characterization of three e'-center variants in x- and γ-irradiated high purity a-sio2

Nuclear Instruments and Methods in Physics Research Bl (1984) 481-488

North-Holland, Amsterdam

481

CHARACTERIZATION OF THREE E’-CENTER VARIANTS IN X- AND y-IRRADIATED HIGH PURITY a-SiO,

David L. GRISCOM

Naval Research Laboratory, Washington, DC 20375, USA

Electron spin resonance (ESR) studies have been carried out on a suite of high purity amorphous silicas (a-SiO,) following irradiation by 100 keV X-rays at 77 K or @‘Co y-rays ( - 1 MeV) near 300 K. Both high-OH ( - 1200 ppm) and low-OH ( < 5 ppm)

samples were investigated. The existence of three E’-center variants is suggested on the bases of their phenomenological production and thermal bleaching behaviors. These variants, Eh, Es, and E; are then shown to differ one from another in aspects of their g

matrices, suggesting structural differences among them. Finally, “Si hyperfine structure ascribable to each of these variants is

reported, demonstrating that each of the three consists of an unpaired electron in a dangling tetrahedral orbital of a single

three-coordinated silicon. Production mechanisms and structural models for these three fundamental point defects are proposed.

1. Introduction

Silicon dioxide is perhaps the second most important material of modern electronics, ranking behind only elemental silicon itself. High purity SiO, in its amorphous form (a-SiO,) is widely used as the core material in low-loss optical fibers and as the gate-oxide layer in metal-oxide-semiconductor (MOS) devices, while its crystalline polymotph a-quartz finds pervasive application in high precision oscillators and frequency standards. It has long been recognized, however, that these various technologies can be seriously degraded in nuclear radiation environments due to defect formation in the SiO, components. Thus, for many envisioned applications, a detailed understanding of radiation damage processes in silicon dioxide has become paramount.

Radiation effects in SiO, have been studied by many different techniques by employing either bulk crystalline or amorphous samples or device structures based on thin-film or fiber technologies. As examples, the following lines of research are reported in the present volume: - Disordering phenomena in a-quartz under the in-

fluence of particle bombardment investigated by nuclear backscattering [l] and diffraction [2] methods.

- Transient volume changes, optical absorption and luminescence in both o-quartz and a-SiO, following pulsed electron irradiation [3].

- Electron spin resonance (ESR), electron nuclear dou- ble resonance (ENDOR), and optical techniques employed to characterize thestructures and production kinetics of point defects in a-quartz [4] and in a-SiO, in the fiber optic geometry [5].

0168-583X/84/$03.00 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

- Optical absorption, etching rate and electron micro- scopic studies of bulk silicas subjected to massive particle bombardment [6].

- Electrical charging effects in electron bombarded a- SiO, [7].

- The influences of isothermal anneals [8] and applied bias [9] on the generation and bleaching of ESR-ac- tive charged defect centers in MOS oxides.

- Transmission electron microscopy (TEM) of the crystal + glass transformation of a-quartz under electron irradiation [lo]. As apparent above, the experimental approaches to

radiation effects in silicon dioxide have been exceed- ingly diverse. Nevertheless, the various interpretations of the observed phenomena all tend to share one com- mon thread, namely, the recognition that point defects play major roles in determining, e.g., the optical colora- tion, the electrical charging, and the initiation of topo- logical changes in bonding configurations. Thus, prime importance can be assigned to understanding the electronic and atomistic structures of these point defects.

Electron spin resonance is generally acknowledged to be most the powerful technique for characterizing point defects in silicon dioxide, and the so called “E’ center” known from ESR is widely mentioned in the present volume [3-lo]. On the basis of early ESR studies of irradiated a-quartz [11,12], the generic “E’ center” can be characterized as an unpaired electron in a dangling tetrahedral orbital of a silicon bonded to just three oxygens in the quartz structure. Careful studies of irradiated fused silicas enriched in the magnetic isotopes 29Si and I70 have permitted the E’ center in y-irradiated a-SiO, to be described in essentially the same language [13-151. A more specific model for the E’ center, that un- doubtedly pertains in certain specific cases, is that of a

VIII. GLASSES

482 DA. Griscom / &hara~terization of three E’-center variants

hole trapped at the site of an asymmetrically relaxed oxygen uacancy [16]. The latter picture has been sup- ported by theoretical calculations [l?] and shown to be consistent with most experimental data for the E{ center in a-quartz (4,11,12,18]. Other known E’-center variants in u-quartz include the E; [lP] and Eb [ZO] centers, each of which involve a near-neighbor proton, and the surface E’ center, E; 1211.

Studies of irradiated bulk a-SiO, have previously revealed two oxygen-associated trapped-hole centers (OHCs), in addition to the generic “E’ center”. These are the nonb~d~ng oxygen hole center (NBOHC) [22] and the peroxy radical [23]. The latter has been shown to comprise an 0; molecular ion [23] bonded to a single silicon in the glass network [24]. An apparent analogue of the NBOHC has recently been described in u-quartz [25]. On the other hand, no obvious counter- parts of the E; or E; centers have ever been reported in amorphous silicon dioxide.

Recent kinetic data (isochronal anneals of “E’ centers” obtained under various conditions) have suggested the existence of more than one distinct E-center variants’in irradiated a-SiO, [26]. Sections 2 and 3 of the present paper describe the experimental and analyti- cal approaches which have been brought to bear in an effort to better characterize these variants. Section 4 summarizes the kinetic data found in ref. 26 and earlier work 122,271, while sec. 5 provides the first ESR spectroscopic evidence for the existence of three distinct E’ wriants in a-SO,. In sec. 6 tentative models are proposed for these three centers, none of which incorpo- rates near-neighbor protons.

2 Experimental details

Samples comprised primarily 4 rmn and 6 mm rods of high purity synthetic silicas, containing < 1 ppm total cationic impurities other than hydrogen. The principal materials investigated were Suprasil 1 and SpectrosiI containing - 1200 ppm OH and Suprasil WI and Spectrosil WF containing < 5 ppm OH. An additional low-OH silica, Coming 7943, was studied in crushed powder form. Samples were irradiated either by @‘Co y-rays ( - 10s rad at - 290 K) or by 100 keV X-rays (- 2 x lo6 rad at 77 K).

ESR spectra were obtained at 100 K (in the case of X-irradiations) or room temperature (in the case of y-irradiations) both before and after isochronal pulse anneals to higher tem~ratur~. The inst~ment was a Varian E-9 spectrometer operating at - 9.1 GHz and employing 100 kHz field modulation when detecting the first derivative of absorption or 50 kHz modulation in the case of second harmonic detection. The E’ center “central line” was generally recorded as the derivative of absorption using a modulation amplitude of 0.016

mT and a power level of 0.2 PW to avoid overmodula- tion or power saturation effects [14]. On the other hand, hyperfine structure (hfs) of the E’ center due to the 4.7% natural abundance of 29Si was observed in an (out- of-phase) second harmonic detection mode at relatively high modulation amplitudes (- 0.8 mT) and microwave powers (- 40-60 mW). Under the latter conditions the detected signal appeared to represent the un~fferenti- ated absorption curve (see discussion in sec. 5). (Sam- ples enriched to 95% 29Si were also investigated by these methods, but their hfs spectra were obscured by impur- ity effects.)

Temperature regulation was by means of a Varian V-4540 nitrogen flow-through device. The exterior of the cavity was insulated with Styrofoam in order to minimize ~uctuations of the microwave cavity temperature, and hence its frequency, during in-situ temperature cycling of the sample. This precaution was important for precise determinations of the relative g values (sets. 3 and 5) of defect centers of differing thermal stabilities occurring in the same sample. Multiple samples were investigated, including some which were initially y- irradiated at 300 K and subsequently exposed to X-rays at 77 K. Spectra were recorded repeatedly before and after each anneal cycle in order to treat statistically the small residual scatter in the measured relative g values. An absolute g value calibration was not attempted.

3. ESR spectral analysis

Spin Hamiltonian parameters for the observed ESR spectra were extracted according to principles previously described [28-311. In particular, the E’ center “central line” was treated as a “powder pattern” with spectral features-determined by an anisotropic g matrix; consistent with previous work [H-15], the electronic and nuclear spins assigned to this spectrum were respectively S = l/2 (a Kramers doublet) and I = 0 (as ex- pected for defects associated with the ?Si and 3oSi isotopes having a combined natural abundance of 95.3%). The principal-axis components of the g matrix (g,, g,, gs) were then calculated from the field positions Hi of the singular features in the “central-line” spectrum according to the formula

gi = g, H,N’l”ri 3 (1)

where g,, and H,, are the g value and field position of a reference spectrum obtained at the same microwave frequency.

In amorphous materials, however, one generally en- counters statistical distributions in spin Hamiltonian parameters due to the inherent site-to-site disorder [28-311. In the present case, the g value dist~butions were estimated by computer simulation methods [32], As shown previously [13], the primary 2gSi hfs of the E

D.L. Griscom / Characterization of three E’-center variants 483

center comprises a - 420 G doublet (I = l/2) whose components are themselves “smeared” by site-to-site variations in the coupling constants, In sec. 5, the techniques of ref. 13 are adapted to extract coupling constant distributions from the 29Si hfs spectra obtained in the second-harmonic mode.

4. Kinetic data

Fig. 1 provides a synopsis of isochronal anneal data for radiation-induced defect centers in high-purity bulk fused silicas obtained under various conditions [22,26,27,33]. Each data point represents the fraction of the initial defect population remaining after a fixed-time (- 5-10 min) anneal to the indicated temperature and retooling to the measurement temperature (either 100 or 300 K). The fully drawn curves pertain to “E’ centers” while the dashed curves indicate the contrasting behaviors of radiolytic atomic hydrogen [26] in X-irradiated high-OH silicas and of the peroxy radical [22] in y- irradiated low-OH materials.

A striking feature of fig. 1 pertinent to the present paper is the fact that the “E’ centers” induced by y-rays bleach at temperatures which are hundreds of degrees higher than the anneal temperatures of “E’ centers” induced by X-rays. (The thermal instabilities of the X-ray-induced centers have been shown to be indepen- dent of irradiation temperature between 77 and 300 K [26].) On the basis of this “process dependent” stability, the y-ray-induced defects - here denoted Eb - have been suggested [34] to be qualitatively different from the species induced by X-rays - designated Ea and Ei

1261.

E’m is observed after X irradiation at 77 K and is reasonably stable at the observation temperature of 100 K selected for these experiments. In low-OH silicas such as Suprasil Wl, Eb, is clearly observed to bleach out as the temperature is raised toward 200 K (see fig. 1). In high-OH silicas such as Suprasil and Spectrosil, a more rapid annealing of Eb, between 100 and - 130 K (not illustrated) has been attributed [26] to a reaction of these defects with radiolytic atomic hydrogen, which also anneals out in this temperature range (fig. 1). However, in most high-OH silicas the bleaching of EL tends to be obscured by the growth of E;P (fig. 1) concomitant with the decay of Ho. Eb has been tentatively ascribed to the reaction of atomic hydrogen with some kind of precursor structure in the glass network [26]. The second-stage growth in EP near 200 K can be explained as the result of a series of diffusion-limited reactions with radiolytic molecular hydrogen [26,35]. Eb is seen in fig. 1 to bleach rapidly as the anneal temperature is raised above 250 K.

The thermal bleaching mechanisms which govern the various anneal curves of fig. 1 are the subject of another paper [36]. However, the three separate anneal curves exhibited by E; in different materials bear at least a passing comment. First, in low-OH silicas, the decay of the E’ center and attendant one-for-one growth of peroxy radical concentration [22] has been explained in terms of a diffusion-limited reaction of interstitial 0, molecules with the E;T, sites [37]. It is tentatively suggested [36] that much of this diffusing 0, is radiolytic - due to dimerization of displaced oxygen atoms. Such displacements can occur with - 1 MeV y-rays but not with X-rays of < 100 keV [38]. In high-OH silicas, it is proposed that the displaced oxygens react with radio-

Fig. 1. ESR isochronal anneal data for curves) in high purity synthetic silicas.

Anneal Temperature (K)

“E’ centers” (unbroken curves) and other radiation-induced paramagnetic species (dashed

VIII. GLASSES

484 D.L. Griscom / Characterization of three E’-center variants

lytic hydrogen to form water. It is the diffusion of these interstitial water molecules which therefore governs the thermal bleaching of E\ in Suprasil 1 or Spectrosil[36]. Finally, Corning 7943, in addition to being low-OH, appears also be oxygen deficient [39]. Thus, in the latter material there are no molecular species to diffuse, and defect annealing may be due to thermal excitation of the holes to the valence band or electrons to the conduc- tion band.

In suck, three possible classes of “I? centers” - denoted EL, I?; and EQ - have been distinguished on the bases of the processes by which they are created and the kinetics of their thermal bleachings. The following section describes ESR spectroscopic me~~ements de- signed to answer the questions: Are these three classes truly distinct? If so, are all of these truly generic “E centers” in the sense of the definition given in sec. I? Finally, what are the details of local structure and bonding which distinguish each one from the others?

5. ‘ESR spttc&~~~py: results ami discussion

Fig. 2 presents the “central line” ESR spectra of E’,, Ep, and E\ obtained in Suprasil 1 under optimal conditions. These spectra are registered with one another on the magnetic field axis by methods described in sec. 2.

Two separate E6, spectra - denoted E’*, and EL, -

3 D

Magnetic Field (mT)

Fig. 2. F center “central-line” spectra of Suprasil 1 measured at 100 K following 100 keV X-irradiation at 77 K: (a) in dark, no warming; (b} exposed to room light, no warming; (c) after 5 min at 210 K and retooling. Also, (d) following 6oCo y irradiation at room temperature. Dotted curves are computer simulations (see text).

Suprasil w 1

I / I I I / I I

324.2 324.4 324.6 324.3

Mapnetb Field tmT)

Fig. 3. E’ center “central-line” spectrum of Suprasil Wl subjected to 1.5 X lo6 rad 100 keV X-rays at 77 K and measured at 100 K. Dotted curve is a computer simulation as a linear combination of the component spectra of fig. 2 assuming [Eh,]:[EJ=1.5.

are illustrated in figs. 2a and 2b, respectively. It should be made clear, however, that the designation “E&” is intended here to refer only to the dotted computer simulation of fig. 2a, and not to the entire spectrum. The experimental spectrum of fig, 2b is tentatively believed to have resulted from an inadvertent optical

bleaching phenomenon (- 2 h exposure to room light), since this sample and numerous other specimens of Suprasil 1 and Spectrosil exhibited the spectrum of fig. la when shielded from room light following X irradiation. Thus, two variants of E’, are proposed to exist: I& and E’_, characterized by the dotted computer simulations of figs. 2a and 2b, respectively. To test this hy pothesis, the Eb, spectra of other samples were computer simulated as linear ~mbinations of the dotted component spectra of figs. 2a and 2b. The result for Suprasil WI is illustrated in fig, 3.

The lower signal-to-noise ratio which characterizes the spectrum of fig. 3 is due to the lower overall E’ center yield per unit X-ray dose in the low-OH silicas (- 3 x lOI cme3 h&ad-‘) vis-a-vis the yield in glasses containing - 1200 ppm OH (- 1.5 x lo*’ cmm3 h&ad-*) [33]. However, the Suprasil Wl spectrum is particularly important to the h~othesis described above for the following reason. Whereas the spectra of figs. 2a and 2b may contain an unknown (though small) E;P admixture (fig. l), the Eh variant is never present in significant numbers in Suprasil Wl at any temperature. Thus, fig. 3 is presumed to represent a pure Eb, spectrum.

The g value distributions which optimized the dotted computer simulations of fig. 2 are displayed in fig. 4. Several features of fig. 4 are particularly striking. First, one can note the effective invariance of g, (gl,) for all samples; only the shift in g,, for E> relative to the values of g, for the other resonances ~+0.~8) is outside of statistical error (s.d. = *0.~6). Second, there is a “trend” from extreme orthorhombic symmetry for E,,

-

(a)

Q

_..-.A.

00

- J 2.002 2.001 2mo Is99

g Value

D. L. Griscom / Characterization of three E ‘- center uariants 485

Fig. 4. g value distributions used for the computer simulations of figs. 2 and 3. Each of the simulations of fig. 2 resulted from’ a linear combination of 2-9 separate spectra characterized by the g values and relative weights indicated by the “data points”. Relative g values were determined by experiment (sec. 2); an absolute value of g, = 2.00176 for J?(v) was taken from the literature.

(fig. 4a) to “near axial” symmetry for E’,, (fig. 4b) to essentially pure axial symmetry for Eh (fig. 4c). Finally, it appears that E& and ET are virtually indistinguisha- ble on the basis of their g matrices alone.

Fig. 5 illustrates the primary hfs of E6, and Eh due to the 4.7% natural abundance of “Si in a sample of Spectrosil following X irradiation at 77 K. Second harmonic detection was employed here (sec. 2), as these weak signals were virtually impossible to observe by conventional absorption or dispersion mode detection schemes such as were used in ref. 14. Fig. Sa also shows the E’, “central line” and the atomic hydrogen doublet as obtained for a very low modulation ~p~tude (0.032 mT) and a slightly reduced gain. Both the high- and low-gain spectra of fig. 5 appear to resemble undifferen- tiated absorption curves, rather than the second derivative of absorption as usually encountered for second harmonic detection. This outcome is probably attributable to “passage effects” [40] in analogy to the case for the high power dispersion mode [41] (previously employed in studies of E; [14]).

The important result of fig. 5 from the standpoint of the present work is the fact that the 29Si hfs spectra of Eh and Es are essentially identical and, in turn, identi- cai to that of El (compare fig. 5 with fig. 6). Thus, all three species - Eb, Eb, and EY - are shown to be generic “E’ centers”. That is, the wavefunction of the

lb)

, 1

230 800 320 340 330

Me~tiC Fmd tmn

Fig. 5. RSR high-power second-harmonic spectra obtained at 100 K for Spectrosil subjected to - 5 x lo6 rad 100 keV X-rays at 77 K: (a) in dark, no warming; (b) after 5 mm at 210 K and retooling. Spectrometer gain for (b) was identical to that for the high-gain trace of (a), so relative spectral amplitudes give a quantitative indication of thermal bleaching effects.

unpaired spin in each case can be expressed as

where c$ = A&/A, r 0.25 and off = A*i~/Ap E 0.65 [12,13]. Here Ai, and A,,,, are the experimental isotropic and anisotropic hyperfine coupling constants and A, = 1710 G and A,, = 34 G are the atomic 3s- and 3p-state coupling constants, respectively [12]. As a rule, the mean field separation of the low- and high-field peaks of the 29Si hfs is to an excellent approximation equal to the average value of Ai,,. On the other hand, for amorphous materials A,, must be estimated by computer sim~ation methods [13].

Given the limited signal-to-noise ratio of the spectra of fig. 5, it is not currently possible to distinguish features which might be separately attributable to E,, and I.!&, For similar reasons, subtle differences in line shape between the E6, and Efi %i hfs cannot presently be translated into detailed structural information. By contrast, samples which were subjected to y-ray doses > lo8 rad exhibited 2gSi hfs with superior signal-to-noise characteristics (fig. 6).

The unbroken curves in fig. 6 represent the second-harmonic mode spectra of a sample of Spectrosil observed 36 d after y i~adiation to 1.65 X lo* rad. The 420 G doublet is presumed to be the 29Si hfs of E;, since any Ek or Ek centers initially present should have

VIII. GLASSES

486 D.L. Griscom / Characterization of three E’- center variants

1 I 1 I 1 L 1 ( I 230 300 320 340 360

Magnetic Field (rnT>

Fig. 6. ESR h&b-power bond-h~rno~c spectra obtained at room temperature for Spectrosil subjected to 1.65 x lO* rad @Co y-rays. 420 G doublet is the primary 29Si hfs of the E; center 1131. Dashed curve is the spectrum obtained at 4 x gain following an anneal at 573 K. Dotted curve is a computer simulation (see text). Weak peaks indicated by arrows may be 29Si hfs of the well-known 74 G doublet, a numerically minor resonance due to a previously unidentified center involving a proton. (Note: 1 G = 0.1 mT.)

thermally bleached during the 5-week “curing’i at room temperature. When the sample was subsequently an- nealed at 573 K for 20 min and remeasured at 4 X spectrometer gain, the resulting spectrum was congruent with the first except in the regions indicated by the dashed curves. This same 573 K anneal also brought about a noticeable change in the shape of the Ei “central line” {not shown) consistent with an evolution of the surviving centers towards an axial “E’+ce” g matrix.

The dotted curves in fig. 6 comprise a computer simulation of the ~~fferen~at~ absorption spectrum of the Et 2q Si hyperfine doublet, using spin Hamilto- nian parameters previowiy determined by computer simulation of the first-derivative-of-absorption spectrum of y irradiated Corning 7943 [13]. In the present case, no parameters were adjusted other than the peak height. From the excellent agreement between the experimental and computed curves, it is concluded that (1) the high- power second-harmonic mode spectra accurately represent the shape of the ~differentiated absorption spectrum and (2) the structure of E!! in Spectrosil is identical to that in Corning 7943, even though the spin concentration in the former is typically about 15 times less than in the latter for comparable y-ray doses ]14J

The computer line shape simulation of fig. 6 in- volved statistical distributions in the parameters Ai, and A,, which are interrelated according to the con- straint 1131

cfs + c$ = wnst. = 0.87. (3)

The (Gaussian) distribution in Ai, which optimized the fits of fig. 6 and ref. 13 is reproduced as the unbroken curve in fig. 7. The dashed curve in fig. 7 portrays a distribution in Ai, which resulted in an equally success- ful simulation (not shown) of the post-anneal spectrum (dashed curve in fig. 6) under the same model con- straints.

It can be recalled that the measured distributions in

Bond An@ (Dee)

109 110 111 112 1

p 0.8 .z 1 0 p’

8 0.4

0.01 1 - I ,

86 40 46 60

Alao

Fig. 7. Dist~butions in isotropic 29Si hyperfine coupling constants for E; centers in fused silicas. Fully drawn curve: determined by computer simulation for unannealed sample of Corning 7943 1131 and used also to achieve the dotted simulation of fig. 6, Dashed curve: used to stimulate the dashed spectrum of Fig 6. Coupling constant distributions are related by a mathematical transformation 1131 to a distribution in the bond angle p, defined in the inset.

D.L. Griscom / Characterization of three E’-center variants 481

29Si Ai, values can be related to corresponding distributions in the bond angles at the defect sites according to 113,421.

2

I( )I l/2

tanp=- 2 l+% , c3s

(4)

where the angle p is defined by the inset to fig. 7. Using eqs. (3) and (4), a bond angle scale has been affixed to the top of fig. ‘7. Since for E; the distribution in bond angles differs in shape only slightly from the distribution in Ai, [13], it is correctly inferred from fig. 7 that the bond angle distributions for Et are peaked at values of p somewhat larger than the tetrahedral angle of 109.47’ {vertical dashed line}. Furthermore, following the 573 K anneal (dashed curve), the low-p sites are seen to bleach more completely than the sites corresponding to larger bond angles. These and other results of this section are interpreted below.

6. structuraI modeIs

In this section, tentative structural models are proposed for E’,, Ei, and ET which are consistent with the observations of sets. 4 and 5. Since all three of these centers were shown in sec. 5 to share nearly identical 29Si hfs, the orbital of the unpaired spin must in each case comprise a dangling “tetrahedral” orbital on a single silicon bonded to three oxygens [12,13].

In the case of EL, one must take into account the following additional facts: (1) EL yields per unit dose at 77 K are comparable to E\ yields [33], despite the fact that the 100 keV X-rays are not suffi~~tly energetic to create oxygen vacancies by “knock-on” processes; (2) EL bleaches below room temperature, whereas Et is stable to 400 K or above (fig. 1); (3) EL, is charactertzed by a large orthorhombic component in its g matrix (fig. 4a). Fact (1) can be explained by postulating a radiolytic mechanism whereby an oxygen atom can be ‘rmoved” out of its normal position under the influence of radiations insufficiently energetic to “displace” the atom over several lattice spacings. Indeed, a great deal of evidence has been amassing for the occurrence of such radiolytic processes in SiO,, e.g., refs. 3, 10, 43, 44. It has been suggested that the radiolytically “moved” oxygen may be stabilixed by bonding to a nei~bo~ng oxygen to form a peroxy linkage [43,44]. Fact (2), above, can be interpreted by assuming that such near-neighbor oxygen vacancy-“crowdion” pairs are thermally unsta- ble above - 150 K. Finally, the oxygen-oxygen linkage could be the source of the off-axial perturbation causing the orthorhombic component in the E& g matrix [fact (3)1. The model suggested here for E&, is represented pictorially in fig. Sa, where the radiolytically “moved” oxygen is shown crosshatched. In this view, the second

(a) EL

fb) Ej

+ Ho--w + H+

I I

j, (f> + e-

@Q3 Fig. 8. Proposed models for the creation of (a) E A, (b) Ej, and (c) Et centers in a-SiO,. Large spheres represent silicons, small spheres oxygens. Mechanism (a) is radiolytic and initiates at a “perfect” lattice site. Mechanism (b) is limited by the diffusion of radiolytic atomic hydrogen to the sites of pre-existing “precursor” defects. Mechanism (c) 116,171 takes place at the site of an oxygen va~~cy, which may have been created by a “knock- on” displacement prows.

EL variant E’,, might result from the transfer of the unpaired spin to the upper silicon of fig. 8a, where the effect of the “moved” oxygen on the g matrix would be greatly reduced.

Fig. 8b portrays the model for Ee already proposed in ref. 26. In effect, it is supposed that a number of isolated, positively charged three-coordinated silicons pre-exist in the silica glass network. These precursor sites then react with diffusing hydrogen atoms to form Ei centers and unbound protons. The presence of a limited number of precursor sites receives some support from an apparent sublinear dependence of the growth of Ek, with X-ray dose [26] and sample-to-sample varia- bility of the yields at fixed dose [33]; additional experiments are planned to confirm these observations. The axial s~et~ of the g matrix of Ei (fig. 4~) is explained by the equivalence of the three basal oxygens in fig. 8b and the absence of a major off-axis perturbation ahead of the orbital of the unpaired spin (vide infra).

The model put forward here for E\ is identical with the asymmetrically relaxed oxygen-vacancy model (fig. SC) proposed by Feigl, Fowler, and Yip 116,171 for the E; center in a quartz. In the present context, the neutral oxygen vacancy represented on the lefthand side of fig. SC is due to a “knock-on” displacement effected by the

VIII. GLASSES

488 D.L. Griscom / Characterization of three E’- center variants

- 1 MeV y-rays. The displaced oxygen is presumed to lie several lattice spacings away (and in fact may be dimerized or converted to H,O by reaction with radiolytic hydrogen [36]). The small orthorhombic component in the EL g matrix (fig. 4d) is presumed to be due to the presence of the “second silicon” at the top of fig. 8c, in an off-axis position with respect to the direction of the orbital of the unpaired spin. This same argument would also apply in the case of EL,. An identical origin for the small orthorhombic component in the g matrix of the E; center in a-quartz has been previously sup- ported by theoretical calculation [45].

The effect of the 573 K anneal on the KY center reported in sec. 5 could be due to a structural relaxation which causes the “second silicon” to draw away even farther from the silicon of the unpaired spin. Such a relaxation could account for both the more axial g matrix and the increased bond angle p (fig. 7) observed after the anneal [17,45]. Similarly, the nearly perfect axial symmetry of the g matrix of Eb (fig. 4c) can be ascribed to the effective absence of a “second silicon” opposite the unpaired spin. In other words, either the dangling orbital of fig. 8b is projecting into a small void in the glass network or the “second silicon” is so distant that its influence on the E;1 g matrix is minimal. Addi- tional light might be shed on these issues by performing full computer simulation analyses of Ei 29Si hfs spectra acquired with higher signal-to-noise ratios than that of fig. 5b.

In summary, the present paper demonstrates the existence of three distinct E’ center variants in high-purity a-SiO,, proposes structural models for each, and suggests future experiments by which they might be further elucidated.

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characterization of three e'-center variants in x- and γ-irradiated high purity a-sio2

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