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Direct formation of crystalline silicon films on an amorphous substrate from chlorinated materials by plasma-enhanced chemical vapor deposition
Journal of Non-Crystalline Solids 299–302 (2002) 118–122 www.elsevier.com/locate/jnoncrysol
Direct formation of crystalline silicon films on an amorphous substrate from chlorinated materials by plasma-enhanced chemical vapor deposition Hajime Shirai a
a,*
, Sughoan Jung a, Yukihiro Fujimura a, Yasutake Toyoshima
b
Department of Functional Materials Science, Faculty of Engineering, Saitama University, 255 Shimo-Okubo, Saitama 338-8570, Japan b National Institute of Advanced Industrial Science and Technology, 1-1-4 Umezono, Tsukuba, Ibaraki 305-8568, Japan
Abstract Low-temperature formation of crystalline silicon (c-Si) is demonstrated by controlling the early stages of a parallel plate rf (13.56 MHz) plasma-enhanced chemical vapor deposition (PE-CVD) in silicon tetrachloride (SiCl4 ) and H2 mixture. The crystal size, height and the number density were directly controlled by rf power, pressure and substrate temperature. The growth mechanism is discussed in terms of the chemical reactivity of the chlorine-terminated surface with atomic hydrogen. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 61.46.+w; 85.40.sz; 68.55.Ac; 61.50.)f
1. Introduction The chlorinated silanes, i.e., SiH2 Cl2 , SiHCl3 and SiCl4 are widely used as a source material for high-temperature crystal silicon epitaxial and polysilane film growth. However, few studies have been performed using the low-temperature plasmaenhanced chemical vapor deposition (PE-CVD) method [1–3]. Thus far, we have studied the lowtemperature growth of crystal silicon films from chlorinated materials using a conventional rf (13.56 MHz) glow-discharge method and identified several specific features of the film growth.
*
Corresponding author. Tel.: +81-48 858 3676; fax: +81-48 858 3676. E-mail address: [email protected] (H. Shirai).
Among them, the direct formation of a crystal silicon film is prominent on glass at low temperatures of 200 °C. However, the crystallite size was 10–30 nm in diameter in the SiH2 Cl2 and H2 mixture plasma, which was almost independent of deposition conditions [4–7]. This originates from a higher sticking probability of the deposition precursor, SiHx Cly ðx þ y < 3Þ and its chemical reactivity on the growing surface. To realize the direct deposition of c-Si with a variety of dot sizes, heights and densities on an amorphous substrate, precise control of the nucleation and grain growth is required. To reduce the nucleation density, it is convenient to use a deposition precursor with lower sticking probability such as SiClx ðx 6 3Þ. In this paper, we investigate the direct formation of c-Si with a variety of dot densities, dot sizes and heights by controlling the early stages of growth of
0022-3093/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 1 1 8 8 - 7
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Fig. 1. AFM images of the Si-dots formed at different rf powers of 5 W for 300 s and 30 W for 30 s on th-SiO2 /c-Si(1 1 1). The inset shows the magnified three-dimensional image.
SiCl4 and H2 plasma, and we discuss the growth mechanism for the low-temperature crystallization. To reveal the influence of chemical bonding on the
oxide surface on the growth kinetics, the Si-dot formation on HF-treated th-SiO2 has also been compared with the case of as-grown SiO2 .
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2. Experimental Crystalline silicon (c-Si) is fabricated on an HFtreated th-SiO2 ð1500 AÞ=nþ -c-Sið1 1 1Þ substrate by controlling the early stages of a parallel plate rf (13.56 MHz) PE-CVD process using a SiCl4 and H2 mixture. The deposition parameters are rf power, Ts , and pressure. The dot densities, sizes and heights were examined by atomic force microscopy (AFM) in air using a c-Si probe. Transmission electron microscopy (TEM) images were taken for nanocrystallites deposited on th-SiO2 . The surface reaction for the Si film growth was monitored as functions of flow rate of H2 FrðH2 ) and Ts using in situ Fourier transform infrared reflection absorption spectroscopy (FTIR-RAS) with an incident angle of 71° and a resolution of 4 cm1 .
3. Results Figs. 1(a) and (b) show AFM images of c-Si dots formed at different rf power of 5 W for 300 s and 30 W for 30 s, respectively, on th-SiO2 =c-Sið1 1 1Þ along with magnified three-dimensional images. Under both conditions. the c-Si dot formation occurs randomly, irrespective of the atomic scale roughness of the SiO2 surface. However, the number density, size and height of Si-dots depends significantly on the rf power. A low rf power of 5 W suppresses the nucleation density and promotes grain growth. On the other hand, a uniform distribution of the c-Si dots is observed at a high rf power of 30 W, along with the decrease of the dot diameter. Thus, they are determined by the speeds of the nucleation and grain growth. In Fig. 2, the Si-dot density is plotted as a function of the deposition pressure. Contrary to the results for an SiH4 plasma, the dot densities and sizes drastically decrease with increasing pressure along with an increase of the dot height. In general, higher pressure promotes a secondary reaction in the SiH4 plasma, which enhances the nucleation density and deposition rate. However, in the SiCl4 and H2 mixture, the Si-dot density decreases exponentially with increasing pressure along with an increase of the dot height up to 15
Fig. 2. The Si-dot density estimated from a 1 lm2 area AFM image plotted as a function of the total pressure. The deposition time of the Si-dot is 80 s under each pressure condition. The flow rates of SiCl4 and H2 are 3 and 100 sccm, respectively.
nm range in the AFM observations up to 600 mTorr, although the size is almost constant at 10– 30 nm in diameter. The dot density also increases with Ts until reaching a value of 3 1011 cm2 , at which point it tends to be saturated because of colarescence as confirmed by AFM and TEM observations. The dot density increases with an activation energy of 0.11 eV, and it is almost independent of Ts above 350 °C. This lower activation energy is due to the chemical reactivities of the deposition precursor and atomic hydrogen. Higher temperature provides wider size and height distributions. The sticking probability of the deposition precursor increases as well as the promotion of the grain growth with increasing Ts by thermal activation. Significant change of the Si-dot density is systematically observed by treating the th-SiO2 surface in 0.5% HF solution for different times before PE-CVD. Figs. 3(a) and (b) show FTIR-ATR spectra of an SiO2 surface after being treated with a 0.5% HF solution at different times and the dot density after 80 s of deposition plotted as a
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Fig. 3. (a) FTIR-ATR spectra of the 0.5% HF-treated th-SiO2 at different times before PE-CVD. (b) The Si-dot density plotted as a function of the 0.5% HF treatment time. The inset shows the AFM image of the Si-dots formed after 80 s on the 30 s-treated th-SiO2 .
function of the treatment time, respectively. The inset shows the AFM image of Si-dots formed after 80 s on the 30 s-treated SiO2 substrate. The Fourier transform total attenuated reflection spectroscopy (FTIR-ATR) spectra of the HF-treated SiO2 exhibit absorption bands centered at 3750 and 3600 cm1 , indicating that the surface is terminated by OH bonds. The OH absorption intensity decreases up to the 30 s of treatment, and subsequently it increases with time. The Si-dot density is also a minimum at the 0.5% HF treatment for 30 s, and subsequently it increases with the HF treatment time. These results suggest that the OH sites are also crucial in determining the nucleation density. To understand the surface reaction for the lowtemperature c-Si dot formation from SiCl4 , in situ FTIR-RAS studies were performed. Fig. 4 shows typical p-polarized FTIR-RAS spectra for the Si film growth under the different Fr(H2 ) conditions. With increasing Fr(H2 ), the SiClx (x ¼ 1–3) absorption intensities at 550–585 cm1 systematically decrease, which suggests that most of atomic hydrogen is consumed in abstracting the surface chlorine. The growing surface is mostly terminated by chlorine as SiClx . No SiHx (x ¼ 1; 2) absorption peaks appear in the 2000–2200 cm1 region. In addition, no significant changes were observed after the post-H plasma exposure, which suggests
Fig. 4. The p-polarized FTIR-RAS spectra for Si film growth under different Fr(H2 ) conditions from an SiCl4 and H2 mixture plasma.
that the growing surface is chemically stable compared with the hydrogenated one.
4. Discussion Thus far, most of studies of nc-Si dot formation have been performed by the thermal decomposition of SiH4 at 500–600 °C. It requires a sufficient thermal energy to promote the diffusion of the deposition precursor such as SiH3 even after nucleation to enlarge the size and height. Thus, a
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higher growth temperature is required. On the other hand, in the case of a plasma process, a precise control of the sticking process of the deposition precursor is required. When SiH4 and SiH2 Cl2 are used as a source material, the sticking process is dominant compared with that of the grain growth [4]. Therefore, a deposition precursor with lower sticking probability such as SiClx is convenient to control the dot density, size, and height distributions. Not only the chemical reactivity of the deposition precursor but also the surface termination species of th-SiO2 are crucial for determining the c-Si dot density as shown in Fig. 3 [8,9]. An in situ FTIR-RAS study reveals that the growing surface is mostly terminated by chlorine, and most of the atomic hydrogen is consumed in abstracting the surface chlorine. The weakly excited Ar plasma exposure experiment shows that both SiCl and SiHx Cly complex absorption intensities at 550 and 585 cm1 , respectively, decrease and tend to saturate with the exposure time. These results suggest that several monolayer consisting of the SiHx Cly complex near the surface are chemically unstable and they strongly contribute to the c-Si network formation. In addition, an in situ UV–Vis ellipsometry study reveals that the thickness of SiHx Cly complex layer is estimated to be 0.5–1 nm [10,11]. The abstraction of chlorine from the growing surface by atomic hydrogen releases a large amount of thermal energy because the Si–Cl bond energy (3.75 eV) is stronger than that of Si–H (3.25 eV) [12]. Therefore, the efficiencies for extracting surface chlorine and for creating a dangling bond site by atomic hydrogen are extremely small in the SiCl4 system, which results in a lower nucleation efficiency and deposition rate, although a sufficient thermal energy is expected per Si atom. Therefore, the cooperative effects, i.e., the creation of a dangling bond and ‘local heating’ over several monolayers region by atomic hydrogen, are crucial for determining the nucleation density.
5. Conclusions The direct deposition of nc-Si dots with a variety of dot densities, sizes and heights is demonstrated on th-SiO2 with a conventional rf PE-CVD method using SiCl4 and H2 . The lower sticking probability of the deposition precursor, lower chemical reactivity of the growing surface and sufficient ‘local heating’ due to the chlorine abstraction by atomic hydrogen are crucial for the direct formation of c-Si dots at low temperatures.
Acknowledgements This work is supported in part by the Venture Small- and Medium-Scale Enterprise (SME)-University Research Promotion Program of Japan Society for the promotion of Science (JSPS).
References [1] G. Bruno, P. Capezzuto, G. Cicala, J. Appl. Phys. 69 (1991) 7256. [2] G. Cilala, M. Losurdom, P. Capezzuto, G. Bruno, Plasma Sources Sci. Technol. 1 (1992) 156. [3] G. Bruno, P. Capezzuto, G. Cicala, F. Cranmarossa, J. Appl. Phys. 62 (1987) 25050. [4] C. Fukai, Y. Fujimura, Y. Moriya, H. Shirai, Jpn. J. Appl. Phys. 38 (1999) L554. [5] H. Shirai, C. Fukai, Y. Moriya, J. Non-Cryst. Solids 266– 269 (2000) 131. [6] H. Liu, S.H. Jung, Y. Fujimura, C. Fukai, Y. Toyoshima, H. Shirai, Jpn. J. Appl. Phys. 40 (2001) 44. [7] H. Liu, S.H. Jung, Y. Fujimura, H. Shirai, Jpn. J. Appl. Phys. 40 (2001) L215. [8] Y. Fujimura, S.H. Jung, H. Shirai, Jpn. J. Appl. Phys. 40 (2001) L1214. [9] Y. Fujimura, S.H. Jung, H. Shirai, Thin Solid Films (in press). [10] S. Kalem, J. Chevallier, S.Al. Dalla, J. Bourneix, J. Phys. (Paris) 42 (1981) 361. [11] H. Shirai, S.H. Jung, Y. Fujimura, J. Appl. Phys., to be published. [12] CRC Handbook of Chemistry and Physics, 59th Ed., CRC, Amlabar, FL, 1978, p. F224.
Direct formation of crystalline silicon films on an amorphous substrate from chlorinated materials by plasma-enhanced chemical vapor deposition Hajime Shirai a
a,*
, Sughoan Jung a, Yukihiro Fujimura a, Yasutake Toyoshima
b
Department of Functional Materials Science, Faculty of Engineering, Saitama University, 255 Shimo-Okubo, Saitama 338-8570, Japan b National Institute of Advanced Industrial Science and Technology, 1-1-4 Umezono, Tsukuba, Ibaraki 305-8568, Japan
Abstract Low-temperature formation of crystalline silicon (c-Si) is demonstrated by controlling the early stages of a parallel plate rf (13.56 MHz) plasma-enhanced chemical vapor deposition (PE-CVD) in silicon tetrachloride (SiCl4 ) and H2 mixture. The crystal size, height and the number density were directly controlled by rf power, pressure and substrate temperature. The growth mechanism is discussed in terms of the chemical reactivity of the chlorine-terminated surface with atomic hydrogen. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 61.46.+w; 85.40.sz; 68.55.Ac; 61.50.)f
1. Introduction The chlorinated silanes, i.e., SiH2 Cl2 , SiHCl3 and SiCl4 are widely used as a source material for high-temperature crystal silicon epitaxial and polysilane film growth. However, few studies have been performed using the low-temperature plasmaenhanced chemical vapor deposition (PE-CVD) method [1–3]. Thus far, we have studied the lowtemperature growth of crystal silicon films from chlorinated materials using a conventional rf (13.56 MHz) glow-discharge method and identified several specific features of the film growth.
*
Corresponding author. Tel.: +81-48 858 3676; fax: +81-48 858 3676. E-mail address: [email protected] (H. Shirai).
Among them, the direct formation of a crystal silicon film is prominent on glass at low temperatures of 200 °C. However, the crystallite size was 10–30 nm in diameter in the SiH2 Cl2 and H2 mixture plasma, which was almost independent of deposition conditions [4–7]. This originates from a higher sticking probability of the deposition precursor, SiHx Cly ðx þ y < 3Þ and its chemical reactivity on the growing surface. To realize the direct deposition of c-Si with a variety of dot sizes, heights and densities on an amorphous substrate, precise control of the nucleation and grain growth is required. To reduce the nucleation density, it is convenient to use a deposition precursor with lower sticking probability such as SiClx ðx 6 3Þ. In this paper, we investigate the direct formation of c-Si with a variety of dot densities, dot sizes and heights by controlling the early stages of growth of
0022-3093/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 1 1 8 8 - 7
H. Shirai et al. / Journal of Non-Crystalline Solids 299–302 (2002) 118–122
119
Fig. 1. AFM images of the Si-dots formed at different rf powers of 5 W for 300 s and 30 W for 30 s on th-SiO2 /c-Si(1 1 1). The inset shows the magnified three-dimensional image.
SiCl4 and H2 plasma, and we discuss the growth mechanism for the low-temperature crystallization. To reveal the influence of chemical bonding on the
oxide surface on the growth kinetics, the Si-dot formation on HF-treated th-SiO2 has also been compared with the case of as-grown SiO2 .
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H. Shirai et al. / Journal of Non-Crystalline Solids 299–302 (2002) 118–122
2. Experimental Crystalline silicon (c-Si) is fabricated on an HFtreated th-SiO2 ð1500 AÞ=nþ -c-Sið1 1 1Þ substrate by controlling the early stages of a parallel plate rf (13.56 MHz) PE-CVD process using a SiCl4 and H2 mixture. The deposition parameters are rf power, Ts , and pressure. The dot densities, sizes and heights were examined by atomic force microscopy (AFM) in air using a c-Si probe. Transmission electron microscopy (TEM) images were taken for nanocrystallites deposited on th-SiO2 . The surface reaction for the Si film growth was monitored as functions of flow rate of H2 FrðH2 ) and Ts using in situ Fourier transform infrared reflection absorption spectroscopy (FTIR-RAS) with an incident angle of 71° and a resolution of 4 cm1 .
3. Results Figs. 1(a) and (b) show AFM images of c-Si dots formed at different rf power of 5 W for 300 s and 30 W for 30 s, respectively, on th-SiO2 =c-Sið1 1 1Þ along with magnified three-dimensional images. Under both conditions. the c-Si dot formation occurs randomly, irrespective of the atomic scale roughness of the SiO2 surface. However, the number density, size and height of Si-dots depends significantly on the rf power. A low rf power of 5 W suppresses the nucleation density and promotes grain growth. On the other hand, a uniform distribution of the c-Si dots is observed at a high rf power of 30 W, along with the decrease of the dot diameter. Thus, they are determined by the speeds of the nucleation and grain growth. In Fig. 2, the Si-dot density is plotted as a function of the deposition pressure. Contrary to the results for an SiH4 plasma, the dot densities and sizes drastically decrease with increasing pressure along with an increase of the dot height. In general, higher pressure promotes a secondary reaction in the SiH4 plasma, which enhances the nucleation density and deposition rate. However, in the SiCl4 and H2 mixture, the Si-dot density decreases exponentially with increasing pressure along with an increase of the dot height up to 15
Fig. 2. The Si-dot density estimated from a 1 lm2 area AFM image plotted as a function of the total pressure. The deposition time of the Si-dot is 80 s under each pressure condition. The flow rates of SiCl4 and H2 are 3 and 100 sccm, respectively.
nm range in the AFM observations up to 600 mTorr, although the size is almost constant at 10– 30 nm in diameter. The dot density also increases with Ts until reaching a value of 3 1011 cm2 , at which point it tends to be saturated because of colarescence as confirmed by AFM and TEM observations. The dot density increases with an activation energy of 0.11 eV, and it is almost independent of Ts above 350 °C. This lower activation energy is due to the chemical reactivities of the deposition precursor and atomic hydrogen. Higher temperature provides wider size and height distributions. The sticking probability of the deposition precursor increases as well as the promotion of the grain growth with increasing Ts by thermal activation. Significant change of the Si-dot density is systematically observed by treating the th-SiO2 surface in 0.5% HF solution for different times before PE-CVD. Figs. 3(a) and (b) show FTIR-ATR spectra of an SiO2 surface after being treated with a 0.5% HF solution at different times and the dot density after 80 s of deposition plotted as a
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Fig. 3. (a) FTIR-ATR spectra of the 0.5% HF-treated th-SiO2 at different times before PE-CVD. (b) The Si-dot density plotted as a function of the 0.5% HF treatment time. The inset shows the AFM image of the Si-dots formed after 80 s on the 30 s-treated th-SiO2 .
function of the treatment time, respectively. The inset shows the AFM image of Si-dots formed after 80 s on the 30 s-treated SiO2 substrate. The Fourier transform total attenuated reflection spectroscopy (FTIR-ATR) spectra of the HF-treated SiO2 exhibit absorption bands centered at 3750 and 3600 cm1 , indicating that the surface is terminated by OH bonds. The OH absorption intensity decreases up to the 30 s of treatment, and subsequently it increases with time. The Si-dot density is also a minimum at the 0.5% HF treatment for 30 s, and subsequently it increases with the HF treatment time. These results suggest that the OH sites are also crucial in determining the nucleation density. To understand the surface reaction for the lowtemperature c-Si dot formation from SiCl4 , in situ FTIR-RAS studies were performed. Fig. 4 shows typical p-polarized FTIR-RAS spectra for the Si film growth under the different Fr(H2 ) conditions. With increasing Fr(H2 ), the SiClx (x ¼ 1–3) absorption intensities at 550–585 cm1 systematically decrease, which suggests that most of atomic hydrogen is consumed in abstracting the surface chlorine. The growing surface is mostly terminated by chlorine as SiClx . No SiHx (x ¼ 1; 2) absorption peaks appear in the 2000–2200 cm1 region. In addition, no significant changes were observed after the post-H plasma exposure, which suggests
Fig. 4. The p-polarized FTIR-RAS spectra for Si film growth under different Fr(H2 ) conditions from an SiCl4 and H2 mixture plasma.
that the growing surface is chemically stable compared with the hydrogenated one.
4. Discussion Thus far, most of studies of nc-Si dot formation have been performed by the thermal decomposition of SiH4 at 500–600 °C. It requires a sufficient thermal energy to promote the diffusion of the deposition precursor such as SiH3 even after nucleation to enlarge the size and height. Thus, a
122
H. Shirai et al. / Journal of Non-Crystalline Solids 299–302 (2002) 118–122
higher growth temperature is required. On the other hand, in the case of a plasma process, a precise control of the sticking process of the deposition precursor is required. When SiH4 and SiH2 Cl2 are used as a source material, the sticking process is dominant compared with that of the grain growth [4]. Therefore, a deposition precursor with lower sticking probability such as SiClx is convenient to control the dot density, size, and height distributions. Not only the chemical reactivity of the deposition precursor but also the surface termination species of th-SiO2 are crucial for determining the c-Si dot density as shown in Fig. 3 [8,9]. An in situ FTIR-RAS study reveals that the growing surface is mostly terminated by chlorine, and most of the atomic hydrogen is consumed in abstracting the surface chlorine. The weakly excited Ar plasma exposure experiment shows that both SiCl and SiHx Cly complex absorption intensities at 550 and 585 cm1 , respectively, decrease and tend to saturate with the exposure time. These results suggest that several monolayer consisting of the SiHx Cly complex near the surface are chemically unstable and they strongly contribute to the c-Si network formation. In addition, an in situ UV–Vis ellipsometry study reveals that the thickness of SiHx Cly complex layer is estimated to be 0.5–1 nm [10,11]. The abstraction of chlorine from the growing surface by atomic hydrogen releases a large amount of thermal energy because the Si–Cl bond energy (3.75 eV) is stronger than that of Si–H (3.25 eV) [12]. Therefore, the efficiencies for extracting surface chlorine and for creating a dangling bond site by atomic hydrogen are extremely small in the SiCl4 system, which results in a lower nucleation efficiency and deposition rate, although a sufficient thermal energy is expected per Si atom. Therefore, the cooperative effects, i.e., the creation of a dangling bond and ‘local heating’ over several monolayers region by atomic hydrogen, are crucial for determining the nucleation density.
5. Conclusions The direct deposition of nc-Si dots with a variety of dot densities, sizes and heights is demonstrated on th-SiO2 with a conventional rf PE-CVD method using SiCl4 and H2 . The lower sticking probability of the deposition precursor, lower chemical reactivity of the growing surface and sufficient ‘local heating’ due to the chlorine abstraction by atomic hydrogen are crucial for the direct formation of c-Si dots at low temperatures.
Acknowledgements This work is supported in part by the Venture Small- and Medium-Scale Enterprise (SME)-University Research Promotion Program of Japan Society for the promotion of Science (JSPS).
References [1] G. Bruno, P. Capezzuto, G. Cicala, J. Appl. Phys. 69 (1991) 7256. [2] G. Cilala, M. Losurdom, P. Capezzuto, G. Bruno, Plasma Sources Sci. Technol. 1 (1992) 156. [3] G. Bruno, P. Capezzuto, G. Cicala, F. Cranmarossa, J. Appl. Phys. 62 (1987) 25050. [4] C. Fukai, Y. Fujimura, Y. Moriya, H. Shirai, Jpn. J. Appl. Phys. 38 (1999) L554. [5] H. Shirai, C. Fukai, Y. Moriya, J. Non-Cryst. Solids 266– 269 (2000) 131. [6] H. Liu, S.H. Jung, Y. Fujimura, C. Fukai, Y. Toyoshima, H. Shirai, Jpn. J. Appl. Phys. 40 (2001) 44. [7] H. Liu, S.H. Jung, Y. Fujimura, H. Shirai, Jpn. J. Appl. Phys. 40 (2001) L215. [8] Y. Fujimura, S.H. Jung, H. Shirai, Jpn. J. Appl. Phys. 40 (2001) L1214. [9] Y. Fujimura, S.H. Jung, H. Shirai, Thin Solid Films (in press). [10] S. Kalem, J. Chevallier, S.Al. Dalla, J. Bourneix, J. Phys. (Paris) 42 (1981) 361. [11] H. Shirai, S.H. Jung, Y. Fujimura, J. Appl. Phys., to be published. [12] CRC Handbook of Chemistry and Physics, 59th Ed., CRC, Amlabar, FL, 1978, p. F224.