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Phase analysis of BiCaSrCuO superconducting films at different growth temperatures from KCl supercooled solutions
j...... oF C R Y S T A L
Journal of Crystal Growth 128 (1993) 729-733 North-Holland
GROWTH
Phase analysis of Bi-Ca-Sr-Cu-O superconducting films at different growth temperatures from KC1 supercooled solutions Kanwal K. Raina, S. Narayanan and R.K. Pandey Center for Electronic Materials, Department of Electrical Engineering, Texas A&M University, College Station, Texas 77843-3128, USA
Films of 80 K phase of BiCaSrCu-oxide superconductor referred as BCSCO have been grown from KCI-Bi2CaSr2Cu2Os+ x solutions at different growth temperature regimes by the LPE process. Twin-free single crystal substrates of NdGaO 3 with (001) orientation are used for growing these films. The temperature range of 850-830°C is found to be the most favorable region for the formation of the 2122 phase of BCSCO. Above 850°C up to the peritectic melting point of the 2122 phase (885°C), formation of the 2122 phase is highly suppressed with the separation of Bi2CaSrzCu208+ x into 2021, calcium copper oxide (CazCuO 3 and CaCu20 3) and other non-superconducting phases. X-ray powder diffraction analysis is used to identify different phases of BCSCO films. Morphological examination of the films is carried out by scanning electron microscopy. The onset of transition of the 2122 phase films is observed at 90 K and zero resistance is reached at 83 K. Post-growth annealing has an adverse effect on the superconducting properties of the films.
1. Introduction
Since the discovery of high-Tc superconducting B i - C a - S r - C u - O class of materials [1-3], much effort has been devoted to preparing high quality BCSCO thin films. The BCSCO system consists mainly of three superconducting phases, i.e., (2021), (2122) and (2223) with Tc's of 10, 80 and 110 K, respectively. Several techniques such as sputtering [4-6], MOCVD [7], MBE [8,9] and electron beam evaporation [10,11] have been extensively used to grow high-Tc films of BCSCO. However, most of the BCSCO films grown from these techniques require a post-growth annealing treatment to form the superconducting phase of 2122. Apparently, because of the accompanying chemical reaction between the substrate and the film as a result of post-growth annealing treatment, there is a deterioration in the quality of the film. This limitation can be overcome by using the LPE film growth method. Recently, we have established the possibility of growing high quality
epitaxial films of BCSCO [12,13] using NdGaO 3 as the substrate material. Soon after the pioneering work [14] that led to bulk crystal growth of BizCaSrzCu2Os+ x phase from KCI supercooled solutions, there have been many reports on the bulk [15-19] and thin film growth [20-23] of this superconducting material. As most of the reports on bulk crystal growth suggest, the formation of 2122 phase takes place by slow cooling of the KC1-BCSCO solution in the temperature range of around 950-750°C, depending on the initial charge composition. Considering this broad growth temperature range, it has not been possible to point out the most favorable temperature region where the 2122 phase is formed. Recently there have been a few attempts [24,25] to explain the melting and reformation behavior of the BizCaSr2Cu2Os+ x phase. These studies have been carried out just below the peritectic melting point of the 2122 phase (885°C) of BCSCO. No such study has, however, been carried out on the LPE grown films. The
0022-0248/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
730
K.K. Raina et al. / Phase analysis of Bi-Ca-Sr-Cu-O superconducting films
purpose of the present study is to analyze the phase formations of Bi2CaSr2Cu208+ x from just below its peritectic melting point to the temperature where complete reformation of the 2122 phase takes place.
ples in warm distilled water. The thickness of the films, which is dependent on the cooling rate of the solution, varied from 0.5 to 1 tzm.
3. Results and discussion 2. Experimental procedure For the preparation of the BCSCO charge, high purity grade powders of B i 2 0 3 (99.9%), CaCO 3 (99.9%), SrCO 3 (99.999%) and CuO (99.9%) were thoroughly mixed in the atomic ratios of 2 : 1 : 2 : 2 (Bi : Ca : Sr : Cu). This was followed by calcination steps at 800°C (twice) and 850°C (once) for 12 h each. The so calcined charge was then pelletized by uniaxial cold pressing at 70 MPa and then annealed at 850°C for 100 h in air. About 10 g of this annealed charge was finely ground and melted at 900°C for 1 h in a 50 cm 3 platinum crucible. Then 40 g of KC1 powder was added at the top of the solidified mass. This ratio of 4 : 1 between KC1 and BCSCO was maintained for all the experiments. The reformation process of BizCaSrzCu208+ x below its peritectic melting point using LPE film processing was accomplished by slow cooling in the following temperature regimes: (i) 880-860°C; (ii) 860-840°C, and (iii) 850-830°C. After sequentially loading the charge and KC1 flux in a platinum crucible, the furnace was heated to about 920°C and left there for 12 h to achieve equilibrium of the solution. The solution was then precooled in one hour to the maximum temperature level of individual growth regimes as described already. The solution was allowed to stay at this t e m p e r a t u r e for 5 h. Then a N d G a O 3 substrate was lowered to a depth of about 5 m m within the solution. The growth period was varied from 7 to 15 h. The seed rotation for all the experiments was set at 40 rpm. After the completion of the film growth the substrates were slowly withdrawn from the solution and spun at about 200 rpm to remove the molten KCI adhering to the film. The samples were then fast cooled to about 350°C in about 5 - 1 0 s and held there for 5 min before exposing them to room temperature. The residual flux from films was removed by washing the sam-
X-ray powder diffraction patterns were recorded on a Scintag PAD5 using C u K a radiation. Fig. 1 shows a diffraction pattern of the film grown in the temperature range of 880-860°C, i.e., just below the peritectic melting point of the 2122 phase. The figure reveals the presence of superconducting phases of 2122 and 2021. In addition, non-superconducting phases of CaCu 2 0 3 and CazCuO 3 are also identified. The unmarked high intensity peaks at about 30.9 ° and 33 ° may be attributed to the presence of c o p p e r strontium-oxide but the exact chemical composition is yet to be determined. The other small unmarked peaks at 16.3 °, 18.2 ° and 29.1 ° can possibly be due to traces of (SrCa)CuO 2 phase as has been observed in the literature at this growth temperature [24]. As the growth t e m p e r a t u r e is lowered, formation of the 2122 phase is enhanced. The films which are grown in the temperature regime of 860-840°C show the elimination of 2021 phases, but the non-superconducting phases observed in the growth regime of 880860°C still persist, although in lesser amount. As the growth temperature is further lowered, i.e., when the films are grown in the t e m p e r a t u r e
2.
--
+
,
lO
20
~
0
30
40
50
2-theta (degrees)
Fig. 1. X-ray powder diffraction pattern of the film grown in the temperature range of 880-860°C. The symbols are identified as: ( * ) 2122; (+) CaCu203; (×) Ca2CuO3; (©) 2021; (* ,)NdGaO 3.
K K. Raina et al. / Phase analysis of Bi-Ca-Sr-Cu-O superconducting films
731
1.2
~,1.0
a
~
o.8
0.6
-2d /0
(
i 20
30
;0
~o
2-theta (degrees)
Fig. 2. X-ray powder diffraction pattern of the film grown in the temperature range of 850-830°C. The peak marks are identified as: ( * ) 2122 phase; ( * * NdGaO3); (#) unidenti fled.
regime of 850-830°C, there is a complete elimination of 2021 and non-superconducting phases. Fig. 2 shows the X-ray powder diffraction pattern of a film grown in this temperature range. All the major peaks in the figure correspond to (001) planes of 2122 phase of BCSCO indicating a highly c-axis oriented epitaxial film. The unidentified peaks at 15.5° and 42° could be attributed to the presence of superconducting phases having slightly different chemical composition than the 2122 phase• This argument is made considering the fact that SEM examination of these films does not show any two distinct regions of the surfaces morphology. The four-point probe resistance measurement was carried out using gold contacts for attaching the current and voltage leads. Fig. 3 shows the resistance behavior of the films grown in the temperature regimes of 880-860, 860-840 and 850-830°C. The zero resistance for the 880-860 and 860-840°C growth regimes is reached at 10 and 45 K, respectively• The temperature regime of 850-830°C is observed to be the most favorable for 2122 phase formation considering the fact that zero resistance is reached at 83 K. These results supplement the findings of X-ray powder diffraction studies described earlier in the text. It has been shown that the 2122 phase decomposes on melting [17,24,25]. Our results conclusively prove that the 2122 phase completely reforms upon return to a temperature (which, in the pre-
/ 0.2 ~ 0.0
0
50 Temperature
100
(K)
150
Fig. 3. Normalized resistance versus temperature plots for the films grown in the temperature ranges of (a) 880-860°C, (b) 860-840°C and (c) 850-830°C.
sent case is 850°C) well below the peritectic melting point of the material. The surface morphology of the grown films was examined with a JEOL 6400 scanning electron microscope• Fig. 4 shows surface morphology of the film grown in the temperature range of 880-860°C. The film shows a typical random crystallite morphology. Fig. 5 depicts the surface morphology of a film grown in the temperature range of 850-830°C. The smoothness of the surface as observed in this figure is an indication of the epitaxial nature of the film. No distinct regions different from the general surface of these films could be observed in fig. 5. Thus the two
Fig. 4. SEM of the film grown in the temperature range of 880-860°C.
732
KK. Raina et al. / Phase analysis of Bi-Ca-Sr-Cu-O superconducting films during annealing it is exposed to air in absence of a medium (like molten KC1) to prevent Ga diffusion. Fig. 6 shows the resistance versus temperature plot of the 2122 phase film annealed at 850°C for 2 h. The nature of the curve is semiconducting, suggesting thereby that there is complete degradation of the superconducting 2122 phase.
4. Conclusion
Fig. 5. Surface morphologyof the 2122 phase grown between 850 and 830°C. unidentified peaks of fig. 2 could not be related to any non-superconducting phase. Post-growth annealing has an adverse effect on the superconducting properties of the films shown in fig. 5. This is attributed to the diffusion of Ga atoms into the film. This is confirmed by E D A X analysis. Recently, a similar result has also been observed for T I - B a - C a - C u - O (2122) superconducting films grown on N d G a O 3 substrate [26]. It is interesting to note that no diffusion of Ga in the film takes place during crystallization. The reason could be that the crystallization occurs within the melt considering of a supersaturated solution of KCI and BCSCO during which the film is not exposed to air atmosphere, whereas
10"
Growth of high quality epitaxial films of the 2122 phase has been optimized from different growth temperature regimes by the LPE method. X-ray powder diffraction analysis, resistance measurement and SEM examination of the films suggest that almost 100% reformation of the 2122 phase takes place at around 850°C after the solution is cooled down from 920°C. Growth temperatures higher than 850°C and up to the peritectic melting point of the 2122 phase favor the formation of 2021, calcium cuprate and other non-superconducting phases. The onset of the resistance transition for 2122 phase is observed at 90 K and zero resistance is reached at 83 K.
Acknowledgment The authors acknowledge the support provided for this work by the National Aeronautics and Space Administration (NASA), Grant No. NAGW-1590; and NASA's Center for Space Power, a division of Texas Engineering Experiment Station (TEES), Grant No. NAGW-1194. We sincerely thank Dr. C.D. Brandle of A T & T Bell Laboratories for providing high quality single crystal N d G a O 3 substrates and Professor A. Clearfield of Texas A & M University for the X-ray diffraction data.
References 0
0
, 50
-
, 100
•
, 150
-
, 200
"
, 250
" 300
Temperature ( K )
Fig. 6. Normalized resistance versus temperature plot of the film annealed at 850°Cfor 2 h.
[1] H. Maeda, Y. Tanaka, M. Fukutomi and T. Asano, Japan. J. Appl. Phys. 27 (1988) L209. [2] M.A. Subramanian, C.C. Torardi, J.C. Calabrese, J. Gopalakrishnan, K.J. Morrissey, T.R. Askew, R.B. Flip-
K.K. Raina et al. / Phase analysis of Bi-Ca-Sr-Cu-O superconducting films pen, U. Chowdhary and A.W. Sleight, Science 239 (1988) 1015. [3] J.M. Tarascon, Y. LePage, P. Barboux, B.G. Bagley, L.H. Greene, W.R. Mckinnon, G.W. Hull, M. Giroud and D.M. Hwang, Phys. Rev. B 37 (1988) 9382. [4] S. Miura, A. Nakai, Y. Shimakawa, T. Yoshitake, Y. Miyasaka and N. Shohata, Appl. Phys. Letters 54 (1989) 1474. [5] K. Nakumara, J. Sato, M. Kaise and K. Ogawa, Japan. J. Appl. Phys. 28 (1989) L437. [6] Y. Hakuraku, S. Higo and T. Ogushi, Appl. Phys. Letters 55 (1989) 1556. [7] J. Zhang, J. Zhao, H.O. Marcy, M. Tonge, B.W. Wessels, TJ. Marks and C.R. Kannewurf, Appl. Phys. Letters 54 (1989) 1166. [8] Y. Nakayama, I. Tsukada, A. Maeda and K. Uchinokura, Japan. J. Appl. Phys. 28 (1989) L1809. [9] H.C. Li, G. Linker, F. Ratzel, R. Smithey and J. Greek, Appl. Phys. Letters 52 (1988) 1089. [10] K. Kuroda, M. Mukaida, M. Yamamoto and S. Miyazawa, Japan. J. Appl. Phys. 27 (1988) L629. [11] J. Steinbeek, B.Y. Tsaur, A.C. Anderson and A.J. Strauss, Appl. Phys. Letters 54 (1989) 466. [12] S. Narayanan, K.K. Raina, R.K. Pandey and C.D. Brandie, Mater. Letters 11 (1991) 212. [13] K.K. Raina, S. Narayanan and R.K. Pandey, J. Mater. Res. 7 (1992) 2303.
733
[14] L.F. Schneemeyer, R.B. Van Dover, S.H. Glarum, S.A. Sunshine, R.M. Fleming, B. Batlogg, T. Siegrist, J.H. Marshall, J.V. Waszczak and L.W. Rupp, Nature 332 (1988) 422. [15] A. Katsui, Japan. J. Appl. Phys. 27 (1988) L844. [16] K.W. Goeking and R.K. Pandey, Mater. Letters 8 (1989) 432. [17] S.C. Gadkari, K.P. Muthe, K.D. Singh, S.C. Sabharwal and M.K. Gupta, J. Crystal Growth 102 (1990) 685. [18] P.D. Han and D.A. Payne, J. Crystal Growth 104 (1990) 201. [19] S. Kishida, H. Tokutaka, S. Nakanisha, H. Fujimoto, K. Neshimori, N. Ishihara, Y. Watanabe and W. Futo, J. Crystal Growth 99 (1990) 937. [20] R.S. Lui, Y.T. Huang, P.T. Wu and J.J. Chu, Japan. J. Appl. Phys. 27 (1988) L1470. [21] G. Balestrino, R.D. Leo, M. Marinelli, E. Milani, A. Paoletti and P. Paroli, Physica C 162-164 (1989) 115. [22] C.S. Yang and A.S. Yue, J. Crystal Growth 99 (1990) 951. [23] J.S. Shin and H. Ozaki, Physica C 173 (1991) 93. [24] H. Komatsu, Y. Kato, S. Miyashita, T. Inoue and S. Hayashi, Physica C 190 (1991) 14. [25] M.F. Garbauskas, R.H. Arendt, J.D. Jorgenson and R.L. Hitterman, Appl. Phys. Letters 58 (1991) 2987. [26] K.H. Young, G.V. Negrete, M.M. Eddy and E.J. Smith, Japan. J. Appl. Phys. 30 (1991) L1359.
Journal of Crystal Growth 128 (1993) 729-733 North-Holland
GROWTH
Phase analysis of Bi-Ca-Sr-Cu-O superconducting films at different growth temperatures from KC1 supercooled solutions Kanwal K. Raina, S. Narayanan and R.K. Pandey Center for Electronic Materials, Department of Electrical Engineering, Texas A&M University, College Station, Texas 77843-3128, USA
Films of 80 K phase of BiCaSrCu-oxide superconductor referred as BCSCO have been grown from KCI-Bi2CaSr2Cu2Os+ x solutions at different growth temperature regimes by the LPE process. Twin-free single crystal substrates of NdGaO 3 with (001) orientation are used for growing these films. The temperature range of 850-830°C is found to be the most favorable region for the formation of the 2122 phase of BCSCO. Above 850°C up to the peritectic melting point of the 2122 phase (885°C), formation of the 2122 phase is highly suppressed with the separation of Bi2CaSrzCu208+ x into 2021, calcium copper oxide (CazCuO 3 and CaCu20 3) and other non-superconducting phases. X-ray powder diffraction analysis is used to identify different phases of BCSCO films. Morphological examination of the films is carried out by scanning electron microscopy. The onset of transition of the 2122 phase films is observed at 90 K and zero resistance is reached at 83 K. Post-growth annealing has an adverse effect on the superconducting properties of the films.
1. Introduction
Since the discovery of high-Tc superconducting B i - C a - S r - C u - O class of materials [1-3], much effort has been devoted to preparing high quality BCSCO thin films. The BCSCO system consists mainly of three superconducting phases, i.e., (2021), (2122) and (2223) with Tc's of 10, 80 and 110 K, respectively. Several techniques such as sputtering [4-6], MOCVD [7], MBE [8,9] and electron beam evaporation [10,11] have been extensively used to grow high-Tc films of BCSCO. However, most of the BCSCO films grown from these techniques require a post-growth annealing treatment to form the superconducting phase of 2122. Apparently, because of the accompanying chemical reaction between the substrate and the film as a result of post-growth annealing treatment, there is a deterioration in the quality of the film. This limitation can be overcome by using the LPE film growth method. Recently, we have established the possibility of growing high quality
epitaxial films of BCSCO [12,13] using NdGaO 3 as the substrate material. Soon after the pioneering work [14] that led to bulk crystal growth of BizCaSrzCu2Os+ x phase from KCI supercooled solutions, there have been many reports on the bulk [15-19] and thin film growth [20-23] of this superconducting material. As most of the reports on bulk crystal growth suggest, the formation of 2122 phase takes place by slow cooling of the KC1-BCSCO solution in the temperature range of around 950-750°C, depending on the initial charge composition. Considering this broad growth temperature range, it has not been possible to point out the most favorable temperature region where the 2122 phase is formed. Recently there have been a few attempts [24,25] to explain the melting and reformation behavior of the BizCaSr2Cu2Os+ x phase. These studies have been carried out just below the peritectic melting point of the 2122 phase (885°C) of BCSCO. No such study has, however, been carried out on the LPE grown films. The
0022-0248/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
730
K.K. Raina et al. / Phase analysis of Bi-Ca-Sr-Cu-O superconducting films
purpose of the present study is to analyze the phase formations of Bi2CaSr2Cu208+ x from just below its peritectic melting point to the temperature where complete reformation of the 2122 phase takes place.
ples in warm distilled water. The thickness of the films, which is dependent on the cooling rate of the solution, varied from 0.5 to 1 tzm.
3. Results and discussion 2. Experimental procedure For the preparation of the BCSCO charge, high purity grade powders of B i 2 0 3 (99.9%), CaCO 3 (99.9%), SrCO 3 (99.999%) and CuO (99.9%) were thoroughly mixed in the atomic ratios of 2 : 1 : 2 : 2 (Bi : Ca : Sr : Cu). This was followed by calcination steps at 800°C (twice) and 850°C (once) for 12 h each. The so calcined charge was then pelletized by uniaxial cold pressing at 70 MPa and then annealed at 850°C for 100 h in air. About 10 g of this annealed charge was finely ground and melted at 900°C for 1 h in a 50 cm 3 platinum crucible. Then 40 g of KC1 powder was added at the top of the solidified mass. This ratio of 4 : 1 between KC1 and BCSCO was maintained for all the experiments. The reformation process of BizCaSrzCu208+ x below its peritectic melting point using LPE film processing was accomplished by slow cooling in the following temperature regimes: (i) 880-860°C; (ii) 860-840°C, and (iii) 850-830°C. After sequentially loading the charge and KC1 flux in a platinum crucible, the furnace was heated to about 920°C and left there for 12 h to achieve equilibrium of the solution. The solution was then precooled in one hour to the maximum temperature level of individual growth regimes as described already. The solution was allowed to stay at this t e m p e r a t u r e for 5 h. Then a N d G a O 3 substrate was lowered to a depth of about 5 m m within the solution. The growth period was varied from 7 to 15 h. The seed rotation for all the experiments was set at 40 rpm. After the completion of the film growth the substrates were slowly withdrawn from the solution and spun at about 200 rpm to remove the molten KCI adhering to the film. The samples were then fast cooled to about 350°C in about 5 - 1 0 s and held there for 5 min before exposing them to room temperature. The residual flux from films was removed by washing the sam-
X-ray powder diffraction patterns were recorded on a Scintag PAD5 using C u K a radiation. Fig. 1 shows a diffraction pattern of the film grown in the temperature range of 880-860°C, i.e., just below the peritectic melting point of the 2122 phase. The figure reveals the presence of superconducting phases of 2122 and 2021. In addition, non-superconducting phases of CaCu 2 0 3 and CazCuO 3 are also identified. The unmarked high intensity peaks at about 30.9 ° and 33 ° may be attributed to the presence of c o p p e r strontium-oxide but the exact chemical composition is yet to be determined. The other small unmarked peaks at 16.3 °, 18.2 ° and 29.1 ° can possibly be due to traces of (SrCa)CuO 2 phase as has been observed in the literature at this growth temperature [24]. As the growth t e m p e r a t u r e is lowered, formation of the 2122 phase is enhanced. The films which are grown in the temperature regime of 860-840°C show the elimination of 2021 phases, but the non-superconducting phases observed in the growth regime of 880860°C still persist, although in lesser amount. As the growth temperature is further lowered, i.e., when the films are grown in the t e m p e r a t u r e
2.
--
+
,
lO
20
~
0
30
40
50
2-theta (degrees)
Fig. 1. X-ray powder diffraction pattern of the film grown in the temperature range of 880-860°C. The symbols are identified as: ( * ) 2122; (+) CaCu203; (×) Ca2CuO3; (©) 2021; (* ,)NdGaO 3.
K K. Raina et al. / Phase analysis of Bi-Ca-Sr-Cu-O superconducting films
731
1.2
~,1.0
a
~
o.8
0.6
-2d /0
(
i 20
30
;0
~o
2-theta (degrees)
Fig. 2. X-ray powder diffraction pattern of the film grown in the temperature range of 850-830°C. The peak marks are identified as: ( * ) 2122 phase; ( * * NdGaO3); (#) unidenti fled.
regime of 850-830°C, there is a complete elimination of 2021 and non-superconducting phases. Fig. 2 shows the X-ray powder diffraction pattern of a film grown in this temperature range. All the major peaks in the figure correspond to (001) planes of 2122 phase of BCSCO indicating a highly c-axis oriented epitaxial film. The unidentified peaks at 15.5° and 42° could be attributed to the presence of superconducting phases having slightly different chemical composition than the 2122 phase• This argument is made considering the fact that SEM examination of these films does not show any two distinct regions of the surfaces morphology. The four-point probe resistance measurement was carried out using gold contacts for attaching the current and voltage leads. Fig. 3 shows the resistance behavior of the films grown in the temperature regimes of 880-860, 860-840 and 850-830°C. The zero resistance for the 880-860 and 860-840°C growth regimes is reached at 10 and 45 K, respectively• The temperature regime of 850-830°C is observed to be the most favorable for 2122 phase formation considering the fact that zero resistance is reached at 83 K. These results supplement the findings of X-ray powder diffraction studies described earlier in the text. It has been shown that the 2122 phase decomposes on melting [17,24,25]. Our results conclusively prove that the 2122 phase completely reforms upon return to a temperature (which, in the pre-
/ 0.2 ~ 0.0
0
50 Temperature
100
(K)
150
Fig. 3. Normalized resistance versus temperature plots for the films grown in the temperature ranges of (a) 880-860°C, (b) 860-840°C and (c) 850-830°C.
sent case is 850°C) well below the peritectic melting point of the material. The surface morphology of the grown films was examined with a JEOL 6400 scanning electron microscope• Fig. 4 shows surface morphology of the film grown in the temperature range of 880-860°C. The film shows a typical random crystallite morphology. Fig. 5 depicts the surface morphology of a film grown in the temperature range of 850-830°C. The smoothness of the surface as observed in this figure is an indication of the epitaxial nature of the film. No distinct regions different from the general surface of these films could be observed in fig. 5. Thus the two
Fig. 4. SEM of the film grown in the temperature range of 880-860°C.
732
KK. Raina et al. / Phase analysis of Bi-Ca-Sr-Cu-O superconducting films during annealing it is exposed to air in absence of a medium (like molten KC1) to prevent Ga diffusion. Fig. 6 shows the resistance versus temperature plot of the 2122 phase film annealed at 850°C for 2 h. The nature of the curve is semiconducting, suggesting thereby that there is complete degradation of the superconducting 2122 phase.
4. Conclusion
Fig. 5. Surface morphologyof the 2122 phase grown between 850 and 830°C. unidentified peaks of fig. 2 could not be related to any non-superconducting phase. Post-growth annealing has an adverse effect on the superconducting properties of the films shown in fig. 5. This is attributed to the diffusion of Ga atoms into the film. This is confirmed by E D A X analysis. Recently, a similar result has also been observed for T I - B a - C a - C u - O (2122) superconducting films grown on N d G a O 3 substrate [26]. It is interesting to note that no diffusion of Ga in the film takes place during crystallization. The reason could be that the crystallization occurs within the melt considering of a supersaturated solution of KCI and BCSCO during which the film is not exposed to air atmosphere, whereas
10"
Growth of high quality epitaxial films of the 2122 phase has been optimized from different growth temperature regimes by the LPE method. X-ray powder diffraction analysis, resistance measurement and SEM examination of the films suggest that almost 100% reformation of the 2122 phase takes place at around 850°C after the solution is cooled down from 920°C. Growth temperatures higher than 850°C and up to the peritectic melting point of the 2122 phase favor the formation of 2021, calcium cuprate and other non-superconducting phases. The onset of the resistance transition for 2122 phase is observed at 90 K and zero resistance is reached at 83 K.
Acknowledgment The authors acknowledge the support provided for this work by the National Aeronautics and Space Administration (NASA), Grant No. NAGW-1590; and NASA's Center for Space Power, a division of Texas Engineering Experiment Station (TEES), Grant No. NAGW-1194. We sincerely thank Dr. C.D. Brandle of A T & T Bell Laboratories for providing high quality single crystal N d G a O 3 substrates and Professor A. Clearfield of Texas A & M University for the X-ray diffraction data.
References 0
0
, 50
-
, 100
•
, 150
-
, 200
"
, 250
" 300
Temperature ( K )
Fig. 6. Normalized resistance versus temperature plot of the film annealed at 850°Cfor 2 h.
[1] H. Maeda, Y. Tanaka, M. Fukutomi and T. Asano, Japan. J. Appl. Phys. 27 (1988) L209. [2] M.A. Subramanian, C.C. Torardi, J.C. Calabrese, J. Gopalakrishnan, K.J. Morrissey, T.R. Askew, R.B. Flip-
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