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Ferroelectric domain configuration and piezoelectric responses in (001)-oriented PMN-PT films
Materials Characterization 48 (2002) 215 – 220
Ferroelectric domain configuration and piezoelectric responses in (001)-oriented PMN-PT films J. Wanga,*, E.Z. Luob, K.H. Wonga, H.L.W. Chana, J.B. Xub, I.H. Wilsonb, C.L. Choyb a
Department of Applied Physics and Materials Research Centre, The Hong Kong Polytechnic University, Hung Hum, Kowloon, Hong Kong, People’s Republic of China b Electronic Engineering Department and Materials Science and Technology Research Center, The Chinese University of Hong Kong, N.T., Hong Kong, People’s Republic of China Received 1 June 2001; accepted 30 September 2001
Abstract (1 x)Pb(Mg1/3Nb2/3) xPbTiO3 (x = 0.3 – 0.35) ferroelectric thin films were prepared by pulsed laser deposition on (001) single crystal LaAlO3 substrates coated with YBa2Cu3O7 electrodes. X-ray diffraction (XRD) measurements showed the as-grown lead magnesium niobate (PMN)-lead titanate (PT) films are epitaxial and have a (001)-oriented perovskite structure. A scanning force microscope (SFM) was employed to probe the domain configuration and piezoelectric response of the films. A comparison of the topographic and domain images showed that the large grains possessed a polydomain configuration. The average value of the piezoresponse signal implied that there was a net spontaneous polarization pointing towards the bottom electrode in the as-grown films, presumably due to the presence of an internal field. The piezo-response hysteresis loop obtained while the films were subjected to a dc bias voltage exhibited a behavior that was dependent on the PbTiO3 content, indicating that the mechanism behind the field-induced strain depended on the composition and structure. D 2002 Elsevier Science Inc. All rights reserved. Keywords: Ferroelectrics; Films; PMN-PT; Domain
1. Introduction The solid solution of lead magnesium niobate (PMN) and lead titanate (PT) is of great importance for numerous technical applications including high-k capacitors, sensors and actuators. In particular, the very high piezoelectric coefficient d33 and electromechanical coupling coefficient recently found in [001]-oriented PMN-PT single crystals makes the materials very promising for applications in micro* Corresponding author. Tel.: +852-2766-4616; fax: +852-2333-7629. E-mail address: [email protected] (J. Wang).
electromechanical systems (MEMS). However, the preparation of PMN-PT thin films still faces many challenges. On one hand, control of the structure and stoichiometry are crucial to obtain good properties. On the other hand, the film properties are also strongly influenced by interface and substrate effects. The characterization of the local ferroelectric and piezoelectric properties at a nanoscale will provide an effective tool to achieve not only insights into fundamental issues such as polarization reversal, domain switching and domain structure but also an optimization of film properties. Recent investigations by various groups [1 – 5] have shown that a contact mode scanning force
1044-5803/02/$ – see front matter D 2002 Elsevier Science Inc. All rights reserved. PII: S 1 0 4 4 - 5 8 0 3 ( 0 2 ) 0 0 2 4 2 - 5
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J. Wang et al. / Materials Characterization 48 (2002) 215–220
microscope (SFM), when combined with the lock-in technique, can be successfully employed to study the domain configuration, domain switching and local piezoelectric properties of thin films. So far, most of the reports have focused on ferroelectric Pb(Zr0.52Ti0.48)TiO3 (PZT) and SrBi2Ta2O9 (SBT) films. Little work has been done on relaxor ferroelectric thin films although these materials exhibit very attractive properties for potential applications in MEMS. We have successfully prepared (001)oriented PMN-PT thin films on conducting oxide electrodes [6]. In this paper, for the first time, ferroelectric domain imaging and local piezoelectric properties are reported for relaxor ferroelectric PMN-PT thin films.
2. Experimental procedures 2.1. Sample preparation and characterization The epitaxial heterostructures PMN-PT/YBCO/ LAO were prepared by pulsed laser deposition using a KrF excimer laser (COMPex 200, Lambda Physik) operating at 248 nm, as described previously [6]. The structure of the thin films was characterized using a Philips X’pert X-ray diffractometer (XRD). 2.2. Piezoelectric mode SFM measurements The principle of the piezoelectric SFM is based on the detection of the local vibration of a ferroelectric material when an ac electrical signal is applied between the conducting tip of the SFM and the bottom electrode of the material. According to the inverse piezoelectric effect, this will result in a change in the thickness of the material. The region under the tip will expand or contract, depending on its polarization state. Using a lock-in technique, the vibration of the SFM tip can be detected. The amplitude (A) of this vibration signal is proportional to the magnitude of the piezoelectric coefficient d33, while the phase difference F between the applied voltage and the detected signal is related to the polarization direction. Regions with opposite polarization directions will have a phase difference of 180. The piezo-response signal, defined as the product of A and cos F, can be processed to an image by scanning the tip across the sample surface. It is well known that, for a ferroelectric material having centrosymmetry in the paraelectric state, the piezoelectric effect can be regarded as an electrostriction biased by the spontaneous polarization P, as given by the formula: d33 ¼ 2Qe33 P
ð1Þ
where Q is the electrostriction coefficient and e33 is the permittivity (Eq. (1)). Therefore, a piezo-response image corresponds to a domain pattern image if the material is ferroelectric. In our experiment, a commercial SFM (Digital Instruments Nanoscope IIIa) was used and a small ac signal of frequency 10 kHz was applied through the TiN-coated conducting tip. The output signal was detected by a lock-in amplifier (EG&G 5210). The functional generator (HP3314A) employed can deliver an ac voltage and a dc voltage simultaneously. If the tip is fixed at one point and the measurement is performed as a function of the dc bias voltage, the so-called piezoelectric hysteresis loop (d33 loop) is obtained. In this measurement, the ac signal is fixed at 0.5 V in amplitude, which is well within the linear part of displacement – ac voltage curve. The measurement can be carried out by placing the tip either on the sample surface or on the top electrode of the sample. In the former case, the tip itself serves as the top electrode. This approach has the advantage that the observed piezoelectric properties can be related to the local domain structure. However, the nonuniform field distribution and strong electrostatic interaction between the tip and the sample surface may lead to strong disturbance of the response. In the present work, the piezoelectric hysteresis loop was determined by placing the tip on the top electrode and averaging the results obtained for several cycles.
3. Results and discussion Thin films of (1 x)PMN xPT (x = 0.3, 0.35) were deposited on YBCO/LAO substrates at temperatures between 600 and 650 C and at an ambient oxygen pressure of around 200 mTorr. They showed ferroelectric behavior with a remnant polarization of Pr = 10 C/cm2. Fig. 1 shows a typical XRD q – 2q scan for the 0.65 PMN – 0.35 PT/YBCO/LAO heterostructure. It can be seen that the PMN-PT film has a perovskite structure and is (001) oriented. The Fscans of the PMN-PT film and LAO substrate are shown in the inset. The fourfold symmetry and the matching of the diffraction peaks of PMN-PT and LAO clearly reveal the epitaxial nature of the PMNPT film. Fig. 2a shows the topographic image of a 400nm-thick 0.65 PMN – 0.35 PT film deposited on a YBCO/LAO substrate. Crystalline grains having an average size of 400 nm are clearly seen. The corresponding piezo-response image (Fig. 2b) shows the domain configuration in the same region. Bright and dark regions correspond to regions with polarization pointing towards the top surface and
J. Wang et al. / Materials Characterization 48 (2002) 215–220
217
Fig. 1. XRD q – 2q scan of the thin film heterostructure PMN-PT/YBCO/LAO. The inset shows the f-scan of the (220) peak of PMN-PT and LAO. The symbols P, Y and L denote the diffraction peaks of 0.65 PMN-0.35 PT, YBCO and LAO, respectively.
bottom electrodes, respectively. Fig. 2c shows the variation of the electric signal along the line drawn
on Fig. 2b. The signal is largely negative, indicating the presence of an internal field. The two
Fig. 2. SFM images of a 0.65 PMN-0.35 PT thin film. (a) topographic image, (b) piezo-response image, (c) electrical signal along the line shown in (b), (d) piezo-response after applying + 10 V dc voltage to the bottom electrode.
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J. Wang et al. / Materials Characterization 48 (2002) 215–220
arrows pointing to the bright and dark areas in Fig. 2b correspond to points with near zero and very negative electrical signals, respectively (see Fig. 2c). In these two areas, the polarizations point two different directions. As this domain image is compared with the topographic one, we can see that many of the grains possess more than one domain. In most cases, each grain contains at least two domains with opposite polarizations and separated by a well-defined U-shape domain wall. Fig. 2d shows the piezo-response image of the same region after applying a positive dc voltage of 10 V to the bottom electrode. We see that many of the dark regions turn white, indicating a change of the polarization vector from the downward to the upward direction. This is a direct evidence of domain switching under an external dc voltage. This confirms again that the piezo-response image reflects the polarization states of the ferroelectric domains. We can see from Fig. 2d that the gray scale of some dark regions remained unchanged after the dc poling. There are at least two mechanisms that may prevent the polar domains from switching: the depolarizing field and the elastic field. These two mechanisms have been proposed by Gruverman et al. [3] to explain the ferroelectric retention behavior in the PZT films. In the present case, the second mechanism seems more plausible because, in our epitaxial films, the lattice mismatch between PMN-PT and the substrate is about 5.8%, such that a large lattice mismatch is expected to result in a significant in-plane compressive stress on the film. Nagarajan et al. [7] reported that PMN-PT epitaxial films are highly stressed due to the lattice mismatch with the substrates. Fig. 3 shows the topographic and domain images of a (1 x)PMN xPT film with a slightly different composition (x = 0.3). This composition is close to
the morphotropic boundary but is still on the rhombohedral side. The domain patterns of pseudocubic (001)-oriented epitaxial films with rhombohedral structure have been analyzed in detailed by Streiffer et al. [8]. They predicted that lamellar-shaped domain patterns would develop along the {100} or {101} domain boundaries. Here, the elongated dark domain patterns in Fig. 3b seem to be consistent with that kind of lamellar structure. The d33 loop for both samples was measured as a function of bias dc voltage. Measurements with the tip as the top electrode were not successful and this was probably due to the nonuniform distribution of the electrical field underneath the tip, as suggested by Ganuple et al. [9], or to electrostatic interaction between the tip and the film surface [2]. Using films with Au top electrodes, d33 hysteresis loops were successfully obtained and the results for films of x = 0.35 and 0.3 are shown in Fig. 4a and b, respectively. The most striking feature is the difference between the shape of these two curves. For the film with x = 0.35, which consists of both tetragonal and rhombohedral phases, d33 increases steadily with increasing bias field and hardly reaches saturation at the highest applied field (25 MV/m). This type of behavior has been previously observed in SBT thin films [9]. The continued increase of d33 with bias field indicates that the field-induced strain is mainly contributed by the lattice displacement and not by the domain wall motion. On the contrary, for the film with x = 0.3, a butterfly-shaped loop is observed (Fig. 4b). The piezoelectric coefficient d33 increases very quickly to a maximum and then decreases slightly. This behavior was also observed previously in Nbdoped PZT [9] and Ca-doped PT [10]. The smaller coercive voltage indicates that domain wall motion is easier in this sample. The saturation of the polarization and a decrease in the permittivity under the bias field may be the main reason for the decrease in d33
Fig. 3. SFM images of a 0.7 PMN-0.3 PT thin film. (a) topographic image, (b) piezo-response image.
J. Wang et al. / Materials Characterization 48 (2002) 215–220
219
Fig. 4. The piezoelectric coefficient d33 of PMN/PT films as a function of dc bias voltage. (a) PMN/PT = 65:35, (b) PMN/ PT = 70:30.
after reaching its maximum, as suggested by Kholkin et al. [10].
Polytechnic University. This work is partly supported by Hong Kong RGC (ref. CUHK4373/99E).
4. Conclusions References (1) Imaging of domains by conducting tip SFM shows that most of the grains in PMN-PT films have a multidomain configuration with domain splitting in the grains. (2) Different domain configurations were observed in PMN-PT thin films, depending on the crystal structure of these films. In particular, lamellarshaped domain patterns were observed in (001)oriented rhombohedral PMN-PT thin films, in agreement with theoretical prediction. (3) The d33 curves of PMN-PT films exhibit hysteresis behavior, reflecting the ferroelectric nature of these films. The variation with composition of the shape of the curves indicates that different mechanisms are responsible for the field-induced strains.
Acknowledgements The authors acknowledge the financial support of the Centre for Smart Materials of The Hong Kong
[1] Hong S, Colla EL, Kim E, Taylor DV, Tagantsev AK, Muralt P, No K, Setter N. High resolution study of domain nucleation and growth during polarization switching in Pb(Zr,Ti)O3 ferroelectric thin film capacitors. J Appl Phys 1999;86(1):607 – 13. [2] Christman JA, Woolcott RR, Kingon AI, Nemanich RJ. Piezoelectric measurements with atomic force microscopy. Appl Phys Lett 1998;73(26):3851 – 3. [3] Gruverman A, Tokumoto H, Prakash AS, Aggarwal S, Yang B, Wuttig M, Ramesh R, Auciello O, Venkatesan T. Nanoscale imaging of domain dynamics and retention in ferroelectric thin films. Appl Phys Lett 1997; 71(24):3492 – 4. [4] Hu GD, Xu JB, Wilson IH. Domain imaging and local piezoelectric properties of the (200)-predominant SrBi2Ta2O9 film. Appl Phys Lett 1999;75(11):1610 – 2. [5] Zavala G, Fendler J, Trolier-McKinstry S. Characterization of ferroelectric lead zirconate titanate films by scanning force microscopy. J Appl Phys 1997;81(11): 7480 – 91. [6] Wang J, Wong KH, Chan HLW, Choy CL. Ferroelectrics 2001;252(11):257 – 64. [7] Nagarajan V, Alpay SP, Ganpule CS, Nagarai BK, Ag-
220
J. Wang et al. / Materials Characterization 48 (2002) 215–220
garwal S, Williams ED, Roytburd AL, Ramesh R. Appl Phys Lett 2000;77(3):438 – 40. [8] Streiffer SK, Parker CB, Romanov AE, Lefevre MJ, Zhao L, Speck JS, Pompe W, Foster CM, Bai GR. Domain patterns in epitaxial rhombohedral ferroelectric films: 1. Geometry and experiments. J Appl Phys 1998;83(5):2742 – 53. [9] Ganpule CS, Stanishevsky A, Aggarwal S, Melngailis J,
Williams E, Ramesh R, Joshi V, De Araujo CP. Scaling of ferroelectric and piezoelectric properties in Pt/ SrBi 2 Ta 2 O 9 /Pt thin films. Appl Phys Lett 2000; 75(24):3874 – 6. [10] Kholkin AL, Calzada ML, Ramos P, Mendiola J, Setter N. Piezoelectric properties of Ca-modified PbTiO3 films. Appl Phys Lett 1996;69(23):3602 – 4.
Ferroelectric domain configuration and piezoelectric responses in (001)-oriented PMN-PT films J. Wanga,*, E.Z. Luob, K.H. Wonga, H.L.W. Chana, J.B. Xub, I.H. Wilsonb, C.L. Choyb a
Department of Applied Physics and Materials Research Centre, The Hong Kong Polytechnic University, Hung Hum, Kowloon, Hong Kong, People’s Republic of China b Electronic Engineering Department and Materials Science and Technology Research Center, The Chinese University of Hong Kong, N.T., Hong Kong, People’s Republic of China Received 1 June 2001; accepted 30 September 2001
Abstract (1 x)Pb(Mg1/3Nb2/3) xPbTiO3 (x = 0.3 – 0.35) ferroelectric thin films were prepared by pulsed laser deposition on (001) single crystal LaAlO3 substrates coated with YBa2Cu3O7 electrodes. X-ray diffraction (XRD) measurements showed the as-grown lead magnesium niobate (PMN)-lead titanate (PT) films are epitaxial and have a (001)-oriented perovskite structure. A scanning force microscope (SFM) was employed to probe the domain configuration and piezoelectric response of the films. A comparison of the topographic and domain images showed that the large grains possessed a polydomain configuration. The average value of the piezoresponse signal implied that there was a net spontaneous polarization pointing towards the bottom electrode in the as-grown films, presumably due to the presence of an internal field. The piezo-response hysteresis loop obtained while the films were subjected to a dc bias voltage exhibited a behavior that was dependent on the PbTiO3 content, indicating that the mechanism behind the field-induced strain depended on the composition and structure. D 2002 Elsevier Science Inc. All rights reserved. Keywords: Ferroelectrics; Films; PMN-PT; Domain
1. Introduction The solid solution of lead magnesium niobate (PMN) and lead titanate (PT) is of great importance for numerous technical applications including high-k capacitors, sensors and actuators. In particular, the very high piezoelectric coefficient d33 and electromechanical coupling coefficient recently found in [001]-oriented PMN-PT single crystals makes the materials very promising for applications in micro* Corresponding author. Tel.: +852-2766-4616; fax: +852-2333-7629. E-mail address: [email protected] (J. Wang).
electromechanical systems (MEMS). However, the preparation of PMN-PT thin films still faces many challenges. On one hand, control of the structure and stoichiometry are crucial to obtain good properties. On the other hand, the film properties are also strongly influenced by interface and substrate effects. The characterization of the local ferroelectric and piezoelectric properties at a nanoscale will provide an effective tool to achieve not only insights into fundamental issues such as polarization reversal, domain switching and domain structure but also an optimization of film properties. Recent investigations by various groups [1 – 5] have shown that a contact mode scanning force
1044-5803/02/$ – see front matter D 2002 Elsevier Science Inc. All rights reserved. PII: S 1 0 4 4 - 5 8 0 3 ( 0 2 ) 0 0 2 4 2 - 5
216
J. Wang et al. / Materials Characterization 48 (2002) 215–220
microscope (SFM), when combined with the lock-in technique, can be successfully employed to study the domain configuration, domain switching and local piezoelectric properties of thin films. So far, most of the reports have focused on ferroelectric Pb(Zr0.52Ti0.48)TiO3 (PZT) and SrBi2Ta2O9 (SBT) films. Little work has been done on relaxor ferroelectric thin films although these materials exhibit very attractive properties for potential applications in MEMS. We have successfully prepared (001)oriented PMN-PT thin films on conducting oxide electrodes [6]. In this paper, for the first time, ferroelectric domain imaging and local piezoelectric properties are reported for relaxor ferroelectric PMN-PT thin films.
2. Experimental procedures 2.1. Sample preparation and characterization The epitaxial heterostructures PMN-PT/YBCO/ LAO were prepared by pulsed laser deposition using a KrF excimer laser (COMPex 200, Lambda Physik) operating at 248 nm, as described previously [6]. The structure of the thin films was characterized using a Philips X’pert X-ray diffractometer (XRD). 2.2. Piezoelectric mode SFM measurements The principle of the piezoelectric SFM is based on the detection of the local vibration of a ferroelectric material when an ac electrical signal is applied between the conducting tip of the SFM and the bottom electrode of the material. According to the inverse piezoelectric effect, this will result in a change in the thickness of the material. The region under the tip will expand or contract, depending on its polarization state. Using a lock-in technique, the vibration of the SFM tip can be detected. The amplitude (A) of this vibration signal is proportional to the magnitude of the piezoelectric coefficient d33, while the phase difference F between the applied voltage and the detected signal is related to the polarization direction. Regions with opposite polarization directions will have a phase difference of 180. The piezo-response signal, defined as the product of A and cos F, can be processed to an image by scanning the tip across the sample surface. It is well known that, for a ferroelectric material having centrosymmetry in the paraelectric state, the piezoelectric effect can be regarded as an electrostriction biased by the spontaneous polarization P, as given by the formula: d33 ¼ 2Qe33 P
ð1Þ
where Q is the electrostriction coefficient and e33 is the permittivity (Eq. (1)). Therefore, a piezo-response image corresponds to a domain pattern image if the material is ferroelectric. In our experiment, a commercial SFM (Digital Instruments Nanoscope IIIa) was used and a small ac signal of frequency 10 kHz was applied through the TiN-coated conducting tip. The output signal was detected by a lock-in amplifier (EG&G 5210). The functional generator (HP3314A) employed can deliver an ac voltage and a dc voltage simultaneously. If the tip is fixed at one point and the measurement is performed as a function of the dc bias voltage, the so-called piezoelectric hysteresis loop (d33 loop) is obtained. In this measurement, the ac signal is fixed at 0.5 V in amplitude, which is well within the linear part of displacement – ac voltage curve. The measurement can be carried out by placing the tip either on the sample surface or on the top electrode of the sample. In the former case, the tip itself serves as the top electrode. This approach has the advantage that the observed piezoelectric properties can be related to the local domain structure. However, the nonuniform field distribution and strong electrostatic interaction between the tip and the sample surface may lead to strong disturbance of the response. In the present work, the piezoelectric hysteresis loop was determined by placing the tip on the top electrode and averaging the results obtained for several cycles.
3. Results and discussion Thin films of (1 x)PMN xPT (x = 0.3, 0.35) were deposited on YBCO/LAO substrates at temperatures between 600 and 650 C and at an ambient oxygen pressure of around 200 mTorr. They showed ferroelectric behavior with a remnant polarization of Pr = 10 C/cm2. Fig. 1 shows a typical XRD q – 2q scan for the 0.65 PMN – 0.35 PT/YBCO/LAO heterostructure. It can be seen that the PMN-PT film has a perovskite structure and is (001) oriented. The Fscans of the PMN-PT film and LAO substrate are shown in the inset. The fourfold symmetry and the matching of the diffraction peaks of PMN-PT and LAO clearly reveal the epitaxial nature of the PMNPT film. Fig. 2a shows the topographic image of a 400nm-thick 0.65 PMN – 0.35 PT film deposited on a YBCO/LAO substrate. Crystalline grains having an average size of 400 nm are clearly seen. The corresponding piezo-response image (Fig. 2b) shows the domain configuration in the same region. Bright and dark regions correspond to regions with polarization pointing towards the top surface and
J. Wang et al. / Materials Characterization 48 (2002) 215–220
217
Fig. 1. XRD q – 2q scan of the thin film heterostructure PMN-PT/YBCO/LAO. The inset shows the f-scan of the (220) peak of PMN-PT and LAO. The symbols P, Y and L denote the diffraction peaks of 0.65 PMN-0.35 PT, YBCO and LAO, respectively.
bottom electrodes, respectively. Fig. 2c shows the variation of the electric signal along the line drawn
on Fig. 2b. The signal is largely negative, indicating the presence of an internal field. The two
Fig. 2. SFM images of a 0.65 PMN-0.35 PT thin film. (a) topographic image, (b) piezo-response image, (c) electrical signal along the line shown in (b), (d) piezo-response after applying + 10 V dc voltage to the bottom electrode.
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J. Wang et al. / Materials Characterization 48 (2002) 215–220
arrows pointing to the bright and dark areas in Fig. 2b correspond to points with near zero and very negative electrical signals, respectively (see Fig. 2c). In these two areas, the polarizations point two different directions. As this domain image is compared with the topographic one, we can see that many of the grains possess more than one domain. In most cases, each grain contains at least two domains with opposite polarizations and separated by a well-defined U-shape domain wall. Fig. 2d shows the piezo-response image of the same region after applying a positive dc voltage of 10 V to the bottom electrode. We see that many of the dark regions turn white, indicating a change of the polarization vector from the downward to the upward direction. This is a direct evidence of domain switching under an external dc voltage. This confirms again that the piezo-response image reflects the polarization states of the ferroelectric domains. We can see from Fig. 2d that the gray scale of some dark regions remained unchanged after the dc poling. There are at least two mechanisms that may prevent the polar domains from switching: the depolarizing field and the elastic field. These two mechanisms have been proposed by Gruverman et al. [3] to explain the ferroelectric retention behavior in the PZT films. In the present case, the second mechanism seems more plausible because, in our epitaxial films, the lattice mismatch between PMN-PT and the substrate is about 5.8%, such that a large lattice mismatch is expected to result in a significant in-plane compressive stress on the film. Nagarajan et al. [7] reported that PMN-PT epitaxial films are highly stressed due to the lattice mismatch with the substrates. Fig. 3 shows the topographic and domain images of a (1 x)PMN xPT film with a slightly different composition (x = 0.3). This composition is close to
the morphotropic boundary but is still on the rhombohedral side. The domain patterns of pseudocubic (001)-oriented epitaxial films with rhombohedral structure have been analyzed in detailed by Streiffer et al. [8]. They predicted that lamellar-shaped domain patterns would develop along the {100} or {101} domain boundaries. Here, the elongated dark domain patterns in Fig. 3b seem to be consistent with that kind of lamellar structure. The d33 loop for both samples was measured as a function of bias dc voltage. Measurements with the tip as the top electrode were not successful and this was probably due to the nonuniform distribution of the electrical field underneath the tip, as suggested by Ganuple et al. [9], or to electrostatic interaction between the tip and the film surface [2]. Using films with Au top electrodes, d33 hysteresis loops were successfully obtained and the results for films of x = 0.35 and 0.3 are shown in Fig. 4a and b, respectively. The most striking feature is the difference between the shape of these two curves. For the film with x = 0.35, which consists of both tetragonal and rhombohedral phases, d33 increases steadily with increasing bias field and hardly reaches saturation at the highest applied field (25 MV/m). This type of behavior has been previously observed in SBT thin films [9]. The continued increase of d33 with bias field indicates that the field-induced strain is mainly contributed by the lattice displacement and not by the domain wall motion. On the contrary, for the film with x = 0.3, a butterfly-shaped loop is observed (Fig. 4b). The piezoelectric coefficient d33 increases very quickly to a maximum and then decreases slightly. This behavior was also observed previously in Nbdoped PZT [9] and Ca-doped PT [10]. The smaller coercive voltage indicates that domain wall motion is easier in this sample. The saturation of the polarization and a decrease in the permittivity under the bias field may be the main reason for the decrease in d33
Fig. 3. SFM images of a 0.7 PMN-0.3 PT thin film. (a) topographic image, (b) piezo-response image.
J. Wang et al. / Materials Characterization 48 (2002) 215–220
219
Fig. 4. The piezoelectric coefficient d33 of PMN/PT films as a function of dc bias voltage. (a) PMN/PT = 65:35, (b) PMN/ PT = 70:30.
after reaching its maximum, as suggested by Kholkin et al. [10].
Polytechnic University. This work is partly supported by Hong Kong RGC (ref. CUHK4373/99E).
4. Conclusions References (1) Imaging of domains by conducting tip SFM shows that most of the grains in PMN-PT films have a multidomain configuration with domain splitting in the grains. (2) Different domain configurations were observed in PMN-PT thin films, depending on the crystal structure of these films. In particular, lamellarshaped domain patterns were observed in (001)oriented rhombohedral PMN-PT thin films, in agreement with theoretical prediction. (3) The d33 curves of PMN-PT films exhibit hysteresis behavior, reflecting the ferroelectric nature of these films. The variation with composition of the shape of the curves indicates that different mechanisms are responsible for the field-induced strains.
Acknowledgements The authors acknowledge the financial support of the Centre for Smart Materials of The Hong Kong
[1] Hong S, Colla EL, Kim E, Taylor DV, Tagantsev AK, Muralt P, No K, Setter N. High resolution study of domain nucleation and growth during polarization switching in Pb(Zr,Ti)O3 ferroelectric thin film capacitors. J Appl Phys 1999;86(1):607 – 13. [2] Christman JA, Woolcott RR, Kingon AI, Nemanich RJ. Piezoelectric measurements with atomic force microscopy. Appl Phys Lett 1998;73(26):3851 – 3. [3] Gruverman A, Tokumoto H, Prakash AS, Aggarwal S, Yang B, Wuttig M, Ramesh R, Auciello O, Venkatesan T. Nanoscale imaging of domain dynamics and retention in ferroelectric thin films. Appl Phys Lett 1997; 71(24):3492 – 4. [4] Hu GD, Xu JB, Wilson IH. Domain imaging and local piezoelectric properties of the (200)-predominant SrBi2Ta2O9 film. Appl Phys Lett 1999;75(11):1610 – 2. [5] Zavala G, Fendler J, Trolier-McKinstry S. Characterization of ferroelectric lead zirconate titanate films by scanning force microscopy. J Appl Phys 1997;81(11): 7480 – 91. [6] Wang J, Wong KH, Chan HLW, Choy CL. Ferroelectrics 2001;252(11):257 – 64. [7] Nagarajan V, Alpay SP, Ganpule CS, Nagarai BK, Ag-
220
J. Wang et al. / Materials Characterization 48 (2002) 215–220
garwal S, Williams ED, Roytburd AL, Ramesh R. Appl Phys Lett 2000;77(3):438 – 40. [8] Streiffer SK, Parker CB, Romanov AE, Lefevre MJ, Zhao L, Speck JS, Pompe W, Foster CM, Bai GR. Domain patterns in epitaxial rhombohedral ferroelectric films: 1. Geometry and experiments. J Appl Phys 1998;83(5):2742 – 53. [9] Ganpule CS, Stanishevsky A, Aggarwal S, Melngailis J,
Williams E, Ramesh R, Joshi V, De Araujo CP. Scaling of ferroelectric and piezoelectric properties in Pt/ SrBi 2 Ta 2 O 9 /Pt thin films. Appl Phys Lett 2000; 75(24):3874 – 6. [10] Kholkin AL, Calzada ML, Ramos P, Mendiola J, Setter N. Piezoelectric properties of Ca-modified PbTiO3 films. Appl Phys Lett 1996;69(23):3602 – 4.