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SrTiO3 (001) films grown by pulsed laser deposition under high oxygen pressure
Thin Solid Films 520 (2012) 2785–2788
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Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Microstructure and ferroelectric properties of epitaxial BaTiO3/SrRuO3/SrTiO3 (001) films grown by pulsed laser deposition under high oxygen pressure X.W. Wang, X. Wang, W.J. Gong, Y.Q. Zhang ⁎, Y.L. Zhu, Z.J. Wang, Z.D. Zhang Shenyang National Laboratory for Materials Science, Institute of Metal Research, and International Center for Materials Physics, Chinese Academic of Science, 72 Wenhua Road, Shenyang 110016, PR China
a r t i c l e
i n f o
Article history: Received 23 February 2011 Received in revised form 14 November 2011 Accepted 8 December 2011 Available online 14 December 2011 Keywords: BaTiO3 Films Pulsed laser deposition Ferroelectric properties
a b s t r a c t BaTiO3 films were epitaxially grown on SrTiO3 (001) substrates buffered with SrRuO3 films as bottom electrode by pulsed laser deposition under high oxygen pressure of 30 Pa. The quality of the BaTiO3/SrRuO3/ SrTiO3 multilayer films was analyzed by means of X-ray diffraction, atomic force microscopy and transmission electron microscopy. BaTiO3 films were found to be highly c-axis-oriented tetragonal phase with c/a = 1.002. The dielectric constant first increased with increasing temperature, and showed a peak at the Curie temperature of about 356 K. The films had well-saturated hysteresis loops with a remnant polarization of 7.3 μC/cm 2 and a coercive field of 29.5 kV/cm at room temperature. © 2011 Elsevier B.V. All rights reserved.
1. Introduction BaTiO3 (BTO) films have been attracting much attention due to many potential applications such as piezoelectric detectors, thin film capacitors, and magnetoelectric devices [1–3]. BTO films have been prepared by sol–gel method, chemical vapor deposition, sputtering and pulsed laser deposition (PLD) [1,4–12]. Among these methods, PLD is a method often used to fabricate epitaxial BTO thin films. It was reported that during the growth of epitaxial BTO films on (001) SrTiO3 (STO) or MgO substrate, oxygen pressure is one of the most important parameters and determines the orientation of the BTO films [13–15]. The films grown under high oxygen pressure (such as 20 Pa, 70 Pa) are a-axis oriented with two equivalent configurations of (100)BTO//(001)STO, [010]BTO//[010]STO and [001]BTO//[100] STO (a1domain) or [010]BTO//[100]STO and [001]BTO//[010]STO (a2 domain) [13–15]. The films grown under lower oxygen pressure (such as 3 Pa, 0.7 Pa, 0.1 Pa) are c-axis oriented with a configuration of (001)BTO//(001)STO, [100]BTO//[100]STO and [010]BTO//[010] STO (c domains) [13–15]. The spontaneous polarization axis of the tetragonal BTO is along c-axis, so the BTO films with c-axis orientation (c domain) are needed to make use of their ferroelectric properties Therefore, many previous studies (including growth mechanism, surface morphology, ferroelectric and dielectric properties etc.) are concerned with BTO films deposited under oxygen pressures lower than 20 Pa [16–18]. However, it was reported that oxygen deficiency is observed in the films grown under oxygen pressure lower than
⁎ Corresponding author. Tel.: + 86 24 23971856; fax: + 86 24 23971215. E-mail address: [email protected] (Y.Q. Zhang). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.12.025
20 Pa [19], which would lead to the degradation of dielectric and ferroelectric properties. In this study, we prepared c-axis orientated BTO films on (001) STO substrates buffered with SrRuO3 (SRO) at high oxygen pressure of 30 Pa. The microstructure and ferroelectric properties were investigated. 2. Experimental details The BTO films of 360 nm thickness were deposited on STO (001) substrates buffered with 60 nm SRO films by pulsed laser deposition using a KrF (λ = 248 nm) excimer laser, under the oxygen pressure of 30 Pa and the substrate temperature of 750 °C during deposition. BTO and SRO targets were purchased from Furuuchi Chemical Corporation, and the distance between the target and the substrate was 3 cm. Prior to deposition, the substrates were ultrasonically cleaned with acetone followed by ethanol. The chamber was evacuated using a turbo-pump down to about 2 × 10 − 5 Pa to remove any extraneous particles. After deposition, the films were annealed at 750 °C for 15 min under 40 kPa and then cooled to room temperature at 2 °C/min. BTO films were also deposited under 20 Pa, 5 Pa and 0.1 Pa for comparison. The microstructure and properties of the bottom SRO electrode can be found in refs. [20,21], its resistivity is about 265 μΩ cm at room temperature. The crystal structure was investigated using by θ–2θ scan using an X-ray diffractometer D8 Discover (CuΚα radiation, λ = 0.15406 nm) equipped with a four circle goniometer. The in plane epitaxy can be checked through X-ray diffraction (XRD) phi-scan (φ scan) at selected diffraction peak such as (202) diffractions of STO substrates and BTO films in our case. In theory, four equivalent planes spaced by 90 degree apart should be observed. Surface morphology was investigated using
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atomic force microscopy (AFM, Digital Instruments, Nanoscope IV) in tapping mode. Cross-sectional specimens for transmission electron microscopy (TEM) were prepared by mechanical grinding and polishing followed by Ar ion milling using a Gatan precision ion polishing system (PIPS 691, Pleasanton, CA), and then the samples were examined using a JEM-2010 electron microscope at 200 kV. The dielectric constant was measured by LCR HiTESTER (HIOKI 3532–50) in the temperature range from 10 K to 395 K in the cavity of a magnetic property measurement system (MPMS-XL, Quantum design) in sweeping mode at 3 K/min. The AC electrical field used in the dielectric constant measurement was 2.8 kV/cm. Ferroelectric properties were measured at room temperature with a ferroelectric analyzer (TF2000E, Aixacct) at 1 kHz. The upper Pt electrodes for electrical measurements were prepared by sputtering through a shadow mask with a diameter of 0.55 mm. 3. Results and discussion
Intensity (arb. units)
Fig. 1 shows XRD patterns of the BTO films grown at 30 Pa. It can be seen clearly that the BTO films are of single phase with (001) or (100) orientation. The φ scans of the (202) diffractions of STO substrates and BTO films, as seen in inset, reveal four symmetric peaks at every 90° rotation angle and all the peaks of the BTO films are found to correspond with those of STO substrates. This indicates that the films are in complete epitaxy. The out-of-plane (c) and in-plane (a) lattice parameters of BTO films are calculated to be 0.4019 nm and 0.4009 nm, respectively, and the c/a ratio is 1.002, implying that the films are c-axis oriented. In addition, the lattice parameters of the BTO films are close to the bulk values (c = 0.4034 nm, a = 0.3994 nm), suggesting that the misfit strain in the films is almost relaxed. We also grew BTO films at lower oxygen pressures of 20 Pa, 5 Pa and 0.1 Pa for 30 min for comparison. The lattice parameters of the BTO films obtained at 20 Pa (c =0.4037 nm, a=0.3996 nm), 5 Pa (c=0.4042 nm, a =0.3997 nm) and 0.1 Pa (c=0.4050 nm, a= 0.3997 nm) indicate that BTO films are all c-axis oriented. Moreover, with oxygen pressure decreasing, the out-of-plane lattice constant increases while in-plane lattice constant almost keeps unchanged, meaning that low oxygen pressure favors BTO films c-axis oriented,
which is in the agreement with previous reports [9,11,12]. In addition, manipulation of the orientation is also achieved by incorporating a buffer layer between the BTO film and the substrate. For example, c-axis oriented epitaxial BTO films grown on Si were realized through insertion of a buffer layer of BaxSr1 − xTiO3 [22]. Compared to the BTO film directly grown on STO under the same oxygen pressure (20 Pa) as in ref. [13], where the BTO film is a-axis oriented, our BTO film with a buffer layer of SRO is c-axis oriented. This possibly indicates that a buffer layer of SRO also contributes to the c-axis orientation in our case. So it seems reasonable that BTO films grown at a pressure of 30 Pa still exhibit c-axis orientation, as found in films grown under lower pressures. Fig. 2(a) shows a low magnification cross-sectional TEM image of the BTO/SRO/STO film, and the image was taken with the incident beam close to [010] direction of STO. The dislocations at the interface between BTO and SRO, and the threading dislocations (with the shape of straight lines along film normal direction), are observed and marked as A and B, respectively, leading to strain relaxation in the BTO films. The threading dislocations were also reported previously in other papers [16–18]. This is in agreement with the XRD results above. Fig. 2(b) and (c) show the [010] zone axis selected area electron diffraction (SAED) patterns of the STO substrate and the BTO/ SRO/STO multilayer. As reported in ref. [20], the diffraction spots, which are marked with circles and triangle, are from the SRO layer with two domain orientations. The splitting of the diffraction spots in Fig. 2(c) results from the lattice difference of three different layers. SAED patterns at different positions in the BTO layer are the same as in Fig. 2(d), indicating an isotropic configuration of the entire BTO film. The lattice constant of the BTO films can be calculated as c ≈ 0.402 nm and a ≈ 0.401 nm using STO as reference, which are consistent with those obtained by XRD. The diffraction spots also verify the epitaxial relationship between BTO and STO. Fig. 3 shows the AFM image of a 5 × 5 μm 2 area. The film surface is mostly composed of uniform small islands of about 50 nm, and few large islands of about 100 nm are observed in the small islands matrix. The root mean square (RMS) roughness of the BTO films is 6.67 nm over a 5 × 5 μm 2 area. Fig. 4 shows the ferroelectric hysteresis loops of the BTO films driven under different electrical fields. Repeatable hysteresis loops imply that
Fig. 1. XRD patterns of the BTO films deposited on STO substrates buffered with SRO films. Insets are the local amplification of XRD pattern and the φ scans of the (202) diffractions of the STO substrates and BTO films.
Fig. 2. (a) Cross-sectional TEM image of the BTO/SRO/STO multilayer film, and the SAED patterns of (b) the STO substrate, (c) the BTO/SRO/STO multilayer, and (d) the BTO layer in [010] direction of STO.
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Fig. 3. AFM image of a 5 × 5 μm2 area.
the films display ferroelectric switching behavior with a remnant polarization (Pr) of 7.3 μC/cm 2 and coercive field of 29.5 kV/cm under the drive field of 278 kV/cm. Under the lower drive field of 56 kV/cm, the remnant polarization is 3.4 μC/cm 2. The films also show a well-saturated hysteresis loop with a saturated polarization of 14.2 μC/cm 2. The BTO films grown at lower oxygen pressure is leakage, and no well-saturated hysteresis loops could be obtained in the leakage BTO films. Fig. 5 shows the temperature dependence of dielectric constant. It is seen that with increasing temperature, the dielectric constant at 1 kHz first increases, reaching a peak at 356 K, which is corresponding to its Curie temperature. However, the peak is not as sharp as that of the ceramic or single crystal BTO, moreover, the Curie temperature of the BTO ferroelectric film in the present study is lower than that of the bulk BTO (393 K) and of films in previous reports [12,23,24]. The lower value for the Curie temperature might be attributed to the lower tetragonal distortion (the smaller c/a value of 1.002 in the present study, but 1.010 for the bulk) [25]. The oxygen pressure during film deposition also can affect the phase transition by varying the elastic properties of the ferroelectric films [25]. The dielectric constant at 290 K is 990 and 820 at the frequency of 1 kHz and 100 kHz, respectively, and the slight dielectric relaxation with
Fig. 5. Temperature dependence of the dielectric constant. Inset presents the frequency dependence of the dielectric constant at 290 K.
frequency could be related to space charge carries and the frequency dependence of dielectric constant. 4. Conclusion BaTiO3 films with a c-axis-oriented tetragonal phase with c = 0.4019 nm and a = 0.4009 nm were prepared on (001) STO buffered with SRO by pulsed laser deposition under oxygen pressure of 30 Pa. The epitaxial relationship between BTO and STO is (001)BTO//(001) STO, [100]BTO//[100]STO and [010]BTO//[010]STO. Columnar structures were obtained in the BTO films as well as 3D islands observed in the surface causes higher surface roughness of BTO films with RMS value of 6.67 nm. The dielectric constant increases slightly with increasing temperature, and shows a peak at its Curie temperature around 356 K. The films have well-saturated hysteresis loops with a remnant polarization of 7.3 μC/cm2 and a coercive field of 29.5 kV/cm. Acknowledgments This work has been supported by the National Basic Research Program No. the 2010CB934603 and the Ministry of Science and Technology of China, National Natural Science Foundation of China (No. 50802098 and No. 51072202), the Hundred Talents Program of Chinese Academy of Sciences. References
Fig. 4. Hysteresis loops of the BTO films under different drive electrical fields.
[1] K.J. Choi, M. Biegalski, Y.L. Li, A. Sharan, J. Schubert, R. Uecker, P. Reiche, Y.B. Chen, X.Q. Pan, V. Gopalan, L.Q. Chen, D.G. Schlom, C.B. Eom, Science 306 (2004) 1005. [2] G. Gerra, A.K. Tagantsev, N. Setter, K. Parlinski, Phys. Rev. Lett. 96 (2006) 107603. [3] H. Zheng, J. Wang, S.E. Lofland, Z. Ma, L. Mohaddes-Ardabili, T. Zhao, L. SalamancaRiba, S.R. Shinde, S.B. Ogale, F. Bai, D. Viehland, Y. Jia, D.G. Schlom, M. Wuttig, A. Roytburd, R. Ramesh, Science 303 (2004) 661. [4] J.B. Xu, B. Shen, J.W. Zhai, Appl. Surf. Sci. 255 (2009) 5922. [5] L.A. Wills, B.W. Wessels, D.S. Richeson, T.J. Marks, Appl. Phys. Lett. 60 (1992) 41. [6] L. Qiao, X.F. Bi, J. Cryst. Growth 310 (2008) 2780. [7] H.P. Sun, W. Tian, X.Q. Pan, J.H. Haeni, D.G. Schlom, Appl. Phys. Lett. 84 (2004) 3298. [8] Y.S. Kim, D.H. Kim, J.D. Kim, Y.J. Chang, T.W. Noh, J.H. Kong, K. Char, Y.D. Park, S.D. Bu, J.G. Yoon, J.S. Chung, Appl. Phys. Lett. 86 (2005) 102907. [9] T. Zhao, F. Chen, H.B. Lu, G.Z. Yang, Z.H. Chen, J. Appl. Phys. 87 (2000) 7442. [10] J. Hiltunen, D. Seneviratne, R. Sun, M. Stolfi, H.L. Tuller, J. Lappalainen, V. Lantto, J. Electroceram. 22 (2009) 416. [11] C.L. Li, Z.H. Chen, Y.L. Zhou, D.F. Cui, J. Phys. Condens. Matter 13 (2001) 5261.
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[12] J. Hiltunen, D. Seneviratne, H.L. Tuller, J. Lappalainen, V. Lantto, J. Electroceram. 22 (2009) 395. [13] C.L. Li, D.F. Cui, Y.L. Zhou, H.B. Lu, Z.H. Chen, D.F. Zhang, F. Wu, Appl. Surf. Sci. 136 (1998) 173. [14] J. Zhang, D.F. Cui, J.L. Zhou, L. Li, Z.H. Chen, M. Szabadi, P. Hess, Thin Solid Films 287 (1996) 101. [15] J. Zhang, M. Szabadi, P. Hess, J. Vac. Sci. Technol. B 14 (1996) 1600. [16] A. Visinoiu, M. Alexe, H.N. Lee, D.N. Zakharov, A. Pignolet, D. Hesse, U. Gosele, J. Appl. Phys. 91 (2002) 10157. [17] J.Q. He, E. Vasco, R. Dittmann, R.H. Wang, Phys. Rev. B 73 (2006) 125413. [18] Y.L. Zhu, S.J. Zheng, D. Chen, X.L. Ma, Thin Solid Films 518 (2010) 3669. [19] W. Li, F. Lu, Z.G. Liu, Y. Zhu, F.X. Wang, X.D. Liu, C.Y. Tan, K.M. Wang, Thin Solid Films 340 (1999) 68.
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Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Microstructure and ferroelectric properties of epitaxial BaTiO3/SrRuO3/SrTiO3 (001) films grown by pulsed laser deposition under high oxygen pressure X.W. Wang, X. Wang, W.J. Gong, Y.Q. Zhang ⁎, Y.L. Zhu, Z.J. Wang, Z.D. Zhang Shenyang National Laboratory for Materials Science, Institute of Metal Research, and International Center for Materials Physics, Chinese Academic of Science, 72 Wenhua Road, Shenyang 110016, PR China
a r t i c l e
i n f o
Article history: Received 23 February 2011 Received in revised form 14 November 2011 Accepted 8 December 2011 Available online 14 December 2011 Keywords: BaTiO3 Films Pulsed laser deposition Ferroelectric properties
a b s t r a c t BaTiO3 films were epitaxially grown on SrTiO3 (001) substrates buffered with SrRuO3 films as bottom electrode by pulsed laser deposition under high oxygen pressure of 30 Pa. The quality of the BaTiO3/SrRuO3/ SrTiO3 multilayer films was analyzed by means of X-ray diffraction, atomic force microscopy and transmission electron microscopy. BaTiO3 films were found to be highly c-axis-oriented tetragonal phase with c/a = 1.002. The dielectric constant first increased with increasing temperature, and showed a peak at the Curie temperature of about 356 K. The films had well-saturated hysteresis loops with a remnant polarization of 7.3 μC/cm 2 and a coercive field of 29.5 kV/cm at room temperature. © 2011 Elsevier B.V. All rights reserved.
1. Introduction BaTiO3 (BTO) films have been attracting much attention due to many potential applications such as piezoelectric detectors, thin film capacitors, and magnetoelectric devices [1–3]. BTO films have been prepared by sol–gel method, chemical vapor deposition, sputtering and pulsed laser deposition (PLD) [1,4–12]. Among these methods, PLD is a method often used to fabricate epitaxial BTO thin films. It was reported that during the growth of epitaxial BTO films on (001) SrTiO3 (STO) or MgO substrate, oxygen pressure is one of the most important parameters and determines the orientation of the BTO films [13–15]. The films grown under high oxygen pressure (such as 20 Pa, 70 Pa) are a-axis oriented with two equivalent configurations of (100)BTO//(001)STO, [010]BTO//[010]STO and [001]BTO//[100] STO (a1domain) or [010]BTO//[100]STO and [001]BTO//[010]STO (a2 domain) [13–15]. The films grown under lower oxygen pressure (such as 3 Pa, 0.7 Pa, 0.1 Pa) are c-axis oriented with a configuration of (001)BTO//(001)STO, [100]BTO//[100]STO and [010]BTO//[010] STO (c domains) [13–15]. The spontaneous polarization axis of the tetragonal BTO is along c-axis, so the BTO films with c-axis orientation (c domain) are needed to make use of their ferroelectric properties Therefore, many previous studies (including growth mechanism, surface morphology, ferroelectric and dielectric properties etc.) are concerned with BTO films deposited under oxygen pressures lower than 20 Pa [16–18]. However, it was reported that oxygen deficiency is observed in the films grown under oxygen pressure lower than
⁎ Corresponding author. Tel.: + 86 24 23971856; fax: + 86 24 23971215. E-mail address: [email protected] (Y.Q. Zhang). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.12.025
20 Pa [19], which would lead to the degradation of dielectric and ferroelectric properties. In this study, we prepared c-axis orientated BTO films on (001) STO substrates buffered with SrRuO3 (SRO) at high oxygen pressure of 30 Pa. The microstructure and ferroelectric properties were investigated. 2. Experimental details The BTO films of 360 nm thickness were deposited on STO (001) substrates buffered with 60 nm SRO films by pulsed laser deposition using a KrF (λ = 248 nm) excimer laser, under the oxygen pressure of 30 Pa and the substrate temperature of 750 °C during deposition. BTO and SRO targets were purchased from Furuuchi Chemical Corporation, and the distance between the target and the substrate was 3 cm. Prior to deposition, the substrates were ultrasonically cleaned with acetone followed by ethanol. The chamber was evacuated using a turbo-pump down to about 2 × 10 − 5 Pa to remove any extraneous particles. After deposition, the films were annealed at 750 °C for 15 min under 40 kPa and then cooled to room temperature at 2 °C/min. BTO films were also deposited under 20 Pa, 5 Pa and 0.1 Pa for comparison. The microstructure and properties of the bottom SRO electrode can be found in refs. [20,21], its resistivity is about 265 μΩ cm at room temperature. The crystal structure was investigated using by θ–2θ scan using an X-ray diffractometer D8 Discover (CuΚα radiation, λ = 0.15406 nm) equipped with a four circle goniometer. The in plane epitaxy can be checked through X-ray diffraction (XRD) phi-scan (φ scan) at selected diffraction peak such as (202) diffractions of STO substrates and BTO films in our case. In theory, four equivalent planes spaced by 90 degree apart should be observed. Surface morphology was investigated using
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atomic force microscopy (AFM, Digital Instruments, Nanoscope IV) in tapping mode. Cross-sectional specimens for transmission electron microscopy (TEM) were prepared by mechanical grinding and polishing followed by Ar ion milling using a Gatan precision ion polishing system (PIPS 691, Pleasanton, CA), and then the samples were examined using a JEM-2010 electron microscope at 200 kV. The dielectric constant was measured by LCR HiTESTER (HIOKI 3532–50) in the temperature range from 10 K to 395 K in the cavity of a magnetic property measurement system (MPMS-XL, Quantum design) in sweeping mode at 3 K/min. The AC electrical field used in the dielectric constant measurement was 2.8 kV/cm. Ferroelectric properties were measured at room temperature with a ferroelectric analyzer (TF2000E, Aixacct) at 1 kHz. The upper Pt electrodes for electrical measurements were prepared by sputtering through a shadow mask with a diameter of 0.55 mm. 3. Results and discussion
Intensity (arb. units)
Fig. 1 shows XRD patterns of the BTO films grown at 30 Pa. It can be seen clearly that the BTO films are of single phase with (001) or (100) orientation. The φ scans of the (202) diffractions of STO substrates and BTO films, as seen in inset, reveal four symmetric peaks at every 90° rotation angle and all the peaks of the BTO films are found to correspond with those of STO substrates. This indicates that the films are in complete epitaxy. The out-of-plane (c) and in-plane (a) lattice parameters of BTO films are calculated to be 0.4019 nm and 0.4009 nm, respectively, and the c/a ratio is 1.002, implying that the films are c-axis oriented. In addition, the lattice parameters of the BTO films are close to the bulk values (c = 0.4034 nm, a = 0.3994 nm), suggesting that the misfit strain in the films is almost relaxed. We also grew BTO films at lower oxygen pressures of 20 Pa, 5 Pa and 0.1 Pa for 30 min for comparison. The lattice parameters of the BTO films obtained at 20 Pa (c =0.4037 nm, a=0.3996 nm), 5 Pa (c=0.4042 nm, a =0.3997 nm) and 0.1 Pa (c=0.4050 nm, a= 0.3997 nm) indicate that BTO films are all c-axis oriented. Moreover, with oxygen pressure decreasing, the out-of-plane lattice constant increases while in-plane lattice constant almost keeps unchanged, meaning that low oxygen pressure favors BTO films c-axis oriented,
which is in the agreement with previous reports [9,11,12]. In addition, manipulation of the orientation is also achieved by incorporating a buffer layer between the BTO film and the substrate. For example, c-axis oriented epitaxial BTO films grown on Si were realized through insertion of a buffer layer of BaxSr1 − xTiO3 [22]. Compared to the BTO film directly grown on STO under the same oxygen pressure (20 Pa) as in ref. [13], where the BTO film is a-axis oriented, our BTO film with a buffer layer of SRO is c-axis oriented. This possibly indicates that a buffer layer of SRO also contributes to the c-axis orientation in our case. So it seems reasonable that BTO films grown at a pressure of 30 Pa still exhibit c-axis orientation, as found in films grown under lower pressures. Fig. 2(a) shows a low magnification cross-sectional TEM image of the BTO/SRO/STO film, and the image was taken with the incident beam close to [010] direction of STO. The dislocations at the interface between BTO and SRO, and the threading dislocations (with the shape of straight lines along film normal direction), are observed and marked as A and B, respectively, leading to strain relaxation in the BTO films. The threading dislocations were also reported previously in other papers [16–18]. This is in agreement with the XRD results above. Fig. 2(b) and (c) show the [010] zone axis selected area electron diffraction (SAED) patterns of the STO substrate and the BTO/ SRO/STO multilayer. As reported in ref. [20], the diffraction spots, which are marked with circles and triangle, are from the SRO layer with two domain orientations. The splitting of the diffraction spots in Fig. 2(c) results from the lattice difference of three different layers. SAED patterns at different positions in the BTO layer are the same as in Fig. 2(d), indicating an isotropic configuration of the entire BTO film. The lattice constant of the BTO films can be calculated as c ≈ 0.402 nm and a ≈ 0.401 nm using STO as reference, which are consistent with those obtained by XRD. The diffraction spots also verify the epitaxial relationship between BTO and STO. Fig. 3 shows the AFM image of a 5 × 5 μm 2 area. The film surface is mostly composed of uniform small islands of about 50 nm, and few large islands of about 100 nm are observed in the small islands matrix. The root mean square (RMS) roughness of the BTO films is 6.67 nm over a 5 × 5 μm 2 area. Fig. 4 shows the ferroelectric hysteresis loops of the BTO films driven under different electrical fields. Repeatable hysteresis loops imply that
Fig. 1. XRD patterns of the BTO films deposited on STO substrates buffered with SRO films. Insets are the local amplification of XRD pattern and the φ scans of the (202) diffractions of the STO substrates and BTO films.
Fig. 2. (a) Cross-sectional TEM image of the BTO/SRO/STO multilayer film, and the SAED patterns of (b) the STO substrate, (c) the BTO/SRO/STO multilayer, and (d) the BTO layer in [010] direction of STO.
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Fig. 3. AFM image of a 5 × 5 μm2 area.
the films display ferroelectric switching behavior with a remnant polarization (Pr) of 7.3 μC/cm 2 and coercive field of 29.5 kV/cm under the drive field of 278 kV/cm. Under the lower drive field of 56 kV/cm, the remnant polarization is 3.4 μC/cm 2. The films also show a well-saturated hysteresis loop with a saturated polarization of 14.2 μC/cm 2. The BTO films grown at lower oxygen pressure is leakage, and no well-saturated hysteresis loops could be obtained in the leakage BTO films. Fig. 5 shows the temperature dependence of dielectric constant. It is seen that with increasing temperature, the dielectric constant at 1 kHz first increases, reaching a peak at 356 K, which is corresponding to its Curie temperature. However, the peak is not as sharp as that of the ceramic or single crystal BTO, moreover, the Curie temperature of the BTO ferroelectric film in the present study is lower than that of the bulk BTO (393 K) and of films in previous reports [12,23,24]. The lower value for the Curie temperature might be attributed to the lower tetragonal distortion (the smaller c/a value of 1.002 in the present study, but 1.010 for the bulk) [25]. The oxygen pressure during film deposition also can affect the phase transition by varying the elastic properties of the ferroelectric films [25]. The dielectric constant at 290 K is 990 and 820 at the frequency of 1 kHz and 100 kHz, respectively, and the slight dielectric relaxation with
Fig. 5. Temperature dependence of the dielectric constant. Inset presents the frequency dependence of the dielectric constant at 290 K.
frequency could be related to space charge carries and the frequency dependence of dielectric constant. 4. Conclusion BaTiO3 films with a c-axis-oriented tetragonal phase with c = 0.4019 nm and a = 0.4009 nm were prepared on (001) STO buffered with SRO by pulsed laser deposition under oxygen pressure of 30 Pa. The epitaxial relationship between BTO and STO is (001)BTO//(001) STO, [100]BTO//[100]STO and [010]BTO//[010]STO. Columnar structures were obtained in the BTO films as well as 3D islands observed in the surface causes higher surface roughness of BTO films with RMS value of 6.67 nm. The dielectric constant increases slightly with increasing temperature, and shows a peak at its Curie temperature around 356 K. The films have well-saturated hysteresis loops with a remnant polarization of 7.3 μC/cm2 and a coercive field of 29.5 kV/cm. Acknowledgments This work has been supported by the National Basic Research Program No. the 2010CB934603 and the Ministry of Science and Technology of China, National Natural Science Foundation of China (No. 50802098 and No. 51072202), the Hundred Talents Program of Chinese Academy of Sciences. References
Fig. 4. Hysteresis loops of the BTO films under different drive electrical fields.
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