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Formation process of CuCl nano-particles in silica glass by ion implantation
Journal of Non-Crystalline Solids 259 (1999) 93±99
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Formation process of CuCl nano-particles in silica glass by ion implantation K. Fukumi *, A. Chayahara, H. Kageyama, K. Kadono, T. Akai, N. Kitamura, H. Mizoguchi, Y. Horino, M. Makihara, K. Fujii, J. Hayakawa Osaka National Research Institute, AIST, 1-8-31, Midorigaoka, Ikeda, Osaka, 563-8577, Japan
Abstract The formation process of CuCl crystals has been studied in (3 MeV 6 ´ 1016 Cl2 ions/cm2 + 3 MeV 6 ´ 1016 Cu2 ions/cm2 )-implanted silica glass by X-ray absorption spectroscopy and secondary ion mass spectroscopy. It was found from X-ray absorption spectroscopy that Cu atoms were mainly coordinated by oxygen atoms in as-implanted glass. Heat-treatment at 600°C caused the formation of Cu±Cl bonds and heat-treatment at 1000°C caused the formation of CuCl crystals in silica. It was deduced that the migration of Cl atoms is a rate-determining step for the formation of CuCl crystal, on the basis of the conventional precipitation model. Ó 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Glasses in which ultra®ne particles of metals or semiconductors are dispersed have various kinds of properties such as optical non-linearity [1] and light emission [2]. The ion implantation method has advantages for the production of glasses in which such particles are dispersed, owing to the control of ion beam position and the control of doping ion concentration, and distribution of particles. Previously, it was shown that particles of elemental substances could be dispersed in silica glass by ion implantation, which is reviewed in Refs. [3,4]. In addition to the particles of elemental substances, it is possible to form particles of compounds by ion implantation [5,6]. In a previous study [5], it was shown that CuCl particles
* Corresponding author. Tel.: +81-727 51 9647; fax: +81-727 51 9627. E-mail address: [email protected] (K. Fukumi)
could be formed in silica glass by ion implantation and subsequent heat treatment. In addition, the formation process of CuCl particles was discussed from ultraviolet and visible absorption spectra. For the preparation of compounds in glasses, it is signi®cant to understand the formation process of compounds in glasses. In this study, the formation process of CuCl particles has been studied in the Cl and Cu ions implanted silica glass by X-ray absorption ®ne structure spectroscopy and secondary ion mass spectroscopy. 2. Experimental Cl2 and Cu2 ions were implanted in an optically ¯at silica glass plate (Type III, Nippon Silica Glass Yamaguchi Co., OH content 1000 wt ppm) with a tandem-type accelerator in a vacuum of 10ÿ7 Torr at room temperature. The glass plate was attached by silver paste on an aluminum sample holder within the sample chamber of the
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K. Fukumi et al. / Journal of Non-Crystalline Solids 259 (1999) 93±99
accelerator. Cl2 ion implantation was performed at an acceleration energy of 3 MeV, a dose of 6 ´ 1016 ions cmÿ2 and a current of 1.7 lA cmÿ2 , followed by Cu2 ion implantation at 3 MeV, 6 ´ 1016 ions cmÿ2 and 1.1 lA cmÿ2 . The ion implanted glass plate was cut into several plates. Then, they were heated in air. Samples A and B were heated at 600°C for 40 min. Sample C was heated at 600°C for 40 min, at 700°C for 40 min, at 800°C for 40 min, at 900°C for 40 min and at 1000°C for 100 min, sequentially. Sample D was heated at 1000°C for 40 min. The ultra violet (UV)±visible absorption spectra of the glasses were measured in the wavelength region from 185 to 800 nm at room temperature. The distribution of the implanted Cu and Cl atoms along depth was measured in an as-implanted sample, sample A (600°C), and sample C (to 1000°C) by secondary ion mass spectroscopy (SIMS). The SIMS measurement of Cu and Cl atoms was performed using O 2 ions as primary ions, simultaneously. An electron beam bombardment technique was applied to avoid the charge-up at the sample surface during the measurement [7]. The depth of crater made by the primary ion bombardment was measured with a surface roughness tester after the SIMS measurement. X-ray absorption ®ne structure (XAFS) spectra around the CuK absorption edge were measured at BL-7C station of Photon Factory, the National Laboratory for High Energy Physics. The energy was de®ned by assigning the pre-edge peak of Cu foil spectrum to 8980.3 eV. XAFS spectra of the as-implanted sample, the sample B, and the sample D were measured by ¯uorescence mode with a Lytle-type ionization chamber equipped with a Ni ®lter. The measurements for the samples were repeated three times at room temperature. XAFS spectra of Cu2 O, CuO and CuCl crystalline powders and Cu foil were measured by the absorption mode at room temperature. The interference function, v(k), obtained from XAFS spectrum was multiplied by k3 (k: photoelectron wave vector) and was Fourier-transformed in the region from 32 to 110 nmÿ1 . The k dependence of phase shifts and backscattering amplitude was ignored in the Fourier-transformation. Single shell ®tting analysis
[8] to the Fourier-®ltered k3 v(k) curves which correspond to the ®rst coordination shell was carried out in the region from 40 to 100 nmÿ1 to obtain interatomic distance and coordination numbers. Normalized near-edge structure (XANES) spectra and normalized dierence edge spectra were calculated according to Kau et al. [10]. For the calculation of the latter, we used the spectrum of Cu2 (L8 ±Et)(N3 )(BF4 )2 shown in Ref. [10]. The measurement and analysis methods have been described elsewhere in detail [9].
3. Results The as-implanted sample and heat-treated samples were colorless. The UV±visible absorption spectra of the glasses are shown in Fig. 1. Although it is known that a band is observed at ca. 370 nm in CuCl crystal [11], no absorption bands due to CuCl crystal were observed in the spectra of the as-implanted sample and the samples heated at 600°C. The spectrum of sample A was the same as that of sample B. A band due to CuCl crystal was observed at 370 nm in the spectra of the glasses after heating at 1000°C. The average radius of
Fig. 1. Ultraviolet±visible absorption spectra of (Cl2 + Cu2 )ions implanted silica glasses.
K. Fukumi et al. / Journal of Non-Crystalline Solids 259 (1999) 93±99
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CuCl particles in sample C was estimated to be 2±3 nm from the peak position of the band at 370 nm measured at 77 K [5]. Although the spectrum of sample C was very similar to that of sample D, the band due to the CuCl crystal was situated at a longer wavelength by 4 nm in sample C than in sample D, owing to the increase in CuCl particle size by the prolonged heat-treatment. Fig. 2 shows the concentration pro®les of the implanted Cu and Cl atoms along depth obtained from the SIMS measurement. Cu and Cl atoms had concentration±depth curves with a single maximum in the as-implanted sample. The curve for Cu atoms had two maxima in the sample heattreated at 600°C, while that for Cl atoms had single maximum. The curves for Cu and Cl atoms almost coincided and had two maxima in the sample heated to 1000°C. Concentration±depth curves of Cu and Cl atoms overlapped one another by 65%, 78% and 92% in area in the as-implanted sample, the sample heat-treated at 600°C, and the sample heated to 1000°C, respectively. Fig. 3(A) shows the normalized XANES spectra. It can be seen that the spectra of the as-implanted sample and the sample heat-treated at 600°C are less structured than those of Cu metal and Cu2 O, CuO and CuCl crystals. The XANES
Fig. 3. (A) Normalized XANES spectra and (B) normalized dierence edge spectra of (Cl2 + Cu2 )-ions implanted silica glasses and reference compounds.
Fig. 2. Concentration of Cu and Cl atoms along depth in (Cl2 + Cu2 )-ions implanted silica glasses. An inset shows the concentration curves in a logarithmic scale.
spectrum of the sample heat-treated at 1000°C was coincident with that of CuCl crystal. Fig. 3(B) depicts the normalized dierence edge spectra which are the dierence in normalized XANES spectra between a sample and Cu(II) complex compounds with an approximately tetragonal coordination. A positive peak at DE (DE: dierence in energy from the ®rst in¯ection point of Cu metal) 4 eV and a negative feature at DE 20 eV were observed in the as-implanted sample, the sample heat-treated at 600°C and Cu2 O crystal. Fig. 4 shows the magnitude of Fourier-transformation (jFTj) curves of the samples and the
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curves of the samples. The results of ®tting analysis are summarized in Table 1. 4. Discussion 4.1. XAFS study: coordination state of implanted Cu atoms
Fig. 4. Magnitude of Fourier-transform of k3 v(k) curves of (Cl2 + Cu2 )-ions implanted silica glasses and reference compounds.
reference materials. Each of these curves depicted a peak due to the ®rst coordination shell around Cu atoms at a distance from 0.10 to 0.22 nm. A peak due to the ®rst coordination shell observed in the as-implanted sample was similar in interatomic distance to that in Cu2 O crystal. The sample heattreated at 1000°C had a peak due to the ®rst coordination shell at the same distance as CuCl crystal had. A peak in the sample heat-treated at 600°C was situated between the peak of the asimplanted sample and the peak of the sample heattreated at 1000°C. The jFTj curve of Cu metal, Cu2 O crystal and CuO crystal had peaks due to the higher coordination shells at distances >0.24 nm. On the other hand, no peaks other than the ®rst coordination shell were observed in the jFTj
The as-implanted sample and Cu2 O crystal had a peak due to the ®rst coordination shell at a similar interatomic distance in the jFTj curve shown in Fig. 4, indicating that the peak in this sample is due to nearest-neighboring Cu±O pairs. No other peaks were observed in this sample, showing that Cu atoms mainly form Cu±O bonds in the as-implanted glass. In other words, Cu±Cu and Cu±Cl bonds were scarcely formed in the asimplanted glass. Copper oxide crystals, however, were not formed in the sample, since the near-edge spectrum of the sample was less structured than Cu2 O and CuO crystals as shown in Fig. 3(A) and the jFTj curve of the sample did not have peaks due to the higher coordination shells which were observed in the jFTj curves of Cu2 O and CuO crystals as shown in Fig. 4. In the normalized dierence edge spectra of the glass shown in Fig. 3(B), a positive peak at DE 4 eV and a negative feature at DE 20 eV was observed. Kau et al. [10] have shown that the normalized dierence edge spectra of Cu(I) complex have a positive peak at DE 2.7±5.7 eV and a broad negative feature at DE 10±20 eV. The spectrum of as-implanted sample agreed with these spectral features, indicating that most implanted Cu atoms are present
Table 1 Results of ®tting analysisa Shell Cu2 O crystal CuO crystal CuCl crystal As-implanted glass Glass heat-treated at 600°C Glass heat-treated at 1000°C a
Cu±O Cu±O Cu±Cl Cu±O Cu±O Cu±Cl Cu±Cl
CN
R/nm
0.184 0.196 0.235 0.187 0.198 0.219 0.234
2.0 4.0 4.0 2.4 (0.06) 3.7 (0.17) 4.7 (0.17) 3.4 (0.16)
(0.0001) (0.0001) (0.0001) (0.0001) (0.0001) (0.0002) (0.0001)
r2 (10ÿ4 nm2 )
k (nm)
0.51 0.57 1.27 0.87 1.06 1.82 1.30
0.79 (0.03) 1.04 (0.06) 0.49 (0.01) 0.79 0.79 0.49 0.49
(0.02) (0.02) (0.02) (0.03) (0.06) (0.05) (0.05)
CN: coordination number, R: interatomic distance, r2 : mean square relative displacement, and k mean free path. Asterisk (*) implies that the parameter was ®xed in the ®tting analysis. The numerical value in parentheses shows the standard deviation of the ®tting analysis.
K. Fukumi et al. / Journal of Non-Crystalline Solids 259 (1999) 93±99
as Cu(I) state. It should be noted, however, that the intensity of the positive peak of the sample, which corresponds to the pre-edge peak due to 1s± 4p transition, was much smaller than that of Cu2 O crystal. According to Kau et al. [10] and Pickering et al. [12], a decrease in intensity of pre-edge peak is caused by an increase in the coordination number and a deviation from ideal geometry. The normalized dierence edge spectrum of the sample heat-treated at 600°C also showed that the implanted Cu atoms are mainly present as Cu(I) after heat-treatment. In the jFTj curve shown in Fig. 4, a peak due to the ®rst coordination shell was observed at longer interatomic distance in the sample heat-treated at 600°C than that in the asimplanted sample. In the ®tting analysis, the experimental k3 v(k) curve of the sample heat-treated at 600°C agreed fairly well with both the theoretical one for chlorine ligands and for oxygen ligands. The region of k3 v(k) curve used for the ®tting analysis was too small to decide which ligands are predominant in the coordination shell around Cu atoms. Nevertheless, comparisons between the present sample and the (O + Cu)-ionsimplanted glass [13] help the interpretation of the peak in the jFTj curve. The distance of a peak due to the ®rst coordination shell was not changed within errors of measurement after heat-treatment at 600°C for 30 min in the (5 ´ 1016 O ions/ cm2 + 1 ´ 1017 Cu ions/cm2 )-implanted silica glass [13], although the peak in this glass was located at the same distance as in the present sample before heat treatment. It was deduced from this dierence after heat treatment that Cu±Cl bonds are formed in the glass after heat treatment and that the formation of Cu±Cl bonds aects the interatomic distance of the ®rst coordination shell. XANES data of the sample, however, diered from that of the CuCl crystal, as shown in Fig. 2. Since XANES data depend not only on the con®guration of the nearest-neighbors but also on the con®guration of the second-and third-nearest neighbors, this disagreement shows that Cu and Cl atoms did not form CuCl crystal in the glass. In fact, an absorption band due to CuCl crystal was not observed in the UV±visible absorption spectra of the sample heat-treated at 600°C, as shown in Fig. 1.
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A peak due to the ®rst coordination shell was observed at the same distance in the jFTj curve of the sample heated to 1000°C as in the curve of CuCl crystal. In addition, no peaks other than this peak were observed in the jFTj curve of the glass. These ®ndings indicate that most of Cu atoms form Cu±Cl bonds in the glass. The XANES data of the sample was consistent with that of CuCl crystal, showing that most Cu atoms form CuCl crystals in the glass. Furthermore, the presence of the band at 370 nm in the UV±visible absorption spectra is indicative of the formation of CuCl crystals in the glass [5]. 4.2. Formation process The concentration±depth curves of Cu and Cl atoms overlapped one another by 65% in area in the as-implanted sample. This overlap implies that 65% of copper atoms might have formed Cu±Cl bonds. The Cu±Cl bonds, however, were not observed, as mentioned above. For the chemical reaction in solids, it is necessary to take into account the diusion of reactants [14]. Since it is known that surface temperature of substrate increases during ion implantation, it is necessary to discuss the diusion of reactants at an elevated temperature. The surface temperature is given by Eq. (1) [15]: DT / Pb ÿ f Tw ÿ g Tw ÿ kA Tw ÿ Twh =Dx; 1 where DT, Pb , k, A, Dx, Tw and Twh represent the temperature change, the input power density, the thermal conductivity of substrate (1.3 W mÿ1 Kÿ1 at 0°C [16]), the cross-sectional area of substrate, the thickness of substrate, the temperature of substrate and the temperature of a substrate holder, respectively. The second and third terms in square brackets in Eq. (1) express the contribution from radiative cooling and the fourth term the contribution from conductive cooling. Since the glass substrate was silver-pasted on a water-cooled sample holder for ion implantation, it is reasonable to assume that the conductive cooling is predominant in these cooling processes in the present experiment. At steady state (DT 0), the
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temperature of glass surface increases up to 332 K during ion implantation as a result of the heat balance of ion beam heating and conductive cooling, according to Eq. (1). The diusion coef®cients of Cu and Cl atoms in silica glass at 332 K were estimated to range from 5.9 ´ 10ÿ29 to 1.9 ´ 10ÿ22 cm2 sÿ1 and from 5.5 ´ 10ÿ35 to 2.0 ´ 10ÿ34 cm2 sÿ1 , respectively, by extrapolating the diusion coecient which had been obtained at higher temperatures experimentally [17±20]. Since the duration of Cu2 -ion implantation was about 5 h, the approximate diusion lengths ((Dt)1=2 , D and t are the diusion coecient and time, respectively) [14] of Cu and Cl atoms during ion implantation were estimated to be from 1.0 ´ 10ÿ5 to 1.9 ´ 10ÿ2 nm and from 5.2 ´ 10ÿ10 to 1.3 ´ 10ÿ9 nm, respectively. Assuming that the diusion length of atoms needed for the reaction between Cu and Cl atoms is equivalent to the average distance between implanted atoms, the approximate diusion length of Cu and Cl atoms during ion implantation is smaller than the average distance between implanted atoms [(FWHM of distribution/dose)1=3 (370 nm/(2 ´ 6 ´ 102 Cu and Cl ions/nm2 ))1=3 0.68 nm]. From the view point of diusion length, it is reasonable that Cu± Cl bonds are scarcely formed during ion implantation. The concentration±depth curves of Cu and Cl atoms overlapped one another by 78% in area in the sample heat-treated at 600°C. The degree of overlap of both the curves increased from 65% to 78% after heat treatment. The increase in overlap is due to the reaction of Cu atoms and Cl atoms in the glass, as deduced from the XAFS data that Cu atoms partially react with Cl atoms during the heat treatment at 600°C. It is expected from the diusion coecients at 600°C that the approximate diusion lengths of Cu and Cl atoms are from 1.6 ´ 102 to 1.4 ´ 103 nm and from 1.2 to 4.0 nm at 600°C for 30 min, respectively, which distances are larger than the average distance between implanted atoms. This implies that Cu and Cl ions can form Cu±Cl bonds in silica glass by the heattreatment at 600°C. CuCl crystal was not detected in the XAFS data in the sample heat-treated at 600°C, although Cu± Cl bonds were formed. According to a simple
model for precipitation [21], the fraction of precipitation was expressed by Eq. (2), on the condition that the number of nuclei is unchanged during precipitation: dW =dt 2 4=3 3 1=3 p 2=3 1=3
n1 0 ÿ n1 = n0 ÿ n1 D 1 ÿ W W 1=3 ;
N 2=3 2
where W and N are the fraction of precipitation completed and the number of particles per unit volume, respectively. n0 , n1 , and n1 (0) are the concentration of solute atom, referred to unit volume, in the precipitate, the concentration of solute atoms in the matrix, which is in equilibrium with the precipitate and the concentration in matrix far away from the precipitation. In the calculation of Eq. (2), n1 (0) was taken as 1 ´ 1021 ions/ cm3 from the SIMS measurement. n1 is negligibly small compared with n1 (0) and n0: N was taken as 1018 /cm3 from the CuCl particle size in the sample heat-treated to 1000°C [5]. When the diusion coecient of Cu atoms is adopted in Eq. (2), the W calculated from Eq. (2) shows that the precipitation of CuCl crystals should almost be complete after heating at 600°C for 30 min. On the other hand, the W shows that the precipitation of CuCl crystals should proceed only by 10% at most, in the case that the diusion coecient of Cl atoms is adopted in Eq. (2). Since CuCl crystals were scarcely formed in the sample heat-treated at 600°C according to XAFS measurement, we infer from the comparison of the results calculated by Eq. (2) that the diusion of Cl atoms was a ratedetermining step for the formation of CuCl particles in silica glass at 600°C. After heat-treatment at 1000°C, the concentration±depth curve of Cl atoms agreed with that of Cu atoms. This result is in accordance with the result obtained from XAFS measurement. The most striking feature is that the distribution of Cl atoms became bimodal according to the Cu atoms. Coincidence of the two concentration±depth curves was caused by the diusion of Cu and Cl atoms toward the same region. Since diusion of atoms takes place in such a way that the chemical potential gradients and the free energy of the sys-
K. Fukumi et al. / Journal of Non-Crystalline Solids 259 (1999) 93±99
tem are reduced, it was deduced that the formation of CuCl crystal in silica glass reduced the free energy of the Cu±Cl±SiO2 system, which leads to the coincidence of the concentration±depth curves of Cu and Cl atoms.
5. Conclusion In (3 MeV 6 ´ 1016 Cl2 ions/cm2 + 3 MeV 6 ´ 1016 Cu2 ions/cm2 )-implanted silica glass, Cu atoms were mainly coordinated by oxygen atoms before heat-treatment. Heat-treatment caused the formation of Cu±Cl bonds. In addition, Cu atoms and Cl atoms gathered together to form CuCl crystals with increasing temperature of heattreatment. It was deduced that diusion of Cl atoms is a rate-determining step for the formation of CuCl crystal. Acknowledgements This work has been performed under the approval of the Photon Factory Program Advisory Committee (Proposal No. 93G019). The authors are grateful to Dr F. Oyama and Dr F. Tojo in Matsushita Techno-research Inc., for the measurement of SIMS. References [1] J.S. Hayden, in: H. Bach, N. Neuroth (Eds.), The Properties of Optical Glass, chap. 2.6, Springer, Berlin, 1995, p. 130. [2] T. Shimizu-Iwayama, K. Fujita, S. Nakao, K. Saitoh, T. Fujita, N. Itoh, J. Appl. Phys. 75 (1994) 75.
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[3] R.A. Weeks, in: J. Zarzycki (Ed.), Materials Science and Technology: A Comprehensive Treatment. Vol. 9, Glasses and Amorphous Materials, chap. 6, VCH, Weinheim, 1991, p. 331. [4] P. Mazzoldi, G.W. Arnold, G. Battaglin, R. Betroncello, F. Gonella, Nucl. Instrum. Meth. B 91 (1994) 478. [5] K. Fukumi, A. Chayahara, N. Kitamura, T. Akai, J. Hayakawa, K. Fujii, M. Satou, J. Non-Cryst. Solids 178 (1994) 155. [6] C.W. White, J.D. Budai, J.G. Zhu, S.P. Withrow, R.A. Zuhr, D.M. Hembree Jr, D.O. Henderson, A. Ueda, Y.S. Tung, R. Mu, R.H. Magruder, J. Appl. Phys. 79 (1996) 1876. [7] A. Benninghoven, F.G. R udenauer, H.W. Wener, Secondary Ion Mass Spectrometry, chap. 5.2, Wiley, New York, 1987, p. 761. [8] B.K. Teo, EXAFS Basic Principles and Data Analysis, chap. 6, Springer, Berlin, 1986, p. 114. [9] K. Fukumi, A. Chayahara, K. Kadono, H. Kageyama, T. Akai, N. Kitamura, M. Makihara, K. Fujii, J. Hayakawa, J. Non-Cryst. Solids 238 (1998) 143. [10] L.-S. Kau, D.J. Spira-Solomon, J.E. Penner-Hahm, K.O. Hodgsen, E.I. Solomon, J. Am. Chem. Soc. 109 (1987) 6433. [11] A.I. Ekimov, Al.L. Efros, A.A. Onushchenko, Solid State Commun. 56 (1985) 921. [12] I.J. Pickering, G.N. George, C.T. Dameron, B. Kurz, D.R. Winge, I.G. Dance, J. Am. Chem. Soc. 115 (1993) 9498. [13] K. Fukumi, unpublished data. [14] W.D. Kingery, H.K. Bowen, D.R. Uhlmann, Introduction to Ceramics, 2nd ed., chap. 6, chap. 9, Wiley, New York, 1976, p. 217, p. 381. [15] P.D. Perry, J. Vac. Sci. Technol. 13 (1976) 622. [16] I. Fanderlik, Silica Glass and its Application, chap. 4.4.5, Elsevier, Amsterdam, 1991, p. 228. [17] J.D. McBrayer, R.M. Swanson, T.W. Sigmon, J. Electrochem. Soc. 133 (1986) 1242. [18] Y. Shacham-Diamond, A. Dedhia, D. Hostetter, W.G. Oldham, J. Electrochem. Soc. 140 (1993) 2427. [19] G. Greeuw, H. Hasper, in: M. Schulz, G. Pensl (Eds.), Insulating Films on Semiconductors, Springer, Berlin, 1981, p. 203. [20] J. Kirchhof, S. Unger, K.-F. Klein, B. Knappe, J. NonCryst. Solids 181 (1995) 266. [21] C. Wert, C. Zener, J. Appl. Phys. 21 (1950) 5.
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Formation process of CuCl nano-particles in silica glass by ion implantation K. Fukumi *, A. Chayahara, H. Kageyama, K. Kadono, T. Akai, N. Kitamura, H. Mizoguchi, Y. Horino, M. Makihara, K. Fujii, J. Hayakawa Osaka National Research Institute, AIST, 1-8-31, Midorigaoka, Ikeda, Osaka, 563-8577, Japan
Abstract The formation process of CuCl crystals has been studied in (3 MeV 6 ´ 1016 Cl2 ions/cm2 + 3 MeV 6 ´ 1016 Cu2 ions/cm2 )-implanted silica glass by X-ray absorption spectroscopy and secondary ion mass spectroscopy. It was found from X-ray absorption spectroscopy that Cu atoms were mainly coordinated by oxygen atoms in as-implanted glass. Heat-treatment at 600°C caused the formation of Cu±Cl bonds and heat-treatment at 1000°C caused the formation of CuCl crystals in silica. It was deduced that the migration of Cl atoms is a rate-determining step for the formation of CuCl crystal, on the basis of the conventional precipitation model. Ó 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Glasses in which ultra®ne particles of metals or semiconductors are dispersed have various kinds of properties such as optical non-linearity [1] and light emission [2]. The ion implantation method has advantages for the production of glasses in which such particles are dispersed, owing to the control of ion beam position and the control of doping ion concentration, and distribution of particles. Previously, it was shown that particles of elemental substances could be dispersed in silica glass by ion implantation, which is reviewed in Refs. [3,4]. In addition to the particles of elemental substances, it is possible to form particles of compounds by ion implantation [5,6]. In a previous study [5], it was shown that CuCl particles
* Corresponding author. Tel.: +81-727 51 9647; fax: +81-727 51 9627. E-mail address: [email protected] (K. Fukumi)
could be formed in silica glass by ion implantation and subsequent heat treatment. In addition, the formation process of CuCl particles was discussed from ultraviolet and visible absorption spectra. For the preparation of compounds in glasses, it is signi®cant to understand the formation process of compounds in glasses. In this study, the formation process of CuCl particles has been studied in the Cl and Cu ions implanted silica glass by X-ray absorption ®ne structure spectroscopy and secondary ion mass spectroscopy. 2. Experimental Cl2 and Cu2 ions were implanted in an optically ¯at silica glass plate (Type III, Nippon Silica Glass Yamaguchi Co., OH content 1000 wt ppm) with a tandem-type accelerator in a vacuum of 10ÿ7 Torr at room temperature. The glass plate was attached by silver paste on an aluminum sample holder within the sample chamber of the
0022-3093/99/$ - see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 5 2 6 - 8
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K. Fukumi et al. / Journal of Non-Crystalline Solids 259 (1999) 93±99
accelerator. Cl2 ion implantation was performed at an acceleration energy of 3 MeV, a dose of 6 ´ 1016 ions cmÿ2 and a current of 1.7 lA cmÿ2 , followed by Cu2 ion implantation at 3 MeV, 6 ´ 1016 ions cmÿ2 and 1.1 lA cmÿ2 . The ion implanted glass plate was cut into several plates. Then, they were heated in air. Samples A and B were heated at 600°C for 40 min. Sample C was heated at 600°C for 40 min, at 700°C for 40 min, at 800°C for 40 min, at 900°C for 40 min and at 1000°C for 100 min, sequentially. Sample D was heated at 1000°C for 40 min. The ultra violet (UV)±visible absorption spectra of the glasses were measured in the wavelength region from 185 to 800 nm at room temperature. The distribution of the implanted Cu and Cl atoms along depth was measured in an as-implanted sample, sample A (600°C), and sample C (to 1000°C) by secondary ion mass spectroscopy (SIMS). The SIMS measurement of Cu and Cl atoms was performed using O 2 ions as primary ions, simultaneously. An electron beam bombardment technique was applied to avoid the charge-up at the sample surface during the measurement [7]. The depth of crater made by the primary ion bombardment was measured with a surface roughness tester after the SIMS measurement. X-ray absorption ®ne structure (XAFS) spectra around the CuK absorption edge were measured at BL-7C station of Photon Factory, the National Laboratory for High Energy Physics. The energy was de®ned by assigning the pre-edge peak of Cu foil spectrum to 8980.3 eV. XAFS spectra of the as-implanted sample, the sample B, and the sample D were measured by ¯uorescence mode with a Lytle-type ionization chamber equipped with a Ni ®lter. The measurements for the samples were repeated three times at room temperature. XAFS spectra of Cu2 O, CuO and CuCl crystalline powders and Cu foil were measured by the absorption mode at room temperature. The interference function, v(k), obtained from XAFS spectrum was multiplied by k3 (k: photoelectron wave vector) and was Fourier-transformed in the region from 32 to 110 nmÿ1 . The k dependence of phase shifts and backscattering amplitude was ignored in the Fourier-transformation. Single shell ®tting analysis
[8] to the Fourier-®ltered k3 v(k) curves which correspond to the ®rst coordination shell was carried out in the region from 40 to 100 nmÿ1 to obtain interatomic distance and coordination numbers. Normalized near-edge structure (XANES) spectra and normalized dierence edge spectra were calculated according to Kau et al. [10]. For the calculation of the latter, we used the spectrum of Cu2 (L8 ±Et)(N3 )(BF4 )2 shown in Ref. [10]. The measurement and analysis methods have been described elsewhere in detail [9].
3. Results The as-implanted sample and heat-treated samples were colorless. The UV±visible absorption spectra of the glasses are shown in Fig. 1. Although it is known that a band is observed at ca. 370 nm in CuCl crystal [11], no absorption bands due to CuCl crystal were observed in the spectra of the as-implanted sample and the samples heated at 600°C. The spectrum of sample A was the same as that of sample B. A band due to CuCl crystal was observed at 370 nm in the spectra of the glasses after heating at 1000°C. The average radius of
Fig. 1. Ultraviolet±visible absorption spectra of (Cl2 + Cu2 )ions implanted silica glasses.
K. Fukumi et al. / Journal of Non-Crystalline Solids 259 (1999) 93±99
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CuCl particles in sample C was estimated to be 2±3 nm from the peak position of the band at 370 nm measured at 77 K [5]. Although the spectrum of sample C was very similar to that of sample D, the band due to the CuCl crystal was situated at a longer wavelength by 4 nm in sample C than in sample D, owing to the increase in CuCl particle size by the prolonged heat-treatment. Fig. 2 shows the concentration pro®les of the implanted Cu and Cl atoms along depth obtained from the SIMS measurement. Cu and Cl atoms had concentration±depth curves with a single maximum in the as-implanted sample. The curve for Cu atoms had two maxima in the sample heattreated at 600°C, while that for Cl atoms had single maximum. The curves for Cu and Cl atoms almost coincided and had two maxima in the sample heated to 1000°C. Concentration±depth curves of Cu and Cl atoms overlapped one another by 65%, 78% and 92% in area in the as-implanted sample, the sample heat-treated at 600°C, and the sample heated to 1000°C, respectively. Fig. 3(A) shows the normalized XANES spectra. It can be seen that the spectra of the as-implanted sample and the sample heat-treated at 600°C are less structured than those of Cu metal and Cu2 O, CuO and CuCl crystals. The XANES
Fig. 3. (A) Normalized XANES spectra and (B) normalized dierence edge spectra of (Cl2 + Cu2 )-ions implanted silica glasses and reference compounds.
Fig. 2. Concentration of Cu and Cl atoms along depth in (Cl2 + Cu2 )-ions implanted silica glasses. An inset shows the concentration curves in a logarithmic scale.
spectrum of the sample heat-treated at 1000°C was coincident with that of CuCl crystal. Fig. 3(B) depicts the normalized dierence edge spectra which are the dierence in normalized XANES spectra between a sample and Cu(II) complex compounds with an approximately tetragonal coordination. A positive peak at DE (DE: dierence in energy from the ®rst in¯ection point of Cu metal) 4 eV and a negative feature at DE 20 eV were observed in the as-implanted sample, the sample heat-treated at 600°C and Cu2 O crystal. Fig. 4 shows the magnitude of Fourier-transformation (jFTj) curves of the samples and the
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K. Fukumi et al. / Journal of Non-Crystalline Solids 259 (1999) 93±99
curves of the samples. The results of ®tting analysis are summarized in Table 1. 4. Discussion 4.1. XAFS study: coordination state of implanted Cu atoms
Fig. 4. Magnitude of Fourier-transform of k3 v(k) curves of (Cl2 + Cu2 )-ions implanted silica glasses and reference compounds.
reference materials. Each of these curves depicted a peak due to the ®rst coordination shell around Cu atoms at a distance from 0.10 to 0.22 nm. A peak due to the ®rst coordination shell observed in the as-implanted sample was similar in interatomic distance to that in Cu2 O crystal. The sample heattreated at 1000°C had a peak due to the ®rst coordination shell at the same distance as CuCl crystal had. A peak in the sample heat-treated at 600°C was situated between the peak of the asimplanted sample and the peak of the sample heattreated at 1000°C. The jFTj curve of Cu metal, Cu2 O crystal and CuO crystal had peaks due to the higher coordination shells at distances >0.24 nm. On the other hand, no peaks other than the ®rst coordination shell were observed in the jFTj
The as-implanted sample and Cu2 O crystal had a peak due to the ®rst coordination shell at a similar interatomic distance in the jFTj curve shown in Fig. 4, indicating that the peak in this sample is due to nearest-neighboring Cu±O pairs. No other peaks were observed in this sample, showing that Cu atoms mainly form Cu±O bonds in the as-implanted glass. In other words, Cu±Cu and Cu±Cl bonds were scarcely formed in the asimplanted glass. Copper oxide crystals, however, were not formed in the sample, since the near-edge spectrum of the sample was less structured than Cu2 O and CuO crystals as shown in Fig. 3(A) and the jFTj curve of the sample did not have peaks due to the higher coordination shells which were observed in the jFTj curves of Cu2 O and CuO crystals as shown in Fig. 4. In the normalized dierence edge spectra of the glass shown in Fig. 3(B), a positive peak at DE 4 eV and a negative feature at DE 20 eV was observed. Kau et al. [10] have shown that the normalized dierence edge spectra of Cu(I) complex have a positive peak at DE 2.7±5.7 eV and a broad negative feature at DE 10±20 eV. The spectrum of as-implanted sample agreed with these spectral features, indicating that most implanted Cu atoms are present
Table 1 Results of ®tting analysisa Shell Cu2 O crystal CuO crystal CuCl crystal As-implanted glass Glass heat-treated at 600°C Glass heat-treated at 1000°C a
Cu±O Cu±O Cu±Cl Cu±O Cu±O Cu±Cl Cu±Cl
CN
R/nm
0.184 0.196 0.235 0.187 0.198 0.219 0.234
2.0 4.0 4.0 2.4 (0.06) 3.7 (0.17) 4.7 (0.17) 3.4 (0.16)
(0.0001) (0.0001) (0.0001) (0.0001) (0.0001) (0.0002) (0.0001)
r2 (10ÿ4 nm2 )
k (nm)
0.51 0.57 1.27 0.87 1.06 1.82 1.30
0.79 (0.03) 1.04 (0.06) 0.49 (0.01) 0.79 0.79 0.49 0.49
(0.02) (0.02) (0.02) (0.03) (0.06) (0.05) (0.05)
CN: coordination number, R: interatomic distance, r2 : mean square relative displacement, and k mean free path. Asterisk (*) implies that the parameter was ®xed in the ®tting analysis. The numerical value in parentheses shows the standard deviation of the ®tting analysis.
K. Fukumi et al. / Journal of Non-Crystalline Solids 259 (1999) 93±99
as Cu(I) state. It should be noted, however, that the intensity of the positive peak of the sample, which corresponds to the pre-edge peak due to 1s± 4p transition, was much smaller than that of Cu2 O crystal. According to Kau et al. [10] and Pickering et al. [12], a decrease in intensity of pre-edge peak is caused by an increase in the coordination number and a deviation from ideal geometry. The normalized dierence edge spectrum of the sample heat-treated at 600°C also showed that the implanted Cu atoms are mainly present as Cu(I) after heat-treatment. In the jFTj curve shown in Fig. 4, a peak due to the ®rst coordination shell was observed at longer interatomic distance in the sample heat-treated at 600°C than that in the asimplanted sample. In the ®tting analysis, the experimental k3 v(k) curve of the sample heat-treated at 600°C agreed fairly well with both the theoretical one for chlorine ligands and for oxygen ligands. The region of k3 v(k) curve used for the ®tting analysis was too small to decide which ligands are predominant in the coordination shell around Cu atoms. Nevertheless, comparisons between the present sample and the (O + Cu)-ionsimplanted glass [13] help the interpretation of the peak in the jFTj curve. The distance of a peak due to the ®rst coordination shell was not changed within errors of measurement after heat-treatment at 600°C for 30 min in the (5 ´ 1016 O ions/ cm2 + 1 ´ 1017 Cu ions/cm2 )-implanted silica glass [13], although the peak in this glass was located at the same distance as in the present sample before heat treatment. It was deduced from this dierence after heat treatment that Cu±Cl bonds are formed in the glass after heat treatment and that the formation of Cu±Cl bonds aects the interatomic distance of the ®rst coordination shell. XANES data of the sample, however, diered from that of the CuCl crystal, as shown in Fig. 2. Since XANES data depend not only on the con®guration of the nearest-neighbors but also on the con®guration of the second-and third-nearest neighbors, this disagreement shows that Cu and Cl atoms did not form CuCl crystal in the glass. In fact, an absorption band due to CuCl crystal was not observed in the UV±visible absorption spectra of the sample heat-treated at 600°C, as shown in Fig. 1.
97
A peak due to the ®rst coordination shell was observed at the same distance in the jFTj curve of the sample heated to 1000°C as in the curve of CuCl crystal. In addition, no peaks other than this peak were observed in the jFTj curve of the glass. These ®ndings indicate that most of Cu atoms form Cu±Cl bonds in the glass. The XANES data of the sample was consistent with that of CuCl crystal, showing that most Cu atoms form CuCl crystals in the glass. Furthermore, the presence of the band at 370 nm in the UV±visible absorption spectra is indicative of the formation of CuCl crystals in the glass [5]. 4.2. Formation process The concentration±depth curves of Cu and Cl atoms overlapped one another by 65% in area in the as-implanted sample. This overlap implies that 65% of copper atoms might have formed Cu±Cl bonds. The Cu±Cl bonds, however, were not observed, as mentioned above. For the chemical reaction in solids, it is necessary to take into account the diusion of reactants [14]. Since it is known that surface temperature of substrate increases during ion implantation, it is necessary to discuss the diusion of reactants at an elevated temperature. The surface temperature is given by Eq. (1) [15]: DT / Pb ÿ f Tw ÿ g Tw ÿ kA Tw ÿ Twh =Dx; 1 where DT, Pb , k, A, Dx, Tw and Twh represent the temperature change, the input power density, the thermal conductivity of substrate (1.3 W mÿ1 Kÿ1 at 0°C [16]), the cross-sectional area of substrate, the thickness of substrate, the temperature of substrate and the temperature of a substrate holder, respectively. The second and third terms in square brackets in Eq. (1) express the contribution from radiative cooling and the fourth term the contribution from conductive cooling. Since the glass substrate was silver-pasted on a water-cooled sample holder for ion implantation, it is reasonable to assume that the conductive cooling is predominant in these cooling processes in the present experiment. At steady state (DT 0), the
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temperature of glass surface increases up to 332 K during ion implantation as a result of the heat balance of ion beam heating and conductive cooling, according to Eq. (1). The diusion coef®cients of Cu and Cl atoms in silica glass at 332 K were estimated to range from 5.9 ´ 10ÿ29 to 1.9 ´ 10ÿ22 cm2 sÿ1 and from 5.5 ´ 10ÿ35 to 2.0 ´ 10ÿ34 cm2 sÿ1 , respectively, by extrapolating the diusion coecient which had been obtained at higher temperatures experimentally [17±20]. Since the duration of Cu2 -ion implantation was about 5 h, the approximate diusion lengths ((Dt)1=2 , D and t are the diusion coecient and time, respectively) [14] of Cu and Cl atoms during ion implantation were estimated to be from 1.0 ´ 10ÿ5 to 1.9 ´ 10ÿ2 nm and from 5.2 ´ 10ÿ10 to 1.3 ´ 10ÿ9 nm, respectively. Assuming that the diusion length of atoms needed for the reaction between Cu and Cl atoms is equivalent to the average distance between implanted atoms, the approximate diusion length of Cu and Cl atoms during ion implantation is smaller than the average distance between implanted atoms [(FWHM of distribution/dose)1=3 (370 nm/(2 ´ 6 ´ 102 Cu and Cl ions/nm2 ))1=3 0.68 nm]. From the view point of diusion length, it is reasonable that Cu± Cl bonds are scarcely formed during ion implantation. The concentration±depth curves of Cu and Cl atoms overlapped one another by 78% in area in the sample heat-treated at 600°C. The degree of overlap of both the curves increased from 65% to 78% after heat treatment. The increase in overlap is due to the reaction of Cu atoms and Cl atoms in the glass, as deduced from the XAFS data that Cu atoms partially react with Cl atoms during the heat treatment at 600°C. It is expected from the diusion coecients at 600°C that the approximate diusion lengths of Cu and Cl atoms are from 1.6 ´ 102 to 1.4 ´ 103 nm and from 1.2 to 4.0 nm at 600°C for 30 min, respectively, which distances are larger than the average distance between implanted atoms. This implies that Cu and Cl ions can form Cu±Cl bonds in silica glass by the heattreatment at 600°C. CuCl crystal was not detected in the XAFS data in the sample heat-treated at 600°C, although Cu± Cl bonds were formed. According to a simple
model for precipitation [21], the fraction of precipitation was expressed by Eq. (2), on the condition that the number of nuclei is unchanged during precipitation: dW =dt 2 4=3 3 1=3 p 2=3 1=3
n1 0 ÿ n1 = n0 ÿ n1 D 1 ÿ W W 1=3 ;
N 2=3 2
where W and N are the fraction of precipitation completed and the number of particles per unit volume, respectively. n0 , n1 , and n1 (0) are the concentration of solute atom, referred to unit volume, in the precipitate, the concentration of solute atoms in the matrix, which is in equilibrium with the precipitate and the concentration in matrix far away from the precipitation. In the calculation of Eq. (2), n1 (0) was taken as 1 ´ 1021 ions/ cm3 from the SIMS measurement. n1 is negligibly small compared with n1 (0) and n0: N was taken as 1018 /cm3 from the CuCl particle size in the sample heat-treated to 1000°C [5]. When the diusion coecient of Cu atoms is adopted in Eq. (2), the W calculated from Eq. (2) shows that the precipitation of CuCl crystals should almost be complete after heating at 600°C for 30 min. On the other hand, the W shows that the precipitation of CuCl crystals should proceed only by 10% at most, in the case that the diusion coecient of Cl atoms is adopted in Eq. (2). Since CuCl crystals were scarcely formed in the sample heat-treated at 600°C according to XAFS measurement, we infer from the comparison of the results calculated by Eq. (2) that the diusion of Cl atoms was a ratedetermining step for the formation of CuCl particles in silica glass at 600°C. After heat-treatment at 1000°C, the concentration±depth curve of Cl atoms agreed with that of Cu atoms. This result is in accordance with the result obtained from XAFS measurement. The most striking feature is that the distribution of Cl atoms became bimodal according to the Cu atoms. Coincidence of the two concentration±depth curves was caused by the diusion of Cu and Cl atoms toward the same region. Since diusion of atoms takes place in such a way that the chemical potential gradients and the free energy of the sys-
K. Fukumi et al. / Journal of Non-Crystalline Solids 259 (1999) 93±99
tem are reduced, it was deduced that the formation of CuCl crystal in silica glass reduced the free energy of the Cu±Cl±SiO2 system, which leads to the coincidence of the concentration±depth curves of Cu and Cl atoms.
5. Conclusion In (3 MeV 6 ´ 1016 Cl2 ions/cm2 + 3 MeV 6 ´ 1016 Cu2 ions/cm2 )-implanted silica glass, Cu atoms were mainly coordinated by oxygen atoms before heat-treatment. Heat-treatment caused the formation of Cu±Cl bonds. In addition, Cu atoms and Cl atoms gathered together to form CuCl crystals with increasing temperature of heattreatment. It was deduced that diusion of Cl atoms is a rate-determining step for the formation of CuCl crystal. Acknowledgements This work has been performed under the approval of the Photon Factory Program Advisory Committee (Proposal No. 93G019). The authors are grateful to Dr F. Oyama and Dr F. Tojo in Matsushita Techno-research Inc., for the measurement of SIMS. References [1] J.S. Hayden, in: H. Bach, N. Neuroth (Eds.), The Properties of Optical Glass, chap. 2.6, Springer, Berlin, 1995, p. 130. [2] T. Shimizu-Iwayama, K. Fujita, S. Nakao, K. Saitoh, T. Fujita, N. Itoh, J. Appl. Phys. 75 (1994) 75.
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[3] R.A. Weeks, in: J. Zarzycki (Ed.), Materials Science and Technology: A Comprehensive Treatment. Vol. 9, Glasses and Amorphous Materials, chap. 6, VCH, Weinheim, 1991, p. 331. [4] P. Mazzoldi, G.W. Arnold, G. Battaglin, R. Betroncello, F. Gonella, Nucl. Instrum. Meth. B 91 (1994) 478. [5] K. Fukumi, A. Chayahara, N. Kitamura, T. Akai, J. Hayakawa, K. Fujii, M. Satou, J. Non-Cryst. Solids 178 (1994) 155. [6] C.W. White, J.D. Budai, J.G. Zhu, S.P. Withrow, R.A. Zuhr, D.M. Hembree Jr, D.O. Henderson, A. Ueda, Y.S. Tung, R. Mu, R.H. Magruder, J. Appl. Phys. 79 (1996) 1876. [7] A. Benninghoven, F.G. R udenauer, H.W. Wener, Secondary Ion Mass Spectrometry, chap. 5.2, Wiley, New York, 1987, p. 761. [8] B.K. Teo, EXAFS Basic Principles and Data Analysis, chap. 6, Springer, Berlin, 1986, p. 114. [9] K. Fukumi, A. Chayahara, K. Kadono, H. Kageyama, T. Akai, N. Kitamura, M. Makihara, K. Fujii, J. Hayakawa, J. Non-Cryst. Solids 238 (1998) 143. [10] L.-S. Kau, D.J. Spira-Solomon, J.E. Penner-Hahm, K.O. Hodgsen, E.I. Solomon, J. Am. Chem. Soc. 109 (1987) 6433. [11] A.I. Ekimov, Al.L. Efros, A.A. Onushchenko, Solid State Commun. 56 (1985) 921. [12] I.J. Pickering, G.N. George, C.T. Dameron, B. Kurz, D.R. Winge, I.G. Dance, J. Am. Chem. Soc. 115 (1993) 9498. [13] K. Fukumi, unpublished data. [14] W.D. Kingery, H.K. Bowen, D.R. Uhlmann, Introduction to Ceramics, 2nd ed., chap. 6, chap. 9, Wiley, New York, 1976, p. 217, p. 381. [15] P.D. Perry, J. Vac. Sci. Technol. 13 (1976) 622. [16] I. Fanderlik, Silica Glass and its Application, chap. 4.4.5, Elsevier, Amsterdam, 1991, p. 228. [17] J.D. McBrayer, R.M. Swanson, T.W. Sigmon, J. Electrochem. Soc. 133 (1986) 1242. [18] Y. Shacham-Diamond, A. Dedhia, D. Hostetter, W.G. Oldham, J. Electrochem. Soc. 140 (1993) 2427. [19] G. Greeuw, H. Hasper, in: M. Schulz, G. Pensl (Eds.), Insulating Films on Semiconductors, Springer, Berlin, 1981, p. 203. [20] J. Kirchhof, S. Unger, K.-F. Klein, B. Knappe, J. NonCryst. Solids 181 (1995) 266. [21] C. Wert, C. Zener, J. Appl. Phys. 21 (1950) 5.