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Effect of substrates on superconductivity and composition of the IR-LPE La2−xSrxCuO4 single crystalline films
Physica C 392–396 (2003) 1302–1305 www.elsevier.com/locate/physc
Effect of substrates on superconductivity and composition of the IR-LPE La2xSrxCuO4 single crystalline films A.T.M.N. Islam *, S. Watauchi, I. Tanaka Center for Crystal Science and Technology, University of Yamanashi, Kofu 400-8511, Japan Received 13 November 2002; accepted 24 March 2003
Abstract We have grown La2x Srx CuO4 (x ¼ 0:1–0:15) single crystalline films on different single crystalline substrates e.g., La2 Cu1y Zny O4 (y ¼ 0:05), La2x Srx Cu1y Zny O4 (x ¼ 0:15, y ¼ 0:05), La2x Bax CuO4 (x ¼ 0:113), La2x Srx CuO4 (x ¼ 0:186), LaSrAlO4 , La2x Srx Cu1y Aly O4 (x ¼ 0:25, y ¼ 0:03) etc., using infrared-heated liquid phase epitaxy technique (IR-LPE). The superconducting properties of the film were found to vary significantly depending on the lattice mismatch with different substrates. Closer lattice matching between film and substrate enhanced the Sr-concentration in film grown under identical conditions. IR-LPE technique was found to be a more effective technique to apply higher epitaxial strain on La2x Srx CuO4 films than other epitaxy techniques using smaller lattice mismatch. Ó 2003 Elsevier B.V. All rights reserved. PACS: 74.76; 81.10; 68.60.B; 81.15 Keywords: IR-LPE film growth; Epitaxial strain; La2x Srx CuO4
1. Introduction The critical temperature, Tc of most of the highTc superconductors are found to be sensitive to application of uniaxial pressure along the different crystallographic axes. For most systems the inplane and out-of-plane Tc derivatives have opposite signs. So under hydrostatic pressure the effect nearly cancel out and the resultant change in Tc is rather small [1]. On the other hand epitaxial strain
* Corresponding author. Tel.: +81-55-220-8625; fax: +81-55254-3035. E-mail address: [email protected] (A.T.M.N. Islam).
could effect a high anisotropic pressure state. An epitaxial growth causing compressive strain in the a-direction and tensile along the c-axis could add up to increase of Tc and vice-versa. One remarkable example is the report by Locquet et al. [2]. They deposited thin films of La1:9 Sr0:1 CuO4 , on a suitable substrate LaSrAlO4 (LSAO) having a smaller a-axis lattice parameter, by molecular beam epitaxy (MBE) technique. The Tc found at 49.1 K in the strained film is twice of that observed in the bulk compound (Tc ¼ 25 K) having the same Sr content. In the same process growth on SrTiO3 (STO) substrate having a larger a-axis had an opposite effect on La1:9 Sr0:1 CuO4 film. Strain tensile along the a- (or b-) axial direction caused Tc to drop to 10 K from 25 K. The
0921-4534/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0921-4534(03)01198-5
A.T.M.N. Islam et al. / Physica C 392–396 (2003) 1302–1305
reason for suppression of Tc due to compression of the c-axis (or expansion of the a-axis) can be explained as similar to the Tc suppression by substitution of Nd in La2xy Srx Ndy CuO4 [3]. The substitution of Nd for La causes compression along the c-axis followed by rotation of the tilt axis of CuO6 octahedra and decrease of Tc . The tilt of CuO6 octahedra beyond a critical value leads to a structural phase transition to low temperature tetragonal (LTT) phase, which is a metallic and magnetically ordered ground state. Epitaxial strain along the c-axis could also lead to similar phase transition and suppression of Tc . Whether the effect is increase or suppression of Tc the choice of substrate is a very important factor. Appropriate substrates have suitable lattice parameters to induce the desired strain. Film growth with optimum effective strain is actually a compromise between the lattice mismatch and thickness of the film. Too large lattice mismatch does not always mean stronger stress, as it could easily lead to misfit dislocations and make the strain ineffective. The optimum thickness using LSAO and STO substrates and MBE technique by Locquet et al. was about 10–15 nm. In some electronic device applications e.g., application of intrinsic Josephsons junctions of La2x Srx CuO4 (LSCO) in low electric power and high speed switching devices, film-thickness of about micrometer order are desired. So what we need here is a growth technique, which uses a small lattice mismatch to minimize misfit dislocations but still can produce a large epitaxial strain. In the present work we want to explore the possible solution in the infrared heated liquid phase epitaxy (IR-LPE) film growth technique, which is preformed at a thermo-equilibrium growth condition rather than lower temperatures. We may compare theoretically the expansion along the c-axis of La2 CuO4 (LCO) due to Sr doping (0.14) with the compression along the c-axis for Nd doping in La2xy Srx Ndy CuO4 (LSNCO). Nd doping of about y ¼ 0:2 results in a change of phase transition to the non-superconducting LTT phase. Interestingly, the change in lattice parameters in Nd doped LSNCO is equivalent (but opposite) to the Sr doping in LCO. So, although substrate having large lattice mismatch
1303
in LSCO) had been e.g., STO (a ¼ 3:905–3:777 A used in MBE film growth, substrate like LCO ) could be sufficient to completely (a ¼ 3:83 A suppress Tc in LSCO, provided the effect of lattice mismatch could completely be transformed to epitaxial strain. In the current work we have grown single crystalline films of LSCO on different substrates e.g., La2 Cu1y Zny O4 (LCZO, y ¼ 0:05), La2x Srx Cu1y Zny O4 (LSCZO, x ¼ 0:15, y ¼ 0:05), La2x Bax CuO4 (LBCO, x ¼ 0:113), La2x Srx CuO4 (LSCO, x ¼ 0:186), LaSrAlO4 , La2x Srx Cu1y Aly O4 (LSCAO, x ¼ 0:25, y ¼ 0:03) etc. using the IRLPE technique. The effect of different substrates on the composition and superconducting transition temperature Tc was investigated by electron probe microanalysis (EPMA) and temperature dependence of resistivity (R(T)) measurements, respectively.
2. Experimental The substrates for film preparation were grown by the travelling solvent floating zone method along the crystallographic c-axis. The as-grown single crystals were cylindrical shaped with 5–5.5 mm in diameter and 40–55 mm in length. The asgrown crystals were sliced to the ac-plane and polished mechanically to mirror-like surface. The substrates were then cleaned using alcohol and ultrasound cleaner. The film growths were performed on different substrates by the IR-LPE method [4]. The flux used for film growth was La2x Srx CuO4 with 80– 85 mol% CuO with x ¼ 0:3–0.65 and were prepared by normal sintering method. After growth, the LPE films on different substrates were checked by Laue X-ray diffraction photograph and polarizing optical microscope for their single crystallinity. EPMA was done to know the film and substrate compositions quantitatively and to check for possible contamination from the substrate. The top surface of the films was polished mechanically carefully to remove any residual flux on the film. Resistance versus temperature measurements were done on the LPE film surface by conventional four probe method using a cryostat.
A.T.M.N. Islam et al. / Physica C 392–396 (2003) 1302–1305
3. Results and discussion
35
1
R N (ohm)
0. 8
0. 6
0. 4
LCZO (y= 0.02) LSCZO (x=0.15, y=0.05) LBCO (x=0.113) LSCO (x=0.186) LSCAO (x=0.23, y=0.02)
0. 2
0 0
50
100
150
200
T(K)
Fig. 1. Temperature dependence of resistance (normalized) for LPE films grown on different substrates. The film thicknesses of LCZO and LSCZO are about 40 lm. The thicknesses of the other films are about 10–15 lm.
5
Tc,onset of films
Fig. 1 shows temperature dependence of resistivity of LSCO films grown on different substrates. We observe that the Tc in films (of thickness 40 lm) grown on LCZO and LSCZO are suppressed. Although the RðT Þ shows Tc;onset like behavior and metallic characteristics below that, the resistance does not go to zero even at low temperatures. On the other hand LSCO (0.1) films grown on LBCO (0.113) and LSCO (0.186) substrates show increase of Tc significantly. The results are summarized and explained through Fig. 2. We find that LCZO and LSCZO lattices have small c=a ratio than the films on it. This clearly is responsible for the epitaxial strain on the films and suppression of Tc However, actually the films on LCZO substrate show thickness dependence of Tc;onset as shown in a separate report and begin to show zero resistance above a critical thickness [5]. LSCO (0.11) films on LBCO and LSCO (0.186) substrates show Tc to increase by about 4.9 and 7.1 K respectively (Fig. 2) from the value of their bulk counterpart having same Sr content. With higher c=a ratio in LSCO (0.186) from LBCO the DTc was also found to decrease to 3.7 from 6.6 K for films grown on LBCO. The films grown on LSCAO substrates failed to show superconducting property (Fig. 1). Compo-
10
30 0 −5
25
− 10
20 − 15
15 3.47
3.48
3.49
3.5
3.51
− 20 3.52
change in Tc,onset from bulk valves
1304
c/a of different substrates
Fig. 2. Tc;onset of LSCO films grown on different substrates (( ) LCZO, (N) LSCZO, (j) LBCO, (d) LSCO (x ¼ 0:186)) having Sr content x ¼ 0:134, 0.155, 0.105 and 0.11, respectively. (}) represents values for bulk LSCO ðx ¼ 0:1Þ. Secondary y-axis shows change in Tc;onset from the value of bulk LSCO having same Sr contents. Lines are drawn to aid the eyes.
sitional analysis by EPMA shows that Al easily contaminated and reacted with the film developing some other non-superconducting phase containing Al. Contamination of Al from substrate was common, similar to a previous report showing formation of La1x Srx Al1y Cuy O3 phase during LPE growth [6]. Films growth on LSAO substrates proved difficult since LSAO is transparent and difficult to heat up using infrared heaters. LSAO substrate was reported to be the most effective to enhance Tc in LSCO films by MBE technique [2,7]. But it turns out to be unusable in our IR-LPE technique. If we compare the substrates used in our work with those in Locquet et al., we observed that we have successfully suppressed Tc in films using a for LCZO smaller lattice mismatch (0.0107 A for STO with LSCO (0.1)). compare to 0.1218 A Strong epitaxial compressive strain along the caxis (or tensile along the a-axis) is responsible for the buckling of CuO6 octahedra and consequently suppression of Tc . Tc is completely suppressed when the CuO6 buckling crosses a critical angle leading to the non-superconducting LTT phase. However other reports show that the LSCO structure having the non-superconducting LTT phase is actually a stable phase [8]. It can be assumed that suitably smaller lattice mismatch dur-
0.16
0
0.12
0.04
LBCO substrate LCZO subsrate LSCZO substrate
0.08
0.08
0.12
0.04
0 0
Sr i n films LSCO, x=0.65 solvent
Sr i n films LSCO, x=0.4 solvent
A.T.M.N. Islam et al. / Physica C 392–396 (2003) 1302–1305
0.16 10
20
30
Approx dist from substrate (µm)
Fig. 3. Sr content in LSCO film grown on LCZO and LSCZO substrates using solvents of two different compositions: LSCO with x ¼ 0:4 (85 mol% CuO) (primary y-axis) and LSCO with x ¼ 0:65 (85 mol% CuO) (secondary y-axis). Values in secondary y-axis were plotted in reverse order to separate from primary y-axis data.
ing the crystal growth led to a more stable growth of the LSCO in the LTT phase. So, although the MBE growth of LSCO on LSAO and STO substrates puts an upper limit on the film thickness (10–15 nm), in case of IR-LPE growth, the use of substrates having smaller lattice mismatch had extended the effect of epitaxial strain effectively and efficiently to several micrometers. Fig. 3 shows results of EPMA measurement of Sr contents in films grown on LBCO, LCZO and LSCZO substrates. Figure shows Sr contents in films grown on all substrates using LSCO, x ¼ 0:4 solvent in the primary y-axis and using LSCO, x ¼ 0:65 solvent in the secondary y-axis. In both cases the solvents were high in CuO (85 mol%). We observe that despite the composition of solvent the Sr content is higher in film on LSCZO substrate.
1305
We think that extra Sr in LSCZO had enhanced the lattice matching between the substrates and the film compared to LCZO and LBCO substrates. Higher epitaxial strain by LCZO and LBCO substrates discouraged Sr doping to the film somewhat. This could be the reason for relatively higher Sr contents in films grown on LSCZO substrates under ideal condition.
4. Conclusion In this work we have grown LSCO single crystalline films on several different substrates using IR-LPE technique. Using the epitaxial strain due to lattice mismatch of different substrates we have succeeded to completely suppress Tc (using LCZO substrate) in LSCO (0.135) and enhance Tc by 7 K (using LSCO (0.186) substrate) in LSCO (0.11) films. We found IR-LPE technique of film growth to be an effective way to apply higher epitaxial strain on LSCO films using small lattice mismatches. We also observed that closer lattice matching between films and substrate enhanced the Sr-content in film grown under identical conditions.
References [1] N. Yamada, M. Ido, Physica C 203 (1992) 340. [2] J.-P. Locquet, J. Perret, J. Fompeyrine, E. Machler, J.W. Seo, G. Van Tendeloo, Nature 394 (1998) 453. [3] B. Buchner, M. Breuer, A. Freimuth, A.P. Kampf, Phys. Rev. Lett. 73 (1994) 1841. [4] I. Tanaka, K. Ashizawa, H. Tanabe, S. Watauchi, J. Yamanaka, Physica C 362 (2001) 180. [5] A.T.M.N. Islam, S. Watauchi, I. Tanaka, Singapore J. Phys. 18 (2002) 233. [6] H. Tanabe, Ph.D. thesis, Yamanashi University, 1999. [7] H. Sato, M. Naito, Physica C 274 (1997) 221. [8] Y. Maeno, A. Odagawa, N. Kakehi, T. Suzuki, T. Fujita, Physica C 173 (1991) 322.
Effect of substrates on superconductivity and composition of the IR-LPE La2xSrxCuO4 single crystalline films A.T.M.N. Islam *, S. Watauchi, I. Tanaka Center for Crystal Science and Technology, University of Yamanashi, Kofu 400-8511, Japan Received 13 November 2002; accepted 24 March 2003
Abstract We have grown La2x Srx CuO4 (x ¼ 0:1–0:15) single crystalline films on different single crystalline substrates e.g., La2 Cu1y Zny O4 (y ¼ 0:05), La2x Srx Cu1y Zny O4 (x ¼ 0:15, y ¼ 0:05), La2x Bax CuO4 (x ¼ 0:113), La2x Srx CuO4 (x ¼ 0:186), LaSrAlO4 , La2x Srx Cu1y Aly O4 (x ¼ 0:25, y ¼ 0:03) etc., using infrared-heated liquid phase epitaxy technique (IR-LPE). The superconducting properties of the film were found to vary significantly depending on the lattice mismatch with different substrates. Closer lattice matching between film and substrate enhanced the Sr-concentration in film grown under identical conditions. IR-LPE technique was found to be a more effective technique to apply higher epitaxial strain on La2x Srx CuO4 films than other epitaxy techniques using smaller lattice mismatch. Ó 2003 Elsevier B.V. All rights reserved. PACS: 74.76; 81.10; 68.60.B; 81.15 Keywords: IR-LPE film growth; Epitaxial strain; La2x Srx CuO4
1. Introduction The critical temperature, Tc of most of the highTc superconductors are found to be sensitive to application of uniaxial pressure along the different crystallographic axes. For most systems the inplane and out-of-plane Tc derivatives have opposite signs. So under hydrostatic pressure the effect nearly cancel out and the resultant change in Tc is rather small [1]. On the other hand epitaxial strain
* Corresponding author. Tel.: +81-55-220-8625; fax: +81-55254-3035. E-mail address: [email protected] (A.T.M.N. Islam).
could effect a high anisotropic pressure state. An epitaxial growth causing compressive strain in the a-direction and tensile along the c-axis could add up to increase of Tc and vice-versa. One remarkable example is the report by Locquet et al. [2]. They deposited thin films of La1:9 Sr0:1 CuO4 , on a suitable substrate LaSrAlO4 (LSAO) having a smaller a-axis lattice parameter, by molecular beam epitaxy (MBE) technique. The Tc found at 49.1 K in the strained film is twice of that observed in the bulk compound (Tc ¼ 25 K) having the same Sr content. In the same process growth on SrTiO3 (STO) substrate having a larger a-axis had an opposite effect on La1:9 Sr0:1 CuO4 film. Strain tensile along the a- (or b-) axial direction caused Tc to drop to 10 K from 25 K. The
0921-4534/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0921-4534(03)01198-5
A.T.M.N. Islam et al. / Physica C 392–396 (2003) 1302–1305
reason for suppression of Tc due to compression of the c-axis (or expansion of the a-axis) can be explained as similar to the Tc suppression by substitution of Nd in La2xy Srx Ndy CuO4 [3]. The substitution of Nd for La causes compression along the c-axis followed by rotation of the tilt axis of CuO6 octahedra and decrease of Tc . The tilt of CuO6 octahedra beyond a critical value leads to a structural phase transition to low temperature tetragonal (LTT) phase, which is a metallic and magnetically ordered ground state. Epitaxial strain along the c-axis could also lead to similar phase transition and suppression of Tc . Whether the effect is increase or suppression of Tc the choice of substrate is a very important factor. Appropriate substrates have suitable lattice parameters to induce the desired strain. Film growth with optimum effective strain is actually a compromise between the lattice mismatch and thickness of the film. Too large lattice mismatch does not always mean stronger stress, as it could easily lead to misfit dislocations and make the strain ineffective. The optimum thickness using LSAO and STO substrates and MBE technique by Locquet et al. was about 10–15 nm. In some electronic device applications e.g., application of intrinsic Josephsons junctions of La2x Srx CuO4 (LSCO) in low electric power and high speed switching devices, film-thickness of about micrometer order are desired. So what we need here is a growth technique, which uses a small lattice mismatch to minimize misfit dislocations but still can produce a large epitaxial strain. In the present work we want to explore the possible solution in the infrared heated liquid phase epitaxy (IR-LPE) film growth technique, which is preformed at a thermo-equilibrium growth condition rather than lower temperatures. We may compare theoretically the expansion along the c-axis of La2 CuO4 (LCO) due to Sr doping (0.14) with the compression along the c-axis for Nd doping in La2xy Srx Ndy CuO4 (LSNCO). Nd doping of about y ¼ 0:2 results in a change of phase transition to the non-superconducting LTT phase. Interestingly, the change in lattice parameters in Nd doped LSNCO is equivalent (but opposite) to the Sr doping in LCO. So, although substrate having large lattice mismatch
1303
in LSCO) had been e.g., STO (a ¼ 3:905–3:777 A used in MBE film growth, substrate like LCO ) could be sufficient to completely (a ¼ 3:83 A suppress Tc in LSCO, provided the effect of lattice mismatch could completely be transformed to epitaxial strain. In the current work we have grown single crystalline films of LSCO on different substrates e.g., La2 Cu1y Zny O4 (LCZO, y ¼ 0:05), La2x Srx Cu1y Zny O4 (LSCZO, x ¼ 0:15, y ¼ 0:05), La2x Bax CuO4 (LBCO, x ¼ 0:113), La2x Srx CuO4 (LSCO, x ¼ 0:186), LaSrAlO4 , La2x Srx Cu1y Aly O4 (LSCAO, x ¼ 0:25, y ¼ 0:03) etc. using the IRLPE technique. The effect of different substrates on the composition and superconducting transition temperature Tc was investigated by electron probe microanalysis (EPMA) and temperature dependence of resistivity (R(T)) measurements, respectively.
2. Experimental The substrates for film preparation were grown by the travelling solvent floating zone method along the crystallographic c-axis. The as-grown single crystals were cylindrical shaped with 5–5.5 mm in diameter and 40–55 mm in length. The asgrown crystals were sliced to the ac-plane and polished mechanically to mirror-like surface. The substrates were then cleaned using alcohol and ultrasound cleaner. The film growths were performed on different substrates by the IR-LPE method [4]. The flux used for film growth was La2x Srx CuO4 with 80– 85 mol% CuO with x ¼ 0:3–0.65 and were prepared by normal sintering method. After growth, the LPE films on different substrates were checked by Laue X-ray diffraction photograph and polarizing optical microscope for their single crystallinity. EPMA was done to know the film and substrate compositions quantitatively and to check for possible contamination from the substrate. The top surface of the films was polished mechanically carefully to remove any residual flux on the film. Resistance versus temperature measurements were done on the LPE film surface by conventional four probe method using a cryostat.
A.T.M.N. Islam et al. / Physica C 392–396 (2003) 1302–1305
3. Results and discussion
35
1
R N (ohm)
0. 8
0. 6
0. 4
LCZO (y= 0.02) LSCZO (x=0.15, y=0.05) LBCO (x=0.113) LSCO (x=0.186) LSCAO (x=0.23, y=0.02)
0. 2
0 0
50
100
150
200
T(K)
Fig. 1. Temperature dependence of resistance (normalized) for LPE films grown on different substrates. The film thicknesses of LCZO and LSCZO are about 40 lm. The thicknesses of the other films are about 10–15 lm.
5
Tc,onset of films
Fig. 1 shows temperature dependence of resistivity of LSCO films grown on different substrates. We observe that the Tc in films (of thickness 40 lm) grown on LCZO and LSCZO are suppressed. Although the RðT Þ shows Tc;onset like behavior and metallic characteristics below that, the resistance does not go to zero even at low temperatures. On the other hand LSCO (0.1) films grown on LBCO (0.113) and LSCO (0.186) substrates show increase of Tc significantly. The results are summarized and explained through Fig. 2. We find that LCZO and LSCZO lattices have small c=a ratio than the films on it. This clearly is responsible for the epitaxial strain on the films and suppression of Tc However, actually the films on LCZO substrate show thickness dependence of Tc;onset as shown in a separate report and begin to show zero resistance above a critical thickness [5]. LSCO (0.11) films on LBCO and LSCO (0.186) substrates show Tc to increase by about 4.9 and 7.1 K respectively (Fig. 2) from the value of their bulk counterpart having same Sr content. With higher c=a ratio in LSCO (0.186) from LBCO the DTc was also found to decrease to 3.7 from 6.6 K for films grown on LBCO. The films grown on LSCAO substrates failed to show superconducting property (Fig. 1). Compo-
10
30 0 −5
25
− 10
20 − 15
15 3.47
3.48
3.49
3.5
3.51
− 20 3.52
change in Tc,onset from bulk valves
1304
c/a of different substrates
Fig. 2. Tc;onset of LSCO films grown on different substrates (( ) LCZO, (N) LSCZO, (j) LBCO, (d) LSCO (x ¼ 0:186)) having Sr content x ¼ 0:134, 0.155, 0.105 and 0.11, respectively. (}) represents values for bulk LSCO ðx ¼ 0:1Þ. Secondary y-axis shows change in Tc;onset from the value of bulk LSCO having same Sr contents. Lines are drawn to aid the eyes.
sitional analysis by EPMA shows that Al easily contaminated and reacted with the film developing some other non-superconducting phase containing Al. Contamination of Al from substrate was common, similar to a previous report showing formation of La1x Srx Al1y Cuy O3 phase during LPE growth [6]. Films growth on LSAO substrates proved difficult since LSAO is transparent and difficult to heat up using infrared heaters. LSAO substrate was reported to be the most effective to enhance Tc in LSCO films by MBE technique [2,7]. But it turns out to be unusable in our IR-LPE technique. If we compare the substrates used in our work with those in Locquet et al., we observed that we have successfully suppressed Tc in films using a for LCZO smaller lattice mismatch (0.0107 A for STO with LSCO (0.1)). compare to 0.1218 A Strong epitaxial compressive strain along the caxis (or tensile along the a-axis) is responsible for the buckling of CuO6 octahedra and consequently suppression of Tc . Tc is completely suppressed when the CuO6 buckling crosses a critical angle leading to the non-superconducting LTT phase. However other reports show that the LSCO structure having the non-superconducting LTT phase is actually a stable phase [8]. It can be assumed that suitably smaller lattice mismatch dur-
0.16
0
0.12
0.04
LBCO substrate LCZO subsrate LSCZO substrate
0.08
0.08
0.12
0.04
0 0
Sr i n films LSCO, x=0.65 solvent
Sr i n films LSCO, x=0.4 solvent
A.T.M.N. Islam et al. / Physica C 392–396 (2003) 1302–1305
0.16 10
20
30
Approx dist from substrate (µm)
Fig. 3. Sr content in LSCO film grown on LCZO and LSCZO substrates using solvents of two different compositions: LSCO with x ¼ 0:4 (85 mol% CuO) (primary y-axis) and LSCO with x ¼ 0:65 (85 mol% CuO) (secondary y-axis). Values in secondary y-axis were plotted in reverse order to separate from primary y-axis data.
ing the crystal growth led to a more stable growth of the LSCO in the LTT phase. So, although the MBE growth of LSCO on LSAO and STO substrates puts an upper limit on the film thickness (10–15 nm), in case of IR-LPE growth, the use of substrates having smaller lattice mismatch had extended the effect of epitaxial strain effectively and efficiently to several micrometers. Fig. 3 shows results of EPMA measurement of Sr contents in films grown on LBCO, LCZO and LSCZO substrates. Figure shows Sr contents in films grown on all substrates using LSCO, x ¼ 0:4 solvent in the primary y-axis and using LSCO, x ¼ 0:65 solvent in the secondary y-axis. In both cases the solvents were high in CuO (85 mol%). We observe that despite the composition of solvent the Sr content is higher in film on LSCZO substrate.
1305
We think that extra Sr in LSCZO had enhanced the lattice matching between the substrates and the film compared to LCZO and LBCO substrates. Higher epitaxial strain by LCZO and LBCO substrates discouraged Sr doping to the film somewhat. This could be the reason for relatively higher Sr contents in films grown on LSCZO substrates under ideal condition.
4. Conclusion In this work we have grown LSCO single crystalline films on several different substrates using IR-LPE technique. Using the epitaxial strain due to lattice mismatch of different substrates we have succeeded to completely suppress Tc (using LCZO substrate) in LSCO (0.135) and enhance Tc by 7 K (using LSCO (0.186) substrate) in LSCO (0.11) films. We found IR-LPE technique of film growth to be an effective way to apply higher epitaxial strain on LSCO films using small lattice mismatches. We also observed that closer lattice matching between films and substrate enhanced the Sr-content in film grown under identical conditions.
References [1] N. Yamada, M. Ido, Physica C 203 (1992) 340. [2] J.-P. Locquet, J. Perret, J. Fompeyrine, E. Machler, J.W. Seo, G. Van Tendeloo, Nature 394 (1998) 453. [3] B. Buchner, M. Breuer, A. Freimuth, A.P. Kampf, Phys. Rev. Lett. 73 (1994) 1841. [4] I. Tanaka, K. Ashizawa, H. Tanabe, S. Watauchi, J. Yamanaka, Physica C 362 (2001) 180. [5] A.T.M.N. Islam, S. Watauchi, I. Tanaka, Singapore J. Phys. 18 (2002) 233. [6] H. Tanabe, Ph.D. thesis, Yamanashi University, 1999. [7] H. Sato, M. Naito, Physica C 274 (1997) 221. [8] Y. Maeno, A. Odagawa, N. Kakehi, T. Suzuki, T. Fujita, Physica C 173 (1991) 322.