Hostname: page-component-74656f9884-rdgjk Total loading time: 0 Render date: 2026-11-08T02:27:03.228Z Has data issue: false hasContentIssue false
  • English
  • Français

Preparation and characterization of La1.8Sr0.2CuO4/La1.9Sr0.1CuO4 superconducting bilayers

Published online by Cambridge University Press:  07 October 2013

Y. Zhang ,
Y.H. An ,
Y. Xing ,
D.Y. Dong ,
S.L. Wang ,
J.Q. Shen ,
P.G. Li  and
W.H. Tang
Y. Zhang
Affiliation:
Center for Optoelectronics Materials and Devices, Department of Physics, Zhejiang Sci-Tech University, Hangzhou 310018, China
Y.H. An
Affiliation:
Center for Optoelectronics Materials and Devices, Department of Physics, Zhejiang Sci-Tech University, Hangzhou 310018, China
Y. Xing
Affiliation:
Center for Optoelectronics Materials and Devices, Department of Physics, Zhejiang Sci-Tech University, Hangzhou 310018, China
D.Y. Dong
Affiliation:
Center for Optoelectronics Materials and Devices, Department of Physics, Zhejiang Sci-Tech University, Hangzhou 310018, China
S.L. Wang
Affiliation:
Center for Optoelectronics Materials and Devices, Department of Physics, Zhejiang Sci-Tech University, Hangzhou 310018, China
J.Q. Shen
Affiliation:
Center for Optoelectronics Materials and Devices, Department of Physics, Zhejiang Sci-Tech University, Hangzhou 310018, China
P.G. Li*
Affiliation:
Center for Optoelectronics Materials and Devices, Department of Physics, Zhejiang Sci-Tech University, Hangzhou 310018, China
W.H. Tang
Affiliation:
State Key Laboratory of Information Photonics and Optical Communication, School of Science, Beijing University of Posts and Telecommunications, Beijing 100876, China
*
a)Author to whom correspondence should be addressed. Electronic mail: pgli@zstu.edu.cn

Abstract

The c-oriented La1.8Sr0.2CuO4 and La1.9Sr0.1CuO4 bilayer films were deposited on (001) SrTiO3 single-crystal substrates by using the pulsed laser deposition technique. The effects of deposition parameters on the quality of thin films were investigated. The crystal structures and surface morphologies were characterized by means of XRD and SEM, and the results showed that an as-prepared film deposited with the optimized parameters has high quality. Then La1.8Sr0.2CuO4/La1.9Sr0.1CuO4 bilayers structure was prepared using the optimized parameters for each corresponding layer, and the electrical transport properties were measured. Interesting rectifying properties were observed at both room and low temperatures, and the rectifying ratio at low temperature was found to be much higher than that at room temperature.

Keywords

La2−xSrxCuO4 thin filmpulsed laser depositionsuperconducting bilayersrectifying property

Information

Type
Technical Articles
Information
Powder Diffraction , Volume 28 , Supplement S1 , September 2013 , pp. S7 - S11
Copyright
Copyright © International Centre for Diffraction Data 2013 

I. INTRODUCTION

Among high-temperature superconducting materials, the La2−xSrxCuO4 (LSCO) system is important to research on the superconductor physical mechanism and the application of the device because its superconductor transition temperature can be modified by varying the carrier concentration over a wide range. Therefore, there have been many research works on La2−xSrxCuO4 single crystal materials and high-quality films. The fabrication process of a high-quality thin film was one of the major research works for the application goal of this high-T c film (Cieplak et al., Reference Cieplak, Berkowski, Guha, Cheng, Vagelos, Rabinowitz, Wu, Trofimov and Lindenfeld1994; Liang et al., Reference Liang, Chen, Shi, Lu, Lu and Kear1995; Geerk et al., Reference Geerk, Zaitsev, Linker, Aidam, Schneider, Ratzel, Fromknecht, Scheerer, Reiner, Gaganidze and Schwab2001). After many years, the study of monolayer film has been moving toward maturity, and many research works have turned to building heterogeneous structures, such as the p–n junction (Xiang et al., Reference Xiang, Qu, Zhang, Lu, Zhou, Li and Li2007; Shi et al., Reference Shi, Zhao, Zhang and Zhang2008; Guo et al., Reference Guo, Guo, Lei, Chen, Tang, Li, Fu and Li2009; Chen et al., Reference Chen, Li, Zhang and Guo2010; Yin et al., Reference Yin, Ding, Wang, Xie, Yu and Li2010). Besides results on building a heterogeneous structure, recently also results on preparing bilayers or multilayer films using different doped LSCO materials and studying the interaction between the films were reported (Caviglia et al., Reference Caviglia, Gariglio, Reyren, Jaccard, Schneider, Gabay, Thiel, Hammerl, Mannhart and Triscone2008; Ueno et al., Reference Ueno, Nakamura, Shimotani, Ohtomo, Kimura, Nojima, Aoki, Iwasa and Kawasaki2008; Biscaras et al., Reference Biscaras, Bergeal, Kushwaha, Wolf, Rastogi, Budhani and Lesueur2010; Koren and Millo, Reference Koren and Millo2010; Rout and Budhani, Reference Rout and Budhani2010; Bollinger et al., Reference Bollinger, Dubuis, Yoon, Pavuna, Misewich and Božović2011). Yuli et al. (Reference Yuli, Asulin, Millo, Orgad, Iomin and Koren2008) reported the observation of a large T c enhancement in bilayers of the non-superconducting, heavily overdoped La1.65Sr0.35CuO4, and superconducting underdoped La1.88Sr0.12CuO4, in comparison with the bare film of the latter (Yuli et al., Reference Yuli, Asulin, Millo, Orgad, Iomin and Koren2008). Following a previous study by Yuli et al. (Reference Yuli, Asulin, Millo, Orgad, Iomin and Koren2008), results on LSCO films with different doping densities that can influence and tune each other due to a conventional proximity effect were reported (Koren and Millo, Reference Koren and Millo2009; Shapiro et al., Reference Shapiro, Shapiro and Atzmon2009). The result that thin films with different doping densities can influence each other is quite interesting, although the specific mechanism has not been particularly clear; these phenomena should be related to the transfer of carrier in the films.

In this paper, we studied the influence of the deposition parameters of the pulsed laser deposition technique (PLD) on the quality of LSCO films, and obtained an optimal process for fabricating LSCO films. Then the optimized parameters were used to fabricate La1.8Sr0.2CuO4/La1.9Sr0.1CuO4 bilayers. The electrical transport properties were characterized, and a rectifying behavior was observed, indicating potential applications of the LSCO bilayers in novel electronic devices.

II. EXPERIMENTAL

The La1.8Sr0.2CuO4 and La1.9Sr0.1CuO4 targets were prepared by the conventional solid-state reaction method. The targets were 3-mm ceramic discs of 30 mm diameter. The chemicals used for preparing the two LSCO targets were polycrystalline powders of La2O3, SrCO3, and CuO. Nominal compositions of the targets were weighed according to the expression La2−xSrxCuO4 with x = 0.1 and 0.2. Each polycrystalline powder of each target was first ground in an agate mortar for 1 h, and dried in a vacuum oven for 2 h. After that, it was pressed into a disc, then sintered at 750 °C for 6 h, and re-ground. Finally, it was pressed with a pressure of 18 MPa and sintered at 1100 °C for 12 h.

In the process of the LSCO thin-film preparation by PLD, the film deposition parameters were changed one at a time to study the effects of laser energy, oxygen atmosphere, substrate temperature, and target–substrate distance on the properties of the films. Under the optimal conditions, the La1.8Sr0.2CuO4/La1.9Sr0.1CuO4 bilayers were firstly deposited on a (001) SrTiO3 single-crystal substrate which was in turn cleaned and dried. And we used the concentrated HNO3 solution that has been diluted to ablate the top film, so the film step structure of two different components was fabricated at the surface and then the electrical transport properties were studied (Huang et al., Reference Huang, Liu, Wu, Jia, Xu and Zhao1999).

X-ray powder diffraction measurements were carried out on a Bruker AXS D8 DISCOVER X-ray diffractometer with Cu Kα radiation. The patterns were taken in the 2θ range from 10 to 80° at a speed of 0.02°/s. The morphology of the as-obtained samples was characterized on a Hitachi 4800 scanning electron microscope (FESEM). The electrical transport properties of bilayers were characterized with a low-temperature electrical measurement system composed of a multifunctional SourceMeter of Keithley 2400, a cooling system of liquid helium, and a Lakeshore 331 temperature controller.

III. RESULTS AND DISCUSSION

A. Characterization of targets

The two La1.8Sr0.2CuO4 and La1.9Sr0.1CuO4 targets were characterized by X-ray diffraction (XRD). The results indicate that the sintered samples with 1150 °C show all the peaks that belong to the different crystal planes of LSCO, while no other visible impurity peaks were detected as shown in Figures 1(a) and 1(b).

Figure 1.

XRD patterns of targets: La1.8Sr0.2CuO4 (a) and La1.9Sr0.1CuO4 (b).

B. Preparation of thin films

The technological conditions for depositing the LSCO films are listed in Table I. Figure 2 shows the XRD patterns for La1.8Sr0.2CuO4 film under different oxygen atmospheres, while other conditions were kept unchanged. We can see that 30 Pa was the optimal parameter in which the film has a strong c-axis orientation growth that confirms a reliable quality without detectable impurity peaks, except those belonging to the (00 l) aligned LSCO film and the (001) oriented STO substrate.

Figure 2.

XRD patterns of La1.8Sr0.2CuO4 thin films deposited at different oxygen atmospheres.

Table I.

Technological parameters for deposition of La1.8Sr0.2CuO4 films by PLD.

The XRD patterns for La1.8Sr0.2CuO4 films under different substrate temperatures are shown in Figure 3. The (002), (004), (060) and (008) diffraction peaks grow stronger with increasing temperature, whereas the peak intensities of the (103), (110), (200) crystal planes become weaker. The reason lies in the fact that the increase of temperature provides energy for deposited particles to migrate to the best possible positions that lead to the higher crystallinity and better quality of the films. This can also be observed from the increase of superconducting transition temperatures, see the right-hand side of Figure 4.

Figure 3.

XRD patterns of La1.8Sr0.2CuO4 thin films deposited at different temperatures.

Figure 4.

R–T curves of La1.8Sr0.2CuO4 thin films deposited at different temperatures.

Figure 5.

XRD patterns of La1.8Sr0.2CuO4 thin films deposited at different laser energies.

Laser energies of 560, 580, 600 and 640 mJ were selected in this work. Figure 5 presents the XRD patterns of the La1.8Sr0.2CuO4 thin film deposited at different laser energies. We can see that the peak intensity of the (00 l) aligned LSCO film at 600 mJ is higher than that at other laser energies.

Figures 6(a)–6(c) show typical SEM images of La1.8Sr0.2CuO4 thin films deposited at different target–substrate distances. Large particles are observed clearly on the film surfaces in Figures 6(a) and 6(c), while the film surface in Figure 6(b) is smoother, which means that 4 cm target–substratedistance is more suitable for the growth of LSCO films.

Figure 6.

SEM images of La1.8Sr0.2CuO4 thin films deposited at target–substrate distances: (a) 3.5 cm, (b) 4.0 cm, and (c) 4.5 cm.

The as-deposited thin film was in-situ annealed in oxygen ambient of 80 Pa at 580 °C for 30 min, and then gradually cooled to room temperature, aiming to supply the oxygen content for film.

Figure 7.

I–V curves of the La1.8Sr0.2CuO4/La1.9Sr0.1CuO4 bilayers. The inset shows the layout of the bilayers.

C. Fabrication and characterization of bilayers

Each of the La1.8Sr0.2CuO4/La1.9Sr0.1CuO4 superconducting bilayers was prepared by PLD in the optimized parameters with which the monolayer was fabricated. Firstly, the bilayers sample was made to adhere to the inside of the Petri dish wall. By adding a dilute HNO3 solution in the Petri dish, three-quarters of the sample was flooded with the solution to ablate in 20 min. Then we took out the sample and used deionized water to wash the surface and dried it in time, so the film step of two different components was fabricated and then the I–V curves of the as-prepared bilayers were measured by the two-probe method.

We can see in Figure 7 that a fine rectifying property was observed at 5 K below the superconductor transition temperature which was better than that in the normal state. We consider that this may be related to the carrier mobility between the top and the bottom layers under the electric field.

IV. CONCLUSION

In summary, the targets of underdoped La1.8Sr0.2CuO4 and overdoped La1.9Sr0.1CuO4 were prepared by the conventional solid state reaction. The XRD analyses reveal that both the targets are single phase without detectable impurity. By exploring technological parameters for depositing La1.8Sr0.2CuO4 films by PLD, the optimal conditions were found to be 30 Pa oxygen pressure, 760 °C substrate temperature, 4 cm target–substrate distance, and 600 mJ laser energy. Using the optimized parameters, La1.8Sr0.2CuO4/La1.9Sr0.1CuO4 superconducting bilayers were successfully fabricated. By using the method of chemical corrosion, the film step of two different components was fabricated at the substrate surface, and then the I–V curves of the bilayers were measured, which show a fine rectifying property at 5 K below the superconductor transition temperature. This is possibly related to the tuning of carrier concentration and mobility in the different doped layers under the electric field.

ACKNOWLEDGMENTS

This work was supported by the National Basic Research Program of China (973 Program) (grant no. 2010CB933501), the National Natural Science Foundation of China (grant nos 51202218, 61274017, 61264007, and 61274009) and the Qianjiang Talent Program of Zhejiang Province (grant no. QJD1202004).

References

Biscaras, J., Bergeal, N., Kushwaha, A., Wolf, T., Rastogi, A., Budhani, R. C., and Lesueur, J. (2010). “Two-dimensional superconductivity at a Mott-Insulator/Band-Insulator interface: LaTiO3/SrTiO3,” Nat. Commun. 1, 1–5.CrossRefGoogle Scholar
Bollinger, A. T., Dubuis, G., Yoon, J., Pavuna, D., Misewich, J., and Božović, I. (2011). “Superconductor–insulator transition in La2−x SrxCuO4 at the pair quantum resistance,” Nature 28, 458–460.CrossRefGoogle Scholar
Caviglia, A. D., Gariglio, S., Reyren, N., Jaccard, D., Schneider, T., Gabay, M., Thiel, S., Hammerl, G., Mannhart, J., and Triscone, J. M. (2008). “Electric field control of the LaAlO3/SrTiO3 interface ground state,” Nature 456, 624–627.CrossRefGoogle ScholarPubMed
Chen, L. M., Li, G. C., Zhang, Y., and Guo, Y. F. (2010). “Film thickness dependence of rectifying properties of La1.85Sr0.15CuO4/Nb–SrTiO3 junctions,” Chin. Phys. Lett. 27, 077401.Google Scholar
Cieplak, M. Z., Berkowski, M., Guha, S., Cheng, E., Vagelos, A. S., Rabinowitz, D. J., Wu, B., Trofimov, I. E., and Lindenfeld, P. (1994). “Thickness dependence of La2−xSrxCuO4 films,” Appl. Phys. Lett. 65, 3383–3385.CrossRefGoogle Scholar
Geerk, J., Zaitsev, A., Linker, G., Aidam, R., Schneider, R., Ratzel, F., Fromknecht, R., Scheerer, B., Reiner, H., Gaganidze, E., and Schwab, R. (2001). “A 3-chamber deposition system for the simultaneous double-sided coating of 5-inch wafers,” IEEE Trans. Appl. Supercond. 11, 3856–3858.CrossRefGoogle Scholar
Guo, Y. F., Guo, X., Lei, M., Chen, L. M., Tang, W. H., Li, P. G., Fu, X. L., and Li, L. H. (2009). “Doping tuned rectifying properties in La2−xSrxCuO4/Nb:SrTiO3 heterojunctions,” Appl. Phys. Lett. 94, 143506.CrossRefGoogle Scholar
Huang, W. W., Liu, B. T., Wu, F., Jia, S. L., Xu, B., and Zhao, B. R. (1999). “Single-crystalline-like La1−xSrxCuO4 thin films and their transport properties,” Supercond. Sci. Technol. 12, 529–532.CrossRefGoogle Scholar
Koren, G. and Millo, O. (2009). “Conventional proximity effect in bilayers of superconducting underdoped La1.88 Sr0.12CuO4 islands coated with nonsuperconducting overdoped La1.65Sr0.35 CuO4,” Phys. Rev. B 80, 054507.CrossRefGoogle Scholar
Koren, G. and Millo, O. (2010). “Enhancement of the superconducting transition temperature of La2−xSrxCuO4 and La1.875Ba0.125CuO4 bilayers: bilayer and reference film prepared on the same wafer,” Phys. Rev. B 81, 134516.CrossRefGoogle Scholar
Liang, S., Chen, C. S., Shi, Z. Q., Lu, P., Lu, Y., and Kear, B. H. (1995). “Control of CeO2 growth by metalorganic chemical vapor deposition with a special source evaporator,” J. Crystal Growth 151, 359–364.CrossRefGoogle Scholar
Rout, P. K. and Budhani, R. C. (2010). “Interface superconductivity in La1.48Nd0.4Sr0.12CuO4/La1.84Sr0.16CuO4 bilayers,” Phys. Rev. B 82, 024518.CrossRefGoogle Scholar
Shi, J. P., Zhao, Y. G., Zhang, H. J., and Zhang, X. P. (2008). “Effect of superconductivity on the electronic transport and capacitance of La1.85Sr0.15CuO4/Nb doped SrTiO3 heterojunction,” Appl. Phys. Lett 92, 132501.CrossRefGoogle Scholar
Shapiro, B.Ya., Shapiro, I., and Atzmon, Y. (2009). “Anomalous proximity effect in a superconductor under a strong electric field,” J. Phys.: Conf. Ser. 150, 052231.Google Scholar
Ueno, K., Nakamura, S., Shimotani, H., Ohtomo, A., Kimura, N., Nojima, T., Aoki, H., Iwasa, Y., and Kawasaki, M. (2008). “Electric-field-induced superconductivity in an insulator,” Nat. Mater. 7, 855–858.CrossRefGoogle Scholar
Xiang, X. Q., Qu, J. F., Zhang, Y. Q., Lu, X. L., Zhou, T. F., Li, G., and Li, X. G. (2007). “I-V characteristics of La1.84Sr0.16CuO4/Nb-doped SrTiO3 heterojunction,” Appl. Phys. Lett. 90, 132513.CrossRefGoogle Scholar
Yin, Y. W., Ding, J. F., Wang, J., Xie, L., Yu, Q. X., and Li, X. G. (2010). “Current-voltage characteristics of La2−xSrxCuO4/Nb-doped SrTiO3 heterojunctions,” J. Appl. Phys. 107, 053915.CrossRefGoogle Scholar
Yuli, O., Asulin, I., Millo, O., Orgad, D., Iomin, L., and Koren, G. (2008). “Enhancement of the superconducting transition temperature of La2-x SrxCuO4 bilayers: role of pairing and phase stiffness,” Phys. Rev. Lett. 101, 057005.CrossRefGoogle Scholar
Figure 0

Figure 1. XRD patterns of targets: La1.8Sr0.2CuO4 (a) and La1.9Sr0.1CuO4 (b).

Figure 1

Figure 2. XRD patterns of La1.8Sr0.2CuO4 thin films deposited at different oxygen atmospheres.

Figure 2

Table I. Technological parameters for deposition of La1.8Sr0.2CuO4 films by PLD.

Figure 3

Figure 3. XRD patterns of La1.8Sr0.2CuO4 thin films deposited at different temperatures.

Figure 4

Figure 4. R–T curves of La1.8Sr0.2CuO4 thin films deposited at different temperatures.

Figure 5

Figure 5. XRD patterns of La1.8Sr0.2CuO4 thin films deposited at different laser energies.

Figure 6

Figure 6. SEM images of La1.8Sr0.2CuO4 thin films deposited at target–substrate distances: (a) 3.5 cm, (b) 4.0 cm, and (c) 4.5 cm.

Figure 7

Figure 7. I–V curves of the La1.8Sr0.2CuO4/La1.9Sr0.1CuO4 bilayers. The inset shows the layout of the bilayers.