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Observation of defect formation process in silica glasses under ion irradiation
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 250 (2006) 174–177 www.elsevier.com/locate/nimb
Observation of defect formation process in silica glasses under ion irradiation M. Watanabe a, T. Yoshida a
b
a,*
, T. Tanabe b, S. Muto a, A. Inoue c, S. Nagata
c
Division of Quantum Science and Energy Engineering, Department of Materials, Physics and Energy Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan Department of Advanced Energy Engineering Science, Interdisciplinary Graduated School of Engineering Sciences, Kyushu University, Hakozaki, Higashi-ku, Hukuoka 812-8581, Japan c Institute for Materials Research, Tohoku University, Katahira, Aoba-ku, Sendai 980-8577, Japan Available online 14 July 2006
Abstract We have performed in situ measurements of luminescence from silica glass induced by H+ beam at various energies to investigate the dynamic defect formation process in a silica glass. The luminescence spectra showed a broad emission band centered at around 2.7 eV assigned to B2a oxygen deficient centers. The intensity of the 2.7 eV band rapidly increased to H+ fluence at first, and then gradually increased to a steady value. We found that the change in the intensity of the 2.7 eV band relates to two processes, fast and slow processes with different reaction rates for producing luminescence centers. The dependences of calculated reaction rates for faster and slower transformation versus H+ energy correspond well to those of electronic and nuclear stopping power, respectively. Consequently, the production of the luminescence centers under sub-MeV H+ irradiation is very likely dominated by electron excitation process at first, and subsequently, by nuclear collision process. 2006 Elsevier B.V. All rights reserved. PACS: 78.55.m; 78.66.Jg
1. Introduction Silica (SiO2) glass is a candidate for a window and an optical fiber in fusion reactors. Its optical properties, however, are easily deteriorated by defects produced in it under such a heavy radiation environment in fusion reactors. It is well known that under the irradiation of high-energy particles and/or photons, both electron excitation and nuclear collision create various defects and/or change the chemical and electronic states of intrinsic defects in silica glass. Therefore, the understanding of the effects of these processes on the defect production is important for improving radiation resistance and lifetime assessment of the silica glass.
*
Corresponding author. Tel.: +81 52 789 5135. E-mail address: [email protected] (T. Yoshida).
0168-583X/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2006.04.103
In situ measurements of radiation-induced luminescence are one of the effective ways to investigate the dynamics of radiation damage in insulators. We had already examined the radiation-induced luminescence of silica glasses under the irradiation of ions [1], c-rays and X-rays [2] and neutrons [3]. Among all, the ion induced-luminescence is the most useful to investigate the dynamics of the damaging process because it is easy to change ion species, incident energies and fluxes, target materials and so on and hence easy to control the ratio of energy deposition due to electron excitation and nuclear collision processes. This allows us a further understanding of damaging process by each energy deposition. In our previous work [4], we measured ion-induced luminescence of silica glass under 10–20 keV H+ irradiation, and found that the ion-induced luminescence is caused by electron excitation and evolved by creation and annihilation of damages produced by atomic displacement
M. Watanabe et al. / Nucl. Instr. and Meth. in Phys. Res. B 250 (2006) 174–177
2. Experimental details Silica samples used in this study were high-OH synthesized silica glasses (T4040, OH content 800 ppm) manufactured by Toshiba Ceramics, Japan. The samples have very few intrinsic defects except hydroxyl radicals and hence show no specific PL (photoluminescence) peak before irradiation. Experiments were carried out in a conventional stainless steel vacuum chamber whose vacuum was maintained below 5.0 · 106 Pa during the ion-induced luminescence measurements. H+ ions accelerated up to 1 MeV were injected with flux of 4.8–10 · 1013 ions/cm2 s at room temperature by a 1.7 MV tandem accelerator at Institute for Materials Research, Tohoku University. The ion-induced luminescence was observed by a spectrometer with a photonic multi-channel analyzer (PMA-11, Hamamatsu Photonics) through an optical fiber. 3. Results and discussions Fig. 1 shows ion-induced luminescence spectra (IL) of a silica glass under 700 keV H+ injection with H+ fluence ranging 1015–17 cm2. The luminescence spectra consist of two main bands centered at 1.9 eV and 2.7 eV, respectively, assigned to NBOHC (non-bridging oxygen hole centers) and B2a oxygen deficient centers [5–8]. Due to the small decrease in intensity of the 1.9 eV band as function of the fluence, we shall not discuss it in this study. The change in the intensity of the 2.7 eV band with fluence is shown in Fig. 2 for different energies of the incident H+ ion. The band intensities are normalized by the H+ ion flux measured by a Faraday cup. The intensity of the 2.7 eV band rapidly increased to H+ fluence at first, and then gradually increased to a steady value. In the present study,
10 1000 keV 8 Intensity (a.u.)
(nuclear collision). In the present work, the incident H+ energy is extended from keV to sub-MeV range, to investigate the defect production process under high-density electron excitation.
175
800 keV
6 700 keV 4
600 keV
2 0 0.0
0.5 1.0 1.5 1017 + H fluence (cm-2)
Fig. 2. Changes in the intensities of the emission band at 2.7 eV with H+ fluence.
we focus on the 2.7 eV band, which must be related to oxygen deficiencies produced by projected ions. Change in the intensity of the 2.7 eV band with fluence did not fit to single exponential saturation. Instead, two exponential saturations below Eq. (1) fit very well IðtÞ ¼ A1 ð1 expðk 1 tÞÞ þ A2 ð1 expðk 2 tÞÞ
ð1Þ
as shown in Fig. 3. This indicates that the time evolution involves two independent first order reaction processes with different rate constants (k1 and k2). For each incident energy, a set of k1 and k2 were determined and plotted on the left axis in Fig. 4. Since the energy deposition into the silica glass is brought by nuclear collision and electron excitation, the two different processes are supposed to correspond to respective energy deposition processes, i.e. the reaction rate constants k1 and k2 could correlate to the nuclear and electronic stopping powers, respectively. The stopping powers averaged over the ion range are plotted against the incident H+ energy on the right axis in Fig. 4. It should be noted that both incident energy dependences are quite consistent with those of k1 and k2, respectively. This agreement supports the above supposition. Since k2 are one or two orders of magnitude larger than k1, the production of the luminescence centers under sub-MeV H+
6 17
1.6
10 (cm-2)
1.6
10 (cm-2)
5
800 600 400
1.6 1.6 1.6
15
10 (cm-2) 16 10 (cm-2) 17 10 (cm-2)
1.6
Intensity (a.u.)
Intensity (a.u.)
1000
16
10
15
(cm-2)
200
4 3 2 1
0 2.0
2.5 3.0 Photon energy (eV)
3.5
4.0
Fig. 1. Luminescence spectra of a silica glass under 700 keV H+ irradiation with three different H+ fluence of 1.6 · 1015, 1.6 · 1016 and 1.6 · 1017 cm2.
0 0.0
data fitting 0.5 1.0 1.5 1017 + H fluence (cm-2)
Fig. 3. Change in the intensity of the 2.7 eV band with H+ fluence under the irradiation of 700 keV H+ ion, and its fitting with Eq. (1).
176
M. Watanabe et al. / Nucl. Instr. and Meth. in Phys. Res. B 250 (2006) 174–177 5
k1 Sn
1.0 10-16
1.2 10
300
Sn Se
500
0.6
150
0.4
100
0.2
50
1.0
400
0.8
300
0.6
200
0.4
100
0.2
Se (eV/μm)
200
Sn (eV/μm)
k 1 (s-1 )
0.8
Sn (eV/μm)
250
(a) 0.0 0.0
0.2
0.4 0.6 0.8 Energy (MeV)
0 1.0
0 0
1.4 10
1.0 10 0.8
0.8
0.6
0.6
0.4
0.4
k2 linear regression of k2 Se
(b)
0.0 0.0
0.2
0.4 0.6 0.8 Energy (MeV)
Se (eV/μm)
1.0
0.2 0.0
1.0
Fig. 4. (a) The plot of calculated values of k1 (on the left axis) and nuclear stopping powers (on the right axis) against incident H+ ion energy. (b) The plot of calculated values of k2 (on the left axis) and electron stopping powers (on the right axis) against incident H+ ion energy.
irradiation is first dominated by the electron excitation and succeeded with gradual increase due to the nuclear collision process. It should be mentioned that the saturated luminescence intensities after the prolong irradiation with different H+ energies showed good linear relationship with the total deposited energy over the whole range from the surface to the projected range by the electron stopping as shown in Fig. 5. This is a clear indication that the IL is caused by the excitation of the luminescence centers (oxygen defi-
Saturated luminescence intensity (a.u.)
0.0 20
5
1.2 k 2 (s-1)
10 15 Depth (μm)
Fig. 6. The depth distribution of the electron stopping power (dotted line) and nuclear stopping power (solid line) under 1 MeV H+ irradiation.
-1 5
0.2
5
10 8 6 4 2 0 6
0.0 0.2 0.4 0.6 0.8 1.0 10 Energy deposition by electron excitation (eV)
Fig. 5. Comparison of saturated luminescence intensity with total energy deposition by electron excitation.
ciencies) by energetic electrons produced by the electron stopping, and the IL intensity increase is caused by the production of the luminescence centers by the electron excitation and the atomic displacement. Still a question remains: why was the time evolution so clearly divided into two processes? Under sub-MeV H+ irradiation, incident H+ ions first deposit their energy by the electron stopping rather homogeneously in a whole range from the surface to the ion projected range, then by the nuclear stopping near the end of the ion trajectory, a little shallower region than the projected range as shown in Fig. 6, which shows the depth distribution of the deposited energy by the electron stopping and nuclear stopping in the silica glass under 1 MeV H+ irradiation calculated by TRIM [9] code. As already noted, one can clearly see that the energy deposition due to the electron stopping Se is much larger than that due to the nuclear stopping Sn. Thus we can conclude that the observed IL change is caused by the superposition of the two processes, (1) the fast process due to the electron excitation over the whole range from the surface to the projected range, and (2) the slow process to the nuclear collision process near end of the ion trajectory. 4. Conclusions We have measured H+ ion induced luminescence of a silica glass under sub-MeV H+ ion irradiation by varying incident H+ energy to investigate the dynamic process of defects production. The ion-induced luminescence (IL) spectra of the silica glass showed a broad emission band centered at 2.7 eV assigned to B2a oxygen deficient center. The intensities of the 2.7 eV band changed with ion fluence; the 2.7 eV band rapidly increased at first with H+ fluence, followed by gradual increase. The change in the 2.7 eV IL band, i.e. the first rapid increase and the subsequent general increase, are probably attributed to production of oxygen deficiency by electron excitation homogeneously given from the surface to the projected range and the nuclear collision, in the narrow damaged region, respectively.
M. Watanabe et al. / Nucl. Instr. and Meth. in Phys. Res. B 250 (2006) 174–177
References [1] M. Fujiwara, T. Tanabe, H. Miyamaru, K. Miyazaki, Nucl. Instr. and Meth. B 116 (1996) 536. [2] T. Yoshida, T. Tanabe, T. Ii, H. Yoshida, Nucl. Instr. and Meth. B 191 (1–4) (2002) 382. [3] T. Yoshida, T. Tanabe, T. Ii, T. Hara, M. Sakai, Y. Inaki, Nucl. Instr. and Meth. B 166–167 (2000) 476. [4] T. Yoshida, T. Tanabe, M. Watanabe, S. Takahara, S. Mizukami, J. Nucl. Mater. 329–333 (2004) 982.
177
[5] M. Goldberg, H.–J. Fitting, A. Trukhin, J. Non-Cryst. Solids 220 (1997) 69. [6] A.J. Miller, R.G. Leisure, Wm.R. Austin, J. Appl. Phys. 86 (1999) 2042. [7] S. Nagata, S. Yamamoto, K. Toh, B. Tsuchiya, N. Ohtsu, T. Shikama, H. Naramoto, J. Nucl. Mater. 329–333 (2004) 1507. [8] L.H. Abu-Hassan, P.D. Townsend, Nucl. Instr. and Meth. B 32 (1988) 293. [9] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Ranges of Ions in Solids, Pergamon Press, New York, 1985.
Nuclear Instruments and Methods in Physics Research B 250 (2006) 174–177 www.elsevier.com/locate/nimb
Observation of defect formation process in silica glasses under ion irradiation M. Watanabe a, T. Yoshida a
b
a,*
, T. Tanabe b, S. Muto a, A. Inoue c, S. Nagata
c
Division of Quantum Science and Energy Engineering, Department of Materials, Physics and Energy Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan Department of Advanced Energy Engineering Science, Interdisciplinary Graduated School of Engineering Sciences, Kyushu University, Hakozaki, Higashi-ku, Hukuoka 812-8581, Japan c Institute for Materials Research, Tohoku University, Katahira, Aoba-ku, Sendai 980-8577, Japan Available online 14 July 2006
Abstract We have performed in situ measurements of luminescence from silica glass induced by H+ beam at various energies to investigate the dynamic defect formation process in a silica glass. The luminescence spectra showed a broad emission band centered at around 2.7 eV assigned to B2a oxygen deficient centers. The intensity of the 2.7 eV band rapidly increased to H+ fluence at first, and then gradually increased to a steady value. We found that the change in the intensity of the 2.7 eV band relates to two processes, fast and slow processes with different reaction rates for producing luminescence centers. The dependences of calculated reaction rates for faster and slower transformation versus H+ energy correspond well to those of electronic and nuclear stopping power, respectively. Consequently, the production of the luminescence centers under sub-MeV H+ irradiation is very likely dominated by electron excitation process at first, and subsequently, by nuclear collision process. 2006 Elsevier B.V. All rights reserved. PACS: 78.55.m; 78.66.Jg
1. Introduction Silica (SiO2) glass is a candidate for a window and an optical fiber in fusion reactors. Its optical properties, however, are easily deteriorated by defects produced in it under such a heavy radiation environment in fusion reactors. It is well known that under the irradiation of high-energy particles and/or photons, both electron excitation and nuclear collision create various defects and/or change the chemical and electronic states of intrinsic defects in silica glass. Therefore, the understanding of the effects of these processes on the defect production is important for improving radiation resistance and lifetime assessment of the silica glass.
*
Corresponding author. Tel.: +81 52 789 5135. E-mail address: [email protected] (T. Yoshida).
0168-583X/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2006.04.103
In situ measurements of radiation-induced luminescence are one of the effective ways to investigate the dynamics of radiation damage in insulators. We had already examined the radiation-induced luminescence of silica glasses under the irradiation of ions [1], c-rays and X-rays [2] and neutrons [3]. Among all, the ion induced-luminescence is the most useful to investigate the dynamics of the damaging process because it is easy to change ion species, incident energies and fluxes, target materials and so on and hence easy to control the ratio of energy deposition due to electron excitation and nuclear collision processes. This allows us a further understanding of damaging process by each energy deposition. In our previous work [4], we measured ion-induced luminescence of silica glass under 10–20 keV H+ irradiation, and found that the ion-induced luminescence is caused by electron excitation and evolved by creation and annihilation of damages produced by atomic displacement
M. Watanabe et al. / Nucl. Instr. and Meth. in Phys. Res. B 250 (2006) 174–177
2. Experimental details Silica samples used in this study were high-OH synthesized silica glasses (T4040, OH content 800 ppm) manufactured by Toshiba Ceramics, Japan. The samples have very few intrinsic defects except hydroxyl radicals and hence show no specific PL (photoluminescence) peak before irradiation. Experiments were carried out in a conventional stainless steel vacuum chamber whose vacuum was maintained below 5.0 · 106 Pa during the ion-induced luminescence measurements. H+ ions accelerated up to 1 MeV were injected with flux of 4.8–10 · 1013 ions/cm2 s at room temperature by a 1.7 MV tandem accelerator at Institute for Materials Research, Tohoku University. The ion-induced luminescence was observed by a spectrometer with a photonic multi-channel analyzer (PMA-11, Hamamatsu Photonics) through an optical fiber. 3. Results and discussions Fig. 1 shows ion-induced luminescence spectra (IL) of a silica glass under 700 keV H+ injection with H+ fluence ranging 1015–17 cm2. The luminescence spectra consist of two main bands centered at 1.9 eV and 2.7 eV, respectively, assigned to NBOHC (non-bridging oxygen hole centers) and B2a oxygen deficient centers [5–8]. Due to the small decrease in intensity of the 1.9 eV band as function of the fluence, we shall not discuss it in this study. The change in the intensity of the 2.7 eV band with fluence is shown in Fig. 2 for different energies of the incident H+ ion. The band intensities are normalized by the H+ ion flux measured by a Faraday cup. The intensity of the 2.7 eV band rapidly increased to H+ fluence at first, and then gradually increased to a steady value. In the present study,
10 1000 keV 8 Intensity (a.u.)
(nuclear collision). In the present work, the incident H+ energy is extended from keV to sub-MeV range, to investigate the defect production process under high-density electron excitation.
175
800 keV
6 700 keV 4
600 keV
2 0 0.0
0.5 1.0 1.5 1017 + H fluence (cm-2)
Fig. 2. Changes in the intensities of the emission band at 2.7 eV with H+ fluence.
we focus on the 2.7 eV band, which must be related to oxygen deficiencies produced by projected ions. Change in the intensity of the 2.7 eV band with fluence did not fit to single exponential saturation. Instead, two exponential saturations below Eq. (1) fit very well IðtÞ ¼ A1 ð1 expðk 1 tÞÞ þ A2 ð1 expðk 2 tÞÞ
ð1Þ
as shown in Fig. 3. This indicates that the time evolution involves two independent first order reaction processes with different rate constants (k1 and k2). For each incident energy, a set of k1 and k2 were determined and plotted on the left axis in Fig. 4. Since the energy deposition into the silica glass is brought by nuclear collision and electron excitation, the two different processes are supposed to correspond to respective energy deposition processes, i.e. the reaction rate constants k1 and k2 could correlate to the nuclear and electronic stopping powers, respectively. The stopping powers averaged over the ion range are plotted against the incident H+ energy on the right axis in Fig. 4. It should be noted that both incident energy dependences are quite consistent with those of k1 and k2, respectively. This agreement supports the above supposition. Since k2 are one or two orders of magnitude larger than k1, the production of the luminescence centers under sub-MeV H+
6 17
1.6
10 (cm-2)
1.6
10 (cm-2)
5
800 600 400
1.6 1.6 1.6
15
10 (cm-2) 16 10 (cm-2) 17 10 (cm-2)
1.6
Intensity (a.u.)
Intensity (a.u.)
1000
16
10
15
(cm-2)
200
4 3 2 1
0 2.0
2.5 3.0 Photon energy (eV)
3.5
4.0
Fig. 1. Luminescence spectra of a silica glass under 700 keV H+ irradiation with three different H+ fluence of 1.6 · 1015, 1.6 · 1016 and 1.6 · 1017 cm2.
0 0.0
data fitting 0.5 1.0 1.5 1017 + H fluence (cm-2)
Fig. 3. Change in the intensity of the 2.7 eV band with H+ fluence under the irradiation of 700 keV H+ ion, and its fitting with Eq. (1).
176
M. Watanabe et al. / Nucl. Instr. and Meth. in Phys. Res. B 250 (2006) 174–177 5
k1 Sn
1.0 10-16
1.2 10
300
Sn Se
500
0.6
150
0.4
100
0.2
50
1.0
400
0.8
300
0.6
200
0.4
100
0.2
Se (eV/μm)
200
Sn (eV/μm)
k 1 (s-1 )
0.8
Sn (eV/μm)
250
(a) 0.0 0.0
0.2
0.4 0.6 0.8 Energy (MeV)
0 1.0
0 0
1.4 10
1.0 10 0.8
0.8
0.6
0.6
0.4
0.4
k2 linear regression of k2 Se
(b)
0.0 0.0
0.2
0.4 0.6 0.8 Energy (MeV)
Se (eV/μm)
1.0
0.2 0.0
1.0
Fig. 4. (a) The plot of calculated values of k1 (on the left axis) and nuclear stopping powers (on the right axis) against incident H+ ion energy. (b) The plot of calculated values of k2 (on the left axis) and electron stopping powers (on the right axis) against incident H+ ion energy.
irradiation is first dominated by the electron excitation and succeeded with gradual increase due to the nuclear collision process. It should be mentioned that the saturated luminescence intensities after the prolong irradiation with different H+ energies showed good linear relationship with the total deposited energy over the whole range from the surface to the projected range by the electron stopping as shown in Fig. 5. This is a clear indication that the IL is caused by the excitation of the luminescence centers (oxygen defi-
Saturated luminescence intensity (a.u.)
0.0 20
5
1.2 k 2 (s-1)
10 15 Depth (μm)
Fig. 6. The depth distribution of the electron stopping power (dotted line) and nuclear stopping power (solid line) under 1 MeV H+ irradiation.
-1 5
0.2
5
10 8 6 4 2 0 6
0.0 0.2 0.4 0.6 0.8 1.0 10 Energy deposition by electron excitation (eV)
Fig. 5. Comparison of saturated luminescence intensity with total energy deposition by electron excitation.
ciencies) by energetic electrons produced by the electron stopping, and the IL intensity increase is caused by the production of the luminescence centers by the electron excitation and the atomic displacement. Still a question remains: why was the time evolution so clearly divided into two processes? Under sub-MeV H+ irradiation, incident H+ ions first deposit their energy by the electron stopping rather homogeneously in a whole range from the surface to the ion projected range, then by the nuclear stopping near the end of the ion trajectory, a little shallower region than the projected range as shown in Fig. 6, which shows the depth distribution of the deposited energy by the electron stopping and nuclear stopping in the silica glass under 1 MeV H+ irradiation calculated by TRIM [9] code. As already noted, one can clearly see that the energy deposition due to the electron stopping Se is much larger than that due to the nuclear stopping Sn. Thus we can conclude that the observed IL change is caused by the superposition of the two processes, (1) the fast process due to the electron excitation over the whole range from the surface to the projected range, and (2) the slow process to the nuclear collision process near end of the ion trajectory. 4. Conclusions We have measured H+ ion induced luminescence of a silica glass under sub-MeV H+ ion irradiation by varying incident H+ energy to investigate the dynamic process of defects production. The ion-induced luminescence (IL) spectra of the silica glass showed a broad emission band centered at 2.7 eV assigned to B2a oxygen deficient center. The intensities of the 2.7 eV band changed with ion fluence; the 2.7 eV band rapidly increased at first with H+ fluence, followed by gradual increase. The change in the 2.7 eV IL band, i.e. the first rapid increase and the subsequent general increase, are probably attributed to production of oxygen deficiency by electron excitation homogeneously given from the surface to the projected range and the nuclear collision, in the narrow damaged region, respectively.
M. Watanabe et al. / Nucl. Instr. and Meth. in Phys. Res. B 250 (2006) 174–177
References [1] M. Fujiwara, T. Tanabe, H. Miyamaru, K. Miyazaki, Nucl. Instr. and Meth. B 116 (1996) 536. [2] T. Yoshida, T. Tanabe, T. Ii, H. Yoshida, Nucl. Instr. and Meth. B 191 (1–4) (2002) 382. [3] T. Yoshida, T. Tanabe, T. Ii, T. Hara, M. Sakai, Y. Inaki, Nucl. Instr. and Meth. B 166–167 (2000) 476. [4] T. Yoshida, T. Tanabe, M. Watanabe, S. Takahara, S. Mizukami, J. Nucl. Mater. 329–333 (2004) 982.
177
[5] M. Goldberg, H.–J. Fitting, A. Trukhin, J. Non-Cryst. Solids 220 (1997) 69. [6] A.J. Miller, R.G. Leisure, Wm.R. Austin, J. Appl. Phys. 86 (1999) 2042. [7] S. Nagata, S. Yamamoto, K. Toh, B. Tsuchiya, N. Ohtsu, T. Shikama, H. Naramoto, J. Nucl. Mater. 329–333 (2004) 1507. [8] L.H. Abu-Hassan, P.D. Townsend, Nucl. Instr. and Meth. B 32 (1988) 293. [9] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Ranges of Ions in Solids, Pergamon Press, New York, 1985.