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Void formation in silica glass induced by thermal oxidation after Zn+ ion implantation
Vacuum 83 (2009) 645–648
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Vacuum journal homepage: www.elsevier.com/locate/vacuum
Void formation in silica glass induced by thermal oxidation after Znþ ion implantation N. Umeda, H. Amekura*, N. Kishimoto Quantum Beam Center, National Institute for Materials Science (NIMS), 3-13 Sakura, Tsukuba, Ibaraki 3050003, Japan
a b s t r a c t Keywords: Ion implantation Void Zn ZnO Nanoparticle Oxidation PACS: 81.16.Pr 82.60.Qr 61.72.Qq 61.72.Ww
Thermal annealing effects on Znþ ion-implanted silica glass (a-SiO2) have been studied in order to control void formation. Void formation in a-SiO2 with Znþ ion implantation and subsequent oxidation has been observed using transmission electron microscopy (TEM). Znþ ions of 60 keV were implanted into a-SiO2 to a fluence of 1.0 1017 ions/cm2. After the implantation, thermal annealing at 600 or 700 C for 1 h in oxygen gas was conducted. In as-implanted state, metal Zn nanoparticles (NPs) of 10–15 nm in diameter are formed in the depth region around the projected range. The size of the Zn nanoparticles increases after the annealing at 600 C in oxygen gas. Annealing in oxygen gas at 700 C for 1 h caused two processes: (1) the migration of Zn atoms which formed Zn NPs in as-implanted state to the surface of the a-SiO2 substrate and (2) the transformation to the oxide phase on the substrate. The transportation of Zn NPs to the surface leaves voids of 10–25 nm in diameter inside the a-SiO2. These results indicate that the oxidation at 700 C for 1 h causes the migration of Zn atoms to the surface without diffusion and recombination of vacancies which form the voids. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Ion implantation technique is a versatile method to create welldefined nanocomposite materials by inherent availability such as spatial controllability and irradiation effects. Void formation in solids via ion beam was reported in various systems, e.g., in Si/SiGe/ Si layers by 1 keV Ge ion implantation and rapid thermal annealing [1], and in sapphire by Zn ion implantation and following annealing in vacuum [2]. Furthermore, voids were formed inside the metal nanoparticles (NPs) themselves by sequential ion implantation [3]. These examples used advantage of ion beam technique such as depth controllability or nuclear energy deposition. The control of void formation can be applicable to form and/or modify nanostructured materials. Microscopically these voids can be used as templates for nano-structure formation. Mesoscopically substrates and sometimes nanoparticles including voids show effectively different physical properties from the counterparts without voids, such as density, dielectric constant, electron concentration, etc. Up to now, we have fabricated metal-oxide NPs such as nickeloxide (NiO) [4,5], cupric oxide (CuO) [6] and zinc oxide (ZnO) [7–9] in silica glasses (a-SiO2) using metal-ion implantation and subsequent thermal oxidation. In these studies, we have found that
* Corresponding author. Tel.: þ81 29 863 5479; fax: þ81 29 863 5599. E-mail address: [email protected] (H. Amekura). 0042-207X/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2008.04.043
voids were formed in a-SiO2 via creating ZnO. The Zn ion is a candidate for the method to create voids because of the high mobility in a-SiO2 and the high susceptibility to oxidation during annealing in oxygen gas. In the present study, Zn ion implantation and subsequent annealing in oxygen gas are applied in order to control the formation of voids in a-SiO2. The formation of high-density and depthcontrolled voids for in a-SiO2 is observed by transmission electron microscopy. 2. Experimental Optical-grade silica glasses of the KU-1 type (OH 820 ppm), 15 mm in diameter and 0.5 mm in thickness, were implanted with 64 Znþ ions of 60 keV up to a fluence of 1.0 1017 ions/cm2. To avoid temperature increase of substrates higher than 100 C during implantation, the ion flux was kept to less than 2 mA/cm2 and the sample holder was cooled by water circulation. Thermal oxidation was carried out at 600 and 700 C for 1 h in a tube furnace under O2 gas flow of w100 sccm at a pressure of w100 kPa. Cross-sectional transmission electron microscopy (XTEM) was conducted to evaluate microstructures in the Zn implanted region. The specimens for XTEM were prepared by mechanical polishing and ion milling technique. A depth profile of 60 keV Zn implants in a-SiO2 was calculated by TRIDYN codes [10]. The TRIDYN code is based on TRIM code [11] and considering sputtering loss and composition changes.
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Fig. 1. XTEM images of silica glasses implanted with Znþ ions of 60 keV up to 1.0 1017 ions/cm2; (a) in as-implanted state and (b) after annealing in oxygen gas flow at 600 C for 1 h. Scale bar of 50 nm in length is shown in each figure.
3. Results and discussion Fig. 1 shows XTEM images of silica glass implanted with 60 keV Znþ ions up to 1.0 1017 ions/cm2; (a) in as-implanted state and (b) after annealing in oxygen gas at 600 C for 1 h. Zn NPs spontaneously form even in the as-implanted state as shown in Fig. 1(a). The shape of the NPs is spherical. In the layer between w10 and w50 nm in depth, NPs of 10–15 nm in diameter are observed. On the other hand, after oxygen annealing at 600 C for 1 h, Zn NPs increase the size. Some NPs become larger than 25 nm in diameter. A small number of ZnO NPs are observed on the surface of a-SiO2 substrate as shown in Fig. 1(b). In the previous study, we have observed by X-ray photoelectron spectroscopy (XPS) that Zn and ZnO particles exist in and on the a-SiO2 substrate, respectively [7]. Fig. 2(a) and (b) shows depth profiles of Zn atomic number density of as-implanted and annealed specimens, respectively. The detailed procedures of determination of a depth profile of implanted atoms from an XTEM image were described elsewhere [12]. The minimum detection size of NPs by XTEM was about 1 nm in diameter. In the annealed specimen (Fig. 2(b)), the NPs on the surface, which consists of ZnO, were also counted into the Zn number density with multiplying 1/2. The projected range of 30 nm
was calculated for Zn ions of 60 keV in a-SiO2 by TRIDYN code [10]. The mean depth of the Zn NPs distribution in the as-implanted state determined from the XTEM images was 27 nm, indicating that the depth distribution of Zn NPs almost coincides with the calculated value by TRIDYN. As shown in Fig. 2(a), the total amount of precipitated Zn atoms is lower than w50% of the totally implanted Zn ions in the asimplanted state. In the previous work [7], we observed by Rutherford backscattering spectrometry (RBS) that the implanted Zn ions of 1.0 1017 ions/cm2 almost stayed inside the a-SiO2 in the asimplanted state. These results indicate that a part of implanted Zn atoms does not precipitate as Zn NPs in as-implanted state. The mean depth of the Zn NPs of the annealed specimen at 600 C, which includes the surface ZnO NPs, was 23 nm in depth. This value is slightly smaller than that of the as-implanted state indicating small shift of the depth distribution toward the surface. As shown in Fig. 2(b), the NP number density increases after the annealing at 600 C comparing with the as-implanted state. The annealing at 600 C enhances the precipitation of Zn NPs without significant shift of the depth profile, and a small amount of Zn atoms migrates to the surface of the a-SiO2 substrate and is oxidized to ZnO NPs.
Fig. 2. Depth profiles of Zn atomic number density in silica glass that was implanted with 60 keV Znþ to 1.0 1017 ions/cm2; (a) in as-implanted state and (b) annealed at 600 C for 1 h in oxygen gas. Histograms show the experimental results determined from XTEM images. Solid line shows calculated results from TRYDIN code. Components shown in the negative values of the depth correspond to NPs located on the surface of the silica substrate.
N. Umeda et al. / Vacuum 83 (2009) 645–648
Fig. 3(a) shows XTEM images of a-SiO2 implanted with 60 keV Znþ ions after annealing at 700 C for 1 h, which were taken within several seconds of electron beam exposure for TEM observation. After the oxygen annealing at 700 C, the larger Zn NPs almost disappear, and droplet-like ZnO particles grow on the surface of the a-SiO2 substrate. In the previous work [7], we observed by XPS that the implanted Zn atoms mainly migrate to the surface of the substrate and form ZnO NPs after the oxygen annealing at 700 C for 1 h. As shown in Fig. 3(a), the larger and spherical voids of 10– 25 nm in diameter form in the depth between w10 and w40 nm. It should be noted again that the XTEM image shown in Fig. 3(a) was taken within several seconds of electron beam exposure. The voids gradually disappear during TEM observation as shown in Fig. 3(b), namely the electron beam irradiation induces recovery of a-SiO2 structure. Fig. 4 shows depth profiles of Zn atom and the void number density in the specimen annealed at 700 C. In derivation of the result shown in Fig. 4, the atomic number density in void was assumed as the same as Zn atoms. The mean depth of the void distribution is 19 nm, namely the voids form in the region which is slightly shallower than the region where once Zn NPs existed after 600 C annealing. The mean diameter of the voids is larger than that of Zn NPs in as-implanted state, but nearly equals to that of Zn NPs after the annealing at 600 C (see Figs. 1 and 3(a)). These results indicate that Zn atoms precipitate and increase the sizes of Zn NPs after annealing at 600 C. However, after annealing at 700 C, the dissolution of Zn atoms from the Zn NPs becomes dominant. Finally the majority of Zn atoms migrate toward the surface leaving the voids. Since the sizes of Zn NPs at 600 C and of voids at 700 C are similar, it is suggested that the voids observed after annealing at 700 C are vacant sites once filled with Zn NPs at 600 C. These results indicate that the migration of Zn atoms by thermal oxidation occurs without significant migration or recombination of vacancies which form the voids. Fig. 3(b) shows XTEM image after the electron beam irradiation for more than 5 min. The voids mostly disappeared. Comparing with Fig. 3(a) and (b), the implanted region is slightly shrunk after the long time electron irradiation. These results indicate that the voids are unstable against the electron beam irradiation and the electron beam recovers a-SiO2 structure. Liu et al. [13] reported the void formation in a-SiO2 implanted with Zn ions of 160 keV to 3 1017 ions/cm2 after oxygen annealing at 700 C for 1 h. They assumed the voids were formed by the internal strain between the ZnO nanograins and the silica surface. However, the strain mechanism does not explain the coincidence
647
Fig. 4. Depth profiles of atomic number density of Zn NPs and voids in silica glass which was implanted with 60 keV Znþ to 1.0 1017 ions/cm2 and annealed at 700 C for 1 h in oxygen gas. Histograms show the experimental distributions of Zn NPs (hatched rectangles) and of voids (open rectangles) determined from XTEM images. The solid line shows the Zn atom distribution in as-implanted state calculated by TRIDYN code. Components shown in the negative values of the depth correspond to NPs located on the surface of the silica substrate.
between the mean size of Zn NPs and that of voids. Void formation was also observed in Zn implanted Al2O3 after annealing at 1000 C for 1 h in vacuum by Marques et al. [2]. They concluded that Kirkendall voids are formed by a significant diffusion of Zn atoms during heat treatment. In their case, the shape of voids is not clear and the volume density of voids is low. It suggests that the oxidation effect, i.e. the attraction of implanted Zn atoms toward the surface, enhances the void formation. In fact, we have also carried out annealing in vacuum of less than 1 103 Pa up to 700 C. Only less clear voids of much lower density were observed. No directional migration of Zn atoms
Fig. 3. XTEM images of silica glasses implanted with Znþ ions of 60 keV up to 1.0 1017 ions/cm2 after annealing in oxygen gas flow at 700 C for 1 h, with electron beam exposure of (a) within several seconds and (b) more than 5 min. Scale bar of 50 nm in length is shown in each figure.
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toward the surface was observed [14]. These facts indicate the importance of oxygen atmosphere for the stable void formation, at least in SiO2. Rapp discussed the role of the internal oxidation in the oxidation of alloys [15]. When a protective layer, which is oxygenated solute element, is formed at the surface in the processes of the oxidation of the alloys, the solute element is oxidized at surface of the matrix rather than inside the matrix. In our case oxidation in composite material, the external oxidation, i.e. oxidation on the surface, of Zn atoms becomes dominant rather than the internal oxidation, i.e. oxidation inside the a-SiO2, of Zn atoms at 700 C for 1 h annealing in oxygen gas. Consequently the void formation inside the a-SiO2 was enhanced in the process of external oxidation of Zn atoms. The oxide layer (ZnO) appears at 600 C and increases the thickness at 700 C (see in Figs. 1(b) and 3(a)). The diffusion length of O2 molecules in a-SiO2 at 700 C for 1 h annealing almost coincides with the project range of Zn ions of 60 keV. On the contrary, the supply of O2 molecules from the surface decreases by growth of ZnO layer thickness on the surface. The diffusion of O2, which is gradually limited by ZnO layer on the surface, may attract Zn atoms toward the surface without the significant internal oxidation of Zn atoms. Hence, the voids are formed by steep migration of Zn atoms toward the surface, and are maintained without significant recovery.
4. Summary Specimens of a-SiO2 were implanted with Zn ions of 60 keV up to 1.0 1017 ions/cm2 and annealed at 600 and 700 C for 1 h in oxygen gas. Effects of thermal annealing on the void formation were evaluated by XTEM observation. Spherical and high-density voids were formed after annealing at 700 C, and located at a certain depth. Both the high mobility of Zn atoms in a-SiO2 and the
high susceptibility to oxidation of Zn may enhance the remarkable void formation. Acknowledgments A part of this study was financially supported by the Budget for Nuclear Research of the Ministry of Education, Culture, Sports, Science and Technology, based on the screening and counseling by the Atomic Energy Commission. This study was also partly supported by JSPS-Kakenhi (No. 18510102) and by Futaba Electronics Memorial Foundation. References [1] Gaiduk PI, Lundsgaard Hansen J, Nylandsted Larsen A. Nucl Instrum Methods 2005;B 230:214–9. [2] Marques C, France N, Alves LC, da Silva RC, Alves E, Safran G, et al. Nucl Instrum Methods 2007;B 257:515–8. [3] Xiao X, Jiang C, Ren F, Wang J, Shi Y. Solid State Commun 2006;137:362–5. [4] Amekura H, Umeda N, Takeda Y, Lu J, Kishimoto N. Appl Phys Lett 2004;85: 1015–7. [5] Amekura H, Umeda N, Takeda Y, Lu J, Kono K, Kishimoto N. Nucl Instrum Methods 2005;B 230:193–7. [6] Amekura H, Kono K, Takeda Y, Kishimoto N. Appl Phys Lett 2005;87: 153105-1–3. [7] Amekura H, Umeda N, Yoshitake M, Kono K, Kishimoto N, Buchal Ch. J Cryst Growth 2006;287:2–6. [8] Amekura H, Umeda N, Boldyryeva H, Kishimoto N. Appl Phys Lett 2007;90: 083102-1–3. [9] Amkura H, Kono K, Kishimoto N, Buchal Ch. Nucl Instrum Methods 2006;B 242:96–9. [10] Moeller W, Eckstein W. Nucl Instrum Methods 1984;B 2:814–8. [11] Ziegler J, Biersack J, Littmark U. The stopping and range of ions in solids. New York: Pergamon; 1985. [12] Umeda N, Kishimoto N, Takeda Y, Lee CG, Gritsyna VT. Nucl Instrum Methods 2000;B 166–167:864–70. [13] Liu YX, Liu YC, Shao CL, Mu R. J Phys D Appl Phys 2004;37:3025–9. [14] Amekura H, Umeda N, Kono K, Takeda Y, Kishimoto N, Buchal Ch. Nanotechnology 2007;18:395707-1–6. [15] Rapp RA. Corrosion 1965;21:382–401.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Void formation in silica glass induced by thermal oxidation after Znþ ion implantation N. Umeda, H. Amekura*, N. Kishimoto Quantum Beam Center, National Institute for Materials Science (NIMS), 3-13 Sakura, Tsukuba, Ibaraki 3050003, Japan
a b s t r a c t Keywords: Ion implantation Void Zn ZnO Nanoparticle Oxidation PACS: 81.16.Pr 82.60.Qr 61.72.Qq 61.72.Ww
Thermal annealing effects on Znþ ion-implanted silica glass (a-SiO2) have been studied in order to control void formation. Void formation in a-SiO2 with Znþ ion implantation and subsequent oxidation has been observed using transmission electron microscopy (TEM). Znþ ions of 60 keV were implanted into a-SiO2 to a fluence of 1.0 1017 ions/cm2. After the implantation, thermal annealing at 600 or 700 C for 1 h in oxygen gas was conducted. In as-implanted state, metal Zn nanoparticles (NPs) of 10–15 nm in diameter are formed in the depth region around the projected range. The size of the Zn nanoparticles increases after the annealing at 600 C in oxygen gas. Annealing in oxygen gas at 700 C for 1 h caused two processes: (1) the migration of Zn atoms which formed Zn NPs in as-implanted state to the surface of the a-SiO2 substrate and (2) the transformation to the oxide phase on the substrate. The transportation of Zn NPs to the surface leaves voids of 10–25 nm in diameter inside the a-SiO2. These results indicate that the oxidation at 700 C for 1 h causes the migration of Zn atoms to the surface without diffusion and recombination of vacancies which form the voids. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Ion implantation technique is a versatile method to create welldefined nanocomposite materials by inherent availability such as spatial controllability and irradiation effects. Void formation in solids via ion beam was reported in various systems, e.g., in Si/SiGe/ Si layers by 1 keV Ge ion implantation and rapid thermal annealing [1], and in sapphire by Zn ion implantation and following annealing in vacuum [2]. Furthermore, voids were formed inside the metal nanoparticles (NPs) themselves by sequential ion implantation [3]. These examples used advantage of ion beam technique such as depth controllability or nuclear energy deposition. The control of void formation can be applicable to form and/or modify nanostructured materials. Microscopically these voids can be used as templates for nano-structure formation. Mesoscopically substrates and sometimes nanoparticles including voids show effectively different physical properties from the counterparts without voids, such as density, dielectric constant, electron concentration, etc. Up to now, we have fabricated metal-oxide NPs such as nickeloxide (NiO) [4,5], cupric oxide (CuO) [6] and zinc oxide (ZnO) [7–9] in silica glasses (a-SiO2) using metal-ion implantation and subsequent thermal oxidation. In these studies, we have found that
* Corresponding author. Tel.: þ81 29 863 5479; fax: þ81 29 863 5599. E-mail address: [email protected] (H. Amekura). 0042-207X/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2008.04.043
voids were formed in a-SiO2 via creating ZnO. The Zn ion is a candidate for the method to create voids because of the high mobility in a-SiO2 and the high susceptibility to oxidation during annealing in oxygen gas. In the present study, Zn ion implantation and subsequent annealing in oxygen gas are applied in order to control the formation of voids in a-SiO2. The formation of high-density and depthcontrolled voids for in a-SiO2 is observed by transmission electron microscopy. 2. Experimental Optical-grade silica glasses of the KU-1 type (OH 820 ppm), 15 mm in diameter and 0.5 mm in thickness, were implanted with 64 Znþ ions of 60 keV up to a fluence of 1.0 1017 ions/cm2. To avoid temperature increase of substrates higher than 100 C during implantation, the ion flux was kept to less than 2 mA/cm2 and the sample holder was cooled by water circulation. Thermal oxidation was carried out at 600 and 700 C for 1 h in a tube furnace under O2 gas flow of w100 sccm at a pressure of w100 kPa. Cross-sectional transmission electron microscopy (XTEM) was conducted to evaluate microstructures in the Zn implanted region. The specimens for XTEM were prepared by mechanical polishing and ion milling technique. A depth profile of 60 keV Zn implants in a-SiO2 was calculated by TRIDYN codes [10]. The TRIDYN code is based on TRIM code [11] and considering sputtering loss and composition changes.
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Fig. 1. XTEM images of silica glasses implanted with Znþ ions of 60 keV up to 1.0 1017 ions/cm2; (a) in as-implanted state and (b) after annealing in oxygen gas flow at 600 C for 1 h. Scale bar of 50 nm in length is shown in each figure.
3. Results and discussion Fig. 1 shows XTEM images of silica glass implanted with 60 keV Znþ ions up to 1.0 1017 ions/cm2; (a) in as-implanted state and (b) after annealing in oxygen gas at 600 C for 1 h. Zn NPs spontaneously form even in the as-implanted state as shown in Fig. 1(a). The shape of the NPs is spherical. In the layer between w10 and w50 nm in depth, NPs of 10–15 nm in diameter are observed. On the other hand, after oxygen annealing at 600 C for 1 h, Zn NPs increase the size. Some NPs become larger than 25 nm in diameter. A small number of ZnO NPs are observed on the surface of a-SiO2 substrate as shown in Fig. 1(b). In the previous study, we have observed by X-ray photoelectron spectroscopy (XPS) that Zn and ZnO particles exist in and on the a-SiO2 substrate, respectively [7]. Fig. 2(a) and (b) shows depth profiles of Zn atomic number density of as-implanted and annealed specimens, respectively. The detailed procedures of determination of a depth profile of implanted atoms from an XTEM image were described elsewhere [12]. The minimum detection size of NPs by XTEM was about 1 nm in diameter. In the annealed specimen (Fig. 2(b)), the NPs on the surface, which consists of ZnO, were also counted into the Zn number density with multiplying 1/2. The projected range of 30 nm
was calculated for Zn ions of 60 keV in a-SiO2 by TRIDYN code [10]. The mean depth of the Zn NPs distribution in the as-implanted state determined from the XTEM images was 27 nm, indicating that the depth distribution of Zn NPs almost coincides with the calculated value by TRIDYN. As shown in Fig. 2(a), the total amount of precipitated Zn atoms is lower than w50% of the totally implanted Zn ions in the asimplanted state. In the previous work [7], we observed by Rutherford backscattering spectrometry (RBS) that the implanted Zn ions of 1.0 1017 ions/cm2 almost stayed inside the a-SiO2 in the asimplanted state. These results indicate that a part of implanted Zn atoms does not precipitate as Zn NPs in as-implanted state. The mean depth of the Zn NPs of the annealed specimen at 600 C, which includes the surface ZnO NPs, was 23 nm in depth. This value is slightly smaller than that of the as-implanted state indicating small shift of the depth distribution toward the surface. As shown in Fig. 2(b), the NP number density increases after the annealing at 600 C comparing with the as-implanted state. The annealing at 600 C enhances the precipitation of Zn NPs without significant shift of the depth profile, and a small amount of Zn atoms migrates to the surface of the a-SiO2 substrate and is oxidized to ZnO NPs.
Fig. 2. Depth profiles of Zn atomic number density in silica glass that was implanted with 60 keV Znþ to 1.0 1017 ions/cm2; (a) in as-implanted state and (b) annealed at 600 C for 1 h in oxygen gas. Histograms show the experimental results determined from XTEM images. Solid line shows calculated results from TRYDIN code. Components shown in the negative values of the depth correspond to NPs located on the surface of the silica substrate.
N. Umeda et al. / Vacuum 83 (2009) 645–648
Fig. 3(a) shows XTEM images of a-SiO2 implanted with 60 keV Znþ ions after annealing at 700 C for 1 h, which were taken within several seconds of electron beam exposure for TEM observation. After the oxygen annealing at 700 C, the larger Zn NPs almost disappear, and droplet-like ZnO particles grow on the surface of the a-SiO2 substrate. In the previous work [7], we observed by XPS that the implanted Zn atoms mainly migrate to the surface of the substrate and form ZnO NPs after the oxygen annealing at 700 C for 1 h. As shown in Fig. 3(a), the larger and spherical voids of 10– 25 nm in diameter form in the depth between w10 and w40 nm. It should be noted again that the XTEM image shown in Fig. 3(a) was taken within several seconds of electron beam exposure. The voids gradually disappear during TEM observation as shown in Fig. 3(b), namely the electron beam irradiation induces recovery of a-SiO2 structure. Fig. 4 shows depth profiles of Zn atom and the void number density in the specimen annealed at 700 C. In derivation of the result shown in Fig. 4, the atomic number density in void was assumed as the same as Zn atoms. The mean depth of the void distribution is 19 nm, namely the voids form in the region which is slightly shallower than the region where once Zn NPs existed after 600 C annealing. The mean diameter of the voids is larger than that of Zn NPs in as-implanted state, but nearly equals to that of Zn NPs after the annealing at 600 C (see Figs. 1 and 3(a)). These results indicate that Zn atoms precipitate and increase the sizes of Zn NPs after annealing at 600 C. However, after annealing at 700 C, the dissolution of Zn atoms from the Zn NPs becomes dominant. Finally the majority of Zn atoms migrate toward the surface leaving the voids. Since the sizes of Zn NPs at 600 C and of voids at 700 C are similar, it is suggested that the voids observed after annealing at 700 C are vacant sites once filled with Zn NPs at 600 C. These results indicate that the migration of Zn atoms by thermal oxidation occurs without significant migration or recombination of vacancies which form the voids. Fig. 3(b) shows XTEM image after the electron beam irradiation for more than 5 min. The voids mostly disappeared. Comparing with Fig. 3(a) and (b), the implanted region is slightly shrunk after the long time electron irradiation. These results indicate that the voids are unstable against the electron beam irradiation and the electron beam recovers a-SiO2 structure. Liu et al. [13] reported the void formation in a-SiO2 implanted with Zn ions of 160 keV to 3 1017 ions/cm2 after oxygen annealing at 700 C for 1 h. They assumed the voids were formed by the internal strain between the ZnO nanograins and the silica surface. However, the strain mechanism does not explain the coincidence
647
Fig. 4. Depth profiles of atomic number density of Zn NPs and voids in silica glass which was implanted with 60 keV Znþ to 1.0 1017 ions/cm2 and annealed at 700 C for 1 h in oxygen gas. Histograms show the experimental distributions of Zn NPs (hatched rectangles) and of voids (open rectangles) determined from XTEM images. The solid line shows the Zn atom distribution in as-implanted state calculated by TRIDYN code. Components shown in the negative values of the depth correspond to NPs located on the surface of the silica substrate.
between the mean size of Zn NPs and that of voids. Void formation was also observed in Zn implanted Al2O3 after annealing at 1000 C for 1 h in vacuum by Marques et al. [2]. They concluded that Kirkendall voids are formed by a significant diffusion of Zn atoms during heat treatment. In their case, the shape of voids is not clear and the volume density of voids is low. It suggests that the oxidation effect, i.e. the attraction of implanted Zn atoms toward the surface, enhances the void formation. In fact, we have also carried out annealing in vacuum of less than 1 103 Pa up to 700 C. Only less clear voids of much lower density were observed. No directional migration of Zn atoms
Fig. 3. XTEM images of silica glasses implanted with Znþ ions of 60 keV up to 1.0 1017 ions/cm2 after annealing in oxygen gas flow at 700 C for 1 h, with electron beam exposure of (a) within several seconds and (b) more than 5 min. Scale bar of 50 nm in length is shown in each figure.
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toward the surface was observed [14]. These facts indicate the importance of oxygen atmosphere for the stable void formation, at least in SiO2. Rapp discussed the role of the internal oxidation in the oxidation of alloys [15]. When a protective layer, which is oxygenated solute element, is formed at the surface in the processes of the oxidation of the alloys, the solute element is oxidized at surface of the matrix rather than inside the matrix. In our case oxidation in composite material, the external oxidation, i.e. oxidation on the surface, of Zn atoms becomes dominant rather than the internal oxidation, i.e. oxidation inside the a-SiO2, of Zn atoms at 700 C for 1 h annealing in oxygen gas. Consequently the void formation inside the a-SiO2 was enhanced in the process of external oxidation of Zn atoms. The oxide layer (ZnO) appears at 600 C and increases the thickness at 700 C (see in Figs. 1(b) and 3(a)). The diffusion length of O2 molecules in a-SiO2 at 700 C for 1 h annealing almost coincides with the project range of Zn ions of 60 keV. On the contrary, the supply of O2 molecules from the surface decreases by growth of ZnO layer thickness on the surface. The diffusion of O2, which is gradually limited by ZnO layer on the surface, may attract Zn atoms toward the surface without the significant internal oxidation of Zn atoms. Hence, the voids are formed by steep migration of Zn atoms toward the surface, and are maintained without significant recovery.
4. Summary Specimens of a-SiO2 were implanted with Zn ions of 60 keV up to 1.0 1017 ions/cm2 and annealed at 600 and 700 C for 1 h in oxygen gas. Effects of thermal annealing on the void formation were evaluated by XTEM observation. Spherical and high-density voids were formed after annealing at 700 C, and located at a certain depth. Both the high mobility of Zn atoms in a-SiO2 and the
high susceptibility to oxidation of Zn may enhance the remarkable void formation. Acknowledgments A part of this study was financially supported by the Budget for Nuclear Research of the Ministry of Education, Culture, Sports, Science and Technology, based on the screening and counseling by the Atomic Energy Commission. This study was also partly supported by JSPS-Kakenhi (No. 18510102) and by Futaba Electronics Memorial Foundation. References [1] Gaiduk PI, Lundsgaard Hansen J, Nylandsted Larsen A. Nucl Instrum Methods 2005;B 230:214–9. [2] Marques C, France N, Alves LC, da Silva RC, Alves E, Safran G, et al. Nucl Instrum Methods 2007;B 257:515–8. [3] Xiao X, Jiang C, Ren F, Wang J, Shi Y. Solid State Commun 2006;137:362–5. [4] Amekura H, Umeda N, Takeda Y, Lu J, Kishimoto N. Appl Phys Lett 2004;85: 1015–7. [5] Amekura H, Umeda N, Takeda Y, Lu J, Kono K, Kishimoto N. Nucl Instrum Methods 2005;B 230:193–7. [6] Amekura H, Kono K, Takeda Y, Kishimoto N. Appl Phys Lett 2005;87: 153105-1–3. [7] Amekura H, Umeda N, Yoshitake M, Kono K, Kishimoto N, Buchal Ch. J Cryst Growth 2006;287:2–6. [8] Amekura H, Umeda N, Boldyryeva H, Kishimoto N. Appl Phys Lett 2007;90: 083102-1–3. [9] Amkura H, Kono K, Kishimoto N, Buchal Ch. Nucl Instrum Methods 2006;B 242:96–9. [10] Moeller W, Eckstein W. Nucl Instrum Methods 1984;B 2:814–8. [11] Ziegler J, Biersack J, Littmark U. The stopping and range of ions in solids. New York: Pergamon; 1985. [12] Umeda N, Kishimoto N, Takeda Y, Lee CG, Gritsyna VT. Nucl Instrum Methods 2000;B 166–167:864–70. [13] Liu YX, Liu YC, Shao CL, Mu R. J Phys D Appl Phys 2004;37:3025–9. [14] Amekura H, Umeda N, Kono K, Takeda Y, Kishimoto N, Buchal Ch. Nanotechnology 2007;18:395707-1–6. [15] Rapp RA. Corrosion 1965;21:382–401.