.., Vitreous silicas are widely used as optical material due

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1 Characteristics of y-ray-induced absorption bands in oxygen deficient silica Yoshizo Kawaguchi a ) Chugoku National Industrial Research Institute, 2-2-2, Hiro-Suehiro, Kure, Hiroshima , Japan Nobu Kuzuu Department of Applied Physics, Faculty of ngineering, Fukui University, 3-9-1, Bunkyo, Fukui 910, Japan (Received 27 June 1996: accepted for publication 14 August 1996) Characteristics of y-ray-induced absorption spectra in oxygen deficient vitreous silica were investigated. The spectra were well fitted with five Gaussian absorption bands, the same bands as those reported before in x-ray- and excimer-iaser-induced absorption spectra. Absorption intensity in oxygen deficient silica was much larger than those in silicas containing Si-OH or Si-Cl units. Creation mechanisms for radiation induced defects and role of terminal structures such as Si-OH and Si-Cl on resisting defect formation have been discussed American Institute of Physics. [S (96) ] I. INTRODUCTION II. XPRIMNTAL PROCDUR We used commercially available oxygen deficient silica.., Vitreous silicas are widely used as optical material due D-B (trademark of Nippon Silica Glass Co. Ltd.); this to their excellent optical transparency in a wide wavelength sample was manufactured by the vapor-phase axial deposiregion. Optical fibers made from high-quality silicas are tion (VAD) method,23 and contains almost no terminal widely used in waveguides for digital telecommunication; groups such as Si-OH, Si-Cl, and Si-F. OH content in the they can be used even under radioactive conditions such as sample was observed by infrared (IR) absorption at 2.7 f-lm.6 in the spacecraft or in the nuclear reactor. In the large scale The hydroxyl content is less than 7.9X 1016 cm - 3 (I ppm. III integration (LSI) manufacturing process, optical lenses made mass) being the detection limit. D-B contains a pre-existing of silica glasses are used in the step and repeat projector absorption band at 5.0 ev. (stepper) for deep ultraviolet (UV) lithography using an ex We also used, for comparison, silica glass samples concimer laser as a light source.. d db UV I' h 1-8 4,5,7,9-11 taining hydroxyl groups or Si-Cl units. The properties of the Defect structures III uce y Ig t, x-ray, samples are described in Table 1. D-C contains no hydroxyl y_ray,4.5.8,]2-16 and mechanical stimulus J7-22 degrade the opgroups and about 3.8X cm-3 (1000 ppm in mass) of tical properties of the vitreous silicas. Many studies have chlorine. D-A and S contain hydroxyl groups of about been executed to clarify the defect formation and relaxation 3 5.5X10 18 and 7.9XI0 19 cm- (70 and 1000 ppm in mass), processes. lectron spin resonance (SR) and induced respectively. S was synthesized directly by flame hydrolyabsorption are very useful tools to detect paramagnetic de sis of SiCl4, type-iii silica glass with which one of the aufects. Imai et al. 3,5,14,]5 investigated excimer-laser- and thors (N.K.) investigated x-ray- and excimer-laser-induced y-ray-induced defects by SR, and discussed the effect of absorption in the previous articles the impurities such as OH and CI and pre-existing precur Samples were cut into rectangles with a dimension of sors. Devine and Arndt 2,13 reported the effect of densification IOX25X2 mm 3, and their surfaces were polished for optical on the y-induced defect formation by SR, On the other measurement. Samples were irradiated with a 60Co y-ray at a hand. photoinduced-absorption provides complimentary in rate of 7.7 kgy/h under atmospheric conditions at about formation to SR: it can provide information related to dia 40 C. Absorption spectra were measured with a spectrophomagnetic defects which cannot be detected by SR. Kuzuu tometer (a Hitachi U-3000) with a spectral range of et al. 7. applied curve fitting analysis to the x-ray- and nm (i.e., ev). excimer-laser-induced absorption in type-iii silica glasses, and reported the defect formation processes resulting in the III. RSULTS AND DISCUSSION absorption bands. In this article. we investigated y-induced absorption A. Defect formation in oxygen deficient silica spectra in oxygen deficient silicas containing almost no hy Figure I shows absorption spectra before y-irradiation. droxyl groups nor other terminal groups such as Si-Cl and D-B had a sharp absorption band at 5.0 ev, while spectra Si-F, Their defect formation processes were discussed. Due for D-A, D-C, and S showed no prominent absorption to the absence of terminal structures, the glass network must bands. There are at least two kinds of absorption bands near be rigid and could have many strained Si-O-Si bonds. We 5.0 ev (B 2 ll' and B 2,B bands).4,lo The peak energy and the full discuss the influence of the rigidness of the glass network on width at half-maximum (FWHM) of the 5.0 ev band for the inherent absorption and y-induced defects. D-B are 5.01 and 0.31 ev, which are in good agreement with those of the B 2 ll' band reported by Tohmon et af. 24 (5.02 ')lectronic mail: kawaguch@cniri.go.jp and 0.35 ev). xciting the 5.0 ev band for D-B by the KrF J. Appl. Phys. 80 (10), 15 November /96/80(10)/5633/6/$ American Institute of Physics 5633

2 Ē.s 0.2 :t:: Q) 0 U vi « "" nergy (ev) FIG. I. Absorption spectra before y irradiation. excimer laser (5.0 ev), two emission bands at 2.7 and 4.4 ev are observed being characteristic for the B 2 a band. I t.l2 Thus, the 5.0 ev band for D-B can be assigned to the B 2 a band which is due to the ground-to-triplet transition in the oxygen deficient center (ODC).2 5 ")I-ray-induced absorption spectra for oxygen deficient silica D-B with different dosage are plotted in Fig. 2. The absorption spectra before irradiation already contain the 5.0 ev band, and the induced absorption spectra are obtained by subtracting the absorption curve before irradiation from that after irradiation. They can be well fitted with five Gaussian curves centered at 4.8, 5.0, 5.4, 5.8, and 6.5 ev. The values of peak energies and FWHMs used here are listed in Table II. They are common in all of the fitting procedures of the absorption curves in this study, and are in good agreement with those by Kuzuu et at. in x-ray- and excimer-iaserinduced absorption spectra for type-iii silica glasses That is, the absorption curves in different samples with different irradiation methods can be fitted with the Gaussian absorption bands with the common values of peak energies and FWHMs, proving the reliability of the curve fitting procedure. The origin of each absorption band is assigned as follows: 4 the 4.8 ev band is assigned to some oxygen related centers like nonbridging oxygen hole center (NBOHC; ==Si-O ), peroxy radical (_Si-O-O ), or dissolved oxygen. The 5.0,5.4, and 5.8 ev bands are assigned to ODC TABL I. Properties of the samples. Concentration in em 3 (ppm) Sample Preparation name method OH Chlorine D-B VAD method :S7.9X10" i[1 <3.8 X10 16 (I) D-C VAD method <7.9X ]0' I[! 3.8X (1000) D-A VAD method 5.5xI0- "'01 <.8X (I) S flame hydrolysis 7.9/1CI -,!I(HI <3.8X10 I6 (I) (-Si"'Si ), center (_Si H-Si I. and center ( Si ). With increasing the accumulated dose. the absorption peak shifts to lower energy. \\hi.:h IS.:aused by the appearance of the 5.4 ev band at higher '::C'<e Figure 3 shows peak absorplc';-: ::-::en,ity for each band versus accumulated dose for D-B B;:nding in the curves is seen at the accumulated dose c':' ::> ':G:. This should suggest tbat there are two stages of de:'e.:: ::;:;,ation. When the accumulated dose exceeds 2(1 :.;G:-. :- Intensities of the absorption bands can be well :=::':'" I::' D U where D is the accumulated dose of ")I-irrad:;::::;-.. ;:::'::,,-0.4 for the 4.8 ev band, a0.6 for the 5.0 e\' ::-;:::'::, ;::-':: a-0.5 for the 5.8 and 6.5 ev bands. On the othe :-,;:<. ::::: 5.4 ev band grows much more rapidly than the::'-::::;::-.d<. The most prominent ab<::-:-::::-, '='Jnd is the 5.8 ev band ascribed to the cent::: c.:-:: :hree mechanisms are proposed: 2,4 (i) Displacement of an : \:. ;:-,.::om: I,, ==Si-O-Si==--S: 5 - -IY. (ii) Trapping of a hole ;::.:,:: C'DC -Si Si==--+-Si S;--, (iii) Cleavage of a Si-O :'::-:: _Si-O-Si_-_S: 0-5:. I Here Si+ represents a planar :!'.ee-. ".:- s::n-coordinated struc / \ ture. Devine and Arndt measur::d 5R <lgn"j, of the ' center and NBOHC induced by 1IT3.dlatmg \\Ith the 5.0 ev laser (a) D-B 10kGy 2 3 :t::: 1 Q) 0 2 U ui..0 <x: I 4 (b) D-B 40kGy 6 (c) D-S 100kGy o o o nergy (ev) nergy (ev) 4 2 FIG. 2. y-ray-induced absorption spectra for D-B: (a) 10 kgy, (b) 40 kgy, (c) 100 kgy J. Appl. Phys., Vol. 80, No. 10, 15 November 1996 Y. Kawaguchi and N. Kuzuu

3 o :: D-B eV '* / ;1>-" #,;... / eV ;I 10-1 /4'" 4.8eV I."I I <1l 0 5.0eV I u 10-2 en D i <l:: I I r /11 5AeV L-...,cu.Lu..uJ DOS (kgy) FIG. 3. Dose dependence of peak absorption intensity for D-B: (A) 4.8 ev, (_I 5.0 ev, (e) 5,4 ev, (.) 5.8 ev, (T) 6.5 ev. and y-ray in densified silica glasses, and showed good correlation between the number of these two centers. 2,13 They reported that the strained bond cleavage created ' centers and NBOHCs simultaneously. Imai et al. reported dose dependence of the ' center and NBOHC concentrations. 5,14,15 They showed [' center]=[nbohc]o::do 5 in silicas with no extrinsic defects, and explained the dose dependence with process (iii). In our result, the absorption intensity of the 5.8 ev band is proportional to DO. 5, and the 4.8 ev band is proportional to DO A. Assuming that the 4.8 ev band is due to NBOHC, the dose dependence in Fig. 3 suggests that the 5.8 and 4.8 ev bands are induced mainly by the cleavage of the Si-O bonds in process (iii), and creation of the ' center and NBOHC,.. similar to those by Devine et al and Imai et al. 5,14.15 It is known that the NBOHC is accompanied by the absorption band at 2.0 ev,-l but no absorption peak was detected around 2.0 ev in our measurement. According to the result of x-rayinduced absorption for type-iii fused silica by Kuzuu and TABL II. Peak energies and FWHMs of the absorption bands. Band (ev) Peak (ev) FWHM (ev) Assignment NBOHC ODC 5,4 5, ij center center Murahara,1O the absorption intensity of the 2.0 ev band related to NBOHC is about two orders smaller than that of the 4.8 ev band. Thus, the absorption intensity of the 2.0 ev band should be too weak to detect in the present measurement. As for the 5.0 ev band, due to the ODCs, the following mechanism is proposed; because the oxygen deficient silica D-B should contain many strained Si-O-Si bonds, part of the bonds can be easily broken by the y-irradiation as follows: (iv) Displacement of the oxygen atom: -Si-O-Si_-> Si.. Si +O(interstitial). On the other hand, part of the ODCs will be changed into ' centers, according to process (ii). There can be a competition of two processes (ii) and (iv); one for the increase of the ODCs and the other for the decrease of ODCs. D-B contains pre-existing ODCs which cause the 5.0 ev absorption band as shown in Fig.!. Peak intensity of the 5.0 ev band for D-B before irradiation is 0.14 cm-i, just comparable to that of the 5.0 ev band in y-induced absorption with the accumulated dose of about 50 kgy. The small value of the y-induced 5.0 ev absorption at low dose might be due to the dissolution of the ODCs (mainly the pre-existing ODCs) according to process (ii), which mostly compensate for the increase of the ODCs. On the other hand, the increasing feature of the 5.8 ev band suggests that the number of ' centers created by process (ii) is rather small in comparison to that by process (iii), cleavage of the Si-O bond. Last, the 5.4 ev band shows the most strange dose dependence. The center causing the 5.4 ev band might be related to the hydrogen with the following formation mechanism: 4 (v) Trapping of a hydrogen atom at an ODe: ;c- S 0.3 o 0.2 U.3 01 <l:: (a) D-C: 10kGy,::.,\.:: 04 (b) D-C: 40kGy 0.4 (c) D-C: 100kGy , nergy (ev) nergy (ev) nergy (ev). FIG. 4. y-ray-induced absorption spectra for D-C: (a) 10 kgy, (b) 40 kgy, (e) 100 kgy. J. Appl. Phys., Vol. 80, No. 10, 15 November 1996 Y. Kawaguchi and N. Kuzuu 5635

4 , rine. Peak intensity of the absorption spectra for D-C was D-C 5.8eV about 1/20 of that for D-B at the same dose. The reason of.. 6.5eV the weak absorption intensity for D-C can be explained as.", X. follows; the Si-CI bonds increase the flexibility of the glass,. /',;;;I...5.4eV 10-1 network and resistivity against y irradiation, as discussed in.'" 1". 4.8eV the next section. "r "," The spectra can be fitted with four Gaussian bands cen o f: II tered at 4.8,5.4,5.8, and 6.5 ev. The values of peak energies ui 10-2 and FWHMs are the same as those for D-B (Table II). Thus, the y-induced defects causing the photoinducedabsorption bands for D-C must be the same as those for 10-3 L-...L_...L '_'_" D-B, though the origin of the 5.4 ev band is not clear, as discussed above. Moreover, the 5.0 ev band is missing in the DOS (kgy) absorption spectra for D-C. Figure 5 shows peak absorption intensity for each band FIG. 5. Dose dependence of peak absorption intensity for D-C: (A) 4.8 versus accumulated dose. The intensity of the 6.5 ev band ev, (e) 5.4 ev, (.) 5.8 ev, (T) 6.5 ev. grows rapidly below 20 kgy. When the accumulated dose exceeds 20 kgy, all of the four bands are almost parallel in doubly logarithmic plot, and grow like D0 7 As for the 5.8 ev band, the absorption intensity for =:Si 'Si=:+ H -+=:Si H-Si=:. D-C is about two orders smaller than that for D-B. In There could be two mechanisms to supply the hydrogen addition to two processes (i) and (iii), the following mecha-.. atom: nism related with the terminal structure Si -CI is proposed (vi) Cleavage of a hydroxyl group: for the creation of the ' center: 4. 5 =:Si-OH-+=:Si-O + HO. (vii) Cleavage of a Si-H bond: (viii) Cleavage of the Si-CI bond. =Si-Cl-+=:Si +Clo. D-C contains about 3.8XlO 19 cm-" of chlorine, therefore, I", =:Si-H-+=:Si + HO. cleavage of the Si-Cl bond should be a dominant process. Assuming that each Si-Cl bond is broken independently and D-B contains no hydroxyl groups, therefore, process (vi) is the released Cl atom will migrate outside the glass, rejected. Thus, it might be that D-B contains some Si-H bonds, and that the Si-H bonds are easily broken by the y d[']ldd = k[si-ci], irradiation and release hydrogen atoms. Miyano et ai. 8 re where ['], [Si-Cl], D, and k are the concentration of the ' ported defect formation processes in various kinds of vitrecenters and Si-CI bonds, the dose of the y ray, and a conous silicas, and proposed the existence of Si-H bonds in stant. If k D< l, D-B. Hydrogen atoms will migrate in the network of Si O-Si bonds, be trapped by the ODCs, and create the ['] = N o[ 1- exp( - k. D )] = Sc. k D x D, centers. where No is the initial concentration of the Si-CI bonds. That is, the ' center will grow in proportion to the dose of B. Defect formation in hydroxyl free silicas the y ray, D. However, the experimental result shows that containing chlorine 7 the defect density grows like D 0., smaller than 1. This Figure 4 shows the y-ray-induced absorption spectra for difference of index on D could be due to the back reaction D-C, containing no OH and about 3.8 Xl 0 19 cm-3 of chlo- between the ' center and the released chlorine atom which 6 :t:: 4 Q) o.2 o 2 8r (a) D-B: 100kGy I"! '\.! / I 0.4 (b) D-A: 100kGy '<";.f;.';';.i. ",;: >., 0 1 (c) S 100kGy «0 i.-_=--...j 00 iif,'.'.'. 0.0.i./.J<.j;j;!:i#!!.{4/':Y:! nergy (ev) nergy (ev) nergy (ev) FIG. 6. y-ray-induced absorption spectra in vitreous silicas after 100 kgy irradiation: (a) D-B, (b) D-A, (c) S J. Appl. Phys., Vol. 80, NO.1 0, 15 November 1996 Y. Kawaguchi and N. Kuzuu

5 1 l, s t 100kGy D-C :g 'D-A a 'e" U 10-1 ui -". S «-<:J r 10J'"'---,-,-,"-,=L content (cm-3) FIG, 7, Peak absorption intensity vs content of OH (e) or Cl(_) after loa kgy irradiation of y-ray, recombine into Si-Cl bond and reduce the net content of the ' center. Griscom 4 reported that about 90% of process (viii) undergoes back reaction at room temperature. C. Role of OH and chlorine on defect formation Absorption spectra for D-B, D-A, and S induced by y irradiation of 100 kgy are shown in Fig. 6. For D-A and S which contained a certain number of hydroxyl groups, absorption intensities were much smaller than that for D-B. Due to the weak absorption intensities for D-A and S for curve fitting analysis, we compared only the peak intensities of the absorption curves directly with those for D-B. The result is plotted as a function of the content of OH in Fig. 7. The peak intensity for D-C, containing chlorine of about 3 3.8X cm-, is also plotted. The absorption intensity decreases rapidly with increasing the OH content in vitreous silicas. Chlorine also weakens the induced absorption, as can be seen by comparing D-C with D-B. It is noted that the intensity for D-C is larger than that in silica containing the corresponding OH content as chlorine for D-C; this is because the Si-Cl is a precursor of the ' center, as discussed in Sec. B. Kobayashi et al. 26 reported that Young's modulus was smaller in larger OH silica glasses, that is, the network of the Si-O-Si bonds was more flexible in larger OH silicas. Thus, the above result must be due to the difference of the flexibility in the network of the Si-O-Si bonds. It also coincides with the result for D-C containing chlorine, where the Si-Cl bond plays a similar role as the Si-OH bonds for increasing the flexibility of the Si-O-Si bonds. This result can be explained as follows. Since silica glass is an amorphous material, the glass network is much disordered and irregular. Thus the glass network could contain a number of strained Si-O-Si bonds, the bond angle of which is different from the equilibrium value. When the silica glass includes some external defects such as hydroxyl group or chlorine, they terminate the unstable silicon bond and relax the network. As a result, hydroxyl group or chlorine increase the flexibility of the Si-O-Si bonds?! and reduce the probability to break by y-irradiation. On the other hand, D-B contains little terminal groups such as Si-OH add Si-Cl, so that many Si-O-Si bonds must be highly strained. Part of the strained Si-O-Si bonds must be broken into ODCs to stabilize the glass network in the manufacturing process, but the glass network should remain brittle and can be easily broken by the y-ray. This situation is similar to that in case of the plastically densified silica glasses by Devine and Arndt,13 where the glass network was highly strained by hydrostatic pressure of 5 GPa, and the number of y-induced ' center became four orders larger than that in undensified silica. Thus, a large number of defects are induced by the scission of the strained bonds for D-B, and the intensity of the absorption increases rapidly. The strained Si-O-Si bonds for D-B should be broken by the y irradiation into ODCs causing the 5.0 ev band, according to process (iv). For D-C, on the other hand, the 5.0 ev band does not appear by y irradiation. This is also by the effect of the terminal structure, Si-Cl, which reduce the strained Si-O-Si bonds, and restrain the creation of the ODe. IV. CONCLUSION y-ray-induced absorption spectra in oxygen deficient vitreous silica were investigated, and compared with those in silicas containing Si-OH or Si-Cl bonds. (i) (ii) (iii) (iv) Absorption spectra in oxygen deficient silica can be well fitted with five Gaussian absorption bands centered at 4.8, 5.0, 5.4, 5.8, and 6.5 ev, the same as those in case of x-ray- and excimer-laser-induced absorption. The first four bands were assigned to nonblidging oxygen hole centers (NBOHCs), oxygen deficient centers (ODCs), centers, and centers. Absorption intensities of the 5.8 and 4.8 ev bands in oxygen deficient silica were nearly in proportion to the square root of the accumulated dose; this result suggests that the two bands were induced by the breaking of the Si-0 bonds and the resulting creation of the centers and NBOHCs. Absorption spectra in vitreous silica containing much chlorine, were fitted with the same absorption bands as in (i), but the 5.0 ev band did not appear. Absorption intensity was about 1120 of that in oxygen deficient silica. This is because the Si-Cl bond relaxes the Si-O-Si bond in the glass network, and reduces the bond-breaking by y irradiation. Absorption intensity becomes much smaller in silicas containing more hydroxyl groups. That is, Si-OH bond also works to increase the flexibility of glass network and resistivity to y irradiation. ACKNOWLDGMNTS The samples were provided from Nippon Silica Glass Co., Ltd. We acknowledge Dr. M. Yamamoto and Dr. H. Matsuda of National Institute of Materials and Chemical Research for their help in y irradiation to the samples. We are indebted to Dr. J. Takatsubo, Dr. J. Sakakibara, and Dr. S. Yamamoto of Chugoku National Industrial Research Institute for their help to our work. J. Appl. Phys., Vol. 80, NO.1 0, 15 November 1996 Y. Kawaguchi and N. Kuzuu 5637

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