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Vacancy-type defects in 6H–SiC caused by N+ and Al+ high fluence co-implantation
Applied Surface Science 194 (2002) 131–135
Vacancy-type defects in 6H–SiC caused by Nþ and Alþ high fluence co-implantation W. Anwand, G. Brauer*, W. Skorupa Institut fu¨r Ionenstrahlphysik und Materialforschung, Forschungszentrum Rossendorf e.V., PF 510119, 01314 Dresden, Germany
Abstract 6H–SiC n-type wafers were implanted with Alþ and Nþ ions in two steps: first Nþ double implantation (65 keV, 5 1016 cm2 and 120 keV, 1:3 1017 cm2) followed by Alþ double implantation (100 keV, 5 1016 cm2 and 160 keV, 1:3 1017 cm2). The implantation was carried out at a substrate temperature of 800 8C, in order to avoid amorphization. In this way, a buried (SiC)1x(AlN)x layer could be created. Variable-energy positron Doppler broadening measurements were performed at room temperature using a magnetic transport beam system in order to characterize the vacancy-type defects created by ion implantation. Depth profiles could be evaluated from the measured Doppler broadening profiles. The defect distribution and the defect size after the complete co-implantation are discussed and the contribution of the different implantation steps to the evolution of this defect structure is shown. # 2002 Elsevier Science B.V. All rights reserved. Keywords: 6H–SiC; Nþ and Alþ co-implantation; Vacancy-type defects; Slow positron implantation spectroscopy
1. Introduction Wide band gap semiconductors are currently attracting increasing attention due to their extremely interesting physical properties, very different from the conventional semiconductors like Si and GaAs. A rapid improvement of the materials quality as well as the knowledge of physical properties is now generating a rapid development in high-power, high-temperature and high-frequency electronics. One of these materials, the (SiC)1x(AlN)x system has been more widely studied because of the full miscibility of SiC and AlN, their good lattice and thermal matches and the possibility of modifying the band gap of the resulting structure over a wide range from 2.9 (6H–SiC) to 6.2 eV (2H–AlN). An *
Corresponding author. Tel.: þ49-351-2602-117; fax: þ49-351-2603-285. E-mail address: [email protected] (G. Brauer).
interesting method of producing buried thin (SiC)1x(AlN)x layers potentially suitable for microelectronic applications is based on high-dose Nþ and Alþ coimplantation into SiC at different substrate temperatures. Much work has been done on selective doping of SiC by either Nþ [1,2] or Alþ [3,4]. First results about Nþ and Alþ co-implantation have been published in [5,6]. It is well known that ion implantation is connected with defect creation and changes of the structure in the range of the implanted ions and maybe beyond it. Positron annihilation spectroscopy (PAS) is above all sensitive to vacancy-type defects and has been used to investigate these defects and their depth depending distribution in 6H–SiC after different implantation and post-implantation annealing processes [7–9]. Investigations of vacancy-type defects in 3C–SiC, separately implanted by either Alþ or Nþ at different substrate temperatures, are described in [10]. Interestingly, in case of co-implantation of Alþ
0169-4332/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 1 1 2 - 5
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W. Anwand et al. / Applied Surface Science 194 (2002) 131–135
and Nþ (two-fold of each ion species) it has been observed [9] that the vacancy-type damage and its depth distribution does depend on the sequence of implantation. To model such a behavior would be a real challenge to theoretical work, but would at least require the knowledge of vacancy-type damage observed after each separate step of implantation for comparison purposes. Therefore, the aim of the present work is to characterize this damage in co-implanted 6H–SiC for the sequence of implanting Nþ first, followed by Alþ implantation, at 800 8C. The PAS technique used is slow positron implantation spectroscopy (SPIS). SPIS results of the opposite sequence, i.e. implanting Alþ first, followed by Nþ implantation, at 800 8C, have already been presented elsewhere [11].
2. Experimental 2.1. Sample preparation (0 0 0 1)-Oriented, n-type 6H–SiC wafers were sequentially implanted with Nþ and Alþ ions at two energies for each species in order to realize an overlap of the respective profiles according to previous calculations. The implantation parameters were chosen in such a way that the produced buried layers of (SiC)1x(AlN)x show a composition of about x 0:2. The sample implantation was scheduled as follows: first Nþ ions were implanted with an energy of 65 keV with a fluence of 5 1016 cm2 and after that with a higher energy of 120 keV with a fluence of 1:3 1017 cm2. Subsequent to the Nþ double implantation, Alþ ions were implanted with energies of 100 and 160 keV, respectively, and to the same fluences as in case of Nþ ions. During implantation the beam current density was kept in the range of 0.6– 1.0 mA cm2 The substrate temperature was chosen to be 800 8C. 2.2. PAS SPIS was performed on the Rossendorf positron beam system with an energy resolution of the Ge detector of 1:09 0:01 keV at 511 keV. Positrons of pre-determined energies (E) varying from 30 eV to 36 keV are implanted at depths up to a few mm in the sample under study. The motion of positron–electron
pairs prior to annihilation causes a Doppler broadening of the photopeak in the measured energy spectrum of the annihilation photons. This Doppler broadening is characterized by the line-shape parameter (S), being the ratio of counts in the central region of the photopeak to the total number of counts in the peak. The limits for the central region of the photopeak are fixed at the annihilation line of the defect free bulk material in such a way that the S parameter of the bulk is equal to Sbulk ¼ 0:5. The measured value of S at each energy (E) is thus X SðEÞ ¼ Ss Fs ðEÞ þ Se Fe ðEÞ þ Sd Fd ðEÞ þ Sb Fb ðEÞ (1) where Ss, Sd and Sb are the S parameters associated with annihilation of positrons with electrons at the surface, in layers containing a typical defect structure and in the unimplanted bulk material, respectively, and Se is associated with the annihilation of epithermal positrons. F(E) are the fractions of positrons annihiP lated in each state; thus, Fn ¼ 1 at each E. 2.3. Results and discussion SPIS results are presented in Fig. 1. The formation of large-sized vacancy-type defects close to the sample surface, together with deeper reaching defects of much smaller size, is already obvious from the S(E) curves after each step of implantation when comparing the experimental data with those obtained for the virgin material. However, only by a careful further analysis of these data using the software program package VEPFIT [12], thereby assuming a box-shaped layer structure for the defects and a positron diffusion length (Lþ) in this box, does allow a deeper insight into the defect structures formed. The positron diffusion length (Lþ) characterizes the distance which a thermalized positron can diffuse in the material before annihilation. In the presence of defects, Lþ is correlated with the defect concentration. In Fig. 2, these fitted vacancy-type layer structures are presented after each step of implantation. The estimation of the size of the defects formed was possible using a scaling curve of the S parameter versus the number of agglomerated Si–C di-vacancies (V2) (Fig. 3) obtained from former investigations on ion implanted 6H–SiC [7,13].
W. Anwand et al. / Applied Surface Science 194 (2002) 131–135
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Fig. 1. Normalized S parameter vs. incident positron energy (E), after the first (a), second (b), third (c) and fourth (d) implantation. The fitted curve through the measured points is given as a full line.
Fig. 2. Fitted box-shape profiles to the measured data (Fig. 1), given as the number of agglomerated V2 defects vs. depth, after the first (a), second (b), third (c) and fourth (d) implantation. The hatched area indicates the implanted region.
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W. Anwand et al. / Applied Surface Science 194 (2002) 131–135
Fig. 3. Scaling curve, given as the normalized S parameter vs. the number of agglomerated V2 defects [7,13].
As a general feature, it is found that large-sized defects are formed in a narrow subsurface layer only. The estimated layer thickness (Lþ) amounts to 28 2 (1:0 0:5), 18 4 (1:0 0:5), 9 3 (1:0 0:5) and 24 2 nm (1:0 þ 0:5 nm) after the first, second, third and fourth step of implantation, respectively. The very short values found for Lþ in each layer can be regarded as an indication for saturation trapping of positrons. This is essential to be certain about the defect size to be found with the help of our scaling curve. It is estimated that after the first implantation the defect size amounts to 11V2 and becomes even larger with every implantation step: 18V2, 23V2 and 26V2 after the second, third and fourth implantation, respectively. Interestingly, vacancy-type defects reaching much deeper than the range of the implanted ions as estimated from TRIM calculations are found up to a depth of 318 6, 321 2, 428 10 and 533 6 nm after the first, second, third and fourth step of implantation, respectively. The corresponding Lþ values are 32 3, 32 2, 42 2 and 22 2 nm, respectively, and need to be compared with the bulk value Lþ ¼ 58 3 nm. It should be mentioned that TRIM results are not really suitable as comparison since channeling is not taken into account. SIMS shows that channeled ions (Alþ, 120 keV) reach deeper than 1 mm into the material.
The damage created by these ions is sufficient to account for the PAS data because of the high fluence. These ‘‘deep’’ defects are routinely found by PAS on implanted SiC. Defects of the size of just 1V2 are observed after the first and second implantation step. The criterion of saturation trapping should be fulfilled for both implantation steps. If not, then the second implantation step should at least result in a further shortage of Lþ compared to the value observed after the first implantation. For the second implantation step, it even seems that filling of already existing V2 defects by implanted ions and formation of new V2 defects due to the implantation does more or less balance each other as almost no change in Lþ and layer width is observed. However, the third implantation step results in a defect size of 0.5–1V2 and it seems that the implanted Alþ ions are filling the existing V2 defects created from the former implantation steps. This interpretation is supported from the increase in positron diffusion length observed here (Lþ ¼ 42 2 nm). The final fourth implantation has the most remarkable impact on the deep reaching defects created. It results not only in a further increase in depth (extension to 533 6 nm) but size of these defects: 4V2 are now observed, according to our scaling curve, to exist in this deep
W. Anwand et al. / Applied Surface Science 194 (2002) 131–135
reaching defected layer. The fact that the positron diffusion length does drop to Lþ ¼ 22 2 nm could point out that not only the large-sized V2 clusters but also smaller-sized defects co-exist in this layer now. However, the latter are obviously invisible to positrons due to preferential and saturation trapping of positrons in the largest-sized vacancy clusters found because otherwise, from our scaling curve, a value smaller than four would have been indicated.
3. Conclusions The evolution of vacancy-type damage after each implantation step when producing a buried layer of (SiC)1x(AlN)x of a composition of about x 0:2 in 6H–SiC by subsequent implantation of Nþ and Alþ ions has been revealed. It shows that a narrow layer of rather large-sized V2 agglomerates at the surface is formed immediately after the first implantation step and remains there with increasing cluster size after each further implantation. Furthermore, the formation of vacancy-type defects of much smaller size could be revealed which reach much deeper into the material than the range of the implanted ions as estimated from TRIM calculations.
Acknowledgements We gratefully acknowledge the support of the Deutsche Forschungsgemeinschaft (DFG), Grants nos. BR 1250/13-1 and BR 1250/13-2, respectively.
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References [1] Y. Hirano, T. Inada, J. Electrochem. Soc. 141 (1994) 3489. [2] R. Lossy, W. Reichert, E. Obermeier, W. Skorupa, J. Electron. Mater. 26 (1997) 123. [3] J.A. Edmond, R.F. Davis, Ceram. Trans. 2 (1987) 479. [4] M. Rao, P. Griffiths, O.W. Holland, G. Kelner, J.A. Freitas Jr., D.S. Simons, P.H. Chi, M. Ghezzo, J. Appl. Phys. 77 (1995) 2479. [5] R.A. Yankov, N. Hatzopoulos, W. Fukarek, M. Voelskow, V. Heera, J. Petzoldt, W. Skorupa, Mater. Res. Soc. Symp. Proc. 438 (1997) 271. [6] J. Pezoldt, R.A. Yankov, M. Voelskow, G. Brauer, W. Anwand, V. Heera, W. Skorupa, P.G. Coleman, in: J. Donecker, I. Rechenberg (Eds.), Proceedings of the Seventh International Conference on Defect Recognition and Image Processing in Semiconductors (DRIP VII), Templin, 1997, Inst. Phys. Conf. Ser. no. 160, IOP Publication, Bristol, 1998, p. 335. [7] G. Brauer, W. Anwand, P.G. Coleman, A.P. Knights, F. Plazaola, Y. Pacaud, W. Skorupa, J. Sto¨ rmer, P. Willutzki, Phys. Rev. B 54 (1996) 3084. [8] G. Brauer, W. Anwand, P.G. Coleman, A. Goodyear, J. Stoermer, F. Plazaola, J.M. Campillo, Y. Pacaud, W. Skorupa, J. Phys. Condens. Matter 10 (1998) 1147. [9] W. Anwand, G. Brauer, P.G. Coleman, R. Yankov, W. Skorupa, Appl. Surf. Sci. 149 (1999) 140. [10] A. Uedono, H. Itoh, T. Oshima, Y. Aoki, M. Yoshikawa, I. Nashiyama, H. Okumura, S. Yoshida, T. Moriya, T. Kawano, S. Tanigawa, Jpn. J. Appl. Phys. 35 (1996) 5986. [11] W. Anwand, G. Brauer, W. Skorupa, Appl. Surf. Sci. 184 (2001) 247. [12] A. van Veen, H. Schut, J. de Vries, R.A. Haakvoort, M.R. Ijpma, Positron beams for solids and surfaces, in: P.J. Schultz, G.R. Massoumi, P.J. Simpson (Eds.), Proceedings of the AIP Conference, Vol. 218, 1990, p. 171. [13] W. Anwand, G. Brauer, P.G. Coleman, W. Skorupa, in: J.C. Barbour, S. Roorda, D. Ila, M. Tsujioka (Eds.), Proceedings of the MRS Fall Meeting (Symposium KK), Boston, 1997, Vol. 504, MRS, Warrendale, PA, 1999, p. 135.
Vacancy-type defects in 6H–SiC caused by Nþ and Alþ high fluence co-implantation W. Anwand, G. Brauer*, W. Skorupa Institut fu¨r Ionenstrahlphysik und Materialforschung, Forschungszentrum Rossendorf e.V., PF 510119, 01314 Dresden, Germany
Abstract 6H–SiC n-type wafers were implanted with Alþ and Nþ ions in two steps: first Nþ double implantation (65 keV, 5 1016 cm2 and 120 keV, 1:3 1017 cm2) followed by Alþ double implantation (100 keV, 5 1016 cm2 and 160 keV, 1:3 1017 cm2). The implantation was carried out at a substrate temperature of 800 8C, in order to avoid amorphization. In this way, a buried (SiC)1x(AlN)x layer could be created. Variable-energy positron Doppler broadening measurements were performed at room temperature using a magnetic transport beam system in order to characterize the vacancy-type defects created by ion implantation. Depth profiles could be evaluated from the measured Doppler broadening profiles. The defect distribution and the defect size after the complete co-implantation are discussed and the contribution of the different implantation steps to the evolution of this defect structure is shown. # 2002 Elsevier Science B.V. All rights reserved. Keywords: 6H–SiC; Nþ and Alþ co-implantation; Vacancy-type defects; Slow positron implantation spectroscopy
1. Introduction Wide band gap semiconductors are currently attracting increasing attention due to their extremely interesting physical properties, very different from the conventional semiconductors like Si and GaAs. A rapid improvement of the materials quality as well as the knowledge of physical properties is now generating a rapid development in high-power, high-temperature and high-frequency electronics. One of these materials, the (SiC)1x(AlN)x system has been more widely studied because of the full miscibility of SiC and AlN, their good lattice and thermal matches and the possibility of modifying the band gap of the resulting structure over a wide range from 2.9 (6H–SiC) to 6.2 eV (2H–AlN). An *
Corresponding author. Tel.: þ49-351-2602-117; fax: þ49-351-2603-285. E-mail address: [email protected] (G. Brauer).
interesting method of producing buried thin (SiC)1x(AlN)x layers potentially suitable for microelectronic applications is based on high-dose Nþ and Alþ coimplantation into SiC at different substrate temperatures. Much work has been done on selective doping of SiC by either Nþ [1,2] or Alþ [3,4]. First results about Nþ and Alþ co-implantation have been published in [5,6]. It is well known that ion implantation is connected with defect creation and changes of the structure in the range of the implanted ions and maybe beyond it. Positron annihilation spectroscopy (PAS) is above all sensitive to vacancy-type defects and has been used to investigate these defects and their depth depending distribution in 6H–SiC after different implantation and post-implantation annealing processes [7–9]. Investigations of vacancy-type defects in 3C–SiC, separately implanted by either Alþ or Nþ at different substrate temperatures, are described in [10]. Interestingly, in case of co-implantation of Alþ
0169-4332/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 1 1 2 - 5
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W. Anwand et al. / Applied Surface Science 194 (2002) 131–135
and Nþ (two-fold of each ion species) it has been observed [9] that the vacancy-type damage and its depth distribution does depend on the sequence of implantation. To model such a behavior would be a real challenge to theoretical work, but would at least require the knowledge of vacancy-type damage observed after each separate step of implantation for comparison purposes. Therefore, the aim of the present work is to characterize this damage in co-implanted 6H–SiC for the sequence of implanting Nþ first, followed by Alþ implantation, at 800 8C. The PAS technique used is slow positron implantation spectroscopy (SPIS). SPIS results of the opposite sequence, i.e. implanting Alþ first, followed by Nþ implantation, at 800 8C, have already been presented elsewhere [11].
2. Experimental 2.1. Sample preparation (0 0 0 1)-Oriented, n-type 6H–SiC wafers were sequentially implanted with Nþ and Alþ ions at two energies for each species in order to realize an overlap of the respective profiles according to previous calculations. The implantation parameters were chosen in such a way that the produced buried layers of (SiC)1x(AlN)x show a composition of about x 0:2. The sample implantation was scheduled as follows: first Nþ ions were implanted with an energy of 65 keV with a fluence of 5 1016 cm2 and after that with a higher energy of 120 keV with a fluence of 1:3 1017 cm2. Subsequent to the Nþ double implantation, Alþ ions were implanted with energies of 100 and 160 keV, respectively, and to the same fluences as in case of Nþ ions. During implantation the beam current density was kept in the range of 0.6– 1.0 mA cm2 The substrate temperature was chosen to be 800 8C. 2.2. PAS SPIS was performed on the Rossendorf positron beam system with an energy resolution of the Ge detector of 1:09 0:01 keV at 511 keV. Positrons of pre-determined energies (E) varying from 30 eV to 36 keV are implanted at depths up to a few mm in the sample under study. The motion of positron–electron
pairs prior to annihilation causes a Doppler broadening of the photopeak in the measured energy spectrum of the annihilation photons. This Doppler broadening is characterized by the line-shape parameter (S), being the ratio of counts in the central region of the photopeak to the total number of counts in the peak. The limits for the central region of the photopeak are fixed at the annihilation line of the defect free bulk material in such a way that the S parameter of the bulk is equal to Sbulk ¼ 0:5. The measured value of S at each energy (E) is thus X SðEÞ ¼ Ss Fs ðEÞ þ Se Fe ðEÞ þ Sd Fd ðEÞ þ Sb Fb ðEÞ (1) where Ss, Sd and Sb are the S parameters associated with annihilation of positrons with electrons at the surface, in layers containing a typical defect structure and in the unimplanted bulk material, respectively, and Se is associated with the annihilation of epithermal positrons. F(E) are the fractions of positrons annihiP lated in each state; thus, Fn ¼ 1 at each E. 2.3. Results and discussion SPIS results are presented in Fig. 1. The formation of large-sized vacancy-type defects close to the sample surface, together with deeper reaching defects of much smaller size, is already obvious from the S(E) curves after each step of implantation when comparing the experimental data with those obtained for the virgin material. However, only by a careful further analysis of these data using the software program package VEPFIT [12], thereby assuming a box-shaped layer structure for the defects and a positron diffusion length (Lþ) in this box, does allow a deeper insight into the defect structures formed. The positron diffusion length (Lþ) characterizes the distance which a thermalized positron can diffuse in the material before annihilation. In the presence of defects, Lþ is correlated with the defect concentration. In Fig. 2, these fitted vacancy-type layer structures are presented after each step of implantation. The estimation of the size of the defects formed was possible using a scaling curve of the S parameter versus the number of agglomerated Si–C di-vacancies (V2) (Fig. 3) obtained from former investigations on ion implanted 6H–SiC [7,13].
W. Anwand et al. / Applied Surface Science 194 (2002) 131–135
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Fig. 1. Normalized S parameter vs. incident positron energy (E), after the first (a), second (b), third (c) and fourth (d) implantation. The fitted curve through the measured points is given as a full line.
Fig. 2. Fitted box-shape profiles to the measured data (Fig. 1), given as the number of agglomerated V2 defects vs. depth, after the first (a), second (b), third (c) and fourth (d) implantation. The hatched area indicates the implanted region.
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W. Anwand et al. / Applied Surface Science 194 (2002) 131–135
Fig. 3. Scaling curve, given as the normalized S parameter vs. the number of agglomerated V2 defects [7,13].
As a general feature, it is found that large-sized defects are formed in a narrow subsurface layer only. The estimated layer thickness (Lþ) amounts to 28 2 (1:0 0:5), 18 4 (1:0 0:5), 9 3 (1:0 0:5) and 24 2 nm (1:0 þ 0:5 nm) after the first, second, third and fourth step of implantation, respectively. The very short values found for Lþ in each layer can be regarded as an indication for saturation trapping of positrons. This is essential to be certain about the defect size to be found with the help of our scaling curve. It is estimated that after the first implantation the defect size amounts to 11V2 and becomes even larger with every implantation step: 18V2, 23V2 and 26V2 after the second, third and fourth implantation, respectively. Interestingly, vacancy-type defects reaching much deeper than the range of the implanted ions as estimated from TRIM calculations are found up to a depth of 318 6, 321 2, 428 10 and 533 6 nm after the first, second, third and fourth step of implantation, respectively. The corresponding Lþ values are 32 3, 32 2, 42 2 and 22 2 nm, respectively, and need to be compared with the bulk value Lþ ¼ 58 3 nm. It should be mentioned that TRIM results are not really suitable as comparison since channeling is not taken into account. SIMS shows that channeled ions (Alþ, 120 keV) reach deeper than 1 mm into the material.
The damage created by these ions is sufficient to account for the PAS data because of the high fluence. These ‘‘deep’’ defects are routinely found by PAS on implanted SiC. Defects of the size of just 1V2 are observed after the first and second implantation step. The criterion of saturation trapping should be fulfilled for both implantation steps. If not, then the second implantation step should at least result in a further shortage of Lþ compared to the value observed after the first implantation. For the second implantation step, it even seems that filling of already existing V2 defects by implanted ions and formation of new V2 defects due to the implantation does more or less balance each other as almost no change in Lþ and layer width is observed. However, the third implantation step results in a defect size of 0.5–1V2 and it seems that the implanted Alþ ions are filling the existing V2 defects created from the former implantation steps. This interpretation is supported from the increase in positron diffusion length observed here (Lþ ¼ 42 2 nm). The final fourth implantation has the most remarkable impact on the deep reaching defects created. It results not only in a further increase in depth (extension to 533 6 nm) but size of these defects: 4V2 are now observed, according to our scaling curve, to exist in this deep
W. Anwand et al. / Applied Surface Science 194 (2002) 131–135
reaching defected layer. The fact that the positron diffusion length does drop to Lþ ¼ 22 2 nm could point out that not only the large-sized V2 clusters but also smaller-sized defects co-exist in this layer now. However, the latter are obviously invisible to positrons due to preferential and saturation trapping of positrons in the largest-sized vacancy clusters found because otherwise, from our scaling curve, a value smaller than four would have been indicated.
3. Conclusions The evolution of vacancy-type damage after each implantation step when producing a buried layer of (SiC)1x(AlN)x of a composition of about x 0:2 in 6H–SiC by subsequent implantation of Nþ and Alþ ions has been revealed. It shows that a narrow layer of rather large-sized V2 agglomerates at the surface is formed immediately after the first implantation step and remains there with increasing cluster size after each further implantation. Furthermore, the formation of vacancy-type defects of much smaller size could be revealed which reach much deeper into the material than the range of the implanted ions as estimated from TRIM calculations.
Acknowledgements We gratefully acknowledge the support of the Deutsche Forschungsgemeinschaft (DFG), Grants nos. BR 1250/13-1 and BR 1250/13-2, respectively.
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References [1] Y. Hirano, T. Inada, J. Electrochem. Soc. 141 (1994) 3489. [2] R. Lossy, W. Reichert, E. Obermeier, W. Skorupa, J. Electron. Mater. 26 (1997) 123. [3] J.A. Edmond, R.F. Davis, Ceram. Trans. 2 (1987) 479. [4] M. Rao, P. Griffiths, O.W. Holland, G. Kelner, J.A. Freitas Jr., D.S. Simons, P.H. Chi, M. Ghezzo, J. Appl. Phys. 77 (1995) 2479. [5] R.A. Yankov, N. Hatzopoulos, W. Fukarek, M. Voelskow, V. Heera, J. Petzoldt, W. Skorupa, Mater. Res. Soc. Symp. Proc. 438 (1997) 271. [6] J. Pezoldt, R.A. Yankov, M. Voelskow, G. Brauer, W. Anwand, V. Heera, W. Skorupa, P.G. Coleman, in: J. Donecker, I. Rechenberg (Eds.), Proceedings of the Seventh International Conference on Defect Recognition and Image Processing in Semiconductors (DRIP VII), Templin, 1997, Inst. Phys. Conf. Ser. no. 160, IOP Publication, Bristol, 1998, p. 335. [7] G. Brauer, W. Anwand, P.G. Coleman, A.P. Knights, F. Plazaola, Y. Pacaud, W. Skorupa, J. Sto¨ rmer, P. Willutzki, Phys. Rev. B 54 (1996) 3084. [8] G. Brauer, W. Anwand, P.G. Coleman, A. Goodyear, J. Stoermer, F. Plazaola, J.M. Campillo, Y. Pacaud, W. Skorupa, J. Phys. Condens. Matter 10 (1998) 1147. [9] W. Anwand, G. Brauer, P.G. Coleman, R. Yankov, W. Skorupa, Appl. Surf. Sci. 149 (1999) 140. [10] A. Uedono, H. Itoh, T. Oshima, Y. Aoki, M. Yoshikawa, I. Nashiyama, H. Okumura, S. Yoshida, T. Moriya, T. Kawano, S. Tanigawa, Jpn. J. Appl. Phys. 35 (1996) 5986. [11] W. Anwand, G. Brauer, W. Skorupa, Appl. Surf. Sci. 184 (2001) 247. [12] A. van Veen, H. Schut, J. de Vries, R.A. Haakvoort, M.R. Ijpma, Positron beams for solids and surfaces, in: P.J. Schultz, G.R. Massoumi, P.J. Simpson (Eds.), Proceedings of the AIP Conference, Vol. 218, 1990, p. 171. [13] W. Anwand, G. Brauer, P.G. Coleman, W. Skorupa, in: J.C. Barbour, S. Roorda, D. Ila, M. Tsujioka (Eds.), Proceedings of the MRS Fall Meeting (Symposium KK), Boston, 1997, Vol. 504, MRS, Warrendale, PA, 1999, p. 135.