- Email: [email protected]
s
ARTICLE IN PRESS
Solar Energy Materials & Solar Cells 90 (2006) 3223–3231 www.elsevier.com/locate/solmat
High rate growth of device-grade microcrystalline silicon films at 8 nm/s Chisato Niikura, Michio Kondo, Akihisa Matsuda National Institute of Advanced Industrial Science and Technology, 1-1-1, Umezono, Tsukuba, Ibaraki 305-8568, Japan Received 24 May 2005; accepted 23 June 2005 Available online 28 August 2006
Abstract We have developed a novel technique for large-area high rate growth of microcrystalline silicon films by plasma-enhanced chemical vapor deposition, designing a novel cathode with interconnected multi-holes, which leads to produce uniformly flat-distributed stable high-density plasma spots near cathode surface. The spatial distribution of plasma at holes on cathode surface was analyzed using optical emission spectroscopy for SiH4/H2 plasma with various pressures with a view to optimizing deposition conditions. Improvement of properties of high-rate-grown films was discussed with regard to silane depletion as well as the temperature of film-growing surface. Microcrystalline silicon films with a low defect density of 5 1015 cm 3 obtained at a high rate approaching 8 nm/s demonstrate the effectiveness of the novel cathode. r 2006 Published by Elsevier B.V. Keywords: Microcrystalline silicon; Plasma-enhanced chemical vapor deposition; Thin film; High rate growth
1. Introduction Microcrystalline silicon, mc-Si:H, attracts intense attention as a promising material for thin film solar cells with high conversion efficiency and high stability. However, mc-Si:H solar cells require a thick intrinsic layer (2.0–3.5 mm) to absorb sufficient amount of sunlight, because of its indirect optical transition. Therefore, high rate growth of mc-Si:H films is a crucial matter for low-cost production of thin-film solar cells. Corresponding author. Tel.: +81 29 861 5080x55387; fax: +81 29 861 3363.
E-mail address: [email protected] (C. Niikura). 0927-0248/$ - see front matter r 2006 Published by Elsevier B.V. doi:10.1016/j.solmat.2006.06.036
ARTICLE IN PRESS 3224
C. Niikura et al. / Solar Energy Materials & Solar Cells 90 (2006) 3223–3231
An approach for high rate growth of mc-Si:H films by increasing the input power under conventional mc-Si:H film deposition conditions with low pressure and high hydrogen (H2) dilution of silane (SiH4) rapidly deteriorates defect density, Ns, with growth rate, Rd [1], owing to enhanced ion bombardment as well as contribution of short-lifetime radicals. We have developed a high-pressure depletion (HPD) method using very high frequency (VHF) plasma [1–3] with low electron temperature and high electron density, where efficient gas dissociation gives rise to SiH4 depletion, enabling to keep sufficient amount of atomic hydrogen. Ns of high-rate-grown mc-Si:H films has been reduced by means of the HPD method, however, further improvement is still required for solar cell application. In this work, we tried to take advantages of high-density plasma realizing efficient gas dissociation, for further development of high-rate mc-Si:H growth technique. For this purpose, we have designed a novel cathode which provides uniformly flat-distributed stable high-density plasma spots near cathode [4,5]. In this study, first, the spatial distribution of plasma at hole on cathode surface is analyzed using optical emission spectroscopy (OES) for optimization of deposition conditions. Then, improvement of quality of high-rate-grown mc-Si:H films is discussed for application to solar cells with regard to SiH4 depletion and thermal heating effect by plasma. 2. Experimental details A conventional diode-type plasma-enhanced chemical vapor deposition (PECVD) system with our novel cathode with an area of 130 cm2 was used for this study. A schematic picture of the novel cathode is shown in Fig. 1. As shown in the inset, on the surface of the new cathode, there are many interconnected holes (hollows) through which source gas is injected to the reactor. The surface pattern size, (a, b, c, d), defined in Fig. 1, was (1, 2, 2, 4) (mm) in this study. 2.1. OES analyses The nature of plasma spots generated at surface-holes of the novel cathode was investigated by OES analyses for VHF SiH4/H2 plasma with an excitation frequency of 60 MHz. Plasma generation conditions are as follows. A gaseous mixture of SiH4 and H2
Fig. 1. A schematic picture of the novel cathode.
ARTICLE IN PRESS C. Niikura et al. / Solar Energy Materials & Solar Cells 90 (2006) 3223–3231
3225
with a SiH4 flow rate, F(SiH4), of 45 sccm and a H2 flow rate, F(H2), of 450 sccm was used, and pressure, Pr, was varied from 0.5 to 9.3 Torr. An input power, Pw, of 225 W was fixed. Sample holder temperature, Tholder, measured by a thermocouple attached to sample holder, was initially set to be 200 1C. The distance between the anode and the cathode was 7 mm. The plasma generation conditions are summarized in Table 1. Two-dimensional spatial distribution of Ha optical emission at cathode-surface holes was observed using a charge coupled device (CCD) camera from the direction parallel to the cathode surface with an interference filter which transmits only the light with the wavelength of 656 (73) nm. 2.2. Optimization of mc-Si:H film properties mc-Si:H films were prepared using a mixture of SiH4 and H2 as source gases under various deposition conditions by VHF-PECVD with an excitation frequency of 60 MHz. The distance between the cathode and the anode was 7 mm. About 0.5 mm-thick quartz, 0.7 mm-thick Corning 7059 glass, and 0.5 mm-thick (1 1 1)-oriented crystalline silicon (c-Si) were used as substrates. A Pw of 525 W and a Pr of 9.3 Torr were used. Tholder was initially set to be 200 1C. Samples of series 1 were prepared varying F(SiH4) from 20 to 40 sccm or from 50 to 70 sccm with fixed F(H2) of 300 or 700 sccm, while samples of series 2 were prepared varying F(H2) from 700 to 1500 sccm with a constant F(SiH4) of 70 sccm. The deposition conditions are also seen in Table 1. The film properties were characterized using Raman scattering spectroscopy, profilometry, electron spin resonance, Fourier-transformed infrared absorption spectroscopy, and conductivity measurements to evaluate film crystallinity (Ic/Ia), growth rate (Rd), defect density (Ns), bonded hydrogen content (CH), and dark conductivity (sd) and photoconductivity (sp), respectively. Raman spectroscopy measurements were performed using a He–Ne excitation source (with a wavelength of 633 nm), and Ic/Ia was estimated from Raman spectra as an intensity ratio of the crystalline peak at around 520 cm 1 (Ic) to the amorphous peak at around 480 cm 1 (Ia). CH was estimated from infrared Si–H stretching-mode absorption, using samples deposited on c-Si substrates. 3. Results and discussion 3.1. OES analyses Two-dimensional spatial distribution images of Ha optical emission at holes on cathode surface, observed from the direction parallel to the cathode surface, are shown in Fig. 2, Table 1 Plasma generation conditions for OES analyses and deposition conditions
Power, Pw (W) Pressure, Pr (Torr) Sample holder temperature, Tholder (1C) Silane flow rate, F(SiH4) (sccm) Hydrogen flow rate, F(H2) (sccm)
OES analyses
Series 1
Series 2
225 0.5–9.3 200 45 450
525 9.3 200 20–70 300, 700
525 9.3 200 70 700–1500
ARTICLE IN PRESS 3226
C. Niikura et al. / Solar Energy Materials & Solar Cells 90 (2006) 3223–3231
Fig. 2. Spatial distribution images of Ha optical emission at cathode-surface hole, for SiH4/H2 plasma with various pressures (Pr).
and the axial distribution of the intensity of the Ha optical emission along the central axis of the cathode-surface hole is shown in Fig. 3, for SiH4/H2 plasma with various Pr. It was revealed that the state of plasma spot at cathode-surface hole depends strongly on Pr. In Pr region above 2 Torr, a stable intense plasma spot was observed at every hole on cathode surface, consisting an uniformly flat-distributed high-density plasma spots on cathode surface, where electrons are likely to be confined realizing high electron density like a hollow discharge and thereby an efficient gas dissociation is realized producing high-density SiHx film precursors and hydrogen atoms. It is likely that the presence of holes and edges is effective for high-density plasma production, and that the connecting slots are useful for uniform production of plasma, though the production mechanism of the intense plasma spots at holes on cathode surface is actually under investigation. As shown in the figures, the Ha OES intensity at cathode-surface hole increases when Pr decreases from 9.3 to 3 Torr, and at low Pr below 1 Torr, there are no intense plasma spots at cathode-surface holes. The state of plasma was similar to that of plasma generated with a flat-plate cathode. The Ha OES intensity at cathode-surface hole takes the maximum at 3–4 Torr, with the position of the maximum intensity close to the hole-edge on cathode surface and just below the Pr, at a Pr of 2 Torr corresponding to the lower limit of plasma spot generation, the position of the maximum intensity goes outside the hole on cathode surface. This behavior could be related to the sheath width that becomes wider for lower Pr, and also to the hole diameter. It is suggested that the Pr corresponding to the maximum OES intensity at the hole depends on the geometry of the hole. A smaller diameter is required for higher Pr, depending on the sheath width. It would be necessary to optimize plasma conditions (such as Pr, excitation frequency, and Pw) relatively to the cathode-surface configuration (such as hole-size, pitch, and pattern). It is favorable that Pr is high enough for obtaining high-density plasma spots at holes on cathode surface. On the other hand, an increase in Tholder was observed during plasma observation with employing relatively high Pr and Pw which favor production of high-density plasma spots at cathode-surface holes as well as high growth
ARTICLE IN PRESS C. Niikura et al. / Solar Energy Materials & Solar Cells 90 (2006) 3223–3231
3227
1 Torr
2 Torr
Position (a.u.)
3 Torr
4 Torr
5 Torr
9.3 Torr
0
20000
40000 60000 80000 100000 Hα intensity (a.u.)
Fig. 3. Axial distribution of Ha optical emission intensity along the central axis of cathode-surface hole, for SiH4/ H2 plasma with various pressures (Pr).
rate. It indicates that gas temperature of the plasma becomes high under high Pr and Pw conditions, like thermal plasma, suggesting that the temperature of film surface is heated up by plasma.
ARTICLE IN PRESS 3228
C. Niikura et al. / Solar Energy Materials & Solar Cells 90 (2006) 3223–3231
3.2. mc-Si:H film deposition During deposition using the novel cathode, a stable intense plasma spot was observed at every hole, providing a uniformly flat-distributed high-density plasma spots near cathode surface. Under these conditions, efficient dissociation of SiH4 and H2 gives rise to highly
Growth rate (nm/s)
10 8 6 4
H2 flow rate
2
300 sccm 700 sccm
0 Defect density (cm-3)
1017
1016
1015 6 5
Ic / Ia
4 3 2 1
µc-Si:H a-Si:H
102
Photo
10-5
Dark
10-6 10-7
101 10-8 100 0
20
40
60
80
Conductivity (S/cm)
σphoto / σdark
0 103
10-9 100
SiH4 flow rate (sccm) Fig. 4. Growth rate, defect density, crystallinity, photoconductivity, dark conductivity, and the ratio as a function of silane flow rate, F(SiH4), for samples of series 1.
ARTICLE IN PRESS C. Niikura et al. / Solar Energy Materials & Solar Cells 90 (2006) 3223–3231
3229
SiH4 depleted state and thereby high-density SiHx film precursors and hydrogen atoms reach the film-growing surface. On the other hand, aforementioned film-surface heating effect by plasma may cause thermal desorption of surface-bonded hydrogen, which can deteriorate quality of deposited films, enhancing the creation of surface dangling bonds [6]. Thus, we tried to optimize properties of the mc-Si:H films grown at high rate with the novel cathode, (1) by optimizing SiH4 depletion state and (2) by alleviating the increase in the temperature of film-growing surface.
Growth rate (nm/s)
10 8 6 4 2 0 Defect density (cm-3)
1017
1016
1015 Hydrogen content (at.%)
8 6 4 2 0 5
Ic /I a
4 3 2 1
µc-Si:H
0 500
1000
1500
a-Si:H
H2 flow rate (sccm) Fig. 5. Growth rate, defect density, hydrogen content, and crystallinity as a function of hydrogen flow rate, F(H2), for samples of series 2.
ARTICLE IN PRESS 3230
C. Niikura et al. / Solar Energy Materials & Solar Cells 90 (2006) 3223–3231
First, we investigated the dependence of film properties on F(SiH4). Rd, Ns, Ic/Ia, sd, sp, and sp/sd ratio of samples of series 1 are shown in Fig. 4 as a function of F(SiH4). Though Ic/Ia is degraded slightly, Rd increases while Ns is reduced when increasing the F(SiH4). The increase in Rd with F(SiH4) is due to an increase in SiH4 partial pressure. The reduction in Ns with F(SiH4) indicates that, by adjusting F(SiH4), SiH4 depletion state, i.e., the ratio of atomic hydrogen to SiHx film precursors arriving on the growing surface is furthermore optimized, and probably that lower electron temperature associated with higher F(SiH4) is beneficial in terms of ion bombardment [7] and contribution of short-lifetime radicals, such as SiHx (x%2), on film-growing surface. sd and sp increase exponentially, whereas sp/sd ratio decreases exponentially with decreasing F(SiH4) improving Ic/Ia, and sp/sd of 101–102 with sd of 10 6–10 7 (S/cm) were obtained for mc-Si:H films. Rapid decrease in Ic/Ia for higher F(SiH4) or for higher total gas flow rate can be due to high partial pressure of SiH4 which scavenges H atoms. It suggests that when increasing the F(SiH4) or total gas flow rate, higher H2-dilution ratio is correspondingly required for obtaining sufficiently high crystallinity. Next, we tried to alleviate the aforementioned surface heating effect by plasma by increasing total gas flow rate. Practically, we increased total gas flow rate by increasing F(H2) from 700 to 1500 sccm with a constant F(SiH4) of 70 sccm (samples of series 2) increasing simultaneously H2-dilution ratio, and improved the crystallinity of the sample of series 1 prepared with an F(SiH4) of 70 sccm. Rd, Ns, CH, and Ic/Ia of samples of series 2 are shown in Fig. 5 as a function of F(H2). When increasing the F(H2), an increase in Ic/Ia and a slight reduction in Ns are simultaneously obtained, while CH increases from 4.7 to 7.2 at% and Rd decreases slightly. An increase in H2-dilution ratio was essential to improve Ic/Ia realizing higher flux ratio of
1018
Defect density (cm-3)
(i) Conventional (Low Pressure)
(ii) High Pressure Depletion
1017
1016
(iii) Novel cathode 1015 0
2
4
6
8
10
Growth rate (nm/s) Fig. 6. Defect density as a function of growth rate, for (i) mc-Si:H samples prepared by conventional low-pressure method, (ii) those prepared by HPD method and (iii) those prepared using the novel cathode with interconnected multi-holes.
ARTICLE IN PRESS C. Niikura et al. / Solar Energy Materials & Solar Cells 90 (2006) 3223–3231
3231
atomic hydrogen to film precursor reaching the film-growing surface. In addition, Ns was furthermore reduced owing to surface cooling effect by increased total gas flow rate, which is confirmed by the higher CH for samples with improved Ic/Ia. Thus, an optimization of SiH4 depletion state as well as an alleviation of surface heating by increasing total gas flow rate was found to be effective for improvement of film properties. As a result, device-grade mc-Si:H films with low Ns of 4.3–5.6 1015 cm 3, Ic/Ia of 2.6–3, and sp/sd of 101–102 were obtained at high Rd of 7.6–7.9 nm/s, by making use of the novel cathode. Ns of these samples prepared using the novel cathode is shown by closed circles as (iii) in Fig. 6, as a function of Rd. That of samples prepared by conventional lowpressure PECVD method and those prepared by the HPD method are also shown in Fig. 6, by open squares as (i) and by open triangles as (ii), respectively [1]. As seen in the figure, Ns of high-rate-grown mc-Si:H films was drastically reduced by means of the novel cathode, demonstrating the effectiveness of this high-rate-growth technique for high-quality mc-Si:H films using the novel cathode. 4. Conclusion We propose a newly designed cathode with interconnected multi-holes (hollows) combined with gas injector, which leads to a production of uniformly flat-distributed stable high-density plasma spots on cathode surface, in order to realize large-area highrate-growth of mc-Si:H films by PECVD. The spatial distribution of plasma at the holes on cathode surface was investigated by OES analyses for SiH4/H2 plasma with various Pr for optimization of deposition conditions. Optimization of SiH4 depletion state as well as alleviation of surface heating effect by plasma was found to be essential for the improvement of properties of mc-Si:H films prepared at high Rd using the novel cathode. By adjusting H2-dilution and increasing total gas flow rate, we succeeded in the reduction of Ns for high-rate-grown mc-Si:H films. As a result, device-grade mc-Si:H films with low Ns of 4–6 1015 cm 3 were obtained at high Rd of 7.6–7.9 nm/s. Acknowledgments The authors would like to thank Prof. N. Sato of Tohoku University for beneficial suggestions and Miss A. Sato for help with the ESR measurements. This work was partially supported by New Energy and Industrial Technology Development Organization (NEDO) as NEDO Industrial Technology Fellowship Program. References [1] M. Kondo, T. Nishimoto, M. Takai, S. Suzuki, Y. Nasuno, A. Matsuda, Sol. Energy Mater. Sol. Cells 78 (2003) 543. [2] L. Guo, M. Kondo, M. Fukawa, K. Saitoh, A. Matsuda, Jpn. J. Appl. Phys. 37 (1998) L1116. [3] M. Fukawa, S. Suzuki, L. Guo, M. Kondo, A. Matsuda, Sol. Energy Mater. Sol. Cells 66 (2001) 217. [4] C. Niikura, N. Itagaki, M. Kondo, Y. Kawai, A. Matsuda, Thin Solid Films, in press. [5] C. Niikura, M. Kondo, A. Matsuda, J. Non-Cryst. Solids, to be published. [6] G. Ganguly, A. Matsuda, Phys. Rev. B 47 (1993) 3661. [7] J. Robertson, J. Non-Cryst. Solids 266–269 (2000) 79.
Solar Energy Materials & Solar Cells 90 (2006) 3223–3231 www.elsevier.com/locate/solmat
High rate growth of device-grade microcrystalline silicon films at 8 nm/s Chisato Niikura, Michio Kondo, Akihisa Matsuda National Institute of Advanced Industrial Science and Technology, 1-1-1, Umezono, Tsukuba, Ibaraki 305-8568, Japan Received 24 May 2005; accepted 23 June 2005 Available online 28 August 2006
Abstract We have developed a novel technique for large-area high rate growth of microcrystalline silicon films by plasma-enhanced chemical vapor deposition, designing a novel cathode with interconnected multi-holes, which leads to produce uniformly flat-distributed stable high-density plasma spots near cathode surface. The spatial distribution of plasma at holes on cathode surface was analyzed using optical emission spectroscopy for SiH4/H2 plasma with various pressures with a view to optimizing deposition conditions. Improvement of properties of high-rate-grown films was discussed with regard to silane depletion as well as the temperature of film-growing surface. Microcrystalline silicon films with a low defect density of 5 1015 cm 3 obtained at a high rate approaching 8 nm/s demonstrate the effectiveness of the novel cathode. r 2006 Published by Elsevier B.V. Keywords: Microcrystalline silicon; Plasma-enhanced chemical vapor deposition; Thin film; High rate growth
1. Introduction Microcrystalline silicon, mc-Si:H, attracts intense attention as a promising material for thin film solar cells with high conversion efficiency and high stability. However, mc-Si:H solar cells require a thick intrinsic layer (2.0–3.5 mm) to absorb sufficient amount of sunlight, because of its indirect optical transition. Therefore, high rate growth of mc-Si:H films is a crucial matter for low-cost production of thin-film solar cells. Corresponding author. Tel.: +81 29 861 5080x55387; fax: +81 29 861 3363.
E-mail address: [email protected] (C. Niikura). 0927-0248/$ - see front matter r 2006 Published by Elsevier B.V. doi:10.1016/j.solmat.2006.06.036
ARTICLE IN PRESS 3224
C. Niikura et al. / Solar Energy Materials & Solar Cells 90 (2006) 3223–3231
An approach for high rate growth of mc-Si:H films by increasing the input power under conventional mc-Si:H film deposition conditions with low pressure and high hydrogen (H2) dilution of silane (SiH4) rapidly deteriorates defect density, Ns, with growth rate, Rd [1], owing to enhanced ion bombardment as well as contribution of short-lifetime radicals. We have developed a high-pressure depletion (HPD) method using very high frequency (VHF) plasma [1–3] with low electron temperature and high electron density, where efficient gas dissociation gives rise to SiH4 depletion, enabling to keep sufficient amount of atomic hydrogen. Ns of high-rate-grown mc-Si:H films has been reduced by means of the HPD method, however, further improvement is still required for solar cell application. In this work, we tried to take advantages of high-density plasma realizing efficient gas dissociation, for further development of high-rate mc-Si:H growth technique. For this purpose, we have designed a novel cathode which provides uniformly flat-distributed stable high-density plasma spots near cathode [4,5]. In this study, first, the spatial distribution of plasma at hole on cathode surface is analyzed using optical emission spectroscopy (OES) for optimization of deposition conditions. Then, improvement of quality of high-rate-grown mc-Si:H films is discussed for application to solar cells with regard to SiH4 depletion and thermal heating effect by plasma. 2. Experimental details A conventional diode-type plasma-enhanced chemical vapor deposition (PECVD) system with our novel cathode with an area of 130 cm2 was used for this study. A schematic picture of the novel cathode is shown in Fig. 1. As shown in the inset, on the surface of the new cathode, there are many interconnected holes (hollows) through which source gas is injected to the reactor. The surface pattern size, (a, b, c, d), defined in Fig. 1, was (1, 2, 2, 4) (mm) in this study. 2.1. OES analyses The nature of plasma spots generated at surface-holes of the novel cathode was investigated by OES analyses for VHF SiH4/H2 plasma with an excitation frequency of 60 MHz. Plasma generation conditions are as follows. A gaseous mixture of SiH4 and H2
Fig. 1. A schematic picture of the novel cathode.
ARTICLE IN PRESS C. Niikura et al. / Solar Energy Materials & Solar Cells 90 (2006) 3223–3231
3225
with a SiH4 flow rate, F(SiH4), of 45 sccm and a H2 flow rate, F(H2), of 450 sccm was used, and pressure, Pr, was varied from 0.5 to 9.3 Torr. An input power, Pw, of 225 W was fixed. Sample holder temperature, Tholder, measured by a thermocouple attached to sample holder, was initially set to be 200 1C. The distance between the anode and the cathode was 7 mm. The plasma generation conditions are summarized in Table 1. Two-dimensional spatial distribution of Ha optical emission at cathode-surface holes was observed using a charge coupled device (CCD) camera from the direction parallel to the cathode surface with an interference filter which transmits only the light with the wavelength of 656 (73) nm. 2.2. Optimization of mc-Si:H film properties mc-Si:H films were prepared using a mixture of SiH4 and H2 as source gases under various deposition conditions by VHF-PECVD with an excitation frequency of 60 MHz. The distance between the cathode and the anode was 7 mm. About 0.5 mm-thick quartz, 0.7 mm-thick Corning 7059 glass, and 0.5 mm-thick (1 1 1)-oriented crystalline silicon (c-Si) were used as substrates. A Pw of 525 W and a Pr of 9.3 Torr were used. Tholder was initially set to be 200 1C. Samples of series 1 were prepared varying F(SiH4) from 20 to 40 sccm or from 50 to 70 sccm with fixed F(H2) of 300 or 700 sccm, while samples of series 2 were prepared varying F(H2) from 700 to 1500 sccm with a constant F(SiH4) of 70 sccm. The deposition conditions are also seen in Table 1. The film properties were characterized using Raman scattering spectroscopy, profilometry, electron spin resonance, Fourier-transformed infrared absorption spectroscopy, and conductivity measurements to evaluate film crystallinity (Ic/Ia), growth rate (Rd), defect density (Ns), bonded hydrogen content (CH), and dark conductivity (sd) and photoconductivity (sp), respectively. Raman spectroscopy measurements were performed using a He–Ne excitation source (with a wavelength of 633 nm), and Ic/Ia was estimated from Raman spectra as an intensity ratio of the crystalline peak at around 520 cm 1 (Ic) to the amorphous peak at around 480 cm 1 (Ia). CH was estimated from infrared Si–H stretching-mode absorption, using samples deposited on c-Si substrates. 3. Results and discussion 3.1. OES analyses Two-dimensional spatial distribution images of Ha optical emission at holes on cathode surface, observed from the direction parallel to the cathode surface, are shown in Fig. 2, Table 1 Plasma generation conditions for OES analyses and deposition conditions
Power, Pw (W) Pressure, Pr (Torr) Sample holder temperature, Tholder (1C) Silane flow rate, F(SiH4) (sccm) Hydrogen flow rate, F(H2) (sccm)
OES analyses
Series 1
Series 2
225 0.5–9.3 200 45 450
525 9.3 200 20–70 300, 700
525 9.3 200 70 700–1500
ARTICLE IN PRESS 3226
C. Niikura et al. / Solar Energy Materials & Solar Cells 90 (2006) 3223–3231
Fig. 2. Spatial distribution images of Ha optical emission at cathode-surface hole, for SiH4/H2 plasma with various pressures (Pr).
and the axial distribution of the intensity of the Ha optical emission along the central axis of the cathode-surface hole is shown in Fig. 3, for SiH4/H2 plasma with various Pr. It was revealed that the state of plasma spot at cathode-surface hole depends strongly on Pr. In Pr region above 2 Torr, a stable intense plasma spot was observed at every hole on cathode surface, consisting an uniformly flat-distributed high-density plasma spots on cathode surface, where electrons are likely to be confined realizing high electron density like a hollow discharge and thereby an efficient gas dissociation is realized producing high-density SiHx film precursors and hydrogen atoms. It is likely that the presence of holes and edges is effective for high-density plasma production, and that the connecting slots are useful for uniform production of plasma, though the production mechanism of the intense plasma spots at holes on cathode surface is actually under investigation. As shown in the figures, the Ha OES intensity at cathode-surface hole increases when Pr decreases from 9.3 to 3 Torr, and at low Pr below 1 Torr, there are no intense plasma spots at cathode-surface holes. The state of plasma was similar to that of plasma generated with a flat-plate cathode. The Ha OES intensity at cathode-surface hole takes the maximum at 3–4 Torr, with the position of the maximum intensity close to the hole-edge on cathode surface and just below the Pr, at a Pr of 2 Torr corresponding to the lower limit of plasma spot generation, the position of the maximum intensity goes outside the hole on cathode surface. This behavior could be related to the sheath width that becomes wider for lower Pr, and also to the hole diameter. It is suggested that the Pr corresponding to the maximum OES intensity at the hole depends on the geometry of the hole. A smaller diameter is required for higher Pr, depending on the sheath width. It would be necessary to optimize plasma conditions (such as Pr, excitation frequency, and Pw) relatively to the cathode-surface configuration (such as hole-size, pitch, and pattern). It is favorable that Pr is high enough for obtaining high-density plasma spots at holes on cathode surface. On the other hand, an increase in Tholder was observed during plasma observation with employing relatively high Pr and Pw which favor production of high-density plasma spots at cathode-surface holes as well as high growth
ARTICLE IN PRESS C. Niikura et al. / Solar Energy Materials & Solar Cells 90 (2006) 3223–3231
3227
1 Torr
2 Torr
Position (a.u.)
3 Torr
4 Torr
5 Torr
9.3 Torr
0
20000
40000 60000 80000 100000 Hα intensity (a.u.)
Fig. 3. Axial distribution of Ha optical emission intensity along the central axis of cathode-surface hole, for SiH4/ H2 plasma with various pressures (Pr).
rate. It indicates that gas temperature of the plasma becomes high under high Pr and Pw conditions, like thermal plasma, suggesting that the temperature of film surface is heated up by plasma.
ARTICLE IN PRESS 3228
C. Niikura et al. / Solar Energy Materials & Solar Cells 90 (2006) 3223–3231
3.2. mc-Si:H film deposition During deposition using the novel cathode, a stable intense plasma spot was observed at every hole, providing a uniformly flat-distributed high-density plasma spots near cathode surface. Under these conditions, efficient dissociation of SiH4 and H2 gives rise to highly
Growth rate (nm/s)
10 8 6 4
H2 flow rate
2
300 sccm 700 sccm
0 Defect density (cm-3)
1017
1016
1015 6 5
Ic / Ia
4 3 2 1
µc-Si:H a-Si:H
102
Photo
10-5
Dark
10-6 10-7
101 10-8 100 0
20
40
60
80
Conductivity (S/cm)
σphoto / σdark
0 103
10-9 100
SiH4 flow rate (sccm) Fig. 4. Growth rate, defect density, crystallinity, photoconductivity, dark conductivity, and the ratio as a function of silane flow rate, F(SiH4), for samples of series 1.
ARTICLE IN PRESS C. Niikura et al. / Solar Energy Materials & Solar Cells 90 (2006) 3223–3231
3229
SiH4 depleted state and thereby high-density SiHx film precursors and hydrogen atoms reach the film-growing surface. On the other hand, aforementioned film-surface heating effect by plasma may cause thermal desorption of surface-bonded hydrogen, which can deteriorate quality of deposited films, enhancing the creation of surface dangling bonds [6]. Thus, we tried to optimize properties of the mc-Si:H films grown at high rate with the novel cathode, (1) by optimizing SiH4 depletion state and (2) by alleviating the increase in the temperature of film-growing surface.
Growth rate (nm/s)
10 8 6 4 2 0 Defect density (cm-3)
1017
1016
1015 Hydrogen content (at.%)
8 6 4 2 0 5
Ic /I a
4 3 2 1
µc-Si:H
0 500
1000
1500
a-Si:H
H2 flow rate (sccm) Fig. 5. Growth rate, defect density, hydrogen content, and crystallinity as a function of hydrogen flow rate, F(H2), for samples of series 2.
ARTICLE IN PRESS 3230
C. Niikura et al. / Solar Energy Materials & Solar Cells 90 (2006) 3223–3231
First, we investigated the dependence of film properties on F(SiH4). Rd, Ns, Ic/Ia, sd, sp, and sp/sd ratio of samples of series 1 are shown in Fig. 4 as a function of F(SiH4). Though Ic/Ia is degraded slightly, Rd increases while Ns is reduced when increasing the F(SiH4). The increase in Rd with F(SiH4) is due to an increase in SiH4 partial pressure. The reduction in Ns with F(SiH4) indicates that, by adjusting F(SiH4), SiH4 depletion state, i.e., the ratio of atomic hydrogen to SiHx film precursors arriving on the growing surface is furthermore optimized, and probably that lower electron temperature associated with higher F(SiH4) is beneficial in terms of ion bombardment [7] and contribution of short-lifetime radicals, such as SiHx (x%2), on film-growing surface. sd and sp increase exponentially, whereas sp/sd ratio decreases exponentially with decreasing F(SiH4) improving Ic/Ia, and sp/sd of 101–102 with sd of 10 6–10 7 (S/cm) were obtained for mc-Si:H films. Rapid decrease in Ic/Ia for higher F(SiH4) or for higher total gas flow rate can be due to high partial pressure of SiH4 which scavenges H atoms. It suggests that when increasing the F(SiH4) or total gas flow rate, higher H2-dilution ratio is correspondingly required for obtaining sufficiently high crystallinity. Next, we tried to alleviate the aforementioned surface heating effect by plasma by increasing total gas flow rate. Practically, we increased total gas flow rate by increasing F(H2) from 700 to 1500 sccm with a constant F(SiH4) of 70 sccm (samples of series 2) increasing simultaneously H2-dilution ratio, and improved the crystallinity of the sample of series 1 prepared with an F(SiH4) of 70 sccm. Rd, Ns, CH, and Ic/Ia of samples of series 2 are shown in Fig. 5 as a function of F(H2). When increasing the F(H2), an increase in Ic/Ia and a slight reduction in Ns are simultaneously obtained, while CH increases from 4.7 to 7.2 at% and Rd decreases slightly. An increase in H2-dilution ratio was essential to improve Ic/Ia realizing higher flux ratio of
1018
Defect density (cm-3)
(i) Conventional (Low Pressure)
(ii) High Pressure Depletion
1017
1016
(iii) Novel cathode 1015 0
2
4
6
8
10
Growth rate (nm/s) Fig. 6. Defect density as a function of growth rate, for (i) mc-Si:H samples prepared by conventional low-pressure method, (ii) those prepared by HPD method and (iii) those prepared using the novel cathode with interconnected multi-holes.
ARTICLE IN PRESS C. Niikura et al. / Solar Energy Materials & Solar Cells 90 (2006) 3223–3231
3231
atomic hydrogen to film precursor reaching the film-growing surface. In addition, Ns was furthermore reduced owing to surface cooling effect by increased total gas flow rate, which is confirmed by the higher CH for samples with improved Ic/Ia. Thus, an optimization of SiH4 depletion state as well as an alleviation of surface heating by increasing total gas flow rate was found to be effective for improvement of film properties. As a result, device-grade mc-Si:H films with low Ns of 4.3–5.6 1015 cm 3, Ic/Ia of 2.6–3, and sp/sd of 101–102 were obtained at high Rd of 7.6–7.9 nm/s, by making use of the novel cathode. Ns of these samples prepared using the novel cathode is shown by closed circles as (iii) in Fig. 6, as a function of Rd. That of samples prepared by conventional lowpressure PECVD method and those prepared by the HPD method are also shown in Fig. 6, by open squares as (i) and by open triangles as (ii), respectively [1]. As seen in the figure, Ns of high-rate-grown mc-Si:H films was drastically reduced by means of the novel cathode, demonstrating the effectiveness of this high-rate-growth technique for high-quality mc-Si:H films using the novel cathode. 4. Conclusion We propose a newly designed cathode with interconnected multi-holes (hollows) combined with gas injector, which leads to a production of uniformly flat-distributed stable high-density plasma spots on cathode surface, in order to realize large-area highrate-growth of mc-Si:H films by PECVD. The spatial distribution of plasma at the holes on cathode surface was investigated by OES analyses for SiH4/H2 plasma with various Pr for optimization of deposition conditions. Optimization of SiH4 depletion state as well as alleviation of surface heating effect by plasma was found to be essential for the improvement of properties of mc-Si:H films prepared at high Rd using the novel cathode. By adjusting H2-dilution and increasing total gas flow rate, we succeeded in the reduction of Ns for high-rate-grown mc-Si:H films. As a result, device-grade mc-Si:H films with low Ns of 4–6 1015 cm 3 were obtained at high Rd of 7.6–7.9 nm/s. Acknowledgments The authors would like to thank Prof. N. Sato of Tohoku University for beneficial suggestions and Miss A. Sato for help with the ESR measurements. This work was partially supported by New Energy and Industrial Technology Development Organization (NEDO) as NEDO Industrial Technology Fellowship Program. References [1] M. Kondo, T. Nishimoto, M. Takai, S. Suzuki, Y. Nasuno, A. Matsuda, Sol. Energy Mater. Sol. Cells 78 (2003) 543. [2] L. Guo, M. Kondo, M. Fukawa, K. Saitoh, A. Matsuda, Jpn. J. Appl. Phys. 37 (1998) L1116. [3] M. Fukawa, S. Suzuki, L. Guo, M. Kondo, A. Matsuda, Sol. Energy Mater. Sol. Cells 66 (2001) 217. [4] C. Niikura, N. Itagaki, M. Kondo, Y. Kawai, A. Matsuda, Thin Solid Films, in press. [5] C. Niikura, M. Kondo, A. Matsuda, J. Non-Cryst. Solids, to be published. [6] G. Ganguly, A. Matsuda, Phys. Rev. B 47 (1993) 3661. [7] J. Robertson, J. Non-Cryst. Solids 266–269 (2000) 79.