- Email: [email protected]
Improved defect profiling with slow positrons
Applied Surface Science 194 (2002) 210–213
Improved defect profiling with slow positrons R. Krause-Rehberga,*, F. Bo¨rnera, F. Redmanna, W. Eggerb, G. Ko¨gelb, P. Sperrb, W. Triftsha¨userb b
a Fachbereich Physik, Martin-Luther-Universita¨t Halle-Wittenberg, 06099 Halle, Germany Universita¨t der Bundeswehr Mu¨nchen, Werner-Heisenberg-Weg 39, 85579 Neubiberg, Germany
Abstract Monoenergetic positrons are widely used to study defects in near-surface regions and buried interfaces of solids. Depth information is usually obtained by varying the positron implantation energy. However, at energies larger than 10 keV the stopping profile becomes much broader than the positron diffusion length. The study shows that optimum depth resolution can be obtained by stepwise removal of the surface and measurement with the smallest possible positron implantation depth. The removal from the surface can be done by ion sputtering or chemical etching. Furthermore, excellent defect depth profiles can be obtained when a sample is wedge-shaped polished (wedge angle about 18). A line scan using a scanning positron microbeam along the wedge with a small positron implantation depth gives then the defect profile with optimum depth resolution. # 2002 Elsevier Science B.V. All rights reserved. PACS: 78.70.Bj Keywords: Positron annihilation; Defect profiling
1. Introduction The variable energy positron annihilation spectroscopy (VEPAS) is a powerful tool for the observation of different crystal lattice defects near the surface of a solid [1]. Defect depth profiles can be observed by doing positron annihilation spectroscopy as a function of positron implantation depth adjusted by the acceleration voltage. The stopping profile of positrons represents a broad depth distribution which can roughly be described by a so-called Makhov profile (details in [1]). As long as the implantation energy is small (<5 keV), the mean implantation depth is *
Corresponding author. Tel.: þ49-345-5525567; fax: þ49-345-5527160. E-mail address: [email protected] (R. Krause-Rehberg).
comparable to the positron diffusion length in defectfree crystalline material (100–300 nm). However, when the defects under investigation are to be found at a depth larger than 1 mm, positron energies >10 keV are necessary, leading to a very wide positron implantation range, and thus, information range. Details of the defect profile are lost, and therefore often only step functions or block profiles have been used to approximate the real profile. In case of thin buried defect layers, such as interface misfit defects, the sensitivity of positron annihilation may vanish completely. To overcome this drawback, the sample surface can be stepwise removed either by ex situ chemical etching or in situ ion sputtering. The measurement can then be performed after each etching step with the lowest possible positron energy, i.e. the optimum depth resolution. This energy is determined by the
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 0 4 - 6
R. Krause-Rehberg et al. / Applied Surface Science 194 (2002) 210–213
positron diffusion length and thus by the defect density itself. This is due to the requirement that positrons should not be able to diffuse back to the surface. Otherwise, they will annihilate there with the surface annihilation making the detailed analysis of the defects in the layer difficult. The positron diffusion length may be rather small (10 nm in defect-rich ionimplanted material) so that the depth resolution can be better than 100 nm. However, when the defect density is low, positrons must be implanted deeper, and the depth resolution will exceed 500 nm. In Chapter 2 we shall demonstrate two examples of defect profiling with depth-resolution enhanced VEPAS using in situ sputtering of the samples. Another excellent possibility to measure defect profiles with optimum depth resolution is to produce a low-angle wedge by polishing of the sample surface through the defect layer to be studied. The depth profile is then obtained as a line scan along the wedge using a scanning positron microbeam (Chapter 3).
2. Defect depth profiling during in situ ion sputtering Fig. 1 shows the comparison of the conventional VEPAS experiment and the in situ sputtered sample of a Si wafer which was twice implanted with B in order
211
to get a double peak defect profile (simulated defect profile plotted as dashed line). The S parameter of the conventional VEPAS measurement exhibits an increase to 1.04 due to positron trapping into small vacancy clusters [2]. However, the twin-peak defect structure is not seen. The second peak at about 1 mm depth is smeared out due to the broad positron implantation profile. The depth profile was also measured after stepwise removal of the sample surface by Arþ ion sputtering [3]. The positron energy was rather low (2.5 keV). This is possible since the defect density is high and thus the fraction of surface annihilations is low. The twin-peak defect structure is now well visible. The depth resolution is improved and allows the differentiation of the two peaks. In another experiment the so-called Rp/2 effect [4] was studied. After high-energy self-implantation of Si (3.5 MeV, 1015 cm2) and subsequent annealing (30 s at 900 8C) two gettering zones appear at Rp and at about Rp/2 (Rp ¼ projected range of implanted Si ions) which capture diffusing impurities. These gettering zones can be seen after intentional Cu contamination of the sample in a SIMS measurement (lower panel of Fig. 2). The defects were identified to be interstitial-type dislocation loops at Rp and small vacancy clusters at Rp/2 (see [4] and references therein). A conventional S(E) measurement did not show a conclusive result. This is not surprising
Fig. 1. Defect depth profiles obtained by the Doppler–broadening method for a Si sample which was twice implanted with B (50 keV, 2:5 1015 cm2 and 300 keV, 5 1015 cm2). The conventional S(Eþ) measurement (*) and the measurement performed with stepwise removal of the sample surface (&) by Arþ ion sputtering (rate 3.3 nm/min) are compared. For the latter experiment, the positron energy was 2.5 keV. The dashed line corresponds to the defect density obtained by a Monte Carlo simulation, given in relative units on the right hand scale. This scale is only valid for the simulations, but does not correspond to the S parameter scale.
212
R. Krause-Rehberg et al. / Applied Surface Science 194 (2002) 210–213
because the defect layers exist at a depths of 1.7 and 2.8 mm where the positron implantation profile is too broad to discriminate between the peaks. The result of an in situ sputter experiment is shown in Fig. 2 [4]. The positrons are implanted with constant energy of 7.5 keV giving a mean implantation depth of about 360 nm. Obviously, the depth resolution is now sufficient to separate between the two defect peaks. From the anomaly of the W parameter in the Rp/2 region, it was concluded that Cu is in the direct vicinity of the positron trap, which was later identified as a vacancy cluster (see Section 3).
3. Defect depth profiling using a positron microbeam and a wedge-shaped sample
Fig. 2. Study of high-energy self-implanted Si. Upper panel: results of Doppler-broadening measurements at a positron energy of 7.5 keV during stepwise sputtering of the sample (implanted, annealed and Cu contaminated). The depth (abscissa) was obtained by adding the mean implantation depth of the positrons (360 nm) to the sputtering depth. The S and W parameter were normalized to the bulk values. The lower panel shows the Cu depth profile obtained by SIMS [4].
Another possibility for depth-resolution-improved defect profiling is to scan a low-angle wedge through the defect layer by a positron microbeam (Figs. 3 and 4). Such an experiment was performed using the Munich Scanning Positron Microscope (positron energy 8 keV) [5]. Forty-five lifetime measurements each separated by 11 mm were performed on a wedge of 0.818 angle. This gives a depth separation of 155 nm between two measurements. The accuracy of the depth profile is
Fig. 3. Scheme of the depth profile measurement performed with the Munich scanning positron microscope using a wedge-shaped sample. The defect depth profile can be obtained from the positron lifetime spectra measured when the beam scans along the wedge. At a wedge angle of 0.68, a 10 mm deep profile corresponds to a distance of 1 mm.
R. Krause-Rehberg et al. / Applied Surface Science 194 (2002) 210–213
213
(mean positron implantation depth). This was checked by comparing a Si sample which was polished completely (no wedge) with an untreated Si reference. The surface annihilation parameters were altered, but the positron diffusion length was found to be Lþ ¼ ð220 15Þ nm for both samples. The same result was found for a sputtered sample after a surface removal of 250 nm. Only the surface S parameter was changed, while Lþ was similar to a reference sample [3].
4. Summary
Fig. 4. Defect depth profile of a self-implanted Si sample (3.5 MeV, 5 1015 cm2, annealed 30 s at 900 8C, Cu contaminated) obtained using the Munich scanning positron microscope. The sample surface was wedge-shaped and polished. The profile was then taken as a line scan along the wedge with constant positron energy of 8 keV. Each positron lifetime measurement is separated by 11 mm corresponding to a depth difference of 155 nm. The depth scale was calculated with the wedge angle of 0.818, and the mean positron implantation depth of 400 nm was added.
obvious. From the defect-related lifetime t2 it was concluded that small vacancy clusters (n 10) are found in the Rp/2 region, while defects with a smaller open volume, of the order of a divacancy, are found in the Rp zone [5]. Although the wedge was produced by mechanical polishing, the layer of grinding defects does not affect the defect identification at a depth of about 400 nm
Very detailed defect depth profiles can be obtained using VEPAS when the sample surface is removed stepwise to ensure optimum depth resolution independent of depth. The removal can be done by ion sputtering or chemical etching. Another possibility is to prepare a low-angle wedge through the defectrich layer and measure a scan with a positron microbeam along the wedge. This method has the advantage that very deep defect profiles can easily be studied with a constant depth resolution. It was shown for both methods for the case of silicon that the surface defect layer created during sputtering and during wedge polishing is small enough not to affect the detection of the defects under investigation.
References [1] R. Krause-Rehberg, H.S. Leipner, Positron Annihilation in Semiconductors, Springer, Berlin, 1999. [2] S. Eichler, J. Gebauer, F. Bo¨ rner, A. Polity, R. Krause-Rehberg, E. Wendler, B. Weber, W. Wesch, H. Bo¨ rner, Phys. Rev. B 56 (1997) 1393. [3] R. Krause-Rehberg, F. Bo¨ rner, in: Proceedings of the Talk Given at PSSD-99, Hamilton, Canada, 20–23 August 1999, http://www.ep3.uni-halle.de/positrons/talks/PSSD99.pdf. [4] R. Krause-Rehberg, F. Bo¨ rner, F. Redmann, Appl. Phys. Lett. 77 (2000) 3932. [5] R. Krause-Rehberg, F. Bo¨ rner, F. Redmann, J. Gebauer, R. Ko¨ gler, R. Kliemann, W. Skorupa, W. Egger, G. Ko¨ gel, W. Triftsha¨ user, Physica B 308–310 (2001) 442.
Improved defect profiling with slow positrons R. Krause-Rehberga,*, F. Bo¨rnera, F. Redmanna, W. Eggerb, G. Ko¨gelb, P. Sperrb, W. Triftsha¨userb b
a Fachbereich Physik, Martin-Luther-Universita¨t Halle-Wittenberg, 06099 Halle, Germany Universita¨t der Bundeswehr Mu¨nchen, Werner-Heisenberg-Weg 39, 85579 Neubiberg, Germany
Abstract Monoenergetic positrons are widely used to study defects in near-surface regions and buried interfaces of solids. Depth information is usually obtained by varying the positron implantation energy. However, at energies larger than 10 keV the stopping profile becomes much broader than the positron diffusion length. The study shows that optimum depth resolution can be obtained by stepwise removal of the surface and measurement with the smallest possible positron implantation depth. The removal from the surface can be done by ion sputtering or chemical etching. Furthermore, excellent defect depth profiles can be obtained when a sample is wedge-shaped polished (wedge angle about 18). A line scan using a scanning positron microbeam along the wedge with a small positron implantation depth gives then the defect profile with optimum depth resolution. # 2002 Elsevier Science B.V. All rights reserved. PACS: 78.70.Bj Keywords: Positron annihilation; Defect profiling
1. Introduction The variable energy positron annihilation spectroscopy (VEPAS) is a powerful tool for the observation of different crystal lattice defects near the surface of a solid [1]. Defect depth profiles can be observed by doing positron annihilation spectroscopy as a function of positron implantation depth adjusted by the acceleration voltage. The stopping profile of positrons represents a broad depth distribution which can roughly be described by a so-called Makhov profile (details in [1]). As long as the implantation energy is small (<5 keV), the mean implantation depth is *
Corresponding author. Tel.: þ49-345-5525567; fax: þ49-345-5527160. E-mail address: [email protected] (R. Krause-Rehberg).
comparable to the positron diffusion length in defectfree crystalline material (100–300 nm). However, when the defects under investigation are to be found at a depth larger than 1 mm, positron energies >10 keV are necessary, leading to a very wide positron implantation range, and thus, information range. Details of the defect profile are lost, and therefore often only step functions or block profiles have been used to approximate the real profile. In case of thin buried defect layers, such as interface misfit defects, the sensitivity of positron annihilation may vanish completely. To overcome this drawback, the sample surface can be stepwise removed either by ex situ chemical etching or in situ ion sputtering. The measurement can then be performed after each etching step with the lowest possible positron energy, i.e. the optimum depth resolution. This energy is determined by the
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 0 4 - 6
R. Krause-Rehberg et al. / Applied Surface Science 194 (2002) 210–213
positron diffusion length and thus by the defect density itself. This is due to the requirement that positrons should not be able to diffuse back to the surface. Otherwise, they will annihilate there with the surface annihilation making the detailed analysis of the defects in the layer difficult. The positron diffusion length may be rather small (10 nm in defect-rich ionimplanted material) so that the depth resolution can be better than 100 nm. However, when the defect density is low, positrons must be implanted deeper, and the depth resolution will exceed 500 nm. In Chapter 2 we shall demonstrate two examples of defect profiling with depth-resolution enhanced VEPAS using in situ sputtering of the samples. Another excellent possibility to measure defect profiles with optimum depth resolution is to produce a low-angle wedge by polishing of the sample surface through the defect layer to be studied. The depth profile is then obtained as a line scan along the wedge using a scanning positron microbeam (Chapter 3).
2. Defect depth profiling during in situ ion sputtering Fig. 1 shows the comparison of the conventional VEPAS experiment and the in situ sputtered sample of a Si wafer which was twice implanted with B in order
211
to get a double peak defect profile (simulated defect profile plotted as dashed line). The S parameter of the conventional VEPAS measurement exhibits an increase to 1.04 due to positron trapping into small vacancy clusters [2]. However, the twin-peak defect structure is not seen. The second peak at about 1 mm depth is smeared out due to the broad positron implantation profile. The depth profile was also measured after stepwise removal of the sample surface by Arþ ion sputtering [3]. The positron energy was rather low (2.5 keV). This is possible since the defect density is high and thus the fraction of surface annihilations is low. The twin-peak defect structure is now well visible. The depth resolution is improved and allows the differentiation of the two peaks. In another experiment the so-called Rp/2 effect [4] was studied. After high-energy self-implantation of Si (3.5 MeV, 1015 cm2) and subsequent annealing (30 s at 900 8C) two gettering zones appear at Rp and at about Rp/2 (Rp ¼ projected range of implanted Si ions) which capture diffusing impurities. These gettering zones can be seen after intentional Cu contamination of the sample in a SIMS measurement (lower panel of Fig. 2). The defects were identified to be interstitial-type dislocation loops at Rp and small vacancy clusters at Rp/2 (see [4] and references therein). A conventional S(E) measurement did not show a conclusive result. This is not surprising
Fig. 1. Defect depth profiles obtained by the Doppler–broadening method for a Si sample which was twice implanted with B (50 keV, 2:5 1015 cm2 and 300 keV, 5 1015 cm2). The conventional S(Eþ) measurement (*) and the measurement performed with stepwise removal of the sample surface (&) by Arþ ion sputtering (rate 3.3 nm/min) are compared. For the latter experiment, the positron energy was 2.5 keV. The dashed line corresponds to the defect density obtained by a Monte Carlo simulation, given in relative units on the right hand scale. This scale is only valid for the simulations, but does not correspond to the S parameter scale.
212
R. Krause-Rehberg et al. / Applied Surface Science 194 (2002) 210–213
because the defect layers exist at a depths of 1.7 and 2.8 mm where the positron implantation profile is too broad to discriminate between the peaks. The result of an in situ sputter experiment is shown in Fig. 2 [4]. The positrons are implanted with constant energy of 7.5 keV giving a mean implantation depth of about 360 nm. Obviously, the depth resolution is now sufficient to separate between the two defect peaks. From the anomaly of the W parameter in the Rp/2 region, it was concluded that Cu is in the direct vicinity of the positron trap, which was later identified as a vacancy cluster (see Section 3).
3. Defect depth profiling using a positron microbeam and a wedge-shaped sample
Fig. 2. Study of high-energy self-implanted Si. Upper panel: results of Doppler-broadening measurements at a positron energy of 7.5 keV during stepwise sputtering of the sample (implanted, annealed and Cu contaminated). The depth (abscissa) was obtained by adding the mean implantation depth of the positrons (360 nm) to the sputtering depth. The S and W parameter were normalized to the bulk values. The lower panel shows the Cu depth profile obtained by SIMS [4].
Another possibility for depth-resolution-improved defect profiling is to scan a low-angle wedge through the defect layer by a positron microbeam (Figs. 3 and 4). Such an experiment was performed using the Munich Scanning Positron Microscope (positron energy 8 keV) [5]. Forty-five lifetime measurements each separated by 11 mm were performed on a wedge of 0.818 angle. This gives a depth separation of 155 nm between two measurements. The accuracy of the depth profile is
Fig. 3. Scheme of the depth profile measurement performed with the Munich scanning positron microscope using a wedge-shaped sample. The defect depth profile can be obtained from the positron lifetime spectra measured when the beam scans along the wedge. At a wedge angle of 0.68, a 10 mm deep profile corresponds to a distance of 1 mm.
R. Krause-Rehberg et al. / Applied Surface Science 194 (2002) 210–213
213
(mean positron implantation depth). This was checked by comparing a Si sample which was polished completely (no wedge) with an untreated Si reference. The surface annihilation parameters were altered, but the positron diffusion length was found to be Lþ ¼ ð220 15Þ nm for both samples. The same result was found for a sputtered sample after a surface removal of 250 nm. Only the surface S parameter was changed, while Lþ was similar to a reference sample [3].
4. Summary
Fig. 4. Defect depth profile of a self-implanted Si sample (3.5 MeV, 5 1015 cm2, annealed 30 s at 900 8C, Cu contaminated) obtained using the Munich scanning positron microscope. The sample surface was wedge-shaped and polished. The profile was then taken as a line scan along the wedge with constant positron energy of 8 keV. Each positron lifetime measurement is separated by 11 mm corresponding to a depth difference of 155 nm. The depth scale was calculated with the wedge angle of 0.818, and the mean positron implantation depth of 400 nm was added.
obvious. From the defect-related lifetime t2 it was concluded that small vacancy clusters (n 10) are found in the Rp/2 region, while defects with a smaller open volume, of the order of a divacancy, are found in the Rp zone [5]. Although the wedge was produced by mechanical polishing, the layer of grinding defects does not affect the defect identification at a depth of about 400 nm
Very detailed defect depth profiles can be obtained using VEPAS when the sample surface is removed stepwise to ensure optimum depth resolution independent of depth. The removal can be done by ion sputtering or chemical etching. Another possibility is to prepare a low-angle wedge through the defectrich layer and measure a scan with a positron microbeam along the wedge. This method has the advantage that very deep defect profiles can easily be studied with a constant depth resolution. It was shown for both methods for the case of silicon that the surface defect layer created during sputtering and during wedge polishing is small enough not to affect the detection of the defects under investigation.
References [1] R. Krause-Rehberg, H.S. Leipner, Positron Annihilation in Semiconductors, Springer, Berlin, 1999. [2] S. Eichler, J. Gebauer, F. Bo¨ rner, A. Polity, R. Krause-Rehberg, E. Wendler, B. Weber, W. Wesch, H. Bo¨ rner, Phys. Rev. B 56 (1997) 1393. [3] R. Krause-Rehberg, F. Bo¨ rner, in: Proceedings of the Talk Given at PSSD-99, Hamilton, Canada, 20–23 August 1999, http://www.ep3.uni-halle.de/positrons/talks/PSSD99.pdf. [4] R. Krause-Rehberg, F. Bo¨ rner, F. Redmann, Appl. Phys. Lett. 77 (2000) 3932. [5] R. Krause-Rehberg, F. Bo¨ rner, F. Redmann, J. Gebauer, R. Ko¨ gler, R. Kliemann, W. Skorupa, W. Egger, G. Ko¨ gel, W. Triftsha¨ user, Physica B 308–310 (2001) 442.