Structural characterization of epitaxial BiSrCaCuO films on MgO substrates

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Physica C 245 (1995) 295-300

Structural characterization of epitaxial BiSrCaCuO films on MgO substrates L. Ranno II

Groupe de Physique des Solide~'J

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J. Perriere

a,

J. Schneck

b

URA 17, UIli!JersitesParis VII el Paris VI. Tour 23, 2. Place Jussieu, 752511'aris Cedex 05. Frallce h CNET, 196 AlIelllleHellri Ral'era, BP 107, 92220 Bag1lC!uxCedex, Frallce Received 13 January 1994; revised manuscript received 23 January 1995

Abstract We have studied the epitaxial growth of thin Bi2Sr2CalCu201l films on (DOl) oriented MgO substrates. These films were grown using the in-situ laser ablation process (700°C, 0.1 mbar oxygen), and they showed a perfect c-axis alignment normal to the substrate, cb,,:'8cterised by narrow mosaic distributions and by channeling effects (low Xmin values) along this direction. Despite the large difference between their lattice parameters (9%), the epitaxial growth of the BiSrCaCuO film on MgO substrate has been found evidence for using X-ray diffraction in transmission geometry, by the observation of three azimuthal adjustments of the a- and b-axes of the films with respect to the a-axis of the substrate (00, 13° and 45°). The possible origins of such epitaxial relationships are presented and discussed.

1. Introduction The new high·~ superconducting materials (Y, Bi or Tl based oxides) have in common a layered structure which is believed to be responsible for their very special properties. Due to this layered structure a large difference of growth rates exists between the direction parall ~l (a-b plane) and perpendicular (c· axis) to the CuD planes. That is why highly textured films are generally grown, with their c·axis normal to the substrate. However, this does not mean lhat epitaxial films are formed. In fact, the films grown by a two·step process (deposition followed by a high-temperature oxygen annealing), were granular textured films without any particular in.plane orien·

• Corresponding author. Present address: Department of Physics, Trinity College, Dublin 2, Republic of Ireland.

tations of the a- and b-axes of the grains. This mosaicity leads to a degradation of the measured transport properties compared to the single crystal ones, in particular the critical currents of non-epitaxial films are one or even several orders of magnitude lower than those of a single crystal or of an epitaxial film. The situation has evolved and nowadays nanostructures, multilayers and even a single layer require the use of the best films one can grow, in order to get the sharpest interfaces and no defects (grain boundaries, roughness ... ). Such a quality requires the use of tri-axiaIly textured or "epitaxially" grown films. For YBaCuO, the situation appears to be well documented, and high-quality epitaxial films (high Te , narrow transition width and high critical current densities) are now routinely obtained [1,2]. On the contrary the growth of BiSrCaCuO films is less developed, and only a few reports of "epitaxy" have

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L. RamlO ct at. / Physico C 245 (1995) 295-300

been made [3,4,9,16,17]. In this letter, we report on the structural characterisations of epitaxiaJ Bi 2Sr 2Ca,Cu 20 Il +li thin films in situ grown on MgO substrates by the pulsed laser derosition technique. These superconducting films were found highly textured, and in the a-b plane, three orientations have been found evidence for (oriented at 0°, ± 13° and ± 45° with respect to the a-axis of the MgO SUbstrate), showing the epitaxial nature of the growth.

2. Experimental The epitaxial BiSrCaCuO films were grown using the in-situ pulsed laser deposition technique [1]. A frequency tripled (354 nm) Nd: YAG laser (BM Industries) was used to ablate a Bi2Sr2Ca1Cu201l+ Ii (j.e. the "2212" pnase) ceramic target. Films, in the 30 to 100 nm thickness range, were grown on MgO (DOl) single crystal substrates heated around 7uO°C under 0.1 mbar oxygen pressure. After deposition the heater was turned off, and the sample was cooled down under 1 bar oxygen. The composition of the films was measured by Rutherford backscattering spectrometry (RBS), and was found to be a few percent off the ideal "2212" composition, but this did not preclude the formation of the crystalline "2212" phase in the films, as has beeJi observed by X-ray diffraction in the standard Bragg-Brentano mode. The superconducting properties of such films were previously reported [5]. Due to a non-optimised oxygen content in the films, the resistivity curves show wide transitions and a low ~, in the 30-50 K range. We have studied the texture of these films and their crystalline quality using X-ray diffraction and RBS in channeling geometry. The mosaic spread (c-axis disorientation) was measured through the FWHM of rocking curves recorded on the (0010) diffraction peak of the film. The crystalline quality was also characterised by the Xmin values, i.e. the ratio of aligned to random yields of backscattered He + ions during channeling experiments. Using a standard X-ray photographic precession camera with a monochromatic Cu Ka X-ray source, we have obtained cleai and quantitative informations about the in-plane epitaxial relationships, between the BiSrCaCuO films and the MgO substrate. This method

gives photographs of Ilkl planes of the film, superimposed with the corresponding planes of the substrate since it is a transmission ar.l\lysis. Using two different masks, "kG and Itk1 plane photographs of the BiSrCaCuO film were easily obtained, without distortion, and a direct measurement of in-plane orientations was performed. 3. Results

The substrates are MgO (00)), 0.5 mm thick, single crystals which have been mechanicalJy and then chemically polished. The properties of the MgO substrates prior to the film deposition were characterised by channeling experiments, and Xmin values lower than 2% were obtained using 2 MeV He + ions. This value indicates a good crystalline quality of these substrates, without any additional annealing treatment. However, the surface peak in the channeled spectra showed the presence of a disturbed layer in the near surface region of the substrate, Le. hbout 1 to 2 nm of MgO are disordered ncar the surface. Although a defect~free surface would have been more suitable for epitaxial-layer formation, we obtained the growth of epitaxial BiSrCaCuO films on such imperfect MgO substrates. The X-ray diffraction patterns of such films showed only the presence of the (OOl) lines of "2212" phase, indicating a textured growth with the c-axis perpendicular to the substrate surface. The crystalline quality of the films was further studied by the rocking curves recorded on the (0010) diffraction peak of the films. For comparison purposes, the rocking curve for the (002) line of the MgO substrate was also recorded, and Figs. 1(a) and (b) represent these two curves. In both cases, a single peak is observed, and although the FWHM appears to be larger for the BiSrCaCuO film (0.5° compared to 0.02°), the absence of wide tails gives evidence of a moderate mosaic spread fo;' the c-axis orientation. This nearly perfect c-axis orientation was checked by RBS analysis in channeling geometry, since Xmin values in the 30 to 50% range were measured for 30 to 100 nm thick BiSrCaCuO films. Such values can be compared to tlte results obtained on pure Bi 2 Sr2Ca,Cu 20 Il single crystal (20%), and they may be related to the incommensurate modulatioJ presence in this material'as previously reported [5].

L. RallIlO C!( al. / PltyJicaC 245 (J/}lJS)295-300

The in-plane structural order of these textured films was studied by the X-fa} precession methorl. and Fig. 2(a) represents a photograph of the hkO plane of the film and its substrate. Fig. 2(b) is the indexed scheme of the photograph. The darker spots are the MgO diffraction spots. One can observe smaller spots on this picture, located on three square lattices corresponding to the 2212 a· and b· values. Thus, this photograph gives evidence for epitaxial relationships betwee.n the BiSrCaCuO film and the substrate, contrary to the case of ex-situ films (deposition at room temperature followed by a hightemperature annealing), for which the diffuse rings for the 2212 diffraction that are observed are typical of a random distribution of the cryHuilites. The picture has a four-fold symmetry but sInc~ the BiSrCaCuO lattice· is almost square it does not imply that the film sym'metry is four-fold. The only three epitaxial relations we have found on our films are: ± 13(1 between BiSrCaCuO [100] axes and MgO

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[I 00], ±45° i.e. a cube-on-cube relationship of BiSrCaCuO [l JOJ on MgO [lOOJ and 00 i.e. cube-an-cube relationship of BiSrCuCuO [lOO] on MgO [100]. In addition to the grain orientation, a Ilk1 plane picture of the film gives information on the incom~ mensurate modulation in the a-b plane of the films. Fig. 3 represents the scheme of a precession photograph of BiSrCaCuO hk1 plane superimposed on MgO (h k 0.13). Satellites are seen on the 2212

L. Ranl/o et al. / Physica C 245 (J99S) 295-300

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diffraction spots, and show the presence of the incommensurate modulation of the 2212 structure. An estimation of its wavelength gives a value in the range of 4a to 5a (i.e. 2.1-2.7 nm) as reported in the literature. Th~ 2212 signal is weaker than in the hkO case (even with a much larger time of exposure) and the MgO signal is still present even if it does no more correspond to a reciprocal plane of the MgO structure. Satellites can be seen in both perpendicular directions for' each orientation so, as expected, the four-fold symmetry of the substrate is respected by the film, contrary to the case of a cleaved substrate where the modulation can be aligned in a specific direction [6].

4. Discussion The epitaxial growth of BiSrCaCuO films on MgO substrates is somewhat surprising, because there

are no simple geometrical relationships between the film and the substrate as shown by the poor lattice match between the two materials (0.42 nm compared to 0.54 nm a·axes parameters). Th~ lattice matching required for this .epitaxial growth is achieved by azimuthal adjustment of the film plane with respect' to the film substrate that we have observt:d, i.e. ± 13°, ±45° or 0° rotations. It is not clear what precisely determines the film orientation in both directions. For the ± 13° direction, such an epitaxial relntionship has been previously observed [6] for the growth by ion beam sputtering of BiSrCaCuO films on cleaved MgO substrates. It was believed that this special orientation was related to the nature nf the surface, i.e. the step edges in the cleaved surface playing an important role, with the incommensurate modulation tend· ing to align its direction in order' to avoid the step edges. In the same way, it is well known that the microstructure and properties of YBaCuO films on MgO are strongly dependent upon the precise state of the substrates [7]. As a matter of fact, various surface preparations (low-energy ion beam treatment [8] for example) promote the growth of YBaCuO films on MgO with different epitaxial relations of the nucleus to the substrate. For the 45° in-plane oricnta~ tion, it has been reported [9] that the growth of BiSrCaCuO films by RF magnetron sputtering at a relatively low temperature (540°C), promotes the alignment of the a-axis of the film with the (110) direction of the MgO substrate. At the same time, the 0° direction is also observed (coincidence of the (100) direction of the film and substrate), with a

Table 1 Values of this misfit () for various misorientations (limited to the configurations with 0"2212 < 10 unit cell of the 2212 phase) of the of the film (F) and of the substrate (S). ~ is evaluated at 700°C using a 0.425 nm MgO parameter Matching vectors F: Film s: Substrate [300]p [31O]f [400]f [300]f [310]F [lOO]f

and [400]s and [420]5 and [51O]s and [41O]s and [400]s and [110]s

Misorientation

Distortion () aMgO = 0.425 nm a2212 = 0,54 nm

[IOO]MgO'

[100]Bi5rCaCuO 0° 8.1 0 11.3° 14° 18.4° 45° 13° 12°

4.4% 10.7% 0.1% 7.4% 0.8% 10.3% ,.

0'2212

Ref.

a~212

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[4,9] [9,13]

9

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L, RaWIO f!t al./ Physica C 245 (1995) 295-300

comparable fraction of domains presenting these 00 and 45 0 orientations. In our case the 00 orientation is a minor orhmtation. In our own work, the MgO substrate was not cleaved, and we did noi try to change th0 MgO surface morphology using a specific treatlnent, there~ fore our results appear as intrinsic of the gmwth of BiSrCaCuO films on MgO by the pulsed laser abla~ tion process. To explain these results, a 'possible approach, classically used in the case of epitaxial high-~ films [10,11], is based on the near coincidence site lattice (NCSL) theory [12J, which allows one to determine the interfacial configurations with the higher densities of coincidence sites. In this frame, one has to superimpose a 2212 cell which surface is CT221 2 (generally larger than the unit cell) on a matching MgO cell (surface CTMgO)' allowing for a distortion of the 2212 lAttice. For BiSrCaCuO on MgO, the degree of coincidence of the two lattices is measured by the misfit 8 given by

8=2

IVO"MgO VUMgO

v~1

(1)

+ VU 2212

Table 1 represents some values of this misfit for various misorientations (limited to the <:onfigurations with 0'2212 < 10 unit cell of the 2212 phase) of the a-axes of the film and of the substrate. Since both a-b planes can be considered as centered square lattices, the coincidence cell surface is a multiple of a2/2, which is the surface of the primitive cell in the a-b plane. First, Table 1 shows that the + 130 orientation observed in this work does not appear as a high-coincidence density one. [n fact, none of the short-range superimpositions gives a 13° angle. The nearest angles are 11.3° and 14°, and taking into account the precision of the X-ray precession measurements (± 10 ), only the 140 orientation could be considered to describe our experimental results. [n a pure application of the NCSL theory, Table 1 shows that the 11.3 0 and 18.4 orientations are the most favorable for the lowest value of the misfit 8. For BiSrCaCuO films grown by RF sputtering the 11.30 orientation has been reported [9,13], while the 18.40 does not seem to have been observed. In this frame the 0°, 14° and 45 0 orientations show high values of the misfit, and therefore do not appear as well adapted for in~plane orientation without an addi0

299

tional hypothesis. [n fact, if we consider that there is a high density of dislocations in the first cell of BiSrCaCuO as has been reported [3], [l40]MgO II [300JBiSrCaCuO (giving the 14° orientation and a 7.4% distortion), or [110]Mgo II [100]BisrCuCuO (giving the 45 0 orientation and a 10.1 % distortion) could be considered. However, in this case, one cannot explain why other in~plane orientations are not observed without including dislocation energy in the model, which leads to an atomic description of the different interfaces. All these remarks clearly show that this geometrical model (NCSL) cannot correctly explain the origin of these in-plane orientations with our present hypotheses. A specific property of the BiSrCaCuO film growth is the nature of the initial deposited layer. In fact, it has been observed by transmission electron microscopy that the BiSrCaCuO film growth starts with a BiO layer on MgO substrates [14]. We have checked by RBS analysis, that a Bi rich compound is formed at the first step of the pulsed laser deposition process. Therefore, we can assume that a BiO plane is present on the MgO substrate at the beginning of the growth. [n this case, one can consider the optimum conditions for an epitaxial orientation. Table 2 shows the value of the BiO lattice parameter to have an ideal misfit (8 = 0). This should be compared to the 0.54 nm parameter of the 2212 structure. Both 45 0 and 14° orientations require an expanded ~ 0.59 nm lattice parameter but on the contrary 11.3 0 and 18.4° orientations require a smaller one, than the 00 minor orientation (0.56 nm). The incommensurate modula~ tion in Bi~2212 is supposed to be related to an intrinsic misfit inside the Bi~2212 cell between the CuD2 plane [15], which imposes its lattice parameter to the structure, and the BiD stressed planes where the main atomic displacements are located. During the first stage of an in-situ growth, the epitaxial orientations could then originate from a NCSL optimization between the MgO lattice and the free, unstressed, BiO layer. This hypothesis discriminated

Table 2 fueal BiO lattice parameter "r 01'(11' In obtain a perfect coincidence (8 == 0) with MgO Rotation anglc(deg.) 0 45 11.3 14 18.4 Ideal BiD parameter (om) 0.567 0.601 0.54 0.584 0.538

300

L. Ranno el al. / Plzyslca C 245 (1995) 295-300

Bi·2212 from YBa 2Cu 30 7 which has nearly the same lattice parameters in the a-b plane but no modulation. In that sense, these in-plane orientations, especially the 14° onc, are specific properties of the Bi compound independently of the precise growth conditions during the pulsed laser ablation process. This means that the oxygen pressure in the ablation chamber, the substrate temperature and the deposition rate would not be determining parameters in the epitaxial orientations between the Bi film and the MgO substrate, and such a conclusion must be checked.

Acknowledgements The authors wish to express their thanks to RM. Defourneau and F. Kerherve for their help during experiments. This work was supported by the EEe (under Contract SC1 *-CT91-0753), by Alcatel Alsthom Recherches, and by the CNRS (GDR 86).

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