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Atomic mixing at metal–oxide interfaces by high energy heavy ions
Nuclear Instruments and Methods in Physics Research B 230 (2005) 234–239 www.elsevier.com/locate/nimb
Atomic mixing at metal–oxide interfaces by high energy heavy ions R. Nakatani a, R. Taniguchi a, Y. Chimi b, N. Ishikawa b, M. Fukuzumi a, Y. Kato a, H. Tsuchida c, N. Matsunami d, A. Iwase a,* a
b
Research Institute for Advanced Science and Technology, Osaka Prefecture University, Gakuen-cho, 1-2, Sakai-shi, Osaka 599-8570, Japan Japan Atomic Energy Research Institute (JAERI-Tokai), Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan c Kyoto University, Sakyo-ku, Kyoto-shi 606-8501, Japan d Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan Available online 22 January 2005
Abstract We study the atomic mixing at metal (Bi or Au)/oxide (SiO2 or Al2O3) interfaces under 150–200 MeV heavy ion irradiation. Irradiation-induced interface mixing state is examined by means of Rutherford backscattering spectrometry (RBS). For Bi/Al2O3 interfaces, the heavy ion irradiations induce a strong atomic mixing and the amount of the mixing increases with increasing the electronic stopping power for heavy ions. By comparing the results with that for 3 MeV Si ion irradiation, we conclude that the strong atomic mixing observed at Bi/Al2O3 interfaces is attributed to the high-density electronic excitation. On the other hand, for other interfaces (Bi/SiO2, Au/Al2O3 and Au/SiO2), atomic mixing is rarely observed after the irradiation. The dependence of atomic mixing on combinations of irradiating ions and interface-forming materials is discussed. 2004 Elsevier B.V. All rights reserved. PACS: 61.80.Jh; 61.82.Bg; 61.82.Ms; 81.40.Wx Keywords: Atomic mixing; Metal/oxide interfaces; Electronic energy deposition
1. Introduction Through a lot of studies concerning the effects of swift heavy ion irradiation on materials, it has *
Corresponding author. Tel.: +81 72 254 9810; fax: +81 72 251 6439. E-mail address: [email protected] (A. Iwase).
been well proven so far that the electronic energy deposition by swift heavy ions plays an important role in atomic displacements in target materials. Even in metallic materials, the effects have been observed as annihilation and production of point defects [1–4], phase transformation [5], modification of magnetic properties [6], precipitation of solute atoms [7,8] and so on.
0168-583X/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2004.12.047
2.72 0.035 2.09 1.60 0.061 3.39 0.94 0.13 3.95 1.51 0.063 2.55 5 · 1015 3 Si
28
Kr Xe 197 Au
150 200 200
1 · 1014 1 · 1014 1 · 1014
16.3 23.5 27.2
0.050 0.12 0.34
14.1 14.9 12.7
31.4 45.1 51.9
0.10 0.25 0.69
7.73 8.23 7.13
17.7 26.2 32.0
0.037 0.089 0.24
12.3 13.2 11.1
10.1 14.9 18.4
0.021 0.051 0.137
21.0 22.3 18.9
235
136
86
Rp (lm) Sn (keV/nm) Se (keV/nm) Se (keV/nm) Sn (keV/nm)
Rp (lm) Se (keV/nm)
Sn (keV/nm)
Rp (lm)
Se (keV/nm)
Sn (keV/nm)
Rp (lm)
SiO2 Al2O3 Au Bi
Target material
Fluence (cm 2)
Specimens for the present study are Bi/Al2O3, Bi/SiO2, Au/Al2O3 and Au/SiO2 systems. Bismuth or gold was evaporated on 10 · 10 · 0.3 mm3 single crystalline Al2O3 (sapphire) substrates or amorphous SiO2 (a-SiO2) substrates in a vacuum of 5 · 10 6 Torr. The thickness of each evaporated layer was about 100–200 nm. The specimens were irradiated with 150 MeV 86Kr, 200 MeV 136Xe or 200 MeV 197Au ions at room temperature up to the fluence of 1 · 1014/cm2 using a 20 MV tandem accelerator at JAERI-Tokai. For heavy ions in the energy range of 100–200 MeV, the value of the nuclear stopping power, Sn, is about two order smaller than that of the electronic stopping power, Se. This fact, however, does not mean that effects of elastic collisions on atomic displacements are negligibly small as compared with those of electronic energy deposition. The elastic collisions transfer the energy of ions directly to lattice system, while energy deposited in electronic system indirectly affects the lattice system. Even though the value of Sn is much smaller than that of Se, elastic collisions may still dominate the atomic displacements.
Energy (MeV)
2. Experimental procedure
Ion
Electronic energy deposition by swift heavy ions also contributes to atomic mixing at the interfaces of bi- and multi layer systems. Recently, much work on atomic mixing induced by swift heavy ions has been reported for several kinds of interfaces; metal–oxide [9], oxide–oxide [9–12], metal– metal [13], metal–semiconductor [10,12] and so on. The previous results show that the effect of swift heavy ions on atomic mixing strongly depends on the sensitivity of interface-forming materials to electronic energy deposition; for oxide–oxide interfaces, atomic mixing due to electronic energy deposition is remarkably observed, while it is scarcely found for metal–metal and metal–oxide interfaces. In this paper, we report the atomic mixing by 150–200 MeV Kr, Xe and Au ion irradiation and 3 MeV Si ion irradiation for some metal–oxide interfaces and discuss its dependence on the interface-forming materials and the electronic stopping power for irradiating ions.
Table 1 Combinations of ion, energy and total fluence, the respective electronic stopping power, Se, nuclear stopping power, Sn and projected range, Rp, for each material
R. Nakatani et al. / Nucl. Instr. and Meth. in Phys. Res. B 230 (2005) 234–239
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R. Nakatani et al. / Nucl. Instr. and Meth. in Phys. Res. B 230 (2005) 234–239
Then, to estimate the effect of elastic collisions on the atomic mixing, we also performed 3 MeV Si ion irradiation using 2 MV tandem accelerator at Nara WomenÔs University. The values of Se, Sn and the projected range, Rp, for each combination of ions and specimen materials are shown in Table 1. The total fluence for each irradiation is also shown. As the thickness of Bi and Au layers on the oxide substrates is much smaller than the projected range of ions, energy loss of ions within the layers is quite small and the values of Se and Sn
around the metal–oxide interfaces are nearly the same as those of incident ions. To examine the irradiation-induced atomic mixing, the concentration profiles at the interfaces for unirradiated and ion-irradiated specimens were obtained by means of Rutherford backscattering spectrometry (RBS) with 2.4 MeV a-particles. The RBS measurements were performed using IBA9900 accelerator at the Research Institute of Advanced Science and Technology (RIAST), Osaka Prefecture University.
200
3000
200 MeV Au
200 MeV Au 150
Bi
O
2000
100
50
unirradiated 14
1x10 /cm
Backscattering Yield
1000
Al
unirradiated 14 2 1x10 /cm
2
0
0
150
2000
200 MeV Xe
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Bi
100
O 1000
14
14
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500
unirradiated 1x10 /cm
unirradiated 1x10 /cm
Al
50 2
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Bi
150 MeV Kr
150 MeV Kr 300
3000
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1x10 /cm
Al 100
0 200
1000
unirradiated 14 2 1x10 /cm 250
2
300
350
Channel Number
400
0 420
440
460
480
Channel Number
Fig. 1. Change in RBS spectra for Bi/Al2O3 system by irradiation with 150 MeV Kr, 200 MeV Xe and 200 MeV Au ions to 1 · 1014/ cm2.
R. Nakatani et al. / Nucl. Instr. and Meth. in Phys. Res. B 230 (2005) 234–239
Fig. 1 shows the change in RBS spectra for Bi/ Al2O3 system by the irradiation with three ion species to the fluence of 1 · 1014/cm2. As the backscattering yield for Bi is much larger than that for Al and O, we plot the spectra for Bi and for Al and O separately with different Y-axis scale. 200 MeV Xe and 200 MeV Au irradiation result in a strong atomic mixing. On the other hand, 150 MeV Kr irradiation scarcely induces the atomic mixing. Fig. 2 shows the result for 3 MeV Si ion irradiation up to 5 · 1015/cm2. Although the energy deposited elastically into specimen, which can be estimated from the values of Sn and the total fluence, is about 10–30 times larger for Si irradiation than for Xe and Au irradiation, we cannot find any atomic mixing by 3 MeV Si irradiation, while Xe and Au ion irradiations induce a large amount of mixing at the interface. This result implies that the atomic mixing at the Bi/Al2O3 interface is induced by high-density electronic excitation due to 200 MeV Xe and Au ions. As can be seen in Fig. 1 and Table 1, the atomic mixing definitely increases with increasing the value of Se for each material. The value of Se for Kr ions is too small to induce any atomic mixing at the Bi/Al2O3 interface. The effects of high-density electronic excitation on Bi and Al2O3 have also been observed as the high-resistivity phase formation [14], and amorphous track formation [15] and surface swelling [16], respectively. The
effects on Bi and Al2O3 become remarkable when the value of Se exceeds about 25 keV/nm and about 20 keV/nm, respectively. Such a trend of the previous results is not inconsistent with the present one. In Fig. 3, we show the RBS spectra for unirradiated Bi/SiO2 specimen and those irradiated with 200 MeV Au ions. In the case of Bi/SiO2 system, even 200 MeV Au ion irradiation cannot induce any atomic mixing. It is worth noting here the effect of surface roughness on the RBS spectrum. If the irradiation induced the roughness of specimen surface, we would obtain the change in RBS spectra, which is similar to the case of interface mixing. As the irradiation-induced surface roughness is attributed to the interaction of irradiating ions with the surface, it does not depend on the substrate material. The present result, however, shows that the change in RBS spectrum for Bi layer occurs for 2500
200 MeV Au 2000
Backscattering Yield
3. Results and discussion
1500 1000
14
500
200 MeV Xe
1500
Backscattering Yield
Backscattering Yield
2
1x10 /cm
1500
3 MeV Si
unirradiated 15
2
5x10 /cm 500
0 420
unirradiated
0
2000
1000
237
440
460
480
Channel Number Fig. 2. RBS spectra for unirradiated Bi/Al2O3 specimen and that irradiated with 3 MeV Si ions to the fluence of 5 · 1015/ cm2. Spectra only for Bi layer are shown.
1000
unirradiated 500
0 420
14
2
1x10 /cm
440
460
480
Channel Number Fig. 3. RBS spectra for unirradiated Bi/SiO2 specimens and for those irradiated with 200 MeV Xe ions or 200 MeV Au ions. Spectra only for Bi layer are shown.
238
R. Nakatani et al. / Nucl. Instr. and Meth. in Phys. Res. B 230 (2005) 234–239
Al2O3 substrate, but not for SiO2 substrate. Therefore, we can conclude that the RBS spectrum change by the swift heavy ion irradiation for Bi/ Al2O3 system is really due to the interface mixing. Recently, Bolse and co-workers have studied the atomic mixing of some oxide–oxide and metal– oxide bi-layers by swift heavy ions and have concluded that atomic mixing occurs only if the values of Se exceed the threshold values of track formation for both interface-forming materials; both sides of the interface must be molten. It has been reported that the threshold value of Se for track formation is 1.8 keV/nm for SiO2 [17], which is much smaller than the threshold value for Al2O3 (about 20 keV/ nm). The larger sensitivity of SiO2 to electronic energy deposition than of Al2O3 has also been observed as the electronic sputtering yield [18]. If BolseÕs criterion could be applied also to the present case, atomic mixing under swift heavy ion irradia6000
Backscattering Yield
200 MeV Au Au/Al O
unirradiated 4000
14
2
2
3
1x10 /cm
4. Summary 2000
0
3000
Backscattering Yield
tion would have occurred at Bi/SiO2 interface as well as at Bi/Al2O3 interface. The present result is, however, not the case. Then, we conclude that the atomic mixing under swift heavy ion irradiation cannot be described only by the sensitivity of each interface-forming material to electronic energy deposition. After the energy transfer from highly excited electronic system to the lattice system, some kind of chemical reaction may occur between interface-forming materials. If such a chemical reaction occurs under the irradiation, it should affect the final state of atomic mixing. To explain the difference in atomic mixing between Bi/Al2O3 and Bi/ SiO2 interfaces, we have to consider some factors which dominate the chemical reaction, such as mixing enthalpy, diffusivity and so on. The effect of 200 MeV Au ion irradiation on RBS spectra of Au/Al2O3 and Au/SiO2 is shown in Fig. 4. The present experiment shows that for Au/oxide interfaces, no atomic mixing occurs irrespective of irradiating ions or species of oxide substrate. These results are quite reasonable since noble metals such as Au and Cu are much less sensitive to electronic energy deposition than Bi.
200 MeV Au Au/SiO
unirradiated 2000
14
2
2
1x10 /cm
1000
0 420
440
460
480
We have found that atomic mixing at Bi/Al2O3 interface is induced by the high-density electronic excitation due to the swift heavy ions. In the case of Bi/SiO2 interface irradiated with the swift heavy ions, although the sensitivity of SiO2 to the electronic energy deposition is much higher than Al2O3, no atomic mixing can be observed. For the precise description of the atomic mixing under swift heavy ion irradiation, we have to consider not only the sensitivity of each interface-forming material to electronic energy deposition, but also other factors which dominate chemical reactions between the materials at the interface under the irradiation.
Acknowledgements
Channel Number Fig. 4. RBS spectra for unirradiated Au/Al2O3 and Au/SiO2 specimens and those irradiated with 200 MeV Au ions. Spectra only for Au layer are shown.
The authors are very grateful to Prof. A. Mizohata and Dr. N. Ito for the RBS measurements. They would like to thank Profs. N. Saka-
R. Nakatani et al. / Nucl. Instr. and Meth. in Phys. Res. B 230 (2005) 234–239
moto and H. Ogawa for the irradiation experiments at Nara WomenÕs University. They greatly thank Prof. G. Szenes for fruitful discussions. They also appreciate the technical staff of the accelerator division at JAERI-Tokai for their great help. This work has been done under the collaboration program between Osaka Prefecture University and Japan Atomic Energy Research Institute. References [1] A. Iwase, T. Iwata, Nucl. Instr. and Meth. B 90 (1994) 322. [2] A. Dunlop, D. Lesueur, P. Legrand, H. Dammak, Nucl. Instr. and Meth. B 90 (1994) 330. [3] A. Dunlop, D. Lesueur, Mat. Sci. Forum 97–99 (1992) 553. [4] Y. Chimi, A. Iwase, N. Ishikawa, T. Kambara, Nucl. Instr. and Meth. B 193 (2002) 248. [5] H. Dammak, A. Barbu, A. Dunlop, D. Lesueur, N. Lorenzelli, Philos. Mag. Lett. 67 (1993) 253. [6] A. Iwase, Y. Hamatani, Y. Mukumoto, N. Ishikawa, Y. Chimi, T. Kambara, C. Mueller, R. Neumann, F. Ono, Nucl. Instr. and Meth. B 209 (2003) 323. [7] A. Barbu, P. Pareige, V. Jacquet, Nucl. Instr. and Meth. B 146 (1998) 278.
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[8] A. Iwase, T. Hasegawa, Y. Chimi, T. Tobita, N. Ishikawa, M. Suzuki, T. Kambara, S. Ishino, Nucl. Instr. and Meth. B 195 (2002) 309. [9] B. Schattat, W. Bolse, S. Klaumuenzer, F. Harbsmeier, A. Jasenek, Nucl. Instr. and Meth. B 191 (2002) 577. [10] D.K. Avashi, W. Assman, H. Nolte, H.D. Mieskes, H. Huber, E.T. Subramaniyam, A. Tripathi, S. Ghosh, Nucl. Instr. and Meth. B 156 (1999) 143. [11] W. Bolse, Sur. Coating Tech. 158–159 (2002) 1. [12] S. Kraft, B. Schattat, W. Bolse, S. Klaumuenzer, F. Harbsmeier, A. Kulinska, A. Loeffl, J. Appl. Phys. 91 (2002) 1129. [13] R. Leguay, A. Dunlop, F. Dunstetter, N. Lorenzelli, A. Braslau, F. Bridou, J. Corno, B. Pardo, J. Chevallier, C. Colliex, A. Menelle, J.L. Rouviere, L. Thome, Nucl. Instr. and Meth. B 122 (1997) 481. [14] C. Dufour, A. Audouard, F. Beuneu, J. Dural, J.P. Girard, A. Hairie, M. Levalois, E. Paumier, M. Toulemonde, J. Phys.: Condens. Matter 5 (1993) 4573. [15] S.M.M. Ramos, N. Bonardi, B. Canut, S. Bouffard, S. Della-Negra, Nucl. Instr. and Meth. B 143 (1998) 319. [16] R. Brenier, B. Canut, S.M.M. Ramos, P. Thevenard, Nucl. Instr. and Meth. B B90 (1994) 339. [17] F. Studer, M. Hervieu, J.M. Constantini, M. Toulemonde, Nucl. Instr. and Meth. B 122 (1997) 449. [18] N. Matsunami, M. Sataka, A. Iwase, S. Okayasu, Nucl. Instr. and Meth. B 209 (2003) 288.
Atomic mixing at metal–oxide interfaces by high energy heavy ions R. Nakatani a, R. Taniguchi a, Y. Chimi b, N. Ishikawa b, M. Fukuzumi a, Y. Kato a, H. Tsuchida c, N. Matsunami d, A. Iwase a,* a
b
Research Institute for Advanced Science and Technology, Osaka Prefecture University, Gakuen-cho, 1-2, Sakai-shi, Osaka 599-8570, Japan Japan Atomic Energy Research Institute (JAERI-Tokai), Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan c Kyoto University, Sakyo-ku, Kyoto-shi 606-8501, Japan d Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan Available online 22 January 2005
Abstract We study the atomic mixing at metal (Bi or Au)/oxide (SiO2 or Al2O3) interfaces under 150–200 MeV heavy ion irradiation. Irradiation-induced interface mixing state is examined by means of Rutherford backscattering spectrometry (RBS). For Bi/Al2O3 interfaces, the heavy ion irradiations induce a strong atomic mixing and the amount of the mixing increases with increasing the electronic stopping power for heavy ions. By comparing the results with that for 3 MeV Si ion irradiation, we conclude that the strong atomic mixing observed at Bi/Al2O3 interfaces is attributed to the high-density electronic excitation. On the other hand, for other interfaces (Bi/SiO2, Au/Al2O3 and Au/SiO2), atomic mixing is rarely observed after the irradiation. The dependence of atomic mixing on combinations of irradiating ions and interface-forming materials is discussed. 2004 Elsevier B.V. All rights reserved. PACS: 61.80.Jh; 61.82.Bg; 61.82.Ms; 81.40.Wx Keywords: Atomic mixing; Metal/oxide interfaces; Electronic energy deposition
1. Introduction Through a lot of studies concerning the effects of swift heavy ion irradiation on materials, it has *
Corresponding author. Tel.: +81 72 254 9810; fax: +81 72 251 6439. E-mail address: [email protected] (A. Iwase).
been well proven so far that the electronic energy deposition by swift heavy ions plays an important role in atomic displacements in target materials. Even in metallic materials, the effects have been observed as annihilation and production of point defects [1–4], phase transformation [5], modification of magnetic properties [6], precipitation of solute atoms [7,8] and so on.
0168-583X/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2004.12.047
2.72 0.035 2.09 1.60 0.061 3.39 0.94 0.13 3.95 1.51 0.063 2.55 5 · 1015 3 Si
28
Kr Xe 197 Au
150 200 200
1 · 1014 1 · 1014 1 · 1014
16.3 23.5 27.2
0.050 0.12 0.34
14.1 14.9 12.7
31.4 45.1 51.9
0.10 0.25 0.69
7.73 8.23 7.13
17.7 26.2 32.0
0.037 0.089 0.24
12.3 13.2 11.1
10.1 14.9 18.4
0.021 0.051 0.137
21.0 22.3 18.9
235
136
86
Rp (lm) Sn (keV/nm) Se (keV/nm) Se (keV/nm) Sn (keV/nm)
Rp (lm) Se (keV/nm)
Sn (keV/nm)
Rp (lm)
Se (keV/nm)
Sn (keV/nm)
Rp (lm)
SiO2 Al2O3 Au Bi
Target material
Fluence (cm 2)
Specimens for the present study are Bi/Al2O3, Bi/SiO2, Au/Al2O3 and Au/SiO2 systems. Bismuth or gold was evaporated on 10 · 10 · 0.3 mm3 single crystalline Al2O3 (sapphire) substrates or amorphous SiO2 (a-SiO2) substrates in a vacuum of 5 · 10 6 Torr. The thickness of each evaporated layer was about 100–200 nm. The specimens were irradiated with 150 MeV 86Kr, 200 MeV 136Xe or 200 MeV 197Au ions at room temperature up to the fluence of 1 · 1014/cm2 using a 20 MV tandem accelerator at JAERI-Tokai. For heavy ions in the energy range of 100–200 MeV, the value of the nuclear stopping power, Sn, is about two order smaller than that of the electronic stopping power, Se. This fact, however, does not mean that effects of elastic collisions on atomic displacements are negligibly small as compared with those of electronic energy deposition. The elastic collisions transfer the energy of ions directly to lattice system, while energy deposited in electronic system indirectly affects the lattice system. Even though the value of Sn is much smaller than that of Se, elastic collisions may still dominate the atomic displacements.
Energy (MeV)
2. Experimental procedure
Ion
Electronic energy deposition by swift heavy ions also contributes to atomic mixing at the interfaces of bi- and multi layer systems. Recently, much work on atomic mixing induced by swift heavy ions has been reported for several kinds of interfaces; metal–oxide [9], oxide–oxide [9–12], metal– metal [13], metal–semiconductor [10,12] and so on. The previous results show that the effect of swift heavy ions on atomic mixing strongly depends on the sensitivity of interface-forming materials to electronic energy deposition; for oxide–oxide interfaces, atomic mixing due to electronic energy deposition is remarkably observed, while it is scarcely found for metal–metal and metal–oxide interfaces. In this paper, we report the atomic mixing by 150–200 MeV Kr, Xe and Au ion irradiation and 3 MeV Si ion irradiation for some metal–oxide interfaces and discuss its dependence on the interface-forming materials and the electronic stopping power for irradiating ions.
Table 1 Combinations of ion, energy and total fluence, the respective electronic stopping power, Se, nuclear stopping power, Sn and projected range, Rp, for each material
R. Nakatani et al. / Nucl. Instr. and Meth. in Phys. Res. B 230 (2005) 234–239
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R. Nakatani et al. / Nucl. Instr. and Meth. in Phys. Res. B 230 (2005) 234–239
Then, to estimate the effect of elastic collisions on the atomic mixing, we also performed 3 MeV Si ion irradiation using 2 MV tandem accelerator at Nara WomenÔs University. The values of Se, Sn and the projected range, Rp, for each combination of ions and specimen materials are shown in Table 1. The total fluence for each irradiation is also shown. As the thickness of Bi and Au layers on the oxide substrates is much smaller than the projected range of ions, energy loss of ions within the layers is quite small and the values of Se and Sn
around the metal–oxide interfaces are nearly the same as those of incident ions. To examine the irradiation-induced atomic mixing, the concentration profiles at the interfaces for unirradiated and ion-irradiated specimens were obtained by means of Rutherford backscattering spectrometry (RBS) with 2.4 MeV a-particles. The RBS measurements were performed using IBA9900 accelerator at the Research Institute of Advanced Science and Technology (RIAST), Osaka Prefecture University.
200
3000
200 MeV Au
200 MeV Au 150
Bi
O
2000
100
50
unirradiated 14
1x10 /cm
Backscattering Yield
1000
Al
unirradiated 14 2 1x10 /cm
2
0
0
150
2000
200 MeV Xe
200 MeV Xe 1500
Bi
100
O 1000
14
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500
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unirradiated 1x10 /cm
Al
50 2
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150 MeV Kr
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3000
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unirradiated 14
1x10 /cm
Al 100
0 200
1000
unirradiated 14 2 1x10 /cm 250
2
300
350
Channel Number
400
0 420
440
460
480
Channel Number
Fig. 1. Change in RBS spectra for Bi/Al2O3 system by irradiation with 150 MeV Kr, 200 MeV Xe and 200 MeV Au ions to 1 · 1014/ cm2.
R. Nakatani et al. / Nucl. Instr. and Meth. in Phys. Res. B 230 (2005) 234–239
Fig. 1 shows the change in RBS spectra for Bi/ Al2O3 system by the irradiation with three ion species to the fluence of 1 · 1014/cm2. As the backscattering yield for Bi is much larger than that for Al and O, we plot the spectra for Bi and for Al and O separately with different Y-axis scale. 200 MeV Xe and 200 MeV Au irradiation result in a strong atomic mixing. On the other hand, 150 MeV Kr irradiation scarcely induces the atomic mixing. Fig. 2 shows the result for 3 MeV Si ion irradiation up to 5 · 1015/cm2. Although the energy deposited elastically into specimen, which can be estimated from the values of Sn and the total fluence, is about 10–30 times larger for Si irradiation than for Xe and Au irradiation, we cannot find any atomic mixing by 3 MeV Si irradiation, while Xe and Au ion irradiations induce a large amount of mixing at the interface. This result implies that the atomic mixing at the Bi/Al2O3 interface is induced by high-density electronic excitation due to 200 MeV Xe and Au ions. As can be seen in Fig. 1 and Table 1, the atomic mixing definitely increases with increasing the value of Se for each material. The value of Se for Kr ions is too small to induce any atomic mixing at the Bi/Al2O3 interface. The effects of high-density electronic excitation on Bi and Al2O3 have also been observed as the high-resistivity phase formation [14], and amorphous track formation [15] and surface swelling [16], respectively. The
effects on Bi and Al2O3 become remarkable when the value of Se exceeds about 25 keV/nm and about 20 keV/nm, respectively. Such a trend of the previous results is not inconsistent with the present one. In Fig. 3, we show the RBS spectra for unirradiated Bi/SiO2 specimen and those irradiated with 200 MeV Au ions. In the case of Bi/SiO2 system, even 200 MeV Au ion irradiation cannot induce any atomic mixing. It is worth noting here the effect of surface roughness on the RBS spectrum. If the irradiation induced the roughness of specimen surface, we would obtain the change in RBS spectra, which is similar to the case of interface mixing. As the irradiation-induced surface roughness is attributed to the interaction of irradiating ions with the surface, it does not depend on the substrate material. The present result, however, shows that the change in RBS spectrum for Bi layer occurs for 2500
200 MeV Au 2000
Backscattering Yield
3. Results and discussion
1500 1000
14
500
200 MeV Xe
1500
Backscattering Yield
Backscattering Yield
2
1x10 /cm
1500
3 MeV Si
unirradiated 15
2
5x10 /cm 500
0 420
unirradiated
0
2000
1000
237
440
460
480
Channel Number Fig. 2. RBS spectra for unirradiated Bi/Al2O3 specimen and that irradiated with 3 MeV Si ions to the fluence of 5 · 1015/ cm2. Spectra only for Bi layer are shown.
1000
unirradiated 500
0 420
14
2
1x10 /cm
440
460
480
Channel Number Fig. 3. RBS spectra for unirradiated Bi/SiO2 specimens and for those irradiated with 200 MeV Xe ions or 200 MeV Au ions. Spectra only for Bi layer are shown.
238
R. Nakatani et al. / Nucl. Instr. and Meth. in Phys. Res. B 230 (2005) 234–239
Al2O3 substrate, but not for SiO2 substrate. Therefore, we can conclude that the RBS spectrum change by the swift heavy ion irradiation for Bi/ Al2O3 system is really due to the interface mixing. Recently, Bolse and co-workers have studied the atomic mixing of some oxide–oxide and metal– oxide bi-layers by swift heavy ions and have concluded that atomic mixing occurs only if the values of Se exceed the threshold values of track formation for both interface-forming materials; both sides of the interface must be molten. It has been reported that the threshold value of Se for track formation is 1.8 keV/nm for SiO2 [17], which is much smaller than the threshold value for Al2O3 (about 20 keV/ nm). The larger sensitivity of SiO2 to electronic energy deposition than of Al2O3 has also been observed as the electronic sputtering yield [18]. If BolseÕs criterion could be applied also to the present case, atomic mixing under swift heavy ion irradia6000
Backscattering Yield
200 MeV Au Au/Al O
unirradiated 4000
14
2
2
3
1x10 /cm
4. Summary 2000
0
3000
Backscattering Yield
tion would have occurred at Bi/SiO2 interface as well as at Bi/Al2O3 interface. The present result is, however, not the case. Then, we conclude that the atomic mixing under swift heavy ion irradiation cannot be described only by the sensitivity of each interface-forming material to electronic energy deposition. After the energy transfer from highly excited electronic system to the lattice system, some kind of chemical reaction may occur between interface-forming materials. If such a chemical reaction occurs under the irradiation, it should affect the final state of atomic mixing. To explain the difference in atomic mixing between Bi/Al2O3 and Bi/ SiO2 interfaces, we have to consider some factors which dominate the chemical reaction, such as mixing enthalpy, diffusivity and so on. The effect of 200 MeV Au ion irradiation on RBS spectra of Au/Al2O3 and Au/SiO2 is shown in Fig. 4. The present experiment shows that for Au/oxide interfaces, no atomic mixing occurs irrespective of irradiating ions or species of oxide substrate. These results are quite reasonable since noble metals such as Au and Cu are much less sensitive to electronic energy deposition than Bi.
200 MeV Au Au/SiO
unirradiated 2000
14
2
2
1x10 /cm
1000
0 420
440
460
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We have found that atomic mixing at Bi/Al2O3 interface is induced by the high-density electronic excitation due to the swift heavy ions. In the case of Bi/SiO2 interface irradiated with the swift heavy ions, although the sensitivity of SiO2 to the electronic energy deposition is much higher than Al2O3, no atomic mixing can be observed. For the precise description of the atomic mixing under swift heavy ion irradiation, we have to consider not only the sensitivity of each interface-forming material to electronic energy deposition, but also other factors which dominate chemical reactions between the materials at the interface under the irradiation.
Acknowledgements
Channel Number Fig. 4. RBS spectra for unirradiated Au/Al2O3 and Au/SiO2 specimens and those irradiated with 200 MeV Au ions. Spectra only for Au layer are shown.
The authors are very grateful to Prof. A. Mizohata and Dr. N. Ito for the RBS measurements. They would like to thank Profs. N. Saka-
R. Nakatani et al. / Nucl. Instr. and Meth. in Phys. Res. B 230 (2005) 234–239
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