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Growth mechanism of PTO on MgO at initial stage
Applied Surface Science 216 (2003) 323–328
Growth mechanism of PTO on MgO at initial stage K. Nishidaa,*, K. Shirakataa, M. Osadab, M. Kakihanac, T. Katodaa a
Department of Electronic and Photonic Systems Engineering, Kochi University of Technology, 185 Miyanokuchi, Tosayamada-cho, Kamigun, Kochi 782-8502, Japan b Precursory Research for Embryonic Science and Technology, Japan Science and Technology Corporation 4259, Nagatsuta, Midori-ku, Yokohama-shi, Kanagawa 226-8503, Japan c Materials and Structures Laboratory, Tokyo Institute of Technology 4259, Nagatsuta, Midori-ku, Yokohama-shi, Kanagawa 226-8503, Japan
Abstract Growth mechanism of a lead titanate (PbTiO3, PTO) thin film at the initial stage has been made clear. The mechanism is strongly affected by accumulation and release of stress. Flatness of the surface and the crystallographic orientation of the PTO film change with growth mode. # 2003 Elsevier Science B.V. All rights reserved. Keywords: PTO; Growth mechanism; Residual stress; Surface roughness; Film orientation
1. Introduction Lead titanate (PbTiO3, PTO) is a well-known ferroelectric material with a prerovskite structure [1]. PTO thin films have many applications such as nonvolatile memory [2], piezoelectric devices [3] and infrared sensors [4,5] because it has ferroelectric, piezoelectric, pyroelectric properties [6] and a high Curie temperature. PTO thin films with high-quality and high-reliability are still requested in order to apply them to devices. There have been many reports on preparation of PTO thin films using rf sputtering [7], sol–gel [8–10], laser ablation [11], ion beam sputtering [12] and chemical vapor deposition (CVD) [13–15]. However, it is difficult at present to control growth and to obtain PTO films with high-reliability because the ferroelectric thin film is *
Corresponding author. Tel.: þ81-887-53-1010; fax: þ81-887-57-2120. E-mail address: [email protected] (K. Nishida).
composed of multi-elements with different vapor pressures and many factors have affected their growth. Especially, the initial stage of growth gives important effects on properties of PTO films. We will report in this paper the growth mechanism of PTO films on MgO substrates.
2. Experiment PTO thin films were grown on MgO(1 0 0) substrates using the plasma enhanced chemical vapor deposition (PE-CVD) method. Organic stain on the substrates was removed followed by rinsing them with de-ionized water. Then, thermal cleaning was done before growth. The crystal growth was carried out at a temperature of 545 8C, a pressure of 10 Pa and a rf power of 170 W. The total growth time was 180 s and properties of film were measured at 15, 30, 45, 90, 135 and 180 s. The film thickness was measured using a surface-profile measuring system (DEKTAK 3).
0169-4332/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(03)00455-0
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K. Nishida et al. / Applied Surface Science 216 (2003) 323–328
The growth rate was 1.12 mm/h. The film thicknesses were estimated to be 5, 9, 14, 28, 42 and 56 nm at the growth times of 15, 30, 45, 90, 135 and 180 s, respectively, with assumption that a growth rate was constant with time. A crystallographic orientation and crystallinity were characterized by the X-ray diffraction (XRD) method. Surface roughness was characterized using an atomic force microscope (AFM). Raman spectroscopy with a T64000 (JOBIN YVON) model was used to determine the structural disorder and the stress accumulated in the films. An Arþ laser of 514.5 nm was used for excitation and a space resolving was 1 mm. A resolution of shift of a Raman peak was about 1 cm1. Fig. 1. XRD spectrum of a PTO thin film grown for 180 s.
3. Results and discussion The XRD spectrum of the PTO thin film grown for 180 s is shown in Fig. 1. The PTO film was only oriented toward (1 0 0) and (0 0 1) axes (a-plane and c-plane). No diffraction peak from other orientation planes and the heterogenous phase was observed
during the growth. An axis orientation ratio a of c-plane was calculated from the results of XRD measurement. Value for a was defined as follows: a¼
Ið0 0 1Þ Ið1 0 0Þ þ Ið0 0 1Þ
Fig. 2. Growth time dependence of the c-axis orientation ratio a of PTO thin film.
(1)
K. Nishida et al. / Applied Surface Science 216 (2003) 323–328
I(1 0 0) and I(0 0 1) are intensities of (1 0 0) and (0 0 1) peaks in the XRD spectrum. Each intensity was evaluated by deconvolution using the Gaussian function. Fig. 2 shows growth time versus c-axis orientation ratio a. A film thickness derived as described above is also shown in the axis on the top. A c-axis orientation ratio of a PTO film was 0.51 at 15 s of growth. Then c-axis orientation ratio decreased until the growth time of 30 s. A c-axis orientation ratio increased again until 45 s and decreased until 90 s. In Fig. 2, the c-axis orientation ratio showed similar behavior from 90 to 180 s of growth. That is the orientation ratio of c-axis changed periodically with growth time.
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Fig. 3(a)–(f) shows AFM images of surfaces of the PTO film of various growth times. A measuring range was 1 mm. AFM measurement was done for over 10 points of the surface of each sample and typical values were plotted in Fig. 3. A height full scale in Fig. 3(a) and (b) was 25 nm and that in Fig. 3(c)–(f) was 50 nm. A root mean square (rms) of surface roughness was calculated from the AFM images and it was shown with a growth time in Fig. 4. At the beginning, crystal nucleus created on a MgO substrate (Fig. 3(a)) and then surface roughness increased with growth of the nucleus (Fig. 3(b)). Then, the surface became flat with combination of the crystal nucleus (Fig. 3(c)). A rms of this sample was 1.7 nm which was smaller
Fig. 3. AFM images of a PTO thin film. Growth time were: (a) 15 s; (b) 30 s; (c) 45 s; (d) 90 s; (e) 135 s; and (f) 180 s.
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Fig. 4. Growth time dependence of surface roughness of a PTO thin film.
than that of the MgO substrate (rms was 6 nm). Then, vertical growth mode occurred at about 90 s of growth accompanying increase of surface roughness (Fig. 3(d)). A rms increased rapidly over four times of that at the initial stage. However, lateral growth mode occurred again after 135 s of growth and the surface
flatness was improved as flat as that of the surface at 45 s (Fig. 3(e)). At the next time, vertical growth started again and surface roughness increased (Fig. 3(f)). The results shown above mean that the PTO film grows in vertical and lateral modes periodically with a growth time at the initial stage.
Fig. 5. Raman spectrum of a PTO thin film grown for 180 s.
K. Nishida et al. / Applied Surface Science 216 (2003) 323–328
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Fig. 6. Growth time vs. a peak shift of the E(1TO) mode in the Raman spectrum.
Raman spectra from the PTO film grown for 180 s are shown in Fig. 5. The A1, E and B1 þ E modes [16– 18] were observed in the spectra in Fig. 5. Intensity of these peaks and the full widths of half maximum (FWHM) of these peaks decreased with a growth time which mean that quality of the crystal improved. A shift of the Raman peak E(1LO) mode was plotted against growth time in Fig. 6. The E(1LO) mode is an important mode called ‘‘soft mode’’ which characterizes ferroelectrics. The relation between the soft
mode and accumulated strain has been made clear and it is possible to estimate accumulated stress from a shift of the E(1LO) peak. The amount of the stress is shown in the axis on right of Fig. 6. Fig. 6 shows that large compressive stress is accumulated in the thin films because the peak of E(1LO) shifts to a lower wavenumber. Stress of about 1.9 GPa is accumulated the PTO thin film grown for 15 s. Stress is released at 30 s at once, but larger compressive stress (2.3 GPa) increased after 45 s until about 90 s of growth.
Fig. 7. Model of growth process of a PTO thin film at initial stage.
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K. Nishida et al. / Applied Surface Science 216 (2003) 323–328
Release of stress begins again about at 90 s and accumulation begins at about 135 s. The result shown above indicates that accumulation and relaxation of stress is repeated with growth time. The results of characterization shown above are summarized in Fig. 7. Nuclei of PTO crystal are formed the first stage (15 s). As the nuclei grow, stress is accumulated with an increase in thickness until 45 s of growth. Flatness is improved during this third stage. Stress is released after the growth of 45 s and decreased from 45 to 90 s of growth. During this fourth stage of growth, surface roughness increased and the c-axis orientation ratio decreased. At the next stage, from 90 to 135 s, stress increased again. Flatness is improved again and the c-axis orientation ratio increased. The surface roughness increased and the stress was released again at the sixth stage. The results mean that stress increased during PTO crystal grew laterally because crystal grains pushed one another. When stress reached at a threshold value, 2.3 or 2.4 GPa in this case, defects were introduced and stress was released. After that stage, the crystal starts to grow vertically. From the start to 180 s of growth, at least, the mechanism is repeated.
4. Summary Growth mechanism of a PTO thin film at the initial stage has been made clear. The mechanism is strongly affected by accumulation and release of stress.
Flatness of the surface and the crystallographic orientation of the PTO film change with growth mode.
References [1] G. Burns, B.A. Scott, Phys. Rev. B7 (1973) 3088. [2] J.F. Scott, C.A. Araujo, Science 246 (1989) 1400. [3] K. Ohtani, M. Okuyama, Y. Hamakawa, Jpn. J. Appl. Phys. 23 (Suppl. 23-1) (1984) 133. [4] M. Okuyama, H. Seto, M. Kojima, Y. Matsui, Y. Hamakawa, Jpn. J. Appl. Phys. 22 (Suppl. 22-1) (1983) 465. [5] R. Takayama, Y. Tomita, K. Iijima, I. Ueda, J. Appl. Phys. 63 (1988) 5868. [6] J.S. Wrigh, L.F. Francis, J. Mater. Res. 8 (1993) 1712. [7] K. Iijima, Y. Tomita, R. Takayama, I. Ueda, J. Appl. Phys. 60 (1986) 361. [8] M. Maeda, H. Ishida, K.K.K. Soe, I. Suzuki, Jpn. J. Appl. Phys. 32 (1993) 4136. [9] S.S. Dana, F.K. Etzod, J. Clabes, J. Appl. Phys. 69 (1991) 4398. [10] C. Chen, D.F. Ryder Jr., J. Am. Ceram. Soc. 72 (1989) 1495. [11] T. Imai, M. Okuyama, Y. Hamakawa, Jpn. J. Appl. Phys. 30 (1991) 2163. [12] I. Kanno, T. Kamada, S. Hayashi, M. Kitagawa, T. Hirao, Jpn. J. Appl. Phys. 32 (1993) L950. [13] B.S. Kwak, E.P. Boyd, A. Erbil, Appl. Phys. Lett. 53 (18) (1988) 1702. [14] T. Li, Y. Zhu, S.B. Desu, C.H. Peng, M. Nagata, Appl. Phys. Lett. 68 (1996) 616. [15] G.R. Bai, H.L.M. Chang, C.M. Foster, Z. Shen, D.J. Lam, J. Mater. Res. 9 (1994) 156. [16] L. Taguchi, A. Pignolet, L. Wand, M. Proctor, F. Levy, P.E. Schmid, J. Appl. Phys. 73 (1993) 394. [17] W.H. Ma, M.S. Zhang, Z. Tin, J. Korean Phys. Soc. 32 (1998) S1137. [18] G. Burns, B.A. Scott, Phys. Rev. B7 (1973) 3088.
Growth mechanism of PTO on MgO at initial stage K. Nishidaa,*, K. Shirakataa, M. Osadab, M. Kakihanac, T. Katodaa a
Department of Electronic and Photonic Systems Engineering, Kochi University of Technology, 185 Miyanokuchi, Tosayamada-cho, Kamigun, Kochi 782-8502, Japan b Precursory Research for Embryonic Science and Technology, Japan Science and Technology Corporation 4259, Nagatsuta, Midori-ku, Yokohama-shi, Kanagawa 226-8503, Japan c Materials and Structures Laboratory, Tokyo Institute of Technology 4259, Nagatsuta, Midori-ku, Yokohama-shi, Kanagawa 226-8503, Japan
Abstract Growth mechanism of a lead titanate (PbTiO3, PTO) thin film at the initial stage has been made clear. The mechanism is strongly affected by accumulation and release of stress. Flatness of the surface and the crystallographic orientation of the PTO film change with growth mode. # 2003 Elsevier Science B.V. All rights reserved. Keywords: PTO; Growth mechanism; Residual stress; Surface roughness; Film orientation
1. Introduction Lead titanate (PbTiO3, PTO) is a well-known ferroelectric material with a prerovskite structure [1]. PTO thin films have many applications such as nonvolatile memory [2], piezoelectric devices [3] and infrared sensors [4,5] because it has ferroelectric, piezoelectric, pyroelectric properties [6] and a high Curie temperature. PTO thin films with high-quality and high-reliability are still requested in order to apply them to devices. There have been many reports on preparation of PTO thin films using rf sputtering [7], sol–gel [8–10], laser ablation [11], ion beam sputtering [12] and chemical vapor deposition (CVD) [13–15]. However, it is difficult at present to control growth and to obtain PTO films with high-reliability because the ferroelectric thin film is *
Corresponding author. Tel.: þ81-887-53-1010; fax: þ81-887-57-2120. E-mail address: [email protected] (K. Nishida).
composed of multi-elements with different vapor pressures and many factors have affected their growth. Especially, the initial stage of growth gives important effects on properties of PTO films. We will report in this paper the growth mechanism of PTO films on MgO substrates.
2. Experiment PTO thin films were grown on MgO(1 0 0) substrates using the plasma enhanced chemical vapor deposition (PE-CVD) method. Organic stain on the substrates was removed followed by rinsing them with de-ionized water. Then, thermal cleaning was done before growth. The crystal growth was carried out at a temperature of 545 8C, a pressure of 10 Pa and a rf power of 170 W. The total growth time was 180 s and properties of film were measured at 15, 30, 45, 90, 135 and 180 s. The film thickness was measured using a surface-profile measuring system (DEKTAK 3).
0169-4332/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(03)00455-0
324
K. Nishida et al. / Applied Surface Science 216 (2003) 323–328
The growth rate was 1.12 mm/h. The film thicknesses were estimated to be 5, 9, 14, 28, 42 and 56 nm at the growth times of 15, 30, 45, 90, 135 and 180 s, respectively, with assumption that a growth rate was constant with time. A crystallographic orientation and crystallinity were characterized by the X-ray diffraction (XRD) method. Surface roughness was characterized using an atomic force microscope (AFM). Raman spectroscopy with a T64000 (JOBIN YVON) model was used to determine the structural disorder and the stress accumulated in the films. An Arþ laser of 514.5 nm was used for excitation and a space resolving was 1 mm. A resolution of shift of a Raman peak was about 1 cm1. Fig. 1. XRD spectrum of a PTO thin film grown for 180 s.
3. Results and discussion The XRD spectrum of the PTO thin film grown for 180 s is shown in Fig. 1. The PTO film was only oriented toward (1 0 0) and (0 0 1) axes (a-plane and c-plane). No diffraction peak from other orientation planes and the heterogenous phase was observed
during the growth. An axis orientation ratio a of c-plane was calculated from the results of XRD measurement. Value for a was defined as follows: a¼
Ið0 0 1Þ Ið1 0 0Þ þ Ið0 0 1Þ
Fig. 2. Growth time dependence of the c-axis orientation ratio a of PTO thin film.
(1)
K. Nishida et al. / Applied Surface Science 216 (2003) 323–328
I(1 0 0) and I(0 0 1) are intensities of (1 0 0) and (0 0 1) peaks in the XRD spectrum. Each intensity was evaluated by deconvolution using the Gaussian function. Fig. 2 shows growth time versus c-axis orientation ratio a. A film thickness derived as described above is also shown in the axis on the top. A c-axis orientation ratio of a PTO film was 0.51 at 15 s of growth. Then c-axis orientation ratio decreased until the growth time of 30 s. A c-axis orientation ratio increased again until 45 s and decreased until 90 s. In Fig. 2, the c-axis orientation ratio showed similar behavior from 90 to 180 s of growth. That is the orientation ratio of c-axis changed periodically with growth time.
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Fig. 3(a)–(f) shows AFM images of surfaces of the PTO film of various growth times. A measuring range was 1 mm. AFM measurement was done for over 10 points of the surface of each sample and typical values were plotted in Fig. 3. A height full scale in Fig. 3(a) and (b) was 25 nm and that in Fig. 3(c)–(f) was 50 nm. A root mean square (rms) of surface roughness was calculated from the AFM images and it was shown with a growth time in Fig. 4. At the beginning, crystal nucleus created on a MgO substrate (Fig. 3(a)) and then surface roughness increased with growth of the nucleus (Fig. 3(b)). Then, the surface became flat with combination of the crystal nucleus (Fig. 3(c)). A rms of this sample was 1.7 nm which was smaller
Fig. 3. AFM images of a PTO thin film. Growth time were: (a) 15 s; (b) 30 s; (c) 45 s; (d) 90 s; (e) 135 s; and (f) 180 s.
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K. Nishida et al. / Applied Surface Science 216 (2003) 323–328
Fig. 4. Growth time dependence of surface roughness of a PTO thin film.
than that of the MgO substrate (rms was 6 nm). Then, vertical growth mode occurred at about 90 s of growth accompanying increase of surface roughness (Fig. 3(d)). A rms increased rapidly over four times of that at the initial stage. However, lateral growth mode occurred again after 135 s of growth and the surface
flatness was improved as flat as that of the surface at 45 s (Fig. 3(e)). At the next time, vertical growth started again and surface roughness increased (Fig. 3(f)). The results shown above mean that the PTO film grows in vertical and lateral modes periodically with a growth time at the initial stage.
Fig. 5. Raman spectrum of a PTO thin film grown for 180 s.
K. Nishida et al. / Applied Surface Science 216 (2003) 323–328
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Fig. 6. Growth time vs. a peak shift of the E(1TO) mode in the Raman spectrum.
Raman spectra from the PTO film grown for 180 s are shown in Fig. 5. The A1, E and B1 þ E modes [16– 18] were observed in the spectra in Fig. 5. Intensity of these peaks and the full widths of half maximum (FWHM) of these peaks decreased with a growth time which mean that quality of the crystal improved. A shift of the Raman peak E(1LO) mode was plotted against growth time in Fig. 6. The E(1LO) mode is an important mode called ‘‘soft mode’’ which characterizes ferroelectrics. The relation between the soft
mode and accumulated strain has been made clear and it is possible to estimate accumulated stress from a shift of the E(1LO) peak. The amount of the stress is shown in the axis on right of Fig. 6. Fig. 6 shows that large compressive stress is accumulated in the thin films because the peak of E(1LO) shifts to a lower wavenumber. Stress of about 1.9 GPa is accumulated the PTO thin film grown for 15 s. Stress is released at 30 s at once, but larger compressive stress (2.3 GPa) increased after 45 s until about 90 s of growth.
Fig. 7. Model of growth process of a PTO thin film at initial stage.
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K. Nishida et al. / Applied Surface Science 216 (2003) 323–328
Release of stress begins again about at 90 s and accumulation begins at about 135 s. The result shown above indicates that accumulation and relaxation of stress is repeated with growth time. The results of characterization shown above are summarized in Fig. 7. Nuclei of PTO crystal are formed the first stage (15 s). As the nuclei grow, stress is accumulated with an increase in thickness until 45 s of growth. Flatness is improved during this third stage. Stress is released after the growth of 45 s and decreased from 45 to 90 s of growth. During this fourth stage of growth, surface roughness increased and the c-axis orientation ratio decreased. At the next stage, from 90 to 135 s, stress increased again. Flatness is improved again and the c-axis orientation ratio increased. The surface roughness increased and the stress was released again at the sixth stage. The results mean that stress increased during PTO crystal grew laterally because crystal grains pushed one another. When stress reached at a threshold value, 2.3 or 2.4 GPa in this case, defects were introduced and stress was released. After that stage, the crystal starts to grow vertically. From the start to 180 s of growth, at least, the mechanism is repeated.
4. Summary Growth mechanism of a PTO thin film at the initial stage has been made clear. The mechanism is strongly affected by accumulation and release of stress.
Flatness of the surface and the crystallographic orientation of the PTO film change with growth mode.
References [1] G. Burns, B.A. Scott, Phys. Rev. B7 (1973) 3088. [2] J.F. Scott, C.A. Araujo, Science 246 (1989) 1400. [3] K. Ohtani, M. Okuyama, Y. Hamakawa, Jpn. J. Appl. Phys. 23 (Suppl. 23-1) (1984) 133. [4] M. Okuyama, H. Seto, M. Kojima, Y. Matsui, Y. Hamakawa, Jpn. J. Appl. Phys. 22 (Suppl. 22-1) (1983) 465. [5] R. Takayama, Y. Tomita, K. Iijima, I. Ueda, J. Appl. Phys. 63 (1988) 5868. [6] J.S. Wrigh, L.F. Francis, J. Mater. Res. 8 (1993) 1712. [7] K. Iijima, Y. Tomita, R. Takayama, I. Ueda, J. Appl. Phys. 60 (1986) 361. [8] M. Maeda, H. Ishida, K.K.K. Soe, I. Suzuki, Jpn. J. Appl. Phys. 32 (1993) 4136. [9] S.S. Dana, F.K. Etzod, J. Clabes, J. Appl. Phys. 69 (1991) 4398. [10] C. Chen, D.F. Ryder Jr., J. Am. Ceram. Soc. 72 (1989) 1495. [11] T. Imai, M. Okuyama, Y. Hamakawa, Jpn. J. Appl. Phys. 30 (1991) 2163. [12] I. Kanno, T. Kamada, S. Hayashi, M. Kitagawa, T. Hirao, Jpn. J. Appl. Phys. 32 (1993) L950. [13] B.S. Kwak, E.P. Boyd, A. Erbil, Appl. Phys. Lett. 53 (18) (1988) 1702. [14] T. Li, Y. Zhu, S.B. Desu, C.H. Peng, M. Nagata, Appl. Phys. Lett. 68 (1996) 616. [15] G.R. Bai, H.L.M. Chang, C.M. Foster, Z. Shen, D.J. Lam, J. Mater. Res. 9 (1994) 156. [16] L. Taguchi, A. Pignolet, L. Wand, M. Proctor, F. Levy, P.E. Schmid, J. Appl. Phys. 73 (1993) 394. [17] W.H. Ma, M.S. Zhang, Z. Tin, J. Korean Phys. Soc. 32 (1998) S1137. [18] G. Burns, B.A. Scott, Phys. Rev. B7 (1973) 3088.