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Microbeam channeling studies of epitaxial titanate films
Nuclear Instruments and Methods in Physics Research B 219–220 (2004) 656–661 www.elsevier.com/locate/nimb
Microbeam channeling studies of epitaxial titanate films Karur R. Padmanabhan
*
Department of Physics and Astronomy, College of Science, Wayne State University, 364 Detroit, Michigan, MI 48202, USA
Abstract PbTiO3 films grown by MOCVD on SrTiO3 substrates were analyzed for epitaxy as a function of thickness by microbeam channeling. Micro RBS/PIXE analysis indicated all films had reasonable stoichiometric composition. The channeling minimum yield (vmin ) was close to 5% of the substrate value for very thin films indicating good epitaxial growth. Thicker films exhibited higher vmin values. Comparison of the experimental and simulated channeling spectra indicates higher defect density in thicker films. Channeling contrast microscopy (CCM) was used to get elemental maps of the surface and interface regions of the epitaxial films. Surface defects were found both in thin and thick films. Ó 2004 Published by Elsevier B.V. PACS: 68.55.Ln; 68.55.Ac; 77.84.Dy; 61.85.+p Keywords: Epitaxial oxide films; Microbeam; Channeling
1. Introduction Ferroelectric ceramic titanate films such as PbTiO3 , and Pb substituted with Zr or Ca have received great attention as promising materials for dielectric, pyroelectric, and electro-optic applications [1]. Generally the structure of PbTiO3 thin films depends on the deposition conditions, substrate type, and deposition method, and it is necessary to obtain epitaxial PbTiO3 thin films with only c domains for most of the practical applications [2]. On the other hand, a-axis oriented films possess higher dielectric constants (e0 ) for capacitor devices. As an optical material, quality epitaxial titanate films offer good transmission, homogeneity, and maximal electro-optical effect
*
Tel.: +1-313-577-3005; fax: +1-313-577-3932. E-mail address: [email protected] (K.R. Padmanabhan). 0168-583X/$ - see front matter Ó 2004 Published by Elsevier B.V. doi:10.1016/j.nimb.2004.01.137
without poling process. Therefore, microstructure study of ferroelectric thin films is of practical importance. Several methods such as MOCVD [3], sputtering [4] and sol–gel [5] deposition have been used for the growth of epitaxial films. While there is seldom any problem in growing good quality thin epitaxial films using any of these methods, it is still not clear how the epitaxy may get affected for thicker films. Due to the epitaxial nature of the growth, the domain structure of the films is closely coupled to the domain structure of the substrate. If the film thickness changes, the stress state is different and hence the domain structure will also change accordingly. This may affect the epitaxy especially at greater distances from the substrate– film interface. In this study, the quality of PbTiO3 thin films grown by MOCVD between 550 and 700 °C on SrTiO3 (1 0 0) substrates have been examined as a function of film thickness. Both the film and the substrate have perovskite structure and there is minimal lattice mismatch between them.
K.R. Padmanabhan / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 656–661
For SrTiO3 , a ¼ 3:905 and that for PbTiO3 , a ¼ 3:899. Epitaxial PbTiO3 films have been successfully grown by MOCVD on various single crystal substrates. Kwak et al. [6] reported 3-D epitaxial PbTiO3 films (a- and c-axis oriented) on KTaO3 (1 0 0) at 500 °C with 90° domain walls. Okada et al. [7] produced highly c-axis oriented films at 500–650 °C on MgO (1 0 0). The film grain size was found to increase with increased film thickness. In this study, we have used micro RBS and microbeam channeling analysis on MOCVD grown films to measure the vmin for films of different thickness. Simulated channeling spectra with different defect densities were used to compare with the experimental results to understand the role of thickness on the epitaxial quality of the films. Channeling contrast microscopy (CCM) was used to map the defective regions of the film.
2. Experimental PbTiO3 (0 0 1) thin films epitaxially grown by a private company using metal organic chemical vapor deposition (MOCVD) method on miscut (0 0 1) SrTiO3 substrate with miscut angle of 1.7° were used for these studies. Deposition of PbTiO3 thin films was carried out in a low-pressure coldwall horizontal quartz reactor using metal-organic precursors, and ultra-high purity nitrogen as the carrier gas. The composition of the film could be
657
modified by adjusting the gas flow rates. Transparent single-phase films were obtained at growth temperatures ranging from 550 to 700 °C. Epitaxial films of different thicknesses were grown and used for ion channeling analysis. Micro RBS/C analysis was carried out with 2 MeV, 4 Heþ . In some cases, for micro PIXE analysis, Hþ beam was employed. CCM was used to map the defective regions of the film. Details of the experimental arrangement and data acquisition system are described elsewhere [8]. The composition and thickness of the epitaxial films were obtained by using DORA and RBS Sim programs [9]. Ion channeling simulations were carried out using a channeling simulation program ISAP [10].
3. Results and discussion Due to the epitaxial nature of the growth, the domain structure of the PbTiO3 films is closely coupled to the domain structure of the substrate. The low lattice mismatch between the film and the substrate is clearly evident from the growth of good quality very thin epitaxial films. Fig. 1 shows the aligned and random spectra for a very thin PbTiO3 film. Analysis of the film with DORA shows that the film is a 30 nm thick stoichiometric film. The simulated channeling and random spectra generated for a defect free film for the same thickness is shown in Fig. 2. It is clear that very thin films of PbTiO3 are almost defect free with a
Fig. 1. Aligned (0 0 1) and random spectra for thin (28 nm) PbTiO3 film on SrTiO3 substrate.
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K.R. Padmanabhan / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 656–661
Fig. 2. Aligned (0 0 1) and random simulated spectra for a defect free thin PbTiO3 film on SrTiO3 substrate.
vmin close to 5% of the substrate value. This was also confirmed from the micro PIXE spectra in aligned and random conditions presented in Fig. 3. Angular scans of Sr from the substrate along with Pb and Ti from the film shown in Fig. 4 further clearly confirm the excellent epitaxial quality for very thin films. The RBS/C and micro PIXE spectra for a thick (400 nm) films are shown in Figs. 5 and 6, respectively. Even for thicker (1 lm) films, the maximum difference in vmin between the substrate and the film does not exceed 31%. It is
well known that the transformation temperature of a material in its thin film form is usually different from that of the bulk material due to the residual strain effect in the film. This strain may result from the epitaxial stress or the differential thermal stress between the film and the substrate during the deposition of the film. This stress may account for the moderate difference in the minimum yield between the substrate and the film. The vmin for films of different thickness are presented in Table 1. Taking the straggling of the
Fig. 3. Micro PIXE spectra for a thin (28 nm) PbTiO3 film on SrTiO3 substrate for aligned (0 0 1) and random orientation.
K.R. Padmanabhan / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 656–661
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Fig. 4. Angular scans of Sr from the SrTiO3 substrate plotted along with Pb and Ti from the PbTiO3 film.
Fig. 5. Aligned (0 0 1) and random spectra for thick (400 nm) PbTiO3 film on SrTiO3 substrate.
ions through the film and substrate lattice into consideration, films up to a maximum thickness of 1.4 lm exhibited some epitaxial growth. Scanning of the microbeam over a limited area in the aligned and random orientation of the sample provided elemental contrast images. Fig. 7
shows the CCM maps for 3 aligned and 1 random spectra. The CCM Pb map (a) for very thin (30 nm) epitaxial film in the aligned position clearly shows defective regions on the surface. The CCM map of the random spectral region for the same sample is shown for comparison. CCM scans in
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K.R. Padmanabhan / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 656–661
the aligned positions for thicker (260 and 400 nm) films ((c) and (d)) also show defective regions. When ferroelectric thin films are prepared, the total stress generated is described by the interaction of four types of stress: epitaxial stress, intrinsic stress, thermal stress and phase transformation Table 1 Typical vmin values for Pb from PbTiO3 film and Sr from SrTiO3 substrate
Fig. 6. Micro PIXE spectra for thick (400 nm) PbTiO3 film on SrTiO3 substrate for aligned (0 0 1) and random orientation.
Film thickness (nm)
Sr
Pb
% difference
28 60 260 400 900
0.1 0.1 0.32 0.59 0.97
0.11 0.12 0.22 0.49 0.80
10 20 31 17 17
Fig. 7. CCM Pb maps obtained in aligned condition for 28 nm thin (a), 260 nm thick (c) and 400 nm thick PbTiO3 film on SrTiO3 substrate. The Pb map for random orientation for the 28 nm film is shown in (b) for comparison.
K.R. Padmanabhan / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 656–661
stress [11]. The dominance of one stress over the other depends on the deposition method. Except in the case of sputter deposition, where intrinsic stress is high, generally the epitaxial stress is dominant for other methods. If the film thickness changes, the stress state is different and the domain structure will also change accordingly. In this study, except for isolated regions of defects, the domain structure of epitaxial PbTiO3 thin films does not appear to be significantly affected by the thickness. This may indicate that the epitaxial stress at the film–substrate interface may be very small. Separate CCM maps taken at the filmsubstrate region for Pb and Sr in the RBS and micro PIXE mode show smooth regions unlike the surface defects seen for thin and thick films. Channeling measurements and CCM maps of Caþ and Zrþ implantations on these epitaxial PbTiO3 films of different thickness provide clear evidence not only of the substitutionality of the dopants but also of additional damaged regions at the interface that is thickness dependent. These results will be presented later.
4. Conclusions Thin (30 nm) and thick (1 lm) films of PbTiO3 deposited by MOCVD on SrTiO3 substrates, show good epitaxial quality. The channeling minimum yield comparable to the substrate ranges from within 10% for very thin and about 31% for thick films. Channeling contrast microscopy of the Pb surface indicates regions of defects
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in the film, which increase with film thickness. However, the film–substrate interface itself appears to be relatively free of defects.
Acknowledgements The author is grateful to Professor Frank Watt of National University of Singapore for the use of microbeam facilities.
References [1] C.M. Foster, Z. Li, M. Buckett, D. Miller, P.M. Baldo, L.E. Rehn, G.R. Bai, D. Guo, H. You, K.L. Merkle, J. Appl. Phys. 78 (1995) 2607. [2] M. Shimizu, H. Fujisawa, H. Niu, Mater. Res. Soc. Symp. Proc. 596 (2000) 259. [3] H. Fujisawa, M. Shimizu, H. Niu, K. Honda, S. Ohtani, Mater. Res. Soc. Symp. Proc. 596 (2000) 321. [4] K. Iijima, Y. Tomita, R. Takayama, I. Ueda, J. Appl. Phys. 60 (1) (1986) 361. [5] K. Kushida, K.R. Udayakumar, S.B. Krupanidhi, L.E. Cross, J. Am. Ceram. Soc. 76 (5) (1993) 1345. [6] B.S. Kwak, A. Erbil, B.J. Wilkens, J.D. Budai, M.F. Chisholm, L.A. Boatner, Ceramic Trans. 25 (1992) 203. [7] M. Okada, S. Takai, M. Amemiya, K. Tominaga, Jpn. J. Appl. Phys. 28 (6) (1989) 1030. [8] F. Watt, I. Orlic, K.K. Loh, C.H. Sow, P. Thong, S.C. Liew, T. Osipowicz, T.F. Choo, S.M. Tang, Nucl. Instr. and Meth. B 85 (1994) 708. [9] S.S. Eskildson, Direct Online RBS Analysis and RBS SimPrograms, Wayne State University, 1989. [10] Y. Kido, T. Koshikawa, J. Appl. Phys. 67 (1990) 187. [11] W.K. Choi, S.K. Choi, H.M. Lee, J. Mater. Res. 14 (12) (1999) 4684.
Microbeam channeling studies of epitaxial titanate films Karur R. Padmanabhan
*
Department of Physics and Astronomy, College of Science, Wayne State University, 364 Detroit, Michigan, MI 48202, USA
Abstract PbTiO3 films grown by MOCVD on SrTiO3 substrates were analyzed for epitaxy as a function of thickness by microbeam channeling. Micro RBS/PIXE analysis indicated all films had reasonable stoichiometric composition. The channeling minimum yield (vmin ) was close to 5% of the substrate value for very thin films indicating good epitaxial growth. Thicker films exhibited higher vmin values. Comparison of the experimental and simulated channeling spectra indicates higher defect density in thicker films. Channeling contrast microscopy (CCM) was used to get elemental maps of the surface and interface regions of the epitaxial films. Surface defects were found both in thin and thick films. Ó 2004 Published by Elsevier B.V. PACS: 68.55.Ln; 68.55.Ac; 77.84.Dy; 61.85.+p Keywords: Epitaxial oxide films; Microbeam; Channeling
1. Introduction Ferroelectric ceramic titanate films such as PbTiO3 , and Pb substituted with Zr or Ca have received great attention as promising materials for dielectric, pyroelectric, and electro-optic applications [1]. Generally the structure of PbTiO3 thin films depends on the deposition conditions, substrate type, and deposition method, and it is necessary to obtain epitaxial PbTiO3 thin films with only c domains for most of the practical applications [2]. On the other hand, a-axis oriented films possess higher dielectric constants (e0 ) for capacitor devices. As an optical material, quality epitaxial titanate films offer good transmission, homogeneity, and maximal electro-optical effect
*
Tel.: +1-313-577-3005; fax: +1-313-577-3932. E-mail address: [email protected] (K.R. Padmanabhan). 0168-583X/$ - see front matter Ó 2004 Published by Elsevier B.V. doi:10.1016/j.nimb.2004.01.137
without poling process. Therefore, microstructure study of ferroelectric thin films is of practical importance. Several methods such as MOCVD [3], sputtering [4] and sol–gel [5] deposition have been used for the growth of epitaxial films. While there is seldom any problem in growing good quality thin epitaxial films using any of these methods, it is still not clear how the epitaxy may get affected for thicker films. Due to the epitaxial nature of the growth, the domain structure of the films is closely coupled to the domain structure of the substrate. If the film thickness changes, the stress state is different and hence the domain structure will also change accordingly. This may affect the epitaxy especially at greater distances from the substrate– film interface. In this study, the quality of PbTiO3 thin films grown by MOCVD between 550 and 700 °C on SrTiO3 (1 0 0) substrates have been examined as a function of film thickness. Both the film and the substrate have perovskite structure and there is minimal lattice mismatch between them.
K.R. Padmanabhan / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 656–661
For SrTiO3 , a ¼ 3:905 and that for PbTiO3 , a ¼ 3:899. Epitaxial PbTiO3 films have been successfully grown by MOCVD on various single crystal substrates. Kwak et al. [6] reported 3-D epitaxial PbTiO3 films (a- and c-axis oriented) on KTaO3 (1 0 0) at 500 °C with 90° domain walls. Okada et al. [7] produced highly c-axis oriented films at 500–650 °C on MgO (1 0 0). The film grain size was found to increase with increased film thickness. In this study, we have used micro RBS and microbeam channeling analysis on MOCVD grown films to measure the vmin for films of different thickness. Simulated channeling spectra with different defect densities were used to compare with the experimental results to understand the role of thickness on the epitaxial quality of the films. Channeling contrast microscopy (CCM) was used to map the defective regions of the film.
2. Experimental PbTiO3 (0 0 1) thin films epitaxially grown by a private company using metal organic chemical vapor deposition (MOCVD) method on miscut (0 0 1) SrTiO3 substrate with miscut angle of 1.7° were used for these studies. Deposition of PbTiO3 thin films was carried out in a low-pressure coldwall horizontal quartz reactor using metal-organic precursors, and ultra-high purity nitrogen as the carrier gas. The composition of the film could be
657
modified by adjusting the gas flow rates. Transparent single-phase films were obtained at growth temperatures ranging from 550 to 700 °C. Epitaxial films of different thicknesses were grown and used for ion channeling analysis. Micro RBS/C analysis was carried out with 2 MeV, 4 Heþ . In some cases, for micro PIXE analysis, Hþ beam was employed. CCM was used to map the defective regions of the film. Details of the experimental arrangement and data acquisition system are described elsewhere [8]. The composition and thickness of the epitaxial films were obtained by using DORA and RBS Sim programs [9]. Ion channeling simulations were carried out using a channeling simulation program ISAP [10].
3. Results and discussion Due to the epitaxial nature of the growth, the domain structure of the PbTiO3 films is closely coupled to the domain structure of the substrate. The low lattice mismatch between the film and the substrate is clearly evident from the growth of good quality very thin epitaxial films. Fig. 1 shows the aligned and random spectra for a very thin PbTiO3 film. Analysis of the film with DORA shows that the film is a 30 nm thick stoichiometric film. The simulated channeling and random spectra generated for a defect free film for the same thickness is shown in Fig. 2. It is clear that very thin films of PbTiO3 are almost defect free with a
Fig. 1. Aligned (0 0 1) and random spectra for thin (28 nm) PbTiO3 film on SrTiO3 substrate.
658
K.R. Padmanabhan / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 656–661
Fig. 2. Aligned (0 0 1) and random simulated spectra for a defect free thin PbTiO3 film on SrTiO3 substrate.
vmin close to 5% of the substrate value. This was also confirmed from the micro PIXE spectra in aligned and random conditions presented in Fig. 3. Angular scans of Sr from the substrate along with Pb and Ti from the film shown in Fig. 4 further clearly confirm the excellent epitaxial quality for very thin films. The RBS/C and micro PIXE spectra for a thick (400 nm) films are shown in Figs. 5 and 6, respectively. Even for thicker (1 lm) films, the maximum difference in vmin between the substrate and the film does not exceed 31%. It is
well known that the transformation temperature of a material in its thin film form is usually different from that of the bulk material due to the residual strain effect in the film. This strain may result from the epitaxial stress or the differential thermal stress between the film and the substrate during the deposition of the film. This stress may account for the moderate difference in the minimum yield between the substrate and the film. The vmin for films of different thickness are presented in Table 1. Taking the straggling of the
Fig. 3. Micro PIXE spectra for a thin (28 nm) PbTiO3 film on SrTiO3 substrate for aligned (0 0 1) and random orientation.
K.R. Padmanabhan / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 656–661
659
Fig. 4. Angular scans of Sr from the SrTiO3 substrate plotted along with Pb and Ti from the PbTiO3 film.
Fig. 5. Aligned (0 0 1) and random spectra for thick (400 nm) PbTiO3 film on SrTiO3 substrate.
ions through the film and substrate lattice into consideration, films up to a maximum thickness of 1.4 lm exhibited some epitaxial growth. Scanning of the microbeam over a limited area in the aligned and random orientation of the sample provided elemental contrast images. Fig. 7
shows the CCM maps for 3 aligned and 1 random spectra. The CCM Pb map (a) for very thin (30 nm) epitaxial film in the aligned position clearly shows defective regions on the surface. The CCM map of the random spectral region for the same sample is shown for comparison. CCM scans in
660
K.R. Padmanabhan / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 656–661
the aligned positions for thicker (260 and 400 nm) films ((c) and (d)) also show defective regions. When ferroelectric thin films are prepared, the total stress generated is described by the interaction of four types of stress: epitaxial stress, intrinsic stress, thermal stress and phase transformation Table 1 Typical vmin values for Pb from PbTiO3 film and Sr from SrTiO3 substrate
Fig. 6. Micro PIXE spectra for thick (400 nm) PbTiO3 film on SrTiO3 substrate for aligned (0 0 1) and random orientation.
Film thickness (nm)
Sr
Pb
% difference
28 60 260 400 900
0.1 0.1 0.32 0.59 0.97
0.11 0.12 0.22 0.49 0.80
10 20 31 17 17
Fig. 7. CCM Pb maps obtained in aligned condition for 28 nm thin (a), 260 nm thick (c) and 400 nm thick PbTiO3 film on SrTiO3 substrate. The Pb map for random orientation for the 28 nm film is shown in (b) for comparison.
K.R. Padmanabhan / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 656–661
stress [11]. The dominance of one stress over the other depends on the deposition method. Except in the case of sputter deposition, where intrinsic stress is high, generally the epitaxial stress is dominant for other methods. If the film thickness changes, the stress state is different and the domain structure will also change accordingly. In this study, except for isolated regions of defects, the domain structure of epitaxial PbTiO3 thin films does not appear to be significantly affected by the thickness. This may indicate that the epitaxial stress at the film–substrate interface may be very small. Separate CCM maps taken at the filmsubstrate region for Pb and Sr in the RBS and micro PIXE mode show smooth regions unlike the surface defects seen for thin and thick films. Channeling measurements and CCM maps of Caþ and Zrþ implantations on these epitaxial PbTiO3 films of different thickness provide clear evidence not only of the substitutionality of the dopants but also of additional damaged regions at the interface that is thickness dependent. These results will be presented later.
4. Conclusions Thin (30 nm) and thick (1 lm) films of PbTiO3 deposited by MOCVD on SrTiO3 substrates, show good epitaxial quality. The channeling minimum yield comparable to the substrate ranges from within 10% for very thin and about 31% for thick films. Channeling contrast microscopy of the Pb surface indicates regions of defects
661
in the film, which increase with film thickness. However, the film–substrate interface itself appears to be relatively free of defects.
Acknowledgements The author is grateful to Professor Frank Watt of National University of Singapore for the use of microbeam facilities.
References [1] C.M. Foster, Z. Li, M. Buckett, D. Miller, P.M. Baldo, L.E. Rehn, G.R. Bai, D. Guo, H. You, K.L. Merkle, J. Appl. Phys. 78 (1995) 2607. [2] M. Shimizu, H. Fujisawa, H. Niu, Mater. Res. Soc. Symp. Proc. 596 (2000) 259. [3] H. Fujisawa, M. Shimizu, H. Niu, K. Honda, S. Ohtani, Mater. Res. Soc. Symp. Proc. 596 (2000) 321. [4] K. Iijima, Y. Tomita, R. Takayama, I. Ueda, J. Appl. Phys. 60 (1) (1986) 361. [5] K. Kushida, K.R. Udayakumar, S.B. Krupanidhi, L.E. Cross, J. Am. Ceram. Soc. 76 (5) (1993) 1345. [6] B.S. Kwak, A. Erbil, B.J. Wilkens, J.D. Budai, M.F. Chisholm, L.A. Boatner, Ceramic Trans. 25 (1992) 203. [7] M. Okada, S. Takai, M. Amemiya, K. Tominaga, Jpn. J. Appl. Phys. 28 (6) (1989) 1030. [8] F. Watt, I. Orlic, K.K. Loh, C.H. Sow, P. Thong, S.C. Liew, T. Osipowicz, T.F. Choo, S.M. Tang, Nucl. Instr. and Meth. B 85 (1994) 708. [9] S.S. Eskildson, Direct Online RBS Analysis and RBS SimPrograms, Wayne State University, 1989. [10] Y. Kido, T. Koshikawa, J. Appl. Phys. 67 (1990) 187. [11] W.K. Choi, S.K. Choi, H.M. Lee, J. Mater. Res. 14 (12) (1999) 4684.