- Email: [email protected]
Lattice site location of clustered boron atoms in silicon
Nuclear
438
LATTICE
SITE LOCATION
P.J.M. SMULDERS,
OF CLUSTERED
D.O. BOERMA,
Instruments
and Methods
in Physics Research
B45 (1990) 438-441 North-Holland
BORON ATOMS IN SILICON
B. BECH NIELSEN
* and M.L. SWANSON
**
General Physics Laboratory (LAN), Materials Science Center, University of Groningen, Westersingel 34, 9718 CM Groningen, The Netherlands
Si crystals were doped with *‘B by ion implantation followed by ruby laser annealing. Displacement of B atoms from lattice sites was induced by proton irradiation at 35 K, followed by thermal annealing at 300 K. The resultant defect configuration was studied by ion channeling. Angular scans for Si and B were measured through several major axes, for irradiated samples, before and after the anneal, using RBS and the “B(p, o)‘Be reaction at Ep = 0.7 MeV. The experiments were compared with simulated scans, calculated with the use of both Monte Carlo and analytical methods, for a variety of assumed lattice sites. The results show that the fraction of displaced B atoms increases from ~15% to = 40% by the annealing process. The lattice site of these B atoms may either be a position displaced by 0.85 A along the (110) direction, or 0.9 A along the (100) direction, or a combination of positions, such as a fraction shifted in the (100) direction and a fraction at the bond center position. Despite considerable differences in the shape of channeling profiles predicted by the two models used, the results regarding the lattice position of the B atoms are consistent.
1. Introduction In previous studies the ion channeling method has been used to measure the substitutional fraction of implanted solute atoms, the clustering of high concentrations of dopant atoms, and the interaction between solute atoms and point defects [l-4]. In the present experiments we have studied the clustering of B atoms in Si as a result of migrating irradiation-induced interstitial B atoms [3,4]. Although B atoms occupy substitutional lattice sites in Si to very high concentrations, up to 5 X lo*’ cmp3 at 1470 K, irradiation creates B interstitial atoms via the interaction of Si self-interstitials with substitutional B atoms [5]. The EPR technique has been used to show this effect, not only for B [5] but also for other group-III solute atoms, such as Al [6]. In the case of B, the low symmetry of the EPR spectrum indicates that the ejected B atoms do not occupy high-symmetry interstitial sites. Several models for the B configuration were suggested [5], including a nearly bond-centered position between two Si atoms and positions displaced from the tetrahedral or hexagonal interstitial sites. Recent experiments indicate that interstitial boron is a negative-U defect, with the neutral charge state (observed by EPR) being unstable [7]. Interstitial B migrates with an activation energy of 0.6 eV [5]. The annealing can be enhanced by
* Institute of Physics, University of Aarhus, DK-8000 Aarhus C, Denmark. ** Department of Physics and Astronomy, University of North Carolina, Chapel Hill, NC 27514, USA. 0168-583X/90/$03.50 (North-Holland)
0 Elsevier Science Publishers
B.V.
minority-carrier injection, suggesting that a change in charge state of interstitial B alters its configuration to the saddle point for migration. One possible set of configurations [5,7] is the bond-centered position for B+, a bent bond-centered position for B” and a (100) split interstitial for B-. Defect complexes arising from the B migration have been observed by EPR, but have not been identified. Previous channeling experiments [3,4] showed that some B atoms were displaced from substitutional lattice sites by proton irradiation at 35 K, and that a considerably larger fraction was displaced during subsequent annealing near 240 K. This annealing stage corresponded to that observed for the disappearance of the EPR defect C28 attributed to interstitial neutral B. Thus the B displacements were due to clustering of interstitial B atoms, perhaps forming B-B split (100) interstitials [S]. The purpose of the present paper is to analyze the previous data [3,4] in more detail, using both analytical [9] and Monte Carlo [lo] simulations of the channeling angular scans.
2. Experimental procedure Si crystals were doped with “B to a concentration of = 6 x 101’ cmm3 in the near-surface region by implantation at 25 keV to a fluence of 1.5 X 1015 cm-‘, followed by ruby laser annealing at 1.6 J/cm2 (melt depth = 250 nm). The crystals were then mounted in a target chamber and irradiated at 35 K with 0.7 MeV H+ ions to a fluence of 4 x 10n’ cm-*. The irradiations were done in a random crystallographic direction over
P.J.M. Smulders et al. / Lattice site location of clustered boron atom
an area of 4 mm*. The samples were annealed in situ. The channeling measurements were performed at 35 K. The B atoms were detected by measuring the yield of alpha particles from the reaction ‘lB(p, a)8Be, which has a broad resonance near a proton energy of 0.67 MeV [ll]. Before irradiation, the B atoms were 99% substitutional; i.e., the minimum yields for channeling in the (110) direction were typically x$r”) = 0.03 and x$l”) = 0.04. After the irradiation, axial scans through the (100) and (110) channels were measured. After the subsequent annealing, scans were measured through the (lOO), (llO), (111) and (211) channels. The (100) scans were taken in a plane 15O from the (100) plane. The (110) scans were either along a (100) or a (110) plane, the (111) scan along a (211) plane. Several tests on the reproducibility of these scans show that the effect of the probing beam dose is negligible.
439
in silicon
1.0 -
+
0.5 -
i 0”
1.0 -
0.5 5
O-
I
3. Computational methods l.O-
Analytical as well as Monte Carlo calculations were carried out to compare the result of the two approaches. The analytical method, which was described recently [9], is based on the continuum string model, with the assumption of statistical equilibrium. The potential used is a multistring (25 strings) Doyle-Turner potential [12], averaged over thermal vibrations. Dechanneling due to scattering from electrons and nuclei is accounted for with a diffusion-like equation for the distribution function of the transverse energy [9]. The Monte Carlo method used is based on a revised version of the program described earlier [lo]. Here the trajectories of ions are followed through the crystal while their path is determined by binary collisions with the thermally vibrating host atoms. The ZBL potential [13] was used. The angular scattering and energy loss due to interaction with electrons are taken into account. In both types of simulations the two-dimensional thermal vibration amplitude was assumed to be 0.07 A for the Si atoms, consistent with a Debye temperature D of 500 K, and 0.12 A for the B atoms. It is well known that the statistical equilibrium hypothesis is an oversimplification of the channeling process [14,15]. In particular it fails to account for any effects that depend on the azimuthal angle of the beam direction with respect to crystallographic planes passing through the string direction. The most favourable application of the model is the case where the yield is measured as an average over the azimuthal angles [9]. Despite the fact that this condition is not met in the present experiment we decided to make a comparison between the two models in order to see the effect on the determination of lattice site locations in a practical case. Some predictions of both models are compared in fig. 1.
> 0.d
O0
Angle
I
I
1.0
2.0
with
(110)
Axis
Fig. 1. Comparison of angular channeling scans predicted by the statistical equilibrium model (solid lines) and the Monte Carlo method, for 0.7 MeV protons scattered from B impurities in Si, for scans through the (110) axis, in a (110) plane (circles), a (100) plane (plusses), or averaged over azimuthal angles (crosses). From top to bottom: (a) B atoms at substitutional sites, (b) B atoms displaced by 0.9 A in the (100) direction, (c) B atoms displaced by 1.0 A in the (110) direction.
It is seen that even when the average over azimuthal angles is taken, the agreement varies from good to poor, depending on the impurity site.
4. Results The channeling dips for silicon show no significant change resulting from the thermal anneal at 300 K. However, they are shallower and narrower than expected, which indicates crystal damage due to the irradiation. Since, unfortunately, these scans had been measured over a much deeper interval (150-500 run) than the boron-doped layer (O-200 nm), we will not attempt to interpret them quantitatively. The (100) VI. ION CHANNELING
P.J.M. &rudders et al. / Lattice site location of clustered boron atoms in silicon
440
”
-1.0 Angle
0 with
(100)
1.0 Axis
Fig. 2. Channeling RBS yield from Si for a scan through the (100) axis, in a direction 15O from the (100) plane, measured after proton irradiation at 35 K, before (squares) and after @hisses) thermal annealing at room temperature.The solid line is the Monte Carlo prediction.
scan is shown in fig. 2. Other scans show similar deviations from the Monte Carlo simulations. The effect of this small amount of damage on the flux density distribution, and thus on the channeling dips for boron, has been ignored in the analysis. The channeling profiles for the B atoms (see figs. 3 and 4), in contrast with the Si profiles, show a dramatic change as a result of the 300 K anneal. We will first consider the measurements after annealing (fig. 3). They were analysed on the assumption that they are due to a substitutional fraction and a fraction due to B atoms at a unique off-substitutional site. If we assume this fraction to be displaced along one of the major axes, the following possibilities were found for the shift D: (100) displacement: D = 0.88 + 0.10 A (Monte Carlo method), 0.9 +O.l A (analytical method); (110) displacement: D = 0.85 f 0.10 A (Monte Carlo method), 1.0 kO.1 A (analytical method). The substitutional fraction is 55-608 in both cases. The third possibility, a shift along the (111) direction, may be rejected. The best fit obtained for such a shift is for a position very close to the bond center. This position gives a good fit to all the data, except the (111) axial scan that should exhibit a peak in the yield at 0 O. Displacements perpendicular to the bond (bent bond configuration) do not improve the fit. Other sites, such as positions close to the tetrahedral or hexagonal interstitial sites, as well as the antibonding site can be safely excluded. As might be expected, the quality of the fits is better for the Monte Carlo simulations than for the analytical calculations. However, the Monte Carlo simulations do
not give a really satisfactory fit to the data either. This suggests there might be more than one off-substitutional position involved. One such possibility was investigated, by the Monte Carlo method only, namely a mixture of (100) shifted positions and bond-centered positions. This gave a reasonable fit (see fig. 3) for a ratio 58: 28: 14 of the substitutional, (100) shifted and bond-centered fractions. The shape of the angular scans measured before the annealing (fig. 4) shows that the lattice sites of the displaced B atoms are similar to those after the annealing. From fits with the same components as described above it follows that the nonsubstitutional fraction is now 15%, while the direction and magnitude of possible displacements are the same. Since no (111) scan was
~~~I-v’ -2.0 -1.0
0
1.0
-2.0
-1.0
0
1D
~~~
-1.0 Angle
0 with
1.0 Axis
0
-1.0 Angle
wtth
1.0 Axis
Fig. 3. Channeling dips of boron, measured after thermal annealing at 300 K, compared to fits from the Monte Carlo method (left) and the analytical method (right). From top to bottom: (a) (110) scan along a (110) plane, (b) (110) scan along a (100) plane, (c) (100) scan, 15 o from a (100) plane, (d) (111) scan along a (211) plane. The lines are for the following nonsubstitutional fractions. Solid lines: 45% shifted in the (100) direction by 0.9 A, dotted lines: 45% shifted in the (110) direction by 0.85 A (left) or 1.0 A (right); dashed lines, only shown on the left: 28% shifted in the (100) direction by 0.9 A, 14% at the bond-center position.
P.J.M.
Smulders et al. / Lattice site location of clustered boron atoms in silicon
M
D-
,
L’
0 1.0 1.0 Anglewth Axis
i
Fig. 4. Channeling dips of boron, measured before thermal annealing, through the (110) axis along a (110) plane (top), and through the (100) axis (bottom) (cf. figs. 2 and 3c), compared with simulated scans from the Monte Carlo method (left) and the analytical method (right). Solid lines: 15% shifted in the (100) direction; dotted lines: 15% shifted in the (110) direction, over the same distances as in fig. 3. The dashed lines, only shown on the left and very close to the dotted line on the top, and hidden by the solid line on the bottom, are for a fraction of 10% at the bond center.
measured we cannot exclude the bond-centered position in this case. For this solution the nonsubstitutional fraction is 10%.
5. Discussion The agreement between the statistical equilibrium model and the more accurate binary collision model in the results obtained for possible boron sites is remarkable. Apparently the changes in the yield as a function of angle due to displacements of the B atoms are more dominant than the differences between the two models in the absolute yield. For instance, the discrepancy seen in fig. lb shows up in fig. 3b but has no effect on the value of the displacement where the best fit occurs. This
441
result should not be generalized on the basis of the present work, but merits further investigation. Of the B interstitial configurations proposed from the EPR data, /he near-tetrahedal and near-hexagonal configurations can be excluded. The analysis restricts the possible sites of the displaced B atoms to positions displaced by about 0.9 A in the (110) or (100) directions from the substitutional site, possibly combined with a smaller component at the bond center. The component displaced in the (100) direction may be due to the formation of B-S+ (100) mixed interstitials by the irradiation, which migrate above 240 K and then form B-B (100) split interstitials.
References [l] M.L. Swanson, Rep. Prog. Phys. 45 (1982) 47. [2] .L.C. Feldman, J.W. Mayer and S.T. Picraux, Materials Analysis by Ion Channeling (Academic Press, New York 1982). [3] M.L. Swanson, L.M. Howe, A.F. QuenneviIIe and F.W. Saris, Radiat. Eff. Lett. 50 (1980) 139. [4] M.L. Swanson, L.M. Howe, F.W. Saris and A.F. Quemieville, in: Defects in Semiconductors, eds. J. Narayan and T.Y. Tan (North-Holland, Amsterdam, 1981) p. 71. [5] G.D. Watkins, Phys. Rev. B12 (1975) 5824. [6] K.L. Brower, Phys. Rev. Bl (1970) 1908. [7] R.D. Harris, J.L. Newton and G.D. Watkins, Phys. Rev. B36 (1987) 1094. [8] M.L. Swanson, Vacuum 39 (1989) 87. [9] B. Beth Nielsen, Phys. Rev. B37 (1988) 6353. [lo] P.J.M. Smulders and D.O. Boerma, Nucl. Instr. Meth. B29 (1987) 471. [ll] 0. Beckman, T. Huus and C. Zupancic, Phys. Rev. 91 (1953) 606. [12] P.A. Doyle and P.S. Turner, Acta CrystaIlogr. A24 (1968) 390. [13] J.F. Ziegler, J.F. Biersack and U. Littmark, The Stopping and Range of Ions in Solids (Pergamon, New York, 1985). [14] J.H. Barrett, Phys. Rev. Lett. 31 (1973) 1542. [15] J.U. Andersen and L.C. Feldman, Phys. Rev. Bl (1970) 2063.
VI. ION CHANNELING
438
LATTICE
SITE LOCATION
P.J.M. SMULDERS,
OF CLUSTERED
D.O. BOERMA,
Instruments
and Methods
in Physics Research
B45 (1990) 438-441 North-Holland
BORON ATOMS IN SILICON
B. BECH NIELSEN
* and M.L. SWANSON
**
General Physics Laboratory (LAN), Materials Science Center, University of Groningen, Westersingel 34, 9718 CM Groningen, The Netherlands
Si crystals were doped with *‘B by ion implantation followed by ruby laser annealing. Displacement of B atoms from lattice sites was induced by proton irradiation at 35 K, followed by thermal annealing at 300 K. The resultant defect configuration was studied by ion channeling. Angular scans for Si and B were measured through several major axes, for irradiated samples, before and after the anneal, using RBS and the “B(p, o)‘Be reaction at Ep = 0.7 MeV. The experiments were compared with simulated scans, calculated with the use of both Monte Carlo and analytical methods, for a variety of assumed lattice sites. The results show that the fraction of displaced B atoms increases from ~15% to = 40% by the annealing process. The lattice site of these B atoms may either be a position displaced by 0.85 A along the (110) direction, or 0.9 A along the (100) direction, or a combination of positions, such as a fraction shifted in the (100) direction and a fraction at the bond center position. Despite considerable differences in the shape of channeling profiles predicted by the two models used, the results regarding the lattice position of the B atoms are consistent.
1. Introduction In previous studies the ion channeling method has been used to measure the substitutional fraction of implanted solute atoms, the clustering of high concentrations of dopant atoms, and the interaction between solute atoms and point defects [l-4]. In the present experiments we have studied the clustering of B atoms in Si as a result of migrating irradiation-induced interstitial B atoms [3,4]. Although B atoms occupy substitutional lattice sites in Si to very high concentrations, up to 5 X lo*’ cmp3 at 1470 K, irradiation creates B interstitial atoms via the interaction of Si self-interstitials with substitutional B atoms [5]. The EPR technique has been used to show this effect, not only for B [5] but also for other group-III solute atoms, such as Al [6]. In the case of B, the low symmetry of the EPR spectrum indicates that the ejected B atoms do not occupy high-symmetry interstitial sites. Several models for the B configuration were suggested [5], including a nearly bond-centered position between two Si atoms and positions displaced from the tetrahedral or hexagonal interstitial sites. Recent experiments indicate that interstitial boron is a negative-U defect, with the neutral charge state (observed by EPR) being unstable [7]. Interstitial B migrates with an activation energy of 0.6 eV [5]. The annealing can be enhanced by
* Institute of Physics, University of Aarhus, DK-8000 Aarhus C, Denmark. ** Department of Physics and Astronomy, University of North Carolina, Chapel Hill, NC 27514, USA. 0168-583X/90/$03.50 (North-Holland)
0 Elsevier Science Publishers
B.V.
minority-carrier injection, suggesting that a change in charge state of interstitial B alters its configuration to the saddle point for migration. One possible set of configurations [5,7] is the bond-centered position for B+, a bent bond-centered position for B” and a (100) split interstitial for B-. Defect complexes arising from the B migration have been observed by EPR, but have not been identified. Previous channeling experiments [3,4] showed that some B atoms were displaced from substitutional lattice sites by proton irradiation at 35 K, and that a considerably larger fraction was displaced during subsequent annealing near 240 K. This annealing stage corresponded to that observed for the disappearance of the EPR defect C28 attributed to interstitial neutral B. Thus the B displacements were due to clustering of interstitial B atoms, perhaps forming B-B split (100) interstitials [S]. The purpose of the present paper is to analyze the previous data [3,4] in more detail, using both analytical [9] and Monte Carlo [lo] simulations of the channeling angular scans.
2. Experimental procedure Si crystals were doped with “B to a concentration of = 6 x 101’ cmm3 in the near-surface region by implantation at 25 keV to a fluence of 1.5 X 1015 cm-‘, followed by ruby laser annealing at 1.6 J/cm2 (melt depth = 250 nm). The crystals were then mounted in a target chamber and irradiated at 35 K with 0.7 MeV H+ ions to a fluence of 4 x 10n’ cm-*. The irradiations were done in a random crystallographic direction over
P.J.M. Smulders et al. / Lattice site location of clustered boron atom
an area of 4 mm*. The samples were annealed in situ. The channeling measurements were performed at 35 K. The B atoms were detected by measuring the yield of alpha particles from the reaction ‘lB(p, a)8Be, which has a broad resonance near a proton energy of 0.67 MeV [ll]. Before irradiation, the B atoms were 99% substitutional; i.e., the minimum yields for channeling in the (110) direction were typically x$r”) = 0.03 and x$l”) = 0.04. After the irradiation, axial scans through the (100) and (110) channels were measured. After the subsequent annealing, scans were measured through the (lOO), (llO), (111) and (211) channels. The (100) scans were taken in a plane 15O from the (100) plane. The (110) scans were either along a (100) or a (110) plane, the (111) scan along a (211) plane. Several tests on the reproducibility of these scans show that the effect of the probing beam dose is negligible.
439
in silicon
1.0 -
+
0.5 -
i 0”
1.0 -
0.5 5
O-
I
3. Computational methods l.O-
Analytical as well as Monte Carlo calculations were carried out to compare the result of the two approaches. The analytical method, which was described recently [9], is based on the continuum string model, with the assumption of statistical equilibrium. The potential used is a multistring (25 strings) Doyle-Turner potential [12], averaged over thermal vibrations. Dechanneling due to scattering from electrons and nuclei is accounted for with a diffusion-like equation for the distribution function of the transverse energy [9]. The Monte Carlo method used is based on a revised version of the program described earlier [lo]. Here the trajectories of ions are followed through the crystal while their path is determined by binary collisions with the thermally vibrating host atoms. The ZBL potential [13] was used. The angular scattering and energy loss due to interaction with electrons are taken into account. In both types of simulations the two-dimensional thermal vibration amplitude was assumed to be 0.07 A for the Si atoms, consistent with a Debye temperature D of 500 K, and 0.12 A for the B atoms. It is well known that the statistical equilibrium hypothesis is an oversimplification of the channeling process [14,15]. In particular it fails to account for any effects that depend on the azimuthal angle of the beam direction with respect to crystallographic planes passing through the string direction. The most favourable application of the model is the case where the yield is measured as an average over the azimuthal angles [9]. Despite the fact that this condition is not met in the present experiment we decided to make a comparison between the two models in order to see the effect on the determination of lattice site locations in a practical case. Some predictions of both models are compared in fig. 1.
> 0.d
O0
Angle
I
I
1.0
2.0
with
(110)
Axis
Fig. 1. Comparison of angular channeling scans predicted by the statistical equilibrium model (solid lines) and the Monte Carlo method, for 0.7 MeV protons scattered from B impurities in Si, for scans through the (110) axis, in a (110) plane (circles), a (100) plane (plusses), or averaged over azimuthal angles (crosses). From top to bottom: (a) B atoms at substitutional sites, (b) B atoms displaced by 0.9 A in the (100) direction, (c) B atoms displaced by 1.0 A in the (110) direction.
It is seen that even when the average over azimuthal angles is taken, the agreement varies from good to poor, depending on the impurity site.
4. Results The channeling dips for silicon show no significant change resulting from the thermal anneal at 300 K. However, they are shallower and narrower than expected, which indicates crystal damage due to the irradiation. Since, unfortunately, these scans had been measured over a much deeper interval (150-500 run) than the boron-doped layer (O-200 nm), we will not attempt to interpret them quantitatively. The (100) VI. ION CHANNELING
P.J.M. &rudders et al. / Lattice site location of clustered boron atoms in silicon
440
”
-1.0 Angle
0 with
(100)
1.0 Axis
Fig. 2. Channeling RBS yield from Si for a scan through the (100) axis, in a direction 15O from the (100) plane, measured after proton irradiation at 35 K, before (squares) and after @hisses) thermal annealing at room temperature.The solid line is the Monte Carlo prediction.
scan is shown in fig. 2. Other scans show similar deviations from the Monte Carlo simulations. The effect of this small amount of damage on the flux density distribution, and thus on the channeling dips for boron, has been ignored in the analysis. The channeling profiles for the B atoms (see figs. 3 and 4), in contrast with the Si profiles, show a dramatic change as a result of the 300 K anneal. We will first consider the measurements after annealing (fig. 3). They were analysed on the assumption that they are due to a substitutional fraction and a fraction due to B atoms at a unique off-substitutional site. If we assume this fraction to be displaced along one of the major axes, the following possibilities were found for the shift D: (100) displacement: D = 0.88 + 0.10 A (Monte Carlo method), 0.9 +O.l A (analytical method); (110) displacement: D = 0.85 f 0.10 A (Monte Carlo method), 1.0 kO.1 A (analytical method). The substitutional fraction is 55-608 in both cases. The third possibility, a shift along the (111) direction, may be rejected. The best fit obtained for such a shift is for a position very close to the bond center. This position gives a good fit to all the data, except the (111) axial scan that should exhibit a peak in the yield at 0 O. Displacements perpendicular to the bond (bent bond configuration) do not improve the fit. Other sites, such as positions close to the tetrahedral or hexagonal interstitial sites, as well as the antibonding site can be safely excluded. As might be expected, the quality of the fits is better for the Monte Carlo simulations than for the analytical calculations. However, the Monte Carlo simulations do
not give a really satisfactory fit to the data either. This suggests there might be more than one off-substitutional position involved. One such possibility was investigated, by the Monte Carlo method only, namely a mixture of (100) shifted positions and bond-centered positions. This gave a reasonable fit (see fig. 3) for a ratio 58: 28: 14 of the substitutional, (100) shifted and bond-centered fractions. The shape of the angular scans measured before the annealing (fig. 4) shows that the lattice sites of the displaced B atoms are similar to those after the annealing. From fits with the same components as described above it follows that the nonsubstitutional fraction is now 15%, while the direction and magnitude of possible displacements are the same. Since no (111) scan was
~~~I-v’ -2.0 -1.0
0
1.0
-2.0
-1.0
0
1D
~~~
-1.0 Angle
0 with
1.0 Axis
0
-1.0 Angle
wtth
1.0 Axis
Fig. 3. Channeling dips of boron, measured after thermal annealing at 300 K, compared to fits from the Monte Carlo method (left) and the analytical method (right). From top to bottom: (a) (110) scan along a (110) plane, (b) (110) scan along a (100) plane, (c) (100) scan, 15 o from a (100) plane, (d) (111) scan along a (211) plane. The lines are for the following nonsubstitutional fractions. Solid lines: 45% shifted in the (100) direction by 0.9 A, dotted lines: 45% shifted in the (110) direction by 0.85 A (left) or 1.0 A (right); dashed lines, only shown on the left: 28% shifted in the (100) direction by 0.9 A, 14% at the bond-center position.
P.J.M.
Smulders et al. / Lattice site location of clustered boron atoms in silicon
M
D-
,
L’
0 1.0 1.0 Anglewth Axis
i
Fig. 4. Channeling dips of boron, measured before thermal annealing, through the (110) axis along a (110) plane (top), and through the (100) axis (bottom) (cf. figs. 2 and 3c), compared with simulated scans from the Monte Carlo method (left) and the analytical method (right). Solid lines: 15% shifted in the (100) direction; dotted lines: 15% shifted in the (110) direction, over the same distances as in fig. 3. The dashed lines, only shown on the left and very close to the dotted line on the top, and hidden by the solid line on the bottom, are for a fraction of 10% at the bond center.
measured we cannot exclude the bond-centered position in this case. For this solution the nonsubstitutional fraction is 10%.
5. Discussion The agreement between the statistical equilibrium model and the more accurate binary collision model in the results obtained for possible boron sites is remarkable. Apparently the changes in the yield as a function of angle due to displacements of the B atoms are more dominant than the differences between the two models in the absolute yield. For instance, the discrepancy seen in fig. lb shows up in fig. 3b but has no effect on the value of the displacement where the best fit occurs. This
441
result should not be generalized on the basis of the present work, but merits further investigation. Of the B interstitial configurations proposed from the EPR data, /he near-tetrahedal and near-hexagonal configurations can be excluded. The analysis restricts the possible sites of the displaced B atoms to positions displaced by about 0.9 A in the (110) or (100) directions from the substitutional site, possibly combined with a smaller component at the bond center. The component displaced in the (100) direction may be due to the formation of B-S+ (100) mixed interstitials by the irradiation, which migrate above 240 K and then form B-B (100) split interstitials.
References [l] M.L. Swanson, Rep. Prog. Phys. 45 (1982) 47. [2] .L.C. Feldman, J.W. Mayer and S.T. Picraux, Materials Analysis by Ion Channeling (Academic Press, New York 1982). [3] M.L. Swanson, L.M. Howe, A.F. QuenneviIIe and F.W. Saris, Radiat. Eff. Lett. 50 (1980) 139. [4] M.L. Swanson, L.M. Howe, F.W. Saris and A.F. Quemieville, in: Defects in Semiconductors, eds. J. Narayan and T.Y. Tan (North-Holland, Amsterdam, 1981) p. 71. [5] G.D. Watkins, Phys. Rev. B12 (1975) 5824. [6] K.L. Brower, Phys. Rev. Bl (1970) 1908. [7] R.D. Harris, J.L. Newton and G.D. Watkins, Phys. Rev. B36 (1987) 1094. [8] M.L. Swanson, Vacuum 39 (1989) 87. [9] B. Beth Nielsen, Phys. Rev. B37 (1988) 6353. [lo] P.J.M. Smulders and D.O. Boerma, Nucl. Instr. Meth. B29 (1987) 471. [ll] 0. Beckman, T. Huus and C. Zupancic, Phys. Rev. 91 (1953) 606. [12] P.A. Doyle and P.S. Turner, Acta CrystaIlogr. A24 (1968) 390. [13] J.F. Ziegler, J.F. Biersack and U. Littmark, The Stopping and Range of Ions in Solids (Pergamon, New York, 1985). [14] J.H. Barrett, Phys. Rev. Lett. 31 (1973) 1542. [15] J.U. Andersen and L.C. Feldman, Phys. Rev. Bl (1970) 2063.
VI. ION CHANNELING