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Enhancement of the ferroelectric properties of Pb(Zr0.53Ti0.47)O3 thin films fabricated by laser ablation
Materials Science and Engineering B 109 (2004) 141–145
Enhancement of the ferroelectric properties of Pb(Zr0.53Ti0.47)O3 thin films fabricated by laser ablation Chang Hoon Jeon, Cheol Su Kim, Kyoung Bo Han, Hee Sauk Jhon, Sang Yeol Lee∗ Department of Electrical and Electronic Engineering, Yonsei University, 134 Shinchondong, Seodaemun-ku, Seoul 120-749, South Korea
Abstract Thin films of phase-pure perovskite Pb(Zr0.53 Ti0.47 )O3 (PZT) were fabricated in situ onto Pt/Ti/SiO2 /Si substrates by pulsed laser deposition (PLD) using a Nd:YAG laser. We have systematically investigated the effect of various parameters, such as substrate temperatures (450–650 ◦ C) and energy densities (1.5–4 J/cm2 ) and annealing time (5–30 min), on the property of PZT thin film. X-ray diffraction (XRD), scanning electron microscope (SEM), C–V measurement and hysteresis were used to investigate the electrical, micro structural properties of the thin films. At the optimized deposition condition, remnant polarization, coercive electric field and dielectric constant of the film were 31.5 C/cm2 , 50.5 kV/cm and 930, respectively. © 2003 Elsevier B.V. All rights reserved. Keywords: Pb(Zr0.53 ,Ti0.47 )O3 ; Pulsed laser deposition; Perovskite structure; Remnant polarization
1. Introduction Ferroelectric thin films of Pb-based perovskite materials have attracted great interest for various applications, such as high dielectric, piezoelectric, pyroelectric, and electro-optic devices and nonvolatile memories [1,2]. Because of their high remnant polarization (Pr ) values, low coercive field (Ec ), and excellent piezoelectric properties, most work has focused on Pb-based perovskite materials as potential candidates for FRAM and microelectromechanical systems (MEMS) applications [1–4]. Among Pb-based thin films, Pb(Zrx ,Ti1−x )O3 (PZT) thin film has been regarded as one of the most promising materials because it exhibits various properties by changing its composition, crystallographic structure, microstructure, orientation, and properties of the substrate [3]. Among the popular thin film preparation techniques, such as sol–gel, sputtering, metal-organic chemical vapor deposition (MOCVD), and pulsed laser deposition (PLD or laser ablation), the laser ablation technique has advantages over other thin film deposition techniques for processing these ∗
Corresponding author. Tel.: +82-2-2123-2776; fax: +82-2-364-9770. E-mail address: [email protected] (S.Y. Lee).
0921-5107/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2003.10.033
thin films in the accuracy in composition control and the simplicity in process control [5–7]. In this paper, we have systematically investigated the effect of various parameters, such as substrate temperature, energy density, and annealing time on the microstructural and electrical properties of PZT thin film fabricated by PLD.
2. Experiment PZT thin films were prepared on Pt/Ti/SiO2 /Si substrate by the pulsed laser deposition technique, using pulsed Nd:YAG laser (Quantel Brilliant Q-switched Nd:YAG laser λ = 355 nm). The laser beam was focused on the target surface with an elliptical spot using a lens and a turning mirror outside the vacuum chamber. The laser power was exactly measured by a powermeter (Quantel portable powermeter: TPM-310B). The laser beam had an incident angle of approximately 45◦ with respect to the normal of the target surface. The ratio of Zr to Ti is 53:47, and this ratio corresponds to the rhombohedral-side of the morphotropic phase boundary (MPB). We have chosen this particular composition because it is known to have a excellent ferroelectric and piezoelectric properties by virtue of its proximity to
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Fig. 1. SEM images of PZT thin films as varying deposition temperature: (a) 450; (b) 500; (c) 550; (d) 600; and (e) 650 ◦ C.
C.H. Jeon et al. / Materials Science and Engineering B 109 (2004) 141–145
the MPB composition [8]. Because in the deposition process much of the lead is lost owing to reduced sticking coefficients at elevated deposition temperature, PZT with excess amounts lead oxide was used as a target. Target was mounted on a rotating multi-target holder inside a vacuum chamber. The chamber was pumped to a base pressure of 3 × 10−5 Torr prior to deposition and the oxygen pressure during deposition was maintained to be about 200 mTorr. 600 nm PZT thin films were prepared at 450–650 ◦ C substrate temperature and the laser energy density was changed systematically from 1.5 to 4 J/cm2 at fixed substrate temperature, 550 ◦ C where we have got the better ferroelectric properties than the others. The crystal structure of the PZT thin films was investigated using X-ray diffraction and the surface morphology of the thin film was examined by the scanning electron-microscopy (SEM). Dotted gold top electrode was thermally evaporated through a shadow mask placed on the top of the thin films to measure electrical properties. Dielectric properties were measured using an HP4280A at 1 kHz. Polarization-field (P-E) properties were evaluated using an RT-66A ferroelectric measurement system.
3. Results and discussion The substrate temperature is one of the most important factors in deposition of thin films. Fig. 1 shows SEM images obtained from laser ablated PZT thin films at various substrate temperatures. The average grain size of PZT thin films was increased 50–300 nm as increasing the substrate temperature 450–650 ◦ C. The mobility of adatom at the substrate surface could be increased by increasing the substrate temperature and larger grain size is possible [9]. At 650 ◦ C of substrate temperature, voids at grains are observed which result in the degradation of electrical property of PZT thin film. The grain size is also strongly influenced not only by substrate temperature but also by laser energy density and annealing treatment in other ferroelectric thin films, such as PLT, SBT and BLT [9–11]. Typical XRD patterns of PZT thin films deposited on Pt/Ti/SiO2 /Si substrate at various substrate temperatures are shown in Fig. 2. They were all polycrystalline with perovskite structure, but also had the pyroclore phase. At the 550 and 600 ◦ C of substrate temperature, the pyroclore phase was most reduced. At the fixed substrate temperature of 550 ◦ C, the structural property has been investigated at various laser energy density points. Fig. 3 shows XRD patterns at various laser energy densities at 550 ◦ C of fixed substrate temperature. Perovskite phase was found through all films. At the energy density of 2.5 J/cm2 , the pyroclore phase was almost disappeared. It is required higher substrate temperature, such as 600–700 ◦ C, to deposit PZT thin films on Pt/Ti/SiO2 /Si substrate [11,12]. At PLD deposition process, the mobility of adatom at the substrate surface could be in-
143
Fig. 2. X-ray diffraction patterns of PZT thin films as varying deposition temperature.
creased by increasing the substrate temperature and the kinetic energy of particles ejected from the target could be increased by increasing laser energy density [9,10]. Therefore, it is possible to deposit thin film at lower substrate temperature. Annealing process is adapted to crystallized ferroelectric thin films. Fig. 4 shows the hysteresis loops depending on annealing time. Except 30 min of annealing time, they show the high quality of ferroelectric property. The remnant polarizations (Pr ) and coercive electric field (Ec ) are about 20–30 C/cm2 and 50–59 kV/cm with applied voltage of 10 V, respectively. They show a good squareness in the P-E hysteresis. At the annealing time of 20 min, 31.5 C/cm2 of the largest remnant polarization was observed and then coercive electric field was 50.5 kV/cm. And dielectric constant was 930. However, at the annealing time of 30 min, remnant polarization was decreased quite drastically. This phenomenon caused by too long annealing pro-
Fig. 3. X-ray diffraction patterns of PZT thin films as varying laser energy density.
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C.H. Jeon et al. / Materials Science and Engineering B 109 (2004) 141–145
Fig. 4. Hysteresis loop of thin films as a function of annealing time of: (a) 5; (b) 10; (c) 20; and (d) 30 min.
cess, which may result in inhomogeneous film morphology with cracks which are not suitable for electrical applications [13].
4. Conclusions In this study, we have deposited Pb(Zrx ,Ti1-x )O3 thin films by pulsed laser deposition. Structural properties and electrical properties of PZT thin films were shown to be strongly influenced by substrate temperature, laser energy density and annealing process. When pyroclore phase was almost reduced at as low temperature as 550 ◦ C, PZT films shows good ferroelectric property. Optimized deposition conditions of PZT thin film by PLD were 550 ◦ C of substrate temperature, 2.5 J/cm2 of laser energy density and 20 min. of annealing time. The remnant polarization, coercive electric field and dielectric constant were 31.5 C/cm2 , 50.5 kV/cm and 930, respectively.
Acknowledgements This research, under the contract project code MS-01-1102, has been supported by the Intelligent Microsystem Center (IMC; http://www.micrsystem.re.kr), which carries out one of the 21st century’s Frontier projects sponsored by the Korea Ministry of Science and Technology.
References [1] J.F. Scott, C.A.P. Araujo, Science 246 (1989) 1400. [2] C.A.P. Araujo, L.D. McMillan, B.M. Melnick, J.D. Cuchiaro, J.F. Scott, Ferroelectric 104 (1990) 241. [3] S.Y. Chen, C.L. Sun, J. Appl. Phys. 90 (2001) 2970. [4] J.D. Olivas, S. Bolin, JOM 50 (1998) 38. [5] K.S. Liu, Y.J. Chen, G. Jamn, Appl. Phys. Lett. 75 (1999) 2647. [6] S. Witanachchi, S.Y. Lee, L.W. Song, Y.K. Kao, D.T. Shaw, Appl. Phys. Lett. 52 (1990) 2133. [7] S.Y. Lee, Q.X. Jia, W.A. Anderson, D.T. Shaw, J. Appl. Phys. 70 (1991) 1991.
C.H. Jeon et al. / Materials Science and Engineering B 109 (2004) 141–145 [8] A.J. Moulson, J.M. Herbert, Electroceramics, Chapman & Hall, New York, 1990 (Chapter 6). [9] J.P. Wang, Y.C. Ling, M.H. Yeh, K.S. Liu, I.N. Lin, Appl. Phys. Lett. 68 (1996) 3401. [10] K.B. Han, C.H. Jeon, H.S. Jhon, S.Y. Lee, Thin Solid Films 437 (2003) 285.
145
[11] S.D. Bu, B.H. Park, B.S. Kang, T.W. Noh, Appl. Phys. Lett. 75 (1999) 1155. [12] M. Aratani, K. Nagashima, H. Funakubo, Jpn. J. Appl. Phys. 40 (2001) 343. [13] C.C. Chang, P.C. Lu, J. Matt, Pro. Tech. 95 (1999) 128.
Enhancement of the ferroelectric properties of Pb(Zr0.53Ti0.47)O3 thin films fabricated by laser ablation Chang Hoon Jeon, Cheol Su Kim, Kyoung Bo Han, Hee Sauk Jhon, Sang Yeol Lee∗ Department of Electrical and Electronic Engineering, Yonsei University, 134 Shinchondong, Seodaemun-ku, Seoul 120-749, South Korea
Abstract Thin films of phase-pure perovskite Pb(Zr0.53 Ti0.47 )O3 (PZT) were fabricated in situ onto Pt/Ti/SiO2 /Si substrates by pulsed laser deposition (PLD) using a Nd:YAG laser. We have systematically investigated the effect of various parameters, such as substrate temperatures (450–650 ◦ C) and energy densities (1.5–4 J/cm2 ) and annealing time (5–30 min), on the property of PZT thin film. X-ray diffraction (XRD), scanning electron microscope (SEM), C–V measurement and hysteresis were used to investigate the electrical, micro structural properties of the thin films. At the optimized deposition condition, remnant polarization, coercive electric field and dielectric constant of the film were 31.5 C/cm2 , 50.5 kV/cm and 930, respectively. © 2003 Elsevier B.V. All rights reserved. Keywords: Pb(Zr0.53 ,Ti0.47 )O3 ; Pulsed laser deposition; Perovskite structure; Remnant polarization
1. Introduction Ferroelectric thin films of Pb-based perovskite materials have attracted great interest for various applications, such as high dielectric, piezoelectric, pyroelectric, and electro-optic devices and nonvolatile memories [1,2]. Because of their high remnant polarization (Pr ) values, low coercive field (Ec ), and excellent piezoelectric properties, most work has focused on Pb-based perovskite materials as potential candidates for FRAM and microelectromechanical systems (MEMS) applications [1–4]. Among Pb-based thin films, Pb(Zrx ,Ti1−x )O3 (PZT) thin film has been regarded as one of the most promising materials because it exhibits various properties by changing its composition, crystallographic structure, microstructure, orientation, and properties of the substrate [3]. Among the popular thin film preparation techniques, such as sol–gel, sputtering, metal-organic chemical vapor deposition (MOCVD), and pulsed laser deposition (PLD or laser ablation), the laser ablation technique has advantages over other thin film deposition techniques for processing these ∗
Corresponding author. Tel.: +82-2-2123-2776; fax: +82-2-364-9770. E-mail address: [email protected] (S.Y. Lee).
0921-5107/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2003.10.033
thin films in the accuracy in composition control and the simplicity in process control [5–7]. In this paper, we have systematically investigated the effect of various parameters, such as substrate temperature, energy density, and annealing time on the microstructural and electrical properties of PZT thin film fabricated by PLD.
2. Experiment PZT thin films were prepared on Pt/Ti/SiO2 /Si substrate by the pulsed laser deposition technique, using pulsed Nd:YAG laser (Quantel Brilliant Q-switched Nd:YAG laser λ = 355 nm). The laser beam was focused on the target surface with an elliptical spot using a lens and a turning mirror outside the vacuum chamber. The laser power was exactly measured by a powermeter (Quantel portable powermeter: TPM-310B). The laser beam had an incident angle of approximately 45◦ with respect to the normal of the target surface. The ratio of Zr to Ti is 53:47, and this ratio corresponds to the rhombohedral-side of the morphotropic phase boundary (MPB). We have chosen this particular composition because it is known to have a excellent ferroelectric and piezoelectric properties by virtue of its proximity to
142
C.H. Jeon et al. / Materials Science and Engineering B 109 (2004) 141–145
Fig. 1. SEM images of PZT thin films as varying deposition temperature: (a) 450; (b) 500; (c) 550; (d) 600; and (e) 650 ◦ C.
C.H. Jeon et al. / Materials Science and Engineering B 109 (2004) 141–145
the MPB composition [8]. Because in the deposition process much of the lead is lost owing to reduced sticking coefficients at elevated deposition temperature, PZT with excess amounts lead oxide was used as a target. Target was mounted on a rotating multi-target holder inside a vacuum chamber. The chamber was pumped to a base pressure of 3 × 10−5 Torr prior to deposition and the oxygen pressure during deposition was maintained to be about 200 mTorr. 600 nm PZT thin films were prepared at 450–650 ◦ C substrate temperature and the laser energy density was changed systematically from 1.5 to 4 J/cm2 at fixed substrate temperature, 550 ◦ C where we have got the better ferroelectric properties than the others. The crystal structure of the PZT thin films was investigated using X-ray diffraction and the surface morphology of the thin film was examined by the scanning electron-microscopy (SEM). Dotted gold top electrode was thermally evaporated through a shadow mask placed on the top of the thin films to measure electrical properties. Dielectric properties were measured using an HP4280A at 1 kHz. Polarization-field (P-E) properties were evaluated using an RT-66A ferroelectric measurement system.
3. Results and discussion The substrate temperature is one of the most important factors in deposition of thin films. Fig. 1 shows SEM images obtained from laser ablated PZT thin films at various substrate temperatures. The average grain size of PZT thin films was increased 50–300 nm as increasing the substrate temperature 450–650 ◦ C. The mobility of adatom at the substrate surface could be increased by increasing the substrate temperature and larger grain size is possible [9]. At 650 ◦ C of substrate temperature, voids at grains are observed which result in the degradation of electrical property of PZT thin film. The grain size is also strongly influenced not only by substrate temperature but also by laser energy density and annealing treatment in other ferroelectric thin films, such as PLT, SBT and BLT [9–11]. Typical XRD patterns of PZT thin films deposited on Pt/Ti/SiO2 /Si substrate at various substrate temperatures are shown in Fig. 2. They were all polycrystalline with perovskite structure, but also had the pyroclore phase. At the 550 and 600 ◦ C of substrate temperature, the pyroclore phase was most reduced. At the fixed substrate temperature of 550 ◦ C, the structural property has been investigated at various laser energy density points. Fig. 3 shows XRD patterns at various laser energy densities at 550 ◦ C of fixed substrate temperature. Perovskite phase was found through all films. At the energy density of 2.5 J/cm2 , the pyroclore phase was almost disappeared. It is required higher substrate temperature, such as 600–700 ◦ C, to deposit PZT thin films on Pt/Ti/SiO2 /Si substrate [11,12]. At PLD deposition process, the mobility of adatom at the substrate surface could be in-
143
Fig. 2. X-ray diffraction patterns of PZT thin films as varying deposition temperature.
creased by increasing the substrate temperature and the kinetic energy of particles ejected from the target could be increased by increasing laser energy density [9,10]. Therefore, it is possible to deposit thin film at lower substrate temperature. Annealing process is adapted to crystallized ferroelectric thin films. Fig. 4 shows the hysteresis loops depending on annealing time. Except 30 min of annealing time, they show the high quality of ferroelectric property. The remnant polarizations (Pr ) and coercive electric field (Ec ) are about 20–30 C/cm2 and 50–59 kV/cm with applied voltage of 10 V, respectively. They show a good squareness in the P-E hysteresis. At the annealing time of 20 min, 31.5 C/cm2 of the largest remnant polarization was observed and then coercive electric field was 50.5 kV/cm. And dielectric constant was 930. However, at the annealing time of 30 min, remnant polarization was decreased quite drastically. This phenomenon caused by too long annealing pro-
Fig. 3. X-ray diffraction patterns of PZT thin films as varying laser energy density.
144
C.H. Jeon et al. / Materials Science and Engineering B 109 (2004) 141–145
Fig. 4. Hysteresis loop of thin films as a function of annealing time of: (a) 5; (b) 10; (c) 20; and (d) 30 min.
cess, which may result in inhomogeneous film morphology with cracks which are not suitable for electrical applications [13].
4. Conclusions In this study, we have deposited Pb(Zrx ,Ti1-x )O3 thin films by pulsed laser deposition. Structural properties and electrical properties of PZT thin films were shown to be strongly influenced by substrate temperature, laser energy density and annealing process. When pyroclore phase was almost reduced at as low temperature as 550 ◦ C, PZT films shows good ferroelectric property. Optimized deposition conditions of PZT thin film by PLD were 550 ◦ C of substrate temperature, 2.5 J/cm2 of laser energy density and 20 min. of annealing time. The remnant polarization, coercive electric field and dielectric constant were 31.5 C/cm2 , 50.5 kV/cm and 930, respectively.
Acknowledgements This research, under the contract project code MS-01-1102, has been supported by the Intelligent Microsystem Center (IMC; http://www.micrsystem.re.kr), which carries out one of the 21st century’s Frontier projects sponsored by the Korea Ministry of Science and Technology.
References [1] J.F. Scott, C.A.P. Araujo, Science 246 (1989) 1400. [2] C.A.P. Araujo, L.D. McMillan, B.M. Melnick, J.D. Cuchiaro, J.F. Scott, Ferroelectric 104 (1990) 241. [3] S.Y. Chen, C.L. Sun, J. Appl. Phys. 90 (2001) 2970. [4] J.D. Olivas, S. Bolin, JOM 50 (1998) 38. [5] K.S. Liu, Y.J. Chen, G. Jamn, Appl. Phys. Lett. 75 (1999) 2647. [6] S. Witanachchi, S.Y. Lee, L.W. Song, Y.K. Kao, D.T. Shaw, Appl. Phys. Lett. 52 (1990) 2133. [7] S.Y. Lee, Q.X. Jia, W.A. Anderson, D.T. Shaw, J. Appl. Phys. 70 (1991) 1991.
C.H. Jeon et al. / Materials Science and Engineering B 109 (2004) 141–145 [8] A.J. Moulson, J.M. Herbert, Electroceramics, Chapman & Hall, New York, 1990 (Chapter 6). [9] J.P. Wang, Y.C. Ling, M.H. Yeh, K.S. Liu, I.N. Lin, Appl. Phys. Lett. 68 (1996) 3401. [10] K.B. Han, C.H. Jeon, H.S. Jhon, S.Y. Lee, Thin Solid Films 437 (2003) 285.
145
[11] S.D. Bu, B.H. Park, B.S. Kang, T.W. Noh, Appl. Phys. Lett. 75 (1999) 1155. [12] M. Aratani, K. Nagashima, H. Funakubo, Jpn. J. Appl. Phys. 40 (2001) 343. [13] C.C. Chang, P.C. Lu, J. Matt, Pro. Tech. 95 (1999) 128.