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Growth mechanisms and structural properties of microcrystalline silicon films deposited by catalytic CVD
Thin Solid Films 395 (2001) 178–183
Growth mechanisms and structural properties of microcrystalline silicon films deposited by catalytic CVD a ´ ´ a C. Niikuraa,*, S.Y. Kimb, B. Drevillon , Y. Poissanta, P. Roca i Cabarrocasa, J.E. Bouree a
Laboratoire de Physique des Interfaces et des Couches Minces, CNRS UMR 7647, Ecole Polytechnique, F-91128 Palaiseau Cedex, France b Department of Physics, Ajou University, Suwon 442-749, South Korea
Abstract Silicon–hydrogen bonding configurations, during or after microcrystalline silicon (mc-Si:H) film deposition by catalytic CVD, have been investigated for the first time by real-time in-situ Fourier transform phase modulated infrared ellipsometry (FTPME). FTPME measurements have been performed during and after mc-Si:H film depositions using high, low or variable dilutions of silane in hydrogen. The silicon–hydrogen bonding configurations of mc-Si:H films have been correlated with their corresponding structural properties as deduced from UV-visible ellipsometry analyses. A 4.6% efficiency has been obtained for mc-Si:H n-i-p solar cells, with the i-layer deposited by catalytic CVD at 2008C on a glass substrate using a variable hydrogen dilution process. Further optimization should improve the performance of catalytic CVD mc-Si:H solar cells. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Chemical vapor deposition (CVD); Ellipsometry; Silicon; Solar cells
1. Introduction Microcrystalline silicon (mc-Si:H) films, deposited by catalytic CVD, are promising materials for low cost and large area thin film devices such as solar cells or thin film transistors (TFT). mc-Si:H solar cells have been deposited entirely by catalytic CVD and by catalytic CVD combined with plasma enhanced CVD (PECVD). Approximately 2% efficiency was obtained for p-i-n or n-i-p solar cells entirely deposited by catalytic CVD w1,2x. A 4.4% efficiency was obtained for n-i-p diodes on stainless steel substrates and a 3.6 % efficiency was reached for n-i-p diodes on glass substrates by combining catalytic CVD with PECVD w3,4x. In order to optimize mc-Si:H solar cells, investigations of mc-Si:H formation mechanisms are required. Infrared vibrational spectroscopy studies have been extremely useful in assessing the different silicon–hydrogen bond* Corresponding author. Tel.: q33-1-69-33-32-17; fax: q33-1-6933-30-06. E-mail address: [email protected] (C. Niikura).
ing configurations in hydrogenated amorphous silicon (a-Si:H) or mc-Si:H films after deposition, as well as for understanding mc-Si:H formation mechanisms w5– 7 x. It is well known that thin (10–30 nm) amorphous layers are unintentionally deposited during the initial growth stage, prior to the nucleation of the crystallites. This amorphous incubation layer influences the electronic properties of the films. Previously, we reported that at very high dilutions of silane (SiH4) in hydrogen (H2), film crystallization starts from the substrate interface after an incubation period, however, the film becomes porous w8x. Then, in order to prevent the formation of an amorphous incubation layer, we proposed a specific deposition process, called a variable hydrogen dilution (VHD) process, where the hydrogen dilution, defined as DHsF(H2) y wF(SiH4 )qF(H2 )x, is high at the start of the deposition w4,8x. It is then reduced with increasing deposition time and is maintained at a lower DH for the rest of the deposition, in order to obtain dense mc-Si:H films. We also reported that the mc-Si:H layers prepared with this VHD process show improved electronic transport along the growth direction w4x.
0040-6090/01/$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 1 . 0 1 2 4 6 - 9
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Table 1 Deposition conditions for mc-Si:H films
SiH4 flow rate F(SiH4) (sccm) H2 flow rate F(H2) (sccm) H2 dilution (DH) Process gas pressure pg (Pa) Nominal substrate temperature Ts (8C) Filament–substrate distance d f (cm) Filament temperature T f (8C) Electric power supplied to the filament (W) Filament surface area (cm2)
In this study, we investigate silicon–hydrogen bonding configurations in mc-Si:H or a-Si:H samples by Fourier transform phase modulated infrared ellipsometry (FTPME), in order to study the correlation with their corresponding structural properties. An advantage of FTPME is that it permits the use of any substrate, whereas attenuated total reflection spectroscopy needs the use of a crystalline silicon substrate. FTPME measurements have been performed, in real-time, during mcSi:H depositions with VHD or with constant DH, in order to obtain information on the deposition mechanisms, as well as after the deposition. Finally, mc-Si:H solar cells, with the i-layer prepared by catalytic CVD VHD, are investigated. The results are shown and are discussed in view of the further improvement of catalytic CVD mc-Si:H based solar cells. 2. Deposition of intrinsic mc-Si:H films by catalytic CVD
A
B
1.5–10 73 0.88–0.98 6 230 3 1500 200 4
1.5 or 4 73 0.98 or 0.95 6 200 6 1500 200 4
may influence the real part of D (ReD), however, this does not influence ImD. mc-Si:H or a-Si:H samples have been prepared by catalytic CVD with silane (SiH4) diluted in hydrogen (H2) on Corning glass substrates or c-Si substrates. The deposition conditions of samples for ex-situ FTPME measurements and those for real-time in-situ FTPME measurements, are shown in Table 1. These are referred to as condition A and condition B, respectively. Realtime in-situ FTPME measurements have also been performed for mc-Si:H film deposition by the VHD process. In the case of VHD, F(SiH4) has been increased from 1.5 to 4 sccm. SiHn (ns1–3) bonding configurations in mc-Si:H samples have been investigated by FTPME analyses, in real-time and also after the deposition. The structural properties of the samples have been studied by UV-visible spectroscopic ellipsometry (SE) w4,8x, Fourier-transform infrared (FT–IR) absorption spectroscopy w8x and electron spin resonance (ESR). Only for FT–IR have samples on c-Si substrates been used.
2.1. Experimental 2.2. Results and discussion The FTPME instrument is coupled to the catalytic CVD reactor w1x, thus enabling the in-situ characterization of film growth. The incidence angle is fixed at 708. FTPME measurements have been performed with a resolution of 8 cmy1 for ex-situ measurements and a resolution of 16 cmy1 for real-time in-situ measurements. An InSb detector (1850–4000 cmy1) has been used. Measurements of the ellipsometric parameters C and D were recorded in-situ as a function of deposition time and also after the deposition, in the range of 1850– 2700 cmy1, to investigate the silicon–hydrogen stretching mode bonding configurations. IR spectra have been analyzed via the complex ellipsometric density D defined by: Dsµln ŽtanCs.ylnŽtanC.∂qiŽDsyD. where (Cs, Ds) correspond to the substrate. We have used the imaginary part of D (ImD) for the analysis, because infrared radiation emitted from the filament
For samples deposited at the condition A, ex-situ FTPME measurements clearly show that Si–H bonding (approx. 2000 cmy1) is dominant in a-Si:H samples prepared with F(SiH4) of 7–10 sccm, whereas SiH2 bonding (approx. 2100 cmy1) is dominant in wellcrystallized mc-Si:H samples. These results confirm the FT–IR results. The ESR analyses revealed that mc-Si:H samples, prepared by the VHD process, have slightly smaller dangling bond (DBs) densities than those prepared with constant DH. The ESR signal of the mc-Si:H sample, prepared with a high DH of 0.98, has a large line width. This result indicates a larger distribution of g-values, which indicates a g-value anisotropy w9x. This sample has a very high DB density, which is correlated with a high porosity as deduced from SE analysis w8x. In the case of the mc-Si:H sample prepared with a DH of 0.98, it is possible that the DBs are situated at the crystallite interfaces, whereas for the other samples, the
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Fig. 1. The FTPME spectra obtained after depositions of (1) a mc-Si:H layer prepared with DH of 0.98, (2) a mc-Si:H layer prepared with variable DH and (3) a mc-Si:H layer prepared with DH of 0.95. Solid lines in figures (a) show fitting results and figures (b) at the right hand show the absorption intensities obtained by fitting.
origin of the ESR signal is mainly considered to be DBs in the amorphous phase. Fig. 1 shows the FTPME ImD spectra of a mc-Si:H layer prepared with a DH of 0.98 (1), a mc-Si:H layer prepared with variable DH (2) and a mc-Si:H layer prepared with a DH of 0.95 (3), which were obtained after deposition. The (ImD — background) in Fig. 1a, at the left hand side of the figure, represent the measured ImD values from which a background curve has been subtracted. The solid lines in Fig. 1a show fitting results obtained with derived Lorentzian functions. Fig. 1b, at the right hand side of the figure show the absorption intensities obtained with the corresponding Lorentzian functions. Fig. 2 shows the FTPME ImD spectra, obtained in real-time during deposition, of (1) a mc-
Si:H layer prepared with a variable DH (at the left hand side of the figure) and of (2) a mc-Si:H layer prepared with a DH of 0.95 (at the right hand side of the figure). Fig. 1 (1) shows that the sample prepared with DH of 0.98 has a strong absorption at 2113 cmy1, due to SiH2 at crystallite surfaces. The strong absorption peaks with a smaller width are due to SiHn (ns1–3) at crystallite surfaces w10x. There is no absorption due to Si–H bonds in the amorphous phase in the bulk layer near 2000 cmy1. These results are correlated with a small amorphous fraction and high void fraction, as deduced from SE analysis w8x. FTPME real-time measurements also showed that absorption near 2100 cmy1 is always dominant for this sample. Since SE analysis indicated that this sample is totally crystallized from the
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181
Fig. 2. The FTPME ImD spectra, obtained, in real-time, during deposition, of (1) a mc-Si:H layer prepared with a variable DH and of (2) a mcSi:H layer prepared with a DH of 0.95.
substrate surface w8x, the absence of the absorption near 2000 cmy1 is also correlated with the absence of an amorphous incubation layer. Fig. 1 (2) shows that the sample prepared by the VHD process, where DH decreases gradually from 0.98 to 0.95, has absorption peaks at 2003 cmy1 due to Si– H bonds in the bulk layer and at 2091 cmy1 due to SiH2 bonds in the bulk layer. Fig. 2 (1) shows that the peak near 2090 cmy1 first appeared at the beginning of the deposition and that the peak near 2000 cmy1 appeared with increasing F(SiH4). The Si–H (bulk) peak intensity increased quickly with film thickness. UV-visible ellipsometry analysis indicated that this sample has no amorphous incubation layer and has a bulk structure similar to the sample prepared with the final DH value (0.95). Thus, no peak observed near 2000 cmy1 at the beginning of the deposition, can be correlated with the crystalline state of the layer at the substrate interface, similar to that of the sample with DHs0.98. The apparition of Si–H bonds with the decrease of DH should be correlated with the amorphous phase, which is observed in the bulk layer. Fig. 1 (3) shows that the sample prepared with a DH of 0.95 has a strong absorption peak at 2006 cmy1 due to Si–H bonding in the bulk layer and a smaller one at 2088 cmy1 due to SiH2 bonding in the bulk layer. Fig. 2 (2) shows that Si–H bonding is always dominant during deposition of this sample and that the Si–H and SiH2 (bulk) peak intensities increased with the thickness. The SE analysis indicated that this sample has approximately a 300-nm thick amorphous incubation layer. Thus, the strong peak near 2000 cmy1 and the
weaker peak near 2090 cmy1, which appeared at the beginning of the deposition, should be correlated with this amorphous incubation layer. There are absorption peaks near 1900 cmy1 in all the cases, which may be tentatively attributed to SiHn complex bonding w7,11x. The contribution of these SiHn peaks to the total integrated intensity is greater for the sample prepared with high DH. Peaks observed near 2176, 2186 and 2250 cmy1 can be attributed to SiH2 (SiO), SiH2 (O2) and SiH (O3), respectively, based on previous studies w12,13x. They may have been formed at the substrate interface by the interaction with the glass substrate. Further works will be needed in order to obtain more detailed information regarding film growth. 3. mc-Si:H solar cells with i-layer deposited by catalytic CVD 3.1. Preparation mc-Si:H solar cells were fabricated on SnO2 coated Asahi glass substrates, with the catalytic CVD intrinsic layer deposited by the VHD process. A catalytic CVD single-chamber reactor has been used for i-layer deposition. The 50–60 nm thick p- and n-type doped layers were deposited by the RF (13.56 MHz) glow discharge PECVD method in a multiplasma-monochamber reactor. Sample configurations and some deposition conditions are shown in Table 2. The thickness t and the crystalline fraction Xc of i-layer are also shown in Table 2. The Xc was estimated, for i-layer deposited on Corning glass
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substrate, by Raman scattering spectroscopy by separating the transverse optical phonon Raman spectra into three components w4,14x. The Ts for the deposition of n-layers has been fixed at 2008C. The i-layers of samples 1, 2 and 3 have been deposited with a d f of 3 cm and with Ts at approximately 2308C. The substrate was heated only by the radiation from filament. In the case of samples 4 and 5, the i-layers have been deposited with a d f of 6 cm and with Ts at 2008C. The substrate was heated up to 2008C, both by the radiation from the filament and by a heater placed inside the substrate holder (because of the larger d f value). Other conditions were fixed (see Table 1). Al or Ag contacts with the area of 0.12 cm2 were formed by vacuum evaporation. 3.2. Results and discussion The cell performance is shown in Table 2. They have been measured under AM1.5 irradiation. When comparing samples 1 and 2, it can be noted that the fill factor (FF) is improved in the case of the n-i-p structure. This is probably due to the diffusion of the boron dopant into the i-layer, which should be more significant in the case of the p-i-n structure when Ts is increased from 100 to 2308C during i-layer deposition. However, sample 1, with a p-i-n structure, shows a higher short circuit current density (Jsc) and thus, has a better response in the low wavelength region, probably due to the higher mobility for electrons than for holes. When increasing the i-layer thickness from 1 to 2 mm, Jsc increases, however, the FF decreases. This probably signifies that the diffusion length in the transverse direction should be superior to that in the lateral direction (0.1 mm), as measured by the steady-state photocarrier grating technique w4x. We tried to improve the interfaces of the cells by decreasing the Ts of the i-layer deposition. Sample 4, deposited at 2008C, shows an increase in FF up to 0.63. We have obtained an efficiency, h, of 4.6% for an active area of 0.12 cm2 for sample 5, in the configuration: Asahi SnO2 coated glass substrate y n-type mc-Si:H (PECVD) y intrinsic mc-Si:H (HWCVD) y p-type mcSi:H (PECVD) y Ag. J–V characteristics of this cell are shown in Fig. 3.
Fig. 3. J–V characteristics of mc-Si:H cell (噛5).
The use of thinner doped layers, irradiation from the side of the p-layer, optimization of the i-layer thickness and the use of a multichamber, that enables us to prepare cells without airbreaks between depositions of i- and doped layers, should further improve the performance of catalytic CVD mc-Si:H solar cells. 4. Conclusions In-situ Fourier transform phase modulated infrared ellipsometry (FTPME) has been found to be a powerful tool in studying the silicon–hydrogen bonding configurations during film deposition by catalytic CVD. It has been shown that a Si–H bonding configuration can be correlated with the amorphous phase in low hydrogen dilution conditions and that strong peaks due to SiHn (ns1–3) bondings at the surface or interface observed in a high hydrogen dilution condition can be correlated with a small amorphous fraction and high void fraction, which was observed through UV-visible ellipsometry. The evolution of Si–H bonding configurations has been observed, for the first time, using this IR analysis, during catalytic CVD mc-Si:H film deposition.
Table 2 Cell configurations and performance Sample 噛
structure
t (mm)
Xc (%)
df (cm)
Ts (8C) i-layer
Ts (8C) p-layer
DH
Contact
FF
Voc (V)
Jsc (mAycm2)
h (%)
1 2 3 4 5
p-i-n n-i-p n-i-p n-i-p
1.2 1.0 1.9 1.1 2.6
65–70
3
f230
f100
0.98–)0.95
Al
f60
6
f200
f150
0.98–)0.94
Al Al Ag
0.45 0.58 0.52 0.63 0.57 0.58
0.38 0.36 0.38 0.41 0.38 0.40
17.9 11.4 18.4 12.4 17.1 19.8
3.1 2.4 3.6 3.2 3.8 4.6
C. Niikura et al. / Thin Solid Films 395 (2001) 178–183
A 4.6% efficiency has been obtained for mc-Si:H ni-p solar cells, with the i-layer deposited by catalytic CVD at 2008C on a glass substrate using the variable hydrogen dilution process. Further optimization should improve the performance of catalytic CVD mc-Si:H solar cells. Acknowledgements The authors would like to thank Prof. K. Morigaki at Hiroshima Institute of Technology for ESR analysis, Mr J.Y. Parey for technical support for FTPME and Mr J.L. Moncel for the design and installation of the load–lock chamber. This work has been supported by the French ADEME-ECODEV (CNRS) program under contact 噛98N33 y 0005. References w1x J. Guillet, S.C. Saha, B. Equer, J.E. Bouree, ´ J.P. Kleider, C. Longeaud, Proceedings of the 2nd World Conference on Photovoltaic Solar Energy Conversion, 1998, p. 826.
183
w2x Q. Wang, E. Iwaniczko, A.H. Mahan, D.L. Williamson, MRS spring, Symp. Proc. 507 (1998) 903. w3x R.E. Schropp, J.K. Rath, IEEE Trans. Electron Devices 46 (10) (1999) 2069. w4x C. Niikura, R. Brenot, R. Vanderhaghen, Y. Poissant, P. Roca i ´ J.P. Kleider, C. Longeaud, to be Cabarrocas, J.E. Bouree, published in Proc. of 16th Eur. Photovoltaic Solar Energy Conf., Glasgow, UK (2000). w5x R. Ossikovski, B. Drevillon, ´ Phys. Rev. B 54 (1996) 10530. w6x B. Drevillon, ´ Thin Solid Films 313-314 (1998) 625. w7x H. Fujiwara, Y. Toyoshima, M. Kondo, A. Matsuda, J. NonCryst. Solids 266-269 (2000) 38. w8x C. Niikura, R. Brenot, J. Guillet, J.E. Bouree, ´ J.P. Kleider, R. ¨ Bruggemann, C. Longeaud, Sol. Energy Mater. Sol. Cells 66 (2001) 421. w9x T. Ehara, T. Ikoma, S. Tero-Kubota, J. Non-Cryst. Solids 266269 (2000) 540. w10x Y.J. Chabal, G.S. Higashi, K. Raghavachari, V.A Burrows, J. Vac. Sci. Technol. A 7 (1989) 2104. w11x T.S. Shi, S.N. Sahu, G.S. Oehrlein, A. Hiraki, J.W. Corbett, Phys. Stat. Sol. A 74 (1982) 329. w12x M. Niwano, J. Kageyama, K. Kurita, K. Kinashi, I. Takahashi, N. Miyamoto, J. Appl. Phys. 76 (1994) 2157. w13x M. Niwano, Surf. Sci. 427-428 (1999) 199. w14x Z. Iqbal, S. Veprek, A.P. Webb, P. Capezzuto, Solid State Commun. 37 (1981) 993.
Growth mechanisms and structural properties of microcrystalline silicon films deposited by catalytic CVD a ´ ´ a C. Niikuraa,*, S.Y. Kimb, B. Drevillon , Y. Poissanta, P. Roca i Cabarrocasa, J.E. Bouree a
Laboratoire de Physique des Interfaces et des Couches Minces, CNRS UMR 7647, Ecole Polytechnique, F-91128 Palaiseau Cedex, France b Department of Physics, Ajou University, Suwon 442-749, South Korea
Abstract Silicon–hydrogen bonding configurations, during or after microcrystalline silicon (mc-Si:H) film deposition by catalytic CVD, have been investigated for the first time by real-time in-situ Fourier transform phase modulated infrared ellipsometry (FTPME). FTPME measurements have been performed during and after mc-Si:H film depositions using high, low or variable dilutions of silane in hydrogen. The silicon–hydrogen bonding configurations of mc-Si:H films have been correlated with their corresponding structural properties as deduced from UV-visible ellipsometry analyses. A 4.6% efficiency has been obtained for mc-Si:H n-i-p solar cells, with the i-layer deposited by catalytic CVD at 2008C on a glass substrate using a variable hydrogen dilution process. Further optimization should improve the performance of catalytic CVD mc-Si:H solar cells. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Chemical vapor deposition (CVD); Ellipsometry; Silicon; Solar cells
1. Introduction Microcrystalline silicon (mc-Si:H) films, deposited by catalytic CVD, are promising materials for low cost and large area thin film devices such as solar cells or thin film transistors (TFT). mc-Si:H solar cells have been deposited entirely by catalytic CVD and by catalytic CVD combined with plasma enhanced CVD (PECVD). Approximately 2% efficiency was obtained for p-i-n or n-i-p solar cells entirely deposited by catalytic CVD w1,2x. A 4.4% efficiency was obtained for n-i-p diodes on stainless steel substrates and a 3.6 % efficiency was reached for n-i-p diodes on glass substrates by combining catalytic CVD with PECVD w3,4x. In order to optimize mc-Si:H solar cells, investigations of mc-Si:H formation mechanisms are required. Infrared vibrational spectroscopy studies have been extremely useful in assessing the different silicon–hydrogen bond* Corresponding author. Tel.: q33-1-69-33-32-17; fax: q33-1-6933-30-06. E-mail address: [email protected] (C. Niikura).
ing configurations in hydrogenated amorphous silicon (a-Si:H) or mc-Si:H films after deposition, as well as for understanding mc-Si:H formation mechanisms w5– 7 x. It is well known that thin (10–30 nm) amorphous layers are unintentionally deposited during the initial growth stage, prior to the nucleation of the crystallites. This amorphous incubation layer influences the electronic properties of the films. Previously, we reported that at very high dilutions of silane (SiH4) in hydrogen (H2), film crystallization starts from the substrate interface after an incubation period, however, the film becomes porous w8x. Then, in order to prevent the formation of an amorphous incubation layer, we proposed a specific deposition process, called a variable hydrogen dilution (VHD) process, where the hydrogen dilution, defined as DHsF(H2) y wF(SiH4 )qF(H2 )x, is high at the start of the deposition w4,8x. It is then reduced with increasing deposition time and is maintained at a lower DH for the rest of the deposition, in order to obtain dense mc-Si:H films. We also reported that the mc-Si:H layers prepared with this VHD process show improved electronic transport along the growth direction w4x.
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Table 1 Deposition conditions for mc-Si:H films
SiH4 flow rate F(SiH4) (sccm) H2 flow rate F(H2) (sccm) H2 dilution (DH) Process gas pressure pg (Pa) Nominal substrate temperature Ts (8C) Filament–substrate distance d f (cm) Filament temperature T f (8C) Electric power supplied to the filament (W) Filament surface area (cm2)
In this study, we investigate silicon–hydrogen bonding configurations in mc-Si:H or a-Si:H samples by Fourier transform phase modulated infrared ellipsometry (FTPME), in order to study the correlation with their corresponding structural properties. An advantage of FTPME is that it permits the use of any substrate, whereas attenuated total reflection spectroscopy needs the use of a crystalline silicon substrate. FTPME measurements have been performed, in real-time, during mcSi:H depositions with VHD or with constant DH, in order to obtain information on the deposition mechanisms, as well as after the deposition. Finally, mc-Si:H solar cells, with the i-layer prepared by catalytic CVD VHD, are investigated. The results are shown and are discussed in view of the further improvement of catalytic CVD mc-Si:H based solar cells. 2. Deposition of intrinsic mc-Si:H films by catalytic CVD
A
B
1.5–10 73 0.88–0.98 6 230 3 1500 200 4
1.5 or 4 73 0.98 or 0.95 6 200 6 1500 200 4
may influence the real part of D (ReD), however, this does not influence ImD. mc-Si:H or a-Si:H samples have been prepared by catalytic CVD with silane (SiH4) diluted in hydrogen (H2) on Corning glass substrates or c-Si substrates. The deposition conditions of samples for ex-situ FTPME measurements and those for real-time in-situ FTPME measurements, are shown in Table 1. These are referred to as condition A and condition B, respectively. Realtime in-situ FTPME measurements have also been performed for mc-Si:H film deposition by the VHD process. In the case of VHD, F(SiH4) has been increased from 1.5 to 4 sccm. SiHn (ns1–3) bonding configurations in mc-Si:H samples have been investigated by FTPME analyses, in real-time and also after the deposition. The structural properties of the samples have been studied by UV-visible spectroscopic ellipsometry (SE) w4,8x, Fourier-transform infrared (FT–IR) absorption spectroscopy w8x and electron spin resonance (ESR). Only for FT–IR have samples on c-Si substrates been used.
2.1. Experimental 2.2. Results and discussion The FTPME instrument is coupled to the catalytic CVD reactor w1x, thus enabling the in-situ characterization of film growth. The incidence angle is fixed at 708. FTPME measurements have been performed with a resolution of 8 cmy1 for ex-situ measurements and a resolution of 16 cmy1 for real-time in-situ measurements. An InSb detector (1850–4000 cmy1) has been used. Measurements of the ellipsometric parameters C and D were recorded in-situ as a function of deposition time and also after the deposition, in the range of 1850– 2700 cmy1, to investigate the silicon–hydrogen stretching mode bonding configurations. IR spectra have been analyzed via the complex ellipsometric density D defined by: Dsµln ŽtanCs.ylnŽtanC.∂qiŽDsyD. where (Cs, Ds) correspond to the substrate. We have used the imaginary part of D (ImD) for the analysis, because infrared radiation emitted from the filament
For samples deposited at the condition A, ex-situ FTPME measurements clearly show that Si–H bonding (approx. 2000 cmy1) is dominant in a-Si:H samples prepared with F(SiH4) of 7–10 sccm, whereas SiH2 bonding (approx. 2100 cmy1) is dominant in wellcrystallized mc-Si:H samples. These results confirm the FT–IR results. The ESR analyses revealed that mc-Si:H samples, prepared by the VHD process, have slightly smaller dangling bond (DBs) densities than those prepared with constant DH. The ESR signal of the mc-Si:H sample, prepared with a high DH of 0.98, has a large line width. This result indicates a larger distribution of g-values, which indicates a g-value anisotropy w9x. This sample has a very high DB density, which is correlated with a high porosity as deduced from SE analysis w8x. In the case of the mc-Si:H sample prepared with a DH of 0.98, it is possible that the DBs are situated at the crystallite interfaces, whereas for the other samples, the
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Fig. 1. The FTPME spectra obtained after depositions of (1) a mc-Si:H layer prepared with DH of 0.98, (2) a mc-Si:H layer prepared with variable DH and (3) a mc-Si:H layer prepared with DH of 0.95. Solid lines in figures (a) show fitting results and figures (b) at the right hand show the absorption intensities obtained by fitting.
origin of the ESR signal is mainly considered to be DBs in the amorphous phase. Fig. 1 shows the FTPME ImD spectra of a mc-Si:H layer prepared with a DH of 0.98 (1), a mc-Si:H layer prepared with variable DH (2) and a mc-Si:H layer prepared with a DH of 0.95 (3), which were obtained after deposition. The (ImD — background) in Fig. 1a, at the left hand side of the figure, represent the measured ImD values from which a background curve has been subtracted. The solid lines in Fig. 1a show fitting results obtained with derived Lorentzian functions. Fig. 1b, at the right hand side of the figure show the absorption intensities obtained with the corresponding Lorentzian functions. Fig. 2 shows the FTPME ImD spectra, obtained in real-time during deposition, of (1) a mc-
Si:H layer prepared with a variable DH (at the left hand side of the figure) and of (2) a mc-Si:H layer prepared with a DH of 0.95 (at the right hand side of the figure). Fig. 1 (1) shows that the sample prepared with DH of 0.98 has a strong absorption at 2113 cmy1, due to SiH2 at crystallite surfaces. The strong absorption peaks with a smaller width are due to SiHn (ns1–3) at crystallite surfaces w10x. There is no absorption due to Si–H bonds in the amorphous phase in the bulk layer near 2000 cmy1. These results are correlated with a small amorphous fraction and high void fraction, as deduced from SE analysis w8x. FTPME real-time measurements also showed that absorption near 2100 cmy1 is always dominant for this sample. Since SE analysis indicated that this sample is totally crystallized from the
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181
Fig. 2. The FTPME ImD spectra, obtained, in real-time, during deposition, of (1) a mc-Si:H layer prepared with a variable DH and of (2) a mcSi:H layer prepared with a DH of 0.95.
substrate surface w8x, the absence of the absorption near 2000 cmy1 is also correlated with the absence of an amorphous incubation layer. Fig. 1 (2) shows that the sample prepared by the VHD process, where DH decreases gradually from 0.98 to 0.95, has absorption peaks at 2003 cmy1 due to Si– H bonds in the bulk layer and at 2091 cmy1 due to SiH2 bonds in the bulk layer. Fig. 2 (1) shows that the peak near 2090 cmy1 first appeared at the beginning of the deposition and that the peak near 2000 cmy1 appeared with increasing F(SiH4). The Si–H (bulk) peak intensity increased quickly with film thickness. UV-visible ellipsometry analysis indicated that this sample has no amorphous incubation layer and has a bulk structure similar to the sample prepared with the final DH value (0.95). Thus, no peak observed near 2000 cmy1 at the beginning of the deposition, can be correlated with the crystalline state of the layer at the substrate interface, similar to that of the sample with DHs0.98. The apparition of Si–H bonds with the decrease of DH should be correlated with the amorphous phase, which is observed in the bulk layer. Fig. 1 (3) shows that the sample prepared with a DH of 0.95 has a strong absorption peak at 2006 cmy1 due to Si–H bonding in the bulk layer and a smaller one at 2088 cmy1 due to SiH2 bonding in the bulk layer. Fig. 2 (2) shows that Si–H bonding is always dominant during deposition of this sample and that the Si–H and SiH2 (bulk) peak intensities increased with the thickness. The SE analysis indicated that this sample has approximately a 300-nm thick amorphous incubation layer. Thus, the strong peak near 2000 cmy1 and the
weaker peak near 2090 cmy1, which appeared at the beginning of the deposition, should be correlated with this amorphous incubation layer. There are absorption peaks near 1900 cmy1 in all the cases, which may be tentatively attributed to SiHn complex bonding w7,11x. The contribution of these SiHn peaks to the total integrated intensity is greater for the sample prepared with high DH. Peaks observed near 2176, 2186 and 2250 cmy1 can be attributed to SiH2 (SiO), SiH2 (O2) and SiH (O3), respectively, based on previous studies w12,13x. They may have been formed at the substrate interface by the interaction with the glass substrate. Further works will be needed in order to obtain more detailed information regarding film growth. 3. mc-Si:H solar cells with i-layer deposited by catalytic CVD 3.1. Preparation mc-Si:H solar cells were fabricated on SnO2 coated Asahi glass substrates, with the catalytic CVD intrinsic layer deposited by the VHD process. A catalytic CVD single-chamber reactor has been used for i-layer deposition. The 50–60 nm thick p- and n-type doped layers were deposited by the RF (13.56 MHz) glow discharge PECVD method in a multiplasma-monochamber reactor. Sample configurations and some deposition conditions are shown in Table 2. The thickness t and the crystalline fraction Xc of i-layer are also shown in Table 2. The Xc was estimated, for i-layer deposited on Corning glass
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substrate, by Raman scattering spectroscopy by separating the transverse optical phonon Raman spectra into three components w4,14x. The Ts for the deposition of n-layers has been fixed at 2008C. The i-layers of samples 1, 2 and 3 have been deposited with a d f of 3 cm and with Ts at approximately 2308C. The substrate was heated only by the radiation from filament. In the case of samples 4 and 5, the i-layers have been deposited with a d f of 6 cm and with Ts at 2008C. The substrate was heated up to 2008C, both by the radiation from the filament and by a heater placed inside the substrate holder (because of the larger d f value). Other conditions were fixed (see Table 1). Al or Ag contacts with the area of 0.12 cm2 were formed by vacuum evaporation. 3.2. Results and discussion The cell performance is shown in Table 2. They have been measured under AM1.5 irradiation. When comparing samples 1 and 2, it can be noted that the fill factor (FF) is improved in the case of the n-i-p structure. This is probably due to the diffusion of the boron dopant into the i-layer, which should be more significant in the case of the p-i-n structure when Ts is increased from 100 to 2308C during i-layer deposition. However, sample 1, with a p-i-n structure, shows a higher short circuit current density (Jsc) and thus, has a better response in the low wavelength region, probably due to the higher mobility for electrons than for holes. When increasing the i-layer thickness from 1 to 2 mm, Jsc increases, however, the FF decreases. This probably signifies that the diffusion length in the transverse direction should be superior to that in the lateral direction (0.1 mm), as measured by the steady-state photocarrier grating technique w4x. We tried to improve the interfaces of the cells by decreasing the Ts of the i-layer deposition. Sample 4, deposited at 2008C, shows an increase in FF up to 0.63. We have obtained an efficiency, h, of 4.6% for an active area of 0.12 cm2 for sample 5, in the configuration: Asahi SnO2 coated glass substrate y n-type mc-Si:H (PECVD) y intrinsic mc-Si:H (HWCVD) y p-type mcSi:H (PECVD) y Ag. J–V characteristics of this cell are shown in Fig. 3.
Fig. 3. J–V characteristics of mc-Si:H cell (噛5).
The use of thinner doped layers, irradiation from the side of the p-layer, optimization of the i-layer thickness and the use of a multichamber, that enables us to prepare cells without airbreaks between depositions of i- and doped layers, should further improve the performance of catalytic CVD mc-Si:H solar cells. 4. Conclusions In-situ Fourier transform phase modulated infrared ellipsometry (FTPME) has been found to be a powerful tool in studying the silicon–hydrogen bonding configurations during film deposition by catalytic CVD. It has been shown that a Si–H bonding configuration can be correlated with the amorphous phase in low hydrogen dilution conditions and that strong peaks due to SiHn (ns1–3) bondings at the surface or interface observed in a high hydrogen dilution condition can be correlated with a small amorphous fraction and high void fraction, which was observed through UV-visible ellipsometry. The evolution of Si–H bonding configurations has been observed, for the first time, using this IR analysis, during catalytic CVD mc-Si:H film deposition.
Table 2 Cell configurations and performance Sample 噛
structure
t (mm)
Xc (%)
df (cm)
Ts (8C) i-layer
Ts (8C) p-layer
DH
Contact
FF
Voc (V)
Jsc (mAycm2)
h (%)
1 2 3 4 5
p-i-n n-i-p n-i-p n-i-p
1.2 1.0 1.9 1.1 2.6
65–70
3
f230
f100
0.98–)0.95
Al
f60
6
f200
f150
0.98–)0.94
Al Al Ag
0.45 0.58 0.52 0.63 0.57 0.58
0.38 0.36 0.38 0.41 0.38 0.40
17.9 11.4 18.4 12.4 17.1 19.8
3.1 2.4 3.6 3.2 3.8 4.6
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A 4.6% efficiency has been obtained for mc-Si:H ni-p solar cells, with the i-layer deposited by catalytic CVD at 2008C on a glass substrate using the variable hydrogen dilution process. Further optimization should improve the performance of catalytic CVD mc-Si:H solar cells. Acknowledgements The authors would like to thank Prof. K. Morigaki at Hiroshima Institute of Technology for ESR analysis, Mr J.Y. Parey for technical support for FTPME and Mr J.L. Moncel for the design and installation of the load–lock chamber. This work has been supported by the French ADEME-ECODEV (CNRS) program under contact 噛98N33 y 0005. References w1x J. Guillet, S.C. Saha, B. Equer, J.E. Bouree, ´ J.P. Kleider, C. Longeaud, Proceedings of the 2nd World Conference on Photovoltaic Solar Energy Conversion, 1998, p. 826.
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