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Effect of yttrium doping on microstructure and ferroelectric properties of Bi4Ti3O12 thin film
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Materials Letters 62 (2008) 3243 – 3245 www.elsevier.com/locate/matlet
Effect of yttrium doping on microstructure and ferroelectric properties of Bi4Ti3O12 thin film M.C. Kao a,⁎, H.Z. Chen b,⁎, S.L. Young b a b
Department of Electronic Engineering, Hsiuping Institute of Technology, Taichung 412, Taiwan Department of Electrical Engineering, Hsiuping Institute of Technology, Taichung 412, Taiwan Received 30 November 2007; accepted 18 February 2008 Available online 7 March 2008
Abstract Bi4 − xYxTi3O12 thin films were successfully deposited on Pt(111)/Ti/SiO2/Si(100) substrates by sol–gel method and rapid thermal annealing. The effects of yttrium-substitution and annealing temperature (500–800 °C) on the microstructure and electrical properties of bismuth titanate thin films were investigated. X-ray diffraction analysis reveals that the degree of (117) orientation increases as the yttrium content increased. The improved ferroelectric properties can be attributed to the enhanced degree of (117) orientation of Bi4 − xYxTi3O12 thin films. The highly (117)-oriented Bi3.2Y0.8Ti3O12 thin films exhibit high remanent polarization (2Pr) of 58 μC/cm2 and low coercive field (2Ec) of 116 kV/cm, with fatigue-free characteristics up to N 108 switching cycles. The corresponding results show that the obtained Bi3.2Y0.8Ti3O12 thin film exhibited excellent ferroelectric properties and, thus, was suitable for application to non-volatile ferroelectric random access memory applications. © 2008 Elsevier B.V. All rights reserved. PACS: 81.20.Fw; 77.84.-s; 81.05.-t; 81.40.Np Keywords: Bismuth titanate; Microstructure; Sol–gel; Ferroelectric
1. Introduction Recently, many researchers have studied ferroelectric thin films for applications in non-volatile ferroelectric random access memory (FeRAM). Among other materials, Pb(ZrxTi1 − x)O3 (PZT) thin films are promising candidates for FeRAM applications because of its superior ferroelectricity [1,2]. However, PZT has a few disadvantages, such as environmentally hazardous lead content and poor fatigue endurance. Lead-free bismuth-layered perovskite ferroelectrics, such as Bi4Ti3O12 (BTO) and (Bi4 − x Lnx)Ti3O12 (Ln = La, Nd, Sm and Pr) thin films, have been reported as alternative materials could improve its ferroelectric and fatigue resistance properties [3–5]. The fatigue-free behavior of these films can be attributed to the enhanced stability of oxygen in the Ti–O octahedron layer, which is caused by the substitution
⁎ Corresponding authors. Tel.: +886 4 24961531; fax: +886 4 24961187. E-mail addresses: [email protected] (M.C. Kao), [email protected] (H.Z. Chen). 0167-577X/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.02.033
of stable rare-earth ions for volatile Bi ions located near the Ti–O octahedron layer. Lanthanide-substituted BTO thin films have been prepared by different deposition techniques, such as r.f. sputtering, pulsed laser deposition (PLD) [6,7], liquid-phase deposition (LPD) [8,9], molecular beam epitaxy (MBE) and the sol–gel process [10,11]. To prepare bismuth titanate thin films with a precise stoichiometric ratio and homogeneous composition distribution, chemical solution methods such as sol–gel technology can be used to give flexible and precise control over the stoichiometry. The sol–gel method has attracted considerable attention in scientific and technological fields because of its advantages of low-temperature processing conditions, easy control of composition and homogeneity, easy fabrication of thin films of large area, and low cost. In this study, we deposited highly (117)-oriented Bi4 − xYxTi3O12 (BYT) thin film onto Pt(111)/Ti/SiO2/Si(100) substrates using the sol–gel process and annealed by rapid thermal annealing (RTA). The structural properties of BYT thin films were examined using X-ray diffraction (XRD) and scanning electron microscopy (SEM).
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M.C. Kao et al. / Materials Letters 62 (2008) 3243–3245
In addition, effects of the yttrium-substitution on the microstructure and ferroelectric properties of BYT thin films were also studied.
promote homogeneity, which yielded a golden-colored stock solution of ∼ 1 M concentration. Inductively coupled plasma mass spectrometry (ICP-MS) was used to confirm that the deviation from stoichiometry was within ±1%. The stock solutions were spin-coated onto Pt/Ti/SiO2/Si (100) substrates at a spin rate of 2500 rpm for 30 s using a commercial photoresist spinner. These substrates were prepared by dc sputtering ∼ 0.1 μm of platinum onto the oxidized silicon substrates. After each coating step, the gel films were pyrolyzed at 300 °C for 2 min on a hot plate before final annealing. The average thickness of a single-coated as-fired layer, measured using an α-step surface profiler, was approximately 0.1 μm. The desired film thickness of approximately 0.7 μm was achieved by repeating the spin-coating and annealing cycles. After multicoating, BYT thin films were subjected to RTA at 500 °C, 600 °C, 700 °C and 800 °C for 2 min at heating rates of 1200 °C/min in an oxygen atmosphere. The crystallization and microstructures of the thin films were analyzed by X-ray diffraction (XRD) with Cu-Kα radiation and scanning electron microscopy (SEM), respectively. Top electrodes with an area of 7.85 × 10− 3 cm2 were prepared by DC sputtering platinum through a mask onto the film surfaces. Measurement of ferroelectric hysteresis loops was carried out using a Sawyer–Tower circuit in a metal-ferroelectric-metal (MFM) configuration.
2. Experimental
3. Results and discussion
Bismuth acetate [Bi(OOCCH3)3; Alfa, 99.99%+ purity], yttrium acetate hydrate [Y(CH3COCHCOCH3)3·2H2O; Aldrich, 99.9%+ purity] and titanium diisopropyl-oxide bis(2,4-pentanedionate) [Ti(OC3H7)2(CH3COCHCOCH3)2; Alfa, 99.9% + purity] were used as the precursors and 2-methoxyethanol (CH3OC2H4OH) was used as the solvent. The gravimetrically assayed bismuth acetate and yttrium acetate hydrate reagents were dissolved in 2-methoxyethanol at a (Bi + Y)/diol molar ratio of 1:5. The mixture was refluxed at 120 °C for 0.5 h at atmospheric pressure and then cooled to 80 °C. After adding TIAA, the solution was further refluxed at 120 °C for 1 h and 80 °C for 2 h to
XRD patterns of Bi3.2Y0.8Ti3O12 thin films and undoped BTO thin films are shown in Fig. 1. The XRD peaks are quite similar to those of standard diffraction patterns of Bi4Ti3O12 on the joint committee on powder diffraction standards (JCPDS) card. The Bi3.2Y0.8Ti3O12 thin films are crystallized with preferred (117) orientation. It is evident that the intensities of the (117) peaks are relatively broad and weak at low annealing temperatures (b 700 °C). As the annealing temperature increased, the XRD peaks become much sharper and stronger. In the bismuth titanate crystals, spontaneous polarizations (Pr) are 4 and 50 μC/cm2, and coercive fields (Ec) are 60 and 50 kV/cm along c- and a-axis, respectively [12]. Therefore, the degree of a-axis orientation is an important factor with respect to obtaining large Pr. In the XRD
Fig. 1. XRD patterns of (a)–(d) Bi3.2Y0.8Ti3O12 thin films annealed at temperatures ranging from 500 °C to 800 °C and (e) BTO thin films annealed at 800 °C.
Fig. 2. Surface microstructure of Bi3.2 Y0.8 Ti3 O12 thin films annealed at various temperatures: (a) 700 °C and (b) 800 °C.
M.C. Kao et al. / Materials Letters 62 (2008) 3243–3245
pattern, the (117) diffraction peak is the most closely related to the a-axis orientation. To quantify the degree of (117) orientation, the Lotgering factor f indicates the degree of (117) orientation as shown in comparison of (117) and (006) reflections. From calculation based on I(117)/[I(006) + I(117)], the degree of (117) orientation f of Bi3.2Y0.8Ti3O12, Bi3.6Y0.4Ti3O12 and undoped Bi4Ti3O12 thin films are 0.9, 0.61 and 0.44, respectively. The results show that the Bi3.2Y0.8Ti3O12 thin films showed higher (117) orientation compared to the undoped Bi4Ti3O12 thin films. The evolution of the surface microstructure of Bi3.2Y0.8Ti3O12 thin films deposited on Pt-coated silicon substrates at annealing temperatures 700 °C and 800 °C is shown in Fig. 2. As the annealing temperature at 700 °C, a grain size of b 30 nm could be identified. A homogeneous microstructure with a uniform grain size of approximately 80 nm could be observed at the annealing temperature of 800 °C. Thus, when the annealing temperature was increased to 800 °C, the grains became irregular and large. The ferroelectric properties of the Bi4 − xYxTi3O12 thin films were investigated using a Sawyer–Tower circuit and monitored using a HP 54502A digitizing oscilloscope. Fig. 3 shows hysteresis loops for Bi3.2Y0.8Ti3O12, Bi3.6Y0.4Ti3O12 and undoped Bi4Ti3O12 thin films annealed at 800 °C. The remanent polarization (2Pr) and coercive field (2Ec) of BTO films were approximately 25 μC/cm2 and 160 kV/cm, respectively. In addition, the Bi3.2Y0.8Ti3O12 and Bi3.6Y0.4Ti3O12 thin films exhibit high remanent polarization (2Pr) of 58 μC/cm2 and 42 μC/cm2, respectively. It is obvious that the values of coercive field (2Ec) decreased from 160 kV/cm to 116 kV/cm and the remanent polarization (2Pr) increased from 25 μC/cm2 to 58 μC/cm2 as the yttrium content (x) increased from 0 to 0.8. The improved ferroelectric properties can be attributed to the enhanced degree of (117) orientation of Bi4 − xYxTi3O12 thin films. These results are consistent with the XRD analysis above. The Bi3.2Y0.8Ti3O12 thin films show high polarization Pr of approximately 29 μC/cm2 and low coercive field Ec of 58 kV/cm, which are higher than the results for Bi3.2Y0.8Ti3O12 thin films annealed at 750 °C by the MOCVD method (2Pr = 15 μC/cm2) [13]. A possible reason for this discrepancy might be differences in microstructure caused by the fabrication methods and deposition temperatures. Higher deposition temperatures will lead to better crystallinity and larger grain sizes, thus decreasing the density of grain boundaries. Normalized polarization is shown as a function of polarization switching cycles at a frequency of 1 MHz for the Bi4 − xYxTi3O12 thin films in Fig. 4. Polarization of the Bi4 − xYxTi3O12 film remained stabile up to N 108 switching cycles. The features of the hysteresis loop remained
3245
Fig. 4. Electrical fatigue characteristics of Bi4Ti3O12 thin films (a), Bi3.6Y0.4 Ti3O12 thin films (b) and Bi3.2Y0.8Ti3O12 thin films (c).
unchanged after the switching cycles, indicating good fatigue-free characteristics.
4. Conclusions Bi4 − xYxTi3O12 thin films were fabricated on Pt(111)/Ti/SiO2/ Si(100) substrates by the sol–gel method. The doped yttrium content in the present study played an important role in the Bi4 − x YxTi3O12 thin films. XRD pattern analysis revealed that the degree of (117) orientation of Bi4 − xYxTi3O12 thin films was enhanced as the yttrium content increased. The improvement in remanent polarization, Pr, and decrease in coercive field, Ec, is due to the improved degree of (117) orientation of Bi4 − xYxTi3O12 thin films. The Bi3.2Y0.8Ti3O12 thin films showed excellent ferroelectric properties, with a remanent polarization (2Pr) of 58 μC/cm2 and a coercive field (2Ec) of 116 kV/cm, which are better than the corresponding values for the Bi4Ti3O12 films. Acknowledgement This study was supported by the National Science Council, Taiwan, R.O.C., under Contract Nos. NSC 96-2112-M-164-002MY2,NSC 96-2112-M-164-003 and NSC 96-2112-M-164-004. References
Fig. 3. Hysteresis loops for Bi4Ti3O12 thin films (a), Bi3.6Y0.4Ti3O12 thin films (b) and Bi3.2Y0.8Ti3O12 thin films (c).
[1] B.H. Park, B.S. Kang, S.D. Bu, T.W. Noh, J. Lee, W. Jo, Nature 401 (1999) 682. [2] D. Wu, A. Li, T. Zhu, Z. Liu, N. Ming, J. Appl. Phys. 88 (2000) 5941. [3] Z. Ye, M.H. Tang, Y.C. Zhou, X.J. Zheng, C.P. Cheng, Z.S. Hu, et al., Appl. Phys. Lett. 90 (2007) 82905. [4] J.H. Kim, J.K. Kim, S.Y. Heo, H.S. Lee, Thin Solid Films 503 (2006) 60. [5] K.T. Kim, C. Kim, D.H. Kang, W. Shim, J. Vac. Sci. Technol. A 21 (2003) 1376. [6] X.L. Zhong, J.B. Wang, L.Z. Sun, C.B. Tan, X.J. Zheng, Y.C. Zhou, Appl. Phys. Lett. 90 (2007) 012906. [7] U. Chon, J.S. Shim, H.M. Jang, J. Appl. Phys. 93 (2003) 4769. [8] M.P. Besland, H.D. Aissa, P.R.J. Barroy, S. Lafane, P.Y. Tessier, B. Angleraud, M.R. Plouet, L. Brohan, M.A. Djouadi, Thin Solid Films 495 (2006) 86. [9] W.T. Lin, C.W. Fan, H.H. Yu, C.S. Wu, Thin Solid Films 471 (2005) 113. [10] S.W. Kang, S.W. Rhee, J. Electrochem. Soc. 150 (2003) C573. [11] S.W. Kang, S.W. Rhee, J. Vac. Sci. Technol. A 21 (2003) 340. [12] S.E. Cummins, L.E. Cross, J. Appl. Phys. 39 (1968) 2268. [13] S.W. Kang, S.W. Rhee, J. Mater. Sci. 15 (2004) 231.
Materials Letters 62 (2008) 3243 – 3245 www.elsevier.com/locate/matlet
Effect of yttrium doping on microstructure and ferroelectric properties of Bi4Ti3O12 thin film M.C. Kao a,⁎, H.Z. Chen b,⁎, S.L. Young b a b
Department of Electronic Engineering, Hsiuping Institute of Technology, Taichung 412, Taiwan Department of Electrical Engineering, Hsiuping Institute of Technology, Taichung 412, Taiwan Received 30 November 2007; accepted 18 February 2008 Available online 7 March 2008
Abstract Bi4 − xYxTi3O12 thin films were successfully deposited on Pt(111)/Ti/SiO2/Si(100) substrates by sol–gel method and rapid thermal annealing. The effects of yttrium-substitution and annealing temperature (500–800 °C) on the microstructure and electrical properties of bismuth titanate thin films were investigated. X-ray diffraction analysis reveals that the degree of (117) orientation increases as the yttrium content increased. The improved ferroelectric properties can be attributed to the enhanced degree of (117) orientation of Bi4 − xYxTi3O12 thin films. The highly (117)-oriented Bi3.2Y0.8Ti3O12 thin films exhibit high remanent polarization (2Pr) of 58 μC/cm2 and low coercive field (2Ec) of 116 kV/cm, with fatigue-free characteristics up to N 108 switching cycles. The corresponding results show that the obtained Bi3.2Y0.8Ti3O12 thin film exhibited excellent ferroelectric properties and, thus, was suitable for application to non-volatile ferroelectric random access memory applications. © 2008 Elsevier B.V. All rights reserved. PACS: 81.20.Fw; 77.84.-s; 81.05.-t; 81.40.Np Keywords: Bismuth titanate; Microstructure; Sol–gel; Ferroelectric
1. Introduction Recently, many researchers have studied ferroelectric thin films for applications in non-volatile ferroelectric random access memory (FeRAM). Among other materials, Pb(ZrxTi1 − x)O3 (PZT) thin films are promising candidates for FeRAM applications because of its superior ferroelectricity [1,2]. However, PZT has a few disadvantages, such as environmentally hazardous lead content and poor fatigue endurance. Lead-free bismuth-layered perovskite ferroelectrics, such as Bi4Ti3O12 (BTO) and (Bi4 − x Lnx)Ti3O12 (Ln = La, Nd, Sm and Pr) thin films, have been reported as alternative materials could improve its ferroelectric and fatigue resistance properties [3–5]. The fatigue-free behavior of these films can be attributed to the enhanced stability of oxygen in the Ti–O octahedron layer, which is caused by the substitution
⁎ Corresponding authors. Tel.: +886 4 24961531; fax: +886 4 24961187. E-mail addresses: [email protected] (M.C. Kao), [email protected] (H.Z. Chen). 0167-577X/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.02.033
of stable rare-earth ions for volatile Bi ions located near the Ti–O octahedron layer. Lanthanide-substituted BTO thin films have been prepared by different deposition techniques, such as r.f. sputtering, pulsed laser deposition (PLD) [6,7], liquid-phase deposition (LPD) [8,9], molecular beam epitaxy (MBE) and the sol–gel process [10,11]. To prepare bismuth titanate thin films with a precise stoichiometric ratio and homogeneous composition distribution, chemical solution methods such as sol–gel technology can be used to give flexible and precise control over the stoichiometry. The sol–gel method has attracted considerable attention in scientific and technological fields because of its advantages of low-temperature processing conditions, easy control of composition and homogeneity, easy fabrication of thin films of large area, and low cost. In this study, we deposited highly (117)-oriented Bi4 − xYxTi3O12 (BYT) thin film onto Pt(111)/Ti/SiO2/Si(100) substrates using the sol–gel process and annealed by rapid thermal annealing (RTA). The structural properties of BYT thin films were examined using X-ray diffraction (XRD) and scanning electron microscopy (SEM).
3244
M.C. Kao et al. / Materials Letters 62 (2008) 3243–3245
In addition, effects of the yttrium-substitution on the microstructure and ferroelectric properties of BYT thin films were also studied.
promote homogeneity, which yielded a golden-colored stock solution of ∼ 1 M concentration. Inductively coupled plasma mass spectrometry (ICP-MS) was used to confirm that the deviation from stoichiometry was within ±1%. The stock solutions were spin-coated onto Pt/Ti/SiO2/Si (100) substrates at a spin rate of 2500 rpm for 30 s using a commercial photoresist spinner. These substrates were prepared by dc sputtering ∼ 0.1 μm of platinum onto the oxidized silicon substrates. After each coating step, the gel films were pyrolyzed at 300 °C for 2 min on a hot plate before final annealing. The average thickness of a single-coated as-fired layer, measured using an α-step surface profiler, was approximately 0.1 μm. The desired film thickness of approximately 0.7 μm was achieved by repeating the spin-coating and annealing cycles. After multicoating, BYT thin films were subjected to RTA at 500 °C, 600 °C, 700 °C and 800 °C for 2 min at heating rates of 1200 °C/min in an oxygen atmosphere. The crystallization and microstructures of the thin films were analyzed by X-ray diffraction (XRD) with Cu-Kα radiation and scanning electron microscopy (SEM), respectively. Top electrodes with an area of 7.85 × 10− 3 cm2 were prepared by DC sputtering platinum through a mask onto the film surfaces. Measurement of ferroelectric hysteresis loops was carried out using a Sawyer–Tower circuit in a metal-ferroelectric-metal (MFM) configuration.
2. Experimental
3. Results and discussion
Bismuth acetate [Bi(OOCCH3)3; Alfa, 99.99%+ purity], yttrium acetate hydrate [Y(CH3COCHCOCH3)3·2H2O; Aldrich, 99.9%+ purity] and titanium diisopropyl-oxide bis(2,4-pentanedionate) [Ti(OC3H7)2(CH3COCHCOCH3)2; Alfa, 99.9% + purity] were used as the precursors and 2-methoxyethanol (CH3OC2H4OH) was used as the solvent. The gravimetrically assayed bismuth acetate and yttrium acetate hydrate reagents were dissolved in 2-methoxyethanol at a (Bi + Y)/diol molar ratio of 1:5. The mixture was refluxed at 120 °C for 0.5 h at atmospheric pressure and then cooled to 80 °C. After adding TIAA, the solution was further refluxed at 120 °C for 1 h and 80 °C for 2 h to
XRD patterns of Bi3.2Y0.8Ti3O12 thin films and undoped BTO thin films are shown in Fig. 1. The XRD peaks are quite similar to those of standard diffraction patterns of Bi4Ti3O12 on the joint committee on powder diffraction standards (JCPDS) card. The Bi3.2Y0.8Ti3O12 thin films are crystallized with preferred (117) orientation. It is evident that the intensities of the (117) peaks are relatively broad and weak at low annealing temperatures (b 700 °C). As the annealing temperature increased, the XRD peaks become much sharper and stronger. In the bismuth titanate crystals, spontaneous polarizations (Pr) are 4 and 50 μC/cm2, and coercive fields (Ec) are 60 and 50 kV/cm along c- and a-axis, respectively [12]. Therefore, the degree of a-axis orientation is an important factor with respect to obtaining large Pr. In the XRD
Fig. 1. XRD patterns of (a)–(d) Bi3.2Y0.8Ti3O12 thin films annealed at temperatures ranging from 500 °C to 800 °C and (e) BTO thin films annealed at 800 °C.
Fig. 2. Surface microstructure of Bi3.2 Y0.8 Ti3 O12 thin films annealed at various temperatures: (a) 700 °C and (b) 800 °C.
M.C. Kao et al. / Materials Letters 62 (2008) 3243–3245
pattern, the (117) diffraction peak is the most closely related to the a-axis orientation. To quantify the degree of (117) orientation, the Lotgering factor f indicates the degree of (117) orientation as shown in comparison of (117) and (006) reflections. From calculation based on I(117)/[I(006) + I(117)], the degree of (117) orientation f of Bi3.2Y0.8Ti3O12, Bi3.6Y0.4Ti3O12 and undoped Bi4Ti3O12 thin films are 0.9, 0.61 and 0.44, respectively. The results show that the Bi3.2Y0.8Ti3O12 thin films showed higher (117) orientation compared to the undoped Bi4Ti3O12 thin films. The evolution of the surface microstructure of Bi3.2Y0.8Ti3O12 thin films deposited on Pt-coated silicon substrates at annealing temperatures 700 °C and 800 °C is shown in Fig. 2. As the annealing temperature at 700 °C, a grain size of b 30 nm could be identified. A homogeneous microstructure with a uniform grain size of approximately 80 nm could be observed at the annealing temperature of 800 °C. Thus, when the annealing temperature was increased to 800 °C, the grains became irregular and large. The ferroelectric properties of the Bi4 − xYxTi3O12 thin films were investigated using a Sawyer–Tower circuit and monitored using a HP 54502A digitizing oscilloscope. Fig. 3 shows hysteresis loops for Bi3.2Y0.8Ti3O12, Bi3.6Y0.4Ti3O12 and undoped Bi4Ti3O12 thin films annealed at 800 °C. The remanent polarization (2Pr) and coercive field (2Ec) of BTO films were approximately 25 μC/cm2 and 160 kV/cm, respectively. In addition, the Bi3.2Y0.8Ti3O12 and Bi3.6Y0.4Ti3O12 thin films exhibit high remanent polarization (2Pr) of 58 μC/cm2 and 42 μC/cm2, respectively. It is obvious that the values of coercive field (2Ec) decreased from 160 kV/cm to 116 kV/cm and the remanent polarization (2Pr) increased from 25 μC/cm2 to 58 μC/cm2 as the yttrium content (x) increased from 0 to 0.8. The improved ferroelectric properties can be attributed to the enhanced degree of (117) orientation of Bi4 − xYxTi3O12 thin films. These results are consistent with the XRD analysis above. The Bi3.2Y0.8Ti3O12 thin films show high polarization Pr of approximately 29 μC/cm2 and low coercive field Ec of 58 kV/cm, which are higher than the results for Bi3.2Y0.8Ti3O12 thin films annealed at 750 °C by the MOCVD method (2Pr = 15 μC/cm2) [13]. A possible reason for this discrepancy might be differences in microstructure caused by the fabrication methods and deposition temperatures. Higher deposition temperatures will lead to better crystallinity and larger grain sizes, thus decreasing the density of grain boundaries. Normalized polarization is shown as a function of polarization switching cycles at a frequency of 1 MHz for the Bi4 − xYxTi3O12 thin films in Fig. 4. Polarization of the Bi4 − xYxTi3O12 film remained stabile up to N 108 switching cycles. The features of the hysteresis loop remained
3245
Fig. 4. Electrical fatigue characteristics of Bi4Ti3O12 thin films (a), Bi3.6Y0.4 Ti3O12 thin films (b) and Bi3.2Y0.8Ti3O12 thin films (c).
unchanged after the switching cycles, indicating good fatigue-free characteristics.
4. Conclusions Bi4 − xYxTi3O12 thin films were fabricated on Pt(111)/Ti/SiO2/ Si(100) substrates by the sol–gel method. The doped yttrium content in the present study played an important role in the Bi4 − x YxTi3O12 thin films. XRD pattern analysis revealed that the degree of (117) orientation of Bi4 − xYxTi3O12 thin films was enhanced as the yttrium content increased. The improvement in remanent polarization, Pr, and decrease in coercive field, Ec, is due to the improved degree of (117) orientation of Bi4 − xYxTi3O12 thin films. The Bi3.2Y0.8Ti3O12 thin films showed excellent ferroelectric properties, with a remanent polarization (2Pr) of 58 μC/cm2 and a coercive field (2Ec) of 116 kV/cm, which are better than the corresponding values for the Bi4Ti3O12 films. Acknowledgement This study was supported by the National Science Council, Taiwan, R.O.C., under Contract Nos. NSC 96-2112-M-164-002MY2,NSC 96-2112-M-164-003 and NSC 96-2112-M-164-004. References
Fig. 3. Hysteresis loops for Bi4Ti3O12 thin films (a), Bi3.6Y0.4Ti3O12 thin films (b) and Bi3.2Y0.8Ti3O12 thin films (c).
[1] B.H. Park, B.S. Kang, S.D. Bu, T.W. Noh, J. Lee, W. Jo, Nature 401 (1999) 682. [2] D. Wu, A. Li, T. Zhu, Z. Liu, N. Ming, J. Appl. Phys. 88 (2000) 5941. [3] Z. Ye, M.H. Tang, Y.C. Zhou, X.J. Zheng, C.P. Cheng, Z.S. Hu, et al., Appl. Phys. Lett. 90 (2007) 82905. [4] J.H. Kim, J.K. Kim, S.Y. Heo, H.S. Lee, Thin Solid Films 503 (2006) 60. [5] K.T. Kim, C. Kim, D.H. Kang, W. Shim, J. Vac. Sci. Technol. A 21 (2003) 1376. [6] X.L. Zhong, J.B. Wang, L.Z. Sun, C.B. Tan, X.J. Zheng, Y.C. Zhou, Appl. Phys. Lett. 90 (2007) 012906. [7] U. Chon, J.S. Shim, H.M. Jang, J. Appl. Phys. 93 (2003) 4769. [8] M.P. Besland, H.D. Aissa, P.R.J. Barroy, S. Lafane, P.Y. Tessier, B. Angleraud, M.R. Plouet, L. Brohan, M.A. Djouadi, Thin Solid Films 495 (2006) 86. [9] W.T. Lin, C.W. Fan, H.H. Yu, C.S. Wu, Thin Solid Films 471 (2005) 113. [10] S.W. Kang, S.W. Rhee, J. Electrochem. Soc. 150 (2003) C573. [11] S.W. Kang, S.W. Rhee, J. Vac. Sci. Technol. A 21 (2003) 340. [12] S.E. Cummins, L.E. Cross, J. Appl. Phys. 39 (1968) 2268. [13] S.W. Kang, S.W. Rhee, J. Mater. Sci. 15 (2004) 231.