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Bubble formation with electron irradiation in SiC implanted with hydrogen or deuterium
Nuclear Instruments and Methods in Physics Research B 191 (2002) 540–543 www.elsevier.com/locate/nimb
Bubble formation with electron irradiation in SiC implanted with hydrogen or deuterium J. Aihara a
a,*
, K. Hojou b, S. Furuno b, M. Ishihara
a
Japan Atomic Energy Research Institute, Oarai-machi, Higashi-ibaraki-gun, Ibaraki-ken, Japan b Japan Atomic Energy Research Institute, Tokai-mura Naka-gun, Ibaraki-ken, Japan
Abstract The bubbles were formed and grew when SiC implanted with high fluence of hydrogen ion was irradiated with electron. In this study we tried to inspect the supposition that the energy deposition of the electron beam to hydrogen caused the migration of hydrogen and gave rise to bubble formation and growth. We used hydrogen (H) or deuterium (D) as implanted ion to change the cross section of the reaction that D or H is given energy by electron beam. The energy deposition cross section in the case of H is 2–3 times as large as that in the case of D. Bubbles were less likely to be formed and grow in the case of D than in the case of H. This result can be explained in terms of the difference of the concentration of the mobile gas atom caused by the difference of the energy deposition cross section, and does not contradict the supposition. Ó 2002 Published by Elsevier Science B.V. Keywords: SiC; Irradiation; Bubble; Isotope
1. Introduction Hojou et al. reported that electron irradiation performed successively after hydrogen ion implantation at room temperature (RT) gave rise to bubble formation and growth in SiC containing BeO (<1 wt.%) [1]. They verified that this phenomenon had not been caused by thermal effect and set up the supposition that the energy deposition of the electron beam to hydrogen atom caused the migration of hydrogen and gave rise to bubble formation and growth. In this study we tried to inspect the supposition mentioned above. To be exact, we investigated *
Corresponding author. Tel.: +86-029-2648711; fax: +86029-2648712. E-mail address: [email protected] (J. Aihara).
whether the phenomena which contradicted the supposition were observed or not. If the bubbles are less likely to be formed and grow in the case of the more concentration of the mobile hydrogen (H) or deuterium (D), the phenomenon is judged to contradict the supposition. We implanted SiC with H or D ion and irradiated the implanted specimen with electron beam to change the energy deposition cross section. If the bubbles are less likely to be formed and grow in the case of the more energy deposition cross section when the specimens are implanted with the same amount of ion and are irradiated with the same flux of electron beam, the phenomenon is judged to contradict the supposition. Because the concentration of the mobile H or D should be higher in the case of the more energy deposition cross section.
0168-583X/02/$ - see front matter Ó 2002 Published by Elsevier Science B.V. PII: S 0 1 6 8 - 5 8 3 X ( 0 2 ) 0 0 6 0 7 - 9
J. Aihara et al. / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 540–543
2. Experimental Specimens were polycrystalline sintered a-SiC. with a purity of about 99.5%. Thin films suitable for observation by transmission electron microscope (TEM) were prepared. Ion irradiation and successive electron irradiations were carried out in the TEM linked with an ion accelerator at RT. However, we took care not to expose specimens to the electron beam to the utmost. The incident angle of the ion beam was about 30° to the observation direction. þ Ion species were 10 keV Hþ 2 or 8.3 keV D2 . Ion 18 2 18 2 fluxes were 3:6 10 /m s and 1:7 10 /m s, respectively. Ion fluence was 6:6 1021 (H or D)/m2 . Irradiation temperature was RT. Ion energy, flux and fluence of Hþ 2 were the same as those used in the experiment by Hojou et al. [1]. Ion energy of Dþ 2 was determined based on the TRIM-95 calculation [2] so that the peak of ion distribution is approximately at the same depth (60 nm) as that of H. We made TRIM-95 calculation, fixing ion energy for 5 keV H and 4.15 keV D. D and H have considerably different dpa distributions. Flux of D was adjusted so that the peak dpa rate of D was the same as that of H. The peak dpa, the peak dpa depth and the peak ion concentration were estimated to be 7.8 dpa, 37 nm and 58 at% for H, and 17 dpa, 30 nm and 54 at% for D by the TRIM-95 calculation. However, sputtering, mixing, and composition change were not taken into account in the estimation. In both cases peak dpa was well above the critical dpa (about 0.4 dpa [3]) necessary for amorphization at RT. After ion irradiation, specimens were continuously irradiated with 200 keV electron in the TEM at RT for 60 min. Electron fluxes were 1:68 1022 /m2 s and 6:72 1022 /m2 s.
541
confirmed. Most of H or D was thought to exist in the amorphized region from the estimation of dpa distribution and ion distribution based on TRIM95, and the amorphous compound of Si, C and (H or D) was thought to be formed. However the structure (including short-range order and chemical bond) of the amorphous compound was not clear. TEM images of the specimen irradiated with the electron flux of 1:68 1022 /m2 s for 60 min (fluence: 6:0 1025 /m2 ) after implantation with D and H are shown in Fig. 1(a) and (b), respectively. Bubble growth was observed to depend on the mass of implanted ion. The bubble coalescence was observed in the case of H implantation, as shown in Fig. 1(b), on the other hand the bubbles were hardly observed in the case of D implantation, as shown in Fig. 1(a). However the bubbles coalesced even in the case of D implantation with the higher flux of electron beam, as shown in Fig. 2. Fig. 2 shows the TEM image of the specimen
3. Results and discussion 3.1. Experimental results Amorphization was observed in both cases of H implantation and D implantation immediately after starting electron irradiation (as ion implanted). The existence of bubbles were not clearly
Fig. 1. TEM image of the specimen irradiated with electron beam with the flux of 1:68 1022 /m2 s for 60 min (fluence: 6:0 1025 /m2 ) at RT. Ion implantation condition: (a) 8.3 keV þ 21 2 21 2 Dþ 2 , 6:6 10 D/m , at RT, (b) 10 keV H2 , 6:6 10 H/m , at RT.
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J. Aihara et al. / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 540–543
Fig. 2. TEM image of the specimen irradiated with electron beam with the flux of 6:72 1022 /m2 s at RT. Ion implantation condition: 21 D/m2 , at RT; electron irradiation time: 60 min (fluence: 2:4 1026 /m2 ). 8.3 keV Dþ 2 , 6:6 10
irradiated with the electron flux of 6:72 1022 /m2 s for 60 min (2:4 1026 /m2 ) after implantation with D. The bubble growth depends also on the thickness of specimen, as shown in Figs. 1(b) and 2. Bubbles were hardly observed thin region near the edge of the specimen, on the other hand they coalesced in the thicker region. It is not clear what gas exists in bubbles. It is assumed that H or D exists in simple substance or as C and/or Si compound [5,6]. The recrystallization in amorphous area with electron irradiation was not observed. 3.2. Energy deposition cross section Eq. (1) gives the approximated cross section (not the differential cross section) of the reaction that nucleus (atomic number Z2 mass ¼ M2 ) receives energy more than E1 and less than E2 ð0 < E1 5 E2 5 Tm Þ by Coulomb scattering with electron (b-value ¼ b, kinetic energy ¼ E) [4]. Tm is the maximum energy which can be deposited to the nucleus and is given in Eq. (3). RðE1 ; E2 Þ ¼ rðE1 Þ rðE2 Þ;
ð1Þ
Tm ¼ 2
m E E þ 2mc2 ; 2 M2 mc
here, Ea : variable ð0 < Ea 5Tm Þ, c: light velocity, m: rest mass of electron, a ¼ Z2 =137, a0 : Bohr radius (¼ 5:29 1011 m), ER : Rydberg energy (¼ 13:6 eV). In order to estimate the energy deposition cross section with Eq. (1) as the standard of the aptness of the formation and/or growth of bubbles, E1 and E2 should be regarded as migration energy and displacement threshold energy, respectively assuming that the formation and/or growth of bubbles is due to radiation-induced diffusion. On the other hand they should be set as displacement threshold energy and Tm , respectively under the assumption that the formation and/or growth of bubbles is due to displacement. Eq. (4) gives the ratio of the cross section in the case of H to the cross section in the case of D. SðE1 ; E2 Þ ¼ RH ðE1 ; E2 Þ=RD ðE1 ; E2 Þ:
Z2 a0 ER ð1 b2 Þ rðEa Þ ¼ 4p mc2 b4 " Tm Tm 1 b2 ln Ea Ea ( " # )# 1=2 Tm Tm þ pab 2 1 ln ; Ea Ea ð2Þ
ð4Þ
RD and RH are the cross section calculated with Eq. (1) in each case of D and H, respectively. Note that SðE1 ; Tm Þ is defined in Eq. (5). SðE1 ; Tm Þ ¼ RH ðE1 ; TmH Þ=RD ðE1 ; TmD Þ:
where
ð3Þ
ð5Þ
Fig. 3 shows the ratio SðE1 ; E2 Þ calculated with Eqs. (4) and (5), setting x-axis as E1 . E2 was set to be 5–100 eV and Tm , and E1 ð
J. Aihara et al. / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 540–543
Fig. 3. The ratio of the cross section in the case of H to the cross section in the case of D.
under the assumption that migration energy and/ or displacement threshold energy are the same in the cases of D and H (in the energy range shown in Fig. 3).
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However, following reasons are supposed other than the difference in the cross section for the implanted ion mass dependence: (1) There is a possibility that D may need larger energy deposition than H to migrate and contribute to the bubble growth and that the thermal diffusivity of D may be smaller than that of H. (2) The dpa in specimen is larger in the case of D than in the case of H, as mentioned in Section 2. This indicates the possibility of the difference in the structure of amorphous compound of Si, C and (H or D). The difference in the structure may cause difference in the migration energy and/or displacement threshold energy, and/or may influence on the other factors controlling the bubble formation and/or growth. There are still some other factors to be determined in order to clarify the details of the mechanism.
3.3. Inspecting the supposition The supposition described in Section 1 was inspected according to the logic mentioned in the same section. The specimen thickness dependence of the bubble growth mentioned in Section 3.1 can be explained by the fact that the amount of implanted H or D is smaller, so the amount of mobile H or D is smaller in thinner area than in thicker area and does not contradict the supposition described in Section 1. The implanted ion mass dependence of the bubble growth mentioned in Section 3.1 can be explained in the term of the difference of the concentration of the mobile gas atom caused by the difference of the energy deposition cross section shown in Fig. 3 and does not contradict the supposition described in Section 1. Moreover, the experimental result that the bubble growth was observed even in the case of D with the electron irradiation with fourfold flux mentioned in Section 3.1, can be explained to be because, the ratio of the cross section in the case of H to that in the case of D was estimated to be no more than 2–3, not 10 or 100, as shown in Fig. 3. Note that the ratio was estimated to be 2–3, independing whether the phenomenon is based on radiationinduced diffusion or displacement.
4. Conclusion (1) Bubble growth with electron irradiation was observed to be dependent on the thickness of specimen and on the mass of implanted ion. (2) The results mentioned in (1) do not contradict the supposition that the energy deposition of the electron beam to hydrogen atom caused the migration of hydrogen and gave rise to bubble formation and growth.
References [1] K. Hojou, S. Furuno, K. Izui, J. Electron Microsc. 40 (1991) 157. [2] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon Press, New York, 1985. [3] S.J. Zinkle, L.L. Snead, Nucl. Instr. and Meth. B 116 (1996) 92. [4] Materials Science Society of Japan (Ed.), Irradiation Effects in Materials, Shokabo, Tokyo, 1994, p. 17. [5] K. Hojou, S. Furuno, T. Soga, K. Izui, J. Nucl. Mater. 179– 181 (1991) 411. [6] K. Hojou, S. Furuno, K.N. Kushita, N. Sasajima, K. Izui, Nucl. Instr. and Meth. B 141 (1998) 148.
Bubble formation with electron irradiation in SiC implanted with hydrogen or deuterium J. Aihara a
a,*
, K. Hojou b, S. Furuno b, M. Ishihara
a
Japan Atomic Energy Research Institute, Oarai-machi, Higashi-ibaraki-gun, Ibaraki-ken, Japan b Japan Atomic Energy Research Institute, Tokai-mura Naka-gun, Ibaraki-ken, Japan
Abstract The bubbles were formed and grew when SiC implanted with high fluence of hydrogen ion was irradiated with electron. In this study we tried to inspect the supposition that the energy deposition of the electron beam to hydrogen caused the migration of hydrogen and gave rise to bubble formation and growth. We used hydrogen (H) or deuterium (D) as implanted ion to change the cross section of the reaction that D or H is given energy by electron beam. The energy deposition cross section in the case of H is 2–3 times as large as that in the case of D. Bubbles were less likely to be formed and grow in the case of D than in the case of H. This result can be explained in terms of the difference of the concentration of the mobile gas atom caused by the difference of the energy deposition cross section, and does not contradict the supposition. Ó 2002 Published by Elsevier Science B.V. Keywords: SiC; Irradiation; Bubble; Isotope
1. Introduction Hojou et al. reported that electron irradiation performed successively after hydrogen ion implantation at room temperature (RT) gave rise to bubble formation and growth in SiC containing BeO (<1 wt.%) [1]. They verified that this phenomenon had not been caused by thermal effect and set up the supposition that the energy deposition of the electron beam to hydrogen atom caused the migration of hydrogen and gave rise to bubble formation and growth. In this study we tried to inspect the supposition mentioned above. To be exact, we investigated *
Corresponding author. Tel.: +86-029-2648711; fax: +86029-2648712. E-mail address: [email protected] (J. Aihara).
whether the phenomena which contradicted the supposition were observed or not. If the bubbles are less likely to be formed and grow in the case of the more concentration of the mobile hydrogen (H) or deuterium (D), the phenomenon is judged to contradict the supposition. We implanted SiC with H or D ion and irradiated the implanted specimen with electron beam to change the energy deposition cross section. If the bubbles are less likely to be formed and grow in the case of the more energy deposition cross section when the specimens are implanted with the same amount of ion and are irradiated with the same flux of electron beam, the phenomenon is judged to contradict the supposition. Because the concentration of the mobile H or D should be higher in the case of the more energy deposition cross section.
0168-583X/02/$ - see front matter Ó 2002 Published by Elsevier Science B.V. PII: S 0 1 6 8 - 5 8 3 X ( 0 2 ) 0 0 6 0 7 - 9
J. Aihara et al. / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 540–543
2. Experimental Specimens were polycrystalline sintered a-SiC. with a purity of about 99.5%. Thin films suitable for observation by transmission electron microscope (TEM) were prepared. Ion irradiation and successive electron irradiations were carried out in the TEM linked with an ion accelerator at RT. However, we took care not to expose specimens to the electron beam to the utmost. The incident angle of the ion beam was about 30° to the observation direction. þ Ion species were 10 keV Hþ 2 or 8.3 keV D2 . Ion 18 2 18 2 fluxes were 3:6 10 /m s and 1:7 10 /m s, respectively. Ion fluence was 6:6 1021 (H or D)/m2 . Irradiation temperature was RT. Ion energy, flux and fluence of Hþ 2 were the same as those used in the experiment by Hojou et al. [1]. Ion energy of Dþ 2 was determined based on the TRIM-95 calculation [2] so that the peak of ion distribution is approximately at the same depth (60 nm) as that of H. We made TRIM-95 calculation, fixing ion energy for 5 keV H and 4.15 keV D. D and H have considerably different dpa distributions. Flux of D was adjusted so that the peak dpa rate of D was the same as that of H. The peak dpa, the peak dpa depth and the peak ion concentration were estimated to be 7.8 dpa, 37 nm and 58 at% for H, and 17 dpa, 30 nm and 54 at% for D by the TRIM-95 calculation. However, sputtering, mixing, and composition change were not taken into account in the estimation. In both cases peak dpa was well above the critical dpa (about 0.4 dpa [3]) necessary for amorphization at RT. After ion irradiation, specimens were continuously irradiated with 200 keV electron in the TEM at RT for 60 min. Electron fluxes were 1:68 1022 /m2 s and 6:72 1022 /m2 s.
541
confirmed. Most of H or D was thought to exist in the amorphized region from the estimation of dpa distribution and ion distribution based on TRIM95, and the amorphous compound of Si, C and (H or D) was thought to be formed. However the structure (including short-range order and chemical bond) of the amorphous compound was not clear. TEM images of the specimen irradiated with the electron flux of 1:68 1022 /m2 s for 60 min (fluence: 6:0 1025 /m2 ) after implantation with D and H are shown in Fig. 1(a) and (b), respectively. Bubble growth was observed to depend on the mass of implanted ion. The bubble coalescence was observed in the case of H implantation, as shown in Fig. 1(b), on the other hand the bubbles were hardly observed in the case of D implantation, as shown in Fig. 1(a). However the bubbles coalesced even in the case of D implantation with the higher flux of electron beam, as shown in Fig. 2. Fig. 2 shows the TEM image of the specimen
3. Results and discussion 3.1. Experimental results Amorphization was observed in both cases of H implantation and D implantation immediately after starting electron irradiation (as ion implanted). The existence of bubbles were not clearly
Fig. 1. TEM image of the specimen irradiated with electron beam with the flux of 1:68 1022 /m2 s for 60 min (fluence: 6:0 1025 /m2 ) at RT. Ion implantation condition: (a) 8.3 keV þ 21 2 21 2 Dþ 2 , 6:6 10 D/m , at RT, (b) 10 keV H2 , 6:6 10 H/m , at RT.
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J. Aihara et al. / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 540–543
Fig. 2. TEM image of the specimen irradiated with electron beam with the flux of 6:72 1022 /m2 s at RT. Ion implantation condition: 21 D/m2 , at RT; electron irradiation time: 60 min (fluence: 2:4 1026 /m2 ). 8.3 keV Dþ 2 , 6:6 10
irradiated with the electron flux of 6:72 1022 /m2 s for 60 min (2:4 1026 /m2 ) after implantation with D. The bubble growth depends also on the thickness of specimen, as shown in Figs. 1(b) and 2. Bubbles were hardly observed thin region near the edge of the specimen, on the other hand they coalesced in the thicker region. It is not clear what gas exists in bubbles. It is assumed that H or D exists in simple substance or as C and/or Si compound [5,6]. The recrystallization in amorphous area with electron irradiation was not observed. 3.2. Energy deposition cross section Eq. (1) gives the approximated cross section (not the differential cross section) of the reaction that nucleus (atomic number Z2 mass ¼ M2 ) receives energy more than E1 and less than E2 ð0 < E1 5 E2 5 Tm Þ by Coulomb scattering with electron (b-value ¼ b, kinetic energy ¼ E) [4]. Tm is the maximum energy which can be deposited to the nucleus and is given in Eq. (3). RðE1 ; E2 Þ ¼ rðE1 Þ rðE2 Þ;
ð1Þ
Tm ¼ 2
m E E þ 2mc2 ; 2 M2 mc
here, Ea : variable ð0 < Ea 5Tm Þ, c: light velocity, m: rest mass of electron, a ¼ Z2 =137, a0 : Bohr radius (¼ 5:29 1011 m), ER : Rydberg energy (¼ 13:6 eV). In order to estimate the energy deposition cross section with Eq. (1) as the standard of the aptness of the formation and/or growth of bubbles, E1 and E2 should be regarded as migration energy and displacement threshold energy, respectively assuming that the formation and/or growth of bubbles is due to radiation-induced diffusion. On the other hand they should be set as displacement threshold energy and Tm , respectively under the assumption that the formation and/or growth of bubbles is due to displacement. Eq. (4) gives the ratio of the cross section in the case of H to the cross section in the case of D. SðE1 ; E2 Þ ¼ RH ðE1 ; E2 Þ=RD ðE1 ; E2 Þ:
Z2 a0 ER ð1 b2 Þ rðEa Þ ¼ 4p mc2 b4 " Tm Tm 1 b2 ln Ea Ea ( " # )# 1=2 Tm Tm þ pab 2 1 ln ; Ea Ea ð2Þ
ð4Þ
RD and RH are the cross section calculated with Eq. (1) in each case of D and H, respectively. Note that SðE1 ; Tm Þ is defined in Eq. (5). SðE1 ; Tm Þ ¼ RH ðE1 ; TmH Þ=RD ðE1 ; TmD Þ:
where
ð3Þ
ð5Þ
Fig. 3 shows the ratio SðE1 ; E2 Þ calculated with Eqs. (4) and (5), setting x-axis as E1 . E2 was set to be 5–100 eV and Tm , and E1 ð
J. Aihara et al. / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 540–543
Fig. 3. The ratio of the cross section in the case of H to the cross section in the case of D.
under the assumption that migration energy and/ or displacement threshold energy are the same in the cases of D and H (in the energy range shown in Fig. 3).
543
However, following reasons are supposed other than the difference in the cross section for the implanted ion mass dependence: (1) There is a possibility that D may need larger energy deposition than H to migrate and contribute to the bubble growth and that the thermal diffusivity of D may be smaller than that of H. (2) The dpa in specimen is larger in the case of D than in the case of H, as mentioned in Section 2. This indicates the possibility of the difference in the structure of amorphous compound of Si, C and (H or D). The difference in the structure may cause difference in the migration energy and/or displacement threshold energy, and/or may influence on the other factors controlling the bubble formation and/or growth. There are still some other factors to be determined in order to clarify the details of the mechanism.
3.3. Inspecting the supposition The supposition described in Section 1 was inspected according to the logic mentioned in the same section. The specimen thickness dependence of the bubble growth mentioned in Section 3.1 can be explained by the fact that the amount of implanted H or D is smaller, so the amount of mobile H or D is smaller in thinner area than in thicker area and does not contradict the supposition described in Section 1. The implanted ion mass dependence of the bubble growth mentioned in Section 3.1 can be explained in the term of the difference of the concentration of the mobile gas atom caused by the difference of the energy deposition cross section shown in Fig. 3 and does not contradict the supposition described in Section 1. Moreover, the experimental result that the bubble growth was observed even in the case of D with the electron irradiation with fourfold flux mentioned in Section 3.1, can be explained to be because, the ratio of the cross section in the case of H to that in the case of D was estimated to be no more than 2–3, not 10 or 100, as shown in Fig. 3. Note that the ratio was estimated to be 2–3, independing whether the phenomenon is based on radiationinduced diffusion or displacement.
4. Conclusion (1) Bubble growth with electron irradiation was observed to be dependent on the thickness of specimen and on the mass of implanted ion. (2) The results mentioned in (1) do not contradict the supposition that the energy deposition of the electron beam to hydrogen atom caused the migration of hydrogen and gave rise to bubble formation and growth.
References [1] K. Hojou, S. Furuno, K. Izui, J. Electron Microsc. 40 (1991) 157. [2] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon Press, New York, 1985. [3] S.J. Zinkle, L.L. Snead, Nucl. Instr. and Meth. B 116 (1996) 92. [4] Materials Science Society of Japan (Ed.), Irradiation Effects in Materials, Shokabo, Tokyo, 1994, p. 17. [5] K. Hojou, S. Furuno, T. Soga, K. Izui, J. Nucl. Mater. 179– 181 (1991) 411. [6] K. Hojou, S. Furuno, K.N. Kushita, N. Sasajima, K. Izui, Nucl. Instr. and Meth. B 141 (1998) 148.