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Epitaxial growth of Bi2Sr2−xLaxCu1O6+δ thin films on vicinal SrTiO3 substrates
Physica C 322 Ž1999. 73–78
Epitaxial growth of Bi 2 Sr2yx La xCu 1O6qd thin films on vicinal SrTiO 3 substrates Y.Z. Zhang b
a,b
, Y.L. Qin b, R. Deltour
a,)
, H.J. Tao b, L. Li b, Z.X. Zhao
b
a Physique des Solides, UniÕersite Libre de Bruxelles, CP 233, B-1050, Brussels, Belgium National Laboratory for SuperconductiÕity, Institute of Physics and Center for Condensed Matter Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100080, China
Received 29 September 1998; received in revised form 8 July 1999; accepted 27 July 1999
Abstract We have studied the epitaxy of Bi 2 Sr2yx La x CuO6q d ŽLa-2201. superconducting thin films deposited by RF sputtering on vicinal SrTiO 3 ŽSTO. substrates. The surface of the single crystal substrate is cut at an angle of 68 with respect to the Ž100. basal plane, being rotated around the w110x axis direction. Contrary to films deposited on untilted single crystal substrate, the AFM surface topographies of the La-2201 films deposited on the tilted substrates show elongated stripe-like shapes. Transmission electron microscopy ŽTEM. and electron diffraction show a film growth with very good crystallographic alignments very similar to what is observed on La-2201 single crystals. The temperature dependency of the a- and c-axis resistivities confirm the high epitaxial quality of the films, as evidenced by atomic force microscopy ŽAFM. and TEM studies. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Epitaxial growth; Bi 2 Sr2yx La x Cu 1O6q d thin films; SrTiO 3 substrates
1. Introduction Bi 2 Sr 2 Ca ny1Cu nO 2 nq4q d ŽBSCCO. materials show mica-like properties with a highly wave modulated crystal structure incommensurate with the lattice. Their high anisotropic structure is known to be highly correlated with quasi two-dimensional superconductivity. The stoichiometric Bi 2 Sr 2 CuO6q d compound is not known to be a superconductor w1x; a deviation of the Bi to Sr Ž1 to 1. ratio is required to
)
Corresponding author. Tel.: q32-2-650-57-52; fax: q32-2650-59-16; E-mail: [email protected]
produce a superconducting ‘‘Bi 2 Sr2 CuO6q d ’’ phase. Previous studies showed that substituting part of the Sr by La made this compound superconducting, with an optimal value for La of x s 0.4 w2–5,13x. The crystal structure of Bi 2 Sr2- x La x CuO6q d ŽLa-2201. is the same as that of superconducting Bi 2 Sr2 CuO6q d with Laq3 substituting for Srq2 . The Bi 2 Sr2 CuO6q d phase has the simplest structure in the series of the superconducting BSCCO materials Žone CuO 2 plane per half-unit-cell. with no intergrowth behavior of 2212 and 2223 structure during the La-2201 thin film growth. In addition, the La-2201 superconductor is an excellent candidate for the observation of transport properties in magnetic fields, as this material is characterised by a low critical temperature Tc0 and a
0921-4534r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 9 9 . 0 0 4 2 9 - 3
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Y.Z. Zhang et al.r Physica C 322 (1999) 73–78
relatively low upper critical field Hc2 Ž0., easier to produce in present-day usual laboratory conditions w6x. The epitaxial BSCCO thin films on a substrate surface with a square symmetry, reflecting the cubic crystal structure of the lattice, such as Ž100. SrTiO 3 ŽSTO., always contain an a–b twinning Ž908 twins. structure w5,13,7–9x; this is because the lattice constants of the a- and b-axes of BSCCO are very close with a nearly equal growth probability along a- and b-axes along the same in-plane direction with respect to the substrate. To prevent screw dislocations and a–b twinning structures w7–9x a slightly tilted substrate, so-called vicinal substrate, can be adopted. With this kind of substrate, the probabilities of epitaxial growth for the a- and b-axes of the BSCCO films are no longer equal w7–9x. Depending on the tilted angle and treatment before deposition, multiple steps Žstep bouncing. can occur, resulting in larger terraces with multiple steps. This stepped surface, dependent on the history before deposition and the deposition temperature, influences the film morphology. The steps on the surface act as sinks for adsorbing mobile ad-atoms or ad-molecules thus allowing the thin film to grow by a step-flow growth mode w7–12x without two-dimensional Ž2D. nucleation. The in- and out-plane resistivities r a b and rc of these films can be extracted from transport measurements. A value for the anisotropic factor g 2 s rcrr a b can then be evaluated. In the following, the results of our measurements and their discussion are given after a brief description of the experimental procedure.
sputtering gas consisted of O 2 :Ar s 1:2 with a total gas pressure of 0.72 Torr. The RF sputtering power was 80 W and the size of the target disc f 40 mm = 4 mm. A deposition rate of about 5 nmrmin was used with the substrate temperature being kept at 8308C during deposition. Atomic force microscopy ŽAFM. analyses of these films were carried out with an Auto Probe CP SPM ŽPark Scientific Instruments. equipment. Transmission electron microscopy ŽTEM. investigations were performed with a H-9000 NA type ŽHitachi. at nominal 300 kV. Cross-sectional specimens for TEM were prepared using the conventional technique of ion milling. The thin films were patterned into a cross-like configuration with voltage and current pick up electrodes shown in Fig. 1. These electrodes Ždenoted as A, B, C, D, E and F in Fig. 1. allow resistive measurements to be performed along the two transverse directions x and y. Current supply electrodes can be shifted from A and B to C and D. When using A and B for the current electrodes, the voltage is measured between D and F, while when using C and D for current electrodes, the voltage electrodes are A and E.
2. Experiments La-2201 thin films are prepared by RF-magnetron sputtering. The preparation procedure and the characteristics for the La-2201 thin films grown on wellaligned Ž100. STO and Ž100. LaAlO 3 ŽLAO. substrates are described elsewhere w5,13x. The surface of the STO substrates Žcommercial. is 68 " 18 misoriented with respect to the Ž100. plane and tilted around the w110x axis. The substrate surface is not annealed at high temperature, a preheating of half an hour at 8308C before deposition being routinely carried on. We observed the presence of terraces with multiple steps on the tilted substrate surface. The
Fig. 1. Patterning shape and in-plane geometric arrangement for the anisotropic measurements of the transport properties.
Y.Z. Zhang et al.r Physica C 322 (1999) 73–78
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3. Results and discussion Fig. 2 shows a 5 = 5 mm2 AFM image of a La-2201 film on a ; 68-tilted STO substrate. The ˚ The image thickness of this thin film is ; 3000 A. clearly shows the presence of nearly parallel stripes with widths between 0.1 and 0.4 mm. The stripes are aligned along the w110x substrate direction which is parallel to the step edges of the substrate. An enlarged image is given in Fig. 3Ža.. The corresponding line scans and the data table are shown in Fig. 3Žb., giving the heights, distances and angular variations of the surface. The stripes are very elongated islands with terraced structure. The line scans show that the heights of the steps, from valleys to plateaus, are less ˚ while the distances between valleys and than 100 A, summits are about ten times larger than their average heights differences. Note that in the data table, heights of half the La-2201 unit-cell steps can be found in the line scans, as also steps with heights of multiple integer of the half-unit-cell. In Fig. 4Ža., we present a topography of the La-2201 film over a 1 = 1 mm2 area on the tilted substrate. One can notice that the edge outlines of the steps are almost parallel to each other. The general parallel outline of the steps is an indication that the film has probably grown by a step-flow mode without 2D nucleation. The stripe-like islands
Fig. 3. Ža. An enlarged AFM image of a 300-nm-thick La-2201 film grown on a ;68-tilted Ž100. STO substrate over 0.4=0.4 mm2 area. Žb. Two corresponding line scans of the stripe-like islands with the corresponding data table.
Fig. 2. An AFM image of a 300-nm-thick La-2201 film grown on a ;68-tilted Ž100. STO substrate over 5=5 mm2 area.
are terraced islands and can be compared to those observed on films grown under similar conditions on well-aligned substrates as shown in Fig. 4Žb.. We observe clear rectangular-like shape patterns for the thin films deposited on well-aligned Žuntilted. STO and LAO substrate surfaces and the widths of the rectangular-like islands are comparable to their lengths w5,13x. For these films, small 2D clusters around 20 nm are also visible in AFM analyses,
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2. This can be compared to the results of a work by Endo w12x who found that the Bi-2223 thin film growth rate along the a-axis is larger than that along the b-axis. Comparing our data with the work of other groups w7–12x, we can conjecture, as shown later, that the thin films grown on the tilted substrates have their a-axis along the long length direc-
Fig. 4. Ža. A topography of the La-2201 thin film grown on tilted substrate. Žb. A topography of the La-2201 thin film grown on a well-aligned Ž100. substrate.
absent on the tilted substrate images. For the films grown on the tilted substrates, the widths of the stripe-like islands are much smaller than their lengths ŽFig. 2. and no visible 2D cluster can be found, as shown in Figs. 3 and 4Ža.. Another great difference is that the lengths of the rectangular-like islands have two growth directions, perpendicular to each other w5,13x, while the lengths of the stripe-like islands show roughly one growth direction as shown in Fig.
Fig. 5. Ža. A SAED pattern of the La-2201 thin film with the incident beam along the w100x azimuth. Žb. A SAED pattern with the incident beam along w110x substrate azimuth Žthe w100x thin film azimuth.. Indexes are only denoted for the diffraction spots of the STO substrate in the SAED pattern.
Y.Z. Zhang et al.r Physica C 322 (1999) 73–78
Fig. 6. A high-resolution lattice image of the film grown on a tilted substrate with the SAED patterns in Fig. 5.
tion of the stripe-like islands with their b-axis at a tilted angle of u ; 68 with respect to the substrate surface. The growth of the film is perpendicular to the stripe direction with layers continuously being added, intersecting the orthorhombic b crystallographic direction at an angle of ; 68, producing steps of multiples of cr2. This does not need the presence of 2D crystallisation nuclei. The step edges of the crystalline plane layers keep moving until they reach the layer edges of the front stripe-like islands. The epitaxial relations are maintained and, as can be seen in Fig. 6 discussed below, there are no observed grain boundaries. To analyse the stripe-like islands, we made a TEM study of a transverse cut through the substrate and overlaying film. Fig. 5Ža. shows the selected area electron diffraction ŽSAED. pattern with the incident beam along a w100x azimuth of the thin film. This SAED pattern corresponds to a cross-section of the stripe-like islands. The distinct satellite spots correspond to the incommensurate modulation and constitute clear evidence of a very good alignment of the thin film crystalline structure. Fig. 5Žb. shows the SAED pattern with the incident beam along the w110x substrate azimuth Žthe w100x azimuth for the thin film.. In Fig. 5Žb., the large spots of the substrate, which overlap some of the main diffraction spots of the La-2201 thin film, show the epitaxial growth of the thin film in coincidence with the substrate. Comparing Fig. 5Ža. and Žb., we can deduce the epitaxial relations as being w100x film 5w110xsubstrate , w010x film 5w110xsubstrate , and w001xfilm 5w001xsubstrate . Taking a lattice constant of 0.39 nm for STO, we can
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derive the lattice constant values of the thin film as 0.54 nm for the b-axis and 2.44 nm for the c-axis. The satellite spots of the incommensurate modulation occurs at non-integral positions. Their positions in the reciprocal lattice space can be written as Q s G " nq, where G is any reciprocal-lattice vector of the superlattice, n is an integer, and q s Ž0, 0.22, 0.70. is the reduced wave vector of the incommensurate modulation. Fig. 6 shows the corresponding high resolution TEM lattice image for the thin film. The image shows the tilted angle of the a–b planes to be ; 68 with respect to the substrate surface. The alignment of atom rows in the b–c plane of the image shows how the single crystal structure is affected by the incommensurate modulation in the b–c plane. Fig. 7 shows another image of the film grown on a well-aligned Ž100. substrate with the a–b planes parallel to the substrate surface without any incommensurate modulation. Considering a thin film thickness d and tilted angle u , the average extension of each a–b plane is rather limited and given by d ctg u along the tilted direction. On the contrary, in the direction of the rotation axis Ž x-axis., the a–b plane layers have a much larger extension. This anisotropic layer extension will have a great influence on the resistivity measurements. We patterned some of these thin films into the shape shown in Fig. 1. In doing so, we adjusted the control cross parallel to the w110x substrate alignment, defining x- and y-axes as shown in Fig. 1. With this configuration, we can obtain the resistivity relations r x s r a , and r y s r b cos 2u q rc sin2u , where
Fig. 7. A high-resolution lattice image of a La-2201 thin film grown on a well-aligned Ž100. STO substrate.
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thin films have a very good growth alignment over the whole substrate surface area. 4. Conclusion
Fig. 8. Anisotropic temperature resistivities of a typical La-2201 thin film over x – y plane. The inset gives the temperature dependence of the anisotropic factor r y r r x .
r x and r y are the resistivities of the film along the x and y-axes, respectively, and r a , r b , and rc are, respectively, the resistivities corresponding to the a-, b- and c-axes of the La-2201 thin film. Since rc 4 r b for the BiSrCaCuO material and u ; 68 then rc ( r yrsin2u . The measured temperature dependence of the resistivities r x and r y for a typical La-2201 thin film is shown in Fig. 8. The resistivity r x of this film exhibits a metallic temperature dependence over the whole temperature range, i.e., from Tc,onset up to 300 K, while r y exhibits a semiconductive temperature behavior. These different temperature dependent features are consistent with the characteristics of the transport properties along the a–b plane and the c-axis direction in BSCCO. The inset of Fig. 8 shows the temperature dependence of the anisotropic factor r yrr x for the x–y plane. The curve exhibits a r yrr x maximum value of 64 at 31.5 K and a r yrr x value of 29 at 300 K. Using the relations r a s r x , rc ( r yrsin2u , and the tilted angle value ; 68, the temperature dependence ratio of rcrr a can be estimated to be approximately 5.8 = 10 3 at 31.5 K and 2.6 = 10 3 at 300 K. A slight misalignment of the bridge direction would give values larger than those mentioned above. All samples measured with a tilted angle ; 68 showed similar anisotropic characteristics, i.e., comparable rcrr a values to those found for single crystals which confirmed that these vicinal
Superconducting Bi 2 Sr2yx La x Cu 1O6q d thin films were deposited on ; 68-misoriented STO substrates by RF-magnetron sputtering. AFM studies show that the growth of the mica-like thin film induces a elongated stripe-like structure. The diffraction patterns together with TEM and measurements of the anisotropic electrical resistivities confirm a very good epitaxy of the films with the substrate, giving rise by a step flow crystallisation mode to a quasi-single crystal structure of the films. Acknowledgements This work has been financially supported by Climb Project of China and PAI 4r10 ŽBelgium.. References w1x Sales, B.C. Chakoumakes, Phys. Rev. B 43 Ž1991. 12994. w2x P.V.P.S.S. Sastry, J.V. Yakhmi, R.M. Iyer, C.K. Subramamian, R. Srinivasan, Physica C 178 Ž1991. 110. w3x Khasanova, E.V. Antipov, Physica C 246 Ž1995. 241. w4x Sales, B.C. Chakoumakes, Phys. Rev. B 43 Ž1991. 12994. w5x Y.Z. Zhang, H.T. Yang, L. Li, D.G. Yang, H.J. Tao, B.R. Zhao, Z.X. Zhao, J. Mater. Sci. Lett. 16 Ž1997. 1905. w6x Y.Z. Zhang, J.-F. de Marneffe, R. Deltour, Y.L. Qin, L. Li, Z.X. Zhao, L. Jansen, P. Wyder, to be published. w7x J.N. Eckstein, I. Bozovic, D.G. Schlom, J.S. Harris Jr., Appl. Phys. Lett. 57 Ž1990. 1049. w8x T. Sugimoto, N. Kubota, Y. Shiohara, S. Tanaka, Appl. Phys. Lett. 63 Ž1993. 2697. w9x Tsukada, K. Uchinokura, J. Appl. Phys. 78 Ž1995. 364. w10x K. Endo, T. Shimizu, H. Matsuhata, F. Hosseini Teherani, S. Yoshida, H. Tokumoto, K. Kajimura, IEEE Trans. Appl. Supercond. J. 5 Ž1995. 1675. w11x K. Koguchi, T. Matsumoto, T. Kawai, S. Kawai, Jpn. J. Appl. Phys. 33 Ž1994. L514. w12x K. Endo, Bismuth-based high-temperature superconductors, in: H. Maeda, K. Togano ŽEds.., Marcel Dekker, 1996, pp. 523–544. w13x Y.Z. Zhang, L. Li, D.G. Yang, B.R. Zhao, H. Chen, C. Dong, H.J. Tao, H.T. Yang, S.L. Jia, B. Yin, J.W. Li, Z.X. Zhao, Physica C 295 Ž1998. 75.
Epitaxial growth of Bi 2 Sr2yx La xCu 1O6qd thin films on vicinal SrTiO 3 substrates Y.Z. Zhang b
a,b
, Y.L. Qin b, R. Deltour
a,)
, H.J. Tao b, L. Li b, Z.X. Zhao
b
a Physique des Solides, UniÕersite Libre de Bruxelles, CP 233, B-1050, Brussels, Belgium National Laboratory for SuperconductiÕity, Institute of Physics and Center for Condensed Matter Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100080, China
Received 29 September 1998; received in revised form 8 July 1999; accepted 27 July 1999
Abstract We have studied the epitaxy of Bi 2 Sr2yx La x CuO6q d ŽLa-2201. superconducting thin films deposited by RF sputtering on vicinal SrTiO 3 ŽSTO. substrates. The surface of the single crystal substrate is cut at an angle of 68 with respect to the Ž100. basal plane, being rotated around the w110x axis direction. Contrary to films deposited on untilted single crystal substrate, the AFM surface topographies of the La-2201 films deposited on the tilted substrates show elongated stripe-like shapes. Transmission electron microscopy ŽTEM. and electron diffraction show a film growth with very good crystallographic alignments very similar to what is observed on La-2201 single crystals. The temperature dependency of the a- and c-axis resistivities confirm the high epitaxial quality of the films, as evidenced by atomic force microscopy ŽAFM. and TEM studies. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Epitaxial growth; Bi 2 Sr2yx La x Cu 1O6q d thin films; SrTiO 3 substrates
1. Introduction Bi 2 Sr 2 Ca ny1Cu nO 2 nq4q d ŽBSCCO. materials show mica-like properties with a highly wave modulated crystal structure incommensurate with the lattice. Their high anisotropic structure is known to be highly correlated with quasi two-dimensional superconductivity. The stoichiometric Bi 2 Sr 2 CuO6q d compound is not known to be a superconductor w1x; a deviation of the Bi to Sr Ž1 to 1. ratio is required to
)
Corresponding author. Tel.: q32-2-650-57-52; fax: q32-2650-59-16; E-mail: [email protected]
produce a superconducting ‘‘Bi 2 Sr2 CuO6q d ’’ phase. Previous studies showed that substituting part of the Sr by La made this compound superconducting, with an optimal value for La of x s 0.4 w2–5,13x. The crystal structure of Bi 2 Sr2- x La x CuO6q d ŽLa-2201. is the same as that of superconducting Bi 2 Sr2 CuO6q d with Laq3 substituting for Srq2 . The Bi 2 Sr2 CuO6q d phase has the simplest structure in the series of the superconducting BSCCO materials Žone CuO 2 plane per half-unit-cell. with no intergrowth behavior of 2212 and 2223 structure during the La-2201 thin film growth. In addition, the La-2201 superconductor is an excellent candidate for the observation of transport properties in magnetic fields, as this material is characterised by a low critical temperature Tc0 and a
0921-4534r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 9 9 . 0 0 4 2 9 - 3
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Y.Z. Zhang et al.r Physica C 322 (1999) 73–78
relatively low upper critical field Hc2 Ž0., easier to produce in present-day usual laboratory conditions w6x. The epitaxial BSCCO thin films on a substrate surface with a square symmetry, reflecting the cubic crystal structure of the lattice, such as Ž100. SrTiO 3 ŽSTO., always contain an a–b twinning Ž908 twins. structure w5,13,7–9x; this is because the lattice constants of the a- and b-axes of BSCCO are very close with a nearly equal growth probability along a- and b-axes along the same in-plane direction with respect to the substrate. To prevent screw dislocations and a–b twinning structures w7–9x a slightly tilted substrate, so-called vicinal substrate, can be adopted. With this kind of substrate, the probabilities of epitaxial growth for the a- and b-axes of the BSCCO films are no longer equal w7–9x. Depending on the tilted angle and treatment before deposition, multiple steps Žstep bouncing. can occur, resulting in larger terraces with multiple steps. This stepped surface, dependent on the history before deposition and the deposition temperature, influences the film morphology. The steps on the surface act as sinks for adsorbing mobile ad-atoms or ad-molecules thus allowing the thin film to grow by a step-flow growth mode w7–12x without two-dimensional Ž2D. nucleation. The in- and out-plane resistivities r a b and rc of these films can be extracted from transport measurements. A value for the anisotropic factor g 2 s rcrr a b can then be evaluated. In the following, the results of our measurements and their discussion are given after a brief description of the experimental procedure.
sputtering gas consisted of O 2 :Ar s 1:2 with a total gas pressure of 0.72 Torr. The RF sputtering power was 80 W and the size of the target disc f 40 mm = 4 mm. A deposition rate of about 5 nmrmin was used with the substrate temperature being kept at 8308C during deposition. Atomic force microscopy ŽAFM. analyses of these films were carried out with an Auto Probe CP SPM ŽPark Scientific Instruments. equipment. Transmission electron microscopy ŽTEM. investigations were performed with a H-9000 NA type ŽHitachi. at nominal 300 kV. Cross-sectional specimens for TEM were prepared using the conventional technique of ion milling. The thin films were patterned into a cross-like configuration with voltage and current pick up electrodes shown in Fig. 1. These electrodes Ždenoted as A, B, C, D, E and F in Fig. 1. allow resistive measurements to be performed along the two transverse directions x and y. Current supply electrodes can be shifted from A and B to C and D. When using A and B for the current electrodes, the voltage is measured between D and F, while when using C and D for current electrodes, the voltage electrodes are A and E.
2. Experiments La-2201 thin films are prepared by RF-magnetron sputtering. The preparation procedure and the characteristics for the La-2201 thin films grown on wellaligned Ž100. STO and Ž100. LaAlO 3 ŽLAO. substrates are described elsewhere w5,13x. The surface of the STO substrates Žcommercial. is 68 " 18 misoriented with respect to the Ž100. plane and tilted around the w110x axis. The substrate surface is not annealed at high temperature, a preheating of half an hour at 8308C before deposition being routinely carried on. We observed the presence of terraces with multiple steps on the tilted substrate surface. The
Fig. 1. Patterning shape and in-plane geometric arrangement for the anisotropic measurements of the transport properties.
Y.Z. Zhang et al.r Physica C 322 (1999) 73–78
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3. Results and discussion Fig. 2 shows a 5 = 5 mm2 AFM image of a La-2201 film on a ; 68-tilted STO substrate. The ˚ The image thickness of this thin film is ; 3000 A. clearly shows the presence of nearly parallel stripes with widths between 0.1 and 0.4 mm. The stripes are aligned along the w110x substrate direction which is parallel to the step edges of the substrate. An enlarged image is given in Fig. 3Ža.. The corresponding line scans and the data table are shown in Fig. 3Žb., giving the heights, distances and angular variations of the surface. The stripes are very elongated islands with terraced structure. The line scans show that the heights of the steps, from valleys to plateaus, are less ˚ while the distances between valleys and than 100 A, summits are about ten times larger than their average heights differences. Note that in the data table, heights of half the La-2201 unit-cell steps can be found in the line scans, as also steps with heights of multiple integer of the half-unit-cell. In Fig. 4Ža., we present a topography of the La-2201 film over a 1 = 1 mm2 area on the tilted substrate. One can notice that the edge outlines of the steps are almost parallel to each other. The general parallel outline of the steps is an indication that the film has probably grown by a step-flow mode without 2D nucleation. The stripe-like islands
Fig. 3. Ža. An enlarged AFM image of a 300-nm-thick La-2201 film grown on a ;68-tilted Ž100. STO substrate over 0.4=0.4 mm2 area. Žb. Two corresponding line scans of the stripe-like islands with the corresponding data table.
Fig. 2. An AFM image of a 300-nm-thick La-2201 film grown on a ;68-tilted Ž100. STO substrate over 5=5 mm2 area.
are terraced islands and can be compared to those observed on films grown under similar conditions on well-aligned substrates as shown in Fig. 4Žb.. We observe clear rectangular-like shape patterns for the thin films deposited on well-aligned Žuntilted. STO and LAO substrate surfaces and the widths of the rectangular-like islands are comparable to their lengths w5,13x. For these films, small 2D clusters around 20 nm are also visible in AFM analyses,
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2. This can be compared to the results of a work by Endo w12x who found that the Bi-2223 thin film growth rate along the a-axis is larger than that along the b-axis. Comparing our data with the work of other groups w7–12x, we can conjecture, as shown later, that the thin films grown on the tilted substrates have their a-axis along the long length direc-
Fig. 4. Ža. A topography of the La-2201 thin film grown on tilted substrate. Žb. A topography of the La-2201 thin film grown on a well-aligned Ž100. substrate.
absent on the tilted substrate images. For the films grown on the tilted substrates, the widths of the stripe-like islands are much smaller than their lengths ŽFig. 2. and no visible 2D cluster can be found, as shown in Figs. 3 and 4Ža.. Another great difference is that the lengths of the rectangular-like islands have two growth directions, perpendicular to each other w5,13x, while the lengths of the stripe-like islands show roughly one growth direction as shown in Fig.
Fig. 5. Ža. A SAED pattern of the La-2201 thin film with the incident beam along the w100x azimuth. Žb. A SAED pattern with the incident beam along w110x substrate azimuth Žthe w100x thin film azimuth.. Indexes are only denoted for the diffraction spots of the STO substrate in the SAED pattern.
Y.Z. Zhang et al.r Physica C 322 (1999) 73–78
Fig. 6. A high-resolution lattice image of the film grown on a tilted substrate with the SAED patterns in Fig. 5.
tion of the stripe-like islands with their b-axis at a tilted angle of u ; 68 with respect to the substrate surface. The growth of the film is perpendicular to the stripe direction with layers continuously being added, intersecting the orthorhombic b crystallographic direction at an angle of ; 68, producing steps of multiples of cr2. This does not need the presence of 2D crystallisation nuclei. The step edges of the crystalline plane layers keep moving until they reach the layer edges of the front stripe-like islands. The epitaxial relations are maintained and, as can be seen in Fig. 6 discussed below, there are no observed grain boundaries. To analyse the stripe-like islands, we made a TEM study of a transverse cut through the substrate and overlaying film. Fig. 5Ža. shows the selected area electron diffraction ŽSAED. pattern with the incident beam along a w100x azimuth of the thin film. This SAED pattern corresponds to a cross-section of the stripe-like islands. The distinct satellite spots correspond to the incommensurate modulation and constitute clear evidence of a very good alignment of the thin film crystalline structure. Fig. 5Žb. shows the SAED pattern with the incident beam along the w110x substrate azimuth Žthe w100x azimuth for the thin film.. In Fig. 5Žb., the large spots of the substrate, which overlap some of the main diffraction spots of the La-2201 thin film, show the epitaxial growth of the thin film in coincidence with the substrate. Comparing Fig. 5Ža. and Žb., we can deduce the epitaxial relations as being w100x film 5w110xsubstrate , w010x film 5w110xsubstrate , and w001xfilm 5w001xsubstrate . Taking a lattice constant of 0.39 nm for STO, we can
77
derive the lattice constant values of the thin film as 0.54 nm for the b-axis and 2.44 nm for the c-axis. The satellite spots of the incommensurate modulation occurs at non-integral positions. Their positions in the reciprocal lattice space can be written as Q s G " nq, where G is any reciprocal-lattice vector of the superlattice, n is an integer, and q s Ž0, 0.22, 0.70. is the reduced wave vector of the incommensurate modulation. Fig. 6 shows the corresponding high resolution TEM lattice image for the thin film. The image shows the tilted angle of the a–b planes to be ; 68 with respect to the substrate surface. The alignment of atom rows in the b–c plane of the image shows how the single crystal structure is affected by the incommensurate modulation in the b–c plane. Fig. 7 shows another image of the film grown on a well-aligned Ž100. substrate with the a–b planes parallel to the substrate surface without any incommensurate modulation. Considering a thin film thickness d and tilted angle u , the average extension of each a–b plane is rather limited and given by d ctg u along the tilted direction. On the contrary, in the direction of the rotation axis Ž x-axis., the a–b plane layers have a much larger extension. This anisotropic layer extension will have a great influence on the resistivity measurements. We patterned some of these thin films into the shape shown in Fig. 1. In doing so, we adjusted the control cross parallel to the w110x substrate alignment, defining x- and y-axes as shown in Fig. 1. With this configuration, we can obtain the resistivity relations r x s r a , and r y s r b cos 2u q rc sin2u , where
Fig. 7. A high-resolution lattice image of a La-2201 thin film grown on a well-aligned Ž100. STO substrate.
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thin films have a very good growth alignment over the whole substrate surface area. 4. Conclusion
Fig. 8. Anisotropic temperature resistivities of a typical La-2201 thin film over x – y plane. The inset gives the temperature dependence of the anisotropic factor r y r r x .
r x and r y are the resistivities of the film along the x and y-axes, respectively, and r a , r b , and rc are, respectively, the resistivities corresponding to the a-, b- and c-axes of the La-2201 thin film. Since rc 4 r b for the BiSrCaCuO material and u ; 68 then rc ( r yrsin2u . The measured temperature dependence of the resistivities r x and r y for a typical La-2201 thin film is shown in Fig. 8. The resistivity r x of this film exhibits a metallic temperature dependence over the whole temperature range, i.e., from Tc,onset up to 300 K, while r y exhibits a semiconductive temperature behavior. These different temperature dependent features are consistent with the characteristics of the transport properties along the a–b plane and the c-axis direction in BSCCO. The inset of Fig. 8 shows the temperature dependence of the anisotropic factor r yrr x for the x–y plane. The curve exhibits a r yrr x maximum value of 64 at 31.5 K and a r yrr x value of 29 at 300 K. Using the relations r a s r x , rc ( r yrsin2u , and the tilted angle value ; 68, the temperature dependence ratio of rcrr a can be estimated to be approximately 5.8 = 10 3 at 31.5 K and 2.6 = 10 3 at 300 K. A slight misalignment of the bridge direction would give values larger than those mentioned above. All samples measured with a tilted angle ; 68 showed similar anisotropic characteristics, i.e., comparable rcrr a values to those found for single crystals which confirmed that these vicinal
Superconducting Bi 2 Sr2yx La x Cu 1O6q d thin films were deposited on ; 68-misoriented STO substrates by RF-magnetron sputtering. AFM studies show that the growth of the mica-like thin film induces a elongated stripe-like structure. The diffraction patterns together with TEM and measurements of the anisotropic electrical resistivities confirm a very good epitaxy of the films with the substrate, giving rise by a step flow crystallisation mode to a quasi-single crystal structure of the films. Acknowledgements This work has been financially supported by Climb Project of China and PAI 4r10 ŽBelgium.. References w1x Sales, B.C. Chakoumakes, Phys. Rev. B 43 Ž1991. 12994. w2x P.V.P.S.S. Sastry, J.V. Yakhmi, R.M. Iyer, C.K. Subramamian, R. Srinivasan, Physica C 178 Ž1991. 110. w3x Khasanova, E.V. Antipov, Physica C 246 Ž1995. 241. w4x Sales, B.C. Chakoumakes, Phys. Rev. B 43 Ž1991. 12994. w5x Y.Z. Zhang, H.T. Yang, L. Li, D.G. Yang, H.J. Tao, B.R. Zhao, Z.X. Zhao, J. Mater. Sci. Lett. 16 Ž1997. 1905. w6x Y.Z. Zhang, J.-F. de Marneffe, R. Deltour, Y.L. Qin, L. Li, Z.X. Zhao, L. Jansen, P. Wyder, to be published. w7x J.N. Eckstein, I. Bozovic, D.G. Schlom, J.S. Harris Jr., Appl. Phys. Lett. 57 Ž1990. 1049. w8x T. Sugimoto, N. Kubota, Y. Shiohara, S. Tanaka, Appl. Phys. Lett. 63 Ž1993. 2697. w9x Tsukada, K. Uchinokura, J. Appl. Phys. 78 Ž1995. 364. w10x K. Endo, T. Shimizu, H. Matsuhata, F. Hosseini Teherani, S. Yoshida, H. Tokumoto, K. Kajimura, IEEE Trans. Appl. Supercond. J. 5 Ž1995. 1675. w11x K. Koguchi, T. Matsumoto, T. Kawai, S. Kawai, Jpn. J. Appl. Phys. 33 Ž1994. L514. w12x K. Endo, Bismuth-based high-temperature superconductors, in: H. Maeda, K. Togano ŽEds.., Marcel Dekker, 1996, pp. 523–544. w13x Y.Z. Zhang, L. Li, D.G. Yang, B.R. Zhao, H. Chen, C. Dong, H.J. Tao, H.T. Yang, S.L. Jia, B. Yin, J.W. Li, Z.X. Zhao, Physica C 295 Ž1998. 75.