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Ultraviolet photon-induced absorption bands and paramagnetic centers in Ge and Sn co-doped SiO2 glass
Journal of Non-Crystalline Solids 318 (2003) 87–94 www.elsevier.com/locate/jnoncrysol
Ultraviolet photon-induced absorption bands and paramagnetic centers in Ge and Sn co-doped SiO2 glass Tetsuya Nakanishi a, Makoto Fujimaki b,c,d, Shin-ichiro Tokuhiro a, Ken-ichi Nomura a,*, Yoshimichi Ohki a,d, Kazuo Imamura e a
Department of Electrical, Electronics, and Computer Engineering, Bldg. 62, Rm. B2-09 (Ohki Lab) 3-4-1 Ohkubo, Waseda University, Shinjuku-ku, Tokyo 169-8555, Japan b Japan Science and Technology Corporation, Kawaguchi-shi 332-0012, Japan c National Institute of Advanced Industrial Science and Technology, Tsukuba-shi 305-8568, Japan d Advanced Research Institute for Science and Engineering, Waseda University, Shinjuku-ku 169-8555, Japan e Mitsubishi Cable Industries, Ltd., Itami-shi 664-0027, Japan Received 12 October 2001; received in revised form 29 July 2002
Abstract Germanium and Sn co-doped SiO2 glass, a material for photosensitive optical fiber core that can be used for fabrication of optical fiber gratings, was exposed to photons from a KrF excimer laser (5.0 eV) or a XeCl lamp (4.0 eV). The photo-induced paramagnetic centers and optical properties of the glass have been examined with electron spin resonance and absorption measurements. Precursors of the paramagnetic centers are also discussed. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 42.70.Ce; 42.79.Dj; 76.30.Mi; 78.40.)q
1. Introduction Germanium-doped SiO2 glass is a typical material for the core of an optical fiber. It has been well known that ultraviolet (uv) photon irradiation induces refractive index changes in Ge-doped SiO2 glass. The photo-induced refractive index changes have been utilized for the fabrication of optical fiber gratings [1] that have been attracting much
*
Corresponding author. Tel./fax: +81-3 3204 1258. E-mail address: [email protected] (K.-i. Nomura).
attention as reflectors [2–6], band rejection filters [7], sensors [8–10], dispersion eliminators [11], and so on. Induction of paramagnetic centers, the Ge E0 center [12,13] and the Ge electron center (GEC) [14–18] with absorption bands in visible-to-uv region, strongly contributes to the photo-induced refractive index changes in Ge-doped SiO2 glass. The Ge E0 center has an absorption band at 6.4 eV [13], and the GEC has two absorption bands at 4.5 and 5.8 eV [14,15]. The induction mechanisms of these centers have been well examined. The neutral oxygen vacancy (NOV), which has an absorption band at 5.06 eV, is known to be the precursor of
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the Ge E0 center [14]. The Ge lone-pair center (GLPC) and the bridging oxygen are reported as electron donors to create the GEC [14,18]. Recently, it was reported that Ge and Sn codoped SiO2 glass shows higher photosensitivity than Ge-doped SiO2 glass [19]. There have been papers on the photosensitivity in Sn-doped SiO2 glasses made by sol–gel and chemical vapor deposition methods [20,21]. However, the formation of defects associated with Sn was not discussed sufficiently there since they stood on a viewpoint that the increase in the photosensitivity was due to the densification induced by uv-photon irradiation. Two kinds of Sn-related paramagnetic cen ters, ðSn4þ Þ , where an electron is trapped at Sn4þ [22], and the Sn E0 center [23] have been observed by electron spin resonance (ESR) in a sodium silicate glass containing 1 mol% of tin oxide and a SnO2 –SiO2 glass, respectively. Therefore, there exists a possibility that the induction of Sn-related defects by the irradiation of uv-photons contributes to the photo-induced refractive index change in a similar manner as Ge-doped SiO2 glass. From this viewpoint, the generation mechanisms, optical properties, and the roles of Sn-related centers in the photo-induced refractive index change are examined in the present paper by observing ESR spectra and absorption bands in the visible-to-uv region induced in Ge and Sn co-doped SiO2 glass by the irradiation of UV photons.
ble-beam spectrophotometer with a wavelength resolution of 2 nm. Paramagnetic centers were detected by ESR at the X-band frequency, and their concentrations were calibrated by comparing their signal intensities with that of a standard 1diphenyl-2-picrylhydrazyl sample of a known weight. The accuracy of the calibration is believed to be 20%. The photon irradiation and measurements were done at room temperature.
3. Results Fig. 1 shows the absorption spectrum of the asprepared Ge–Sn–SiO2 glass. The absorption has a peak around 4.9 eV. As shown in Fig. 2, the 4.9 eV absorption is divided into three spectral components at 4.83, 5.06, and 5.16 eV indicated by the broken curves, which are obtained by the least-squares fitting with Gaussian shapes. The intensities and the values of the FWHM of these absorption components are shown in Table 1. The 5.06 eV peak with the FWHM of 0.38 eV and the 5.16 eV peak with the FWHM of 0.48 eV are the absorption bands of the NOV and GLPC, respectively [12]. The 4.83 eV peak with the FWHM of 0.42 eV, which is the main component of the 4.9 eV absorption, has not been reported in Ge-doped SiO2 glass. Fitting with several Gaussian curves might be considered to have diverse sets of results. However, the result obtained by fitting with the
2. Experimental details The sample used was a Ge and Sn co-doped SiO2 glass (hereafter abbreviated as Ge–Sn–SiO2 glass) rod containing about 1.5 mol% Sn and 10 mol% Ge and was prepared by the vapor-phase axial deposition method and solution-doping technique. The glass rod was cut into plates with a thickness of 0.2 mm. The sample was polished for optical measurements. A KrF excimer laser (248 nm ¼ 5:0 eV, 80 mJ/ cm2 per pulse, pulse duration of 25 ns) and a XeCl excimer lamp (308 nm ¼ 4:0 eV, 6.4 mW/cm2 , full width at half maximum (FWHM) of 0.03 eV) were used as photon sources. Absorption spectra from the visible-to-uv region were measured by a dou-
Fig. 1. Absorption spectrum of the as-prepared Ge–Sn–SiO2 glass.
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Fig. 2. Absorption components around 5 eV. The solid curve denotes the absorption of the Ge–Sn–SiO2 glass, while the broken curves are the components obtained by the least-squares fitting with Gaussian shapes. The inset shows the difference between the observed spectrum and the fitting result. The components at 5.06 and 5.16 eV are due to the NOV and the GLPC, respectively. The origin of the component at 4.83 eV is to be discussed in the present paper.
Table 1 Peak positions, intensities, and the values FWHM of the absorption components in the as-prepared Ge–Sn–SiO2 glass Peak position (eV)
Intensity (cm1 )
FWHM (eV)
4.83 0.03 5.06 0.03 5.16 0.03
88.7 5 29.9 5 23.7 5
0.42 0.03 0.38 0.03 0.48 0.03
values of the peak position and the FWHM, which are thought reasonable as the absorption components [12] of the present sample, was almost completely unique. Namely, the error is smaller than 0.03 eV for both the peak position and the FWHM, and is within 5 cm1 for the intensity. The calculation about the remaining slope with its peak around 6 eV contains uncertainty to some extent, since the observed absorption intensity around 6 eV is beyond the experimental limit. However, this uncertainty scarcely affects the calculation results of the other components, and the influence is within the errors. Fig. 3(a) shows the absorption spectra of the asprepared glass (i), after the laser irradiation of 102 pulses (ii), and after 104 pulses (iii), while Fig. 3(b) indicates photo-induced absorption spectra ob-
Fig. 3. (a) Absorption spectra of the as-prepared Ge–Sn–SiO2 glass (i), after the KrF excimer laser irradiation of 102 pulses (ii), and after 104 pulses (iii). (b) Differential absorption spectra after the irradiation of laser photons, obtained by subtracting the spectrum (i) from the spectrum (ii) (solid curve) and from the spectrum (iii) (broken curve).
tained by subtracting the spectrum (i) from the spectrum (ii) (solid curve) and from the spectrum (iii) (broken curve). The absorption decreases around 4.9 eV with the photon irradiation, while it increases around 4.5 and 5.4 eV up to the irradiation of 102 pulses. Further irradiation scarcely changes the absorption intensity around 4.5 and 5.4 eV, but it increases the absorption around 6 eV continuously. Fig. 4 denotes the ESR spectra induced in the Ge–Sn–SiO2 glass by the laser photon irradiation of 102 pulses (a) and 104 pulses (b). No paramagnetic centers are observed in the as-prepared glass. As shown in Fig. 4, four components due to the GEC, Ge E0 center, ðSn4þ Þ , and Sn E0 center are recognized in the spectra. Fig. 5 shows the changes in the concentrations of the induced paramagnetic
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Fig. 4. ESR spectra of the Ge–Sn–SiO2 glass after the laser photon irradiation of 102 pulses (a) and 104 pulses (b).
ðSn4þ Þ shows a saturation at 102 pulses, while the Ge E0 center and the Sn E0 center are continuously induced during the irradiation. Isochronal annealing was applied to the Ge– Sn–SiO2 glass after the laser irradiation of 104 pulses. The annealing was performed under 1 atm N2 atmosphere for 30 min at each temperature. Fig. 6 shows the change in the concentration of each paramagnetic center observed by ESR during the annealing, which was normalized by the concentration observed before the annealing. The meaning of each symbol is the same as that of Fig. 5. The GEC and ðSn4þ Þ almost disappear at 400 °C, while the Ge E0 center and the Sn E0 center are still observable at 600 °C. Fig. 7 compares the photo-induced absorption spectra in the Ge–Sn– SiO2 glass before the annealing (a) and after the 600 °C annealing (b). The photo-induced absorption change is diminished around 4.5 and 5.4 eV by the annealing at 600 °C, but the absorption induced around 6 eV is still observed. Fig. 8 shows the induced absorption spectrum in the Ge–Sn–SiO2 glass by the irradiation of photons from the XeCl excimer lamp for 20 h. The 4.9 eV absorption decreases by the irradiation, and the absorption around 6 eV is induced. Fig. 9 shows the ESR spectrum observed in the Ge–Sn– SiO2 after the 20 h photon irradiation by the lamp.
Fig. 5. Concentration of each paramagnetic center induced in the Ge–Sn–SiO2 glass as a function of the number of irradiated laser pulses. The solid circle, solid triangle, open circle, and open triangle denote the concentrations of the GEC, Ge E0 center, ðSn4þ Þ , and Sn E0 center, respectively. The vertical bars indicate possible calculation errors.
centers with the number of irradiated laser pulses. The solid circle, solid triangle, open circle, and open triangle denote the concentrations of the GEC, Ge E0 center, ðSn4þ Þ , and Sn E0 center, respectively. The induction of the GEC and
Fig. 6. Change in the concentration of each paramagnetic center during the annealing that followed the laser photon irradiation of 104 pulses in the Ge–Sn–SiO2 glass. The concentrations are normalized by the values before the annealing, and the meaning of each symbol is the same as that of Fig. 5.
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Fig. 7. Differential absorption spectra of the Ge–Sn–SiO2 glass before the annealing (a) and after the 600 °C annealing (b) obtained by subtracting the spectrum of the as-prepared sample. Fig. 9. ESR spectrum of the Ge–Sn–SiO2 glass after the 20 h irradiation of photons from the XeCl excimer lamp.
Fig. 8. Induced absorption spectrum in the Ge–Sn–SiO2 glass by the irradiation of photons from the XeCl excimer lamp for 20 h.
The Ge E0 center and the Sn E0 center are induced by the irradiation, while the GEC and ðSn4þ Þ are not induced. The photon irradiation by the XeCl excimer lamp following the irradiation of photons from the KrF excimer laser is considered to bleach the absorption components and paramagnetic centers induced by the laser irradiation. The solid curve in Fig. 10 indicates the induced absorption spectrum in the Ge–Sn–SiO2 glass caused by the laser photon irradiation of 104 pulses, and the broken curve
Fig. 10. Induced absorption spectra of the Ge–Sn–SiO2 glass. The solid curve and broken curve denote spectra after the laser photon irradiation of 104 pulses and after the 2 h irradiation of photons from the XeCl excimer lamp following the laser photon irradiation, respectively, obtained by subtracting the spectrum of the as-prepared sample. Inset: difference between the two absorption spectra obtained by subtracting the solid curve from the broken curve.
denotes the one after 2 h irradiation of photons from the XeCl excimer lamp following the laser photon irradiation. The inset shows the spectral change induced by the lamp irradiation obtained by subtracting the solid curve from the broken
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curve. As seen in Fig. 10, the 4.9 eV absorption that has decreased by the laser irradiation recovers with the lamp irradiation, while the absorption components around 4.5 and 5.4 eV are bleached. The absorption around 6 eV is still observed after the lamp irradiation. Fig. 11 shows the change in the concentration of each paramagnetic center during the lamp irradiation, which was normalized by the concentration before the lamp irradiation. The meaning of each symbol is the same as those of Figs. 5 and 6. The GEC and ðSn4þ Þ are bleached with the lamp irradiation, while the Ge E0 center and the Sn E0 center are not bleached at all. The as-prepared Ge–Sn–SiO2 glass was annealed under 1 atm O2 or N2 atmosphere at 900 °C. The curve (a) in Fig. 12 is the absorption spectrum of the as-prepared glass, and the curves (b)–(d) are the absorption spectra of the glass after 50, 100, and 150 h annealing in the O2 atmosphere, respectively. The 4.9 eV absorption is bleached with the O2 annealing. The decrease in the 4.9 eV absorption was not observed with the annealing in the N2 atmosphere. The KrF excimer laser photon irradiation did not induce any paramagnetic centers after the glass had been annealed in the O2 atmosphere for 150 h, by which the 4.9 eV absorption had been diminished.
Fig. 11. Changes in the concentrations of the paramagnetic centers in Ge–Sn–SiO2 induced by the laser photon irradiation of 104 pulses as a function of the time of bleaching by the XeCl lamp, which are normalized by the values before the bleaching. The meaning of each symbol is same as those of Figs. 5 and 6.
Fig. 12. Absorption spectra of the Ge–Sn–SiO2 glass annealed in O2 atmosphere at 900 °C. Curve (a) denotes the absorption spectrum of the as-prepared glass without O2 annealing, while curves (b)–(d) denote the spectra after the 50, 100, and 150 h annealing, respectively.
4. Discussion As shown in Fig. 1, the Ge–Sn–SiO2 glass has the 4.9 eV absorption, which is divided into three components at 4.83, 5.06, and 5.16 eV. As mentioned above, the 5.06 and 5.16 eV bands are due to NOV and GLPC, respectively. The 4.83 eV absorption band, which is the dominant component of the 4.9 eV absorption, is considered to be due to a Sn-related defect, since such a strong absorption band around 4.8 eV has not been reported in pure SiO2 glass, nor in Ge-doped SiO2 glass. The O2 treatment at 900 °C decreases the 4.83 eV band as shown in Fig. 12, while the N2 treatment at 900 °C does not. This result indicates that the 4.83 eV band is due to an oxygen deficient center relating to Sn. It has been reported that twofold coordinated Sn has an absorption band at 4.9 eV [24], which indicates that the 4.83 eV band is likely to be due to the twofold coordinated Sn. The absorption increases around 4.4 and 5.4 eV by the irradiation of 102 pulses of laser photons as shown in Fig. 3(b), and the GEC and ðSn4þ Þ are simultaneously induced as seen in the ESR spectra shown in Figs. 4(a) and 5. It has been known that the GEC has absorption bands at 4.5 and 5.8 eV [14–18]. The correlation between the concentration of GEC (NGEC ) and the absorption intensity
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at 4.5 eV (I4:5 ) and the one between NGEC and the absorption intensity at 5.8 eV (I5:8 ) are expressed as follows [18]: I4:5 ffi 2:3 1017 NGEC ;
ð1Þ
I5:8 ffi 4:3 1017 NGEC :
ð2Þ
As shown in Fig. 5, NGEC induced by 102 laser pulses is 5 1017 cm3 , which results in the increase of I4:5 by 11 cm1 and I5:8 by 21 cm1 according to Eqs. (1) and (2). These values are smaller than the values of absorption increase shown in Fig. 3(b). Furthermore, the peaks of the induced absorption at 4.4 and 5.4 eV are slightly different from the absorption peaks due to the GEC. These results indicate that the induced absorption peaks around 4.4 and 5.4 eV are both due to the overlap of two absorption bands by the GEC and by some other defect. The absorption that overlaps with the GEC absorption is bleached by irradiation of photons from the XeCl excimer lamp as shown in Fig. 10. Four photo-induced absorption bands around this region, 5.8 eV due to the Si E0 center [25], 5.8 eV due to Ge E0 center [26], 4.66 eV due to an unknown defect [26], and 4.8 eV due to another unknown defect [14,27], have been reported in SiO2 glass and/or Ge-doped SiO2 glass. These four bands are induced by uvphoton irradiation. Therefore, they are not those overlapping with the GEC absorption. The simultaneous increase and decrease in the absorption bands at 4.4 and 5.4 eV, and those due to GEC and ðSn4þ Þ with the laser photon irradiation, with the thermal treatment, and with the photo-bleaching, shown in Figs. 3(b), 4(a), 5–7, 10, and 11, indicate that the overlapping absorption is due to the ðSn4þ Þ . As shown in Figs. 3 and 4, the irradiation of KrF excimer laser photons induces the Ge E0 center, the Sn E0 center, and the absorption around 6 eV in the Ge–Sn–SiO2 glass, simultaneously. The XeCl excimer lamp photons also induce the Ge E0 center, the Sn E0 center, and the absorption around 6 eV, while no other increase is observed in the absorption spectrum in the range of 1.9–6.0 eV as seen in Fig. 8. Furthermore, the Ge E0 center, the Sn E0 center, and the absorption around 6 eV behave similarly during the annealing and photo-
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bleaching experiments as shown in Figs. 6, 7, 10, and 11. From these results, it is expected that the Sn E0 center has an absorption band around 6 eV on the assumption that the center has an absorption band in the range of 1.9–6.0 eV. However, since the Ge E0 center also has strong absorption around 6.0 eV [13] and since the intensity of the absorption of the Ge–Sn–SiO2 glass above 6.0 eV is as large as the observation limit of our measurement system, further research is required to deduce details of the absorption band of the Sn E0 center. The induction of the Sn-related paramagnetic centers is always accompanied by the decrease in the 4.9 eV absorption. The decrease of the paramagnetic centers with the thermal annealing or with the photo-bleaching by the XeCl excimer lamp results in the recovery of the 4.9 eV absorption. Furthermore, the irradiation of photons by the KrF excimer laser does not induce paramagnetic centers in the Ge–Sn–SiO2 glass, if the 4.9 eV absorption has been bleached by the O2 annealing. From these results, it is concluded that the Sn-related oxygen deficient center that has the 4.83 eV absorption band is the precursor of the Snrelated paramagnetic centers. The density of Sn is much smaller than that of Ge in the present Ge–Sn–SiO2 glass. Nevertheless, the concentrations of the Sn-related paramagnetic centers are larger than those of the Ge-related paramagnetic centers as shown in Fig. 5. This result indicates that the number of the Sn-related oxygen deficient centers, which are the precursors of the Sn-related paramagnetic centers, is more than that of the Ge oxygen deficient centers. In fact, the 4.83 eV absorption is much larger than the 5.06 or 5.16 eV absorption band. This is probably due to the fact that Sn is more easily reduced than Ge, which results in the high photosensitivity of the Ge–Sn–SiO2 glass. It has been reported that a precipitate of Ge or Sn occurs in Ge-doped SiO2 glass or in Sn-doped SiO2 glass with the Sn content of more than 1 mol%. It has been also reported that the precipitate of Ge is a primary factor of the defect formation and photosensitivity in Ge-doped SiO2 glass [28]. Since the Sn content of the present sample is 1.5 mol%, the precipitate of Sn may
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exist, although we were unable to observe a clear precipitate by scanning electron microscopy. 5. Conclusions
The Sn E0 center and ðSn4þ Þ were induced by the irradiation of KrF excimer laser photons in Ge–Sn–SiO2 glass. The absorption of ðSn4þ Þ is around 4.4 and 5.4 eV. The absorption band of the Sn E0 center is probably around 6 eV. It was also found that the Sn-related oxygen deficient center has an absorption band around 4.83 eV. It is suggested that the Sn-related oxygen deficient center is the precursor of the paramagnetic centers and that the Sn-related oxygen deficient center contributes to the high photosensitivity of the Ge– Sn–SiO2 glass. Acknowledgements This work was partly supported by a Grant-inAid for Scientific Research (B) (no. 12450132) from Japan Society for the Promotion of Science. A High-Tech Research Grant from the Ministry of Education, Culture, Sports, Science, and Technology of Japan is also appreciated.
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[6] R.M. Atkins, V. Mizrahi, Electron. Lett. 28 (1992) 1743. [7] A.M. Vengsarkar, P.J. Lemaire, J.B. Judkins, V. Bhatia, T. Erdogan, J.E. Sipe, J. Lightwave Technol. 14 (1996) 58. [8] W.W. Morey, G. Meltz, W.H. Glenn, SPIE 1169, Fiber Optics Sensors VII 98 (1989). [9] R. Kashyap, J.R. Armitage, R.J. Campbell, D.L. Williams, G.D. Maxwell, B.J. Ainsle, C.A. Millar, BT Technol. J. 11 (1993) 150. [10] S.M. Melle, L. Kexing, R.M. Measures, IEEE Phot. Tech. Lett. 4 (1992) 516. [11] F. Ouellette, Opt. Lett. 12 (1987) 847. [12] H. Hosono, Y. Abe, D.L. Kinser, R.A. Weeks, K. Muta, H. Kawazoe, Phys. Rev. B 46 (1992) 11 445. [13] H. Hosono, M. Mizuguchi, H. Kawazoe, J. Nishi, Jpn. J. Appl. Phys. 35 (1996) L234. [14] J. Nishii, K. Fukumi, H. Yamanaka, K. Kawamura, H. Hosono, H. Kawazoe, Phys. Rev. B 52 (1995) 1661. [15] M. Fujimaki, K. Yagi, Y. Ohki, H. Nishikawa, K. Awazu, Phys. Rev. B 53 (1996) 9859. [16] T.E. Tsai, D.L. Griscom, E.J. Friebele, Diffus. Defect Data B 53&54 (1987) 469. [17] E.V. Anoikin, A.N. Guryanov, D.D. Gusovskii, V.M. Mashinskii, S.I. Miroshnichenko, V.B. Neustruev, V.A. Tikhomirov, Yu.B. Zverev, Sov. Lightwave Commun. 1 (1991) 123. [18] M. Fujimaki, T. Watanabe, T. Katoh, T. Kasahara, N. Miyazaki, Y. Ohki, H. Nishikawa, Phys. Rev. B 57 (1998) 3920. [19] L. Dong, J.L. Curuz, L. Reekie, M.G. Xu, D.N. Payne, IEEE Phot. Tech. Lett. 7 (9) (1995) 1048. [20] A. Anedda, C.M. Carbonaro, A. Serpi, N. Chiodini, A. Paleari, R. Scotti, G. Spinolo, G. Brambilla, V. Pruneri, J. Non-Cryst. Solids 280 (2001) 287. [21] N. Chiodini, S. Ghidini, A. Paleari, Phys. Rev. B 64 (073102) (2001) 1. [22] H. Kawazoe, J. Nishii, H. Hosono, T. Kanazawa, H. Imagawa, J. Physique colloq. 43 (1982) C9-155. [23] N. Chiodini, F. Meinardi, F. Morazzoni, A. Paleari, R. Scotti, G. Spinolo, Phys. Rev. B 58 (1998) 9615. [24] L. Skuja, J. Non-Cryst. Solids 149 (1992) 77. [25] D.L. Griscom, J. Ceram. Soc. Jpn. 99 (1991) 899. [26] B. Crivelli, M. Martini, F. Meinardi, A. Paleari, G. Spinolo, Phys. Rev. B 54 (1996) 16 637. [27] M. Fujimaki, T. Kasahara, S. Shimoto, N. Miyazaki, S. Tokuhiro, K.S. Seol, Y. Ohki, Phys. Rev. B 60 (1999) 4682. [28] K. Kawamura, H. Hosono, H. Kawazoe, J. Appl. Phys. 30 (1996) 1357.
Ultraviolet photon-induced absorption bands and paramagnetic centers in Ge and Sn co-doped SiO2 glass Tetsuya Nakanishi a, Makoto Fujimaki b,c,d, Shin-ichiro Tokuhiro a, Ken-ichi Nomura a,*, Yoshimichi Ohki a,d, Kazuo Imamura e a
Department of Electrical, Electronics, and Computer Engineering, Bldg. 62, Rm. B2-09 (Ohki Lab) 3-4-1 Ohkubo, Waseda University, Shinjuku-ku, Tokyo 169-8555, Japan b Japan Science and Technology Corporation, Kawaguchi-shi 332-0012, Japan c National Institute of Advanced Industrial Science and Technology, Tsukuba-shi 305-8568, Japan d Advanced Research Institute for Science and Engineering, Waseda University, Shinjuku-ku 169-8555, Japan e Mitsubishi Cable Industries, Ltd., Itami-shi 664-0027, Japan Received 12 October 2001; received in revised form 29 July 2002
Abstract Germanium and Sn co-doped SiO2 glass, a material for photosensitive optical fiber core that can be used for fabrication of optical fiber gratings, was exposed to photons from a KrF excimer laser (5.0 eV) or a XeCl lamp (4.0 eV). The photo-induced paramagnetic centers and optical properties of the glass have been examined with electron spin resonance and absorption measurements. Precursors of the paramagnetic centers are also discussed. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 42.70.Ce; 42.79.Dj; 76.30.Mi; 78.40.)q
1. Introduction Germanium-doped SiO2 glass is a typical material for the core of an optical fiber. It has been well known that ultraviolet (uv) photon irradiation induces refractive index changes in Ge-doped SiO2 glass. The photo-induced refractive index changes have been utilized for the fabrication of optical fiber gratings [1] that have been attracting much
*
Corresponding author. Tel./fax: +81-3 3204 1258. E-mail address: [email protected] (K.-i. Nomura).
attention as reflectors [2–6], band rejection filters [7], sensors [8–10], dispersion eliminators [11], and so on. Induction of paramagnetic centers, the Ge E0 center [12,13] and the Ge electron center (GEC) [14–18] with absorption bands in visible-to-uv region, strongly contributes to the photo-induced refractive index changes in Ge-doped SiO2 glass. The Ge E0 center has an absorption band at 6.4 eV [13], and the GEC has two absorption bands at 4.5 and 5.8 eV [14,15]. The induction mechanisms of these centers have been well examined. The neutral oxygen vacancy (NOV), which has an absorption band at 5.06 eV, is known to be the precursor of
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the Ge E0 center [14]. The Ge lone-pair center (GLPC) and the bridging oxygen are reported as electron donors to create the GEC [14,18]. Recently, it was reported that Ge and Sn codoped SiO2 glass shows higher photosensitivity than Ge-doped SiO2 glass [19]. There have been papers on the photosensitivity in Sn-doped SiO2 glasses made by sol–gel and chemical vapor deposition methods [20,21]. However, the formation of defects associated with Sn was not discussed sufficiently there since they stood on a viewpoint that the increase in the photosensitivity was due to the densification induced by uv-photon irradiation. Two kinds of Sn-related paramagnetic cen ters, ðSn4þ Þ , where an electron is trapped at Sn4þ [22], and the Sn E0 center [23] have been observed by electron spin resonance (ESR) in a sodium silicate glass containing 1 mol% of tin oxide and a SnO2 –SiO2 glass, respectively. Therefore, there exists a possibility that the induction of Sn-related defects by the irradiation of uv-photons contributes to the photo-induced refractive index change in a similar manner as Ge-doped SiO2 glass. From this viewpoint, the generation mechanisms, optical properties, and the roles of Sn-related centers in the photo-induced refractive index change are examined in the present paper by observing ESR spectra and absorption bands in the visible-to-uv region induced in Ge and Sn co-doped SiO2 glass by the irradiation of UV photons.
ble-beam spectrophotometer with a wavelength resolution of 2 nm. Paramagnetic centers were detected by ESR at the X-band frequency, and their concentrations were calibrated by comparing their signal intensities with that of a standard 1diphenyl-2-picrylhydrazyl sample of a known weight. The accuracy of the calibration is believed to be 20%. The photon irradiation and measurements were done at room temperature.
3. Results Fig. 1 shows the absorption spectrum of the asprepared Ge–Sn–SiO2 glass. The absorption has a peak around 4.9 eV. As shown in Fig. 2, the 4.9 eV absorption is divided into three spectral components at 4.83, 5.06, and 5.16 eV indicated by the broken curves, which are obtained by the least-squares fitting with Gaussian shapes. The intensities and the values of the FWHM of these absorption components are shown in Table 1. The 5.06 eV peak with the FWHM of 0.38 eV and the 5.16 eV peak with the FWHM of 0.48 eV are the absorption bands of the NOV and GLPC, respectively [12]. The 4.83 eV peak with the FWHM of 0.42 eV, which is the main component of the 4.9 eV absorption, has not been reported in Ge-doped SiO2 glass. Fitting with several Gaussian curves might be considered to have diverse sets of results. However, the result obtained by fitting with the
2. Experimental details The sample used was a Ge and Sn co-doped SiO2 glass (hereafter abbreviated as Ge–Sn–SiO2 glass) rod containing about 1.5 mol% Sn and 10 mol% Ge and was prepared by the vapor-phase axial deposition method and solution-doping technique. The glass rod was cut into plates with a thickness of 0.2 mm. The sample was polished for optical measurements. A KrF excimer laser (248 nm ¼ 5:0 eV, 80 mJ/ cm2 per pulse, pulse duration of 25 ns) and a XeCl excimer lamp (308 nm ¼ 4:0 eV, 6.4 mW/cm2 , full width at half maximum (FWHM) of 0.03 eV) were used as photon sources. Absorption spectra from the visible-to-uv region were measured by a dou-
Fig. 1. Absorption spectrum of the as-prepared Ge–Sn–SiO2 glass.
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Fig. 2. Absorption components around 5 eV. The solid curve denotes the absorption of the Ge–Sn–SiO2 glass, while the broken curves are the components obtained by the least-squares fitting with Gaussian shapes. The inset shows the difference between the observed spectrum and the fitting result. The components at 5.06 and 5.16 eV are due to the NOV and the GLPC, respectively. The origin of the component at 4.83 eV is to be discussed in the present paper.
Table 1 Peak positions, intensities, and the values FWHM of the absorption components in the as-prepared Ge–Sn–SiO2 glass Peak position (eV)
Intensity (cm1 )
FWHM (eV)
4.83 0.03 5.06 0.03 5.16 0.03
88.7 5 29.9 5 23.7 5
0.42 0.03 0.38 0.03 0.48 0.03
values of the peak position and the FWHM, which are thought reasonable as the absorption components [12] of the present sample, was almost completely unique. Namely, the error is smaller than 0.03 eV for both the peak position and the FWHM, and is within 5 cm1 for the intensity. The calculation about the remaining slope with its peak around 6 eV contains uncertainty to some extent, since the observed absorption intensity around 6 eV is beyond the experimental limit. However, this uncertainty scarcely affects the calculation results of the other components, and the influence is within the errors. Fig. 3(a) shows the absorption spectra of the asprepared glass (i), after the laser irradiation of 102 pulses (ii), and after 104 pulses (iii), while Fig. 3(b) indicates photo-induced absorption spectra ob-
Fig. 3. (a) Absorption spectra of the as-prepared Ge–Sn–SiO2 glass (i), after the KrF excimer laser irradiation of 102 pulses (ii), and after 104 pulses (iii). (b) Differential absorption spectra after the irradiation of laser photons, obtained by subtracting the spectrum (i) from the spectrum (ii) (solid curve) and from the spectrum (iii) (broken curve).
tained by subtracting the spectrum (i) from the spectrum (ii) (solid curve) and from the spectrum (iii) (broken curve). The absorption decreases around 4.9 eV with the photon irradiation, while it increases around 4.5 and 5.4 eV up to the irradiation of 102 pulses. Further irradiation scarcely changes the absorption intensity around 4.5 and 5.4 eV, but it increases the absorption around 6 eV continuously. Fig. 4 denotes the ESR spectra induced in the Ge–Sn–SiO2 glass by the laser photon irradiation of 102 pulses (a) and 104 pulses (b). No paramagnetic centers are observed in the as-prepared glass. As shown in Fig. 4, four components due to the GEC, Ge E0 center, ðSn4þ Þ , and Sn E0 center are recognized in the spectra. Fig. 5 shows the changes in the concentrations of the induced paramagnetic
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Fig. 4. ESR spectra of the Ge–Sn–SiO2 glass after the laser photon irradiation of 102 pulses (a) and 104 pulses (b).
ðSn4þ Þ shows a saturation at 102 pulses, while the Ge E0 center and the Sn E0 center are continuously induced during the irradiation. Isochronal annealing was applied to the Ge– Sn–SiO2 glass after the laser irradiation of 104 pulses. The annealing was performed under 1 atm N2 atmosphere for 30 min at each temperature. Fig. 6 shows the change in the concentration of each paramagnetic center observed by ESR during the annealing, which was normalized by the concentration observed before the annealing. The meaning of each symbol is the same as that of Fig. 5. The GEC and ðSn4þ Þ almost disappear at 400 °C, while the Ge E0 center and the Sn E0 center are still observable at 600 °C. Fig. 7 compares the photo-induced absorption spectra in the Ge–Sn– SiO2 glass before the annealing (a) and after the 600 °C annealing (b). The photo-induced absorption change is diminished around 4.5 and 5.4 eV by the annealing at 600 °C, but the absorption induced around 6 eV is still observed. Fig. 8 shows the induced absorption spectrum in the Ge–Sn–SiO2 glass by the irradiation of photons from the XeCl excimer lamp for 20 h. The 4.9 eV absorption decreases by the irradiation, and the absorption around 6 eV is induced. Fig. 9 shows the ESR spectrum observed in the Ge–Sn– SiO2 after the 20 h photon irradiation by the lamp.
Fig. 5. Concentration of each paramagnetic center induced in the Ge–Sn–SiO2 glass as a function of the number of irradiated laser pulses. The solid circle, solid triangle, open circle, and open triangle denote the concentrations of the GEC, Ge E0 center, ðSn4þ Þ , and Sn E0 center, respectively. The vertical bars indicate possible calculation errors.
centers with the number of irradiated laser pulses. The solid circle, solid triangle, open circle, and open triangle denote the concentrations of the GEC, Ge E0 center, ðSn4þ Þ , and Sn E0 center, respectively. The induction of the GEC and
Fig. 6. Change in the concentration of each paramagnetic center during the annealing that followed the laser photon irradiation of 104 pulses in the Ge–Sn–SiO2 glass. The concentrations are normalized by the values before the annealing, and the meaning of each symbol is the same as that of Fig. 5.
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Fig. 7. Differential absorption spectra of the Ge–Sn–SiO2 glass before the annealing (a) and after the 600 °C annealing (b) obtained by subtracting the spectrum of the as-prepared sample. Fig. 9. ESR spectrum of the Ge–Sn–SiO2 glass after the 20 h irradiation of photons from the XeCl excimer lamp.
Fig. 8. Induced absorption spectrum in the Ge–Sn–SiO2 glass by the irradiation of photons from the XeCl excimer lamp for 20 h.
The Ge E0 center and the Sn E0 center are induced by the irradiation, while the GEC and ðSn4þ Þ are not induced. The photon irradiation by the XeCl excimer lamp following the irradiation of photons from the KrF excimer laser is considered to bleach the absorption components and paramagnetic centers induced by the laser irradiation. The solid curve in Fig. 10 indicates the induced absorption spectrum in the Ge–Sn–SiO2 glass caused by the laser photon irradiation of 104 pulses, and the broken curve
Fig. 10. Induced absorption spectra of the Ge–Sn–SiO2 glass. The solid curve and broken curve denote spectra after the laser photon irradiation of 104 pulses and after the 2 h irradiation of photons from the XeCl excimer lamp following the laser photon irradiation, respectively, obtained by subtracting the spectrum of the as-prepared sample. Inset: difference between the two absorption spectra obtained by subtracting the solid curve from the broken curve.
denotes the one after 2 h irradiation of photons from the XeCl excimer lamp following the laser photon irradiation. The inset shows the spectral change induced by the lamp irradiation obtained by subtracting the solid curve from the broken
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curve. As seen in Fig. 10, the 4.9 eV absorption that has decreased by the laser irradiation recovers with the lamp irradiation, while the absorption components around 4.5 and 5.4 eV are bleached. The absorption around 6 eV is still observed after the lamp irradiation. Fig. 11 shows the change in the concentration of each paramagnetic center during the lamp irradiation, which was normalized by the concentration before the lamp irradiation. The meaning of each symbol is the same as those of Figs. 5 and 6. The GEC and ðSn4þ Þ are bleached with the lamp irradiation, while the Ge E0 center and the Sn E0 center are not bleached at all. The as-prepared Ge–Sn–SiO2 glass was annealed under 1 atm O2 or N2 atmosphere at 900 °C. The curve (a) in Fig. 12 is the absorption spectrum of the as-prepared glass, and the curves (b)–(d) are the absorption spectra of the glass after 50, 100, and 150 h annealing in the O2 atmosphere, respectively. The 4.9 eV absorption is bleached with the O2 annealing. The decrease in the 4.9 eV absorption was not observed with the annealing in the N2 atmosphere. The KrF excimer laser photon irradiation did not induce any paramagnetic centers after the glass had been annealed in the O2 atmosphere for 150 h, by which the 4.9 eV absorption had been diminished.
Fig. 11. Changes in the concentrations of the paramagnetic centers in Ge–Sn–SiO2 induced by the laser photon irradiation of 104 pulses as a function of the time of bleaching by the XeCl lamp, which are normalized by the values before the bleaching. The meaning of each symbol is same as those of Figs. 5 and 6.
Fig. 12. Absorption spectra of the Ge–Sn–SiO2 glass annealed in O2 atmosphere at 900 °C. Curve (a) denotes the absorption spectrum of the as-prepared glass without O2 annealing, while curves (b)–(d) denote the spectra after the 50, 100, and 150 h annealing, respectively.
4. Discussion As shown in Fig. 1, the Ge–Sn–SiO2 glass has the 4.9 eV absorption, which is divided into three components at 4.83, 5.06, and 5.16 eV. As mentioned above, the 5.06 and 5.16 eV bands are due to NOV and GLPC, respectively. The 4.83 eV absorption band, which is the dominant component of the 4.9 eV absorption, is considered to be due to a Sn-related defect, since such a strong absorption band around 4.8 eV has not been reported in pure SiO2 glass, nor in Ge-doped SiO2 glass. The O2 treatment at 900 °C decreases the 4.83 eV band as shown in Fig. 12, while the N2 treatment at 900 °C does not. This result indicates that the 4.83 eV band is due to an oxygen deficient center relating to Sn. It has been reported that twofold coordinated Sn has an absorption band at 4.9 eV [24], which indicates that the 4.83 eV band is likely to be due to the twofold coordinated Sn. The absorption increases around 4.4 and 5.4 eV by the irradiation of 102 pulses of laser photons as shown in Fig. 3(b), and the GEC and ðSn4þ Þ are simultaneously induced as seen in the ESR spectra shown in Figs. 4(a) and 5. It has been known that the GEC has absorption bands at 4.5 and 5.8 eV [14–18]. The correlation between the concentration of GEC (NGEC ) and the absorption intensity
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at 4.5 eV (I4:5 ) and the one between NGEC and the absorption intensity at 5.8 eV (I5:8 ) are expressed as follows [18]: I4:5 ffi 2:3 1017 NGEC ;
ð1Þ
I5:8 ffi 4:3 1017 NGEC :
ð2Þ
As shown in Fig. 5, NGEC induced by 102 laser pulses is 5 1017 cm3 , which results in the increase of I4:5 by 11 cm1 and I5:8 by 21 cm1 according to Eqs. (1) and (2). These values are smaller than the values of absorption increase shown in Fig. 3(b). Furthermore, the peaks of the induced absorption at 4.4 and 5.4 eV are slightly different from the absorption peaks due to the GEC. These results indicate that the induced absorption peaks around 4.4 and 5.4 eV are both due to the overlap of two absorption bands by the GEC and by some other defect. The absorption that overlaps with the GEC absorption is bleached by irradiation of photons from the XeCl excimer lamp as shown in Fig. 10. Four photo-induced absorption bands around this region, 5.8 eV due to the Si E0 center [25], 5.8 eV due to Ge E0 center [26], 4.66 eV due to an unknown defect [26], and 4.8 eV due to another unknown defect [14,27], have been reported in SiO2 glass and/or Ge-doped SiO2 glass. These four bands are induced by uvphoton irradiation. Therefore, they are not those overlapping with the GEC absorption. The simultaneous increase and decrease in the absorption bands at 4.4 and 5.4 eV, and those due to GEC and ðSn4þ Þ with the laser photon irradiation, with the thermal treatment, and with the photo-bleaching, shown in Figs. 3(b), 4(a), 5–7, 10, and 11, indicate that the overlapping absorption is due to the ðSn4þ Þ . As shown in Figs. 3 and 4, the irradiation of KrF excimer laser photons induces the Ge E0 center, the Sn E0 center, and the absorption around 6 eV in the Ge–Sn–SiO2 glass, simultaneously. The XeCl excimer lamp photons also induce the Ge E0 center, the Sn E0 center, and the absorption around 6 eV, while no other increase is observed in the absorption spectrum in the range of 1.9–6.0 eV as seen in Fig. 8. Furthermore, the Ge E0 center, the Sn E0 center, and the absorption around 6 eV behave similarly during the annealing and photo-
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bleaching experiments as shown in Figs. 6, 7, 10, and 11. From these results, it is expected that the Sn E0 center has an absorption band around 6 eV on the assumption that the center has an absorption band in the range of 1.9–6.0 eV. However, since the Ge E0 center also has strong absorption around 6.0 eV [13] and since the intensity of the absorption of the Ge–Sn–SiO2 glass above 6.0 eV is as large as the observation limit of our measurement system, further research is required to deduce details of the absorption band of the Sn E0 center. The induction of the Sn-related paramagnetic centers is always accompanied by the decrease in the 4.9 eV absorption. The decrease of the paramagnetic centers with the thermal annealing or with the photo-bleaching by the XeCl excimer lamp results in the recovery of the 4.9 eV absorption. Furthermore, the irradiation of photons by the KrF excimer laser does not induce paramagnetic centers in the Ge–Sn–SiO2 glass, if the 4.9 eV absorption has been bleached by the O2 annealing. From these results, it is concluded that the Sn-related oxygen deficient center that has the 4.83 eV absorption band is the precursor of the Snrelated paramagnetic centers. The density of Sn is much smaller than that of Ge in the present Ge–Sn–SiO2 glass. Nevertheless, the concentrations of the Sn-related paramagnetic centers are larger than those of the Ge-related paramagnetic centers as shown in Fig. 5. This result indicates that the number of the Sn-related oxygen deficient centers, which are the precursors of the Sn-related paramagnetic centers, is more than that of the Ge oxygen deficient centers. In fact, the 4.83 eV absorption is much larger than the 5.06 or 5.16 eV absorption band. This is probably due to the fact that Sn is more easily reduced than Ge, which results in the high photosensitivity of the Ge–Sn–SiO2 glass. It has been reported that a precipitate of Ge or Sn occurs in Ge-doped SiO2 glass or in Sn-doped SiO2 glass with the Sn content of more than 1 mol%. It has been also reported that the precipitate of Ge is a primary factor of the defect formation and photosensitivity in Ge-doped SiO2 glass [28]. Since the Sn content of the present sample is 1.5 mol%, the precipitate of Sn may
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exist, although we were unable to observe a clear precipitate by scanning electron microscopy. 5. Conclusions
The Sn E0 center and ðSn4þ Þ were induced by the irradiation of KrF excimer laser photons in Ge–Sn–SiO2 glass. The absorption of ðSn4þ Þ is around 4.4 and 5.4 eV. The absorption band of the Sn E0 center is probably around 6 eV. It was also found that the Sn-related oxygen deficient center has an absorption band around 4.83 eV. It is suggested that the Sn-related oxygen deficient center is the precursor of the paramagnetic centers and that the Sn-related oxygen deficient center contributes to the high photosensitivity of the Ge– Sn–SiO2 glass. Acknowledgements This work was partly supported by a Grant-inAid for Scientific Research (B) (no. 12450132) from Japan Society for the Promotion of Science. A High-Tech Research Grant from the Ministry of Education, Culture, Sports, Science, and Technology of Japan is also appreciated.
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