions Cooling rate a n d c o m p o s i t i o n variations from the
n o m i n a l 2212 composition in B i - S r - C a - C u - O thin films having the 2212 structure are o f great importance for achieving Tc..... values above the 90 K range. R a p i d cooling from annealing t e m p e r a t u r e ( 8 2 0 ° C ) and slightly lower S r + C a content in the films cause Tc..... increase. Better knowledge concerning the interaction between point defects, such as excess oxygen, oxygen vacancies and substituted a t o m s in strained BiO planes, seems to be necessary to u n d e r s t a n d the origin o f unusually high Tc..... values o b t a i n e d for the thin films.
Acknowledgement This work was s u p p o r t e d by the Swedish Natural Science Research Council a n d the Swedish Research Council for the Engineering Sciences within the project for thin film synthesis and surface spectroscopy o f high-To superconductors.
References [ 1] R.G. Buckley, J.L. Tallon, I.W.M. Brown, M.R. Presland, N.E. Flower, P.W. Gilberd, M. Bowden and N.B. Milestone, Physica C 156 (1988) 629. [2] N. Knauf, J. Harnischmacher, R. Mfiller, R. Borowski, B. Roden and D. Wohlleben, Physica C 173 ( 1991 ) 414. [3] M. Nagoshi, T. Suzuki, Y. Fukuda, K. Terashima, Y. Nakanishi, M. Ogita, A. Tokiwa, Y. Syono and M. Tachiki, Phys. Rev. B 43 ( 1991 ) 10445. [4] W.A. Groen, D.M. de Leeuw and L.F. Feiner, Physica C 165 (1990) 55. [ 5 ] V.A. Murashov, S.N. Gordeev, I.S. Dubenko, V.M. lonov, A.Y. Martinkin, A.V. Titov, Y.V. Trofimov and A.M. Frolov, Supercond. Phys. Chem. Techn. 3 (1990) 963 (in Russian ). [6] D. Huang, B. Zhu, H. Ming, G. Gu, W. Cai and Y. Ruan, Appl. Phys. Lett. 58 (1991) 1098. [7] K. Hirochi, K. Setsune, S. Hayashi, K. Mizuno, T. Matsushima, Y. Ichikawa, H. Adachi and K. Wasa, Physica B 165&166 (1990) 1255. [ 8 ] S. Martin, A.T. Fiori, R.M. Fleming, L.F. Schneemeyer and J.V. Waszczak, Phys. Rev. B 41 (1990) 846. [ 9 ] S. Tajima, Electronic States of High-ToCuprates Investigated by Optical Measurements Proc. 3rd Inter. Symp. of Superconductivity, November 1990, Sendal, Japan. [ 10] R.J. Cava, A.W. Hewat, E.A. Hewat, B. Batlogg,M. Marezio, K.M. Rabe, J.J. Krajewski, W.F. Peck Jr. and UW. Rupp Jr., Physica C 165 (1990) 419. [11] H.W. Zandbergen, W.A. Groen, A. Smit and G. van Tendeloo, Physica C 168 (1990) 426.