- Email: [email protected]
High-rate deposition of highly crystallized silicon films from inductively coupled plasma
Thin Solid Films 435 (2003) 39–43
High-rate deposition of highly crystallized silicon films from inductively coupled plasma Nihan Kosku*, Fumitada Kurisu, Miwako Takegoshi, Hiroshi Takahashi, Seiichi Miyazaki Department of Electrical Engineering, Graduate School of Advanced Sciences of Matter, Hiroshima University, Kagamiyama 1-3-1, Higashi-Hiroshima 739-8530, Japan
Abstract We have studied microcrystalline silicon (mc-Si) film growth from inductively coupled RF plasma (ICP) of monosilane (SiH4) and hydrogen gas mixtures and demonstrated the feasibility of ICP for a high rate growth of highly crystallized Si films at a temperature as low as 250 8C. Using 15% SiH4 diluted with H2 , a deposition rate of ;1.0 nm sy1 was achieved for crystalline films, for which Raman scattering spectra show the TO phonon peak intensity ratio for the crystallineydisordered phase is )5 for film thickness )1 mm. By measuring optical emission from the SiH4 ICP, we suggest that the relative flux of hydrogen radicals to the growing film surface with respect to the flux of film precursors during monatomic layer growth is a key parameter in obtaining highly crystallized films at a higher growth rate. Strong influence of the crystallinity on the optical and electrical properties has also been confirmed for films prepared with different SiH4 concentrations or different film thickness. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Inductively coupled plasma; Microcrystalline silicon; High-rate deposition; Optical emission spectroscopy
1. Introduction Microcrystalline silicon-based (mc-Si) thin films are attracting much attention because of their potential importance in further improving the device performance of amorphous Si-based (a-Si) thin-film transistors and solar cells w1,2x. In particular, requirements for stable and high-efficiency solar cells, which are needed to overcome the light-induced degradation observed a-Si based films and to improve carrier transport and collection, led us to study mc-Si:H intensively w3x. For the application of mc-Si-based films to such large-area electronic devices, one of the major research issues is to increase the deposition rate as high as possible, while keeping the crystallinity high at substrate temperatures lower than 250 8C. To date, mc-Si–H deposition at rates higher than ;2 nm sy1 has been demonstrated using a very-high-frequency (VHF; 60 MHz) plasma w4x and a high-density microwave plasma w5x. In addition to these methods, the use of an inductively coupled plasma (ICP) w6x is another promising method because a uniform and high-density plasma is sustained under a relatively low *Corresponding author. E-mail address: [email protected] (N. Kosku).
pressure at low substrate temperature without any external magnetic field. It has recently been demonstrated that, from a modified ICP of H2-diluted SiH4, highly crystallized Si films can be prepared at temperatures below 300 8C at a deposition rate of ;0.15 nm sy1. In that regard, uniform and high-rate deposition of highly crystallized Si films from ICP is still a matter of research at temperatures below 250 8C. Previously, we reported that from a pure SiH4 RFICP, device-quality a-Si:H films could be prepared at 150 8C at a deposition rate of approximately 4 nm sy1 w7x and found that the distance between the RF antenna and the substrate is one of the crucial factors for highrate deposition with a low defect density. In this work we extended our research to the high-rate deposition of highly crystallized films from H2-diluted SiH4 RF-ICP. We have studied how the SiH4 concentration, gas pressure and distance between the RF antenna and the substrate influence the deposition rate, crystallinity, and optical and electrical properties of the films. In addition, optical emission spectroscopy (OES) of the ICP plasma was carried out to gain a better understanding of the flux of atomic hydrogen and precursors incident to the growing film surface.
0040-6090/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0040-6090(03)00374-2
40
N. Kosku et al. / Thin Solid Films 435 (2003) 39–43
Fig. 1. Raman scattering spectra for ;1-mm-thick Si:H films grown from ICP with different SiH4 concentrations. The distance between the antenna and the substrate, the RF power and the pressure were kept constant at 45 mm, 3.1 W cmy2 and 60 mTorr, respectively.
2. Experimental An external single-turn antenna, which was connected to a 13.56-MHz generator through a matching box, was placed on a quartz cooling tube in contact with a quartz plate attached to a deposition chamber w8x. In the ICP system, 0.27–1.6-mm-thick Si:H films were deposited on quartz, Cr-evaporated Si(100) and H-terminated Si(100) substrates at 250 8C. At a constant gas flow rate of 110 sccm, the SiH4 concentration {wSiH4x y (wH2xqwSiH4 x)} and the distance between the antenna and the grounded substrate susceptor were varied in the ranges of 5–25% and 40–60 mm, respectively. The gas pressure and RF power were changed in the ranges of 60–90 mTorr and 0.9–3.1 W cmy2, respectively. For direct characterization of the network structure of the films thus prepared, Raman scattering spectra were measured under a right-angle scattering geometry, in which a p-polarized 441.6-nm light from a He–Cd laser was incident to the sample surface in an Ar atmosphere at a glancing angle of approximately 108. Further structural characterization was performed using a transmission electron microscope (TEM) system operated at 200 kV. Optical emission spectroscopy (OES) was employed in the wavelength range of 200–700 nm to diagnose the ICP and ascertain ways of increasing the growth rate without degrading the crystallinity.
which is characteristic of the TO phonon mode in the crystalline phase, as represented in Fig. 1. Obviously, highly crystallized Si films are obtained in the SiH4 concentration range of 8–15%, and at higher and lower SiH4 concentrations the growth of the disordered phase becomes significant. This result is quite different from the growth of mc-Si:H film in capacitively coupled plasma (CCP) of SiH4, for which H2 dilution higher than 0.5% is favorable for microcrystallization w9x. Considering the fact that for SiH4 concentrations below 7%, an increase in the deposition rate is rather small (Fig. 2), ion-assisted etching reactions due to atomic hydrogen and energetic ions are responsible for the poor crystallinity at such low SiH4 concentrations, because H2 and SiH4 dissociation in the ICP is thought to be more efficient than that in the CCP generated at a higher pressure, as discussed later. For SiH4 concentrations above 15%, the formation of an amorphous network can be interpreted in terms of the flux of atomic hydrogen incident to the growing film surface with respect to the flux of film precursors being insufficient to promote structural relaxation for microcrystallite formation, as suggested from the OES measurements (also discussed later). The evolution of the crystalline phase with progressive film growth was examined by Raman scattering measurements and cross-sectional TEM observations, as shown in Fig. 3. The result indicates that an amorphous incubation layer with a thickness of 0.2 mm is formed on quartz before the microcrystallites start to grow. After ;1.0-mm-thick film growth, microcrystallites are compacted and a highly crystallized network is formed in the surface region. To obtain clear insight into the microcrystalline film growth, we monitored radiative species such as atomic
3. Results and discussion In the case of the SiH4 concentration ranging from 5 to 15%, Raman scattering spectra for ;1-mm-thick films show a strong feature peaked at ;517 cmy1,
Fig. 2. The SiH4 concentration dependence of the deposition rate and crystallinity, indicated by the TO-phonon intensity ratio of the 517cmy1 signal to the 480-cmy1 signal for the films shown in Fig. 1.
N. Kosku et al. / Thin Solid Films 435 (2003) 39–43
41
Fig. 3. Raman scattering spectra for Si:H films grown at 15% SiH4 with different film thickness. A cross-sectional TEM image is also shown in the inset. The other conditions for the ICP are the same as in the cases of Figs. 1 and 2.
Si, SiH, atomic H and H2 from optical emission spectra. From the intense emission due to atomic Si, SiH and H compared with the emission from H2 in the range of 200–350 nm, we confirmed that the dissociation of SiH4 and H2 in ICP is more efficient than in the CCP case. The SiH emission intensity increased linearly with the SiH4 concentration, while the Si emission tends to saturate at SiH4 concentration )10%, as observed in Fig. 4, indicating that SiH emission is a good indicator of the deposition rate, as in the case of deposition from conventional SiH4 RF-CCP. In other words, reactive species generated by one-electron impact, as in the case of SiH, play an important role in film growth. As for the emission intensity of Ha, it decreases with increasing SiH4 concentration up to 15%, and then tends to increase again at higher SiH4 concentrations. The reduction observed in Ha for SiH4 concentrations below 15% is attributable to a decrease in H2 concentration in the source gas. In contrast, a slight increase in Ha for higher SiH4 concentrations can be interpreted in terms of an increase in the contribution of SiH4 dissociation to generate atomic hydrogen. Since the emission intensity of Hb is almost constant, the Hb yHa ratio, approximately corresponding to the electron temperature, reaches as maximum at a SiH4 concentration of 15%. The dramatic suppression of crystallization for SiH4 concentrations ) 15% is likely to be related in part to this decrease in electron temperature. For SiH4 concentration higher than 10%, the importance of the incidence of hydrogen radicals to the growing film surface on microcrystallization is realized when the crystallinity evaluated from the Raman scat-
tering ratio of crystalline TO phonons to amorphous ones is plotted as a function of wHax y wSiHx normalized by the deposition rate (Fig. 5); the value reflects the relative flux of H radicals to the flux of film precursors during monatomic layer growth, also expressed by the ratio of wHax y wSiHx2 . When the (wHax y wSiHx)y(growth rate) ratio is less than 0.2, nucleation and growth of microcrystallites hardly occur and we can suggest that
Fig. 4. The emission intensity of Si (288 nm), SiH (413 nm) and H2 (;250 nm), and the ratio of Hb (486 nm)yHa (656 nm) as a function of the SiH4 concentration at 3.1 W cmy2. The antenna– substrate distance and gas pressure were kept constant at 45 mm and 60 mTorr, respectively.
42
N. Kosku et al. / Thin Solid Films 435 (2003) 39–43
Fig. 5. The TO-phonon peak intensity ratio of crystallineyamorphous phase as a function of the emission intensity ratio of Ha (656 nm)ySiH (413 nm) normalized by the deposition rate.
the incidence of hydrogen radicals to the growing film surface plays an important role in microcrystallization at a deposition rate higher than 0.3 nm sy1. To gain a better understanding of the degrading crystallinity with decreasing SiH4 concentration below 10%, the influence of the RF antenna–substrate distance on the deposition rate and film crystallinity was examined. Under ICP conditions of 12% SiH4 at 2.6 W cmy2 and 60 mTorr, when the antenna–substrate distance is decreased from 50 to 40 mm, the deposition rate is decreased by a factor of eight and the crystallinity determined from the TO-phonon intensity ratio for crystalline and amorphous phases is markedly degraded. For antenna–substrate distance of 40 mm, an amorphous network is predominantly formed. For antenna–substrate distance greater than 50 mm, no significant increase in the deposition rate is observed. Considering the fact that the mean-free-path length of atomic hydrogen at 60 mTorr is of the order of 2 mm, these results indicate that an ion-assisted etching reaction becomes significant for antenna–substrate distance of -50 mm. At the fixed antenna–substrate distance of 45 mm, as the pressure of 12% SiH4 is increased from 60 to 90 mTorr with the RF power constant at 2.6 W cmy2, the deposition rate is increased by a factor of three and the crystallinity is increased by ;30%. This result also suggests that, for antenna–substrate distance as short as 45 mm at pressure as low as 60 mTorr, both the deposition rate and crystallinity are suppressed by ion-assisted etching reactions. Figs. 6 and 7 show optical and electrical properties of Si:H films as a function of the SiH4 concentration and film thickness, respectively. As shown in Fig. 6, the
Fig. 6. Optical bandgap Eopt, activation energy Ea, dark conductivity sd and AM1 photoconductivity sp for Si:H films shown in Fig. 1 as a function of SiH4 concentration.
evolution of microcrystallites causes a dramatic decrease in activation energy of the dark conductivity, and a resultant increase in the dark conductivity accompanied by a slightly reducing trend for the optical bandgap. The highly crystallized films prepared with SiH4 concentrations in the range of 10–15% exhibit photoconductivity of ;10y5 S cmy1 and photosensitivity as low as 3 under AM1 (100 mW cmy2) illumination (Fig. 6).
Fig. 7. Optical bandgap Eopt, activation energy Ea, dark conductivity sd and photoconductivity sp of Si:H films shown in Fig. 2 as a function of the film thickness.
N. Kosku et al. / Thin Solid Films 435 (2003) 39–43
Note that for the highly crystallized films thicker than 1 mm, the photosensitivity is markedly improved, as observed in Fig. 7, which is attributed to the further improvement in the crystallinity, namely a reduction in defects in grains andyor at the grain boundaries.
43
in reduced damage and enhanced crystallinity. We have demonstrated that an increase in photosensitivity for mcSi:H films thicker than 1 mm is a good indication of the improved crystallinity. References
4. Conclusions We have demonstrated that the use of ICP is a promising method to achieve high deposition rates of mc-Si:H. Control of the SiH4 concentration is of great importance for higher crystallinity at higher deposition rates. From OES results, we have confirmed that the SiH emission intensity enables us to predict the film deposition rate. From a combination of the results of Raman spectroscopy and OES, we conclude that highly crystallized films at a growth rate as high as ;1.0 nm sy1 can be deposited when the relative flux of H radicals to the flux of film precursors for monatomic layer growth is kept constant. In addition, we have found that the ion-assisted etching reaction on the growing film surface is significantly suppressed by adjusting the pressure and the antenna–electrode distance, resulting
w1x J. Meier, R. Fluckiger, H. Keppner, A. Shah, Appl. Phys. Lett. 65 (1994) 860. w2x R.B. Wehrspohn, S.C. Deane, I.D. French, M.J. Powell, Thin Solid Films 383 (2001) 117. w3x M. Goerlitzer, N. Beck, P. Torres, U. Kroll, H. Keppner, J. Meier, J. Koehler, N. Wyrsch, A. Shah, Mater. Res. Soc. Symp.Proc. 467 (1997) 301. w4x M. Kondo, M. Fukawa, L. Guo, A. Matsuda, J. Non-Cryst. Solids 266-269 (2000) 84. w5x Y. Sakuma, L. Haiping, H. Ueyama, H. Shirai, Thin Solid Films 386 (2001) 261. w6x K. Goshima, H. Toyoda, T. Kojima, M. Nishitani, M. Kitagawa, H. Yamazoe, H. Sugai, Jpn. J. Appl. Phys. 38 (1999) 3655. w7x N. Sakikawa, Y. Shishida, S. Miyazaki, M. Hirose, Sol. Energy Mater. Sol. Cells 66 (2001) 337–343. w8x Y. Okazaki, S. Miyazaki, M. Hirose, J. Non-Cryst. Solids 266– 269 (2000) 54. w9x S. Miyazaki, Y. Osaka, M. Hirose, Sol. Energy Mater. 11 (1984) 85.
High-rate deposition of highly crystallized silicon films from inductively coupled plasma Nihan Kosku*, Fumitada Kurisu, Miwako Takegoshi, Hiroshi Takahashi, Seiichi Miyazaki Department of Electrical Engineering, Graduate School of Advanced Sciences of Matter, Hiroshima University, Kagamiyama 1-3-1, Higashi-Hiroshima 739-8530, Japan
Abstract We have studied microcrystalline silicon (mc-Si) film growth from inductively coupled RF plasma (ICP) of monosilane (SiH4) and hydrogen gas mixtures and demonstrated the feasibility of ICP for a high rate growth of highly crystallized Si films at a temperature as low as 250 8C. Using 15% SiH4 diluted with H2 , a deposition rate of ;1.0 nm sy1 was achieved for crystalline films, for which Raman scattering spectra show the TO phonon peak intensity ratio for the crystallineydisordered phase is )5 for film thickness )1 mm. By measuring optical emission from the SiH4 ICP, we suggest that the relative flux of hydrogen radicals to the growing film surface with respect to the flux of film precursors during monatomic layer growth is a key parameter in obtaining highly crystallized films at a higher growth rate. Strong influence of the crystallinity on the optical and electrical properties has also been confirmed for films prepared with different SiH4 concentrations or different film thickness. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Inductively coupled plasma; Microcrystalline silicon; High-rate deposition; Optical emission spectroscopy
1. Introduction Microcrystalline silicon-based (mc-Si) thin films are attracting much attention because of their potential importance in further improving the device performance of amorphous Si-based (a-Si) thin-film transistors and solar cells w1,2x. In particular, requirements for stable and high-efficiency solar cells, which are needed to overcome the light-induced degradation observed a-Si based films and to improve carrier transport and collection, led us to study mc-Si:H intensively w3x. For the application of mc-Si-based films to such large-area electronic devices, one of the major research issues is to increase the deposition rate as high as possible, while keeping the crystallinity high at substrate temperatures lower than 250 8C. To date, mc-Si–H deposition at rates higher than ;2 nm sy1 has been demonstrated using a very-high-frequency (VHF; 60 MHz) plasma w4x and a high-density microwave plasma w5x. In addition to these methods, the use of an inductively coupled plasma (ICP) w6x is another promising method because a uniform and high-density plasma is sustained under a relatively low *Corresponding author. E-mail address: [email protected] (N. Kosku).
pressure at low substrate temperature without any external magnetic field. It has recently been demonstrated that, from a modified ICP of H2-diluted SiH4, highly crystallized Si films can be prepared at temperatures below 300 8C at a deposition rate of ;0.15 nm sy1. In that regard, uniform and high-rate deposition of highly crystallized Si films from ICP is still a matter of research at temperatures below 250 8C. Previously, we reported that from a pure SiH4 RFICP, device-quality a-Si:H films could be prepared at 150 8C at a deposition rate of approximately 4 nm sy1 w7x and found that the distance between the RF antenna and the substrate is one of the crucial factors for highrate deposition with a low defect density. In this work we extended our research to the high-rate deposition of highly crystallized films from H2-diluted SiH4 RF-ICP. We have studied how the SiH4 concentration, gas pressure and distance between the RF antenna and the substrate influence the deposition rate, crystallinity, and optical and electrical properties of the films. In addition, optical emission spectroscopy (OES) of the ICP plasma was carried out to gain a better understanding of the flux of atomic hydrogen and precursors incident to the growing film surface.
0040-6090/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0040-6090(03)00374-2
40
N. Kosku et al. / Thin Solid Films 435 (2003) 39–43
Fig. 1. Raman scattering spectra for ;1-mm-thick Si:H films grown from ICP with different SiH4 concentrations. The distance between the antenna and the substrate, the RF power and the pressure were kept constant at 45 mm, 3.1 W cmy2 and 60 mTorr, respectively.
2. Experimental An external single-turn antenna, which was connected to a 13.56-MHz generator through a matching box, was placed on a quartz cooling tube in contact with a quartz plate attached to a deposition chamber w8x. In the ICP system, 0.27–1.6-mm-thick Si:H films were deposited on quartz, Cr-evaporated Si(100) and H-terminated Si(100) substrates at 250 8C. At a constant gas flow rate of 110 sccm, the SiH4 concentration {wSiH4x y (wH2xqwSiH4 x)} and the distance between the antenna and the grounded substrate susceptor were varied in the ranges of 5–25% and 40–60 mm, respectively. The gas pressure and RF power were changed in the ranges of 60–90 mTorr and 0.9–3.1 W cmy2, respectively. For direct characterization of the network structure of the films thus prepared, Raman scattering spectra were measured under a right-angle scattering geometry, in which a p-polarized 441.6-nm light from a He–Cd laser was incident to the sample surface in an Ar atmosphere at a glancing angle of approximately 108. Further structural characterization was performed using a transmission electron microscope (TEM) system operated at 200 kV. Optical emission spectroscopy (OES) was employed in the wavelength range of 200–700 nm to diagnose the ICP and ascertain ways of increasing the growth rate without degrading the crystallinity.
which is characteristic of the TO phonon mode in the crystalline phase, as represented in Fig. 1. Obviously, highly crystallized Si films are obtained in the SiH4 concentration range of 8–15%, and at higher and lower SiH4 concentrations the growth of the disordered phase becomes significant. This result is quite different from the growth of mc-Si:H film in capacitively coupled plasma (CCP) of SiH4, for which H2 dilution higher than 0.5% is favorable for microcrystallization w9x. Considering the fact that for SiH4 concentrations below 7%, an increase in the deposition rate is rather small (Fig. 2), ion-assisted etching reactions due to atomic hydrogen and energetic ions are responsible for the poor crystallinity at such low SiH4 concentrations, because H2 and SiH4 dissociation in the ICP is thought to be more efficient than that in the CCP generated at a higher pressure, as discussed later. For SiH4 concentrations above 15%, the formation of an amorphous network can be interpreted in terms of the flux of atomic hydrogen incident to the growing film surface with respect to the flux of film precursors being insufficient to promote structural relaxation for microcrystallite formation, as suggested from the OES measurements (also discussed later). The evolution of the crystalline phase with progressive film growth was examined by Raman scattering measurements and cross-sectional TEM observations, as shown in Fig. 3. The result indicates that an amorphous incubation layer with a thickness of 0.2 mm is formed on quartz before the microcrystallites start to grow. After ;1.0-mm-thick film growth, microcrystallites are compacted and a highly crystallized network is formed in the surface region. To obtain clear insight into the microcrystalline film growth, we monitored radiative species such as atomic
3. Results and discussion In the case of the SiH4 concentration ranging from 5 to 15%, Raman scattering spectra for ;1-mm-thick films show a strong feature peaked at ;517 cmy1,
Fig. 2. The SiH4 concentration dependence of the deposition rate and crystallinity, indicated by the TO-phonon intensity ratio of the 517cmy1 signal to the 480-cmy1 signal for the films shown in Fig. 1.
N. Kosku et al. / Thin Solid Films 435 (2003) 39–43
41
Fig. 3. Raman scattering spectra for Si:H films grown at 15% SiH4 with different film thickness. A cross-sectional TEM image is also shown in the inset. The other conditions for the ICP are the same as in the cases of Figs. 1 and 2.
Si, SiH, atomic H and H2 from optical emission spectra. From the intense emission due to atomic Si, SiH and H compared with the emission from H2 in the range of 200–350 nm, we confirmed that the dissociation of SiH4 and H2 in ICP is more efficient than in the CCP case. The SiH emission intensity increased linearly with the SiH4 concentration, while the Si emission tends to saturate at SiH4 concentration )10%, as observed in Fig. 4, indicating that SiH emission is a good indicator of the deposition rate, as in the case of deposition from conventional SiH4 RF-CCP. In other words, reactive species generated by one-electron impact, as in the case of SiH, play an important role in film growth. As for the emission intensity of Ha, it decreases with increasing SiH4 concentration up to 15%, and then tends to increase again at higher SiH4 concentrations. The reduction observed in Ha for SiH4 concentrations below 15% is attributable to a decrease in H2 concentration in the source gas. In contrast, a slight increase in Ha for higher SiH4 concentrations can be interpreted in terms of an increase in the contribution of SiH4 dissociation to generate atomic hydrogen. Since the emission intensity of Hb is almost constant, the Hb yHa ratio, approximately corresponding to the electron temperature, reaches as maximum at a SiH4 concentration of 15%. The dramatic suppression of crystallization for SiH4 concentrations ) 15% is likely to be related in part to this decrease in electron temperature. For SiH4 concentration higher than 10%, the importance of the incidence of hydrogen radicals to the growing film surface on microcrystallization is realized when the crystallinity evaluated from the Raman scat-
tering ratio of crystalline TO phonons to amorphous ones is plotted as a function of wHax y wSiHx normalized by the deposition rate (Fig. 5); the value reflects the relative flux of H radicals to the flux of film precursors during monatomic layer growth, also expressed by the ratio of wHax y wSiHx2 . When the (wHax y wSiHx)y(growth rate) ratio is less than 0.2, nucleation and growth of microcrystallites hardly occur and we can suggest that
Fig. 4. The emission intensity of Si (288 nm), SiH (413 nm) and H2 (;250 nm), and the ratio of Hb (486 nm)yHa (656 nm) as a function of the SiH4 concentration at 3.1 W cmy2. The antenna– substrate distance and gas pressure were kept constant at 45 mm and 60 mTorr, respectively.
42
N. Kosku et al. / Thin Solid Films 435 (2003) 39–43
Fig. 5. The TO-phonon peak intensity ratio of crystallineyamorphous phase as a function of the emission intensity ratio of Ha (656 nm)ySiH (413 nm) normalized by the deposition rate.
the incidence of hydrogen radicals to the growing film surface plays an important role in microcrystallization at a deposition rate higher than 0.3 nm sy1. To gain a better understanding of the degrading crystallinity with decreasing SiH4 concentration below 10%, the influence of the RF antenna–substrate distance on the deposition rate and film crystallinity was examined. Under ICP conditions of 12% SiH4 at 2.6 W cmy2 and 60 mTorr, when the antenna–substrate distance is decreased from 50 to 40 mm, the deposition rate is decreased by a factor of eight and the crystallinity determined from the TO-phonon intensity ratio for crystalline and amorphous phases is markedly degraded. For antenna–substrate distance of 40 mm, an amorphous network is predominantly formed. For antenna–substrate distance greater than 50 mm, no significant increase in the deposition rate is observed. Considering the fact that the mean-free-path length of atomic hydrogen at 60 mTorr is of the order of 2 mm, these results indicate that an ion-assisted etching reaction becomes significant for antenna–substrate distance of -50 mm. At the fixed antenna–substrate distance of 45 mm, as the pressure of 12% SiH4 is increased from 60 to 90 mTorr with the RF power constant at 2.6 W cmy2, the deposition rate is increased by a factor of three and the crystallinity is increased by ;30%. This result also suggests that, for antenna–substrate distance as short as 45 mm at pressure as low as 60 mTorr, both the deposition rate and crystallinity are suppressed by ion-assisted etching reactions. Figs. 6 and 7 show optical and electrical properties of Si:H films as a function of the SiH4 concentration and film thickness, respectively. As shown in Fig. 6, the
Fig. 6. Optical bandgap Eopt, activation energy Ea, dark conductivity sd and AM1 photoconductivity sp for Si:H films shown in Fig. 1 as a function of SiH4 concentration.
evolution of microcrystallites causes a dramatic decrease in activation energy of the dark conductivity, and a resultant increase in the dark conductivity accompanied by a slightly reducing trend for the optical bandgap. The highly crystallized films prepared with SiH4 concentrations in the range of 10–15% exhibit photoconductivity of ;10y5 S cmy1 and photosensitivity as low as 3 under AM1 (100 mW cmy2) illumination (Fig. 6).
Fig. 7. Optical bandgap Eopt, activation energy Ea, dark conductivity sd and photoconductivity sp of Si:H films shown in Fig. 2 as a function of the film thickness.
N. Kosku et al. / Thin Solid Films 435 (2003) 39–43
Note that for the highly crystallized films thicker than 1 mm, the photosensitivity is markedly improved, as observed in Fig. 7, which is attributed to the further improvement in the crystallinity, namely a reduction in defects in grains andyor at the grain boundaries.
43
in reduced damage and enhanced crystallinity. We have demonstrated that an increase in photosensitivity for mcSi:H films thicker than 1 mm is a good indication of the improved crystallinity. References
4. Conclusions We have demonstrated that the use of ICP is a promising method to achieve high deposition rates of mc-Si:H. Control of the SiH4 concentration is of great importance for higher crystallinity at higher deposition rates. From OES results, we have confirmed that the SiH emission intensity enables us to predict the film deposition rate. From a combination of the results of Raman spectroscopy and OES, we conclude that highly crystallized films at a growth rate as high as ;1.0 nm sy1 can be deposited when the relative flux of H radicals to the flux of film precursors for monatomic layer growth is kept constant. In addition, we have found that the ion-assisted etching reaction on the growing film surface is significantly suppressed by adjusting the pressure and the antenna–electrode distance, resulting
w1x J. Meier, R. Fluckiger, H. Keppner, A. Shah, Appl. Phys. Lett. 65 (1994) 860. w2x R.B. Wehrspohn, S.C. Deane, I.D. French, M.J. Powell, Thin Solid Films 383 (2001) 117. w3x M. Goerlitzer, N. Beck, P. Torres, U. Kroll, H. Keppner, J. Meier, J. Koehler, N. Wyrsch, A. Shah, Mater. Res. Soc. Symp.Proc. 467 (1997) 301. w4x M. Kondo, M. Fukawa, L. Guo, A. Matsuda, J. Non-Cryst. Solids 266-269 (2000) 84. w5x Y. Sakuma, L. Haiping, H. Ueyama, H. Shirai, Thin Solid Films 386 (2001) 261. w6x K. Goshima, H. Toyoda, T. Kojima, M. Nishitani, M. Kitagawa, H. Yamazoe, H. Sugai, Jpn. J. Appl. Phys. 38 (1999) 3655. w7x N. Sakikawa, Y. Shishida, S. Miyazaki, M. Hirose, Sol. Energy Mater. Sol. Cells 66 (2001) 337–343. w8x Y. Okazaki, S. Miyazaki, M. Hirose, J. Non-Cryst. Solids 266– 269 (2000) 54. w9x S. Miyazaki, Y. Osaka, M. Hirose, Sol. Energy Mater. 11 (1984) 85.