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Characterisation by various microscopic techniques of the damage created by MeV C60 ions in amorphous Ni3B
Nuclear Instruments and Methods in Physics Research B 146 (1998) 222±232
Characterisation by various microscopic techniques of the damage created by MeV C60 ions in amorphous Ni3 B q A. Dunlop a
a,*
, J. Henry b, G. Jaskierowicz
a
Laboratoire des Solides Irradi es, Commissariat a lÕEnergie Atomique/Ecole Polytechnique, 91128 Palaiseau Cedex, France b CEA/CEREM, Service de Recherches M etallurgiques Appliqu ees, 91191 Gif-sur-Yvette, France
Abstract In the work presented here, we study the damage resulting from the high rates of energy deposition in electronic processes during the slowing down of energetic projectiles in amorphous Ni3 B. More precisely, we study the structural modi®cations of the targets irradiated at 300 K in a wide range of ¯uences (109 ±5 ´ 1011 cmÿ2 ) with 30 MeV C60 ions. The experimental results mainly concern the surface features and were obtained by Transmission Electron Microscopy (TEM) using in particular phase contrast and topographical contrast imaging. In the case of C60 ions, TEM reveals that surface damage consists of approximatively 20 nm diameter hollows, which are surrounded by protuding rims. This seems to indicate that the anisotropic growth process, which is speci®c of amorphous structures, is induced by each individual C60 cluster ion. Previous results obtained using GeV heavy monoatomic projectiles showed that the plastic deformation of the targets started only above an incubation ¯uence, i.e. after a signi®cant spatial overlap of the damaged regions. Ó 1998 Elsevier Science B.V. All rights reserved. Keywords: Radiation damage; Electronic excitation; Fullerene; Anisotropic growth; Transmission electron microscopy; Amorphous targets
1. Introduction This paper deals with the so-called ``anisotropic growth'' phenomenon, which was shown to take place in all amorphous targets irradiated with very energetic projectiles [1,2]. More precisely, the dimension of the sample parallel to the projectile direction shrinks, whereas its dimensions perpen-
q
The irradiations were performed on the tandem accelerator, Orsay, France. * Corresponding author.
dicular to the incident beam direction increase. These macroscopic deformations occur without noticeable volume change [3]. They only appear above a threshold in the linear rate of energy deposition in electronic processes (dE/dx)e and at suciently high irradiation ¯uences. At low ¯uences, disorder is introduced in the amorphous structure (point defects, additional free volume) in the vicinity of the ion path, which generates ``disordered amorphous tracks''. Above an incubation ¯uence Ui , the value of which depends on (dE/dx)e [4], the anisotropic growth takes place. This phenomenon was studied in detail
0168-583X/98/$ ± see front matter Ó 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 4 6 9 - 8
A. Dunlop et al. / Nucl. Instr. and Meth. in Phys. Res. B 146 (1998) 222±232
using in situ electrical resistance measurements during low temperature irradiations with GeV heavy ions. Using phenomenological models, the diameters of the ``disordered tracks'' could be estimated [4,5]. Recent experiments allowed the visualisation of the impacts of: (i) GeV Pb in amorphous Fe85 B15 using scanning tunneling microscopy [6] and (ii) GeV U and 30 MeV C60 in amorphous Ni3 B using transmission electron microscopy [7] and the comparison of the diameters of the observed surface features with the diameters of the `` disordered amorphous tracks '' deduced from the phenomenological models. In the study described below, we show how the combined use of various investigation methods (bright ®eld, phase contrast and topographical imaging techniques in an electron microscope) allows a detailed characterisation of the defect structures and in particular of local surface deformations.
223
surrounding matrix matching between simulated and experimental micrographs recorded at dierent defocus values, is required. (iii) Micrographs obtained using the topographical contrast technique exhibit localised bright±dark contrasts which, in the case of a thin amorphous sample, correspond to the positions of surface displacements. The images look similar to those obtained using a scanning electron microscope, although the two physical processes involved in image formation are totally dierent. It must be emphasized that the interpretation of topographical contrast images is not always straightforward (for a detailed description of the technique, see [8±10]).
3. TEM results 3.1. Irradiations at low ¯uences
2. Experimental Three millimeters discs were electrochemically thinned for electron microscopy observations. They were irradiated at 300 K by 30 MeV fullerene ions delivered by the tandem accelerator in Orsay. Irradiations were performed at very low ¯uences (109 cmÿ2 ) at normal and tilted incidences in order to visualise individual impacts, and at higher ¯uences (>1010 cmÿ2 ) for which spatial overlap of the damaged zones occurs. Observations of the irradiated samples were performed by Transmission Electron Microscopy (TEM) using in particular phase contrast and topographical contrast imaging: (i) When performing TEM observations in bright ®eld conditions and at focus, the transmitted intensity through an amorphous sample only depends on the mass thickness (local thickness multiplied by the density of the material [11,12]). (ii) In very thin regions of an amorphous sample, local thickness variations can be detected by defocusing the objective lens which gives rise to the so-called phase contrast. However, to obtain quantitative information about the topography of the observed region and even to determine whether the sample is locally thicker or thinner than the
3.1.1. Irradiations at normal incidence Fig. 1 presents TEM micrographs of isolated C60 impacts obtained using various imaging conditions. In Fig. 1(a), taken in focused conditions, the impacts appear as bright discs surrounded by dark rings. In Fig. 1(b) and (c), white or dark halos, respectively appear in overfocused and underfocused imaging conditions. They are most probably due to local thickness variations. In Fig. 1(d), the impacts are imaged using the topographical contrast technique. They appear as hollows surrounded by a protuding rim. 3.1.2. Irradiations at tilted incidence Fig. 2 shows a sample irradiated at an angle of incidence of 60° respective to the sample normal. Fig. 2(a), taken in focused conditions, shows that the features occuring at the entrance and exit sample surfaces consist of elongated bright spots partially surrounded by dark rims. In Fig. 2(b) and (c), corresponding to defocused conditions, the rims appear with a much higher contrast. In Fig. 2(a), the average target thickness slowly increases going from right to left, so that the distance between the features generated by one projectile on the entrance and exit surfaces gradually
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Fig. 1. Transmission electron micrographs of the impacts of 30 MeV C60 ions in amorphous Ni3 B irradiated at perpendicular incidence and at 300 K up to a ¯uence of 109 ions/cm2 ; (a) bright ®eld imaging in focused conditions; (b), (c) phase contrast imaging, respectively in overfocused and underfocused conditions; (d) topological contrast imaging conditions.
increases. On the right side (very thin regions) the projections of the two features are almost superimposed, whereas they overlap only slightly on the left side, and are completely separated in thicker regions (not visible in Fig. 2). In the left part, the contrasts mainly consist of a sucession of white± black±white regions, which can be interpreted as resulting from the contribution of the deformations occuring on both sample surfaces. In the right part of the micrograph corresponding to very thin sample regions, the dark contrasts are weaker along the direction of propagation of the ion beam and mostly visible on the sides. This phenomenon, which is not well understood up to now, is con®rmed by the micrograph presented in Fig. 3 (topographical contrast imaging in a very thin sample region). On the entrance and exit surfaces, the projectile creates a crater which is partially surrounded by protuding matter. This matter is located in the forward direction on the entrance surface and in the backward direction on the exit surface.
3.2. Irradiations at high ¯uences 3.2.1. Irradiations at normal incidence The following observations are all relative to irradiations at normal incidence with 30 MeV C60 ions at a ¯uence of 1010 ions/cm2 . Fig. 4 shows a group of impacts imaged in focused and defocused conditions: it can be noticed that a signi®cant spatial overlap of the objects takes place. Here again, at focus, the projections of the damaged regions appear with a bright contrast and are surrounded by dark rings. In Fig. 5, two types of regions are visible. They are attributed to a very speci®c phenomenon, the ``sample desquamation'' which is only observed after high ¯uence irradiations. Regions of type 1 (Fig. 5(b)), which appear as the brighter regions, correspond to parts of the sample in which super®cial layers were pealed o as a consequence of irradiation. In regions of type 2, appearing with a dark contrast, this ``desquamation'' phenomenon did not take place. In type 2 regions, the impact
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Fig. 2. Transmission electron micrographs of the impacts of 30 MeV C60 ions in amorphous Ni3 B irradiated at 300 K up to a ¯uence of 109 ions/cm2 at an angle of incidence of 60° respective to the sample normal: (a) bright ®eld imaging in focused conditions; (b), (c) phase contrast imaging respectively in overfocused and underfocused conditions.
features are identical to those observed in Fig. 4 and their density corresponds to the impinging ion ¯uence. At the opposite, in type 1 regions, the
observed ``objects'' have a similar density, but they appear with a fainter contrast and with a slightly smaller diameter.
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Fig. 3. Transmission electron micrograph taken in topographical imaging conditions of an impact of a 30 MeV C60 ion in amorphous Ni3 B irradiated at 300 K up to a ¯uence of 109 ions/cm2 at an angle of incidence of 60° respective to the sample normal.
A more detailed examination of this process is presented in Fig. 6. Fig. 6(a) is an image in focused conditions showing both types of regions. Fig. 6(b) and (c) give topographical contrast images of some impacts, respectively, located in region 1 and in region 2. In region 2, except for the frequent spatial overlap, the observed features are very similar to those previously described at low ¯uences: hollows are surrounded by protuding rims. In region 1, the topographical contrast of the objects is clearly less pronounced. In the left hand side of Fig. 5(a), bright circular contrasts (of typical sizes much larger than the individual impact sizes) are observed; they correspond to regions which are thinner than the surrounding matrix. This could indicate that ``desquamation'' occurs by successive ``pealing'' of small size regions. The faint contrasts observed in the ``desquamed'' regions (Fig. 6(b)) could be attributed to surface deformations related
Fig. 4. Transmission electron micrographs of impacts of 30 MeV C60 ions in amorphous Ni3 B irradiated at perpendicular incidence and at 300 K up to a ¯uence of 1010 ions/cm2 : (a) bright ®eld imaging in focused conditions; (b), (c) phase contrast imaging respectively in overfocused and underfocused conditions.
A. Dunlop et al. / Nucl. Instr. and Meth. in Phys. Res. B 146 (1998) 222±232
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Fig. 5. Two regions of the same sample as for Fig. 4. The desquamation process, described in the text, is seen in type 1 regions, but did not occur in type 2 regions.
to the ``disordered amorphous'' tracks generated in the bulk of the target. It has to be noted that a similar desquamation has already been observed after an irradiation of prethinned amorphous Ni3 B with GeV uranium at a high ¯uence (1011 cmÿ2 ) [Fig. 6 in Ref. 7]. In order to understand better this surprising ``desquamation'' eect, we also irradiated 30 lm thick amorphous Ni3 B targets at 300 K with 30 MeV C60 ions at two ¯uences: 5 ´ 1010 and 5 ´ 1011 ions/cm2 . The entrance surface of these massive targets on which the fullerene ions were sent, was protected by a lacquer during electrochemical
thinning after irradiation. The sample is thus thinned from the rear, the lacquer is then dissolved, so that the impinging entrance surface of the projectiles in the ®nal sample can be observed by TEM. The sample irradiated at the ¯uence of 5 ´ 1010 ions/cm2 (Fig. 7) presents damage features which are very similar to those shown in Fig. 5: the desquamation occurs and two types of features which strong or faint contrast are respectively found in the undesquamed and desquamed regions. This important result shows that the ``desquamation'' occurs as well in samples irradiated in the prethinned or bulk states.
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Fig. 6. Border between a ``desquamed'' and nondesquamed regions, observed in amorphous Ni3 B irradiated at high ¯uences with 30 MeV C60 projectiles: (a) bright ®eld image; (b), (c) topographical contrast imaging, respectively in desquamed and nondesquamed regions.
Fig. 8 shows typical features observed in a 30 lm thick sample irradiated up to a ¯uence of 5 ´ 1011 ions/cm2 and thinned for electron microscopy observations after irradiation as described above. Here again strong local thicknesses variations are observed in the totality of the sample. But now, due to the higher ion ¯uence, the regions
which peel o are subsequently irradiated with numerous fullerene ions. The complete surface is now uniformly covered with pustules. 3.2.2. Irradiations at tilted incidence The experimental set-up in the accelerator does not allow an extremely precise determination of
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Fig. 7. Transmission electron micrographs of a 30 lm thick amorphous Ni3 B sample irradiated with 30 MeV C60 ions at perpendicular incidence and at 300 K up to a ¯uence of 5 ´ 1010 ions/cm2 . The sample was thinned from the rear after irradiation for electron microscopy observations. The border between ``desquamed'' and nondesquamed regions is shown in: (a) bright ®eld imaging in focused conditions; (b), (c) phase contrast imaging respectively in overfocused and underfocused conditions.
the sample orientation respective to the projectile incidence direction. Moreover, some local sample deformations lead to an additional uncertainty in the exact beam incidence angle. Fig. 9 features a thin region of a sample irradiated with a beam disoriented by a few degrees respective to the sample normal. The topographical contrast image clearly shows a lack of symmetry in the outer rims. Returning to individual impact features, the main conclusion of the TEM study is that they always appear as hollows surrounded by protuding rings. 4. Discussion The dierent characterisation techniques lead to the same conclusion concerning the topography of the individual impacts. The images of the impacts obtained by TEM in bright ®eld conditions at focus are in perfect agreement with the results of topographical contrast imaging. As recalled in the
experimental section, in focused conditions, the transmitted intensity through an amorphous specimen depends on the mass thickness, so that a hollow should appear brighter than the surrounding matrix which is consistent with the experimental results. Let us now return to the origin of the anisotropic growth process, which was very rapidly summarised in the introduction. Numerous studies of the damage induced by GeV monoatomic ions (Ar to U) in all types of amorphous targets have led various groups of researchers to propose the following interpretations: · for a given material, the anisotropic growth is induced above a threshold in (dE/dx)e · above this threshold, at low ¯uences (i.e. during the incubation period), disorder is induced in the vicinity of the ion path, as a consequence of electronic energy deposition [13]. At higher ¯uences, i.e. above Ui , radial correlated atomic movements take place and lead to the observed macroscopic deformation of the sample.
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Fig. 8. Transmission electron micrographs of a 30 lm thick amorphous Ni3 B sample irradiated with 30 MeV C60 ions at perpendicular incidence and at 300 K up to a ¯uence of 5 ´ 1011 ions/cm2 . The sample was thinned from the rear after irradiation for electron microscopy observations: (a) bright ®eld imaging in focused conditions; (b), (c) phase contrast imaging respectively in overfocused and underfocused conditions.
A. Dunlop et al. / Nucl. Instr. and Meth. in Phys. Res. B 146 (1998) 222±232
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Fig. 10. Schematic representation of the radial stresses acting in the vicinity of an excited cylinder in the wake of the projectile (a) and of the resulting deformations after relaxation (b) of the excitation in very thin target regions.
Fig. 9. Transmission electron micrograph in topographical contrast conditions of impacts of 30 MeV C60 ions in amorphous Ni3 B irradiated at 300 K up to a ¯uence of 1010 ions/cm2 . The ion beam incidence direction is tilted by a few degrees respective to the sample normal.
Due to the high rates of energy deposition in electronic processes, there is ®rst a very localized region around the ion path in which electronic excitation and ionisation take place: showers of excited d-electrons are ejected. This might well lead, even in metallic targets, to a Coulomb explosion [14], to energy transfers to the neighboring target atoms and to the generation of a shock wave which originates from the projectile wake and propagates radially outwards [15]. Part of the kinetic energy transmitted to the d-electrons might also be converted into atomic motion through electron±phonon coupling [16]. More recently, a visco-elastic model based on the relaxation of the local stresses initially con®ned in an excited cylinder was proposed to account for the anisotropic growth phenomenon [17]. Whatever the process which is considered to describe the chronology of the events, there is always in a second step, a relaxation of the energy deposited in the track core: a radial extension of the excited region occurs and leads to the local deformations schematised by the dashed lines in
Fig. 10(a). The anisotropic growth results from irreversible outwards atomic displacements. These are only possible in an amorphous disordered state (after an incubation ¯uence) considered either as containing a large amount of free volume [14] or having a modi®ed viscosity [17]. The higher the rate of energy deposition (dE/dx)e by the projectile, the larger the outward impulses given to the target atoms and thus the lower the incubation ¯uence. As predicted in [17], it is quite possible to suppose that the incubation ¯uence might well disappear if sucient rates of energy deposition are reached. Coming back to Fig. 10, one can then predict that the relaxation of the dashed volume in Fig. 10(a) will then, in a thin target lead to local surface deformations as indicated in Fig. 10(b). At the vertical of the ion trajectory, a depletion is formed, whereas the matter pushed outwards gives rise to circular rims on the surfaces of very thin samples. As already mentioned [18], the speci®city of cluster projectiles such as 30 MeV C60 , is that they give similar or higher (dE/dx)e than monoatomic GeV ions, their velocity being one order of magnitude smaller. This results in a very high localisation of the energy: the density of deposited energy can reach values as high as a few 100 eV/atom in the vicinity of the ion wake. This seems thus very favorable to induce anisotropic growth of an amorphous target in a single track. The surface
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features that we observed after isolated impacts are in perfect agreement with the schematic description of Fig. 10(b). 5. Conclusion The TEM experiments reported in this paper demonstrate that anisotropic growth takes place in amorphous Ni3 B irradiated at 300 K with 30 MeV C60 , and that this process occurs in individual impacts. This is clearly related to the high density of energy deposited in electronic processes by the fullerene ions during their slowing down in the targets. The ``sample desquamation'' process, which was observed here to occur systematically after irradiations at high ¯uences of thin or massive amorphous targets could result from very high mechanical stresses accumulating in the vicinity of the sample surface. In order to con®rm these results and to extract some quantitative informations concerning: (i) the depths and heights of the depletions and rims and (ii) the height of the step when desquamation appears, examinations using atomic force microscopy are in progress. Acknowledgements We are grateful to S. Della-Negra for fruitful discussions and for delivering the fullerene beam.
References [1] S. Klaum unzer, C. Li, S. L oer, M. Rammensee, G. Schumacher, C.H. Neitzert, Radiation Eects and Defects in Solids 108 (1989) 131. [2] M.-D. Hou, S. Klaum unzer, G. Schumacher, Phys. Review B 41 (1990) 1144. [3] A. Audouard, E. Balanzat, G. Fuchs, J.C. Jousset, D. Lesueur, L. Thome, Europhysics Letters 3 (1987) 327. [4] A. Audouard, E. Balanzat, J.C. Jousset, D. Lesueur, L. Thome, J. Phys.: Condens. Matter 5 (1993) 995. [5] A. Audouard, J. Dural, M. Toulemonde, A. Lovas, G. Szenes, L. Thome, Phys. Review B 54 (1996) 15690. [6] A. Audouard, R. Mamy, M. Toulemonde, G. Szenes, L. Thome, Europhysics Letters 40 (1997) 527. [7] A. Dunlop, G. Jaskierowicz, S. Della-Negra, C.R. Acad. Sci. Paris IIb 325 (1997) 397. [8] A.G. Cullis, D.M. Maher, Phil. Mag. 30 (1974) 447. [9] A.G. Cullis, D.M. Maher, Ultramicroscopy 1 (1975) 97. [10] J. Henry, A. Dunlop, S. Della-Negra, these Proceedings (SHIM-98), Nucl. Instr. and Meth. B 146 (1998) 405. [11] L. Reimer, In: Transmission Electron Microscopy, Springer, 1997, p. 198. [12] D.B. Williams, C.B. Carter, In: Transmission Electron Microscopy, Plenum Press, New York, 1996, p. 357. [13] A. Audouard, E. Balanzat, J.C. Jousset, G. Fuchs, D. Lesueur, L. Thome, Radiation Eects and Defects in Solids 110 (1989) 109. [14] S. Klaum unzer, M.-D. Hou, G. Schumacher, Phys. Rev. Lett. 57 (1986) 850. [15] D. Lesueur, A. Dunlop, Radiation Eects and Defects in Solids 126 (1993) 163. [16] M. Toulemonde, C. Dufour, E. Paumier, Phys. Rev. B 46 (1992) 14362. [17] H. Trinkhaus, A.I. Ryazanov, Phys. Rev. Lett. 74 (1995) 5072. [18] H. Dammak, A. Dunlop, D. Lesueur, A. Brunelle, S. Della-Negra, Y. Le Beyec, Phys. Rev. Lett. 74 (1995) 1135.
Characterisation by various microscopic techniques of the damage created by MeV C60 ions in amorphous Ni3 B q A. Dunlop a
a,*
, J. Henry b, G. Jaskierowicz
a
Laboratoire des Solides Irradi es, Commissariat a lÕEnergie Atomique/Ecole Polytechnique, 91128 Palaiseau Cedex, France b CEA/CEREM, Service de Recherches M etallurgiques Appliqu ees, 91191 Gif-sur-Yvette, France
Abstract In the work presented here, we study the damage resulting from the high rates of energy deposition in electronic processes during the slowing down of energetic projectiles in amorphous Ni3 B. More precisely, we study the structural modi®cations of the targets irradiated at 300 K in a wide range of ¯uences (109 ±5 ´ 1011 cmÿ2 ) with 30 MeV C60 ions. The experimental results mainly concern the surface features and were obtained by Transmission Electron Microscopy (TEM) using in particular phase contrast and topographical contrast imaging. In the case of C60 ions, TEM reveals that surface damage consists of approximatively 20 nm diameter hollows, which are surrounded by protuding rims. This seems to indicate that the anisotropic growth process, which is speci®c of amorphous structures, is induced by each individual C60 cluster ion. Previous results obtained using GeV heavy monoatomic projectiles showed that the plastic deformation of the targets started only above an incubation ¯uence, i.e. after a signi®cant spatial overlap of the damaged regions. Ó 1998 Elsevier Science B.V. All rights reserved. Keywords: Radiation damage; Electronic excitation; Fullerene; Anisotropic growth; Transmission electron microscopy; Amorphous targets
1. Introduction This paper deals with the so-called ``anisotropic growth'' phenomenon, which was shown to take place in all amorphous targets irradiated with very energetic projectiles [1,2]. More precisely, the dimension of the sample parallel to the projectile direction shrinks, whereas its dimensions perpen-
q
The irradiations were performed on the tandem accelerator, Orsay, France. * Corresponding author.
dicular to the incident beam direction increase. These macroscopic deformations occur without noticeable volume change [3]. They only appear above a threshold in the linear rate of energy deposition in electronic processes (dE/dx)e and at suciently high irradiation ¯uences. At low ¯uences, disorder is introduced in the amorphous structure (point defects, additional free volume) in the vicinity of the ion path, which generates ``disordered amorphous tracks''. Above an incubation ¯uence Ui , the value of which depends on (dE/dx)e [4], the anisotropic growth takes place. This phenomenon was studied in detail
0168-583X/98/$ ± see front matter Ó 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 4 6 9 - 8
A. Dunlop et al. / Nucl. Instr. and Meth. in Phys. Res. B 146 (1998) 222±232
using in situ electrical resistance measurements during low temperature irradiations with GeV heavy ions. Using phenomenological models, the diameters of the ``disordered tracks'' could be estimated [4,5]. Recent experiments allowed the visualisation of the impacts of: (i) GeV Pb in amorphous Fe85 B15 using scanning tunneling microscopy [6] and (ii) GeV U and 30 MeV C60 in amorphous Ni3 B using transmission electron microscopy [7] and the comparison of the diameters of the observed surface features with the diameters of the `` disordered amorphous tracks '' deduced from the phenomenological models. In the study described below, we show how the combined use of various investigation methods (bright ®eld, phase contrast and topographical imaging techniques in an electron microscope) allows a detailed characterisation of the defect structures and in particular of local surface deformations.
223
surrounding matrix matching between simulated and experimental micrographs recorded at dierent defocus values, is required. (iii) Micrographs obtained using the topographical contrast technique exhibit localised bright±dark contrasts which, in the case of a thin amorphous sample, correspond to the positions of surface displacements. The images look similar to those obtained using a scanning electron microscope, although the two physical processes involved in image formation are totally dierent. It must be emphasized that the interpretation of topographical contrast images is not always straightforward (for a detailed description of the technique, see [8±10]).
3. TEM results 3.1. Irradiations at low ¯uences
2. Experimental Three millimeters discs were electrochemically thinned for electron microscopy observations. They were irradiated at 300 K by 30 MeV fullerene ions delivered by the tandem accelerator in Orsay. Irradiations were performed at very low ¯uences (109 cmÿ2 ) at normal and tilted incidences in order to visualise individual impacts, and at higher ¯uences (>1010 cmÿ2 ) for which spatial overlap of the damaged zones occurs. Observations of the irradiated samples were performed by Transmission Electron Microscopy (TEM) using in particular phase contrast and topographical contrast imaging: (i) When performing TEM observations in bright ®eld conditions and at focus, the transmitted intensity through an amorphous sample only depends on the mass thickness (local thickness multiplied by the density of the material [11,12]). (ii) In very thin regions of an amorphous sample, local thickness variations can be detected by defocusing the objective lens which gives rise to the so-called phase contrast. However, to obtain quantitative information about the topography of the observed region and even to determine whether the sample is locally thicker or thinner than the
3.1.1. Irradiations at normal incidence Fig. 1 presents TEM micrographs of isolated C60 impacts obtained using various imaging conditions. In Fig. 1(a), taken in focused conditions, the impacts appear as bright discs surrounded by dark rings. In Fig. 1(b) and (c), white or dark halos, respectively appear in overfocused and underfocused imaging conditions. They are most probably due to local thickness variations. In Fig. 1(d), the impacts are imaged using the topographical contrast technique. They appear as hollows surrounded by a protuding rim. 3.1.2. Irradiations at tilted incidence Fig. 2 shows a sample irradiated at an angle of incidence of 60° respective to the sample normal. Fig. 2(a), taken in focused conditions, shows that the features occuring at the entrance and exit sample surfaces consist of elongated bright spots partially surrounded by dark rims. In Fig. 2(b) and (c), corresponding to defocused conditions, the rims appear with a much higher contrast. In Fig. 2(a), the average target thickness slowly increases going from right to left, so that the distance between the features generated by one projectile on the entrance and exit surfaces gradually
224
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Fig. 1. Transmission electron micrographs of the impacts of 30 MeV C60 ions in amorphous Ni3 B irradiated at perpendicular incidence and at 300 K up to a ¯uence of 109 ions/cm2 ; (a) bright ®eld imaging in focused conditions; (b), (c) phase contrast imaging, respectively in overfocused and underfocused conditions; (d) topological contrast imaging conditions.
increases. On the right side (very thin regions) the projections of the two features are almost superimposed, whereas they overlap only slightly on the left side, and are completely separated in thicker regions (not visible in Fig. 2). In the left part, the contrasts mainly consist of a sucession of white± black±white regions, which can be interpreted as resulting from the contribution of the deformations occuring on both sample surfaces. In the right part of the micrograph corresponding to very thin sample regions, the dark contrasts are weaker along the direction of propagation of the ion beam and mostly visible on the sides. This phenomenon, which is not well understood up to now, is con®rmed by the micrograph presented in Fig. 3 (topographical contrast imaging in a very thin sample region). On the entrance and exit surfaces, the projectile creates a crater which is partially surrounded by protuding matter. This matter is located in the forward direction on the entrance surface and in the backward direction on the exit surface.
3.2. Irradiations at high ¯uences 3.2.1. Irradiations at normal incidence The following observations are all relative to irradiations at normal incidence with 30 MeV C60 ions at a ¯uence of 1010 ions/cm2 . Fig. 4 shows a group of impacts imaged in focused and defocused conditions: it can be noticed that a signi®cant spatial overlap of the objects takes place. Here again, at focus, the projections of the damaged regions appear with a bright contrast and are surrounded by dark rings. In Fig. 5, two types of regions are visible. They are attributed to a very speci®c phenomenon, the ``sample desquamation'' which is only observed after high ¯uence irradiations. Regions of type 1 (Fig. 5(b)), which appear as the brighter regions, correspond to parts of the sample in which super®cial layers were pealed o as a consequence of irradiation. In regions of type 2, appearing with a dark contrast, this ``desquamation'' phenomenon did not take place. In type 2 regions, the impact
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Fig. 2. Transmission electron micrographs of the impacts of 30 MeV C60 ions in amorphous Ni3 B irradiated at 300 K up to a ¯uence of 109 ions/cm2 at an angle of incidence of 60° respective to the sample normal: (a) bright ®eld imaging in focused conditions; (b), (c) phase contrast imaging respectively in overfocused and underfocused conditions.
features are identical to those observed in Fig. 4 and their density corresponds to the impinging ion ¯uence. At the opposite, in type 1 regions, the
observed ``objects'' have a similar density, but they appear with a fainter contrast and with a slightly smaller diameter.
226
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Fig. 3. Transmission electron micrograph taken in topographical imaging conditions of an impact of a 30 MeV C60 ion in amorphous Ni3 B irradiated at 300 K up to a ¯uence of 109 ions/cm2 at an angle of incidence of 60° respective to the sample normal.
A more detailed examination of this process is presented in Fig. 6. Fig. 6(a) is an image in focused conditions showing both types of regions. Fig. 6(b) and (c) give topographical contrast images of some impacts, respectively, located in region 1 and in region 2. In region 2, except for the frequent spatial overlap, the observed features are very similar to those previously described at low ¯uences: hollows are surrounded by protuding rims. In region 1, the topographical contrast of the objects is clearly less pronounced. In the left hand side of Fig. 5(a), bright circular contrasts (of typical sizes much larger than the individual impact sizes) are observed; they correspond to regions which are thinner than the surrounding matrix. This could indicate that ``desquamation'' occurs by successive ``pealing'' of small size regions. The faint contrasts observed in the ``desquamed'' regions (Fig. 6(b)) could be attributed to surface deformations related
Fig. 4. Transmission electron micrographs of impacts of 30 MeV C60 ions in amorphous Ni3 B irradiated at perpendicular incidence and at 300 K up to a ¯uence of 1010 ions/cm2 : (a) bright ®eld imaging in focused conditions; (b), (c) phase contrast imaging respectively in overfocused and underfocused conditions.
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Fig. 5. Two regions of the same sample as for Fig. 4. The desquamation process, described in the text, is seen in type 1 regions, but did not occur in type 2 regions.
to the ``disordered amorphous'' tracks generated in the bulk of the target. It has to be noted that a similar desquamation has already been observed after an irradiation of prethinned amorphous Ni3 B with GeV uranium at a high ¯uence (1011 cmÿ2 ) [Fig. 6 in Ref. 7]. In order to understand better this surprising ``desquamation'' eect, we also irradiated 30 lm thick amorphous Ni3 B targets at 300 K with 30 MeV C60 ions at two ¯uences: 5 ´ 1010 and 5 ´ 1011 ions/cm2 . The entrance surface of these massive targets on which the fullerene ions were sent, was protected by a lacquer during electrochemical
thinning after irradiation. The sample is thus thinned from the rear, the lacquer is then dissolved, so that the impinging entrance surface of the projectiles in the ®nal sample can be observed by TEM. The sample irradiated at the ¯uence of 5 ´ 1010 ions/cm2 (Fig. 7) presents damage features which are very similar to those shown in Fig. 5: the desquamation occurs and two types of features which strong or faint contrast are respectively found in the undesquamed and desquamed regions. This important result shows that the ``desquamation'' occurs as well in samples irradiated in the prethinned or bulk states.
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Fig. 6. Border between a ``desquamed'' and nondesquamed regions, observed in amorphous Ni3 B irradiated at high ¯uences with 30 MeV C60 projectiles: (a) bright ®eld image; (b), (c) topographical contrast imaging, respectively in desquamed and nondesquamed regions.
Fig. 8 shows typical features observed in a 30 lm thick sample irradiated up to a ¯uence of 5 ´ 1011 ions/cm2 and thinned for electron microscopy observations after irradiation as described above. Here again strong local thicknesses variations are observed in the totality of the sample. But now, due to the higher ion ¯uence, the regions
which peel o are subsequently irradiated with numerous fullerene ions. The complete surface is now uniformly covered with pustules. 3.2.2. Irradiations at tilted incidence The experimental set-up in the accelerator does not allow an extremely precise determination of
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Fig. 7. Transmission electron micrographs of a 30 lm thick amorphous Ni3 B sample irradiated with 30 MeV C60 ions at perpendicular incidence and at 300 K up to a ¯uence of 5 ´ 1010 ions/cm2 . The sample was thinned from the rear after irradiation for electron microscopy observations. The border between ``desquamed'' and nondesquamed regions is shown in: (a) bright ®eld imaging in focused conditions; (b), (c) phase contrast imaging respectively in overfocused and underfocused conditions.
the sample orientation respective to the projectile incidence direction. Moreover, some local sample deformations lead to an additional uncertainty in the exact beam incidence angle. Fig. 9 features a thin region of a sample irradiated with a beam disoriented by a few degrees respective to the sample normal. The topographical contrast image clearly shows a lack of symmetry in the outer rims. Returning to individual impact features, the main conclusion of the TEM study is that they always appear as hollows surrounded by protuding rings. 4. Discussion The dierent characterisation techniques lead to the same conclusion concerning the topography of the individual impacts. The images of the impacts obtained by TEM in bright ®eld conditions at focus are in perfect agreement with the results of topographical contrast imaging. As recalled in the
experimental section, in focused conditions, the transmitted intensity through an amorphous specimen depends on the mass thickness, so that a hollow should appear brighter than the surrounding matrix which is consistent with the experimental results. Let us now return to the origin of the anisotropic growth process, which was very rapidly summarised in the introduction. Numerous studies of the damage induced by GeV monoatomic ions (Ar to U) in all types of amorphous targets have led various groups of researchers to propose the following interpretations: · for a given material, the anisotropic growth is induced above a threshold in (dE/dx)e · above this threshold, at low ¯uences (i.e. during the incubation period), disorder is induced in the vicinity of the ion path, as a consequence of electronic energy deposition [13]. At higher ¯uences, i.e. above Ui , radial correlated atomic movements take place and lead to the observed macroscopic deformation of the sample.
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Fig. 8. Transmission electron micrographs of a 30 lm thick amorphous Ni3 B sample irradiated with 30 MeV C60 ions at perpendicular incidence and at 300 K up to a ¯uence of 5 ´ 1011 ions/cm2 . The sample was thinned from the rear after irradiation for electron microscopy observations: (a) bright ®eld imaging in focused conditions; (b), (c) phase contrast imaging respectively in overfocused and underfocused conditions.
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Fig. 10. Schematic representation of the radial stresses acting in the vicinity of an excited cylinder in the wake of the projectile (a) and of the resulting deformations after relaxation (b) of the excitation in very thin target regions.
Fig. 9. Transmission electron micrograph in topographical contrast conditions of impacts of 30 MeV C60 ions in amorphous Ni3 B irradiated at 300 K up to a ¯uence of 1010 ions/cm2 . The ion beam incidence direction is tilted by a few degrees respective to the sample normal.
Due to the high rates of energy deposition in electronic processes, there is ®rst a very localized region around the ion path in which electronic excitation and ionisation take place: showers of excited d-electrons are ejected. This might well lead, even in metallic targets, to a Coulomb explosion [14], to energy transfers to the neighboring target atoms and to the generation of a shock wave which originates from the projectile wake and propagates radially outwards [15]. Part of the kinetic energy transmitted to the d-electrons might also be converted into atomic motion through electron±phonon coupling [16]. More recently, a visco-elastic model based on the relaxation of the local stresses initially con®ned in an excited cylinder was proposed to account for the anisotropic growth phenomenon [17]. Whatever the process which is considered to describe the chronology of the events, there is always in a second step, a relaxation of the energy deposited in the track core: a radial extension of the excited region occurs and leads to the local deformations schematised by the dashed lines in
Fig. 10(a). The anisotropic growth results from irreversible outwards atomic displacements. These are only possible in an amorphous disordered state (after an incubation ¯uence) considered either as containing a large amount of free volume [14] or having a modi®ed viscosity [17]. The higher the rate of energy deposition (dE/dx)e by the projectile, the larger the outward impulses given to the target atoms and thus the lower the incubation ¯uence. As predicted in [17], it is quite possible to suppose that the incubation ¯uence might well disappear if sucient rates of energy deposition are reached. Coming back to Fig. 10, one can then predict that the relaxation of the dashed volume in Fig. 10(a) will then, in a thin target lead to local surface deformations as indicated in Fig. 10(b). At the vertical of the ion trajectory, a depletion is formed, whereas the matter pushed outwards gives rise to circular rims on the surfaces of very thin samples. As already mentioned [18], the speci®city of cluster projectiles such as 30 MeV C60 , is that they give similar or higher (dE/dx)e than monoatomic GeV ions, their velocity being one order of magnitude smaller. This results in a very high localisation of the energy: the density of deposited energy can reach values as high as a few 100 eV/atom in the vicinity of the ion wake. This seems thus very favorable to induce anisotropic growth of an amorphous target in a single track. The surface
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features that we observed after isolated impacts are in perfect agreement with the schematic description of Fig. 10(b). 5. Conclusion The TEM experiments reported in this paper demonstrate that anisotropic growth takes place in amorphous Ni3 B irradiated at 300 K with 30 MeV C60 , and that this process occurs in individual impacts. This is clearly related to the high density of energy deposited in electronic processes by the fullerene ions during their slowing down in the targets. The ``sample desquamation'' process, which was observed here to occur systematically after irradiations at high ¯uences of thin or massive amorphous targets could result from very high mechanical stresses accumulating in the vicinity of the sample surface. In order to con®rm these results and to extract some quantitative informations concerning: (i) the depths and heights of the depletions and rims and (ii) the height of the step when desquamation appears, examinations using atomic force microscopy are in progress. Acknowledgements We are grateful to S. Della-Negra for fruitful discussions and for delivering the fullerene beam.
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