- Email: [email protected]
Particle formation and a-Si:H film deposition in narrow-gap RF plasma CVD
Thin Solid Films 345 (1999) 80±84
Particle formation and a-Si:H ®lm deposition in narrow-gap RF plasma CVD Yoko Maemura a,*, Hiroshi Fujiyama a, Tomoko Takagi b, Ryo Hayashi c, Wataru Futako b, Michio Kondo b, Akihisa Matsuda b b
a Department of Electrical Engineering, Nagasaki university, 1-14 Bunkyo-machi, Nagasaki-shi, 852-8521, Japan Thin Film Silicon Solar Cells Super Lab. Electrotechnical Lab., 1-1-4 Umezono, Tsukuba-shi, Ibaraki 305-8568, Japan c Canon Inc., Ecology R&D Center, 4-1-1 Kizugawadai, Kizu-cho, Souraku-gun, Kyoto 619-0281, Japan
Abstract The effects of electrode distance are discussed in a diode type plasma enhanced chemical vapor deposition (PECVD) system as an important external control parameter for the preparation of hydrogenated amorphous silicon (a-Si:H) using an RF silane (SiH4) plasma. The electron temperature is increased by shortening the electrode distance due to a plasma self-organization mechanisms, leading to an increase in the growth rate of a-Si:H ®lms. Furthermore, shortening the distance between the heated electrodes (anode and/or cathode) gives rise to decrease in SiH4 density in the discharge space near the electrodes resulting in suppression of the particle formation. Through the control of particle formation by changing the electrode distance, the defect properties in the resulting ®lms are successfully controlled. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Plasma enhanced chemical vapor deposition; Amorphous silicon; Silane
1. Introduction Hydrogenated amorphous silicon (a-Si:H) grown by silane (SiH4) plasma is an important material for their applications to large area, thin ®lm devices, and it is therefore necessary for the increase in the production throughput to achieve higher deposition rate. The deposition rate of a-Si:H is increased with an increase in the partial pressure of silane and/or the RF power density, however, the particle formation due to the gas phase polymerization occurs, which results in an overall reduction in the ®lm opto-electronic properties and the deterioration of the device performance. It is therefore important to suppress the generation and growth of harmful particles, and extensive studies have been performed with respect to particle suppression by electrode heating [1,2], modulation of RF discharge [3±6] and so on. In this work, we investigated the effects of electrode distance on the particle formation, the plasma parameters such as electron temperature and electron density, and the properties of the resulting ®lms, and suggested the electrode distance as an effective external control parameter for the
* Corresponding author. E-mail address: [email protected] (Y. Maemura)
suppression of particle formation at high deposition rate. Furthermore, from a viewpoint of surface reaction kinetics [7,8], these results are useful to understand the relationship between the defect density in the resulting a-Si:H ®lms and the particles and/or radicals related to higher order silanes.
2. Experimental The CVD system used in this study is a conventional diode type reactor equipped with two parallel electrodes of 10 cm in diameter as shown in Fig. 1, together with the setup for the Langmuir probe and the particle measurements. The distance between the electrodes was changed between 2.5 and 4 cm. Either Ar or silane (SiH4) plasma was generated by RF (13.56 MHz) power. Corning 7059 glass and crystalline silicon substrates were placed on the grounded electrode (anode), and the temperature of the substrate was changed from room temperature up to 3508C. The electron temperature and the electron density for different electrode distances were measured by a cylindrical Langmuir probe in order to understand the correlation between the electrode distance and the plasma parameters. The Langmuir probe was located at a distance of 1 cm from the substrates. The probe diagnostics were performed for
0040-6090/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S00 40-6090(99)0010 0-5
Y. Maemura et al. / Thin Solid Films 345 (1999) 80±84
81
Fig. 1. Experimental set-up for PECVD, probe measurement and LMS observation.
pure Ar and pure SiH4 plasma at the substrate temperature of room temperature. For Ar plasma, the pressure was set at 20 and 100 mTorr, and the RF power density at 60, 190 and 320 mW/cm 2. The probe measurements in SiH4 plasma were performed only under a condition with no serious particle formation, that is at 20 mTorr and 127 mW/cm 2. The particle in the discharge space near the cathode was measured by laser Mie scattering light (LMS) method using a He-Ne laser (5 mW at 632.8 nm). A sheet beam type laser of 15 mm in width and 1 mm in thickness, introduced into the chamber from a view port, was passed through the discharge space near the cathode with a clearance between the beam and cathode of 1 mm. Mie scattering light from particles was detected by a CCD camera with an interference ®lter (center wave length 632.8 nm, full width of 1 nm at half maximum), which were set at an angle of 458 to the incident beam. For the deposition of a-Si:H, pure SiH4 gas with a constant ¯ow rate of 30 sccm was introduced and the pressure was kept constant at 100 mTorr. The ®lms were deposited with the electrode distance of 2.5 and 4 cm. The ®lm thickness, optical gap, refractive index, and the electronic conductivities were measured for the ®lms on the glass substrates.
Fig. 2. Electron temperature, Te, (a) and electron density, ne, (b) versus electrode distances (ED), as a parameter of RF power density.
Fig. 3. Spatially integrated Mie scattering int for different electrode distances of 4 and 2.5 cm at different substrate temperatures, Ts.
3. Results and discussion Fig. 2a,b show the electrode distance dependence of the electron temperature and the electron density, respectively, in pure Ar plasma for different RF power densities at 20 and 100 mTorr. A decrease in the electrode distance enhances the electron temperature for both pressures as shown in Fig. 2a. Similar tendency was also seen in silane plasma (open circles). Shortening the electrode distance leads to an increase in the electron temperature since the plasma self-organization mechanisms make up the loss of electrons to the wall (electrodes). On the other hand, Fig. 2b shows that the electron density is rather independent of the electrode distance and it increases with RF power density. Particle observations were carried out under the condition with the SiH4 pressure of 150 mTorr at a ¯ow rate of 30 sccm and RF power density of 95 mW/cm 2. Fig. 3 shows the substrate temperature dependence of the spatially integrated Mie scattering light intensity for the different electrode distances of 4 and 2.5 cm. The particle density tends to increase with an increase in the temperature for both electrode distances. While the particles at 2.5 cm is lower than 4 cm above 2008C, the value for 2.5 cm becomes twice as much high as 4 cm at room temperature. This predicts that shortening the electrode distance is not the main cause to suppress the particle formation, but significantly enhances it because of the high electron temperature mentioned in Fig. 2. The plasma potential increases with an increase in the electron temperature. This is expected to enhance the coagulation of negatively charged particles with the positively charged particles which are generated
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Y. Maemura et al. / Thin Solid Films 345 (1999) 80±84
Fig. 4. Deposition rate (DR), as a function of the substrate temperatures, Ts, for different electrode distances of 4 and 2.5 cm.
by the collision of g electrons accelerated by the sheath electric ®eld [9]. However, we observed the opposite result when the anode was heated. In this case, the gas molecules near the cathode at shorter electrode distances must be heated to higher temperature by the radiation from the heated anode. Therefore, it is suggested that the particle formation is suppressed as a result of the decrease in the SiH4 density near the cathode which is heated by the anode. The substrate temperature dependence of the deposition rate of a-Si:H ®lms for electrode distances of 2.5 and 4 cm are shown in Fig. 4. All the conditions were kept constant except for the electrode distance and the substrate temperature. The deposition rate was higher for 2.5 cm over the whole temperature range shown here. This correlates with the higher electron temperature for the shorter distance case as shown in Fig. 2a. To study how the electrode distance affects the resulting a-Si:H ®lm property, a-Si:H ®lms were deposited under the Ê /s by changing RF same deposition rate condition of 20 A power density. The RF power densities were 95 and 111 W/ cm 2 for the electrode distances of 2.5 and 4 cm, respectively, and the pressure was 100 mTorr at a SiH4 ¯ow rate of 30 sccm. The substrate temperature dependence of optical (Tauc) gap for the ®lms prepared at electrode distances of 2.5 and 4 cm is shown in Fig. 5. The optical gap obtained in the case of shorter electrode distance, which is effective for suppressing particle formation at higher substrate temperature range, was smaller than that of longer electrode distance. This may be due to less contribution of higher-silane related radicals during ®lm growth.
Fig. 6 shows the dependence of the photo and dark electric conductivities, Ds p and s d, on substrate temperature. The variation of photo-conductivity re¯ects the optical band gap, simply because the optical absorption coef®cient in the ®lms of wide optical band gap is small. Good opto-electronic properties were obtained from a substrate temperature between 250 and 3008C for both electrode distances. The optical subband-gap absorption spectrum has been measured using a constant photo-current method (CPM) to determine the defect density in the resulting ®lms. The absorption coef®cient at 1.2 eV re¯ects the defect density [10], and the reciprocal of the slope of the Urbach edge in the optical absorption spectrum (Urbach energy) re¯ects the disorder of the network structure in the ®lm. Figs. 7 and 8 show the absorption coef®cient at 1.2 eV and the Urbach energy as a function of the substrate temperature at two different electrode distances, respectively. Both the defect density and the disorder parameter are lower for 2.5 cm in the whole temperature range, showing minima at 2508C for 4 cm and at 3008C for 2.5 cm. On the basis of surface reaction model [7,8], the balance equation of the generation and annihilation rates of defects on the steady-state growing surface is expressed as dNs =dt Ca T NH SiH3 1 CH T NH2 2 Cs T Ns SiH3 0 1 where Ca and Cs are the temperature dependent rate constants for the creation and saturation reaction of dangling bonds by the SiH3 radicals, respectively and CH is the hydrogen desorption reaction rate constant at high temperature. Ns, NH and [SiH3] are the surface density of dangling bond
Fig. 5. Optical (Tauc) gap versus substrate temperatures, Ts, for ®lms prepared at electrode distances of 4 and 2.5 cm.
Y. Maemura et al. / Thin Solid Films 345 (1999) 80±84
Fig. 6. Photo and dark conductivities, Ds p and s d, versus substrate temperature, Ts, for ®lms prepared at electrode distances of 4 and 2.5 cm.
sites, hydrogen terminated sites and physisorbed SiH3 radicals, respectively. Eq. (1) shows that the generation and annihilation rates of defect density are dependent on the substrate temperature and on the ¯ux of radicals to ®lm surface. This explains the temperature dependence of defect density and disorder parameter, Eu. Namely, when increasing the substrate temperature, steady state surface defect density decreases by an
83
Fig. 8. The slope of the exponential absorption tail plotted (Urbach energy) as a function of substrate temperature, Ts, for different electrode distances of 4 and 2.5 cm.
enhancement of defect annihilation rate due to an enhanced diffusion of SiH3 radicals, resulting in the decrease of defect density in the resulting ®lms. Further increase of substrate temperature gives rise to a thermal creation of defects on the surface (second term in Eq. (1)), leading to the increase of defect density in the resulting ®lms. However, as shown in Figs. 7 and 8, when the electrode distance is short, the defect density and the disorder parameter is always smaller compared to the case of longer electrode distance. These results can be explained by taking into account the in¯uence of the higher order silane related radicals on the ®lm growth. It is suggested that the radicals related to higher silane, reaching the ®lm growing surface, cause a steric hindrance effect on the surface diffusion of physisorbed SiH3 radicals, resulting in an increase in the steady state surface defect density through a decrease of defect annihilation rate in the third term of Eq. (1). 4. Conclusions We have investigated the effect of electrode distance in the preparation process of a-Si:H using pure silane RF PECVD and observed the results as follows;
Fig. 7. Absorption coef®cient at 1.2 eV (defect density) measured by CPM in a-Si:H ®lms prepared at different substrate temperatures, Ts, for different of electrode distances of 4 and 2.5 cm.
1. The electron temperature was increased by shortening the electrode distance since plasma self-organization mechanisms make up the loss of electrons to wall. As a result, the deposition rate of a-Si:H ®lm increased by shortening the electrode distance. 2. When the anode is heated, the particle formation was suppressed by shortening the electrode distance. This is caused by the decrease in SiH4 density near the cathode
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Y. Maemura et al. / Thin Solid Films 345 (1999) 80±84
which is heated by the in¯uence of heated anode to higher temperature at shorter distance. 3. Good opto-electronic property was obtained even at high deposition rate in the case of shortened electrode distance by increasing the substrate temperature between 250 and 3008C. The suppression of particle formation is effective to the reduction of defect density in the resulting ®lms. When the electrode distance is shortened, the higher silane related radicals, causing the steric hindrance effect on the surface diffusion of physisorbed SiH3 radicals, is suppressed, resulting in the reduction of the defect density in the resulting ®lm. We discussed the phenomena of the increase in deposition rate and the suppression of the particle formation due to shortening of the electrode distance. Preliminary results shown here predict the possibility of obtaining a-Si:H with much better opto-electronic properties at much high deposition rate by the control of deposition parameters including the electrode distance. Acknowledgements This work has been carried out at Thin Film Silicon Solar
Cells Super Lab, Electrotechnical Laboratory. One of the authors (Yoko Maemura) would like to gratefully thank all of members at Super Lab for their heartfull assistance.
References [1] S. Yokoyama, M. Tanaka, T. Yamaoka, M. Ueda, A. Matsuda, Proc. 5th (1988) 219. [2] G.M. Jellam, D.B. Graves, J. Appl. Phys. 67 (1990) 6490. [3] Y. Watanabe, M. Shiratani, Y. Kubo, I. Ogawa, S. Ogi, Appl. Phys. Lett. 53 (1988) 1263. [4] L. Bouchoule, A. Plain, L. Boufendi, J.Ph. Blondeau, C. Laure, M. Toogood, J. Appl. Phys. 70 (1991) 1991. [5] A.A. Howling, Ch. Hollenstein, P.J. Paris, Appl. Phys. Lett. 59 (1991) 1409. [6] Y. Watanabe, M. Shiratani, Plasma Sources Sci, Technol. 3 (1994) 286. [7] A. Matsuda, J. Non-Cryst, Solids 59 (1983) 767. [8] G. Ganguly, A. Matsuda, Phys. Rev. B 47 (1993) 3661. [9] M. Shiratani, H. Kawasaki, T. Fukuzawa, T. Yoshikawa, Y. Ueda, S. Singh, Y. Watanabe, J. Appl. Phys. 79 (1) (1996) 104. [10] Z.E. Smith, V. Chu, K. Shepard, S. Alijisji, D. Slobodin, J. Kolodzey, S. Wagner, T.L. Chu, Appl. Phys. Lett. 50 (1987) 1521.
Particle formation and a-Si:H ®lm deposition in narrow-gap RF plasma CVD Yoko Maemura a,*, Hiroshi Fujiyama a, Tomoko Takagi b, Ryo Hayashi c, Wataru Futako b, Michio Kondo b, Akihisa Matsuda b b
a Department of Electrical Engineering, Nagasaki university, 1-14 Bunkyo-machi, Nagasaki-shi, 852-8521, Japan Thin Film Silicon Solar Cells Super Lab. Electrotechnical Lab., 1-1-4 Umezono, Tsukuba-shi, Ibaraki 305-8568, Japan c Canon Inc., Ecology R&D Center, 4-1-1 Kizugawadai, Kizu-cho, Souraku-gun, Kyoto 619-0281, Japan
Abstract The effects of electrode distance are discussed in a diode type plasma enhanced chemical vapor deposition (PECVD) system as an important external control parameter for the preparation of hydrogenated amorphous silicon (a-Si:H) using an RF silane (SiH4) plasma. The electron temperature is increased by shortening the electrode distance due to a plasma self-organization mechanisms, leading to an increase in the growth rate of a-Si:H ®lms. Furthermore, shortening the distance between the heated electrodes (anode and/or cathode) gives rise to decrease in SiH4 density in the discharge space near the electrodes resulting in suppression of the particle formation. Through the control of particle formation by changing the electrode distance, the defect properties in the resulting ®lms are successfully controlled. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Plasma enhanced chemical vapor deposition; Amorphous silicon; Silane
1. Introduction Hydrogenated amorphous silicon (a-Si:H) grown by silane (SiH4) plasma is an important material for their applications to large area, thin ®lm devices, and it is therefore necessary for the increase in the production throughput to achieve higher deposition rate. The deposition rate of a-Si:H is increased with an increase in the partial pressure of silane and/or the RF power density, however, the particle formation due to the gas phase polymerization occurs, which results in an overall reduction in the ®lm opto-electronic properties and the deterioration of the device performance. It is therefore important to suppress the generation and growth of harmful particles, and extensive studies have been performed with respect to particle suppression by electrode heating [1,2], modulation of RF discharge [3±6] and so on. In this work, we investigated the effects of electrode distance on the particle formation, the plasma parameters such as electron temperature and electron density, and the properties of the resulting ®lms, and suggested the electrode distance as an effective external control parameter for the
* Corresponding author. E-mail address: [email protected] (Y. Maemura)
suppression of particle formation at high deposition rate. Furthermore, from a viewpoint of surface reaction kinetics [7,8], these results are useful to understand the relationship between the defect density in the resulting a-Si:H ®lms and the particles and/or radicals related to higher order silanes.
2. Experimental The CVD system used in this study is a conventional diode type reactor equipped with two parallel electrodes of 10 cm in diameter as shown in Fig. 1, together with the setup for the Langmuir probe and the particle measurements. The distance between the electrodes was changed between 2.5 and 4 cm. Either Ar or silane (SiH4) plasma was generated by RF (13.56 MHz) power. Corning 7059 glass and crystalline silicon substrates were placed on the grounded electrode (anode), and the temperature of the substrate was changed from room temperature up to 3508C. The electron temperature and the electron density for different electrode distances were measured by a cylindrical Langmuir probe in order to understand the correlation between the electrode distance and the plasma parameters. The Langmuir probe was located at a distance of 1 cm from the substrates. The probe diagnostics were performed for
0040-6090/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S00 40-6090(99)0010 0-5
Y. Maemura et al. / Thin Solid Films 345 (1999) 80±84
81
Fig. 1. Experimental set-up for PECVD, probe measurement and LMS observation.
pure Ar and pure SiH4 plasma at the substrate temperature of room temperature. For Ar plasma, the pressure was set at 20 and 100 mTorr, and the RF power density at 60, 190 and 320 mW/cm 2. The probe measurements in SiH4 plasma were performed only under a condition with no serious particle formation, that is at 20 mTorr and 127 mW/cm 2. The particle in the discharge space near the cathode was measured by laser Mie scattering light (LMS) method using a He-Ne laser (5 mW at 632.8 nm). A sheet beam type laser of 15 mm in width and 1 mm in thickness, introduced into the chamber from a view port, was passed through the discharge space near the cathode with a clearance between the beam and cathode of 1 mm. Mie scattering light from particles was detected by a CCD camera with an interference ®lter (center wave length 632.8 nm, full width of 1 nm at half maximum), which were set at an angle of 458 to the incident beam. For the deposition of a-Si:H, pure SiH4 gas with a constant ¯ow rate of 30 sccm was introduced and the pressure was kept constant at 100 mTorr. The ®lms were deposited with the electrode distance of 2.5 and 4 cm. The ®lm thickness, optical gap, refractive index, and the electronic conductivities were measured for the ®lms on the glass substrates.
Fig. 2. Electron temperature, Te, (a) and electron density, ne, (b) versus electrode distances (ED), as a parameter of RF power density.
Fig. 3. Spatially integrated Mie scattering int for different electrode distances of 4 and 2.5 cm at different substrate temperatures, Ts.
3. Results and discussion Fig. 2a,b show the electrode distance dependence of the electron temperature and the electron density, respectively, in pure Ar plasma for different RF power densities at 20 and 100 mTorr. A decrease in the electrode distance enhances the electron temperature for both pressures as shown in Fig. 2a. Similar tendency was also seen in silane plasma (open circles). Shortening the electrode distance leads to an increase in the electron temperature since the plasma self-organization mechanisms make up the loss of electrons to the wall (electrodes). On the other hand, Fig. 2b shows that the electron density is rather independent of the electrode distance and it increases with RF power density. Particle observations were carried out under the condition with the SiH4 pressure of 150 mTorr at a ¯ow rate of 30 sccm and RF power density of 95 mW/cm 2. Fig. 3 shows the substrate temperature dependence of the spatially integrated Mie scattering light intensity for the different electrode distances of 4 and 2.5 cm. The particle density tends to increase with an increase in the temperature for both electrode distances. While the particles at 2.5 cm is lower than 4 cm above 2008C, the value for 2.5 cm becomes twice as much high as 4 cm at room temperature. This predicts that shortening the electrode distance is not the main cause to suppress the particle formation, but significantly enhances it because of the high electron temperature mentioned in Fig. 2. The plasma potential increases with an increase in the electron temperature. This is expected to enhance the coagulation of negatively charged particles with the positively charged particles which are generated
82
Y. Maemura et al. / Thin Solid Films 345 (1999) 80±84
Fig. 4. Deposition rate (DR), as a function of the substrate temperatures, Ts, for different electrode distances of 4 and 2.5 cm.
by the collision of g electrons accelerated by the sheath electric ®eld [9]. However, we observed the opposite result when the anode was heated. In this case, the gas molecules near the cathode at shorter electrode distances must be heated to higher temperature by the radiation from the heated anode. Therefore, it is suggested that the particle formation is suppressed as a result of the decrease in the SiH4 density near the cathode which is heated by the anode. The substrate temperature dependence of the deposition rate of a-Si:H ®lms for electrode distances of 2.5 and 4 cm are shown in Fig. 4. All the conditions were kept constant except for the electrode distance and the substrate temperature. The deposition rate was higher for 2.5 cm over the whole temperature range shown here. This correlates with the higher electron temperature for the shorter distance case as shown in Fig. 2a. To study how the electrode distance affects the resulting a-Si:H ®lm property, a-Si:H ®lms were deposited under the Ê /s by changing RF same deposition rate condition of 20 A power density. The RF power densities were 95 and 111 W/ cm 2 for the electrode distances of 2.5 and 4 cm, respectively, and the pressure was 100 mTorr at a SiH4 ¯ow rate of 30 sccm. The substrate temperature dependence of optical (Tauc) gap for the ®lms prepared at electrode distances of 2.5 and 4 cm is shown in Fig. 5. The optical gap obtained in the case of shorter electrode distance, which is effective for suppressing particle formation at higher substrate temperature range, was smaller than that of longer electrode distance. This may be due to less contribution of higher-silane related radicals during ®lm growth.
Fig. 6 shows the dependence of the photo and dark electric conductivities, Ds p and s d, on substrate temperature. The variation of photo-conductivity re¯ects the optical band gap, simply because the optical absorption coef®cient in the ®lms of wide optical band gap is small. Good opto-electronic properties were obtained from a substrate temperature between 250 and 3008C for both electrode distances. The optical subband-gap absorption spectrum has been measured using a constant photo-current method (CPM) to determine the defect density in the resulting ®lms. The absorption coef®cient at 1.2 eV re¯ects the defect density [10], and the reciprocal of the slope of the Urbach edge in the optical absorption spectrum (Urbach energy) re¯ects the disorder of the network structure in the ®lm. Figs. 7 and 8 show the absorption coef®cient at 1.2 eV and the Urbach energy as a function of the substrate temperature at two different electrode distances, respectively. Both the defect density and the disorder parameter are lower for 2.5 cm in the whole temperature range, showing minima at 2508C for 4 cm and at 3008C for 2.5 cm. On the basis of surface reaction model [7,8], the balance equation of the generation and annihilation rates of defects on the steady-state growing surface is expressed as dNs =dt Ca T NH SiH3 1 CH T NH2 2 Cs T Ns SiH3 0 1 where Ca and Cs are the temperature dependent rate constants for the creation and saturation reaction of dangling bonds by the SiH3 radicals, respectively and CH is the hydrogen desorption reaction rate constant at high temperature. Ns, NH and [SiH3] are the surface density of dangling bond
Fig. 5. Optical (Tauc) gap versus substrate temperatures, Ts, for ®lms prepared at electrode distances of 4 and 2.5 cm.
Y. Maemura et al. / Thin Solid Films 345 (1999) 80±84
Fig. 6. Photo and dark conductivities, Ds p and s d, versus substrate temperature, Ts, for ®lms prepared at electrode distances of 4 and 2.5 cm.
sites, hydrogen terminated sites and physisorbed SiH3 radicals, respectively. Eq. (1) shows that the generation and annihilation rates of defect density are dependent on the substrate temperature and on the ¯ux of radicals to ®lm surface. This explains the temperature dependence of defect density and disorder parameter, Eu. Namely, when increasing the substrate temperature, steady state surface defect density decreases by an
83
Fig. 8. The slope of the exponential absorption tail plotted (Urbach energy) as a function of substrate temperature, Ts, for different electrode distances of 4 and 2.5 cm.
enhancement of defect annihilation rate due to an enhanced diffusion of SiH3 radicals, resulting in the decrease of defect density in the resulting ®lms. Further increase of substrate temperature gives rise to a thermal creation of defects on the surface (second term in Eq. (1)), leading to the increase of defect density in the resulting ®lms. However, as shown in Figs. 7 and 8, when the electrode distance is short, the defect density and the disorder parameter is always smaller compared to the case of longer electrode distance. These results can be explained by taking into account the in¯uence of the higher order silane related radicals on the ®lm growth. It is suggested that the radicals related to higher silane, reaching the ®lm growing surface, cause a steric hindrance effect on the surface diffusion of physisorbed SiH3 radicals, resulting in an increase in the steady state surface defect density through a decrease of defect annihilation rate in the third term of Eq. (1). 4. Conclusions We have investigated the effect of electrode distance in the preparation process of a-Si:H using pure silane RF PECVD and observed the results as follows;
Fig. 7. Absorption coef®cient at 1.2 eV (defect density) measured by CPM in a-Si:H ®lms prepared at different substrate temperatures, Ts, for different of electrode distances of 4 and 2.5 cm.
1. The electron temperature was increased by shortening the electrode distance since plasma self-organization mechanisms make up the loss of electrons to wall. As a result, the deposition rate of a-Si:H ®lm increased by shortening the electrode distance. 2. When the anode is heated, the particle formation was suppressed by shortening the electrode distance. This is caused by the decrease in SiH4 density near the cathode
84
Y. Maemura et al. / Thin Solid Films 345 (1999) 80±84
which is heated by the in¯uence of heated anode to higher temperature at shorter distance. 3. Good opto-electronic property was obtained even at high deposition rate in the case of shortened electrode distance by increasing the substrate temperature between 250 and 3008C. The suppression of particle formation is effective to the reduction of defect density in the resulting ®lms. When the electrode distance is shortened, the higher silane related radicals, causing the steric hindrance effect on the surface diffusion of physisorbed SiH3 radicals, is suppressed, resulting in the reduction of the defect density in the resulting ®lm. We discussed the phenomena of the increase in deposition rate and the suppression of the particle formation due to shortening of the electrode distance. Preliminary results shown here predict the possibility of obtaining a-Si:H with much better opto-electronic properties at much high deposition rate by the control of deposition parameters including the electrode distance. Acknowledgements This work has been carried out at Thin Film Silicon Solar
Cells Super Lab, Electrotechnical Laboratory. One of the authors (Yoko Maemura) would like to gratefully thank all of members at Super Lab for their heartfull assistance.
References [1] S. Yokoyama, M. Tanaka, T. Yamaoka, M. Ueda, A. Matsuda, Proc. 5th (1988) 219. [2] G.M. Jellam, D.B. Graves, J. Appl. Phys. 67 (1990) 6490. [3] Y. Watanabe, M. Shiratani, Y. Kubo, I. Ogawa, S. Ogi, Appl. Phys. Lett. 53 (1988) 1263. [4] L. Bouchoule, A. Plain, L. Boufendi, J.Ph. Blondeau, C. Laure, M. Toogood, J. Appl. Phys. 70 (1991) 1991. [5] A.A. Howling, Ch. Hollenstein, P.J. Paris, Appl. Phys. Lett. 59 (1991) 1409. [6] Y. Watanabe, M. Shiratani, Plasma Sources Sci, Technol. 3 (1994) 286. [7] A. Matsuda, J. Non-Cryst, Solids 59 (1983) 767. [8] G. Ganguly, A. Matsuda, Phys. Rev. B 47 (1993) 3661. [9] M. Shiratani, H. Kawasaki, T. Fukuzawa, T. Yoshikawa, Y. Ueda, S. Singh, Y. Watanabe, J. Appl. Phys. 79 (1) (1996) 104. [10] Z.E. Smith, V. Chu, K. Shepard, S. Alijisji, D. Slobodin, J. Kolodzey, S. Wagner, T.L. Chu, Appl. Phys. Lett. 50 (1987) 1521.