- Email: [email protected]
High-resolution depth profiling of soda-lime silicate glass using high-resolution RBS and ERDA
Nuclear Inst. and Methods in Physics Research B 440 (2019) 60–63
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
High-resolution depth profiling of soda-lime silicate glass using highresolution RBS and ERDA
T
Hiroki Sakataa, Yuichi Yamamotob, Koya Satoa, Kaoru Nakajimaa, Kenji Kimuraa,
⁎
a b
Department of Micro Engineering, Kyoto University, Kyoto 615-8540, Japan Innovative Technology Research Center, ASAHI Glass Co., Ltd., Hazawa-cho, Kanagawa-ku, Yokohama 221-8755, Japan
ARTICLE INFO
ABSTRACT
Keywords: High-resolution RBS High-resolution ERDA Soda-lime silicate glass Depth profiling
Composition depth profiles of soda-lime silicate glasses were measured using high-resolution Rutherford backscattering spectroscopy (HRBS) and high-resolution elastic recoil detection analysis (HERDA). Surface enrichment of modifier cations (Na+ and Ca2+) followed by a depletion in the deeper regions was observed using HRBS. The observed surface enrichment was attributed to the effect of radiation damage caused by the He+ irradiation. It was shown that the precise depth profiling of glass surfaces can be performed using HRBS when the fluence is limited below 3 × 1014 He ions cm−2. HERDA measurements demonstrated that there is a substantial amount of hydrogen (7.3 ± 1.3 × 1021 hydrogen cm−3) in the subsurface region as well as on the surface (1.6 ± 0.2 × 1015 hydrogen cm−2), which is very different from the silica glass surface where no subsurface hydrogen was observed.
1. Introduction Glasses have been used in a variety of applications for more than four thousand years. Although the traditional applications, such as buildings and automobiles are still major applications, glasses are also used in new fields, for example, used as substrates for solar cells and display devices. In these new applications characterization and control of glass surfaces and interfaces are of prime importance. Secondary ion mass spectrometry (SIMS) is often used to measure elemental depth profiling. SIMS, however, suffers from artificial effects, that is, the matrix effect and the surface transient effect. Particularly for the analysis of glass surfaces, it is known that the SIMS measurement strongly disturbs the distribution of modifier elements, such as sodium, calcium and magnesium [1,2]. Recent advancement of X-ray photoelectron spectroscopy (XPS) with the help of C60 ion sputtering allows elemental depth profiling of glass surfaces without such artifacts [3–7]. Unfortunately, however, XPS cannot measure hydrogen. In addition, the depth resolution of this method is a few nm and the improvement of the depth resolution is still desired for precise analysis. In this respect, high-resolution Rutherford backscattering spectroscopy (HRBS) and high-resolution elastic recoil detection analysis (HERDA) could be promising candidates because the depth resolutions of these techniques are sub-nm. Soda-lime silicate (SLS) glass is one of the most commonly used glasses. The most popular production process of the SLS glass is a so-
⁎
called float process in which molten glass is floated on a bath of molten tin during cooling. This process allows to produce flat glass sheets of large size. In this work, the surfaces of the SLS glass produced by the float process are observed using HRBS and HERDA. We examine if the precise composition depth profiling of the SLS glass, including hydrogen and modifier ions, is possible using HRBS and HERDA or not. 2. Experimental procedure In this study, commercial SLS glasses provided by Asahi Glass Co., Ltd. were analyzed using HRBS and HERDA. The details of the HRBS/ HERDA measurements were described elsewhere [8,9]. The scattering geometry was the IBM geometry for both HRBS and HERDA. For HRBS measurements, a beam of 400 keV He+ was produced by a 400 kV accelerator and collimated to 2 × 2 mm2 (typical beam current was 50 nA). The beam was sent to an ultra-high vacuum (UHV) scattering chamber (base pressure 1 × 10−8 Pa) via a differential pumping system. The glass sample was mounted on a 5-axis goniometer in the UHV chamber and irradiated with the He+ ion beam. Energy spectra of He+ ions scattered from the sample were measured by a 90° sector magnetic spectrometer. The UHV chamber had an electron flood gun (EFG, biased at −3 V) to prevent possible charging. For HERDA measurements, a collimated beam of 300 keV C+ ions (beam size was 2 × 2 mm2 and typical beam current was 5 nA) was
Corresponding author. E-mail address: [email protected] (K. Kimura).
https://doi.org/10.1016/j.nimb.2018.11.006 Received 18 July 2018; Received in revised form 7 October 2018; Accepted 5 November 2018 Available online 04 December 2018 0168-583X/ © 2018 Elsevier B.V. All rights reserved.
Nuclear Inst. and Methods in Physics Research B 440 (2019) 60–63
H. Sakata et al.
Fig. 1. HRBS spectra of silica glass observed under operation of an electron flood gun with various electric currents. The solid line shows the expected spectrum when there is no charging. The observed spectra agrees with the expected spectrum when the electric current is larger than 1.5 A.
Fig. 2. HRBS spectra measured for (a) top and (b) bottom surfaces of SLS glass. The dashed line shows the calculated spectrum for the SLS glass having a uniform and stoichiometric composition. In addition to the constituent elements, there are small Cu and Fe peaks, which may be surface-segregated trace elements. The amount of these elements are only 1.2 × 1013 cm−2 and 1.4 × 1013 cm−2, respectively.
used as a primary beam (the incident angle was 68°). Energy spectra of hydrogen ions recoiled from the sample at 25° were measured by the same spectrometer used in HRBS. A thin Mylar foil of 0.5 μm thickness coated with an aluminum layer (20 nm) was placed in front of the detector to reject carbon ions scattered from the sample [10]. A Kapton® thin film (Dupont-TORAY Co., Ltd.) with a thickness of 8 μm was used as a calibration standard for the hydrogen quantification.
glass. The incident angle was 40° and the fluence was 5.4 × 1015 ions cm−2. The top (bottom) surface means the surface which was not (was) contacted with the molten tin during the float process. The depth scales for tin and sodium are shown in the upper abscissa for reference. It is seen that the bottom surface contains a large amount of tin which was incorporated during the float process while the top surface does not. Except for this difference, both spectra are similar. The dashed line shows the calculated spectrum for the SLS glass having a uniform and stoichiometric composition (O – 60.4 at%, Si – 24.8 at%, Na – 9.3 at%, Ca – 3.0 at%, Mg – 2.2 at%, Al – 0.3 at%). The observed spectra deviate from the calculated one, especially around the leading edge of sodium. The composition depth profiles were derived from the observed HRBS spectra through spectrum simulations and the result of the top surface is shown in Fig. 3. Surface depletion of silicon and oxygen is clearly seen. On the other hand, sodium and calcium show surface enrichment followed by a slight depletion in deeper regions. The surface enrichment of sodium was often observed for the SLS glasses exposed to humid environment. The origin of the surface enrichment was attributed to the formation of sodium carbonate (Na2CO3) at the surface [11]. If this is the case, one sodium atom is accompanied by one and a half oxygen atoms. We estimated the oxygen profile from the observed silicon, tin, aluminum, calcium, magnesium and sodium profiles assuming that these elements are originating from SiO2, SnO2, Al2O3, CaO, MgO and Na2CO3, respectively. The result is shown by a thin solid line in Fig. 3. There is a large discrepancy between thus estimated profile and the observed one, especially in the surface region where sodium is enriched. On the other hand, if Na2O was formed instead of Na2CO3, the estimated oxygen profile agrees with the observed one very well as shown by a thin dashed line. This indicates that the observed surface enrichment of sodium cannot be ascribed to the formation of Na2CO3. Another possible origin of the observed surface enrichment of sodium could be radiation damage. It is known that the modifier cations, including Na+, are attached to non-bridging oxygen (NBO, Si–O−) in glass. If the Si–O–Si network is broken by the He+ irradiation, Na+ ions may move towards the newly created NBOs. In order to see if the radiation damage play a role or not, a series of short HRBS measurements were performed. Fig. 4 shows the measured yield in the energy region from 302 to 308 keV as a function of the irradiation time. This energy
3. Results and discussion 3.1. Suppression of charging using an electron flood gun Before analyzing the SLS glass surfaces, the performance of the electron flood gun was examined using a silica glass as a target. Fig. 1 shows HRBS spectrum observed without the electron flood gun (closed circles). The incident angle was 55° and the scattering angle was 50°. The arrows show the calculated energies of He+ ions scattered from surface silicon and oxygen atoms. The observed leading edges of these elements are located at larger energies than the calculated energies by several keV and the edges are very broad (note that the energy resolution of the present system is better than 1 keV). These observations are clear signatures of the charging due to the He+ irradiation. In order to prevent the charging, the electron flood gun was turned on and the EFG current was gradually increased. The HRBS spectra observed at various EFG currents are shown in Fig. 1. The observed spectrum did not change up to 1.0 A. After that the leading edges started to move toward lower energies and the yield decreased with increasing current. At 1.5 A, the observed spectrum agreed almost perfectly with the spectrum calculated using a simulation code (shown by a solid line), indicating that the charging was completely prevented by the electron flood gun with an adequate current. The measurements of both HRBS and HERDA shown in the following sections were done with the electron flood gun operating at 1.9 A. In these measurements, we also checked that the electron flood gun can successfully prevent the charging. It is noteworthy that the simulation code, which was used to calculate HRBS spectra, is based on the single scattering model. The scattering cross sections, stopping powers and energy loss straggling were calculated using the universal potential, SRIM code and the Yang’s energy loss straggling formula, respectively. 3.2. Results of HRBS measurements Fig. 2 shows the energy spectra of He+ ions scattered at 75° from the top (closed circles) and bottom (open circles) surfaces of the SLS 61
Nuclear Inst. and Methods in Physics Research B 440 (2019) 60–63
H. Sakata et al.
Fig. 5. HRBS spectrum obtained by summing up the results of several short measurements for different sample positions (fluence of each measurement is 3 × 1014 He ions cm−2 and the total fluence is 1.5 × 1015 He ions cm−2). The solid line shows the spectrum calculated for the uniform composition. The effect of the radiation damage on the sample composition is negligibly small in this low fluence measurement.
Fig. 3. Composition depth profiles of the top surface of SLS glass derived from the observed HRBS spectrum. The thin dashed line shows the oxygen profile estimated from the observed profiles of Si, Sn, Al, Ca, Mg and Na by assuming formation of SiO2, SnO2, Al2O3, CaO, MgO and Na2O, respectively. The thin solid line shows a similar result but assuming formation of Na2CO3 instead of Na2O (see text).
In passing, we note that due to the He+ irradiation sodium is depleted in the region from ∼14 nm to ∼32 nm and enriched in the region from the surface to ∼14 nm while calcium is depleted in the region from ∼1 nm to ∼15 nm and enriched in the very surface region (Fig. 3). This indicates that the radiation enhanced diffusion coefficient of sodium is several times larger than that of calcium. This may be attributed to the fact that Ca2+ needs two neighboring NBOs while Na+ needs only one NBO to create a new site to which they can move to. Another possibility is the mass difference between Ca and Na ions. The lighter mass might enhance the diffusion process. 3.3. Results of HERDA measurements Fig. 6 shows examples of energy spectra of recoiled H+ observed in a series of short HERDA measurements for the bottom surface of SLS glass. All spectra have a sharp peak at ∼70.5 keV, which corresponds to the hydrogen sitting on the surface. It is noteworthy that the sharpness of the observed peak demonstrates that the charging was successfully suppressed using the electron flood gun similarly to HRBS. In addition to the surface peak, there is a certain amount of hydrogen in the subsurface region at least up to 3 nm (see the depth scale shown in the
Fig. 4. Observed HRBS yield in the energy region from 302 to 308 keV as a function of irradiation time. This energy region corresponds to the depth region from the surface to 6 nm for sodium.
region corresponds to the depth region from the surface to 6 nm for sodium where sodium enrichment was observed (see Fig. 2). The observed yield increases with increasing irradiation time t, indicating that the observed surface enrichment of sodium is caused by the radiation damage. The result was fitted by Y(t) = Y1 − Y2 e−t/τ as shown by a dashed line. From this fitting, it can be concluded that if one wants to suppress the yield change less than 1%, the measurement time should be shorter than 50 s, which corresponds to a fluence of 3 × 1014 He ions cm−2. In order to confirm this conclusion, the HRBS measurements of the SLS glass were performed with a fluence of 3 × 1014 He ions cm−2 on several sample points. The measured HRBS spectra were summed up and shown in Fig. 5. The solid line shows the calculated spectrum for the uniform composition. Differently from the spectra shown in Fig. 2, the agreement between the observed and calculated spectra is very good. This clearly demonstrates that the precise depth profiling of the SLS glasses can be performed when the fluence is limited below 3 × 1014 He ions cm−2.
Fig. 6. Examples of HERDA spectra observed in a series of short time measurements for the bottom surface of SLS glass. 62
Nuclear Inst. and Methods in Physics Research B 440 (2019) 60–63
H. Sakata et al.
modifier ions play an important role in the hydrogen incorporation. In passing, we note that recent nuclear reaction analysis (NRA) also showed that the hydrogen concentration of SLS glass is as high as 4 × 1021 cm−3 at ∼20 nm from the surface and rapidly decreases with increasing depth [13]. 4. Conclusion The composition depth profiles of the SLS glass surfaces were measured using HRBS and HERDA. The charging caused by the irradiation of primary ion beams was successfully suppressed using an electron flood gun for both HRBS and HERDA. The HRBS measurements showed surface enrichment of sodium and calcium. The origin of the observed surface enrichment was attributed to the effect of radiation damage caused by the He+ irradiation. It was shown that the effect of the radiation damage on the HRBS spectrum is negligibly small when the fluence of the HRBS measurement is less than 3 × 1014 He ions cm−2. The HERDA measurements revealed that there is a substantial amount of hydrogen, about 10 at%, in the subsurface region in addition to the hydrogen on the outermost surface. The surface hydrogen decreased very rapidly during the measurement, while the hydrogen in the subsurface region is rather radiation resistant. The present results indicates that the suppression of both the charging and the radiation damage is crucial for the precise HRBS/HERDA analysis of the SLS glass surfaces.
Fig. 7. Surface density and subsurface concentration of hydrogen as a function of irradiation time. The hydrogen density and the subsurface concentration before irradiation can be estimated to 1.6 ± 0.2 × 1015 cm−2 and 7.3 ± 1.3 × 1021 cm−3 by extrapolating the fitting curves to t = 0 (see text).
upper abscissa). The observed surface peak becomes smaller with irradiation, while the hydrogen yield in the subsurface region seems constant. The surface hydrogen density and the hydrogen concentration in the subsurface region were estimated from the observed spectra. Fig. 7 shows the observed surface hydrogen density (solid circles) and the hydrogen concentration in a depth region from 2 nm to 3 nm (open circles) as a function of the irradiation time. As was mentioned above, the surface hydrogen density decreases rapidly with increasing irradiation time, while the hydrogen concentration in the subsurface region decreases much more slowly. These results were fitted by Y (t) = Y1 + Y2 e−t/τ as is shown by dashed lines. By extrapolating the fitting curves to t = 0, the surface hydrogen density and the hydrogen concentration in the subsurface region before the C+ irradiation were determined to be 1.6 ± 0.2 × 1015 cm−2 and 7.3 ± 1.3 × 1021 cm−3, respectively. The observed surface hydrogen density is at the same level as that of non-treated silica glass surfaces, which was mainly attributed to physically adsorbed water molecules [12]. The present result showed that there is a substantial amount of hydrogen (∼10 at%) also in the subsurface region of the SLS glass. This is very different from the silica glass surfaces, where hydrogen exists only on the surface and no subsurface hydrogen was observed by HERDA [12]. This suggests that the
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B.M.J. Smets, T.P.A. Lommen, J. Am. Ceram. Soc. 65 (1982) c-80. R.K. Brow, J. Non-Cryst. Solids 107 (1988) 1. Y. Yamamoto, K. Yamamoto, J. Non-Cryst. Solids 356 (2010) 14. Y. Yamamoto, K. Yamamoto, Opt. Mater. 33 (2011) 1927. D. Kobayashi, Y. Yamamoto, T. Isemura, Surf. Interface Anal. 45 (2013) 113. T. Sekine, T. Suzuki, K. Yamamoto, J. Am. Ceram. Soc. 98 (2015) 1464. D. Sakai, K. Harada, Y. Hara, H. Ikeda, S. Funatsu, K. Uraji, T. Suzuki, Y. Yamamoto, K. Yamamoto, N. Ikutame, K. Kawaguchi, H. Kaiju, J. Nishii, Sci. Rep. 6 (2016) 27767. K. Kimura, S. Joumori, Y. Oota, K. Nakajima, M. Suzuki, Nucl. Instrum. Methods Phys. Res., Sect. B 219–220 (2004) 351. K. Kimura, K. Nakajima, S. Yamanaka, M. Hasegawa, H. Okushi, Appl. Phys. Lett. 78 (2001) 1679. H. Hashimoto, S. Fujita, K. Nakajima, M. Suzuki, K. Sasakawa, K. Kimura, Nucl. Instrum. Methods Phys. Res., Sect. B 273 (2012) 241. J. Banerjee, V. Bojan, C.G. Pantano, S.H. Kim, J. Am. Ceram. Soc. 101 (2018) 644. Y. Yamamoto, K. Sato, K. Nakajima, K. Kimura, J. Non-Cryst. Solids 499 (2018) 408. T. Suzuki, J. Konishi, K. Yamamoto, S. Ogura, K. Fukutani, J. Am. Ceram. Soc. 98 (2015) 1794.
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
High-resolution depth profiling of soda-lime silicate glass using highresolution RBS and ERDA
T
Hiroki Sakataa, Yuichi Yamamotob, Koya Satoa, Kaoru Nakajimaa, Kenji Kimuraa,
⁎
a b
Department of Micro Engineering, Kyoto University, Kyoto 615-8540, Japan Innovative Technology Research Center, ASAHI Glass Co., Ltd., Hazawa-cho, Kanagawa-ku, Yokohama 221-8755, Japan
ARTICLE INFO
ABSTRACT
Keywords: High-resolution RBS High-resolution ERDA Soda-lime silicate glass Depth profiling
Composition depth profiles of soda-lime silicate glasses were measured using high-resolution Rutherford backscattering spectroscopy (HRBS) and high-resolution elastic recoil detection analysis (HERDA). Surface enrichment of modifier cations (Na+ and Ca2+) followed by a depletion in the deeper regions was observed using HRBS. The observed surface enrichment was attributed to the effect of radiation damage caused by the He+ irradiation. It was shown that the precise depth profiling of glass surfaces can be performed using HRBS when the fluence is limited below 3 × 1014 He ions cm−2. HERDA measurements demonstrated that there is a substantial amount of hydrogen (7.3 ± 1.3 × 1021 hydrogen cm−3) in the subsurface region as well as on the surface (1.6 ± 0.2 × 1015 hydrogen cm−2), which is very different from the silica glass surface where no subsurface hydrogen was observed.
1. Introduction Glasses have been used in a variety of applications for more than four thousand years. Although the traditional applications, such as buildings and automobiles are still major applications, glasses are also used in new fields, for example, used as substrates for solar cells and display devices. In these new applications characterization and control of glass surfaces and interfaces are of prime importance. Secondary ion mass spectrometry (SIMS) is often used to measure elemental depth profiling. SIMS, however, suffers from artificial effects, that is, the matrix effect and the surface transient effect. Particularly for the analysis of glass surfaces, it is known that the SIMS measurement strongly disturbs the distribution of modifier elements, such as sodium, calcium and magnesium [1,2]. Recent advancement of X-ray photoelectron spectroscopy (XPS) with the help of C60 ion sputtering allows elemental depth profiling of glass surfaces without such artifacts [3–7]. Unfortunately, however, XPS cannot measure hydrogen. In addition, the depth resolution of this method is a few nm and the improvement of the depth resolution is still desired for precise analysis. In this respect, high-resolution Rutherford backscattering spectroscopy (HRBS) and high-resolution elastic recoil detection analysis (HERDA) could be promising candidates because the depth resolutions of these techniques are sub-nm. Soda-lime silicate (SLS) glass is one of the most commonly used glasses. The most popular production process of the SLS glass is a so-
⁎
called float process in which molten glass is floated on a bath of molten tin during cooling. This process allows to produce flat glass sheets of large size. In this work, the surfaces of the SLS glass produced by the float process are observed using HRBS and HERDA. We examine if the precise composition depth profiling of the SLS glass, including hydrogen and modifier ions, is possible using HRBS and HERDA or not. 2. Experimental procedure In this study, commercial SLS glasses provided by Asahi Glass Co., Ltd. were analyzed using HRBS and HERDA. The details of the HRBS/ HERDA measurements were described elsewhere [8,9]. The scattering geometry was the IBM geometry for both HRBS and HERDA. For HRBS measurements, a beam of 400 keV He+ was produced by a 400 kV accelerator and collimated to 2 × 2 mm2 (typical beam current was 50 nA). The beam was sent to an ultra-high vacuum (UHV) scattering chamber (base pressure 1 × 10−8 Pa) via a differential pumping system. The glass sample was mounted on a 5-axis goniometer in the UHV chamber and irradiated with the He+ ion beam. Energy spectra of He+ ions scattered from the sample were measured by a 90° sector magnetic spectrometer. The UHV chamber had an electron flood gun (EFG, biased at −3 V) to prevent possible charging. For HERDA measurements, a collimated beam of 300 keV C+ ions (beam size was 2 × 2 mm2 and typical beam current was 5 nA) was
Corresponding author. E-mail address: [email protected] (K. Kimura).
https://doi.org/10.1016/j.nimb.2018.11.006 Received 18 July 2018; Received in revised form 7 October 2018; Accepted 5 November 2018 Available online 04 December 2018 0168-583X/ © 2018 Elsevier B.V. All rights reserved.
Nuclear Inst. and Methods in Physics Research B 440 (2019) 60–63
H. Sakata et al.
Fig. 1. HRBS spectra of silica glass observed under operation of an electron flood gun with various electric currents. The solid line shows the expected spectrum when there is no charging. The observed spectra agrees with the expected spectrum when the electric current is larger than 1.5 A.
Fig. 2. HRBS spectra measured for (a) top and (b) bottom surfaces of SLS glass. The dashed line shows the calculated spectrum for the SLS glass having a uniform and stoichiometric composition. In addition to the constituent elements, there are small Cu and Fe peaks, which may be surface-segregated trace elements. The amount of these elements are only 1.2 × 1013 cm−2 and 1.4 × 1013 cm−2, respectively.
used as a primary beam (the incident angle was 68°). Energy spectra of hydrogen ions recoiled from the sample at 25° were measured by the same spectrometer used in HRBS. A thin Mylar foil of 0.5 μm thickness coated with an aluminum layer (20 nm) was placed in front of the detector to reject carbon ions scattered from the sample [10]. A Kapton® thin film (Dupont-TORAY Co., Ltd.) with a thickness of 8 μm was used as a calibration standard for the hydrogen quantification.
glass. The incident angle was 40° and the fluence was 5.4 × 1015 ions cm−2. The top (bottom) surface means the surface which was not (was) contacted with the molten tin during the float process. The depth scales for tin and sodium are shown in the upper abscissa for reference. It is seen that the bottom surface contains a large amount of tin which was incorporated during the float process while the top surface does not. Except for this difference, both spectra are similar. The dashed line shows the calculated spectrum for the SLS glass having a uniform and stoichiometric composition (O – 60.4 at%, Si – 24.8 at%, Na – 9.3 at%, Ca – 3.0 at%, Mg – 2.2 at%, Al – 0.3 at%). The observed spectra deviate from the calculated one, especially around the leading edge of sodium. The composition depth profiles were derived from the observed HRBS spectra through spectrum simulations and the result of the top surface is shown in Fig. 3. Surface depletion of silicon and oxygen is clearly seen. On the other hand, sodium and calcium show surface enrichment followed by a slight depletion in deeper regions. The surface enrichment of sodium was often observed for the SLS glasses exposed to humid environment. The origin of the surface enrichment was attributed to the formation of sodium carbonate (Na2CO3) at the surface [11]. If this is the case, one sodium atom is accompanied by one and a half oxygen atoms. We estimated the oxygen profile from the observed silicon, tin, aluminum, calcium, magnesium and sodium profiles assuming that these elements are originating from SiO2, SnO2, Al2O3, CaO, MgO and Na2CO3, respectively. The result is shown by a thin solid line in Fig. 3. There is a large discrepancy between thus estimated profile and the observed one, especially in the surface region where sodium is enriched. On the other hand, if Na2O was formed instead of Na2CO3, the estimated oxygen profile agrees with the observed one very well as shown by a thin dashed line. This indicates that the observed surface enrichment of sodium cannot be ascribed to the formation of Na2CO3. Another possible origin of the observed surface enrichment of sodium could be radiation damage. It is known that the modifier cations, including Na+, are attached to non-bridging oxygen (NBO, Si–O−) in glass. If the Si–O–Si network is broken by the He+ irradiation, Na+ ions may move towards the newly created NBOs. In order to see if the radiation damage play a role or not, a series of short HRBS measurements were performed. Fig. 4 shows the measured yield in the energy region from 302 to 308 keV as a function of the irradiation time. This energy
3. Results and discussion 3.1. Suppression of charging using an electron flood gun Before analyzing the SLS glass surfaces, the performance of the electron flood gun was examined using a silica glass as a target. Fig. 1 shows HRBS spectrum observed without the electron flood gun (closed circles). The incident angle was 55° and the scattering angle was 50°. The arrows show the calculated energies of He+ ions scattered from surface silicon and oxygen atoms. The observed leading edges of these elements are located at larger energies than the calculated energies by several keV and the edges are very broad (note that the energy resolution of the present system is better than 1 keV). These observations are clear signatures of the charging due to the He+ irradiation. In order to prevent the charging, the electron flood gun was turned on and the EFG current was gradually increased. The HRBS spectra observed at various EFG currents are shown in Fig. 1. The observed spectrum did not change up to 1.0 A. After that the leading edges started to move toward lower energies and the yield decreased with increasing current. At 1.5 A, the observed spectrum agreed almost perfectly with the spectrum calculated using a simulation code (shown by a solid line), indicating that the charging was completely prevented by the electron flood gun with an adequate current. The measurements of both HRBS and HERDA shown in the following sections were done with the electron flood gun operating at 1.9 A. In these measurements, we also checked that the electron flood gun can successfully prevent the charging. It is noteworthy that the simulation code, which was used to calculate HRBS spectra, is based on the single scattering model. The scattering cross sections, stopping powers and energy loss straggling were calculated using the universal potential, SRIM code and the Yang’s energy loss straggling formula, respectively. 3.2. Results of HRBS measurements Fig. 2 shows the energy spectra of He+ ions scattered at 75° from the top (closed circles) and bottom (open circles) surfaces of the SLS 61
Nuclear Inst. and Methods in Physics Research B 440 (2019) 60–63
H. Sakata et al.
Fig. 5. HRBS spectrum obtained by summing up the results of several short measurements for different sample positions (fluence of each measurement is 3 × 1014 He ions cm−2 and the total fluence is 1.5 × 1015 He ions cm−2). The solid line shows the spectrum calculated for the uniform composition. The effect of the radiation damage on the sample composition is negligibly small in this low fluence measurement.
Fig. 3. Composition depth profiles of the top surface of SLS glass derived from the observed HRBS spectrum. The thin dashed line shows the oxygen profile estimated from the observed profiles of Si, Sn, Al, Ca, Mg and Na by assuming formation of SiO2, SnO2, Al2O3, CaO, MgO and Na2O, respectively. The thin solid line shows a similar result but assuming formation of Na2CO3 instead of Na2O (see text).
In passing, we note that due to the He+ irradiation sodium is depleted in the region from ∼14 nm to ∼32 nm and enriched in the region from the surface to ∼14 nm while calcium is depleted in the region from ∼1 nm to ∼15 nm and enriched in the very surface region (Fig. 3). This indicates that the radiation enhanced diffusion coefficient of sodium is several times larger than that of calcium. This may be attributed to the fact that Ca2+ needs two neighboring NBOs while Na+ needs only one NBO to create a new site to which they can move to. Another possibility is the mass difference between Ca and Na ions. The lighter mass might enhance the diffusion process. 3.3. Results of HERDA measurements Fig. 6 shows examples of energy spectra of recoiled H+ observed in a series of short HERDA measurements for the bottom surface of SLS glass. All spectra have a sharp peak at ∼70.5 keV, which corresponds to the hydrogen sitting on the surface. It is noteworthy that the sharpness of the observed peak demonstrates that the charging was successfully suppressed using the electron flood gun similarly to HRBS. In addition to the surface peak, there is a certain amount of hydrogen in the subsurface region at least up to 3 nm (see the depth scale shown in the
Fig. 4. Observed HRBS yield in the energy region from 302 to 308 keV as a function of irradiation time. This energy region corresponds to the depth region from the surface to 6 nm for sodium.
region corresponds to the depth region from the surface to 6 nm for sodium where sodium enrichment was observed (see Fig. 2). The observed yield increases with increasing irradiation time t, indicating that the observed surface enrichment of sodium is caused by the radiation damage. The result was fitted by Y(t) = Y1 − Y2 e−t/τ as shown by a dashed line. From this fitting, it can be concluded that if one wants to suppress the yield change less than 1%, the measurement time should be shorter than 50 s, which corresponds to a fluence of 3 × 1014 He ions cm−2. In order to confirm this conclusion, the HRBS measurements of the SLS glass were performed with a fluence of 3 × 1014 He ions cm−2 on several sample points. The measured HRBS spectra were summed up and shown in Fig. 5. The solid line shows the calculated spectrum for the uniform composition. Differently from the spectra shown in Fig. 2, the agreement between the observed and calculated spectra is very good. This clearly demonstrates that the precise depth profiling of the SLS glasses can be performed when the fluence is limited below 3 × 1014 He ions cm−2.
Fig. 6. Examples of HERDA spectra observed in a series of short time measurements for the bottom surface of SLS glass. 62
Nuclear Inst. and Methods in Physics Research B 440 (2019) 60–63
H. Sakata et al.
modifier ions play an important role in the hydrogen incorporation. In passing, we note that recent nuclear reaction analysis (NRA) also showed that the hydrogen concentration of SLS glass is as high as 4 × 1021 cm−3 at ∼20 nm from the surface and rapidly decreases with increasing depth [13]. 4. Conclusion The composition depth profiles of the SLS glass surfaces were measured using HRBS and HERDA. The charging caused by the irradiation of primary ion beams was successfully suppressed using an electron flood gun for both HRBS and HERDA. The HRBS measurements showed surface enrichment of sodium and calcium. The origin of the observed surface enrichment was attributed to the effect of radiation damage caused by the He+ irradiation. It was shown that the effect of the radiation damage on the HRBS spectrum is negligibly small when the fluence of the HRBS measurement is less than 3 × 1014 He ions cm−2. The HERDA measurements revealed that there is a substantial amount of hydrogen, about 10 at%, in the subsurface region in addition to the hydrogen on the outermost surface. The surface hydrogen decreased very rapidly during the measurement, while the hydrogen in the subsurface region is rather radiation resistant. The present results indicates that the suppression of both the charging and the radiation damage is crucial for the precise HRBS/HERDA analysis of the SLS glass surfaces.
Fig. 7. Surface density and subsurface concentration of hydrogen as a function of irradiation time. The hydrogen density and the subsurface concentration before irradiation can be estimated to 1.6 ± 0.2 × 1015 cm−2 and 7.3 ± 1.3 × 1021 cm−3 by extrapolating the fitting curves to t = 0 (see text).
upper abscissa). The observed surface peak becomes smaller with irradiation, while the hydrogen yield in the subsurface region seems constant. The surface hydrogen density and the hydrogen concentration in the subsurface region were estimated from the observed spectra. Fig. 7 shows the observed surface hydrogen density (solid circles) and the hydrogen concentration in a depth region from 2 nm to 3 nm (open circles) as a function of the irradiation time. As was mentioned above, the surface hydrogen density decreases rapidly with increasing irradiation time, while the hydrogen concentration in the subsurface region decreases much more slowly. These results were fitted by Y (t) = Y1 + Y2 e−t/τ as is shown by dashed lines. By extrapolating the fitting curves to t = 0, the surface hydrogen density and the hydrogen concentration in the subsurface region before the C+ irradiation were determined to be 1.6 ± 0.2 × 1015 cm−2 and 7.3 ± 1.3 × 1021 cm−3, respectively. The observed surface hydrogen density is at the same level as that of non-treated silica glass surfaces, which was mainly attributed to physically adsorbed water molecules [12]. The present result showed that there is a substantial amount of hydrogen (∼10 at%) also in the subsurface region of the SLS glass. This is very different from the silica glass surfaces, where hydrogen exists only on the surface and no subsurface hydrogen was observed by HERDA [12]. This suggests that the
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