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Electron-beam-induced-current study of artificial twist boundaries in bonded Si wafers
Journal of Crystal Growth 210 (2000) 90}93
Electron-beam-induced-current study of arti"cial twist boundaries in bonded Si wafers K. Ikeda!,1, T. Sekiguchi!,*, S. Ito!, M. Takebe", M. Suezawa! !Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan "Technology Research Laboratory, Shimadzu Corp., Kyoto 604-8511, Japan
Abstract Several pairs of (1 0 0)Si wafers were directly bonded with known misorientation angles, between 0.1 and 113, to form arti"cial twist boundaries. The structure and electrical property of these boundaries were studied by means of transmission electron microscopy (TEM) and electron-beam-induced-current (EBIC) method. TEM images showed that the dominant defects at boundaries were screw dislocations which compensate the twist component. All the twist boundaries were EBIC active at room temperature and their contrast values have increased with the increase in the twist angle. The EBIC pro"les of twist boundaries were well "tted with the Donolato's model. Although EBIC contrasts of all the boundaries increased with decreasing temperature, the increase of 0.13 boundary was the most signi"cant. Such temperature dependencies indicates that shallow levels are dominant in the smaller angle boundaries, while certain amounts of deep levels are introduced in larger angle boundaries due to dislocation interaction. ( 2000 Elsevier Science B.V. All rights reserved. PACS: 71.55.Cn; 73.20.Hb Keywords: Si; Bonding; Twist boundary; EBIC; Dislocation
1. Introduction The interfaces of directly bonded silicon wafers can be regarded as the model of grain boundaries. We can fabricate arti"cial grain boundaries of any character and free from metallic contamination with such a bonding technique [1,2]. In the study of electrical activity of actual grain boundaries in low-grade polycrystalline Si, it is * Corresponding author. Tel.: #81-22-215-2042; fax: #8122-215-2041. E-mail address: [email protected] (T. Sekiguchi) 1 Present address. Central R&D Laboratories, Taiyo-Yuden Co. Ltd. Haruna-Machi 370-3347, Japan.
hard to estimate the amount of metallic impurities incorporated in the grain boundaries. Since a small amount of metallic impurities, like Cu or Fe, strongly a!ects the electrical activities of grain boundaries [3] and/or dislocations [4,5], it is di$cult to study the inherent electrical activity of grain boundaries with such specimens. In this work, we fabricated arti"cial (1 0 0) twist boundaries in Si free from metallic contamination by direct bonding technique and studied the structure and electrical property of these boundaries by means of transmission electron microscope (TEM) and electron-beam-induced-current (EBIC) method. The EBIC activity of grain boundaries was discussed in terms of the dislocation density of twist boundaries.
0022-0248/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 9 ) 0 0 6 5 3 - 3
K. Ikeda et al. / Journal of Crystal Growth 210 (2000) 90}93
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2. Experimental procedure Arti"cial twist boundaries were fabricated by direct bonding technique. The (1 0 0) Cz-grown Si wafers of 6A in size were prepared. They were n-type doped with P at around 1015 cm~3. Prior to the bonding, the wafers were dipped in the conc. HF solution (50%) to remove surface oxide layer. Then, two Si wafers were put together at room temperature with the twist angles of 0.1, 1, 5 and 113. They were annealed at 11003C for 2 h in N ambient to 2 improve the bonding strength, afterwards. TEM specimens were prepared by the mechanical grinding on both sides of bonded wafers followed by the ion thinning technique. Plan-views of the boundaries were observed by 200 kV TEM. For EBIC observation, slices of the (0 1 1) cross sections were cut to place the grain boundary perpendicular to the surface. Schottky contacts were made by evaporation of Au with 250 As thick. EBIC measurements were done with a scanning electron microscope (TOPCON DS-130) in an EBIC mode [6]. The geometry of EBIC observation is shown in Fig. 1. EBIC images were taken with an electron beam of 20 kV and 1 nA. The electron beam was scanned digitally over 320]320 points and the corresponding EBIC signals are stored in a memory of the computer. The EBIC contrast of the defect was de"ned by
Fig. 1. Geometry of the EBIC observation.
C"(I !I )/I , (1) " $ " where I and I are the EBIC currents at the " $ background and at the interface, respectively.
3. Results and discussion Fig. 2. TEM images of (a) 0.13 and (b) 13 twist boundaries.
The plan-views of 0.1 and 13 twist boundaries are shown in Fig. 2. In 0.13 twist boundary, a regularly arranged square network of the b/2S0 1 1T screw dislocations was observed. The interval of dislocation is 200 nm, corresponding with the twist angle of 0.13. The dominant defects at the boundary were dislocations and other defects such as oxygen precipitates were not so densely existed. In 13 twist boundary, a network of screw dislocations existed at 20 nm spacing, but the network was not so regular compared with that of the 0.13 boundary.
We did not succeed to observe dislocation networks in 5 and 113 twist boundaries probably due to small spacing. Estimated dislocation densities of these specimens are 1]105, 1]106, 4]106, and 1]107 cm~1, respectively. In EBIC observation, all the twist boundaries were seen as dark lines. Fig. 3a shows an EBIC image of 0.13 twist boundary at room temperature. Each dislocation on the boundary was
92
K. Ikeda et al. / Journal of Crystal Growth 210 (2000) 90}93
Fig. 4. Temperature dependence of EBIC contrasts at the twist boundaries.
Fig. 3. (a) EBIC image of 0.13 twist boundary at room temperature and (b) its contrast pro"le. Solid line is a "tting curve according to the Donolato's model.
not distinguished due to low resolution of EBIC image. The pro"le of EBIC contrast away from the boundary is shown in Fig. 3b. EBIC contrast at the boundary was 2.9%, and it gradually decreased to zero. This pro"le was "tted with the Donolato's model [7,8] assuming point source generation. The solid line is a "tting curve using the least-squares method. The "tting parameters were determined as v "1.5]105 cm/s, a"0.64, and ¸"26 lm, 4 where v , a, ¸ are grain boundary recombination 4 velocity, source depth in the unit of electron range and di!usion length, respectively. In the previous paper [9], we "tted these pro"les with two exponential functions taking into account of two types of defects. However, this "tting is much better than the previous model. This result suggests that only one parameter v is enough to represent the 4 recombination phenomena at the grain boundary. More re"ned "tting taking account of the "nite size of carrier generation volume is now in progress. Fig. 4 shows the temperature dependence of EBIC contrasts at the twist boundaries. The EBIC
Fig. 5. EBIC contrasts at room temperature (C (295 K)), the di!erence between the maximum EBIC contrast and that of 295 K (*C), and their ratio (*C/C (295 K)) plotted against dislocation density.
contrasts at room temperature was increased with the twist angle, in other words, the dislocation density at the twist boundary. This dependence is replotted against the dislocation density, which is shown as circles in Fig. 5. The EBIC contrast at room temperature seems to saturate at the dislocation density of 4]106 cm~1 (53 boundary). The overlap of carrier capture radius of adjacent dislocations may be the reason of this saturation. At lower temperatures, EBIC contrasts in all the specimens increased and became maximum
K. Ikeda et al. / Journal of Crystal Growth 210 (2000) 90}93
at 70}170 K. No clear dependence of EBIC contrast on the twist angle was found at lower temperatures. According to the study of the EBIC contrast of dislocations [10}12], the shallow levels are EBIC active at lower temperatures while deep ones at higher temperatures. In this paper, we tentatively assume that the density of the deep levels and shallow levels are proportional to the temperature independent and dependent component of EBIC contrast. These values are roughly represented by the EBIC contrast at room temperature (C(295 K)) and the di!erence between the maximum EBIC contrast and that at room temperature (*C), respectively. Thus, *C/C (295 K) is a tentative measure of the ratio of `shallowa to `deepa level concentration. These values are plotted in Fig. 5 against the dislocation density at the boundary. The result indicates that shallow levels are dominant only in the 0.13 twist boundary and considerable amounts of deep levels are introduced in larger angle twist boundaries. The dislocation network of 13 twist boundary was more irregular than that of 0.13 boundary in Fig. 1. It indicates that the amount of the defects produced by the dislocation interaction increases with the twist angle. Such defects may be the origin of deep levels in larger angle twist boundaries.
4. Conclusions Arti"cial (1 0 0) twist boundaries in Si were fabricated by direct bonding technique. The networks of two sets of screw dislocations were observed in the 0.1 and 13 twist boundaries. EBIC pro"les across the twist boundaries were well "tted with the Donolato's model. EBIC contrast at room temperature was higher in larger angle twist boundaries. Simple analysis of temperature dependence of EBIC contrast showed that shallow levels were
93
dominant in 0.13 twist boundary and considerable amount of deep levels was introduced in larger angle boundaries probably by the dislocation interaction.
Acknowledgements The authors express their gratitude to Mr. K. Fujimoto and Mr. H. Furukawa of Komatsu Electric Metals Co. Ltd. for the preparation of the bonded Si wafers. They also thanks to the Materials Information Science Group of the IMR, Tohoku University for their support of the computing facilities.
References [1] U. Gosele, H. Stenzel, M. Reiche, T. Martini, H. Steinkirchner, Q.-Y. Tong, Solid State Phenomenon 47}48 (1996) 33. [2] M. Benamara, A. Rocher, P. Sopena, A. Claverie, A. Laporte, G. Sarrabayrouse, L. Lescouzeres, A. PeyreLavigne, Mater. Sci. Eng. B 42 (1996) 164. [3] S. Martinuzzi, I. Perichaud, S. McHugo, Solid State Phenom. 63}64 (1998) 53. [4] B. Shen, T. Sekiguchi, J. Jablonski, K. Sumino, J. Appl. Phys. 76 (1994) 4540. [5] T. Sekiguchi, S. Kusanagi, B. Shen, K. Sumino, in: H.R. Hu!, W. Bergholtz, K. Sumino (Eds.), Semiconductor Silicon/1994, Electrochemical Society, 1994, p. 659. [6] T. Sekiguchi, K. Sumino, Rev. Sci. Instr. 66 (1995) 4277. [7] C. Donolato, J. Appl. Phys. 54 (1983) 1314. [8] R. Corkisch, T. Puzzer, A.B. Sproul, K.L. Luke, J. Appl. Phys. 84 (1998) 5473. [9] K. Ikeda, T. Sekiguchi, S. Ito, M. Suezawa, Solid State Phenom. 63}64 (1998) 481. [10] Z.J. Radzimski, T.Q. Zhou, A. Buczkowski, G.A. Rozgonyi, D. Flinn, L.G. Hellwig, J.A. Ross, Appl. Phys. Lett. 60 (1992) 1096. [11] M. Kittler, W. Seifert, Phys. Stat. Sol. (a) 138 (1993) 687. [12] S. Kusanagi, T. Sekiguchi, B. Shen, K. Sumino, Mater. Sci. Technol. 11 (1995) 685.
Electron-beam-induced-current study of arti"cial twist boundaries in bonded Si wafers K. Ikeda!,1, T. Sekiguchi!,*, S. Ito!, M. Takebe", M. Suezawa! !Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan "Technology Research Laboratory, Shimadzu Corp., Kyoto 604-8511, Japan
Abstract Several pairs of (1 0 0)Si wafers were directly bonded with known misorientation angles, between 0.1 and 113, to form arti"cial twist boundaries. The structure and electrical property of these boundaries were studied by means of transmission electron microscopy (TEM) and electron-beam-induced-current (EBIC) method. TEM images showed that the dominant defects at boundaries were screw dislocations which compensate the twist component. All the twist boundaries were EBIC active at room temperature and their contrast values have increased with the increase in the twist angle. The EBIC pro"les of twist boundaries were well "tted with the Donolato's model. Although EBIC contrasts of all the boundaries increased with decreasing temperature, the increase of 0.13 boundary was the most signi"cant. Such temperature dependencies indicates that shallow levels are dominant in the smaller angle boundaries, while certain amounts of deep levels are introduced in larger angle boundaries due to dislocation interaction. ( 2000 Elsevier Science B.V. All rights reserved. PACS: 71.55.Cn; 73.20.Hb Keywords: Si; Bonding; Twist boundary; EBIC; Dislocation
1. Introduction The interfaces of directly bonded silicon wafers can be regarded as the model of grain boundaries. We can fabricate arti"cial grain boundaries of any character and free from metallic contamination with such a bonding technique [1,2]. In the study of electrical activity of actual grain boundaries in low-grade polycrystalline Si, it is * Corresponding author. Tel.: #81-22-215-2042; fax: #8122-215-2041. E-mail address: [email protected] (T. Sekiguchi) 1 Present address. Central R&D Laboratories, Taiyo-Yuden Co. Ltd. Haruna-Machi 370-3347, Japan.
hard to estimate the amount of metallic impurities incorporated in the grain boundaries. Since a small amount of metallic impurities, like Cu or Fe, strongly a!ects the electrical activities of grain boundaries [3] and/or dislocations [4,5], it is di$cult to study the inherent electrical activity of grain boundaries with such specimens. In this work, we fabricated arti"cial (1 0 0) twist boundaries in Si free from metallic contamination by direct bonding technique and studied the structure and electrical property of these boundaries by means of transmission electron microscope (TEM) and electron-beam-induced-current (EBIC) method. The EBIC activity of grain boundaries was discussed in terms of the dislocation density of twist boundaries.
0022-0248/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 9 ) 0 0 6 5 3 - 3
K. Ikeda et al. / Journal of Crystal Growth 210 (2000) 90}93
91
2. Experimental procedure Arti"cial twist boundaries were fabricated by direct bonding technique. The (1 0 0) Cz-grown Si wafers of 6A in size were prepared. They were n-type doped with P at around 1015 cm~3. Prior to the bonding, the wafers were dipped in the conc. HF solution (50%) to remove surface oxide layer. Then, two Si wafers were put together at room temperature with the twist angles of 0.1, 1, 5 and 113. They were annealed at 11003C for 2 h in N ambient to 2 improve the bonding strength, afterwards. TEM specimens were prepared by the mechanical grinding on both sides of bonded wafers followed by the ion thinning technique. Plan-views of the boundaries were observed by 200 kV TEM. For EBIC observation, slices of the (0 1 1) cross sections were cut to place the grain boundary perpendicular to the surface. Schottky contacts were made by evaporation of Au with 250 As thick. EBIC measurements were done with a scanning electron microscope (TOPCON DS-130) in an EBIC mode [6]. The geometry of EBIC observation is shown in Fig. 1. EBIC images were taken with an electron beam of 20 kV and 1 nA. The electron beam was scanned digitally over 320]320 points and the corresponding EBIC signals are stored in a memory of the computer. The EBIC contrast of the defect was de"ned by
Fig. 1. Geometry of the EBIC observation.
C"(I !I )/I , (1) " $ " where I and I are the EBIC currents at the " $ background and at the interface, respectively.
3. Results and discussion Fig. 2. TEM images of (a) 0.13 and (b) 13 twist boundaries.
The plan-views of 0.1 and 13 twist boundaries are shown in Fig. 2. In 0.13 twist boundary, a regularly arranged square network of the b/2S0 1 1T screw dislocations was observed. The interval of dislocation is 200 nm, corresponding with the twist angle of 0.13. The dominant defects at the boundary were dislocations and other defects such as oxygen precipitates were not so densely existed. In 13 twist boundary, a network of screw dislocations existed at 20 nm spacing, but the network was not so regular compared with that of the 0.13 boundary.
We did not succeed to observe dislocation networks in 5 and 113 twist boundaries probably due to small spacing. Estimated dislocation densities of these specimens are 1]105, 1]106, 4]106, and 1]107 cm~1, respectively. In EBIC observation, all the twist boundaries were seen as dark lines. Fig. 3a shows an EBIC image of 0.13 twist boundary at room temperature. Each dislocation on the boundary was
92
K. Ikeda et al. / Journal of Crystal Growth 210 (2000) 90}93
Fig. 4. Temperature dependence of EBIC contrasts at the twist boundaries.
Fig. 3. (a) EBIC image of 0.13 twist boundary at room temperature and (b) its contrast pro"le. Solid line is a "tting curve according to the Donolato's model.
not distinguished due to low resolution of EBIC image. The pro"le of EBIC contrast away from the boundary is shown in Fig. 3b. EBIC contrast at the boundary was 2.9%, and it gradually decreased to zero. This pro"le was "tted with the Donolato's model [7,8] assuming point source generation. The solid line is a "tting curve using the least-squares method. The "tting parameters were determined as v "1.5]105 cm/s, a"0.64, and ¸"26 lm, 4 where v , a, ¸ are grain boundary recombination 4 velocity, source depth in the unit of electron range and di!usion length, respectively. In the previous paper [9], we "tted these pro"les with two exponential functions taking into account of two types of defects. However, this "tting is much better than the previous model. This result suggests that only one parameter v is enough to represent the 4 recombination phenomena at the grain boundary. More re"ned "tting taking account of the "nite size of carrier generation volume is now in progress. Fig. 4 shows the temperature dependence of EBIC contrasts at the twist boundaries. The EBIC
Fig. 5. EBIC contrasts at room temperature (C (295 K)), the di!erence between the maximum EBIC contrast and that of 295 K (*C), and their ratio (*C/C (295 K)) plotted against dislocation density.
contrasts at room temperature was increased with the twist angle, in other words, the dislocation density at the twist boundary. This dependence is replotted against the dislocation density, which is shown as circles in Fig. 5. The EBIC contrast at room temperature seems to saturate at the dislocation density of 4]106 cm~1 (53 boundary). The overlap of carrier capture radius of adjacent dislocations may be the reason of this saturation. At lower temperatures, EBIC contrasts in all the specimens increased and became maximum
K. Ikeda et al. / Journal of Crystal Growth 210 (2000) 90}93
at 70}170 K. No clear dependence of EBIC contrast on the twist angle was found at lower temperatures. According to the study of the EBIC contrast of dislocations [10}12], the shallow levels are EBIC active at lower temperatures while deep ones at higher temperatures. In this paper, we tentatively assume that the density of the deep levels and shallow levels are proportional to the temperature independent and dependent component of EBIC contrast. These values are roughly represented by the EBIC contrast at room temperature (C(295 K)) and the di!erence between the maximum EBIC contrast and that at room temperature (*C), respectively. Thus, *C/C (295 K) is a tentative measure of the ratio of `shallowa to `deepa level concentration. These values are plotted in Fig. 5 against the dislocation density at the boundary. The result indicates that shallow levels are dominant only in the 0.13 twist boundary and considerable amounts of deep levels are introduced in larger angle twist boundaries. The dislocation network of 13 twist boundary was more irregular than that of 0.13 boundary in Fig. 1. It indicates that the amount of the defects produced by the dislocation interaction increases with the twist angle. Such defects may be the origin of deep levels in larger angle twist boundaries.
4. Conclusions Arti"cial (1 0 0) twist boundaries in Si were fabricated by direct bonding technique. The networks of two sets of screw dislocations were observed in the 0.1 and 13 twist boundaries. EBIC pro"les across the twist boundaries were well "tted with the Donolato's model. EBIC contrast at room temperature was higher in larger angle twist boundaries. Simple analysis of temperature dependence of EBIC contrast showed that shallow levels were
93
dominant in 0.13 twist boundary and considerable amount of deep levels was introduced in larger angle boundaries probably by the dislocation interaction.
Acknowledgements The authors express their gratitude to Mr. K. Fujimoto and Mr. H. Furukawa of Komatsu Electric Metals Co. Ltd. for the preparation of the bonded Si wafers. They also thanks to the Materials Information Science Group of the IMR, Tohoku University for their support of the computing facilities.
References [1] U. Gosele, H. Stenzel, M. Reiche, T. Martini, H. Steinkirchner, Q.-Y. Tong, Solid State Phenomenon 47}48 (1996) 33. [2] M. Benamara, A. Rocher, P. Sopena, A. Claverie, A. Laporte, G. Sarrabayrouse, L. Lescouzeres, A. PeyreLavigne, Mater. Sci. Eng. B 42 (1996) 164. [3] S. Martinuzzi, I. Perichaud, S. McHugo, Solid State Phenom. 63}64 (1998) 53. [4] B. Shen, T. Sekiguchi, J. Jablonski, K. Sumino, J. Appl. Phys. 76 (1994) 4540. [5] T. Sekiguchi, S. Kusanagi, B. Shen, K. Sumino, in: H.R. Hu!, W. Bergholtz, K. Sumino (Eds.), Semiconductor Silicon/1994, Electrochemical Society, 1994, p. 659. [6] T. Sekiguchi, K. Sumino, Rev. Sci. Instr. 66 (1995) 4277. [7] C. Donolato, J. Appl. Phys. 54 (1983) 1314. [8] R. Corkisch, T. Puzzer, A.B. Sproul, K.L. Luke, J. Appl. Phys. 84 (1998) 5473. [9] K. Ikeda, T. Sekiguchi, S. Ito, M. Suezawa, Solid State Phenom. 63}64 (1998) 481. [10] Z.J. Radzimski, T.Q. Zhou, A. Buczkowski, G.A. Rozgonyi, D. Flinn, L.G. Hellwig, J.A. Ross, Appl. Phys. Lett. 60 (1992) 1096. [11] M. Kittler, W. Seifert, Phys. Stat. Sol. (a) 138 (1993) 687. [12] S. Kusanagi, T. Sekiguchi, B. Shen, K. Sumino, Mater. Sci. Technol. 11 (1995) 685.