Oxygen-associated trapped-hole centers in high-purity fused silicas

Journal of Non-Crystalline Solids 32 (1979) 313--326 © North-Holland Publishing Company

OXYGEN-ASSOCIATED TRAPPED-HOLE CENTERS IN HIGH-PURITY FUSED SILICAS M. STAPELBROEK *, D.L. GRISCOM, E.J. FRIEBELE and G.H. SIGEL, Jr. Optical Sciences Division, Naval Research Laboratory, Washington, DC 20375, USA Received 11 August 1978

Two distinct oxygen-associated trapped-hole centers (OHCs) are identified in samples of room-temperature y-irradiated, high-purity fused silica. One, which we label the "wet" OHC, predominates in the high-OH-content (wet) silicas while the other, the "dry" OHC, is more prevalent in low-OH (dry) silicas. Excellent computer simulations of the low-temperature electronspin-resonance spectra are obtained for both wet and dry silicas using only the relative abundance of the "wet" and "dry" OHCs as an adjustable parameter. Analysis of the 170_hyper. fine structure which occurs in samples of wet silica enriched in 170 provides direct confirmation that the "wet" OHC is a hole trapped in a single nonbonding 2p-orbital of an oxygen (presumed nonbridging). Correlation of optical absorption and electron spin resonance via isochronal pulse anneals indicates that the "dry" OHC has an optical transition ay 7.6 eV. In addition, it is reported that the "dry" OHC can be induced in the dry silicas by the fiber drawing process. From the present results, an 0 2 molecular ion model appears most attractive for the "dry" OHC.

1. Introduction The development of low-loss fiber optic waveguides has stimulated a renewed interest in the study of "color centers" that occur in highly-transparent glasses as a result of irradiation. Because of the long optical path-length inherent in the fiber geometry, light-absorbing centers, even at very low concentrations, can seriously degrade fiber performance. Radiation-induced defect centers in high-purity ** fused silica are of particular interest not only because of the importance of fused silica in fiber optics and other applications but also because fused silica is a prototype for many glassy materials and its defect structure is of intrinsic scientific interest. Historically, one of the most powerful techniques for the identification and characterization of defect centers has been electron spin resonance (ESR), and the ESR spectra of radiation-induced defect centers in pure fused silicas have been known for over two decades. Weeks [1] studied neutron-irradiated silica samples by ESR as * NRC-NRL Resident Research Associate. ** The term "high-purity" in this paper refers only to the concentration of metallic impurities, typically <1 ppm total. 313

314

M. Stapelbroek et al. / Trapped-hole centers

early as 1956 and, on the basis of g-shifts, he tentatively assigned a sharp, easily saturable resonance as due to trapped-electron centers (now well-known as E'-centers). In addition, a broad resonance peaking near g = 2.009 with a long low-field tail extending to g ~ 2.06 was assigned by Weeks [1], again on the basis of the g-shifts, to trapped-hole centers (referred to as oxygen-associated trapped-hole centers or OHCs in the rest of this paper [2]). Since that initial investigation numerous ESR [ 3 - 9 ] , optical [ 10-14], and theoretical [ 15-18] studies have appeared concerning the E'-centers in both crystalline a-quartz and fused silica. These studies have correlated [6] the E'-center ESR signal with an optical absorption band at ~5.8 eV and have firmly established the model for the E'-center [5], [8] and [ 15-18] as an unpaired electron residing in a dangling sp3-orbital of a silicon which is bonded to only three oxygens. The broad OHC resonance has, in contrast, been neglected in the literature and a definitive model for the defect(s) responsible for this resonance has not been established. Nelson and Weeks [13] concluded that the 7.6 eV optical absorption band which is induced in "),-irradiated fused silica is associated "with an electron-spinresonance system with a g-value characteristic of a trapped hole" (presumably the broad OHC resonance). More recently, however, Friebele et al. [19] reported that the room-temperature OHC ESR is a superposition of signals from several different centers and an extension of these studies [20] showed that the 7.6 eV band may correlate with only part of the OHC resonance. These authors also observed significant differences Ln the annealing behavior of the OHCs between high-OH-content (wet) and low-OH-content (dry) silicas [20]. In particular, a large increase in both the hole-center ESR signal and the 7.6 eV optical band was observed in room-temperature "/-irradiated dry silicas following a 300°C anneal [20]. In this paper new results, some of which were included in a recent brief report [21], are presented concerning the OHC-type centers in high-purity wet and dry fused silicas. These new results include: (i) resolution of the broad OHC ESR signal into components due to two distinct defect centers (which have been labelled the "wet" OHC and the "dry" OHC); (ii) observation that the "wet" OHC predominates in high-OH-content silicas while the "dry" OHC is more prevalent in low-OHcontent silicas; (iii) correlation via isochronal pulse annealing experiments of the "dry" OHC with the 7.6 eV absorption band in dry silicas indicating that the "dry" OHC has an optical transition at 7.6 eV; (iv) direct confirmation that the "wet" OHC corresponds to an unpaired spin in a nonbonding oxygen 2p-orbital based on the 170-hyperfine structure in isotopically enriched samples; and (v) determination that "dry" OHCs are among the drawing-induced defects [22] found in fibers drawn from high-purity dry silicas. 2. Experimental The samples studied were either commercially available, high-purity wet (Suprasil 1, Spectrosil A, and Dynasil 1000; all ~1200 ppmw OH) or dry (Suprasil W-1

M. Stapelbroek et al. / Trapped-hole centers

315

and Spectrosil WF; 5 - 1 0 ppmw OH) fused silicas or wet silicas prepared in-house with and without 17O enrichment. These "homemade" precipitated silicas were prepared from the reaction of water with a large excess of SIC14. After evaporating away the excess SIC14, the SiO2 powder was sintered in air at ~1200°C and fused in an O2-H2 flame. 170.enriched water obtained from Miles Laboratories was combined with the appropriate proportions of distilled "normal" water to produce SiO2 samples with target ' 7 0 enrichments of 0%, 18%, and 36%. ESR experiments to be described support the conclusion that negligible amounts of ZTO were lost by isotopic exchange with the atmosphere during the brief sintering and fusing steps. All ESR spectra were recorded using a Varian E-9 spectrometer operating at 9 GHz. Optical absorption measurements were obtained at room temperature on thin (<~1 mm) samples using a McPherson model 225 one-meter vacuum-uv scanning monochromator which was operated in a double beam mode. 6°Co "r-irradiations were performed at room temperature and all samples were irradiated to doses between 1 and 2 × l0 s rad (Si). Isochronal pulse anneals were performed in air; ESR samples for these experiments were rods, 4 - 5 mm in diameter. The samples were held at the anneal temperature for 10 min and returned to 120 K. for ESR measurement or room temperature for optical measurement. Anneal data were obtained in 50°C increments.

3. Results

The hole-center ESR spectra in high-purity fused silicas which have been 7-irradiated at room temperature have a dramatic temperature dependence. At low temperatures there is a marked "sharpening" of the spectra and a number of additional features become apparent. The spectra that are observed in a typical high-purity dry fused silica (Suprasil W-l) at room-temperature and 120 K are shown in fig. 1. The additional sharp features around 3250 G in the low-temperature spectrum are evident and have been reported previously [9]. The spectra shown in fig. 1 are from a sample that had been aged for several months at room temperature after irradiation. Using computer simulation techniques it was found possible to simulate the low-temperature OHC resonance in all the silicas studied by using a superposition of resonances from only two distinct types of hole centers [21]. One center (the "wet" OHC) dominates in the wet silicas while the other (the "dry" OHC) is more prevalent in the dry silicas. Appropriate annealing (see below) of 7-irradiated dry silicas results in an ESR spectrum that is almost completely due to "dry" OHCs with very little interference from either the "wet" OHC or the E'-center resonances. Also, spectra almost completely due to "wet" OHCs have been obtained in samples of Suprasil 1. These characteristic spectra and their computer simulations are shown in fig. 2, and the ESR parameters for the "wet" and "dry" OHCs are listed in table 1. Details of the computer simulations, including the anomalous g3-distribution required to simulate the

hf. Stapelbroek

316

(a)

T = 300’K

(bl

T = 120 OK

et al. f Trapped-hole centers

.~./-----_-_--_.--

1

I

3150

I

3200 MAGNETIC

I

3258‘ FIELD

3300

( GAUSS)

Fig. 1. ESR spectra of Suprash W-l several months after room-temperature y-irradiation. The spectrum sharpens markedly as the sample temperature is lowered from 300 K (trace a) to 120 K (trace b). Similar sharpening was observed in all wet and dry silicas studied.

Table 1 ESR parameters for the “wet” and “dry” OHCs and Al center in fused silica. The unique axis of the 170 hypcrfine tensor is parallel to the axis of g r . Parameter g1 g2

(gs) a) A II (‘70) Al (‘70) Aiso (27A1)

“wet” OHC 2.0010 2.0095 2.078 -109.5 G +16.0 G b) n.a.

“dry” OHC 2.0014 2.0074 2.067 c) c) n.a.

Al center 2.0043 2.0120 2.034 -109.5 G b) +16.0 G b) 8.4 G

a) Weighted average of gs-values used in line shape simulations of figs. 2 and 3. b) Could not be measured directly, but implied by theoretical considerations assumed in carrying out the line shape simulations of fig. 3. c, Not determined due to nonavailability of t70enriched dry silica.

and therefore

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M. Stapelbroek et al. / Trapped-hole centers

(a) "Dry" OHG

-g2 = 2.0074/

"°T I

0.0 ~

2.08

(b)

2.06

g3 VALUE

2.04

I

L_gl = 2.0014

"Wet = OHC

:. __..___.-..... .--:_:,.-: ;~-- ;~;:- ~;',.- ,. = 2.00/ ~ a5 Pmox 0.

2. I0 I

5100

2.08

2.06 g3 VALUE I

2.04 Igl

= 2.0010

I

3150 3200 MAGNETIC FIELD (GAUSS)

5250

Fig. 2. Characteristic ESR spectra at 100 K and computer simulations (dots) of the "dry" and "wet" OHCs. The ga-distributions used for the simulations are shown in the insets. (a) The "dry" OHC spectrum from a sample of Suprasil W-1 which was annealed for 10 min at 500°C following 10a rad v-irradiation. The small bump marked by the arrow is due to residual "wet" OHCs. The reverse skew in g3 is evident in the high-gain trace. (b) The "wet" OHC spectrum in Suprasil 1 several months after 3,-irradiation to 108 tad. The small discrepancies between simulated and experimental spectra are due to small concentrations of "dry" OHCs and E-centers. Lorentzian convolution functions with peak-to-peak derivative widths of 0.58 and 1.33 G were used in the simulations of (a) and (b), respectively. " d r y " OHC spectrum, are presented in the discussion. Additional information on the nature of the "wet" OHC in pure fused silicas has been obtained through the use of "homemade" silicas enriched to 0%, 18%, and 36% in the oxygen isotope 170 (1= 5/2). The low-temperature ESR spectrum and the associated computer simulation are shown in fig. 3 for a sample enriched to 36% in 170 and 7-irradiated at room temperature. The 6-1ine hyperfine structure expected for interaction of the unpaired spin with a single I = 5/2 nucleus of 170 is evident. Unfortunately, AI contamination of the enriched samples resulted in the

318

M. Stapelbroek et al. / Trapped.hole centers

A

. . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

iji"

EXPERIMENTAL .......... SIMULATION, ASSUMING: [170]/[1601 : 36/64 [OHC]/[A1 CENTER] = 2 / I

i

i

I

L

a

2800

3000

:5200

5400

3600

MAGNETIC FIELD

(GAUSS)

Fig. 3. Experimental and simulated (dots) ESR spectra of 36% 170_enriched fused silica. The simulation was achieved under the constraints discussed in the text. The microwave frequency was 9.11 GHz and the sample was at 110 K. Due to the high microwave power (200 mW) used, both the "wet" OHC and the A1 center are somewhat saturated in the experimental trace. A Gaussian convolution function with a peak-to-peak derivative width of 12 G was employed in simulating the 170.hyperfine components. appearance of the well-known A1 center [ 2 3 - 2 6 ] . The computer simulation shown in fig. 3 includes these A1 centers. Parameters used in the simulation for the "wet" OHC and the A1 center are given in table I. A familiar optical absorption band at 7.6 eV has been associated previously with the trapped-hole centers in irradiated fused silicas [ 13], [ 19] and [20]. Further evidence of this association has been obtained in this investigation by monitoring the intensities of the "wet" OHC, " d r y " OHC, and E'-center ESR signals and the peak height of the 7.6 eV optical band as functions of annealing temperature during a a series of isochronal pulse anneals in both wet and dry silicas. Fig. 4 shows the results for the wet and dry Suprasils which were typical of the silicas studied. In the wet silicas all the paramagnetic centers and the 7.6 eV band anneal out together in the temperature range from 200 to 400°C. The dry silicas, on the other hand, have a large increase in both the " d r y " OHC concentration and in the intensity of the 7.6 eV absorption in the region around 300°C. This is strong evidence that the " d r y " OHC has an optical transition centered at 7.6 eV. The possibility that the "wet" OHC also contributes to the optical absorption at this energy cannot be eliminated, however, because the total ("wet" + "dry") OHC concentration also correlates very well with the 7.6 eV peak height and because an overlapping band centered near ~8 eV appears in the wet silicas where the concentration o f " d r y " OHCs is low. The total OHC concentration in the Suprasil W-1 sample that was used for the optical pulse anneal was measured by double numerical integration of the room-

M. Stapelbroek et al. / Trapped-hole centers (b) SUPRASIL 1

(a) SUPRASIL W-t

1.0 r ' m . . r J k ~

1.0 A

o

LIJ N --I

319

0.9

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7.6 IN OPTICAL

0.91 I

OPTICAL

O.al

< 0.8 :E a.. o

0.7 I

z 0.7 >-

0.6!

~- 0.6

TOTAL 0HC

Z tAJ

ta 0.5

0.51

-I

8

0.4

3.41

0.3

0.31

't

O2

O.Zl

E' CENTER\

0

w I,-

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z_ 0.1

5

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0

100 2 0 0

3,00 4 0 0 5 0 0 6 0 0 7 0 0 ANNEALING TEMPERATURE

0.0 0

v

\

0.1

if)

\

\ I00

200

300

400

500

(°C)

Fig. 4. Optical and ESR isochronal pulse anneals of "),-irradiated Suprasil 1 and Suprasil W-1. The samples were annealed for 10 min at each temperature. ESR measurements for Suprasil W-1 were made at 77 K, enabling the "wet" and "dry" OHC components to be separated; those for Suprasil 1 were performed at room temperature, yielding only a measure of the total OHC population. Due to an erratic baseline, the optical data are considered unreliable at relative optical densities below ~0.4.

temperature ESR signal and comparison with a known standard. The measured total OHC concentration was 1.5 X 1016 cm -a for a 6°Co ")'-ray dose o f 10 a tad (Si). Irradiation is not the only mechanism which produces defects in high-purity dry fused silica. It has recently been shown [27,28] that an optical absorption band centered at about 630 nm is induced in dry silicas b y the process o f fiber drawing. An ESR signal is also observed in these as-drawn fibers which seems to intensify when the fiber is annealed at 300°C [22]. The present results indicate that this drawing-induced paramagnetic defect center is the " d r y " OHC. The annealing curves shown in fig. 5 show no correlation between the 630 nm absorption and the " d r y " OHC concentration, however. The generation o f drawing-induced absorption bands in the ultraviolet region is presently being investigated.

M. Stapelbroek et al. / Trapped-hole centers

320

I /

400~-

1.---o,,. ~

/

"

~300

,57 d

~

36

~200

29

o/

ESR OPTICAL -o-4SSW-I -*"~ SPWF

/

1.0

o,8

o

t°.i •

0

I00

200

300 400 500 ANNEALING TEMPERATURE (*C)

,

600

700

Fig. 5. Optical and ESR isochronal pulse anneal of drawing-induced defects in dry silica fibers. The vertical line at 20°C shows the growth of the paramagnetic center as the fibers age at room temperature. Aging times are indicated in days. The "dry" OHC concentration was measured at room temperature. Optical data are the peak height of the 630 nm band. Samples were annealed for 10 min at each temperature and quenched back to room temperature for measurement.

4. Discussion

4.1. Computer simulations o f the ESR spectra The techniques involved in the computer simulation o f ESR spectra which occur in glasses have been reviewed in several papers [ 2 9 - 3 1 ]. An additional complication in the glass spectra, which is not found in the "powder p a t t e r n " spectra that are observed for polycrystalline materials, is that the randomness o f the glass network introduces a site-to-site variation in the local environment o f the defects. As a consequence, in glass spectra distributions are found for those ESR parameters that are sensitive to the local environment o f the defect. The simulations for the " w e t " and " d r y " OHC spectra that are shown in fig. 2 were achieved using the indicated distributions in the g3-component. To our knowledge, all simulations o f glass spectra to date have used g-distributions skewed outward from the free electron g-value [ 2 9 - 3 1 ]. By careful fitting o f spectra obtained at high gain, it was determined that the low-field shoulder o f the " d r y " OHC could

M. Stapelbroek et al. / Trapped-hole centers

321

be adequately simulated only by using a g3-distribution that was skewed inward (see fig. 2a). The physical significance of this reverse skew is not yet clear. The simulation of the ESR spectrum of 36% ~70-enriched fused silica was achieved with the following constraints: (i) the previously determined g-values and distributions for the "wet" OHC and the A1 center were not changed nor was the AI center hyperfine interaction; (ii) the ~70-hyperfine interaction was assumed to be axial and the anisotropic part ~ (AII - A O was fixed at the value expected for an unpaired spin in a 2p nonbonding orbital of an O- ion (see Section 4.2); and (iii) identical ~70-hyperfine structure was included in the spectra of 36% of the "wet" OHCs and the AI centers. The only adjustable parameters used in the simulation were the isotropic part of the hyperfine interaction, the ratio of "wet" OHC to A1 center, and the width of the Gaussian convolution function which approximated the effect of a slight distribution in coupling constants. "Dry" OHCs were not included because they were observed to have negligible intensity in unenriched samples of "homemade" fused silica. In view of the rigid constraints, the agreement between simulated and experimental spectra is excellent. It should be remarked that the hyperfine peaks in the spectrum shown in fig. 3 are sharp only because of the "wet" OHC contribution; the A1 center has broad peaks because of the additional interaction with 27A1" 4.2. The 170.hyperfine interaction

The hyperfine tensor components for a system with axial symmetry can be represented in terms of isotropic and anisotropic parts as [32] All=a + 2b

(1)

A± = a - b .

(2)

and

The isotropic part, a, arises from the presence of unpaired spin density at the magnetic nucleus because of the Fermi contact interaction [32], and the anisotropic part, b, is due to the dipole-dipole interaction between the electronic and nuclear magnetic moments [32]. These contributions to the hyperfine interaction can be written as [32] a = -~ gN{3N(47r I~b(0)12 + X} ,

(3)

and for a p-orbital b = ~ gN/3N(r-a)

•

(4)

Here gN and/3N are the nuclear g-facto/and magneton, respectively, I ~b(0)12 is the unpaired spin density at the nucleus due to admixture of s-orbital in the wavefunction, × represents the unpaired spin density at the nucleus due to core polarization, and {r-3) is taken over the wavefunction of the unpaired spin [32].

322

M. Stapelbroek et al. / Trapped-hole centers

Clementi [33] has calculated wavefunctions for a number of free atoms and ions including O-. Using his 2p-wavefunction for O-, we calculate = 4.055 a.u. and eq. (4) gives b = - 4 2 G (the negative sign arises because gN = --0.7573 for 170) [34]. From the observed value A Jl= - 1 1 0 G (Ail is negative because gN is negative and all other quantities in eqs. (3) and (4) are positive [34]), the isotropic part of the hyperfine interaction is deduced from eq. (1) as a = - 2 6 G. This predictsA± = +16 G which is the value used in the simulation shown in fig. 3. The consistency of these results can be checked by a consideration of the corepolarization contribution to the isotropic part of the hyperfine interaction. Harvey [35] has measured the core polarization of neutral O and F atoms. Using Harvey's result X = 0.901 a.u. (for F atoms because they are isoelectronic with O-), the corepolarization contribution to the isotropic hyperfine interaction is - 1 5 G. The remaining - 1 1 G of isotropic hyperfine is satisfactorily explained by a very small (~0.007) admixture of s-orbital into the 2p nonbonding orbital since the isotropic hyperfine interaction expected for an unpaired spin fully localized in an oxygen 2sorbital is calculated to be over 1600 G. The hyperfine interaction for the "wet" OHC is direct evidence that the hole is trapped in a single nonbonding oxygen 2p-orbital. In particular, the ~ O results eliminate the possibility of more complex oxygen radicals (e.g. the peroxy radical O~; see Section 4.3) as a model for the "wet" OHC. 4. 3. Tentative models for the " w e t " and "dry" OHCs

The association of the broad ESR signals in high-purity fused silicas with the oxygens is well established. As noted previously [1], at high radiation doses the defect concentration is well above the levels of any foreign elements in these glasses except for H and C1 [36]. However, the lack of any observable hyperfine structure with 1H or 3Sc1 and 37C1 nuclei eliminates the possibility of any appreciable unpaired spin density at these species. In addition, the g-values for these broad resonances are in the range characteristic of oxygen-associated centers in other materials [30], [37-39]. The 170.hyperfin e structure observed for the "wet" OHC has definitely confirmed the association of this defect with oxygen. In view of the prevalence of the "wet" OHCs in the high-OH-content fused silicas, however, it seems likely that the "wet" OHCs are also associated with OH. Calculations based on extended Hfickel theory indicate that paired OH groups formed by the addition of an H20 molecule to an S i - O - S i bond are favored energetically over isolated OH groups in silica [16]. Similarly, thermodynamic data indicate that nonbridging oxygens are paired in alkali silicate glasses [40]. Assuming paired OH groups are present, the scenario illustrated in fig. 6 is suggested for formation of the "wet" OHC via irradiation. The absence of resolved proton hyperfine structure for the "wet" OHC, however, requires a distance of at least 2.5 A between the unpaired spin and the proton. This distance is not unreasonable, and unresolved proton hyperfine structure may account for the

323

M. Stapelbroek et al. / Trapped-hole centers

0 --O-Si-OH/ o

O O--Si-OI o

I

irradiation 0 --=-

-O-Si-O I 0

0 I O-Si-OI

I trapped hole

+ I-t°

i

Fig. 6. Suggested model for the formation of the "wet" OHC.

somewhat wider linewidth observed in the "wet" OHC spectrum compared to the "dry" OHC spectrum (c.f. fig. 2). Although its association with oxygen is unquestioned, the absence of hyperfine data for the "dry" OHC makes the model less certain. Weeks [41] has suggested an 02 defect model for a resonance observed in neutron-irradiated silica that had been annealed to ~550°C. The room-temperature ESR spectrum observed by Weeks is virtually identical to the room-temperature spectrum shown in fig. la (i.e. mostly "dry" OHC spectrum). The O~ model for the "dry" OHC appears attractive for several reasons discussed below. The theory for the g-values of O~ molecular ions is well understood [42] and the equations for gl, g2, and g3 are given in various places, e.g. refs. [30], [37], [39] and [42]. The computer simulation shown in fig. 2 for the "dry" OHC was achieved using g-values consistent with these equations (the parameters that were used in the equations were l = 1.04, X = 0.014 eV, and E = 4.7 eV; see ref. [37] .) The experimentally determined distribution in g3 was used to calculate distributions in g~ and g2 through the equations. As seen in fig. 2, the calculated distributions in g~ and g2 turned out to be very narrow, and a very satisfactory simulation of the "dry" OHC spectrum is achieved within the constraints of this model. Preliminary observations on the temperature dependence of the "dry" OHC spectrum suggest that a motional effect is responsible for the broadened spectrum at room temperature. As the temperature of observation is increased from 100 K, it has been shown by computer simulation that the g3-distribution shifts inward toward the free electron value while the g=- and g2-distributions shift outward. This behavior suggests that the g-tensor may be motionally averaged at higher temperatures. For an O~ molecule bonded to a single silicon such an effect can be envisioned as due to a librational motion about the Si-O2 bond. For a single nonbridging oxygen model, such as that proposed for the "wet" OHC, a mechanism for motional averaging is more difficult to imagine. The total OHC concentration measured for the Suprasil W-1 optical sample allows an estimate to be made for the oscillator strength of the 7.6 eV band. Assuming that the 7.6 eV band is due to both "wet" and "dry" OHCs and that the oscillator strengths are the same for these centers, the application of Smakula's equation [43] results in an oscillator strength f = 0.65 -+ 50%. This large oscillator strength indicates that the 7.6 eV transition of the "dry" OHC is highly allowed

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M. Stapelbroek et al. / Trapped-hole centers

since the calculated value o f f would increase if the "wet" OHCs were assumed not to contribute this band. While direct evidence of the O5 model for the "dry" OHC is lacking, the model appears attractive and is highly consistent with the present observations. Confirmation of this model by way of 170-hyperfine structure is desirable and attempts to produce dry silica enriched in 170 are in progress. 4.4. Trapped electrons or trapped holes?

The pulse anneal data of fig. 4, together with the ESR spectroscopic analyses of the "wet" and "dry" OttCs, permit some heuristic arguments regarding which defects are actually trapped hole centers and which are trapped electron centers in the high purity wet and dry fused silicas investigated here. The keystone of this development is the 170 hyperfine results which show the "wet" OHC to comprise an unpaired spin in a pure 2p-orbital of a single oxygen. Since in any oxide insulator all oxygen 2p-orbitals are normally filled, the "wet" OHC can only be a trapped hole center. While the "dry" OHC has not been as firmly identified, the g-tensor analysis has been strongly supportive of an O5 molecular ion model. As a radiationinduced defect, O~ could conceivably result from an electron trapping on an interstitial 02 molecule or from a hole trapping on a peroxy group (05-) substitutional for a bridging oxygen. Whereas the former possibility seems remote, the latter is highly reasonable. In the absence of 170 hyperfine data for the "dry" OHC, one cannot exclude the possibility that this center, like the "wet" OHC, consists of a hole trapped on a single nonbridging oxygen. But in either case, both the "wet" and "dry" OHCs are concluded to be trapped hole centers. As discussed elsewhere [26], the E'-center is generally thought of as a trapped hole c enterin crystalline a-quartz, but the possibility of its being an electron center in silica glass cannot be discounted. In fig. 4a, it can be noted that the E'-center in Suprasil W-1 anneals out around 200°C, in contrast with the behavior of Coming 7943 fused silica where the E'-center signal is stable to ~500°C [20]. In view of the general absence [20] of OHCs in irradiated Coming 7943, this material can be inferred to be oxygen deficient, and the intense induced E'-center resonance can be logically ascribed to holes trapped at pre-existing Si-Si bonds. If the trappedhole E'-center is stable to 500°C in Coming 7943, then it would also be stable to the same temperature in Suprasil W-1 (if it exists there), unless it were to recombine with free electrons released from another site. The present interpretation of the "dry" OHC as a trapped hole center, coupled with the observed increase in "dry" OHC population on annealing to 200°C (fig. 4a), means that either (i) free holes are released at this temperature from an unspecified third site or (ii) free electrons are released from the precursor site of the "dry" OHC. If the former should be true, then the E'-center could only be a trapped electron center in Suprasil W-1 (and in Spectrosil WF, which behaves similarly). But if the latter situation should prevail, then the E'-center must be a trapped hole defect

M. Stapelbroek et al. / Trapped-hole centers

325

in all of the low-OH silicas. Fig. 4a shows that the "wet" OHC could conceivably be a source of free holes at 200°C, but this source would be incapable of accounting for the increase in total OHC concentration at this temperature. Moreover, the annealing growth of the "dry" OHC in unirradiated fibers (fig. 5) cannot be explained in this way since no "wet" OHCs are present. While it is not necessary to assume that the "dry" OHC forms by the same process in as-drawn fibers as it does in the irradiated bulk material, the similar annealing curves in figs. 4a and 5 strongly suggest that this may be so. On this basis, it is tentatively concluded that the "dry" OHC forms when its pre-existing precursor site releases a free electron. A corollary of this conclusion is that most of the E'-centers in irradiated low-OH silicas arise from holes trapped at pre-existing Si-Si bonds.

5. Summary The trapped-hole center ESR spectra in room-temperature 7-irradiated, highpurity fused silicas have been separated into components due to two distinct centers. The center that predominates in the wet silicas has been termed the "wet" OHC and, from the hyperfine interaction with 170, has been shown to comprise a hole trapped in a single nonbonding 2p orbital of an oxygen (the oxygen is presumed to be nonbridging). The center that is prevalent in the dry silicas, the "dry" OHC, was shown to be consistent with an O5 molecular ion model. This interpretation of the "dry" OHC is tentative pending the availability of XTO-hyperfine data. Strong evidence that the "dry" OHC has an optical transition at 7.6 eV was presented and it was shown that the "dry" OHC occurs in as-drawn dry silica fibers as a result of the fiber drawing process.

Acknowledgements We would like to thank R.D. Kirk and R.J. Ginther for their help in the preparation of the "homemade" silicas. The NRL Radiation Technology Division is thanked for the use of their 6°Co source.

N o t e added in proof." 170.hyperfin e spectra have now been obtained for the "dry"

OHC, unambiguously confirming the suggestion made above that this defect is of the O~ type [44].

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