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Processing effects on the microstructure and ferroelectric properties of Pb(Zr,Ti)O3 thin films prepared by sol–gel process
Surface and Coatings Technology 161 (2002) 169–173
Processing effects on the microstructure and ferroelectric properties of Pb(Zr,Ti)O3 thin films prepared by sol–gel process X.G. Tanga,b,*, H.L.W. Chana, A.L. Dingb, Q.R. Yinb a
Department of Applied Physics and Materials Research Centre, The Hong Kong Polytechnic University, Kowloon, Hong Kong, PR China b The State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China Received 10 March 2002; accepted in revised form 23 July 2002
Abstract Lead zirconate titanate Pb(Zr0.53 Ti0.47 )O3 (PZT) thin films grown on Pt(1 1 1)yTiySiO2 ySi(1 0 0) substrates by a simple sol– gel process use various thermally decompose temperatures from 320 to 400 8C. Then PZT films were annealed at various temperatures from 550 to 600 8C in oxygen atmosphere by a rapid thermal annealing process, and highly (1 1 1)-oriented PZT thin films have been obtained. The microstructure and surface morphologies, and root mean square (RMS) roughness of the thin films were studied by X-ray diffraction and atomic force microscopy (AFM). AFM images shown that the higher (1 1 1) orientation present in the PZT thin films, the smaller grain size, and the lower RMS roughness. PZT films on PtyTiySiO2 ySi substrates were initially heated at 400 and 320 8C, then annealed at 550 8C, the remanent polarization (Pr ) and coercive electric field (Ec) were 16.1 mCycm2 and 105 kVycm, 32.2 mCycm2 and 79.9 kVycm; at 100 kHz, the dielectric constant and dissipation factor were 331 and 0.045, 539 and 0.066, respectively. The highly (1 1 1) oriented PZT films, have smooth surface, good ferroelectric and dielectric properties. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Heat decompose temperature; Sol–gel; Ferroelectric property; Dielectric property; PZT thin films
1. Introduction In the past decades, a considerable amount of research has been focused on the growth and device fabrication of ferroelectric thin films for application to piezoelectric, pyroelectric and non-volatile random access memory device w1–3x. Lead zirconate titanate Pb(Zr0.53Ti0.47)O3 (PZT) is considered to be one of the very promising materials because of the large remanent polarization and relatively low temperature deposition process. To deposit PZT films, various techniques have been used, such as sol–gel w4,5x, sputtering w6x, laser deposition w7x, and metal organic chemical vapour deposition w8x. Among these methods, chemical methods such as sol–gel are promising for producing PZT thin films for their easy *Corresponding author. Fax: q852-2333-7629. E-mail address: [email protected] (X.G. Tang).
composition control and low processing temperatures when compared with other techniques w9,10x. Usually, to prepare PZT thin films by sol–gel process, a reflux or high temperature distillation is needed to remove water w11x. Recently, we have developed a simple sol–gel route with rapid thermal annealing (RTA) process to prepare the PZT thin films. The technique uses lead acetate, zirconium-n-propoxide, and tetrabutyl titanate as the starting materials, 2-methoxyethanol as a solvent and acetic acid as a chelating agent. Without reflux and high temperature distillation of water, the precursor solution is stable under ambient laboratory conditions w12x. In this present work, we report on the effects of processing conditions on the microstructure and ferroelectric of PZT thin films on Pt(1 1 1)yTiy SiO2 ySi(1 0 0) substrates by the simple sol–gel technique.
0257-8972/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 2 . 0 0 5 2 0 - 0
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2. Experimental Ferroelectric PZT thin films were prepared by a simple sol–gel route with spin-on process modified from former reports in Refs. w9,12x. Lead acetate hydrate Pb(CH3COO)2Ø3H2O, zirconium-n-propoxide Zr(O(CH2)2CH3)4 are initially dissolved in 2-methoxyethanol C3H8O2, and stirring for 30 min at 70 8C, the precursor with 10% excess Pb composition was prepared on purpose to compensate for the lead loss in the deposition processing. After cooling to room temperature, the required quantity of tetrabutyl titanate Ti(OC4H9)4 is added to the solution and mixed in the flask at 70 8C. Then the solution is stirred for 30 min, without reflux or high temperature distillation to remove water. The solution is stable with no crystallite formation after 10 months storage. The concentration of the final solution can be adjusted to 0.3 M and pH value to 2–4 by adding 2-methoxyethanol and acetic acid CH3COOH. The whole process of the preparation of the precursor solution is performed in an ambient atmosphere. Before spin-coating on the substrates, the solution is filtered with filter paper to avoid particulate contamination. The thermal treatment process for the samples is completed in a hot plate and a RTA furnace. The coating solution of PZT films was deposited onto Pt(1 1 1)yTiy SiO2 ySi(1 0 0) substrates by spin-coating at 3000 rpm for 30 s. After each spin-coating process, sample was heat-treated at 320 or 400 8C for 10 min in air atmosphere using a hot plate. This step is repeated several times to obtain the desired thickness of the films. The PZT films on PtyTiySiO2 ySi substrates annealed at 550–600 8C for 5 min by RTA in oxygen atmosphere. The thickness of the annealed PZT films with four and six layers, as measured by Dektak Stylus Profilers (Digital Instruments, USA), were 0.21 and 0.30 mm, respectively, for the films thermally decomposed at 400 and 320 8C. The structure and surface morphology of PZT thin films deposited on PtyTiySiO2 ySi substrates were analysed by an DyMax 2550V (Rakagu, Japan) X-ray diffraction (XRD), an NanoScope IIIa atomic force microscopy (AFM) (Digital Instruments) or a SPM9500J AFM (Shimadzu, Japan). To investigate the electrical properties of prepared PZT thin films, the top electrodes of gold (Au) of 0.2 mm diameter were prepared on the top surface of the PZT films through a shadow mask in a vacuum evaporation system. The polarization vs. electric field (P–E) loop of the specimens was measured using a Radiant Technologies RT66 ferroelectrics instrument. The dielectric permittivity of the specimens was calculated from the study of capacitance vs. frequency (C–F) characteristic. The C–F characteristics were measured by an HP4192A LCR impedance analyser with a small a.c. signal of 50 mV.
Fig. 1. XRD patterns of PZT films on Pt(1 1 1)yTiySiO2 ySi(1 0 0) substrates annealed at 550 and 600 8C for 5 min in oxygen atmosphere by RTA, before thermal decompose at (a) 400 and (b) 320 8C, respectively. Pyro, pyrochlore phase.
3. Results and discussion Fig. 1a and b show the XRD patterns of the PZT thin films annealed for 5 min in oxygen atmosphere by RTA at various temperatures range from 550 to 600 8C, before the thermally decompose temperature were 320 and 400 8C, respectively. The XRD results also exhibit that the films’ thermally decomposition temperature was 400 8C, and after subsequent annealing at various temperature ranges from 550 to 600 8C, the crystalline PZT sample displayed strong (1 1 1) orientation (Fig. 1a). The peaks corresponding to (1 0 0), (1 1 0), (1 1 1), (2 0 0) and (2 1 1) were observed in the films. The relative peak intensity of I(1 1 1)ySI(h k l) was 75 and 86% for the films annealed at 550 and 600 8C, respectively. For films produced using an initial thermally decomposition temperature of 320 8C, then subsequently annealed at a temperature of 550 8C, only the peaks corresponding to (1 1 0) and (1 1 1) were observed in the film, with the relative peak intensity of I(1 1 1)y SI(h k l) being 98% (Fig. 1b). The (1 1 1) peak indicated that the (1 1 1) orientation increased appreciably
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(Fig. 1a), we know that there are pyrochlore phases in the film. Brooks et al. reported that there are metastable pyrochlore phases observed by the TEM and XRD w13x. When the annealing temperature is increased to 600 8C, the nanocrystalline on the surface of clusters disappear (Fig. 2b), it indicated that the metastable pyrochlore phase crystallization to PZT perovskite phase. The result agrees with XRD. The root mean square (RMS) roughness of the films surfaces are 7.4 and 9.7 nm, respectively for the films annealed at 550 and 600 8C. During RTA the PZT perovskite nuclei form on the Pt and crystallize on the bottom electrode and then growth proceeds from the interface uniformly to the surface in columnar grains that push excess Pb and inclusions to the film surface. The excess Pb on the surface evaporates leaving a fine-grained dense film (Fig. 2a and b). As the annealing progresses the nuclei grow into half-dome shapes pushing out excess Pb in front of the perovskite grain as well as some Zr and inclusions w14x. From the XRD and AFM results, we can infer that heterogeneous nucleation is plays a dominant role in the nucleation of the perovskite phase. Fig. 3 show the AFM image of highly (1 1 1)-oriented PZT thin films grown on PtyTiySiO2 ySi substrate, thermally decompose temperature was 320 8C, then annealed at 550 8C for 5 min in an oxygen atmosphere by RTA. The surface of the film is smooth with grain size of approximately 200–300 nm, and RMS roughness of 0.92 nm. This suggests that the higher the orientation, the smaller the roughness. Fig. 4 shows a typical polarization–electric field (P– E) hysteresis loop for the PZT films on PtyTiySiO2 ySi substrates, thermally decompose temperature was 400
Fig. 2. The AFM images of PZT thin films on PtyTiySiO2ySi substrates thermally decomposed at 400 8C, then annealed at (a) 550 and (b) 600 8C for 5 min in oxygen atmosphere by RTA, respectively.
with increasing annealing temperature due to further enhancement in crystallization. The surface morphology of the films was characterized by a NanoScope IIIa (Digital Instruments) AFM. Fig. 2 shows the AFM image of (1 1 1)-oriented PZT thin films on PtyTiySiO2 ySi substrates thermally decomposed at a temperature of 400 8C, then annealed at 550 and 600 8C for 5 min in an oxygen atmosphere by RTA. On the surface image of PZT thin films annealed at 550 8C (Fig. 2a), many clusters are found, which are composed of nano-sized particles of approximately 70– 120 nm surrounding larger particles of 0.6–1.0 mm in size. At the surface of the clusters, there are nanocrystalline particles with size of 30–50 nm. From the XRD
Fig. 3. The AFM image of PZT thin film on PtyTiySiO2 ySi substrate thermally decomposed at 320 8C, then annealed at 550 8C for 5 min in oxygen atmosphere by RTA.
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Fig. 4. The typical P–E hysteresis loops for PZT thin films deposited on PtyTiySiO2ySi substrates thermally decomposed at 400 8C, then annealed at (a) 550 and (b) 600 8C for 5 min in oxygen atmosphere by RTA.
Fig. 6. The variation of dielectric constant with the frequency for the PZT films deposited on PtyTiySiO2ySi substrates thermally decomposed at 400 8C, then annealed at (a) 550 and (b) 600 8C for 5 min in oxygen atmosphere by RTA.
8C, then annealed at 550–600 8C for 5 min in the oxygen atmosphere by a RTA process. The average remanent polarization (Pr) and the coercive electric field (Ec) obtained from the P–E hysteresis loops, are 16.1 mCycm2 and 105 kVycm, 21.3 mCycm2 and 105 kVy cm, respectively, for PZT thin films annealed at 550 and 600 8C. Fig. 5 shows a typical polarization–electric field (P– E) hysteresis loop for the PZT films on PtyTiySiO2 y Si(1 0 0) substrates. Here the thermally decomposition temperature was 320 8C, with annealing at 550 8C for 5 min in the oxygen atmosphere by the RTA process. The average remanent polarization (Pr ) and the coercive electric field (Ec) obtained from the P–E hysteresis loops, are 32.2 mCycm2 and 79.9 kVycm, respectively. This shows a good ferroelectricity of the prepared PZT films on PtyTiySiO2 ySi substrates by the simple sol– gel process. The remanent polarization value is higher
than that of PLD epitaxial PZT films (6.5 mCycm2) w15x, and compare with that of sol–gel derived (1 1 1) oriented PZT(65y35) (31.3 mCycm2) and PZT(35y65) films (20.9 mCycm2) w16x. In general, it was found that the higher orientation, the better the ferroelectric properties obtained. The dielectric constant and dissipation factor measurements were made at room temperature as a function of frequency in the range of 100 Hz to 1 MHz for the film deposited on PtyTiySiO2 ySi substrates. Figs. 6 and 7 show the variation of dielectric constant and dissipation factor with the frequency for PZT thin films deposited on PtyTiySiO2 ySi substrates, with thermally decomposition temperature of 400 and 320 8C, respectively, followed by annealing at 550–600 8C and 550 8C for 5 min in an oxygen atmosphere by RTA. From
Fig. 5. The typical P–E hysteresis loops for PZT thin film deposited on PtyTiySiO2ySi substrate thermally decomposed at 320 8C, then annealed at 550 8C for 5 min in oxygen atmosphere by RTA.
Fig. 7. The variation of dissipation factor with the frequency for PZT film deposited on PtyTiySiO2ySi substrate thermally decomposed at 320 8C, then annealed at 550 8C for 5 min in oxygen atmosphere by RTA.
X.G. Tang et al. / Surface and Coatings Technology 161 (2002) 169–173
Fig. 6, the dielectric constant and dissipation factor were 331 and 0.045, 380 and 0.034, respectively, at 100 kHz for the films annealed at 550 and 600 8C for 5 min. The PZT thin films annealed at 600 8C, exhibit good ferroelectric and dielectric properties. From Fig. 7, it can be seen that the dielectric constant and dissipation factor were 539 and 0.066, respectively, at 100 kHz for the films annealed at 550 8C for 5 min. The PZT thin films grown on PtyTiySiO2 ySi substrates decomposed at 320 8C, then annealed at 550 8C for 5 min in the oxygen atmosphere by RTA process, exhibit high relative peak intensity. Compared with that of films decomposed at 400 8C, then annealed at 550 8C, the highly (1 1 1) oriented PZT film has good ferroelectric and dielectric properties. 4. Conclusions The role of processing effect on the microstructure and ferroelectric properties was investigated for PZT films were grown on Pt(1 1 1)yTiySiO2 ySi(1 0 0) substrates. PZT films prepared by the sol–gel route with various thermally decomposition temperatures and RTA processing. Highly (1 1 1)-oriented PZT thin films have been obtained. Using the same annealing temperature, but changing the thermally decomposition temperatures from 320 to 400 8C, produces differences in the microstructure and surface images of the PZT films obtained. The PZT film grown on PtyTiySiO2 ySi substrate with thermally decomposition temperature of 320 8C, then annealed at 550 8C gave a dense, smooth surface, and exhibited remanent polarization (Pr) and coercive electric field (Ec) values of 32.2 mCycm2 and 79.9 kVycm, respectively, at an applied electric voltage of 10 V; the typical small signal dielectric constant and dissipation factor at a frequency of 100 kHz were 539 and 0.066, respectively. By the sol–gel route with various thermally decompose temperature and low temperature annealing
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processing, we obtained the highly (1 1 1) oriented PZT thins film with high dielectric constant and good ferroelectric properties. Acknowledgments This work is supported by the Centre for Smart Materials of the Hong Kong Polytechnic University, the K.-C. Wong Education Foundation of Hong Kong, and the National Natural Science Foundation of China (No. 59995520). References w1x J.F. Scott, C.A. Paz de Araujo, Science 246 (1989) 1400. w2x D.L. Polla, L.F. Francis, Annu. Rev. Mater. Sci. 28 (1998) 563. w3x D.L. Polla, L.F. Francis, MRS Bull. 21 (1996) 59. w4x X.H. Pu, W.G. Luo, A.L. Ding, H.Y. Tian, P.S. Qiu, Phys. Stat. Sol. (a) 182 (2000) R10. w5x Y.S. Yang, S.J. Lee, S. Yi, et al., App. Phys. Lett. 76 (2000) 774. w6x G. Suchaneck, R. Koehler, P. Padmini, T. Sandner, J. Frey, G. Gerlach, Surf. Coat. Technol. 116–119 (1999) 1238. w7x A. Husmann, D.A. Wesner, J. Schmidt, T. Klotzbucher, M. Mergens, E.W. Kreutz, Surf. Coat. Technol. 97 (1997) 420. w8x D.H. Kim, J.S. Na, S.W. Rhee, J. Electrochem. Soc. 148 (2001) C668. w9x X.G. Tang, A.L. Ding, Y. Ye, W.G. Luo, Ferroelectrics 264 (2001) 297. w10x C.K. Kwok, S.B. Desu, J. Mater. Res. 8 (1993) 339. w11x B.M. Xu, Y.H. Ye, L.E. Cross, Appl. Phys. Lett. 74 (1999) 3549. w12x X.G. Tang, A.L. Ding, W.G. Luo, Appl. Surf. Sci. 174 (2001) 148. w13x K.G. Brooks, I.M. Reaney, R. Klissurska, Y. Huang, L. Bursill, N. Setter, J. Mater. Res. 9 (1994) 2540. w14x J.S. Cross, M. Fujiki, M. Tsukada, Y. Kotaka, Y. Goto, J. Mater. Res. 14 (1999) 4366. w15x C. Guerrero, J. Roldan, C. Ferrater, M.V. Garcia-Cuenca, F. Sanchez, M. Varela, Solid State Electron. 45 (2001) 1433. w16x S.-Y. Chen, C.-L. Sun, J. Appl. Phys. 90 (2001) 2970.
Processing effects on the microstructure and ferroelectric properties of Pb(Zr,Ti)O3 thin films prepared by sol–gel process X.G. Tanga,b,*, H.L.W. Chana, A.L. Dingb, Q.R. Yinb a
Department of Applied Physics and Materials Research Centre, The Hong Kong Polytechnic University, Kowloon, Hong Kong, PR China b The State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China Received 10 March 2002; accepted in revised form 23 July 2002
Abstract Lead zirconate titanate Pb(Zr0.53 Ti0.47 )O3 (PZT) thin films grown on Pt(1 1 1)yTiySiO2 ySi(1 0 0) substrates by a simple sol– gel process use various thermally decompose temperatures from 320 to 400 8C. Then PZT films were annealed at various temperatures from 550 to 600 8C in oxygen atmosphere by a rapid thermal annealing process, and highly (1 1 1)-oriented PZT thin films have been obtained. The microstructure and surface morphologies, and root mean square (RMS) roughness of the thin films were studied by X-ray diffraction and atomic force microscopy (AFM). AFM images shown that the higher (1 1 1) orientation present in the PZT thin films, the smaller grain size, and the lower RMS roughness. PZT films on PtyTiySiO2 ySi substrates were initially heated at 400 and 320 8C, then annealed at 550 8C, the remanent polarization (Pr ) and coercive electric field (Ec) were 16.1 mCycm2 and 105 kVycm, 32.2 mCycm2 and 79.9 kVycm; at 100 kHz, the dielectric constant and dissipation factor were 331 and 0.045, 539 and 0.066, respectively. The highly (1 1 1) oriented PZT films, have smooth surface, good ferroelectric and dielectric properties. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Heat decompose temperature; Sol–gel; Ferroelectric property; Dielectric property; PZT thin films
1. Introduction In the past decades, a considerable amount of research has been focused on the growth and device fabrication of ferroelectric thin films for application to piezoelectric, pyroelectric and non-volatile random access memory device w1–3x. Lead zirconate titanate Pb(Zr0.53Ti0.47)O3 (PZT) is considered to be one of the very promising materials because of the large remanent polarization and relatively low temperature deposition process. To deposit PZT films, various techniques have been used, such as sol–gel w4,5x, sputtering w6x, laser deposition w7x, and metal organic chemical vapour deposition w8x. Among these methods, chemical methods such as sol–gel are promising for producing PZT thin films for their easy *Corresponding author. Fax: q852-2333-7629. E-mail address: [email protected] (X.G. Tang).
composition control and low processing temperatures when compared with other techniques w9,10x. Usually, to prepare PZT thin films by sol–gel process, a reflux or high temperature distillation is needed to remove water w11x. Recently, we have developed a simple sol–gel route with rapid thermal annealing (RTA) process to prepare the PZT thin films. The technique uses lead acetate, zirconium-n-propoxide, and tetrabutyl titanate as the starting materials, 2-methoxyethanol as a solvent and acetic acid as a chelating agent. Without reflux and high temperature distillation of water, the precursor solution is stable under ambient laboratory conditions w12x. In this present work, we report on the effects of processing conditions on the microstructure and ferroelectric of PZT thin films on Pt(1 1 1)yTiy SiO2 ySi(1 0 0) substrates by the simple sol–gel technique.
0257-8972/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 2 . 0 0 5 2 0 - 0
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2. Experimental Ferroelectric PZT thin films were prepared by a simple sol–gel route with spin-on process modified from former reports in Refs. w9,12x. Lead acetate hydrate Pb(CH3COO)2Ø3H2O, zirconium-n-propoxide Zr(O(CH2)2CH3)4 are initially dissolved in 2-methoxyethanol C3H8O2, and stirring for 30 min at 70 8C, the precursor with 10% excess Pb composition was prepared on purpose to compensate for the lead loss in the deposition processing. After cooling to room temperature, the required quantity of tetrabutyl titanate Ti(OC4H9)4 is added to the solution and mixed in the flask at 70 8C. Then the solution is stirred for 30 min, without reflux or high temperature distillation to remove water. The solution is stable with no crystallite formation after 10 months storage. The concentration of the final solution can be adjusted to 0.3 M and pH value to 2–4 by adding 2-methoxyethanol and acetic acid CH3COOH. The whole process of the preparation of the precursor solution is performed in an ambient atmosphere. Before spin-coating on the substrates, the solution is filtered with filter paper to avoid particulate contamination. The thermal treatment process for the samples is completed in a hot plate and a RTA furnace. The coating solution of PZT films was deposited onto Pt(1 1 1)yTiy SiO2 ySi(1 0 0) substrates by spin-coating at 3000 rpm for 30 s. After each spin-coating process, sample was heat-treated at 320 or 400 8C for 10 min in air atmosphere using a hot plate. This step is repeated several times to obtain the desired thickness of the films. The PZT films on PtyTiySiO2 ySi substrates annealed at 550–600 8C for 5 min by RTA in oxygen atmosphere. The thickness of the annealed PZT films with four and six layers, as measured by Dektak Stylus Profilers (Digital Instruments, USA), were 0.21 and 0.30 mm, respectively, for the films thermally decomposed at 400 and 320 8C. The structure and surface morphology of PZT thin films deposited on PtyTiySiO2 ySi substrates were analysed by an DyMax 2550V (Rakagu, Japan) X-ray diffraction (XRD), an NanoScope IIIa atomic force microscopy (AFM) (Digital Instruments) or a SPM9500J AFM (Shimadzu, Japan). To investigate the electrical properties of prepared PZT thin films, the top electrodes of gold (Au) of 0.2 mm diameter were prepared on the top surface of the PZT films through a shadow mask in a vacuum evaporation system. The polarization vs. electric field (P–E) loop of the specimens was measured using a Radiant Technologies RT66 ferroelectrics instrument. The dielectric permittivity of the specimens was calculated from the study of capacitance vs. frequency (C–F) characteristic. The C–F characteristics were measured by an HP4192A LCR impedance analyser with a small a.c. signal of 50 mV.
Fig. 1. XRD patterns of PZT films on Pt(1 1 1)yTiySiO2 ySi(1 0 0) substrates annealed at 550 and 600 8C for 5 min in oxygen atmosphere by RTA, before thermal decompose at (a) 400 and (b) 320 8C, respectively. Pyro, pyrochlore phase.
3. Results and discussion Fig. 1a and b show the XRD patterns of the PZT thin films annealed for 5 min in oxygen atmosphere by RTA at various temperatures range from 550 to 600 8C, before the thermally decompose temperature were 320 and 400 8C, respectively. The XRD results also exhibit that the films’ thermally decomposition temperature was 400 8C, and after subsequent annealing at various temperature ranges from 550 to 600 8C, the crystalline PZT sample displayed strong (1 1 1) orientation (Fig. 1a). The peaks corresponding to (1 0 0), (1 1 0), (1 1 1), (2 0 0) and (2 1 1) were observed in the films. The relative peak intensity of I(1 1 1)ySI(h k l) was 75 and 86% for the films annealed at 550 and 600 8C, respectively. For films produced using an initial thermally decomposition temperature of 320 8C, then subsequently annealed at a temperature of 550 8C, only the peaks corresponding to (1 1 0) and (1 1 1) were observed in the film, with the relative peak intensity of I(1 1 1)y SI(h k l) being 98% (Fig. 1b). The (1 1 1) peak indicated that the (1 1 1) orientation increased appreciably
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(Fig. 1a), we know that there are pyrochlore phases in the film. Brooks et al. reported that there are metastable pyrochlore phases observed by the TEM and XRD w13x. When the annealing temperature is increased to 600 8C, the nanocrystalline on the surface of clusters disappear (Fig. 2b), it indicated that the metastable pyrochlore phase crystallization to PZT perovskite phase. The result agrees with XRD. The root mean square (RMS) roughness of the films surfaces are 7.4 and 9.7 nm, respectively for the films annealed at 550 and 600 8C. During RTA the PZT perovskite nuclei form on the Pt and crystallize on the bottom electrode and then growth proceeds from the interface uniformly to the surface in columnar grains that push excess Pb and inclusions to the film surface. The excess Pb on the surface evaporates leaving a fine-grained dense film (Fig. 2a and b). As the annealing progresses the nuclei grow into half-dome shapes pushing out excess Pb in front of the perovskite grain as well as some Zr and inclusions w14x. From the XRD and AFM results, we can infer that heterogeneous nucleation is plays a dominant role in the nucleation of the perovskite phase. Fig. 3 show the AFM image of highly (1 1 1)-oriented PZT thin films grown on PtyTiySiO2 ySi substrate, thermally decompose temperature was 320 8C, then annealed at 550 8C for 5 min in an oxygen atmosphere by RTA. The surface of the film is smooth with grain size of approximately 200–300 nm, and RMS roughness of 0.92 nm. This suggests that the higher the orientation, the smaller the roughness. Fig. 4 shows a typical polarization–electric field (P– E) hysteresis loop for the PZT films on PtyTiySiO2 ySi substrates, thermally decompose temperature was 400
Fig. 2. The AFM images of PZT thin films on PtyTiySiO2ySi substrates thermally decomposed at 400 8C, then annealed at (a) 550 and (b) 600 8C for 5 min in oxygen atmosphere by RTA, respectively.
with increasing annealing temperature due to further enhancement in crystallization. The surface morphology of the films was characterized by a NanoScope IIIa (Digital Instruments) AFM. Fig. 2 shows the AFM image of (1 1 1)-oriented PZT thin films on PtyTiySiO2 ySi substrates thermally decomposed at a temperature of 400 8C, then annealed at 550 and 600 8C for 5 min in an oxygen atmosphere by RTA. On the surface image of PZT thin films annealed at 550 8C (Fig. 2a), many clusters are found, which are composed of nano-sized particles of approximately 70– 120 nm surrounding larger particles of 0.6–1.0 mm in size. At the surface of the clusters, there are nanocrystalline particles with size of 30–50 nm. From the XRD
Fig. 3. The AFM image of PZT thin film on PtyTiySiO2 ySi substrate thermally decomposed at 320 8C, then annealed at 550 8C for 5 min in oxygen atmosphere by RTA.
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Fig. 4. The typical P–E hysteresis loops for PZT thin films deposited on PtyTiySiO2ySi substrates thermally decomposed at 400 8C, then annealed at (a) 550 and (b) 600 8C for 5 min in oxygen atmosphere by RTA.
Fig. 6. The variation of dielectric constant with the frequency for the PZT films deposited on PtyTiySiO2ySi substrates thermally decomposed at 400 8C, then annealed at (a) 550 and (b) 600 8C for 5 min in oxygen atmosphere by RTA.
8C, then annealed at 550–600 8C for 5 min in the oxygen atmosphere by a RTA process. The average remanent polarization (Pr) and the coercive electric field (Ec) obtained from the P–E hysteresis loops, are 16.1 mCycm2 and 105 kVycm, 21.3 mCycm2 and 105 kVy cm, respectively, for PZT thin films annealed at 550 and 600 8C. Fig. 5 shows a typical polarization–electric field (P– E) hysteresis loop for the PZT films on PtyTiySiO2 y Si(1 0 0) substrates. Here the thermally decomposition temperature was 320 8C, with annealing at 550 8C for 5 min in the oxygen atmosphere by the RTA process. The average remanent polarization (Pr ) and the coercive electric field (Ec) obtained from the P–E hysteresis loops, are 32.2 mCycm2 and 79.9 kVycm, respectively. This shows a good ferroelectricity of the prepared PZT films on PtyTiySiO2 ySi substrates by the simple sol– gel process. The remanent polarization value is higher
than that of PLD epitaxial PZT films (6.5 mCycm2) w15x, and compare with that of sol–gel derived (1 1 1) oriented PZT(65y35) (31.3 mCycm2) and PZT(35y65) films (20.9 mCycm2) w16x. In general, it was found that the higher orientation, the better the ferroelectric properties obtained. The dielectric constant and dissipation factor measurements were made at room temperature as a function of frequency in the range of 100 Hz to 1 MHz for the film deposited on PtyTiySiO2 ySi substrates. Figs. 6 and 7 show the variation of dielectric constant and dissipation factor with the frequency for PZT thin films deposited on PtyTiySiO2 ySi substrates, with thermally decomposition temperature of 400 and 320 8C, respectively, followed by annealing at 550–600 8C and 550 8C for 5 min in an oxygen atmosphere by RTA. From
Fig. 5. The typical P–E hysteresis loops for PZT thin film deposited on PtyTiySiO2ySi substrate thermally decomposed at 320 8C, then annealed at 550 8C for 5 min in oxygen atmosphere by RTA.
Fig. 7. The variation of dissipation factor with the frequency for PZT film deposited on PtyTiySiO2ySi substrate thermally decomposed at 320 8C, then annealed at 550 8C for 5 min in oxygen atmosphere by RTA.
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Fig. 6, the dielectric constant and dissipation factor were 331 and 0.045, 380 and 0.034, respectively, at 100 kHz for the films annealed at 550 and 600 8C for 5 min. The PZT thin films annealed at 600 8C, exhibit good ferroelectric and dielectric properties. From Fig. 7, it can be seen that the dielectric constant and dissipation factor were 539 and 0.066, respectively, at 100 kHz for the films annealed at 550 8C for 5 min. The PZT thin films grown on PtyTiySiO2 ySi substrates decomposed at 320 8C, then annealed at 550 8C for 5 min in the oxygen atmosphere by RTA process, exhibit high relative peak intensity. Compared with that of films decomposed at 400 8C, then annealed at 550 8C, the highly (1 1 1) oriented PZT film has good ferroelectric and dielectric properties. 4. Conclusions The role of processing effect on the microstructure and ferroelectric properties was investigated for PZT films were grown on Pt(1 1 1)yTiySiO2 ySi(1 0 0) substrates. PZT films prepared by the sol–gel route with various thermally decomposition temperatures and RTA processing. Highly (1 1 1)-oriented PZT thin films have been obtained. Using the same annealing temperature, but changing the thermally decomposition temperatures from 320 to 400 8C, produces differences in the microstructure and surface images of the PZT films obtained. The PZT film grown on PtyTiySiO2 ySi substrate with thermally decomposition temperature of 320 8C, then annealed at 550 8C gave a dense, smooth surface, and exhibited remanent polarization (Pr) and coercive electric field (Ec) values of 32.2 mCycm2 and 79.9 kVycm, respectively, at an applied electric voltage of 10 V; the typical small signal dielectric constant and dissipation factor at a frequency of 100 kHz were 539 and 0.066, respectively. By the sol–gel route with various thermally decompose temperature and low temperature annealing
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processing, we obtained the highly (1 1 1) oriented PZT thins film with high dielectric constant and good ferroelectric properties. Acknowledgments This work is supported by the Centre for Smart Materials of the Hong Kong Polytechnic University, the K.-C. Wong Education Foundation of Hong Kong, and the National Natural Science Foundation of China (No. 59995520). References w1x J.F. Scott, C.A. Paz de Araujo, Science 246 (1989) 1400. w2x D.L. Polla, L.F. Francis, Annu. Rev. Mater. Sci. 28 (1998) 563. w3x D.L. Polla, L.F. Francis, MRS Bull. 21 (1996) 59. w4x X.H. Pu, W.G. Luo, A.L. Ding, H.Y. Tian, P.S. Qiu, Phys. Stat. Sol. (a) 182 (2000) R10. w5x Y.S. Yang, S.J. Lee, S. Yi, et al., App. Phys. Lett. 76 (2000) 774. w6x G. Suchaneck, R. Koehler, P. Padmini, T. Sandner, J. Frey, G. Gerlach, Surf. Coat. Technol. 116–119 (1999) 1238. w7x A. Husmann, D.A. Wesner, J. Schmidt, T. Klotzbucher, M. Mergens, E.W. Kreutz, Surf. Coat. Technol. 97 (1997) 420. w8x D.H. Kim, J.S. Na, S.W. Rhee, J. Electrochem. Soc. 148 (2001) C668. w9x X.G. Tang, A.L. Ding, Y. Ye, W.G. Luo, Ferroelectrics 264 (2001) 297. w10x C.K. Kwok, S.B. Desu, J. Mater. Res. 8 (1993) 339. w11x B.M. Xu, Y.H. Ye, L.E. Cross, Appl. Phys. Lett. 74 (1999) 3549. w12x X.G. Tang, A.L. Ding, W.G. Luo, Appl. Surf. Sci. 174 (2001) 148. w13x K.G. Brooks, I.M. Reaney, R. Klissurska, Y. Huang, L. Bursill, N. Setter, J. Mater. Res. 9 (1994) 2540. w14x J.S. Cross, M. Fujiki, M. Tsukada, Y. Kotaka, Y. Goto, J. Mater. Res. 14 (1999) 4366. w15x C. Guerrero, J. Roldan, C. Ferrater, M.V. Garcia-Cuenca, F. Sanchez, M. Varela, Solid State Electron. 45 (2001) 1433. w16x S.-Y. Chen, C.-L. Sun, J. Appl. Phys. 90 (2001) 2970.