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Effect of Mn doping on the microstructures and dielectric properties of Bi3.15Nd0.85Ti3O12 thin films
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Thin Solid Films 516 (2008) 8240 – 8243
www.elsevier.com/locate/tsf
Effect of Mn doping on the microstructures and dielectric properties of Bi3.15Nd0.85Ti3O12 thin films X.L. Zhong, J.B. Wang, M. Liao, C.B. Tan, H.B. Shu, Y.C. Zhou ⁎ Institute of Modern Physics, Xiangtan University, Hunan, 411105, China Key Laboratory of Low Dimensional Materials and Application Technology of Ministry of Education, Xiangtan University, Hunan, 411105, China Received 8 April 2007; received in revised form 19 January 2008; accepted 2 March 2008 Available online 8 March 2008
Abstract Mn-doped Bi3.15Nd0.85Ti3O12 (BNT), i.e., Bi3.15Nd0.85Ti3 − xMnxO12 (BNTM, x = 0, 0.005, 0.01, 0.03, 0.05, and 0.1) thin films with bismuthlayered perovskite structure were prepared on Pt/Ti/SiO2/Si(100) substrates by a chemical solution deposition method at 700 °C. The crystal structures of BNTM films were analyzed by X-ray diffraction and the surface morphologies were observed by field emission scanning electron microscopy. The effects of Mn contents on the microstructures and dielectric properties of BNTM films are investigated in detail. Among these BNTM films, it is found that the BNTM01 (x = 0.01) film exhibits the highest dielectric tunability and dielectric constant but the lowest dielectric loss. Compared with BNT film, the BNTM01 film has lower leakage current density. Eventually, the mechanism involved in the Mn doping effect on the electrical properties of the BNT films is discussed. © 2008 Elsevier B.V. All rights reserved. PACS: 77.55.+f; 77.84.−s; 81.20.Fw Keywords: Ferroelectric films; Electrical properties; Chemical solution deposition
1. Introduction Ferroelectric thin films have potential applications on nonvolatile random access memories, room temperature pyroelectric devices, electro-optic devices, and micro-electromechanical systems, etc. Bismuth titanate (Bi4Ti3O12, BIT), a typical kind of Aurivillius bismuth-layer-structured ferroelectrics (BLSFs), has been studied intensively in the past decade due to its large remanent polarization (Pr), low crystallization temperature, and high Curie temperature. Ion substitution in the BIT film is known to be an effective technique for improving its electrical properties, such as Pr and fatigue characteristics [1–12]. In the case of A-site substitution in the BIT structure, Park et al. reported that Lasubstituted BIT (BLT) films exhibited an enhanced Pr with excellent fatigue endurance [1]. Other lanthanides, such as Nd, Sm, Pr, Eu, etc., resulted in similar results [2–5]. Especially, Chon et al. had prepared Bi3.15Nd0.85Ti3O12 (BNT) films with Pr ⁎ Corresponding author. Institute of Modern Physics, Xiangtan University, Hunan, 411105, China. Fax: +86 732 8292 468. E-mail address: [email protected] (Y.C. Zhou). 0040-6090/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.03.004
above 50 μC/cm2 and a fatigue free characteristic [2]. In the case of B-site substitution in BIT, some ions such as V5+, Nb5+, W6+, could also improve the ferroelectric properties effectively [6–8]. Furthermore, A- and B-sites cosubstituted BIT films were studied, such as Bi3.25La0.75Ti2.8Zr0.2O12 (BLTZ), Bi3.25La0.75 (Ti1 − xMox)3O12 (BLTM), Bi4 − yNdyTi3 − xVxO12 (BNTV), and Bi3.15Nd0.85Ti3 − xWxO12 (BNTW), which indicates that A- and B-sites cosubstitution is an effective way for improving the electrical properties [9–12]. However, although the effect of A-site substitution on structure and electrical property of BIT film has been well studied, there are few reports concerning the effects of B-site substitution, or both A- and B-sites cosubstitution on the electrical property so far. Recently, it has been demonstrated that Mn doping in BLSFs and ABO3 perovskite ferroelectric thin films could significantly reduce leakage current and improve ferroelectric and dielectric properties of the films [13–18]. Moreover, we have also reported that Mn doping improves the ferroelectric properties of BNT thin films [19]. In this study, we systematically investigate the effect of Mn content on the microstructure, dielectric properties and leakage current characteristic of BNT films.
X.L. Zhong et al. / Thin Solid Films 516 (2008) 8240–8243
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2. Experimental details The Bi3.15Nd0.85(Ti3 − xMnx)O12 (x = 0, 0.005, 0.01, 0.03, 0.05, and 0.1, the samples are denoted as BNT, BNTM005, BNTM01, BNTM03, BNTM05, and BNTM1, respectively) thin films were fabricated on Pt/Ti/SiO2/Si(100) substrates by a chemical solution deposition technique according to the procedure described in previous study [19]. The crystal structures of the films were characterized by X-ray diffraction (XRD) using a D/max-rA X-ray diffractometer (Rigaku, Japan) with Cu Kα radiation. The surface morphologies and thickness of the film samples were measured via a field emission scanning electron microscope (FESEM, ZEISS SUPRA 55) with the voltage of 10 kV. The thickness of the derived films is about 500 nm by a cross-sectional view of FESEM. To investigate the electrical properties, Pt dot electrodes with diameters of 200 μm were deposited on the top surface of the films by direct current sputtering through a shadow mask. The capacitance–voltage characteristics and frequency dependent dielectric properties were measured via a HP4194A impedance analyzer (Hewlett Packard, America). The leakage current characteristics and ferroelectric properties were studied by a Radiant Technologies Precision Workstation ferroelectric test system. 3. Results and discussion The XRD patterns of the BNTM films with various Mn contents annealed at 700 °C for 5 min are shown in Fig. 1. The diffraction peaks were identified by using the standard powder diffraction data of BIT. It is found that all of the films consist of a single phase of bismuth-layered perovskite structure. Fig. 2 displays the surface morphologies of the samples with various Mn contents analyzed by FESEM. The BNT film (Fig. 2a) is mostly composed of plate-like grains and the approximate diameter of grains is 200 nm. While the grains of BNTM films seem to change apparently to an elliptical shape and become smaller, compared with those of BNT film.
Fig. 1. XRD patterns of the BNTM thin films with various Mn contents (from Ref. [19]).
Fig. 2. Surface morphologies of (a) BNT, (b) BNTM005, (c) BNTM01, (d) BNTM03, (e) BNTM05, and (f) BNTM1 thin films.
Fig. 3 shows the capacitance–voltage (C–V) curves of the BNTM thin films with various Mn contents. The measurement was carried out with a sine wave with an amplitude of 200 mVat a frequency of 1 MHz at room temperature. The butterfly shape of the C–V curves confirms the ferroelectric characteristics of both the BNT and BNTM thin films. Furthermore, the capacitance exhibits a strong nonlinear dependence on bias voltage, which is the typical nature for ferroelectric thin films. From
Fig. 3. Capacitance–voltage characteristics of the BNTM thin films with various Mn contents.
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Fig. 4. The dielectric constants and dissipation factors as a function of frequency for BNTM films with various Mn contents.
Fig. 6. P–E hysteresis loops of the BNTM01 thin film at various applied electric field.
Fig. 3, the sharp and symmetric switching peaks are observed in Pt/BNTM/Pt capacitors. The two maximum values of the curves correspond to the domain-switching voltages where the polarization reversals take place. The capacitance tunabilities [100% × ( C max − C min ) / C ma x ] of the BNT, BNTM005, BNTM01, BNTM03, BNTM05, and BNTM1 are calculated from Fig. 3 to be 6.4%, 18.7%, 23%, 20.6%, 16.5%, and 18.5%, respectively. Clearly, in the case of Mn doping, dielectric tunability increases greatly, similarly to the result of Mn-doped (Ba, Sr)TiO3 [17]. Fig. 4 displays the dielectric constant (εr) and dissipation factor (tanδ) as a function of frequency for BNTM films with various Mn contents. There is no sudden change of εr in the frequency range up to 1 MHz. The εr exhibits a slight decline while the applied frequency increases from 1 kHz to 1 MHz. The dissipation factors are below 0.04 for all films. The values of εr and tanδ at 100 kHz were estimated to be 250, 343, 422, 356, 321, and 315; 0.022, 0.017, 0.019, 0.020, 0.018 and 0.017 for BNT, BNTM005, BNTM01, BNTM03, BNTM05, and BNTM1, respectively. Compared with BNT film, BNTM films have larger εr but lower tanδ, which is similar to the results of other Mn-doped BLSFs and ABO3 perovskite ferroelectric thin
films [15,16]. The lower tanδ implies the Mn-doped BNT films are better insulators. Fig. 5 shows the typical electric field dependence of the leakage current density of the BNT and BNTM01 thin films. It is noticeable that the BNTM01 film sample shows lower leakage current, compared with BNT film. The leakage current density of both the films increases gradually with the applied electric field. There is no significant difference in leakage when the voltage polarity is reversed. The value of current density at 150 kV/cm is 1.1 × 10− 7 A/cm2 in BNTM01 film, which is comparable with other A- and B-sites cosubstituted BIT films such as BLTM [10]. Fig. 6 shows the polarization–electric field (P–E) hysteresis loops of the prepared BNTM01 films measured under various applied electric field. The characterized BNTM01 film capacitor exhibits well-saturated P–E switching curves. At an applied electric field of 520 kV/cm, the remanent polarization 2Pr and the coercive field 2Ec are about 78 μC/cm2 and 205 kV/cm, respectively. Table 1 summarizes the values of 2Pr and 2Ec of bismuth-layer-structured ferroelectric thin films prepared by other groups, which are obtained under the applied electric field of 250 kV/cm. From Table 1, one can see that the 2Pr value of BNTM is higher than those of BNT, BLT, BIT, BLTZ and BLTG thin films [9,20–23]. Table 1 Comparison of 2Pr and 2Ec values of BIT-based ferroelectric thin films under the applied electric field of 250 kV/cm
Fig. 5. Leakage current characteristics of the BNT and BNTM01 films.
Thin film
2Pr(μC/cm2)
2Ec(kV/cm)
Literature
BNTM BNT BLT BNT BIT BLT BTZ BLTZ BLT BLT BLTG BGT
55 34 23 43 5 24 25 24 19 26 24 42
165 208 160 252 120 270 139 109 188 160 132 235
Present work Ref. [20] Ref. [20] Ref. [21] Ref. [21] Ref. [21] Ref. [9] Ref. [9] Ref. [9] Ref. [22] Ref. [22] Ref. [23]
X.L. Zhong et al. / Thin Solid Films 516 (2008) 8240–8243
Apparently, the electrical properties of the BNT thin films are strongly influenced by Mn doping. These results are partially in agreement with the results of other Mn-doped BLSFs and ABO3 perovskite ferroelectric thin films [13–18]. The mechanism responsible for improving the dielectric and insulating properties of BNTM films could be explained as follows. One of the key factors in affecting the electrical properties of BNT thin films is that the electrons (space charge) generated from the oxygen vacancies can hop between different titanium ions, i.e., Ti4+ and Ti3+. Because of the substitution of Ti4+ by the low-valance Mn ions (act as “B” site acceptor dopants in the perovskite-type units of BNT lattices), extrinsic oxygen vacancies are created in order to maintain the charge balance. Thus, the acceptor dopants prevent the reduction of Ti4+ to Ti3+ by neutralizing the donor action of the oxygen vacancies [17]. This further reduces the pinning of domain walls and stabilizes the electrical conductivity, resulting in the improvement of electrical properties. 4. Conclusions BNTM ferroelectric thin films with various Mn concentrations were fabricated by chemical solution deposition on Pt/Ti/ SiO2/Si(100) substrates. The grain of BNTM film shows different shape and the grain size becomes smaller, compared with those of BNT film. The electrical properties of BNTM film depend on Mn doping content. A low concentration substitution with Mn in BNT films could enhance dielectric tunability and dielectric constant, and reduce the dielectric loss and leakage current density, which may be due to preventing the reduction of Ti4+ to Ti3+ by neutralizing the donor action of the oxygen vacancies. Acknowledgements This work was financially supported by the National Natural Science Foundation of China for Distinguished Young Scholars (No. 10525211), the National Science Found of China (No.
8243
50772093, 50702048 and 10732100), the Cultivation Fund of the Key Scientific and Technical Innovation Project, Ministry of Education of China (No. 076044), and the Hunan Provincial Natural Science Foundation of China (No. 06JJ30022 and No. 05JJ30126). References [1] B.H. Park, B.S. Kang, S.D. Bu, T.W. Noh, J. Lee, W. Jo, Nature (London) 401 (1999) 682. [2] U. Chon, H.M. Jang, M.G. Kim, C.H. Chang, Phys. Rev. Lett. 89 (2002) 087601. [3] U. Chon, K.B. Kim, H.M. Jang, G.C. Yi, Appl. Phys. Lett. 79 (2001) 3137. [4] U. Chon, J.S. Shim, H.M. Jang, J. Appl. Phys. 93 (2003) 4769. [5] K.T. Kim, C.I. Kim, D.H. Kang, I.W. Shim, Thin Solid Films 422 (2002) 230. [6] Y. Noguchi, M. Miyayama, Appl. Phys. Lett. 78 (2001) 1903. [7] J.K. Kim, J. Kim, T.K. Song, S.S. Kim, Thin Solid Films 419 (2002) 225. [8] T. Sakai, T. Watanabe, M. Osada, M. Kakihana, Y. Noguchi, M. Miyayama, H. Funakubo, Jpn. J. Appl. Phys. Part 1 42 (2003) 2850. [9] S.T. Zhang, Y.F. Chen, J. Wang, G.X. Cheng, Z.G. Liu, N.B. Ming, Appl. Phys. Lett. 84 (2004) 3660. [10] X.S. Wang, H. Ishiwara, Appl. Phys. Lett. 82 (2003) 2479. [11] H. Uchida, H. Yoshikawa, I. Okada, H. Matsuda, T. lijima, T. Watanabe, T. Kojima, H. Funakubo, Appl. Phys. Lett. 81 (2002) 2229. [12] W. Li, Y. Yin, D. Su, J.S. Zhu, J. Appl. Phys. 97 (2005) 084102. [13] S.K. Singh, H. Ishiwara, Solid State Commun. 140 (2006) 430. [14] J.P. Kim, C.R. Cho, M.S. Jang, S.Y. Jeong, Integr. Ferroelectr. 79 (2006) 55. [15] G.R. Li, L.Y. Zheng, Q.R. Yin, J. Appl. Phys. 98 (2005) 064108. [16] M. Jain, S.B. Majumder, R.S. Katiyar, F.A. Miranda, F.W. Van Keuls, Appl. Phys. Lett. 82 (2003) 1911. [17] Z. Yuan, Y. Lin, J. Weaver, X. Chen, C.L. Chen, G. Subramanyam, J.C. Jiang, E.I. Meletis, Appl. Phys. Lett. 87 (2005) 152901. [18] S.S. Kim, C. Park, Appl. Phys. Lett. 75 (1999) 2554. [19] X.L. Zhong, J.B. Wang, L.Z. Sun, C.B. Tan, X.J. Zheng, Y.C. Zhou, Appl. Phys. Lett. 90 (2006) 012906. [20] S.K. Lee, D. Hesse, U. Gösele, J. Appl. Phys. 100 (2006) 044108. [21] T. Kojima, T. Sakai, T. Watanabe, H. Funakubo, K. Saito, M. Osada, Appl. Phys. Lett. 80 (2002) 2746. [22] W. Sakamoto, Y. Mizutani, N. Iizawa, T. Yogo, T. Hayashi, S. Hirano, J. Eur. Ceram. Soc. 25 (2005) 2305. [23] S.S. Kim, J.C. Bae, W.J. Kim, J. Cryst. Growth 274 (2005) 394.
Thin Solid Films 516 (2008) 8240 – 8243
www.elsevier.com/locate/tsf
Effect of Mn doping on the microstructures and dielectric properties of Bi3.15Nd0.85Ti3O12 thin films X.L. Zhong, J.B. Wang, M. Liao, C.B. Tan, H.B. Shu, Y.C. Zhou ⁎ Institute of Modern Physics, Xiangtan University, Hunan, 411105, China Key Laboratory of Low Dimensional Materials and Application Technology of Ministry of Education, Xiangtan University, Hunan, 411105, China Received 8 April 2007; received in revised form 19 January 2008; accepted 2 March 2008 Available online 8 March 2008
Abstract Mn-doped Bi3.15Nd0.85Ti3O12 (BNT), i.e., Bi3.15Nd0.85Ti3 − xMnxO12 (BNTM, x = 0, 0.005, 0.01, 0.03, 0.05, and 0.1) thin films with bismuthlayered perovskite structure were prepared on Pt/Ti/SiO2/Si(100) substrates by a chemical solution deposition method at 700 °C. The crystal structures of BNTM films were analyzed by X-ray diffraction and the surface morphologies were observed by field emission scanning electron microscopy. The effects of Mn contents on the microstructures and dielectric properties of BNTM films are investigated in detail. Among these BNTM films, it is found that the BNTM01 (x = 0.01) film exhibits the highest dielectric tunability and dielectric constant but the lowest dielectric loss. Compared with BNT film, the BNTM01 film has lower leakage current density. Eventually, the mechanism involved in the Mn doping effect on the electrical properties of the BNT films is discussed. © 2008 Elsevier B.V. All rights reserved. PACS: 77.55.+f; 77.84.−s; 81.20.Fw Keywords: Ferroelectric films; Electrical properties; Chemical solution deposition
1. Introduction Ferroelectric thin films have potential applications on nonvolatile random access memories, room temperature pyroelectric devices, electro-optic devices, and micro-electromechanical systems, etc. Bismuth titanate (Bi4Ti3O12, BIT), a typical kind of Aurivillius bismuth-layer-structured ferroelectrics (BLSFs), has been studied intensively in the past decade due to its large remanent polarization (Pr), low crystallization temperature, and high Curie temperature. Ion substitution in the BIT film is known to be an effective technique for improving its electrical properties, such as Pr and fatigue characteristics [1–12]. In the case of A-site substitution in the BIT structure, Park et al. reported that Lasubstituted BIT (BLT) films exhibited an enhanced Pr with excellent fatigue endurance [1]. Other lanthanides, such as Nd, Sm, Pr, Eu, etc., resulted in similar results [2–5]. Especially, Chon et al. had prepared Bi3.15Nd0.85Ti3O12 (BNT) films with Pr ⁎ Corresponding author. Institute of Modern Physics, Xiangtan University, Hunan, 411105, China. Fax: +86 732 8292 468. E-mail address: [email protected] (Y.C. Zhou). 0040-6090/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.03.004
above 50 μC/cm2 and a fatigue free characteristic [2]. In the case of B-site substitution in BIT, some ions such as V5+, Nb5+, W6+, could also improve the ferroelectric properties effectively [6–8]. Furthermore, A- and B-sites cosubstituted BIT films were studied, such as Bi3.25La0.75Ti2.8Zr0.2O12 (BLTZ), Bi3.25La0.75 (Ti1 − xMox)3O12 (BLTM), Bi4 − yNdyTi3 − xVxO12 (BNTV), and Bi3.15Nd0.85Ti3 − xWxO12 (BNTW), which indicates that A- and B-sites cosubstitution is an effective way for improving the electrical properties [9–12]. However, although the effect of A-site substitution on structure and electrical property of BIT film has been well studied, there are few reports concerning the effects of B-site substitution, or both A- and B-sites cosubstitution on the electrical property so far. Recently, it has been demonstrated that Mn doping in BLSFs and ABO3 perovskite ferroelectric thin films could significantly reduce leakage current and improve ferroelectric and dielectric properties of the films [13–18]. Moreover, we have also reported that Mn doping improves the ferroelectric properties of BNT thin films [19]. In this study, we systematically investigate the effect of Mn content on the microstructure, dielectric properties and leakage current characteristic of BNT films.
X.L. Zhong et al. / Thin Solid Films 516 (2008) 8240–8243
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2. Experimental details The Bi3.15Nd0.85(Ti3 − xMnx)O12 (x = 0, 0.005, 0.01, 0.03, 0.05, and 0.1, the samples are denoted as BNT, BNTM005, BNTM01, BNTM03, BNTM05, and BNTM1, respectively) thin films were fabricated on Pt/Ti/SiO2/Si(100) substrates by a chemical solution deposition technique according to the procedure described in previous study [19]. The crystal structures of the films were characterized by X-ray diffraction (XRD) using a D/max-rA X-ray diffractometer (Rigaku, Japan) with Cu Kα radiation. The surface morphologies and thickness of the film samples were measured via a field emission scanning electron microscope (FESEM, ZEISS SUPRA 55) with the voltage of 10 kV. The thickness of the derived films is about 500 nm by a cross-sectional view of FESEM. To investigate the electrical properties, Pt dot electrodes with diameters of 200 μm were deposited on the top surface of the films by direct current sputtering through a shadow mask. The capacitance–voltage characteristics and frequency dependent dielectric properties were measured via a HP4194A impedance analyzer (Hewlett Packard, America). The leakage current characteristics and ferroelectric properties were studied by a Radiant Technologies Precision Workstation ferroelectric test system. 3. Results and discussion The XRD patterns of the BNTM films with various Mn contents annealed at 700 °C for 5 min are shown in Fig. 1. The diffraction peaks were identified by using the standard powder diffraction data of BIT. It is found that all of the films consist of a single phase of bismuth-layered perovskite structure. Fig. 2 displays the surface morphologies of the samples with various Mn contents analyzed by FESEM. The BNT film (Fig. 2a) is mostly composed of plate-like grains and the approximate diameter of grains is 200 nm. While the grains of BNTM films seem to change apparently to an elliptical shape and become smaller, compared with those of BNT film.
Fig. 1. XRD patterns of the BNTM thin films with various Mn contents (from Ref. [19]).
Fig. 2. Surface morphologies of (a) BNT, (b) BNTM005, (c) BNTM01, (d) BNTM03, (e) BNTM05, and (f) BNTM1 thin films.
Fig. 3 shows the capacitance–voltage (C–V) curves of the BNTM thin films with various Mn contents. The measurement was carried out with a sine wave with an amplitude of 200 mVat a frequency of 1 MHz at room temperature. The butterfly shape of the C–V curves confirms the ferroelectric characteristics of both the BNT and BNTM thin films. Furthermore, the capacitance exhibits a strong nonlinear dependence on bias voltage, which is the typical nature for ferroelectric thin films. From
Fig. 3. Capacitance–voltage characteristics of the BNTM thin films with various Mn contents.
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Fig. 4. The dielectric constants and dissipation factors as a function of frequency for BNTM films with various Mn contents.
Fig. 6. P–E hysteresis loops of the BNTM01 thin film at various applied electric field.
Fig. 3, the sharp and symmetric switching peaks are observed in Pt/BNTM/Pt capacitors. The two maximum values of the curves correspond to the domain-switching voltages where the polarization reversals take place. The capacitance tunabilities [100% × ( C max − C min ) / C ma x ] of the BNT, BNTM005, BNTM01, BNTM03, BNTM05, and BNTM1 are calculated from Fig. 3 to be 6.4%, 18.7%, 23%, 20.6%, 16.5%, and 18.5%, respectively. Clearly, in the case of Mn doping, dielectric tunability increases greatly, similarly to the result of Mn-doped (Ba, Sr)TiO3 [17]. Fig. 4 displays the dielectric constant (εr) and dissipation factor (tanδ) as a function of frequency for BNTM films with various Mn contents. There is no sudden change of εr in the frequency range up to 1 MHz. The εr exhibits a slight decline while the applied frequency increases from 1 kHz to 1 MHz. The dissipation factors are below 0.04 for all films. The values of εr and tanδ at 100 kHz were estimated to be 250, 343, 422, 356, 321, and 315; 0.022, 0.017, 0.019, 0.020, 0.018 and 0.017 for BNT, BNTM005, BNTM01, BNTM03, BNTM05, and BNTM1, respectively. Compared with BNT film, BNTM films have larger εr but lower tanδ, which is similar to the results of other Mn-doped BLSFs and ABO3 perovskite ferroelectric thin
films [15,16]. The lower tanδ implies the Mn-doped BNT films are better insulators. Fig. 5 shows the typical electric field dependence of the leakage current density of the BNT and BNTM01 thin films. It is noticeable that the BNTM01 film sample shows lower leakage current, compared with BNT film. The leakage current density of both the films increases gradually with the applied electric field. There is no significant difference in leakage when the voltage polarity is reversed. The value of current density at 150 kV/cm is 1.1 × 10− 7 A/cm2 in BNTM01 film, which is comparable with other A- and B-sites cosubstituted BIT films such as BLTM [10]. Fig. 6 shows the polarization–electric field (P–E) hysteresis loops of the prepared BNTM01 films measured under various applied electric field. The characterized BNTM01 film capacitor exhibits well-saturated P–E switching curves. At an applied electric field of 520 kV/cm, the remanent polarization 2Pr and the coercive field 2Ec are about 78 μC/cm2 and 205 kV/cm, respectively. Table 1 summarizes the values of 2Pr and 2Ec of bismuth-layer-structured ferroelectric thin films prepared by other groups, which are obtained under the applied electric field of 250 kV/cm. From Table 1, one can see that the 2Pr value of BNTM is higher than those of BNT, BLT, BIT, BLTZ and BLTG thin films [9,20–23]. Table 1 Comparison of 2Pr and 2Ec values of BIT-based ferroelectric thin films under the applied electric field of 250 kV/cm
Fig. 5. Leakage current characteristics of the BNT and BNTM01 films.
Thin film
2Pr(μC/cm2)
2Ec(kV/cm)
Literature
BNTM BNT BLT BNT BIT BLT BTZ BLTZ BLT BLT BLTG BGT
55 34 23 43 5 24 25 24 19 26 24 42
165 208 160 252 120 270 139 109 188 160 132 235
Present work Ref. [20] Ref. [20] Ref. [21] Ref. [21] Ref. [21] Ref. [9] Ref. [9] Ref. [9] Ref. [22] Ref. [22] Ref. [23]
X.L. Zhong et al. / Thin Solid Films 516 (2008) 8240–8243
Apparently, the electrical properties of the BNT thin films are strongly influenced by Mn doping. These results are partially in agreement with the results of other Mn-doped BLSFs and ABO3 perovskite ferroelectric thin films [13–18]. The mechanism responsible for improving the dielectric and insulating properties of BNTM films could be explained as follows. One of the key factors in affecting the electrical properties of BNT thin films is that the electrons (space charge) generated from the oxygen vacancies can hop between different titanium ions, i.e., Ti4+ and Ti3+. Because of the substitution of Ti4+ by the low-valance Mn ions (act as “B” site acceptor dopants in the perovskite-type units of BNT lattices), extrinsic oxygen vacancies are created in order to maintain the charge balance. Thus, the acceptor dopants prevent the reduction of Ti4+ to Ti3+ by neutralizing the donor action of the oxygen vacancies [17]. This further reduces the pinning of domain walls and stabilizes the electrical conductivity, resulting in the improvement of electrical properties. 4. Conclusions BNTM ferroelectric thin films with various Mn concentrations were fabricated by chemical solution deposition on Pt/Ti/ SiO2/Si(100) substrates. The grain of BNTM film shows different shape and the grain size becomes smaller, compared with those of BNT film. The electrical properties of BNTM film depend on Mn doping content. A low concentration substitution with Mn in BNT films could enhance dielectric tunability and dielectric constant, and reduce the dielectric loss and leakage current density, which may be due to preventing the reduction of Ti4+ to Ti3+ by neutralizing the donor action of the oxygen vacancies. Acknowledgements This work was financially supported by the National Natural Science Foundation of China for Distinguished Young Scholars (No. 10525211), the National Science Found of China (No.
8243
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