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Structural and electrical properties of highly oriented Pb(Zr,Ti)O3 thin films deposited by facing target sputtering
Sensors and Actuators 82 Ž2000. 265–269 www.elsevier.nlrlocatersna
Structural and electrical properties of highly oriented Pb žZr,Ti/ O 3 thin films deposited by facing target sputtering Xin-Shan Li a,) , Kaoru Yamashita b, Tsunehisa Tanaka c , Yoshihiko Suzuki c , Masanori Okuyama b a
c
Shanghai UniÕersity, Shanghai, People’s Republic of China b Osaka UniÕersity, Osaka, Japan Super-Eye Image Sensor Project, Technology Research Institute of Osaka Prefecture, Ayumino 2-7-1, Izumi, Osaka 594-1157, Japan Received 7 June 1999; received in revised form 27 October 1999; accepted 1 November 1999
Abstract The PbŽZr,Ti.O 3 thin films with single Ž111. oriented perovskite phase and excellent electrical properties have been prepared by annealing the as-deposited samples. The orientation of crystals depends strongly on both the deposition temperature and annealing temperature. The sample annealed at 6068C has the best Ž111. orientation. When the sample is annealed at lower temperature, the relative content of Ž111. oriented perovskite phase decreases quickly with increase of deposition temperature. The deposition temperature has little effects on the orientation when the sample is annealed at higher temperature. The effects of deposition temperature and annealing temperature on the crystallographic structures and electrical properties of PZT thin films were investigated in this paper. The sample deposited at 2858C and annealed at 6068C has the maximum value of polarization, which displays excellent ferroelectric properties. The typical Pr and Ps values are 57 and 101 mCrcm2 , respectively. q 2000 Elsevier Science S.A. All rights reserved. Keywords: PZT thin film; Facing target sputtering; Crystalline orientation; Ferroelectric property
1. Introduction Lead zirconate titanate ŽPZT. thin film is one of the best candidates for ultrasonic sensors because of its excellent piezoelectric properties. PZT thin films have been attracting extra attention recently w1–8x. As known, PZT ferroelectric material has a polar axis, hence it shows different electrical properties depending on the orientation of crystals. In order to improve the electrical properties of PZT thin films, it has been reported that several techniques have been used for fabricating epitaxial or preferred oriented PZT thin films w5–8x. Being used as a transducer in acoustic imaging systems, PZT thin films should be Ž111. oriented or Ž100. oriented. However, there are few reports on highly oriented PZT thin films with excellent electrical properties. Among numerous kinds of sputtering techniques, facing target sputtering ŽFTS. is the unique one. It has been ) Corresponding author. Tel.: q81-725-51-2534; fax: q81-725-530423. E-mail address: [email protected] ŽX-S. Li..
reported that almost all kinds of materials can be sputtered by FTS at low substrate temperature, and that the as-deposited thin films have excellent crystallograghic structure and good uniformity in a large area w9x. Moreover the sputtering process with RF discharge is very suitable for the deposition of dielectric thin films under stable conditions. For PZT ceramic targets, RF discharge is very stable in the experimental conditions of our work. On the other hand, the substrate with on-board circuitry can not be damaged by the plasma with high-energy particles and high temperature, because the substrate is placed at the outside of the plasma, and almost not bombarded by high-energy particles such as g electrons, ions, and reflected atoms. In our early experimental work, the perovskite phase was obtained in the as-deposited PZT thin films at very low substrate temperature by facing target sputtering. However, not only was there perovskite phase with different planar orientation, but also pyrochlore phase and lead oxide and titanium oxide phases existed in all as-deposited samples. In order to change the phase composition and get highly oriented perovskite phase of PZT thin films, post-
0924-4247r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 Ž 9 9 . 0 0 3 0 7 - 6
266
X.-S. Li et al.r Sensors and Actuators 82 (2000) 265–269
deposition annealing process was used. By controlling the conditions of deposition and annealing process, the PZT thin films with single Ž111. oriented perovskite phase and excellent electrical properties have been prepared in the present work.
2. Experimental procedure The instrument used for sputtering was FTS-1CB ŽOsaka Vacuum.. The distance between the two targets was fixed at 140 mm, and the distance between the surface of substrate and the central line of the two targets was controlled at 138 mm. Two hot-pressed sintered ceramic plates ŽPb1.2 Zr0.58Ti 0.42 O x . were used as the targets. All PZT thin films were deposited on PtrTirSiO2rSi substrate at 200–5508C, 0.8 Pa under RF700W for 120 min in the ambience of oxygen and argon Ž4:1.. The thickness of samples was about 400 nm. The as-deposited thin films were annealed at 453–6568C for 50 min in air atmosphere, in which the heating rate was controlled in 308Crmin. The crystallization characteristics were determined by X-ray diffraction ŽXRD. analysis with Cu K a radiation of 50 kV, 150 mA ŽRigaku RINT-2500.. The micrographs of the surfaces and cross-sections of PZT thin films were observed by using scanning electron microscopy ŽJSM6301F.. The ferroelectric properties were measured by means of RT6000 system ŽRadiant..
3. Results and discussion Fig. 1 shows the XRD patterns of PZT thin films deposited at 3608C, and annealed at different temperature. It is found that the perovskite phase can be obtained in the as-deposited sample at such low substrate temperature, which has never been reported in the previous literatures. However, the pyrochlore phase exists simultaneously in the as-deposited sample, and post-deposition annealing
Fig. 1. XRD patterns of PZT thin films deposited by RF700W at 3608C, and annealed at different temperature for 50 min in air.
Fig. 2. XRD patterns of 6068C-annealed PZT thin films deposited at different temperature.
process is needed for getting highly oriented perovskite phase in PZT thin films. It is found from Fig. 1 that the crystallization process of PZT thin films depends strongly on annealing temperature. There is almost only Ž111. oriented perovskite phase in the sample annealed at 6068C. With a decrease in annealing temperature, more and more impure phases such as pyrochlore phase and lead oxide phase appear in the samples. However, if the annealing temperature increase is too high, such as above 6568C, there is enough energy to make the grains of PZT thin film grow along with the other orientations. That means the sample annealed at 6068C displays the best Ž111. orientation; this temperature is not so high when compared with that reported in the previous works w10–12x. On the other hand, the crystallization process of annealed samples depends strongly on the substrate temperature at which the thin films were deposited. Fig. 2 shows the XRD spectra of 6068C-annealed samples deposited at different temperature. It is found that there is almost no other peaks except for that of Ž111. oriented perovskite phase in the samples deposited in the range 285–4358C. However, the other peaks of perovskite phase such as Ž100., Ž110. and Ž200. peak as well as the strong peaks of pyrochlore phase exist in the sample deposited at 5508C. In the case of sample deposited at very low substrate temperature, the peak of Ž222. pyrochlore phase appears in the XRD pattern. It indicates that too high and too low deposition temperature are not advantageous to formation of Ž111. oriented perovskite phase in the annealed PZT thin films. Fig. 3 summarizes the correlation among deposition temperature, annealing temperature and the relative contents of Ž111. oriented perovskite phase in the total phases of the samples. When the thin films were annealed at 6568C, the deposition temperature has a very little effect on formation of phases, because the relative contents are not changed obviously in this case. For the thin films annealed at 6068C, it has the best Ž111. orientation of
X.-S. Li et al.r Sensors and Actuators 82 (2000) 265–269
Fig. 3. Relative intensity of PZT Ž111. peak in XRD patterns of samples deposited at different temperature.
267
Fig. 5. The cross-sectional SEM micrograph of the sample deposited at 2858C and annealed at 6068C.
perovskite phase when the deposition temperature is ranged in 360 " 808C, and the relative content of Ž111. perovskite phase decreases rapidly in the sample deposited at 5508C, which is shown clearly in Fig. 2. When the samples were annealed at 5568C, the crystallization process depends strongly on the deposition temperature, and the relative contents of Ž111. oriented perovskite phase decreases quickly with increase of deposition temperature, in which the relative content of Ž111. perovskite phase decreases from 79% to 6%. Because the sample deposited at higher temperature has lower reactivity, it needs more energy to transfer the crystallographic structures and phase composition, which means the sample has to be annealed at higher temperature. It is found from Fig. 4 that the surface status of annealed sample depends strongly on deposition temperature. The grain size of the 6068C-annealed samples increases from about 30 to 200 nm with increase of deposition temperature from 2008C to 5508C, while the density decreases relatively when the deposition temperature is so
high as 3608C to 5508C, because more and more large cracks and pores appear in the samples. Especially in the case of photo Žd., all of the grains are almost independent because of an excessive growth. On the other hand, the grain size increases quickly with increase of annealing temperature. It is concluded that lower deposition temperature and lower annealing temperature are advantageous to formation of good surface status in the annealed samples. It is very important for fabrication of sensors and actuators to prepare PZT films with excellent surface and cross-section status. Fig. 5 shows the cross-sectional SEM micrograph of the sample deposited at 2858C and annealed at 6068C, in which the surface and interface are very smooth. That means the sample is very suitable for fabrication of sensors or actuators. And it is seen clearly that the oriented grains in the PZT film grew. The microstructure of thin films can effect strongly on the electrical properties such as insulation, dielectrics, ferroelectrics and piezoelectrics. Fig. 6 shows the leakage current characteristics of 6068C-annealed PZT thin films deposited at different temperature. It is seen that there is an optimum range of
Fig. 4. The micrographs of 6068C-annealed PZT thin films deposited at Ža. 2008C, Žb. 2858C, Žc. 3608C, Žd. 5508C.
Fig. 6. The leakage current of 6068C-annealed PZT thin films deposited at different temperature. Here, the applied voltage is 5 V.
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deposition temperature from 2858C to 3608C, in which the leakage current of samples are much lower than that of samples deposited at higher temperature. This phenomenon is corresponding to the crystallographic structures of PZT thin films deposited at different temperature Žsee Fig. 4.. There are more and more large cracks and pores in the samples with increase of deposition temperature from 3608C to 5508C, more and more particles of up-electrode entered the cracks and pores when the up-electrode was deposited. So the leakage currents of the samples increase relatively with increase of deposition temperature. The ferroelectric properties of PZT thin films depend strongly on the deposition temperature. It is found from Fig. 7 that the sample deposited at 2858C and annealed at 6068C has the maximum value of polarization, which displays excellent ferroelectric properties. The typical Pr and Ps values are 57 and 101 mCrcm2 , respectively, which are larger than all of the results of Ž111. oriented PZT samples reported before w1–8x. The Pr and Ps decrease gradually with increase of deposition temperature from 2858C to 5508C. It has been said that large grain is advantageous to ferroelectric property of PZT thin film w13,14x. Therefore, the sample deposited at 2008C displays very weak ferroelectric property because of its too small grains, which may hinder the ferroelectric domains. It is seen from Fig. 4 that the grain size of 6068C-annealed samples increases with increasing the deposition temperature. It seems that the polarization of samples might increase with increase of deposition temperature. However, more and more micro-cracks and pores appear in the samples with increase of deposition temperature, which causes higher and higher leakage current. Meanwhile, the relative contents of perovskite phase in the 6068C-annealed PZT thin films deposited at too high and too low substrate temperature are less than that of samples deposited at the
Fig. 7. The P – E hysteresis loops of PZT thin films deposited at different substrate temperature, annealed at 6068C for 50 min in air.
Fig. 8. The coercive electric field of the samples deposited at different temperature and annealed at 6068C. Here, the applied voltage is 10 V.
optimum temperature. So there is a maximum value of polarization in the samples deposited at the optimum substrate temperature in this case. Fig. 8 shows the effect of deposition temperature on the coercive electric field of 6068C-annealed samples, in which the Ec values were obtained with the negative values on Fig. 7. It is found that the Ec values of samples are between 58 and 78 kVrcm, except for the sample deposited at 2008C, which Ec is about 150 kVrcm. Because the 2008C-deposited sample has very small grains, it displays a very high value of coercive electric field.
4. Conclusions By controlling the conditions of deposition and annealing process, the PZT thin films with single Ž111. oriented perovskite phase have been prepared in this work. The crystallization process depends strongly on the deposition temperature and annealing temperature. The samples deposited in the range 285–4358C and annealed at 6068C have the best Ž111. orientation. When the sample is annealed at 5568C, the relative content of Ž111. oriented perovskite phase decreases quickly with increase of deposition temperature. The deposition temperature has little effects on the orientation when the sample is annealed at 6568C. The grain size increases quickly with increase of deposition temperature and annealing temperature, and the density decreases relatively. The micro-structure of thin films effects strongly on the electrical properties. The sample deposited at 2858C and annealed at 6068C has the maximum value of polarization, and then the polarization decreases with increase of deposition temperature from 2858C to 5508C. The Ec values of samples are between 58 and 78 kVrcm in the range between 2858C and 5508C.
X.-S. Li et al.r Sensors and Actuators 82 (2000) 265–269
Acknowledgements Support of this work by Foundation for the Osaka Research Enterprise is gratefully acknowledged. Li also acknowledges support from the Japan Science and Technology ŽJST. through the STA Fellowship.
References w1x G.A.C.M. Spierings, M.J.E. Ulenaers, G.L.M. Kampschoer, H.A.M. van Hal, P.K. Larsen, Preparation and ferroelectric properties of PbZr0.53Ti 0.47 O 3 thin films by spin coating and metallorganic decomposition, J. Appl. Phys. 70 Ž1991. 2290–2298. w2x K. Hirata, N. Hosokawa, T. Hase, T. Sakuma, Y. Miyasaka, PbŽZr,Ti. thin film preparation by multitarget magnetron sputtering, Jpn. J. Appl. Phys. 31 Ž1992. 3021–3024. w3x S.-Y. Chen, I.-W. Chen, Temperature–time texture transition of PbŽZr1y xTi x .O 3 thin films: II, Heat treatment and compositional effects, J. Am. Ceram. Soc. 77 Ž1994. 2337–2344. w4x I.M. Reaney, K.G. Brooks, R. Klissurska, C. Pawlaczyk, N. Setter, Use of transmission electron microscope for the characterization of rapid thermally annealed, solution–gel, lead zirconate titanate films, J. Am. Ceram. Soc. 77 Ž1994. 1209–1216. w5x Y. Masuda, A. Baba, Oxidation and heat treatment effect on crystal structure and electrical conductivity of ferroelectric PbŽZr,Ti.O 3 films, Jpn. J. Appl. Phys. 35 Ž1996. 5002–5007. w6x Y.J. Song, Y. Zhu, S. Desu, Low temperature fabrication and properties of sol–gel derived Ž111. oriented PbŽZr1y xTi x .O 3 thin films, Appl. Phys. Lett. 72 Ž1998. 2686–2688. w7x C.J. Kim, W.J. Lee, K. No, Control of preferred orientation in sol–gel lead–zirconate–titanate film on PtrTirglass substrate, Thin Solid Films 312 Ž1998. 130–134. w8x B.A. Tuttle, J.A. Voigt, D.C. Goognow et al., Highly oriented, chemically prepared PbŽZr,Ti.O 3 thin films, J. Am. Ceram. Soc. 76 Ž1993. 1537–1544. w9x M. Matsuoka, Y. Hoshi, M. Naoe, Rf and dc discharge characteristics for opposed-targets sputtering, J. Appl. Phys. 60 Ž1986. 2096– 2102.
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w10x B. Ea-Kim, P. Aubert, F. Ayguavives, R. Bisaro, F. Varniere, J. Olivier, M. Puech, B. Agius, Growth and characterization of radiofrequency magnetron sputtered lead zirconate titanate thin films deposited on Ž111. Pt electrodes, J. Vac. Sci. Technol. A 16 Ž1998. 2876–2884. w11x Y. Fukuda, K. Aoki, Effects of excess Pb and substrate on crystallization processes of amorphous PbŽZr,Ti.O 3 thin films prepared by RF magnetron sputtering, Jpn. J. Appl. Phys. 36 Ž1997. 5793–5798. w12x H.-J. Nam, H.-H. Kim, W.-J. Lee, The effects of the preparation conditions and heat-treatment conditions of PtrTirSiO2 rSi substrates on the nucleation and growth of PbŽZr,Ti.O 3 films, Jpn. J. Appl. Phys. 37 Ž1998. 3462–3470. w13x C.A. Randall, N. Kim, J.-P. Kucera, W. Cao, T.R. Shrout, Intrinsic and extrinsic size effects in fine-grained morphotropic-phaseboundary lead zirconate titanate ceramics, J. Am. Ceram. Soc. 81 Ž1998. 677–688. w14x K.R. Udayakumar, P.J. Schuele, J. Chen, S.B. Krupanidhi, L.E. Cross, Thickness-dependent electrical characteristics of lead zirconate titanate thin films, J. Appl. Phys. 77 Ž1995. 3981–3986.
Biographies X.S. Li, PhD, Associate professor, School of Materials Science and Engineering, Shanghai University, China; Fellow researcher of Science and Technology Agency in Japan Government and visiting researcher in Technology Research Institute of Osaka Prefecture, Japan. K. Yamashita, Eng.D., Research associate, Department of Physical Science, Graduate School of Engineering Science, Osaka University, Japan. T. Tanaka, Researcher, Electronic Devices Group, Materials Technology Division, Technology Research Institute of Osaka Prefecture, Japan. Y. Suzuki, Eng.D., Executive senior researcher, Materials Technology Division, Technology Research Institute of Osaka Prefecture, Japan. M. Okuyama, Eng.D., Professor, Department of Physical Science, Graduate School of Engineering Science, Osaka University, Japan.
Structural and electrical properties of highly oriented Pb žZr,Ti/ O 3 thin films deposited by facing target sputtering Xin-Shan Li a,) , Kaoru Yamashita b, Tsunehisa Tanaka c , Yoshihiko Suzuki c , Masanori Okuyama b a
c
Shanghai UniÕersity, Shanghai, People’s Republic of China b Osaka UniÕersity, Osaka, Japan Super-Eye Image Sensor Project, Technology Research Institute of Osaka Prefecture, Ayumino 2-7-1, Izumi, Osaka 594-1157, Japan Received 7 June 1999; received in revised form 27 October 1999; accepted 1 November 1999
Abstract The PbŽZr,Ti.O 3 thin films with single Ž111. oriented perovskite phase and excellent electrical properties have been prepared by annealing the as-deposited samples. The orientation of crystals depends strongly on both the deposition temperature and annealing temperature. The sample annealed at 6068C has the best Ž111. orientation. When the sample is annealed at lower temperature, the relative content of Ž111. oriented perovskite phase decreases quickly with increase of deposition temperature. The deposition temperature has little effects on the orientation when the sample is annealed at higher temperature. The effects of deposition temperature and annealing temperature on the crystallographic structures and electrical properties of PZT thin films were investigated in this paper. The sample deposited at 2858C and annealed at 6068C has the maximum value of polarization, which displays excellent ferroelectric properties. The typical Pr and Ps values are 57 and 101 mCrcm2 , respectively. q 2000 Elsevier Science S.A. All rights reserved. Keywords: PZT thin film; Facing target sputtering; Crystalline orientation; Ferroelectric property
1. Introduction Lead zirconate titanate ŽPZT. thin film is one of the best candidates for ultrasonic sensors because of its excellent piezoelectric properties. PZT thin films have been attracting extra attention recently w1–8x. As known, PZT ferroelectric material has a polar axis, hence it shows different electrical properties depending on the orientation of crystals. In order to improve the electrical properties of PZT thin films, it has been reported that several techniques have been used for fabricating epitaxial or preferred oriented PZT thin films w5–8x. Being used as a transducer in acoustic imaging systems, PZT thin films should be Ž111. oriented or Ž100. oriented. However, there are few reports on highly oriented PZT thin films with excellent electrical properties. Among numerous kinds of sputtering techniques, facing target sputtering ŽFTS. is the unique one. It has been ) Corresponding author. Tel.: q81-725-51-2534; fax: q81-725-530423. E-mail address: [email protected] ŽX-S. Li..
reported that almost all kinds of materials can be sputtered by FTS at low substrate temperature, and that the as-deposited thin films have excellent crystallograghic structure and good uniformity in a large area w9x. Moreover the sputtering process with RF discharge is very suitable for the deposition of dielectric thin films under stable conditions. For PZT ceramic targets, RF discharge is very stable in the experimental conditions of our work. On the other hand, the substrate with on-board circuitry can not be damaged by the plasma with high-energy particles and high temperature, because the substrate is placed at the outside of the plasma, and almost not bombarded by high-energy particles such as g electrons, ions, and reflected atoms. In our early experimental work, the perovskite phase was obtained in the as-deposited PZT thin films at very low substrate temperature by facing target sputtering. However, not only was there perovskite phase with different planar orientation, but also pyrochlore phase and lead oxide and titanium oxide phases existed in all as-deposited samples. In order to change the phase composition and get highly oriented perovskite phase of PZT thin films, post-
0924-4247r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 Ž 9 9 . 0 0 3 0 7 - 6
266
X.-S. Li et al.r Sensors and Actuators 82 (2000) 265–269
deposition annealing process was used. By controlling the conditions of deposition and annealing process, the PZT thin films with single Ž111. oriented perovskite phase and excellent electrical properties have been prepared in the present work.
2. Experimental procedure The instrument used for sputtering was FTS-1CB ŽOsaka Vacuum.. The distance between the two targets was fixed at 140 mm, and the distance between the surface of substrate and the central line of the two targets was controlled at 138 mm. Two hot-pressed sintered ceramic plates ŽPb1.2 Zr0.58Ti 0.42 O x . were used as the targets. All PZT thin films were deposited on PtrTirSiO2rSi substrate at 200–5508C, 0.8 Pa under RF700W for 120 min in the ambience of oxygen and argon Ž4:1.. The thickness of samples was about 400 nm. The as-deposited thin films were annealed at 453–6568C for 50 min in air atmosphere, in which the heating rate was controlled in 308Crmin. The crystallization characteristics were determined by X-ray diffraction ŽXRD. analysis with Cu K a radiation of 50 kV, 150 mA ŽRigaku RINT-2500.. The micrographs of the surfaces and cross-sections of PZT thin films were observed by using scanning electron microscopy ŽJSM6301F.. The ferroelectric properties were measured by means of RT6000 system ŽRadiant..
3. Results and discussion Fig. 1 shows the XRD patterns of PZT thin films deposited at 3608C, and annealed at different temperature. It is found that the perovskite phase can be obtained in the as-deposited sample at such low substrate temperature, which has never been reported in the previous literatures. However, the pyrochlore phase exists simultaneously in the as-deposited sample, and post-deposition annealing
Fig. 1. XRD patterns of PZT thin films deposited by RF700W at 3608C, and annealed at different temperature for 50 min in air.
Fig. 2. XRD patterns of 6068C-annealed PZT thin films deposited at different temperature.
process is needed for getting highly oriented perovskite phase in PZT thin films. It is found from Fig. 1 that the crystallization process of PZT thin films depends strongly on annealing temperature. There is almost only Ž111. oriented perovskite phase in the sample annealed at 6068C. With a decrease in annealing temperature, more and more impure phases such as pyrochlore phase and lead oxide phase appear in the samples. However, if the annealing temperature increase is too high, such as above 6568C, there is enough energy to make the grains of PZT thin film grow along with the other orientations. That means the sample annealed at 6068C displays the best Ž111. orientation; this temperature is not so high when compared with that reported in the previous works w10–12x. On the other hand, the crystallization process of annealed samples depends strongly on the substrate temperature at which the thin films were deposited. Fig. 2 shows the XRD spectra of 6068C-annealed samples deposited at different temperature. It is found that there is almost no other peaks except for that of Ž111. oriented perovskite phase in the samples deposited in the range 285–4358C. However, the other peaks of perovskite phase such as Ž100., Ž110. and Ž200. peak as well as the strong peaks of pyrochlore phase exist in the sample deposited at 5508C. In the case of sample deposited at very low substrate temperature, the peak of Ž222. pyrochlore phase appears in the XRD pattern. It indicates that too high and too low deposition temperature are not advantageous to formation of Ž111. oriented perovskite phase in the annealed PZT thin films. Fig. 3 summarizes the correlation among deposition temperature, annealing temperature and the relative contents of Ž111. oriented perovskite phase in the total phases of the samples. When the thin films were annealed at 6568C, the deposition temperature has a very little effect on formation of phases, because the relative contents are not changed obviously in this case. For the thin films annealed at 6068C, it has the best Ž111. orientation of
X.-S. Li et al.r Sensors and Actuators 82 (2000) 265–269
Fig. 3. Relative intensity of PZT Ž111. peak in XRD patterns of samples deposited at different temperature.
267
Fig. 5. The cross-sectional SEM micrograph of the sample deposited at 2858C and annealed at 6068C.
perovskite phase when the deposition temperature is ranged in 360 " 808C, and the relative content of Ž111. perovskite phase decreases rapidly in the sample deposited at 5508C, which is shown clearly in Fig. 2. When the samples were annealed at 5568C, the crystallization process depends strongly on the deposition temperature, and the relative contents of Ž111. oriented perovskite phase decreases quickly with increase of deposition temperature, in which the relative content of Ž111. perovskite phase decreases from 79% to 6%. Because the sample deposited at higher temperature has lower reactivity, it needs more energy to transfer the crystallographic structures and phase composition, which means the sample has to be annealed at higher temperature. It is found from Fig. 4 that the surface status of annealed sample depends strongly on deposition temperature. The grain size of the 6068C-annealed samples increases from about 30 to 200 nm with increase of deposition temperature from 2008C to 5508C, while the density decreases relatively when the deposition temperature is so
high as 3608C to 5508C, because more and more large cracks and pores appear in the samples. Especially in the case of photo Žd., all of the grains are almost independent because of an excessive growth. On the other hand, the grain size increases quickly with increase of annealing temperature. It is concluded that lower deposition temperature and lower annealing temperature are advantageous to formation of good surface status in the annealed samples. It is very important for fabrication of sensors and actuators to prepare PZT films with excellent surface and cross-section status. Fig. 5 shows the cross-sectional SEM micrograph of the sample deposited at 2858C and annealed at 6068C, in which the surface and interface are very smooth. That means the sample is very suitable for fabrication of sensors or actuators. And it is seen clearly that the oriented grains in the PZT film grew. The microstructure of thin films can effect strongly on the electrical properties such as insulation, dielectrics, ferroelectrics and piezoelectrics. Fig. 6 shows the leakage current characteristics of 6068C-annealed PZT thin films deposited at different temperature. It is seen that there is an optimum range of
Fig. 4. The micrographs of 6068C-annealed PZT thin films deposited at Ža. 2008C, Žb. 2858C, Žc. 3608C, Žd. 5508C.
Fig. 6. The leakage current of 6068C-annealed PZT thin films deposited at different temperature. Here, the applied voltage is 5 V.
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X.-S. Li et al.r Sensors and Actuators 82 (2000) 265–269
deposition temperature from 2858C to 3608C, in which the leakage current of samples are much lower than that of samples deposited at higher temperature. This phenomenon is corresponding to the crystallographic structures of PZT thin films deposited at different temperature Žsee Fig. 4.. There are more and more large cracks and pores in the samples with increase of deposition temperature from 3608C to 5508C, more and more particles of up-electrode entered the cracks and pores when the up-electrode was deposited. So the leakage currents of the samples increase relatively with increase of deposition temperature. The ferroelectric properties of PZT thin films depend strongly on the deposition temperature. It is found from Fig. 7 that the sample deposited at 2858C and annealed at 6068C has the maximum value of polarization, which displays excellent ferroelectric properties. The typical Pr and Ps values are 57 and 101 mCrcm2 , respectively, which are larger than all of the results of Ž111. oriented PZT samples reported before w1–8x. The Pr and Ps decrease gradually with increase of deposition temperature from 2858C to 5508C. It has been said that large grain is advantageous to ferroelectric property of PZT thin film w13,14x. Therefore, the sample deposited at 2008C displays very weak ferroelectric property because of its too small grains, which may hinder the ferroelectric domains. It is seen from Fig. 4 that the grain size of 6068C-annealed samples increases with increasing the deposition temperature. It seems that the polarization of samples might increase with increase of deposition temperature. However, more and more micro-cracks and pores appear in the samples with increase of deposition temperature, which causes higher and higher leakage current. Meanwhile, the relative contents of perovskite phase in the 6068C-annealed PZT thin films deposited at too high and too low substrate temperature are less than that of samples deposited at the
Fig. 7. The P – E hysteresis loops of PZT thin films deposited at different substrate temperature, annealed at 6068C for 50 min in air.
Fig. 8. The coercive electric field of the samples deposited at different temperature and annealed at 6068C. Here, the applied voltage is 10 V.
optimum temperature. So there is a maximum value of polarization in the samples deposited at the optimum substrate temperature in this case. Fig. 8 shows the effect of deposition temperature on the coercive electric field of 6068C-annealed samples, in which the Ec values were obtained with the negative values on Fig. 7. It is found that the Ec values of samples are between 58 and 78 kVrcm, except for the sample deposited at 2008C, which Ec is about 150 kVrcm. Because the 2008C-deposited sample has very small grains, it displays a very high value of coercive electric field.
4. Conclusions By controlling the conditions of deposition and annealing process, the PZT thin films with single Ž111. oriented perovskite phase have been prepared in this work. The crystallization process depends strongly on the deposition temperature and annealing temperature. The samples deposited in the range 285–4358C and annealed at 6068C have the best Ž111. orientation. When the sample is annealed at 5568C, the relative content of Ž111. oriented perovskite phase decreases quickly with increase of deposition temperature. The deposition temperature has little effects on the orientation when the sample is annealed at 6568C. The grain size increases quickly with increase of deposition temperature and annealing temperature, and the density decreases relatively. The micro-structure of thin films effects strongly on the electrical properties. The sample deposited at 2858C and annealed at 6068C has the maximum value of polarization, and then the polarization decreases with increase of deposition temperature from 2858C to 5508C. The Ec values of samples are between 58 and 78 kVrcm in the range between 2858C and 5508C.
X.-S. Li et al.r Sensors and Actuators 82 (2000) 265–269
Acknowledgements Support of this work by Foundation for the Osaka Research Enterprise is gratefully acknowledged. Li also acknowledges support from the Japan Science and Technology ŽJST. through the STA Fellowship.
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Biographies X.S. Li, PhD, Associate professor, School of Materials Science and Engineering, Shanghai University, China; Fellow researcher of Science and Technology Agency in Japan Government and visiting researcher in Technology Research Institute of Osaka Prefecture, Japan. K. Yamashita, Eng.D., Research associate, Department of Physical Science, Graduate School of Engineering Science, Osaka University, Japan. T. Tanaka, Researcher, Electronic Devices Group, Materials Technology Division, Technology Research Institute of Osaka Prefecture, Japan. Y. Suzuki, Eng.D., Executive senior researcher, Materials Technology Division, Technology Research Institute of Osaka Prefecture, Japan. M. Okuyama, Eng.D., Professor, Department of Physical Science, Graduate School of Engineering Science, Osaka University, Japan.