- Email: [email protected]
Improved initial epitaxial growth of superconducting YBa2Cu3O7 thin films on Y–ZrO2 substrates with a La1.85Sr0.15CuO4 buffer layer
Physica C 330 Ž2000. 160–164 www.elsevier.nlrlocaterphysc
Improved initial epitaxial growth of superconducting YBa 2 Cu 3 O 7 thin films on Y–ZrO 2 substrates with a La 1.85 Sr0.15 CuO4 buffer layer J. Gao b
a,)
, G.J. Lian b, G.C. Xiong
b
a Department of Physics, The UniÕersity of Hong Kong, Pokfulam Road, Hong Kong, People’s Republic of China Department of Physics and Mesoscopic Physics National State Key Laboratory, Peking UniÕersity, 100871 Beijing, People’s Republic of China
Received 20 October 1999; accepted 16 November 1999
Abstract Epitaxial thin films of YBa 2 Cu 3 O 7 ŽYBCO. have been deposited on Y–ZrO 2 ŽYSZ. substrates by means of the pulse laser deposition technique. It has been found that the initial epitaxy of YBCO thin films grown on YSZ can be significantly improved by using La 1.85 Sr0.15 CuO4 ŽLSCO. as a buffer layer. X-ray diffraction measurements show that the epitaxial YBCO films have single in-plane orientation with YBCO w100x I LSCO w100x and LSCO w100x I YSZ w110x. The real-time resistance measurements reveal that with LSCO buffer layers the initial formation of the YBCO ultra-thin films changes from the island growth to the layer-by-layer growth. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Electrical resistivity; Superconductivity; Thin films
1. Introduction Yttrium-stabilized zirconia ŽYSZ. is one of the most common substrate materials used to prepare high temperature superconducting thin films. YSZ substrates are much cheaper than other substrates like SrTiO 3 ŽSTO. and LaAlO 3 ŽLAO.. YBCO thin films with high critical current density JC have been grown on YSZ substrates by using many techniques such as the magnetron sputtering w1–3x and the pulsed laser deposition w4,5x. YSZ has also been used successfully as a diffusion barrier on other more reac) Corresponding author. Tel.: q852-2859-7948; fax: q8522559-9152. E-mail address: [email protected] ŽJ. Gao..
tive substrates such as Si, Al 2 O 3 and flexible metallic substrates w6,7x. However, YBa 2 Cu 3 O 7 ŽYBCO. thin films grown directly on YSZ substrates can have different orientations in the a–b plane w8x. Using STO bicrystals, Dimos et al. w9x demonstrated the detrimental effect of high angle grain boundaries on Jc in c-axis-oriented epitaxial YBCO thin films. Therefore, single-oriented epitaxial YBCO films are preferred in physics research and applications, which limits the potential of applications for using YSZ substrates. Beside the orientation problem, it has been recognized by transmission electron microscopy ŽTEM. and in-situ resistance measurement ŽISRM. that the initial growth of YBCO films on YSZ substrates showed significant island formation w10–12x. The island growth mechanism will have
0921-4534r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 9 9 . 0 0 6 0 5 - X
J. Gao et al.r Physica C 330 (2000) 160–164
direct effect on the property of ultra-thin YBCO films deposited on YSZ substrates. In addition, the initial epitaxy of YBCO on YSZ is often puzzled by the occurrence of an intermediate layer between film and YSZ. Such an intermediate layer is mainly formed as BaZrO 3 below the substrate surface due to the diffusion of Ba into YSZ. The loss of Ba in the YBCO would results in imperfection and discontinuities in the initial grown film w13x. YSZ has a lattice constant Ž a s 0.516 nm. which is quite different from that of YBCO Ž a s 0.382 nm and b s 0.389 nm.. Comparing with STO Ž a s 0.391 nm. and LAO Ž a s 0.379 nm., YBCO and YSZ have large lattice mismatch about D s 4.5–6.2%. The dissimilarity of crystal structure and lattice constants between YBCO and YSZ results in the feature that epitaxial c-axis YBCO films grown on YSZ have several in-plane orientations with island growth mode. Two in-plane orientations, YBCOw100x I YSZw110x and YBCOw100x I YSZw100x, have been observed by Wu et al. w7x in YBCO thin films grown directly on YSZ substrates. Experimental results indicates that seed buffer layers deposited prior to YBCO may be an effective method for improving the initial epitaxial growth of YBCO on YSZ substrates. La 1.85 Sr0.15 CuO4 ŽLSCO. has a K 2 NiF4-type structure with the lattice constant of Ž a s 0.3779 nm. just located in between that of YSZ and YBCO. Using pulsed laser deposition ŽPLD. we have been achieved epitaxial growth of LSCO on YSZ. In this paper, we report our experimental results on the epitaxial growth of LSCO buffer layers, YBCO films and the ISRM. Epitaxial YBCO thin films deposited on YSZ substrates with seed LSCO buffer layers show good superconducting properties and only one in-plane orientation. In situ real-time resistance measurements reveal that the epitaxial YBCO on LSCO buffer layers were grown layer-by-layer ŽFrank-van der Merwe mode. from beginning of the deposition.
2. Experimental The YBCO films and LSCO buffer layers were prepared by PLD. The PLD system and deposition process have been described previously w14x. In brief, a KrF excimer laser with a wavelength of 248 nm
161
was used. Sintered ceramic targets with nominal compositions of LSCO and YBCO were mounted in the chamber at an angle of 458 to the laser beam and can be in site changed as required. The laser beam was set to 2 Jrcm2 at the target surface with 4–6 Hz pulse rate. During deposition, the oxygen partial pressure was kept at 25 Pa. After deposition the as-grown films were annealed at 4508C for 10 min in oxygen. The superconducting YBCO films were grown at the substrate temperature of Ts s 7868C. A Philips X pert four-circle X-ray diffractometer with Cu K a radiation was used to investigate the structures and orientations of the buffer layers and films.
3. Results Epitaxial growth of LSCO films can be achieved on Ž100. STO and Ž100. LAO substrates by using similar deposition conditions as that used for fabricating superconducting YBCO thin films. However, we found that the X-ray diffraction patterns of LSCO films deposited on YSZ substrates at TS s 7868C show strong c-axis textured feature with a very weak Ž110. peak Žsee Fig. 1Ža... YBCO thin films deposited on these c-axis textured LSCO layers show good superconducting properties with Tc Ž R s 0. in the range of 89–91 K. It is interesting that only one in-plane orientation was observed for the YBCO film. To determine the in-plane orientations of the YBCO and LSCO, f-scan measurements were carried out. Fig. 1Žb. demonstrates the f-scan spectra for the Ž202. family of the YSZ substrate, the Ž103. family of the LSCO buffer layer and the Ž103. family of the YBCO film, respectively. The f-scans of the YBCO film and the LSCO buffer layer indicate in-plane single orientation with four-fold symmetry, which characterize the achievement of heteroepitaxial growth of the YBCO film. The epitaxial orientation is c-axis perpendicular to the substrate surface and YBCOw100x I LSCOw100x. The 458 shift between the LSCO Ž103. peaks and YSZ Ž202. peaks in Fig. 1Žb. indicates that the heteroepitaxial relationship is LSCOw100x I YSZw110x. To obtain a fully c-axis orientation in the LSCO layer, we increased the deposition temperature to 8868C and found no Ž110. peaks in the X-ray diffractions. All LSCO films deposited on YSZ at TS s
162
J. Gao et al.r Physica C 330 (2000) 160–164
Fig. 1. X-ray diffraction spectrum and the f-scan patterns of an YBCO film deposited at Ts s 7868C on an YSZ substrate with a LSCO buffer layer.
8868C present a highly c-axis epitaxial feature. Fig. 2 shows the X-ray diffraction pattern and the f-scans of a 150 nm YBCO film deposited on Ž100. YSZ substrate with a 150 nm LSCO buffer layer. In Fig. 2Ža., the u –2 u scan clearly shows the c-axis orientation of the YBCO film and the LSCO buffer layers with c s 1.17 and 1.32 nm, respectively. The heteroepitaxial orientations of the LSCO buffer layer and the YBCO film are c-axis perpendicular to the substrate surface, YBCOw100x I LSCOw100x and LSCO w100x I YSZ w110x. The lattice constants of the LSCO buffer layer are a s 0.378 and c s 1.32 nm. Good superconducting transitions were observed in the YBCO films deposited on the LSCO buffered YSZ substrates. In order to distinguish the benefit of LSCO buffer layers on the initial growth of epitaxial YBCO films deposited on YSZ substrates, real-time ISRM was performed on samples with and without LSCO buffer layers. Because the resistance of YSZ substrate at the deposited temperature is smaller than that of ultra thin YBCO layers Ž1102., sapphire substrates with 200 nm epitaxial YSZ buffer layer Žprepared by PLD
as described by reference w15x. were used in such measurements. X-ray analysis and superconducting measurement of YBCO films deposited on the sapphire substrates indicated that the buffer layers were of good quality. Two YBCO electrodes were deposited onto both ends of the substrates. The area between the electrodes was about 2.5 mm wide and 4 mm long. Platinum wires and silver glue were used to connect the YBCO electrodes with the digital multimeter ŽHP 3478A.. Such an arrangement allowed us to measure the film resistance in real time. At room temperature the bare substrates showed very high resistance over 100 GV. At the deposition temperature the resistance of the Al 2 O 3 substrates with YSZ layers was about 10 M V. Fig. 3 shows data of a real-time resistance measurement for YBCO on the epitaxial YSZ layers without LSCO buffer. At the beginning of the deposition, the sample resistance decreased slowly from 8.45 to 3 M V. After several tens of laser pulses, the value of the resistance drooped rapidly. At the end of deposition process, oxygen was filled into the chamber to a pressure of 1 atm, which accompanied a drastic
Fig. 2. X-ray diffraction u –2 u pattern Ža. and f-scan Žb. of an YBCO film deposited at Ts s 7868C on an YSZ substrate with LSCO buffer layers deposited at Ts s 8868C.
J. Gao et al.r Physica C 330 (2000) 160–164
resistance change. The inset of Fig. 3 is the R–t curve of this film, which shows the superconducting transition at 90 K. In the data analysis, if the YBCO film was grown layer-by-layer the real-time conductance of s can be described by s s s 0 q s YBCO . Fig. 4 shows the real-time conductance as a function of the deposition time for YBCO grown on YSZ with or without a LSCO buffer. In Fig. 4, the real-time conductance data for YBCO grown on YSZ without LSCO buffer is presented as curve Ža.. It can be seen that in the first twenty seconds there is no increase in the conductance. As observed by Shen and Olsan et al. the initial depositions have little contribution to the conductivity of the film, due to the island growth mechanism. In contrast, the real time conductance vs. the deposition time for YBCO on YSZ with a LSCO buffer exhibits a linear dependence Žcurve Žb... This clearly indicates a layer-bylayer initial growth of the YBCO film on LSCO buffer layer. It is interesting that with a LSCO buffer on YSZ, the real-time conductance data of YBCO shows the layer-by-layer growth mechanism from the beginning of the deposition as observed in Fig. 4Žb.. The inset of Fig. 4 shows the real-time resistance measurement for YBCO on the epitaxial LSCOrYSZrAl 2 O 3 , the initial resistance is 3.3 M V. YBCO ultra-thin films grown directly on YSZ substrates show the island growth mechanism with different in-plane orientations, which was attributed
163
Fig. 4. Real-time conductance data for YBCO deposited on the epitaxial YSZ layers Ža. without LSCO buffer and Žb. with a LSCO buffer. The inset shows data of a real-time resistance for YBCO deposited with LSCO buffer.
to the large lattice mismatch and stains on the substrate-film interface. By using the LSCO buffer layers, these two problems have been overcome, which reveal the benefit of suitable buffer layers on the technique of epitaxial growth. The layer-by-layer growth and single in-plane orientation of YBCO can be obtained on YSZ substrates. Moreover, the formation of the intermediate layer due to interaction between YBCO and YSZ could also be prevented.
4. Conclusions Significantly improved initial epitaxy of YBCO films with single in-plane orientation has been obtained on YSZ substrates by using a LSCO buffer layer. The epitaxial orientations of the YBCO on LSCO buffered YSZ substrates are YBCOw100xI LSCO w100x and LSCO w100x I YSZ w110x. The measurements of real-time conductance show that the initial growth of YBCO ultra-thin films deposited on the LSCO buffer layers is formed layer-by-layer.
Acknowledgements Fig. 3. Data of a real-time resistance for YBCO deposited on the epitaxial YSZ layers without LSCO buffer. The inset shows the superconducting measurement for this film.
This work has been supported by Research Grants Council ŽRGC. of Hong Kong ŽProject HKU7102r 97P..
164
J. Gao et al.r Physica C 330 (2000) 160–164
References w1x C.B. Eom, J.Z. Sun, K. Yamamoto, A.F. Masshall, K.E. Luther, T.H. Oeballe, S.S. Laderman, Appl. Phys. Lett 55 Ž1989. 595. w2x Q. Li, O. Meyer, X.X. Xi, J. Greek, G. Linker, Appl. Phys. Lett. 55 Ž1989. 1792. w3x J. Gao, B. Hauser, H. Rogalla, J. Appl. Phys. 67 Ž1990. ¨ 2512. w4x J.P. Zheng, H.S. Kim, Q.Y. Ying, R. Barone, P. Bush, D.T. Shaw, H.S. Kwok, Appl. Phys. Lett. 55 Ž1989. 1044. w5x G. Koren, E. Polturak, B. Fisher, D. Cohen, G. Kimel, Appil. Phys. Lett. 53 Ž1988. 2330. w6x D.K. Fork, D.B. Fennerr, R.W. Barton, J.M. Phillips, G.A.N. Connell, J.B. Boyce, T.H. Geballe, Appl. Phys. Lett. 57 Ž1990. 1161. w7x X.D. Wu, S.R. Foltyn, P.N. Arendt, W.R. Blumenthal, I.H.
w8x w9x w10x w11x w12x w13x w14x w15x
Campbell, J.D. Cotton, J.Y. Coulter, W.L. Hults, M.P. Maley, H.F. Safar, J.L. Smith, Appl. Phys. Lett. 67 Ž1995. 2397. S.M. Garrison, N. Newman, B.F. Cole, K. Char, R.W. Barton, Appl. Phys. Lett. 58 Ž1991. 2168. D. Dimos, P. Chaudhari, J. Mannhart, F.K. LeGouse, Phys. Rev. Lett. 61 Ž1988. 219. M.G. Norton, C.B. Carter, J. Cryst. Growth 110 Ž1991. 2684. W.P. Shen, C. Lenane, J.P. Zhengm, H.S. Kwok, Appl. Phys. Lett. 64 Ž1994. 3175. V. Olsan, M. Jelinek, Physica C 207 Ž1993. 391. J. Gao, T.C. Chui, W.H. Tamg, IEEE Trans. Appl. Supercon. 9 Ž1999. 1661. G.C. Xiong, G.J. Lian, Y.J. Li, Z.Z. Gan, Physica C 282–287 Ž1997. 551. X.D. Wu, R.E. Muenchausen, N.S. Nogar, A. Pique, R. Edwards, B. Wilkens, T.S. Ravi, D.M. Hwang, C.Y. Chen, Appl. Phys. Lett. 58 Ž1991. 304.
Improved initial epitaxial growth of superconducting YBa 2 Cu 3 O 7 thin films on Y–ZrO 2 substrates with a La 1.85 Sr0.15 CuO4 buffer layer J. Gao b
a,)
, G.J. Lian b, G.C. Xiong
b
a Department of Physics, The UniÕersity of Hong Kong, Pokfulam Road, Hong Kong, People’s Republic of China Department of Physics and Mesoscopic Physics National State Key Laboratory, Peking UniÕersity, 100871 Beijing, People’s Republic of China
Received 20 October 1999; accepted 16 November 1999
Abstract Epitaxial thin films of YBa 2 Cu 3 O 7 ŽYBCO. have been deposited on Y–ZrO 2 ŽYSZ. substrates by means of the pulse laser deposition technique. It has been found that the initial epitaxy of YBCO thin films grown on YSZ can be significantly improved by using La 1.85 Sr0.15 CuO4 ŽLSCO. as a buffer layer. X-ray diffraction measurements show that the epitaxial YBCO films have single in-plane orientation with YBCO w100x I LSCO w100x and LSCO w100x I YSZ w110x. The real-time resistance measurements reveal that with LSCO buffer layers the initial formation of the YBCO ultra-thin films changes from the island growth to the layer-by-layer growth. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Electrical resistivity; Superconductivity; Thin films
1. Introduction Yttrium-stabilized zirconia ŽYSZ. is one of the most common substrate materials used to prepare high temperature superconducting thin films. YSZ substrates are much cheaper than other substrates like SrTiO 3 ŽSTO. and LaAlO 3 ŽLAO.. YBCO thin films with high critical current density JC have been grown on YSZ substrates by using many techniques such as the magnetron sputtering w1–3x and the pulsed laser deposition w4,5x. YSZ has also been used successfully as a diffusion barrier on other more reac) Corresponding author. Tel.: q852-2859-7948; fax: q8522559-9152. E-mail address: [email protected] ŽJ. Gao..
tive substrates such as Si, Al 2 O 3 and flexible metallic substrates w6,7x. However, YBa 2 Cu 3 O 7 ŽYBCO. thin films grown directly on YSZ substrates can have different orientations in the a–b plane w8x. Using STO bicrystals, Dimos et al. w9x demonstrated the detrimental effect of high angle grain boundaries on Jc in c-axis-oriented epitaxial YBCO thin films. Therefore, single-oriented epitaxial YBCO films are preferred in physics research and applications, which limits the potential of applications for using YSZ substrates. Beside the orientation problem, it has been recognized by transmission electron microscopy ŽTEM. and in-situ resistance measurement ŽISRM. that the initial growth of YBCO films on YSZ substrates showed significant island formation w10–12x. The island growth mechanism will have
0921-4534r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 9 9 . 0 0 6 0 5 - X
J. Gao et al.r Physica C 330 (2000) 160–164
direct effect on the property of ultra-thin YBCO films deposited on YSZ substrates. In addition, the initial epitaxy of YBCO on YSZ is often puzzled by the occurrence of an intermediate layer between film and YSZ. Such an intermediate layer is mainly formed as BaZrO 3 below the substrate surface due to the diffusion of Ba into YSZ. The loss of Ba in the YBCO would results in imperfection and discontinuities in the initial grown film w13x. YSZ has a lattice constant Ž a s 0.516 nm. which is quite different from that of YBCO Ž a s 0.382 nm and b s 0.389 nm.. Comparing with STO Ž a s 0.391 nm. and LAO Ž a s 0.379 nm., YBCO and YSZ have large lattice mismatch about D s 4.5–6.2%. The dissimilarity of crystal structure and lattice constants between YBCO and YSZ results in the feature that epitaxial c-axis YBCO films grown on YSZ have several in-plane orientations with island growth mode. Two in-plane orientations, YBCOw100x I YSZw110x and YBCOw100x I YSZw100x, have been observed by Wu et al. w7x in YBCO thin films grown directly on YSZ substrates. Experimental results indicates that seed buffer layers deposited prior to YBCO may be an effective method for improving the initial epitaxial growth of YBCO on YSZ substrates. La 1.85 Sr0.15 CuO4 ŽLSCO. has a K 2 NiF4-type structure with the lattice constant of Ž a s 0.3779 nm. just located in between that of YSZ and YBCO. Using pulsed laser deposition ŽPLD. we have been achieved epitaxial growth of LSCO on YSZ. In this paper, we report our experimental results on the epitaxial growth of LSCO buffer layers, YBCO films and the ISRM. Epitaxial YBCO thin films deposited on YSZ substrates with seed LSCO buffer layers show good superconducting properties and only one in-plane orientation. In situ real-time resistance measurements reveal that the epitaxial YBCO on LSCO buffer layers were grown layer-by-layer ŽFrank-van der Merwe mode. from beginning of the deposition.
2. Experimental The YBCO films and LSCO buffer layers were prepared by PLD. The PLD system and deposition process have been described previously w14x. In brief, a KrF excimer laser with a wavelength of 248 nm
161
was used. Sintered ceramic targets with nominal compositions of LSCO and YBCO were mounted in the chamber at an angle of 458 to the laser beam and can be in site changed as required. The laser beam was set to 2 Jrcm2 at the target surface with 4–6 Hz pulse rate. During deposition, the oxygen partial pressure was kept at 25 Pa. After deposition the as-grown films were annealed at 4508C for 10 min in oxygen. The superconducting YBCO films were grown at the substrate temperature of Ts s 7868C. A Philips X pert four-circle X-ray diffractometer with Cu K a radiation was used to investigate the structures and orientations of the buffer layers and films.
3. Results Epitaxial growth of LSCO films can be achieved on Ž100. STO and Ž100. LAO substrates by using similar deposition conditions as that used for fabricating superconducting YBCO thin films. However, we found that the X-ray diffraction patterns of LSCO films deposited on YSZ substrates at TS s 7868C show strong c-axis textured feature with a very weak Ž110. peak Žsee Fig. 1Ža... YBCO thin films deposited on these c-axis textured LSCO layers show good superconducting properties with Tc Ž R s 0. in the range of 89–91 K. It is interesting that only one in-plane orientation was observed for the YBCO film. To determine the in-plane orientations of the YBCO and LSCO, f-scan measurements were carried out. Fig. 1Žb. demonstrates the f-scan spectra for the Ž202. family of the YSZ substrate, the Ž103. family of the LSCO buffer layer and the Ž103. family of the YBCO film, respectively. The f-scans of the YBCO film and the LSCO buffer layer indicate in-plane single orientation with four-fold symmetry, which characterize the achievement of heteroepitaxial growth of the YBCO film. The epitaxial orientation is c-axis perpendicular to the substrate surface and YBCOw100x I LSCOw100x. The 458 shift between the LSCO Ž103. peaks and YSZ Ž202. peaks in Fig. 1Žb. indicates that the heteroepitaxial relationship is LSCOw100x I YSZw110x. To obtain a fully c-axis orientation in the LSCO layer, we increased the deposition temperature to 8868C and found no Ž110. peaks in the X-ray diffractions. All LSCO films deposited on YSZ at TS s
162
J. Gao et al.r Physica C 330 (2000) 160–164
Fig. 1. X-ray diffraction spectrum and the f-scan patterns of an YBCO film deposited at Ts s 7868C on an YSZ substrate with a LSCO buffer layer.
8868C present a highly c-axis epitaxial feature. Fig. 2 shows the X-ray diffraction pattern and the f-scans of a 150 nm YBCO film deposited on Ž100. YSZ substrate with a 150 nm LSCO buffer layer. In Fig. 2Ža., the u –2 u scan clearly shows the c-axis orientation of the YBCO film and the LSCO buffer layers with c s 1.17 and 1.32 nm, respectively. The heteroepitaxial orientations of the LSCO buffer layer and the YBCO film are c-axis perpendicular to the substrate surface, YBCOw100x I LSCOw100x and LSCO w100x I YSZ w110x. The lattice constants of the LSCO buffer layer are a s 0.378 and c s 1.32 nm. Good superconducting transitions were observed in the YBCO films deposited on the LSCO buffered YSZ substrates. In order to distinguish the benefit of LSCO buffer layers on the initial growth of epitaxial YBCO films deposited on YSZ substrates, real-time ISRM was performed on samples with and without LSCO buffer layers. Because the resistance of YSZ substrate at the deposited temperature is smaller than that of ultra thin YBCO layers Ž1102., sapphire substrates with 200 nm epitaxial YSZ buffer layer Žprepared by PLD
as described by reference w15x. were used in such measurements. X-ray analysis and superconducting measurement of YBCO films deposited on the sapphire substrates indicated that the buffer layers were of good quality. Two YBCO electrodes were deposited onto both ends of the substrates. The area between the electrodes was about 2.5 mm wide and 4 mm long. Platinum wires and silver glue were used to connect the YBCO electrodes with the digital multimeter ŽHP 3478A.. Such an arrangement allowed us to measure the film resistance in real time. At room temperature the bare substrates showed very high resistance over 100 GV. At the deposition temperature the resistance of the Al 2 O 3 substrates with YSZ layers was about 10 M V. Fig. 3 shows data of a real-time resistance measurement for YBCO on the epitaxial YSZ layers without LSCO buffer. At the beginning of the deposition, the sample resistance decreased slowly from 8.45 to 3 M V. After several tens of laser pulses, the value of the resistance drooped rapidly. At the end of deposition process, oxygen was filled into the chamber to a pressure of 1 atm, which accompanied a drastic
Fig. 2. X-ray diffraction u –2 u pattern Ža. and f-scan Žb. of an YBCO film deposited at Ts s 7868C on an YSZ substrate with LSCO buffer layers deposited at Ts s 8868C.
J. Gao et al.r Physica C 330 (2000) 160–164
resistance change. The inset of Fig. 3 is the R–t curve of this film, which shows the superconducting transition at 90 K. In the data analysis, if the YBCO film was grown layer-by-layer the real-time conductance of s can be described by s s s 0 q s YBCO . Fig. 4 shows the real-time conductance as a function of the deposition time for YBCO grown on YSZ with or without a LSCO buffer. In Fig. 4, the real-time conductance data for YBCO grown on YSZ without LSCO buffer is presented as curve Ža.. It can be seen that in the first twenty seconds there is no increase in the conductance. As observed by Shen and Olsan et al. the initial depositions have little contribution to the conductivity of the film, due to the island growth mechanism. In contrast, the real time conductance vs. the deposition time for YBCO on YSZ with a LSCO buffer exhibits a linear dependence Žcurve Žb... This clearly indicates a layer-bylayer initial growth of the YBCO film on LSCO buffer layer. It is interesting that with a LSCO buffer on YSZ, the real-time conductance data of YBCO shows the layer-by-layer growth mechanism from the beginning of the deposition as observed in Fig. 4Žb.. The inset of Fig. 4 shows the real-time resistance measurement for YBCO on the epitaxial LSCOrYSZrAl 2 O 3 , the initial resistance is 3.3 M V. YBCO ultra-thin films grown directly on YSZ substrates show the island growth mechanism with different in-plane orientations, which was attributed
163
Fig. 4. Real-time conductance data for YBCO deposited on the epitaxial YSZ layers Ža. without LSCO buffer and Žb. with a LSCO buffer. The inset shows data of a real-time resistance for YBCO deposited with LSCO buffer.
to the large lattice mismatch and stains on the substrate-film interface. By using the LSCO buffer layers, these two problems have been overcome, which reveal the benefit of suitable buffer layers on the technique of epitaxial growth. The layer-by-layer growth and single in-plane orientation of YBCO can be obtained on YSZ substrates. Moreover, the formation of the intermediate layer due to interaction between YBCO and YSZ could also be prevented.
4. Conclusions Significantly improved initial epitaxy of YBCO films with single in-plane orientation has been obtained on YSZ substrates by using a LSCO buffer layer. The epitaxial orientations of the YBCO on LSCO buffered YSZ substrates are YBCOw100xI LSCO w100x and LSCO w100x I YSZ w110x. The measurements of real-time conductance show that the initial growth of YBCO ultra-thin films deposited on the LSCO buffer layers is formed layer-by-layer.
Acknowledgements Fig. 3. Data of a real-time resistance for YBCO deposited on the epitaxial YSZ layers without LSCO buffer. The inset shows the superconducting measurement for this film.
This work has been supported by Research Grants Council ŽRGC. of Hong Kong ŽProject HKU7102r 97P..
164
J. Gao et al.r Physica C 330 (2000) 160–164
References w1x C.B. Eom, J.Z. Sun, K. Yamamoto, A.F. Masshall, K.E. Luther, T.H. Oeballe, S.S. Laderman, Appl. Phys. Lett 55 Ž1989. 595. w2x Q. Li, O. Meyer, X.X. Xi, J. Greek, G. Linker, Appl. Phys. Lett. 55 Ž1989. 1792. w3x J. Gao, B. Hauser, H. Rogalla, J. Appl. Phys. 67 Ž1990. ¨ 2512. w4x J.P. Zheng, H.S. Kim, Q.Y. Ying, R. Barone, P. Bush, D.T. Shaw, H.S. Kwok, Appl. Phys. Lett. 55 Ž1989. 1044. w5x G. Koren, E. Polturak, B. Fisher, D. Cohen, G. Kimel, Appil. Phys. Lett. 53 Ž1988. 2330. w6x D.K. Fork, D.B. Fennerr, R.W. Barton, J.M. Phillips, G.A.N. Connell, J.B. Boyce, T.H. Geballe, Appl. Phys. Lett. 57 Ž1990. 1161. w7x X.D. Wu, S.R. Foltyn, P.N. Arendt, W.R. Blumenthal, I.H.
w8x w9x w10x w11x w12x w13x w14x w15x
Campbell, J.D. Cotton, J.Y. Coulter, W.L. Hults, M.P. Maley, H.F. Safar, J.L. Smith, Appl. Phys. Lett. 67 Ž1995. 2397. S.M. Garrison, N. Newman, B.F. Cole, K. Char, R.W. Barton, Appl. Phys. Lett. 58 Ž1991. 2168. D. Dimos, P. Chaudhari, J. Mannhart, F.K. LeGouse, Phys. Rev. Lett. 61 Ž1988. 219. M.G. Norton, C.B. Carter, J. Cryst. Growth 110 Ž1991. 2684. W.P. Shen, C. Lenane, J.P. Zhengm, H.S. Kwok, Appl. Phys. Lett. 64 Ž1994. 3175. V. Olsan, M. Jelinek, Physica C 207 Ž1993. 391. J. Gao, T.C. Chui, W.H. Tamg, IEEE Trans. Appl. Supercon. 9 Ž1999. 1661. G.C. Xiong, G.J. Lian, Y.J. Li, Z.Z. Gan, Physica C 282–287 Ž1997. 551. X.D. Wu, R.E. Muenchausen, N.S. Nogar, A. Pique, R. Edwards, B. Wilkens, T.S. Ravi, D.M. Hwang, C.Y. Chen, Appl. Phys. Lett. 58 Ž1991. 304.