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Thickness effects of Bi0.89Ti0.11FeO3 thin films deposited on PbZr0.2Ti0.79Nb0.01O3 buffer layers
Journal of Alloys and Compounds 509 (2011) 431–434
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Thickness effects of Bi0.89 Ti0.11 FeO3 thin films deposited on PbZr0.2 Ti0.79 Nb0.01 O3 buffer layers X.M. Chen, G.D. Hu ∗ , J.C. Wang, L. Cheng, C.H. Yang, W.B. Wu School of Materials Science and Engineering, University of Jinan, Jinan 250022, China
a r t i c l e
i n f o
Article history: Received 11 June 2010 Received in revised form 5 September 2010 Accepted 8 September 2010 Available online 17 September 2010 Keywords: Ferroelectrics Thin films Chemical synthesis Electronic properties
a b s t r a c t Bi0.89 Ti0.11 FeO3 thin films with the thicknesses of 200–440 nm were fabricated on the 40-nm-thick PbZr0.2 Ti0.79 Nb0.01 O3 (PZTN)-buffered Pt(1 1 1)/Ti/SiO2 /Si substrates using a metal organic decomposition process. As a result of the good insulating property and high breakdown characteristic of the PZTN buffer layer, the leakage currents in the Bi0.89 Tb0.11 FeO3 films are significantly reduced. All the films show well-saturated and rectangular P–E hysteresis loops without any evident leaky behavior. The remnant polarization Pr and coercive field Ec for all Bi0.89 Ti0.11 FeO3 films are around 45–50 C/cm2 and 200 kV/cm, respectively, and show weak dependent on the film thickness. The 200-nm-thick Bi0.89 Ti0.11 FeO3 film exhibits better fatigue-free characteristic and charge-retaining ability, and the domain backswitching is significantly restrained due to the strong anti-aging ability of the PZTN buffer layer. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In recent years, BiFeO3 (BFO) thin films have been extensively investigated for the applications of multi-state data storage and microelectromechanical systems due to its simultaneously ferroelectric ordering (Curie temperature TC = 850 ◦ C) and antiferromagnetic ordering (Néel temperature, TN = 370 ◦ C) above room temperature [1–4]. However, the fatal drawback of BFO is the high leakage current, making the ferroelectric properties of BFO films non-measurable before breakdown. In addition, the polycrystalline BFO thin film has a large coercive field (Ec ) (300–500 kV/cm) [5–6]. Therefore, the reductions of thickness and Ec for the BFO-based ferroelectric thin films are imperative for the low-voltage operation of the future high-density FeRAMs. It has been demonstrated that the BFO films with the thickness thinner than 300 nm and Ec less than 200 kV/cm can be readily obtained by using the pulsed laser deposition technique [7–9]. However, for the case of BFO films prepared using the chemical solution deposition method, saturated P–E hysteresis loops can only be obtained in the films thicker than 300 nm [10–11], which cannot meet the requirement for the application of FeRAMs. Recently, we have demonstrated that the Bi0.89 Ti0.11 FeO3 thin film exhibits better multiferroic properties [12]. However, the thickness and Ec of the Bi0.89 Ti0.11 FeO3 film were too high. Since introducing a ferroelectric thin film to form a double-layered struc-
∗ Corresponding author. Tel.: +86 531 88374857; fax: +86 531 87974453. E-mail address: [email protected] (G.D. Hu). 0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2010.09.049
ture has been verified as an effective route to improve the electric properties of BFO-based thin films, therefore, our present work related to reduce the thickness and Ec of the Bi0.89 Ti0.11 FeO3 film was focused on introducing a PbZr0.2 Ti0.79 Nb0.01 O3 (PZTN) buffer layer, because the PZTN film has a smaller Ec (<100 kV/cm) and larger breakdown voltage [13]. In this work, Bi0.89 Ti0.11 FeO3 films with the thicknesses ranging from 200 to 440 nm were fabricated on the 40-nm-thick PZTN-buffered Pt(1 1 1)/Ti/SiO2 /Si substrates. The results show that the Ec for all the Bi0.89 Ti0.11 FeO3 films are reduced to about 200 kV/cm as a result of the introduction of the PZTN buffer layer. The 200-nm-thick Bi0.89 Ti0.11 FeO3 film exhibits better anti-fatigue and retention properties. 2. Experimental procedure Both Bi0.89 Ti0.11 FeO3 and PZTN layers were fabricated using a metal organic decomposition process. The preparation of Bi0.89 Ti0.11 FeO3 precursor solution has been described elsewhere [12]. The PZTN precursor solution was prepared by dissolving lead acetate trihydrate, titanium isopropoxide, zirconium n-propoxide, and niobium ethoxide as raw materials and 2-methoxyethanol as the solvent. Lead of 5 mol % excess was added in PZTN precursor to compensate the lead loss during the heating treatment process. Prior to depositing Bi0.89 Ti0.11 FeO3 films, four PZTN thin films with the thickness of 40 nm were span-coated at 8000 rpm for 30 s on Pt(1 1 1)/Ti/SiO2 /Si substrates and then crystallized at 550 ◦ C. Subsequently, Bi0.89 Ti0.11 FeO3 films with the thickness of about 200-, 280-, 360-, and 440 nm were coated on the PZTN/Pt(1 1 1)/Ti/SiO2 /Si substrates. These Bi0.89 Ti0.11 FeO3 /PZTN bilayered thin films were crystallized at 525 ◦ C for 2 min in a N2 atmosphere. Au top electrodes were deposited on the films using a sputtering system through a shadow mask with a diameter of 200 m for electrical measurements. The crystal structure of the films was characterized by X-ray diffraction (XRD) using a Bruker D8 diffractormeter with Cu K␣ radiation. A standard ferroelectric tester (Precision Pro. Radiant Technologies) was used to investigate the detailed ferroelectric properties of the films. The piezoelectric properties of the films in the
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Fig. 2. The Leakage current density as a function of electric field for 200-, 280-, 360-, and 440-nm-thick Bi0.89 Ti0.11 FeO3 thin films.
Fig. 1. XRD patterns of (a) PZTN buffer layer, (b) 200-, (c) 280-, (d) 360-, and (e) 440-nm-thick Bi0.89 Ti0.11 FeO3 thin films.
micrometer scale were investigated using the AFM in the piezoresponse mode. A hard Pt-coated Si AFM tip with a force constant of 40 N/m was chosen to eliminate the electrostatic effect. The frequency of the ac driving electric field is 10 kHz, which is far below the resonant frequency of the cantilever and fast enough not to disturb the feed-back loop of the AFM system. A scanning frequency of 0.5 Hz is utilized to ensure that the ferroelectric domains in the film can be fully switched.
3. Results and discussion Fig. 1 shows the XRD patterns of the PZTN buffer layer and Bi0.89 Ti0.11 FeO3 thin films with various thicknesses (200 nm, 280 nm, 360 nm, and 440 nm). All the Bi0.89 Ti0.11 FeO3 thin films show similar polycrystalline structure without any secondary phases and evident preferential orientation. The detectable diffraction peaks for the Bi0.89 Ti0.11 FeO3 films match well to those of the distorted rhombohedral R3c structure of BFO [14]. Fig. 2 plots the leakage currents as a function of electric field measured at room temperature. As a result of the good insulating property and high breakdown characteristic of the PZTN buffer layer, the leakage currents in the Bi0.89 Tb0.11 FeO3 films deposited on the PZTN buffer layer are about two or three orders of magnitude lower than that in the BFO film deposited directly on Pt(1 1 1)/Ti/SiO2 /Si substrate [15]. When the applied electric field is about 200 kV/cm, the leakage currents in the 200-, 280-, 360-, and 440-nm-thick Bi0.89 Tb0.11 FeO3 films are around 3.95 × 10−9 A, 1.22 × 10−8 A, 3.77 × 10−8 A, 1.26 × 10−8 A, respectively. One can see that the 200-nm-thick Bi0.89 Tb0.11 FeO3 film has the lowest leakage current, the leakage current does not decrease with the increase of the film thicknesses. This should be due to the fact that with the increase of the Bi0.89 Tb0.11 FeO3 film thickness, the proportion of PZTN buffer layer gradually becomes smaller in the
Bi0.89 Tb0.11 FeO3 /PZTN bi-layered film, thus, the effect of the PZTN buffer layer to reduce the leakage current of the total bi-layered film gets weaker. The room temperature hysteresis loops for Bi0.89 Ti0.11 FeO3 bilayered films with various thicknesses measured at 5 kHz are plotted in Fig. 3. All the films show well-saturated and rectangular P–E hysteresis loops without any evident leaky characteristics. To confirm the saturated degree for the Bi0.89 Ti0.11 FeO3 films, the electric field dependences of remnant polarization Pr and Ec are summarized in Fig. 4. One can see that the Ec for all the films increase sharply at the electric fields around 200 kV/cm, suggesting that the grain sizes for all the films are highly uniform. Normally, the Ec for ferroelectric thin film gets decreased with the increase of the film thickness. However, Ec for all Bi0.89 Ti0.11 FeO3 films in the present work show weak dependence on the film thickness. The values of Ec are in the range of 200–220 kV/cm, which are much smaller than those for the pure [10] and doped BFO films [15], as well as double-layered ones [16,17]. This should be due to the fact that the PZTN film has a small Ec . It is worth noting that the Pr for the films also show weak dependent on the film thickness, the values are in the range of 45–50 C/cm2 , which are much larger than those of the BFO/Pub(Zr0.5 Ti0.5 )O3 (11.9 C/cm2 ),
Fig. 3. P–E hysteresis loops for (a) 200-, (b) 280-, (c) 360-, and (d) 440-nm-thick Bi0.89 Ti0.11 FeO3 thin films.
X.M. Chen et al. / Journal of Alloys and Compounds 509 (2011) 431–434
Fig. 4. The Pr and Ec as a function of the electric field for (a) 200-, (b) 280-, (c) 360-, and (d) 440-nm-thick Bi0.89 Ti0.11 FeO3 thin films.
BFO/Bi3.25 Sm0.75 Ti2.98 V0.02 O12 (35.9 C/cm2 ) double-layered films [16,17]. This phenomenon should be related to the large Pr value of the PZTN film (∼35 C/cm2 , not shown here) in comparison with those of the Pub(Zr0.5 Ti0.5 )O3 and Bi3.25 Sm0.75 Ti2.98 V0.02 O12 films. Now we discuss the asymmetric coercive fields of the asymmetric Bi0.89 Ti0.11 FeO3 /PZTN bi-layered structures. Generally, the asymmetric coercivities can be frequently observed in BFO-based thin films prepared using the pulsed laser deposition (PLD) technique and metal organic decomposition (MOD) method [10,18,19]. Our recent work demonstrated that the asymmetric coercive fields are related to the preferential polarization direction formed in the BFO-based films [19]. The aging effect can render this asymmetric behavior more severe. As can be seen in Fig. 3, the P–E hysteresis loops for the Bi0.89 Ti0.11 FeO3 films shift towards the positive field. To evaluate the degree of the shift, Ec [Ec = (| Ecp | − | Ecn |)/2, Ecp denotes the positive Ec , while Ecn denotes the negative one] is introduced in the following discussion. Fig. 5 shows the Ec as a function of the film thickness. One can see that the Ec gets decreased with the increase of the film thickness. This phenomenon can be explained by the formations of stresses including residual stress, thermal stress, and epitaxial stress in the films prepared using the MOD technique. These stresses can induce the preferential orientation of ferroelectric domains, resulting in the asymmetric coercivity of the films. Evidently, these stresses will get partially relaxed with the increase of the film thickness. Note that even the largest Ec of the present samples is much smaller than those of the BFO films prepared by the PLD and MOD techniques [15,18,19]. This may be due to the fact that the high valence element, Nb, is doped in the
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Fig. 5. The asymmetry of the coercive field (Ec ) as a function of the film thickness.
PZT buffer layer, thus, the oxygen vacancies (VO2− )•• can be greatly eliminated, which in turn leads to that the PZTN film has strong anti-aging ability. Fig. 6(a) illustrates the fatigue characteristic of the 200-nmthick Bi0.89 Ti0.11 FeO3 film. One can see that the normalized loss of the pulsed polarization P [P = P* (switched polarization)−Pˆ (nonswitched polarization)] for the film after 109 switching cycles is about 11%, which is lower than that of the BFO/Bi3.25 Sm0.75 Ti2.98 V0.02 O12 film [17]. It is well known that the fatigue behavior of the film is strongly dependent on the content of (VO2− )•• . Therefore, the better fatigue resistant property shown in the present work should result from the high crystallinity of the Bi0.89 Ti0.11 FeO3 film. The normalized reduction of P for 200nm-thick Bi0.89 Ti0.11 FeO3 film in retention is given in Fig. 6(b). The reduction of P for retention time up to 104 s is only 1.4%. The good charge-retaining ability should also be ascribed to the strong anti-aging ability of the PZTN film, which leads to that the domains with opposite directions locating at the interface between the PZTN layer and Pt(1 1 1)/Ti/SiO2 /Si substrate are difficult to nucleate. To investigate the piezoelectric properties of the films on the micrometer scale, a background region was formed by scanning with a negative dc bias (−10 V). Subsequently, a 5 × 5 m2 square inside the background area was polarized using an opposite voltage (10 V). A 10 kHz ac voltage of 4 V (peak-to-peak) was adopted to realize the nondestructive readout of the piezoresponse images of the films. Fig. 7 shows the two-dimensional piezoresponse image of the polarized 200-nm-thick Bi0.89 Ti0.11 FeO3 film. A large piezoresponse with complete reversible switching can be observed in
Fig. 6. (a) The pulsed remnant polarization (normalized P) as a function of switching cycles. (b) The normalized P as a function of retention time for 200-nm-thick Bi0.89 Ti0.11 FeO3 film.
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200 kV/cm, and shows weak dependence on the film thickness. The 200-nm-thick Bi0.89 Tb0.11 FeO3 film exhibits the lowest leakage current, strong fatigue resistance as well as good charge-retaining ability, which render it a promising candidate for the future high-density FeRAMs. The work related to further decreasing the thickness of the BFO film and PZTN buffer layer is in process. Acknowledgements This work was supported by funding from the National Natural Science Foundation of China (50972049) and Natural Science Foundation of Shangdong Province (ZR2009FZ008). References
Fig. 7. Two-dimensional piezoresponse image of the polarized 200-nm-thick Bi0.89 Ti0.11 FeO3 film.
the films, suggesting the film can be uniformly polarized. Generally, the domain backswitching induced by the local internal fields associated with the (Fe2+3+ ) − VO•• complexes can be frequently Fe seen in the BFO-based thin films [18,19]. However, one can see from Fig. 7 that the domains in the background region are almost completely switched, indicating the domain backswitching is significantly restrained. This should result from the anti-aging ability of the PZTN film as mentioned above. 4. Conclusions In conclusion, the Bi0.89 Ti0.11 FeO3 thin films with the thicknesses ranging from 200 to 440 nm were fabricated on PZTNbuffered Pt(1 1 1)/Ti/SiO2 /Si substrates using a metal organic decomposition process. As a result of the small coercive field and strong anti-aging property of PbZr0.2 Ti0.79 Nb0.01 O3 buffer layer, the leakage currents, coercive fields, as well as the asymmetry coercivities of the Bi0.89 Tb0.11 FeO3 films are remarkably decreased. The coercive field Ec for all the Bi0.89 Ti0.11 FeO3 films is around
[1] J. Wang, J.B. Neaton, H. Zheng, V. Nagarajan, S.B. Ogale, B. Liu, D. Viehland, V. Vaithyanathan, D.G. Schlom, U.V. Waghmare, N.A. Spaldin, K.M. Rabe, M. Wuttig, R. Ramesh, Science 299 (2003) 1719–1722. [2] T. Zhao, A. Scholl, F. Zavaliche, K. Lee, M. Barry, A. Doran, M.P. Cruz, Y.H. Chu, C. Ederer, N.A. Spaldin, R.R. Das, D.M. Kim, S.H. Baek, C.B. Eom, R. Ramesh, Nat. Mater. 5 (2006) 823–829. [3] H.W. Jang, D. Ortiz, S.H. Baek, C.M. Folkman, R.R. Das, P. Shafer, Y. Chen, C.T. Nelson, X. Pan, R. Ramesh, C.B. Eom, Adv. Mater. (Weinheim. Ger.) 21 (2009) 817–823. [4] Z.Q. Hu, M.Y. Li, B.F. Yu, L. Pei, J. Liu, J. Wang, X.Z. Zhao, J. Phys. D: Appl. Phys. 42 (2009) 185010-1–185010-5. [5] A.Z. Simões, A.H.M. Gonzalez, L.S. Cavalcante, C.S. Riccardi, E. Longo, J.A. Varela, J. Appl. Phys. 101 (2007) 074108-1–1074108-6. [6] S.K. Singh, H. Ishiwara, K. Maruyama, Appl. Phys. Lett. 88 (2006) 2629081–262908-3. [7] Y.H. Chu, Q. Zhan, C.-H. Yang, M.P. Cruz, L.W. Martin, T. Zhao, P. Yu, R. Ramesh, P.T. Joseph, I.N. Lin, W. Tian, D.G. Schlom, Appl. Phys. Lett. 92 (2008) 1029091–102909-3. [8] R.R. Das, D.M. Kim, S.H. Baek, C.B. Eom, F. Zavaliche, S.Y. Yang, R. Ramesh, Y.B. Chen, X.Q. Pan, X. Ke, M.S. Rzchowski, S.K. Streiffer, Appl. Phys. Lett. 88 (2006) 242904–1242904-3. [9] G.W. Pabst, L.W. Martin, Y.H. Chu, R. Ramesh, Appl. Phys. Lett. 90 (2007) 072902-1–072902-3. [10] H.R. Liu, X.Z. Wang, J. Phys. D: Appl. Phys. 41 (2008) 175411-1–175411-5. [11] J. Liu, M.Y. Li, L. Pei, J. Wang, B.F. Yu, X. Wang, X.Z. Zhao, J. Alloy. Compd. 493 (2010) 544–548. [12] X.M. Chen, G.D. Hu, W.B. Wu, C.H. Yang, X. Wang, J. Am. Ceram. Soc. 93 (2010) 948–950. [13] T. Haccart, D. Remiens, E. Cattan, Thin Solid Films 423 (2003) 235–242. [14] JCPDS Card No. 86-1518. [15] G.D. Hu, S.H. Fan, C.H. Yang, W.B. Wu, Appl. Phys. Lett. 92 (2008) 1929051–192905-3. [16] Y.W. Li, J.L. Sun, J. Chen, X.J. Meng, J.H. Chu, Phys. Lett. 87 (2005) 1829021–182902-3. [17] Z.X. Cheng, X.L. Wang, C.V. Kannan, K. Ozawa, H. Kimura, T. Nishida, S.J. Zhang, T.R. Shrout, Appl. Phys. Lett. 88 (2006) 132909-1–132909-3. [18] J. Dho, X. Qi, H. Kim, J.L. MacManus-Driscoll, M.G. Blamire, Adv. Mater. (Weinheim Ger.) 18 (2006) 1445–1448. [19] Z. Wen, G. Hu, C. Yang, W. Wu, Appl. Phys. A 97 (2009) 937–941.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Thickness effects of Bi0.89 Ti0.11 FeO3 thin films deposited on PbZr0.2 Ti0.79 Nb0.01 O3 buffer layers X.M. Chen, G.D. Hu ∗ , J.C. Wang, L. Cheng, C.H. Yang, W.B. Wu School of Materials Science and Engineering, University of Jinan, Jinan 250022, China
a r t i c l e
i n f o
Article history: Received 11 June 2010 Received in revised form 5 September 2010 Accepted 8 September 2010 Available online 17 September 2010 Keywords: Ferroelectrics Thin films Chemical synthesis Electronic properties
a b s t r a c t Bi0.89 Ti0.11 FeO3 thin films with the thicknesses of 200–440 nm were fabricated on the 40-nm-thick PbZr0.2 Ti0.79 Nb0.01 O3 (PZTN)-buffered Pt(1 1 1)/Ti/SiO2 /Si substrates using a metal organic decomposition process. As a result of the good insulating property and high breakdown characteristic of the PZTN buffer layer, the leakage currents in the Bi0.89 Tb0.11 FeO3 films are significantly reduced. All the films show well-saturated and rectangular P–E hysteresis loops without any evident leaky behavior. The remnant polarization Pr and coercive field Ec for all Bi0.89 Ti0.11 FeO3 films are around 45–50 C/cm2 and 200 kV/cm, respectively, and show weak dependent on the film thickness. The 200-nm-thick Bi0.89 Ti0.11 FeO3 film exhibits better fatigue-free characteristic and charge-retaining ability, and the domain backswitching is significantly restrained due to the strong anti-aging ability of the PZTN buffer layer. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In recent years, BiFeO3 (BFO) thin films have been extensively investigated for the applications of multi-state data storage and microelectromechanical systems due to its simultaneously ferroelectric ordering (Curie temperature TC = 850 ◦ C) and antiferromagnetic ordering (Néel temperature, TN = 370 ◦ C) above room temperature [1–4]. However, the fatal drawback of BFO is the high leakage current, making the ferroelectric properties of BFO films non-measurable before breakdown. In addition, the polycrystalline BFO thin film has a large coercive field (Ec ) (300–500 kV/cm) [5–6]. Therefore, the reductions of thickness and Ec for the BFO-based ferroelectric thin films are imperative for the low-voltage operation of the future high-density FeRAMs. It has been demonstrated that the BFO films with the thickness thinner than 300 nm and Ec less than 200 kV/cm can be readily obtained by using the pulsed laser deposition technique [7–9]. However, for the case of BFO films prepared using the chemical solution deposition method, saturated P–E hysteresis loops can only be obtained in the films thicker than 300 nm [10–11], which cannot meet the requirement for the application of FeRAMs. Recently, we have demonstrated that the Bi0.89 Ti0.11 FeO3 thin film exhibits better multiferroic properties [12]. However, the thickness and Ec of the Bi0.89 Ti0.11 FeO3 film were too high. Since introducing a ferroelectric thin film to form a double-layered struc-
∗ Corresponding author. Tel.: +86 531 88374857; fax: +86 531 87974453. E-mail address: [email protected] (G.D. Hu). 0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2010.09.049
ture has been verified as an effective route to improve the electric properties of BFO-based thin films, therefore, our present work related to reduce the thickness and Ec of the Bi0.89 Ti0.11 FeO3 film was focused on introducing a PbZr0.2 Ti0.79 Nb0.01 O3 (PZTN) buffer layer, because the PZTN film has a smaller Ec (<100 kV/cm) and larger breakdown voltage [13]. In this work, Bi0.89 Ti0.11 FeO3 films with the thicknesses ranging from 200 to 440 nm were fabricated on the 40-nm-thick PZTN-buffered Pt(1 1 1)/Ti/SiO2 /Si substrates. The results show that the Ec for all the Bi0.89 Ti0.11 FeO3 films are reduced to about 200 kV/cm as a result of the introduction of the PZTN buffer layer. The 200-nm-thick Bi0.89 Ti0.11 FeO3 film exhibits better anti-fatigue and retention properties. 2. Experimental procedure Both Bi0.89 Ti0.11 FeO3 and PZTN layers were fabricated using a metal organic decomposition process. The preparation of Bi0.89 Ti0.11 FeO3 precursor solution has been described elsewhere [12]. The PZTN precursor solution was prepared by dissolving lead acetate trihydrate, titanium isopropoxide, zirconium n-propoxide, and niobium ethoxide as raw materials and 2-methoxyethanol as the solvent. Lead of 5 mol % excess was added in PZTN precursor to compensate the lead loss during the heating treatment process. Prior to depositing Bi0.89 Ti0.11 FeO3 films, four PZTN thin films with the thickness of 40 nm were span-coated at 8000 rpm for 30 s on Pt(1 1 1)/Ti/SiO2 /Si substrates and then crystallized at 550 ◦ C. Subsequently, Bi0.89 Ti0.11 FeO3 films with the thickness of about 200-, 280-, 360-, and 440 nm were coated on the PZTN/Pt(1 1 1)/Ti/SiO2 /Si substrates. These Bi0.89 Ti0.11 FeO3 /PZTN bilayered thin films were crystallized at 525 ◦ C for 2 min in a N2 atmosphere. Au top electrodes were deposited on the films using a sputtering system through a shadow mask with a diameter of 200 m for electrical measurements. The crystal structure of the films was characterized by X-ray diffraction (XRD) using a Bruker D8 diffractormeter with Cu K␣ radiation. A standard ferroelectric tester (Precision Pro. Radiant Technologies) was used to investigate the detailed ferroelectric properties of the films. The piezoelectric properties of the films in the
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X.M. Chen et al. / Journal of Alloys and Compounds 509 (2011) 431–434
Fig. 2. The Leakage current density as a function of electric field for 200-, 280-, 360-, and 440-nm-thick Bi0.89 Ti0.11 FeO3 thin films.
Fig. 1. XRD patterns of (a) PZTN buffer layer, (b) 200-, (c) 280-, (d) 360-, and (e) 440-nm-thick Bi0.89 Ti0.11 FeO3 thin films.
micrometer scale were investigated using the AFM in the piezoresponse mode. A hard Pt-coated Si AFM tip with a force constant of 40 N/m was chosen to eliminate the electrostatic effect. The frequency of the ac driving electric field is 10 kHz, which is far below the resonant frequency of the cantilever and fast enough not to disturb the feed-back loop of the AFM system. A scanning frequency of 0.5 Hz is utilized to ensure that the ferroelectric domains in the film can be fully switched.
3. Results and discussion Fig. 1 shows the XRD patterns of the PZTN buffer layer and Bi0.89 Ti0.11 FeO3 thin films with various thicknesses (200 nm, 280 nm, 360 nm, and 440 nm). All the Bi0.89 Ti0.11 FeO3 thin films show similar polycrystalline structure without any secondary phases and evident preferential orientation. The detectable diffraction peaks for the Bi0.89 Ti0.11 FeO3 films match well to those of the distorted rhombohedral R3c structure of BFO [14]. Fig. 2 plots the leakage currents as a function of electric field measured at room temperature. As a result of the good insulating property and high breakdown characteristic of the PZTN buffer layer, the leakage currents in the Bi0.89 Tb0.11 FeO3 films deposited on the PZTN buffer layer are about two or three orders of magnitude lower than that in the BFO film deposited directly on Pt(1 1 1)/Ti/SiO2 /Si substrate [15]. When the applied electric field is about 200 kV/cm, the leakage currents in the 200-, 280-, 360-, and 440-nm-thick Bi0.89 Tb0.11 FeO3 films are around 3.95 × 10−9 A, 1.22 × 10−8 A, 3.77 × 10−8 A, 1.26 × 10−8 A, respectively. One can see that the 200-nm-thick Bi0.89 Tb0.11 FeO3 film has the lowest leakage current, the leakage current does not decrease with the increase of the film thicknesses. This should be due to the fact that with the increase of the Bi0.89 Tb0.11 FeO3 film thickness, the proportion of PZTN buffer layer gradually becomes smaller in the
Bi0.89 Tb0.11 FeO3 /PZTN bi-layered film, thus, the effect of the PZTN buffer layer to reduce the leakage current of the total bi-layered film gets weaker. The room temperature hysteresis loops for Bi0.89 Ti0.11 FeO3 bilayered films with various thicknesses measured at 5 kHz are plotted in Fig. 3. All the films show well-saturated and rectangular P–E hysteresis loops without any evident leaky characteristics. To confirm the saturated degree for the Bi0.89 Ti0.11 FeO3 films, the electric field dependences of remnant polarization Pr and Ec are summarized in Fig. 4. One can see that the Ec for all the films increase sharply at the electric fields around 200 kV/cm, suggesting that the grain sizes for all the films are highly uniform. Normally, the Ec for ferroelectric thin film gets decreased with the increase of the film thickness. However, Ec for all Bi0.89 Ti0.11 FeO3 films in the present work show weak dependence on the film thickness. The values of Ec are in the range of 200–220 kV/cm, which are much smaller than those for the pure [10] and doped BFO films [15], as well as double-layered ones [16,17]. This should be due to the fact that the PZTN film has a small Ec . It is worth noting that the Pr for the films also show weak dependent on the film thickness, the values are in the range of 45–50 C/cm2 , which are much larger than those of the BFO/Pub(Zr0.5 Ti0.5 )O3 (11.9 C/cm2 ),
Fig. 3. P–E hysteresis loops for (a) 200-, (b) 280-, (c) 360-, and (d) 440-nm-thick Bi0.89 Ti0.11 FeO3 thin films.
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Fig. 4. The Pr and Ec as a function of the electric field for (a) 200-, (b) 280-, (c) 360-, and (d) 440-nm-thick Bi0.89 Ti0.11 FeO3 thin films.
BFO/Bi3.25 Sm0.75 Ti2.98 V0.02 O12 (35.9 C/cm2 ) double-layered films [16,17]. This phenomenon should be related to the large Pr value of the PZTN film (∼35 C/cm2 , not shown here) in comparison with those of the Pub(Zr0.5 Ti0.5 )O3 and Bi3.25 Sm0.75 Ti2.98 V0.02 O12 films. Now we discuss the asymmetric coercive fields of the asymmetric Bi0.89 Ti0.11 FeO3 /PZTN bi-layered structures. Generally, the asymmetric coercivities can be frequently observed in BFO-based thin films prepared using the pulsed laser deposition (PLD) technique and metal organic decomposition (MOD) method [10,18,19]. Our recent work demonstrated that the asymmetric coercive fields are related to the preferential polarization direction formed in the BFO-based films [19]. The aging effect can render this asymmetric behavior more severe. As can be seen in Fig. 3, the P–E hysteresis loops for the Bi0.89 Ti0.11 FeO3 films shift towards the positive field. To evaluate the degree of the shift, Ec [Ec = (| Ecp | − | Ecn |)/2, Ecp denotes the positive Ec , while Ecn denotes the negative one] is introduced in the following discussion. Fig. 5 shows the Ec as a function of the film thickness. One can see that the Ec gets decreased with the increase of the film thickness. This phenomenon can be explained by the formations of stresses including residual stress, thermal stress, and epitaxial stress in the films prepared using the MOD technique. These stresses can induce the preferential orientation of ferroelectric domains, resulting in the asymmetric coercivity of the films. Evidently, these stresses will get partially relaxed with the increase of the film thickness. Note that even the largest Ec of the present samples is much smaller than those of the BFO films prepared by the PLD and MOD techniques [15,18,19]. This may be due to the fact that the high valence element, Nb, is doped in the
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Fig. 5. The asymmetry of the coercive field (Ec ) as a function of the film thickness.
PZT buffer layer, thus, the oxygen vacancies (VO2− )•• can be greatly eliminated, which in turn leads to that the PZTN film has strong anti-aging ability. Fig. 6(a) illustrates the fatigue characteristic of the 200-nmthick Bi0.89 Ti0.11 FeO3 film. One can see that the normalized loss of the pulsed polarization P [P = P* (switched polarization)−Pˆ (nonswitched polarization)] for the film after 109 switching cycles is about 11%, which is lower than that of the BFO/Bi3.25 Sm0.75 Ti2.98 V0.02 O12 film [17]. It is well known that the fatigue behavior of the film is strongly dependent on the content of (VO2− )•• . Therefore, the better fatigue resistant property shown in the present work should result from the high crystallinity of the Bi0.89 Ti0.11 FeO3 film. The normalized reduction of P for 200nm-thick Bi0.89 Ti0.11 FeO3 film in retention is given in Fig. 6(b). The reduction of P for retention time up to 104 s is only 1.4%. The good charge-retaining ability should also be ascribed to the strong anti-aging ability of the PZTN film, which leads to that the domains with opposite directions locating at the interface between the PZTN layer and Pt(1 1 1)/Ti/SiO2 /Si substrate are difficult to nucleate. To investigate the piezoelectric properties of the films on the micrometer scale, a background region was formed by scanning with a negative dc bias (−10 V). Subsequently, a 5 × 5 m2 square inside the background area was polarized using an opposite voltage (10 V). A 10 kHz ac voltage of 4 V (peak-to-peak) was adopted to realize the nondestructive readout of the piezoresponse images of the films. Fig. 7 shows the two-dimensional piezoresponse image of the polarized 200-nm-thick Bi0.89 Ti0.11 FeO3 film. A large piezoresponse with complete reversible switching can be observed in
Fig. 6. (a) The pulsed remnant polarization (normalized P) as a function of switching cycles. (b) The normalized P as a function of retention time for 200-nm-thick Bi0.89 Ti0.11 FeO3 film.
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200 kV/cm, and shows weak dependence on the film thickness. The 200-nm-thick Bi0.89 Tb0.11 FeO3 film exhibits the lowest leakage current, strong fatigue resistance as well as good charge-retaining ability, which render it a promising candidate for the future high-density FeRAMs. The work related to further decreasing the thickness of the BFO film and PZTN buffer layer is in process. Acknowledgements This work was supported by funding from the National Natural Science Foundation of China (50972049) and Natural Science Foundation of Shangdong Province (ZR2009FZ008). References
Fig. 7. Two-dimensional piezoresponse image of the polarized 200-nm-thick Bi0.89 Ti0.11 FeO3 film.
the films, suggesting the film can be uniformly polarized. Generally, the domain backswitching induced by the local internal fields associated with the (Fe2+3+ ) − VO•• complexes can be frequently Fe seen in the BFO-based thin films [18,19]. However, one can see from Fig. 7 that the domains in the background region are almost completely switched, indicating the domain backswitching is significantly restrained. This should result from the anti-aging ability of the PZTN film as mentioned above. 4. Conclusions In conclusion, the Bi0.89 Ti0.11 FeO3 thin films with the thicknesses ranging from 200 to 440 nm were fabricated on PZTNbuffered Pt(1 1 1)/Ti/SiO2 /Si substrates using a metal organic decomposition process. As a result of the small coercive field and strong anti-aging property of PbZr0.2 Ti0.79 Nb0.01 O3 buffer layer, the leakage currents, coercive fields, as well as the asymmetry coercivities of the Bi0.89 Tb0.11 FeO3 films are remarkably decreased. The coercive field Ec for all the Bi0.89 Ti0.11 FeO3 films is around
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