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Polarization properties of praseodymium-modified SrBi2Ta2O9 ceramics and thin films prepared by sol–gel method
Materials Letters 58 (2004) 1815 – 1818 www.elsevier.com/locate/matlet
Polarization properties of praseodymium-modified SrBi2Ta2O9 ceramics and thin films prepared by sol–gel method Atsushi Kitamura a, Yuji Noguchi a,b, Masaru Miyayama a,* a
b
Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro, Tokyo 153-8505, Japan PRESTO, Japan Science and Technology Corporation (JST), 4-1-8 Honcho Kawaguchi, Saitama, 332-0012, Japan Received 6 September 2003; received in revised form 10 October 2003; accepted 15 November 2003
Abstract Pr0.14Sr0.8Bi2.1Ta2O9 (Pr-SBT) thin films were prepared by sol – gel method, and the polarization properties were measured and compared with that of Pr-SBT ceramics. The Rietveld analysis of powder X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) revealed that praseodymium ions are substituted at the Sr site as Pr3 + with Sr vacancies. While Pr-SBT ceramics showed a remanent polarization (2Pr) of 21 AC/cm2, the 2Pr of Pr-SBT thin films with the thickness of 280 nm was 15 AC/cm2. The value of coercive voltage (2Vc) of the films was 1.4 V (coercive field, 2Ec was 52 kV/cm), which was lower that that of SBT thin films. It is shown that Pr-SBT is a promising candidate material for low-voltage operating ferroelectric memories. D 2004 Elsevier B.V. All rights reserved. Keywords: Ferroelectrics; Perovskites; Defects, SrBi2Ta2O9
1. Introduction Ferroelectric thin films have been intensively studied for use in non-volatile random access memory (NvRAM). The films in the transistor-type memory are required to possess the following properties: a large remanent polarization (2Pr), a low coercive field (2Ec), sufficient fatigue endurance against repetitive polarization switching and so on. Ferroelectric SrBi2Ta2O9 (SBT), a kind of bismuth layer-structured ferroelectrics, is considered as a promising candidate material for NvRAM due to its excellent fatigue endurance in the form of thin films with Pt electrodes [1] and lower 2Ec than Pb(Zr,Ti)O3 films. However, a major problem of SBT is small 2Pr of about 14 AC/cm2 [1]. The modification of composition in SBT has been conducted for improving polarization properties, mainly for a larger 2Pr [2– 4]. It is widely recognized that SBT with Sr-deficient and Bi-excess composition (denoted as Bi-SBT) has an improved 2Pr compared with stoichiometric SBT [2 – 11], and Bi-SBT thin films with the composition of Sr0.8Bi2.2Ta2O9 have
* Corresponding author. Tel.: +81-3-5452-6339; fax: +81-3-54526341. E-mail address: [email protected] (M. Miyayama). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2003.11.012
been extensively studied. Iijima [11] reported that a-axisoriented Sr0.8Bi2.2Ta2O9 thin films prepared by a chemical solution deposition method showed a large 2Pr of 30 AC/ cm2. Neutron diffraction studies demonstrated that the excess Bi is substituted for divalent Sr ions at the perovskite A-site as Bi3 + [5,6] and the composition is expressed by Sr1 x5x/3Bi2 + 2x/3Ta2O9 (5 indicates Sr vacancies). It was confirmed that a larger 2Pr of 26 AC/cm2 was obtained for Bi-SBT (x = 0.27) dense ceramics with random orientation [6,7]. Recently, it has been reported that praseodymium-modified SBT (Pr-SBT) ceramics exhibited a larger 2Pr ( f 22 AC/cm2) and a smaller 2Ec ( f 60 kV/cm) [8]. In the present study, Pr-SBT thin films were prepared by a sol – gel method employing mixed metal alkoxides, and the polarization properties were compared with those of Pr-SBT ceramics.
2. Experiments Ceramic samples of Pr 2x/3Sr 1 xBi 2Ta2O 9 (Pr-SBT: x = 0.2) were prepared by a solid-state reaction using raw materials of Pr6O11, SrCO3, Bi2O3 and Ta2O5. The ground powder was fired at 1100 jC for 4 h for powder diffraction measurements. To obtain dense samples for electrical meas-
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urements, excess Bi2O3 of 2 at.% of the stoichiometric composition was added to the starting powder, and then sintered at 1250 jC for 1– 2 h. Powder X-ray diffraction (XRD) data were analyzed by the Rietveld method using the program RIETAN-2000 [12] based on A21am orthorhombic symmetry. The X-ray photoelectron spectroscopy (XPS) study was performed using a Quantum-2000 (ULVACPHI) spectrometer (Al Ka X-ray source). For the XPS measurements, a considerable care was paid to dealing with Pr2O3 powder (99.9% purity, Kojundo Chemical Lab.). After the powder was scooped out of the bottle in a grove box, the powder was set to the XPS holder as quickly as possible. Since the XPS measurements were conducted on the clean surface of the fractured ceramics, the spectra were obtained without any disturbance of surface caused by a sputter beam and so on. Calibration of the binding energy scale was carried out using the C1s line that appears in the spectra due to the usual carbon contamination. Thin films of Pr-SBT (x = 0.2) were prepared by a sol – gel method. Praseodymium acetylacetate, strontium di-ipropoxide, bismuth tri-n-butoxide and tantalum pentaeth-
Fig. 2. X-ray photoelectron spectra of Pr-SBT (x = 0.2) powder, Pr2O3, and Pr6O11.
oxide were selected as precursors and methoxypropanol was used as a solvent. The composition of the precursor solution was chosen as Pr0.14Sr0.8Bi2.1Ta2O9. Mixed alkoxides were spin-coated on Pt (200 nm)/TiOx (100 nm)/SiO2/Si substrates. The thin film gels were calcined at 400 jC for 5 min in air, and then heated rapidly to 800 jC in a high purity O2 (>99.9999%) atmosphere. The preheating treatment at 800 jC was done for 5 min. After repeating the process of spincoating/preheating several times, the final annealing was carried out at 800 jC for 1 h in the O2 atmosphere for crystallization. The thickness of Pr-SBT thin films with three and six times coating was 140 and 280 nm, respectively. Pt top electrodes were sputtered, and post-annealing after the Pt deposition was performed at 800 jC for 30 min in the O2 atmosphere. The crystalline phase of films was investigated by XRD. The surface morphology was observed using atomic force microscopy (AFM). The polarization hysteresis properties were measured using an FCE2STD (Toyo Technica, Japan).
3. Results and discussion
Fig. 1. XRD patterns of (a) Pr-SBT powder, and Pr-SBT films with the thickness of (b) 140 and (c) 280 nm. The result of the Rietveld analysis of the powder XRD is also indicated in (a). Delta (D) means the difference between observed (cross) and calculated values (solid line). The R-weighted pattern (Rwp) was 15% and the goodness of fit (S) was 1.4.
We have already demonstrated that Bi and rare-earth elements (La, Nd, etc.) are substituted at the A-site as trivalent ions and that the charge neutrality is satisfied through the formation of Sr vacancies (V) [6 –8]. In this study, XRD data of Pr-SBT (x = 0.2) powder were analyzed by the Rietveld method in the same manner, and the result is indicated in Fig. 1(a). The calculated pattern assuming that Pr ions occupy the A-site with Sr vacancies fitted fairly well to the observed data and any impurity peak did not appeared. While the lattice parameters of SBT were a = 0.552 30(2), b = 0.552 44(2) and c = 2.503 66(4) nm [8], Pr-SBT (x = 0.2) had smaller lattice parameters of a = 0.551 7 (1), b = 0.551 8 (1) and c = 2.501(2) nm. The
A. Kitamura et al. / Materials Letters 58 (2004) 1815–1818
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Fig. 3. AFM images of Pr-SBT films with the thickness of (a) 140 and (b) 280 nm.
Pr substitution led to a 0.1% isotropic shrinkage in unit cell. The orthorhombic distortion expressed by b/a did not change at around 1.0003. The XRD patterns for Pr-SBT (x = 0.2) thin films with different thickness are indicated in Fig. 1(b) and (c). A single phase of Pr-SBT with the layeredperovskite structure was obtained for both films. While the 140-nm-thick films showed a random orientation with a high (115) diffraction intensity, the 280-nm-thick films exhibited (00l) preferred orientation. Fig. 2 indicates the XPS spectrum of Pr-SBT (x = 0.2), and those of Pr2O3 and Pr6O11 are given as references. For Pr2O3, the peak attributed to Pr3 + appeared at around 935 and 955 eV. In the spectrum of Pr6O11 that contains both of Pr3 + and Pr4 + (the molar ratio of Pr3 + to Pr4 + is 2 to 1), the apparent peaks originating in Pr4 + were observed at the lower binding energy of Pr3 + peaks. The peak-fitting
analysis of the Pr-SBT spectrum suggested that Pr4 + was not present. The results of the XRD Rietveld analysis and XPS spectra lead to the conclusion that Pr is substituted at the A-site as Pr3 + and that the formation of V compensates the charge deference between Sr2 + and Pr3 +. The substitution of Pr at the A-site is expressed by the formula: Pr2O3 ! 2PrSr + 3OOx + VSrW, where PrSr indicates Pr3 + at the Sr2 + site (A-site), and OOx denotes O2 at the oxygen site. Fig. 3 indicates the AFM images for Pr-SBT (x = 0.2) thin films with the thickness of (a) 140 and (b) 280 nm. The microstructure of 140-nm-thick film was not uniform, and small grains with diameter 10– 30 nm as well as larger grains were observed. The AFM image of 280-nm-thick films showed a uniform microstructure with large grains with 50– 200 nm diameter.
Fig. 4. Polarization hysteresis loops measured at 25 jC of (a) Pr-SBT dense ceramics, and the films with the thickness of (b) 140 and (c) 280 nm.
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that the 2Pr of Sr0.8Bi2.2Ta2O9 was about 3 AC/cm2 at around 50 kV/cm [4]. These results indicate that the 2Pr of Pr-SBT films starts to increase at a smaller electric field than that of Sr0.8Bi2.2Ta2O9 thin films.
4. Summary
Fig. 5. 2Pr as a function of maximum applied voltage (Vm) measured at 25 jC for the films with the thickness of 280 nm.
Fig. 4 shows the results of polarization measurements at 25 jC. The polarization hysteresis loop of Pr-SBT (x = 0.2) ceramics at a maximum applied field (Em) of 200 kV/cm (Fig. 4(a)) exhibited a 2Pr of 21 AC/cm2, which was much larger than that of SBT (2Pr = 14 AC/cm2) [6]. Fig. 4(b) and (c) indicates the loops for Pr-SBT films with the thickness of 140 and 280 nm, respectively. The 2Pr of 140-nm-thick films was 6.5 AC/cm2. On the other hand, 2Pr was enhanced to be 15 AC/cm2 for 280-nm-thick films. In the SBT system, while the ionic displacements along the c-axis are cancelled each other by the presence of mirror plane normal to the caxis, constituent ions are displaced cooperatively along the a-axis, leading to the spontaneous polarization along the aaxis. Thus, the films with random orientation should show a larger Pr compared with that with (00l) preferred orientation, in principle. Whereas the thinner films had a random orientation, these films exhibited a lower Pr. A smaller grain as well as the nonuniform microstructure found in the 140nm-thick film would be partially responsible for the smaller Pr . The 2Pr of 15 AC/cm2 of 280-nm-thick films was smaller than that of Pr-SBT dense ceramics (2P f 21 AC/cm2) with random orientation [8]. It is expected that (00l) preferred orientation led to a smaller 2Pr in comparison with that of Pr-SBT dense ceramics. On the other hand, the 2Ec of 280nm-thick films was 52 kV/cm, which was a little smaller than the case of the dense ceramics. Fig. 5 indicates 2Pr as a function of Em for Pr-SBT films with the thickness of 280 nm. A 2Pr value as high as 5.4 AC/ cm2 was observed at 37 kV/cm (1 V). It has been reported
Thin films and dense ceramics of Pr-SBT (x = 0.2) were prepared, and the Rietveld refinement of powder XRD data, XPS analysis and polarization measurements were performed. The structural and XPS analyses revealed that praseodymium ions are substituted as Pr3 + for the Sr site (perovskite A-site) and that the charge difference between Sr2 + and Pr3 + is compensated by the formation of Sr vacancies. The thin film with the thickness of 280 nm had a (00l) preferred orientation, and showed a smaller 2Pr of 15 AC/cm2 than dense ceramics (2Pr f 21 AC/cm2). On the other hand, the 2Ec for the films was about 52 kV/cm at Em of 185 kV/cm and the 2Pr was 5.4 AC/cm2 at Em of 37 kV/ cm. It was found that Pr-SBT is a promising candidate material for NvRAM operating at low voltage.
Acknowledgements We thank Hokko Chemical for the preparation of mixed alkoxides and Prof. Funakubo (TIT) for fruitful discussion.
References [1] C.A.-P. de Araujo, J.D. Cuchiaro, L.D. Memillan, M.C. Scott, J.F. Scott, Nature (London) 374 (1995) 627. [2] T. Atsuki, N. Soyama, T. Yonezawa, K. Ogi, Jpn. J. Appl. Phys. 34 (1995) 5096. [3] M. Noda, Y. Matsumuro, H. Sugiyama, M. Okuyama, Jpn. J. Appl. Phys. 38 (1999) 2275. [4] T. Noguchi, T. Hase, Y. Miyasaka, Jpn. J. Appl. Phys. 35 (1996) 4900. [5] Y. Shimakawa, Y. Kudo, Y. Nakagawa, T. Kamiyama, H. Asano, F. Izumi, Appl. Phys. Lett. 74 (1999) 1904. [6] Y. Noguchi, M. Miyayama, T. Kudo, Phys. Rev., B 63 (2001) 214102. [7] Y. Noguchi, M. Miyayama, Trans. Mater. Res. Soc. Jpn. 26 (2001) 7. [8] Y. Noguchi, M. Miyayama, K. Oikawa, T. Kamiyama, M. Osada, M. Kakihana, Jpn. J. Appl. Phys. 41 (2002) 7062. [9] T. Hirai, Y. Fujisaki, K. Nagashima, H. Koike, Y. Tarui, Jpn. J. Appl. Phys. 36 (1997) 5908. [10] T. Hayashi, T. Hara, H. Takahashi, Jpn. J. Appl. Phys. 36 (1997) 5900. [11] R. Iijima, Appl. Phys. Lett. 79 (2001) 2240. [12] F. Izumi, T. Ikeda, Mat. Sci. Forum 321 – 324 (2000) 198 – 203.
Polarization properties of praseodymium-modified SrBi2Ta2O9 ceramics and thin films prepared by sol–gel method Atsushi Kitamura a, Yuji Noguchi a,b, Masaru Miyayama a,* a
b
Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro, Tokyo 153-8505, Japan PRESTO, Japan Science and Technology Corporation (JST), 4-1-8 Honcho Kawaguchi, Saitama, 332-0012, Japan Received 6 September 2003; received in revised form 10 October 2003; accepted 15 November 2003
Abstract Pr0.14Sr0.8Bi2.1Ta2O9 (Pr-SBT) thin films were prepared by sol – gel method, and the polarization properties were measured and compared with that of Pr-SBT ceramics. The Rietveld analysis of powder X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) revealed that praseodymium ions are substituted at the Sr site as Pr3 + with Sr vacancies. While Pr-SBT ceramics showed a remanent polarization (2Pr) of 21 AC/cm2, the 2Pr of Pr-SBT thin films with the thickness of 280 nm was 15 AC/cm2. The value of coercive voltage (2Vc) of the films was 1.4 V (coercive field, 2Ec was 52 kV/cm), which was lower that that of SBT thin films. It is shown that Pr-SBT is a promising candidate material for low-voltage operating ferroelectric memories. D 2004 Elsevier B.V. All rights reserved. Keywords: Ferroelectrics; Perovskites; Defects, SrBi2Ta2O9
1. Introduction Ferroelectric thin films have been intensively studied for use in non-volatile random access memory (NvRAM). The films in the transistor-type memory are required to possess the following properties: a large remanent polarization (2Pr), a low coercive field (2Ec), sufficient fatigue endurance against repetitive polarization switching and so on. Ferroelectric SrBi2Ta2O9 (SBT), a kind of bismuth layer-structured ferroelectrics, is considered as a promising candidate material for NvRAM due to its excellent fatigue endurance in the form of thin films with Pt electrodes [1] and lower 2Ec than Pb(Zr,Ti)O3 films. However, a major problem of SBT is small 2Pr of about 14 AC/cm2 [1]. The modification of composition in SBT has been conducted for improving polarization properties, mainly for a larger 2Pr [2– 4]. It is widely recognized that SBT with Sr-deficient and Bi-excess composition (denoted as Bi-SBT) has an improved 2Pr compared with stoichiometric SBT [2 – 11], and Bi-SBT thin films with the composition of Sr0.8Bi2.2Ta2O9 have
* Corresponding author. Tel.: +81-3-5452-6339; fax: +81-3-54526341. E-mail address: [email protected] (M. Miyayama). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2003.11.012
been extensively studied. Iijima [11] reported that a-axisoriented Sr0.8Bi2.2Ta2O9 thin films prepared by a chemical solution deposition method showed a large 2Pr of 30 AC/ cm2. Neutron diffraction studies demonstrated that the excess Bi is substituted for divalent Sr ions at the perovskite A-site as Bi3 + [5,6] and the composition is expressed by Sr1 x5x/3Bi2 + 2x/3Ta2O9 (5 indicates Sr vacancies). It was confirmed that a larger 2Pr of 26 AC/cm2 was obtained for Bi-SBT (x = 0.27) dense ceramics with random orientation [6,7]. Recently, it has been reported that praseodymium-modified SBT (Pr-SBT) ceramics exhibited a larger 2Pr ( f 22 AC/cm2) and a smaller 2Ec ( f 60 kV/cm) [8]. In the present study, Pr-SBT thin films were prepared by a sol – gel method employing mixed metal alkoxides, and the polarization properties were compared with those of Pr-SBT ceramics.
2. Experiments Ceramic samples of Pr 2x/3Sr 1 xBi 2Ta2O 9 (Pr-SBT: x = 0.2) were prepared by a solid-state reaction using raw materials of Pr6O11, SrCO3, Bi2O3 and Ta2O5. The ground powder was fired at 1100 jC for 4 h for powder diffraction measurements. To obtain dense samples for electrical meas-
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urements, excess Bi2O3 of 2 at.% of the stoichiometric composition was added to the starting powder, and then sintered at 1250 jC for 1– 2 h. Powder X-ray diffraction (XRD) data were analyzed by the Rietveld method using the program RIETAN-2000 [12] based on A21am orthorhombic symmetry. The X-ray photoelectron spectroscopy (XPS) study was performed using a Quantum-2000 (ULVACPHI) spectrometer (Al Ka X-ray source). For the XPS measurements, a considerable care was paid to dealing with Pr2O3 powder (99.9% purity, Kojundo Chemical Lab.). After the powder was scooped out of the bottle in a grove box, the powder was set to the XPS holder as quickly as possible. Since the XPS measurements were conducted on the clean surface of the fractured ceramics, the spectra were obtained without any disturbance of surface caused by a sputter beam and so on. Calibration of the binding energy scale was carried out using the C1s line that appears in the spectra due to the usual carbon contamination. Thin films of Pr-SBT (x = 0.2) were prepared by a sol – gel method. Praseodymium acetylacetate, strontium di-ipropoxide, bismuth tri-n-butoxide and tantalum pentaeth-
Fig. 2. X-ray photoelectron spectra of Pr-SBT (x = 0.2) powder, Pr2O3, and Pr6O11.
oxide were selected as precursors and methoxypropanol was used as a solvent. The composition of the precursor solution was chosen as Pr0.14Sr0.8Bi2.1Ta2O9. Mixed alkoxides were spin-coated on Pt (200 nm)/TiOx (100 nm)/SiO2/Si substrates. The thin film gels were calcined at 400 jC for 5 min in air, and then heated rapidly to 800 jC in a high purity O2 (>99.9999%) atmosphere. The preheating treatment at 800 jC was done for 5 min. After repeating the process of spincoating/preheating several times, the final annealing was carried out at 800 jC for 1 h in the O2 atmosphere for crystallization. The thickness of Pr-SBT thin films with three and six times coating was 140 and 280 nm, respectively. Pt top electrodes were sputtered, and post-annealing after the Pt deposition was performed at 800 jC for 30 min in the O2 atmosphere. The crystalline phase of films was investigated by XRD. The surface morphology was observed using atomic force microscopy (AFM). The polarization hysteresis properties were measured using an FCE2STD (Toyo Technica, Japan).
3. Results and discussion
Fig. 1. XRD patterns of (a) Pr-SBT powder, and Pr-SBT films with the thickness of (b) 140 and (c) 280 nm. The result of the Rietveld analysis of the powder XRD is also indicated in (a). Delta (D) means the difference between observed (cross) and calculated values (solid line). The R-weighted pattern (Rwp) was 15% and the goodness of fit (S) was 1.4.
We have already demonstrated that Bi and rare-earth elements (La, Nd, etc.) are substituted at the A-site as trivalent ions and that the charge neutrality is satisfied through the formation of Sr vacancies (V) [6 –8]. In this study, XRD data of Pr-SBT (x = 0.2) powder were analyzed by the Rietveld method in the same manner, and the result is indicated in Fig. 1(a). The calculated pattern assuming that Pr ions occupy the A-site with Sr vacancies fitted fairly well to the observed data and any impurity peak did not appeared. While the lattice parameters of SBT were a = 0.552 30(2), b = 0.552 44(2) and c = 2.503 66(4) nm [8], Pr-SBT (x = 0.2) had smaller lattice parameters of a = 0.551 7 (1), b = 0.551 8 (1) and c = 2.501(2) nm. The
A. Kitamura et al. / Materials Letters 58 (2004) 1815–1818
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Fig. 3. AFM images of Pr-SBT films with the thickness of (a) 140 and (b) 280 nm.
Pr substitution led to a 0.1% isotropic shrinkage in unit cell. The orthorhombic distortion expressed by b/a did not change at around 1.0003. The XRD patterns for Pr-SBT (x = 0.2) thin films with different thickness are indicated in Fig. 1(b) and (c). A single phase of Pr-SBT with the layeredperovskite structure was obtained for both films. While the 140-nm-thick films showed a random orientation with a high (115) diffraction intensity, the 280-nm-thick films exhibited (00l) preferred orientation. Fig. 2 indicates the XPS spectrum of Pr-SBT (x = 0.2), and those of Pr2O3 and Pr6O11 are given as references. For Pr2O3, the peak attributed to Pr3 + appeared at around 935 and 955 eV. In the spectrum of Pr6O11 that contains both of Pr3 + and Pr4 + (the molar ratio of Pr3 + to Pr4 + is 2 to 1), the apparent peaks originating in Pr4 + were observed at the lower binding energy of Pr3 + peaks. The peak-fitting
analysis of the Pr-SBT spectrum suggested that Pr4 + was not present. The results of the XRD Rietveld analysis and XPS spectra lead to the conclusion that Pr is substituted at the A-site as Pr3 + and that the formation of V compensates the charge deference between Sr2 + and Pr3 +. The substitution of Pr at the A-site is expressed by the formula: Pr2O3 ! 2PrSr + 3OOx + VSrW, where PrSr indicates Pr3 + at the Sr2 + site (A-site), and OOx denotes O2 at the oxygen site. Fig. 3 indicates the AFM images for Pr-SBT (x = 0.2) thin films with the thickness of (a) 140 and (b) 280 nm. The microstructure of 140-nm-thick film was not uniform, and small grains with diameter 10– 30 nm as well as larger grains were observed. The AFM image of 280-nm-thick films showed a uniform microstructure with large grains with 50– 200 nm diameter.
Fig. 4. Polarization hysteresis loops measured at 25 jC of (a) Pr-SBT dense ceramics, and the films with the thickness of (b) 140 and (c) 280 nm.
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that the 2Pr of Sr0.8Bi2.2Ta2O9 was about 3 AC/cm2 at around 50 kV/cm [4]. These results indicate that the 2Pr of Pr-SBT films starts to increase at a smaller electric field than that of Sr0.8Bi2.2Ta2O9 thin films.
4. Summary
Fig. 5. 2Pr as a function of maximum applied voltage (Vm) measured at 25 jC for the films with the thickness of 280 nm.
Fig. 4 shows the results of polarization measurements at 25 jC. The polarization hysteresis loop of Pr-SBT (x = 0.2) ceramics at a maximum applied field (Em) of 200 kV/cm (Fig. 4(a)) exhibited a 2Pr of 21 AC/cm2, which was much larger than that of SBT (2Pr = 14 AC/cm2) [6]. Fig. 4(b) and (c) indicates the loops for Pr-SBT films with the thickness of 140 and 280 nm, respectively. The 2Pr of 140-nm-thick films was 6.5 AC/cm2. On the other hand, 2Pr was enhanced to be 15 AC/cm2 for 280-nm-thick films. In the SBT system, while the ionic displacements along the c-axis are cancelled each other by the presence of mirror plane normal to the caxis, constituent ions are displaced cooperatively along the a-axis, leading to the spontaneous polarization along the aaxis. Thus, the films with random orientation should show a larger Pr compared with that with (00l) preferred orientation, in principle. Whereas the thinner films had a random orientation, these films exhibited a lower Pr. A smaller grain as well as the nonuniform microstructure found in the 140nm-thick film would be partially responsible for the smaller Pr . The 2Pr of 15 AC/cm2 of 280-nm-thick films was smaller than that of Pr-SBT dense ceramics (2P f 21 AC/cm2) with random orientation [8]. It is expected that (00l) preferred orientation led to a smaller 2Pr in comparison with that of Pr-SBT dense ceramics. On the other hand, the 2Ec of 280nm-thick films was 52 kV/cm, which was a little smaller than the case of the dense ceramics. Fig. 5 indicates 2Pr as a function of Em for Pr-SBT films with the thickness of 280 nm. A 2Pr value as high as 5.4 AC/ cm2 was observed at 37 kV/cm (1 V). It has been reported
Thin films and dense ceramics of Pr-SBT (x = 0.2) were prepared, and the Rietveld refinement of powder XRD data, XPS analysis and polarization measurements were performed. The structural and XPS analyses revealed that praseodymium ions are substituted as Pr3 + for the Sr site (perovskite A-site) and that the charge difference between Sr2 + and Pr3 + is compensated by the formation of Sr vacancies. The thin film with the thickness of 280 nm had a (00l) preferred orientation, and showed a smaller 2Pr of 15 AC/cm2 than dense ceramics (2Pr f 21 AC/cm2). On the other hand, the 2Ec for the films was about 52 kV/cm at Em of 185 kV/cm and the 2Pr was 5.4 AC/cm2 at Em of 37 kV/ cm. It was found that Pr-SBT is a promising candidate material for NvRAM operating at low voltage.
Acknowledgements We thank Hokko Chemical for the preparation of mixed alkoxides and Prof. Funakubo (TIT) for fruitful discussion.
References [1] C.A.-P. de Araujo, J.D. Cuchiaro, L.D. Memillan, M.C. Scott, J.F. Scott, Nature (London) 374 (1995) 627. [2] T. Atsuki, N. Soyama, T. Yonezawa, K. Ogi, Jpn. J. Appl. Phys. 34 (1995) 5096. [3] M. Noda, Y. Matsumuro, H. Sugiyama, M. Okuyama, Jpn. J. Appl. Phys. 38 (1999) 2275. [4] T. Noguchi, T. Hase, Y. Miyasaka, Jpn. J. Appl. Phys. 35 (1996) 4900. [5] Y. Shimakawa, Y. Kudo, Y. Nakagawa, T. Kamiyama, H. Asano, F. Izumi, Appl. Phys. Lett. 74 (1999) 1904. [6] Y. Noguchi, M. Miyayama, T. Kudo, Phys. Rev., B 63 (2001) 214102. [7] Y. Noguchi, M. Miyayama, Trans. Mater. Res. Soc. Jpn. 26 (2001) 7. [8] Y. Noguchi, M. Miyayama, K. Oikawa, T. Kamiyama, M. Osada, M. Kakihana, Jpn. J. Appl. Phys. 41 (2002) 7062. [9] T. Hirai, Y. Fujisaki, K. Nagashima, H. Koike, Y. Tarui, Jpn. J. Appl. Phys. 36 (1997) 5908. [10] T. Hayashi, T. Hara, H. Takahashi, Jpn. J. Appl. Phys. 36 (1997) 5900. [11] R. Iijima, Appl. Phys. Lett. 79 (2001) 2240. [12] F. Izumi, T. Ikeda, Mat. Sci. Forum 321 – 324 (2000) 198 – 203.