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Latent tracks formation in silicon single crystals irradiated with fullerenes in the electronic regime
Nuclear Instruments and Methods in Physics Research B 146 (1998) 296±301
Latent tracks formation in silicon single crystals irradiated with fullerenes in the electronic regime B. Canut a
a,*
, N. Bonardi a, S.M.M. Ramos a, S. Della-Negra
b
D epartement de Physique des Mat eriaux (UMR CNRS 5586), Universit e Claude Bernard Lyon I, 69622 Villeurbanne C edex, France b Institut de Physique Nucl eaire, CNRS IN2P3, 91406 Orsay, France
Abstract Silicon targets of (1 0 0) orientation were prepared for transmission electron microscopy observations and then ir3 radiated with either 30 MeV C2 60 or 40 MeV C60 at normal incidence. All the irradiations were performed at room 9 temperature, up to ¯uences of a few 10 clusters cmÿ2 . The incident electronic stopping powers were 48 and 57 keV 3 nmÿ1 for C2 60 and C60 projectiles, respectively. High resolution observations at normal incidence evidenced amorphous zones of circular shape at each projectile impact. The track diameters near the target surface were 8.4 and 10.5 nm for irradiations at 30 and 40 MeV, respectively. This eect, which was never observed in silicon single crystals bombarded with swift heavy ions, was ascribed to the high density of electronic energy associated with the correlated electronic stopping of the cluster components. Observations at conventional resolution of samples tilted in the microscope allowed to follow the depth evolution of the entire tracks. The damage extends from the surface to a maximum depth L which depends on the incident energy of the clusters (L 160 nm and L 190 nm for irradiations at 30 and 40 MeV, respectively). This progressive extinction of the radiation-induced disorder was linked to the decorrelation process of the C60 ions during their slowing-down in the target. Ó 1998 Elsevier Science B.V. All rights reserved. Keywords: Irradiation; Cluster; Silicon; Latent track
1. Introduction During the last decade, many experiments showed that energy deposition through high electronic excitations can induce stable defects in many radiolysis-resistant materials. By using heavy ions (up to 238 U) accelerated up to a few tens of GeV, the response of a given target can be studied
* Corresponding author. Tel.: +33 4 72 43 12 18; fax: +33 4 78 89 74 10; e-mail: [email protected]
in a large electronic stopping power range (1 keV nmÿ1 < Se < 100 keV nmÿ1 ). This allowed systematical investigations of the Se -induced eects. For instance: (i) Latent tracks formation in insulating [1] and conducting [2] oxides, (ii) Anisotropic plastic deformation in amorphous metallic alloys [3], (iii) Defect creation in pure metals [4]. Despite the increasing number of materials found to be sensitive to collective electronic excitations induced by high energy heavy ion irradiation, an intriguing exception remains as regards semiconductors. In amorphous silicon and germanium
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 5 1 2 - 6
B. Canut et al. / Nucl. Instr. and Meth. in Phys. Res. B 146 (1998) 296±301
prepared by vacuum evaporation and subsequently irradiated with swift ions (from 100 MeV 16 O to 207 MeV 197 Au), Izui et al. [5] evidenced tracks which consist of small recrystallized particles. This eect occurs above a Se threshold dependent on the target (Seth 5 keV nmÿ1 in Ge and Seth 15 keV nmÿ1 in Si) and was interpreted in the framework of the thermal spike model [6]. Besides, silicon and germanium single crystals are up to now considered as insensitive to collective electronic excitations. The irradiations of these materials at the highest electronic stopping powers available on the high energy ion accelerators [7±10] did not evidence any defect creation via inelastic processes. However, recent works devoted to the irradiation eects of energetic clusters require to revisit the insensitivity of single crystalline semiconductors. As a matter of fact, the use as projectiles of clusters (like Aun and Cn ) accelerated to a total energy of a few tens of MeV allows to reach energy densities via electronic processes which exceed those obtained with ions [11]. In such irradiation conditions, new experimental results have been evidenced. For instance: (i) Non-linear sputtering eects in organic and inorganic ®lms [12]; (ii) Dissolution of alkaline precipitates embedded in MgO matrix [13]; (iii) Latent tracks registration in pure metals [14] and in sapphire [15]. In this paper, we present preliminary results concerning the lattice disorder induced in silicon single crystals by fullerene irradiations in the MeV range. Our purpose can be summarized by the following question: Is it possible to create amorphous latent tracks in silicon by means of very high con®nements of electronic energy?
297
2. Experimental procedure The starting material was a p-type boron-doped silicon wafer of (1 0 0) orientation purchased from Siltronix. The irradiated targets consist of small pieces (2 mm diameter), cut out from the original wafer, and prepared for TEM observations in plane view con®guration. These specimen were mechanically thinned down to about 50 lm and then ion milled using 5 keV argon bombardment of 1 mA beam current and a milling angle of 15°. The surface amorphization due to the milling does not extend above a few nanometers thickness. The targets were then irradiated at room temperature 3 with either 30 MeV C2 60 or 40 MeV C60 clusters provided by the tandem accelerator at Institut de Physique Nucleaire d'Orsay. The ¯ux was about 106 clusters cmÿ2 sÿ1 on a 7 mm2 irradiated surface. All the irradiations were performed at normal incidence up to ¯uences of a few 109 clusters cmÿ2 , in order to avoid any spatial overlap of the expected tracks. The main irradiation parameters are listed in Table 1 in conjunction with the data related to a swift heavy ion (1 GeV 238 U) which are given for comparison. The stopping powers of the clusters were deduced from TRIM97 code [16] calculations performed with either 500 or 667 keV monoatomic carbon. According to recent results evidenced by Baudin et al. [17], it has been assumed that the incident energy loss of the projectile is the sum of the energy losses of the individual carbon ions. In order to compare the radial extensions of the electronic energy deposited by the two kinds of projectiles (cluster or ion), we have added in Table 1 the maximum range rm of the d electrons emitted by the target. This parameter
Table 1 Main features of the cluster irradiations in Si: incident energy (E), relative velocity (b) of the projectile, projected range (Rp ), electronic (Se ) and nuclear (Sn ) stopping powers Projectile C60 C60 238 U
E (MeV) 30 40 1000
b (%)
Rp (lm)
Se (keV/nm)
Sn (keV/nm)
0.9 1.1 9.5
0.99 1.18 53.6
47.9 57.2 25.0
0.88 0.71 0.06
rm (nm) 1.9 2.7 1050
An (nm2 ) 0.14 0.11 0.01
rm is the maximum range of the d electrons and An is the cross-section for disordering the target surface via elastic processes (see text). The same data related to a swift ion (238 U) have been also listed for comparison.
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was calculated from the maximum energy transferred to the lattice electrons (Em 2me v2 , where me is the electron mass and v the projectile velocity) and by using a semi empirical range±energy relationship covering the 10±10 keV range [18]. The nuclear damage cross-sections An , listed in the last column of Table 1, correspond to elastic displacements induced per one incident projectile at the sample surface. They were calculated assuming a displacement energy of 21 eV [19] for silicon atoms. The irradiated samples were observed with a TOP CON 002B transmission microscope, operating at 200 kV and oering a resolution of 0.18 nm. The images were taken in bright ®eld, using two dierent con®gurations: (i) with the electron beam direction normal to the target surface and (ii) with the sample tilted 42° in the microscope. 3. Results and discussion The TEM micrographs presented in Fig. 1a and b exhibit arrays of contrasted zones which are normal to the sample surface (upper part of the ®gures). The mean distance D between two adjacent dark areas is about 100 nm, a value consistent with the irradiation ¯uence (U 1/D2 109 ±1010 cmÿ2 ). These features allow to conclude to the formation of a latent track at each projectile impact. The track diameters d remain constant in the ®rst 50 nm below the target surface and are of 8.4 3 and 10.5 nm for 30 MeV C2 60 and 40 MeV C60 irradiations, respectively. Assuming an axial symmetry, the straightforward relation Ad pd2 /4 allows to determine the damage cross-section Ad . The values obtained for this latter parameter are 3 55 and 87 nm2 for 30 MeV C2 60 and 40 MeV C60 irradiations, respectively. Such damage cross-sections are necessarily ascribable to the high density of electronic energy deposited by the clusters in the near surface of the target. As a matter of fact, the cross-sections An for displacement events via nuclear processes reported in Table 1 are by three orders of magnitude lower than Ad . In order to precise the type of defects created in the wake of the cluster, observations at high resolution were performed under normal incidence. An example of HRTEM image, recorded on a sample irradiated
Fig. 1. Bright ®eld images of (1 0 0) Si irradiated at 300 K with C60 projectiles at normal incidence. (a) 30 MeV incident energy. (b) 40 MeV incident energy. In both cases the sample is tilted 42° in the microscope.
B. Canut et al. / Nucl. Instr. and Meth. in Phys. Res. B 146 (1998) 296±301
at 30 MeV, is displayed in Fig. 2. The circular shape of the cylindrical tracks and the presence of an amorphous core are clearly evidenced. This amorphization of silicon has been con®rmed by a numerical Fourier transform of the direct image. It has to be noticed that the diameter of the track visualized at high resolution (5.7 nm) is signi®cantly lower than the diameter of 8.4 nm which was measured at conventional magni®cation. This discrepancy, which increases with the duration of the HRTEM observations [20], can be explained as follows: The observations at low magni®cation were performed on a sample area suciently thick (250 nm) to include the entire track. In these conditions, the latent track formation occurs in a medium which, like a bulk sample, may be regarded as semi-in®nite. As a consequence, the only available routes for dissipating the deposited energy are the radial directions inside the target. The observations at high resolution were performed on a very thin region of the specimen whose thickness (typically 20 nm) is lower than the range of the ``cluster zone'' (that is the depth range where the
Fig. 2. High resolution transmission electron micrograph of a sample irradiated with 30 MeV C2 60 . The amorphization of silicon via electronic processes is clearly evidenced.
299
radial density of energy is sucient to induce lattice disorder). A signi®cant amount of the deposited energy can thus relax longitudinally at both the entrance and the exit of the sample. This phenomenon, analoguous to an ``outdiusion process'', should lower the eciency of the damage creation in the radial directions and consequently should reduce the track diameter. The two damage cross-sections Ad mentioned above are plotted as a function of the incident electronic stopping power in Fig. 3. A linear ®tting of the data intercepts the horizontal axis at Seth 30 keV nmÿ1 . Within the experimental errors, this value may be regarded as the threshold for defect creation in silicon irradiated in the electronic regime with slow (b < 1%) projectiles. As a comparison, the threshold related to LiNbO3 [21,22] bombarded with MeV Cn clusters (4 6 n 6 60) in the same velocity range than in Si is only 3 keV nmÿ1 . According to a description proposed recently [22], the dierence between Seth (Si) and Seth (LiNbO3 ) could result mainly from the dierent thermal diusivities of these two materials. The results presented above allow to understand why no Se -induced defects can be observed in single crystalline silicon submitted to swift heavy ion bombardment. The maximum electronic stopping power attainable by a monoatomic projectile in Si does not exceed 25 keV nmÿ1
Fig. 3. Damage cross-sections, measured at the surface of silicon, versus the incident electronic stopping power (squares). The data related to LiNbO3 (circles) irradiated in similar conditions are also given for comparison. In both cases, the continuous straight lines are drawn to guide the eye.
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(irradiation with 238 U ions of 1 GeV energy; see Table 1). This value is lower than the threshold of 30 keV nmÿ1 mentioned previously. In addition, the higher velocity b of a GeV ion, compared to a MeV cluster, is responsible for a larger radial extension of the deposited energy. In the example given in Table 1, the maximum range rm of the d electrons emitted by a 238 U ion of 1 GeV exceeds the one corresponding to C60 irradiations at a few tens of MeV. These two characteristics (too low Se + too high b) of swift heavy ion bombardment in Si lead to densities of electronic energy which are not sucient enough to create lattice disorder in the target. The high eciency of MeV clusters to damage a sample via electronic processes is depth-limited. The mean lengths of the tracks evidenced in Fig. 1a and b are L 160 nm and L 190 nm for 30 3 MeV C2 60 and 40 MeV C60 irradiations, respectively. The decrease of the electronic stopping power versus depth cannot account for these two measurements. As a matter of fact, the Se values calculated from TRIM97 code at the end of the tracks are 39 and 48 keV nmÿ1 , for L 160 nm and L 190 nm, respectively. In both cases the remaining electronic stopping power exceeds the threshold for defect creation in silicon (Seth 30 keV nmÿ1 ). Actually, the extinction in depth of the radiation-induced disorder results from the fragmentation of the projectile during its slowingdown in the target. Two eects contribute to decorrelate the cluster components: (i) The scattering of the incident particles, due to the nuclear interactions at close impact parameter with host atoms and (ii) the Coulomb repulsion between the carbon ions, due to their eective charge at equilibrium state in the solid. As pointed out elsewhere [15,21], in the energy range used here (<1 MeV per carbon), the decorrelation of the projectile is mainly controlled by the nuclear scattering. This eect causes a radial spreading of the incident trajectories, characterized by a mean radial range Rr . From TRIM simulations, performed with single carbon ions of either 500 or 667 keV incident energies (corresponding to 30 and 40 MeV C60 clusters, respectively), one obtains Rr 7 nm in both cases. This mean radial range, signi®cantly higher than the range of the d electrons (see Table
1), is thus responsible for a large decrease of the radial density of energy near the track end. On the other hand, the depth dependence of Rr is much more pronounced than the depth dependence of Se (by denoting the depth x, Rr increases as x2 whereas Se decreases as x1 ). This con®rms that the radial density of deposited energy is a better parameter than the lineic energy loss for scaling the radiation-induced disorder in the electronic regime. 4. Conclusion In opposition to what has been usually believed, we have shown that single crystalline silicon is sensitive to collective electronic excitations. By using C60 clusters accelerated in the 10 MeV range, the formation of amorphous latent tracks was evidenced for the ®rst time in this material. This was ascribed to the very high density of electronic energy deposited by the correlated components of the projectile. The limited depth of the observed tracks is mainly due to the nuclear scattering of the cluster components with the target atoms. From the two experimental damage cross-sections (Ad 55 and 87 nm2 for 30 MeV C2 60 and 40 MeV C3 60 irradiations, respectively), the disorder creation in Si via electronic processes should occur above an electronic stopping power threshold estimated at 30 keV nmÿ1 . This high value, which is valid only at low projectile velociy (b 1%), allows to understand why no Se -induced defects can be evidenced in silicon single crystals irradiated with swift heavy ions. The extension of this study to other semiconductors, like Ge and GaAs, is at present underway.
References [1] F. Studer, C. Houpert, H. Pascard, R. Spohr, J. Vetter, J. Yun Fan, M. Toulemonde, Radiat. E. Def. Sol. 116 (1991) 59. [2] B. Roas, B. Hensel, S. Henke, S. Klaum unzer, B. Kabius, W. Watanabe, G. Saemann-Ischenko, L. Schultz, K. Urban, Europhys. Lett. 11 (1990) 669. [3] S. Klaum unzer, Ming-Dong Hou, G. Schumacher, Phys. Rev. Lett. 57 (1986) 850.
B. Canut et al. / Nucl. Instr. and Meth. in Phys. Res. B 146 (1998) 296±301 [4] A. Iwase, S. Sasaki, T. Iwata, T. Nihira, Phys. Rev. Lett. 58 (1987) 2450. [5] K. Izui, S. Furuno, in: T. Imura, S. Maruse and T. Suzuki (Ed.), Proceedings of the XIth International Congress on Electron Microscopy, Kyoto, 1986, The Japanese Society of Electron Microscopy, Tokyo, 1986, p. 1299. [6] M. Toulemonde, C. Dufour, E. Paumier, Phys. Rev. B 46 (1993) 14362. [7] M. Toulemonde, J. Dural, G. Nouet, P. Mary, J.F. Hamet, M.F. Beaufort, J.C. Desoyer, C. Blanchard, J. Auleytner, Phys. Stat. Sol. (a) 114 (1989) 467. [8] M. Levalois, P. Bogdanski, M. Toulemonde, Nucl. Instr. and Meth. B 63 (1992) 14. [9] P. Marie, M. Levalois, E. Paumier, J. Appl. Phys. 79 (10) (1996) 7555. [10] M. Mikou, R. Carin, P. Bogdanski, P. Marie, J. Phys. (France) III (1997) 1661. [11] S. Della Negra, A. Brunelle, Y. Le Beyec, J.M. Curaudeau, J.P. Mouron, B. Waast, P. H akansson, B.U.R. Sundqvist, E. Parilis, Nucl. Instr. and Meth. B 74 (1993) 453. [12] S. Della Negra, P. H akansson, Y. Le Beyec, Int. J. Mass. Spectr. Ion Proc. 73 (1994) 130. [13] M. Beranger, P. Thevenard, R. Brenier, B. Canut, S.M.M. Ramos, A. Brunelle, S. Della Negra, Y. Le Beyec,
[14] [15] [16] [17]
[18] [19] [20] [21] [22]
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E. Balanzat, T. Tombrello, Phys. Rev. B 53 (1996) 14773. H. Dammak, A. Dunlop, D. Lesueur, Phys. Rev. Lett. 74 (1995) 1135. S.M.M. Ramos, N. Bonardi, B. Canut, S. Della Negra, Phys. Rev. B 57 (1998) 189. J.F. Ziegler, J.P. Biersack, U. Littmark, in: Stopping Power and Ranges of Ions in Matter, vol. I, Pergamon Press, New York, 1985. K. Baudin, A. Brunelle, M. Chabot, S. Della Negra, J. Depauw, D. Gardes, P. Hakansson, Y. Le Beyec, A. Billebaud, M. Fallavier, J. Remillieux, J.C. Poizat, J.P. Thomas, Nucl. Instr. and Meth. B 94 (1994) 341. M.P.R. Waligorski, R.N. Hamm, R. Katz, Nucl. Tracks. Radiat. Meas. 11 (1986) 309. J. Bourgoin, M. Lannoo, in: Point Defects in Semiconductors, vol. I, Springer, Berlin, 1983. A. Dunlop, G. Jaskierowicz, S. Della Negra, these Proceedings (SHIM-98), Nucl. Instr. and Meth. B 146 (1998) 302. B. Canut, S. Ramos, N. Bonardi, J. Chaumont, H. Bernas, E. Cottereau, Nucl. Instr. and Meth. B 122 (1997) 335. B. Canut, S.M.M. Ramos, Radiat. E. Def. Sol. 145 (1998) 1.
Latent tracks formation in silicon single crystals irradiated with fullerenes in the electronic regime B. Canut a
a,*
, N. Bonardi a, S.M.M. Ramos a, S. Della-Negra
b
D epartement de Physique des Mat eriaux (UMR CNRS 5586), Universit e Claude Bernard Lyon I, 69622 Villeurbanne C edex, France b Institut de Physique Nucl eaire, CNRS IN2P3, 91406 Orsay, France
Abstract Silicon targets of (1 0 0) orientation were prepared for transmission electron microscopy observations and then ir3 radiated with either 30 MeV C2 60 or 40 MeV C60 at normal incidence. All the irradiations were performed at room 9 temperature, up to ¯uences of a few 10 clusters cmÿ2 . The incident electronic stopping powers were 48 and 57 keV 3 nmÿ1 for C2 60 and C60 projectiles, respectively. High resolution observations at normal incidence evidenced amorphous zones of circular shape at each projectile impact. The track diameters near the target surface were 8.4 and 10.5 nm for irradiations at 30 and 40 MeV, respectively. This eect, which was never observed in silicon single crystals bombarded with swift heavy ions, was ascribed to the high density of electronic energy associated with the correlated electronic stopping of the cluster components. Observations at conventional resolution of samples tilted in the microscope allowed to follow the depth evolution of the entire tracks. The damage extends from the surface to a maximum depth L which depends on the incident energy of the clusters (L 160 nm and L 190 nm for irradiations at 30 and 40 MeV, respectively). This progressive extinction of the radiation-induced disorder was linked to the decorrelation process of the C60 ions during their slowing-down in the target. Ó 1998 Elsevier Science B.V. All rights reserved. Keywords: Irradiation; Cluster; Silicon; Latent track
1. Introduction During the last decade, many experiments showed that energy deposition through high electronic excitations can induce stable defects in many radiolysis-resistant materials. By using heavy ions (up to 238 U) accelerated up to a few tens of GeV, the response of a given target can be studied
* Corresponding author. Tel.: +33 4 72 43 12 18; fax: +33 4 78 89 74 10; e-mail: [email protected]
in a large electronic stopping power range (1 keV nmÿ1 < Se < 100 keV nmÿ1 ). This allowed systematical investigations of the Se -induced eects. For instance: (i) Latent tracks formation in insulating [1] and conducting [2] oxides, (ii) Anisotropic plastic deformation in amorphous metallic alloys [3], (iii) Defect creation in pure metals [4]. Despite the increasing number of materials found to be sensitive to collective electronic excitations induced by high energy heavy ion irradiation, an intriguing exception remains as regards semiconductors. In amorphous silicon and germanium
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 5 1 2 - 6
B. Canut et al. / Nucl. Instr. and Meth. in Phys. Res. B 146 (1998) 296±301
prepared by vacuum evaporation and subsequently irradiated with swift ions (from 100 MeV 16 O to 207 MeV 197 Au), Izui et al. [5] evidenced tracks which consist of small recrystallized particles. This eect occurs above a Se threshold dependent on the target (Seth 5 keV nmÿ1 in Ge and Seth 15 keV nmÿ1 in Si) and was interpreted in the framework of the thermal spike model [6]. Besides, silicon and germanium single crystals are up to now considered as insensitive to collective electronic excitations. The irradiations of these materials at the highest electronic stopping powers available on the high energy ion accelerators [7±10] did not evidence any defect creation via inelastic processes. However, recent works devoted to the irradiation eects of energetic clusters require to revisit the insensitivity of single crystalline semiconductors. As a matter of fact, the use as projectiles of clusters (like Aun and Cn ) accelerated to a total energy of a few tens of MeV allows to reach energy densities via electronic processes which exceed those obtained with ions [11]. In such irradiation conditions, new experimental results have been evidenced. For instance: (i) Non-linear sputtering eects in organic and inorganic ®lms [12]; (ii) Dissolution of alkaline precipitates embedded in MgO matrix [13]; (iii) Latent tracks registration in pure metals [14] and in sapphire [15]. In this paper, we present preliminary results concerning the lattice disorder induced in silicon single crystals by fullerene irradiations in the MeV range. Our purpose can be summarized by the following question: Is it possible to create amorphous latent tracks in silicon by means of very high con®nements of electronic energy?
297
2. Experimental procedure The starting material was a p-type boron-doped silicon wafer of (1 0 0) orientation purchased from Siltronix. The irradiated targets consist of small pieces (2 mm diameter), cut out from the original wafer, and prepared for TEM observations in plane view con®guration. These specimen were mechanically thinned down to about 50 lm and then ion milled using 5 keV argon bombardment of 1 mA beam current and a milling angle of 15°. The surface amorphization due to the milling does not extend above a few nanometers thickness. The targets were then irradiated at room temperature 3 with either 30 MeV C2 60 or 40 MeV C60 clusters provided by the tandem accelerator at Institut de Physique Nucleaire d'Orsay. The ¯ux was about 106 clusters cmÿ2 sÿ1 on a 7 mm2 irradiated surface. All the irradiations were performed at normal incidence up to ¯uences of a few 109 clusters cmÿ2 , in order to avoid any spatial overlap of the expected tracks. The main irradiation parameters are listed in Table 1 in conjunction with the data related to a swift heavy ion (1 GeV 238 U) which are given for comparison. The stopping powers of the clusters were deduced from TRIM97 code [16] calculations performed with either 500 or 667 keV monoatomic carbon. According to recent results evidenced by Baudin et al. [17], it has been assumed that the incident energy loss of the projectile is the sum of the energy losses of the individual carbon ions. In order to compare the radial extensions of the electronic energy deposited by the two kinds of projectiles (cluster or ion), we have added in Table 1 the maximum range rm of the d electrons emitted by the target. This parameter
Table 1 Main features of the cluster irradiations in Si: incident energy (E), relative velocity (b) of the projectile, projected range (Rp ), electronic (Se ) and nuclear (Sn ) stopping powers Projectile C60 C60 238 U
E (MeV) 30 40 1000
b (%)
Rp (lm)
Se (keV/nm)
Sn (keV/nm)
0.9 1.1 9.5
0.99 1.18 53.6
47.9 57.2 25.0
0.88 0.71 0.06
rm (nm) 1.9 2.7 1050
An (nm2 ) 0.14 0.11 0.01
rm is the maximum range of the d electrons and An is the cross-section for disordering the target surface via elastic processes (see text). The same data related to a swift ion (238 U) have been also listed for comparison.
298
B. Canut et al. / Nucl. Instr. and Meth. in Phys. Res. B 146 (1998) 296±301
was calculated from the maximum energy transferred to the lattice electrons (Em 2me v2 , where me is the electron mass and v the projectile velocity) and by using a semi empirical range±energy relationship covering the 10±10 keV range [18]. The nuclear damage cross-sections An , listed in the last column of Table 1, correspond to elastic displacements induced per one incident projectile at the sample surface. They were calculated assuming a displacement energy of 21 eV [19] for silicon atoms. The irradiated samples were observed with a TOP CON 002B transmission microscope, operating at 200 kV and oering a resolution of 0.18 nm. The images were taken in bright ®eld, using two dierent con®gurations: (i) with the electron beam direction normal to the target surface and (ii) with the sample tilted 42° in the microscope. 3. Results and discussion The TEM micrographs presented in Fig. 1a and b exhibit arrays of contrasted zones which are normal to the sample surface (upper part of the ®gures). The mean distance D between two adjacent dark areas is about 100 nm, a value consistent with the irradiation ¯uence (U 1/D2 109 ±1010 cmÿ2 ). These features allow to conclude to the formation of a latent track at each projectile impact. The track diameters d remain constant in the ®rst 50 nm below the target surface and are of 8.4 3 and 10.5 nm for 30 MeV C2 60 and 40 MeV C60 irradiations, respectively. Assuming an axial symmetry, the straightforward relation Ad pd2 /4 allows to determine the damage cross-section Ad . The values obtained for this latter parameter are 3 55 and 87 nm2 for 30 MeV C2 60 and 40 MeV C60 irradiations, respectively. Such damage cross-sections are necessarily ascribable to the high density of electronic energy deposited by the clusters in the near surface of the target. As a matter of fact, the cross-sections An for displacement events via nuclear processes reported in Table 1 are by three orders of magnitude lower than Ad . In order to precise the type of defects created in the wake of the cluster, observations at high resolution were performed under normal incidence. An example of HRTEM image, recorded on a sample irradiated
Fig. 1. Bright ®eld images of (1 0 0) Si irradiated at 300 K with C60 projectiles at normal incidence. (a) 30 MeV incident energy. (b) 40 MeV incident energy. In both cases the sample is tilted 42° in the microscope.
B. Canut et al. / Nucl. Instr. and Meth. in Phys. Res. B 146 (1998) 296±301
at 30 MeV, is displayed in Fig. 2. The circular shape of the cylindrical tracks and the presence of an amorphous core are clearly evidenced. This amorphization of silicon has been con®rmed by a numerical Fourier transform of the direct image. It has to be noticed that the diameter of the track visualized at high resolution (5.7 nm) is signi®cantly lower than the diameter of 8.4 nm which was measured at conventional magni®cation. This discrepancy, which increases with the duration of the HRTEM observations [20], can be explained as follows: The observations at low magni®cation were performed on a sample area suciently thick (250 nm) to include the entire track. In these conditions, the latent track formation occurs in a medium which, like a bulk sample, may be regarded as semi-in®nite. As a consequence, the only available routes for dissipating the deposited energy are the radial directions inside the target. The observations at high resolution were performed on a very thin region of the specimen whose thickness (typically 20 nm) is lower than the range of the ``cluster zone'' (that is the depth range where the
Fig. 2. High resolution transmission electron micrograph of a sample irradiated with 30 MeV C2 60 . The amorphization of silicon via electronic processes is clearly evidenced.
299
radial density of energy is sucient to induce lattice disorder). A signi®cant amount of the deposited energy can thus relax longitudinally at both the entrance and the exit of the sample. This phenomenon, analoguous to an ``outdiusion process'', should lower the eciency of the damage creation in the radial directions and consequently should reduce the track diameter. The two damage cross-sections Ad mentioned above are plotted as a function of the incident electronic stopping power in Fig. 3. A linear ®tting of the data intercepts the horizontal axis at Seth 30 keV nmÿ1 . Within the experimental errors, this value may be regarded as the threshold for defect creation in silicon irradiated in the electronic regime with slow (b < 1%) projectiles. As a comparison, the threshold related to LiNbO3 [21,22] bombarded with MeV Cn clusters (4 6 n 6 60) in the same velocity range than in Si is only 3 keV nmÿ1 . According to a description proposed recently [22], the dierence between Seth (Si) and Seth (LiNbO3 ) could result mainly from the dierent thermal diusivities of these two materials. The results presented above allow to understand why no Se -induced defects can be observed in single crystalline silicon submitted to swift heavy ion bombardment. The maximum electronic stopping power attainable by a monoatomic projectile in Si does not exceed 25 keV nmÿ1
Fig. 3. Damage cross-sections, measured at the surface of silicon, versus the incident electronic stopping power (squares). The data related to LiNbO3 (circles) irradiated in similar conditions are also given for comparison. In both cases, the continuous straight lines are drawn to guide the eye.
300
B. Canut et al. / Nucl. Instr. and Meth. in Phys. Res. B 146 (1998) 296±301
(irradiation with 238 U ions of 1 GeV energy; see Table 1). This value is lower than the threshold of 30 keV nmÿ1 mentioned previously. In addition, the higher velocity b of a GeV ion, compared to a MeV cluster, is responsible for a larger radial extension of the deposited energy. In the example given in Table 1, the maximum range rm of the d electrons emitted by a 238 U ion of 1 GeV exceeds the one corresponding to C60 irradiations at a few tens of MeV. These two characteristics (too low Se + too high b) of swift heavy ion bombardment in Si lead to densities of electronic energy which are not sucient enough to create lattice disorder in the target. The high eciency of MeV clusters to damage a sample via electronic processes is depth-limited. The mean lengths of the tracks evidenced in Fig. 1a and b are L 160 nm and L 190 nm for 30 3 MeV C2 60 and 40 MeV C60 irradiations, respectively. The decrease of the electronic stopping power versus depth cannot account for these two measurements. As a matter of fact, the Se values calculated from TRIM97 code at the end of the tracks are 39 and 48 keV nmÿ1 , for L 160 nm and L 190 nm, respectively. In both cases the remaining electronic stopping power exceeds the threshold for defect creation in silicon (Seth 30 keV nmÿ1 ). Actually, the extinction in depth of the radiation-induced disorder results from the fragmentation of the projectile during its slowingdown in the target. Two eects contribute to decorrelate the cluster components: (i) The scattering of the incident particles, due to the nuclear interactions at close impact parameter with host atoms and (ii) the Coulomb repulsion between the carbon ions, due to their eective charge at equilibrium state in the solid. As pointed out elsewhere [15,21], in the energy range used here (<1 MeV per carbon), the decorrelation of the projectile is mainly controlled by the nuclear scattering. This eect causes a radial spreading of the incident trajectories, characterized by a mean radial range Rr . From TRIM simulations, performed with single carbon ions of either 500 or 667 keV incident energies (corresponding to 30 and 40 MeV C60 clusters, respectively), one obtains Rr 7 nm in both cases. This mean radial range, signi®cantly higher than the range of the d electrons (see Table
1), is thus responsible for a large decrease of the radial density of energy near the track end. On the other hand, the depth dependence of Rr is much more pronounced than the depth dependence of Se (by denoting the depth x, Rr increases as x2 whereas Se decreases as x1 ). This con®rms that the radial density of deposited energy is a better parameter than the lineic energy loss for scaling the radiation-induced disorder in the electronic regime. 4. Conclusion In opposition to what has been usually believed, we have shown that single crystalline silicon is sensitive to collective electronic excitations. By using C60 clusters accelerated in the 10 MeV range, the formation of amorphous latent tracks was evidenced for the ®rst time in this material. This was ascribed to the very high density of electronic energy deposited by the correlated components of the projectile. The limited depth of the observed tracks is mainly due to the nuclear scattering of the cluster components with the target atoms. From the two experimental damage cross-sections (Ad 55 and 87 nm2 for 30 MeV C2 60 and 40 MeV C3 60 irradiations, respectively), the disorder creation in Si via electronic processes should occur above an electronic stopping power threshold estimated at 30 keV nmÿ1 . This high value, which is valid only at low projectile velociy (b 1%), allows to understand why no Se -induced defects can be evidenced in silicon single crystals irradiated with swift heavy ions. The extension of this study to other semiconductors, like Ge and GaAs, is at present underway.
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