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YBaCuO matrix interfaces: HREM study
Physica C 235-240 (1994)617-618 North-Holland
PUVS[gA
Y203 nanoprecipitate/YBaCuO matrix interfaces : HREM study D. Dorignaca, S. Schamm a, C. Grigis a, J. S6vely~, J. Santisob, A. Figueras b • CEMES-LOE/CNRS, BP4347, 31055 Toulouse Cedex, France blnstitut de Ciencia de Materials de Barcelona, Bellatera, Spain Y203 inclusions locally embedded in MOCVD YBCO films have been unambiguously identified by HREM and associated image calculations. The nanoprecipitates exhibit rectangular shapes, with their largest interfacial facets lying parallel to the (001) plane. The interfaces features can be interpreted at an atomic scale on the basis of lattice-matching arguments.
1. INTRODUCTION Defects play an essential role in controlling the growth mechanisms and the critical current density of high-Te superconductors. This particularly motivates a detailed characterization of the nanostructure of slightly nonstoichiometric films containing impurity phases, which may act as effective pinning centres. We report here first highresolution electron-microscopy (HREM) results concerning Y203 inclusions in YBa2Cu3OT.x (YBCO) compounds, which should elucidate their epitaxial relationships on an atomic scale. 2. EXPERIMENTAL DETAILS Thin films of a Ba-deficient YBCO (1:1.6:3) compound were elaborated by metal organic chemical vapour deposition (MOCVD) on a SrTiO3 (100) substrate [1]. Suitable HREM samples were then prepared from the above films by standard mechanical polishing and ion milling with liquid nitrogen cooling. We used a Philips CM30ST electron microscope at 300kV with point resolution 0.2nm, axial-illumination bright-field operation, no objective aperture and calibrated magnification 840k. The simulated images were computed using the Mac Tempas suite of multi-slice programs, with spherical aberration 1.2mm, rms focal spread 8nm, beam divergence semi-angle 1.2mrad, rms vibration parameter 0.02nm and "effective" aperture 10nm-1. 3. RESULTS AND DISCUSSION Plane-view and cross-section observations showed that the YBCO film growth is epitaxial, with a c-axis preferential orientation, and that the film structure is predominantly the "1:2:3" characteristic one, with interesting planar defects in some small a-axis oriented domains [2].
The recorded HREM images also revealed the presence of rectangular-shaped Y203 inclusions, which are randomly dispersed and lead to slightly strained and misaligned regions. Their epitaxial relationship is always found to be [110] Y203 // [010] YBCO and (001)Y203//(001) YBCO. This is probably because of the favourable influence of the very low lattice mismatch in the interfacial (001) plane (da40Y203 = 0.375nm, dl00YBCO = 0.383nm: 2.2% resulting misfi0. Such an orientation relation has also been observed in sputtered films [3, 4] and in plasma-enhanced MOCVD films [5]. Figure 1 shows for example three atomic structure images of a typical nanoprecipitate with a size of about (4x30)nm 2. They correspond to different defocus values which produce highcontrast easily-recognizable images in an experiment. It is clearly evidenced that the (001) interfaces are flat, with no interracial steps, and that cristallinity continues up to the interfaces. The image computations demonstrate that the (001) Y203 interracial facets are exactly located in place of the Y layers of the matrix. It can also easily be seen, by looking at a grazing angle, that the over-grown YBCO planes as well as the Y203 planes are distorted in the vicinity of all the interfaces, acc0modating the two-dimensionnal lattice misfit. Along the [001] axis the lattice matching is poor (doo4Y2O3 = 0.265nm, d001YBCO= 1.172nm: 9.5% resulting misfit). Nevertheless, since 9xdo04Y2O3 ~- 2xd001YBCO, local correspondances and some coincidences between the metallic layers in the matrix and the (004) planes in the precipitate can be consistently interpreted. In the interfacial (001) plane the lattices are nearly coherent, except in particular sites where the low mismatch seems accomodated by (222) extra lattice planes in the precipitate. Extensive HREM studies are in progress to clarify these last points, in particular to determine the core structure of the defects that are created at the termination of the corresponding (222) planes.
0921-4534/94/$07.00 © 1994 - ElsevierScience B.V. All rights reserved. SSDI 0921-4534(94)00870-I
618
l ) L)ori~nclc' et ~l/ Phv,sic~l C' 235 240 (1~)94) nl 7 h i s
i ,~z = ,-87 nm-
Figure 1. HREM images showing transverse cross-sectional views of Y203 nanoprecipitate, in l110] direction, locally embedded in YBCO matrix, in [010] orientation, for three different microscope defocus values. Simulated images of the matrix, the precipitate and the (001) interface are inserted (metal atom-columns arc black for defocus Az = -35nm, white for -76 and -87nm; thickness is close to 3nm). Note that the (001) Y203 interfacial facets coincide with Y-matrix layers. Note also the distortion of the atomic arrangements in the vicinity of the interfaces due to lattice misfit and the presence of accomodating defects at the end of two particular (222) Y203 planes (arrowed).
REFERENCES 1. J. Santiso, A. Figueras, S. Schamm, Ch. Grigis, D. Dorignac and J. Sdvely, this conference. 2. D. Dorignac, S. Schamm, Ch. Grigis and J. S6vely, in ICEM13 (1994), to be published.
3. A. Catana, R.F. Broom, J.G. Berdnorz, J. Mannhart and D.G. Schlom, Appl. Phys. Lett., 60(8) (1992) 1016. 4. A. Catana, D.G. Schlom, J. Mannhart and J.G. Berdnorz, Appl. Phys. Lett., 61(6) (1992) 720. 5. P. Lu, J. Zhao, C.S. Chern et al., J. Mater. Res., 7(8) (1992) 1993.
PUVS[gA
Y203 nanoprecipitate/YBaCuO matrix interfaces : HREM study D. Dorignaca, S. Schamm a, C. Grigis a, J. S6vely~, J. Santisob, A. Figueras b • CEMES-LOE/CNRS, BP4347, 31055 Toulouse Cedex, France blnstitut de Ciencia de Materials de Barcelona, Bellatera, Spain Y203 inclusions locally embedded in MOCVD YBCO films have been unambiguously identified by HREM and associated image calculations. The nanoprecipitates exhibit rectangular shapes, with their largest interfacial facets lying parallel to the (001) plane. The interfaces features can be interpreted at an atomic scale on the basis of lattice-matching arguments.
1. INTRODUCTION Defects play an essential role in controlling the growth mechanisms and the critical current density of high-Te superconductors. This particularly motivates a detailed characterization of the nanostructure of slightly nonstoichiometric films containing impurity phases, which may act as effective pinning centres. We report here first highresolution electron-microscopy (HREM) results concerning Y203 inclusions in YBa2Cu3OT.x (YBCO) compounds, which should elucidate their epitaxial relationships on an atomic scale. 2. EXPERIMENTAL DETAILS Thin films of a Ba-deficient YBCO (1:1.6:3) compound were elaborated by metal organic chemical vapour deposition (MOCVD) on a SrTiO3 (100) substrate [1]. Suitable HREM samples were then prepared from the above films by standard mechanical polishing and ion milling with liquid nitrogen cooling. We used a Philips CM30ST electron microscope at 300kV with point resolution 0.2nm, axial-illumination bright-field operation, no objective aperture and calibrated magnification 840k. The simulated images were computed using the Mac Tempas suite of multi-slice programs, with spherical aberration 1.2mm, rms focal spread 8nm, beam divergence semi-angle 1.2mrad, rms vibration parameter 0.02nm and "effective" aperture 10nm-1. 3. RESULTS AND DISCUSSION Plane-view and cross-section observations showed that the YBCO film growth is epitaxial, with a c-axis preferential orientation, and that the film structure is predominantly the "1:2:3" characteristic one, with interesting planar defects in some small a-axis oriented domains [2].
The recorded HREM images also revealed the presence of rectangular-shaped Y203 inclusions, which are randomly dispersed and lead to slightly strained and misaligned regions. Their epitaxial relationship is always found to be [110] Y203 // [010] YBCO and (001)Y203//(001) YBCO. This is probably because of the favourable influence of the very low lattice mismatch in the interfacial (001) plane (da40Y203 = 0.375nm, dl00YBCO = 0.383nm: 2.2% resulting misfi0. Such an orientation relation has also been observed in sputtered films [3, 4] and in plasma-enhanced MOCVD films [5]. Figure 1 shows for example three atomic structure images of a typical nanoprecipitate with a size of about (4x30)nm 2. They correspond to different defocus values which produce highcontrast easily-recognizable images in an experiment. It is clearly evidenced that the (001) interfaces are flat, with no interracial steps, and that cristallinity continues up to the interfaces. The image computations demonstrate that the (001) Y203 interracial facets are exactly located in place of the Y layers of the matrix. It can also easily be seen, by looking at a grazing angle, that the over-grown YBCO planes as well as the Y203 planes are distorted in the vicinity of all the interfaces, acc0modating the two-dimensionnal lattice misfit. Along the [001] axis the lattice matching is poor (doo4Y2O3 = 0.265nm, d001YBCO= 1.172nm: 9.5% resulting misfit). Nevertheless, since 9xdo04Y2O3 ~- 2xd001YBCO, local correspondances and some coincidences between the metallic layers in the matrix and the (004) planes in the precipitate can be consistently interpreted. In the interfacial (001) plane the lattices are nearly coherent, except in particular sites where the low mismatch seems accomodated by (222) extra lattice planes in the precipitate. Extensive HREM studies are in progress to clarify these last points, in particular to determine the core structure of the defects that are created at the termination of the corresponding (222) planes.
0921-4534/94/$07.00 © 1994 - ElsevierScience B.V. All rights reserved. SSDI 0921-4534(94)00870-I
618
l ) L)ori~nclc' et ~l/ Phv,sic~l C' 235 240 (1~)94) nl 7 h i s
i ,~z = ,-87 nm-
Figure 1. HREM images showing transverse cross-sectional views of Y203 nanoprecipitate, in l110] direction, locally embedded in YBCO matrix, in [010] orientation, for three different microscope defocus values. Simulated images of the matrix, the precipitate and the (001) interface are inserted (metal atom-columns arc black for defocus Az = -35nm, white for -76 and -87nm; thickness is close to 3nm). Note that the (001) Y203 interfacial facets coincide with Y-matrix layers. Note also the distortion of the atomic arrangements in the vicinity of the interfaces due to lattice misfit and the presence of accomodating defects at the end of two particular (222) Y203 planes (arrowed).
REFERENCES 1. J. Santiso, A. Figueras, S. Schamm, Ch. Grigis, D. Dorignac and J. Sdvely, this conference. 2. D. Dorignac, S. Schamm, Ch. Grigis and J. S6vely, in ICEM13 (1994), to be published.
3. A. Catana, R.F. Broom, J.G. Berdnorz, J. Mannhart and D.G. Schlom, Appl. Phys. Lett., 60(8) (1992) 1016. 4. A. Catana, D.G. Schlom, J. Mannhart and J.G. Berdnorz, Appl. Phys. Lett., 61(6) (1992) 720. 5. P. Lu, J. Zhao, C.S. Chern et al., J. Mater. Res., 7(8) (1992) 1993.