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Sputtering of high Tc superconductor YBa2Cu3O7−δ by high energy heavy ions
Nuclear Instruments and Methods in Physics Research B 175±177 (2001) 56±61
Sputtering of high Tc superconductor YBa2 Cu3 O7 heavy ions Noriaki Matsunami a
a,*
www.elsevier.nl/locate/nimb
d
by high energy
, Masao Sataka b, Akihiro Iwase
b
Energy Engineering and Science, School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan b Department of Materials Science, Japan Atomic Energy Research Institute, Tokai-mura, Ibaraki 319-1195, Japan
Abstract To investigate the electronic excitation eect on ion-induced atomic displacement, we have applied a carbon (C)-®lm collector method to measure the sputtering yields of YBa2 Cu3 O7 d (YBCO) by high energy heavy ions. The collection eciency of the C-®lms is calibrated by using 120 keV Ne ions and the values are obtained as 0.048, 0.076, 0.061 and 0.3 for Y, Ba, Cu and O, respectively. The total sputtering yields of YBCO induced by 198 MeV I, 69 MeV Ni and 80 MeV S ions are evaluated as 1030, 580 and 100, respectively, including oxygen contribution. These values are larger by a factor of 1000 than the calculated sputtering yield due to the elastic collision cascade, indicating a signi®cant contribution of the electronic excitation to the sputtering yields. It also appears that the total sputtering yields scale reasonably with the square of the electronic stopping power. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 79.20; 74.76 Keywords: Electronic excitation eects; Sputtering; High Tc superconductor; High energy heavy ion
1. Introduction It has been suggested that the electronic excitation by ion irradiation contributes to the increase in the resistivity and lattice parameter (c-axis) elongation of high Tc superconductors [1,2]. The relation between the electronic stopping power Se and the track radii has been also discussed [3]. It would be interesting to observe ``di-
* Corresponding author. Tel.: +81-52-789-3777; fax: +81-52789-3847. E-mail address: [email protected] (N. Matsunami).
rectly'' the atomic displacements due to the electronic energy deposition (EED). However, no direct observations of atomic displacements due to EED have been done yet for high Tc superconductors. This paper reports the sputtering yields of YBa2 Cu3 O7 d (YBCO) induced by high energy heavy ions. It should be noticed here that sputtering yield measurement is ``a direct method for observation of atomic displacements''. In this study, we adopt a carbon (C)-®lm collector method to measure the sputtering yield. Collection eciency of the C-®lm and the relation between the sputtering yields and Se are described.
0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 1 ) 0 0 3 3 7 - 8
N. Matsunami et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 56±61
2. Experimental YBCO samples were prepared on MgO by an o-axis rf-magnetron sputter-deposition method [4] and the sample thickness was 50±90 nm. The samples were irradiated with high energy heavy ions, 200 MeV 127 I12 , 70 MeV 58 Ni6 and 80 MeV 32 7 S obtained from the tandem accelerator at JAERI at a normal incidence through C-®lms (100 nm thick) placed in front of the samples in vacuum of 10 5 Pa. The C-®lm was supported on an Al plate (0.4 mm) with double holes of 4 mm in diameter. A sketch of the sample and the C-®lm collector assembly is shown in Fig. 1. The distance between the sample surface and the C-®lm was 2 mm. The ion dose was up to 2 1014 cm 2 . The
57
energy loss in the C-®lm for 200 MeV I, 70 MeV Ni and 80 MeV S was calculated as 1.6, 0.8 and 0.4 MeV, respectively [5]. The ion energy after transmission through the C-foil is given in Table 1. Bohr energy straggling in the C-®lm was calculated as 0.07, 0.035 and 0.02 MeV for I, Ni and S ions, respectively. The energy loss and the energy straggling in the C-®lm are very small, and do not disturb the present experimental study. The ion charge incident on YBCO samples, which diers from that on the C-®lms, may aect sputtering and it is assumed to reach the equilibrium during transmission of the C-®lm. The mean charge Q of ions after transmission through the C-®lm is estimated from [6] and is also given in Table 1. Sputtered atoms, which were collected in the C®lms, were analyzed by MeV He Rutherford backscattering spectroscopy (RBS) using a Van de Graa accelerator at Nagoya University (NU).
3. Results and discussion 3.1. Atoms collected in C-®lm during ion irradiation
Fig. 1. Sketch of C-®lm collector and sample assembly for ion irradiation. C-®lm is supported on Al plate (0.4 mm thick) with holes of 4 mm in diameter. Samples are supported on Al (2 mm).
Fig. 2 shows RBS spectra of the C-®lm collector after irradiation of 69 MeV Ni at a dose of 0:8 1014 cm 2 . Be spectrum appears, because the C-®lms were supported by Be for RBS analysis. Cu, Y and Ba atoms are seen around the channel number of 680, 730 and 780, respectively. O atoms are noticed around the channel number of 310 in the inset (indicated by ///). RBS analysis shows that O of 1±2% exists in the virgin C-®lms and that O of 20±50 1015 cm 2 and C of 3±30 1015 cm 2 exist in Be support. RBS and 12 C d; p13 C nuclear
Table 1 Sputtering yields Y (atoms per ion) of YBCO samples by high energy heavy ion irradiationa Ion
E (MeV)
Q
Y
Ba
Cu
Yexp
Se (keV/nm)
Sn (keV/nm)
Yc
127 12
198 69 80
30 18 13
73 25 6.3
375 290 22
279 164 12
727 479 40
26.1 14.1 6.68
0.10 0.0372 0.00687
0.42 0.16 0.029
I Ni6 32 7 S 58
a
E and Q are the mean energy and mean charge after transmission through C-®lm (100 nm). Yexp is the sum of Y, Ba and Cu sputtering yields. Se and Sn are the electronic (inelastic) and nuclear (elastic) energy losses, respectively. Yc is the calculated sputtering yield due to the elastic collisions.
58
N. Matsunami et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 56±61
Fig. 2. RBS spectra of C-®lm collector after irradiation with 69 MeV Ni on YBCO sample at a dose of 0:8 1014 cm 2 . RBS analysis was performed using 1.8 MeV He with normal incidence and 160° scattering angle. O peaks in an expanded scale is shown in the inset(o), together with RBS spectra of unirradiated C-®lm(+). O(Be) peak and O(///) indicate O in Be and sputtered O atoms in the C-®lm from the YBCO sample, respectively.
reaction analysis with 1.2 MeV deuterium show no appreciable increase of C impurities in YBCO samples and reduction of C impurities in Be samples by 198 MeV I ion irradiation at a dose of 3 1014 cm 2 , indicating no recoil eects of C from the C-®lm into samples. The numbers of collected atoms in the C-®lm are plotted against dose in Fig. 3. Atoms collected in the C-®lm are nearly proportional to the dose. At small doses, these are somewhat larger than the average values indicated by the dashed lines. This could be partly due to errors of dose and RBS intensity for small dose, and also surface contamination eects. Another possibility is that the collection eciency of the C-®lm is larger at smaller dose than that at larger dose. Ion irradiation eects on the collection eciency will be discussed in the next section. Similar dose dependence was observed for 198 MeV I and 80
Fig. 3. Y , Ba M, Cu and O } atoms collected in C®lm vs dose for 79 MeV Ni irradiation on YBCO ®lms on MgO. Also plotted are Y , Ba N, Cu and O atoms from thick polycrystalline YBCO sample. Average of Y is drawn by dashed lines.
N. Matsunami et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 56±61
MeV S ion irradiation. The data in the region of <1 1014 cm 2 was used to derive the sputtering yields.
59
®lm collector dose not aect the collection eciency of sputtered atoms. 3.3. Sputtering yield
3.2. Collection eciency of C-®lm The collection eciency fc , which is given by fc atoms collected in C-®lm/(Ne dose sputtering yield composition ratio), was calibrated using the same C-®lm and sample assembly shown in Fig. 1, and 120 keV Ne ions obtained from a 200 kV Cockcroft-Walton accelerator at NU. The energy loss and straggling of Ne in the C-®lms were estimated to be 60 and 13 keV, respectively. RBS spectra similar to those in Fig. 2 were obtained. Sputtering yield of YBCO by 120 keV Ne through the C-®lm was experimentally determined as 1.1 from the thickness decrease of YBCO ®lms, using the method in [7]. The composition ratios are taken as 1/13, 2/13, 3/13 and 7/13 for Y, Ba, Cu and O, respectively, assuming stoichiometric sputtering [7]. The value of fc were obtained as 0.048, 0.076, 0.061 and 0.3 for Y, Ba, Cu and O, respectively, at Ne dose of 5±23 1015 cm 2 . An estimated error of fc for Y, Ba and Cu is 20%, and fc for O may have uncertainty of a factor of 2±3, due to large background correction as shown in the inset in Fig. 2. We have also calibrated fc for Au and it is found that values of fc for Y, Ba and Cu are much smaller than those for O and Au (fc 0:2) [8]. The sputtering yields of polycrystalline Au by high energy heavy ions have been measured by using the C-®lm collector method and it appears that the yields agree well with the calculated sputtering yields due to the elastic collision cascade [8], which is expected to be dominant for polycrystalline Au. This con®rms the validity of the C-®lm collector method. The collection eciency might be modi®ed by ion irradiation and this can be inferred from the part of C-®lm supported on Al. The eciency under no ion irradiation was obtained as 1/3 to 1 of fc mentioned above and this variation can be explained by angular dependence of the sputtering yields. This indicates that ion irradiation on the C-
The sputtering yield of each component was derived from the slope in the dose (0.1± 1 1014 cm 2 ) as shown in Fig. 3 and fc . The results are summarized in Table 1. The oxygen sputtering yields were obtained as roughly 300, 100 and 60 for 198 MeV I, 69 MeV Ni and 80 MeV S ion irradiation, respectively. The sputtering yields of thick-polycrystalline YBCO samples were also measured and agree reasonably with those of YBCO samples, except for the Ba yield at a small dose. The reason for this dierence is not understood at present. Excluding the exception, the above result leads to the fact that ion beam mixing of YBCO ®lms and MgO substrate, which will be described in Section 3.4, does not in¯uence the sputtering of YBCO ®lms. The sputtering yields do not follow the composition ratio. The sputtering yield Yc is calculated assuming that Yc is proportional to the nuclear (elastic) energy loss, Sn , with the experimental values of 2.5 and 1.1 for 100 keV Ar and Ne, respectively [6]. It appears that TRIM (both 1992 and 1997 versions) simulation results are smaller by a factor of 3 than Yc . The experimental sputtering yields are larger by 1000 than
Fig. 4. Sputtering yield Yexp versus electronic stopping power, Se . Closed and open circles show the sum of Y, Ba and Cu yields, and sum of Y, Ba, Cu and O yields, respectively.
60
N. Matsunami et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 56±61
Yc , indicating the considerable contribution of EED. Fig. 4 shows the experimental sputtering yields Yexp versus the electronic stopping power Se . Yexp is nearly proportional to Sen with n 2. The exponent is smaller than fourth power for lattice parameter elongation [2]. The square dependence is much stronger than the Se dependence of track radii [3]. These indicate that additional and/or dierent processes are involved in lattice parameter elongation and track formation. One notices that variation of Yexp is much stronger than that of the mean charge Q, implying no eects of multiple charges carried by ions on the sputtering. Finally, the dose of superconductor to normal phase transition are estimated
to be roughly 1, 3 and 10 1011 cm 2 for 198 MeV I, 69 MeV Ni and 80 MeV S, respectively, considering the factor of 1000 and displacement per atoms due to the elastic collision cascade (DPA) are 0.033, 0.012 and 0.0029 at 1014 cm 2 , respectively (DPA for the transition 0.03 [9]). The sputtering yields obtained above are not for YBCO at superconductor, but normal phase. The details are under investigation. 3.4. Ion beam mixing and composition modi®cation Fig. 5 shows the RBS spectra of YBCO before and after 69 MeV Ni ion irradiation at a dose of 2 1014 cm 2 . One can see mixing of YBCO ®lm
Fig. 5. RBS spectra of YBCO ®lm on MgO: (a) unirradiated and (b) after 69 MeV Ni ion irradiation at a dose of 2 1014 cm 2 , illustrating mixing of YBCO ®lm and MgO substrate, and composition modi®cation. RBS was performed using 2.5 MeV He at normal incidence and 160° scattering angle.
N. Matsunami et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 56±61
and MgO substrate, and composition modi®cation of YBCO ®lms. Ion beam mixing and composition modi®cation were observed for 198 MeV I at dose >0:5 1014 cm 2 , 69 MeV Ni at dose >0:8 1014 cm 2 . These were not observed for 198 MeV I at dose <0:17 1014 cm 2 , 69 MeV Ni at dose <0:2 1014 cm 2 , and 80 MeV S at dose <0:8 1014 cm 2 . Ion beam mixing and composition modi®cation at small dose are insigni®cant, and hence do not aect the sputtering yields discussed above. 4. Conclusion We have measured the sputtering yields from YBCO samples with high energy heavy ions, by using C-®lm collector method. The total sputtering yields are larger by 103 than those of calculations based on the nuclear energy deposition, indicating considerable electronic excitation effects on atomic displacements. The total sputtering yields scale well with the square of the electronic stopping power. Ion beam mixing and composition modi®cation are observed only for high irradiation doses and do not aect the present results.
61
Acknowledgements The authors thank to Prof. Y. Takai for helpful discussion and sample preparation, and Mr. T. Masuda for technical assistance of RBS at NU. References [1] B. Hensel, B. Roas, S. Henke, R. Hopfengartner, M. Lippert, J.P. Strobel, M. Vildic, G.S. Ischenko, Phys. Rev. B 42 (1990) 4135. [2] A. Iwase, N. Ishikawa, Y. Chimi, K. Tsuru, H. Wakana, O. Michikami, T. Kambara, Nucl. Instr. and Meth. B 146 (1998) 557. [3] M. Toulemonde, S. Bouard, F. Studer, Nucl. Instr. and Meth. B 91 (1994) 108. [4] J. Oketani, Thesis of Master's Degree, Nagoya University, Engineering, 1999. [5] J.F. Ziegler, J.P. Biersack, U. Littmark, in: The Stopping and Range of Ions in Solids, Pergamon, New York, 1985. [6] K. Shima, N. Kuno, M. Yamanouchi, H. Tawara, At. Data Nucl. Data Tables 51 (1992) 173. [7] N. Matsunami, Nucl. Instr. and Meth. B 134 (1998) 346 and Ne results (private communication). [8] N. Matsunami, M. Sataka, A. Iwase, T. Inami, M. Kobiyama, in: Proceedings of the International Conference on Nanostructured Materials, Sendai, Japan, 2000. [9] J.C. Barbour, E.L. Venturini, D.S. Ginley, J.F. Kwak, Nucl. Instr. and Meth. B 65 (1992) 531.
Sputtering of high Tc superconductor YBa2 Cu3 O7 heavy ions Noriaki Matsunami a
a,*
www.elsevier.nl/locate/nimb
d
by high energy
, Masao Sataka b, Akihiro Iwase
b
Energy Engineering and Science, School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan b Department of Materials Science, Japan Atomic Energy Research Institute, Tokai-mura, Ibaraki 319-1195, Japan
Abstract To investigate the electronic excitation eect on ion-induced atomic displacement, we have applied a carbon (C)-®lm collector method to measure the sputtering yields of YBa2 Cu3 O7 d (YBCO) by high energy heavy ions. The collection eciency of the C-®lms is calibrated by using 120 keV Ne ions and the values are obtained as 0.048, 0.076, 0.061 and 0.3 for Y, Ba, Cu and O, respectively. The total sputtering yields of YBCO induced by 198 MeV I, 69 MeV Ni and 80 MeV S ions are evaluated as 1030, 580 and 100, respectively, including oxygen contribution. These values are larger by a factor of 1000 than the calculated sputtering yield due to the elastic collision cascade, indicating a signi®cant contribution of the electronic excitation to the sputtering yields. It also appears that the total sputtering yields scale reasonably with the square of the electronic stopping power. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 79.20; 74.76 Keywords: Electronic excitation eects; Sputtering; High Tc superconductor; High energy heavy ion
1. Introduction It has been suggested that the electronic excitation by ion irradiation contributes to the increase in the resistivity and lattice parameter (c-axis) elongation of high Tc superconductors [1,2]. The relation between the electronic stopping power Se and the track radii has been also discussed [3]. It would be interesting to observe ``di-
* Corresponding author. Tel.: +81-52-789-3777; fax: +81-52789-3847. E-mail address: [email protected] (N. Matsunami).
rectly'' the atomic displacements due to the electronic energy deposition (EED). However, no direct observations of atomic displacements due to EED have been done yet for high Tc superconductors. This paper reports the sputtering yields of YBa2 Cu3 O7 d (YBCO) induced by high energy heavy ions. It should be noticed here that sputtering yield measurement is ``a direct method for observation of atomic displacements''. In this study, we adopt a carbon (C)-®lm collector method to measure the sputtering yield. Collection eciency of the C-®lm and the relation between the sputtering yields and Se are described.
0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 1 ) 0 0 3 3 7 - 8
N. Matsunami et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 56±61
2. Experimental YBCO samples were prepared on MgO by an o-axis rf-magnetron sputter-deposition method [4] and the sample thickness was 50±90 nm. The samples were irradiated with high energy heavy ions, 200 MeV 127 I12 , 70 MeV 58 Ni6 and 80 MeV 32 7 S obtained from the tandem accelerator at JAERI at a normal incidence through C-®lms (100 nm thick) placed in front of the samples in vacuum of 10 5 Pa. The C-®lm was supported on an Al plate (0.4 mm) with double holes of 4 mm in diameter. A sketch of the sample and the C-®lm collector assembly is shown in Fig. 1. The distance between the sample surface and the C-®lm was 2 mm. The ion dose was up to 2 1014 cm 2 . The
57
energy loss in the C-®lm for 200 MeV I, 70 MeV Ni and 80 MeV S was calculated as 1.6, 0.8 and 0.4 MeV, respectively [5]. The ion energy after transmission through the C-foil is given in Table 1. Bohr energy straggling in the C-®lm was calculated as 0.07, 0.035 and 0.02 MeV for I, Ni and S ions, respectively. The energy loss and the energy straggling in the C-®lm are very small, and do not disturb the present experimental study. The ion charge incident on YBCO samples, which diers from that on the C-®lms, may aect sputtering and it is assumed to reach the equilibrium during transmission of the C-®lm. The mean charge Q of ions after transmission through the C-®lm is estimated from [6] and is also given in Table 1. Sputtered atoms, which were collected in the C®lms, were analyzed by MeV He Rutherford backscattering spectroscopy (RBS) using a Van de Graa accelerator at Nagoya University (NU).
3. Results and discussion 3.1. Atoms collected in C-®lm during ion irradiation
Fig. 1. Sketch of C-®lm collector and sample assembly for ion irradiation. C-®lm is supported on Al plate (0.4 mm thick) with holes of 4 mm in diameter. Samples are supported on Al (2 mm).
Fig. 2 shows RBS spectra of the C-®lm collector after irradiation of 69 MeV Ni at a dose of 0:8 1014 cm 2 . Be spectrum appears, because the C-®lms were supported by Be for RBS analysis. Cu, Y and Ba atoms are seen around the channel number of 680, 730 and 780, respectively. O atoms are noticed around the channel number of 310 in the inset (indicated by ///). RBS analysis shows that O of 1±2% exists in the virgin C-®lms and that O of 20±50 1015 cm 2 and C of 3±30 1015 cm 2 exist in Be support. RBS and 12 C d; p13 C nuclear
Table 1 Sputtering yields Y (atoms per ion) of YBCO samples by high energy heavy ion irradiationa Ion
E (MeV)
Q
Y
Ba
Cu
Yexp
Se (keV/nm)
Sn (keV/nm)
Yc
127 12
198 69 80
30 18 13
73 25 6.3
375 290 22
279 164 12
727 479 40
26.1 14.1 6.68
0.10 0.0372 0.00687
0.42 0.16 0.029
I Ni6 32 7 S 58
a
E and Q are the mean energy and mean charge after transmission through C-®lm (100 nm). Yexp is the sum of Y, Ba and Cu sputtering yields. Se and Sn are the electronic (inelastic) and nuclear (elastic) energy losses, respectively. Yc is the calculated sputtering yield due to the elastic collisions.
58
N. Matsunami et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 56±61
Fig. 2. RBS spectra of C-®lm collector after irradiation with 69 MeV Ni on YBCO sample at a dose of 0:8 1014 cm 2 . RBS analysis was performed using 1.8 MeV He with normal incidence and 160° scattering angle. O peaks in an expanded scale is shown in the inset(o), together with RBS spectra of unirradiated C-®lm(+). O(Be) peak and O(///) indicate O in Be and sputtered O atoms in the C-®lm from the YBCO sample, respectively.
reaction analysis with 1.2 MeV deuterium show no appreciable increase of C impurities in YBCO samples and reduction of C impurities in Be samples by 198 MeV I ion irradiation at a dose of 3 1014 cm 2 , indicating no recoil eects of C from the C-®lm into samples. The numbers of collected atoms in the C-®lm are plotted against dose in Fig. 3. Atoms collected in the C-®lm are nearly proportional to the dose. At small doses, these are somewhat larger than the average values indicated by the dashed lines. This could be partly due to errors of dose and RBS intensity for small dose, and also surface contamination eects. Another possibility is that the collection eciency of the C-®lm is larger at smaller dose than that at larger dose. Ion irradiation eects on the collection eciency will be discussed in the next section. Similar dose dependence was observed for 198 MeV I and 80
Fig. 3. Y , Ba M, Cu and O } atoms collected in C®lm vs dose for 79 MeV Ni irradiation on YBCO ®lms on MgO. Also plotted are Y , Ba N, Cu and O atoms from thick polycrystalline YBCO sample. Average of Y is drawn by dashed lines.
N. Matsunami et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 56±61
MeV S ion irradiation. The data in the region of <1 1014 cm 2 was used to derive the sputtering yields.
59
®lm collector dose not aect the collection eciency of sputtered atoms. 3.3. Sputtering yield
3.2. Collection eciency of C-®lm The collection eciency fc , which is given by fc atoms collected in C-®lm/(Ne dose sputtering yield composition ratio), was calibrated using the same C-®lm and sample assembly shown in Fig. 1, and 120 keV Ne ions obtained from a 200 kV Cockcroft-Walton accelerator at NU. The energy loss and straggling of Ne in the C-®lms were estimated to be 60 and 13 keV, respectively. RBS spectra similar to those in Fig. 2 were obtained. Sputtering yield of YBCO by 120 keV Ne through the C-®lm was experimentally determined as 1.1 from the thickness decrease of YBCO ®lms, using the method in [7]. The composition ratios are taken as 1/13, 2/13, 3/13 and 7/13 for Y, Ba, Cu and O, respectively, assuming stoichiometric sputtering [7]. The value of fc were obtained as 0.048, 0.076, 0.061 and 0.3 for Y, Ba, Cu and O, respectively, at Ne dose of 5±23 1015 cm 2 . An estimated error of fc for Y, Ba and Cu is 20%, and fc for O may have uncertainty of a factor of 2±3, due to large background correction as shown in the inset in Fig. 2. We have also calibrated fc for Au and it is found that values of fc for Y, Ba and Cu are much smaller than those for O and Au (fc 0:2) [8]. The sputtering yields of polycrystalline Au by high energy heavy ions have been measured by using the C-®lm collector method and it appears that the yields agree well with the calculated sputtering yields due to the elastic collision cascade [8], which is expected to be dominant for polycrystalline Au. This con®rms the validity of the C-®lm collector method. The collection eciency might be modi®ed by ion irradiation and this can be inferred from the part of C-®lm supported on Al. The eciency under no ion irradiation was obtained as 1/3 to 1 of fc mentioned above and this variation can be explained by angular dependence of the sputtering yields. This indicates that ion irradiation on the C-
The sputtering yield of each component was derived from the slope in the dose (0.1± 1 1014 cm 2 ) as shown in Fig. 3 and fc . The results are summarized in Table 1. The oxygen sputtering yields were obtained as roughly 300, 100 and 60 for 198 MeV I, 69 MeV Ni and 80 MeV S ion irradiation, respectively. The sputtering yields of thick-polycrystalline YBCO samples were also measured and agree reasonably with those of YBCO samples, except for the Ba yield at a small dose. The reason for this dierence is not understood at present. Excluding the exception, the above result leads to the fact that ion beam mixing of YBCO ®lms and MgO substrate, which will be described in Section 3.4, does not in¯uence the sputtering of YBCO ®lms. The sputtering yields do not follow the composition ratio. The sputtering yield Yc is calculated assuming that Yc is proportional to the nuclear (elastic) energy loss, Sn , with the experimental values of 2.5 and 1.1 for 100 keV Ar and Ne, respectively [6]. It appears that TRIM (both 1992 and 1997 versions) simulation results are smaller by a factor of 3 than Yc . The experimental sputtering yields are larger by 1000 than
Fig. 4. Sputtering yield Yexp versus electronic stopping power, Se . Closed and open circles show the sum of Y, Ba and Cu yields, and sum of Y, Ba, Cu and O yields, respectively.
60
N. Matsunami et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 56±61
Yc , indicating the considerable contribution of EED. Fig. 4 shows the experimental sputtering yields Yexp versus the electronic stopping power Se . Yexp is nearly proportional to Sen with n 2. The exponent is smaller than fourth power for lattice parameter elongation [2]. The square dependence is much stronger than the Se dependence of track radii [3]. These indicate that additional and/or dierent processes are involved in lattice parameter elongation and track formation. One notices that variation of Yexp is much stronger than that of the mean charge Q, implying no eects of multiple charges carried by ions on the sputtering. Finally, the dose of superconductor to normal phase transition are estimated
to be roughly 1, 3 and 10 1011 cm 2 for 198 MeV I, 69 MeV Ni and 80 MeV S, respectively, considering the factor of 1000 and displacement per atoms due to the elastic collision cascade (DPA) are 0.033, 0.012 and 0.0029 at 1014 cm 2 , respectively (DPA for the transition 0.03 [9]). The sputtering yields obtained above are not for YBCO at superconductor, but normal phase. The details are under investigation. 3.4. Ion beam mixing and composition modi®cation Fig. 5 shows the RBS spectra of YBCO before and after 69 MeV Ni ion irradiation at a dose of 2 1014 cm 2 . One can see mixing of YBCO ®lm
Fig. 5. RBS spectra of YBCO ®lm on MgO: (a) unirradiated and (b) after 69 MeV Ni ion irradiation at a dose of 2 1014 cm 2 , illustrating mixing of YBCO ®lm and MgO substrate, and composition modi®cation. RBS was performed using 2.5 MeV He at normal incidence and 160° scattering angle.
N. Matsunami et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 56±61
and MgO substrate, and composition modi®cation of YBCO ®lms. Ion beam mixing and composition modi®cation were observed for 198 MeV I at dose >0:5 1014 cm 2 , 69 MeV Ni at dose >0:8 1014 cm 2 . These were not observed for 198 MeV I at dose <0:17 1014 cm 2 , 69 MeV Ni at dose <0:2 1014 cm 2 , and 80 MeV S at dose <0:8 1014 cm 2 . Ion beam mixing and composition modi®cation at small dose are insigni®cant, and hence do not aect the sputtering yields discussed above. 4. Conclusion We have measured the sputtering yields from YBCO samples with high energy heavy ions, by using C-®lm collector method. The total sputtering yields are larger by 103 than those of calculations based on the nuclear energy deposition, indicating considerable electronic excitation effects on atomic displacements. The total sputtering yields scale well with the square of the electronic stopping power. Ion beam mixing and composition modi®cation are observed only for high irradiation doses and do not aect the present results.
61
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