- Email: [email protected]
Study of swift heavy ion tracks on crystalline quartz surfaces
Nuclear Instruments and Methods in Physics Research B 209 (2003) 165–169 www.elsevier.com/locate/nimb
Study of swift heavy ion tracks on crystalline quartz surfaces N. Khalfaoui *, C.C. Rotaru, S. Bouffard, E. Jacquet, H. Lebius, M. Toulemonde Centre interdisciplinaire de recherche ions laser (CEA-CNRS-ISMRA), CIRIL-GANIL, rue Claude Bloch, BP 5133, 14070 Caen Cedex 5, France
Abstract Ion irradiation of SiO2 quartz at medium energy induces damage in the bulk as well as modification on the surface. To make a correlation between the bulk and the surface phenomena, AFM investigations on the SiO2 quartz surfaces, irradiated with Ni, Kr and Pb ions were performed. To avoid the overlap of the defects, the fluences were 5 109 , 1 1010 and 6 1010 ions/cm2 . The specific energies were between 2 and 6 MeV/u. The ion-induced damage surface was studied using a Digital Nanoscope III working in Tapping Mode, at ambient conditions. We observed hillocks induced by the irradiation with the different ions. The radius of hillocks has been compared with the radius of the tracks induced in the bulk. At high energy loss for dE=dx larger than 7 keV/nm, there is one hillock per ion, contrary to the observations of Wilson et al. Ó 2003 Published by Elsevier B.V. PACS: 61.80.Jh; 79.20.Rf Keywords: Tapping mode; Scanning force microscopy; Heavy ions; Hillocks; SiO2 quartz
1. Introduction Fast heavy ions (specific energy larger than 1 MeV/u) predominantly lose their energy in a solid by electronic excitation and ionization of the target atoms. Now it is well established that during bombardment with such an ion beam, insulators are subject to structural modification and damage in the bulk [1] and at the surface [2]. For four decades several techniques were used to characterise bulk damage like transmission electron microscopy (TEM) [3], channelling Rutherford backscattering (C-RBS) [4], small angle X-rays
*
Corresponding author. Tel.: +33-231-45-44-23; fax: +33231-45-47-14. E-mail address: [email protected] (N. Khalfaoui). 0168-583X/$ - see front matter Ó 2003 Published by Elsevier B.V. doi:10.1016/S0168-583X(02)02014-1
scattering (SAXS) [5] and surface profilometry for swelling determination [6]. Near-field microscopy techniques, namely scanning tunnelling and atomic force microscopy (AFM), were developed to probe the surface topography of materials. Due to their high spatial atomic resolution, in favourable cases a few tenths of Angstrom, they constitute ideal tools for the study of individual surface ion tracks. In recent years, several studies have been carried out to investigate surface tracks on various materials [2]. Depending on the conditions of irradiation and the type of material, the damaged surface observed by AFM exhibits hillocks [7] and/or craters [8] attributed to local plastic deformation. An extensive study of damage SiO2 quartz induced by swift heavy ions using TEM [9], RBS [9], swelling [6], has been done, leading to a threshold
166
N. Khalfaoui et al. / Nucl. Instr. and Meth. in Phys. Res. B 209 (2003) 165–169
of an electronic energy loss (dE=dx) of amorphisation at about 1.5 keV/nm. With increasing energy loss the damage along the ion track increases: small islands of amorphous material created at small energy losses start to grow together. At high energy losses the islands along the ion track overlap and form continuously damaged amorphous cylinders and, from electron microscopy, it was evident that each ion in the bulk creates a latent track in the regime of continuous tracks [9]. Wilson et al. [10] studied an individual radiation damage on crystalline quartz surfaces by means of AFM in contact mode. They observed hillocks whose number was smaller than the number of incident ions in the case of lead irradiation. In this energy regime these results are difficult to understand because for the quoted beams there is one track per ion in the bulk. In order to investigate these contradictory observations between surface and bulk damage, we have undertaken studies of crystalline quartz in approximately the same conditions as Wilson et al. [10]. The irradiated surface was studied preferentially using AFM in Tapping Mode, in ambient conditions instead of a contact mode. This kind of investigation can give more information to understand the mechanism of the formation of tracks in the bulk and at the surface.
2. Experimental setup Single crystals of a SiO2 quartz in optical quality with the dimensions (10 10 0:5) mm3 were obtained from Kristalltechnologie. They were irradiated at room temperature at GANIL (Caen).
The samples were irradiated at normal incidence with different Ni, Kr and Pb ions in their respective equilibrium charge state. Part of the surface has been masked in order to compare virgin and irradiated areas. The ion beam was scanned over the target surface by means of horizontal and vertical sweeping magnets in order to ensure a homogenous irradiation [11]. The ion beam flux was in the order of 6 107 ions/cm2 /s. The fluences of 5 109 , 1 1010 and 6 1010 ions/cm2 were applied in order to avoid the overlapping of neighbouring tracks. In some cases, thin foils of aluminium or copper were placed in front of the sample in order to decrease the energy and therefore vary the energy loss of the ions impinging on the sample. Details of the irradiation parameters are presented in Table 1. The projected ion range Rp and the electronic energy loss dE=dx were calculated with the code TRIM 95 [12]. In the energy domain explored here, the nuclear stopping is negligible over the major part of the ion trajectory and consequently at the surface. On the same sample, both the irradiated and nonirradiated part of the surface were probed under ambient conditions using Nanoscope III Digital Instruments in tapping mode atomic force microscopy (TM-AFM), with nanosensors Si tips with nominal radius of curvature (5–10 nm) and cone angle of 20°. The cantilever resonance frequency was between 313 and 382 kHz and the oscillation amplitude was 22.5 nm. When probing a sample, the damping of the free oscillation of the cantilever was kept at a minimum value compatible with a stable imaging of the surface. Images were taken with different tips. The samples were re-examined after ageing during six months at room temperature in order to check the stability of the irradiation induced damage.
Table 1 Irradiation parameters and yields of production of hillocks Ion type
Energy (MeV/u)
dE=dx (keV/nm)
Rp (lm)
Fluences (ions/cm2 )
Yield of hillocks
58
6.1 3 6 2 5
7 8.6 10 26.3 27.6
43 21.6 50 23.9 46.8
1 1010 1 1010 1 1010 5 109 6 1010
0.8 0.1 0.9 0.1 0.7 0.1 0.8 0.2 0.80 0.2
Ni 58 Ni 86 Kr 208 Pb 208 Pb
N. Khalfaoui et al. / Nucl. Instr. and Meth. in Phys. Res. B 209 (2003) 165–169
167
3. Results and discussion The image of virgin SiO2 quartz (Fig. 1(a)) has a roughness of 0.1 nm over an area of (0:5 0:5 lm2 ) determined by AFM. The irradiated surfaces observed a few hours after the irradiation by TM-AFM in constant height mode exhibit conical-shaped hillocks with essentially circular bases (Figs. 1(b) and 2). Several authors have vigorously
Fig. 2. AFM image of quartz irradiated with Pb at 6.5 MeV/u, fluence 6 1010 Pb/cm2 , X : 0.1 lm/div, Z: 2 nm/div.
argued that hillocks observed on organic and inorganic targets result from a surface topography modification [13,14]. The use of low fluences allows us to count the number of impacts on the surface and therefore to analyse the result of each impact. A profile line scan over an exposed area is shown in Fig. 3, where the marks indicate the border between the damaged and the virgin zone. We have used a Gaussian curve to fit the surface profiles, in order to determine the height and the diameter of hillocks. In our experiments, similar to Thibaudau et al. [15] and Bouffard et al. [2], we have assumed that diameter corresponds to twice the standard
Fig. 1. (a) AFM image of virgin quartz. (b) AFM image of quartz irradiated with Kr at 6 MeV/u, fluence 1 1010 Kr/cm2 , X : 0.1 lm/div, Z: 4 nm/div.
Fig. 3. Cut of AFM image in the case of Kr irradiation at 6 MeV/u.
168
N. Khalfaoui et al. / Nucl. Instr. and Meth. in Phys. Res. B 209 (2003) 165–169
Radius (nm)
15
10
5
0 0
5
10
15
20
25
30
dE/dx (KeV/nm) Fig. 4. Distribution of radii in the case of Kr irradiation E ¼ 6 MeV/u.
Fig. 6. Mean radii versus electronic energy loss.
3
Height (nm)
2.5 2 1.5 1 0.5 0 0
5
10
15
20
25
30
dE/dx (KeV/nm) Fig. 5. Distribution of heights in the case of Kr irradiation, E ¼ 6 MeV/u.
Fig. 7. Mean heights versus electronic energy loss.
deviation of the Gaussian. For each energy, several images were done and analysed. The mean track radius and height for each energy were extracted from the histograms of the hillock radii and heights. Figs. 4 and 5 show the case of an irradiation with Kr ions at energies equal to 6 MeV/u. The deduced mean radii and heights were plotted versus the electronic stopping power in Figs. 6 and 7. Each error bar represents one standard deviation of the mean value. We do not observe any change of the radii and the heights with the electronic energy loss.
Ageing of SiO2 Quartz irradiated with nickel ions, at room temperature during six months does not lead to a change of the number of hillocks and of their characteristics. In the case of mica, several groups [16,17] observed the evolution of the diameter of tracks with the electronic energy loss, in our case, we do not observe this evolution. A comparative analysis of experimental data obtained from AFM for surface tracks and RBS for bulk tracks [9], shows that diameters obtained by AFM are greater than diameters determined by RBS. However, at the
N. Khalfaoui et al. / Nucl. Instr. and Meth. in Phys. Res. B 209 (2003) 165–169
present stage we did not account for the effect of convolution of the actual diameter of the hillock with the size of the AFM tip [18]. On contrary to the result of Wilson et al. [10] the efficiency of hillock formation is nearly one within the experimental error in our case and larger than the one determined previously [10] under approximately the same irradiation conditions. The only difference between our work and Wilson et al. [10] is the operating mode of the AFM (tapping and contact mode, respectively). It is very difficult to approach the SiO2 quartz surface in contact mode and therefore, we could not observe the irradiated surface. This prevented us from determining if the measured yield is influenced by the operating mode of AFM.
4. Conclusion We have shown that tapping mode scanning force microscopy provides a non-damaging way of imaging surface defects induced by incident swift heavy ions. The damage tracks induced on SiO2 quartz by ions irradiated at normal incidence appear as conical hillocks having circular bases. In our case like in the bulk, each ion creates a defect on the surface, so our yields of hillocks production are much higher than those observed by Wilson et al. [10].
Acknowledgement One of us (C.C.R) is supported by the EuNITT research and training network number HPRNCT-2000-00047.
169
References [1] M. Toulemonde, S. Bouffard, F. Studer, Nucl. Instr. and Meth. B 91 (1994) 108. [2] S. Bouffard, J. Cousty, Y. Pennec, F. Thibaudau, Radiat. Eff. Def. Solids 126 (1993) 225. [3] J.M. Costantini, F. Ravel, F. Brisard, M. Caput, C. Cluzeau, Nucl. Instr. and Meth. B 80/81 (1993) 1249. [4] A. Meftah, F. Brisard, J.M. Costantini, M. Hage-Ali, J.P. Stoquert, M. Toulemonde, Phys. Rev. B 48 (1993) 920. [5] R. Spohr, P. Armbruster, K. Schaupert, Radiat. Eff. Def. Solids 110 (1989) 27. [6] C. Trautmann, M. Boccanfuso, A. Benyagoub, S. Klaumunzer, K. Schwartz, M. Toulemonde, Nucl. Instr. and Meth. B 191 (2002) 144. [7] A. Muller, C. Muller, R. Neumann, F. Ohnesorge, Nucl. Instr. and Meth. B 166–167 (2000) 581. [8] R.M. Papaleo, L.S. Farenzena, M.A. de Araujo, R.P. Livi, Nucl. Instr. and Meth. B 151 (1999) 135. [9] A. Meftah, F. Brisard, J.M. Costantini, M. Hage-Ali, M. Hervieu, J.P. Stoquert, F. Studer, M. Toulemonde, Phys. Rev. B 49 (1994) 18. [10] I.H. Wilson, J.B. Xu, R.A.B. Devine, R.P. Webb, Nucl. Instr. and Meth. B 118 (1996) 473. [11] S. Bouffard, J. Dural, F. Lesvesque, J.M. Ramillon, Ann. Phys. 14 (1989) 385. [12] J.P. Biersack, L.G. Haggmark, Nucl. Instr. and Meth. B 174 (1994) 12457. [13] D.D.N. Barlo Daya, A. Hallen, P. Hakansson, B.U.R. Sundqvist, C.T. Reimann, Nucl. Instr. and Meth. B 103 (1995) 454. [14] D.D.N. Barlo Daya, A. Hallen, J. Eriksson, J. Kopniczky, R. Papaleo, C.T. Reimann, P. Hakansson, B.U.R. Sundqvist, A. Brunelle, S. Della-Negra, Y. Le Beyec, Nucl. Instr. and Meth. B 106 (1995) 38. [15] F. Thibaudau, J. Cousty, E. Balanzat, S. Bouffard, Phys. Rev. Lett. 67 (1991) 1582. [16] S. Bouffard, C. Leroy, S. Della-Negra, A. Brunelle, J.M. Costantini, Philos. Mag. A 81 (2001) 2841. [17] J. Ackermann, N. Angert, R. Neumann, C. Trautmann, M. Dischner, T. Hagen, M. Sedlacek, Nucl. Instr. and Meth. B 107 (1996) 181. [18] C. Odin, J.P. Aime, Z. El Kaakour, T. Bouhacina, Surf. Sci. 317 (1994) 321.
Study of swift heavy ion tracks on crystalline quartz surfaces N. Khalfaoui *, C.C. Rotaru, S. Bouffard, E. Jacquet, H. Lebius, M. Toulemonde Centre interdisciplinaire de recherche ions laser (CEA-CNRS-ISMRA), CIRIL-GANIL, rue Claude Bloch, BP 5133, 14070 Caen Cedex 5, France
Abstract Ion irradiation of SiO2 quartz at medium energy induces damage in the bulk as well as modification on the surface. To make a correlation between the bulk and the surface phenomena, AFM investigations on the SiO2 quartz surfaces, irradiated with Ni, Kr and Pb ions were performed. To avoid the overlap of the defects, the fluences were 5 109 , 1 1010 and 6 1010 ions/cm2 . The specific energies were between 2 and 6 MeV/u. The ion-induced damage surface was studied using a Digital Nanoscope III working in Tapping Mode, at ambient conditions. We observed hillocks induced by the irradiation with the different ions. The radius of hillocks has been compared with the radius of the tracks induced in the bulk. At high energy loss for dE=dx larger than 7 keV/nm, there is one hillock per ion, contrary to the observations of Wilson et al. Ó 2003 Published by Elsevier B.V. PACS: 61.80.Jh; 79.20.Rf Keywords: Tapping mode; Scanning force microscopy; Heavy ions; Hillocks; SiO2 quartz
1. Introduction Fast heavy ions (specific energy larger than 1 MeV/u) predominantly lose their energy in a solid by electronic excitation and ionization of the target atoms. Now it is well established that during bombardment with such an ion beam, insulators are subject to structural modification and damage in the bulk [1] and at the surface [2]. For four decades several techniques were used to characterise bulk damage like transmission electron microscopy (TEM) [3], channelling Rutherford backscattering (C-RBS) [4], small angle X-rays
*
Corresponding author. Tel.: +33-231-45-44-23; fax: +33231-45-47-14. E-mail address: [email protected] (N. Khalfaoui). 0168-583X/$ - see front matter Ó 2003 Published by Elsevier B.V. doi:10.1016/S0168-583X(02)02014-1
scattering (SAXS) [5] and surface profilometry for swelling determination [6]. Near-field microscopy techniques, namely scanning tunnelling and atomic force microscopy (AFM), were developed to probe the surface topography of materials. Due to their high spatial atomic resolution, in favourable cases a few tenths of Angstrom, they constitute ideal tools for the study of individual surface ion tracks. In recent years, several studies have been carried out to investigate surface tracks on various materials [2]. Depending on the conditions of irradiation and the type of material, the damaged surface observed by AFM exhibits hillocks [7] and/or craters [8] attributed to local plastic deformation. An extensive study of damage SiO2 quartz induced by swift heavy ions using TEM [9], RBS [9], swelling [6], has been done, leading to a threshold
166
N. Khalfaoui et al. / Nucl. Instr. and Meth. in Phys. Res. B 209 (2003) 165–169
of an electronic energy loss (dE=dx) of amorphisation at about 1.5 keV/nm. With increasing energy loss the damage along the ion track increases: small islands of amorphous material created at small energy losses start to grow together. At high energy losses the islands along the ion track overlap and form continuously damaged amorphous cylinders and, from electron microscopy, it was evident that each ion in the bulk creates a latent track in the regime of continuous tracks [9]. Wilson et al. [10] studied an individual radiation damage on crystalline quartz surfaces by means of AFM in contact mode. They observed hillocks whose number was smaller than the number of incident ions in the case of lead irradiation. In this energy regime these results are difficult to understand because for the quoted beams there is one track per ion in the bulk. In order to investigate these contradictory observations between surface and bulk damage, we have undertaken studies of crystalline quartz in approximately the same conditions as Wilson et al. [10]. The irradiated surface was studied preferentially using AFM in Tapping Mode, in ambient conditions instead of a contact mode. This kind of investigation can give more information to understand the mechanism of the formation of tracks in the bulk and at the surface.
2. Experimental setup Single crystals of a SiO2 quartz in optical quality with the dimensions (10 10 0:5) mm3 were obtained from Kristalltechnologie. They were irradiated at room temperature at GANIL (Caen).
The samples were irradiated at normal incidence with different Ni, Kr and Pb ions in their respective equilibrium charge state. Part of the surface has been masked in order to compare virgin and irradiated areas. The ion beam was scanned over the target surface by means of horizontal and vertical sweeping magnets in order to ensure a homogenous irradiation [11]. The ion beam flux was in the order of 6 107 ions/cm2 /s. The fluences of 5 109 , 1 1010 and 6 1010 ions/cm2 were applied in order to avoid the overlapping of neighbouring tracks. In some cases, thin foils of aluminium or copper were placed in front of the sample in order to decrease the energy and therefore vary the energy loss of the ions impinging on the sample. Details of the irradiation parameters are presented in Table 1. The projected ion range Rp and the electronic energy loss dE=dx were calculated with the code TRIM 95 [12]. In the energy domain explored here, the nuclear stopping is negligible over the major part of the ion trajectory and consequently at the surface. On the same sample, both the irradiated and nonirradiated part of the surface were probed under ambient conditions using Nanoscope III Digital Instruments in tapping mode atomic force microscopy (TM-AFM), with nanosensors Si tips with nominal radius of curvature (5–10 nm) and cone angle of 20°. The cantilever resonance frequency was between 313 and 382 kHz and the oscillation amplitude was 22.5 nm. When probing a sample, the damping of the free oscillation of the cantilever was kept at a minimum value compatible with a stable imaging of the surface. Images were taken with different tips. The samples were re-examined after ageing during six months at room temperature in order to check the stability of the irradiation induced damage.
Table 1 Irradiation parameters and yields of production of hillocks Ion type
Energy (MeV/u)
dE=dx (keV/nm)
Rp (lm)
Fluences (ions/cm2 )
Yield of hillocks
58
6.1 3 6 2 5
7 8.6 10 26.3 27.6
43 21.6 50 23.9 46.8
1 1010 1 1010 1 1010 5 109 6 1010
0.8 0.1 0.9 0.1 0.7 0.1 0.8 0.2 0.80 0.2
Ni 58 Ni 86 Kr 208 Pb 208 Pb
N. Khalfaoui et al. / Nucl. Instr. and Meth. in Phys. Res. B 209 (2003) 165–169
167
3. Results and discussion The image of virgin SiO2 quartz (Fig. 1(a)) has a roughness of 0.1 nm over an area of (0:5 0:5 lm2 ) determined by AFM. The irradiated surfaces observed a few hours after the irradiation by TM-AFM in constant height mode exhibit conical-shaped hillocks with essentially circular bases (Figs. 1(b) and 2). Several authors have vigorously
Fig. 2. AFM image of quartz irradiated with Pb at 6.5 MeV/u, fluence 6 1010 Pb/cm2 , X : 0.1 lm/div, Z: 2 nm/div.
argued that hillocks observed on organic and inorganic targets result from a surface topography modification [13,14]. The use of low fluences allows us to count the number of impacts on the surface and therefore to analyse the result of each impact. A profile line scan over an exposed area is shown in Fig. 3, where the marks indicate the border between the damaged and the virgin zone. We have used a Gaussian curve to fit the surface profiles, in order to determine the height and the diameter of hillocks. In our experiments, similar to Thibaudau et al. [15] and Bouffard et al. [2], we have assumed that diameter corresponds to twice the standard
Fig. 1. (a) AFM image of virgin quartz. (b) AFM image of quartz irradiated with Kr at 6 MeV/u, fluence 1 1010 Kr/cm2 , X : 0.1 lm/div, Z: 4 nm/div.
Fig. 3. Cut of AFM image in the case of Kr irradiation at 6 MeV/u.
168
N. Khalfaoui et al. / Nucl. Instr. and Meth. in Phys. Res. B 209 (2003) 165–169
Radius (nm)
15
10
5
0 0
5
10
15
20
25
30
dE/dx (KeV/nm) Fig. 4. Distribution of radii in the case of Kr irradiation E ¼ 6 MeV/u.
Fig. 6. Mean radii versus electronic energy loss.
3
Height (nm)
2.5 2 1.5 1 0.5 0 0
5
10
15
20
25
30
dE/dx (KeV/nm) Fig. 5. Distribution of heights in the case of Kr irradiation, E ¼ 6 MeV/u.
Fig. 7. Mean heights versus electronic energy loss.
deviation of the Gaussian. For each energy, several images were done and analysed. The mean track radius and height for each energy were extracted from the histograms of the hillock radii and heights. Figs. 4 and 5 show the case of an irradiation with Kr ions at energies equal to 6 MeV/u. The deduced mean radii and heights were plotted versus the electronic stopping power in Figs. 6 and 7. Each error bar represents one standard deviation of the mean value. We do not observe any change of the radii and the heights with the electronic energy loss.
Ageing of SiO2 Quartz irradiated with nickel ions, at room temperature during six months does not lead to a change of the number of hillocks and of their characteristics. In the case of mica, several groups [16,17] observed the evolution of the diameter of tracks with the electronic energy loss, in our case, we do not observe this evolution. A comparative analysis of experimental data obtained from AFM for surface tracks and RBS for bulk tracks [9], shows that diameters obtained by AFM are greater than diameters determined by RBS. However, at the
N. Khalfaoui et al. / Nucl. Instr. and Meth. in Phys. Res. B 209 (2003) 165–169
present stage we did not account for the effect of convolution of the actual diameter of the hillock with the size of the AFM tip [18]. On contrary to the result of Wilson et al. [10] the efficiency of hillock formation is nearly one within the experimental error in our case and larger than the one determined previously [10] under approximately the same irradiation conditions. The only difference between our work and Wilson et al. [10] is the operating mode of the AFM (tapping and contact mode, respectively). It is very difficult to approach the SiO2 quartz surface in contact mode and therefore, we could not observe the irradiated surface. This prevented us from determining if the measured yield is influenced by the operating mode of AFM.
4. Conclusion We have shown that tapping mode scanning force microscopy provides a non-damaging way of imaging surface defects induced by incident swift heavy ions. The damage tracks induced on SiO2 quartz by ions irradiated at normal incidence appear as conical hillocks having circular bases. In our case like in the bulk, each ion creates a defect on the surface, so our yields of hillocks production are much higher than those observed by Wilson et al. [10].
Acknowledgement One of us (C.C.R) is supported by the EuNITT research and training network number HPRNCT-2000-00047.
169
References [1] M. Toulemonde, S. Bouffard, F. Studer, Nucl. Instr. and Meth. B 91 (1994) 108. [2] S. Bouffard, J. Cousty, Y. Pennec, F. Thibaudau, Radiat. Eff. Def. Solids 126 (1993) 225. [3] J.M. Costantini, F. Ravel, F. Brisard, M. Caput, C. Cluzeau, Nucl. Instr. and Meth. B 80/81 (1993) 1249. [4] A. Meftah, F. Brisard, J.M. Costantini, M. Hage-Ali, J.P. Stoquert, M. Toulemonde, Phys. Rev. B 48 (1993) 920. [5] R. Spohr, P. Armbruster, K. Schaupert, Radiat. Eff. Def. Solids 110 (1989) 27. [6] C. Trautmann, M. Boccanfuso, A. Benyagoub, S. Klaumunzer, K. Schwartz, M. Toulemonde, Nucl. Instr. and Meth. B 191 (2002) 144. [7] A. Muller, C. Muller, R. Neumann, F. Ohnesorge, Nucl. Instr. and Meth. B 166–167 (2000) 581. [8] R.M. Papaleo, L.S. Farenzena, M.A. de Araujo, R.P. Livi, Nucl. Instr. and Meth. B 151 (1999) 135. [9] A. Meftah, F. Brisard, J.M. Costantini, M. Hage-Ali, M. Hervieu, J.P. Stoquert, F. Studer, M. Toulemonde, Phys. Rev. B 49 (1994) 18. [10] I.H. Wilson, J.B. Xu, R.A.B. Devine, R.P. Webb, Nucl. Instr. and Meth. B 118 (1996) 473. [11] S. Bouffard, J. Dural, F. Lesvesque, J.M. Ramillon, Ann. Phys. 14 (1989) 385. [12] J.P. Biersack, L.G. Haggmark, Nucl. Instr. and Meth. B 174 (1994) 12457. [13] D.D.N. Barlo Daya, A. Hallen, P. Hakansson, B.U.R. Sundqvist, C.T. Reimann, Nucl. Instr. and Meth. B 103 (1995) 454. [14] D.D.N. Barlo Daya, A. Hallen, J. Eriksson, J. Kopniczky, R. Papaleo, C.T. Reimann, P. Hakansson, B.U.R. Sundqvist, A. Brunelle, S. Della-Negra, Y. Le Beyec, Nucl. Instr. and Meth. B 106 (1995) 38. [15] F. Thibaudau, J. Cousty, E. Balanzat, S. Bouffard, Phys. Rev. Lett. 67 (1991) 1582. [16] S. Bouffard, C. Leroy, S. Della-Negra, A. Brunelle, J.M. Costantini, Philos. Mag. A 81 (2001) 2841. [17] J. Ackermann, N. Angert, R. Neumann, C. Trautmann, M. Dischner, T. Hagen, M. Sedlacek, Nucl. Instr. and Meth. B 107 (1996) 181. [18] C. Odin, J.P. Aime, Z. El Kaakour, T. Bouhacina, Surf. Sci. 317 (1994) 321.