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PbO thin films prepared by radio frequency magnetron sputtering
Materials Research Bulletin 43 (2008) 556–560 www.elsevier.com/locate/matresbu
Enhanced ferroelectric properties of Pb(Zr0.80Ti0.20)O3/PbO thin films prepared by radio frequency magnetron sputtering Jiagang Wu a,*, Dingquan Xiao a, Junzhe Tan a, Jiliang Zhu a, Jianguo Zhu a, Yunfei Tian a,b a
Department of Materials Science, College of Materials Science and Engineering, Sichuan University, Chengdu 610064, China b Analysis and Test Center, Sichuan University, Chengdu 610064, China Received 4 February 2007; received in revised form 10 April 2007; accepted 12 April 2007 Available online 18 April 2007
Abstract The Pb(Zr0.80Ti0.20)O3 (PZT) thin films with and without a PbO buffer layer were deposited on the Pt(1 1 1)/Ti/SiO2/Si(1 0 0) substrates by radio frequency (rf) magnetron sputtering method. The PbO buffer layer improves the microstructure and electrical properties of the PZT thin films. High phase purity and good microstructure of the PZT thin films with a PbO buffer layer were obtained. The effect of the PbO buffer layer on the ferroelectric properties of the PZT thin films was also investigated. The PZT thin films with a PbO buffer layer possess better ferroelectric properties with higher remnant polarization (Pr = 25.6 mC/cm2), and lower coercive field (Ec = 60.5 kV/cm) than that of the films without a PbO buffer layer (Pr = 9.4 mC/cm2, Ec = 101.3 kV/cm). Enhanced ferroelectric properties of the PZT thin films with a PbO buffer layer is attributed to high phase purity and good microstructure. # 2007 Elsevier Ltd. All rights reserved. Keywords: A. Thin films; B. Sputtering; D. Ferroelectricity
1. Introduction Ferroelectric thin films have attracted much attention in recent years, because of their practical or potential applications in various fields, especially for ferroelectric random access memories (FeRAM) [1,2]. Among the researchers, lead zirconate titanate [Pb(Zr1 xTix)O3 (PZT)] thin films have been one of the most extensively studied materials for nonvolatile random access memory application due to large remnant polarization and low coercive field [3,4]. Recently, a few papers [5,6] reported that the rhombohedral (x < 0.48) PZT films possessed several important ferroelectric properties including low coercive field, large remnant polarization, low leakage current density, which are correlative to FeRAM capacitors, moreover, are superior as compared with those in the MPB region or in the tetragonal side of the MPB. The phase transformation temperature of PZT thin films usually depends on the Zr/Ti ratio, so the PZT films with higher Zr/Ti ratio require higher transformation temperature [7,8]. However, high temperature processing is easy to cause the diffusion of Pb from the films to the substrate, and results in undesirable pyrochlore phase. As a result, it is very important for fabricating Zr-rich PZT thin films to control the perovskite phase purity.
* Corresponding author. Tel.: +86 28 85412415; fax: +86 28 85415045. E-mail address: [email protected] (J. Wu). 0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2007.04.017
J. Wu et al. / Materials Research Bulletin 43 (2008) 556–560
557
In this work, the Pb(Zr0.80Ti0.20)O3 (PZT) thin films with and without a PbO buffer layer were deposited on the Pt(1 1 1)/Ti/SiO2/Si(1 1 1) substrate by radio frequency (rf) magnetron sputtering method. The effect of PbO buffer layer on the microstructure and ferroelectric properties of the PZT thin films is mainly investigated. 2. Experimental In this work, we report the preparation of Pb(Zr0.80Ti0.20)O3 thin films with and without a PbO buffer layer on the Pt(1 1 1)/Ti/SiO2/Si(1 0 0) substrate by rf magnetron sputtering at the substrate temperature of 450 8C, and followed by annealing in air by conventional furnace annealing (CFA). Sputtering targets of PbO and PZT thin films were made by a conventional mixing-oxides method, 10 mol% excess PbO was added to PZT target for compensating for the loss of Pb in PZT targets. Sputtering of PbO buffer layer and PZT thin films was carried out at a power of 50 and 80 W, respectively. A high purity sputtering gas is composed of 20% O2 + 80% Ar under a working pressure of 2.0 Pa. The PZT thin films without a PbO buffer layer were sputtered with the PZT targets under the same conditions as the PZT films with a PbO buffer layer for comparison. ˚ ) in X-ray diffraction (XRD) characterization of the films was performed by using Cu Ka radiation (l = 1.54178 A the mode of u–2u scan (DX-1000, Dandong, China). The cross-sectional images of the films were observed by scanning electron microscopy (SEM, JSM-5900, Japan). The surface morphology was studied by atomic force microscope (AFM, SPA-400, Japan). The chemical composition analysis of the films was carried out by energy dispersive X-ray (EDX) in field emission scanning electron microscope (FE-SEM, HITACHI S4800, Japan). In order to measure the ferroelectric properties of the films, dot-type gold electrodes with an area of 20 10 4 cm2 were deposited by dc sputtering, which form stacked capacitors. The hysteresis loops were evaluated by using Radiant Precision Ferroelectric Measurement System (Precision Workstation Tester, USA). 3. Results and discussion The preparation of Zr-rich PZT thin films on Pt-coated Si substrate is not easily straight to form perovskite phase, but the pyrochlore phase always forms preferentially during heat treatment of film [9]. In order to improve the phase purity of PZT thin films, in this work, the PbO was used for the buffer layer, the PZT thin films with a PbO buffer layer were deposited at the substrate temperature of 450 8C, and followed by annealing at 650 8C in air by conventional furnace annealing. Fig. 1(a and b) shows the XRD patterns of the PZT thin films with and without a PbO buffer layer. It is found that the PZT thin films with a PbO buffer layer possess pure perovskite structure with random orientation. However, the PZT thin films without a PbO buffer layer possess a mixture of pyrochlore and perovskite structure. Such a phenomenon is because the lead element of the PbO buffer layer is also one of the main elements of PZT thin films, and the lead element of the films may be partially volatile during film’s deposition. We regard in this case that the lead
Fig. 1. X-ray diffraction patterns of the PZT thin films with (a) and without (b) a PbO buffer layer.
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Fig. 2. Cross-section of the PZT thin films with (a) and without (b) a PbO buffer layer.
element of the PbO buffer layer enters the lattice of PZT thin films compensating for the loss of the lead during heat treatment of film. As a result, the PbO buffer layer improves perovskite phase purity of the PZT thin films. Fig. 2(a and b) shows the SEM images of the cross-sections of the PZT thin films with and without a PbO buffer layer. From Fig. 2, it can be seen that the thickness of the PZT thin films deposited is approximately 0.75–0.85 mm. It will be noted that there are no evidences showing the PbO buffer layer at the interface between the PZT thin film and the platinum electrode in Fig. 2(a). This is because the lead element of the PbO buffer layer enters the lattice of PZT thin films compensating for the loss of the lead during the processing. As compared with the PZT thin films without a PbO buffer layer, the PZT thin films with a PbO buffer layer possess dense, crack-free, and well adhered on the substrates. Fig. 3 shows the AFM patterns of PZT thin films with and without a PbO buffer layer. The PZT thin films with a PbO buffer layer possess better microstructure with good developed grains (<100 nm). As shown in Fig. 3, there is an increase in the grain size of the PZT thin films without a PbO buffer layer compared with the films with a PbO buffer layer. It is also found that the PZT thin films with a PbO buffer layer possess lower root mean square (RMS) surface roughness than that of the films without a PbO buffer layer. As a result, the PZT thin films with good microstructure can be obtained by inserting the PbO buffer layer between the PZT thin films and Pt(1 1 1)/Ti/SiO2/Si(1 0 0) substrate. The chemical composition analysis of the films was carried out by EDX in field emission scanning electron microscope (FE-SEM, HITACHI S4800, Japan) for identifying the chemical composition of the PZT thin films with a PbO buffer layer. The EDX spectra of the PZT thin films with a PbO buffer layer is shown in Fig. 4. The film is
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Fig. 3. AFM patterns of the PZT thin films with (a) and without (b) a PbO buffer layer.
composed of Pb, Ti, Zr, O, and C element, the C element functions as the conducting layer in the investigation of FESEM. From the EDX measurement, the ratio of Zr and Ti in the PZT thin films with a PbO buffer layer is approached to 4:1, the PZT thin films with desired compositions were obtained, and the PbO buffer layer helps to maintain chemical composition of the PZT thin films. Fig. 5(a and b) gives the typical P–E hysteresis loops of the PZT thin films with and without a PbO buffer layer, which were measured at room temperature by using Radiant Precision Ferroelectric Measurement System (Precision Workstation Tester, USA). As shown in Fig. 5, the PZT thin films with a PbO buffer layer possess good ferroelectric properties (Pr = 25.6 mC/cm2, Ec = 60.5 kV/cm) which is better as compared with the PZT thin films without a PbO buffer layer (Pr = 9.4 mC/cm2, Ec = 101.3 kV/cm), and also larger compared with the literature values of 10–20 mC/ cm2 for PZT thin films with the compositions in the rhombohedral side [5,7,10]. Enhanced ferroelectric properties of the PZT thin films in this work are due to the PbO buffer layer, because the PbO buffer layer improves perovskite phase purity and microstructure of the PZT thin films. The fact that the Zr-rich PZT thin films with a PbO buffer layer in
Fig. 4. EDX spectra of the PZT thin films with a PbO buffer layer.
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Fig. 5. P–E hysteresis loops of the PZT thin films with (a) and without (b) a PbO buffer layer.
present work possess large remnant polarization together with low coercive field indicates that the Zr-rich PZT thin film with a PbO buffer layer is a good candidate for FeRAM, especially for high-density memories application. 4. Conclusion The Pb(Zr0.80Ti0.20)O3 thin films with and without a PbO buffer layer were deposited on the Pt(1 1 1)/Ti/SiO2/ Si(1 0 0) substrates by rf magnetron sputtering method. The PbO buffer layer improves the perovskite phase purity and microstructure of the films. The PZT thin films with a PbO buffer layer possess good ferroelectric properties, the values of Pr and Ec are 25.6 mC/cm2 and 60.5 kV/cm, respectively. The result shows the Zr-rich PZT thin film with a PbO buffer layer is a good candidate for FeRAM, especially for high-density memories application. Acknowledgements The authors gratefully acknowledge the support of the National Science Foundation of China (Grant No. 50132020) and the National Key Fundamental Research Program (Grant No. Z0601). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
J.F. Scott, C.A.P. de Araujo, Science 246 (1989) 1400. O. Auciello, J.F. Scott, R. Ramesh, Phys. Today 51 (1998) 22. Y.J. Yu, H.L.W. Chan, F.P. Wang, L.C. Zhao, Mater. Lett. 58 (2004) 1885. C.H. Jeon, C.S. Kim, K.B. Han, H.S. Jhon, S.Y. Lee, Mater. Sci. Eng. B 109 (2004) 141. S. Kalpat, K. Uchino, J. Appl. Phys. 90 (2001) 2703. W. Gong, J.F. Li, X.C. Chu, Z.L. Gui, L.T. Li, Appl. Phys. Lett. 85 (2004) 3818. D.J. Kim, J.P. Maria, A.I. Kingon, S.K. Streiffer, J. Appl. Phys. 93 (2003) 5568. C.C. Chang, P.C. Lu, J. Mater. Process. Technol. 95 (1999) 128. H.S. Kang, W.J. Lee, J. Vac. Sci. Technol. A 20 (2002) 1498. W.S. Wang, T. Fujii, T. Karaki, M. Adachi, Jpn. J. Appl. Phys. 38 (1999) 6807.
Enhanced ferroelectric properties of Pb(Zr0.80Ti0.20)O3/PbO thin films prepared by radio frequency magnetron sputtering Jiagang Wu a,*, Dingquan Xiao a, Junzhe Tan a, Jiliang Zhu a, Jianguo Zhu a, Yunfei Tian a,b a
Department of Materials Science, College of Materials Science and Engineering, Sichuan University, Chengdu 610064, China b Analysis and Test Center, Sichuan University, Chengdu 610064, China Received 4 February 2007; received in revised form 10 April 2007; accepted 12 April 2007 Available online 18 April 2007
Abstract The Pb(Zr0.80Ti0.20)O3 (PZT) thin films with and without a PbO buffer layer were deposited on the Pt(1 1 1)/Ti/SiO2/Si(1 0 0) substrates by radio frequency (rf) magnetron sputtering method. The PbO buffer layer improves the microstructure and electrical properties of the PZT thin films. High phase purity and good microstructure of the PZT thin films with a PbO buffer layer were obtained. The effect of the PbO buffer layer on the ferroelectric properties of the PZT thin films was also investigated. The PZT thin films with a PbO buffer layer possess better ferroelectric properties with higher remnant polarization (Pr = 25.6 mC/cm2), and lower coercive field (Ec = 60.5 kV/cm) than that of the films without a PbO buffer layer (Pr = 9.4 mC/cm2, Ec = 101.3 kV/cm). Enhanced ferroelectric properties of the PZT thin films with a PbO buffer layer is attributed to high phase purity and good microstructure. # 2007 Elsevier Ltd. All rights reserved. Keywords: A. Thin films; B. Sputtering; D. Ferroelectricity
1. Introduction Ferroelectric thin films have attracted much attention in recent years, because of their practical or potential applications in various fields, especially for ferroelectric random access memories (FeRAM) [1,2]. Among the researchers, lead zirconate titanate [Pb(Zr1 xTix)O3 (PZT)] thin films have been one of the most extensively studied materials for nonvolatile random access memory application due to large remnant polarization and low coercive field [3,4]. Recently, a few papers [5,6] reported that the rhombohedral (x < 0.48) PZT films possessed several important ferroelectric properties including low coercive field, large remnant polarization, low leakage current density, which are correlative to FeRAM capacitors, moreover, are superior as compared with those in the MPB region or in the tetragonal side of the MPB. The phase transformation temperature of PZT thin films usually depends on the Zr/Ti ratio, so the PZT films with higher Zr/Ti ratio require higher transformation temperature [7,8]. However, high temperature processing is easy to cause the diffusion of Pb from the films to the substrate, and results in undesirable pyrochlore phase. As a result, it is very important for fabricating Zr-rich PZT thin films to control the perovskite phase purity.
* Corresponding author. Tel.: +86 28 85412415; fax: +86 28 85415045. E-mail address: [email protected] (J. Wu). 0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2007.04.017
J. Wu et al. / Materials Research Bulletin 43 (2008) 556–560
557
In this work, the Pb(Zr0.80Ti0.20)O3 (PZT) thin films with and without a PbO buffer layer were deposited on the Pt(1 1 1)/Ti/SiO2/Si(1 1 1) substrate by radio frequency (rf) magnetron sputtering method. The effect of PbO buffer layer on the microstructure and ferroelectric properties of the PZT thin films is mainly investigated. 2. Experimental In this work, we report the preparation of Pb(Zr0.80Ti0.20)O3 thin films with and without a PbO buffer layer on the Pt(1 1 1)/Ti/SiO2/Si(1 0 0) substrate by rf magnetron sputtering at the substrate temperature of 450 8C, and followed by annealing in air by conventional furnace annealing (CFA). Sputtering targets of PbO and PZT thin films were made by a conventional mixing-oxides method, 10 mol% excess PbO was added to PZT target for compensating for the loss of Pb in PZT targets. Sputtering of PbO buffer layer and PZT thin films was carried out at a power of 50 and 80 W, respectively. A high purity sputtering gas is composed of 20% O2 + 80% Ar under a working pressure of 2.0 Pa. The PZT thin films without a PbO buffer layer were sputtered with the PZT targets under the same conditions as the PZT films with a PbO buffer layer for comparison. ˚ ) in X-ray diffraction (XRD) characterization of the films was performed by using Cu Ka radiation (l = 1.54178 A the mode of u–2u scan (DX-1000, Dandong, China). The cross-sectional images of the films were observed by scanning electron microscopy (SEM, JSM-5900, Japan). The surface morphology was studied by atomic force microscope (AFM, SPA-400, Japan). The chemical composition analysis of the films was carried out by energy dispersive X-ray (EDX) in field emission scanning electron microscope (FE-SEM, HITACHI S4800, Japan). In order to measure the ferroelectric properties of the films, dot-type gold electrodes with an area of 20 10 4 cm2 were deposited by dc sputtering, which form stacked capacitors. The hysteresis loops were evaluated by using Radiant Precision Ferroelectric Measurement System (Precision Workstation Tester, USA). 3. Results and discussion The preparation of Zr-rich PZT thin films on Pt-coated Si substrate is not easily straight to form perovskite phase, but the pyrochlore phase always forms preferentially during heat treatment of film [9]. In order to improve the phase purity of PZT thin films, in this work, the PbO was used for the buffer layer, the PZT thin films with a PbO buffer layer were deposited at the substrate temperature of 450 8C, and followed by annealing at 650 8C in air by conventional furnace annealing. Fig. 1(a and b) shows the XRD patterns of the PZT thin films with and without a PbO buffer layer. It is found that the PZT thin films with a PbO buffer layer possess pure perovskite structure with random orientation. However, the PZT thin films without a PbO buffer layer possess a mixture of pyrochlore and perovskite structure. Such a phenomenon is because the lead element of the PbO buffer layer is also one of the main elements of PZT thin films, and the lead element of the films may be partially volatile during film’s deposition. We regard in this case that the lead
Fig. 1. X-ray diffraction patterns of the PZT thin films with (a) and without (b) a PbO buffer layer.
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J. Wu et al. / Materials Research Bulletin 43 (2008) 556–560
Fig. 2. Cross-section of the PZT thin films with (a) and without (b) a PbO buffer layer.
element of the PbO buffer layer enters the lattice of PZT thin films compensating for the loss of the lead during heat treatment of film. As a result, the PbO buffer layer improves perovskite phase purity of the PZT thin films. Fig. 2(a and b) shows the SEM images of the cross-sections of the PZT thin films with and without a PbO buffer layer. From Fig. 2, it can be seen that the thickness of the PZT thin films deposited is approximately 0.75–0.85 mm. It will be noted that there are no evidences showing the PbO buffer layer at the interface between the PZT thin film and the platinum electrode in Fig. 2(a). This is because the lead element of the PbO buffer layer enters the lattice of PZT thin films compensating for the loss of the lead during the processing. As compared with the PZT thin films without a PbO buffer layer, the PZT thin films with a PbO buffer layer possess dense, crack-free, and well adhered on the substrates. Fig. 3 shows the AFM patterns of PZT thin films with and without a PbO buffer layer. The PZT thin films with a PbO buffer layer possess better microstructure with good developed grains (<100 nm). As shown in Fig. 3, there is an increase in the grain size of the PZT thin films without a PbO buffer layer compared with the films with a PbO buffer layer. It is also found that the PZT thin films with a PbO buffer layer possess lower root mean square (RMS) surface roughness than that of the films without a PbO buffer layer. As a result, the PZT thin films with good microstructure can be obtained by inserting the PbO buffer layer between the PZT thin films and Pt(1 1 1)/Ti/SiO2/Si(1 0 0) substrate. The chemical composition analysis of the films was carried out by EDX in field emission scanning electron microscope (FE-SEM, HITACHI S4800, Japan) for identifying the chemical composition of the PZT thin films with a PbO buffer layer. The EDX spectra of the PZT thin films with a PbO buffer layer is shown in Fig. 4. The film is
J. Wu et al. / Materials Research Bulletin 43 (2008) 556–560
559
Fig. 3. AFM patterns of the PZT thin films with (a) and without (b) a PbO buffer layer.
composed of Pb, Ti, Zr, O, and C element, the C element functions as the conducting layer in the investigation of FESEM. From the EDX measurement, the ratio of Zr and Ti in the PZT thin films with a PbO buffer layer is approached to 4:1, the PZT thin films with desired compositions were obtained, and the PbO buffer layer helps to maintain chemical composition of the PZT thin films. Fig. 5(a and b) gives the typical P–E hysteresis loops of the PZT thin films with and without a PbO buffer layer, which were measured at room temperature by using Radiant Precision Ferroelectric Measurement System (Precision Workstation Tester, USA). As shown in Fig. 5, the PZT thin films with a PbO buffer layer possess good ferroelectric properties (Pr = 25.6 mC/cm2, Ec = 60.5 kV/cm) which is better as compared with the PZT thin films without a PbO buffer layer (Pr = 9.4 mC/cm2, Ec = 101.3 kV/cm), and also larger compared with the literature values of 10–20 mC/ cm2 for PZT thin films with the compositions in the rhombohedral side [5,7,10]. Enhanced ferroelectric properties of the PZT thin films in this work are due to the PbO buffer layer, because the PbO buffer layer improves perovskite phase purity and microstructure of the PZT thin films. The fact that the Zr-rich PZT thin films with a PbO buffer layer in
Fig. 4. EDX spectra of the PZT thin films with a PbO buffer layer.
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J. Wu et al. / Materials Research Bulletin 43 (2008) 556–560
Fig. 5. P–E hysteresis loops of the PZT thin films with (a) and without (b) a PbO buffer layer.
present work possess large remnant polarization together with low coercive field indicates that the Zr-rich PZT thin film with a PbO buffer layer is a good candidate for FeRAM, especially for high-density memories application. 4. Conclusion The Pb(Zr0.80Ti0.20)O3 thin films with and without a PbO buffer layer were deposited on the Pt(1 1 1)/Ti/SiO2/ Si(1 0 0) substrates by rf magnetron sputtering method. The PbO buffer layer improves the perovskite phase purity and microstructure of the films. The PZT thin films with a PbO buffer layer possess good ferroelectric properties, the values of Pr and Ec are 25.6 mC/cm2 and 60.5 kV/cm, respectively. The result shows the Zr-rich PZT thin film with a PbO buffer layer is a good candidate for FeRAM, especially for high-density memories application. Acknowledgements The authors gratefully acknowledge the support of the National Science Foundation of China (Grant No. 50132020) and the National Key Fundamental Research Program (Grant No. Z0601). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
J.F. Scott, C.A.P. de Araujo, Science 246 (1989) 1400. O. Auciello, J.F. Scott, R. Ramesh, Phys. Today 51 (1998) 22. Y.J. Yu, H.L.W. Chan, F.P. Wang, L.C. Zhao, Mater. Lett. 58 (2004) 1885. C.H. Jeon, C.S. Kim, K.B. Han, H.S. Jhon, S.Y. Lee, Mater. Sci. Eng. B 109 (2004) 141. S. Kalpat, K. Uchino, J. Appl. Phys. 90 (2001) 2703. W. Gong, J.F. Li, X.C. Chu, Z.L. Gui, L.T. Li, Appl. Phys. Lett. 85 (2004) 3818. D.J. Kim, J.P. Maria, A.I. Kingon, S.K. Streiffer, J. Appl. Phys. 93 (2003) 5568. C.C. Chang, P.C. Lu, J. Mater. Process. Technol. 95 (1999) 128. H.S. Kang, W.J. Lee, J. Vac. Sci. Technol. A 20 (2002) 1498. W.S. Wang, T. Fujii, T. Karaki, M. Adachi, Jpn. J. Appl. Phys. 38 (1999) 6807.