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Effect of implanted silicon on hydrogen behavior in aluminum and nickel
Nuclear Instruments and Methods in Physics Research B 161±163 (2000) 401±405
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Eect of implanted silicon on hydrogen behavior in aluminum and nickel Nobutsugu Imanishi *, Masahiko Ogura 1, Mitsuharu Ikeda, Ryuta Mitsusue, Akio Itoh Department of Nuclear Engineering, Kyoto University, Sakyo, Kyoto 606-8501, Japan
Abstract We have used elastic recoil detection (ERD) to study the behavior of hydrogen in Si-implanted Al and Ni samples, with a particular emphasis on the eect of the grain boundaries formed by implanting additive elements on hydrogen precipitation. From the measured H depth pro®les and their thermal behavior, it is shown that the presence of an Al±Si layer in the near-surface region obstructs the annihilation of vacancies formed by hydrogen implantation in the pure Al region. In the case of Si-implanted Ni samples, hydrogen hardly precipitates except for the case of annealing procedure done at a relatively low temperature at which silicides are not formed but vacancies produced by the Si implantation are removed from the samples. These hydrogen trap sites were identi®ed and the observed facts are explained well by taking into account the interface formed by the Si implantation. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 61.72.Ss; 66.30.Jt Keywords: Hydrogen-trapping; Thermal behavior; Implantation; Nickel; Aluminum; Silicon
1. Introduction Hydrogen in metals plays bene®cial and detrimental roles in structural alloys and electronic materials. Trapping of hydrogen by irradiation damages caused by ion implantation has been a great concern of many groups for several decades [1±4]. It was pointed out by Myers et al. [1,2] that * Corresponding author. Tel.: +81-75-7535846; fax: +81-757535821. E-mail address: [email protected] (N. Imanishi). 1 Present address: Electrotechnical Laboratory (Agency of Industrial Science and Technology).
impurity elements added in metals could also aect the hydrogen behavior. So far, however, experimental studies are scarce. We found recently that a small amount of an additive element drastically aects the hydrogen trapping [5±8]. That is, hydrogen was trapped at room temperature by vacancies in the pure Al region behind the Al±Si layer formed by implanting Si ions to a dose as low as 3 1014 Si/cm2 equivalent to the solution limit of 0.05 Si at.% in Al. At a dose of 1 1017 Si/cm2 , hydrogen was trapped forming H2 bubbles in the pure Al region. Hydrogen hardly stays at room temperature in pure Al with no Al±Si layer and the observed hydrogen trapping is caused because Si grains in the Al±Si layer obstruct the annihilation
0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 6 8 4 - 9
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N. Imanishi et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 401±405
of vacancies formed by the hydrogen itself in the pure Al region [7,8]. The aim of the present study is to clarify the in¯uence of the grain boundary on the trapping system of hydrogen. For this purpose, H ions were implanted into two extreme cases of the physical state of implanted Si realized in Al and Ni samples. That is, Si tends to precipitate and form grains in the Al sample above a concentration of 0.05 at.%, but it easily makes silicide in the Ni sample depending on its temperature. For the purpose, we ®rst implanted Si ions in the Al and Ni samples and subsequently H ions in the pure Al and Ni regions behind the end-of-range of the implanted Si. Measurements of hydrogen pro®les, retention and thermal annealing behavior were done at various conditions of dose and temperature both for the Si and H implantations. 2. Experimental procedures Samples were prepared by the following procedures. 4N-purity electropolished Ni plates were mounted on a sample holder in a vacuum of 10ÿ5 Pa and annealed at a temperature of 1223 K for 30 min. Then, 75-keV Si ions were implanted into the Ni plates kept at a temperature of 423 K to doses of 1 1016 ±1 1017 Si/cm2 at a beam current density of about 1 lA/cm2 . The Si-implanted samples were kept at the same temperature for 24 h to eliminate irradiation damage. 20-keV H ions with a beam current density of about 1.5 lA/cm2 were then implanted into the samples to a dose of 1 1017 H/cm2 at 100 and 300 K. As shown in Fig. 1, most of the implanted H ions stopped beyond the 75-keV Si range and the rest stopped at the same region of the implanted Si [9]. The procedure for the Al specimens was described in our previous reports [7,8] and here the H implantation was done at 300 K. For comparison, H ions were implanted into the 4N-purity electropolished Ni plate at 100 and 300 K without the Si pre-implantation. After the H implantation, the depth pro®les of hydrogen were measured by the elastic recoil detection (ERD) method using a 2-MeV 4 He ion beam [10,11]. The 4 He beam was collimated to 1 mm square in size and incident on the target at 75°
Fig. 1. Simulated depth pro®les of 75-keV Si (the chained line) and 20-keV H and vacancy (the solid and dashed lines, respectively) in Ni [10].
to the surface normal. The hydrogen energy spectra were taken with a surface barrier detector set at an angle of 30° using a 4 He-absorbing ®lm. The 4 He beam current was monitored with another detector set at 160° and was in the order of 5 nA/mm2 on the target. The spectra were taken every 10 min during a ramp annealing from 300 to 600 K at a rate of 1 K/min. The depth pro®les of hydrogen were deduced from the energy spectra using the TRIM simulation code [9]. 3. Results and discussion Examples of the depth pro®les of hydrogen measured during ramp annealing are shown in Fig. 2 for the Si-implanted Al and Ni samples. Data of the Ni sample without Si-implant are also shown for comparison. The doses of the implanted Si and H ions were both 1 1017 =cm2 for the Al and Ni samples. Most of the implanted H ions stopped beyond the Si/metal alloys. The strong surface peak is observed even in samples without H-implant and is attributable to hydrogen tightly bound in surface oxide/bulk interface. In the Al sample the pro®le peak of hydrogen was unambiguously observed in the bulk region at the implantation temperature condition of 300 K. The observed pro®le locates 30 nm this side of the projected range of 30-keV hydrogen and are in accordance
N. Imanishi et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 401±405
Fig. 2. Change of the depth pro®le of H during annealing. (a) 78-keV Si-implanted Al, (b) 75-keV Si-implanted Ni and (c) Ni without Si-implant. The doses of Si and H are both 1 1017 =cm2 . The H implantation was done at temperatures of 100 and 300 K for the Ni and Al samples, respectively.
with the calculated vacancy distribution [8]. In the case of Ni sample, no hydrogen was trapped when it was implanted at 300 K. Even for the hydrogen
403
implantation at 100 K, it is rather dicult to recognize the peak in the bulk region. The stopped hydrogen diused broadly and was trapped over a wide region ranging from the hydrogen end-ofrange to the surface before the pro®le measurement done at room temperature. A similar observation was encountered in the Ni sample without Si-implant. It is generally accepted that vacancies formed by H implantation disappear before trapping hydrogen at 300 K in pure Al [6]. The observed hydrogen trapping in the pure Al region in the Siimplanted Al was, therefore, speculated to result from Si±Al grain boundaries in the near-surface region obstructing the annihilation of vacancies formed by the hydrogen implantation itself in the pure Al region [7,8]. Hydrogen trapping at grain boundaries of the Si/Al alloy was con®rmed unambiguously when hydrogen was implanted directly into the Si/Al alloy layer [7]. That is, the presence of Si as an added element contributes to the retention of vacancies and promotes the formation of V±H complexes that in turn lead to the formation of H2 bubbles at the 1 1017 Si=cm2 implantation. Si forms a range of silicides in Ni depending on temperature. Therefore, the Si-implanted Ni samples were carefully annealed below or above the temperature of 470 K for forming Ni2 Si to eliminate only the radiation damage produced by the Si implantation. The elimination of radiation damage is a very important process because the damage is extensive after heavy ion irradiation and the implanted hydrogen easily escapes from the sample, resulting in a low retention of hydrogen [12]. When the Si-implanted Ni sample was pre-annealed above 470 K, no hydrogen was trapped in the bulk region. However, for samples annealed at a temperature below 470 K, hydrogen was retained, forming a broad pro®le peak as shown in Fig. 2. This suggests that annealing eliminated the radiation damage without forming a silicide. Fig. 2 includes the change of the depth pro®les of hydrogen during the ramp-annealing process. In the case of Ni sample without Si-implant the broadly distributed hydrogen peak disappeared wholly by raising a temperature of about only 100 K. For both the Si-implanted Al and Ni samples,
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N. Imanishi et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 401±405
however, hydrogen near the Si-implanted region seems to stay longer than the deeper region. This annealing behavior is not caused by indiusion of hydrogen trapped in the surface region, because the surface peak for the Si-implanted samples without H-implant decreases keeping the same shape with increasing temperature. Fig. 3 shows the annealing-temperature dependence of the hydrogen retention in the bulk region of the Si-implanted Ni samples. The total hydrogen retention is normalized at 300 K. The fractional retention is shown for hydrogen trapped in the region containing the implanted Si and for that in the pure Ni region. The data for the pure Al samples were taken from our previous study and show typical hydrogen behaviors [5]. That is, based on the con®rmation by Myers et al. [1,4] hydrogen implanted into pure Al at 125 K is trapped as monovacancy (V)±H complexes with a binding enthalpy of 0.52 eV and as hydrogen bubbles with 0.71 eV for 3 1016 and 1 1017 H=cm2 hydrogen doses, respectively. Fig. 3 shows that the concentration of the hydrogen in the Ni sample without Si-implant began a rapid decrease at 300 K. This trap site has a trapping enthalpy of 0.43 eV and was assigned as a vacancy cluster-H complex formation in pure Ni [13,14]. As shown in Fig. 3, the hydrogen retention curve in the Si-implanted Ni samples also contains the rapid-decrease component for both the 1 1016 and 1 1017 Si=cm2 Si implantation doses. This indicates that the main candidate for the H-trap in the Si-implanted Ni is the V cluster-H complex. Besides the rapid component, a more stable component was observed in the annealing curves and can be extracted by taking the rapid component o the total hydrogen retention curve of the Si-implanted Ni. This trap site has a binding enthalpy higher than 0.71 eV of the hydrogen bubble formation in Al and is more stable than the heliumassociated trap site observed in He-implanted Ni samples, where hydrogen is bound to the walls formed by implanted He with a trap strength of 0.55 eV [14]. In the present experiment, Si ions were implanted before the hydrogen implantation and the produced vacancies were eliminated at a temperature of about 423 K. At the stage of hydrogen
Fig. 3. The thermal behavior of H implanted into the 75-keV Siimplanted Ni with doses of 1 1017 =cm2 (a) and 1 1016 =cm2 (c), (b) and (d) are the annealing curves taken the rapid components o the corresponding curves in (a) and (c). Marks indicate the total, the fractional (25±75 nm in depth) and (75±125 nm) (the circles, triangles and squares, respectively). The H thermal behavior in the pure Ni sample is denoted with the crosses. Those of H implanted into pure Al at 125 K to doses of 1 1017 and 3 1016 =cm2 are shown with the solid and dashed lines, respectively.
implantation, hydrogen itself produces vacancies. At room temperature, vacancies start to move [15]. Hydrogen was not observed in the room temperature implantation but it was retained in the 100 K implantation. This suggests that vacancies produced by the hydrogen implantation disappeared
N. Imanishi et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 401±405
before trapping hydrogen for room temperature implantation. For the 100 K implantation, vacancies hardly moved and combined with hydrogen during implantation. The Si-rich layer could play an active role in promoting the formation of vacancy clusters. As to the trap site with the higher binding enthalpy, a similar tight trap site was observed for Ca-, Rb- and Cs- implanted Ni samples. Myers et al. [1] proposed that the strong interaction is ascribed to a combination of reduced local electron density together with other electronic effects due to the reactive impurities. In the present experiment, as shown in Fig. 3, the trap site with the higher binding enthalpy is more prominent in the Si-containing region than deep inside the pure Ni region. Therefore the added Si is important in understanding the trap mechanism. Combining these results for the Ni and Al samples stresses again the importance of grain boundaries in forming traps sites for hydrogen. 4. Conclusion We have studied the eects of grain boundaries formed by implanting Si on H precipitation. For this purpose we implanted Al and Ni samples prior to H implantation. Si tends to precipitate and form grains in Al above a concentration of 0.05 at.%, but it forms a silicide in Ni depending on temperature. It was found that for the Si-implanted Al, hydrogen implanted into the pure Al region far beyond the Al±Si thin alloy layer is unexpectedly retained in the sample. This shows that the presence of the Al±Si layer in a near-surface region obstructs the annihilation of vacancies formed by hydrogen itself in the pure Al region. In the case of the Si-implanted Ni sample, hydrogen hardly precipitates except for the case of annealing procedure done at a relatively low temperature at which silicides are not formed but vacancies produced by the Si implantation are removed from the samples.
405
Acknowledgements This work was done with a 250-keV CockcroftWalton Accelerator, a 30-kV Ion Implanter and a 4-MV Van de Graa Accelerator at Kyoto University. We thank Mr. K. Yoshida, Mr. K. Norizawa and Mr. M. Imai for their useful advice and technical support during the experiments. We are much obliged to Dr. S. Hattori for the target preparation.
References [1] S.M. Myers, P.M. Richards, W.R. Wampler, J. Nucl. Mater. 165 (1989) 9. [2] S.M. Myers et al., Rev. Mod. Phys. 64 (1992) 559. [3] J.P. Bugeat, A.C. Chami, E. Ligeon, Phys. Lett. 58 A (1976) 127. [4] S.M. Myers, F. Besenbacher, J.K. Nùrskov, J. Appl. Phys. 58 (1985) 1841. [5] M. Ogura, N. Nakatani, N. Yamaji, M. Imai, A. Itoh, N. Imanishi, Radiat. Phys. Chem. 49 (1997) 645. [6] M. Ogura, N. Yamaji, M. Imai, A. Itoh, N. Imanishi, Nucl. Instr. and Meth. B 121 (1997) 470. [7] M. Ogura, T. Higuchi, M. Imai, A. Itoh, N. Imanishi, Nucl. Instr. and Meth. B 136±138 (1998) 478. [8] M. Ogura, T. Higuchi, M. Ikeda, A. Itoh, N. Imanishi, in: Proc. of the 12th Internat. Conf. on Ion Implantation Technology, Kyoto, Japan, 1999, p. 606. [9] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon, New York, 1985. [10] S. Nagata, S. Yamaguchi, Y. Fujino, Y. Hori, N. Sugiyama, K. Kamada, Nucl. Instr. and Meth. B 6 (1985) 53. [11] W. Hong, Z.G. Qing, Nucl. Instr. and Meth. B 34 (1988) 145. [12] M. Ogura, N. Yamaji, M. Imani, A. Itoh, N. Imanishi, in: Proc. of Internat. Conference on Accelerator Application in Research and Industry, Denton, 1997, p. 953. [13] F. Besenbacher, J. Bùttiger, S.M. Myers, J. Appl. Phys. 53 (1982) 3536. [14] F. Besenbacher, J. Bùttiger, S.M. Myers, J. Appl. Phys. 53 (1982) 3547. [15] W. Schilling, G. Burger, K. Isebeck, H. Wenzl, Vacancies and Interstitials in Metals, North-Holland, Amsterdam, 1969, p. 318.
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Eect of implanted silicon on hydrogen behavior in aluminum and nickel Nobutsugu Imanishi *, Masahiko Ogura 1, Mitsuharu Ikeda, Ryuta Mitsusue, Akio Itoh Department of Nuclear Engineering, Kyoto University, Sakyo, Kyoto 606-8501, Japan
Abstract We have used elastic recoil detection (ERD) to study the behavior of hydrogen in Si-implanted Al and Ni samples, with a particular emphasis on the eect of the grain boundaries formed by implanting additive elements on hydrogen precipitation. From the measured H depth pro®les and their thermal behavior, it is shown that the presence of an Al±Si layer in the near-surface region obstructs the annihilation of vacancies formed by hydrogen implantation in the pure Al region. In the case of Si-implanted Ni samples, hydrogen hardly precipitates except for the case of annealing procedure done at a relatively low temperature at which silicides are not formed but vacancies produced by the Si implantation are removed from the samples. These hydrogen trap sites were identi®ed and the observed facts are explained well by taking into account the interface formed by the Si implantation. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 61.72.Ss; 66.30.Jt Keywords: Hydrogen-trapping; Thermal behavior; Implantation; Nickel; Aluminum; Silicon
1. Introduction Hydrogen in metals plays bene®cial and detrimental roles in structural alloys and electronic materials. Trapping of hydrogen by irradiation damages caused by ion implantation has been a great concern of many groups for several decades [1±4]. It was pointed out by Myers et al. [1,2] that * Corresponding author. Tel.: +81-75-7535846; fax: +81-757535821. E-mail address: [email protected] (N. Imanishi). 1 Present address: Electrotechnical Laboratory (Agency of Industrial Science and Technology).
impurity elements added in metals could also aect the hydrogen behavior. So far, however, experimental studies are scarce. We found recently that a small amount of an additive element drastically aects the hydrogen trapping [5±8]. That is, hydrogen was trapped at room temperature by vacancies in the pure Al region behind the Al±Si layer formed by implanting Si ions to a dose as low as 3 1014 Si/cm2 equivalent to the solution limit of 0.05 Si at.% in Al. At a dose of 1 1017 Si/cm2 , hydrogen was trapped forming H2 bubbles in the pure Al region. Hydrogen hardly stays at room temperature in pure Al with no Al±Si layer and the observed hydrogen trapping is caused because Si grains in the Al±Si layer obstruct the annihilation
0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 6 8 4 - 9
402
N. Imanishi et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 401±405
of vacancies formed by the hydrogen itself in the pure Al region [7,8]. The aim of the present study is to clarify the in¯uence of the grain boundary on the trapping system of hydrogen. For this purpose, H ions were implanted into two extreme cases of the physical state of implanted Si realized in Al and Ni samples. That is, Si tends to precipitate and form grains in the Al sample above a concentration of 0.05 at.%, but it easily makes silicide in the Ni sample depending on its temperature. For the purpose, we ®rst implanted Si ions in the Al and Ni samples and subsequently H ions in the pure Al and Ni regions behind the end-of-range of the implanted Si. Measurements of hydrogen pro®les, retention and thermal annealing behavior were done at various conditions of dose and temperature both for the Si and H implantations. 2. Experimental procedures Samples were prepared by the following procedures. 4N-purity electropolished Ni plates were mounted on a sample holder in a vacuum of 10ÿ5 Pa and annealed at a temperature of 1223 K for 30 min. Then, 75-keV Si ions were implanted into the Ni plates kept at a temperature of 423 K to doses of 1 1016 ±1 1017 Si/cm2 at a beam current density of about 1 lA/cm2 . The Si-implanted samples were kept at the same temperature for 24 h to eliminate irradiation damage. 20-keV H ions with a beam current density of about 1.5 lA/cm2 were then implanted into the samples to a dose of 1 1017 H/cm2 at 100 and 300 K. As shown in Fig. 1, most of the implanted H ions stopped beyond the 75-keV Si range and the rest stopped at the same region of the implanted Si [9]. The procedure for the Al specimens was described in our previous reports [7,8] and here the H implantation was done at 300 K. For comparison, H ions were implanted into the 4N-purity electropolished Ni plate at 100 and 300 K without the Si pre-implantation. After the H implantation, the depth pro®les of hydrogen were measured by the elastic recoil detection (ERD) method using a 2-MeV 4 He ion beam [10,11]. The 4 He beam was collimated to 1 mm square in size and incident on the target at 75°
Fig. 1. Simulated depth pro®les of 75-keV Si (the chained line) and 20-keV H and vacancy (the solid and dashed lines, respectively) in Ni [10].
to the surface normal. The hydrogen energy spectra were taken with a surface barrier detector set at an angle of 30° using a 4 He-absorbing ®lm. The 4 He beam current was monitored with another detector set at 160° and was in the order of 5 nA/mm2 on the target. The spectra were taken every 10 min during a ramp annealing from 300 to 600 K at a rate of 1 K/min. The depth pro®les of hydrogen were deduced from the energy spectra using the TRIM simulation code [9]. 3. Results and discussion Examples of the depth pro®les of hydrogen measured during ramp annealing are shown in Fig. 2 for the Si-implanted Al and Ni samples. Data of the Ni sample without Si-implant are also shown for comparison. The doses of the implanted Si and H ions were both 1 1017 =cm2 for the Al and Ni samples. Most of the implanted H ions stopped beyond the Si/metal alloys. The strong surface peak is observed even in samples without H-implant and is attributable to hydrogen tightly bound in surface oxide/bulk interface. In the Al sample the pro®le peak of hydrogen was unambiguously observed in the bulk region at the implantation temperature condition of 300 K. The observed pro®le locates 30 nm this side of the projected range of 30-keV hydrogen and are in accordance
N. Imanishi et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 401±405
Fig. 2. Change of the depth pro®le of H during annealing. (a) 78-keV Si-implanted Al, (b) 75-keV Si-implanted Ni and (c) Ni without Si-implant. The doses of Si and H are both 1 1017 =cm2 . The H implantation was done at temperatures of 100 and 300 K for the Ni and Al samples, respectively.
with the calculated vacancy distribution [8]. In the case of Ni sample, no hydrogen was trapped when it was implanted at 300 K. Even for the hydrogen
403
implantation at 100 K, it is rather dicult to recognize the peak in the bulk region. The stopped hydrogen diused broadly and was trapped over a wide region ranging from the hydrogen end-ofrange to the surface before the pro®le measurement done at room temperature. A similar observation was encountered in the Ni sample without Si-implant. It is generally accepted that vacancies formed by H implantation disappear before trapping hydrogen at 300 K in pure Al [6]. The observed hydrogen trapping in the pure Al region in the Siimplanted Al was, therefore, speculated to result from Si±Al grain boundaries in the near-surface region obstructing the annihilation of vacancies formed by the hydrogen implantation itself in the pure Al region [7,8]. Hydrogen trapping at grain boundaries of the Si/Al alloy was con®rmed unambiguously when hydrogen was implanted directly into the Si/Al alloy layer [7]. That is, the presence of Si as an added element contributes to the retention of vacancies and promotes the formation of V±H complexes that in turn lead to the formation of H2 bubbles at the 1 1017 Si=cm2 implantation. Si forms a range of silicides in Ni depending on temperature. Therefore, the Si-implanted Ni samples were carefully annealed below or above the temperature of 470 K for forming Ni2 Si to eliminate only the radiation damage produced by the Si implantation. The elimination of radiation damage is a very important process because the damage is extensive after heavy ion irradiation and the implanted hydrogen easily escapes from the sample, resulting in a low retention of hydrogen [12]. When the Si-implanted Ni sample was pre-annealed above 470 K, no hydrogen was trapped in the bulk region. However, for samples annealed at a temperature below 470 K, hydrogen was retained, forming a broad pro®le peak as shown in Fig. 2. This suggests that annealing eliminated the radiation damage without forming a silicide. Fig. 2 includes the change of the depth pro®les of hydrogen during the ramp-annealing process. In the case of Ni sample without Si-implant the broadly distributed hydrogen peak disappeared wholly by raising a temperature of about only 100 K. For both the Si-implanted Al and Ni samples,
404
N. Imanishi et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 401±405
however, hydrogen near the Si-implanted region seems to stay longer than the deeper region. This annealing behavior is not caused by indiusion of hydrogen trapped in the surface region, because the surface peak for the Si-implanted samples without H-implant decreases keeping the same shape with increasing temperature. Fig. 3 shows the annealing-temperature dependence of the hydrogen retention in the bulk region of the Si-implanted Ni samples. The total hydrogen retention is normalized at 300 K. The fractional retention is shown for hydrogen trapped in the region containing the implanted Si and for that in the pure Ni region. The data for the pure Al samples were taken from our previous study and show typical hydrogen behaviors [5]. That is, based on the con®rmation by Myers et al. [1,4] hydrogen implanted into pure Al at 125 K is trapped as monovacancy (V)±H complexes with a binding enthalpy of 0.52 eV and as hydrogen bubbles with 0.71 eV for 3 1016 and 1 1017 H=cm2 hydrogen doses, respectively. Fig. 3 shows that the concentration of the hydrogen in the Ni sample without Si-implant began a rapid decrease at 300 K. This trap site has a trapping enthalpy of 0.43 eV and was assigned as a vacancy cluster-H complex formation in pure Ni [13,14]. As shown in Fig. 3, the hydrogen retention curve in the Si-implanted Ni samples also contains the rapid-decrease component for both the 1 1016 and 1 1017 Si=cm2 Si implantation doses. This indicates that the main candidate for the H-trap in the Si-implanted Ni is the V cluster-H complex. Besides the rapid component, a more stable component was observed in the annealing curves and can be extracted by taking the rapid component o the total hydrogen retention curve of the Si-implanted Ni. This trap site has a binding enthalpy higher than 0.71 eV of the hydrogen bubble formation in Al and is more stable than the heliumassociated trap site observed in He-implanted Ni samples, where hydrogen is bound to the walls formed by implanted He with a trap strength of 0.55 eV [14]. In the present experiment, Si ions were implanted before the hydrogen implantation and the produced vacancies were eliminated at a temperature of about 423 K. At the stage of hydrogen
Fig. 3. The thermal behavior of H implanted into the 75-keV Siimplanted Ni with doses of 1 1017 =cm2 (a) and 1 1016 =cm2 (c), (b) and (d) are the annealing curves taken the rapid components o the corresponding curves in (a) and (c). Marks indicate the total, the fractional (25±75 nm in depth) and (75±125 nm) (the circles, triangles and squares, respectively). The H thermal behavior in the pure Ni sample is denoted with the crosses. Those of H implanted into pure Al at 125 K to doses of 1 1017 and 3 1016 =cm2 are shown with the solid and dashed lines, respectively.
implantation, hydrogen itself produces vacancies. At room temperature, vacancies start to move [15]. Hydrogen was not observed in the room temperature implantation but it was retained in the 100 K implantation. This suggests that vacancies produced by the hydrogen implantation disappeared
N. Imanishi et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 401±405
before trapping hydrogen for room temperature implantation. For the 100 K implantation, vacancies hardly moved and combined with hydrogen during implantation. The Si-rich layer could play an active role in promoting the formation of vacancy clusters. As to the trap site with the higher binding enthalpy, a similar tight trap site was observed for Ca-, Rb- and Cs- implanted Ni samples. Myers et al. [1] proposed that the strong interaction is ascribed to a combination of reduced local electron density together with other electronic effects due to the reactive impurities. In the present experiment, as shown in Fig. 3, the trap site with the higher binding enthalpy is more prominent in the Si-containing region than deep inside the pure Ni region. Therefore the added Si is important in understanding the trap mechanism. Combining these results for the Ni and Al samples stresses again the importance of grain boundaries in forming traps sites for hydrogen. 4. Conclusion We have studied the eects of grain boundaries formed by implanting Si on H precipitation. For this purpose we implanted Al and Ni samples prior to H implantation. Si tends to precipitate and form grains in Al above a concentration of 0.05 at.%, but it forms a silicide in Ni depending on temperature. It was found that for the Si-implanted Al, hydrogen implanted into the pure Al region far beyond the Al±Si thin alloy layer is unexpectedly retained in the sample. This shows that the presence of the Al±Si layer in a near-surface region obstructs the annihilation of vacancies formed by hydrogen itself in the pure Al region. In the case of the Si-implanted Ni sample, hydrogen hardly precipitates except for the case of annealing procedure done at a relatively low temperature at which silicides are not formed but vacancies produced by the Si implantation are removed from the samples.
405
Acknowledgements This work was done with a 250-keV CockcroftWalton Accelerator, a 30-kV Ion Implanter and a 4-MV Van de Graa Accelerator at Kyoto University. We thank Mr. K. Yoshida, Mr. K. Norizawa and Mr. M. Imai for their useful advice and technical support during the experiments. We are much obliged to Dr. S. Hattori for the target preparation.
References [1] S.M. Myers, P.M. Richards, W.R. Wampler, J. Nucl. Mater. 165 (1989) 9. [2] S.M. Myers et al., Rev. Mod. Phys. 64 (1992) 559. [3] J.P. Bugeat, A.C. Chami, E. Ligeon, Phys. Lett. 58 A (1976) 127. [4] S.M. Myers, F. Besenbacher, J.K. Nùrskov, J. Appl. Phys. 58 (1985) 1841. [5] M. Ogura, N. Nakatani, N. Yamaji, M. Imai, A. Itoh, N. Imanishi, Radiat. Phys. Chem. 49 (1997) 645. [6] M. Ogura, N. Yamaji, M. Imai, A. Itoh, N. Imanishi, Nucl. Instr. and Meth. B 121 (1997) 470. [7] M. Ogura, T. Higuchi, M. Imai, A. Itoh, N. Imanishi, Nucl. Instr. and Meth. B 136±138 (1998) 478. [8] M. Ogura, T. Higuchi, M. Ikeda, A. Itoh, N. Imanishi, in: Proc. of the 12th Internat. Conf. on Ion Implantation Technology, Kyoto, Japan, 1999, p. 606. [9] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon, New York, 1985. [10] S. Nagata, S. Yamaguchi, Y. Fujino, Y. Hori, N. Sugiyama, K. Kamada, Nucl. Instr. and Meth. B 6 (1985) 53. [11] W. Hong, Z.G. Qing, Nucl. Instr. and Meth. B 34 (1988) 145. [12] M. Ogura, N. Yamaji, M. Imani, A. Itoh, N. Imanishi, in: Proc. of Internat. Conference on Accelerator Application in Research and Industry, Denton, 1997, p. 953. [13] F. Besenbacher, J. Bùttiger, S.M. Myers, J. Appl. Phys. 53 (1982) 3536. [14] F. Besenbacher, J. Bùttiger, S.M. Myers, J. Appl. Phys. 53 (1982) 3547. [15] W. Schilling, G. Burger, K. Isebeck, H. Wenzl, Vacancies and Interstitials in Metals, North-Holland, Amsterdam, 1969, p. 318.