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Interaction between Nd0.4Sr0.6Al0.7Ta0.3O3 substrates and YBa2Cu3O7 films deposited by sputtering
MaterialsLetters North-Holland
15 (1992) 146-148
Interaction between Ndo.4Sro.sAlo.7Tao.30~substrates and YBa2Cu307 films deposited by sputtering A. Zaitsev
‘, K.H.
Klatt,
P. Reiche
2 and P.M. Meuffels
Institut ftir Festkiirperforschung, KFA Forschungszentrum Jiilich. W-5 170 Jiilich, Germany Received 3 August 1992; in final form 27 August 1992
Nd&rO.&lO.,Ta,,sO~ substrates with a lattice constant of 0.384 nm were used for the deposition of YBa&uXO,_, thin films by means of dc magnetron sputtering. This substrate material influences the film properties due to excessive interface reactions which cause a decrease in the superconducting transition temperature of the films and an apparent deterioration of the substrate surface.
One of the main requirements for the epitaxial growth of superconducting YBazCu30, films is the close match of the substrate lattice constants to those of the films. To meet this requirement single crystals of SrTi03, MgO, yttrium-stabilized ZrOz, A120,, LaA103, LaGaO,, LiNb03 are widely used [ l-31. These substrates can provide specific problems due to, for example, chemical reactions between film and substrate or their incompatibility with certain technical applications. The number of potentially useful substrates was increased considerably by the recent development of a series of mixed-perovskite crystals [ 41. Detailed studies are required before a final conclusion on their suitability for high-T, film manufacturing can be made. This Letter reports the first results obtained for in situ deposited YBa2Cu307_-x thin films on Nd~,~Sr~,~Al~.,Tao.~03 substrates. Nd0.4Sr0.6A1,-,7Ta0,J03 is one of the mixed-perovskite crystals [4] with a lattice constant close to that of YBa$&O,. A dc planar magnetron sputtering system with a 50 mm diameter superconducting YBa2Cu90, target was used for the film preparation. The substrate was located 25 mm below the target centre to protect the ’ On leave from Electrical Engineering Institute, Department EIVT, 197376 St. Petersburg, Russia. * Institut ftir Kristallzilchtung Berlin-Adlershof, 0- 1199 Berlin, Germany.
146
film from negative ion bombardment during the “low-pressure” sputtering [ 21. A silicon carbide heater was employed for the substrate heating. The Nd~.~Sro.~Al~.,Ta~.~O~ substrates were provided by the Institut ftir Kristallziichtung, BerlinAdlershof. The crystals were violet pieces of size 1 x 1 cm’ and a thickness of 0.5 mm. One side of the substrates with ( 100) orientation was polished (see fig. 2a below). The crystals had a cubic structure with a lattice constant of 0.384 nm. The sputtering parameters were: a pressure of 20 Pa of a 1: 1 Ar-O2 mixture, a substrate temperature of 720’ C and a discharge current of 150 mA. From our experience with MgO and SrTi03 substrates, these parameters were expected to produce in situ superconducting films with a transition temperature to superconductivity, T,, of at least 85 K. All the YBa2Cu307_-x films on Ndo.4Sro.sAlo.,Tao.30~ substrates, however, exhibited maximum T, values of 81 K. The variation of the sputtering parameters around the reported values did not produce any significant improvement in film quality. A factor affecting the quality of the films was found to be the chemical reaction between the film and substrate material. Fig. 1 shows the results of a SIMS analysis (CAMECA, type IMS4S) carried out on a typical YBaZCu307_-x film on Ndo.4Sro.sAlo.,Tao.~O~. The depth profiles of the substrate and film elements were measured during the bombardment with 5.5
0167-577x/92/$05.00
0 1992 Elsevier Science Publishers B.V. All rights reserved.
Volume 15, number
3
MATERIALS
Nij,, ,/----..
,.
LETTERS
November
1992
I
--.-__B?_ ‘--.__ ---.____ ---__ .__ Y ---._ -----v._-.._=
c
I-\
/
fO’L-.----,~-. 0
\
1 10
20
30
Sputter Time (min)
Fig. 1. SIMS depth profiles measured on a YBa2Cu307_-x deposited onto a Ndo.4Sr(l.6Alo.7Tao.303 substrate.
film
keV Cs+ ions. The film thickness was 200 nm, its surface morphology is presented in fig. 2b. Although the data in fig. 1 must be viewed as qualitative, they indicate substantial concentrations of Al atoms inside the film. The highest intensities measured for Ta+, Y+ and Ba’ ions were registered from the interface region between the film and the substrate. As the elemental signal strengths are highly dependent on the local chemical environment, this results from a specific kind of matrix formed in the interface region owing to chemical reactions between both materials. It is well known that interface reactions can debase the film properties by contaminating it or changing its composition [ 5 1. The reaction between film and substrate material during the deposition process also caused a severe deterioration of the substrate surface as observed after removal of the film by etching (20% water solution of HNO,). Most apparent was the change in colour from violet to dark grey. The change in surface morphology is illustrated in fig. 2c, where SEM (Philips, type 525) pictures are shown. EDAX (Philips 9900) measurements indicated that the surface composition of the used substrates no longer corresponded to the original one. Relative changes in composition of about 10% were observed. Furthermore, examination of different points of the treated substrate surface showed that the composition changes were not homogeneous. It should be noted that neither a heating at 720°C under a pressure of 20 Pa of the Ar-O2 mixture nor a treatment with 20% water solution of HNO3 were
Fig. 2. SEM micrographs of Nd&&Alo.,T~.~O~ substrates: (a) surface of an unused substrate, (b) surface of a YBazCu107_, film on Ndo.4Sro.sAla7Ta0.303, (c) substrate surface after removal of a deposited YBa$Zuj07_x film by etching.
147
Volume 15, number 3
MATERIALS LETTERS
able to cause similar changes of the surface of virgin samples. This means that the contact between film and substrate gives rise to the formation of interface compounds which are preferentially etched. One can conclude that, compared to certain other substrate materials f S-71, Ndo.~Sro.~Alo.7Ta~.303 is not suited for the direct deposition of YBazCu30, thin films because of excessive interface reactions. In order to use this substrate material it is likely to be necessary to either decrease the deposition temperature or to produce a buffer layer. Otherwise, the interface reactions will reduce the benefit gained from the close lattice match to YBa2Cu307 attainable with this material. It should be noted that the pure perovskites NdGaO,, LaGa03 [ 1,7] and LaA103 [ 2,8] which are closely related to Ndo..lSro.6Al,,,Tao.303are suitable for the preparation of high-quality YBazCu30, films under deposition conditions comparable to those used by us. Negligible or no interface reactions seem to take place between these pure perovskites and YBa2Cu307. The formation of a mixed-perovskite by partially substituting the parent elements thus enhances the reactivity with YBazCu307. We suppose that significant interface reactions might also be observed when other mixed-perovs~te substrates of the series proposed by Mateika et al. [4] are employed. This is of general significance with respect to the search for new substrate materials. Nevertheless, each material should be examined in a way as described in this work because it seems to us very difficult to make a priori decisions on its suitability. As an example, one should compare the
148
November 1992
susceptibility to interface reaction problems of the compounds LiNb03 and KTaO, which are closely related to each other. Whereas KTaO, is relatively inert chemically to YBa$&O, [ 9 1, LiNb03 distinguishes itself by a strong reactivity [ 5 1. The authors are grateful to H. Holzbrecher and E.M. Wuertz for their technical assistance. AZ wishes to thank the Deutscher Akademischer Austauschdienst for financial support. References [ I] C. Koren, A. Gupta, E.A. Giess, A. Segmtiller and R.B. Laibowitz, Appl. Phys. Letters 54 (1989) 1054.
[ 21 J.R. Gavaler, J. Talvacchio, T.T. Braggins, M.G. Forrester and J. Greggi, J. Appl. Phys. 70 (1991) 4383.
[ 31 SM. Garrison, N. Newman, B.F. Cole, K. Char and R.W. Barton, Appl. Phys. Letters 58 (1991) 2168. [4] D. Mateika, H. Kohler, H. Laudan and E. vijlkel, J. Cryst. Growth 109 (1991) 447. f5] T. Venkatesan, C.C. Chang, D. Dijkkamp, S.B. Ogale, E.W. Chase, L.A. Farrow, D.M. Hwang, P.F. Mice& S.A. Schwarz, J.M. Tarascon, X.D. Wu and A. Inam, J. Appl. Phys. 63 (1988) 4591. ] P. Madakson, J.J. Cuomo, D.S. Yee, R.A. Roy and G. Scilla, J. Appt. Phys. 63 (1988) 2046. ] E.A. Gies, R.L. Sandstrom, W.J. Gallagher, A. Gupta, S.L. Shinde, R.F. Cook, E.I. Cooper, E.J.M. ~Sulliv~, J.M. Roldan, A.P. Segmtiller and J. Angilello, IBM J. Res. Develop. 34 (1990) 916. ] R.W. Simon, C.E. Platt,A.E. Lee, G.S. Lee, K.P. Daly, MS. Wire, J.A. Luine and M. Urbanik, Appl. Phys. Letters 53 (1988) 2677. [9] J.W. McCamy, D.P. Norton, D.H. Lowndes, L.A. Boatner, D.K. Christen, R. Feenstra and E. Sonder, Mater. Res. Sot. Symp. Proc. 169 (1990) 469.
15 (1992) 146-148
Interaction between Ndo.4Sro.sAlo.7Tao.30~substrates and YBa2Cu307 films deposited by sputtering A. Zaitsev
‘, K.H.
Klatt,
P. Reiche
2 and P.M. Meuffels
Institut ftir Festkiirperforschung, KFA Forschungszentrum Jiilich. W-5 170 Jiilich, Germany Received 3 August 1992; in final form 27 August 1992
Nd&rO.&lO.,Ta,,sO~ substrates with a lattice constant of 0.384 nm were used for the deposition of YBa&uXO,_, thin films by means of dc magnetron sputtering. This substrate material influences the film properties due to excessive interface reactions which cause a decrease in the superconducting transition temperature of the films and an apparent deterioration of the substrate surface.
One of the main requirements for the epitaxial growth of superconducting YBazCu30, films is the close match of the substrate lattice constants to those of the films. To meet this requirement single crystals of SrTi03, MgO, yttrium-stabilized ZrOz, A120,, LaA103, LaGaO,, LiNb03 are widely used [ l-31. These substrates can provide specific problems due to, for example, chemical reactions between film and substrate or their incompatibility with certain technical applications. The number of potentially useful substrates was increased considerably by the recent development of a series of mixed-perovskite crystals [ 41. Detailed studies are required before a final conclusion on their suitability for high-T, film manufacturing can be made. This Letter reports the first results obtained for in situ deposited YBa2Cu307_-x thin films on Nd~,~Sr~,~Al~.,Tao.~03 substrates. Nd0.4Sr0.6A1,-,7Ta0,J03 is one of the mixed-perovskite crystals [4] with a lattice constant close to that of YBa$&O,. A dc planar magnetron sputtering system with a 50 mm diameter superconducting YBa2Cu90, target was used for the film preparation. The substrate was located 25 mm below the target centre to protect the ’ On leave from Electrical Engineering Institute, Department EIVT, 197376 St. Petersburg, Russia. * Institut ftir Kristallzilchtung Berlin-Adlershof, 0- 1199 Berlin, Germany.
146
film from negative ion bombardment during the “low-pressure” sputtering [ 21. A silicon carbide heater was employed for the substrate heating. The Nd~.~Sro.~Al~.,Ta~.~O~ substrates were provided by the Institut ftir Kristallziichtung, BerlinAdlershof. The crystals were violet pieces of size 1 x 1 cm’ and a thickness of 0.5 mm. One side of the substrates with ( 100) orientation was polished (see fig. 2a below). The crystals had a cubic structure with a lattice constant of 0.384 nm. The sputtering parameters were: a pressure of 20 Pa of a 1: 1 Ar-O2 mixture, a substrate temperature of 720’ C and a discharge current of 150 mA. From our experience with MgO and SrTi03 substrates, these parameters were expected to produce in situ superconducting films with a transition temperature to superconductivity, T,, of at least 85 K. All the YBa2Cu307_-x films on Ndo.4Sro.sAlo.,Tao.30~ substrates, however, exhibited maximum T, values of 81 K. The variation of the sputtering parameters around the reported values did not produce any significant improvement in film quality. A factor affecting the quality of the films was found to be the chemical reaction between the film and substrate material. Fig. 1 shows the results of a SIMS analysis (CAMECA, type IMS4S) carried out on a typical YBaZCu307_-x film on Ndo.4Sro.sAlo.,Tao.~O~. The depth profiles of the substrate and film elements were measured during the bombardment with 5.5
0167-577x/92/$05.00
0 1992 Elsevier Science Publishers B.V. All rights reserved.
Volume 15, number
3
MATERIALS
Nij,, ,/----..
,.
LETTERS
November
1992
I
--.-__B?_ ‘--.__ ---.____ ---__ .__ Y ---._ -----v._-.._=
c
I-\
/
fO’L-.----,~-. 0
\
1 10
20
30
Sputter Time (min)
Fig. 1. SIMS depth profiles measured on a YBa2Cu307_-x deposited onto a Ndo.4Sr(l.6Alo.7Tao.303 substrate.
film
keV Cs+ ions. The film thickness was 200 nm, its surface morphology is presented in fig. 2b. Although the data in fig. 1 must be viewed as qualitative, they indicate substantial concentrations of Al atoms inside the film. The highest intensities measured for Ta+, Y+ and Ba’ ions were registered from the interface region between the film and the substrate. As the elemental signal strengths are highly dependent on the local chemical environment, this results from a specific kind of matrix formed in the interface region owing to chemical reactions between both materials. It is well known that interface reactions can debase the film properties by contaminating it or changing its composition [ 5 1. The reaction between film and substrate material during the deposition process also caused a severe deterioration of the substrate surface as observed after removal of the film by etching (20% water solution of HNO,). Most apparent was the change in colour from violet to dark grey. The change in surface morphology is illustrated in fig. 2c, where SEM (Philips, type 525) pictures are shown. EDAX (Philips 9900) measurements indicated that the surface composition of the used substrates no longer corresponded to the original one. Relative changes in composition of about 10% were observed. Furthermore, examination of different points of the treated substrate surface showed that the composition changes were not homogeneous. It should be noted that neither a heating at 720°C under a pressure of 20 Pa of the Ar-O2 mixture nor a treatment with 20% water solution of HNO3 were
Fig. 2. SEM micrographs of Nd&&Alo.,T~.~O~ substrates: (a) surface of an unused substrate, (b) surface of a YBazCu107_, film on Ndo.4Sro.sAla7Ta0.303, (c) substrate surface after removal of a deposited YBa$Zuj07_x film by etching.
147
Volume 15, number 3
MATERIALS LETTERS
able to cause similar changes of the surface of virgin samples. This means that the contact between film and substrate gives rise to the formation of interface compounds which are preferentially etched. One can conclude that, compared to certain other substrate materials f S-71, Ndo.~Sro.~Alo.7Ta~.303 is not suited for the direct deposition of YBazCu30, thin films because of excessive interface reactions. In order to use this substrate material it is likely to be necessary to either decrease the deposition temperature or to produce a buffer layer. Otherwise, the interface reactions will reduce the benefit gained from the close lattice match to YBa2Cu307 attainable with this material. It should be noted that the pure perovskites NdGaO,, LaGa03 [ 1,7] and LaA103 [ 2,8] which are closely related to Ndo..lSro.6Al,,,Tao.303are suitable for the preparation of high-quality YBazCu30, films under deposition conditions comparable to those used by us. Negligible or no interface reactions seem to take place between these pure perovskites and YBa2Cu307. The formation of a mixed-perovskite by partially substituting the parent elements thus enhances the reactivity with YBazCu307. We suppose that significant interface reactions might also be observed when other mixed-perovs~te substrates of the series proposed by Mateika et al. [4] are employed. This is of general significance with respect to the search for new substrate materials. Nevertheless, each material should be examined in a way as described in this work because it seems to us very difficult to make a priori decisions on its suitability. As an example, one should compare the
148
November 1992
susceptibility to interface reaction problems of the compounds LiNb03 and KTaO, which are closely related to each other. Whereas KTaO, is relatively inert chemically to YBa$&O, [ 9 1, LiNb03 distinguishes itself by a strong reactivity [ 5 1. The authors are grateful to H. Holzbrecher and E.M. Wuertz for their technical assistance. AZ wishes to thank the Deutscher Akademischer Austauschdienst for financial support. References [ I] C. Koren, A. Gupta, E.A. Giess, A. Segmtiller and R.B. Laibowitz, Appl. Phys. Letters 54 (1989) 1054.
[ 21 J.R. Gavaler, J. Talvacchio, T.T. Braggins, M.G. Forrester and J. Greggi, J. Appl. Phys. 70 (1991) 4383.
[ 31 SM. Garrison, N. Newman, B.F. Cole, K. Char and R.W. Barton, Appl. Phys. Letters 58 (1991) 2168. [4] D. Mateika, H. Kohler, H. Laudan and E. vijlkel, J. Cryst. Growth 109 (1991) 447. f5] T. Venkatesan, C.C. Chang, D. Dijkkamp, S.B. Ogale, E.W. Chase, L.A. Farrow, D.M. Hwang, P.F. Mice& S.A. Schwarz, J.M. Tarascon, X.D. Wu and A. Inam, J. Appl. Phys. 63 (1988) 4591. ] P. Madakson, J.J. Cuomo, D.S. Yee, R.A. Roy and G. Scilla, J. Appt. Phys. 63 (1988) 2046. ] E.A. Gies, R.L. Sandstrom, W.J. Gallagher, A. Gupta, S.L. Shinde, R.F. Cook, E.I. Cooper, E.J.M. ~Sulliv~, J.M. Roldan, A.P. Segmtiller and J. Angilello, IBM J. Res. Develop. 34 (1990) 916. ] R.W. Simon, C.E. Platt,A.E. Lee, G.S. Lee, K.P. Daly, MS. Wire, J.A. Luine and M. Urbanik, Appl. Phys. Letters 53 (1988) 2677. [9] J.W. McCamy, D.P. Norton, D.H. Lowndes, L.A. Boatner, D.K. Christen, R. Feenstra and E. Sonder, Mater. Res. Sot. Symp. Proc. 169 (1990) 469.