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Atomic force microscopy and scanning tunneling microscopy studies on the growth mechanism of a-Si:H film on graphite substrate
J O U R N A l . OF
ELSEVIER
Journal of Non-Crystalline Solids 198-200 (1996) 787-791
Atomic force microscopy and scanning tunneling microscopy studies on the growth mechanism of a-Si:H film on graphite substrate Mitsutaka Matsuse *, Seiji Tsuboi, Takashi Arakane, Masashi Kawasaki, Hideomi Koinuma Research Laboratory of Engineering Materials, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan
Abstract Based on analysis by atomic force microscopy, a nucleation and growth model is presented for the initial growth of hydrogenated amorphous silicon on a graphite surface. On an as-cleaved graphite surface, hydrogenated amorphous silicon islands were formed only along the graphite steps, indicating that the precursors can migrate on the atomically flat graphite surface until they are pinned by the islands nucleated along the steps. Hydrogen plasma treatment of graphite surfaces gave fine hydrogenated amorphous silicon islands on atomically flat terraces as well as homogeneous hydrogenated amorphous silicon films. On the surface of graphite treated with the hydrogen plasma, a high density of bright spots was observed by scanning tunneling microscopy. These spots were suggested to act as pinning sites for the aggregation of surface migrating precursors into hydrogenated amorphous silicon nucleation centers.
1. Introduction
Reactions at the growing film surface in plasma chemical vapor deposition (CVD) play a decisive role on the resulting film quality. Especially important is the initial growth of a-Si:H on the substrate where the hetero-interface formation takes place, since this process determines the atomic scale structure of the hetero-junction and the performance of a-Si:H based devices. Optical reflection methods, such as ellipsometry [1] and infrared reflection absorption spectroscopy [2] have been employed for
* Corresponding author. Tel.: + 81-45 921 5313; fax: + 81-45 921 6953; e-mail: matsusel @rlem.titech.ac.jp.
observing the initial growth and understanding the growth mechanism of a-Si:H thin films. We reported recently that such real space observation methods as scanning tunneling microscopy (STM) and atomic force microscopy (AFM) could be powerful tools for determining the initial growth mechanism. The substrate material and deposition temperature strongly influenced the initial growth of the a-Si:H film [3,4]. The growth on Coming #7059 glass was homogeneous as observed by in-situ XPS and ex-situ AFM. On an SnO 2 substrate, the growth mode changed from homogeneous to island growth with an increase of the deposition temperature. On a highly oriented pyrolytic graphite (HOPG) substrate, the initial growth was strongly influenced by the deposition temperature. When the film was deposited below
0022-3093/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 3 0 9 3 ( 9 6 ) 0 0 0 2 3 - 3
788
M. Matsuse et al. / Journal of Non-Co'stalline Solids 198-200 (1996) 787-791
l l 0 ° C from pure S i l l 4, a-Si:H islands were formed both on terraces and along steps. When the deposition temperature was increased to 230°C, a-Si:H was lined up only along the steps ( > 1 nm), indicating that the migration of adsorbed precursors was thermally activated and nucleation took place only along the steps. For interpreting these observations, we proposed a pinning mechanism which consists of the adsorption, migration, and pinning of precursors on the surface and of subsequent nucleation and growth of the film. In this paper, we report that the initial growth of a-Si:H on H O P G can be controlled by hydrogen plasma treatment prior to the a-Si:H deposition. STM observation of the H O P G surface suggests a chemical or morphological change of the H O P G surface by the hydrogen plasma treatment which generated pinning sites for the precursor on the terraces. Fig. 1. AFM image (2×2 /,zm2) of a-Si:H film (2 nm thick) deposited on a HOPG substrate at a temperature of 230°C.
2. Experiment A capacitively coupled rf (13.56 M H z ) plasma CVD apparatus was employed to deposit ultra-thin a-Si:H on H O P G (Panasonic graphite PGX04) surfaces. H O P G substrates were cleaved in-situ prior to the experiments in an ultra-high vacuum ( < 10 -8 Torr). Some of the freshly prepared surface were exposed to a hydrogen plasma just prior to the a-Si:H deposition. The treatment conditions were: rf power density, 200 m W / c m 2 ; H 2 gas flow rate, 10
sccm; and reaction pressure, 1.5 Torr. The exposure time was varied from 0 to 5 s. The growth conditions of a-Si:H were: rf power density, 40 m W / c m 2 ; S i l l 4 gas flow rate, 3 sccm; reaction pressure, 100 mTorr; and deposition temperature, 230°C. The sample was taken out of the chamber and subjected quickly to observation by A F M (SEIKO-SPI-3700) in air. The A F M was operated in a non-contact mode.
Fig. 2. AFM images (2 × 2 p.m 2) of a-Si:H films (2 nm thick) deposited on HOPG substrates treated with hydrogen plasma. The treatment time was (a) 0.05, (b) 0.5 and (c) 5 s, respectively.
M. Matsuse et al. / Journal of Non-Crystalline Solids 198-200 (1996) 787-791
3. Results First we checked the deposition rate and the properties of thick ( ~ 1 p~m) a-Si:H films deposited on Corning # 7 0 5 9 glass and crystalline silicon substrates. The film deposition rate was deduced to be 0.13 n m / s from the thickness measurement by the stylus method. The films showed device grade quality with a hydrogen content of 9% and a photoconductivity of 1 X 10 - 4 S / c m under the illumination of 100 m W / c m 2 Xe lamp. The AFM image was taken on the surface of as-cleaved HOPG to show atomically flat terraces which were 1 to 2 p,m wide and 0.3 to 2 nm high. Fig. 1 shows the AFM image ( 2 x 2 p,m 2) of a-Si:H deposits on an as-cleaved H O P G substrate. Nominally 2 nm thick a-Si:H film should be de-
(a)
precursor(Sill3) -- ~ C~
(b)
(*)
Fig. 3. Schematic illustration of precursor pinning mechanism for the initial growth of a-Si:H on HOPG substrate. The nucleation and growth of a-Si:H islands should take place at the pinning sites (see text) located atomically flat terraces as well as along steps ( > 1 nm). From pure Sill4, a-Si:H was deposited only along the steps (a). When the HOPG surface was treated by hydrogen plasma, the growth mode could change to (b) and (c) with the treatment duration.
789
posited by taking the product of deposition rate and time into account. However, a-Si:H was deposited only along the steps, as already reported in our previous papers [4]. The height of the islands was about 5 nm and the width ranged from 70 to 500 nm. As the nominal thickness of a-Si:H was increased from 0.5 to 2 nm, the height of the islands scarcely changed but the width increased proportionally. Along the steps adjacent to the wider terraces, more deposits of a-Si:H were formed as shown in Fig. 1. It must be noted that the area free from the deposits is composed not only of terraces, where the atomic image of graphite can be seen, but also of steps with their heights less than 1 nm (1 to 3 atomic layers height). Fig. 2 shows AFM images of nominally 2 nm thick a-Si:H deposited on a HOPG substrate treated with an rf excited hydrogen plasma. When a HOPG substrate was exposed to the hydrogen plasma for 0.05 s prior to a-Si:H deposition, a-Si:H islands were formed on the atomically flat terraces as well as along the steps (Fig. 2(a)). By increasing the exposure time to 0.5 s, the density of a-Si:H islands was increased and the size of the islands was decreased (Fig. 2(b)). The nominal thickness dependence of the a-Si:H islands formed on these substrates indicated that the density of a-Si:H islands scarcely changed but their average size increased. When the HOPG surface was exposed to a hydrogen plasma for 5 s, a uniform coverage of a-Si:H was achieved as shown in Fig. 2(c).
4. Discussion We schematically summarize the growth behavior of a-Si:H films on graphite substrate as shown in Fig. 3. The results shown in Fig. 1 clearly indicate that on the as-cleaved surface, silicon containing precursors adsorbed on the HOPG surface can migrate on the atomically flat terraces as well as across steps as high as 1 nm until they encounter such strong pinning sites as steps higher than 1 nm (Fig. 3(a)). Silicon containing radicals which act as precursors to a-Si:H are presumed not to interact so strongly with the ~- electrons of the graphite surface as to form S i - C chemical bonds. In the subsequent period, precursors adsorbed on the bare HOPG sur-
790
M. Matsuse et al. / Journal of Non-C~stalline Solids 198-200 (1996) 787-791
face migrate until they encounter a-Si:H islands, which are also strong pinning sites for precursors migrating on the surface. Since the coverage is low, the existing a-Si:H islands grow by the pinning of radicals migrating from the adjacent bare HOPG surface. From the series of pictures shown in Fig. 2, the hydrogen plasma treatment is considered to generate pinning sites, which induce the nucleation and subsequent growth of a-Si:H islands, on the atomically flat terraces of HOPG. As the hydrogen plasma treatment time is prolonged (from Fig. 2(a) to Fig. 2(b)), the density of pinning sites is increased (from Fig. 3(a) to Fig. 3(b)). When the density of pinning sites becomes large (Fig. 2(c)), fine a-Si:H islands cannot be identified and the film shows homogeneous coverage as schematically illustrated in Fig. 3(c). To further determine the effect of hydrogen plasma treatment, the HOPG substrate exposed to the hydrogen plasma for 0.05 s was measured by AFM and ultra high vacuum (UHV) STM. An AFM image for the treated surface showed no visible change as compared with that of an as-cleaved surface. In contrast, the STM image in Fig. 4 clearly shows bright spots of about 3 nm diameter on the plasmatreated surface. In the STM image taken in a nar-
rower scan mode, we could see clear atomic images of graphite on the terrace surface (except for the spot areas). These bright spots presumably act as pinning sites for migrating precursors of a-Si:H. The following two explanation are plausible for the bright spots. One is hydrogen termination of the HOPG surface and the other is generation of surface defects with the hydrogen plasma treatment. Our recent quantum chemical calculation supports the former explanation [5]. Hydrogen radical arriving on a graphite sheet can form a C - H bond in a configuration having a local minimum in the total energy. The orbital hybridaization at the C atom changes its nature from pure sp 2 to the mixture of sp 2 and sp 3. However, the latter explanation is supported by the observation that atomic hydrogen can etch sp 2 carbon preferentially in CVD diamond film growth [6]. Etch pits produced by the hydrogen plasma on HOPG may act as nucleation or pinning sites. In either case, the hydrogen plasma-treated HOPG should have an interaction with migrating precursors different from as-cleaved HOPG. Further research is in progress to specify the origin of these pinning sites and to generate an atomically defined image of the surface reaction involved in a-Si:H film growth.
5. Conclusion Hydrogen plasma treatment of a HOPG surface is shown to provide new sites for the pinning of migrating precursors on atomically flat HOPG terraces. Prolonged exposure to a hydrogen plasma for more than 5 s drastically changed the growth mode from island formation to homogeneous coverage of the surface.
Acknowledgements
Fig. 4. UHV-STM image (50×50 nm2) of a HOPG substrate treated with hydrogen plasma obtained with a constant tunneling current 0.10 nA and a sample-bias voltage of 0.24 V.
The authors would like to thank Mitsui Toatsu Chemicals Inc. for supplying Sill 4 gas. This work was supported in part by PVTEC (Photovoltaic Power Generation Technology Research Association), the Sasakawa Scientific Research Grant and a Grant-inAid for Scientific Research from the Ministry of Education, Science and Culture.
M. Matsuse et al. / Journal of Non.Crystalline Solids 198-200 (1996) 787-791
References [1] I. An, Y. Li, C.R. Wronski and R.W. Collins, Mater. Soc. Symp. Proc. 258 (1992) 27. [2] Y. Toyoshima, K. Araki, A. Matsuda and K. Tanaka, Appl. Phys. Lett. 57 (1991) 1028. [3] M. Kawasaki, M. Matsuse, M. Eguchi, M. Sumiya, H. Sakata and H. Koinuma, in: Proc. 2nd Int. Conf. on Reactive Plasma
791
and 1 lth Symp. on Plasma Processing, ed. T. Goto (Organizing Committee ICRP-2/SPP-11, Yokohama, 1993) p. 637. [4] M. Matsuse, M. Kawasaki and H. Koinuma, in: Proc. 1st World Conf. on Photovoltaic Energy Conversion, Hawaii, USA, 1994, ed. D.J. Flood, p. 425. [5] K. Nakajima, K. Sato and H. Koinuma, unpublished. [6] Y. Saito, K. Sato, H. Tanaka and H. Miyadera, J. Mater. Sci. 24 (1989) 293.
ELSEVIER
Journal of Non-Crystalline Solids 198-200 (1996) 787-791
Atomic force microscopy and scanning tunneling microscopy studies on the growth mechanism of a-Si:H film on graphite substrate Mitsutaka Matsuse *, Seiji Tsuboi, Takashi Arakane, Masashi Kawasaki, Hideomi Koinuma Research Laboratory of Engineering Materials, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan
Abstract Based on analysis by atomic force microscopy, a nucleation and growth model is presented for the initial growth of hydrogenated amorphous silicon on a graphite surface. On an as-cleaved graphite surface, hydrogenated amorphous silicon islands were formed only along the graphite steps, indicating that the precursors can migrate on the atomically flat graphite surface until they are pinned by the islands nucleated along the steps. Hydrogen plasma treatment of graphite surfaces gave fine hydrogenated amorphous silicon islands on atomically flat terraces as well as homogeneous hydrogenated amorphous silicon films. On the surface of graphite treated with the hydrogen plasma, a high density of bright spots was observed by scanning tunneling microscopy. These spots were suggested to act as pinning sites for the aggregation of surface migrating precursors into hydrogenated amorphous silicon nucleation centers.
1. Introduction
Reactions at the growing film surface in plasma chemical vapor deposition (CVD) play a decisive role on the resulting film quality. Especially important is the initial growth of a-Si:H on the substrate where the hetero-interface formation takes place, since this process determines the atomic scale structure of the hetero-junction and the performance of a-Si:H based devices. Optical reflection methods, such as ellipsometry [1] and infrared reflection absorption spectroscopy [2] have been employed for
* Corresponding author. Tel.: + 81-45 921 5313; fax: + 81-45 921 6953; e-mail: matsusel @rlem.titech.ac.jp.
observing the initial growth and understanding the growth mechanism of a-Si:H thin films. We reported recently that such real space observation methods as scanning tunneling microscopy (STM) and atomic force microscopy (AFM) could be powerful tools for determining the initial growth mechanism. The substrate material and deposition temperature strongly influenced the initial growth of the a-Si:H film [3,4]. The growth on Coming #7059 glass was homogeneous as observed by in-situ XPS and ex-situ AFM. On an SnO 2 substrate, the growth mode changed from homogeneous to island growth with an increase of the deposition temperature. On a highly oriented pyrolytic graphite (HOPG) substrate, the initial growth was strongly influenced by the deposition temperature. When the film was deposited below
0022-3093/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 3 0 9 3 ( 9 6 ) 0 0 0 2 3 - 3
788
M. Matsuse et al. / Journal of Non-Co'stalline Solids 198-200 (1996) 787-791
l l 0 ° C from pure S i l l 4, a-Si:H islands were formed both on terraces and along steps. When the deposition temperature was increased to 230°C, a-Si:H was lined up only along the steps ( > 1 nm), indicating that the migration of adsorbed precursors was thermally activated and nucleation took place only along the steps. For interpreting these observations, we proposed a pinning mechanism which consists of the adsorption, migration, and pinning of precursors on the surface and of subsequent nucleation and growth of the film. In this paper, we report that the initial growth of a-Si:H on H O P G can be controlled by hydrogen plasma treatment prior to the a-Si:H deposition. STM observation of the H O P G surface suggests a chemical or morphological change of the H O P G surface by the hydrogen plasma treatment which generated pinning sites for the precursor on the terraces. Fig. 1. AFM image (2×2 /,zm2) of a-Si:H film (2 nm thick) deposited on a HOPG substrate at a temperature of 230°C.
2. Experiment A capacitively coupled rf (13.56 M H z ) plasma CVD apparatus was employed to deposit ultra-thin a-Si:H on H O P G (Panasonic graphite PGX04) surfaces. H O P G substrates were cleaved in-situ prior to the experiments in an ultra-high vacuum ( < 10 -8 Torr). Some of the freshly prepared surface were exposed to a hydrogen plasma just prior to the a-Si:H deposition. The treatment conditions were: rf power density, 200 m W / c m 2 ; H 2 gas flow rate, 10
sccm; and reaction pressure, 1.5 Torr. The exposure time was varied from 0 to 5 s. The growth conditions of a-Si:H were: rf power density, 40 m W / c m 2 ; S i l l 4 gas flow rate, 3 sccm; reaction pressure, 100 mTorr; and deposition temperature, 230°C. The sample was taken out of the chamber and subjected quickly to observation by A F M (SEIKO-SPI-3700) in air. The A F M was operated in a non-contact mode.
Fig. 2. AFM images (2 × 2 p.m 2) of a-Si:H films (2 nm thick) deposited on HOPG substrates treated with hydrogen plasma. The treatment time was (a) 0.05, (b) 0.5 and (c) 5 s, respectively.
M. Matsuse et al. / Journal of Non-Crystalline Solids 198-200 (1996) 787-791
3. Results First we checked the deposition rate and the properties of thick ( ~ 1 p~m) a-Si:H films deposited on Corning # 7 0 5 9 glass and crystalline silicon substrates. The film deposition rate was deduced to be 0.13 n m / s from the thickness measurement by the stylus method. The films showed device grade quality with a hydrogen content of 9% and a photoconductivity of 1 X 10 - 4 S / c m under the illumination of 100 m W / c m 2 Xe lamp. The AFM image was taken on the surface of as-cleaved HOPG to show atomically flat terraces which were 1 to 2 p,m wide and 0.3 to 2 nm high. Fig. 1 shows the AFM image ( 2 x 2 p,m 2) of a-Si:H deposits on an as-cleaved H O P G substrate. Nominally 2 nm thick a-Si:H film should be de-
(a)
precursor(Sill3) -- ~ C~
(b)
(*)
Fig. 3. Schematic illustration of precursor pinning mechanism for the initial growth of a-Si:H on HOPG substrate. The nucleation and growth of a-Si:H islands should take place at the pinning sites (see text) located atomically flat terraces as well as along steps ( > 1 nm). From pure Sill4, a-Si:H was deposited only along the steps (a). When the HOPG surface was treated by hydrogen plasma, the growth mode could change to (b) and (c) with the treatment duration.
789
posited by taking the product of deposition rate and time into account. However, a-Si:H was deposited only along the steps, as already reported in our previous papers [4]. The height of the islands was about 5 nm and the width ranged from 70 to 500 nm. As the nominal thickness of a-Si:H was increased from 0.5 to 2 nm, the height of the islands scarcely changed but the width increased proportionally. Along the steps adjacent to the wider terraces, more deposits of a-Si:H were formed as shown in Fig. 1. It must be noted that the area free from the deposits is composed not only of terraces, where the atomic image of graphite can be seen, but also of steps with their heights less than 1 nm (1 to 3 atomic layers height). Fig. 2 shows AFM images of nominally 2 nm thick a-Si:H deposited on a HOPG substrate treated with an rf excited hydrogen plasma. When a HOPG substrate was exposed to the hydrogen plasma for 0.05 s prior to a-Si:H deposition, a-Si:H islands were formed on the atomically flat terraces as well as along the steps (Fig. 2(a)). By increasing the exposure time to 0.5 s, the density of a-Si:H islands was increased and the size of the islands was decreased (Fig. 2(b)). The nominal thickness dependence of the a-Si:H islands formed on these substrates indicated that the density of a-Si:H islands scarcely changed but their average size increased. When the HOPG surface was exposed to a hydrogen plasma for 5 s, a uniform coverage of a-Si:H was achieved as shown in Fig. 2(c).
4. Discussion We schematically summarize the growth behavior of a-Si:H films on graphite substrate as shown in Fig. 3. The results shown in Fig. 1 clearly indicate that on the as-cleaved surface, silicon containing precursors adsorbed on the HOPG surface can migrate on the atomically flat terraces as well as across steps as high as 1 nm until they encounter such strong pinning sites as steps higher than 1 nm (Fig. 3(a)). Silicon containing radicals which act as precursors to a-Si:H are presumed not to interact so strongly with the ~- electrons of the graphite surface as to form S i - C chemical bonds. In the subsequent period, precursors adsorbed on the bare HOPG sur-
790
M. Matsuse et al. / Journal of Non-C~stalline Solids 198-200 (1996) 787-791
face migrate until they encounter a-Si:H islands, which are also strong pinning sites for precursors migrating on the surface. Since the coverage is low, the existing a-Si:H islands grow by the pinning of radicals migrating from the adjacent bare HOPG surface. From the series of pictures shown in Fig. 2, the hydrogen plasma treatment is considered to generate pinning sites, which induce the nucleation and subsequent growth of a-Si:H islands, on the atomically flat terraces of HOPG. As the hydrogen plasma treatment time is prolonged (from Fig. 2(a) to Fig. 2(b)), the density of pinning sites is increased (from Fig. 3(a) to Fig. 3(b)). When the density of pinning sites becomes large (Fig. 2(c)), fine a-Si:H islands cannot be identified and the film shows homogeneous coverage as schematically illustrated in Fig. 3(c). To further determine the effect of hydrogen plasma treatment, the HOPG substrate exposed to the hydrogen plasma for 0.05 s was measured by AFM and ultra high vacuum (UHV) STM. An AFM image for the treated surface showed no visible change as compared with that of an as-cleaved surface. In contrast, the STM image in Fig. 4 clearly shows bright spots of about 3 nm diameter on the plasmatreated surface. In the STM image taken in a nar-
rower scan mode, we could see clear atomic images of graphite on the terrace surface (except for the spot areas). These bright spots presumably act as pinning sites for migrating precursors of a-Si:H. The following two explanation are plausible for the bright spots. One is hydrogen termination of the HOPG surface and the other is generation of surface defects with the hydrogen plasma treatment. Our recent quantum chemical calculation supports the former explanation [5]. Hydrogen radical arriving on a graphite sheet can form a C - H bond in a configuration having a local minimum in the total energy. The orbital hybridaization at the C atom changes its nature from pure sp 2 to the mixture of sp 2 and sp 3. However, the latter explanation is supported by the observation that atomic hydrogen can etch sp 2 carbon preferentially in CVD diamond film growth [6]. Etch pits produced by the hydrogen plasma on HOPG may act as nucleation or pinning sites. In either case, the hydrogen plasma-treated HOPG should have an interaction with migrating precursors different from as-cleaved HOPG. Further research is in progress to specify the origin of these pinning sites and to generate an atomically defined image of the surface reaction involved in a-Si:H film growth.
5. Conclusion Hydrogen plasma treatment of a HOPG surface is shown to provide new sites for the pinning of migrating precursors on atomically flat HOPG terraces. Prolonged exposure to a hydrogen plasma for more than 5 s drastically changed the growth mode from island formation to homogeneous coverage of the surface.
Acknowledgements
Fig. 4. UHV-STM image (50×50 nm2) of a HOPG substrate treated with hydrogen plasma obtained with a constant tunneling current 0.10 nA and a sample-bias voltage of 0.24 V.
The authors would like to thank Mitsui Toatsu Chemicals Inc. for supplying Sill 4 gas. This work was supported in part by PVTEC (Photovoltaic Power Generation Technology Research Association), the Sasakawa Scientific Research Grant and a Grant-inAid for Scientific Research from the Ministry of Education, Science and Culture.
M. Matsuse et al. / Journal of Non.Crystalline Solids 198-200 (1996) 787-791
References [1] I. An, Y. Li, C.R. Wronski and R.W. Collins, Mater. Soc. Symp. Proc. 258 (1992) 27. [2] Y. Toyoshima, K. Araki, A. Matsuda and K. Tanaka, Appl. Phys. Lett. 57 (1991) 1028. [3] M. Kawasaki, M. Matsuse, M. Eguchi, M. Sumiya, H. Sakata and H. Koinuma, in: Proc. 2nd Int. Conf. on Reactive Plasma
791
and 1 lth Symp. on Plasma Processing, ed. T. Goto (Organizing Committee ICRP-2/SPP-11, Yokohama, 1993) p. 637. [4] M. Matsuse, M. Kawasaki and H. Koinuma, in: Proc. 1st World Conf. on Photovoltaic Energy Conversion, Hawaii, USA, 1994, ed. D.J. Flood, p. 425. [5] K. Nakajima, K. Sato and H. Koinuma, unpublished. [6] Y. Saito, K. Sato, H. Tanaka and H. Miyadera, J. Mater. Sci. 24 (1989) 293.