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Effects of post-annealing on the microstructure and ferroelectric properties of YMnO3 thin films on Si
Journal of Crystal Growth 233 (2001) 243–247
Effects of post-annealing on the microstructure and ferroelectric properties of YMnO3 thin films on Si D.C. Yooa, J.Y. Leea,*, I.S. Kimb, Y.T. Kimb a
Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, 373-1, Guseong Dong, Yuseong Gu, Daejon 305-701, South Korea b Semiconductor Materials Laboratory, Korea Institute of Science and Technology, Seoul 136-791, South Korea Received 25 May 2001; accepted 21 June 2001 Communicated by M. Schieber
Abstract YMnO3 thin films deposited on Si (1 0 0) substrate by rf-sputtering were annealed with various conditions. YMnO3 films annealed in a furnace had a c-axis preferred orientation and the films annealed in a rapid thermal processor (RTP) had random orientations. However, cracks were observed in the highly c-axis oriented YMnO3 films. YMnO3 films annealed in the furnace showed poor ferroelectric characteristics. However, YMnO3 films annealed in the RTP showed a ferroelectric C2V hysteresis with 1.5 V memory window at 0.2 V/s sweep rate. Since the thermal expansion of a-axis is five times higher than that of c-axis in the YMnO3 thin films, the c-axis oriented thin films are expected to be easily cracked during the post-annealing process. Moreover, the rapid thermal annealing process effectively suppressed the increase of a native SiO2 thickness in the YMnO3/Si structure. r 2001 Published by Elsevier Science B.V. PACS: 77.80; 81.40.G; 61.16.B Keywords: A1. Transmission electron microscopy; B1. Yittrium compounds; B2. Ferroelectric materials
1. Introduction Ferroelectric thin films have attracted much attention for use in nonvolatile memories [1–4]. Nonvolatile ferroelectric memories using Si substrates have been investigated due to the advantages such as shrinking memory cell size and saving electrical power [5].
*Corresponding author. Tel.: +82-42-869-4216; fax: +8242-869-3310. E-mail address: [email protected] (J.Y. Lee).
The ferroelectric films are used as a gate insulator of a metal-ferroelectric-semiconductor field effect transistor (MFSFET) structure. The ferroelectric thin films do not need to have large spontaneous polarization, because the spontaneous polarization directly controls the channel conductivity of the field effect transistor. For the MFSFET structure, it is necessary for the film to have not only a ferroelectric property but also a small interface state density between Si and ferroelectric films. However, the interdiffusion between ferroelectric films and Si and the formation of a SiO2 layer with a low dielectric constant
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at the film/Si interface makes it very difficult [6,7]. Therefore, it is very important to reduce the thickness of amorphous layers at the film/Si interface for MFSFET structure. As the ferroelectric materials, we have used YMnO3 thin film that has a hexagonal structure with an unipolarization axis along [0 0 0 1]. Since YMnO3 has a low permittivity and does not include volatile elements such as Bi and Pb, it is expected to have several advantages over Pb(ZrxTi1x)O3 and SrBi2Ta2O9 [8,9]. Moreover, YMnO3 with low dielectric permittivity (e ¼ 20 at RT) has the greatest advantage for a device directly on Si, because the applied voltage for ferroelectric thin films does not decrease so much due to the existence of the layer with low dielectric permittivity [10,11]. In some reports, the crystallization of YMnO3 thin films is expected to be very much dependent upon the post-annealing processes [12–14]. To investigate the relationships between the crystallization and the electrical properties of YMnO3 films with the annealing processes is necessary for evaluating metal-ferroelectric-semiconductor (MFS) structure and to characterize the microstructure at the YMnO3/Si interface is expected to be important for enhancing the electrical properties. In this study, we annealed YMnO3 thin films on Si (1 0 0) with various conditions and investigated the dependence of ferroelectric characteristics upon the microstructure using a high-resolution transmission electron microscopy (HRTEM).
films were examined with an X-ray diffraction (XRD) (Rigaku-D/MAX-IIIC). The microstructures of YMnO3 films were characterized by a HRTEM with a JEOL JEM-2000EX microscope operating at 200 kV. Capacitance–voltage (C2V) characteristics were measured to investigate the ferroelectric memory effects by using a HP 4280A in an MFS structure with a circular Pt electrode (f ¼ 160 mm) as a gate. Probing AC signal frequency and constant sweep rate were 1 MHz and 0.2 V/s, respectively.
3. Results and discussion Fig. 1 shows the XRD patterns of YMnO3 thin films on Si (1 0 0) substrate annealed with various conditions. YMnO3 thin film annealed in the furnace for 1 h has a c-axis preferred orientation in Fig. 1(a). As YMnO3 thin film has unipolarization axis along [0 0 0 1], the c-axis oriented films are very favorable in view of the microstructure. However, we could find that a second phase such as Y2O3 was formed in the YMnO3 thin films after the annealing process for 4 h. Fig. 1(c) shows that YMnO3 film annealed in the RTP has random orientations. To characterize the ferroelectric properties of the YMnO3/Si (1 0 0) gate structure, MFS
2. Experiments YMnO3 thin films were deposited on p-Si (1 0 0) substrate by rf-sputtering by using a YMnO3 single target mixed with Y2O3 and Mn2O3 at a ratio of 1 : 1. During the deposition in an Ar ambient, the rf power density and chamber pressure were fixed at 1.85 W/cm2 and 5 103 Torr. The thickness of the as-deposited YMnO3 thin films was 300 nm. In N2 atmosphere, the YMnO3 thin films were post-annealed in a furnace at 8701C for 1–4 h or in a rapid thermal processor (RTP) at 8501C for 3 min. After the post-annealing processes, the orientations of the
Fig. 1. XRD patterns of YMnO3 thin films on Si (1 0 0): (a) annealed in the furnace for 1 h; (b) for 4 h at 8701C, and (c) annealed in the RTP for 3 min at 8501C.
D.C. Yoo et al. / Journal of Crystal Growth 233 (2001) 243–247
capacitor was prepared by the deposition of a top Pt electrode. Fig. 2 represents C2V measurements at RT for the Pt/YMnO3 (300 nm)/Si (1 0 0) gate capacitor. The C2V measurements were accomplished in the voltage regions from 5 to 5 V, sweeping at 0.2 V/s from a negative bias to a positive bias and reversing it again. In Fig. 2(a), YMnO3 thin film annealed in the furnace shows poor ferroelectric properties in spite of having a c-axis preferred orientation. However, YMnO3 thin film annealed in the RTP which has random orientation, shows good C2V characteristics in Fig. 2(b). Moreover, a memory window of
Fig. 2. C2V characteristics of the Pt/YMnO3/Si capacitor, where the YMnO3 film was: (a) annealed in the furnace and (b) annealed in the RTP.
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YMnO3 thin film increases on increasing the bias sweep rate from 0.1 to 0.2 V/s. The increase of a memory window as the bias sweep rate implies that more time is needed for rotating a polarization direction on increasing the bias sweep rate because the YMnO3 thin film has random orientation. The memory window at 0.2 V/s sweep rate is about 1.5 V, which is high enough to use for MFS gate capacitor. Fig. 3 shows optical micrographs of a surface in the post-annealed YMnO3 films. In Fig. 3, cracks are observed only in the highly c-axis oriented YMnO3 films. Although a crack density of the YMnO3 film annealed for 4 h was fairly reduced, most of cracks remained, as shown in Figs. 3(a) and (b). However, YMnO3 film annealed in the RTP has a smooth surface without cracks. Since the crack is only observed in the highly c-axis oriented films, the formation of crack is expected to relate with the orientation of YMnO3 thin films. The formation of crack in thin films is generally related with the difference of a thermal expansion between films and substrate. The thermal expansion coefficients of a-axis, c-axis in YMnO3, SiO2 and Si are about 18.5 106, 3.5 106, 5.5 107 and 2.3 106/1C, respectively [15]. The thermal expansion coefficient of a-axis is five times higher than that of c-axis in the YMnO3 films. Therefore, c-axis oriented YMnO3 films have a more stress than the random oriented films during the post-annealing process. These results show that the poor ferroelectric properties of the highly c-axis oriented YMnO3 thin films annealed in the furnace are due to the crack in the YMnO3 films. As the microstructure at the YMnO3/Si interface is very important for a MFS structure, we investigated the microstructure at the YMnO3/Si interface using a HRTEM. Fig. 4 shows HRTEM micrographs of the YMnO3/Si interface. Figs. 4(a) and (b) show the c-axis oriented YMnO3 thin films on the Si (1 0 0) substrate. In Fig. 4(c), YMnO3 thin film has random orientations. These results are well consistent with the XRD results. In the HRTEM micrographs, no second phase was observed due to the interdiffusion at the YMnO3/ Si interface. The interface layers are divided into two regions, i.e., the amorphous YMnO3 and
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Fig. 4. HRTEM micrographs of the YMnO3/Si interface which was: (a) annealed in the furnace for 1 h; (b) for 4 h and (c) annealed in the RTP for 3 min. Fig. 3. Optical micrographs of the surface in the YMnO3 films: (a) annealed in the furnace for 1 h; (b) for 4 h and (c) annealed in the RTP for 3 min.
the native SiO2. In Fig. 4, the thickness of the interfacial amorphous YMnO3 layer is about 11–12 nm regardless of the post-annealing conditions. However, the thickness of the native
SiO2 is varied with the post-annealing conditions. In the as-deposited YMnO3 film, the thickness of the native SiO2 was about 3 nm. The thickness of the native SiO2 is about 7 nm in the furnace annealed YMnO3 films, while the thickness of the native SiO2 is about 3.5 nm in the RTP annealed YMnO3 film. The increase of native SiO2 is due to the diffusion of oxygen atoms from the amorphous
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YMnO3 to Si substrate. Owing to a very fast annealing time, the increase of the native SiO2 thickness in YMnO3 film annealed in the RTP is negligible. Since an applied voltage for the YMnO3 films is reduced on increasing the native SiO2 thickness in MFS structure, the rapid thermal annealing process is favorable as the post-annealing process for the YMnO3/Si structure.
4. Conclusions Crystallization behaviors and ferroelectric properties of YMnO3 thin films on YMnO3/Si (1 0 0) structure were investigated after various postannealing conditions. Although the YMnO3 thin films annealed in the furnace have a c-axis preferred orientation, the films were cracked due to the large difference of a thermal expansion between a-axis of YMnO3 and Si substrate. YMnO3 thin film annealed in the RTP has random orientations; however, it showed good C2V characteristics with 1.5 V memory window. Moreover, using a high-resolution transmission electron microscopy, we could find that the rapid thermal annealing process effectively suppresses the increase of the native SiO2 thickness in the YMnO3/ Si structure.
Acknowledgements We acknowledge the support of this research by the Ministry of Science and Technology of Korea
through the Program.
National
Research
Laboratory
References [1] W.C. Yi, J.S. Choe, C.R. Moon, S.I. Kwun, J.G. Yoon, Appl. Phys. Lett. 73 (7) (1998) 903. [2] N. Fujimura, S.I. Azuma, N. Aoki, T. Yoshimura, J. Appl. Phys. 80 (12) (1996) 7084. [3] S. Imada, S. Shouriki, E. Tokumitsu, H. Ishiwara, Jpn. J. Appl. Phys. 37 (1998) 6497. [4] M.N. Iliev, H.G. Lee, V.N. Popov, M.V. Abrashev, A. Hamed, R.L. Meng, C.W. Chu, Phys. Rev. B 56 (5) (1997) 2488. [5] E. Rokuta, Y. Hotta, H. Tabata, H. Kobayashi, T. Kawai, J. Appl. Phys. 88 (11) (2000) 6598. [6] T. Yoshimura, N. Fujimura, T. Ito, Appl. Phys. Lett. 73 (3) (1998) 414. [7] W.C. Yi, J.S. Choe, C.R. Moon, S.I. Kwun, J.G. Yoon, Appl. Phys. Lett. 77 (7) (2000) 1044. [8] T. Yoshimura, N. Fujimura, D. Ito, T. Ito, J. Appl. Phys. 87 (7) (2000) 3444. [9] H.N. Lee, Y.T. Kim, S.H. Choh, Appl. Phys. Lett. 76 (8) (2000) 1066. [10] N. Fujimura, T. Ishida, T. Yoshimura, T. Ito, Appl. Phys. Lett. 69 (7) (1996) 1011. [11] N. Aoki, N. Fujimura, T. Yoshimura, T. Ito, J. Crystal Growth 174 (1997) 796. [12] H. Kitahata, K. Tadanaga, T. Minami, N. Fujimura, T. Ito, Appl. Phys. Lett. 75 (5) (1999) 719. [13] H.N. Lee, Y.T. Kim, Y.K. Park, Appl. Phys. Lett. 74 (25) (1999) 3887. [14] K.J. Choi, W.C. Shin, S.G. Yoon, Thin Solid Films 384 (2001) 146. [15] T. Mitsui, S. Nomura, Numerical Data and Functional Relationships in Science and Technology, Springer, Berlin; New York, 1981, p. 506.
Effects of post-annealing on the microstructure and ferroelectric properties of YMnO3 thin films on Si D.C. Yooa, J.Y. Leea,*, I.S. Kimb, Y.T. Kimb a
Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, 373-1, Guseong Dong, Yuseong Gu, Daejon 305-701, South Korea b Semiconductor Materials Laboratory, Korea Institute of Science and Technology, Seoul 136-791, South Korea Received 25 May 2001; accepted 21 June 2001 Communicated by M. Schieber
Abstract YMnO3 thin films deposited on Si (1 0 0) substrate by rf-sputtering were annealed with various conditions. YMnO3 films annealed in a furnace had a c-axis preferred orientation and the films annealed in a rapid thermal processor (RTP) had random orientations. However, cracks were observed in the highly c-axis oriented YMnO3 films. YMnO3 films annealed in the furnace showed poor ferroelectric characteristics. However, YMnO3 films annealed in the RTP showed a ferroelectric C2V hysteresis with 1.5 V memory window at 0.2 V/s sweep rate. Since the thermal expansion of a-axis is five times higher than that of c-axis in the YMnO3 thin films, the c-axis oriented thin films are expected to be easily cracked during the post-annealing process. Moreover, the rapid thermal annealing process effectively suppressed the increase of a native SiO2 thickness in the YMnO3/Si structure. r 2001 Published by Elsevier Science B.V. PACS: 77.80; 81.40.G; 61.16.B Keywords: A1. Transmission electron microscopy; B1. Yittrium compounds; B2. Ferroelectric materials
1. Introduction Ferroelectric thin films have attracted much attention for use in nonvolatile memories [1–4]. Nonvolatile ferroelectric memories using Si substrates have been investigated due to the advantages such as shrinking memory cell size and saving electrical power [5].
*Corresponding author. Tel.: +82-42-869-4216; fax: +8242-869-3310. E-mail address: [email protected] (J.Y. Lee).
The ferroelectric films are used as a gate insulator of a metal-ferroelectric-semiconductor field effect transistor (MFSFET) structure. The ferroelectric thin films do not need to have large spontaneous polarization, because the spontaneous polarization directly controls the channel conductivity of the field effect transistor. For the MFSFET structure, it is necessary for the film to have not only a ferroelectric property but also a small interface state density between Si and ferroelectric films. However, the interdiffusion between ferroelectric films and Si and the formation of a SiO2 layer with a low dielectric constant
0022-0248/01/$ - see front matter r 2001 Published by Elsevier Science B.V. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 1 5 6 3 - 9
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D.C. Yoo et al. / Journal of Crystal Growth 233 (2001) 243–247
at the film/Si interface makes it very difficult [6,7]. Therefore, it is very important to reduce the thickness of amorphous layers at the film/Si interface for MFSFET structure. As the ferroelectric materials, we have used YMnO3 thin film that has a hexagonal structure with an unipolarization axis along [0 0 0 1]. Since YMnO3 has a low permittivity and does not include volatile elements such as Bi and Pb, it is expected to have several advantages over Pb(ZrxTi1x)O3 and SrBi2Ta2O9 [8,9]. Moreover, YMnO3 with low dielectric permittivity (e ¼ 20 at RT) has the greatest advantage for a device directly on Si, because the applied voltage for ferroelectric thin films does not decrease so much due to the existence of the layer with low dielectric permittivity [10,11]. In some reports, the crystallization of YMnO3 thin films is expected to be very much dependent upon the post-annealing processes [12–14]. To investigate the relationships between the crystallization and the electrical properties of YMnO3 films with the annealing processes is necessary for evaluating metal-ferroelectric-semiconductor (MFS) structure and to characterize the microstructure at the YMnO3/Si interface is expected to be important for enhancing the electrical properties. In this study, we annealed YMnO3 thin films on Si (1 0 0) with various conditions and investigated the dependence of ferroelectric characteristics upon the microstructure using a high-resolution transmission electron microscopy (HRTEM).
films were examined with an X-ray diffraction (XRD) (Rigaku-D/MAX-IIIC). The microstructures of YMnO3 films were characterized by a HRTEM with a JEOL JEM-2000EX microscope operating at 200 kV. Capacitance–voltage (C2V) characteristics were measured to investigate the ferroelectric memory effects by using a HP 4280A in an MFS structure with a circular Pt electrode (f ¼ 160 mm) as a gate. Probing AC signal frequency and constant sweep rate were 1 MHz and 0.2 V/s, respectively.
3. Results and discussion Fig. 1 shows the XRD patterns of YMnO3 thin films on Si (1 0 0) substrate annealed with various conditions. YMnO3 thin film annealed in the furnace for 1 h has a c-axis preferred orientation in Fig. 1(a). As YMnO3 thin film has unipolarization axis along [0 0 0 1], the c-axis oriented films are very favorable in view of the microstructure. However, we could find that a second phase such as Y2O3 was formed in the YMnO3 thin films after the annealing process for 4 h. Fig. 1(c) shows that YMnO3 film annealed in the RTP has random orientations. To characterize the ferroelectric properties of the YMnO3/Si (1 0 0) gate structure, MFS
2. Experiments YMnO3 thin films were deposited on p-Si (1 0 0) substrate by rf-sputtering by using a YMnO3 single target mixed with Y2O3 and Mn2O3 at a ratio of 1 : 1. During the deposition in an Ar ambient, the rf power density and chamber pressure were fixed at 1.85 W/cm2 and 5 103 Torr. The thickness of the as-deposited YMnO3 thin films was 300 nm. In N2 atmosphere, the YMnO3 thin films were post-annealed in a furnace at 8701C for 1–4 h or in a rapid thermal processor (RTP) at 8501C for 3 min. After the post-annealing processes, the orientations of the
Fig. 1. XRD patterns of YMnO3 thin films on Si (1 0 0): (a) annealed in the furnace for 1 h; (b) for 4 h at 8701C, and (c) annealed in the RTP for 3 min at 8501C.
D.C. Yoo et al. / Journal of Crystal Growth 233 (2001) 243–247
capacitor was prepared by the deposition of a top Pt electrode. Fig. 2 represents C2V measurements at RT for the Pt/YMnO3 (300 nm)/Si (1 0 0) gate capacitor. The C2V measurements were accomplished in the voltage regions from 5 to 5 V, sweeping at 0.2 V/s from a negative bias to a positive bias and reversing it again. In Fig. 2(a), YMnO3 thin film annealed in the furnace shows poor ferroelectric properties in spite of having a c-axis preferred orientation. However, YMnO3 thin film annealed in the RTP which has random orientation, shows good C2V characteristics in Fig. 2(b). Moreover, a memory window of
Fig. 2. C2V characteristics of the Pt/YMnO3/Si capacitor, where the YMnO3 film was: (a) annealed in the furnace and (b) annealed in the RTP.
245
YMnO3 thin film increases on increasing the bias sweep rate from 0.1 to 0.2 V/s. The increase of a memory window as the bias sweep rate implies that more time is needed for rotating a polarization direction on increasing the bias sweep rate because the YMnO3 thin film has random orientation. The memory window at 0.2 V/s sweep rate is about 1.5 V, which is high enough to use for MFS gate capacitor. Fig. 3 shows optical micrographs of a surface in the post-annealed YMnO3 films. In Fig. 3, cracks are observed only in the highly c-axis oriented YMnO3 films. Although a crack density of the YMnO3 film annealed for 4 h was fairly reduced, most of cracks remained, as shown in Figs. 3(a) and (b). However, YMnO3 film annealed in the RTP has a smooth surface without cracks. Since the crack is only observed in the highly c-axis oriented films, the formation of crack is expected to relate with the orientation of YMnO3 thin films. The formation of crack in thin films is generally related with the difference of a thermal expansion between films and substrate. The thermal expansion coefficients of a-axis, c-axis in YMnO3, SiO2 and Si are about 18.5 106, 3.5 106, 5.5 107 and 2.3 106/1C, respectively [15]. The thermal expansion coefficient of a-axis is five times higher than that of c-axis in the YMnO3 films. Therefore, c-axis oriented YMnO3 films have a more stress than the random oriented films during the post-annealing process. These results show that the poor ferroelectric properties of the highly c-axis oriented YMnO3 thin films annealed in the furnace are due to the crack in the YMnO3 films. As the microstructure at the YMnO3/Si interface is very important for a MFS structure, we investigated the microstructure at the YMnO3/Si interface using a HRTEM. Fig. 4 shows HRTEM micrographs of the YMnO3/Si interface. Figs. 4(a) and (b) show the c-axis oriented YMnO3 thin films on the Si (1 0 0) substrate. In Fig. 4(c), YMnO3 thin film has random orientations. These results are well consistent with the XRD results. In the HRTEM micrographs, no second phase was observed due to the interdiffusion at the YMnO3/ Si interface. The interface layers are divided into two regions, i.e., the amorphous YMnO3 and
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Fig. 4. HRTEM micrographs of the YMnO3/Si interface which was: (a) annealed in the furnace for 1 h; (b) for 4 h and (c) annealed in the RTP for 3 min. Fig. 3. Optical micrographs of the surface in the YMnO3 films: (a) annealed in the furnace for 1 h; (b) for 4 h and (c) annealed in the RTP for 3 min.
the native SiO2. In Fig. 4, the thickness of the interfacial amorphous YMnO3 layer is about 11–12 nm regardless of the post-annealing conditions. However, the thickness of the native
SiO2 is varied with the post-annealing conditions. In the as-deposited YMnO3 film, the thickness of the native SiO2 was about 3 nm. The thickness of the native SiO2 is about 7 nm in the furnace annealed YMnO3 films, while the thickness of the native SiO2 is about 3.5 nm in the RTP annealed YMnO3 film. The increase of native SiO2 is due to the diffusion of oxygen atoms from the amorphous
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YMnO3 to Si substrate. Owing to a very fast annealing time, the increase of the native SiO2 thickness in YMnO3 film annealed in the RTP is negligible. Since an applied voltage for the YMnO3 films is reduced on increasing the native SiO2 thickness in MFS structure, the rapid thermal annealing process is favorable as the post-annealing process for the YMnO3/Si structure.
4. Conclusions Crystallization behaviors and ferroelectric properties of YMnO3 thin films on YMnO3/Si (1 0 0) structure were investigated after various postannealing conditions. Although the YMnO3 thin films annealed in the furnace have a c-axis preferred orientation, the films were cracked due to the large difference of a thermal expansion between a-axis of YMnO3 and Si substrate. YMnO3 thin film annealed in the RTP has random orientations; however, it showed good C2V characteristics with 1.5 V memory window. Moreover, using a high-resolution transmission electron microscopy, we could find that the rapid thermal annealing process effectively suppresses the increase of the native SiO2 thickness in the YMnO3/ Si structure.
Acknowledgements We acknowledge the support of this research by the Ministry of Science and Technology of Korea
through the Program.
National
Research
Laboratory
References [1] W.C. Yi, J.S. Choe, C.R. Moon, S.I. Kwun, J.G. Yoon, Appl. Phys. Lett. 73 (7) (1998) 903. [2] N. Fujimura, S.I. Azuma, N. Aoki, T. Yoshimura, J. Appl. Phys. 80 (12) (1996) 7084. [3] S. Imada, S. Shouriki, E. Tokumitsu, H. Ishiwara, Jpn. J. Appl. Phys. 37 (1998) 6497. [4] M.N. Iliev, H.G. Lee, V.N. Popov, M.V. Abrashev, A. Hamed, R.L. Meng, C.W. Chu, Phys. Rev. B 56 (5) (1997) 2488. [5] E. Rokuta, Y. Hotta, H. Tabata, H. Kobayashi, T. Kawai, J. Appl. Phys. 88 (11) (2000) 6598. [6] T. Yoshimura, N. Fujimura, T. Ito, Appl. Phys. Lett. 73 (3) (1998) 414. [7] W.C. Yi, J.S. Choe, C.R. Moon, S.I. Kwun, J.G. Yoon, Appl. Phys. Lett. 77 (7) (2000) 1044. [8] T. Yoshimura, N. Fujimura, D. Ito, T. Ito, J. Appl. Phys. 87 (7) (2000) 3444. [9] H.N. Lee, Y.T. Kim, S.H. Choh, Appl. Phys. Lett. 76 (8) (2000) 1066. [10] N. Fujimura, T. Ishida, T. Yoshimura, T. Ito, Appl. Phys. Lett. 69 (7) (1996) 1011. [11] N. Aoki, N. Fujimura, T. Yoshimura, T. Ito, J. Crystal Growth 174 (1997) 796. [12] H. Kitahata, K. Tadanaga, T. Minami, N. Fujimura, T. Ito, Appl. Phys. Lett. 75 (5) (1999) 719. [13] H.N. Lee, Y.T. Kim, Y.K. Park, Appl. Phys. Lett. 74 (25) (1999) 3887. [14] K.J. Choi, W.C. Shin, S.G. Yoon, Thin Solid Films 384 (2001) 146. [15] T. Mitsui, S. Nomura, Numerical Data and Functional Relationships in Science and Technology, Springer, Berlin; New York, 1981, p. 506.