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Extended-type defects created by high temperature helium implantation into silicon
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 242 (2006) 565–567 www.elsevier.com/locate/nimb
Extended-type defects created by high temperature helium implantation into silicon M.F. Beaufort *, S.E. Donnelly, S. Rousselet, M.L. David, J.F. Barbot Laboratoire de Me´tallurgie Physique, UMR 6630, Universite´ de Poitiers, SP2MI, BP 30179, 86962 Chasseneuil-Futuroscope cedex, France Institute for Materials Research, University of Salford, Manchester M5 4WT, UK Available online 6 October 2005
Abstract Following helium implantation (50 keV, 5 · 1016 cm2) at 800 C in silicon, only {1 1 3} defects are present spreading out beyond the maximum of the damage distribution. Both linear rod-like defects as well as so-called ribbon-like {1 1 3} defects are observed. During annealing at 800 C, the number of interstitials in ribbon-like defects appears to increase at the expense of the rod-like defects. After annealing at 1000 C, only a row of dislocation loops is observed. These results suggest that the formation energy of the ribbon-like defects may be lower than that of the rod-like defects. 2005 Elsevier B.V. All rights reserved. PACS: 61.72.V; 61.72.C; 68.37.L Keywords: Defects; Ion implantation; Helium; Silicon; TEM
1. Introduction Cavities can be formed in silicon by high fluence helium implantation followed by thermal annealing at high temperature [1]. However, depending on the implantation parameters, interstitial type defects ({1 1 3} defects and dislocations) can also be created [2,3]. Such defects have been extensively studied for the case of B, Si or P implantation [4–6]. In a recent paper, we have reported the effect of implantation temperature on defects created by high fluence He implantation [7]. We have found that, depending on the implantation temperature, cavities are formed in different stages related to the mobility and stability of the helium and vacancies involved in their formation. Moreover, we have observed that high temperature implantation leads * Corresponding author. Address: Laboratoire de Me´tallurgie Physique, UMR 6630, Universite´ de Poitiers, SP2MI, BP 30179, 86962 ChasseneuilFuturoscope cedex, France. Tel.: +33 5 49 49 68 34; fax: +33 5 49 49 66 92. E-mail address: [email protected] (M.F. Beaufort).
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.08.145
to the formation of extended interstitial-type defects namely, rod-like defects (RdDs), ribbon-like defects (RbnDs) and dislocation loops (DLs). For implantation temperatures below 400 C, only small clusters of interstitials and dislocations are formed. Above 400 C, small RdDs are created and from 600 C, RbnDs are formed in addition to RdDs. In this article, we report on the defects created by an 800 C helium implantation at high fluence into silicon, and some preliminary results are presented regarding their evolution with annealing temperature. 2. Experiments All the experiments were performed on n–n+ silicon wafers. The n-type layer (1014 P cm3) was epitaxially grown on a h1 1 1i oriented n+ substrate of Czochralski silicon. The specimens were implanted at 800 C. The fluence and energy were kept constant at 5 · 1016 He/cm2 and 50 keV respectively (Rp = 500 nm, DRp = 140 nm according to SRIM calculations [8]). The ion beam current was in the range 20–40 lA leading to an average implantation time of 50 min. Specimens were annealed either at 800 C or
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M.F. Beaufort et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) 565–567
1000 C in a tubular furnace under high vacuum. The structure of the implantation damage was studied by means of cross-sectional transmission electron microscopy (TEM) using a JEOL 200 CX microscope. 3. Results and discussion After helium implantation at 50 keV to a fluence of 5 · 1016 cm2 at 800 C, no cavities are found in the sample as most of vacancies and He escape from the sample during implantation [7]. The weak beam dark field (WBDF) image, Fig. 1(a), reveals extended defects located between 500 and 900 nm from the surface, i.e. beyond the maximum of the damage distribution. Three types of interstitial de-
fects are observed in this layer: defects with a faint contrast and with a non-geometrical shape (not as yet identified) located at the beginning of the damage layer, a few RbnDs present in the middle of the damaged zone, and numerous well-known RdDs that are observed at the end of the layer. Note that these two types of defect lie on {1 1 3} planes [9]: RdDs are elongated in a h1 1 0i direction, whereas the RbnDs are two-dimensional interstitial platelets and as such are essentially interstitial-type dislocation loops on {1 1 3} planes. However, in order to distinguish this defect from (perfect or Frank) loops which lie on {1 1 1} planes, we continue to use the nomenclature ribbon-like defects (RbnDs). A very small number of faulted dislocation loops (DLs) is also found [10], but are none present in the figure. All these defects have a mean size of about 100 nm. Upon annealing at 800 C for 30 min, Fig. 1(b), the vast majority of defects are RbnDs and only a few RdDs are observed. Their mean size is now about 200 nm. We note that the defects with the faint contrast are no longer observed. On increasing the anneal time to 1 h 30 min (see Fig. 1(c)), the {1 1 3} defects are still observed but their size has slightly increased (up to 500 nm for a few of them) whereas their density has decreased. After annealing at 1000 C for 1 h 30 min, only a row of perfect dislocation loops located at 750 nm from the surface is observed as shown in Fig 1(d). Their mean size is about 250 nm. From TEM observations, it appears that the number of RbnDs increases at the expense of the RdDs upon annealing, which would indicate that the formation energy of RbnDs is lower than that of the RdDs. A similar evolution has been previously reported for implantation at 600 C followed by an annealing for 3 h 30 min at 800 C [10]. The dissolution and/or transformation of the defects giving rise to the faint contrast suggest that these defects are metastable and perhaps were in the process of slowly evolving into RbnDs and RdDs during the 800 C implantation. After annealing at 1000 C, as expected, RbnDs and RdDs spontaneously transform into dislocation loops as the formation energy of DLs is lower than that of {1 1 3} defects [11]. Once created, DLs are strong sinks for the atoms emitted by RdDs and RbnDs. Calvo et al. [12] have shown that, in the presence of dislocation loops, the atoms lost by the {1 1 3}s, are captured by the DLs and the whole population undergoes a quasi-conservative Ostwald ripening. Our preliminary results suggest that the formation energy of RbnDs is lower than that of RdDs but higher than the formation energy of the loops. The extended defect hierarchy, starting with the highest energy formation energy, is thus: clusters, RdDs, RbnDs and DLs. Work is in progress to confirm these results. 4. Conclusions
Fig. 1. Cross-section TEM images of a specimen implanted at an energy of 50 keV to a fluence of 5 · 1016 cm2 at 800 C, (a) WBDF image, g: (1 1 1), as implanted; (b) WBDF image, g: (1 1 1), 30 min anneal at 800 C; (c) WBDF image, g: (1 1 1), 90 min anneal at 800 C; (d) Bright-field, 2beam image, g: (2 2 0), 90 min anneal at 1000 C.
Conventional transmission electron microscopy has been used to study the defects created by high fluence helium implantation of silicon at 800 C and their evolution after thermal annealing. No cavities are found in the as-im-
M.F. Beaufort et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) 565–567
planted sample, only extended defects are observed. During annealing at 800 C, our preliminary results suggest that rod-like defects evolve into ribbon-like defects. After an anneal at 1000 C, {1 1 3}s undergo a transformation into dislocation loops. Acknowledgements This collaborative project has been greatly facilitated by funding from the Alliance programme (French Ministe`re des Affaires Etrange`res and the British Council). One of us (SED) acknowledges funding from the University of Poitiers for a one-month visit to the Laboratoire de Me´tallurgie Physique. References [1] C.C. Griffioen, J.H. Evans, P.C. De Jong, A. van Veen, Nucl. Instr. and Meth. B 27 (1987) 417. [2] E. Oliviero, M.F. Beaufort, J.F. Barbot, J. Appl. Phys. 90 (2001) 1718.
567
[3] F. Roqueta, Ph.D. Thesis, University of Tours, France, 2000. [4] A. Claverie, B. Colombeau, G. Ben Assayag, C. Bonafos, F. Cristiano, M. Omri, B. de Mauduit, Mat. Sci. Semi. Proc. 3 (2000) 269. [5] D.J. Eaglesham, P.A. Stolk, H.J. Gossmann, T.E. Haynes, J.M. Poate, Nucl. Instr. and Meth. B 106 (1995) 191. [6] P.A. Stolk, H.J. Grossmann, D.J. Eaglesham, D.C. Jacobson, C.S. Rafferty, G.H. Gilmer, M. Jaraiz, J.M. Poate, H.S. Luftman, T.E. Haynes, J. Appl. Phys. 81 (1997) 6031. [7] M.L. David, M.F. Beaufort, J.F. Barbot, J. Appl. Phys. 93 (2003) 1438. [8] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon, New York, 1985. [9] A.L. Aseev, L.I. Fedina, D. Hoehl, H. Bartsch, Clusters of Interstitials Atoms in Silicon and Germanium, Akademie Verlag, Berlin, 1994. [10] M.L. David, M.F. Beaufort, J.F. Barbot, Nucl. Instr. and Meth. B 226 (2004) 531. [11] F. Cristiano, N. Cherkashin, X. Hebras, P. Calvo, A. Claverie, Y. Lamrani, E. Scheid, B. De Mauduit, B. Colombeau, W. Lerch, S. Paul, A. Claverie, Nucl. Instr. and Meth. B 216 (2004) 46. [12] P. Calvo, A. Claverie, N. Cherkashin, B. Colombeau, Y. Lamrani, B. De Mauduit, F. Cristiano, Nucl. Instr. and Meth. B 226 (2004) 173.
Nuclear Instruments and Methods in Physics Research B 242 (2006) 565–567 www.elsevier.com/locate/nimb
Extended-type defects created by high temperature helium implantation into silicon M.F. Beaufort *, S.E. Donnelly, S. Rousselet, M.L. David, J.F. Barbot Laboratoire de Me´tallurgie Physique, UMR 6630, Universite´ de Poitiers, SP2MI, BP 30179, 86962 Chasseneuil-Futuroscope cedex, France Institute for Materials Research, University of Salford, Manchester M5 4WT, UK Available online 6 October 2005
Abstract Following helium implantation (50 keV, 5 · 1016 cm2) at 800 C in silicon, only {1 1 3} defects are present spreading out beyond the maximum of the damage distribution. Both linear rod-like defects as well as so-called ribbon-like {1 1 3} defects are observed. During annealing at 800 C, the number of interstitials in ribbon-like defects appears to increase at the expense of the rod-like defects. After annealing at 1000 C, only a row of dislocation loops is observed. These results suggest that the formation energy of the ribbon-like defects may be lower than that of the rod-like defects. 2005 Elsevier B.V. All rights reserved. PACS: 61.72.V; 61.72.C; 68.37.L Keywords: Defects; Ion implantation; Helium; Silicon; TEM
1. Introduction Cavities can be formed in silicon by high fluence helium implantation followed by thermal annealing at high temperature [1]. However, depending on the implantation parameters, interstitial type defects ({1 1 3} defects and dislocations) can also be created [2,3]. Such defects have been extensively studied for the case of B, Si or P implantation [4–6]. In a recent paper, we have reported the effect of implantation temperature on defects created by high fluence He implantation [7]. We have found that, depending on the implantation temperature, cavities are formed in different stages related to the mobility and stability of the helium and vacancies involved in their formation. Moreover, we have observed that high temperature implantation leads * Corresponding author. Address: Laboratoire de Me´tallurgie Physique, UMR 6630, Universite´ de Poitiers, SP2MI, BP 30179, 86962 ChasseneuilFuturoscope cedex, France. Tel.: +33 5 49 49 68 34; fax: +33 5 49 49 66 92. E-mail address: [email protected] (M.F. Beaufort).
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.08.145
to the formation of extended interstitial-type defects namely, rod-like defects (RdDs), ribbon-like defects (RbnDs) and dislocation loops (DLs). For implantation temperatures below 400 C, only small clusters of interstitials and dislocations are formed. Above 400 C, small RdDs are created and from 600 C, RbnDs are formed in addition to RdDs. In this article, we report on the defects created by an 800 C helium implantation at high fluence into silicon, and some preliminary results are presented regarding their evolution with annealing temperature. 2. Experiments All the experiments were performed on n–n+ silicon wafers. The n-type layer (1014 P cm3) was epitaxially grown on a h1 1 1i oriented n+ substrate of Czochralski silicon. The specimens were implanted at 800 C. The fluence and energy were kept constant at 5 · 1016 He/cm2 and 50 keV respectively (Rp = 500 nm, DRp = 140 nm according to SRIM calculations [8]). The ion beam current was in the range 20–40 lA leading to an average implantation time of 50 min. Specimens were annealed either at 800 C or
566
M.F. Beaufort et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) 565–567
1000 C in a tubular furnace under high vacuum. The structure of the implantation damage was studied by means of cross-sectional transmission electron microscopy (TEM) using a JEOL 200 CX microscope. 3. Results and discussion After helium implantation at 50 keV to a fluence of 5 · 1016 cm2 at 800 C, no cavities are found in the sample as most of vacancies and He escape from the sample during implantation [7]. The weak beam dark field (WBDF) image, Fig. 1(a), reveals extended defects located between 500 and 900 nm from the surface, i.e. beyond the maximum of the damage distribution. Three types of interstitial de-
fects are observed in this layer: defects with a faint contrast and with a non-geometrical shape (not as yet identified) located at the beginning of the damage layer, a few RbnDs present in the middle of the damaged zone, and numerous well-known RdDs that are observed at the end of the layer. Note that these two types of defect lie on {1 1 3} planes [9]: RdDs are elongated in a h1 1 0i direction, whereas the RbnDs are two-dimensional interstitial platelets and as such are essentially interstitial-type dislocation loops on {1 1 3} planes. However, in order to distinguish this defect from (perfect or Frank) loops which lie on {1 1 1} planes, we continue to use the nomenclature ribbon-like defects (RbnDs). A very small number of faulted dislocation loops (DLs) is also found [10], but are none present in the figure. All these defects have a mean size of about 100 nm. Upon annealing at 800 C for 30 min, Fig. 1(b), the vast majority of defects are RbnDs and only a few RdDs are observed. Their mean size is now about 200 nm. We note that the defects with the faint contrast are no longer observed. On increasing the anneal time to 1 h 30 min (see Fig. 1(c)), the {1 1 3} defects are still observed but their size has slightly increased (up to 500 nm for a few of them) whereas their density has decreased. After annealing at 1000 C for 1 h 30 min, only a row of perfect dislocation loops located at 750 nm from the surface is observed as shown in Fig 1(d). Their mean size is about 250 nm. From TEM observations, it appears that the number of RbnDs increases at the expense of the RdDs upon annealing, which would indicate that the formation energy of RbnDs is lower than that of the RdDs. A similar evolution has been previously reported for implantation at 600 C followed by an annealing for 3 h 30 min at 800 C [10]. The dissolution and/or transformation of the defects giving rise to the faint contrast suggest that these defects are metastable and perhaps were in the process of slowly evolving into RbnDs and RdDs during the 800 C implantation. After annealing at 1000 C, as expected, RbnDs and RdDs spontaneously transform into dislocation loops as the formation energy of DLs is lower than that of {1 1 3} defects [11]. Once created, DLs are strong sinks for the atoms emitted by RdDs and RbnDs. Calvo et al. [12] have shown that, in the presence of dislocation loops, the atoms lost by the {1 1 3}s, are captured by the DLs and the whole population undergoes a quasi-conservative Ostwald ripening. Our preliminary results suggest that the formation energy of RbnDs is lower than that of RdDs but higher than the formation energy of the loops. The extended defect hierarchy, starting with the highest energy formation energy, is thus: clusters, RdDs, RbnDs and DLs. Work is in progress to confirm these results. 4. Conclusions
Fig. 1. Cross-section TEM images of a specimen implanted at an energy of 50 keV to a fluence of 5 · 1016 cm2 at 800 C, (a) WBDF image, g: (1 1 1), as implanted; (b) WBDF image, g: (1 1 1), 30 min anneal at 800 C; (c) WBDF image, g: (1 1 1), 90 min anneal at 800 C; (d) Bright-field, 2beam image, g: (2 2 0), 90 min anneal at 1000 C.
Conventional transmission electron microscopy has been used to study the defects created by high fluence helium implantation of silicon at 800 C and their evolution after thermal annealing. No cavities are found in the as-im-
M.F. Beaufort et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) 565–567
planted sample, only extended defects are observed. During annealing at 800 C, our preliminary results suggest that rod-like defects evolve into ribbon-like defects. After an anneal at 1000 C, {1 1 3}s undergo a transformation into dislocation loops. Acknowledgements This collaborative project has been greatly facilitated by funding from the Alliance programme (French Ministe`re des Affaires Etrange`res and the British Council). One of us (SED) acknowledges funding from the University of Poitiers for a one-month visit to the Laboratoire de Me´tallurgie Physique. References [1] C.C. Griffioen, J.H. Evans, P.C. De Jong, A. van Veen, Nucl. Instr. and Meth. B 27 (1987) 417. [2] E. Oliviero, M.F. Beaufort, J.F. Barbot, J. Appl. Phys. 90 (2001) 1718.
567
[3] F. Roqueta, Ph.D. Thesis, University of Tours, France, 2000. [4] A. Claverie, B. Colombeau, G. Ben Assayag, C. Bonafos, F. Cristiano, M. Omri, B. de Mauduit, Mat. Sci. Semi. Proc. 3 (2000) 269. [5] D.J. Eaglesham, P.A. Stolk, H.J. Gossmann, T.E. Haynes, J.M. Poate, Nucl. Instr. and Meth. B 106 (1995) 191. [6] P.A. Stolk, H.J. Grossmann, D.J. Eaglesham, D.C. Jacobson, C.S. Rafferty, G.H. Gilmer, M. Jaraiz, J.M. Poate, H.S. Luftman, T.E. Haynes, J. Appl. Phys. 81 (1997) 6031. [7] M.L. David, M.F. Beaufort, J.F. Barbot, J. Appl. Phys. 93 (2003) 1438. [8] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon, New York, 1985. [9] A.L. Aseev, L.I. Fedina, D. Hoehl, H. Bartsch, Clusters of Interstitials Atoms in Silicon and Germanium, Akademie Verlag, Berlin, 1994. [10] M.L. David, M.F. Beaufort, J.F. Barbot, Nucl. Instr. and Meth. B 226 (2004) 531. [11] F. Cristiano, N. Cherkashin, X. Hebras, P. Calvo, A. Claverie, Y. Lamrani, E. Scheid, B. De Mauduit, B. Colombeau, W. Lerch, S. Paul, A. Claverie, Nucl. Instr. and Meth. B 216 (2004) 46. [12] P. Calvo, A. Claverie, N. Cherkashin, B. Colombeau, Y. Lamrani, B. De Mauduit, F. Cristiano, Nucl. Instr. and Meth. B 226 (2004) 173.