- Email: [email protected]
C60 multilayers
Nuclear Instruments and Methods in Physics Research B 219–220 (2004) 815–819 www.elsevier.com/locate/nimb
Swift heavy ion induced modification of Si/C60 multilayers S.K. Srivastava a
a,*
, D. Kabiraj b, B. Schattat c, H.D. Carstanjen a, D.K. Avasthi
b
Max-Planck-Institut f€ur Metallforschung, Heisenbergstr. 3, Baden-Wurttemberg, 70569 Stuttgart, Germany b Nuclear Science Centre, Aruna Asaf Ali Marg, PO Box 10502, New Delhi, 110067, India c Institut f€ur Strahlenphysik, Universit€at Stuttgart, Allmandring 3, D-70569 Stuttgart, Germany
Abstract Modifications induced by 120 and 350 MeV Au ions at the interfaces of a Si/C60 multilayer system have been investigated. High resolution Rutherford backscattering spectrometry using 2 MeV Nþ ions and detecting N3þ backscatters from Si was performed for characterization. A significant smearing out of the peaks as a consequence of mixing of Si and C atoms across the interfaces is observed, which increases noticeably with depth. The amount of mixing for 350 MeV ions is found to be higher than that for 120 MeV ions and is, thus, dependent on the electronic energy deposited in the samples. The mixing has been inferred to be a result of interdiffusion in a transient molten state. 2004 Elsevier B.V. All rights reserved. Keywords: SHI; Thermal spike; SiC; HRBS
1. Introduction Currently, there is an increasing drive to reduce the synthesis temperature of thin SiC films, which have alluring superior properties, mainly their wide band gap semi-conductivity and excellent thermal, chemical and corrosional stability. SiC thin films are being grown by epitaxy [1], chemical vapour deposition (CVD) [2], laser ablation [3], ion implantation [4,5] and ion beam mixing [6]. All these processes require a high temperature (>1000 C), which restricts their application to device fabrication processes capable of withstanding such temperatures. Apart from the stable SiC phase,
*
Corresponding author. Tel.: +49-711-689-1853; fax: +49711-689-1932. E-mail address: [email protected] (S.K. Srivastava).
Si1x Cx random alloys are also very appealing to device designers, because there is a conduction band offset between Si and Si1x Cx , making it ideal for semiconductor electronics [7]. The CVD technique employed to prepare these alloys, however, also requires higher temperatures. A solution approaching the formation of Si–C alloys at room temperature (RT) can be sought in the application of ion beam mixing to a Si/C-allotrope multilayer. RT ion beam mixing of Si/C multilayers by low energy ions (e.g. 400 keV Xe), where the nuclear stopping Sn is dominant, has been reported by Harbsmeier et al. [8]. The mixing in this case is governed by the kinetic processes of collision cascades and recoil implantation. In the light of the requirement of very high Si–C reaction temperatures, however, swift heavy ions (SHI) would be more interesting as the production of high temperature conditions (thermal spikes) is a well
0168-583X/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2004.01.169
816
S.K. Srivastava et al. / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 815–819
established SHI–matter interaction mechanism. One of our recent publications [9] is the first report of SHI induced mixing of Si/C multilayers in which the formation of oriented b-SiC(1 1 1) has been shown by 120 MeV Au irradiation at RT. Recently, carbonization of C60 on Si substrates has been adopted as a technique to produce epitaxial b-SiC films at temperatures ranging from 800 to 1000 C [10–12]. The mechanism involved is the dissociation, and subsequent reaction with Si, of C60 molecules at these temperatures. Swift heavy ions are known to produce multi-fragmentation of C60 molecules [13–15] apart from the creation of high temperature conditions. Therefore, it is interesting to explore the possibility of SHI induced reactions between Si and C atoms at Si/C60 interfaces. In this contribution, we present the results of 120 and 350 MeV Au ion induced interface modifications at RT in a Si/C60 multilayer system. In order to observe even very small changes at the interfaces, we use high resolution Rutherford backscattering spectrometry (HRBS) as the main tool of characterization.
2. Experiment The [Si/C60 ] · 5 multilayer sample was deposited on a Si(1 0 0) substrate by sequential evaporation of Si (e-beam evaporation) and C60 (resistive heating) in a cryo-pumped ultra-high vacuum deposition system. The substrate was kept at RT during multilayer deposition. The deposition was made at a pressure of 2 · 108 Torr and at a rate The samples were ex situ placed in the of 0.3 A/s. irradiation chamber. RT irradiation at a fluence of 1 · 1014 ions/cm2 by 120 MeV Au ions was performed at NSC, New Delhi, while that by 350 MeV Au ions was performed at HMI, Berlin. The electronic (nuclear) energy losses Se (Sn ) as calculated by TRIM [16] for the two ion energies are 13.3 (0.2) keV/nm and 19.0 (0.1) keV/nm, respectively. The HRBS measurements were performed at the Pelletron Accelerator Laboratory of the MPI-MF, Stuttgart, using the electrostatic spectrometer available there. 2 MeV Nþ ions were incident at an angle of 11 and N3þ ions backscattered from Si were detected at a scattering
angle of 38 by a micro-channel plate detector with an energy resolution of 1 keV. For discrimination between backscattered and recoiled particles and between different charge states, a time of flight technique was used for the data collection by taking the start and stop signals from the detector and a chopper before the scattering chamber, respectively. Atomic force microscopy (AFM) was also performed at MPI-MF in order to see the changes in surface roughness after irradiation. The HRBS spectra were fitted with the code SIMNRA [17] incorporating the surface roughness measured by AFM.
3. Results and discussion Fig. 1 shows the HRBS spectra and SIMNRA fits for the pristine, 120 and 350 MeV irradiated samples. The spectrum of the pristine sample exhibits well separated peaks, which shows the good quality of the multilayer. After irradiation, a significant smearing out of these peaks is observed for both ion energies, indicating a change in surface roughness and/or intermixing of Si and C atoms at the interfaces. From AFM measurements, the rms surface roughnesses for the three respectively. These samples are 3.3, 5.8 and 4.8 A,
Fig. 1. HRBS spectra of pristine and 1 · 1014 ions/cm2 120 and 350 MeV Au irradiated samples. The experimental data are shown by symbols + broken lines. The continuous lines are the corresponding SIMNRA fits. The base lines of the three spectra are shifted along the y-axis for better clarity.
S.K. Srivastava et al. / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 815–819
values have been taken as inputs to the SIMNRA fit in order to quantify the pristine multilayer structure and the amounts of intermixing after irradiations. For the fitting, different layers of fixed atomic ratios of Si, C and O are taken and the thicknesses and compositions are allowed to vary. The fitted layered structures are then plotted as step function depth profiles for the atomic fractions of Si and O in Fig. 2. The following conclusions can readily be drawn from the figure: (i) there is an apparently small amount of Si–C intermixing in the as-deposited sample, (ii) irradiations induce considerable intermixing which increases with depth and (iii) the amount of intermixing is higher for higher Se . The apparent intermixing in the pristine sample arises probably from the interface roughness introduced during deposition. The presence of oxygen (which normally hampers intermixing) at the sample surface and at the sample/substrate interface (see Fig. 2) is not of much concern in our case as we are interested in the modifications of interfaces. The observation of a significant amount of interdiffusion after irradiation can be understood on the following basis. It is known that chemi-
1.0
Atomic fraction
0.8 Si pristine
0.6
Si irr. 120 MeV Si irr. 350 MeV O pristine
0.4
O irr. 120 MeV O irr. 350 MeV
0.2 0.0 0
100
200
300
400
500
Depth (x1015 atoms/cm2)
Fig. 2. Step-function depth profiles of concentration for pristine and 1 · 1014 ions/cm2 120 and 350 MeV Au irradiated samples plotted according to the fit values from the SIMNRA simulations. The line curves show the Si concentration profiles, while the curves with lines + symbols show the oxygen concentration profiles. It is evident that oxygen stays at the sample surface and sample/substrate interface.
817
sorption of C60 molecules by a Si surface starts at 600 C forming Si–C covalent bonds [18]. The formation of the SiC phase sets in at even higher temperatures (700 C) when the C60 molecules start decomposing [12,19]. After the formation of an interfacial layer of SiC, it is the diffusion of either Si or fragmented C through this layer which promotes the reaction. Thus, at a given time, as long as not all the Si and C atoms have been consumed by SiC formation, one should obtain interdiffusion-like depth profiles of the Si and C concentrations, if the temperature is sufficiently high. For a multilayer stack with very thin layers, the diffusion profiles may be approximated as shown in Fig. 2, i.e. by step function depth profiles. The observation of SHI induced interdiffusion, therefore, suggests that there are high temperature conditions established in the material as a result of irradiation. It is also evident from the grazing angle X-ray diffraction results which show the formation of 4H–SiC phases after irradiation, as the formation of this phase requires high temperatures. This is well established by the thermal spike model of defect creation in materials by swift heavy ions. According to this model, the deposited electronic energy is transferred to the lattice atoms via electron–phonon coupling. It creates a transient (picosecond) high temperature (1000 C) condition in a narrow (nanometer) cylindrical region around the ion path. Lotha et al. [14] report the formation of such SHI induced tracks composed of fragments of C60 , and that the track diameter d for Se 11 keV/nm is 12 nm. For the Se values in the present experiment, such tracks will clearly be formed with even larger radii. Since the resistivity of C60 (1010 X m) is close to the resistivities of oxide materials, the rate of heat dissipation, and hence the duration of the existence of the high temperature phase ts of these tracks, will be similar to that in oxide materials, which is 10–100 ps [20]. Similar transient high temperature conditions exist also in the Si layers. We postulate that the observed interdiffusion takes place in these transient ion tracks. SHI induced diffusion in the transient melt phase has been shown earlier [20,21]. Overlapping of these tracks leads to mixing covering the whole sample area.
818
S.K. Srivastava et al. / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 815–819
Let us now examine whether the high temperature track in C60 also attains a molten state. Obviously, the track temperatures must exceed 700 C to allow interdiffusion, which has experimentally been observed. Since the C60 layers are sandwiched between Si layers, they do not sublimate at the sublimation temperature of 430 C. Instead, this results in the development of a (vapour) pressure in the C60 layers. It has been conjectured by Aschroft [22] that a modest application of pressure to C60 gives rise to a liquid phase at elevated temperatures of 1250 C [23]. Such temperatures are attainable in a SHI track [24] and a transient molten state of the track can be assumed to exist. In addition, the gradients of the acquired electronic and lattice energies across a Si/C60 interface lead to energy exchange across the interface and may result in the molten state of Si in the interfacial region [25]. According to the fittings for 120 MeV Au irradiation (see Fig. 2), 3.5 · 1015 Si atoms/cm2 have, on the average, diffused into the C60 layer, which is equivalent to a The diffusion coeffidiffusion length r of 7 A. cient D of Si in C60 can thus be estimated (by r2 ¼ 2Dd 2 /ts ) to be 108 –109 m2 /s under these conditions. This is of the order of liquid state diffusivities, thereby confirming the existence of the transient molten state of the SHI induced tracks. This also leads to the inference that the observed interdiffusion has taken place in the transient molten state of the tracks. The increasing amount of intermixing towards deeper layers can perhaps be explained in the following way. In the first stage of the distribution of the SHI deposited electronic energy in the electronic subsystem, about two-third of it is used up in the production of convoy and d-electrons with average velocities equal to and twice that of the ion energy, respectively. There is a finite probability of these electrons to escape through the surface. This probability decreases with the depth of generation of these electrons. Moreover, the angular distribution of the number of these electrons peaks in the forward (beam) direction [26]. In effect, there is a successively increasing number of the energetic electrons confined around the ion path as one goes deeper in the sample. This results in successively higher track temperatures, leading to an increasing
amount of intermixing, with depth. A detailed study of the increase in mixing with depth, however, needs to be pursued. The reason for the higher amount of intermixing for 350 MeV Au irradiation than that for 120 MeV Au irradiation is simple. Since Se is higher in the former case, the molten track radius, its duration, and its temperature will be higher. So, for a particular fluence, the diffusion time and the temperature of diffusion will be higher, leading to longer diffusion lengths. 4. Conclusion We have shown that RT 120 and 350 MeV Au ion irradiations at 1 · 1014 ions/cm2 fluence induce considerable amount of intermixing at Si/C60 interface. The amount of mixing has been shown to increase with depth and Se . The requirement of high temperature conditions for this to take place proves the applicability of the thermal spike model as the responsible SHI–matter interaction mechanism. The very high experimental diffusivity (109 m2 /s) establishes that the mixing is due to interdiffusion in transient melt phase.
Acknowledgements We acknowledge the consent of Dr. S. Klaum€ unzer, HMI, Berlin for 350 MeV Au irradiation and Dr. U. T€affner and Dr. Hannes-P. Lamparter of the MPI-MF, Stuttgart to perform the AFM and grazing angle X-ray diffraction measurements, respectively.
References [1] A. Fissel, B. Schroter, W. Richter, Appl. Phys. Lett. 66 (1995) 3182. [2] O. Kordina, A. Henry, J.P. Bergman, N.T. Son, W.M. Chen, C. Hallin, E. Janzen, Appl. Phys. Lett. 66 (1995) 1373. [3] S. Boily, M. Chaker, H. Pepin, T. Kerdja, J. Voyer, A. Jean, J.C. Kieffer, P. Lieung, F. Cerrina, G. Wells, J. Vac. Sci. Technol. B 9 (1991) 3254. [4] D. Chen, W.Y. Cheung, S.P. Wong, Nucl. Instr. and Meth. B 148 (1999) 589.
S.K. Srivastava et al. / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 815–819 Barna, [5] V. Heera, F. Fontaine, W. Skorupa, B. Pecz, A. Appl. Phys. Lett. 77 (2000) 226. [6] J.P. Riviere, M. Zaytouni, M.F. Denanot, J. Allain, Mater. Sci. Eng. B 29 (1995) 105. [7] M.S. Carroll, Ph.D. Thesis, Princeton University, June 2001. [8] F. Harbsmeier, W. Bolse, A.-M. Flank, Nucl. Instr. and Meth. B 166–167 (2000) 385. [9] K. Asokan, S.K. Srivastava, D. Kabiraj, S. Mookerjee, D.K. Avasthi, J.C. Jan, J.W. Chiou, W.F. Pong, L.C. Ting, F.Z. Chien, Nucl. Instr. and Meth. B 193 (2002) 324. [10] A.V. Hamza, M. Balooch, M. Moalem, Surf. Sci. 317 (1994) L1129. [11] K. Sakamoto, T. Suzuki, M. Harada, T. Wakita, S. Suto, A. Kasuya, Phys. Rev. B 57 (1998) 9003. [12] L. Moro, A. Paul, D.C. Lorents, R. Malhotra, R.S. Ruoff, P. Lazzeri, L. Vanzetti, A. Lui, S. Subramoney, J. Appl. Phys. 81 (1997) 6141. [13] Ch. Dufour, E. Paumier, M. Toulemonde, Nucl. Instr. and Meth. B 122 (1997) 445. [14] S. Lotha, A. Ingale, D.K. Avasthi, V.K. Mittal, S. Mishra, K.C. Rustagi, A. Gupta, V.N. Kulkarni, D.T. Khathing, Solid State Commun. 111 (1999) 55.
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[15] A. Itoh, T. Majima, H. Tsuchida, Y. Nakai, T. Kambara, Phys. Scr. T 92 (2001) 176. [16] J.P. Biersack, Nucl. Instr. and Meth. B 27 (1987) 21. [17] J. Mayer, ‘‘SIMNRA’’ User guide, Technical Report IPP 9/113, MPI f€ ur Plasmaphysik, Garching, 1997. [18] D. Chen, D. Sarid, Phys. Rev. B 49 (1994) 7612. [19] D. Chen, R. Workman, D. Sarid, Surf. Sci. 344 (1995) 23. [20] S. Kraft, B. Schattat, W. Bolse, S. Klaum€ unzer, F. Harbsmeier, A. Kulinska, A. L€ offl, J. Appl. Phys. 91 (2002) 1129. [21] D.K. Avasthi, W. Assmann, H. Nolte, H.D. Mieskes, S. Ghosh, N.C. Mishra, Nucl. Instr. and Meth. B 166–167 (2000) 345. [22] N.W. Aschroft, Europhys. Lett. 16 (1991) 355. [23] J.Q. Broughton, J.V. Lill, J.K. Johnson, Phys. Rev. B 55 (1997) 2808. [24] Z.G. Wang, C. Dufour, E. Paumier, M. Toulemonde, J. Phys.: Condens. Matter 6 (1994) 6733. [25] Z.G. Wang et al., Private communications, SHIM2002, 22– 25 May 2002, Catania, Italy. [26] U. Bechthold, S. Hagmann, J. Ullrich, B. Bathelt, G. Kraft, H.S. B€ ocking, Nucl. Instr. and Meth. B 143 (1998) 441.
Swift heavy ion induced modification of Si/C60 multilayers S.K. Srivastava a
a,*
, D. Kabiraj b, B. Schattat c, H.D. Carstanjen a, D.K. Avasthi
b
Max-Planck-Institut f€ur Metallforschung, Heisenbergstr. 3, Baden-Wurttemberg, 70569 Stuttgart, Germany b Nuclear Science Centre, Aruna Asaf Ali Marg, PO Box 10502, New Delhi, 110067, India c Institut f€ur Strahlenphysik, Universit€at Stuttgart, Allmandring 3, D-70569 Stuttgart, Germany
Abstract Modifications induced by 120 and 350 MeV Au ions at the interfaces of a Si/C60 multilayer system have been investigated. High resolution Rutherford backscattering spectrometry using 2 MeV Nþ ions and detecting N3þ backscatters from Si was performed for characterization. A significant smearing out of the peaks as a consequence of mixing of Si and C atoms across the interfaces is observed, which increases noticeably with depth. The amount of mixing for 350 MeV ions is found to be higher than that for 120 MeV ions and is, thus, dependent on the electronic energy deposited in the samples. The mixing has been inferred to be a result of interdiffusion in a transient molten state. 2004 Elsevier B.V. All rights reserved. Keywords: SHI; Thermal spike; SiC; HRBS
1. Introduction Currently, there is an increasing drive to reduce the synthesis temperature of thin SiC films, which have alluring superior properties, mainly their wide band gap semi-conductivity and excellent thermal, chemical and corrosional stability. SiC thin films are being grown by epitaxy [1], chemical vapour deposition (CVD) [2], laser ablation [3], ion implantation [4,5] and ion beam mixing [6]. All these processes require a high temperature (>1000 C), which restricts their application to device fabrication processes capable of withstanding such temperatures. Apart from the stable SiC phase,
*
Corresponding author. Tel.: +49-711-689-1853; fax: +49711-689-1932. E-mail address: [email protected] (S.K. Srivastava).
Si1x Cx random alloys are also very appealing to device designers, because there is a conduction band offset between Si and Si1x Cx , making it ideal for semiconductor electronics [7]. The CVD technique employed to prepare these alloys, however, also requires higher temperatures. A solution approaching the formation of Si–C alloys at room temperature (RT) can be sought in the application of ion beam mixing to a Si/C-allotrope multilayer. RT ion beam mixing of Si/C multilayers by low energy ions (e.g. 400 keV Xe), where the nuclear stopping Sn is dominant, has been reported by Harbsmeier et al. [8]. The mixing in this case is governed by the kinetic processes of collision cascades and recoil implantation. In the light of the requirement of very high Si–C reaction temperatures, however, swift heavy ions (SHI) would be more interesting as the production of high temperature conditions (thermal spikes) is a well
0168-583X/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2004.01.169
816
S.K. Srivastava et al. / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 815–819
established SHI–matter interaction mechanism. One of our recent publications [9] is the first report of SHI induced mixing of Si/C multilayers in which the formation of oriented b-SiC(1 1 1) has been shown by 120 MeV Au irradiation at RT. Recently, carbonization of C60 on Si substrates has been adopted as a technique to produce epitaxial b-SiC films at temperatures ranging from 800 to 1000 C [10–12]. The mechanism involved is the dissociation, and subsequent reaction with Si, of C60 molecules at these temperatures. Swift heavy ions are known to produce multi-fragmentation of C60 molecules [13–15] apart from the creation of high temperature conditions. Therefore, it is interesting to explore the possibility of SHI induced reactions between Si and C atoms at Si/C60 interfaces. In this contribution, we present the results of 120 and 350 MeV Au ion induced interface modifications at RT in a Si/C60 multilayer system. In order to observe even very small changes at the interfaces, we use high resolution Rutherford backscattering spectrometry (HRBS) as the main tool of characterization.
2. Experiment The [Si/C60 ] · 5 multilayer sample was deposited on a Si(1 0 0) substrate by sequential evaporation of Si (e-beam evaporation) and C60 (resistive heating) in a cryo-pumped ultra-high vacuum deposition system. The substrate was kept at RT during multilayer deposition. The deposition was made at a pressure of 2 · 108 Torr and at a rate The samples were ex situ placed in the of 0.3 A/s. irradiation chamber. RT irradiation at a fluence of 1 · 1014 ions/cm2 by 120 MeV Au ions was performed at NSC, New Delhi, while that by 350 MeV Au ions was performed at HMI, Berlin. The electronic (nuclear) energy losses Se (Sn ) as calculated by TRIM [16] for the two ion energies are 13.3 (0.2) keV/nm and 19.0 (0.1) keV/nm, respectively. The HRBS measurements were performed at the Pelletron Accelerator Laboratory of the MPI-MF, Stuttgart, using the electrostatic spectrometer available there. 2 MeV Nþ ions were incident at an angle of 11 and N3þ ions backscattered from Si were detected at a scattering
angle of 38 by a micro-channel plate detector with an energy resolution of 1 keV. For discrimination between backscattered and recoiled particles and between different charge states, a time of flight technique was used for the data collection by taking the start and stop signals from the detector and a chopper before the scattering chamber, respectively. Atomic force microscopy (AFM) was also performed at MPI-MF in order to see the changes in surface roughness after irradiation. The HRBS spectra were fitted with the code SIMNRA [17] incorporating the surface roughness measured by AFM.
3. Results and discussion Fig. 1 shows the HRBS spectra and SIMNRA fits for the pristine, 120 and 350 MeV irradiated samples. The spectrum of the pristine sample exhibits well separated peaks, which shows the good quality of the multilayer. After irradiation, a significant smearing out of these peaks is observed for both ion energies, indicating a change in surface roughness and/or intermixing of Si and C atoms at the interfaces. From AFM measurements, the rms surface roughnesses for the three respectively. These samples are 3.3, 5.8 and 4.8 A,
Fig. 1. HRBS spectra of pristine and 1 · 1014 ions/cm2 120 and 350 MeV Au irradiated samples. The experimental data are shown by symbols + broken lines. The continuous lines are the corresponding SIMNRA fits. The base lines of the three spectra are shifted along the y-axis for better clarity.
S.K. Srivastava et al. / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 815–819
values have been taken as inputs to the SIMNRA fit in order to quantify the pristine multilayer structure and the amounts of intermixing after irradiations. For the fitting, different layers of fixed atomic ratios of Si, C and O are taken and the thicknesses and compositions are allowed to vary. The fitted layered structures are then plotted as step function depth profiles for the atomic fractions of Si and O in Fig. 2. The following conclusions can readily be drawn from the figure: (i) there is an apparently small amount of Si–C intermixing in the as-deposited sample, (ii) irradiations induce considerable intermixing which increases with depth and (iii) the amount of intermixing is higher for higher Se . The apparent intermixing in the pristine sample arises probably from the interface roughness introduced during deposition. The presence of oxygen (which normally hampers intermixing) at the sample surface and at the sample/substrate interface (see Fig. 2) is not of much concern in our case as we are interested in the modifications of interfaces. The observation of a significant amount of interdiffusion after irradiation can be understood on the following basis. It is known that chemi-
1.0
Atomic fraction
0.8 Si pristine
0.6
Si irr. 120 MeV Si irr. 350 MeV O pristine
0.4
O irr. 120 MeV O irr. 350 MeV
0.2 0.0 0
100
200
300
400
500
Depth (x1015 atoms/cm2)
Fig. 2. Step-function depth profiles of concentration for pristine and 1 · 1014 ions/cm2 120 and 350 MeV Au irradiated samples plotted according to the fit values from the SIMNRA simulations. The line curves show the Si concentration profiles, while the curves with lines + symbols show the oxygen concentration profiles. It is evident that oxygen stays at the sample surface and sample/substrate interface.
817
sorption of C60 molecules by a Si surface starts at 600 C forming Si–C covalent bonds [18]. The formation of the SiC phase sets in at even higher temperatures (700 C) when the C60 molecules start decomposing [12,19]. After the formation of an interfacial layer of SiC, it is the diffusion of either Si or fragmented C through this layer which promotes the reaction. Thus, at a given time, as long as not all the Si and C atoms have been consumed by SiC formation, one should obtain interdiffusion-like depth profiles of the Si and C concentrations, if the temperature is sufficiently high. For a multilayer stack with very thin layers, the diffusion profiles may be approximated as shown in Fig. 2, i.e. by step function depth profiles. The observation of SHI induced interdiffusion, therefore, suggests that there are high temperature conditions established in the material as a result of irradiation. It is also evident from the grazing angle X-ray diffraction results which show the formation of 4H–SiC phases after irradiation, as the formation of this phase requires high temperatures. This is well established by the thermal spike model of defect creation in materials by swift heavy ions. According to this model, the deposited electronic energy is transferred to the lattice atoms via electron–phonon coupling. It creates a transient (picosecond) high temperature (1000 C) condition in a narrow (nanometer) cylindrical region around the ion path. Lotha et al. [14] report the formation of such SHI induced tracks composed of fragments of C60 , and that the track diameter d for Se 11 keV/nm is 12 nm. For the Se values in the present experiment, such tracks will clearly be formed with even larger radii. Since the resistivity of C60 (1010 X m) is close to the resistivities of oxide materials, the rate of heat dissipation, and hence the duration of the existence of the high temperature phase ts of these tracks, will be similar to that in oxide materials, which is 10–100 ps [20]. Similar transient high temperature conditions exist also in the Si layers. We postulate that the observed interdiffusion takes place in these transient ion tracks. SHI induced diffusion in the transient melt phase has been shown earlier [20,21]. Overlapping of these tracks leads to mixing covering the whole sample area.
818
S.K. Srivastava et al. / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 815–819
Let us now examine whether the high temperature track in C60 also attains a molten state. Obviously, the track temperatures must exceed 700 C to allow interdiffusion, which has experimentally been observed. Since the C60 layers are sandwiched between Si layers, they do not sublimate at the sublimation temperature of 430 C. Instead, this results in the development of a (vapour) pressure in the C60 layers. It has been conjectured by Aschroft [22] that a modest application of pressure to C60 gives rise to a liquid phase at elevated temperatures of 1250 C [23]. Such temperatures are attainable in a SHI track [24] and a transient molten state of the track can be assumed to exist. In addition, the gradients of the acquired electronic and lattice energies across a Si/C60 interface lead to energy exchange across the interface and may result in the molten state of Si in the interfacial region [25]. According to the fittings for 120 MeV Au irradiation (see Fig. 2), 3.5 · 1015 Si atoms/cm2 have, on the average, diffused into the C60 layer, which is equivalent to a The diffusion coeffidiffusion length r of 7 A. cient D of Si in C60 can thus be estimated (by r2 ¼ 2Dd 2 /ts ) to be 108 –109 m2 /s under these conditions. This is of the order of liquid state diffusivities, thereby confirming the existence of the transient molten state of the SHI induced tracks. This also leads to the inference that the observed interdiffusion has taken place in the transient molten state of the tracks. The increasing amount of intermixing towards deeper layers can perhaps be explained in the following way. In the first stage of the distribution of the SHI deposited electronic energy in the electronic subsystem, about two-third of it is used up in the production of convoy and d-electrons with average velocities equal to and twice that of the ion energy, respectively. There is a finite probability of these electrons to escape through the surface. This probability decreases with the depth of generation of these electrons. Moreover, the angular distribution of the number of these electrons peaks in the forward (beam) direction [26]. In effect, there is a successively increasing number of the energetic electrons confined around the ion path as one goes deeper in the sample. This results in successively higher track temperatures, leading to an increasing
amount of intermixing, with depth. A detailed study of the increase in mixing with depth, however, needs to be pursued. The reason for the higher amount of intermixing for 350 MeV Au irradiation than that for 120 MeV Au irradiation is simple. Since Se is higher in the former case, the molten track radius, its duration, and its temperature will be higher. So, for a particular fluence, the diffusion time and the temperature of diffusion will be higher, leading to longer diffusion lengths. 4. Conclusion We have shown that RT 120 and 350 MeV Au ion irradiations at 1 · 1014 ions/cm2 fluence induce considerable amount of intermixing at Si/C60 interface. The amount of mixing has been shown to increase with depth and Se . The requirement of high temperature conditions for this to take place proves the applicability of the thermal spike model as the responsible SHI–matter interaction mechanism. The very high experimental diffusivity (109 m2 /s) establishes that the mixing is due to interdiffusion in transient melt phase.
Acknowledgements We acknowledge the consent of Dr. S. Klaum€ unzer, HMI, Berlin for 350 MeV Au irradiation and Dr. U. T€affner and Dr. Hannes-P. Lamparter of the MPI-MF, Stuttgart to perform the AFM and grazing angle X-ray diffraction measurements, respectively.
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