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PrBa2Cu3O7 superlattices deposited on (1 0 0) SrTiO3 single crystals
Physica C 172 (1990) 365-372 North-Holland
Microstructure of YBa2Cu307/PrBa2Cu307 superlattices deposited on ( 1 0 0) SrTiO3 single crystals O. Eibl Siemens Research Laboratories, Otto tlahn Ring 6, W-8000 Munich 83, Germany
H.E. Hoenig Siemens Research Laboratories, Paul Gossen Strafle 100, I+'-8520 Erlangen, Germany
J.-M. Triscone, t0. Fischer, L. Antognazza and O. Brunner Universite de Genbve, DPMC, 24 Quai E. Ansermet, 1211 Genbve, Switzerland
Received 25 July 1990 Revised manuscript received 16 October 1990
High-quality,t-oriented YBa2CusOT/PrBa2Cu307superlattices have been grown on ( 10 0 ) SrTiO3 substrates. TEM cross sections revealed that the sharpness of the (0 0 1) interfaces was of the order of the lattice parameter ( 1 nm ). The films were deposited at 790°C and no evidence of Y/Pr interdiffusion could be detected. The orthorhombic ( I I 0) twins were 20 nm wide and the orthorhombic distortion in the YBa2Cu307layer yielded a ( b - a ) / b ratio of 1.6%. Surface steps and inclined surface areas at the SrTiOs-YBa~Cu307interface were imaged by high-resolutionTEM. Apart from a 2-6 nm wide layer at the interface, which contained extended defects accounting for the mismatch due to the imperfect surface, the film grew perltectlyon top of this layer. (0 0 I ) lattice planes passed continuously from regions with perfect substrate surfaces to regions containing a larger density of surface steps.
1. Introduction The deposition and the electrical properties of superlattices based on the alternating sequence of YBa2Cu3OT/MBa2Cu307 (M = Dy, Pr) layers have been the subject of a n u m b e r of studies [ 1-4]. In particular, YBa2Cu3OT/PrBa2Cu307 heterostructures are of interest because of the different electrical properties of the two compounds, the first being a superconductor and the second a semiconductor. A n u m b e r of applications seem to be in reach, e.g. tunnel j u n c t i o n s for SQUIDs. We have deposited superlattices by DC magnetron sputtering on ( 1 0 0) SrTiO3 and MgO single crystal substrates [ 1-4]. The mismatch between substrate and film is smaller for SrTiO3 ( ~ 3 % ) and significantly larger for MgO ( ~ 8 . 5 % ) . However. YBa2Cu307 thin films can be grown on both substratcs epitaxially with the c-axis perpendicular to the substrate surface.
In a recent study [ 1 ] a two-unit-cell superlattice was grown on ( 1 0 0) MgO with a periodicity 0 f 2 . 3 nm, i.e. the sequence of unit cells was intended to be 1.17 nm-YBa2Cu3OT/1.17 nm-DyBa2Cu3OT/1.17 nm-YBa2Cu307 perpendicular to the substrate surface. Calculations of the X-ray diffraction pattern for this sample showed that as much as 87% of the rare earth species were arranged in the structure in an ordered way [1 ]. For superlattices with layer thicknesses larger than 5 n m even t h i r d order satellite reflections were observed, which appeared due to the sharp interfaces between the YBa2Cu307 and PrBa2Cu307 layers [3]. Compared to single crystalline YBa2Cu30 7 thin films, the YBa2Cu3OT/PrBa2Cu307 superlattices have markedly different superconducting properties: (i) for ultrathin YBa2Cu307 layers ~t" depends on the PrBa2Cu307 layer thickness, and (ii) the electrical resistivity at the superconducting transition depends
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O. Eibl et al. I Mtcro.st,ucture qf YBa2(u.¢O,lt'rBa2f'u~O, s tperlattices"
less sensitively upon an external magnetic field when applied parallel to the substrate and the YBa2Cu~OT/ PrBa_,Cu307 interfaces. The results obtained on 2,3 nm ( Y ) / I . 2 nm (Pr), 2.3 nm (Y)/2.3 nm (Pr) and 2.3 nm (Y)/14.4 nm (Pr) superlattices can be interpreted such that the magnetic flux vortices arc predominantly locked in the semiconducting PrBa,Cu307 layers, thus suppressing the magnetic flux flow [3]. Some of these striking results have now been confirmed by other groups [5,6]. More recently, Nd,_ , C e . , C u O 4 / Y B a 2 C u 3 0 7 supcrlatticcs were realized successfully [7], illustrating the growing interest in artificially layered high-temperature superconductor structures. In this paper we characterized the microstrueture of YBa2Cu3OT/PrBa2Cu~O7 superlattices by transmission electron microscopy. The SrTiO3-superconductor interface and the sharpness of the interfaces between the various layers are of special interest.
2. Experimental Thin film superlattices were deposited by DC planar magnetron sputtering using two stoichiometric targets of composition YBa2Cu307 and PrBa2Cu307. For the multilayer deposition two magnetron sputtering guns are placed opposite each other.
In the middle, between the two guns. the substrate is mounted on a platform which can bc rotated by a step motor. The platform (substrate) faces one of the two guns (targets) and can bc rotated by the step motor to face the second target. After a layer of certain thickness is grown the step motor rotates the substratc by 180 = to face the other target. This proccdurc is continued until all layers arc deposited and the total thickness of the film is reached, which is usually 120 to 150 nm. The substratc to target distancc is 2 to 3 cm. The total pressure during deposition is approximately 300 mTorr (0.4 mbar) and the oxygen partial pressure is 20 mTorr (2.7X 10 -2 mbar). A more complete description of the deposition process is given in refs. [ 1,8 ]. The sample studied by TEM had a 10.5 nm ( Y ) / 10.5 nm (Pr) superlattice. During deposition the substratc temperature was 790 :C and the deposition rate was 0.05 nm/s. The first and last layer consisted of YBa2Cu30~_,. The X-ray diffraction pattern of the film (fig. I ) clearly shows the satellite reflections due to the superlattice up to third order. For the TEM analysis cross sections were prepared by cutting bars out of the substrate after the film deposition, gluing two bars face-to-face such that the thin films were adjacent to each other and separated by the glue. The subsequent preparation was equivalent to that of plan view samples. It included me-
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O. Eibl et al. / Microstructure of YBae('u.~O,/PrBa,Cu~07 superlattwes
chanical grinding and polishing, d i m p l i n g and ion milling with effective liquid nitrogen cooling. The samples were put into the microscope i m m e d i a t e l y after ion milling to a v o i d degradation o f the samples by exposure to the atmosphere. The high-resolution study was carried out in a 400 kV microscope e q u i p p e d with a high-resolution pole piece ( p o i n t - t o - p o i n t resolution 0.17 n m ) . Diffraction contrast images were o b t a i n e d in an analytical microscope at 200 kV. YBa2Cu307 has an o r l h o r h o m b i c crystal structure (space group P m m m ) and the lattice p a r a m e t e r s a = 0 . 3 8 2 n m and b = 0 . 3 8 8 n m differ only slightly. PrBa2Cu307 is i s o m o r p h i c with YBa2Cu307 but the difference between the lattice p a r a m e t e r s a and b is evcn smaller than for YBa2Cu307 [9].
367
3. Results and discussion The sequence o f the different layers can be seen in the low-magnification, high-resolution image o f fig. 2 ( a ) . The intcrfaces between the various layers appear sharp, however, the layers are corrugated with a period o f a p p r o x i m a t e l y 1 lain due to a eorrugatcd substrate. The cross sectional sample p e r m i t t e d the film to bc imaged along several 10 lam, and patterns as revealcd in fig, 2a were always obscrved. The contrast between the different layers originates p r e d o m i n a n t l y from the difference in scattering strength between Y ( Z = 3 9 ) and Pr ( Z = 5 9 ) . The YBa,Cu~O7 layers a p p e a r brighter than the PrBa~_Cu~O7 layers in bright-field (high-resolution) images. The extinction distances (which are the rel-
Fig. 2. (a) High-resolution, low-magnification image of the YBa~Cu~Ov/PrBa2Cu307 superlattice (cross section). The interfaces between the various layers are indicated, the SrTiO~ interface is indicated by arrows. (b) [ 110] diffraction pattern at the SrTiO3-thin film cross section. Reflections of the substrate and film are superimposed. In the left insert the (002 ), (003 ) and (004) reflections are shown enlarged and the satellite reflections are indicated by single arrows. Fundamental reflections are indicated by double arrows.
368
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cvant physical quantities used in electron diffraction thco~') differ only slightly (e.g. 7% for the ( 0 0 6 ) reflection at 400 kV) between YBa2Cu307 and the isomorphic c o m p o u n d PrBa2Cu307 a more detailed analysis of dynamical electron diffraction in YBa2Cu307 and PrBa2Cu307 will be given elsewhere [ 12 ]. The rather weak contrast between the layers is often invisible due to superimposed contrast originating from the strain field of dislocations and the thickness dependence o f the lattice fringe contrast in high-resolution images. The presence of the supcrlattice is also observed in electron diffraction patterns obtained with a highly parallel beam (fig. 2 ( b ) ) . First-order satellite diffraction spots are observed corresponding to a period of 21.1 nm and are indicated in the left part o f fig. 2 ( b ) . A high-resolution image of the SrTiO3-YBa2Cu307 interface is shown in fig. 3. A step of one unit cell height is observed. The change in position of the interface is indicated by the two arrows. Close to the step a second phase particle approximately 10 nm in diameter is found. By imaging larger areas of the film in diffraction contrast, second phase particles of the same size could be identified by a Moir6 fringe contrast. In YBa2Cu307 thin films deposited on ( 1 0 0 ) SrTiO3, precipitates of similar morphology and size,
also showing a Moir6 fringe contrast were identified as cubic Y203 [ 10]. Therefore, it is assumed that the particle in fig. 4 is a Y203 precipitate. In spite of a step at the SrTiO3 surface and a second phase particlc at the interface, the film was deposited without any observable extended defects: the surface step does not extend into the film by forming a stacking fault and the (0 0 1 ) lattice planes arc continued without shear from the left to the right part of the image. Presumably, in the very initial state o f growth a complete unit cell was deposited in the left part o f the image and only 2/3 of the unit cell was deposited in the right part. A similar surface step configuration was observed by Jia et al. [ I 1 ] in a DC sputtered film which was deposited at a lower substrate temperature (T~=700-'C). In this case, however, the surface step gave rise to an extended defect in the YBazCu307 film. We explain the different results by the enhanced surface mobility o f the atoms at 790:C, which would not allow the formation of an extended defect plus a surface step in the YBa2Cu307 thin film. The microstructure of the films can be studied over large distances (about 1 m m ) in plan view samples. Plan view samples are also valuable to determine the thickness of the ( 1 1 0) twins which is found to be approximately 20 nm (fig. 4). In the upper part of the fig. 4 the sample is thin and the SrTiO3 substrate
Fig. 3. High-resolution image of the SrTiO3-YBa2Cu307interface. The arrows indicate the position of the interface and are positioned at a surface step of the substrate.
O. Eibl et al. / Microstructure of YBa2Cu3OT/PrBa~Cu307 superlattices
369
Fig.4. Diffraction contrast image (under two beam conditions) in plan view. "'s'" indicates a second phase particle and "m'" indicates small precipitates revealing a Moire contrast. (110) twin boundaries are best visible in the upper part of the image and a few are indicated by arrows. was completely r e m o v e d by the ion milling procedure. In the lower-part o f the image a thin layer o f SrTiO3 is still beneath the YBa2Cu307 thin film, giving rise to a Moir6 fringe contrast. A n o t h e r type o f Moir6 fringe contrast is observed from small ( 10 n m ) second phase particles ( " m " in fig. 4) which were also observed in the high-resolution image (fig. 3). A n o t h e r second phase particle 40 nm in d i a m e t e r is m a r k e d by "s". The second phase particles exhibiting Moir6 contrast are observed only in the thicker regions o f the plan view sample, since they are located at or very close to the SrTiO3 interface. The presence o f twins is also evident from split diffraction spots in [0 0 1 ] diffraction patterns (fig. 5). The (1 1 0 ) , ( 2 2 0 ) a n d ( 3 3 0 ) reflections are shown at the b o t t o m eight times enlarged. The splitting o f spots is best visible for the (3 3 0) reflections. Diffraction patterns o f single crystal YBa2Cu307 thin Fig. 5.[00;1] diffraction pattern of the YBa2Cu3OT/Pr- b. Ba2Cu~O7 superlattice (plan view ). The presence of orthorhombic ( 1 I 0) twins leads to spot splitting which is best seen for the (330) reflections (insert).
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films revealed a similar amount of spot splitting, corresponding to a ( b - a ) / b ratio of ~1.6% [10]. However, for the films studied in ref. [ 10] the two diffraction spots (one belonging to the matrix, the other to the twin) were clearly separated, in fig. 5 diffracted intensity is also observed between the two (3 3 0) spots which are indicated by arrows. This diffracted intensity is due to the PrBa2Cu307 layers and to the slightly different lattice parameters of this compound, which yields a smaller ( b - a ) / b ratio than YBa2Cu3OT. Also, a contribution of the underlying SrTiO3 substrate cannot be ruled out. The ( h - a ) / b ratio o f 1.6% indicates an oxygen content of larger or equal to YBa2Cu306.8 for the deposited layers [ I 0 ]. Transmission electron microscopy o f the cross section also revealed that the surface structure of the substrate was not perfect. Some areas showed a high density of surface steps, yielding inclined surfaces with respect to the crystallographic ( 0 0 1 ) plane of the SrTiO3. Such an area is shown in the.high-resolution image of fig. 6. The arrows were placed at the approximate position o f the surface steps and indicate the plane of the interface. The average SrTiO3 surface would have an inclination such that it is higher in the left part and lower in the right part of the image. The reaction of the YBa2Cu307 thin film to such an imperfect surface is very moderate: a zone approximately 2-6 nm wide is formed at the interface which accommodates for the steps by introducing stacking faults and dislocations. At a distance of 4-6 nm from the interface the ( 0 0 1 ) lattice planes are already continuous from one margin of the image to the order. The ( 0 0 I ) lattice planes are parallel to the average surface of the substrate which deviates from a crystallographic (0 0 1) plane by about 5 °. In the region "A" o f fig. 6 where the distance between two adjacent steps is 16 nm, the (0 0 1 ) ,planes are even parallel to the crystallographic (0 0 1 ) planes of the SrTiO3 substrate. Figure 7 shows a region o f an inclined substratefilm interface, the ( 1 1 0) lattice planes are continuous across the interface and no extended defects can be observed in the film. Steps at the SrTiO3 interface are indicated by arrows. A larger area containing the region of fig. 7 is shown in fig. 8. The tilt angle between the average surface and the SrTiO3 ( 0 0 1 ) planes is 4 ° . This can be seen from the lines indicating the projection of the ( 0 0 1 ) planes of SrTiO3
372
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Fig. 8. A larger area of an inclined region of the SrTiO3 substrate. The projection of (00 1 ) lattice planes of SrTiO~ is indicated by single arrows and the projection of (0 0 I ) YBa2Cu307 lattice planes is indicated by double arrows. They enclose an angle of approximately 4 ': .
(single arrows) and the projection of the YBa2Cu307 (00 I ) planes (double arrows).
4. Conclusion and summary
In this paper we have shown that the deposition of YBa2Cu307/PrBa2Cu307 superlattices yields c-oriented, high-quality thin films with highly ordered layers consisting of either Y B a 2 C u 3 0 7 o r PrB a 2 C u 3 0 7 . The sharpness of the YBa2Cu3OT/PrB a 2 C u 3 0 7 interface is of the order of the lattice parameter ( ~ 1 nm). Satellite reflections could be clearly resolved by X-ray and electron diffraction. These results are especially important since the substrafe temperature during deposition was as high as 790~C. Even at this high temperatures no evidence of Y / P r interdiffusion could be detected. The activation energy for Y / P r diffusion in Y/PrBa2Cu307 must be therefore considerably larger than 0.1 eV. The thickness of the ( 1 I 0) twins was found to be 20 nm. The (b-a)/h ratio was 1.6%. Second phase particles were identified by a Moir6 contrast at the SrTiO3/YBaaCu307 interface. High-resolution images showed surface steps and areas inclined to the crystallographic ( 0 0 1 ) plane of the SrTiO3 substrate. However, surface steps did not lead to the formation of stacking faults in the films. At inclined areas of the substrate surface, a very thin layer approximately 2-6 nm in thickness was formed which contained defects accounting for the misfit (tilt). At distances further away from the interface the (0 0 1 ) planes of the Y B a 2 f u 3 0 7 w e r e parallel to the average direction of the surface and (0 0 1 ) lattice planes were continuous from areas with an atomically planar surface to areas containing a high density of steps. It is assumed that the high deposition temperature and
thus the high mobility of the atoms on the film surface during the deposition is necessary to avoid defect formation in the film in areas with a high density of steps at the surface of the substratc.
References [ I ] J.M. Triscone, M.G. Karkut, L. Antognazza, O. Brunner and O. Fischer, Phys. Rev. Lett. 63 (1989) 1016. [ 2 ] J.M. Triscone, O. Fischer, O. Brunner, L. Antognazza, A.D. Kent and M.G. Karkut, Phys. Rev. Len. 64 (1990) 804. [ 3 ] J.M. Triscone, O. Fischer, O. Brunner, L. Antognazza, A.D. Kent, L. Mi~ville and M.G. Karkut, in: Science and Technology of Thin Film Superconductors II, eds. R D . McDonnet and S.A. Wolf (Plenum Press, 1990). [4] P. Brunner, J.M. Triscone, L. Antognazza, L. Mi6ville, M.G. Karkut and O. Fischer, The magnetic field induced broadening of the resistive transitions in YBa2CU~OT/ PrBa2Cu~O7 superlattices, E-MRS Conf., Strasbourg, spring 1990, J. Less Common Metals 164-165 (1990) 1186. O. Fischer, J.M. Triscone, L. Antognazza, O. Brunner, A.D. Kent, L. Mi~ville and M.G. Karkut, Artificially prepared YBa2Cu~O7/PrBa2Cu~07 superlattices: growth and superconducting properties, ibid., J. Less Common Metals 164-165 (1990) 257. [5] D.H. Lownde, D.P. Norton, J.D. Budai, S.J. Pennycook, D.K. Christen, B.C. Sales and R. Feenstra, Proc. MRS spring meeting, Symposium N, to be published. [6] Q. Li, X.X. Xi, X.D. Wu, A. lnam, S. Vadlamannati, W.L. McLean, T. Venkatesan, R. Ramesh, D.M. Hwang, J.A. Marlinez and L. Nazar, Phys. Rev. Lett. 64 (1990) 3086. [ 7 ] A. Gupta, R. Gross, E. Olsson, A. Segmiiller, G. Koren and C.C. Tsuei, Phys. Rev. Left. 64 (1990) 3191. [8] J.M. Triscone, M.G. Karkut O. Brunner, L. Antognazza, M. Decroux and O. Fischer, Physica C 158 (1990) 293. [9] J.L. Peng, P. KJavins, R . N Shehon, H.B. Radousky, P.A. Hahn and L. Bernandez, Phys. Rev. B 40 (1989) 4517. [ l 0 ] O. Eibl and B. Roas, submitted to J. Mater. Res. ( 1990 ). [ 1 l ] C.L. Jia, B. Kabius, H. Soltner, U. Poppe, Ch. Buchal, J. Schubert and K. Urban, Physica C 167 (1990) 463. [ 12] O. Eibl, J.M. Triscone and O. Fischer, Phys. Stat. Sol. (a) 122 (1990) in print.
Microstructure of YBa2Cu307/PrBa2Cu307 superlattices deposited on ( 1 0 0) SrTiO3 single crystals O. Eibl Siemens Research Laboratories, Otto tlahn Ring 6, W-8000 Munich 83, Germany
H.E. Hoenig Siemens Research Laboratories, Paul Gossen Strafle 100, I+'-8520 Erlangen, Germany
J.-M. Triscone, t0. Fischer, L. Antognazza and O. Brunner Universite de Genbve, DPMC, 24 Quai E. Ansermet, 1211 Genbve, Switzerland
Received 25 July 1990 Revised manuscript received 16 October 1990
High-quality,t-oriented YBa2CusOT/PrBa2Cu307superlattices have been grown on ( 10 0 ) SrTiO3 substrates. TEM cross sections revealed that the sharpness of the (0 0 1) interfaces was of the order of the lattice parameter ( 1 nm ). The films were deposited at 790°C and no evidence of Y/Pr interdiffusion could be detected. The orthorhombic ( I I 0) twins were 20 nm wide and the orthorhombic distortion in the YBa2Cu307layer yielded a ( b - a ) / b ratio of 1.6%. Surface steps and inclined surface areas at the SrTiOs-YBa~Cu307interface were imaged by high-resolutionTEM. Apart from a 2-6 nm wide layer at the interface, which contained extended defects accounting for the mismatch due to the imperfect surface, the film grew perltectlyon top of this layer. (0 0 I ) lattice planes passed continuously from regions with perfect substrate surfaces to regions containing a larger density of surface steps.
1. Introduction The deposition and the electrical properties of superlattices based on the alternating sequence of YBa2Cu3OT/MBa2Cu307 (M = Dy, Pr) layers have been the subject of a n u m b e r of studies [ 1-4]. In particular, YBa2Cu3OT/PrBa2Cu307 heterostructures are of interest because of the different electrical properties of the two compounds, the first being a superconductor and the second a semiconductor. A n u m b e r of applications seem to be in reach, e.g. tunnel j u n c t i o n s for SQUIDs. We have deposited superlattices by DC magnetron sputtering on ( 1 0 0) SrTiO3 and MgO single crystal substrates [ 1-4]. The mismatch between substrate and film is smaller for SrTiO3 ( ~ 3 % ) and significantly larger for MgO ( ~ 8 . 5 % ) . However. YBa2Cu307 thin films can be grown on both substratcs epitaxially with the c-axis perpendicular to the substrate surface.
In a recent study [ 1 ] a two-unit-cell superlattice was grown on ( 1 0 0) MgO with a periodicity 0 f 2 . 3 nm, i.e. the sequence of unit cells was intended to be 1.17 nm-YBa2Cu3OT/1.17 nm-DyBa2Cu3OT/1.17 nm-YBa2Cu307 perpendicular to the substrate surface. Calculations of the X-ray diffraction pattern for this sample showed that as much as 87% of the rare earth species were arranged in the structure in an ordered way [1 ]. For superlattices with layer thicknesses larger than 5 n m even t h i r d order satellite reflections were observed, which appeared due to the sharp interfaces between the YBa2Cu307 and PrBa2Cu307 layers [3]. Compared to single crystalline YBa2Cu30 7 thin films, the YBa2Cu3OT/PrBa2Cu307 superlattices have markedly different superconducting properties: (i) for ultrathin YBa2Cu307 layers ~t" depends on the PrBa2Cu307 layer thickness, and (ii) the electrical resistivity at the superconducting transition depends
0921-4534/90/$ 03.50 © 1990 - Elsevier Science Publishers B.V. ( North-Holland )
366
O. Eibl et al. I Mtcro.st,ucture qf YBa2(u.¢O,lt'rBa2f'u~O, s tperlattices"
less sensitively upon an external magnetic field when applied parallel to the substrate and the YBa2Cu~OT/ PrBa_,Cu307 interfaces. The results obtained on 2,3 nm ( Y ) / I . 2 nm (Pr), 2.3 nm (Y)/2.3 nm (Pr) and 2.3 nm (Y)/14.4 nm (Pr) superlattices can be interpreted such that the magnetic flux vortices arc predominantly locked in the semiconducting PrBa,Cu307 layers, thus suppressing the magnetic flux flow [3]. Some of these striking results have now been confirmed by other groups [5,6]. More recently, Nd,_ , C e . , C u O 4 / Y B a 2 C u 3 0 7 supcrlatticcs were realized successfully [7], illustrating the growing interest in artificially layered high-temperature superconductor structures. In this paper we characterized the microstrueture of YBa2Cu3OT/PrBa2Cu~O7 superlattices by transmission electron microscopy. The SrTiO3-superconductor interface and the sharpness of the interfaces between the various layers are of special interest.
2. Experimental Thin film superlattices were deposited by DC planar magnetron sputtering using two stoichiometric targets of composition YBa2Cu307 and PrBa2Cu307. For the multilayer deposition two magnetron sputtering guns are placed opposite each other.
In the middle, between the two guns. the substrate is mounted on a platform which can bc rotated by a step motor. The platform (substrate) faces one of the two guns (targets) and can bc rotated by the step motor to face the second target. After a layer of certain thickness is grown the step motor rotates the substratc by 180 = to face the other target. This proccdurc is continued until all layers arc deposited and the total thickness of the film is reached, which is usually 120 to 150 nm. The substratc to target distancc is 2 to 3 cm. The total pressure during deposition is approximately 300 mTorr (0.4 mbar) and the oxygen partial pressure is 20 mTorr (2.7X 10 -2 mbar). A more complete description of the deposition process is given in refs. [ 1,8 ]. The sample studied by TEM had a 10.5 nm ( Y ) / 10.5 nm (Pr) superlattice. During deposition the substratc temperature was 790 :C and the deposition rate was 0.05 nm/s. The first and last layer consisted of YBa2Cu30~_,. The X-ray diffraction pattern of the film (fig. I ) clearly shows the satellite reflections due to the superlattice up to third order. For the TEM analysis cross sections were prepared by cutting bars out of the substrate after the film deposition, gluing two bars face-to-face such that the thin films were adjacent to each other and separated by the glue. The subsequent preparation was equivalent to that of plan view samples. It included me-
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Fig. 1. X-ray diffraction pattern of the YBa2Cu30,/PrBa_,Cu307 superlattice. Third-order satellite reflections are clearly identified in the diffraction pattern on the right-hand side obtained with higher resolution.
O. Eibl et al. / Microstructure of YBae('u.~O,/PrBa,Cu~07 superlattwes
chanical grinding and polishing, d i m p l i n g and ion milling with effective liquid nitrogen cooling. The samples were put into the microscope i m m e d i a t e l y after ion milling to a v o i d degradation o f the samples by exposure to the atmosphere. The high-resolution study was carried out in a 400 kV microscope e q u i p p e d with a high-resolution pole piece ( p o i n t - t o - p o i n t resolution 0.17 n m ) . Diffraction contrast images were o b t a i n e d in an analytical microscope at 200 kV. YBa2Cu307 has an o r l h o r h o m b i c crystal structure (space group P m m m ) and the lattice p a r a m e t e r s a = 0 . 3 8 2 n m and b = 0 . 3 8 8 n m differ only slightly. PrBa2Cu307 is i s o m o r p h i c with YBa2Cu307 but the difference between the lattice p a r a m e t e r s a and b is evcn smaller than for YBa2Cu307 [9].
367
3. Results and discussion The sequence o f the different layers can be seen in the low-magnification, high-resolution image o f fig. 2 ( a ) . The intcrfaces between the various layers appear sharp, however, the layers are corrugated with a period o f a p p r o x i m a t e l y 1 lain due to a eorrugatcd substrate. The cross sectional sample p e r m i t t e d the film to bc imaged along several 10 lam, and patterns as revealcd in fig, 2a were always obscrved. The contrast between the different layers originates p r e d o m i n a n t l y from the difference in scattering strength between Y ( Z = 3 9 ) and Pr ( Z = 5 9 ) . The YBa,Cu~O7 layers a p p e a r brighter than the PrBa~_Cu~O7 layers in bright-field (high-resolution) images. The extinction distances (which are the rel-
Fig. 2. (a) High-resolution, low-magnification image of the YBa~Cu~Ov/PrBa2Cu307 superlattice (cross section). The interfaces between the various layers are indicated, the SrTiO~ interface is indicated by arrows. (b) [ 110] diffraction pattern at the SrTiO3-thin film cross section. Reflections of the substrate and film are superimposed. In the left insert the (002 ), (003 ) and (004) reflections are shown enlarged and the satellite reflections are indicated by single arrows. Fundamental reflections are indicated by double arrows.
368
O. Ethl et al. /Microstructure of YBa :('u.~OJt'rBa:('u~O, superlatttces
cvant physical quantities used in electron diffraction thco~') differ only slightly (e.g. 7% for the ( 0 0 6 ) reflection at 400 kV) between YBa2Cu307 and the isomorphic c o m p o u n d PrBa2Cu307 a more detailed analysis of dynamical electron diffraction in YBa2Cu307 and PrBa2Cu307 will be given elsewhere [ 12 ]. The rather weak contrast between the layers is often invisible due to superimposed contrast originating from the strain field of dislocations and the thickness dependence o f the lattice fringe contrast in high-resolution images. The presence of the supcrlattice is also observed in electron diffraction patterns obtained with a highly parallel beam (fig. 2 ( b ) ) . First-order satellite diffraction spots are observed corresponding to a period of 21.1 nm and are indicated in the left part o f fig. 2 ( b ) . A high-resolution image of the SrTiO3-YBa2Cu307 interface is shown in fig. 3. A step of one unit cell height is observed. The change in position of the interface is indicated by the two arrows. Close to the step a second phase particle approximately 10 nm in diameter is found. By imaging larger areas of the film in diffraction contrast, second phase particles of the same size could be identified by a Moir6 fringe contrast. In YBa2Cu307 thin films deposited on ( 1 0 0 ) SrTiO3, precipitates of similar morphology and size,
also showing a Moir6 fringe contrast were identified as cubic Y203 [ 10]. Therefore, it is assumed that the particle in fig. 4 is a Y203 precipitate. In spite of a step at the SrTiO3 surface and a second phase particlc at the interface, the film was deposited without any observable extended defects: the surface step does not extend into the film by forming a stacking fault and the (0 0 1 ) lattice planes arc continued without shear from the left to the right part of the image. Presumably, in the very initial state o f growth a complete unit cell was deposited in the left part o f the image and only 2/3 of the unit cell was deposited in the right part. A similar surface step configuration was observed by Jia et al. [ I 1 ] in a DC sputtered film which was deposited at a lower substrate temperature (T~=700-'C). In this case, however, the surface step gave rise to an extended defect in the YBazCu307 film. We explain the different results by the enhanced surface mobility o f the atoms at 790:C, which would not allow the formation of an extended defect plus a surface step in the YBa2Cu307 thin film. The microstructure of the films can be studied over large distances (about 1 m m ) in plan view samples. Plan view samples are also valuable to determine the thickness of the ( 1 1 0) twins which is found to be approximately 20 nm (fig. 4). In the upper part of the fig. 4 the sample is thin and the SrTiO3 substrate
Fig. 3. High-resolution image of the SrTiO3-YBa2Cu307interface. The arrows indicate the position of the interface and are positioned at a surface step of the substrate.
O. Eibl et al. / Microstructure of YBa2Cu3OT/PrBa~Cu307 superlattices
369
Fig.4. Diffraction contrast image (under two beam conditions) in plan view. "'s'" indicates a second phase particle and "m'" indicates small precipitates revealing a Moire contrast. (110) twin boundaries are best visible in the upper part of the image and a few are indicated by arrows. was completely r e m o v e d by the ion milling procedure. In the lower-part o f the image a thin layer o f SrTiO3 is still beneath the YBa2Cu307 thin film, giving rise to a Moir6 fringe contrast. A n o t h e r type o f Moir6 fringe contrast is observed from small ( 10 n m ) second phase particles ( " m " in fig. 4) which were also observed in the high-resolution image (fig. 3). A n o t h e r second phase particle 40 nm in d i a m e t e r is m a r k e d by "s". The second phase particles exhibiting Moir6 contrast are observed only in the thicker regions o f the plan view sample, since they are located at or very close to the SrTiO3 interface. The presence o f twins is also evident from split diffraction spots in [0 0 1 ] diffraction patterns (fig. 5). The (1 1 0 ) , ( 2 2 0 ) a n d ( 3 3 0 ) reflections are shown at the b o t t o m eight times enlarged. The splitting o f spots is best visible for the (3 3 0) reflections. Diffraction patterns o f single crystal YBa2Cu307 thin Fig. 5.[00;1] diffraction pattern of the YBa2Cu3OT/Pr- b. Ba2Cu~O7 superlattice (plan view ). The presence of orthorhombic ( 1 I 0) twins leads to spot splitting which is best seen for the (330) reflections (insert).
370
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films revealed a similar amount of spot splitting, corresponding to a ( b - a ) / b ratio of ~1.6% [10]. However, for the films studied in ref. [ 10] the two diffraction spots (one belonging to the matrix, the other to the twin) were clearly separated, in fig. 5 diffracted intensity is also observed between the two (3 3 0) spots which are indicated by arrows. This diffracted intensity is due to the PrBa2Cu307 layers and to the slightly different lattice parameters of this compound, which yields a smaller ( b - a ) / b ratio than YBa2Cu3OT. Also, a contribution of the underlying SrTiO3 substrate cannot be ruled out. The ( h - a ) / b ratio o f 1.6% indicates an oxygen content of larger or equal to YBa2Cu306.8 for the deposited layers [ I 0 ]. Transmission electron microscopy o f the cross section also revealed that the surface structure of the substrate was not perfect. Some areas showed a high density of surface steps, yielding inclined surfaces with respect to the crystallographic ( 0 0 1 ) plane of the SrTiO3. Such an area is shown in the.high-resolution image of fig. 6. The arrows were placed at the approximate position o f the surface steps and indicate the plane of the interface. The average SrTiO3 surface would have an inclination such that it is higher in the left part and lower in the right part of the image. The reaction of the YBa2Cu307 thin film to such an imperfect surface is very moderate: a zone approximately 2-6 nm wide is formed at the interface which accommodates for the steps by introducing stacking faults and dislocations. At a distance of 4-6 nm from the interface the ( 0 0 1 ) lattice planes are already continuous from one margin of the image to the order. The ( 0 0 I ) lattice planes are parallel to the average surface of the substrate which deviates from a crystallographic (0 0 1) plane by about 5 °. In the region "A" o f fig. 6 where the distance between two adjacent steps is 16 nm, the (0 0 1 ) ,planes are even parallel to the crystallographic (0 0 1 ) planes of the SrTiO3 substrate. Figure 7 shows a region o f an inclined substratefilm interface, the ( 1 1 0) lattice planes are continuous across the interface and no extended defects can be observed in the film. Steps at the SrTiO3 interface are indicated by arrows. A larger area containing the region of fig. 7 is shown in fig. 8. The tilt angle between the average surface and the SrTiO3 ( 0 0 1 ) planes is 4 ° . This can be seen from the lines indicating the projection of the ( 0 0 1 ) planes of SrTiO3
372
O. Eibl et al. / Mtcrostructure of YBa zCu 3Oz/PrBae('u 307 superlatttces
Fig. 8. A larger area of an inclined region of the SrTiO3 substrate. The projection of (00 1 ) lattice planes of SrTiO~ is indicated by single arrows and the projection of (0 0 I ) YBa2Cu307 lattice planes is indicated by double arrows. They enclose an angle of approximately 4 ': .
(single arrows) and the projection of the YBa2Cu307 (00 I ) planes (double arrows).
4. Conclusion and summary
In this paper we have shown that the deposition of YBa2Cu307/PrBa2Cu307 superlattices yields c-oriented, high-quality thin films with highly ordered layers consisting of either Y B a 2 C u 3 0 7 o r PrB a 2 C u 3 0 7 . The sharpness of the YBa2Cu3OT/PrB a 2 C u 3 0 7 interface is of the order of the lattice parameter ( ~ 1 nm). Satellite reflections could be clearly resolved by X-ray and electron diffraction. These results are especially important since the substrafe temperature during deposition was as high as 790~C. Even at this high temperatures no evidence of Y / P r interdiffusion could be detected. The activation energy for Y / P r diffusion in Y/PrBa2Cu307 must be therefore considerably larger than 0.1 eV. The thickness of the ( 1 I 0) twins was found to be 20 nm. The (b-a)/h ratio was 1.6%. Second phase particles were identified by a Moir6 contrast at the SrTiO3/YBaaCu307 interface. High-resolution images showed surface steps and areas inclined to the crystallographic ( 0 0 1 ) plane of the SrTiO3 substrate. However, surface steps did not lead to the formation of stacking faults in the films. At inclined areas of the substrate surface, a very thin layer approximately 2-6 nm in thickness was formed which contained defects accounting for the misfit (tilt). At distances further away from the interface the (0 0 1 ) planes of the Y B a 2 f u 3 0 7 w e r e parallel to the average direction of the surface and (0 0 1 ) lattice planes were continuous from areas with an atomically planar surface to areas containing a high density of steps. It is assumed that the high deposition temperature and
thus the high mobility of the atoms on the film surface during the deposition is necessary to avoid defect formation in the film in areas with a high density of steps at the surface of the substratc.
References [ I ] J.M. Triscone, M.G. Karkut, L. Antognazza, O. Brunner and O. Fischer, Phys. Rev. Lett. 63 (1989) 1016. [ 2 ] J.M. Triscone, O. Fischer, O. Brunner, L. Antognazza, A.D. Kent and M.G. Karkut, Phys. Rev. Len. 64 (1990) 804. [ 3 ] J.M. Triscone, O. Fischer, O. Brunner, L. Antognazza, A.D. Kent, L. Mi~ville and M.G. Karkut, in: Science and Technology of Thin Film Superconductors II, eds. R D . McDonnet and S.A. Wolf (Plenum Press, 1990). [4] P. Brunner, J.M. Triscone, L. Antognazza, L. Mi6ville, M.G. Karkut and O. Fischer, The magnetic field induced broadening of the resistive transitions in YBa2CU~OT/ PrBa2Cu~O7 superlattices, E-MRS Conf., Strasbourg, spring 1990, J. Less Common Metals 164-165 (1990) 1186. O. Fischer, J.M. Triscone, L. Antognazza, O. Brunner, A.D. Kent, L. Mi~ville and M.G. Karkut, Artificially prepared YBa2Cu~O7/PrBa2Cu~07 superlattices: growth and superconducting properties, ibid., J. Less Common Metals 164-165 (1990) 257. [5] D.H. Lownde, D.P. Norton, J.D. Budai, S.J. Pennycook, D.K. Christen, B.C. Sales and R. Feenstra, Proc. MRS spring meeting, Symposium N, to be published. [6] Q. Li, X.X. Xi, X.D. Wu, A. lnam, S. Vadlamannati, W.L. McLean, T. Venkatesan, R. Ramesh, D.M. Hwang, J.A. Marlinez and L. Nazar, Phys. Rev. Lett. 64 (1990) 3086. [ 7 ] A. Gupta, R. Gross, E. Olsson, A. Segmiiller, G. Koren and C.C. Tsuei, Phys. Rev. Left. 64 (1990) 3191. [8] J.M. Triscone, M.G. Karkut O. Brunner, L. Antognazza, M. Decroux and O. Fischer, Physica C 158 (1990) 293. [9] J.L. Peng, P. KJavins, R . N Shehon, H.B. Radousky, P.A. Hahn and L. Bernandez, Phys. Rev. B 40 (1989) 4517. [ l 0 ] O. Eibl and B. Roas, submitted to J. Mater. Res. ( 1990 ). [ 1 l ] C.L. Jia, B. Kabius, H. Soltner, U. Poppe, Ch. Buchal, J. Schubert and K. Urban, Physica C 167 (1990) 463. [ 12] O. Eibl, J.M. Triscone and O. Fischer, Phys. Stat. Sol. (a) 122 (1990) in print.