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High-temperature growth of thin film microcrystalline silicon on silicon carbide using EBEP-CVD
Solar Energy Materials & Solar Cells 74 (2002) 561–566
High-temperature growth of thin film microcrystalline silicon on silicon carbide using EBEP-CVD Matt Borelanda,*, Masayuki Isogamib a
Toyota Technological Institute, 2-12-1 Hisakata, Tempaku-ku, 468 Nagoya, Japan b Toyota Central R&D Labs, Aisin Chemical Co. Ltd., Nagoya, Japan
Abstract Thin silicon films were deposited onto silicon carbide (SiC) substrates at high temperatures using electron beam excited plasma chemical vapour deposition. The film quality was characterised using X-ray diffraction and Raman measurements. Grain size, growth rate and crystal fraction were all seen to improve at the higher deposition temperature. The use of thin film silicon has the potential to reduce the material cost of silicon solar cells. However, the compatibility of the SiC substrates with high temperatures would also allow the use of existing, industrially proven bulk silicon processing techniques, which are incompatible with low-temperature glass, to be used for thin film devices. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Thin film silicon; EBEP-CVD; Silicon carbide
1. Introduction The cost of photovoltaic (PV) production must be reduced in order to increase the use of PV in the energy generation market. In bulk silicon cells about half the cost of the solar cells is the silicon itself, but only the first few microns of the silicon wafer makes a significant contribution to the light generated current. Therefore, one option for significant cost reduction is to move to thin film silicon to reduce the materials cost of the PV module.
*Corresponding author E-mail address: [email protected] (M. Boreland). 0927-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 4 8 ( 0 2 ) 0 0 0 7 7 - 6
562
M. Boreland, M. Isogami / Solar Energy Materials & Solar Cells 74 (2002) 561–566
Typically low-cost, low-temperature glass, such as Corning 1737, is used as the supporting substrate for the silicon film. Whilst, the thin film silicon on glass substrates reduces material costs, it also has the added technological problem of reducing the maximum process temperature to o6001C. The low-temperature limitation of glass disallows the use of high-temperature fabrication techniques, such as junction diffusion and contact sintering techniques, that have become highly developed and characterised over many years of industrial production of bulk silicon solar cells. As a result, new processes would have to be developed, optimised and tested for low-temperature PV devices. Furthermore, low-temperature deposition of the silicon films is also required, which generally results in lower quality films. Deposition of silicon on a high-temperature substrate, such as silicon carbide (SiC), would allow the advantage of thin film cost reductions without losing access to industrially proven fabrication processes. Access to high temperatures during the silicon deposition may also improve the film quality. This paper reports on the deposition of thin film silicon on high-temperature SiC substrates using electron beam excited plasma chemical vapour deposition (EBEP-CVD). The SiC substrates used in this experimental study where produced by Aisin Chemical Co. Ltd., using a proprietary process, which they envisage will be compatible with low-cost production of the substrates.
2. EBEP-CVD EBEP-CVD is a recently developed CVD method that is capable of direct nc-Si growth without the need for hydrogen dilution [1], and combines the potential for high growth rates [2]. A high current e-beam, derived from a DC argon plasma, is used to excite the silane by electron–silane collision interactions. Both the e-beam energy and current are separately controlled, allowing independent optimisation of beam parameters and reaction gas pressures. There is also an absence of electrical and magnetic fields used in the plasma chamber that is also seen as beneficial. A schematic of the EBEP-CVD system is shown in Fig. 1.
3. Experimental method The SiC samples, which were nominally 20 mm in diameter and 1.1 mm thick, were cleaned with RCA1 (NH4OH:H2O2:H2O@85–901C) and RCA2 (HCl:H2O2: H2O@85–901C) immediately prior to deposition. Each wafer was then heated to 8001C, slowly ramped up from room temperature over a period of 60 min. Deposition by EBEP-CVD was carried out at a silane flow rate of 16 sccm, with pressures ranging from 1–3 Pa for 180 min. In each case an e-beam energy of 70 V was used. After deposition, each sample was analysed by Raman spectroscopy and X-ray diffraction (XRD) to evaluate relative crystallinity, grain size and orientation. Grain sizes were extracted from XRD by the Scherer’s method [3].
M. Boreland, M. Isogami / Solar Energy Materials & Solar Cells 74 (2002) 561–566
gate valve
load-lock chamber
563
substrate heater TMP
substrate
SiH4
plasma
magnetic coil
TMP electron beam A
Ar DC plasma Ar LaB6 filament
Fig. 1. Schematic of the EBEP-CVD deposition system.
During deposition the sample was held in a sample carrier, which masked the edge of the sample leaving an undeposited area around the edge of the sample. The presence of the undeposited region allowed the film thickness to be measured using a Dektak profilometer, from which the deposition rate was calculated.
4. Results Higher growth rates were the first noticeable improvement gained by using the high-temperature deposition afforded by the SiC substrates. Under similar plasma conditions, but lower substrate temperatures, we have typically achieved deposition rates between 1.2 and 1.3 mm/h. However, the high temperatures used in this study, increased the deposition rate to 2.2 mm/h, as shown in Table 1. Whilst, the deposition rate was increased, the crystal quality of the film was maintained. Moreover, crystal quality improvements were observed. The XRD measurements show that silicon grain sizes of 16–24 nm where achieved, demonstrating a notable improvement over the 5 nm grain sizes previously reported using low-temperature, direct EBEP-CVD deposition on glass at 400–6001C [4]. The Raman spectra in Fig. 2 also shows a high level of crystallinity, the lack of a shoulder on the low wave number of the peak indicating a high crystal fraction. For the most part the results were similar at each of the silane deposition pressures used, but there is a slight improvement in grain size at 2 Pa silane pressure, indicating a marginally preferred silane pressure of 2 Pa. Detailed measurement of the FWHM
M. Boreland, M. Isogami / Solar Energy Materials & Solar Cells 74 (2002) 561–566
564
Table 1 Summary of grain sizes and deposition rates achieved Silane gas pressure and flow
XRD measured grain size (nm)
Dektak measured film thickness (mm)
Deposition rate (mm/h)
1 Pa, 16 sccm 2 Pa, 16 sccm 3 Pa, 16 sccm
19 24 16
5.7 6.5 5.6
1.9 2.2 1.8
Raman Peak FWHM
Grain size was measured using XRD, and the film thicknesses where measured using a Dektak profilometer. All depositions were done at 8001C.
Raman Signal (a.u.)
3 P a 1 6 scc m
8 7 6 5 4 0
1 2 3 Silane Pressure (Pa)
4
0
1
4
2 P a 1 6 scc m
Raman Peak Centre
521
1 P a 1 6 scc m
400
450
500
5 2 0 .5
519
517 550
Raman Shift (cm-1 )
600
650
700
2
3
Silane Pressure (Pa)
Fig. 2. Raman spectra of silicon deposited by EBEP-CVD for a range of silane pressures. The lack of a prominent shoulder on the peak indicates a high crystal fraction in the sample. The plots of FWHM and peak centre indicate that the best grain fraction is achieved under the 2 Pa silane pressure regime.
of the Raman spectra also showed a slightly shaper peak at the 2 Pa pressure, indicating a higher crystal fraction. The Raman peak centre was also shifted closer to the 520.5 cm 1 peak of single crystal silicon, showing both decreased stress and larger grains at 2 Pa. Interestingly the XRD spectra, shown in Fig. 3, also shows an anomalous shift to a (3 1 1) preferred orientation at silane pressures above 2 Pa. 5. Summary and conclusions The results show that improved film quality can be achieved using hightemperature EBEP-CVD on SiC substrates. Furthermore, the improved film quality was achieved at higher deposition rates. Any increase in deposition rate, without loss of crystal quality, would be beneficial in terms of throughput when producing PV devices.
M. Boreland, M. Isogami / Solar Energy Materials & Solar Cells 74 (2002) 561–566
565
(311) 3Pa 16sccm (111) (220)
(311)
2Pa 16sccm (111) (220)
(111) 1Pa 16sccm
20
25
(311)
(220)
30
35
40
45
50
55
60
65
70
2θ Fig. 3. XRD spectra of EBEP-CVD silicon deposited on SiC over a range of silane pressures.
When combined with the thermal compatibility of SiC with standard hightemperature silicon solar cell processing, the SiC substrates become an attractive candidate as an alternative to low-temperature glass for thin film silicon solar cells. The current experimental set-up limits the deposition temperature to 8001C, but the improvements in crystal growth with the increase in temperature afforded by the SiC substrates suggest that further improvements may be achievable at higher temperatures.
Acknowledgements The financial and scientific support of The Japan Society for the Promotion of Science (JSPS), Toyota Technological Institute, and Professor Yamaguchi are gratefully acknowledged.
References [1] M. Imaizumi, K. Okitsu, M. Yamaguchi, T. Hara, T. Ito, I. Konomi, M. Ban, M. Tokai, K. Kawamura, Growth of microcrystalline silicon film by electron beam excited plasma chemical vapor
566
M. Boreland, M. Isogami / Solar Energy Materials & Solar Cells 74 (2002) 561–566
deposition without hydrogen dilution, J. Vac. Sci. Technol. A-Vac. Surf. Films 16 (5) (1998) 3134–3137. [2] T. Sasaki, M. Ryoji, Y. Ichikawa, M. Tohkai, Deposition of microcrystalline silicon by electron beam excited plasma, Sol. Energy Mater. Sol. Cells 49 (1–4) (1997) 81–88. [3] C. Suryanarayana, M.G. Norton, X-Ray Diffraction: A Practical Approach, Plenum Press, New York, 1998. [4] M. Boreland, K. Yamaguchi, Y. Oshita, M. Yamaguchi, Migration of the aluminium enhanced crystal growth process from quartz to glass using EBEP-CVD. 28th IEEE Photovoltaic Specialists Conference, Anchorage Hilton, Anchorage, Alaska, 17–22 September, 2000.
High-temperature growth of thin film microcrystalline silicon on silicon carbide using EBEP-CVD Matt Borelanda,*, Masayuki Isogamib a
Toyota Technological Institute, 2-12-1 Hisakata, Tempaku-ku, 468 Nagoya, Japan b Toyota Central R&D Labs, Aisin Chemical Co. Ltd., Nagoya, Japan
Abstract Thin silicon films were deposited onto silicon carbide (SiC) substrates at high temperatures using electron beam excited plasma chemical vapour deposition. The film quality was characterised using X-ray diffraction and Raman measurements. Grain size, growth rate and crystal fraction were all seen to improve at the higher deposition temperature. The use of thin film silicon has the potential to reduce the material cost of silicon solar cells. However, the compatibility of the SiC substrates with high temperatures would also allow the use of existing, industrially proven bulk silicon processing techniques, which are incompatible with low-temperature glass, to be used for thin film devices. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Thin film silicon; EBEP-CVD; Silicon carbide
1. Introduction The cost of photovoltaic (PV) production must be reduced in order to increase the use of PV in the energy generation market. In bulk silicon cells about half the cost of the solar cells is the silicon itself, but only the first few microns of the silicon wafer makes a significant contribution to the light generated current. Therefore, one option for significant cost reduction is to move to thin film silicon to reduce the materials cost of the PV module.
*Corresponding author E-mail address: [email protected] (M. Boreland). 0927-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 4 8 ( 0 2 ) 0 0 0 7 7 - 6
562
M. Boreland, M. Isogami / Solar Energy Materials & Solar Cells 74 (2002) 561–566
Typically low-cost, low-temperature glass, such as Corning 1737, is used as the supporting substrate for the silicon film. Whilst, the thin film silicon on glass substrates reduces material costs, it also has the added technological problem of reducing the maximum process temperature to o6001C. The low-temperature limitation of glass disallows the use of high-temperature fabrication techniques, such as junction diffusion and contact sintering techniques, that have become highly developed and characterised over many years of industrial production of bulk silicon solar cells. As a result, new processes would have to be developed, optimised and tested for low-temperature PV devices. Furthermore, low-temperature deposition of the silicon films is also required, which generally results in lower quality films. Deposition of silicon on a high-temperature substrate, such as silicon carbide (SiC), would allow the advantage of thin film cost reductions without losing access to industrially proven fabrication processes. Access to high temperatures during the silicon deposition may also improve the film quality. This paper reports on the deposition of thin film silicon on high-temperature SiC substrates using electron beam excited plasma chemical vapour deposition (EBEP-CVD). The SiC substrates used in this experimental study where produced by Aisin Chemical Co. Ltd., using a proprietary process, which they envisage will be compatible with low-cost production of the substrates.
2. EBEP-CVD EBEP-CVD is a recently developed CVD method that is capable of direct nc-Si growth without the need for hydrogen dilution [1], and combines the potential for high growth rates [2]. A high current e-beam, derived from a DC argon plasma, is used to excite the silane by electron–silane collision interactions. Both the e-beam energy and current are separately controlled, allowing independent optimisation of beam parameters and reaction gas pressures. There is also an absence of electrical and magnetic fields used in the plasma chamber that is also seen as beneficial. A schematic of the EBEP-CVD system is shown in Fig. 1.
3. Experimental method The SiC samples, which were nominally 20 mm in diameter and 1.1 mm thick, were cleaned with RCA1 (NH4OH:H2O2:H2O@85–901C) and RCA2 (HCl:H2O2: H2O@85–901C) immediately prior to deposition. Each wafer was then heated to 8001C, slowly ramped up from room temperature over a period of 60 min. Deposition by EBEP-CVD was carried out at a silane flow rate of 16 sccm, with pressures ranging from 1–3 Pa for 180 min. In each case an e-beam energy of 70 V was used. After deposition, each sample was analysed by Raman spectroscopy and X-ray diffraction (XRD) to evaluate relative crystallinity, grain size and orientation. Grain sizes were extracted from XRD by the Scherer’s method [3].
M. Boreland, M. Isogami / Solar Energy Materials & Solar Cells 74 (2002) 561–566
gate valve
load-lock chamber
563
substrate heater TMP
substrate
SiH4
plasma
magnetic coil
TMP electron beam A
Ar DC plasma Ar LaB6 filament
Fig. 1. Schematic of the EBEP-CVD deposition system.
During deposition the sample was held in a sample carrier, which masked the edge of the sample leaving an undeposited area around the edge of the sample. The presence of the undeposited region allowed the film thickness to be measured using a Dektak profilometer, from which the deposition rate was calculated.
4. Results Higher growth rates were the first noticeable improvement gained by using the high-temperature deposition afforded by the SiC substrates. Under similar plasma conditions, but lower substrate temperatures, we have typically achieved deposition rates between 1.2 and 1.3 mm/h. However, the high temperatures used in this study, increased the deposition rate to 2.2 mm/h, as shown in Table 1. Whilst, the deposition rate was increased, the crystal quality of the film was maintained. Moreover, crystal quality improvements were observed. The XRD measurements show that silicon grain sizes of 16–24 nm where achieved, demonstrating a notable improvement over the 5 nm grain sizes previously reported using low-temperature, direct EBEP-CVD deposition on glass at 400–6001C [4]. The Raman spectra in Fig. 2 also shows a high level of crystallinity, the lack of a shoulder on the low wave number of the peak indicating a high crystal fraction. For the most part the results were similar at each of the silane deposition pressures used, but there is a slight improvement in grain size at 2 Pa silane pressure, indicating a marginally preferred silane pressure of 2 Pa. Detailed measurement of the FWHM
M. Boreland, M. Isogami / Solar Energy Materials & Solar Cells 74 (2002) 561–566
564
Table 1 Summary of grain sizes and deposition rates achieved Silane gas pressure and flow
XRD measured grain size (nm)
Dektak measured film thickness (mm)
Deposition rate (mm/h)
1 Pa, 16 sccm 2 Pa, 16 sccm 3 Pa, 16 sccm
19 24 16
5.7 6.5 5.6
1.9 2.2 1.8
Raman Peak FWHM
Grain size was measured using XRD, and the film thicknesses where measured using a Dektak profilometer. All depositions were done at 8001C.
Raman Signal (a.u.)
3 P a 1 6 scc m
8 7 6 5 4 0
1 2 3 Silane Pressure (Pa)
4
0
1
4
2 P a 1 6 scc m
Raman Peak Centre
521
1 P a 1 6 scc m
400
450
500
5 2 0 .5
519
517 550
Raman Shift (cm-1 )
600
650
700
2
3
Silane Pressure (Pa)
Fig. 2. Raman spectra of silicon deposited by EBEP-CVD for a range of silane pressures. The lack of a prominent shoulder on the peak indicates a high crystal fraction in the sample. The plots of FWHM and peak centre indicate that the best grain fraction is achieved under the 2 Pa silane pressure regime.
of the Raman spectra also showed a slightly shaper peak at the 2 Pa pressure, indicating a higher crystal fraction. The Raman peak centre was also shifted closer to the 520.5 cm 1 peak of single crystal silicon, showing both decreased stress and larger grains at 2 Pa. Interestingly the XRD spectra, shown in Fig. 3, also shows an anomalous shift to a (3 1 1) preferred orientation at silane pressures above 2 Pa. 5. Summary and conclusions The results show that improved film quality can be achieved using hightemperature EBEP-CVD on SiC substrates. Furthermore, the improved film quality was achieved at higher deposition rates. Any increase in deposition rate, without loss of crystal quality, would be beneficial in terms of throughput when producing PV devices.
M. Boreland, M. Isogami / Solar Energy Materials & Solar Cells 74 (2002) 561–566
565
(311) 3Pa 16sccm (111) (220)
(311)
2Pa 16sccm (111) (220)
(111) 1Pa 16sccm
20
25
(311)
(220)
30
35
40
45
50
55
60
65
70
2θ Fig. 3. XRD spectra of EBEP-CVD silicon deposited on SiC over a range of silane pressures.
When combined with the thermal compatibility of SiC with standard hightemperature silicon solar cell processing, the SiC substrates become an attractive candidate as an alternative to low-temperature glass for thin film silicon solar cells. The current experimental set-up limits the deposition temperature to 8001C, but the improvements in crystal growth with the increase in temperature afforded by the SiC substrates suggest that further improvements may be achievable at higher temperatures.
Acknowledgements The financial and scientific support of The Japan Society for the Promotion of Science (JSPS), Toyota Technological Institute, and Professor Yamaguchi are gratefully acknowledged.
References [1] M. Imaizumi, K. Okitsu, M. Yamaguchi, T. Hara, T. Ito, I. Konomi, M. Ban, M. Tokai, K. Kawamura, Growth of microcrystalline silicon film by electron beam excited plasma chemical vapor
566
M. Boreland, M. Isogami / Solar Energy Materials & Solar Cells 74 (2002) 561–566
deposition without hydrogen dilution, J. Vac. Sci. Technol. A-Vac. Surf. Films 16 (5) (1998) 3134–3137. [2] T. Sasaki, M. Ryoji, Y. Ichikawa, M. Tohkai, Deposition of microcrystalline silicon by electron beam excited plasma, Sol. Energy Mater. Sol. Cells 49 (1–4) (1997) 81–88. [3] C. Suryanarayana, M.G. Norton, X-Ray Diffraction: A Practical Approach, Plenum Press, New York, 1998. [4] M. Boreland, K. Yamaguchi, Y. Oshita, M. Yamaguchi, Migration of the aluminium enhanced crystal growth process from quartz to glass using EBEP-CVD. 28th IEEE Photovoltaic Specialists Conference, Anchorage Hilton, Anchorage, Alaska, 17–22 September, 2000.