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Structural properties of directionally grown polycrystalline SiGe for solar cells
ARTICLE IN PRESS
Journal of Crystal Growth 275 (2005) 467–473 www.elsevier.com/locate/jcrysgro
Structural properties of directionally grown polycrystalline SiGe for solar cells Kozo Fujiwara, Wugen Pan, Noritaka Usami, Kohei Sawada, Akiko Nomura, Toru Ujihara, Toetsu Shishido, Kazuo Nakajima Institute for Materials Research (IMR), Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan Received 5 June 2004; accepted 7 December 2004 Communicated by K.W. Benz Available online 23 January 2005
Abstract We investigated structural properties of polycrystalline SiGe grown by directional growth method for solar cells. The average Ge composition was systematically changed in the range between 0 and 10 at%. Distributions of concentration and crystallographic orientation in the SiGe crystal were measured and the preferential growth orientation was found to change from (1 1 1) to (1 1 0) with increasing average Ge composition. Misfit dislocation was observed using transmission electron microscope. There was few dislocations in SiGe crystals when the average Ge composition was between 0 and 5 at% though a lot of dislocations existed at the average Ge composition of 10 at%. We concluded that the optimum Ge composition is around 5 at% for solar cells. r 2004 Elsevier B.V. All rights reserved. PACS: 61.66.Dk; 61.72.Ff; 81.10.Fq Keywords: A1. Misfit dislocation; A1. Preferential orientation; B1. Silicon–germanium; B3. Solar cell
1. Introduction In the solar cell industrial world, low-cost materials showing a high energy conversion efficiency have been demanded. At the moment, Corresponding author. Tel.: +81 22 215 2013;
fax: +81 22 215 2011. E-mail address: [email protected] (K. Fujiwara).
polycrystalline and single-crystalline silicon are majorly used for solar cells. In particular, polycrystalline Si (poly-Si) solar cell covered more than 50% of the solar cell market because poly-Si grown by cast method has an advantage over the single crystalline silicon in terms of the low production cost. However, the limited conversion efficiency of poly-Si solar cells has been the most important problem. If we can make high-quality
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.12.023
ARTICLE IN PRESS 468
K. Fujiwara et al. / Journal of Crystal Growth 275 (2005) 467–473
poly-Si, which has few defects, few metallic impurities, and uniform crystallographic orientation on the wafer surface, its solar cell properties may achieve the same level as single-crystalline solar cells. However, such a growth technique has not been established yet. Therefore, it is imperative to develop a new material, which exhibits high performance exceeding poly-Si while keeping low production cost. Polycrystalline SiGe has a possibility to satisfy the two demands of high efficiency and low production cost. Electrical and optical properties of SiGe crystal can be controlled by changing its composition [1,2]. The onset of the absorption is extended to longer wavelengths with increasing Ge concentration. Such a bandgap reduction will be more pronounced if the strain exist in the SiGe material [3]. It is expected such a bandgap reduction lead to increase of the photo-current owing to the increased absorption of near-infrared light. Nakajima et al. had proposed polycrystalline SiGe with microscopic compositional distribution for a new solar cell material [4]. They calculated a short circuit photo-current of solar cells and showed that a solar cell based on SiGe with microscopic compositional distribution exhibits larger conversion efficiency than that based on SiGe with uniform composition or pure Si. Raue et al. [5] and Geiger et al. [6] reported the electrical properties of polycrystalline SiGe grown by the Bridgman technique. However, the conversion efficiency of their SiGe solar cells was poor, as less than 4.8%. Unfortunately, the information of crystal structures such as defects, which influence the solar cell efficiency, has not been reported. Basic investigations of the structural properties of directionally grown polycrystalline SiGe have been lacking by now. In this work, we investigated the characteristic of structural properties of directionally grown polycrystalline SiGe with various average Ge composition. We discuss the range of Ge composition applicable to solar cells.
2. Experimental procedures Polycrystalline SiGe was grown by directional growth method using a Bridgman-type vertical
1723 K melt
Pull down
20 K/cm
temperature Fig. 1. Schematic image and temperature distribution inside the furnace.
furnace as shown in Fig. 1. Source materials of Si (p-type, 1–5 O cm) and Ge (non-doped, high purity of 99.9999%) were put in the silica crucible with 70 mm diameter. To avoid sticking and cracking of the crystal during cooling down, the wall of the crucible was coated with a Si3N4 layer. The sample was melted at 1723 K in argon gas atmosphere in the furnace. After that, the crucible was pulled down at a constant rate of 0.2 mm/min in the temperature gradient zone of 20 K/cm. This growth condition is similar to the commercially used cast method for solar cell materials. Initial Ge concentration in Si–Ge melt was changed between 0 and 10 at%. The crystals were cut parallel to the growth direction in wafers as shown in Fig. 2, and polished to mirror with diamond slurries. Before the orientation analysis or compositional analysis, the surface damaged layer was removed by chemical etching with HF:HNO3 ¼ 1:6 solution. The distribution of Ge concentration in the wafer was measured about 100 points on the polished wafer surface using energy-dispersive X-ray (EDX) analysis. Crystallographic orientation analysis was performed using electron back scattering diffraction pattern (EBSP) method. The specimen for transmission electron microscope (TEM) was obtained from the wafers and dislocations inside the grain were observed.
ARTICLE IN PRESS K. Fujiwara et al. / Journal of Crystal Growth 275 (2005) 467–473
469
ingot 70 mm
top
30 mm
bottom
Fig. 3. Cross-sectional image of a polycrystalline SiGe ingot cut parallel to the growth direction. Columnar grains were formed from bottom to top.
17 mm 15 mm
15 mm
each sample, microscopic compositional nonuniformity existed because of the curvature of solidus line of silicon–germanium binary phase diagram. The minimum and maximum concentrations in each sample are also shown in Fig. 4. It is usual that the range of concentration distribution is getting wider with increasing average Ge composition due to the shape of solidus curve [4,7]. 3.2. Dominant plane in the growth direction
wafers Fig. 2. Fabrication processes of SiGe and Si wafers.
3. Results and discussions 3.1. Grain structure and compositional distribution Fig. 3 shows a cross-sectional image of typical polycrystalline SiGe cut parallel to the growth direction. It is shown that the columnar grains were directionally grown from bottom to top. Such columnar structures were obtained in all samples. The concentration distributions of Ge on the wafers were measured by SEM–EDX and the results are summarized as histograms in Fig. 4. In
Preferential growth orientation of polycrystalline SiGe and polycrystalline Si was investigated using EBSP method. Orientations in the growth direction of each sample are shown in Figs. 5(a)–(d). Each color corresponds to the orientation in the colored standard stereo-triangle. In pure silicon, (1 1 1) or near (1 1 1) planes (blue color) majority dominate as shown in Fig. 5(a). This result agrees with the results of direct observation of crystal growth behaviors from silicon melt [8]. On the other hand, in SiGe crystal, fraction of (1 1 0) plane is increasing with increasing the average Ge composition. Similar tendency concerning with a preferential growth orientation were reported for GexSi1x crystal (x is more than 0.9) by Azuma et al. [9]. (1 1 0) plane was also dominant in such a Ge-rich crystal though their growth rate was two orders in magnitude slower
ARTICLE IN PRESS K. Fujiwara et al. / Journal of Crystal Growth 275 (2005) 467–473
470
60
40 30
10
20
40 20
10
0
0
0 0
(a)
Number
Number
Number
30 20
1 2 3 4 5 Ge concentration (at %) maximum : 3.8 at % minimum : 2.3 at %
0
(b)
2 4 6 8 10 Ge concentration (at %) maximum : 6.4 at % minimum : 3.8 at %
0
(c)
5 10 15 20 Ge concentration (at %) maximum : 14.8 at % minimum : 5.1 at %
Fig. 4. Histograms of concentration distribution on SiGe wafer. Average Ge composition is (a) 3 at%, (b) 5 at % and (c) 10 at%.
Fig. 5. Distribution of crystallographic orientation in polycrystalline Si and SiGe. Orientation analysis was performed vertical to the growth direction. Orientation of each color corresponds to the standard stereo-triangle. The average Ge composition is (a) pure Si, (b) 3 at%, (c) 5 at% and (d) 10 at%.
than our growth. If the crystal grows in the equilibrium condition, (1 1 1) plane should be dominant as shown in Fig. 5(a) because of the lowest surface energy of a material with diamond structure. Therefore, these results of (1 1 0) dominance in polycrystalline SiGe imply that a characteristic mechanism exists in crystal growth from binary melt.
3.3. Dislocation density in the crystals Figs. 6(a)–(d) show typical TEM images of each sample. There are few dislocations when the average Ge composition is between 0 and 5 at% as shown in Figs. 6(a)–(c). However, there exist much quantity of dislocations at average Ge composition of 10 at%. It is natural that
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471
Fig. 6. TEM images of each sample. The average Ge composition is (a) pure Si, (b) 3 at%, (c) 5 at% and (d) 10 at%.
compositional nonuniformity exist in SiGe crystals grown from melt and the distribution range of concentration extends with increasing Ge concentration in Si–Ge melt due to its phase diagram. The deviation between maximum and minimum concentration was as large as 10 at% in the crystal with average Ge composition of 10 at% as shown in Fig. 4(c). Such a large concentration deviation is considered as the reason for introduction of a large amount of misfit dislocations. Geiger et al. reported the solar cell property of polycrystalline SiGe showing low energy conversion efficiency of 4.8% [6]. It is considerable that their crystal would include many dislocations because their average Ge compositions were more than 11 at%.
4. Discussion Optimum Ge composition for solar cell: We grew polycrystalline SiGe using directional growth
method at commonly used growth rate of 0.2 mm/ min for applying solar cell. We investigated the structural properties of polycrystalline SiGe changing the average Ge composition. Firstly, it was found that the dominant orientation changes with Ge composition. As shown in Figs. 5, (1 1 0) plane was the dominant orientation in SiGe crystals though (1 1 1) plane is dominant in the pure Si. In a usual solar cell process, a surface textural structure is formed on the wafer surface by chemical etching for light trapping to achieve a higher energy conversion efficiency [10,11]. Fig. 7(a) shows the surface shape on (1 1 1) and (1 1 0) planes after etching process. Textured structure with many dents and pimples is formed on (1 1 0) surface though the (1 1 1) surface is smooth. Reflection spectra were measured on both surfaces as shown in Fig. 7(b). It can be found that (1 1 0) texture is more effective to reduce the surface reflectance compared with (1 1 1) surface. This character of (1 1 0) dominance
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Fig. 7. (a) Surface structure of (1 1 1) and (1 1 0) surface after chemical etching. Texture structure formed on the (1 1 0) surface. (b) Surface reflectivity on (1 1 1) and textured (1 1 0) surface.
in polycrystalline SiGe is one of the merits for solar cells. Secondly, we found that few dislocations were introduced even if average Ge composition increased up to 5 at% though many dislocations existed at the average Ge composition of 10 at%. Such dislocations will act as recombination centers of photogenerated carriers [12] and cause low solar cell performance. Therefore, the amount of Ge should be kept lower than the critical value to cause introduction of dislocations. In addition, Nakajima et al. suggested that SiGe crystals with microscopic compositional distribution have an advantage of increasing the absorption coefficient even though the amount of Ge is small. Totally considering these results, optimum average Ge composition is concluded to be around 5 at% for solar cells. Actually, Pan et al. fabricated SiGe solar cells using our crystals and showed higher conversion efficiency exceeding pure-Si solar cells [13].
5. Conclusions We investigated the structural properties of polycrystalline SiGe for solar cell grown by directional growth method at typically used
growth rate. It was found that the preferential orientation changed from (1 1 1) to (1 1 0) with increasing Ge concentration. Misfit dislocation is hardly increased, even if increasing Ge concentration up to around 5 at% at this growth condition. The optimum Ge composition is concluded to be about 5 at%.
Acknowledgements The authors would like to acknowledge Dr. G. Sazaki for helpful discussions. The authors also would like to acknowledge Mr. M. Kishimoto and Mr. S. Itoh for their technical supports. This work was supported by New Energy and industrial Technology Development Organization (NEDO) of Japan and by a Grant-in-Aid for Scientific Research No. 16686001 from the Ministry of Education, Culture, Sports, Science and Technology of Japan. References [1] J.M. Ruiz, J. Casado, A. Luque, Proceedings of the 12th European Photovoltaic Solar Energy Conference, Amsterdam, 1994, p. 572.
ARTICLE IN PRESS K. Fujiwara et al. / Journal of Crystal Growth 275 (2005) 467–473 [2] N. Usami, K. Fujiwara, T. Ujihara, G. Sazaki, H. Yaguchi, Y. Murakami, K. Nakajima, Jpn. J. Appl. Phys. 41 (2002) 37. [3] N. Usami, T. Ichitsubo, T. Ujihara, T. Takahashi, K. Fujiwara, G. Sazaki, K. Nakajima, J. Appl. Phys. 94 (2003) 916. [4] K. Nakajima, N. Usami, K. Fujiwara, Y. murakami, T. Ujihara, G. Sazaki, T. Shishido, Sol. Energy Mater. Sol. Cells 73 (2002) 305. [5] P. Raue, A. Lawerenz, L. Long, M. Rinio, E. Buhrig, H.J. Mo¨ller, Proceedings of the 14th European Photovoltaic Solar Energy Conference, Barcelona, 1997, p. 1791. [6] P. Geiger, P. Raue, G. Hahn, P. Fath, E. Bucher, E. Buhrig, H.J. Mo¨ller, Proceedings of the 16th European Photovoltaic Solar Energy Conference, Glasgow, 2000.
473
[7] J. Schilz, V.N. Romanenko, J. Mater. Sci.: Mater. Electr. 6 (1995) 265. [8] K. Fujiwara, Y. Obinata, T. Ujihara, N. Usami, G. Sazaki, K. Nakajima, J. Crystal Growth 266 (2004) 441. [9] Y. Azuma, N. Usami, K. Fujiwara, T. Ujihara, K. Nakajima, J. Crystal Growth, in press. [10] F. Restrepo, C.E. Backus, IEEE Trans. Electron Dev. ED23 (1976) 1195. [11] P. Campbell, M.A. Green, Sol. Energy Mater. Sol. Cells 65 (2001) 369. [12] K. Yang, G.H. Schwuttke, T.F. Ciszek, J. Crystal Growth 50 (1980) 301. [13] W. Pan, K. Fujiwara, N. Usami, T. Ujihara, K. Nakajima, R. Shimokawa, J. Appl. Phys. 96 (2004) 1238.
Journal of Crystal Growth 275 (2005) 467–473 www.elsevier.com/locate/jcrysgro
Structural properties of directionally grown polycrystalline SiGe for solar cells Kozo Fujiwara, Wugen Pan, Noritaka Usami, Kohei Sawada, Akiko Nomura, Toru Ujihara, Toetsu Shishido, Kazuo Nakajima Institute for Materials Research (IMR), Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan Received 5 June 2004; accepted 7 December 2004 Communicated by K.W. Benz Available online 23 January 2005
Abstract We investigated structural properties of polycrystalline SiGe grown by directional growth method for solar cells. The average Ge composition was systematically changed in the range between 0 and 10 at%. Distributions of concentration and crystallographic orientation in the SiGe crystal were measured and the preferential growth orientation was found to change from (1 1 1) to (1 1 0) with increasing average Ge composition. Misfit dislocation was observed using transmission electron microscope. There was few dislocations in SiGe crystals when the average Ge composition was between 0 and 5 at% though a lot of dislocations existed at the average Ge composition of 10 at%. We concluded that the optimum Ge composition is around 5 at% for solar cells. r 2004 Elsevier B.V. All rights reserved. PACS: 61.66.Dk; 61.72.Ff; 81.10.Fq Keywords: A1. Misfit dislocation; A1. Preferential orientation; B1. Silicon–germanium; B3. Solar cell
1. Introduction In the solar cell industrial world, low-cost materials showing a high energy conversion efficiency have been demanded. At the moment, Corresponding author. Tel.: +81 22 215 2013;
fax: +81 22 215 2011. E-mail address: [email protected] (K. Fujiwara).
polycrystalline and single-crystalline silicon are majorly used for solar cells. In particular, polycrystalline Si (poly-Si) solar cell covered more than 50% of the solar cell market because poly-Si grown by cast method has an advantage over the single crystalline silicon in terms of the low production cost. However, the limited conversion efficiency of poly-Si solar cells has been the most important problem. If we can make high-quality
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.12.023
ARTICLE IN PRESS 468
K. Fujiwara et al. / Journal of Crystal Growth 275 (2005) 467–473
poly-Si, which has few defects, few metallic impurities, and uniform crystallographic orientation on the wafer surface, its solar cell properties may achieve the same level as single-crystalline solar cells. However, such a growth technique has not been established yet. Therefore, it is imperative to develop a new material, which exhibits high performance exceeding poly-Si while keeping low production cost. Polycrystalline SiGe has a possibility to satisfy the two demands of high efficiency and low production cost. Electrical and optical properties of SiGe crystal can be controlled by changing its composition [1,2]. The onset of the absorption is extended to longer wavelengths with increasing Ge concentration. Such a bandgap reduction will be more pronounced if the strain exist in the SiGe material [3]. It is expected such a bandgap reduction lead to increase of the photo-current owing to the increased absorption of near-infrared light. Nakajima et al. had proposed polycrystalline SiGe with microscopic compositional distribution for a new solar cell material [4]. They calculated a short circuit photo-current of solar cells and showed that a solar cell based on SiGe with microscopic compositional distribution exhibits larger conversion efficiency than that based on SiGe with uniform composition or pure Si. Raue et al. [5] and Geiger et al. [6] reported the electrical properties of polycrystalline SiGe grown by the Bridgman technique. However, the conversion efficiency of their SiGe solar cells was poor, as less than 4.8%. Unfortunately, the information of crystal structures such as defects, which influence the solar cell efficiency, has not been reported. Basic investigations of the structural properties of directionally grown polycrystalline SiGe have been lacking by now. In this work, we investigated the characteristic of structural properties of directionally grown polycrystalline SiGe with various average Ge composition. We discuss the range of Ge composition applicable to solar cells.
2. Experimental procedures Polycrystalline SiGe was grown by directional growth method using a Bridgman-type vertical
1723 K melt
Pull down
20 K/cm
temperature Fig. 1. Schematic image and temperature distribution inside the furnace.
furnace as shown in Fig. 1. Source materials of Si (p-type, 1–5 O cm) and Ge (non-doped, high purity of 99.9999%) were put in the silica crucible with 70 mm diameter. To avoid sticking and cracking of the crystal during cooling down, the wall of the crucible was coated with a Si3N4 layer. The sample was melted at 1723 K in argon gas atmosphere in the furnace. After that, the crucible was pulled down at a constant rate of 0.2 mm/min in the temperature gradient zone of 20 K/cm. This growth condition is similar to the commercially used cast method for solar cell materials. Initial Ge concentration in Si–Ge melt was changed between 0 and 10 at%. The crystals were cut parallel to the growth direction in wafers as shown in Fig. 2, and polished to mirror with diamond slurries. Before the orientation analysis or compositional analysis, the surface damaged layer was removed by chemical etching with HF:HNO3 ¼ 1:6 solution. The distribution of Ge concentration in the wafer was measured about 100 points on the polished wafer surface using energy-dispersive X-ray (EDX) analysis. Crystallographic orientation analysis was performed using electron back scattering diffraction pattern (EBSP) method. The specimen for transmission electron microscope (TEM) was obtained from the wafers and dislocations inside the grain were observed.
ARTICLE IN PRESS K. Fujiwara et al. / Journal of Crystal Growth 275 (2005) 467–473
469
ingot 70 mm
top
30 mm
bottom
Fig. 3. Cross-sectional image of a polycrystalline SiGe ingot cut parallel to the growth direction. Columnar grains were formed from bottom to top.
17 mm 15 mm
15 mm
each sample, microscopic compositional nonuniformity existed because of the curvature of solidus line of silicon–germanium binary phase diagram. The minimum and maximum concentrations in each sample are also shown in Fig. 4. It is usual that the range of concentration distribution is getting wider with increasing average Ge composition due to the shape of solidus curve [4,7]. 3.2. Dominant plane in the growth direction
wafers Fig. 2. Fabrication processes of SiGe and Si wafers.
3. Results and discussions 3.1. Grain structure and compositional distribution Fig. 3 shows a cross-sectional image of typical polycrystalline SiGe cut parallel to the growth direction. It is shown that the columnar grains were directionally grown from bottom to top. Such columnar structures were obtained in all samples. The concentration distributions of Ge on the wafers were measured by SEM–EDX and the results are summarized as histograms in Fig. 4. In
Preferential growth orientation of polycrystalline SiGe and polycrystalline Si was investigated using EBSP method. Orientations in the growth direction of each sample are shown in Figs. 5(a)–(d). Each color corresponds to the orientation in the colored standard stereo-triangle. In pure silicon, (1 1 1) or near (1 1 1) planes (blue color) majority dominate as shown in Fig. 5(a). This result agrees with the results of direct observation of crystal growth behaviors from silicon melt [8]. On the other hand, in SiGe crystal, fraction of (1 1 0) plane is increasing with increasing the average Ge composition. Similar tendency concerning with a preferential growth orientation were reported for GexSi1x crystal (x is more than 0.9) by Azuma et al. [9]. (1 1 0) plane was also dominant in such a Ge-rich crystal though their growth rate was two orders in magnitude slower
ARTICLE IN PRESS K. Fujiwara et al. / Journal of Crystal Growth 275 (2005) 467–473
470
60
40 30
10
20
40 20
10
0
0
0 0
(a)
Number
Number
Number
30 20
1 2 3 4 5 Ge concentration (at %) maximum : 3.8 at % minimum : 2.3 at %
0
(b)
2 4 6 8 10 Ge concentration (at %) maximum : 6.4 at % minimum : 3.8 at %
0
(c)
5 10 15 20 Ge concentration (at %) maximum : 14.8 at % minimum : 5.1 at %
Fig. 4. Histograms of concentration distribution on SiGe wafer. Average Ge composition is (a) 3 at%, (b) 5 at % and (c) 10 at%.
Fig. 5. Distribution of crystallographic orientation in polycrystalline Si and SiGe. Orientation analysis was performed vertical to the growth direction. Orientation of each color corresponds to the standard stereo-triangle. The average Ge composition is (a) pure Si, (b) 3 at%, (c) 5 at% and (d) 10 at%.
than our growth. If the crystal grows in the equilibrium condition, (1 1 1) plane should be dominant as shown in Fig. 5(a) because of the lowest surface energy of a material with diamond structure. Therefore, these results of (1 1 0) dominance in polycrystalline SiGe imply that a characteristic mechanism exists in crystal growth from binary melt.
3.3. Dislocation density in the crystals Figs. 6(a)–(d) show typical TEM images of each sample. There are few dislocations when the average Ge composition is between 0 and 5 at% as shown in Figs. 6(a)–(c). However, there exist much quantity of dislocations at average Ge composition of 10 at%. It is natural that
ARTICLE IN PRESS K. Fujiwara et al. / Journal of Crystal Growth 275 (2005) 467–473
471
Fig. 6. TEM images of each sample. The average Ge composition is (a) pure Si, (b) 3 at%, (c) 5 at% and (d) 10 at%.
compositional nonuniformity exist in SiGe crystals grown from melt and the distribution range of concentration extends with increasing Ge concentration in Si–Ge melt due to its phase diagram. The deviation between maximum and minimum concentration was as large as 10 at% in the crystal with average Ge composition of 10 at% as shown in Fig. 4(c). Such a large concentration deviation is considered as the reason for introduction of a large amount of misfit dislocations. Geiger et al. reported the solar cell property of polycrystalline SiGe showing low energy conversion efficiency of 4.8% [6]. It is considerable that their crystal would include many dislocations because their average Ge compositions were more than 11 at%.
4. Discussion Optimum Ge composition for solar cell: We grew polycrystalline SiGe using directional growth
method at commonly used growth rate of 0.2 mm/ min for applying solar cell. We investigated the structural properties of polycrystalline SiGe changing the average Ge composition. Firstly, it was found that the dominant orientation changes with Ge composition. As shown in Figs. 5, (1 1 0) plane was the dominant orientation in SiGe crystals though (1 1 1) plane is dominant in the pure Si. In a usual solar cell process, a surface textural structure is formed on the wafer surface by chemical etching for light trapping to achieve a higher energy conversion efficiency [10,11]. Fig. 7(a) shows the surface shape on (1 1 1) and (1 1 0) planes after etching process. Textured structure with many dents and pimples is formed on (1 1 0) surface though the (1 1 1) surface is smooth. Reflection spectra were measured on both surfaces as shown in Fig. 7(b). It can be found that (1 1 0) texture is more effective to reduce the surface reflectance compared with (1 1 1) surface. This character of (1 1 0) dominance
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Fig. 7. (a) Surface structure of (1 1 1) and (1 1 0) surface after chemical etching. Texture structure formed on the (1 1 0) surface. (b) Surface reflectivity on (1 1 1) and textured (1 1 0) surface.
in polycrystalline SiGe is one of the merits for solar cells. Secondly, we found that few dislocations were introduced even if average Ge composition increased up to 5 at% though many dislocations existed at the average Ge composition of 10 at%. Such dislocations will act as recombination centers of photogenerated carriers [12] and cause low solar cell performance. Therefore, the amount of Ge should be kept lower than the critical value to cause introduction of dislocations. In addition, Nakajima et al. suggested that SiGe crystals with microscopic compositional distribution have an advantage of increasing the absorption coefficient even though the amount of Ge is small. Totally considering these results, optimum average Ge composition is concluded to be around 5 at% for solar cells. Actually, Pan et al. fabricated SiGe solar cells using our crystals and showed higher conversion efficiency exceeding pure-Si solar cells [13].
5. Conclusions We investigated the structural properties of polycrystalline SiGe for solar cell grown by directional growth method at typically used
growth rate. It was found that the preferential orientation changed from (1 1 1) to (1 1 0) with increasing Ge concentration. Misfit dislocation is hardly increased, even if increasing Ge concentration up to around 5 at% at this growth condition. The optimum Ge composition is concluded to be about 5 at%.
Acknowledgements The authors would like to acknowledge Dr. G. Sazaki for helpful discussions. The authors also would like to acknowledge Mr. M. Kishimoto and Mr. S. Itoh for their technical supports. This work was supported by New Energy and industrial Technology Development Organization (NEDO) of Japan and by a Grant-in-Aid for Scientific Research No. 16686001 from the Ministry of Education, Culture, Sports, Science and Technology of Japan. References [1] J.M. Ruiz, J. Casado, A. Luque, Proceedings of the 12th European Photovoltaic Solar Energy Conference, Amsterdam, 1994, p. 572.
ARTICLE IN PRESS K. Fujiwara et al. / Journal of Crystal Growth 275 (2005) 467–473 [2] N. Usami, K. Fujiwara, T. Ujihara, G. Sazaki, H. Yaguchi, Y. Murakami, K. Nakajima, Jpn. J. Appl. Phys. 41 (2002) 37. [3] N. Usami, T. Ichitsubo, T. Ujihara, T. Takahashi, K. Fujiwara, G. Sazaki, K. Nakajima, J. Appl. Phys. 94 (2003) 916. [4] K. Nakajima, N. Usami, K. Fujiwara, Y. murakami, T. Ujihara, G. Sazaki, T. Shishido, Sol. Energy Mater. Sol. Cells 73 (2002) 305. [5] P. Raue, A. Lawerenz, L. Long, M. Rinio, E. Buhrig, H.J. Mo¨ller, Proceedings of the 14th European Photovoltaic Solar Energy Conference, Barcelona, 1997, p. 1791. [6] P. Geiger, P. Raue, G. Hahn, P. Fath, E. Bucher, E. Buhrig, H.J. Mo¨ller, Proceedings of the 16th European Photovoltaic Solar Energy Conference, Glasgow, 2000.
473
[7] J. Schilz, V.N. Romanenko, J. Mater. Sci.: Mater. Electr. 6 (1995) 265. [8] K. Fujiwara, Y. Obinata, T. Ujihara, N. Usami, G. Sazaki, K. Nakajima, J. Crystal Growth 266 (2004) 441. [9] Y. Azuma, N. Usami, K. Fujiwara, T. Ujihara, K. Nakajima, J. Crystal Growth, in press. [10] F. Restrepo, C.E. Backus, IEEE Trans. Electron Dev. ED23 (1976) 1195. [11] P. Campbell, M.A. Green, Sol. Energy Mater. Sol. Cells 65 (2001) 369. [12] K. Yang, G.H. Schwuttke, T.F. Ciszek, J. Crystal Growth 50 (1980) 301. [13] W. Pan, K. Fujiwara, N. Usami, T. Ujihara, K. Nakajima, R. Shimokawa, J. Appl. Phys. 96 (2004) 1238.