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Anomalous dielectric properties in (BaSr)TiO3 films fabricated by pulsed-laser deposition in N2 atmosphere
Solid State Communications 135 (2005) 707–710 www.elsevier.com/locate/ssc
Anomalous dielectric properties in (BaSr)TiO3 films fabricated by pulsed-laser deposition in N2 atmosphere Qiuwen Tang, Mingrong Shen*, Liang Fang Department of Physics, Jiangsu Key Laboratory of Thin Films, Suzhou University, Suzhou 215006, People’s Republic of China Received 6 April 2005; accepted 4 June 2005 by A. Pinczuk Available online 21 June 2005
Abstract A high dielectric constant of 2500 at 10 kHz near room temperature was observed in (Ba0.5Sr0.5)TiO3 (BST) films prepared by pulsed-laser deposition (PLD) at 550 8C in N2 atmosphere. The dielectric constant is weakly temperature-dependent above 200 K. However, there is about one hundred fold drop below 200 K, which can be characterized by a thermally excited relaxation process. The temperature and frequency-dependent dielectric response of the film is very similar to that reported for the so-called ‘colossal’ dielectric constant material, such as CaCu3Ti4O12. The dielectric properties are sensitive to the metal contacts with different work functions, however insensitive to the film thicknesses. Such an anomalous dielectric response for the BST films is ascribed to the formation of the Schottky barrier between the metallic electrode and the film surface, and the existence of the mobile charges in the film as indicated by the frequency characteristics of dielectric constant changing as the applied dc bias. q 2005 Elsevier Ltd. All rights reserved. PACS: 77.55.Cf; 81.15.Fg Keywords: A. Thin films; B. Laser processing; D. Dielectric response
1. Introduction The materials with high dielectric constant have received ever-increasing attention for their uses in important microelectronic devices such as capacitors and memory devices. Recently, perovskite-related material CaCu3Ti4O12 (CCTO) [1] and non-perovskite material Li0.05Ti0.02Ni0.93O (LTNO) [2] and Ta2O5 [3] have been reported for their unusual high dielectric constant of w103–105 at room temperature and very small temperature-dependence near room temperature. These materials show a similar dielectric response at low temperature, etc, a strong frequency-dependent steep decrease of the dielectric constant by two or three orders of magnitude and the
* Corresponding author. Tel.: C86 512 6511 2251; fax: C86 512 6511 2597. E-mail address: [email protected] (M. Shen).
0038-1098/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2005.06.007
associated peaks in the dielectric loss. The mechanism of such an anomalous dielectric response in these materials is mainly ascribed to the extrinsic effect including the formation of internal boundary-layer capacitors (IBLCs) [4] and interfacial space charge polarization between the metallic electrodes and film surfaces [5]. Most commercial IBLCs are based on (Ba,Sr)TiO3 (BST) ceramics [6–12], which are generally prepared in an O2 atmosphere and show a normal ferroelectric phase transition, which can be fitted by a Curie–Weis law [13,14]. The dielectric constant of these films is usually about 1000. In this paper we show that, depending on the experimental conditions in pulsed-laser deposition (PLD), conventional perovskite material BST films exhibit an anomalous dielectric response similar to that observed in the materials mentioned above. The dielectric constant of more than 2500 at 10 kHz near room temperature was observed in BST film prepared in 1 Pa N2 atmosphere at a relative low
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temperature of 550 8C. We investigate the frequency, temperature, thickness and electrode dependence of the dielectric response of the BST films, and conclude that the anomalous dielectric response is closely related to the mobile charges in the films and the electrode polarization effect.
2. Experimental details BST films were deposited by a 248-nm KrF excimer laser (Lambda Physik 105i) with the repetition rate of 5 Hz. The laser beam was focused on a BST ceramic target through a quartz window with the energy density of 1.6 J cmK2. Ceramic target with 2 cm in diameter was prepared by a conventional powder processing route. In this experiment, Pt/Ti/SiO2/Si (100) substrate was selected to place parallel to the target at a distance of 4.0 cm. Prior to the deposition, the chamber was firstly pumped down to 2.66!10K3 Pa, and then high purity nitrogen gas was introduced into the chamber. During the deposition, the substrate temperature was heated to 550 8C and the nitrogen pressure is controlled at 1 Pa. For carrying out the electrical measurement, different metal (Pt, Al, Ag) dots of 0.28 mm in diameter were deposited onto the BST film at room temperature through a shadow mask. The frequencydependence of the dielectric properties was measured using a HP4294A impedance analyzer over a frequency range of 100 Hz–2 MHz. The temperature-dependence of the dielectric properties was measured by a HP4284 LCR meter from 120–330 K in a computer-controlled Delta 9023 oven. The structure of the films was analyzed by a Rigaku D/MAX 3C XRD diffractometer using Cu Ka radiation at 40 kV. The surface morphology of the film was investigated using a Hitachi S-5750 scanning electron microscope. The film thickness was determined by an ET350 Talysurf profilometer (Kosaka Laboratory Ltd).
Fig. 1. XRD pattern for the BST film deposited in 1 Pa N2 atmosphere with a substrate temperature of 550 8C. The insert shows the surface morphology of the corresponding film.
corresponding to the sharp drops in the dielectric constant. And the temperature peaks at the loss shift to lower temperatures with decreasing frequency. The dielectric constant is larger than 2500 above 200 K at frequencies lower than 10 kHz, and is weakly temperature-dependent over the measured temperature range between 200 and 330 K. Such an abnormal phenomenon of dielectric response is obviously not a Curie–Weiss behavior as presented in a normal ferroelectric or ferroelectric-based relaxor. Instead, the dependence of dielectric properties on temperature under different frequencies are similar to that observed in the perovskite-related material CaCu3Ti4O12 (CCTO) [1,16,17] and non-perovskite material Li0.05Ti0.02Ni0.93O (LTNO) [2] and Ta2O5 [3], indicating that the dielectric anomaly in the Fig. 1 is not due to a thermally driven phase transition, but a thermally excited relaxation process [2,18]. An Arrhenius plot of the dielectric relaxation rate as a function of inverse temperature is shown in the inset of Fig. 2(a), which is obtained from the dissipation
3. Results and discussion Fig. 1 shows the XRD pattern of the as-deposited BST film with a thickness of 331 nm. It can be observed that the BST film shows (110) and (211) peaks, which match the characteristic of the corresponding BST ceramics. Under a temperature of 550 8C, the crystallization of the BST film is very difficult [12,15], so the intensities of (110) and (211) peaks are weak and broad. The surface morphology of the film is shown in the inset of Fig. 1, which is quite smooth with small grains, indicating the poor crystallization of the film, and is consistent with the XRD results. Fig. 2(a) and (b) presents the temperature-dependence of the dielectric constant and loss, respectively, under different frequencies for the as-deposited Pt/BST/Pt capacitor. The dielectric constant decreases rapidly with decreasing temperature in the low temperature region. There show broad peaks in the loss for the different frequencies,
Fig. 2. The temperature dependence of (a) the dielectric constant and (b) the loss for the BST film at different frequencies. The insert presents the relaxation time t versus 1/T obtained from the maxima of the measured loss peaks (the separated squares). The dotted straight line is the fitted curve.
Q. Tang et al. / Solid State Communications 135 (2005) 707–710
peaks. According to the Debye model, the dielectric relaxation time t can be expressed as: U t Z t0 exp kB T Where UZ0.20 eV is the activation energy required for the relaxation and t0Z5.08!10K10 second is a preexponential factor. The calculated data based on above equation are fitted well with the experimental ones, as shown in the insert of Fig. 2(a), demonstrating that the anomalous dielectric behavior below 200 K in present BST films can well be characterized by a thermally excited relaxation process. In order to understand the origin of the anomalous behavior of the BST films prepared in low-pressure N2 atmosphere at 550 8C, we added a dc bias on top electrode during the measurements on the frequency-dependent dielectric constant. The results are shown in Fig. 3. We found that the dielectric constant below 10 kHz can be enhanced by either negative or positive dc bias. The larger the dc voltage is, the larger the dielectric enhancement can be obtained. We also note that the process of the dielectric enhancement is reversible as shown in the inset of Fig. 3. The dielectric constant slowly reduces back to their original values after the dc bias is turned off. It was found that the dielectric loss has a similar behavior. These results indicate that there are mobile charges in the BST film, and the film behaves as a semiconductor rather than an insulator. When the film was deposited or annealed at nitrogen, it is generally believed that oxygen vacancies are produced by the following process: Oo ðoxygen ion at its normal siteÞ/ VoCCðoxygen vacancyÞ C 2eK C 1=2O2
Fig. 3. Change of frequency characteristics of dielectric constant in BST film under different dc bias. The insert shows the change of frequency characteristics of dielectric constant in BST film at different time after the dc bias (K500 mV) being turned off.
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We suggest that the mobile charges in the present BST film are related to the oxygen vacancies and/or the defects with the incorporation of nitrogen in the BST. If the concentration of the mobile space charges acting as dopants is sufficiently high so that the depletion region of the Schottky barrier is much thinner than the film thickness, the measured capacitance will be dominated by that of the Schottky barrier. Here we performed two crucial experiments to check for the electrode contributions, namely measurements of a capacitor with varying contact material and with varying film thickness [5]. Fig. 4 shows the dielectric properties as a function of frequency for the samples with different types of top contacts. It can be found that the dielectric constant is very sensitive to the top contacts. When the contacts of Al and Ag with low work functions (4.28 eV for Al, and 4.26 eV for Ag) are used, the values of the dielectric constant in the testing frequency range are significantly lowered, comparing with those with Pt contact which has a much higher work function of 5.65 eV. This finding clearly proves that the high dielectric constant is related to the contact contributions. To further corroborate this finding, we also tested the capacitance of the samples with different thicknesses, as presented in the insert of Fig. 4. It can be found that the capacitance is nearly independent of the film thicknesses. This again demonstrates that the large enhancement in present Pt/BST/Pt capacitance can not be attributed to a property of the bulk BST material described by a dielectric constant, instead, originates from a surface effect at the interface between the Pt electrode and the BST surface.
4. Conclusion BST film with high room temperature dielectric constant
Fig. 4. Frequency-dependent (a) dielectric constant and (b) dielectric loss at room temperature of BST film, coated with (1) platinum, (2) aluminum, and (3) silver top contacts. The insert shows the frequency-dependence of the Pt/BST/Pt capacitances with two different thicknesses.
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of 2500 at 10 kHz is prepared on Pt/Ti/SiO2/Si (100) substrate by PLD at 550 8C in N2 atmosphere. The temperature and frequency-dependent dielectric response of the film is very similar to that reported for the so-called ‘colossal’ dielectric constant material, such as CaCu3Ti4O12. Through the measurements of the dielectric properties under dc bias, and with varying contact materials and film thicknesses, the high dielectric constant for the BST film is ascribed to the formation of the Schottky barrier between the metallic electrode and the film surface, and the existence of the mobile charges in the film.
Acknowledgements This research was sponsored by Natural Science Foundation of China (Grant No. 10204016).
References [1] C.C. Homes, T. Vogt, S.M. Shapiro, S. Wakimoto, A.P. Ramirez, Science 293 (2001) 673. [2] J. Wu, C.W. Nan, Y.H. Lin, Y. Deng, Phys. Rev. Lett. 89 (2002) 217601. [3] N.N. Losad, G.J. Ruis, E.V. Morks, A.F. Morpurgo, J. Appl. Phys. 95 (2004) 8087.
[4] T.B. Adams, D.C. Sinclair, A.R. West, Adv. Mater. 14 (2002) 1321. [5] P. Lunkenheimer, R. Fichtl, S.G. Ebbinghaus, A. Loidl, Phys. Rev. B 70 (2004) 172102. [6] A.J. Moulson, J.M. Herbert, Electroceramics: Materials, Properties and Applications, Chapman and Hall, London, 1990. [7] S. Waku, Rev. Electr. Commun. Lab. 15 (1967) 689. [8] S. Waku, M. Uchdata, K. Kiuch, Rev. Electr. Commun. Lab. 18 (1970) 681. [9] M. Fujimoto, W.D. Kingery, J. Am. Ceram. Soc. 68 (1985) 169. [10] B.S. Chiou, S.T. Lin, J.G. Duh, P.H. Chang, J. Am. Ceram. Soc. 72 (1989) 1967. [11] C. Yang, Jpn. J. Appl. Phys. 36 (1997) 188. [12] L. Goux, M. Gervais, A. Catherinot, F. Sabary, J. Non-Cryst. Solids 303 (2002) 194. [13] X. Zhu, N. Chong, H.L. Chan, C. Choy, K. Wong, Appl. Phys. Lett. 80 (2002) 3376. [14] B. Su, J.E. Holmes, C. Meggs, T.W. Button, J. Eur. Ceram. Soc. 23 (2003) 2699. [15] S. Saha, S.B. Krupanidhi, J. Appl. Phys. 88 (2000) 3506. [16] L. Fang, M.R. Shen, Thin Solid Films 440 (2003) 60. [17] W. Si, E.M. Cruz, P.D. Johnson, P.W. Barnes, P. Woodward, A.P. Ramirez, Appl. Phys. Lett. 81 (2002) 2056. [18] A.P. Ramirez, M.A. Subramanian, M. Gardela, G. Blumberg, D. Li, T. Vogt, S.M. Shapiro, Solid State Commun. 115 (2000) 217.
Anomalous dielectric properties in (BaSr)TiO3 films fabricated by pulsed-laser deposition in N2 atmosphere Qiuwen Tang, Mingrong Shen*, Liang Fang Department of Physics, Jiangsu Key Laboratory of Thin Films, Suzhou University, Suzhou 215006, People’s Republic of China Received 6 April 2005; accepted 4 June 2005 by A. Pinczuk Available online 21 June 2005
Abstract A high dielectric constant of 2500 at 10 kHz near room temperature was observed in (Ba0.5Sr0.5)TiO3 (BST) films prepared by pulsed-laser deposition (PLD) at 550 8C in N2 atmosphere. The dielectric constant is weakly temperature-dependent above 200 K. However, there is about one hundred fold drop below 200 K, which can be characterized by a thermally excited relaxation process. The temperature and frequency-dependent dielectric response of the film is very similar to that reported for the so-called ‘colossal’ dielectric constant material, such as CaCu3Ti4O12. The dielectric properties are sensitive to the metal contacts with different work functions, however insensitive to the film thicknesses. Such an anomalous dielectric response for the BST films is ascribed to the formation of the Schottky barrier between the metallic electrode and the film surface, and the existence of the mobile charges in the film as indicated by the frequency characteristics of dielectric constant changing as the applied dc bias. q 2005 Elsevier Ltd. All rights reserved. PACS: 77.55.Cf; 81.15.Fg Keywords: A. Thin films; B. Laser processing; D. Dielectric response
1. Introduction The materials with high dielectric constant have received ever-increasing attention for their uses in important microelectronic devices such as capacitors and memory devices. Recently, perovskite-related material CaCu3Ti4O12 (CCTO) [1] and non-perovskite material Li0.05Ti0.02Ni0.93O (LTNO) [2] and Ta2O5 [3] have been reported for their unusual high dielectric constant of w103–105 at room temperature and very small temperature-dependence near room temperature. These materials show a similar dielectric response at low temperature, etc, a strong frequency-dependent steep decrease of the dielectric constant by two or three orders of magnitude and the
* Corresponding author. Tel.: C86 512 6511 2251; fax: C86 512 6511 2597. E-mail address: [email protected] (M. Shen).
0038-1098/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2005.06.007
associated peaks in the dielectric loss. The mechanism of such an anomalous dielectric response in these materials is mainly ascribed to the extrinsic effect including the formation of internal boundary-layer capacitors (IBLCs) [4] and interfacial space charge polarization between the metallic electrodes and film surfaces [5]. Most commercial IBLCs are based on (Ba,Sr)TiO3 (BST) ceramics [6–12], which are generally prepared in an O2 atmosphere and show a normal ferroelectric phase transition, which can be fitted by a Curie–Weis law [13,14]. The dielectric constant of these films is usually about 1000. In this paper we show that, depending on the experimental conditions in pulsed-laser deposition (PLD), conventional perovskite material BST films exhibit an anomalous dielectric response similar to that observed in the materials mentioned above. The dielectric constant of more than 2500 at 10 kHz near room temperature was observed in BST film prepared in 1 Pa N2 atmosphere at a relative low
708
Q. Tang et al. / Solid State Communications 135 (2005) 707–710
temperature of 550 8C. We investigate the frequency, temperature, thickness and electrode dependence of the dielectric response of the BST films, and conclude that the anomalous dielectric response is closely related to the mobile charges in the films and the electrode polarization effect.
2. Experimental details BST films were deposited by a 248-nm KrF excimer laser (Lambda Physik 105i) with the repetition rate of 5 Hz. The laser beam was focused on a BST ceramic target through a quartz window with the energy density of 1.6 J cmK2. Ceramic target with 2 cm in diameter was prepared by a conventional powder processing route. In this experiment, Pt/Ti/SiO2/Si (100) substrate was selected to place parallel to the target at a distance of 4.0 cm. Prior to the deposition, the chamber was firstly pumped down to 2.66!10K3 Pa, and then high purity nitrogen gas was introduced into the chamber. During the deposition, the substrate temperature was heated to 550 8C and the nitrogen pressure is controlled at 1 Pa. For carrying out the electrical measurement, different metal (Pt, Al, Ag) dots of 0.28 mm in diameter were deposited onto the BST film at room temperature through a shadow mask. The frequencydependence of the dielectric properties was measured using a HP4294A impedance analyzer over a frequency range of 100 Hz–2 MHz. The temperature-dependence of the dielectric properties was measured by a HP4284 LCR meter from 120–330 K in a computer-controlled Delta 9023 oven. The structure of the films was analyzed by a Rigaku D/MAX 3C XRD diffractometer using Cu Ka radiation at 40 kV. The surface morphology of the film was investigated using a Hitachi S-5750 scanning electron microscope. The film thickness was determined by an ET350 Talysurf profilometer (Kosaka Laboratory Ltd).
Fig. 1. XRD pattern for the BST film deposited in 1 Pa N2 atmosphere with a substrate temperature of 550 8C. The insert shows the surface morphology of the corresponding film.
corresponding to the sharp drops in the dielectric constant. And the temperature peaks at the loss shift to lower temperatures with decreasing frequency. The dielectric constant is larger than 2500 above 200 K at frequencies lower than 10 kHz, and is weakly temperature-dependent over the measured temperature range between 200 and 330 K. Such an abnormal phenomenon of dielectric response is obviously not a Curie–Weiss behavior as presented in a normal ferroelectric or ferroelectric-based relaxor. Instead, the dependence of dielectric properties on temperature under different frequencies are similar to that observed in the perovskite-related material CaCu3Ti4O12 (CCTO) [1,16,17] and non-perovskite material Li0.05Ti0.02Ni0.93O (LTNO) [2] and Ta2O5 [3], indicating that the dielectric anomaly in the Fig. 1 is not due to a thermally driven phase transition, but a thermally excited relaxation process [2,18]. An Arrhenius plot of the dielectric relaxation rate as a function of inverse temperature is shown in the inset of Fig. 2(a), which is obtained from the dissipation
3. Results and discussion Fig. 1 shows the XRD pattern of the as-deposited BST film with a thickness of 331 nm. It can be observed that the BST film shows (110) and (211) peaks, which match the characteristic of the corresponding BST ceramics. Under a temperature of 550 8C, the crystallization of the BST film is very difficult [12,15], so the intensities of (110) and (211) peaks are weak and broad. The surface morphology of the film is shown in the inset of Fig. 1, which is quite smooth with small grains, indicating the poor crystallization of the film, and is consistent with the XRD results. Fig. 2(a) and (b) presents the temperature-dependence of the dielectric constant and loss, respectively, under different frequencies for the as-deposited Pt/BST/Pt capacitor. The dielectric constant decreases rapidly with decreasing temperature in the low temperature region. There show broad peaks in the loss for the different frequencies,
Fig. 2. The temperature dependence of (a) the dielectric constant and (b) the loss for the BST film at different frequencies. The insert presents the relaxation time t versus 1/T obtained from the maxima of the measured loss peaks (the separated squares). The dotted straight line is the fitted curve.
Q. Tang et al. / Solid State Communications 135 (2005) 707–710
peaks. According to the Debye model, the dielectric relaxation time t can be expressed as: U t Z t0 exp kB T Where UZ0.20 eV is the activation energy required for the relaxation and t0Z5.08!10K10 second is a preexponential factor. The calculated data based on above equation are fitted well with the experimental ones, as shown in the insert of Fig. 2(a), demonstrating that the anomalous dielectric behavior below 200 K in present BST films can well be characterized by a thermally excited relaxation process. In order to understand the origin of the anomalous behavior of the BST films prepared in low-pressure N2 atmosphere at 550 8C, we added a dc bias on top electrode during the measurements on the frequency-dependent dielectric constant. The results are shown in Fig. 3. We found that the dielectric constant below 10 kHz can be enhanced by either negative or positive dc bias. The larger the dc voltage is, the larger the dielectric enhancement can be obtained. We also note that the process of the dielectric enhancement is reversible as shown in the inset of Fig. 3. The dielectric constant slowly reduces back to their original values after the dc bias is turned off. It was found that the dielectric loss has a similar behavior. These results indicate that there are mobile charges in the BST film, and the film behaves as a semiconductor rather than an insulator. When the film was deposited or annealed at nitrogen, it is generally believed that oxygen vacancies are produced by the following process: Oo ðoxygen ion at its normal siteÞ/ VoCCðoxygen vacancyÞ C 2eK C 1=2O2
Fig. 3. Change of frequency characteristics of dielectric constant in BST film under different dc bias. The insert shows the change of frequency characteristics of dielectric constant in BST film at different time after the dc bias (K500 mV) being turned off.
709
We suggest that the mobile charges in the present BST film are related to the oxygen vacancies and/or the defects with the incorporation of nitrogen in the BST. If the concentration of the mobile space charges acting as dopants is sufficiently high so that the depletion region of the Schottky barrier is much thinner than the film thickness, the measured capacitance will be dominated by that of the Schottky barrier. Here we performed two crucial experiments to check for the electrode contributions, namely measurements of a capacitor with varying contact material and with varying film thickness [5]. Fig. 4 shows the dielectric properties as a function of frequency for the samples with different types of top contacts. It can be found that the dielectric constant is very sensitive to the top contacts. When the contacts of Al and Ag with low work functions (4.28 eV for Al, and 4.26 eV for Ag) are used, the values of the dielectric constant in the testing frequency range are significantly lowered, comparing with those with Pt contact which has a much higher work function of 5.65 eV. This finding clearly proves that the high dielectric constant is related to the contact contributions. To further corroborate this finding, we also tested the capacitance of the samples with different thicknesses, as presented in the insert of Fig. 4. It can be found that the capacitance is nearly independent of the film thicknesses. This again demonstrates that the large enhancement in present Pt/BST/Pt capacitance can not be attributed to a property of the bulk BST material described by a dielectric constant, instead, originates from a surface effect at the interface between the Pt electrode and the BST surface.
4. Conclusion BST film with high room temperature dielectric constant
Fig. 4. Frequency-dependent (a) dielectric constant and (b) dielectric loss at room temperature of BST film, coated with (1) platinum, (2) aluminum, and (3) silver top contacts. The insert shows the frequency-dependence of the Pt/BST/Pt capacitances with two different thicknesses.
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Q. Tang et al. / Solid State Communications 135 (2005) 707–710
of 2500 at 10 kHz is prepared on Pt/Ti/SiO2/Si (100) substrate by PLD at 550 8C in N2 atmosphere. The temperature and frequency-dependent dielectric response of the film is very similar to that reported for the so-called ‘colossal’ dielectric constant material, such as CaCu3Ti4O12. Through the measurements of the dielectric properties under dc bias, and with varying contact materials and film thicknesses, the high dielectric constant for the BST film is ascribed to the formation of the Schottky barrier between the metallic electrode and the film surface, and the existence of the mobile charges in the film.
Acknowledgements This research was sponsored by Natural Science Foundation of China (Grant No. 10204016).
References [1] C.C. Homes, T. Vogt, S.M. Shapiro, S. Wakimoto, A.P. Ramirez, Science 293 (2001) 673. [2] J. Wu, C.W. Nan, Y.H. Lin, Y. Deng, Phys. Rev. Lett. 89 (2002) 217601. [3] N.N. Losad, G.J. Ruis, E.V. Morks, A.F. Morpurgo, J. Appl. Phys. 95 (2004) 8087.
[4] T.B. Adams, D.C. Sinclair, A.R. West, Adv. Mater. 14 (2002) 1321. [5] P. Lunkenheimer, R. Fichtl, S.G. Ebbinghaus, A. Loidl, Phys. Rev. B 70 (2004) 172102. [6] A.J. Moulson, J.M. Herbert, Electroceramics: Materials, Properties and Applications, Chapman and Hall, London, 1990. [7] S. Waku, Rev. Electr. Commun. Lab. 15 (1967) 689. [8] S. Waku, M. Uchdata, K. Kiuch, Rev. Electr. Commun. Lab. 18 (1970) 681. [9] M. Fujimoto, W.D. Kingery, J. Am. Ceram. Soc. 68 (1985) 169. [10] B.S. Chiou, S.T. Lin, J.G. Duh, P.H. Chang, J. Am. Ceram. Soc. 72 (1989) 1967. [11] C. Yang, Jpn. J. Appl. Phys. 36 (1997) 188. [12] L. Goux, M. Gervais, A. Catherinot, F. Sabary, J. Non-Cryst. Solids 303 (2002) 194. [13] X. Zhu, N. Chong, H.L. Chan, C. Choy, K. Wong, Appl. Phys. Lett. 80 (2002) 3376. [14] B. Su, J.E. Holmes, C. Meggs, T.W. Button, J. Eur. Ceram. Soc. 23 (2003) 2699. [15] S. Saha, S.B. Krupanidhi, J. Appl. Phys. 88 (2000) 3506. [16] L. Fang, M.R. Shen, Thin Solid Films 440 (2003) 60. [17] W. Si, E.M. Cruz, P.D. Johnson, P.W. Barnes, P. Woodward, A.P. Ramirez, Appl. Phys. Lett. 81 (2002) 2056. [18] A.P. Ramirez, M.A. Subramanian, M. Gardela, G. Blumberg, D. Li, T. Vogt, S.M. Shapiro, Solid State Commun. 115 (2000) 217.