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Characterization of H-related defects in H-implanted Si with slow positrons
Applied Surface Science 149 Ž1999. 188–192
Characterization of H-related defects in H-implanted Si with slow positrons M. Fujinami a
a,)
, R. Suzuki b, T. Ohdaira b, T. Mikado
b
AdÕanced Technology Research Laboratories, Nippon Steel Corporation, 3-35-1 Ida, Nakahara, Kawasaki 211, Japan b Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba 305, Japan
Abstract The thermal behavior of H-related defects in H-implanted Si has been investigated by positron beams. It is found that positrons are sensitive to the H-related defects and give a relatively low S parameter, equivalent to that of bulk Si, and a long lifetime. It is found that the defects terminated by H are stabilized up to 4008C and that high void density can be achieved at the narrow layer around the peak of H-implantation profile. q 1999 Elsevier Science B.V. All rights reserved. PACS: 61.72.Ji; 61.72.Tt; 68.55.Ln; 78.70.Bj Keywords: H ion implantation; H–V defects; Annealing behavior; Positron beams
1. Introduction It has been discovered that hydrogen in silicon greatly changes the electrical property of the resultant device by passivating shallow-level and deeplevel defects w1x. It has been further reported that high dose implantation of hydrogen and subsequent anneal induce a splitting of Si, which is utilized by a fabrication of silicon on insulator materials w2x. Thus, the physical and chemical state of hydrogen in Si is currently of scientific and technological interest. In spite of this, there is not much direct information on the behavior of the H-related defects. Positron annihilation spectroscopy is one of the most powerful techniques to detect the vacancy-type defects and the monoenergetic positron beam has )
Corresponding author. Department of Applied Chemistry, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan. Tel.: q81-3-5841-7237; fax: q81-3-5841-6037; E-mail: [email protected]
been extensively applied to the measurement of the defect profiles in semiconductors during the last decade w3x. Some works w4–6x on the positron study of the H-related defects in Si have been reported so far, but the Doppler broadening parameter Ž S parameter. vs. positron energy Ž S–E curve. is very complicated and the conclusive explanation for the interaction between positron and H-related defects has never been obtained. The aim of this work is to confirm that positrons are sensitive to the H-related defects by measurement of the S parameter and the positron lifetime using a positron beam. Further, the thermal evolution of the H-related defects has been investigated and an origin of splitting of Si is discussed.
2. Experimental Samples were implanted with 1 = 10 16 and 5 = 10 Hqrcm2 at an energy of 60 keV to p-type 16
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 9 . 0 0 1 9 8 - 1
M. Fujinami et al.r Applied Surface Science 149 (1999) 188–192
189
The S–E curves were taken by Nippon Steel positron beam at RT and analyzed to determine the vacancy profile in depth using POSTRAP4 w8x. The value of S was normalized to that for bulk Si. The Electrotechnical Laboratory pulsed positron beam w9x was used to measure the positron lifetime at the positron energies of 3 and 7 keV, which coincide with the near-surface Žy100 nm. and the peak of the H-implantation profile, respectively. The lifetime spectra were analyzed using RESOLUSION w10x with one or two components. Fig. 1. TDS spectra of hydrogen escaped from the Si samples implanted with 1=10 16 and 5=10 16 Hqrcm2 .
3. Results and discussion
Czochralski-SiŽ100. wafers at RT. The implantations were carried out by tilting them by 78 with respect to the beam. The mean projected range and the standard deviation were calculated to be 560 and 76 nm, respectively w7x. A furnace anneal was performed in N2 atmosphere at various temperatures.
The desorption of H from the samples implanted with 1 = 10 16 and 5 = 10 16 Hqrcm2 was measured by thermal desorption spectroscopy ŽTDS., whose spectra are shown in Fig. 1. Hydrogen is hardly mobile up to 4008C and most of hydrogen is released around 7008C. The peak around 4608C is observed in
Fig. 2. Cross-sectional transmission electron micrographs of the Si samples implanted with 1 = 10 16 and 5 = 10 16 Hqrcm2 and subsequently annealed at 6008C and 7008C.
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M. Fujinami et al.r Applied Surface Science 149 (1999) 188–192
Fig. 3. The S – E curves for the 1=10 16 Hqrcm2 sample annealed at various temperatures.
both samples, while only the 5 = 10 16 Hqrcm2 sample has the second peak around 6408C, indicating that two steps exist in the anneal stage. Cross-sectional transmission electron micrographs for the samples annealed at 600 and 7008C are exhibited in Fig. 2. For the sample implanted with 1 = 10 16 Hqrcm2 , a splitting in Si cannot be observed, but the dislocation loops are recognized around the peak of H-implantation profile. On the other hand, there are some cracks in the 5 = 10 16 Hqrcm2 sample annealed at 6008C and a clear splitting at the peak of H profile is seen by annealing at 7008C. It looks that no damage is present from the top surface to 500-nm depth. Figs. 3 and 4 show the S–E curves for the samples implanted with 1 = 10 16 and 5 = 10 16 Hqrcm2 , respectively. The results of the lifetime measurement are summarized in Table 1.
that no vacancy-type defects are induced around the peak of H profile, which is agreement with the previous positron studies. However, it is inconsistent to the Kinchin-Pease model w7x and is not correct. According to IR results w12x, the defects induced near the peak of H profile are associated with H and various types of V–H defects are formed and evolved. The Doppler broadening reflects the momentum distribution of the electrons which annihilate positrons at the traps. When the positrons are trapped at vacancy-type defects coupled with impurities, the value of S greatly depends on the configuration of the defects. For example, the defects coupled with oxygen w13x or fluorine w14x give much lower S than that of the bulk Si. Hence, it is not unreasonable that S value is as low as that of the free positron when positrons are trapped at the V–H defects. In order to confirm a positron trap to V–H defects, the positron lifetime was measured using a positron beam. The positron lifetime, 270 ps Žintensity: 100%., obtained at the energy of 7 keV corresponds to that of monovacancy in Si, indicating that positrons are trapped at vacancy-type defects. In the previous papers w4,5x, only the positron energy dependence of S was measured, so that it was misunderstood that positrons were insensitive to V–H defects. However, at the moment, it is difficult to calculate the content of the
3.1. 1 = 10 16 H qr cm 2 sample [11] For the as-implanted sample, the S parameter becomes larger than that for the bulk and the defect profile is derived from the S–E curve using the computer code of POSTRAP4, as shown in Fig. 5. The result indicates that the defects induced are located 350 nm below the top surface and it looks
Fig. 4. The S – E curves for the 5=10 16 Hqrcm2 sample annealed at various temperatures.
M. Fujinami et al.r Applied Surface Science 149 (1999) 188–192
191
Table 1 Positron lifetimes at the incident positron energies of 3 keV and 7 keV for the Si samples implanted with 1 = 10 16 and 5 = 10 16 Hqrcm2 at 60 keV after implantation and isochronal annealing 1 = 10 16 Hqrcm2
5 = 10 16 Hqrcm2
3 keV Ž100 nm.
7 keV Ž400–500 nm.
3 keV Ž100 nm.
7 keV Ž400–500 nm.
S
Lifetime Žps.
Intensity Ž%.
S
Lifetime Žps.
Intensity Ž%.
S
Lifetime Žps.
Intensity Ž%.
S
Lifetime Žps.
Intensity Ž%.
As-implanted 4008C 5008C
1.050 1.019 1.015
298 322 319
100 100 100
1.027 1.012 1.017
322 344 352
100 100 100
1.034 1.020 1.028
1.027
370
100
1.035
1.035
371
100
1.040
7008C
1.027
373
100
1.030
100 100 60 40 38 62 44 56
1.047 1.013 1.019
6008C
281 283 232 427 230 445 190 473
1.037
385
100
1.067
294 302 244 378 268 431 215 435
100 100 36 64 51 49 26 74
H–V defects, since the specific trapping rates for them are unknown. From these results, it is concluded that simple vacancy defects Že.g., divacancy. mainly exist up to 350 nm below the surface and that V–H complexes are predominant around the peak of the H-implantation profile. Annealing at 4008C results in a decrease of S even at the positron energy below 5 keV in the S–E curve. The lifetimes at energies of 3 and 7 keV become 322 and 283 ps, respectively. Generally speaking, simple divacancies in Si are mobile above 2008C and stabilized by the formation of larger
Fig. 5. The depth distribution of the defects derived from the S – E curve of the as-implanted 1=10 16 Hqrcm2 sample. The implanted H and vacancy profiles calculated by TRIM are also shown.
vacancy clusters. In this specimen, the predominant divacancies near the top surface are transformed into larger defects and, simultaneously, hydrogen which diffuses towards the surface is associated with them. That is an explanation why the low S and the long lifetime are observed in the vicinity of the surface. On the other hand, both of the S and the lifetime at the positron energy of 7 keV are immutable. It is considered that the defects which are located near the peak of H-implantation profile are stabilized and immobilized by terminating H up to 4008C. Annealing at 5008C leads to a release of most of the bound H. The lifetime at the energy of 3 keV is unchanged and the lifetime spectrum at 7 keV is decomposed into two components: a very long lifetime Ž427 ps. and the bulk one Ž232 ps.. The S–E curve at 5008C is almost similar to that at 4008C. In the vicinity of the surface, both of the S and the lifetime are unvaried, so that it is concluded that the defects near the surface are still terminated by H and immobile. On the contrary, in the region of the H-implantation profile, low S and longer lifetime are recognized, indicating that the larger voids are formed and also terminated by H. An increase in S is observed by annealing at 6008C again. The maximum value in S is taken around the positron energy of 7 keV, which coincides with the peak of H profile. This fact means that the strongly bound H is released from the voids in this annealing stage. The positron lifetime at the positron energy 7 keV is almost similar to that of the
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sample annealed at 5008C, indicating that the size of the defects is unchanged. During the evolution of defects induced by the Siw15x and the P- w16x implantation to Si, large vacancy clusters are formed and diffuse towards the surface, which plays a roll of sink of defects. In the H-related defects, V–H defects are stabilized and immobilized up to relatively high temperature, 4008C, so that it is possible that the formation of large vacancy clusters, or voids, takes place at the narrow layer inside the bulk Si, where a high defect density is achieved. These vacancy clusters are stable up to 7008C, but annealing at 8008C gets rid of any positron trap sites. 3.2. 5 = 10 16 H qr cm 2 sample This dose is larger than the critical one required for the splitting in Si. Up to 5008C, the anneal temperature dependencies of S–E curve and positron lifetime are almost identical to those of the 1 = 10 16 Hqrcm2 sample. In the samples annealed at 700 and 8008C, larger S value around the peak of H profile is observed, but their positron lifetimes are not different from those for the 1 = 10 16 Hqrcm2 sample. Further a positronium formation was not observed. If the positrons are trapped at the cracks in Si, the positronium should be formed. Therefore, this origin is considered not to be the cracks in Si, but to be the remaining microvoids. To shear the Si wafer, hydrogen acts physically as an internal pressure source and it is necessary that there are a lot of voids in the narrow region and that hydrogen content is enough to raise the internal pressure w17x. From a view of the positron results, the thermal behavior of the H-related defects of the 1 = 10 16 Hqrcm2 sample is not very different from that of the 5 = 10 16 Hqrcm2 sample and the number and the size of voids are seemed to be same. It is considered that, in the 1 = 10 16 Hqrcm2 sample, the concentration of defects is enough to produce the hydrogen sinks, but that of H itself is shortage. That is why the splitting of Si is not caused in the 1 = 10 16 Hqrcm2 sample. 4. Summary From the measurements of positron lifetime and Doppler broadening, it is proved that positrons are sensitive to V–H defects in Si. The vacancies cou-
pled with H give a similar S value to that of the bulk Si, although the lifetime is much longer than that of the bulk Si. Further, the thermal evolution of the V–H defects induced by H implantation to Si has been investigated. It is found that the defects are stabilized by H up to high temperature, 4008C, and that the large vacancy clusters, or voids, can be formed in the narrow layer, around the peak of H profile. References w1x S.J. Pearton, J.W. Corbett, M. Stavola ŽEds.., Hydrogen in Crystalline Semiconductors, Springer, Berlin, 1992, and references therein. w2x A.J. Auberton-Herve, ´ J.M. Lamure, T. Barge, M. Bruel, B. Aspar, J.L. Pelloie, Silicon-on-Insulator Technology, SemiCon West, 1995. w3x A. Dupasquier, A.P. Mills Jr. ŽEds.., Positron Spectroscopy of Solids, IOS Press, Amsterdam, 1995. w4x J. Keinonen, M. Hautala, E. Rauhala, V. Karttunen, A. Kuronen, J. Raisanen, J. Lahtinen, A. Vehanen, E. Punkka, ¨ ¨ P. Hautojarvi, Phys. Rev. B 37 Ž1988. 8269. ¨ w5x R.S. Brusa, M.D. Naia, A. Zecca, C. Nobili, G. Ottaviani, R. Tonini, A. Dupasquier, Phys. Rev. B 49 Ž1994. 7271. w6x R.A. Hakvoort, A. van Veen, P.E. Mijnarends, H. Schut, Appl. Surf. Sci. 85 Ž1995. 271. w7x J.P. Biersack, L.G. Haggmark, Nucl. Instrum. Methods 174 Ž1980. 257. w8x G.C. Aers, in: P.J. Schultz, G.R. Massoumi, P.J. Simpson ŽEds.., Positron Beams for Solids and Surfaces, Proceedings of the 4th International Workshop on Slow Positron Beam Techniques for Solids and Surfaces, AIP Conf. Proc. No. 218, AIP, New York, 1990, p. 162. w9x R. Suzuki, Y. Kobayashi, T. Mikado, H. Ohgaki, M. Chiwaki, T. Yamazaki, T. Tomimasu, Jpn. J. Appl. Phys. 30 Ž1991. L532. w10x P. Kirkegaard, M. Eldrup, O.E. Mogensen, N.J. Pedersen, Comput. Phys. Commun. 23 Ž1981. 307. w11x M. Fujinami, R. Suzuki, T. Ohdaira, T. Mikado, Phys. Rev. B 58 Ž1998. 12599. w12x B. Bech Nielsen, H.G. Grimmeiss, Phys. Rev. B 40 Ž1989. 12403. w13x M. Fujinami, Phys. Rev. B 53 Ž1996. 13047. w14x A. Uedono, T. Kitano, M. Watanabe, T. Moriya, N. Komuro, T. Kawano, S. Tanigawa, R. Suzuki, T. Ohdaira, T. Mikado, Jpn. J. Appl. Phys. 36 Ž1997. 969. w15x M. Fujinami, T. Tsuge, K. Tanaka, J. Appl. Phys. 79 Ž1996. 9017. w16x M. Fujinami, S. Hayashi, Mater. Sci. Forum 196–201 Ž1995. 1165. w17x G.F. Cerofolini, L. Meda, C. Volpones, G. Ottaviani, J. DeFayette, R. Dierckx, D. Donelli, M. Orlandini, M. Anderle, R. Canteri, C. Claeys, J. Vanhellemont, Phys. Rev. B 41 Ž1990. 12607.
Characterization of H-related defects in H-implanted Si with slow positrons M. Fujinami a
a,)
, R. Suzuki b, T. Ohdaira b, T. Mikado
b
AdÕanced Technology Research Laboratories, Nippon Steel Corporation, 3-35-1 Ida, Nakahara, Kawasaki 211, Japan b Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba 305, Japan
Abstract The thermal behavior of H-related defects in H-implanted Si has been investigated by positron beams. It is found that positrons are sensitive to the H-related defects and give a relatively low S parameter, equivalent to that of bulk Si, and a long lifetime. It is found that the defects terminated by H are stabilized up to 4008C and that high void density can be achieved at the narrow layer around the peak of H-implantation profile. q 1999 Elsevier Science B.V. All rights reserved. PACS: 61.72.Ji; 61.72.Tt; 68.55.Ln; 78.70.Bj Keywords: H ion implantation; H–V defects; Annealing behavior; Positron beams
1. Introduction It has been discovered that hydrogen in silicon greatly changes the electrical property of the resultant device by passivating shallow-level and deeplevel defects w1x. It has been further reported that high dose implantation of hydrogen and subsequent anneal induce a splitting of Si, which is utilized by a fabrication of silicon on insulator materials w2x. Thus, the physical and chemical state of hydrogen in Si is currently of scientific and technological interest. In spite of this, there is not much direct information on the behavior of the H-related defects. Positron annihilation spectroscopy is one of the most powerful techniques to detect the vacancy-type defects and the monoenergetic positron beam has )
Corresponding author. Department of Applied Chemistry, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan. Tel.: q81-3-5841-7237; fax: q81-3-5841-6037; E-mail: [email protected]
been extensively applied to the measurement of the defect profiles in semiconductors during the last decade w3x. Some works w4–6x on the positron study of the H-related defects in Si have been reported so far, but the Doppler broadening parameter Ž S parameter. vs. positron energy Ž S–E curve. is very complicated and the conclusive explanation for the interaction between positron and H-related defects has never been obtained. The aim of this work is to confirm that positrons are sensitive to the H-related defects by measurement of the S parameter and the positron lifetime using a positron beam. Further, the thermal evolution of the H-related defects has been investigated and an origin of splitting of Si is discussed.
2. Experimental Samples were implanted with 1 = 10 16 and 5 = 10 Hqrcm2 at an energy of 60 keV to p-type 16
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 9 . 0 0 1 9 8 - 1
M. Fujinami et al.r Applied Surface Science 149 (1999) 188–192
189
The S–E curves were taken by Nippon Steel positron beam at RT and analyzed to determine the vacancy profile in depth using POSTRAP4 w8x. The value of S was normalized to that for bulk Si. The Electrotechnical Laboratory pulsed positron beam w9x was used to measure the positron lifetime at the positron energies of 3 and 7 keV, which coincide with the near-surface Žy100 nm. and the peak of the H-implantation profile, respectively. The lifetime spectra were analyzed using RESOLUSION w10x with one or two components. Fig. 1. TDS spectra of hydrogen escaped from the Si samples implanted with 1=10 16 and 5=10 16 Hqrcm2 .
3. Results and discussion
Czochralski-SiŽ100. wafers at RT. The implantations were carried out by tilting them by 78 with respect to the beam. The mean projected range and the standard deviation were calculated to be 560 and 76 nm, respectively w7x. A furnace anneal was performed in N2 atmosphere at various temperatures.
The desorption of H from the samples implanted with 1 = 10 16 and 5 = 10 16 Hqrcm2 was measured by thermal desorption spectroscopy ŽTDS., whose spectra are shown in Fig. 1. Hydrogen is hardly mobile up to 4008C and most of hydrogen is released around 7008C. The peak around 4608C is observed in
Fig. 2. Cross-sectional transmission electron micrographs of the Si samples implanted with 1 = 10 16 and 5 = 10 16 Hqrcm2 and subsequently annealed at 6008C and 7008C.
190
M. Fujinami et al.r Applied Surface Science 149 (1999) 188–192
Fig. 3. The S – E curves for the 1=10 16 Hqrcm2 sample annealed at various temperatures.
both samples, while only the 5 = 10 16 Hqrcm2 sample has the second peak around 6408C, indicating that two steps exist in the anneal stage. Cross-sectional transmission electron micrographs for the samples annealed at 600 and 7008C are exhibited in Fig. 2. For the sample implanted with 1 = 10 16 Hqrcm2 , a splitting in Si cannot be observed, but the dislocation loops are recognized around the peak of H-implantation profile. On the other hand, there are some cracks in the 5 = 10 16 Hqrcm2 sample annealed at 6008C and a clear splitting at the peak of H profile is seen by annealing at 7008C. It looks that no damage is present from the top surface to 500-nm depth. Figs. 3 and 4 show the S–E curves for the samples implanted with 1 = 10 16 and 5 = 10 16 Hqrcm2 , respectively. The results of the lifetime measurement are summarized in Table 1.
that no vacancy-type defects are induced around the peak of H profile, which is agreement with the previous positron studies. However, it is inconsistent to the Kinchin-Pease model w7x and is not correct. According to IR results w12x, the defects induced near the peak of H profile are associated with H and various types of V–H defects are formed and evolved. The Doppler broadening reflects the momentum distribution of the electrons which annihilate positrons at the traps. When the positrons are trapped at vacancy-type defects coupled with impurities, the value of S greatly depends on the configuration of the defects. For example, the defects coupled with oxygen w13x or fluorine w14x give much lower S than that of the bulk Si. Hence, it is not unreasonable that S value is as low as that of the free positron when positrons are trapped at the V–H defects. In order to confirm a positron trap to V–H defects, the positron lifetime was measured using a positron beam. The positron lifetime, 270 ps Žintensity: 100%., obtained at the energy of 7 keV corresponds to that of monovacancy in Si, indicating that positrons are trapped at vacancy-type defects. In the previous papers w4,5x, only the positron energy dependence of S was measured, so that it was misunderstood that positrons were insensitive to V–H defects. However, at the moment, it is difficult to calculate the content of the
3.1. 1 = 10 16 H qr cm 2 sample [11] For the as-implanted sample, the S parameter becomes larger than that for the bulk and the defect profile is derived from the S–E curve using the computer code of POSTRAP4, as shown in Fig. 5. The result indicates that the defects induced are located 350 nm below the top surface and it looks
Fig. 4. The S – E curves for the 5=10 16 Hqrcm2 sample annealed at various temperatures.
M. Fujinami et al.r Applied Surface Science 149 (1999) 188–192
191
Table 1 Positron lifetimes at the incident positron energies of 3 keV and 7 keV for the Si samples implanted with 1 = 10 16 and 5 = 10 16 Hqrcm2 at 60 keV after implantation and isochronal annealing 1 = 10 16 Hqrcm2
5 = 10 16 Hqrcm2
3 keV Ž100 nm.
7 keV Ž400–500 nm.
3 keV Ž100 nm.
7 keV Ž400–500 nm.
S
Lifetime Žps.
Intensity Ž%.
S
Lifetime Žps.
Intensity Ž%.
S
Lifetime Žps.
Intensity Ž%.
S
Lifetime Žps.
Intensity Ž%.
As-implanted 4008C 5008C
1.050 1.019 1.015
298 322 319
100 100 100
1.027 1.012 1.017
322 344 352
100 100 100
1.034 1.020 1.028
1.027
370
100
1.035
1.035
371
100
1.040
7008C
1.027
373
100
1.030
100 100 60 40 38 62 44 56
1.047 1.013 1.019
6008C
281 283 232 427 230 445 190 473
1.037
385
100
1.067
294 302 244 378 268 431 215 435
100 100 36 64 51 49 26 74
H–V defects, since the specific trapping rates for them are unknown. From these results, it is concluded that simple vacancy defects Že.g., divacancy. mainly exist up to 350 nm below the surface and that V–H complexes are predominant around the peak of the H-implantation profile. Annealing at 4008C results in a decrease of S even at the positron energy below 5 keV in the S–E curve. The lifetimes at energies of 3 and 7 keV become 322 and 283 ps, respectively. Generally speaking, simple divacancies in Si are mobile above 2008C and stabilized by the formation of larger
Fig. 5. The depth distribution of the defects derived from the S – E curve of the as-implanted 1=10 16 Hqrcm2 sample. The implanted H and vacancy profiles calculated by TRIM are also shown.
vacancy clusters. In this specimen, the predominant divacancies near the top surface are transformed into larger defects and, simultaneously, hydrogen which diffuses towards the surface is associated with them. That is an explanation why the low S and the long lifetime are observed in the vicinity of the surface. On the other hand, both of the S and the lifetime at the positron energy of 7 keV are immutable. It is considered that the defects which are located near the peak of H-implantation profile are stabilized and immobilized by terminating H up to 4008C. Annealing at 5008C leads to a release of most of the bound H. The lifetime at the energy of 3 keV is unchanged and the lifetime spectrum at 7 keV is decomposed into two components: a very long lifetime Ž427 ps. and the bulk one Ž232 ps.. The S–E curve at 5008C is almost similar to that at 4008C. In the vicinity of the surface, both of the S and the lifetime are unvaried, so that it is concluded that the defects near the surface are still terminated by H and immobile. On the contrary, in the region of the H-implantation profile, low S and longer lifetime are recognized, indicating that the larger voids are formed and also terminated by H. An increase in S is observed by annealing at 6008C again. The maximum value in S is taken around the positron energy of 7 keV, which coincides with the peak of H profile. This fact means that the strongly bound H is released from the voids in this annealing stage. The positron lifetime at the positron energy 7 keV is almost similar to that of the
192
M. Fujinami et al.r Applied Surface Science 149 (1999) 188–192
sample annealed at 5008C, indicating that the size of the defects is unchanged. During the evolution of defects induced by the Siw15x and the P- w16x implantation to Si, large vacancy clusters are formed and diffuse towards the surface, which plays a roll of sink of defects. In the H-related defects, V–H defects are stabilized and immobilized up to relatively high temperature, 4008C, so that it is possible that the formation of large vacancy clusters, or voids, takes place at the narrow layer inside the bulk Si, where a high defect density is achieved. These vacancy clusters are stable up to 7008C, but annealing at 8008C gets rid of any positron trap sites. 3.2. 5 = 10 16 H qr cm 2 sample This dose is larger than the critical one required for the splitting in Si. Up to 5008C, the anneal temperature dependencies of S–E curve and positron lifetime are almost identical to those of the 1 = 10 16 Hqrcm2 sample. In the samples annealed at 700 and 8008C, larger S value around the peak of H profile is observed, but their positron lifetimes are not different from those for the 1 = 10 16 Hqrcm2 sample. Further a positronium formation was not observed. If the positrons are trapped at the cracks in Si, the positronium should be formed. Therefore, this origin is considered not to be the cracks in Si, but to be the remaining microvoids. To shear the Si wafer, hydrogen acts physically as an internal pressure source and it is necessary that there are a lot of voids in the narrow region and that hydrogen content is enough to raise the internal pressure w17x. From a view of the positron results, the thermal behavior of the H-related defects of the 1 = 10 16 Hqrcm2 sample is not very different from that of the 5 = 10 16 Hqrcm2 sample and the number and the size of voids are seemed to be same. It is considered that, in the 1 = 10 16 Hqrcm2 sample, the concentration of defects is enough to produce the hydrogen sinks, but that of H itself is shortage. That is why the splitting of Si is not caused in the 1 = 10 16 Hqrcm2 sample. 4. Summary From the measurements of positron lifetime and Doppler broadening, it is proved that positrons are sensitive to V–H defects in Si. The vacancies cou-
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