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Track formation in amorphous Fe0.55Zr0.45 alloys irradiated by MeV C60 ions: Influence of intrinsic stress on induced surface deformations
Nuclear Instruments and Methods in Physics Research B 209 (2003) 85–92 www.elsevier.com/locate/nimb
Track formation in amorphous Fe0:55Zr0:45 alloys irradiated by MeV C60 ions: Influence of intrinsic stress on induced surface deformations J.C. Girard
a,b
, A. Michel
a,*
, C. Tromas a, C. Jaouen a, S. Della-Negra
c
a
b
Laboratoire de M etallurgie Physique, UMR 6630 CNRS, Universit e de Poitiers, Av. M. et P. Curie, 86962 Futuroscope-Chasseneuil, France Laboratoire de Photonique et Nanostructures, UPR 020 CNRS, route de Nozay, 91460 Marcoussis, France c Institut de Physique Nucl eaire, CNRS-IN2P3, 91406 Orsay Cedex, France
Abstract Amorphous Fe0:55 Zr0:45 films, having thickness of 400 nm, were grown on silicon substrates by co-deposition using ion beam sputtering. Limited surface roughness makes this system particularly suitable for fine-scale scanning force microscopy analysis and nano-indentation. The samples were irradiated with MeV C60 clusters, and the surface morphology of single impacts was found to have a ‘‘doughnut’’ shape, i.e. hillocks having a central crater. Quantitative evaluation of the deformation was achieved by measuring their height and diameter. When C60 projectiles deviate from normal incidence, a tail emerges along the direction of the incident beam. The height of the hillock and length of the tail are increasing with the incidence angle, and the magnitude of the deformation indicates that the damage mainly occurs due to a radial coherent mass transport outwards from the track core by a compression shockwave-like mechanism. The residual compressive in-plane stress, )0.4 GPa for the as-deposited films, was found to notably influence the C60 induced plastic deformations. Indeed, stress relaxation results in a marked decrease in height combined with a significant widening of the surface features. This ‘‘flat’’ surface morphology is attributed to an enhanced radial efficiency of the pressure pulse, owing to a significant reduction of the hardness of the amorphous film after stress relaxation. The overall picture outlined from our observations suggests that the surface damage induced by single MeV C60 ions possibly is the signature of plastic deformation induced at large distances by an energetic radial pressure pulse. This unsteady shockwave allows the energy transfer outwards from the localised region along the ion path that experiences a sudden transient heating. Ó 2003 Elsevier B.V. All rights reserved. PACS: 61.80.Jh; 79.20.Rf; 61.43.Dq; 61.16.Ch; 83.50.By; 68.60.Bs Keywords: Radiation damage; Ion tracks; Surface defects; Amorphous metals and alloys; Atomic force microscopy; Mechanical properties
1. Introduction *
Corresponding author. Tel.: +33-5-49-49-67-55; fax: +33-549-49-66-92. E-mail address: [email protected] (A. Michel).
The interaction of swift heavy ions with a target material is initially governed by the energy transfer to the electronic system ðdE=dxÞe . Structural
0168-583X/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0168-583X(02)01998-5
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modifications, often associated with phase changes, located along the ion wake, the so-called iontracks, have now been observed in a large variety of materials ranging from insulators to metallic systems, with very different ðdE=dxÞe threshold values [1]. In the case of high electronic excitation in solids, the transfer of energy into atomic motion remains a controversial question, especially for free electron systems. At present, it is generally assumed that the space charge is screened in a quasi-instantaneous way and that a fast spreading of the deposited energy is achieved, arising from the high mobility of electrons and thus from the high electronic conductibility of the solid. The use of MeV C60 cluster ion beams offers a new insight into particle–solid interactions [2]. While the electronic stopping power ðdE=dxÞe values are comparable to those of GeV heavy monoatomic ions, the very small range of the d electrons emitted during the ionisation process – related to the projectile velocity – leads to a strong localisation of the electronic energy. As a result, a large density of deposited energy (100 eV/at.) is reached, a value much greater than those previously obtained with monoatomic swift heavy ions (up to several eV/at.). A few experiments carried out in metallic systems with MeV fullerene ion beams show that a drastic damage is therefore induced along the ion path [3–5]. This situation is quite favourable to study on an atomic scale the track created by a single cluster impact. The present work reports results of our investigations by tapping-mode scanning force microscopy (TM-SFM) of surface tracks induced by MeV C60 ions in metallic Fe55 Zr45 amorphous films. This technique allows to record the ÔfingerprintsÕ of single ion impacts, and indeed, large impacts are identified. We show that the surface ion track is associated with the creation not only of a hillock, but also of a central crater, in good agreement with observations performed in amorphous metallic Fe0:69 Zr0:31 films [6]. The dependence of the surface features on the projectile parameters – energy and incidence angle – is consistent with the interpretation that surface deformations result mainly from a transient and efficient pressure pulse. This work mainly focuses on the influence of the internal in-plane compres-
sive stress of the amorphous films deposited onto Si substrates on the morphology of surface deformation related to a single cluster impact. The results are discussed in relation with the mechanical properties – elastic modulus and nanohardness – of the substrate-supported thin film determined by the use of a nano-indentation technique.
2. Experiment Amorphous Fe0:55 Zr0:45 films were grown at room temperature using a high-vacuum (base pressure 6 108 Torr) NORDIKO-3000 sputtering chamber equipped with a RF-plasma ion gun. The films were deposited on natural-oxidised (0 0 1) Si substrates using a 1.2 kV Ar ion beam /s. The atomic and with a deposition rate of 0.7 A composition was measured by energy dispersive X-ray spectroscopy with an uncertainty lower than 1 at.%. The total thickness of the films was 400 nm. The amorphous structure was confirmed by X-ray diffraction in the Bragg-Brentano geometry using a Bruker D5005 diffractometer. The amorphous metallic films were irradiated at room temperature with either 30 MeV C2þ 60 or 40 MeV C3þ 60 fullerene ions delivered by the tandem accelerator at Institut de Physique Nucleaire dÕOrsay. A detailed description of the experimental set-up, which allows producing and accelerating molecular beams of Cnþ 60 to MeV energies is given in [2]. The fluence was limited to 2 109 clusters/ cm2 to study the damage induced by single impacts either at normal (0°) or at tilted (30–75°) incidence. The beam intensity on the target was in the range of 105 –106 clusters per second on a selected area of 7 mm2 . The main irradiation parameters are listed in Table 1. The energy loss values are deduced from calculations using the SRIM-2000 code [7]. Simulation was performed with monoatomic carbon atoms, taking into account the additive rule of the individual stopping powers introduced by Baudin et al. [8]. After the clustersÕ breaking-up within the first atomic layers, the projectile constituents deposit their energy simultaneously and their trajectories remain strongly correlated over some distance. Previous experimental observations
J.C. Girard et al. / Nucl. Instr. and Meth. in Phys. Res. B 209 (2003) 85–92 Table 1 Characteristic values for the C60 cluster ion irradiation in Fe0:55 Zr0:45 amorphous film: incident energy, velocity (in units of the Bohr velocity, vB ), linear rates of energy deposition in electronic processes and in atomic collisions at the incident energy and maximum energy Em transferred to the d electrons Particle
Incident energy (MeV)
Velocity (v=vB )
ðdE=dxÞe (keV/nm)
ðdE=dxÞn (keV/nm)
Em (eV)
C2þ 60 C3þ 60
30 40
1.3 1.7
72 86
1.5 1.4
90 120
in insulators allowed an estimation on the correlation length of 120–150 nm [9,10]: because this value is much lower than the total thickness of the film, influence of a damage induced in the silicon substrates can be ruled out. The surface tracks were studied in air by scanning force microscopy (SFM) using a Nanoscope III multimode-head equipment (Digital Instruments, CA, USA) and performing the investigations in an intermittent contact or tapping mode (TM-SFM) with either Nano-probe Si tips with a nominal radius of curvature of 10 nm and cone angle 36° or ultrasharp Nano-sensor Si tips with nominal radius of 2 nm and cone angle 20°. In this mode, the tip oscillates with a frequency of about 300 kHz, and we used the lower setpoint amplitude to follow the surface (0.5 V RMS). Owing to the specificity of the sputtering technique and to the film-substrate interaction, the film is submitted to large in-plane compressive elastic stresses. A biaxial compressive stress of )0.4 GPa was deduced using StoneyÕs formula [11] from the measurement of the curvature radii before and after film deposition onto a 100 lm thick silicon substrate using an optical in-house developed setup based on the measurement of the deflection of a laser beam when a scan is performed along the Si substrate. In order to examine the influence of the intrinsic stress on the local response of the material to a strong energy deposition, we performed a preirradiation at room temperature using 380 keV Ar2þ ions at a fluence as low as 1014 ions/cm2 , which corresponds to a calculated ‘‘dpa’’ dose of 0.12 displacements per atom or higher, using the specific threshold displacement energies for crystalline iron and zirconium. A nearly complete re-
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lease of the initial elastic stresses was observed on a depth comparable to the projected range of the Ar ions, namely 300 nm. The measured biaxial stress was reduced to )0.03 GPa for a Fe0:55 Zr0:45 film having this thickness. Additionally, in order to investigate the role of residual elastic stresses on the elasto-plastic behaviour of the amorphous films, the hardness and the elastic modulus of the films were determined using a NHT nano-indenter (CSEM, Switzerland), with a diamond Berkovich indenter. A 100 100 lm2 array of indents was made on each sample using maximum loads ranging from 0.5 to 90 mN. The hardness and elastic modulus values were determined from the unloading curves, according to the Oliver and Pharr method [12].
3. Results and discussion TM-SFM images of an amorphous Fe0:55 Zr0:45 film surface irradiated at normal incidence by 30 MeV C60 ions are displayed in Fig. 1. Large surface damage features can be seen (Fig. 1(a)). The single impacts present a characteristic ‘‘doughnut’’ shape, i.e. a hillock with a central shallow hole (Fig. 1(b)). The depth of this apparent crater is very limited compared to the height of the hillock. The quantitative evaluation of the deformation was achieved by measuring its height, its basal width, so-called external diameter De and its internal diameter at the top of the rim, Di . For the data given here, no correction for the tip radius was made: the shape of the tip is known to have an effect on the evaluated diameter, but not on the measured height of the features. Moreover, the diameter Di is independent of the tip radius and gives a lower boundary for the absolute width of the deformed region. A typical cross-sectional profile is shown in Fig. 1(c). The dimensions, as determined from the average values over 10 impacts, are h ¼ 3:1 0:1 nm for the height of the hillock, De ¼ 42 2 nm and Di ¼ 10:8 0:3 nm for the mean external and interior diameters respectively. It is worth noting that the shallow character of the central hole is significantly different from previous observations on an amorphous Fe0:69 Zr0:31 alloy irradiated under similar
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Fig. 1. Tapping mode-SFM images of surface tracks on an as-deposited amorphous Fe0:55 Zr0:45 thin film irradiated at normal incidence using 30 MeV C60 ions: (a) top view of a scan area 500 500 nm2 ; (b) typical three dimensional surface morphology of a single surface track; (c) top view of a selected surface track and surface profile along the indicated line.
conditions. In the current study, TM-SFM images even display a flat part inside a more burst structure, i.e. the splitting of the surrounding rim in several lobes. This feature does not seem to be
influenced by the probe tip. Some surface tracks even suggest the emergence of a slight bump at the core of the impacts. Such a swelling effect is confirmed by observations performed on samples ir-
Fig. 2. TM-SFM images of surface tracks induced on amorphous Fe0:55 Zr0:45 film irradiated at normal incidence by 40 MeV C60 ions: (a) three dimensional surface morphology of a single surface track; (b) top view and corresponding surface profile.
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Fig. 3. TM-SFM images of surface tracks induced on an amorphous Fe0:55 Zr0:45 thin film irradiated by 30 MeV C60 clusters having incidence angles of 30°, 60° and 75° with respect to the surface normal. The arrow indicates the entry point of the C60 cluster. Typical three-dimensional surface morphology of a surface track (a)–(c), corresponding surface profile along the incidence azimuth (d)–(f) and transversal cross-section along the indicated white lines (g)–(i).
radiated with 40 MeV C60 ions (Fig. 2). The overall picture of these surface tracks, i.e. their ‘‘doughnut’’ shape, is quite consistent with the TEM im-
ages obtained by Dunlop and co-workers [4,5] in the Ni0:75 B0:25 system. The track diameters are also in agreement. The comparison with the SFM
Fig. 4. TM-SFM images of surface tracks induced by irradiation at normal incidence with 30 MeV C60 ions on an amorphous stressrelaxed Fe0:55 Zr0:45 film pre-irradiated with 380 keV Ar ions: (a) top view of a scan area 500 500 nm2 ; (b) typical three dimensional surface morphology of a single surface track; (c) top view of a selected surface track and surface profile along the indicated line.
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observations carried out in a Fe0:85 B0:15 amorphous alloy irradiated with 0.85 GeV Pb ions confirms the crucial role of the electronically deposited energy density since very small hillocks (h 1 nm) were observed by Audouard et al. [13], for a very similar (56 keV nm1 ) value of ðdE=dxÞe . When the cluster beam deviates from normal incidence, surface defects become more and more asymmetrical (Fig. 3). A tail emerges along the direction of the incident beam in front of each hillock. For a near surface incidence of 75°, a thin and elongated groove, limited by two raised edges, is visible before each hillock. This drastic matter redistribution at the surface, possibly accompanied by the ejection of matter, can be attributed to the furrow bored by the entrance of the C60 cluster in the solid. Furthermore, the profiles along the incidence azimuth show that the height of the hillock and the length of the tail are increasing with increasing angle of incidence. This provides some evidence for a radial expansion along the ion path due to mechanical forces associated with the high transient energy density within the ion track. Moreover, the magnitude of the permanent surface deformations indicates that this radial atomic motion is probably driven by a compression shockwave-like mechanism. This trend will be more extensively discussed in a forthcoming paper [6] in the light of additional observations by high resolution electron microscopy and with reference to quantitative surface deformations predicted by a thermo-elastic model [14]. This conclusion is in full agreement with the model proposed to explain the desorption of large organic molecules excited by swift heavy ions through a pressure pulse [15], or as a consequence of a shockwave-like mechanism, generated by the steep radial energy gradient [16,17]. A similar conclusion was drawn from the few observations performed by SFM on surface tracks induced by MeV clusters: in this aspect, the studies carried out on muscovite mica and organic single-crystals by Barlo Daya and co-workers [18,19] are noteworthy. Nevertheless, in these cases, surface tracks were associated with a bulk phase change. Fig. 4 displays a typical surface defect induced by 30 MeV C60 ions in the stress-free amorphous
film. Stress relaxation was achieved by a previous irradiation with 380 keV Ar at a fluence of 1014 ions/cm2 . A decrease in height of about 25% combined with a slight, but significant, widening of the surface features is observed: after stress relaxation, the measured parameters are h ¼ 2:3 0:2 nm, De ¼ 45 3 nm and Di ¼ 11:4 0:3 nm respectively. Note that for a valid comparison, an effort was made to use the same tip throughout the investigation of the samples before and after stress-relaxation. This ‘‘flatness’’ of the feature cannot be explained by an enhanced thermal diffusivity involving a more prompt energy transport by collisional effects from the hot region to the matrix. Indeed, a very weak dependence of the thermodynamical properties, namely the heat capacity and the thermal conductivity, with stress and related elastic strains is expected. The effect of the in-plane compressive residual stress, 0.4 GPa in the as-deposited film, on the near surface deformations is nevertheless considerable. If the surface damage originates from a radial pressure pulse, the ‘‘flat’’ surface morphology seen after stress relaxation points to an enhanced radial efficiency of the pressure pulse, concurrently with a pronounced lowering in the axial direction. This is consistent with rules of yielding under biaxial loading, and implies that the stress relaxation involves a significant reduction of the yield stress. In order to confirm this interpretation, depth sensing nano-indentation experiments were undertaken to obtain an accurate characterisation of the elasto-plastic properties of the films. It is well known that continuous indentation test measures the resistance of a solid to local deformation. Moreover, nano-indentation offers the possibility, through continuous load-controlled experiments to determine the hardness and the effective elastic Young modulus of the film. Fig. 5 illustrates how the hardness and the effective Young modulus depend on the indentation depth for both the stressed film and for the stressrelaxed one. The load–displacement curves, not displayed here, were analysed according to the Oliver and Pharr method [12]. It is worth to note that each data point reported in Fig. 5 is obtained from ten indentation tests carried out within a
J.C. Girard et al. / Nucl. Instr. and Meth. in Phys. Res. B 209 (2003) 85–92
Fig. 5. Hardness of Fe0:55 Zr0:45 films, under biaxial compressive stress and stress-relaxed, plotted as a function of the indentation depth normalised by the film thickness. The effective Young elastic moduli of the films (assuming that the Poisson ratio of the amorphous film is equal to 0.31) are also shown in the inset.
100 100 lm2 array. Therefore, the values of hardness and elastic modulus refer to the average and the given uncertainties to the standarddeviations on the average. The variation of hardness and Young modulus versus indentation depth shows the influence of the silicon substrate. In other words, the deformation field is not confined within the film thickness. Thus, intrinsic values for the film can be only estimated at low loading, i.e. for a depth of at the most the tenth of the total thickness of the film. Finally, two trends can be described: (i) the elastic modulus seems insensitive to the stress field level. The expected value of the elastic modulus of the Fe0:55 Zr0:45 alloy can be determined using a simple rule-of-mixing on the Young moduli of the elemental crystalline materials, and taking into account the usually observed decrease ()25%) when the material is amorphous [20]. The experimental value of the Young modulus, 125 GPa, is in good agreement with this model; (ii) the hardness of the thin film under a compressive stress is significantly increased (10%), compared to that of the stressfree film. Thus, these results give clear indication of an apparent hardening of the amorphous film under biaxial compressive stress while in turn re-
91
laxation promotes yielding and reduces the hardness. It is however worth mentioning that this sensitivity to a stress of a few tenths of GPa must be compared to the yield strength ry of the amorphous metallic material that may be deduced from the measured hardness H through the approximate relation H 3ry , that is ry 3 GPa. To summarise, this study of the material response to nano-indentation proves that stress appreciably influences the yield stress required for appearance of plastic deformation. That information yields the picture that the surface damage would be mainly achieved by a propagating ‘‘pressure pulse’’. Therefore, ion induced near-surface plastic deformation would involve a coherent atomic motion leading to a permanent decrease of the atomic density at the centre of the impact. In a recent study [21], a significant increase of the chemical etchability of tracks of 2.4 GeV Pb in amorphous metallic samples has been observed when a compressive in-plane stress was introduced locally. This was interpreted as the result of the variation of the chemical potential with the elastic energy stored in the ion track under the thermal expansion of the transient hot phase produced during the thermal spike. This explanation can be founded for impact events induced by GeV monoatomic ions, nevertheless if a high energy density is initially deposited within a cylindrical region, recent calculations by molecular dynamics predict that a significant part of energy may also be transported in the solid matrix by an unsteady shockwave [22]. Concluding, the overall picture outlined from our observations suggests that surface damage induced by single MeV C60 ions is the signature of plastic deformations induced at large distances by an energetic radial pressure pulse: an unsteady shock wave emanating from the sudden and transient heating of a localised region along the ion path. It is worth to note that a compressive stress as low as 0.4 GPa is enough to influence appreciably the mechanical response of the amorphous Fe0:55 Zr0:45 matrix following the prompt energy deposition in the ion wake. This suggests that transient induced stresses would be in the GPa range. The anisotropic growth phenomenon observed particularly in amorphous materials
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[23–26] – interpreted with a visco-elastic model based on the relaxation of shear stresses [27] – could also derive from this damage mechanism. The possible occurrence of some additional relaxation mechanisms leading to surface damage has also to be considered. Indeed, the flattening in the middle of the central shallow hole and the more complex structure observed with 40 MeV C60 ion irradiation possibly hint to a relaxation by a viscous axial outflow arising from the very hot region generated in the core of the ion track. Such an effect has been shown for near-surface cascades induced by keV recoil atoms first by means of molecular dynamic calculations [28], then experimentally using in situ electron microscopy [29]. In so far, one can stress the striking resemblance of this emergence with the sub-structure observed in the central part of some large impacts induced by meteorites. More detailed investigations are clearly required to confirm this feature and to determine the real underlying mechanism. Acknowledgements The authors would like to thank the technical staff of the tandem accelerator at IPN-Orsay for providing the fullerene beam and for their efficient collaboration. C.J., A.M. and C.T. (LMP-Poitiers) gratefully acknowledge the financial support of the Ministere de la Recherche through an ‘‘Action Concertee Incitative’’: Surfaces, Interfaces and Conception of New Materials for the development of the in situ stress measurement set-up. References [1] For a review, see for example: R. Spohr, Ion Tracks and Microtechnology, Vieweg, Braunschweig, 1990; M. Toulemonde, F. Studer, Solid State Phenom. 30/31 (1993) 477; A. Barbu, H. Dammak, A. Dunlop, D. Lesueur, MRS Bull. Dec. (1995) 29. [2] S. Della-Negra, A. Brunelle, Y. Le Beyec, J.M. Cureaudeau, J.P. Mouffron, B. Waast, P. H akanson, B.U.R. Sunqvist, E. Parilis, Nucl. Instr. and Meth. B 74 (1993) 453. [3] H. Dammak, A. Dunlop, D. Lesueur, A. Brunelle, S. Della-Negra, Y. Le Beyec, Phys. Rev. Lett. 74 (1995) 1135.
[4] A. Dunlop, G. Jaskierowicz, S. Della-Negra, C.R. Acad. Sci. Paris t. 325 (serie II b) (1997) 397. [5] A. Dunlop, J. Henry, G. Jaskierowicz, Nucl. Instr. and Meth. B 146 (1998) 222. [6] A. Michel, C. Jaouen, J.C. Girard, S. Della-Negra, in preparation. [7] J.F. Ziegler, J.P. Biersack, U. Littmark, Stopping Power and Ranges of Ions in Matter, Vol. I, Pergamon Press, New York, 1985. [8] K. Baudin, A. Brunelle, M. Chabot, S. Della-Negra, J. Depaw, D. Gardes, P. H akanson, Y. Le Beyec, A. Billebaud, M. Fallavier, J. Remilleux, J.C. Poizat, J.P. Thomas, Nucl. Instr. and Meth. B 94 (1994) 341. [9] A. Dunlop, G. Jaskierowicz, J. Jensen, S. Della-Negra, Nucl. Instr. and Meth. B 132 (1997) 93. [10] S.M.M. Ramos, N. Bonardi, B. Canut, S. Della-Negra, Phys. Rev. B 57 (1998) 189. [11] G.G. Stoney, Roy. Soc. Ser. A 82 (1909) 172. [12] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564. [13] A. Audouard, R. Mamy, M. Toulemonde, G. Szenes, L. Thome, Europhys. Lett. 40 (1997) 527. [14] J. Colin, D. Lesueur, J. Grilhe, Philos. Mag. A 81 (2001) 857. [15] R.E. Johnson, B.U.R. Sundqvist, A. Hedin, D. Feny€ o, Phys. Rev. B 40 (1989) 49. [16] I.S. Bitensky, E.S. Parilis, Nucl. Instr. and Meth. B 21 (1987) 26. [17] Y. Kitazoe, N. Hiraoka, Y. Yamamura, Surf. Sci. 111 (1981) 381. [18] D.D.N. Barlo Daya, A. Hallen, J. Eriksson, J. Kopniczisky, R. Papaleo, C.T. Reimann, P. H akanson, B.U.R. Sunqvist, A. Brunelle, S. Della-Negra, Y. Le Beyec, Nucl. Instr. and Meth. B 106 (1995) 38. [19] D.D.N. Barlo Daya, C.T. Reimann, P. H akanson, B.U.R. Sunqvist, A. Brunelle, S. Della-Negra, Y. Le Beyec, Nucl. Instr. and Meth. B 124 (1997) 484. [20] H.U. K€ unzi, Glassy Metals, Springer Verlag, 1983, p. 163. [21] C. Trautmann, S. Klaum€ unzer, H. Trinkaus, Phys. Rev. Lett. 85 (2000) 3648. [22] E.M. Bringa, R.E. Johnson, Nucl. Instr. and Meth. B 143 (1998) 513. [23] S. Klaum€ unzer, G. Schumacher, S. Rentzsch, G. Vogl, L. S€ oldner, H. Bieger, Acta Met. 30 (1982) 1493. [24] S. Klaum€ unzer, M.-D. Hou, G. Schumacher, Phys. Rev. Lett. 57 (1986) 850. [25] S. Klaum€ unzer, C. Li, S. L€ offler, M. Rammensee, G. Schumacher, C.H. Neitzert, Radiat. Effects Def. Solids 108 (1989) 131. [26] A. Audouard, E. Balanzat, G. Fuchs, J.C. Jousset, D. Lesueur, L. Thome, Europhys. Lett. 3 (1987) 327. [27] H. Trinkaus, A.I. Ryazanov, Phys. Rev. Lett. 74 (1995) 5072. [28] M. Ghaly, R.S. Averback, Phys. Rev. Lett. 72 (1994) 364. [29] R.C. Birtcher, S.E. Donnelly, S. Schlutig, Phys. Rev. Lett. 85 (2000) 4968.
Track formation in amorphous Fe0:55Zr0:45 alloys irradiated by MeV C60 ions: Influence of intrinsic stress on induced surface deformations J.C. Girard
a,b
, A. Michel
a,*
, C. Tromas a, C. Jaouen a, S. Della-Negra
c
a
b
Laboratoire de M etallurgie Physique, UMR 6630 CNRS, Universit e de Poitiers, Av. M. et P. Curie, 86962 Futuroscope-Chasseneuil, France Laboratoire de Photonique et Nanostructures, UPR 020 CNRS, route de Nozay, 91460 Marcoussis, France c Institut de Physique Nucl eaire, CNRS-IN2P3, 91406 Orsay Cedex, France
Abstract Amorphous Fe0:55 Zr0:45 films, having thickness of 400 nm, were grown on silicon substrates by co-deposition using ion beam sputtering. Limited surface roughness makes this system particularly suitable for fine-scale scanning force microscopy analysis and nano-indentation. The samples were irradiated with MeV C60 clusters, and the surface morphology of single impacts was found to have a ‘‘doughnut’’ shape, i.e. hillocks having a central crater. Quantitative evaluation of the deformation was achieved by measuring their height and diameter. When C60 projectiles deviate from normal incidence, a tail emerges along the direction of the incident beam. The height of the hillock and length of the tail are increasing with the incidence angle, and the magnitude of the deformation indicates that the damage mainly occurs due to a radial coherent mass transport outwards from the track core by a compression shockwave-like mechanism. The residual compressive in-plane stress, )0.4 GPa for the as-deposited films, was found to notably influence the C60 induced plastic deformations. Indeed, stress relaxation results in a marked decrease in height combined with a significant widening of the surface features. This ‘‘flat’’ surface morphology is attributed to an enhanced radial efficiency of the pressure pulse, owing to a significant reduction of the hardness of the amorphous film after stress relaxation. The overall picture outlined from our observations suggests that the surface damage induced by single MeV C60 ions possibly is the signature of plastic deformation induced at large distances by an energetic radial pressure pulse. This unsteady shockwave allows the energy transfer outwards from the localised region along the ion path that experiences a sudden transient heating. Ó 2003 Elsevier B.V. All rights reserved. PACS: 61.80.Jh; 79.20.Rf; 61.43.Dq; 61.16.Ch; 83.50.By; 68.60.Bs Keywords: Radiation damage; Ion tracks; Surface defects; Amorphous metals and alloys; Atomic force microscopy; Mechanical properties
1. Introduction *
Corresponding author. Tel.: +33-5-49-49-67-55; fax: +33-549-49-66-92. E-mail address: [email protected] (A. Michel).
The interaction of swift heavy ions with a target material is initially governed by the energy transfer to the electronic system ðdE=dxÞe . Structural
0168-583X/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0168-583X(02)01998-5
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modifications, often associated with phase changes, located along the ion wake, the so-called iontracks, have now been observed in a large variety of materials ranging from insulators to metallic systems, with very different ðdE=dxÞe threshold values [1]. In the case of high electronic excitation in solids, the transfer of energy into atomic motion remains a controversial question, especially for free electron systems. At present, it is generally assumed that the space charge is screened in a quasi-instantaneous way and that a fast spreading of the deposited energy is achieved, arising from the high mobility of electrons and thus from the high electronic conductibility of the solid. The use of MeV C60 cluster ion beams offers a new insight into particle–solid interactions [2]. While the electronic stopping power ðdE=dxÞe values are comparable to those of GeV heavy monoatomic ions, the very small range of the d electrons emitted during the ionisation process – related to the projectile velocity – leads to a strong localisation of the electronic energy. As a result, a large density of deposited energy (100 eV/at.) is reached, a value much greater than those previously obtained with monoatomic swift heavy ions (up to several eV/at.). A few experiments carried out in metallic systems with MeV fullerene ion beams show that a drastic damage is therefore induced along the ion path [3–5]. This situation is quite favourable to study on an atomic scale the track created by a single cluster impact. The present work reports results of our investigations by tapping-mode scanning force microscopy (TM-SFM) of surface tracks induced by MeV C60 ions in metallic Fe55 Zr45 amorphous films. This technique allows to record the ÔfingerprintsÕ of single ion impacts, and indeed, large impacts are identified. We show that the surface ion track is associated with the creation not only of a hillock, but also of a central crater, in good agreement with observations performed in amorphous metallic Fe0:69 Zr0:31 films [6]. The dependence of the surface features on the projectile parameters – energy and incidence angle – is consistent with the interpretation that surface deformations result mainly from a transient and efficient pressure pulse. This work mainly focuses on the influence of the internal in-plane compres-
sive stress of the amorphous films deposited onto Si substrates on the morphology of surface deformation related to a single cluster impact. The results are discussed in relation with the mechanical properties – elastic modulus and nanohardness – of the substrate-supported thin film determined by the use of a nano-indentation technique.
2. Experiment Amorphous Fe0:55 Zr0:45 films were grown at room temperature using a high-vacuum (base pressure 6 108 Torr) NORDIKO-3000 sputtering chamber equipped with a RF-plasma ion gun. The films were deposited on natural-oxidised (0 0 1) Si substrates using a 1.2 kV Ar ion beam /s. The atomic and with a deposition rate of 0.7 A composition was measured by energy dispersive X-ray spectroscopy with an uncertainty lower than 1 at.%. The total thickness of the films was 400 nm. The amorphous structure was confirmed by X-ray diffraction in the Bragg-Brentano geometry using a Bruker D5005 diffractometer. The amorphous metallic films were irradiated at room temperature with either 30 MeV C2þ 60 or 40 MeV C3þ 60 fullerene ions delivered by the tandem accelerator at Institut de Physique Nucleaire dÕOrsay. A detailed description of the experimental set-up, which allows producing and accelerating molecular beams of Cnþ 60 to MeV energies is given in [2]. The fluence was limited to 2 109 clusters/ cm2 to study the damage induced by single impacts either at normal (0°) or at tilted (30–75°) incidence. The beam intensity on the target was in the range of 105 –106 clusters per second on a selected area of 7 mm2 . The main irradiation parameters are listed in Table 1. The energy loss values are deduced from calculations using the SRIM-2000 code [7]. Simulation was performed with monoatomic carbon atoms, taking into account the additive rule of the individual stopping powers introduced by Baudin et al. [8]. After the clustersÕ breaking-up within the first atomic layers, the projectile constituents deposit their energy simultaneously and their trajectories remain strongly correlated over some distance. Previous experimental observations
J.C. Girard et al. / Nucl. Instr. and Meth. in Phys. Res. B 209 (2003) 85–92 Table 1 Characteristic values for the C60 cluster ion irradiation in Fe0:55 Zr0:45 amorphous film: incident energy, velocity (in units of the Bohr velocity, vB ), linear rates of energy deposition in electronic processes and in atomic collisions at the incident energy and maximum energy Em transferred to the d electrons Particle
Incident energy (MeV)
Velocity (v=vB )
ðdE=dxÞe (keV/nm)
ðdE=dxÞn (keV/nm)
Em (eV)
C2þ 60 C3þ 60
30 40
1.3 1.7
72 86
1.5 1.4
90 120
in insulators allowed an estimation on the correlation length of 120–150 nm [9,10]: because this value is much lower than the total thickness of the film, influence of a damage induced in the silicon substrates can be ruled out. The surface tracks were studied in air by scanning force microscopy (SFM) using a Nanoscope III multimode-head equipment (Digital Instruments, CA, USA) and performing the investigations in an intermittent contact or tapping mode (TM-SFM) with either Nano-probe Si tips with a nominal radius of curvature of 10 nm and cone angle 36° or ultrasharp Nano-sensor Si tips with nominal radius of 2 nm and cone angle 20°. In this mode, the tip oscillates with a frequency of about 300 kHz, and we used the lower setpoint amplitude to follow the surface (0.5 V RMS). Owing to the specificity of the sputtering technique and to the film-substrate interaction, the film is submitted to large in-plane compressive elastic stresses. A biaxial compressive stress of )0.4 GPa was deduced using StoneyÕs formula [11] from the measurement of the curvature radii before and after film deposition onto a 100 lm thick silicon substrate using an optical in-house developed setup based on the measurement of the deflection of a laser beam when a scan is performed along the Si substrate. In order to examine the influence of the intrinsic stress on the local response of the material to a strong energy deposition, we performed a preirradiation at room temperature using 380 keV Ar2þ ions at a fluence as low as 1014 ions/cm2 , which corresponds to a calculated ‘‘dpa’’ dose of 0.12 displacements per atom or higher, using the specific threshold displacement energies for crystalline iron and zirconium. A nearly complete re-
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lease of the initial elastic stresses was observed on a depth comparable to the projected range of the Ar ions, namely 300 nm. The measured biaxial stress was reduced to )0.03 GPa for a Fe0:55 Zr0:45 film having this thickness. Additionally, in order to investigate the role of residual elastic stresses on the elasto-plastic behaviour of the amorphous films, the hardness and the elastic modulus of the films were determined using a NHT nano-indenter (CSEM, Switzerland), with a diamond Berkovich indenter. A 100 100 lm2 array of indents was made on each sample using maximum loads ranging from 0.5 to 90 mN. The hardness and elastic modulus values were determined from the unloading curves, according to the Oliver and Pharr method [12].
3. Results and discussion TM-SFM images of an amorphous Fe0:55 Zr0:45 film surface irradiated at normal incidence by 30 MeV C60 ions are displayed in Fig. 1. Large surface damage features can be seen (Fig. 1(a)). The single impacts present a characteristic ‘‘doughnut’’ shape, i.e. a hillock with a central shallow hole (Fig. 1(b)). The depth of this apparent crater is very limited compared to the height of the hillock. The quantitative evaluation of the deformation was achieved by measuring its height, its basal width, so-called external diameter De and its internal diameter at the top of the rim, Di . For the data given here, no correction for the tip radius was made: the shape of the tip is known to have an effect on the evaluated diameter, but not on the measured height of the features. Moreover, the diameter Di is independent of the tip radius and gives a lower boundary for the absolute width of the deformed region. A typical cross-sectional profile is shown in Fig. 1(c). The dimensions, as determined from the average values over 10 impacts, are h ¼ 3:1 0:1 nm for the height of the hillock, De ¼ 42 2 nm and Di ¼ 10:8 0:3 nm for the mean external and interior diameters respectively. It is worth noting that the shallow character of the central hole is significantly different from previous observations on an amorphous Fe0:69 Zr0:31 alloy irradiated under similar
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Fig. 1. Tapping mode-SFM images of surface tracks on an as-deposited amorphous Fe0:55 Zr0:45 thin film irradiated at normal incidence using 30 MeV C60 ions: (a) top view of a scan area 500 500 nm2 ; (b) typical three dimensional surface morphology of a single surface track; (c) top view of a selected surface track and surface profile along the indicated line.
conditions. In the current study, TM-SFM images even display a flat part inside a more burst structure, i.e. the splitting of the surrounding rim in several lobes. This feature does not seem to be
influenced by the probe tip. Some surface tracks even suggest the emergence of a slight bump at the core of the impacts. Such a swelling effect is confirmed by observations performed on samples ir-
Fig. 2. TM-SFM images of surface tracks induced on amorphous Fe0:55 Zr0:45 film irradiated at normal incidence by 40 MeV C60 ions: (a) three dimensional surface morphology of a single surface track; (b) top view and corresponding surface profile.
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Fig. 3. TM-SFM images of surface tracks induced on an amorphous Fe0:55 Zr0:45 thin film irradiated by 30 MeV C60 clusters having incidence angles of 30°, 60° and 75° with respect to the surface normal. The arrow indicates the entry point of the C60 cluster. Typical three-dimensional surface morphology of a surface track (a)–(c), corresponding surface profile along the incidence azimuth (d)–(f) and transversal cross-section along the indicated white lines (g)–(i).
radiated with 40 MeV C60 ions (Fig. 2). The overall picture of these surface tracks, i.e. their ‘‘doughnut’’ shape, is quite consistent with the TEM im-
ages obtained by Dunlop and co-workers [4,5] in the Ni0:75 B0:25 system. The track diameters are also in agreement. The comparison with the SFM
Fig. 4. TM-SFM images of surface tracks induced by irradiation at normal incidence with 30 MeV C60 ions on an amorphous stressrelaxed Fe0:55 Zr0:45 film pre-irradiated with 380 keV Ar ions: (a) top view of a scan area 500 500 nm2 ; (b) typical three dimensional surface morphology of a single surface track; (c) top view of a selected surface track and surface profile along the indicated line.
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observations carried out in a Fe0:85 B0:15 amorphous alloy irradiated with 0.85 GeV Pb ions confirms the crucial role of the electronically deposited energy density since very small hillocks (h 1 nm) were observed by Audouard et al. [13], for a very similar (56 keV nm1 ) value of ðdE=dxÞe . When the cluster beam deviates from normal incidence, surface defects become more and more asymmetrical (Fig. 3). A tail emerges along the direction of the incident beam in front of each hillock. For a near surface incidence of 75°, a thin and elongated groove, limited by two raised edges, is visible before each hillock. This drastic matter redistribution at the surface, possibly accompanied by the ejection of matter, can be attributed to the furrow bored by the entrance of the C60 cluster in the solid. Furthermore, the profiles along the incidence azimuth show that the height of the hillock and the length of the tail are increasing with increasing angle of incidence. This provides some evidence for a radial expansion along the ion path due to mechanical forces associated with the high transient energy density within the ion track. Moreover, the magnitude of the permanent surface deformations indicates that this radial atomic motion is probably driven by a compression shockwave-like mechanism. This trend will be more extensively discussed in a forthcoming paper [6] in the light of additional observations by high resolution electron microscopy and with reference to quantitative surface deformations predicted by a thermo-elastic model [14]. This conclusion is in full agreement with the model proposed to explain the desorption of large organic molecules excited by swift heavy ions through a pressure pulse [15], or as a consequence of a shockwave-like mechanism, generated by the steep radial energy gradient [16,17]. A similar conclusion was drawn from the few observations performed by SFM on surface tracks induced by MeV clusters: in this aspect, the studies carried out on muscovite mica and organic single-crystals by Barlo Daya and co-workers [18,19] are noteworthy. Nevertheless, in these cases, surface tracks were associated with a bulk phase change. Fig. 4 displays a typical surface defect induced by 30 MeV C60 ions in the stress-free amorphous
film. Stress relaxation was achieved by a previous irradiation with 380 keV Ar at a fluence of 1014 ions/cm2 . A decrease in height of about 25% combined with a slight, but significant, widening of the surface features is observed: after stress relaxation, the measured parameters are h ¼ 2:3 0:2 nm, De ¼ 45 3 nm and Di ¼ 11:4 0:3 nm respectively. Note that for a valid comparison, an effort was made to use the same tip throughout the investigation of the samples before and after stress-relaxation. This ‘‘flatness’’ of the feature cannot be explained by an enhanced thermal diffusivity involving a more prompt energy transport by collisional effects from the hot region to the matrix. Indeed, a very weak dependence of the thermodynamical properties, namely the heat capacity and the thermal conductivity, with stress and related elastic strains is expected. The effect of the in-plane compressive residual stress, 0.4 GPa in the as-deposited film, on the near surface deformations is nevertheless considerable. If the surface damage originates from a radial pressure pulse, the ‘‘flat’’ surface morphology seen after stress relaxation points to an enhanced radial efficiency of the pressure pulse, concurrently with a pronounced lowering in the axial direction. This is consistent with rules of yielding under biaxial loading, and implies that the stress relaxation involves a significant reduction of the yield stress. In order to confirm this interpretation, depth sensing nano-indentation experiments were undertaken to obtain an accurate characterisation of the elasto-plastic properties of the films. It is well known that continuous indentation test measures the resistance of a solid to local deformation. Moreover, nano-indentation offers the possibility, through continuous load-controlled experiments to determine the hardness and the effective elastic Young modulus of the film. Fig. 5 illustrates how the hardness and the effective Young modulus depend on the indentation depth for both the stressed film and for the stressrelaxed one. The load–displacement curves, not displayed here, were analysed according to the Oliver and Pharr method [12]. It is worth to note that each data point reported in Fig. 5 is obtained from ten indentation tests carried out within a
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Fig. 5. Hardness of Fe0:55 Zr0:45 films, under biaxial compressive stress and stress-relaxed, plotted as a function of the indentation depth normalised by the film thickness. The effective Young elastic moduli of the films (assuming that the Poisson ratio of the amorphous film is equal to 0.31) are also shown in the inset.
100 100 lm2 array. Therefore, the values of hardness and elastic modulus refer to the average and the given uncertainties to the standarddeviations on the average. The variation of hardness and Young modulus versus indentation depth shows the influence of the silicon substrate. In other words, the deformation field is not confined within the film thickness. Thus, intrinsic values for the film can be only estimated at low loading, i.e. for a depth of at the most the tenth of the total thickness of the film. Finally, two trends can be described: (i) the elastic modulus seems insensitive to the stress field level. The expected value of the elastic modulus of the Fe0:55 Zr0:45 alloy can be determined using a simple rule-of-mixing on the Young moduli of the elemental crystalline materials, and taking into account the usually observed decrease ()25%) when the material is amorphous [20]. The experimental value of the Young modulus, 125 GPa, is in good agreement with this model; (ii) the hardness of the thin film under a compressive stress is significantly increased (10%), compared to that of the stressfree film. Thus, these results give clear indication of an apparent hardening of the amorphous film under biaxial compressive stress while in turn re-
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laxation promotes yielding and reduces the hardness. It is however worth mentioning that this sensitivity to a stress of a few tenths of GPa must be compared to the yield strength ry of the amorphous metallic material that may be deduced from the measured hardness H through the approximate relation H 3ry , that is ry 3 GPa. To summarise, this study of the material response to nano-indentation proves that stress appreciably influences the yield stress required for appearance of plastic deformation. That information yields the picture that the surface damage would be mainly achieved by a propagating ‘‘pressure pulse’’. Therefore, ion induced near-surface plastic deformation would involve a coherent atomic motion leading to a permanent decrease of the atomic density at the centre of the impact. In a recent study [21], a significant increase of the chemical etchability of tracks of 2.4 GeV Pb in amorphous metallic samples has been observed when a compressive in-plane stress was introduced locally. This was interpreted as the result of the variation of the chemical potential with the elastic energy stored in the ion track under the thermal expansion of the transient hot phase produced during the thermal spike. This explanation can be founded for impact events induced by GeV monoatomic ions, nevertheless if a high energy density is initially deposited within a cylindrical region, recent calculations by molecular dynamics predict that a significant part of energy may also be transported in the solid matrix by an unsteady shockwave [22]. Concluding, the overall picture outlined from our observations suggests that surface damage induced by single MeV C60 ions is the signature of plastic deformations induced at large distances by an energetic radial pressure pulse: an unsteady shock wave emanating from the sudden and transient heating of a localised region along the ion path. It is worth to note that a compressive stress as low as 0.4 GPa is enough to influence appreciably the mechanical response of the amorphous Fe0:55 Zr0:45 matrix following the prompt energy deposition in the ion wake. This suggests that transient induced stresses would be in the GPa range. The anisotropic growth phenomenon observed particularly in amorphous materials
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[23–26] – interpreted with a visco-elastic model based on the relaxation of shear stresses [27] – could also derive from this damage mechanism. The possible occurrence of some additional relaxation mechanisms leading to surface damage has also to be considered. Indeed, the flattening in the middle of the central shallow hole and the more complex structure observed with 40 MeV C60 ion irradiation possibly hint to a relaxation by a viscous axial outflow arising from the very hot region generated in the core of the ion track. Such an effect has been shown for near-surface cascades induced by keV recoil atoms first by means of molecular dynamic calculations [28], then experimentally using in situ electron microscopy [29]. In so far, one can stress the striking resemblance of this emergence with the sub-structure observed in the central part of some large impacts induced by meteorites. More detailed investigations are clearly required to confirm this feature and to determine the real underlying mechanism. Acknowledgements The authors would like to thank the technical staff of the tandem accelerator at IPN-Orsay for providing the fullerene beam and for their efficient collaboration. C.J., A.M. and C.T. (LMP-Poitiers) gratefully acknowledge the financial support of the Ministere de la Recherche through an ‘‘Action Concertee Incitative’’: Surfaces, Interfaces and Conception of New Materials for the development of the in situ stress measurement set-up. References [1] For a review, see for example: R. Spohr, Ion Tracks and Microtechnology, Vieweg, Braunschweig, 1990; M. Toulemonde, F. Studer, Solid State Phenom. 30/31 (1993) 477; A. Barbu, H. Dammak, A. Dunlop, D. Lesueur, MRS Bull. Dec. (1995) 29. [2] S. Della-Negra, A. Brunelle, Y. Le Beyec, J.M. Cureaudeau, J.P. Mouffron, B. Waast, P. H akanson, B.U.R. Sunqvist, E. Parilis, Nucl. Instr. and Meth. B 74 (1993) 453. [3] H. Dammak, A. Dunlop, D. Lesueur, A. Brunelle, S. Della-Negra, Y. Le Beyec, Phys. Rev. Lett. 74 (1995) 1135.
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