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Polytypoidic superlattices in Y-Ba-Cu-O thin films
PhysicaC 171 (1990) 14-18 North-Holland
Polytypoidic superlattices in Y-Ba-Cu-O thin films R. Ramesh, D.M. Hwang, T.S. R a v i a n d A. I n a m Bellcore, Red Bank, NJ 07701, USA
X.D. W u a n d T. V e n k a t e s a n Rutgers University, Piscataway, NJ 08854, USA Received 4 August 1990
We report, for the first time, the observation of natural, polytypoidic superlattice structures in pulsed laser deposited superconducting Y - B a - C u - O thin films. These structures correspond to ordered stacking sequences of the "123", "124" and "224" cationic ratios. Such structurally coherent, natural superlattice structures, either in the stable or metastable form, may be an attractive alternative for the artificial superlaUices suggested for novel electronic applications. The results suggest the existence of a family of polytypoidic phases with different cationic compositions in the Y - B a - C u - O phase diagram.
1. Introduction There has been considerable interest in the fabrication of structurally stable heterostructures and superlattices that have novel electronic properties, e.g., junction devices, from the newly discovered cuprate superconductors [ 1,2]. Typically, the heterostructures (or superlattices) consist of a layer of Y B a - C u - O and a layer of either a superconducting or a non-superconducting lattice matched material, such as PrBa2Cu307_x. The structural and electrical transport results obtained on such artificially layered materials have been very encouraging [3]. It is, however, equally important to explore the possibility of growing natural superlattice structures using the polytypoidic variations in the stacking sequences observed in the Y - B a - C u - O system. Three of the possible polytypoidic variants are shown in fig. la-c. While the "123" and "248" structures have been synthesized and studied in bulk form, the "224" structure has been observed only as a growth defect [ 3,4 ]. Recently, we presented the results of high-resolution electron microscopy ( H R E M ) studies on Y - B a - C u - O films on single crystal MgO substrates [ 3 ]. In this study, the "224" structure was observed to exist in extended regions for the first time. It has, however, not yet been syn-
BaO CuO2 Y CuO2 BaO
CuO C
Bat) CuO2 Y CuO2 BaO
CuO CC CuO
Bat) CuO2
y CuO2 BaO
CuO ~ doping YO CYC planes CuO
BaO CuO2 Y CuO2 BaO
Bat) CuO2 Y CuO2 BaO
CuO2 y CuO2
YBa2Cu307
YBa2Cu408
Y2Ba2Cu409
• 123"
"124" or "24g"
Tc-91K
Tc-81K
Perovskite Blocks, P
BaO Perovskite Blocks
BaO
"224"
Tc =7
Fig. 1. (a-c) Schematic illustrations of the "123", "248" and "224" structures. Note that the perovskite block, P, is common to all the three structures while the layers connecting two such perovskite blocks are different for each structure.
thesized in bulk form and its superconducting properties are unknown. Among the several types of structural defects, the polytypoidic stacking faults are prominent in thin films due to local fluctuations in the composition during deposition. It may be noted that in all three structures illustrated in fig. 1a-c, the
0921-4534/90/$03.50 © 1990 - Elsevier Science Publishers B.V. (North-Holland)
R. Ramesh et aL / Polytypoidic superlattices in YBCO thin films
"perovskite block" is common, while the planes between two "perovskite" blocks change from one polytypoid to another. The superconducting transition temperature, To, is sensitive to the changes illustrated in this figure. The simplest polytypoidic superstructures consist of repeat units of " 1 2 3 " + " 1 2 4 " that is now identified as the "247" phase [5], or " 1 2 4 " + " 1 2 4 " which is the "248" structure [6 ]. The crystal structure and the nature of the superstructure are defined by the planes that intercalate between two perovskite blocks. Clearly, long-period superstructures comprised of various combinations of these individual polytypoids can be obtained by suitable modifications in the repeat sequence. For example, long-period superstructure have been observed in sintered samples of the T1 based superconducting oxides [ 7 ]. In the case of the Y - B a - C u - O system, ordering of the oxygen atoms .(and oxygen vacancies) and the consequent superstructure formation have been observed frequently and studied extensively [ 8 ]. In this paper we report, for the first time, the observation of superlattice structures based upon different ordered stacking sequences of the polytypoidic variants in laser deposited thin films of Y - B a - C u O. For clarity, the perovskite unit is termed "P", one CuO chain is termed "C", two CuO chains "CC" and a C u O - Y O - C u O stacking (as in the "224" structure) is termed "CYC". thus, for example, the "123" structure would correspond to "PC" while the "248" structure would correspond to "PCC-PCC".
2. Experimental Details of the thin film laser deposition process have already been reported elsewhere [ 9 ]. Briefly, a pulsed excimer laser was used to evaporate the Y B a - C u - O target. The films were prepared at a nominal frequency of 1 Hz with a deposition rate of about 1 A/s. Typically, films of about 1500-2000 A thickness were grown. The films were deposited at a substrate heater temperature of 750 °C in an oxygen ambient of 100 mTorr of oxygen. A single crystal LaGaO3 in the [001 ] zone axis was used as the substrate. The sample exhibits a resistive superconducting transition at 89 K and critical current densities have been measured to be greater than l 06 A /
15
cm 2 at 77 K. Cross-section samples were prepared and were ion milled for a few minutes prior to examination so as to reduce the degradation due to moisture in the atmosphere. High-resolution electron microscopy (HREM) was carried out in the Berkeley Atomic Resolution Microscope (ARM) with a point-to-point resolution of 1.6 A. HREM images were obtained under conditions close to Scherzer defocus such that the projections of the atomic columns appear black on a white background. Images were obtained within a few minutes of exposure to the electron beam to minimize the beam induced damage. All the regions observed in the cross-sectioned sample had the c-axis normal to the substrate surface. No large angle grain boundaries were observed. Figure 2 is a HREM image in which a new superstructure, designated as "3 6 10", consisting of the repeat stacking sequence of "PCPCPCC... " is observed. This structure is the next higher h o m o l o g u e of the "247" structure, arising due to the addition of one more "PC" unit. In this image, the "247" structure can also be observed. For the sake of clarity, the inset to this image shows one unit cell of the "248" structure, illustrating the two CuO chain layers. The "247" phase is known to have a superconducting transition temperature, Tc in the range of 50-55 K, while that of the "248" structure is about 80 K. Another n e w superstructure that is observed in the thin films is similar to the "247" structure, but the "124" unit of the "247" structure is replaced by a "224" unit. This yields a cationic composition of "347" and the stacking repeat o f " P C P C Y C ... ". This is illustrated in the HREM image in fig. 3a. The inset to this image shows an atomic resolution image of the "224" structure. In fig. 3b, is shown an example that is analogous to the structure shown in fig. 2, except that the "PCC" unit is replaced by a "PCYC" unit. This superstructure is the next higher homologue of the "347" structure, obtained by the insertion of one more "PC" unit into the "347" structure to form a "4 6 l 0" structure. Neither of these superstructures have been reported before. Finally, in fig. 4 an example of a long range superstructure is shown. In this image, the superstructure consists of two repeating blocks. One of them is the "PCPCYC" and the other is "PCPCPCYC". Since these two structures have 2 and 3 perovskite
16
R. Ramesh et al. /Polytypoidic superlattices in YBCO thin films
Fig. 2. HREM image showingthe "247" structure and the next higher homologue, i.e., the "3 6 10" cationic stoichiometry. The inset shows, for clarity, one half unit cell of the "248" structure. blocks respectively, they are identified as "2" and "3" in the image in fig. 4. Stacking defects, such as that indicated in this image alter the repeat sequence of this superstructure. Although stacking defects have been observed in other films grown on various substrates, this is the first instance when such polytypoidic superstructures in the Y - B a - C u - O system have been observed in these films. It is thus essential to carry out systematic studies of the effect of processing conditions such as oxygen partial pressure, substrate temperature in order to obtain films in which such superstructures can be repeatably observed and characterized over regions significantly larger than that observed in this study. The observation of these polytypoidic superstructures raises several interesting questions. The first concerns the nature of the superconducting properties of these structures. Upon progressing from the "12Y' structure to the "248" structure, the Tc drops to 80 K, illustrating the adverse effect of the extra
CuO chain layer. However, the superconducting properties of the "224" structure and the superstructures derived from this structure are not known. The potential for the study of the physics of these anisotropic superconductors and for the fabrication of novel heterostructures similar to the semiconductor superlattices has been recognized in recent studies on artificially layered superlattices [1,2]. Our observations of such natural superlattices, in which one of the layers either has a lower Tc or is non-superconducting, raises exciting possibilities for the fabrication of novel superlattice-based heterostructures through the control of the processing conditions. The second aspect is related to the phase equilibria existing in this system. In sintered bulk samples prepared with the "123" nominal composition, generally the "123" phase, the "211" phase and sometimes CuO or BaCuO2 are observed. In the thin films, produced by the laser deposition technique, our studies are progressively revealing a plethora of structural defects related to the layered structure of
R. Ramesh et al. / Polytypoidic superlattices in YBCO thin films
17
Fig. 3. (a) A new superstructure, consisting of the repeat unit of"123" + "224"= "347", with the inset showing an atomic resolution image of the "224" structure for clarity; (b) HREM image showing a superstructure made up of two "123" units and one "224" unit with the cationic stoichiometry of"4 6 10". This superstructure is the next higher homologous to the "347" superstructure.
these c o m p o u n d s . The o b s e r v a t i o n o f p o l y t y p o i d i c variants a n d such superstructures suggests the existence o f a series o f either stable or metastable phases as with invariant cationic compositions, with small differences in the free energy o f formation. These different c o m p o u n d s would have different fixed ratios o f the various cations. In this sense, the structural evolution in this system is likely to be similar
to the Y - S i - A I - O - N system [ 10 ]. The laser deposition technique, in which thin film deposition is carried out under conditions very far from t h e r m o d y n a m i c equilibrium, is a powerful processing route to obtain such structures. G r o w t h o f layered structures, such as the Bi cuprate superconductor, by MBE has also been shown recently, suggesting another possible route for obtaining such novel superlattices
18
R. Ramesh et al. / Polytypoidic superlattices in YBCO thin films
[ 11 ] a n d for o b t a i n i n g samples large e n o u g h to m a k e definitive measurements.
Acknowledgements T h e s u p p o r t a n d e n c o u r a g e m e n t o f P.L. Key, J.H. W e r n i c k , M.J. B o w d e n , V.G. K e r a m i d a s a n d P.F. L i a o is greatly a p p r e c i a t e d . S t i m u l a t i n g discussions w i t h J . M . T a r a s c o n , B.G. Bagley, T. Sands, T.R. D i n g e r ( I B M ) a n d A.F. M a r s h a l l ( S t a n f o r d U n i v e r s i t y ) are gratefully a c k n o w l e d g e d . T h e s u p p o r t o f P r o f e s s o r G. T h o m a s a n d the s t a f f o f the N a t i o n a l C e n t e r for E l e c t r o n M i c r o s c o p y , L a w r e n c e Berkeley L a b o r a t o r y is gratefully a c k n o w l e d g e d .
References
Fig. 4. Low-magnification HREM image showing a long range superstructure consisting of alternating units of the "347" block and the "4 6 10" block. Since these two structures consist of 2 and 3 perovskite blocks respectively, they are numbered as "2" and "3" in the image.
[ 1 ] J.M. Triscone et al., Phys. Rev. Lett., in press. [2] X.D. Wu et al., Appl. Phys. Lett., in press; Q. Li et al., submitted to Phys. Rev. Len. [ 3 ] R. Ramesh et al., Science, in press. [4] A. Ourmazd et al., Nature 327 (1987 ) 308. [5] P. Border et al., Nature 334 (1988) 596. [6] A.F. Marshall et al., Phys. Rev. B 37 (1988) 9353. [7] M. Verwenft, G. van Tendeloo and S. Amelinckx, Physica C 156 (1988) 607. [8] J. Reyes-Gasga et al., Physica C 159 (1989) 831, and the references quoted therein. [9] A. Inam et al., Appl. Phys. Lett., in press. [10] K.H. Jack, J. Mater. Sci. 11 (1976) 1135. [ 11 ] D.G. Schlom et al., in: Proc. MRS Fall Meeting, Symposium M, Boston, MA (1989).
Polytypoidic superlattices in Y-Ba-Cu-O thin films R. Ramesh, D.M. Hwang, T.S. R a v i a n d A. I n a m Bellcore, Red Bank, NJ 07701, USA
X.D. W u a n d T. V e n k a t e s a n Rutgers University, Piscataway, NJ 08854, USA Received 4 August 1990
We report, for the first time, the observation of natural, polytypoidic superlattice structures in pulsed laser deposited superconducting Y - B a - C u - O thin films. These structures correspond to ordered stacking sequences of the "123", "124" and "224" cationic ratios. Such structurally coherent, natural superlattice structures, either in the stable or metastable form, may be an attractive alternative for the artificial superlaUices suggested for novel electronic applications. The results suggest the existence of a family of polytypoidic phases with different cationic compositions in the Y - B a - C u - O phase diagram.
1. Introduction There has been considerable interest in the fabrication of structurally stable heterostructures and superlattices that have novel electronic properties, e.g., junction devices, from the newly discovered cuprate superconductors [ 1,2]. Typically, the heterostructures (or superlattices) consist of a layer of Y B a - C u - O and a layer of either a superconducting or a non-superconducting lattice matched material, such as PrBa2Cu307_x. The structural and electrical transport results obtained on such artificially layered materials have been very encouraging [3]. It is, however, equally important to explore the possibility of growing natural superlattice structures using the polytypoidic variations in the stacking sequences observed in the Y - B a - C u - O system. Three of the possible polytypoidic variants are shown in fig. la-c. While the "123" and "248" structures have been synthesized and studied in bulk form, the "224" structure has been observed only as a growth defect [ 3,4 ]. Recently, we presented the results of high-resolution electron microscopy ( H R E M ) studies on Y - B a - C u - O films on single crystal MgO substrates [ 3 ]. In this study, the "224" structure was observed to exist in extended regions for the first time. It has, however, not yet been syn-
BaO CuO2 Y CuO2 BaO
CuO C
Bat) CuO2 Y CuO2 BaO
CuO CC CuO
Bat) CuO2
y CuO2 BaO
CuO ~ doping YO CYC planes CuO
BaO CuO2 Y CuO2 BaO
Bat) CuO2 Y CuO2 BaO
CuO2 y CuO2
YBa2Cu307
YBa2Cu408
Y2Ba2Cu409
• 123"
"124" or "24g"
Tc-91K
Tc-81K
Perovskite Blocks, P
BaO Perovskite Blocks
BaO
"224"
Tc =7
Fig. 1. (a-c) Schematic illustrations of the "123", "248" and "224" structures. Note that the perovskite block, P, is common to all the three structures while the layers connecting two such perovskite blocks are different for each structure.
thesized in bulk form and its superconducting properties are unknown. Among the several types of structural defects, the polytypoidic stacking faults are prominent in thin films due to local fluctuations in the composition during deposition. It may be noted that in all three structures illustrated in fig. 1a-c, the
0921-4534/90/$03.50 © 1990 - Elsevier Science Publishers B.V. (North-Holland)
R. Ramesh et aL / Polytypoidic superlattices in YBCO thin films
"perovskite block" is common, while the planes between two "perovskite" blocks change from one polytypoid to another. The superconducting transition temperature, To, is sensitive to the changes illustrated in this figure. The simplest polytypoidic superstructures consist of repeat units of " 1 2 3 " + " 1 2 4 " that is now identified as the "247" phase [5], or " 1 2 4 " + " 1 2 4 " which is the "248" structure [6 ]. The crystal structure and the nature of the superstructure are defined by the planes that intercalate between two perovskite blocks. Clearly, long-period superstructures comprised of various combinations of these individual polytypoids can be obtained by suitable modifications in the repeat sequence. For example, long-period superstructure have been observed in sintered samples of the T1 based superconducting oxides [ 7 ]. In the case of the Y - B a - C u - O system, ordering of the oxygen atoms .(and oxygen vacancies) and the consequent superstructure formation have been observed frequently and studied extensively [ 8 ]. In this paper we report, for the first time, the observation of superlattice structures based upon different ordered stacking sequences of the polytypoidic variants in laser deposited thin films of Y - B a - C u O. For clarity, the perovskite unit is termed "P", one CuO chain is termed "C", two CuO chains "CC" and a C u O - Y O - C u O stacking (as in the "224" structure) is termed "CYC". thus, for example, the "123" structure would correspond to "PC" while the "248" structure would correspond to "PCC-PCC".
2. Experimental Details of the thin film laser deposition process have already been reported elsewhere [ 9 ]. Briefly, a pulsed excimer laser was used to evaporate the Y B a - C u - O target. The films were prepared at a nominal frequency of 1 Hz with a deposition rate of about 1 A/s. Typically, films of about 1500-2000 A thickness were grown. The films were deposited at a substrate heater temperature of 750 °C in an oxygen ambient of 100 mTorr of oxygen. A single crystal LaGaO3 in the [001 ] zone axis was used as the substrate. The sample exhibits a resistive superconducting transition at 89 K and critical current densities have been measured to be greater than l 06 A /
15
cm 2 at 77 K. Cross-section samples were prepared and were ion milled for a few minutes prior to examination so as to reduce the degradation due to moisture in the atmosphere. High-resolution electron microscopy (HREM) was carried out in the Berkeley Atomic Resolution Microscope (ARM) with a point-to-point resolution of 1.6 A. HREM images were obtained under conditions close to Scherzer defocus such that the projections of the atomic columns appear black on a white background. Images were obtained within a few minutes of exposure to the electron beam to minimize the beam induced damage. All the regions observed in the cross-sectioned sample had the c-axis normal to the substrate surface. No large angle grain boundaries were observed. Figure 2 is a HREM image in which a new superstructure, designated as "3 6 10", consisting of the repeat stacking sequence of "PCPCPCC... " is observed. This structure is the next higher h o m o l o g u e of the "247" structure, arising due to the addition of one more "PC" unit. In this image, the "247" structure can also be observed. For the sake of clarity, the inset to this image shows one unit cell of the "248" structure, illustrating the two CuO chain layers. The "247" phase is known to have a superconducting transition temperature, Tc in the range of 50-55 K, while that of the "248" structure is about 80 K. Another n e w superstructure that is observed in the thin films is similar to the "247" structure, but the "124" unit of the "247" structure is replaced by a "224" unit. This yields a cationic composition of "347" and the stacking repeat o f " P C P C Y C ... ". This is illustrated in the HREM image in fig. 3a. The inset to this image shows an atomic resolution image of the "224" structure. In fig. 3b, is shown an example that is analogous to the structure shown in fig. 2, except that the "PCC" unit is replaced by a "PCYC" unit. This superstructure is the next higher homologue of the "347" structure, obtained by the insertion of one more "PC" unit into the "347" structure to form a "4 6 l 0" structure. Neither of these superstructures have been reported before. Finally, in fig. 4 an example of a long range superstructure is shown. In this image, the superstructure consists of two repeating blocks. One of them is the "PCPCYC" and the other is "PCPCPCYC". Since these two structures have 2 and 3 perovskite
16
R. Ramesh et al. /Polytypoidic superlattices in YBCO thin films
Fig. 2. HREM image showingthe "247" structure and the next higher homologue, i.e., the "3 6 10" cationic stoichiometry. The inset shows, for clarity, one half unit cell of the "248" structure. blocks respectively, they are identified as "2" and "3" in the image in fig. 4. Stacking defects, such as that indicated in this image alter the repeat sequence of this superstructure. Although stacking defects have been observed in other films grown on various substrates, this is the first instance when such polytypoidic superstructures in the Y - B a - C u - O system have been observed in these films. It is thus essential to carry out systematic studies of the effect of processing conditions such as oxygen partial pressure, substrate temperature in order to obtain films in which such superstructures can be repeatably observed and characterized over regions significantly larger than that observed in this study. The observation of these polytypoidic superstructures raises several interesting questions. The first concerns the nature of the superconducting properties of these structures. Upon progressing from the "12Y' structure to the "248" structure, the Tc drops to 80 K, illustrating the adverse effect of the extra
CuO chain layer. However, the superconducting properties of the "224" structure and the superstructures derived from this structure are not known. The potential for the study of the physics of these anisotropic superconductors and for the fabrication of novel heterostructures similar to the semiconductor superlattices has been recognized in recent studies on artificially layered superlattices [1,2]. Our observations of such natural superlattices, in which one of the layers either has a lower Tc or is non-superconducting, raises exciting possibilities for the fabrication of novel superlattice-based heterostructures through the control of the processing conditions. The second aspect is related to the phase equilibria existing in this system. In sintered bulk samples prepared with the "123" nominal composition, generally the "123" phase, the "211" phase and sometimes CuO or BaCuO2 are observed. In the thin films, produced by the laser deposition technique, our studies are progressively revealing a plethora of structural defects related to the layered structure of
R. Ramesh et al. / Polytypoidic superlattices in YBCO thin films
17
Fig. 3. (a) A new superstructure, consisting of the repeat unit of"123" + "224"= "347", with the inset showing an atomic resolution image of the "224" structure for clarity; (b) HREM image showing a superstructure made up of two "123" units and one "224" unit with the cationic stoichiometry of"4 6 10". This superstructure is the next higher homologous to the "347" superstructure.
these c o m p o u n d s . The o b s e r v a t i o n o f p o l y t y p o i d i c variants a n d such superstructures suggests the existence o f a series o f either stable or metastable phases as with invariant cationic compositions, with small differences in the free energy o f formation. These different c o m p o u n d s would have different fixed ratios o f the various cations. In this sense, the structural evolution in this system is likely to be similar
to the Y - S i - A I - O - N system [ 10 ]. The laser deposition technique, in which thin film deposition is carried out under conditions very far from t h e r m o d y n a m i c equilibrium, is a powerful processing route to obtain such structures. G r o w t h o f layered structures, such as the Bi cuprate superconductor, by MBE has also been shown recently, suggesting another possible route for obtaining such novel superlattices
18
R. Ramesh et al. / Polytypoidic superlattices in YBCO thin films
[ 11 ] a n d for o b t a i n i n g samples large e n o u g h to m a k e definitive measurements.
Acknowledgements T h e s u p p o r t a n d e n c o u r a g e m e n t o f P.L. Key, J.H. W e r n i c k , M.J. B o w d e n , V.G. K e r a m i d a s a n d P.F. L i a o is greatly a p p r e c i a t e d . S t i m u l a t i n g discussions w i t h J . M . T a r a s c o n , B.G. Bagley, T. Sands, T.R. D i n g e r ( I B M ) a n d A.F. M a r s h a l l ( S t a n f o r d U n i v e r s i t y ) are gratefully a c k n o w l e d g e d . T h e s u p p o r t o f P r o f e s s o r G. T h o m a s a n d the s t a f f o f the N a t i o n a l C e n t e r for E l e c t r o n M i c r o s c o p y , L a w r e n c e Berkeley L a b o r a t o r y is gratefully a c k n o w l e d g e d .
References
Fig. 4. Low-magnification HREM image showing a long range superstructure consisting of alternating units of the "347" block and the "4 6 10" block. Since these two structures consist of 2 and 3 perovskite blocks respectively, they are numbered as "2" and "3" in the image.
[ 1 ] J.M. Triscone et al., Phys. Rev. Lett., in press. [2] X.D. Wu et al., Appl. Phys. Lett., in press; Q. Li et al., submitted to Phys. Rev. Len. [ 3 ] R. Ramesh et al., Science, in press. [4] A. Ourmazd et al., Nature 327 (1987 ) 308. [5] P. Border et al., Nature 334 (1988) 596. [6] A.F. Marshall et al., Phys. Rev. B 37 (1988) 9353. [7] M. Verwenft, G. van Tendeloo and S. Amelinckx, Physica C 156 (1988) 607. [8] J. Reyes-Gasga et al., Physica C 159 (1989) 831, and the references quoted therein. [9] A. Inam et al., Appl. Phys. Lett., in press. [10] K.H. Jack, J. Mater. Sci. 11 (1976) 1135. [ 11 ] D.G. Schlom et al., in: Proc. MRS Fall Meeting, Symposium M, Boston, MA (1989).