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Epitaxial multilayers of YBa2Cu3O7 and PrBa2Cu3O7 as a possible basis for superconducting electronic devices
Solid State Communications, Vol. 71, No. 7, pp. 569-572,1989. Printedin Great Britain.
0038-io98/89$3.00+.00 MaxwellPergamonMacmillanplc
EPITAXIAL MULTILAYERS OF YBa2Cu307 and PrBa2Cu307 AS A POSSIBLE BASIS FOR SUPERCONDUCTING ELECTRONIC DEVICES U. Poppe, P. Prieto*, J. Schubert, H. Soltner, K. Urban Institut fiirFestkdrperforschung, and Ch. Buchal Institut fiirSchicht- und Ionentechnik, Kernforschungsanlage Jiilich GmbH, Postfach 19 13, D-5178 Jiilich, Federal Republic of Germany (* On leave from Universidad de1 Valle, Cali, Colombia, Alexander von Humboldt Foundation fellow) (Received 13 June 1989 by P.H. Dederichs) ABSTRACT structural and The superconducting properties of ‘f~a~~s30~/PrBa2Cu307 heterostructures on SrTi03 were studied. ound that the epitaxy is maintained throughout the layer system. Interdiffusion and possible chemical reaction close to the interfaces can be neglected. Within measurement accuracy no effect of the semiconducting PrBa Cu O7 film on and the critical current density in YBa $u$7 could be TC detected. The results indicate that 'YBa2 u3 7/PrBa2Cu O7 heterostructures provide a promising basis 2or superconducting electronic devices.
INTRODUCTION Since the discovery of the high T, perovskite with superconductors structure /1,2/, considerable progress in the production of thin films of these materials has been made. Thin films of YBa2Cu307 grow epitaxially on SrTi03 single crystals. They are fully superconducting at temperatures between cri ical 80 and 90 K and exhibi current densities above 10 i A/cm'2 at 77 K. Possible future electronic applications of such films concern SIS or SQUIDS, tunnel junctions in SNS threedevices or transmission-line the For terminal devices /3-5/. realization of such devices it will be crucial to know whether techniques can be developed allowing the production of multilayers of superconducting and nonsuperconducting or insulating films. It is well established that the superconducting properties depend very and structure the sensitively on stoichiometry of the films. Furthermore, it is known that grain boundaries YBa2Cu307 act as in polycrystalline
.
multiple weak links in SQUID structures /6/ and that critical current densities in polycrystalline material are too low. defines a number of This requirements which have to be met by a suitable materials process and combination. The epitaxy has to be maintained throughout the multilayer system. There should be little or no interdiffusion and the nonsuperconducting layers must not react chemically with the superconducting ones at the usual deposition temperatures between 650' and 750 'C. In particular, the oxygen content in the superconductor should remain at its optimum value. Furthermore, nonsuperconducting the layer should not have a strong pair breaking effect, e. g. by magnetic scattering. A possible depression of the order parameter or the critical temperature via proximity effect in the superconductor should be negligible. In a tunneling junction, the sampling depths of the electrons or Cooper pairs are in the order of the coherence length, which is known to be extremely short in the high T, superconductors. between the Thus the interface 569
570
Vol. 71, No. 7
SUPERCONDUCTING ELECTRONICDEVICES
nonsuperconand the superconductor ducting material should be very sharp. devices like transmission In other lines the semiconducting or insulating layer acts as a dielectric separation between the superconducting stripline and the superconducting ground plane. the case the demands on In this interface quality are less stringent. Possible candidates for combination with YBa2Cu307 are the semiconducting perovskite like compounds PrBa2Cu 07 or Ml+xBa2_xCu307 (M = Nd, Eu, La,.. 7 /7/. For the present study we have chosen PrBa2Cu 07 (in the following abbreviated i 0 Pr*). The properties of bulk ceramic PrBa2~o~~ou,",'v;a~e;n sk;z;;d /8-lo/. The rhombic perovskite structure isomorphic ;;b;i;;a,o,', ~oa+~)307 (in the following EXPERIMENTAL In the experiments, (001) orisnted SrriO substrates were used. The Y and Pr t2. in films were deposited at a rate of about 100 rim/h on substrates heated to a temperature of about 700 OC. A dcsputtering technique in pure oxygen at a pressure of about 4 mbar was used /li,lZ/. For the productio? of sandwich films the structures of Y and Pr target heads were changed after opening the sputtering chamber. During this the substrate temperature procedure, was kept at a value above 300 “C and exposure of the films to the atmosphere was avoided by maintaining a strong oxygen flow through the chamber. RESULTS AND DISCUSSION The properties of single Y* and Pr* thin films were studied first. By using improved surface substrates with and targets of quality employing stoichiometric composition instead of copperrich ones as in our earlier work /11,13/, we were able to obtain highquality Y* films homogeneous over a circular area of 3 cm in diameter. The films were almost free of precipitates and flat down to the nanometer scale as shown by scanning electron microscopy and scanning tunneling microscopy. As indicated by X-ray diffractometry the grew with films epitaxially the crystallographic c-axis parallel to the substrate normal. The superconducting transitions were sharp, with complete zero resistance between about 85 and 90 K. Films thicker than about 80 nm ex ibite critical currents of about 1 A/cm_!! in lo_ zero and of field lo5 A/cm2 in a magnetic field of 3 T. Since for epitaxy the surface of the Y* films has to be of good structural quality, we investigated a film by LEED quick transfer from the after a sputtering system to an UHV chamber. The diffraction spots indicated that
surfaces with a minimum of degradation sputtering by the can be obtained technique used. Figure 1 shows the resistivity of a Pr* film on SrTi03. The film thickness is about 100 nm. The resistivity at 77 K is quite high. It is higher by a factor of 10000 than that of a Y* film before it becomes superconducting. We X-ray found by diffractometry, Rutherford Backscattering Spectroscopy (RBS) and He ion channeling that the Pr* films grow epitaxially on SrTi03 the the with c-axis parallel to substrate normal. The properties of Y*/Pr* bilayers on SrTi03 were studied by - X-ray diffractometrv and RBS. In the X-rav patterns only reflections of the {OOLj family could be observed indicating that the c-axis in the bilayer system is parallel to the substrate normal. A He beam of 2 MeV energy and 1.5 mm in diameter was used for RBS and channeling. In a part of the sample protected from the Pr* film deposition, RBS yielded a value of 15 nm for the thickness of the Y* film. A RBS spectrum of the bilayer is shown in Fig. 2. The computer simulation /14/ indicated a thickness of the Pr* film of 55 nm and confirmed the stoichiometry of the film system. In Fig. 2 the upper curve was obtained for a random orientation of the incident He beam with respect to the lattice while the lower curve was obtained by aligning the beam parallel to the c-axis. We find that channeling (through the whole Pr*/Y*/SrTi03 system) strongly reduces the backscattering yield, e. g. the maximum due to Pr and Ba at 1.18 MeV is reduced to 10 % of the random yield. This is indicative of the very good epitaxial quality of the heterostructure. We
PrBaPCuS07 E 0 1.5 c
ZTiOS
0.0
Temperature Fig.
1
/ K
Temperature dependence of the . resistivity ot an epitaxial ;;;4.-&u307 (Pr*) film on (001) ?'
SUPERCONDUCTING ELECTRONICDEVICES
Vo1..71,No. 7 Energy(MeV) 50
1.2 1
660
1.4
I
1.6
I
1
.a I
760
Channel Fig.
2 RBS and channeling measurement of a bilayer of Pr* (thickness 55 nm) on Y* (thickness 15 run) epitaxially grown on (001) SrTi03 substrate. In the upper curve the incident 2 MeV He-ion beam has a random direction, in the lower curve the beam is aligned normal to the substrate.
could also observe channeling aligning the incident beam with the [Oil] axis. However, in this case the yield was reduced to 30 % of the random value and was dechanneling pronounced a more observed with increasing depth. This result of explained as a can be twinning in the a-b plane. The presence of such twins in Y* films is well the documented in literature /e.g. 15,16/. The quality of Y*/Pr*/Y* trilayer systems on SrTiO was also investigated by RBS. The thicj,ness of each of these layers was about 100 nm. In this case induced channeling along [0011 reduction of the Pr/Ba yield to 30 % 0: extent of The the random value. interdiffusion was estimated from the sharpness of the Pr-signal on top of the Ba-signal in the RBS spectrum. The computer simulation of the RBS spectrum did not indicate any interdiffusion between the individual layers within the depth resolution of the technique of 5 to 10 nm. It is possible to grow ultrathin Y* superconducting with good films properties on (001) SrTi03 /13,17-19/. It is not known whether these films are However, pin-hole-free. entirely down to constant resisti-vity thickness of 10 nm suggests that thz films are continuous. We have used such films to study the effect of the Pr* films on the superconducting properties of the Y* layers. A 15 nm thick y film was deposited substrate. SrTiO 1 cm on alx e film was Subsequently, half of t;1
571
convered by a 55 nm thick Pr* film (this sample was used for the RBS study of bilayers described above). On each half of the substrate a bridge 80 pm wide and 0.5 mm long was structured by laser ablation. In a 4-point measurement the critical current density I, of both of these Y* films, one covered with a Pr* film and on uncovered, was determined as a function of temperature. The results are depicted in Fig. 3. We find that in;l;th4erAa;$ ;f is of the order of 2 77 K. The same value for a film of equal thickness was measured earlier in a study of the thickness dependence of I in Y* fiims /13/. This means that &e Pr* film leaves critical the density in current the Y* film essentially unaffected. Whether the difference in the absolute values of I represents Fig. 3 physica E i:operty of the Y*/Pr* bilaayer is at not present clear. growth Local inhomogeneities resulting from a few scratches on small the substrate surface could induce critical current density variations of the same order of magnitude /20/. We note that both the covered and uncovered Y* exhibit a the film relatively high T, greater than 83 K. The critical temperature of the covered Y* film is even higher than that of the uncovered one. In /8,9/ the composition dependence of Tc in the Ylr~PrxaBsa2;;?;J system was investigated. that already for x = 0.3 T, is lowered to 60 K and that superconductivity vanishes at x = 0.6. Since in our results there is no evidence for a reduction of T in the Y* film, we can conclude thaz at the deposition
Fig.
3
Temperature dependence of the current density of two microbridges, one placed on a bilayer of a 55 nm Pr* and 15 nm Y* and the other one placed on a 15 nm thick single layer of Y*.
ELECTRONICDEVICES SUPERCONDUCTING
572
temperature of about 700 'C possible interdiffusion must be limited to a thinner than the range considerably film thickness of 15 nm. In addition from this experiment substantial pair breaking by magnetic Pr ions and strong superconductor by weakening of the proximity effect close to the interface can be excluded. CONCLUSIONS We have studied the structural and superconducting properties of heterostructures with YBa Cu307/PrBa2Cu307 on SrTiO3. We found t%Iat the epitaxy is maintained throughout the layer system. chemical Interdiffusion and possible reaction close to the interfaces can be neglected even in the case of ultrathin superconducting films. Within measueffect of the rement accuracy, no PrBa2Cu307 film on TfB;2-$13;y ~~~&~.cEI~ current density in detected. indicate that Our results heterostructures ;;z%;;;07;PrBa2Cu3?7 promising basis for many electronic devices. For example, using
Vol. 71, No.
the measured critical current density value and taking into account typical inductivities of small superconducting loops, it can be estimated that it should be possible to use narrow bridqes (width n< 5 urn) of eoitaxial ultrathin YBa2Cu307 films together with PrBa Cu307 to produce weak-link type SQUIss operating at 77 K. For SIS tunnel junctions with areas of a few micrometers in diameter which are typical for SQUIDS or other electronic devices the resistance of the PrBa2Cu307 barrier will be high enough to guarantee a significant voltage drop in the resistive state of the junction. ACKNOWLEDGEMENTS The authors wish to thank E. Gomez, C. Heiden, B. Kabius, B. Stritzker, J. Wittig, M. Zinke-Allmang and W. Zinn for stimulating discussions, G. Cox and D. Szynka for the STM and LEED investigations, C.H. Freiburg and W. Reichert for X-ray diffractometry, W. Sybertz for the SEM work, and W. Evers for technical assistance.
REFERENCES J.G. Bednorz and K.A. Miiller, Z. Phys. B64 189 (1986) 2 M.K. Wu, J.R. Ashburn, C.T. Torng, P.H. Hor, R.L. Meng, L. Gao, Z.J. Huang, Y.Q. Wang and C.W. Chu , Phys. Rev. Lett. 58 908 (1987) J.-Clarke, Nature 333 29‘(1988) A.I. Braginski, Physica C 153-155 1598 (1988) A.P. Malozemoff, Physica C 153-155 1049 (1988) R.H. ’ Koch, C.P. Umbach, M.M. Oprysko, J.D. Mannhart, B. Bumble, Gallagher, A. G.J. Clark, W.J. R.B. Gupta, A. Kleinsasser, Laibowitz, R.B. Sandstrom and M.R. Scheuermann, Physica C 153-155 1685 (1988) 7 I\. Zhang, B. Dabrowski, C.U. Segre, D.G. Hinks, I.K. Schuller, J.D. Jorgensen and M. Slaski, submitted to J. Phys. C 8 L. Soderholm, K. Zhang, D.G. Hinks, M.A. Beno, J.D. Jorgensen, C.U. Segre and I.K. Schuller, Nature 328 604 (1988) Y. Dalichaouch, M.S. Torikachvili, 9 E.A. Early, B.W. Lee, C.L. Seaman, K.N. Yang, H. Zhou and M.B. Maple; Solid State Comm. 65 1001 (1988) Kinoshita, A. Matsuda, H. 10 K. Shibata. T. Ishii. T. Watanabe and T. Yamaha; Jpn. J. Appl. Phys. _2J L1642 (1988) 11 U. Poppe, J. Schubert, R.R. Arons, W. Freiburg, W. Evers, C.H. 1
12 13
14 15
16 17 18
19 20
Reichert, K. Schmidt, W. Sybertz and K. Urban, Solid State Comm. 66 661 (1988) J. Schubert, U. and W. Poppe Sybertz; to be published in J. Less Common Metals (1989) P. Prieto and U. J. Schubert, Proceedings of the 7th Poor=, International Conference on Ion and Plasma Assisted Techniaues 9 11988) RBS simulation was done with RUMP provided by Larry Doolittle, Cornell University, Ithaca, N.Y. Y. Syono, M. Kikuchi, K. Ohishi, K. Hiraga, H. Arai, Y. Matsui, N. Kobayashi, T. Sasaoka and Y. Muto, Jpn. J. Appl. Phys. 26 1498 (1987) H.W. Zandbergen, G. van Tendeloo, Okabe and S. Amelinckx, Phys. gtat. Sol.(a) 103 45 (1987) X.X. Xi, J. Geerk, G. Linker, Q. Li and 0. Meyer, accepted at Appl. Phys. Lett. T. Venkatesan, X.D. Wu, B. Dutta, A. Inam, M.S. Hegde, D. M. Hwang, Chang, L. Nazar and B. C.C. Wilkens, Appl. Phys. Lett. 54 581 (1989) J. Frohlingsdorf, W. Zander and B. Stritzker, Solid State Comm. 67 965 (1988) R. Gross, K. Hipler, J. Mannhart, R.P. Huebener, P. Chaudhari, D. Dimos, C.C. Tsuei, J. Schubert and U. Poppe, submitted to Appl. Phys. Lett. (1988)
0038-io98/89$3.00+.00 MaxwellPergamonMacmillanplc
EPITAXIAL MULTILAYERS OF YBa2Cu307 and PrBa2Cu307 AS A POSSIBLE BASIS FOR SUPERCONDUCTING ELECTRONIC DEVICES U. Poppe, P. Prieto*, J. Schubert, H. Soltner, K. Urban Institut fiirFestkdrperforschung, and Ch. Buchal Institut fiirSchicht- und Ionentechnik, Kernforschungsanlage Jiilich GmbH, Postfach 19 13, D-5178 Jiilich, Federal Republic of Germany (* On leave from Universidad de1 Valle, Cali, Colombia, Alexander von Humboldt Foundation fellow) (Received 13 June 1989 by P.H. Dederichs) ABSTRACT structural and The superconducting properties of ‘f~a~~s30~/PrBa2Cu307 heterostructures on SrTi03 were studied. ound that the epitaxy is maintained throughout the layer system. Interdiffusion and possible chemical reaction close to the interfaces can be neglected. Within measurement accuracy no effect of the semiconducting PrBa Cu O7 film on and the critical current density in YBa $u$7 could be TC detected. The results indicate that 'YBa2 u3 7/PrBa2Cu O7 heterostructures provide a promising basis 2or superconducting electronic devices.
INTRODUCTION Since the discovery of the high T, perovskite with superconductors structure /1,2/, considerable progress in the production of thin films of these materials has been made. Thin films of YBa2Cu307 grow epitaxially on SrTi03 single crystals. They are fully superconducting at temperatures between cri ical 80 and 90 K and exhibi current densities above 10 i A/cm'2 at 77 K. Possible future electronic applications of such films concern SIS or SQUIDS, tunnel junctions in SNS threedevices or transmission-line the For terminal devices /3-5/. realization of such devices it will be crucial to know whether techniques can be developed allowing the production of multilayers of superconducting and nonsuperconducting or insulating films. It is well established that the superconducting properties depend very and structure the sensitively on stoichiometry of the films. Furthermore, it is known that grain boundaries YBa2Cu307 act as in polycrystalline
.
multiple weak links in SQUID structures /6/ and that critical current densities in polycrystalline material are too low. defines a number of This requirements which have to be met by a suitable materials process and combination. The epitaxy has to be maintained throughout the multilayer system. There should be little or no interdiffusion and the nonsuperconducting layers must not react chemically with the superconducting ones at the usual deposition temperatures between 650' and 750 'C. In particular, the oxygen content in the superconductor should remain at its optimum value. Furthermore, nonsuperconducting the layer should not have a strong pair breaking effect, e. g. by magnetic scattering. A possible depression of the order parameter or the critical temperature via proximity effect in the superconductor should be negligible. In a tunneling junction, the sampling depths of the electrons or Cooper pairs are in the order of the coherence length, which is known to be extremely short in the high T, superconductors. between the Thus the interface 569
570
Vol. 71, No. 7
SUPERCONDUCTING ELECTRONICDEVICES
nonsuperconand the superconductor ducting material should be very sharp. devices like transmission In other lines the semiconducting or insulating layer acts as a dielectric separation between the superconducting stripline and the superconducting ground plane. the case the demands on In this interface quality are less stringent. Possible candidates for combination with YBa2Cu307 are the semiconducting perovskite like compounds PrBa2Cu 07 or Ml+xBa2_xCu307 (M = Nd, Eu, La,.. 7 /7/. For the present study we have chosen PrBa2Cu 07 (in the following abbreviated i 0 Pr*). The properties of bulk ceramic PrBa2~o~~ou,",'v;a~e;n sk;z;;d /8-lo/. The rhombic perovskite structure isomorphic ;;b;i;;a,o,', ~oa+~)307 (in the following EXPERIMENTAL In the experiments, (001) orisnted SrriO substrates were used. The Y and Pr t2. in films were deposited at a rate of about 100 rim/h on substrates heated to a temperature of about 700 OC. A dcsputtering technique in pure oxygen at a pressure of about 4 mbar was used /li,lZ/. For the productio? of sandwich films the structures of Y and Pr target heads were changed after opening the sputtering chamber. During this the substrate temperature procedure, was kept at a value above 300 “C and exposure of the films to the atmosphere was avoided by maintaining a strong oxygen flow through the chamber. RESULTS AND DISCUSSION The properties of single Y* and Pr* thin films were studied first. By using improved surface substrates with and targets of quality employing stoichiometric composition instead of copperrich ones as in our earlier work /11,13/, we were able to obtain highquality Y* films homogeneous over a circular area of 3 cm in diameter. The films were almost free of precipitates and flat down to the nanometer scale as shown by scanning electron microscopy and scanning tunneling microscopy. As indicated by X-ray diffractometry the grew with films epitaxially the crystallographic c-axis parallel to the substrate normal. The superconducting transitions were sharp, with complete zero resistance between about 85 and 90 K. Films thicker than about 80 nm ex ibite critical currents of about 1 A/cm_!! in lo_ zero and of field lo5 A/cm2 in a magnetic field of 3 T. Since for epitaxy the surface of the Y* films has to be of good structural quality, we investigated a film by LEED quick transfer from the after a sputtering system to an UHV chamber. The diffraction spots indicated that
surfaces with a minimum of degradation sputtering by the can be obtained technique used. Figure 1 shows the resistivity of a Pr* film on SrTi03. The film thickness is about 100 nm. The resistivity at 77 K is quite high. It is higher by a factor of 10000 than that of a Y* film before it becomes superconducting. We X-ray found by diffractometry, Rutherford Backscattering Spectroscopy (RBS) and He ion channeling that the Pr* films grow epitaxially on SrTi03 the the with c-axis parallel to substrate normal. The properties of Y*/Pr* bilayers on SrTi03 were studied by - X-ray diffractometrv and RBS. In the X-rav patterns only reflections of the {OOLj family could be observed indicating that the c-axis in the bilayer system is parallel to the substrate normal. A He beam of 2 MeV energy and 1.5 mm in diameter was used for RBS and channeling. In a part of the sample protected from the Pr* film deposition, RBS yielded a value of 15 nm for the thickness of the Y* film. A RBS spectrum of the bilayer is shown in Fig. 2. The computer simulation /14/ indicated a thickness of the Pr* film of 55 nm and confirmed the stoichiometry of the film system. In Fig. 2 the upper curve was obtained for a random orientation of the incident He beam with respect to the lattice while the lower curve was obtained by aligning the beam parallel to the c-axis. We find that channeling (through the whole Pr*/Y*/SrTi03 system) strongly reduces the backscattering yield, e. g. the maximum due to Pr and Ba at 1.18 MeV is reduced to 10 % of the random yield. This is indicative of the very good epitaxial quality of the heterostructure. We
PrBaPCuS07 E 0 1.5 c
ZTiOS
0.0
Temperature Fig.
1
/ K
Temperature dependence of the . resistivity ot an epitaxial ;;;4.-&u307 (Pr*) film on (001) ?'
SUPERCONDUCTING ELECTRONICDEVICES
Vo1..71,No. 7 Energy(MeV) 50
1.2 1
660
1.4
I
1.6
I
1
.a I
760
Channel Fig.
2 RBS and channeling measurement of a bilayer of Pr* (thickness 55 nm) on Y* (thickness 15 run) epitaxially grown on (001) SrTi03 substrate. In the upper curve the incident 2 MeV He-ion beam has a random direction, in the lower curve the beam is aligned normal to the substrate.
could also observe channeling aligning the incident beam with the [Oil] axis. However, in this case the yield was reduced to 30 % of the random value and was dechanneling pronounced a more observed with increasing depth. This result of explained as a can be twinning in the a-b plane. The presence of such twins in Y* films is well the documented in literature /e.g. 15,16/. The quality of Y*/Pr*/Y* trilayer systems on SrTiO was also investigated by RBS. The thicj,ness of each of these layers was about 100 nm. In this case induced channeling along [0011 reduction of the Pr/Ba yield to 30 % 0: extent of The the random value. interdiffusion was estimated from the sharpness of the Pr-signal on top of the Ba-signal in the RBS spectrum. The computer simulation of the RBS spectrum did not indicate any interdiffusion between the individual layers within the depth resolution of the technique of 5 to 10 nm. It is possible to grow ultrathin Y* superconducting with good films properties on (001) SrTi03 /13,17-19/. It is not known whether these films are However, pin-hole-free. entirely down to constant resisti-vity thickness of 10 nm suggests that thz films are continuous. We have used such films to study the effect of the Pr* films on the superconducting properties of the Y* layers. A 15 nm thick y film was deposited substrate. SrTiO 1 cm on alx e film was Subsequently, half of t;1
571
convered by a 55 nm thick Pr* film (this sample was used for the RBS study of bilayers described above). On each half of the substrate a bridge 80 pm wide and 0.5 mm long was structured by laser ablation. In a 4-point measurement the critical current density I, of both of these Y* films, one covered with a Pr* film and on uncovered, was determined as a function of temperature. The results are depicted in Fig. 3. We find that in;l;th4erAa;$ ;f is of the order of 2 77 K. The same value for a film of equal thickness was measured earlier in a study of the thickness dependence of I in Y* fiims /13/. This means that &e Pr* film leaves critical the density in current the Y* film essentially unaffected. Whether the difference in the absolute values of I represents Fig. 3 physica E i:operty of the Y*/Pr* bilaayer is at not present clear. growth Local inhomogeneities resulting from a few scratches on small the substrate surface could induce critical current density variations of the same order of magnitude /20/. We note that both the covered and uncovered Y* exhibit a the film relatively high T, greater than 83 K. The critical temperature of the covered Y* film is even higher than that of the uncovered one. In /8,9/ the composition dependence of Tc in the Ylr~PrxaBsa2;;?;J system was investigated. that already for x = 0.3 T, is lowered to 60 K and that superconductivity vanishes at x = 0.6. Since in our results there is no evidence for a reduction of T in the Y* film, we can conclude thaz at the deposition
Fig.
3
Temperature dependence of the current density of two microbridges, one placed on a bilayer of a 55 nm Pr* and 15 nm Y* and the other one placed on a 15 nm thick single layer of Y*.
ELECTRONICDEVICES SUPERCONDUCTING
572
temperature of about 700 'C possible interdiffusion must be limited to a thinner than the range considerably film thickness of 15 nm. In addition from this experiment substantial pair breaking by magnetic Pr ions and strong superconductor by weakening of the proximity effect close to the interface can be excluded. CONCLUSIONS We have studied the structural and superconducting properties of heterostructures with YBa Cu307/PrBa2Cu307 on SrTiO3. We found t%Iat the epitaxy is maintained throughout the layer system. chemical Interdiffusion and possible reaction close to the interfaces can be neglected even in the case of ultrathin superconducting films. Within measueffect of the rement accuracy, no PrBa2Cu307 film on TfB;2-$13;y ~~~&~.cEI~ current density in detected. indicate that Our results heterostructures ;;z%;;;07;PrBa2Cu3?7 promising basis for many electronic devices. For example, using
Vol. 71, No.
the measured critical current density value and taking into account typical inductivities of small superconducting loops, it can be estimated that it should be possible to use narrow bridqes (width n< 5 urn) of eoitaxial ultrathin YBa2Cu307 films together with PrBa Cu307 to produce weak-link type SQUIss operating at 77 K. For SIS tunnel junctions with areas of a few micrometers in diameter which are typical for SQUIDS or other electronic devices the resistance of the PrBa2Cu307 barrier will be high enough to guarantee a significant voltage drop in the resistive state of the junction. ACKNOWLEDGEMENTS The authors wish to thank E. Gomez, C. Heiden, B. Kabius, B. Stritzker, J. Wittig, M. Zinke-Allmang and W. Zinn for stimulating discussions, G. Cox and D. Szynka for the STM and LEED investigations, C.H. Freiburg and W. Reichert for X-ray diffractometry, W. Sybertz for the SEM work, and W. Evers for technical assistance.
REFERENCES J.G. Bednorz and K.A. Miiller, Z. Phys. B64 189 (1986) 2 M.K. Wu, J.R. Ashburn, C.T. Torng, P.H. Hor, R.L. Meng, L. Gao, Z.J. Huang, Y.Q. Wang and C.W. Chu , Phys. Rev. Lett. 58 908 (1987) J.-Clarke, Nature 333 29‘(1988) A.I. Braginski, Physica C 153-155 1598 (1988) A.P. Malozemoff, Physica C 153-155 1049 (1988) R.H. ’ Koch, C.P. Umbach, M.M. Oprysko, J.D. Mannhart, B. Bumble, Gallagher, A. G.J. Clark, W.J. R.B. Gupta, A. Kleinsasser, Laibowitz, R.B. Sandstrom and M.R. Scheuermann, Physica C 153-155 1685 (1988) 7 I\. Zhang, B. Dabrowski, C.U. Segre, D.G. Hinks, I.K. Schuller, J.D. Jorgensen and M. Slaski, submitted to J. Phys. C 8 L. Soderholm, K. Zhang, D.G. Hinks, M.A. Beno, J.D. Jorgensen, C.U. Segre and I.K. Schuller, Nature 328 604 (1988) Y. Dalichaouch, M.S. Torikachvili, 9 E.A. Early, B.W. Lee, C.L. Seaman, K.N. Yang, H. Zhou and M.B. Maple; Solid State Comm. 65 1001 (1988) Kinoshita, A. Matsuda, H. 10 K. Shibata. T. Ishii. T. Watanabe and T. Yamaha; Jpn. J. Appl. Phys. _2J L1642 (1988) 11 U. Poppe, J. Schubert, R.R. Arons, W. Freiburg, W. Evers, C.H. 1
12 13
14 15
16 17 18
19 20
Reichert, K. Schmidt, W. Sybertz and K. Urban, Solid State Comm. 66 661 (1988) J. Schubert, U. and W. Poppe Sybertz; to be published in J. Less Common Metals (1989) P. Prieto and U. J. Schubert, Proceedings of the 7th Poor=, International Conference on Ion and Plasma Assisted Techniaues 9 11988) RBS simulation was done with RUMP provided by Larry Doolittle, Cornell University, Ithaca, N.Y. Y. Syono, M. Kikuchi, K. Ohishi, K. Hiraga, H. Arai, Y. Matsui, N. Kobayashi, T. Sasaoka and Y. Muto, Jpn. J. Appl. Phys. 26 1498 (1987) H.W. Zandbergen, G. van Tendeloo, Okabe and S. Amelinckx, Phys. gtat. Sol.(a) 103 45 (1987) X.X. Xi, J. Geerk, G. Linker, Q. Li and 0. Meyer, accepted at Appl. Phys. Lett. T. Venkatesan, X.D. Wu, B. Dutta, A. Inam, M.S. Hegde, D. M. Hwang, Chang, L. Nazar and B. C.C. Wilkens, Appl. Phys. Lett. 54 581 (1989) J. Frohlingsdorf, W. Zander and B. Stritzker, Solid State Comm. 67 965 (1988) R. Gross, K. Hipler, J. Mannhart, R.P. Huebener, P. Chaudhari, D. Dimos, C.C. Tsuei, J. Schubert and U. Poppe, submitted to Appl. Phys. Lett. (1988)