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High-rate deposition of polycrystalline silicon thin films by hot wire cell method using disilane
Solar Energy Materials & Solar Cells 66 (2001) 225}230
High-rate deposition of polycrystalline silicon thin "lms by hot wire cell method using disilane Mitsuru Ichikawa*, Takeshi Tsushima, Akira Yamada, Makoto Konagai Department of Electrical and Electronic Engineering, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan
Abstract A new process, hot wire cell method, was developed and successfully used to grow polycrystalline silicon thin "lms at a low-temperature and high deposition rate. In the hot wire cell method, reactant gases are decomposed by a heated tungsten "lament. Polycrystalline silicon "lms can be deposited at deposition rates of 1.2 nm/s for mono-silane (SiH ) and 2.8 nm/s for disilane (Si H ). By using disilane as a reactant gas, it is possible to achieve a high deposition rate without any change in the quality of the "lms. 2001 Elsevier Science B.V. All rights reserved. Keywords: Silicon materials; Polycrystalline; Hot wire
1. Introduction The hot wire cell method is a new and very promising process for depositing polycrystalline silicon thin "lms for photovoltaic applications. In this process, a heated tungsten "lament is used to induce the catalytic and/or pyrolytic dissociation of reactant gases such as mono-silane (SiH ), disilane (Si H ) and hydrogen (H ) and produce atomic hydrogen as a reaction by-product, which then reacts with silane molecules to produce SiH radicals. In this work, since the "lament is positioned perpendicular to the substrate holder, the reactant gas is e$ciently decomposed during its passage through the "lament thus enhancing its decomposition rate. We have previously obtained polycrystalline silicon thin "lms without hydrogen dilution * Corresponding author. Tel.: #81-3-5734-2661; fax: #81-3-5734-2897. E-mail address: [email protected] (M. Ichikawa). 0927-0248/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 4 8 ( 0 0 ) 0 0 1 7 7 - X
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and demonstrated that polycrystalline silicon "lms could be deposited at low temperatures of 175}4003C at a relatively high-deposition rate of 1.0 nm/s [1]. Furthermore, we found that the deposition rate increases linearly with SiH #ow rate and the deposition rate does not saturate implying that higher deposition rate is achievable at higher SiH #ow rates [2]. However, increasing the deposition rate by merely increasing the SiH #ow rate leads to degradation of the "lament by generating plenty of silicide on the "lament. Thus, we adopt other approaches, such as the use of Si H to increase the deposition rate. In this study, we present our experimental results on the deposition rate and structural properties of polycrystalline silicon "lms deposited from Si H . 2. Experimental procedure The details of the deposition apparatus are described elsewhere [1]. Since the hot wire cell consists of a gas inlet and a tungsten "lament that was arranged to be parallel to the gas #ow, reactant gas is e$ciently decomposed during its passage through the "lament. We have four deposition parameters that we readily vary in our depositions: chamber pressure, Si H #ow rate, substrate temperature, and "lament temperature. It has been found in our previous works that chamber pressure strongly in#uences on crystallinity of the "lms [2] and that polycrystalline silicon "lms are obtained at a pressure of 0.1 Torr. In this study, we investigate the deposition of polycrystalline silicon "lms with high growth rate conditions. Therefore, we kept chamber pressure as 0.1 Torr and varied the other three parameters shown in Table 1. The distance between the "lament and substrate was kept at 6 cm. Thin "lms were deposited on Corning 7059 glass substrate.
3. Results and discussion The deposition rate was signi"cantly in#uenced by the Si H #ow rate. Fig. 1 shows the deposition rate as a function of the gas #ow rate for both SiH and Si H as a reactant gas. In the previous work, it was found that the deposition rate increases linearly with SiH #ow rate. The deposition rate also increases linearly with Si H #ow rate. We found that the deposition rate reaches 2.1 nm/s at a Si H #ow rate of 15 sccm and the deposition rate does not saturate implying that higher deposition rate Table 1 Deposition conditions Deposition parameter Si H #ow rate Filament temperature Substrate temperature
5}15 sccm 1800}22003C 175}3503C
M. Ichikawa et al. / Solar Energy Materials & Solar Cells 66 (2001) 225}230
227
Fig. 1. The deposition rate of the "lms deposited from SiH and Si H as a function of #ow rate.
Fig. 2. XRD pattern of the "lms deposited from SiH and Si H which have the same "lm thickness.
is achievable at higher Si H #ow rates. This result suggests that Si H gas is decomposed su$ciently on the surface of "lament and e!ectively utilized for the deposition of "lms. Since the deposition rate was 1.0 nm/s at a SiH #ow rate of 15 sccm, the deposition rate at the same Si H #ow rate is two times larger. This relationship of deposition rate is held throughout the range of the #ow rate in this experiment. These results suggest that by using Si H as a reactant gas it is possible to achieve higher deposition rate in the hot wire cell method. Fig. 2 shows the XRD pattern of the "lms deposited from SiH and Si H at the same deposition condition, which have the same "lm thickness. In general, high-rate
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M. Ichikawa et al. / Solar Energy Materials & Solar Cells 66 (2001) 225}230
deposition makes crystallinity of "lms worse. However, we found that polycrystalline "lms were obtained and that the "lms deposited from Si H have a (2 2 0) prefer ential orientation, which showed the same pattern as that of the "lm deposited from SiH . The intensity of each peak and FWHM do not make a di!erence for the two patterns. Furthermore, we observed that polycrystalline thin "lms deposited from Si H have a columnar structure similar to that from SiH . These results suggest that by using Si H as a reactant gas it is possible to achieve a high deposition rate without any change in the quality of the "lms. Since it was demonstrated in our previous experimental and theoretical works that the SiH radicals on the growing surface play a key role in the growth of high-quality amorphous and crystal silicon at a low temperature [3], we assume that su$cient SiH radicals were also supplied to the growth surface in the case where Si H was used as a reactant gas. Next, we deposited silicon "lms by varying the substrate temperature. Fig. 3 shows the deposition rate of the "lms as a function of the substrate temperature. It shows that the deposition rate varies from 2.4 nm/s at a substrate temperature of 3503C to 2.8 nm/s at a substrate temperature of 1753C. Thus, there is little change in the deposition rate on varying the substrate temperature. We assume that dissociation of Si H gas is mostly occurring on the surface of "lament and a hardly a!ected by the temperature of the growing surface. However, a little increase of the deposition rate was observed with the substrate temperature decrease. This increase of the deposition rate may be due to the change of the e!ective surface reaction probability of the radicals at the growing surface [4]. Up to now a deposition rate of 2.8 nm/s was the maximum value in our experiment. We also observed XRD patterns of those "lms. The "lms deposited at a high substrate temperature have a (2 2 0) preferential orientation. However, for the "lm deposited at 1753C the XRD pattern shows a di!erent orientation: the intensity of the di!raction peak of the (1 1 0) plane decreases to half of the (1 1 1) di!raction peak indicating that the "lm had a random orientation. Thus, to
Fig. 3. The deposition rate of the "lms as a function of substrate temperature.
M. Ichikawa et al. / Solar Energy Materials & Solar Cells 66 (2001) 225}230
229
achieve high-quality polycrystalline silicon "lms the substrate temperature of over 2503C is required. Fig. 4 shows the deposition rate of the "lms deposited from SiH and Si H as a function of "lament temperature. The deposition time of each sample was 1 h for SiH and 30 min for Si H .There is no signi"cant change in the deposition rate at the low "lament temperature condition; however, the rate decreased gradually at the "lament temperatures over 20003C. When SiH is used as a reactant gas, polycrystal line silicon "lms are observed at any condition of the "lament temperature without hydrogen dilution. However, in the case of Si H , the "lm is crystallized at "lament temperatures over 20003C without hydrogen dilution. We assume that this transition
Fig. 4. The deposition rate of the "lms deposited from SiH and Si H as a function of "lament temperature.
Fig. 5. XRD pattern as a function of hydrogen #ow rate at a "lament temperature of 18003C.
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of crystallinity was due to the #ux of the atomic hydrogen at the growing surface. The main channel for Si H dissociation is "rmly believed to be the elimination of SiH [5]. In this Si H decomposition process, secondary and/or tertiary chemical reac tions are required for generating hydrogen molecule or atomic hydrogen. Consequently, #ux of the atomic hydrogen in Si H dissociation may decrease in compari son with that of SiH dissociation. This assumption was established by the results of hydrogen dilution. In the case of hydrogen addition, hydrogen gas was also passed through the heated "lament. The #ow rate of hydrogen was varied from 0 to 60 sccm. Fig. 5 shows the XRD pattern as a function of hydrogen #ow rate at a "lament temperature of 18003C. Within the low hydrogen #ow rate, no di!raction peak was observed, which indicates that the "lm was amorphous. When Si H gas is diluted with hydrogen at a #ow rate of 45 sccm, the "lm is crystallized at a low "lament temperature of 18003C. These results indicate the importance of atomic hydrogen during the growth.
4. Conclusions We investigated the use of Si H to achieve higher deposition rate of the polycrys talline silicon "lms. On using Si H as a reactant gas, the deposition rate becomes twice larger than that from SiH without any change in the quality of the "lms. Polycrystalline "lms were deposited at a deposition rate of 2.8 nm/s on glass substrates by the hot wire cell method using Si H . Acknowledgements This work was supported in part by NEDO as a part of the New Sunshine Program under the Ministry of International Trade and Industry of Japan.
References [1] [2] [3] [4]
M. Ichikawa, J. Takeshita, A. Yamada, M. Konagai, Jpn. J. Appl. Phys. 38 (1999) L24. M. Ichikawa, J. Takeshita, A. Yamada, M. Konagai, MRS Proc. 536 (1998) 487. T. Oshima, A. Yamada, M. Konagai, Jpn. J. Appl. Phys. 36 (1997) 6481. J. Perrin, M. Shiratani, P. Kae-Nune, H. Videlot, J. Jolly, J. Guillon, J. Vac. Sci. Technol. A 16 (1998) 278. [5] H.K. Mo!at, K.F. Jensen, R.W. Carr, J. Phys. Chem. 96 (1992) 7683.
High-rate deposition of polycrystalline silicon thin "lms by hot wire cell method using disilane Mitsuru Ichikawa*, Takeshi Tsushima, Akira Yamada, Makoto Konagai Department of Electrical and Electronic Engineering, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan
Abstract A new process, hot wire cell method, was developed and successfully used to grow polycrystalline silicon thin "lms at a low-temperature and high deposition rate. In the hot wire cell method, reactant gases are decomposed by a heated tungsten "lament. Polycrystalline silicon "lms can be deposited at deposition rates of 1.2 nm/s for mono-silane (SiH ) and 2.8 nm/s for disilane (Si H ). By using disilane as a reactant gas, it is possible to achieve a high deposition rate without any change in the quality of the "lms. 2001 Elsevier Science B.V. All rights reserved. Keywords: Silicon materials; Polycrystalline; Hot wire
1. Introduction The hot wire cell method is a new and very promising process for depositing polycrystalline silicon thin "lms for photovoltaic applications. In this process, a heated tungsten "lament is used to induce the catalytic and/or pyrolytic dissociation of reactant gases such as mono-silane (SiH ), disilane (Si H ) and hydrogen (H ) and produce atomic hydrogen as a reaction by-product, which then reacts with silane molecules to produce SiH radicals. In this work, since the "lament is positioned perpendicular to the substrate holder, the reactant gas is e$ciently decomposed during its passage through the "lament thus enhancing its decomposition rate. We have previously obtained polycrystalline silicon thin "lms without hydrogen dilution * Corresponding author. Tel.: #81-3-5734-2661; fax: #81-3-5734-2897. E-mail address: [email protected] (M. Ichikawa). 0927-0248/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 4 8 ( 0 0 ) 0 0 1 7 7 - X
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and demonstrated that polycrystalline silicon "lms could be deposited at low temperatures of 175}4003C at a relatively high-deposition rate of 1.0 nm/s [1]. Furthermore, we found that the deposition rate increases linearly with SiH #ow rate and the deposition rate does not saturate implying that higher deposition rate is achievable at higher SiH #ow rates [2]. However, increasing the deposition rate by merely increasing the SiH #ow rate leads to degradation of the "lament by generating plenty of silicide on the "lament. Thus, we adopt other approaches, such as the use of Si H to increase the deposition rate. In this study, we present our experimental results on the deposition rate and structural properties of polycrystalline silicon "lms deposited from Si H . 2. Experimental procedure The details of the deposition apparatus are described elsewhere [1]. Since the hot wire cell consists of a gas inlet and a tungsten "lament that was arranged to be parallel to the gas #ow, reactant gas is e$ciently decomposed during its passage through the "lament. We have four deposition parameters that we readily vary in our depositions: chamber pressure, Si H #ow rate, substrate temperature, and "lament temperature. It has been found in our previous works that chamber pressure strongly in#uences on crystallinity of the "lms [2] and that polycrystalline silicon "lms are obtained at a pressure of 0.1 Torr. In this study, we investigate the deposition of polycrystalline silicon "lms with high growth rate conditions. Therefore, we kept chamber pressure as 0.1 Torr and varied the other three parameters shown in Table 1. The distance between the "lament and substrate was kept at 6 cm. Thin "lms were deposited on Corning 7059 glass substrate.
3. Results and discussion The deposition rate was signi"cantly in#uenced by the Si H #ow rate. Fig. 1 shows the deposition rate as a function of the gas #ow rate for both SiH and Si H as a reactant gas. In the previous work, it was found that the deposition rate increases linearly with SiH #ow rate. The deposition rate also increases linearly with Si H #ow rate. We found that the deposition rate reaches 2.1 nm/s at a Si H #ow rate of 15 sccm and the deposition rate does not saturate implying that higher deposition rate Table 1 Deposition conditions Deposition parameter Si H #ow rate Filament temperature Substrate temperature
5}15 sccm 1800}22003C 175}3503C
M. Ichikawa et al. / Solar Energy Materials & Solar Cells 66 (2001) 225}230
227
Fig. 1. The deposition rate of the "lms deposited from SiH and Si H as a function of #ow rate.
Fig. 2. XRD pattern of the "lms deposited from SiH and Si H which have the same "lm thickness.
is achievable at higher Si H #ow rates. This result suggests that Si H gas is decomposed su$ciently on the surface of "lament and e!ectively utilized for the deposition of "lms. Since the deposition rate was 1.0 nm/s at a SiH #ow rate of 15 sccm, the deposition rate at the same Si H #ow rate is two times larger. This relationship of deposition rate is held throughout the range of the #ow rate in this experiment. These results suggest that by using Si H as a reactant gas it is possible to achieve higher deposition rate in the hot wire cell method. Fig. 2 shows the XRD pattern of the "lms deposited from SiH and Si H at the same deposition condition, which have the same "lm thickness. In general, high-rate
228
M. Ichikawa et al. / Solar Energy Materials & Solar Cells 66 (2001) 225}230
deposition makes crystallinity of "lms worse. However, we found that polycrystalline "lms were obtained and that the "lms deposited from Si H have a (2 2 0) prefer ential orientation, which showed the same pattern as that of the "lm deposited from SiH . The intensity of each peak and FWHM do not make a di!erence for the two patterns. Furthermore, we observed that polycrystalline thin "lms deposited from Si H have a columnar structure similar to that from SiH . These results suggest that by using Si H as a reactant gas it is possible to achieve a high deposition rate without any change in the quality of the "lms. Since it was demonstrated in our previous experimental and theoretical works that the SiH radicals on the growing surface play a key role in the growth of high-quality amorphous and crystal silicon at a low temperature [3], we assume that su$cient SiH radicals were also supplied to the growth surface in the case where Si H was used as a reactant gas. Next, we deposited silicon "lms by varying the substrate temperature. Fig. 3 shows the deposition rate of the "lms as a function of the substrate temperature. It shows that the deposition rate varies from 2.4 nm/s at a substrate temperature of 3503C to 2.8 nm/s at a substrate temperature of 1753C. Thus, there is little change in the deposition rate on varying the substrate temperature. We assume that dissociation of Si H gas is mostly occurring on the surface of "lament and a hardly a!ected by the temperature of the growing surface. However, a little increase of the deposition rate was observed with the substrate temperature decrease. This increase of the deposition rate may be due to the change of the e!ective surface reaction probability of the radicals at the growing surface [4]. Up to now a deposition rate of 2.8 nm/s was the maximum value in our experiment. We also observed XRD patterns of those "lms. The "lms deposited at a high substrate temperature have a (2 2 0) preferential orientation. However, for the "lm deposited at 1753C the XRD pattern shows a di!erent orientation: the intensity of the di!raction peak of the (1 1 0) plane decreases to half of the (1 1 1) di!raction peak indicating that the "lm had a random orientation. Thus, to
Fig. 3. The deposition rate of the "lms as a function of substrate temperature.
M. Ichikawa et al. / Solar Energy Materials & Solar Cells 66 (2001) 225}230
229
achieve high-quality polycrystalline silicon "lms the substrate temperature of over 2503C is required. Fig. 4 shows the deposition rate of the "lms deposited from SiH and Si H as a function of "lament temperature. The deposition time of each sample was 1 h for SiH and 30 min for Si H .There is no signi"cant change in the deposition rate at the low "lament temperature condition; however, the rate decreased gradually at the "lament temperatures over 20003C. When SiH is used as a reactant gas, polycrystal line silicon "lms are observed at any condition of the "lament temperature without hydrogen dilution. However, in the case of Si H , the "lm is crystallized at "lament temperatures over 20003C without hydrogen dilution. We assume that this transition
Fig. 4. The deposition rate of the "lms deposited from SiH and Si H as a function of "lament temperature.
Fig. 5. XRD pattern as a function of hydrogen #ow rate at a "lament temperature of 18003C.
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of crystallinity was due to the #ux of the atomic hydrogen at the growing surface. The main channel for Si H dissociation is "rmly believed to be the elimination of SiH [5]. In this Si H decomposition process, secondary and/or tertiary chemical reac tions are required for generating hydrogen molecule or atomic hydrogen. Consequently, #ux of the atomic hydrogen in Si H dissociation may decrease in compari son with that of SiH dissociation. This assumption was established by the results of hydrogen dilution. In the case of hydrogen addition, hydrogen gas was also passed through the heated "lament. The #ow rate of hydrogen was varied from 0 to 60 sccm. Fig. 5 shows the XRD pattern as a function of hydrogen #ow rate at a "lament temperature of 18003C. Within the low hydrogen #ow rate, no di!raction peak was observed, which indicates that the "lm was amorphous. When Si H gas is diluted with hydrogen at a #ow rate of 45 sccm, the "lm is crystallized at a low "lament temperature of 18003C. These results indicate the importance of atomic hydrogen during the growth.
4. Conclusions We investigated the use of Si H to achieve higher deposition rate of the polycrys talline silicon "lms. On using Si H as a reactant gas, the deposition rate becomes twice larger than that from SiH without any change in the quality of the "lms. Polycrystalline "lms were deposited at a deposition rate of 2.8 nm/s on glass substrates by the hot wire cell method using Si H . Acknowledgements This work was supported in part by NEDO as a part of the New Sunshine Program under the Ministry of International Trade and Industry of Japan.
References [1] [2] [3] [4]
M. Ichikawa, J. Takeshita, A. Yamada, M. Konagai, Jpn. J. Appl. Phys. 38 (1999) L24. M. Ichikawa, J. Takeshita, A. Yamada, M. Konagai, MRS Proc. 536 (1998) 487. T. Oshima, A. Yamada, M. Konagai, Jpn. J. Appl. Phys. 36 (1997) 6481. J. Perrin, M. Shiratani, P. Kae-Nune, H. Videlot, J. Jolly, J. Guillon, J. Vac. Sci. Technol. A 16 (1998) 278. [5] H.K. Mo!at, K.F. Jensen, R.W. Carr, J. Phys. Chem. 96 (1992) 7683.