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Online monitoring of ion induced modifications by ERDA using a large area position-sensitive detector telescope
Nuclear Instruments and Methods in Physics Research B 142 (1998) 117±121
Online monitoring of ion induced modi®cations by ERDA using a large area position-sensitive detector telescope D.K. Avasthi a, W. Assmann b
b,*
, H. Huber b, H.D. Mieskes b, H. Nolte
b
a Nuclear Science Centre, Aruna Asaf Ali Marg, Post Box 10502, New Delhi 110076, India Sektion Physik, Ludwig-Maximilians-Universit at M unchen, Beschleunigerlabor, D-85748 Garching, Germany
Received 13 November 1997; received in revised form 23 January 1998
Abstract Heavy ion induced materials modi®cations, seen as composition or depth pro®le changes, can be measured in situ by ERDA (elastic recoil detection analysis). The necessity and importance of a large area position sensitive detector telescope, which combines high depth resolution and large detection eciency, is pointed out for this technique by two examples. One is an ERDA experiment carried out during 243 MeV Au ion irradiation of a thin Fe ®lm on Si. The dynamic measurement revealed changes in the depth pro®le of Fe and O due to ion induced oxygen absorption. A comparison is made of the present result with the result of a ®ctitious conventional detector telescope whose solid angle is considerably smaller. The other example is a blocking-ERDA experiment with a 165 MeV Ni beam on a Ge crystal where the damage was followed as a function of ion ¯uence. This work demonstrates the suitability of the experimental setup used for online monitoring of ion induced modi®cations. Ó 1998 Elsevier Science B.V. All rights reserved. Keywords: In-situ analysis; ERDA; High energetic heavy ion irradiation
1. Introduction High-energy, heavy ions and DE±E detector telescopes are quite well suited for the ERDA [1± 4] technique of simultaneous depth pro®ling of several elements in materials. The choice of using heavier ions such as Au has several advantages such as (1) the maximum scattering angle of the beam is reduced, (2) the cross-section is larger,
* Corresponding author. Tel.: 49 89 2891 4283; fax: 49 89 2891 4280; e-mail: [email protected].
(3) depth pro®ling of elements up to mass 150 amu can be carried out. Energetic heavy ions can also cause materials modi®cations because of their ability to impart large electronic excitation by inelastic collisions with the atoms of the material. It is therefore essential to know whether the material under study has preserved its original elemental composition and depth pro®le or has been changed during the ERDA irradiation. There have been several reports which have shown out-diusion of volatile components such as, e.g. hydrogen [5±7], sputtering of sample components or compounds such as hydrocarbons and
0168-583X/98/$19.00 Ó 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 2 0 9 - 2
118
D.K. Avasthi et al. / Nucl. Instr. and Meth. in Phys. Res. B 142 (1998) 117±121
carbon clusters [8±11] during ion irradiation. Therefore, for ERDA with heavy ions it is necessary to make dynamic measurements in which the depth pro®le can be followed as a function of ion dose. This kind of measurement is also of interest in studies where materials modi®cations are intentionally produced by heavy ion irradiation. A detector with a large solid angle is required for such measurements so that a sucient number of recoils can be recorded for a depth pro®le with reasonable statistics for a small beam dose. However, the use of a detector with a large solid angle and consequentially a large opening angle results in a large kinematic broadening and hence poor depth resolution. This can be overcome if the detector is position sensitive so that the kinematic energy shift can be corrected by software [12]. This method improves the detection sensitivity without sacri®cing the depth resolution. In the present work we show how such a large area, position sensitive detector can be used to monitor ion-beam induced changes. The absorption of oxygen in a thin (50 nm) Fe ®lm on Si, during irradiation, is demonstrated and compared with the result that would be obtained using a conventional small solid-angle detector. A second example is the radiation damage in Ge during heavy ion irradiation which has been followed by blocking-ERDA measurements. 2. Experimental conditions A beam of 243 MeV 197 Au ions from the Munich 15 MV tandem accelerator was used to bombard a thin (50 nm) Fe layer evaporated on a silicon substrate. A current of 0.2 particle nA was limited in size to about 0.75 mm2 by collimation. Recoils were detected at an angle of 37.5° to the beam direction with the sample surface tilted at 10° to the beam. The vacuum in the scattering chamber was 8 ´ 10ÿ7 mbar during the measurement. The detector used in these experiments was a DE±Erest ionization chamber which has been described in detail in Ref. [12]. Brie¯y, particle identi®cation was deduced from the DE- and Erest -signals of the subdivided detector anode. The position information was derived from the
two backgammon±shaped cathode halves in the scattering plane and perpendicular to this from the ratio of the total anode to the total cathode pulse heights. The angular resolution of better than 0.1° allowed a complete correction of the kinematic energy shift within the energy resolution of the ionization detector of about 1%. The two energy signals, two position signals and the integrated current from the sample were stored event-by-event on magnetic tape. The integrated current was calibrated with a suppressed Faraday cup for dose measurements. 3. Irradiation induced oxidation of Fe The data from a total ion dose on the sample of 4.6 ´ 1014 ions/cm2 were divided into twelve equal bins of ion dose and kinematically corrected recoil energy spectra of Fe, Si, O and C were accumulated for each bin from tape. Energy spectra for the ®rst and last bins of ion dose for Fe and O recoils are shown in Figs. 1 and 2, respectively. Inspection of the spectra shows that the low energy edge of the Fe and O recoils, and the high energy side of the Si recoils (not plotted) are shifted to lower energies. In addition, the high energy edge of the Fe spectrum, representing the surface of the ®lm, is also skewed. This change in recoil spectra suggests a change of composition during irradiation which is con®rmed in the oxygen recoil spectra by the
Fig. 1. Recoil spectrum of Fe from the initial and ®nal part of the irradiation.
D.K. Avasthi et al. / Nucl. Instr. and Meth. in Phys. Res. B 142 (1998) 117±121
119
Fig. 2. Recoil spectra of O from dierent parts of the irradiation as indicated.
surface oxygen increase between the beginning and end of the irradiation. We note that there is a small amount of oxygen at the interface to the Si substrate which is not changed during the irradiation. Also, no change was measured in a weak carbon surface contamination. The increase in oxygen corresponds to a number of 250 90 atoms per incident Au ion. The quoted error is mainly due to the uncertainty in dose measurement. This unusual large increase in oxygen is attributed to the high electronic energy deposition of 47 keV/nm by the Au ions near the Fe surface. 4. Comparison with a conventional small solid-angle detector From the data collected in this experiment we can compare the result that would be obtained using a conventional small solid-angle detector. In order to have the same kinematic spread in a standard detector as obtained with the detector used in the present experiment, one would have to use a detector with an opening angle of 0.22°. A typical detector would have a 2 mm ´ 4 mm aperture situated at 520 mm to give an opening angle of 0.22° and would have a solid angle of 0.3 msr. This solid angle is 206 times smaller than the solid angle of the detector used in the present experiment. We therefore played back our data tapes sorting only one in every 206 events to simulate
Fig. 3. The Fe recoil spectra which would be obtained using a conventional small-angle detector telescope at the initial part of the irradiation (above) and the ®nal part of the irradiation (below).
the smaller solid angle detector. The resulting recoil energy spectra are shown in Figs. 3 and 4 for Fe and O recoils, respectively, for the otherwise same conditions as that shown in Figs. 1 and 2. The resulting poor statistical accuracy of the data makes it impossible to make any quantitative statements about damage under irradiation. Fig. 5 shows the energy spectrum for Fe recoils from all twelve data bins analyzed in this manner, and the pro®le is obviously misleading. 5. Irradiation damage of Ge In the second example we make use of the twodimensional position sensitive feature of the ionization chamber to observe blocking patterns as a function of the irradiation dose. A beam of 165 MeV 58 Ni ions was used to bombard a Ge crystal
120
D.K. Avasthi et al. / Nucl. Instr. and Meth. in Phys. Res. B 142 (1998) 117±121
Fig. 4. Same as Fig. 3 for oxygen recoils.
with a h1 0 0i axis normal to its surface. Blocking patterns were recorded on tape of Ge recoils at a scattering angle of 50.2° along a h1 1 1i axis which has an angle of 54.7° to the surface normal. The total ¯uence of 5 ´ 1015 ions/cm2 was subdivided for data evaluation into bins of 2 ´ 1014 ions/ cm2 . For each bin the minimum yield, vmin , of the blocking pattern was derived by taking circular scans around the minimum in the blocking pattern. The radiation damage was determined from the normalized value vrad / 1 ÿ 1 ÿ vmin /= 1 ÿ vmin 0. Fig. 6 shows the damage rate, vrad , as a function of beam dose. As can be seen, the advantage of a large solid-angle detector is invaluable for such measurements in order to follow the characteristic damage development with adequate statistics. Thus two dierent dose ranges can be distinguished, an almost linear increase in vrad is observed up to around 1.5 ´ 1015 ions/cm2 , followed by an almost constant vrad level of 0.4± 0.5. This behavior can be interpreted as an annealing of pre-damaged volumes. Only the ®rst linear part of the damage rate can be used for comparison with simulation codes to extract electronic stopping eects. Our systematic study for dierent ions and energies shows a steady decrease of the
Fig. 5. The recoil spectrum for Fe which would be obtained from a conventional small-angle detector telescope for the same total dose as that used in the present experiment.
D.K. Avasthi et al. / Nucl. Instr. and Meth. in Phys. Res. B 142 (1998) 117±121
121
this detection eciency essential dose dependent eects can be missed. Finally, because of the large solid angle, shorter analysis times are required for a particular sample measurement with the consequent reduction in accelerator beam time and sample damage. Acknowledgements
Fig. 6. The damage rate of 165 MeV 58 Ni ions in Ge measured by blocking along the h1 1 1i axis.
One of us (D.K.A.) would like to acknowledge the ®nancial support of the German academic exchange program (DAAD). We would also like to thank J.S. Forster for his help in preparing the manuscript. References
damage rate, normalized to the calculated nuclear damage rate, with increasing electronic stopping power [13]. Blocking-ERDA seems to be a well suited method to measure online radiation induced crystal damage. As in the preceding example the heavy ion beam both creates and monitors the sample change at the same time. An alternative possibility would be channeling-RBS with He to see the radiation damage which is produced in separate random irradiations by heavy ions. Resistance and Hall mobility of n- and p-doped Ge have been measured in situ as a function of the ion ¯uence by Levalois et al. [14] in the dose range of up to 1012 ions/cm2 . Although both parameters are not a direct measure of the number of displaced atoms a partial electronic annealing of defects has been found for large electronic stopping powers. 6. Conclusions The availability of a large solid-angle detector with particle and position resolution has several advantages in ERDA measurements using beams of very heavy ions. Such detectors have high sensitivity as well as excellent depth resolution, characteristics which are usually mutually exclusive. As shown in this work it is an ideal tool for monitoring ion-beam induced sample changes. Without
[1] J. L'Ecuyer, C. Brassard, C. Cardinal, B. Terrault, Nucl. Instr. and Meth. 149 (1978) 271. [2] J.P. Stoquert, G. Guillaume, M. Hage-Ali, J.J. Grob, C. Ganter, P. Siert, Nucl. Instr. and Meth. B 44 (1989) 184. [3] W. Assmann, H. Huber, Ch. Steinhausen, M. Dobler, H. Gl uckler, A. Weidinger, Nucl. Instr. and Meth. B 89 (1994) 131. [4] D.K. Avasthi, D. Kabiraj, A. Bhagwat, G.K. Metha, V.D. Vankar, S.B. Ogale, Nucl. Instr. and Meth. B 93 (1994) 480. [5] F. Fujimoto, M. Tanaka, Y. Iwata, A. Ootuka, M. Haba, K. Kobayashi, Nucl. Instr. and Meth. B 33 (1988) 792. [6] D.K. Avasthi, Vacuum 47 (1996) 1249. [7] R. Behrisch, V.M. Prozesky, H. Huber, W. Assmann, Nucl. Instr. and Meth. B 118 (1996) 262. [8] G. Brinkmalm, P. Demirev, D. Fenyo, P. Hakansson, B.U.R. Sundqvist, Phys. Rev. B 47 (1993) 7560. [9] R.M. Papaleo, A. Hallen, P. Demirev, G. Brinkmalm, J.E. Eriksson, P. Hakansson, B.U.R. Sundqvist, Nucl. Instr. and Meth. B 91 (1994) 667. [10] W. Assmann, J.A. Davies, G. Dollinger, J.S. Forster, H. Huber, Th. Reichelt, R. Siegele, Nucl. Instr. and Meth. B 118 (1996) 242. [11] G. Dollinger, M. Boulouedine, A. Bergmaier, T. Faestermann, C.M. Frey, Nucl. Instr. and Meth. B 118 (1996) 291. [12] W. Assmann, P. Hartung, H. Huber, P. Staat, H. Steens, Ch. Steinhausen, Nucl. Instr. and Meth. B 85 (1994) 726. [13] H. Nolte, W. Assmann, H. Huber, S.A. Karamian, H.D. Mieskes, Proc. Thirteenth Int. Conf. on Ion Beam Analysis, August 1997, Lisboa, Portugal, Nucl. Instr. and Meth. B 136±138 (1998) 587. [14] M. Levalois, J.P. Girard, A. Allais, A. Hairie, M.N. Metzner, E. Paumier, Nucl. Instr. and Meth. B 63 (1992) 25.
Online monitoring of ion induced modi®cations by ERDA using a large area position-sensitive detector telescope D.K. Avasthi a, W. Assmann b
b,*
, H. Huber b, H.D. Mieskes b, H. Nolte
b
a Nuclear Science Centre, Aruna Asaf Ali Marg, Post Box 10502, New Delhi 110076, India Sektion Physik, Ludwig-Maximilians-Universit at M unchen, Beschleunigerlabor, D-85748 Garching, Germany
Received 13 November 1997; received in revised form 23 January 1998
Abstract Heavy ion induced materials modi®cations, seen as composition or depth pro®le changes, can be measured in situ by ERDA (elastic recoil detection analysis). The necessity and importance of a large area position sensitive detector telescope, which combines high depth resolution and large detection eciency, is pointed out for this technique by two examples. One is an ERDA experiment carried out during 243 MeV Au ion irradiation of a thin Fe ®lm on Si. The dynamic measurement revealed changes in the depth pro®le of Fe and O due to ion induced oxygen absorption. A comparison is made of the present result with the result of a ®ctitious conventional detector telescope whose solid angle is considerably smaller. The other example is a blocking-ERDA experiment with a 165 MeV Ni beam on a Ge crystal where the damage was followed as a function of ion ¯uence. This work demonstrates the suitability of the experimental setup used for online monitoring of ion induced modi®cations. Ó 1998 Elsevier Science B.V. All rights reserved. Keywords: In-situ analysis; ERDA; High energetic heavy ion irradiation
1. Introduction High-energy, heavy ions and DE±E detector telescopes are quite well suited for the ERDA [1± 4] technique of simultaneous depth pro®ling of several elements in materials. The choice of using heavier ions such as Au has several advantages such as (1) the maximum scattering angle of the beam is reduced, (2) the cross-section is larger,
* Corresponding author. Tel.: 49 89 2891 4283; fax: 49 89 2891 4280; e-mail: [email protected].
(3) depth pro®ling of elements up to mass 150 amu can be carried out. Energetic heavy ions can also cause materials modi®cations because of their ability to impart large electronic excitation by inelastic collisions with the atoms of the material. It is therefore essential to know whether the material under study has preserved its original elemental composition and depth pro®le or has been changed during the ERDA irradiation. There have been several reports which have shown out-diusion of volatile components such as, e.g. hydrogen [5±7], sputtering of sample components or compounds such as hydrocarbons and
0168-583X/98/$19.00 Ó 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 2 0 9 - 2
118
D.K. Avasthi et al. / Nucl. Instr. and Meth. in Phys. Res. B 142 (1998) 117±121
carbon clusters [8±11] during ion irradiation. Therefore, for ERDA with heavy ions it is necessary to make dynamic measurements in which the depth pro®le can be followed as a function of ion dose. This kind of measurement is also of interest in studies where materials modi®cations are intentionally produced by heavy ion irradiation. A detector with a large solid angle is required for such measurements so that a sucient number of recoils can be recorded for a depth pro®le with reasonable statistics for a small beam dose. However, the use of a detector with a large solid angle and consequentially a large opening angle results in a large kinematic broadening and hence poor depth resolution. This can be overcome if the detector is position sensitive so that the kinematic energy shift can be corrected by software [12]. This method improves the detection sensitivity without sacri®cing the depth resolution. In the present work we show how such a large area, position sensitive detector can be used to monitor ion-beam induced changes. The absorption of oxygen in a thin (50 nm) Fe ®lm on Si, during irradiation, is demonstrated and compared with the result that would be obtained using a conventional small solid-angle detector. A second example is the radiation damage in Ge during heavy ion irradiation which has been followed by blocking-ERDA measurements. 2. Experimental conditions A beam of 243 MeV 197 Au ions from the Munich 15 MV tandem accelerator was used to bombard a thin (50 nm) Fe layer evaporated on a silicon substrate. A current of 0.2 particle nA was limited in size to about 0.75 mm2 by collimation. Recoils were detected at an angle of 37.5° to the beam direction with the sample surface tilted at 10° to the beam. The vacuum in the scattering chamber was 8 ´ 10ÿ7 mbar during the measurement. The detector used in these experiments was a DE±Erest ionization chamber which has been described in detail in Ref. [12]. Brie¯y, particle identi®cation was deduced from the DE- and Erest -signals of the subdivided detector anode. The position information was derived from the
two backgammon±shaped cathode halves in the scattering plane and perpendicular to this from the ratio of the total anode to the total cathode pulse heights. The angular resolution of better than 0.1° allowed a complete correction of the kinematic energy shift within the energy resolution of the ionization detector of about 1%. The two energy signals, two position signals and the integrated current from the sample were stored event-by-event on magnetic tape. The integrated current was calibrated with a suppressed Faraday cup for dose measurements. 3. Irradiation induced oxidation of Fe The data from a total ion dose on the sample of 4.6 ´ 1014 ions/cm2 were divided into twelve equal bins of ion dose and kinematically corrected recoil energy spectra of Fe, Si, O and C were accumulated for each bin from tape. Energy spectra for the ®rst and last bins of ion dose for Fe and O recoils are shown in Figs. 1 and 2, respectively. Inspection of the spectra shows that the low energy edge of the Fe and O recoils, and the high energy side of the Si recoils (not plotted) are shifted to lower energies. In addition, the high energy edge of the Fe spectrum, representing the surface of the ®lm, is also skewed. This change in recoil spectra suggests a change of composition during irradiation which is con®rmed in the oxygen recoil spectra by the
Fig. 1. Recoil spectrum of Fe from the initial and ®nal part of the irradiation.
D.K. Avasthi et al. / Nucl. Instr. and Meth. in Phys. Res. B 142 (1998) 117±121
119
Fig. 2. Recoil spectra of O from dierent parts of the irradiation as indicated.
surface oxygen increase between the beginning and end of the irradiation. We note that there is a small amount of oxygen at the interface to the Si substrate which is not changed during the irradiation. Also, no change was measured in a weak carbon surface contamination. The increase in oxygen corresponds to a number of 250 90 atoms per incident Au ion. The quoted error is mainly due to the uncertainty in dose measurement. This unusual large increase in oxygen is attributed to the high electronic energy deposition of 47 keV/nm by the Au ions near the Fe surface. 4. Comparison with a conventional small solid-angle detector From the data collected in this experiment we can compare the result that would be obtained using a conventional small solid-angle detector. In order to have the same kinematic spread in a standard detector as obtained with the detector used in the present experiment, one would have to use a detector with an opening angle of 0.22°. A typical detector would have a 2 mm ´ 4 mm aperture situated at 520 mm to give an opening angle of 0.22° and would have a solid angle of 0.3 msr. This solid angle is 206 times smaller than the solid angle of the detector used in the present experiment. We therefore played back our data tapes sorting only one in every 206 events to simulate
Fig. 3. The Fe recoil spectra which would be obtained using a conventional small-angle detector telescope at the initial part of the irradiation (above) and the ®nal part of the irradiation (below).
the smaller solid angle detector. The resulting recoil energy spectra are shown in Figs. 3 and 4 for Fe and O recoils, respectively, for the otherwise same conditions as that shown in Figs. 1 and 2. The resulting poor statistical accuracy of the data makes it impossible to make any quantitative statements about damage under irradiation. Fig. 5 shows the energy spectrum for Fe recoils from all twelve data bins analyzed in this manner, and the pro®le is obviously misleading. 5. Irradiation damage of Ge In the second example we make use of the twodimensional position sensitive feature of the ionization chamber to observe blocking patterns as a function of the irradiation dose. A beam of 165 MeV 58 Ni ions was used to bombard a Ge crystal
120
D.K. Avasthi et al. / Nucl. Instr. and Meth. in Phys. Res. B 142 (1998) 117±121
Fig. 4. Same as Fig. 3 for oxygen recoils.
with a h1 0 0i axis normal to its surface. Blocking patterns were recorded on tape of Ge recoils at a scattering angle of 50.2° along a h1 1 1i axis which has an angle of 54.7° to the surface normal. The total ¯uence of 5 ´ 1015 ions/cm2 was subdivided for data evaluation into bins of 2 ´ 1014 ions/ cm2 . For each bin the minimum yield, vmin , of the blocking pattern was derived by taking circular scans around the minimum in the blocking pattern. The radiation damage was determined from the normalized value vrad / 1 ÿ 1 ÿ vmin /= 1 ÿ vmin 0. Fig. 6 shows the damage rate, vrad , as a function of beam dose. As can be seen, the advantage of a large solid-angle detector is invaluable for such measurements in order to follow the characteristic damage development with adequate statistics. Thus two dierent dose ranges can be distinguished, an almost linear increase in vrad is observed up to around 1.5 ´ 1015 ions/cm2 , followed by an almost constant vrad level of 0.4± 0.5. This behavior can be interpreted as an annealing of pre-damaged volumes. Only the ®rst linear part of the damage rate can be used for comparison with simulation codes to extract electronic stopping eects. Our systematic study for dierent ions and energies shows a steady decrease of the
Fig. 5. The recoil spectrum for Fe which would be obtained from a conventional small-angle detector telescope for the same total dose as that used in the present experiment.
D.K. Avasthi et al. / Nucl. Instr. and Meth. in Phys. Res. B 142 (1998) 117±121
121
this detection eciency essential dose dependent eects can be missed. Finally, because of the large solid angle, shorter analysis times are required for a particular sample measurement with the consequent reduction in accelerator beam time and sample damage. Acknowledgements
Fig. 6. The damage rate of 165 MeV 58 Ni ions in Ge measured by blocking along the h1 1 1i axis.
One of us (D.K.A.) would like to acknowledge the ®nancial support of the German academic exchange program (DAAD). We would also like to thank J.S. Forster for his help in preparing the manuscript. References
damage rate, normalized to the calculated nuclear damage rate, with increasing electronic stopping power [13]. Blocking-ERDA seems to be a well suited method to measure online radiation induced crystal damage. As in the preceding example the heavy ion beam both creates and monitors the sample change at the same time. An alternative possibility would be channeling-RBS with He to see the radiation damage which is produced in separate random irradiations by heavy ions. Resistance and Hall mobility of n- and p-doped Ge have been measured in situ as a function of the ion ¯uence by Levalois et al. [14] in the dose range of up to 1012 ions/cm2 . Although both parameters are not a direct measure of the number of displaced atoms a partial electronic annealing of defects has been found for large electronic stopping powers. 6. Conclusions The availability of a large solid-angle detector with particle and position resolution has several advantages in ERDA measurements using beams of very heavy ions. Such detectors have high sensitivity as well as excellent depth resolution, characteristics which are usually mutually exclusive. As shown in this work it is an ideal tool for monitoring ion-beam induced sample changes. Without
[1] J. L'Ecuyer, C. Brassard, C. Cardinal, B. Terrault, Nucl. Instr. and Meth. 149 (1978) 271. [2] J.P. Stoquert, G. Guillaume, M. Hage-Ali, J.J. Grob, C. Ganter, P. Siert, Nucl. Instr. and Meth. B 44 (1989) 184. [3] W. Assmann, H. Huber, Ch. Steinhausen, M. Dobler, H. Gl uckler, A. Weidinger, Nucl. Instr. and Meth. B 89 (1994) 131. [4] D.K. Avasthi, D. Kabiraj, A. Bhagwat, G.K. Metha, V.D. Vankar, S.B. Ogale, Nucl. Instr. and Meth. B 93 (1994) 480. [5] F. Fujimoto, M. Tanaka, Y. Iwata, A. Ootuka, M. Haba, K. Kobayashi, Nucl. Instr. and Meth. B 33 (1988) 792. [6] D.K. Avasthi, Vacuum 47 (1996) 1249. [7] R. Behrisch, V.M. Prozesky, H. Huber, W. Assmann, Nucl. Instr. and Meth. B 118 (1996) 262. [8] G. Brinkmalm, P. Demirev, D. Fenyo, P. Hakansson, B.U.R. Sundqvist, Phys. Rev. B 47 (1993) 7560. [9] R.M. Papaleo, A. Hallen, P. Demirev, G. Brinkmalm, J.E. Eriksson, P. Hakansson, B.U.R. Sundqvist, Nucl. Instr. and Meth. B 91 (1994) 667. [10] W. Assmann, J.A. Davies, G. Dollinger, J.S. Forster, H. Huber, Th. Reichelt, R. Siegele, Nucl. Instr. and Meth. B 118 (1996) 242. [11] G. Dollinger, M. Boulouedine, A. Bergmaier, T. Faestermann, C.M. Frey, Nucl. Instr. and Meth. B 118 (1996) 291. [12] W. Assmann, P. Hartung, H. Huber, P. Staat, H. Steens, Ch. Steinhausen, Nucl. Instr. and Meth. B 85 (1994) 726. [13] H. Nolte, W. Assmann, H. Huber, S.A. Karamian, H.D. Mieskes, Proc. Thirteenth Int. Conf. on Ion Beam Analysis, August 1997, Lisboa, Portugal, Nucl. Instr. and Meth. B 136±138 (1998) 587. [14] M. Levalois, J.P. Girard, A. Allais, A. Hairie, M.N. Metzner, E. Paumier, Nucl. Instr. and Meth. B 63 (1992) 25.