- Email: [email protected]
Guiding principles for device-grade hydrogenated amorphous silicon films and design of catalytic chemical vapor deposition apparatus
Thin Solid Films 395 (2001) 112–115
Guiding principles for device-grade hydrogenated amorphous silicon films and design of catalytic chemical vapor deposition apparatus Atsushi Masuda*, Hideki Matsumura School of Materials Science, Japan Advanced Institute of Science and Technology (JAIST), Asahidai, Tatsunokuchi, Ishikawa 923-1292, Japan
Abstract The guiding principles for obtaining device-grade hydrogenated amorphous silicon (a-Si:H) films with high deposition rate on large-area substrates by catalytic chemical vapor deposition (Cat-CVD) are presented. The most important points are controlling both the heat flow and the atomic H. Other points of note are the suppression of silicide formation on the catalyzer and control of the gas flow. Solar cells and thin-film transistors using a-Si:H films thus obtained show excellent device performance. Largearea deposition, high deposition rate and high efficiency of gas use are also promising for the application of Cat-CVD a-Si:H films to these devices. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Catalytic chemical vapor deposition; Hydrogenated amorphous silicon; Heat control; Atomic hydrogen; Efficiency of gas use
1. Introduction Catalytic chemical vapor deposition (Cat-CVD) w1x, often called hot-wire CVD, is one of the promising methods for obtaining hydrogenated amorphous silicon (a-Si:H) films for solar cells and thin-film transistors (TFTs), since high deposition rate, large-area deposition and high efficiency of gas use are expected. The H content in such a-Si:H films was also confirmed to be approximately 2–3 at.% w2,3x, which is the possible reason why light-induced degradation is quite small in Cat-CVD a-Si:H films. However, it is not necessarily easy to obtain devicegrade a-Si:H films without guidance, since there are few commercial Cat-CVD apparatuses. Here, some guiding principles for obtaining device-grade a-Si:H films are presented. The most important points are controlling both the heat flow and the atomic H generated on the catalyzer. Other points of note are the suppression of silicide formation on the catalyzer and control of the gas flow. Cat-CVD apparatuses designed by the guiding principles thus presented are required for industrial * Corresponding author. Tel.: q81-761-51-1563; fax: q81-76151-1149. E-mail address: [email protected] (A. Masuda).
application of Cat-CVD a-Si:H films for solar cells and TFTs. 2. Heat control Heat from the catalyzer is transported to the growing surface of a-Si:H films by both heat radiation and gas molecules or decomposed species, causing H desorption and a high dangling-bond (DB) density. Of course, a decrease in the catalyzer surface area leads to a decrease in the heat transferred to the growing surface. However, the deposition rate also decreases with a decrease in the catalyzer surface area. A tungsten plate perpendicular to the substrate holder, termed a ‘catalytic plate’, was proposed for solving this trade-off problem w4x. It was found that anisotropic thermal radiation is realized using this catalytic plate. In comparison with a conventional hot wire with the same surface area, the same deposition ˚ y s is obtained with lower heating of the rate of 18 A substrate surface by 708C, as shown in Fig. 1. However, it is inevitable that a large amount of heat is transferred to the growing surface in order to increase the deposition rate or area. In such a case, control of the heat flow is a key factor in suppressing degradation of the film quality. Heat transferred to the sample on the substrate holder should be immediately transported to the sub-
0040-6090/01/$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 1 . 0 1 2 2 4 - X
A. Masuda, H. Matsumura / Thin Solid Films 395 (2001) 112–115
Fig. 1. Relationship between the deposition rate of a-Si:H films and the substrate surface temperature at various catalyzer temperatures for catalytic plate and conventional hot-wire catalyzers.
strate holder, which should be cooled by air or water, and the substrate temperature should be controlled by the heater in the substrate holder, independently of the heat from the catalyzer. In order to realize the rapid transport of heat from the sample to the sample holder, chucking the sample on the substrate holder is very important for enhancing the heat contact between them. The development of a chucking method for large-size insulating glass wafers is urgently required, as electrostatic chucking brings about a dramatic improvement in heat control in a Cat-CVD apparatus for semiconductor substrates, such as Si or GaAs w5x. 3. Control of atomic H and chamber cleaning Atomic H also plays an important role for a-Si:H film growth, not only by governing the reaction on the growing surface, but also by generating unexpected species. It is thought that a large amount of atomic H is generated in Cat-CVD, since the decomposition probability of gas molecules is much higher than that in plasma-enhanced CVD (PECVD). It was also reported by our group that atomic H generated on the catalyzer etches even single-crystalline Si with an etch rate greater ˚ y min w6x, and that SiH4 and Si2H6 molethan 2000 A cules are generated by the reaction between atomic H and Si films covering the chamber wall before deposition w7x. Fig. 2 shows the DB density and linewidth of the electron spin resonance (ESR) signal of DBs for a-Si:H films with a thickness of approximately 1 mm on mechanically chucked fused-quartz substrates as a function of cumulative deposited film thickness on the chamber wall w8x. It is clearly shown that a-Si:H film quality is degraded with an increase in film thickness on the chamber wall to over 5 mm. Therefore, chamber cleaning is one of the key issues in obtaining high reproducibility of a-Si:H film quality. The chamber for a-Si:H film deposition is easily cleaned using atomic H
113
generated on the catalyzer from H2 gas. Recently, chamber cleaning using NF3 gas was also reported in a Cat-CVD apparatus w9x, and is also applicable to SiNx film deposition systems. The chamber design is also essential for decreasing the interaction between atomic H and the previously deposited films on the walls. For example, heating the chamber wall is useful to suppress this reaction, since the etch rate of Si by atomic H is drastically suppressed above approximately 1008C w10x. The influence of atomic H in a-Si:H deposition onto a transparent conducting oxide (TCO) substrate is also a concern for the fabrication of superstrate-type solar cells using Cat-CVD a-Si:H films. However, a minimal decrease in the transmittance was observed by covering the SnO2 with a layer of ZnO, showing that atomic H generated during a-Si:H deposition induces little damage to the TCO substrates w11x. 4. Suppression of silicide formation on catalyzer and large-area deposition Low catalyzer temperatures of approximately 1600– 17008C and high SiH4 flow rates are often employed for a-Si:H deposition; however, silicide formation on the catalyzer often occurs under such conditions. It is thus necessary to pay attention to the electrical or mechanical connections to the catalyzer, since the catalyzer temperature is easily lowered at the cold connections, inducing silicide formation. Recently, a gas showerhead equipped with a catalyzer has been developed w9x. The flow paths of SiH4 and those of H2 or noble gases are separated in the gas showerhead, from which these gases are separately induced. The electrical connections to the catalyzer exist within a confined space, inside the gas showerhead, filled with H2 or a noble gas. Therefore, the cool areas of the catalyzer are protected by H2 or a noble gas, eliminating exposure to SiH4, such that no silicide formation occurs under the conditions for a-Si:H deposition w12x.
Fig. 2. DB density and linewidth of ESR signal of DBs for a-Si:H films with a thickness of approximately 1 mm as a function of cumulative deposited film thickness on the chamber wall.
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A. Masuda, H. Matsumura / Thin Solid Films 395 (2001) 112–115
5. Efficiency of gas use
Fig. 3. Deposition-rate distribution for a-Si:H films as a function of the distance from the catalyzer. Arrow shows the gas-flow direction, perpendicular to the catalyzer.
The newly developed gas showerhead also brings about a drastic revolution in large-area deposition for Cat-CVD. To date, the fixed end points of the catalyzer should not be set within the line-of-sight of the substrate, since if silicide formation occurs at these points, film quality is degraded because of either a change in the decomposed species or an increase in the contamination with an increase in the vapor pressure. Therefore, a long wire is needed for large-area deposition and the wire often hangs down or changes shape. As a result, largearea deposition in Cat-CVD has, until now, been limited by these reasons. On the other hand, since no silicide formation occurs at the fixed end points of the catalyzer using the newly developed gas showerhead, short wires are employed, so that the catalyzer y substrate spacing does not change. Very large-area deposition is easily realized by simply combining several gas showerheads with favorable forms, considering their weight or the shape of the substrate. Gas-flow control from the gas showerhead to the substrate through the catalyzer is also important for large-area deposition. Fig. 3 shows the deposition rate distribution for a-Si:H films as a function of the lateral distance from the catalyzer w13x. The catalyzer is set at the position xs0 mm in the reactor tube and is oriented perpendicular to the gas-flow direction. Gas flow along the center axis of the tube is shown by an arrow in Fig. 3. Note the increased deposition rate downstream from the catalyzer as the total gas-flow rate of SiH4 and H2 increases. It was concluded from these experiments that to realize uniform film growth, attention should be paid to both realizing uniform gas flow in the case of gas pressure above approximately 10 Pa, and a gas-flow velocity of greater than several m y s.
It has been shown that the decomposition probability of SiH4 gas on the catalyzer at 20008C is approximately 40% in the conditions often used for a-Si:H deposition w13x. Therefore, for efficient gas use above 70%, the required number of collisions of one SiH4 molecule with the catalyzer surface is only two or three, since the decomposition probability is so large. For both high deposition rate and high gas use efficiency, it is important to balance the amount of SiH4 supplied with the catalyzer surface area. Fig. 4 shows the estimated relationship between the flow rate of SiH4 and the catalyzer length at various gas pressures during deposition to realize a gas-use efficiency of 60% w14x. Here, the catalyzer diameter and temperature are 0.5 mm and 2300 K, respectively. It is suggested that a gas-use efficiency of 60% is maintained by increasing the catalyzer surface area or the gas pressure, even if the flow rate of SiH4 increases for high deposition rate. 6. Application to thin-film devices and conclusions Using the guiding principles discussed above enabled us to deposit Cat-CVD a-Si:H films at a high deposition ˚ y s that exhibit low DB density of rate of 10–20 A 16 3 3=10 y cm after light soaking w3,4x, thus showing the importance of these principles with regard to the deposition of high quality Cat-CVD a-Si:H films. In addition, superstrate-type solar cells (glass y SnO2 y p y buffer y i y n y ZnO y Ag y Al) using Cat-CVD a-Si:H ilayers and photo-CVD a-Si:H p- and n-layers were fabricated. These devices used a B-doped a-SiCx:H player of 10–15 nm, an undoped a-SiCx:H buffer layer of 10 nm, an i-layer of 230 nm and a P-doped a-Si:H n-layer of 60 nm, and were deposited on a SnO2-covered glass substrate. Both the p y i and n y i interfaces were exposed to air to allow transport between deposition chambers; the deposition chamber for the i-layer was at our institute and those for the p- and n-layers were at
Fig. 4. Estimated relationship between the flow rate of SiH4 and the catalyzer length at various gas pressures for maintaining a fixed efficiency of gas use of 60%.
A. Masuda, H. Matsumura / Thin Solid Films 395 (2001) 112–115
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laboratory for fabricating solar cells and for stimulating discussions. They also wish to express their sincere thanks to N. Honda, Y. Ishibashi, M. Sakai, K. Imamori and Dr K. Ishibashi for their collaborative study presented in this paper, as well as fruitful discussions. This work was supported in part by the R&D Projects in Cooperation with Academic Institutions ‘Cat-CVD Fabrication Processes for Semiconductor Devices’ entrusted by the New Energy and Industrial Technology Development Organization (NEDO) to the Ishikawa Sunrise Industries Creation Organization (ISICO) and carried out at the Japan Advanced Institute of Science and Technology (JAIST). References Fig. 5. Typical current density–voltage characteristics for solar cells with Cat-CVD a-Si:H i-layer.
the Tokyo Institute of Technology. The i-layer was ˚ y s. prepared at 3408C with a deposition rate of 11 A Fig. 5 shows the typical current density–voltage characteristics. The initial efficiency is 5.0% and the efficiency after light soaking is 4.6%. It was also confirmed that the quantum efficiency in the region above 700 nm is quite high, probably because the optical band gap of Cat-CVD i layer is relatively small, approximately 1.6 eV, due to a low H content of 2–3 at.%. Recently Sakai et al. w15x also reported that bottomgate TFTs with a field effect mobility of approximately 0.85 cm2 y V s have been obtained by Cat-CVD. In these TFTs, not only was the a-Si:H active layer prepared by Cat-CVD, but also the SiNx gate insulator and the nqa-Si:H contact layer. These examinations reveal that Cat-CVD a-Si:H films are promising for industrial applications, such as solar cells and TFTs. The authors hope that Cat-CVD apparatuses for the manufacture of solar cells or TFTs will be developed following the guiding principles discussed above. It is hoped that a large number of researchers in universities and academic institutes can thus prepare device-grade a-Si:H films by Cat-CVD. Acknowledgements The authors are very grateful to Prof M. Konagai of the Tokyo Institute of Technology and the staff of his
w1x H. Matsumura, Jpn. J. Appl. Phys. 37 (1998) 3175. w2x A.H. Mahan, J. Carapella, B.P. Nelson, R.S. Crandall, I. Balberg, J. Appl. Phys. 69 (1991) 6728. w3x A. Masuda, C. Niikura, Y. Ishibashi, H. Matsumura, Sol. Energy Mater. Sol. Cells 66 (2001) 259. w4x A. Masuda, Y. Ishibashi, H. Matsumura, Mater. Res. Soc. Symp. Proc. 609 (2000) A6.3.1. w5x M. Karasawa, A. Masuda, K. Ishibashi, H. Matsumura, Extended Abstracts of the 1st International Conference on Cat-CVD (HotWire CVD) Processes, Kanazawa, 2000, p. 117. w6x A. Izumi, H. Sato, S. Hashioka, M. Kudo, H. Matsumura, Microelectron. Eng. 51y52 (2000) 495. w7x Y. Nozaki, K. Kongo, T. Miyazaki, M. Kitazoe, K. Horii, H. Umemoto, A. Masuda, H. Matsumura, J. Appl. Phys. 88 (2000) 5437. w8x A. Masuda, Y. Ishibashi, A. Izumi, H. Matsumura: Dig. Tech. Papers, International Workshop on Active-Matrix Liquid-Crystal Displays — TFT Technologies and Related Materials — Tokyo, 2000, p. 219. w9x K. Ishibashi, Extended Abstracts of the 1st International Conference on Cat-CVD (Hot-Wire CVD) Processes, Kanazawa, 2000, p. 45. w10x K. Kamesaki, A. Masuda, A. Izumi, H. Matsumura, Extended Abstracts of the 1st International Conference on Cat-CVD (HotWire CVD) Processes, Kanazawa, 2000, p. 85. w11x K. Imamori, A. Masuda, H. Matsumura, Extended Abstracts of the 1st International Conference on Cat-CVD (Hot-Wire CVD) Processes, Kanazawa, 2000, p. 147. w12x M. Karasawa, A. Masuda, K. Ishibashi, M. Tanaka and H. Matsumura, in: Extended Abstracts of the 48th Spring Meeting of the Japan Society of Applied Physics, Tokyo, 2001, in p. 949 win Japanesex. w13x N. Honda, A. Masuda, H. Matsumura, J. Non-Cryst. Solids 266–269 (2000) 100. w14x N. Honda, A. Masuda and H. Matsumura, unpublished. w15x M. Sakai, T. Tsutsumi, T. Yoshioka, A. Masuda, H. Matsumura, Extended Abstracts of the 1st International Conference on CatCVD (Hot-Wire CVD) Processes, Kanazawa, 2000, p. 311.
Guiding principles for device-grade hydrogenated amorphous silicon films and design of catalytic chemical vapor deposition apparatus Atsushi Masuda*, Hideki Matsumura School of Materials Science, Japan Advanced Institute of Science and Technology (JAIST), Asahidai, Tatsunokuchi, Ishikawa 923-1292, Japan
Abstract The guiding principles for obtaining device-grade hydrogenated amorphous silicon (a-Si:H) films with high deposition rate on large-area substrates by catalytic chemical vapor deposition (Cat-CVD) are presented. The most important points are controlling both the heat flow and the atomic H. Other points of note are the suppression of silicide formation on the catalyzer and control of the gas flow. Solar cells and thin-film transistors using a-Si:H films thus obtained show excellent device performance. Largearea deposition, high deposition rate and high efficiency of gas use are also promising for the application of Cat-CVD a-Si:H films to these devices. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Catalytic chemical vapor deposition; Hydrogenated amorphous silicon; Heat control; Atomic hydrogen; Efficiency of gas use
1. Introduction Catalytic chemical vapor deposition (Cat-CVD) w1x, often called hot-wire CVD, is one of the promising methods for obtaining hydrogenated amorphous silicon (a-Si:H) films for solar cells and thin-film transistors (TFTs), since high deposition rate, large-area deposition and high efficiency of gas use are expected. The H content in such a-Si:H films was also confirmed to be approximately 2–3 at.% w2,3x, which is the possible reason why light-induced degradation is quite small in Cat-CVD a-Si:H films. However, it is not necessarily easy to obtain devicegrade a-Si:H films without guidance, since there are few commercial Cat-CVD apparatuses. Here, some guiding principles for obtaining device-grade a-Si:H films are presented. The most important points are controlling both the heat flow and the atomic H generated on the catalyzer. Other points of note are the suppression of silicide formation on the catalyzer and control of the gas flow. Cat-CVD apparatuses designed by the guiding principles thus presented are required for industrial * Corresponding author. Tel.: q81-761-51-1563; fax: q81-76151-1149. E-mail address: [email protected] (A. Masuda).
application of Cat-CVD a-Si:H films for solar cells and TFTs. 2. Heat control Heat from the catalyzer is transported to the growing surface of a-Si:H films by both heat radiation and gas molecules or decomposed species, causing H desorption and a high dangling-bond (DB) density. Of course, a decrease in the catalyzer surface area leads to a decrease in the heat transferred to the growing surface. However, the deposition rate also decreases with a decrease in the catalyzer surface area. A tungsten plate perpendicular to the substrate holder, termed a ‘catalytic plate’, was proposed for solving this trade-off problem w4x. It was found that anisotropic thermal radiation is realized using this catalytic plate. In comparison with a conventional hot wire with the same surface area, the same deposition ˚ y s is obtained with lower heating of the rate of 18 A substrate surface by 708C, as shown in Fig. 1. However, it is inevitable that a large amount of heat is transferred to the growing surface in order to increase the deposition rate or area. In such a case, control of the heat flow is a key factor in suppressing degradation of the film quality. Heat transferred to the sample on the substrate holder should be immediately transported to the sub-
0040-6090/01/$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 1 . 0 1 2 2 4 - X
A. Masuda, H. Matsumura / Thin Solid Films 395 (2001) 112–115
Fig. 1. Relationship between the deposition rate of a-Si:H films and the substrate surface temperature at various catalyzer temperatures for catalytic plate and conventional hot-wire catalyzers.
strate holder, which should be cooled by air or water, and the substrate temperature should be controlled by the heater in the substrate holder, independently of the heat from the catalyzer. In order to realize the rapid transport of heat from the sample to the sample holder, chucking the sample on the substrate holder is very important for enhancing the heat contact between them. The development of a chucking method for large-size insulating glass wafers is urgently required, as electrostatic chucking brings about a dramatic improvement in heat control in a Cat-CVD apparatus for semiconductor substrates, such as Si or GaAs w5x. 3. Control of atomic H and chamber cleaning Atomic H also plays an important role for a-Si:H film growth, not only by governing the reaction on the growing surface, but also by generating unexpected species. It is thought that a large amount of atomic H is generated in Cat-CVD, since the decomposition probability of gas molecules is much higher than that in plasma-enhanced CVD (PECVD). It was also reported by our group that atomic H generated on the catalyzer etches even single-crystalline Si with an etch rate greater ˚ y min w6x, and that SiH4 and Si2H6 molethan 2000 A cules are generated by the reaction between atomic H and Si films covering the chamber wall before deposition w7x. Fig. 2 shows the DB density and linewidth of the electron spin resonance (ESR) signal of DBs for a-Si:H films with a thickness of approximately 1 mm on mechanically chucked fused-quartz substrates as a function of cumulative deposited film thickness on the chamber wall w8x. It is clearly shown that a-Si:H film quality is degraded with an increase in film thickness on the chamber wall to over 5 mm. Therefore, chamber cleaning is one of the key issues in obtaining high reproducibility of a-Si:H film quality. The chamber for a-Si:H film deposition is easily cleaned using atomic H
113
generated on the catalyzer from H2 gas. Recently, chamber cleaning using NF3 gas was also reported in a Cat-CVD apparatus w9x, and is also applicable to SiNx film deposition systems. The chamber design is also essential for decreasing the interaction between atomic H and the previously deposited films on the walls. For example, heating the chamber wall is useful to suppress this reaction, since the etch rate of Si by atomic H is drastically suppressed above approximately 1008C w10x. The influence of atomic H in a-Si:H deposition onto a transparent conducting oxide (TCO) substrate is also a concern for the fabrication of superstrate-type solar cells using Cat-CVD a-Si:H films. However, a minimal decrease in the transmittance was observed by covering the SnO2 with a layer of ZnO, showing that atomic H generated during a-Si:H deposition induces little damage to the TCO substrates w11x. 4. Suppression of silicide formation on catalyzer and large-area deposition Low catalyzer temperatures of approximately 1600– 17008C and high SiH4 flow rates are often employed for a-Si:H deposition; however, silicide formation on the catalyzer often occurs under such conditions. It is thus necessary to pay attention to the electrical or mechanical connections to the catalyzer, since the catalyzer temperature is easily lowered at the cold connections, inducing silicide formation. Recently, a gas showerhead equipped with a catalyzer has been developed w9x. The flow paths of SiH4 and those of H2 or noble gases are separated in the gas showerhead, from which these gases are separately induced. The electrical connections to the catalyzer exist within a confined space, inside the gas showerhead, filled with H2 or a noble gas. Therefore, the cool areas of the catalyzer are protected by H2 or a noble gas, eliminating exposure to SiH4, such that no silicide formation occurs under the conditions for a-Si:H deposition w12x.
Fig. 2. DB density and linewidth of ESR signal of DBs for a-Si:H films with a thickness of approximately 1 mm as a function of cumulative deposited film thickness on the chamber wall.
114
A. Masuda, H. Matsumura / Thin Solid Films 395 (2001) 112–115
5. Efficiency of gas use
Fig. 3. Deposition-rate distribution for a-Si:H films as a function of the distance from the catalyzer. Arrow shows the gas-flow direction, perpendicular to the catalyzer.
The newly developed gas showerhead also brings about a drastic revolution in large-area deposition for Cat-CVD. To date, the fixed end points of the catalyzer should not be set within the line-of-sight of the substrate, since if silicide formation occurs at these points, film quality is degraded because of either a change in the decomposed species or an increase in the contamination with an increase in the vapor pressure. Therefore, a long wire is needed for large-area deposition and the wire often hangs down or changes shape. As a result, largearea deposition in Cat-CVD has, until now, been limited by these reasons. On the other hand, since no silicide formation occurs at the fixed end points of the catalyzer using the newly developed gas showerhead, short wires are employed, so that the catalyzer y substrate spacing does not change. Very large-area deposition is easily realized by simply combining several gas showerheads with favorable forms, considering their weight or the shape of the substrate. Gas-flow control from the gas showerhead to the substrate through the catalyzer is also important for large-area deposition. Fig. 3 shows the deposition rate distribution for a-Si:H films as a function of the lateral distance from the catalyzer w13x. The catalyzer is set at the position xs0 mm in the reactor tube and is oriented perpendicular to the gas-flow direction. Gas flow along the center axis of the tube is shown by an arrow in Fig. 3. Note the increased deposition rate downstream from the catalyzer as the total gas-flow rate of SiH4 and H2 increases. It was concluded from these experiments that to realize uniform film growth, attention should be paid to both realizing uniform gas flow in the case of gas pressure above approximately 10 Pa, and a gas-flow velocity of greater than several m y s.
It has been shown that the decomposition probability of SiH4 gas on the catalyzer at 20008C is approximately 40% in the conditions often used for a-Si:H deposition w13x. Therefore, for efficient gas use above 70%, the required number of collisions of one SiH4 molecule with the catalyzer surface is only two or three, since the decomposition probability is so large. For both high deposition rate and high gas use efficiency, it is important to balance the amount of SiH4 supplied with the catalyzer surface area. Fig. 4 shows the estimated relationship between the flow rate of SiH4 and the catalyzer length at various gas pressures during deposition to realize a gas-use efficiency of 60% w14x. Here, the catalyzer diameter and temperature are 0.5 mm and 2300 K, respectively. It is suggested that a gas-use efficiency of 60% is maintained by increasing the catalyzer surface area or the gas pressure, even if the flow rate of SiH4 increases for high deposition rate. 6. Application to thin-film devices and conclusions Using the guiding principles discussed above enabled us to deposit Cat-CVD a-Si:H films at a high deposition ˚ y s that exhibit low DB density of rate of 10–20 A 16 3 3=10 y cm after light soaking w3,4x, thus showing the importance of these principles with regard to the deposition of high quality Cat-CVD a-Si:H films. In addition, superstrate-type solar cells (glass y SnO2 y p y buffer y i y n y ZnO y Ag y Al) using Cat-CVD a-Si:H ilayers and photo-CVD a-Si:H p- and n-layers were fabricated. These devices used a B-doped a-SiCx:H player of 10–15 nm, an undoped a-SiCx:H buffer layer of 10 nm, an i-layer of 230 nm and a P-doped a-Si:H n-layer of 60 nm, and were deposited on a SnO2-covered glass substrate. Both the p y i and n y i interfaces were exposed to air to allow transport between deposition chambers; the deposition chamber for the i-layer was at our institute and those for the p- and n-layers were at
Fig. 4. Estimated relationship between the flow rate of SiH4 and the catalyzer length at various gas pressures for maintaining a fixed efficiency of gas use of 60%.
A. Masuda, H. Matsumura / Thin Solid Films 395 (2001) 112–115
115
laboratory for fabricating solar cells and for stimulating discussions. They also wish to express their sincere thanks to N. Honda, Y. Ishibashi, M. Sakai, K. Imamori and Dr K. Ishibashi for their collaborative study presented in this paper, as well as fruitful discussions. This work was supported in part by the R&D Projects in Cooperation with Academic Institutions ‘Cat-CVD Fabrication Processes for Semiconductor Devices’ entrusted by the New Energy and Industrial Technology Development Organization (NEDO) to the Ishikawa Sunrise Industries Creation Organization (ISICO) and carried out at the Japan Advanced Institute of Science and Technology (JAIST). References Fig. 5. Typical current density–voltage characteristics for solar cells with Cat-CVD a-Si:H i-layer.
the Tokyo Institute of Technology. The i-layer was ˚ y s. prepared at 3408C with a deposition rate of 11 A Fig. 5 shows the typical current density–voltage characteristics. The initial efficiency is 5.0% and the efficiency after light soaking is 4.6%. It was also confirmed that the quantum efficiency in the region above 700 nm is quite high, probably because the optical band gap of Cat-CVD i layer is relatively small, approximately 1.6 eV, due to a low H content of 2–3 at.%. Recently Sakai et al. w15x also reported that bottomgate TFTs with a field effect mobility of approximately 0.85 cm2 y V s have been obtained by Cat-CVD. In these TFTs, not only was the a-Si:H active layer prepared by Cat-CVD, but also the SiNx gate insulator and the nqa-Si:H contact layer. These examinations reveal that Cat-CVD a-Si:H films are promising for industrial applications, such as solar cells and TFTs. The authors hope that Cat-CVD apparatuses for the manufacture of solar cells or TFTs will be developed following the guiding principles discussed above. It is hoped that a large number of researchers in universities and academic institutes can thus prepare device-grade a-Si:H films by Cat-CVD. Acknowledgements The authors are very grateful to Prof M. Konagai of the Tokyo Institute of Technology and the staff of his
w1x H. Matsumura, Jpn. J. Appl. Phys. 37 (1998) 3175. w2x A.H. Mahan, J. Carapella, B.P. Nelson, R.S. Crandall, I. Balberg, J. Appl. Phys. 69 (1991) 6728. w3x A. Masuda, C. Niikura, Y. Ishibashi, H. Matsumura, Sol. Energy Mater. Sol. Cells 66 (2001) 259. w4x A. Masuda, Y. Ishibashi, H. Matsumura, Mater. Res. Soc. Symp. Proc. 609 (2000) A6.3.1. w5x M. Karasawa, A. Masuda, K. Ishibashi, H. Matsumura, Extended Abstracts of the 1st International Conference on Cat-CVD (HotWire CVD) Processes, Kanazawa, 2000, p. 117. w6x A. Izumi, H. Sato, S. Hashioka, M. Kudo, H. Matsumura, Microelectron. Eng. 51y52 (2000) 495. w7x Y. Nozaki, K. Kongo, T. Miyazaki, M. Kitazoe, K. Horii, H. Umemoto, A. Masuda, H. Matsumura, J. Appl. Phys. 88 (2000) 5437. w8x A. Masuda, Y. Ishibashi, A. Izumi, H. Matsumura: Dig. Tech. Papers, International Workshop on Active-Matrix Liquid-Crystal Displays — TFT Technologies and Related Materials — Tokyo, 2000, p. 219. w9x K. Ishibashi, Extended Abstracts of the 1st International Conference on Cat-CVD (Hot-Wire CVD) Processes, Kanazawa, 2000, p. 45. w10x K. Kamesaki, A. Masuda, A. Izumi, H. Matsumura, Extended Abstracts of the 1st International Conference on Cat-CVD (HotWire CVD) Processes, Kanazawa, 2000, p. 85. w11x K. Imamori, A. Masuda, H. Matsumura, Extended Abstracts of the 1st International Conference on Cat-CVD (Hot-Wire CVD) Processes, Kanazawa, 2000, p. 147. w12x M. Karasawa, A. Masuda, K. Ishibashi, M. Tanaka and H. Matsumura, in: Extended Abstracts of the 48th Spring Meeting of the Japan Society of Applied Physics, Tokyo, 2001, in p. 949 win Japanesex. w13x N. Honda, A. Masuda, H. Matsumura, J. Non-Cryst. Solids 266–269 (2000) 100. w14x N. Honda, A. Masuda and H. Matsumura, unpublished. w15x M. Sakai, T. Tsutsumi, T. Yoshioka, A. Masuda, H. Matsumura, Extended Abstracts of the 1st International Conference on CatCVD (Hot-Wire CVD) Processes, Kanazawa, 2000, p. 311.