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Onset of microcrystallinity in silicon thin films
Thin Solid Films 403 – 404 (2002) 81–85
Onset of microcrystallinity in silicon thin films C. Das, S. Ray* Energy Research Unit, Indian Association for the Cultivation of Science, Jadavpur, Calcutta 700 032, India
Abstract The onset of microcrystallinity in silicon thin films was realized via an amorphous-to-microcrystalline phase transition. Undoped films have been deposited by the plasma-enhanced chemical vapor deposition (PECVD) technique from silane diluted with hydrogen. Substrate temperature was set as the variable parameter in the deposition. The films were characterized by Raman spectroscopy, Fourier-transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM). The presence of both amorphous and crystalline phases is indicated in the Raman spectra and the signature of grain boundary regions is not prominent for the transition films. The dominance of monohydride bonding in the amorphous matrix is revealed by FTIR spectra. TEM ˚ embedded in the amorphous matrix for the films prepared at the transition region. confirms the presence of small grains (;50 A) At the onset of crystallinity, films have a higher order of dark conductivity than a typical amorphous film, but are still photosensitive. Better stability under illumination is observed compared to amorphous silicon. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Silicon thin film; Microcrystallinity; Raman spectra; Photosensitivity; Stability
1. Introduction The potential of silicon thin-film technology has boosted several research works for the last decades in the field of solar cells, thin-film transistors (TFTs), sensors, etc. w1–3x. Microcrystalline silicon (mc-Si:H) thin film has drawn the main focus of attention, since it can almost avoid light-induced degradation compared to amorphous silicon, which degrades considerably w4,5x. In the last few years, mc-Si:H has been successfully used as the active layer (i.e. i-layer) of solar cells due to its high dark conductivity and very low light-induced degradation w6,7x. However, the poor photosensitivity of mc-Si:H is a disadvantage for its use as an absorber layer. Very recent studies on undoped silicon thin films have reported the best performance of a solar cell achieved with the i-layers prepared at the onset of crystallinity, and Vetterl et al. have shown that both the efficiency and fill factor attain their maximum value * Corresponding author. Tel.: q91-33-473-6612; fax: q91-33-4736612. E-mail address: [email protected] (S. Ray).
when the pertinent layer is composed of small crystallites with a large volume fraction of amorphous material, rather than the highly microcrystalline materials w8–10x. Thus, it is important to correlate the structural properties of the material with their effects on performance of solar cells. In the present work, an onset of crystallinity in undoped silicon thin film has been realized through the amorphous-to-microcrystalline phase transition initiated by the variation of substrate temperature, which has been much less studied so far. The structural properties of the films are discussed and correlated with the transport properties. 2. Experimental The films have been deposited by the conventional 13.56-MHz (radio frequency) PECVD technique using silane (SiH4) and hydrogen (H2) as source gases with a constant flow ratio of 1:9. The substrate temperature was varied from 170 to 3708C, keeping the chamber pressure and RF power density constant at 0.2 torr and 35 mW cmy2, respectively.
0040-6090/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 1 . 0 1 5 5 1 - 6
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C. Das, S. Ray / Thin Solid Films 403 – 404 (2002) 81–85
Fig. 1. Raman spectra of the films deposited at a substrate temperature of: (a) 200; (b) 270; (c) 300; and (d) 3408C. Deconvolutions are shown by broken curves.
3. Results and discussion 3.1. Structural properties 3.1.1. Raman spectra The Raman spectra of the films deposited at different substrate temperatures are shown in Fig. 1. The transverse optical (TO) phonon mode of the Raman spectra of the samples have been mainly deconvoluted into two peaks: Ia (;480 cmy1) for the amorphous fraction and Ic (;520 cmy1) for the crystalline fraction w11x. However, in cases for which the whole data set is best fitted with three peaks, a third peak Igb (;510 cmy1) has been included in the deconvolution. The peak at 510 cmy1 arises due to the presence of tensile strained Si–Si bonds in defective regions w12,13x. The crystalline volume fraction Xc has been calculated from Xcs(IcqIgb) y(IcqIgbqIa), where I stands for the integrated intensity of a peak. The film deposited at 2008C shows only a broad peak at 480 cmy1 in Fig. 1a. In Fig. 1b, the spectrum is shown for the film deposited at 2708C. Here, in addition to the principal peak at 480 cmy1, a peak of very low intensity arises at 520 cmy1. The crystalline volume fraction is only 3.6%. At the next higher step of substrate temperature, i.e. 3008C, the intensity of the peak at 520 cmy1 increases (Xcs12%), which is shown in Fig. 1c. For the films deposited at 340 and 3708C, a new feature is added to the structural property. The presence of the grain boundary is notable in these two cases, since another peak at 510 cmy1 also contributes to the spectral intensity, as shown in Fig. 1d. The crystalline volume fraction increases with temperature and values of Xcs42 and 65% are obtained for the films deposited at 340 and 3708C, respectively. From the spectral analysis, it is evident that a structural transformation takes place during the increase in
substrate temperature. This transformation initiates the amorphous-to-microcrystalline phase transition in the films. At lower substrate temperature, the film possesses a disordered amorphous structure. With the increase in substrate temperature, the structural order increases. For the films deposited at higher substrate temperature (340 and 3708C), the intense peak at 520 cmy1 indicates the large volume fraction of the crystalline part. Beside the crystallites and small amorphous fraction, there is also a different structural constituent in the film: grain boundaries, where mainly tensile, strained Si–Si bonds, hydrogen in the form of Si–H2 and defects are present. Again, in the films deposited at intermediate temperatures (270 and 3008C), the signature of grain boundaries is not prominent in the spectra. The films obviously contain a large volume fraction of amorphous matrix, along with comparatively ordered regions, considered as crystallites, which, however, reflect the very low crystalline volume fraction of the films. 3.1.2. FTIR spectra The Fourier-transform infrared absorption spectra are shown in Fig. 2 for the stretching mode of vibration. Here also, the main curves have been fitted to their best profile fitting the deconvoluted curves. In Fig. 2a, there is only one peak at 2000 cmy1 for the film deposited at 2008C. At 2708C, the total peak profile can be deconvoluted into two peaks at 2000 and 2100 cmy1. However, the intensity of the higher frequency peak is much less compared to that at the lower frequency, as shown in Fig. 2b. The doublet formation is retained for the films deposited at higher substrate temperatures (Fig.
Fig. 2. FTIR spectra of the films deposited at a substrate temperature of: (a) 200; (b) 270; (c) 300; and (d) 3408C. Deconvolutions are shown by broken curves.
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3.2. TEM studies The morphology of the samples was studied by transmission electron microscopy (TEM) and transmission electron diffraction (TED) patterns. Fig. 3a,b shows TEM and TED micrographs, respectively, taken for the sample deposited at 3008C. Very tiny and closely spaced grains are observed in the TEM, which are distributed ˚ uniformly. The average size of the grains is ;50 A. Some elongated grains are also apparent on the surface. In the TED pattern, clear halos corresponding to 111, 220 and 311 crystalline silicon planes indicate the very large volume fraction of amorphous matrix in the film. 4. Transport properties 4.1. Electrical conductivity
Fig. 3. TEM pictures of the film deposited at 3008C: (a) cross-sectional view of TEM; and (b) transmission electron diffraction.
2c,d), and the contribution from the higher frequency peak increases. The stretching-mode peak positions can reveal information about the nature of hydrogen bonding with silicon in the films. Generally, in IR spectra, the peak at 2000 cmy1 is considered to arise from the stretching mode of vibration of Si–H-type bonds, whereas the peak at 2100 cmy1 is due to that of Si–H2 andyor Si–Hn (n)2) types of bonds w11x. From the above-discussed analysis of IR spectra, the dominance of monohydride bonding is observed for all the films deposited at various substrate temperatures. At lower temperature (2008C), the bonds are completely monohydride in nature, where the film is completely amorphous as well. Even for the films with higher crystallinity deposited at the higher temperature at 3408C, the monohydride nature is predominant. However, the di andyor higher hydrides have a notable contribution at this stage. It is interesting to note that for the films deposited in the transition region (at 270 and 3008C), the di andyor higher hydrides content is very low. It should be mentioned here that Si–H2 andyor Si–Hn (n)2) types of bonds are commonly formed at the grain boundaries, the main constituents of which are defects, and distorted and strained bonds w13x. The contribution from the grain boundaries is negligibly small in the films deposited at 270 and 3008C. The effect of grain boundaries is pronounced at higher temperature (340 and 3708C), where the crystallinity is comparatively high.
The structural properties of the films are correlated with their transport properties. Fig. 4 shows the variation in dark conductivity of the films deposited at various substrate temperatures. The low values of dark conductivity (;10y10 S cmy1) at lower substrate temperature (F2408C) indicate the amorphous nature of the films. At the transition region, the films have a higher order of dark conductivity (;10y8 S cmy1) than typical amorphous films, which is an outcome of the improvement in structural order of the films. The films deposited at higher substrate temperatures (G3408C) show high values for dark conductivity (;10y5 S cmy1), since these films are microcrystalline with a higher crystalline volume fraction. The photosensitivity (defined as the ratio of the electrical conductivity with and without illumination, G) of the films is also shown in Fig. 4 as a function of the substrate temperature. It offers one of the most interesting results in the present study. High values of photosensitivity (;105) for the amorphous films deposited at lower substrate temperature (F2408C) and low values (;100) for the microcrystalline films deposited at higher
Fig. 4. Variation in dark conductivity and photosensitivity of the films as a function of substrate temperature. The encircled portion approximately indicates the amorphous-to-microcrystalline transition region in the present study.
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C. Das, S. Ray / Thin Solid Films 403 – 404 (2002) 81–85
Fig. 5. The changes in photosensitivity and photosensitivity after 103 h of light soaking are plotted for the films deposited at various substrate temperatures. The dotted lines are guides for the eye.
substrate temperature (G3408C) are quite typical. However, the films deposited at the transition region have a higher value for dark conductivity than amorphous films, but are still photosensitive (G;103 ). This is a necessary requirement for undoped films to be used as an active layer in solar cells. This notable photosensitivity and higher dark conductivity may be attributed to the presence of nanocrystalline silicon in the high volume fraction of amorphous matrix in the film. 4.2. Light-induced degradation It is important to study light-induced degradation of the films deposited at different substrate temperatures. When exposed to illumination, photosensitivity (G) of the film degrades to different extents, depending on the crystallinity of the film w14x. In Fig. 5, changes in photosensitivity from its initial value after 103 h of light soaking are plotted for different substrate temperatures. The amorphous films deposited at lower substrate temperatures degrade to a great extent. They do not attain a stable value, even after 103 h. The microcrystalline samples prepared at 340 and 3708C, with different volume fraction of crystallinity (42 and 65%) degrade very little, but they are not photosensitive. Degradation of the film deposited at 3008C is also very low. However, the photosensitivity after degradation is reasonably high (102). These properties make the sample prepared at 3008C suitable for application as the active layer in solar cells. 5. Conclusions The structural and transport properties of silicon films deposited with different substrate temperatures have
been investigated. The transport properties of the films are a perfect reflection of their structural properties. The evolution of a microcrystalline structure from a completely amorphous structure has been observed. The film prepared in the transition region contains small crystal˚ embedded in an amorphous matrix. Mainly lites (50 A) monohydride bonds of hydrogen are observed in the matrix and the grain boundaries are not prominent. The films have notable photosensitivity compared to microcrystalline films. In addition to these properties, the films have higher stability under light exposure than amorphous films. This can be attributed to the presence of small crystallites and a better structural relaxation in the film. These nanocrystalline films have high potential for application in solar cells. References w1x J. Meier, P. Torres, R. Platz, S. Dubail, U. Kroll, J.A. Anna Selvan, N. Pellaton Vaucher, C. Hof, D Fischer, H. Keppner, A. Shah, K.D. Ufert, P. Giannoules, J. Koehler, Mater. Res. Soc. Symp. Proc. 420 (1996) 3. w2x S.C. Saha, S. Ray, J. Appl. Phys. 78 (1995) 5713. w3x R.A. Street, Hydrogenated Amorphous Silicon, Cambridge University Press, Cambridge, 1991. w4x C.C. Tsai, in: H. Frittzsche (Ed.), Amorphous Silicon and Related Materials, World Scientific, Singapore, 1998, pp. 123– 147. w5x D.L. Staebler, C.R. Wronski, Appl. Phys. Lett. 31 (1977) 292. w6x R.J. Koval, J. Koh, Z. Lu, R.W. Collins, C.R. Wronski, G. Ganguly, Technical Digest of the 11th Photovoltaic Scientist and Engineers Conference, Sapporo, Japan, 1999, p. 217. w7x K. Saitoh, N. Ishiguro, N. Yanagawa, H. Tanaka, S. Fukuda, M. Sadamato, Y. Ashida, N. Fukuda, J. Non-Cryst. Solids 198-200 (1996) 1093. w8x O. Vetterl, F. Finger, R. Carius, P. Hapke, L. Houben, O. Kluth, A. Lambertz, A. Muck, B. Rech, H. Wagner, Sol. Energy Mater. Sol. Cells 62 (2000) 97.
C. Das, S. Ray / Thin Solid Films 403 – 404 (2002) 81–85 w9x M. Vieira, A. Fantoni, M. Fernandes, R. Schwarz, Philos. Mag. B 80 (2000) 755. w10x G. Ambrosone, G. Bruno, P. Capezzuto, G. Cicala, U. Coscia, Philos. Mag. B 80 (2000) 487. w11x M.H. Brodsky, M. Cardona, J.J. Cuomo, Phys. Rev. B 16 (1977) 3556.
85
w12x T. Kaneko, M. Wakagi, K. Onisawa, T. Minemura, Appl. Phys. Lett. 64 (1994) 1865. w13x S. Veprek, F.A. Sarrot, Z. Iqbal, Phys. Rev. B 36 (1987) 3344. w14x M. Isomura, N. Hata, S. Wagner, Jpn. J. Appl. Phys. 31 (1992) 3500.
Onset of microcrystallinity in silicon thin films C. Das, S. Ray* Energy Research Unit, Indian Association for the Cultivation of Science, Jadavpur, Calcutta 700 032, India
Abstract The onset of microcrystallinity in silicon thin films was realized via an amorphous-to-microcrystalline phase transition. Undoped films have been deposited by the plasma-enhanced chemical vapor deposition (PECVD) technique from silane diluted with hydrogen. Substrate temperature was set as the variable parameter in the deposition. The films were characterized by Raman spectroscopy, Fourier-transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM). The presence of both amorphous and crystalline phases is indicated in the Raman spectra and the signature of grain boundary regions is not prominent for the transition films. The dominance of monohydride bonding in the amorphous matrix is revealed by FTIR spectra. TEM ˚ embedded in the amorphous matrix for the films prepared at the transition region. confirms the presence of small grains (;50 A) At the onset of crystallinity, films have a higher order of dark conductivity than a typical amorphous film, but are still photosensitive. Better stability under illumination is observed compared to amorphous silicon. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Silicon thin film; Microcrystallinity; Raman spectra; Photosensitivity; Stability
1. Introduction The potential of silicon thin-film technology has boosted several research works for the last decades in the field of solar cells, thin-film transistors (TFTs), sensors, etc. w1–3x. Microcrystalline silicon (mc-Si:H) thin film has drawn the main focus of attention, since it can almost avoid light-induced degradation compared to amorphous silicon, which degrades considerably w4,5x. In the last few years, mc-Si:H has been successfully used as the active layer (i.e. i-layer) of solar cells due to its high dark conductivity and very low light-induced degradation w6,7x. However, the poor photosensitivity of mc-Si:H is a disadvantage for its use as an absorber layer. Very recent studies on undoped silicon thin films have reported the best performance of a solar cell achieved with the i-layers prepared at the onset of crystallinity, and Vetterl et al. have shown that both the efficiency and fill factor attain their maximum value * Corresponding author. Tel.: q91-33-473-6612; fax: q91-33-4736612. E-mail address: [email protected] (S. Ray).
when the pertinent layer is composed of small crystallites with a large volume fraction of amorphous material, rather than the highly microcrystalline materials w8–10x. Thus, it is important to correlate the structural properties of the material with their effects on performance of solar cells. In the present work, an onset of crystallinity in undoped silicon thin film has been realized through the amorphous-to-microcrystalline phase transition initiated by the variation of substrate temperature, which has been much less studied so far. The structural properties of the films are discussed and correlated with the transport properties. 2. Experimental The films have been deposited by the conventional 13.56-MHz (radio frequency) PECVD technique using silane (SiH4) and hydrogen (H2) as source gases with a constant flow ratio of 1:9. The substrate temperature was varied from 170 to 3708C, keeping the chamber pressure and RF power density constant at 0.2 torr and 35 mW cmy2, respectively.
0040-6090/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 1 . 0 1 5 5 1 - 6
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C. Das, S. Ray / Thin Solid Films 403 – 404 (2002) 81–85
Fig. 1. Raman spectra of the films deposited at a substrate temperature of: (a) 200; (b) 270; (c) 300; and (d) 3408C. Deconvolutions are shown by broken curves.
3. Results and discussion 3.1. Structural properties 3.1.1. Raman spectra The Raman spectra of the films deposited at different substrate temperatures are shown in Fig. 1. The transverse optical (TO) phonon mode of the Raman spectra of the samples have been mainly deconvoluted into two peaks: Ia (;480 cmy1) for the amorphous fraction and Ic (;520 cmy1) for the crystalline fraction w11x. However, in cases for which the whole data set is best fitted with three peaks, a third peak Igb (;510 cmy1) has been included in the deconvolution. The peak at 510 cmy1 arises due to the presence of tensile strained Si–Si bonds in defective regions w12,13x. The crystalline volume fraction Xc has been calculated from Xcs(IcqIgb) y(IcqIgbqIa), where I stands for the integrated intensity of a peak. The film deposited at 2008C shows only a broad peak at 480 cmy1 in Fig. 1a. In Fig. 1b, the spectrum is shown for the film deposited at 2708C. Here, in addition to the principal peak at 480 cmy1, a peak of very low intensity arises at 520 cmy1. The crystalline volume fraction is only 3.6%. At the next higher step of substrate temperature, i.e. 3008C, the intensity of the peak at 520 cmy1 increases (Xcs12%), which is shown in Fig. 1c. For the films deposited at 340 and 3708C, a new feature is added to the structural property. The presence of the grain boundary is notable in these two cases, since another peak at 510 cmy1 also contributes to the spectral intensity, as shown in Fig. 1d. The crystalline volume fraction increases with temperature and values of Xcs42 and 65% are obtained for the films deposited at 340 and 3708C, respectively. From the spectral analysis, it is evident that a structural transformation takes place during the increase in
substrate temperature. This transformation initiates the amorphous-to-microcrystalline phase transition in the films. At lower substrate temperature, the film possesses a disordered amorphous structure. With the increase in substrate temperature, the structural order increases. For the films deposited at higher substrate temperature (340 and 3708C), the intense peak at 520 cmy1 indicates the large volume fraction of the crystalline part. Beside the crystallites and small amorphous fraction, there is also a different structural constituent in the film: grain boundaries, where mainly tensile, strained Si–Si bonds, hydrogen in the form of Si–H2 and defects are present. Again, in the films deposited at intermediate temperatures (270 and 3008C), the signature of grain boundaries is not prominent in the spectra. The films obviously contain a large volume fraction of amorphous matrix, along with comparatively ordered regions, considered as crystallites, which, however, reflect the very low crystalline volume fraction of the films. 3.1.2. FTIR spectra The Fourier-transform infrared absorption spectra are shown in Fig. 2 for the stretching mode of vibration. Here also, the main curves have been fitted to their best profile fitting the deconvoluted curves. In Fig. 2a, there is only one peak at 2000 cmy1 for the film deposited at 2008C. At 2708C, the total peak profile can be deconvoluted into two peaks at 2000 and 2100 cmy1. However, the intensity of the higher frequency peak is much less compared to that at the lower frequency, as shown in Fig. 2b. The doublet formation is retained for the films deposited at higher substrate temperatures (Fig.
Fig. 2. FTIR spectra of the films deposited at a substrate temperature of: (a) 200; (b) 270; (c) 300; and (d) 3408C. Deconvolutions are shown by broken curves.
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3.2. TEM studies The morphology of the samples was studied by transmission electron microscopy (TEM) and transmission electron diffraction (TED) patterns. Fig. 3a,b shows TEM and TED micrographs, respectively, taken for the sample deposited at 3008C. Very tiny and closely spaced grains are observed in the TEM, which are distributed ˚ uniformly. The average size of the grains is ;50 A. Some elongated grains are also apparent on the surface. In the TED pattern, clear halos corresponding to 111, 220 and 311 crystalline silicon planes indicate the very large volume fraction of amorphous matrix in the film. 4. Transport properties 4.1. Electrical conductivity
Fig. 3. TEM pictures of the film deposited at 3008C: (a) cross-sectional view of TEM; and (b) transmission electron diffraction.
2c,d), and the contribution from the higher frequency peak increases. The stretching-mode peak positions can reveal information about the nature of hydrogen bonding with silicon in the films. Generally, in IR spectra, the peak at 2000 cmy1 is considered to arise from the stretching mode of vibration of Si–H-type bonds, whereas the peak at 2100 cmy1 is due to that of Si–H2 andyor Si–Hn (n)2) types of bonds w11x. From the above-discussed analysis of IR spectra, the dominance of monohydride bonding is observed for all the films deposited at various substrate temperatures. At lower temperature (2008C), the bonds are completely monohydride in nature, where the film is completely amorphous as well. Even for the films with higher crystallinity deposited at the higher temperature at 3408C, the monohydride nature is predominant. However, the di andyor higher hydrides have a notable contribution at this stage. It is interesting to note that for the films deposited in the transition region (at 270 and 3008C), the di andyor higher hydrides content is very low. It should be mentioned here that Si–H2 andyor Si–Hn (n)2) types of bonds are commonly formed at the grain boundaries, the main constituents of which are defects, and distorted and strained bonds w13x. The contribution from the grain boundaries is negligibly small in the films deposited at 270 and 3008C. The effect of grain boundaries is pronounced at higher temperature (340 and 3708C), where the crystallinity is comparatively high.
The structural properties of the films are correlated with their transport properties. Fig. 4 shows the variation in dark conductivity of the films deposited at various substrate temperatures. The low values of dark conductivity (;10y10 S cmy1) at lower substrate temperature (F2408C) indicate the amorphous nature of the films. At the transition region, the films have a higher order of dark conductivity (;10y8 S cmy1) than typical amorphous films, which is an outcome of the improvement in structural order of the films. The films deposited at higher substrate temperatures (G3408C) show high values for dark conductivity (;10y5 S cmy1), since these films are microcrystalline with a higher crystalline volume fraction. The photosensitivity (defined as the ratio of the electrical conductivity with and without illumination, G) of the films is also shown in Fig. 4 as a function of the substrate temperature. It offers one of the most interesting results in the present study. High values of photosensitivity (;105) for the amorphous films deposited at lower substrate temperature (F2408C) and low values (;100) for the microcrystalline films deposited at higher
Fig. 4. Variation in dark conductivity and photosensitivity of the films as a function of substrate temperature. The encircled portion approximately indicates the amorphous-to-microcrystalline transition region in the present study.
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C. Das, S. Ray / Thin Solid Films 403 – 404 (2002) 81–85
Fig. 5. The changes in photosensitivity and photosensitivity after 103 h of light soaking are plotted for the films deposited at various substrate temperatures. The dotted lines are guides for the eye.
substrate temperature (G3408C) are quite typical. However, the films deposited at the transition region have a higher value for dark conductivity than amorphous films, but are still photosensitive (G;103 ). This is a necessary requirement for undoped films to be used as an active layer in solar cells. This notable photosensitivity and higher dark conductivity may be attributed to the presence of nanocrystalline silicon in the high volume fraction of amorphous matrix in the film. 4.2. Light-induced degradation It is important to study light-induced degradation of the films deposited at different substrate temperatures. When exposed to illumination, photosensitivity (G) of the film degrades to different extents, depending on the crystallinity of the film w14x. In Fig. 5, changes in photosensitivity from its initial value after 103 h of light soaking are plotted for different substrate temperatures. The amorphous films deposited at lower substrate temperatures degrade to a great extent. They do not attain a stable value, even after 103 h. The microcrystalline samples prepared at 340 and 3708C, with different volume fraction of crystallinity (42 and 65%) degrade very little, but they are not photosensitive. Degradation of the film deposited at 3008C is also very low. However, the photosensitivity after degradation is reasonably high (102). These properties make the sample prepared at 3008C suitable for application as the active layer in solar cells. 5. Conclusions The structural and transport properties of silicon films deposited with different substrate temperatures have
been investigated. The transport properties of the films are a perfect reflection of their structural properties. The evolution of a microcrystalline structure from a completely amorphous structure has been observed. The film prepared in the transition region contains small crystal˚ embedded in an amorphous matrix. Mainly lites (50 A) monohydride bonds of hydrogen are observed in the matrix and the grain boundaries are not prominent. The films have notable photosensitivity compared to microcrystalline films. In addition to these properties, the films have higher stability under light exposure than amorphous films. This can be attributed to the presence of small crystallites and a better structural relaxation in the film. These nanocrystalline films have high potential for application in solar cells. References w1x J. Meier, P. Torres, R. Platz, S. Dubail, U. Kroll, J.A. Anna Selvan, N. Pellaton Vaucher, C. Hof, D Fischer, H. Keppner, A. Shah, K.D. Ufert, P. Giannoules, J. Koehler, Mater. Res. Soc. Symp. Proc. 420 (1996) 3. w2x S.C. Saha, S. Ray, J. Appl. Phys. 78 (1995) 5713. w3x R.A. Street, Hydrogenated Amorphous Silicon, Cambridge University Press, Cambridge, 1991. w4x C.C. Tsai, in: H. Frittzsche (Ed.), Amorphous Silicon and Related Materials, World Scientific, Singapore, 1998, pp. 123– 147. w5x D.L. Staebler, C.R. Wronski, Appl. Phys. Lett. 31 (1977) 292. w6x R.J. Koval, J. Koh, Z. Lu, R.W. Collins, C.R. Wronski, G. Ganguly, Technical Digest of the 11th Photovoltaic Scientist and Engineers Conference, Sapporo, Japan, 1999, p. 217. w7x K. Saitoh, N. Ishiguro, N. Yanagawa, H. Tanaka, S. Fukuda, M. Sadamato, Y. Ashida, N. Fukuda, J. Non-Cryst. Solids 198-200 (1996) 1093. w8x O. Vetterl, F. Finger, R. Carius, P. Hapke, L. Houben, O. Kluth, A. Lambertz, A. Muck, B. Rech, H. Wagner, Sol. Energy Mater. Sol. Cells 62 (2000) 97.
C. Das, S. Ray / Thin Solid Films 403 – 404 (2002) 81–85 w9x M. Vieira, A. Fantoni, M. Fernandes, R. Schwarz, Philos. Mag. B 80 (2000) 755. w10x G. Ambrosone, G. Bruno, P. Capezzuto, G. Cicala, U. Coscia, Philos. Mag. B 80 (2000) 487. w11x M.H. Brodsky, M. Cardona, J.J. Cuomo, Phys. Rev. B 16 (1977) 3556.
85
w12x T. Kaneko, M. Wakagi, K. Onisawa, T. Minemura, Appl. Phys. Lett. 64 (1994) 1865. w13x S. Veprek, F.A. Sarrot, Z. Iqbal, Phys. Rev. B 36 (1987) 3344. w14x M. Isomura, N. Hata, S. Wagner, Jpn. J. Appl. Phys. 31 (1992) 3500.