- Email: [email protected]
High-quality [1 0 0] and [1 1 0] YBa2Cu3O7−δ films for Josephson tunnelling
Journal of Crystal Growth 249 (2003) 186–190
High-quality [1 0 0] and [1 1 0] YBa2Cu3O7d films for Josephson tunnelling S.J. Kima,1, X. Grisona, G. Passerieuxa, J. Ayachea, J. Lesueurb,*, F. Lalua b
a # 108, 91405 Orsay Campus, France CSNSM, IN2P3-CNRS, Bat Laboratoire de Physique Quantique, CNRS—ESPCI, 10 rue Vauquelin, 75 231 Paris Cedex, France
Received 9 October 2002; accepted 18 October 2002 Communicated by R. Kern
Abstract YBa2Cu3O7d/PrBa2Cu3O7d (YBCO/PBCO) bilayers [1 0 0] and [1 1 0] oriented are in situ grown by a reactive codeposition technique on SrTiO3 single-crystal substrates. Atomic force microscopy and transmission electron microscopy images, X-rays diffraction patterns have been used to investigate their structural properties and surface morphology, in order to optimize their characteristics. To obtain high-quality YBCO [1 0 0] and [1 1 0] layers, we first deposit a PBCO template layer at 6301C, followed by a YBCO film at 630–7501C, with a growth rate of 0.063 nm/s. They display a very high crystallinity and a surface roughness as low as 0.5 nm for YBCO/PBCO [1 0 0] bilayer at a 1 mm2 scale. Reproducible Josephson junctions have been made on these films. r 2002 Elsevier Science B.V. All rights reserved. PACS: 74.72.Bk; 74.76.w; 74.76. Bz; 74.80. Dm; 68.55.a; 81.15.z Keywords: A1. Characterization; A3. Molecular beam epitaxy; B1. Yttrium compounds; B2. Superconducting materials
One of the intended applications of high Tc superconducting thin films is the fabrication of Josephson and tunnel junctions for electronic applications [1,2]. In oxide superconductors, the coherence length in the basal plane xab is much bigger than along the normal direction c: It is therefore better to fabricate thin films with a crystallographic orientation along the a or b directions to make stacked junctions [3]. [1 0 0] and [1 1 0] oriented thin films has to be prepared at *Corresponding author. E-mail address: [email protected] (J. Lesueur). 1 Now at ONERA-DMMP, BP 72-29, Avenue de la Division Leclerc, 92322 Ch#atillon Cedex, France.
a temperature roughly 1001C below that for c-axis ones [4]. Atomic surface mobility is then reduced, and a-axis films have smaller grains than c-axis ones together with a large amount of outgrowths, and therefore poor superconducting properties [5]. Inam et al. [6] have shown that a PrBa2Cu3O7d (PBCO) film, which has the same crystalline structure as YBa2Cu3O7d (YBCO), can be used as a template layer between the SrTiO3 (STO) substrate and the YBCO film to obtain high quality epitaxial growth in the [1 0 0] and [1 1 0] directions. In this paper we present a complete investigation of the structure, the microstructure of YBCO and PBCO films grown along [1 0 0] and [1 1 0]
0022-0248/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 2 ) 0 2 0 7 0 - 5
S.J. Kim et al. / Journal of Crystal Growth 249 (2003) 186–190
directions by co-deposition on STO. Such films have been recently used to make in situ planar tunnel junctions with very good electronic properties [7]. The evaporation system for oxides has been described in details elsewhere [8]. Regular shadow masks operating at room temperature have been setup to in situ deposit contacts and other useful layers (like Ag, Al, Pb and SiO) to make junctions. Contact resistances as low as 107 O cm2 are directly obtained with silver, with no further annealing. Films with a typical thickness of 50 nm have been studied. Combined Rutherford backscattering spectrometry (RBS), X-rays diffraction (XRD), atomic force microscopy (AFM) and analytical transmission electron microscopy (TEM//EDX) have been carried out to study their microstructure and surface morphology. XRD patterns have been recorded with an X’pert Philips diffractometer. Thin film topography has been observed using a Park-Science AFM system. The microstructure has been investigated using a 120 kV CM 12 and 200 kV Akashi Topcon electron transmission microscopes equipped with an EDX microanalyser. TEM cross-section foils were prepared using soft mechanical thinning to avoid chemical diffusion during preparation. With our growth system, the optimal temperature for depositing [0 0 1] YBCO on STO is 7501C [9]. In order to determine the optimum conditions for [1 0 0] and [1 1 0] axis growth of PBCO films, the substrate temperature has been varied between 6151C and 6501C, while the growth rate has been kept constant at 0.21 nm/s first, and then changed to 0.063 nm/s. Let us first focus on [1 0 0] PBCO/STO thin films growth. [1 0 0] STO substrates have been used. Y 2Y XRD diagrams display a mixed orientation [1 0 0] and [0 0 1] for the films grown between 6151C and 6501C at 0.21 nm/s, as measured by the surface ratio of the (0 0 5) and (0 0 2) peaks, with a minimum for 6301C. This corresponds still to 35% of [0 0 1] phase. Once the thermodynamical conditions for a-axis growth have been fulfilled (substrate temperature 6301C), we have reduced the growth rate to investigate the role of kinetics. While the (2 0 0) reflexion is clearly seen in Fig. 1, the [0 0 5] line (inset Fig. 1) corresponds to the
187
Fig. 1. y2y scan of a [1 0 0]PBCO/STO film deposited at ( The (2 0 0) line is found while the (0 0 5) one 6301C and 0.63 A/s. not (see inset).
lowest remaining [0 0 1] oriented phase (namely less than 2%) for what we consider as the optimal growth rate: 0. 063 nm/s. This corresponds to pure a-axis PBCO growth within experimental errors. But why does the growth rate influence the orientation of the layer? It is well known [10] that PBCO grows on STO following a 2D mechanism due to the very good lattice match (0.1%). According to Miletto et al. [11], the energy cost for making a nucleus of given orientation depends on its number of unit cells N: small nuclei (No10) will grow [1 0 0] and big ones will grow (N > 20) [0 0 1]. Given a first incomplete 2D layer deposited, if the diffusion time is high compared to the average time between the deposition of new incoming atoms (i.e. at low rate), the system will keep growing 2D because atoms will have time to reach energetically favorable sites, namely steps of the initial islands. This prevents growing big nuclei, and therefore favors [1 0 0] oriented growth. Estimates of these typical times by Mamutin [12] are compatible with this interpretation. The best conditions for pure a-axis PBCO growth corresponds to low surface diffusion rates (low temperature) and therefore to poor crystallinity [5]. The trick is to slowly increase the substrate temperature while growing the YBCO film, in order to keep advantage of both the a-axis seed layer and the high temperature diffusion rate. In our case, we raised the temperature from 6301C
188
S.J. Kim et al. / Journal of Crystal Growth 249 (2003) 186–190
to 7501C (the optimal temperature for c-axis growth) in 360 s, and maintained it for 440 s, keeping the same evaporation rate. The thickness of the PBCO template layer is typically 15 nm and that of the YBCO film 50 nm. Fig. 2 shows a Y 2Y diagram centered on (2 0 0) and (0 0 5) peaks for a typical YBCO/ PBCO/STO bilayer grown in optimum conditions. It is clear that this bilayer is pure a-axis oriented. This is confirmed by TEM investigations. A crosssection micrograph in high resolution mode (HREM) (Fig. 3) indicates a very coherent growth along the whole thickness of the bilayer. The diffraction diagrams obtained on the YBCO layer are pure, with very punctual reflexions; the absence of diffuse spots indicates a good crystalline quality of the sample. The top layer appears very flat at this nanometer scale. The very same film has been studied by AFM (Fig. 4): the surface is homogeneous and the RMS roughness at 1 mm scale is as low as 0.5 nm, i.e. one unit cell and a half in height. Such characteristics are favorable to make good and reproducible Josephson junctions. Let us focus now on the [1 1 0] oriented films. The STO substrate is now [1 1 0] oriented, and a low temperature [1 1 0] PBCO template layer has been used. The overall best conditions to grow pure [1 1 0] YBCO/PBCO/STO bilayer are the same that for [1 0 0] growth. There is a competition between [1 1 0] and [1 0 3] orientations [13], quite difficult to evidence in a standard XRD Y 2Y scan, since the corresponding diffraction peaks are
Fig. 2. y2y scan of a [1 0 0]YBCO/PBCO/STO bilayer deposited at optimal conditions. The (2 0 0) line is found while the (0 0 5) one not (see inset).
Fig. 3. TEM image and associated diffraction pattern for the [1 0 0] YBCO/PBCO/STO bilayer.
Fig. 4. AFM image of the [1 0 0] YBCO/PBCO/STO bilayer. Vertical scale is 4 nm
very close [14,15]. However, by tilting the sample around the axis intersection of the Y 2Y plane and the substrate surface (C tilt), we were able to distinguish between the two orientations. With C ¼ 01; a Y 2Y scan displays a peak corresponding either to [1 1 0] or [1 0 3] (Fig. 5). If [1 0 3] oriented grains are present, their c-axis will be perpendicular to the [1 0 0] STO planes; therefore, a Y 2Y scan with C ¼ 451 will display [0 0 l] lines provided a proper in-plane orientation of the substrate has been made (j scan). As shown in the frames Fig. 5, we have no signal from the (0 0 5) peaks whatever the j value is, which means that the epitaxial growth of this bilayer is only [1 1 0] oriented, without mixture
S.J. Kim et al. / Journal of Crystal Growth 249 (2003) 186–190
189
Fig. 5. y 2y scan of a [1 1 0] YBCO/PBCO/STO bilayer deposited at optimal conditions with c ¼ 01 (see text). The insets show no peak corresponding to (0 0 5) lines with c ¼ 451 (see text).
with [1 0 3] phases. This is confirmed by TEM investigations. Fig. 6 shows a HREM cross-section micrograph and the electron diffraction associated with the YBCO film of this YBCO/PBCO/STO [1 1 0] bilayer. High quality columnar growth can be seen in the [1 1 0] direction. Continuous atomic planes are seen from the substrate, through the template layer, up to the top surface. Very few interface dislocations are observed, as expected from the very good lattice match between YBCO and PBCO (0.34%). The 1 mm scale RMS roughness value obtained by AFM is 2.8 nm, higher than in the [1 0 0] direction, due to the columnar growth of this orientation. Such smooth films have been used to make in situ Josephson Junctions with lead as a second superconductor. The idea was to study the coupling between a BCS order parameter and an unconventional one (namely dx2 y2 in YBCO) along the nodal directions ([1 1 0] in this case), and to test theoretical predictions about the role of the Andreev Bound States expected at such oriented surface [16]. Such junctions have been successfully made for the first time, which display a hysteretic behavior at low temperature (see inset Fig. 7). The critical current as a function of temperature shows a standard downward curvature (Fig. 7), unlike the prediction of Tanaka et al. This may be in favor of a more complex order parameter (as d þ is or d þ id 0 ) in YBCO.
Fig. 6. TEM image and associated diffraction pattern of the [1 1 0] YBCO/PBCO/STO bilayer.
Fig. 7. Temperature dependence of a [1 1 0] YBCO/Pb Josephson junction. Inset: typical hysteretic I V characteristics at 1.3 K.
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S.J. Kim et al. / Journal of Crystal Growth 249 (2003) 186–190
In conclusion, [1 0 0] and [1 1 0] oriented YBCO thin films of very high quality have been obtained by a reactive codeposition technique. Their low roughness allows making Josephson Junctions suitable for superconducting electronics. Authors would like to thank D. Debarre at IEF, the ARAMIS team at CSNSM, and Z.Z. Li at LPS Orsay for their technical support. M. Aprili and H. Bernas are acknowledged for stimulating discussions.
References [1] J. Lesueur, M. Aprili, X. Grison, J. Ayache, S.J. Kim, A. Thorel, J. Phys. IV France 9 (Part 4) (1999) 207. [2] T.P. Sheahen, Introduction to High Temperature Superconductivity, Plenum Press, New York, 1994. [3] C.B. Eom, A.F. Marshall, S.S. Laderman, R.D. Jacowitz, T.H. Gerballe, Science 249 (1990) 1549. [4] J.F. Hamet, B. Mercey, M. Hervieu, B. Raveau, Physica C 193 (1992) 465. [5] R. Ramesh, T.S. Ravi, D.M. Hwang, C.C. Chang, A. Inam, T. Venkatesan, X.D. Wu, R.E. Muenchansen, S. Foltyn, N.S. Nogar, Physica C 173 (1991) 163.
[6] A. Inam, R. Ramesh, C.T. Rogers, B. Wilkens, K. Remschning, D. Hart, J. Barner, IEEE Trans. Magnetics 27 (2) (1991) 1603. [7] J. Lesueur, M. Aprili, A. Goulon, T.J. Horton, L. Dumoulin, Phys. Rev. B 55 (1997) R3398; J. Lesueur, B. Leridon, M. Aprili, X. Grison, in: Bok et al. (Ed.), The Gap Symmetry and Fluctuations in High Tc superconductors, Plenun Press, New York, 1998; X. Grison, Ph.D. Thesis, Ecole polytechnique, France, 2000. [8] J. Lesueur, M. Aprili, T.J. Horton, F. Lalu, M. GuillouxViry, A. Perrin, L. Dumoulin, J. Alloys Compounds 251 (1997) 156. [9] S.J. Kim, X. Grison, M.G. Passerieux, J. Ayache, J. Lesueur, to be published. [10] I. Utke, C. Klemenz, H.J. Scheel, M. Sasaura, S. Miyarawa, J. Crystal Growth 174 (1997) 806. [11] F. Miletto Granozio, U. Scotti di Uccio, J. Crystal Growth 174 (1997) 409. [12] V.V. Mamutin, J. Crystal Growth 153 (1995) 140. [13] J.Z. Wu, P.Y. Hsieh, A.V. McGuire, D.L. Schmidt, L.T. Wood, Y. Shen, W.K. Chu, Phys. Rev. B 44 (22) (1991) 12643. [14] D.W. Susnitzky, Y.K. Simpson, B.C. De Cooman, C.B. Carter, Mat. Res. Soc. Proc. No.6 (1986) 219. [15] C.B. Eom, A.F. Marshall, Y. Suzuki, T.H. Geballe, B. Boyer, R.F.W. Pease, R.B. van Dover, .Phillips Julia M, Phys. Rev. B 46 (18) (1992) 11902. [16] Y. Tanaka, S. Kashiwaya, Phys. Rev. B 58 (6) (1997) 892.
High-quality [1 0 0] and [1 1 0] YBa2Cu3O7d films for Josephson tunnelling S.J. Kima,1, X. Grisona, G. Passerieuxa, J. Ayachea, J. Lesueurb,*, F. Lalua b
a # 108, 91405 Orsay Campus, France CSNSM, IN2P3-CNRS, Bat Laboratoire de Physique Quantique, CNRS—ESPCI, 10 rue Vauquelin, 75 231 Paris Cedex, France
Received 9 October 2002; accepted 18 October 2002 Communicated by R. Kern
Abstract YBa2Cu3O7d/PrBa2Cu3O7d (YBCO/PBCO) bilayers [1 0 0] and [1 1 0] oriented are in situ grown by a reactive codeposition technique on SrTiO3 single-crystal substrates. Atomic force microscopy and transmission electron microscopy images, X-rays diffraction patterns have been used to investigate their structural properties and surface morphology, in order to optimize their characteristics. To obtain high-quality YBCO [1 0 0] and [1 1 0] layers, we first deposit a PBCO template layer at 6301C, followed by a YBCO film at 630–7501C, with a growth rate of 0.063 nm/s. They display a very high crystallinity and a surface roughness as low as 0.5 nm for YBCO/PBCO [1 0 0] bilayer at a 1 mm2 scale. Reproducible Josephson junctions have been made on these films. r 2002 Elsevier Science B.V. All rights reserved. PACS: 74.72.Bk; 74.76.w; 74.76. Bz; 74.80. Dm; 68.55.a; 81.15.z Keywords: A1. Characterization; A3. Molecular beam epitaxy; B1. Yttrium compounds; B2. Superconducting materials
One of the intended applications of high Tc superconducting thin films is the fabrication of Josephson and tunnel junctions for electronic applications [1,2]. In oxide superconductors, the coherence length in the basal plane xab is much bigger than along the normal direction c: It is therefore better to fabricate thin films with a crystallographic orientation along the a or b directions to make stacked junctions [3]. [1 0 0] and [1 1 0] oriented thin films has to be prepared at *Corresponding author. E-mail address: [email protected] (J. Lesueur). 1 Now at ONERA-DMMP, BP 72-29, Avenue de la Division Leclerc, 92322 Ch#atillon Cedex, France.
a temperature roughly 1001C below that for c-axis ones [4]. Atomic surface mobility is then reduced, and a-axis films have smaller grains than c-axis ones together with a large amount of outgrowths, and therefore poor superconducting properties [5]. Inam et al. [6] have shown that a PrBa2Cu3O7d (PBCO) film, which has the same crystalline structure as YBa2Cu3O7d (YBCO), can be used as a template layer between the SrTiO3 (STO) substrate and the YBCO film to obtain high quality epitaxial growth in the [1 0 0] and [1 1 0] directions. In this paper we present a complete investigation of the structure, the microstructure of YBCO and PBCO films grown along [1 0 0] and [1 1 0]
0022-0248/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 2 ) 0 2 0 7 0 - 5
S.J. Kim et al. / Journal of Crystal Growth 249 (2003) 186–190
directions by co-deposition on STO. Such films have been recently used to make in situ planar tunnel junctions with very good electronic properties [7]. The evaporation system for oxides has been described in details elsewhere [8]. Regular shadow masks operating at room temperature have been setup to in situ deposit contacts and other useful layers (like Ag, Al, Pb and SiO) to make junctions. Contact resistances as low as 107 O cm2 are directly obtained with silver, with no further annealing. Films with a typical thickness of 50 nm have been studied. Combined Rutherford backscattering spectrometry (RBS), X-rays diffraction (XRD), atomic force microscopy (AFM) and analytical transmission electron microscopy (TEM//EDX) have been carried out to study their microstructure and surface morphology. XRD patterns have been recorded with an X’pert Philips diffractometer. Thin film topography has been observed using a Park-Science AFM system. The microstructure has been investigated using a 120 kV CM 12 and 200 kV Akashi Topcon electron transmission microscopes equipped with an EDX microanalyser. TEM cross-section foils were prepared using soft mechanical thinning to avoid chemical diffusion during preparation. With our growth system, the optimal temperature for depositing [0 0 1] YBCO on STO is 7501C [9]. In order to determine the optimum conditions for [1 0 0] and [1 1 0] axis growth of PBCO films, the substrate temperature has been varied between 6151C and 6501C, while the growth rate has been kept constant at 0.21 nm/s first, and then changed to 0.063 nm/s. Let us first focus on [1 0 0] PBCO/STO thin films growth. [1 0 0] STO substrates have been used. Y 2Y XRD diagrams display a mixed orientation [1 0 0] and [0 0 1] for the films grown between 6151C and 6501C at 0.21 nm/s, as measured by the surface ratio of the (0 0 5) and (0 0 2) peaks, with a minimum for 6301C. This corresponds still to 35% of [0 0 1] phase. Once the thermodynamical conditions for a-axis growth have been fulfilled (substrate temperature 6301C), we have reduced the growth rate to investigate the role of kinetics. While the (2 0 0) reflexion is clearly seen in Fig. 1, the [0 0 5] line (inset Fig. 1) corresponds to the
187
Fig. 1. y2y scan of a [1 0 0]PBCO/STO film deposited at ( The (2 0 0) line is found while the (0 0 5) one 6301C and 0.63 A/s. not (see inset).
lowest remaining [0 0 1] oriented phase (namely less than 2%) for what we consider as the optimal growth rate: 0. 063 nm/s. This corresponds to pure a-axis PBCO growth within experimental errors. But why does the growth rate influence the orientation of the layer? It is well known [10] that PBCO grows on STO following a 2D mechanism due to the very good lattice match (0.1%). According to Miletto et al. [11], the energy cost for making a nucleus of given orientation depends on its number of unit cells N: small nuclei (No10) will grow [1 0 0] and big ones will grow (N > 20) [0 0 1]. Given a first incomplete 2D layer deposited, if the diffusion time is high compared to the average time between the deposition of new incoming atoms (i.e. at low rate), the system will keep growing 2D because atoms will have time to reach energetically favorable sites, namely steps of the initial islands. This prevents growing big nuclei, and therefore favors [1 0 0] oriented growth. Estimates of these typical times by Mamutin [12] are compatible with this interpretation. The best conditions for pure a-axis PBCO growth corresponds to low surface diffusion rates (low temperature) and therefore to poor crystallinity [5]. The trick is to slowly increase the substrate temperature while growing the YBCO film, in order to keep advantage of both the a-axis seed layer and the high temperature diffusion rate. In our case, we raised the temperature from 6301C
188
S.J. Kim et al. / Journal of Crystal Growth 249 (2003) 186–190
to 7501C (the optimal temperature for c-axis growth) in 360 s, and maintained it for 440 s, keeping the same evaporation rate. The thickness of the PBCO template layer is typically 15 nm and that of the YBCO film 50 nm. Fig. 2 shows a Y 2Y diagram centered on (2 0 0) and (0 0 5) peaks for a typical YBCO/ PBCO/STO bilayer grown in optimum conditions. It is clear that this bilayer is pure a-axis oriented. This is confirmed by TEM investigations. A crosssection micrograph in high resolution mode (HREM) (Fig. 3) indicates a very coherent growth along the whole thickness of the bilayer. The diffraction diagrams obtained on the YBCO layer are pure, with very punctual reflexions; the absence of diffuse spots indicates a good crystalline quality of the sample. The top layer appears very flat at this nanometer scale. The very same film has been studied by AFM (Fig. 4): the surface is homogeneous and the RMS roughness at 1 mm scale is as low as 0.5 nm, i.e. one unit cell and a half in height. Such characteristics are favorable to make good and reproducible Josephson junctions. Let us focus now on the [1 1 0] oriented films. The STO substrate is now [1 1 0] oriented, and a low temperature [1 1 0] PBCO template layer has been used. The overall best conditions to grow pure [1 1 0] YBCO/PBCO/STO bilayer are the same that for [1 0 0] growth. There is a competition between [1 1 0] and [1 0 3] orientations [13], quite difficult to evidence in a standard XRD Y 2Y scan, since the corresponding diffraction peaks are
Fig. 2. y2y scan of a [1 0 0]YBCO/PBCO/STO bilayer deposited at optimal conditions. The (2 0 0) line is found while the (0 0 5) one not (see inset).
Fig. 3. TEM image and associated diffraction pattern for the [1 0 0] YBCO/PBCO/STO bilayer.
Fig. 4. AFM image of the [1 0 0] YBCO/PBCO/STO bilayer. Vertical scale is 4 nm
very close [14,15]. However, by tilting the sample around the axis intersection of the Y 2Y plane and the substrate surface (C tilt), we were able to distinguish between the two orientations. With C ¼ 01; a Y 2Y scan displays a peak corresponding either to [1 1 0] or [1 0 3] (Fig. 5). If [1 0 3] oriented grains are present, their c-axis will be perpendicular to the [1 0 0] STO planes; therefore, a Y 2Y scan with C ¼ 451 will display [0 0 l] lines provided a proper in-plane orientation of the substrate has been made (j scan). As shown in the frames Fig. 5, we have no signal from the (0 0 5) peaks whatever the j value is, which means that the epitaxial growth of this bilayer is only [1 1 0] oriented, without mixture
S.J. Kim et al. / Journal of Crystal Growth 249 (2003) 186–190
189
Fig. 5. y 2y scan of a [1 1 0] YBCO/PBCO/STO bilayer deposited at optimal conditions with c ¼ 01 (see text). The insets show no peak corresponding to (0 0 5) lines with c ¼ 451 (see text).
with [1 0 3] phases. This is confirmed by TEM investigations. Fig. 6 shows a HREM cross-section micrograph and the electron diffraction associated with the YBCO film of this YBCO/PBCO/STO [1 1 0] bilayer. High quality columnar growth can be seen in the [1 1 0] direction. Continuous atomic planes are seen from the substrate, through the template layer, up to the top surface. Very few interface dislocations are observed, as expected from the very good lattice match between YBCO and PBCO (0.34%). The 1 mm scale RMS roughness value obtained by AFM is 2.8 nm, higher than in the [1 0 0] direction, due to the columnar growth of this orientation. Such smooth films have been used to make in situ Josephson Junctions with lead as a second superconductor. The idea was to study the coupling between a BCS order parameter and an unconventional one (namely dx2 y2 in YBCO) along the nodal directions ([1 1 0] in this case), and to test theoretical predictions about the role of the Andreev Bound States expected at such oriented surface [16]. Such junctions have been successfully made for the first time, which display a hysteretic behavior at low temperature (see inset Fig. 7). The critical current as a function of temperature shows a standard downward curvature (Fig. 7), unlike the prediction of Tanaka et al. This may be in favor of a more complex order parameter (as d þ is or d þ id 0 ) in YBCO.
Fig. 6. TEM image and associated diffraction pattern of the [1 1 0] YBCO/PBCO/STO bilayer.
Fig. 7. Temperature dependence of a [1 1 0] YBCO/Pb Josephson junction. Inset: typical hysteretic I V characteristics at 1.3 K.
190
S.J. Kim et al. / Journal of Crystal Growth 249 (2003) 186–190
In conclusion, [1 0 0] and [1 1 0] oriented YBCO thin films of very high quality have been obtained by a reactive codeposition technique. Their low roughness allows making Josephson Junctions suitable for superconducting electronics. Authors would like to thank D. Debarre at IEF, the ARAMIS team at CSNSM, and Z.Z. Li at LPS Orsay for their technical support. M. Aprili and H. Bernas are acknowledged for stimulating discussions.
References [1] J. Lesueur, M. Aprili, X. Grison, J. Ayache, S.J. Kim, A. Thorel, J. Phys. IV France 9 (Part 4) (1999) 207. [2] T.P. Sheahen, Introduction to High Temperature Superconductivity, Plenum Press, New York, 1994. [3] C.B. Eom, A.F. Marshall, S.S. Laderman, R.D. Jacowitz, T.H. Gerballe, Science 249 (1990) 1549. [4] J.F. Hamet, B. Mercey, M. Hervieu, B. Raveau, Physica C 193 (1992) 465. [5] R. Ramesh, T.S. Ravi, D.M. Hwang, C.C. Chang, A. Inam, T. Venkatesan, X.D. Wu, R.E. Muenchansen, S. Foltyn, N.S. Nogar, Physica C 173 (1991) 163.
[6] A. Inam, R. Ramesh, C.T. Rogers, B. Wilkens, K. Remschning, D. Hart, J. Barner, IEEE Trans. Magnetics 27 (2) (1991) 1603. [7] J. Lesueur, M. Aprili, A. Goulon, T.J. Horton, L. Dumoulin, Phys. Rev. B 55 (1997) R3398; J. Lesueur, B. Leridon, M. Aprili, X. Grison, in: Bok et al. (Ed.), The Gap Symmetry and Fluctuations in High Tc superconductors, Plenun Press, New York, 1998; X. Grison, Ph.D. Thesis, Ecole polytechnique, France, 2000. [8] J. Lesueur, M. Aprili, T.J. Horton, F. Lalu, M. GuillouxViry, A. Perrin, L. Dumoulin, J. Alloys Compounds 251 (1997) 156. [9] S.J. Kim, X. Grison, M.G. Passerieux, J. Ayache, J. Lesueur, to be published. [10] I. Utke, C. Klemenz, H.J. Scheel, M. Sasaura, S. Miyarawa, J. Crystal Growth 174 (1997) 806. [11] F. Miletto Granozio, U. Scotti di Uccio, J. Crystal Growth 174 (1997) 409. [12] V.V. Mamutin, J. Crystal Growth 153 (1995) 140. [13] J.Z. Wu, P.Y. Hsieh, A.V. McGuire, D.L. Schmidt, L.T. Wood, Y. Shen, W.K. Chu, Phys. Rev. B 44 (22) (1991) 12643. [14] D.W. Susnitzky, Y.K. Simpson, B.C. De Cooman, C.B. Carter, Mat. Res. Soc. Proc. No.6 (1986) 219. [15] C.B. Eom, A.F. Marshall, Y. Suzuki, T.H. Geballe, B. Boyer, R.F.W. Pease, R.B. van Dover, .Phillips Julia M, Phys. Rev. B 46 (18) (1992) 11902. [16] Y. Tanaka, S. Kashiwaya, Phys. Rev. B 58 (6) (1997) 892.