Radiation damage in vitreous fused silica induced by MeV ion implantation

Journal of Non-Crystalline Solids 104 (1988) 85-94 North-Holland, A m s t e r d a m

85

R A D I A T I O N D A M A G E IN V I T R E O U S F U S E D SILICA I N D U C E D BY MeV I O N I M P L A N T A T I O N * SHI Chengru **, T A N Manqi t and T.A. T O M B R E L L O Division of Physics, Mathematics, and Astronomy, 301-38, California Institute of Technology, Pasadena, CA 91125. USA Received 8 September 1987 Revised manuscript received 22 February 1988

The nature of E~ defects in vitreous fused silica induced by high energy (1-17 MeV) CI and F ion implantation in terms of ion fluence, ion energy, saturation behavior and annealing feature, has been studied and compared with results obtained after 2 MeV proton and 0.633 MeV y-ray irradiation using the electron spin resonance (ESR) method. The ESR spectra of E~ defects created by ),-ray, proton, and MeV ion (at low ion fluence) irradiation are similar, with a mean g factor of 2.0005 + 0.0004. For MeV heavy ion implantation the density of E~ defects reaches a m a x i m u m at a deposited energy density of the order of 1021 keV cm, and the average yield of E( defects per unit energy increases with increasing ion mass and decreasing ion energy for a given ion fluence. The influence of ion flux on defect creation as seen in low-energy heavy ion implanted crystal GaAs was not observed. The E~ defects caused by either y-ray or MeV heavy ion irradiation anneal at a unique stage and bleach at temperatures above 450 ° C. The annealing feature is dependent on the ion fluence. For 2 MeV proton implantation, the E~ defects decay more slowly and bleach at temperatures above 600 ° C. The surface cracking produced by MeV heavy-ion implantation has been observed. The crack density increases with ion fluence, reaches a maximum, and then declines. The cracks close up for subsequent higher ion fluences. No surface cracks were seen for 2 MeV proton implantation in the ion fluence range of l x 1014 to 2 × 1017 protons cm 2.

1. Introduction The effects of radiation or implantation on amorphous and crystalline S i O 2 have been studied for m a n y years. This steady interest reflects the importance of their practical applications. For example, radiation can cause absorption in the region of the desired transmission of fiber-optics systems; Si integrated circuits can be seriously degraded in nuclear radiation environments due to defect formation in the SiO 2 and its interface. On the other hand, there is a possibility of using radiation-induced density changes in silica for the formation of optical waveguides [1-3].

* Supported in part by the National Science Foundation [DMR83-18274 and its continuation, DMR86-15641]. ** Permanent address: Nuclear Research Institute, Lanzhou University, China. *** Permanent address: Inst. of Biophysics, Academia Sinica, Beijing, China. 0022-3093/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

Ion implantation in the semiconductor industry has become as important technology both as a means of doping silica [4,5], and as a potential method of hardening or predamaging oxide layers to enhance radiation tolerance [6,7]. Recently, MeV ion implantation has been known to offer an attractive variety of process applications for Si IC devices [8,9]. High-energy heavy-ion b o m b a r d ment represents a valuable method by which to study the long-term stability of glasses, which might be made for nuclear waste storage, under irradiation so as to simulate in a short time what would occur in many years. All these applications are dependent on a clear understanding of basic damage mechanisms. Defects have been known to be introduced into SiO 2 exposed to X-rays, -/-rays, electrons, neutrons, and energetic ions [10-14]. Recently, Antonini et al. measured the irradiation effects in vitreous fused silica (V-SiO2) under 46.5 MeV Ni-ion b o m b a r d m e n t [15]. But studies of particle

86

Shi Chengru et al. / Radiation damage in vitreous fused silica

type, time (dose), saturation, and annealing feature, and their relation to defects produced by X-rays, y-rays, electrons, neutrons, etc, are still rare. The E~ defect has been found in silica and the SiO 2 interface region of MOS devices [16], and in both cases is believed to be one of the major features of the damage structure of the material. M a n y studies have been made to investigate its production mechanisms; however, they have not been completely understood. Systematic experiments are needed to get a better understanding of the processes. In this paper we present our experimental results on surface cracking and E~ defects in V-SiO 2 induced by MeV C1, F, and H ion implantation observed with electron spin resonance (ESR). The aim of this work was to investigate the production, saturation, and annealing feature of E~ defects created by MeV heavy-ion implantation, and to compare these results with those caused by MeV proton, and 0.633 MeV y-ray irradiation.

ture result [18]. Both spectrometers operated in the X-band at 9.3740 G H z with the field modulation frequency of 100 kHz. The microwave power and field modulation used were 0.05 mW and 0.25 G, respectively, in order to avoid ESR signal saturation effects. Absolute spin density was determined by numerical double integration of the unsaturated first derivative spectra with a computer and by comparing the unknown with a standard reference (DPPH) run at the same time. The absolute accuracy of defect density was estimated to be of the order of +30%, and the accuracy from sample to sample was better than _+10% in our measurement. Isothermal annealings were performed in flowing N 2 gas. All samples to be compared were heated at the same time for a period of 20 min at the temperature desired, then quenched to room temperature and measured.

3. Results and discussion 3.1. E S R spectra

2. Experimental The vitreous fused silica Suprasil-2 from Heraeus Amersil Inc. was used in this experiment. Sample dimensions were 25.4 × 9.5 x 1 m m 3. After a thorough cleaning the samples were coated with 500 A Au or Ag, in order to reduce thermal gradients and to prevent charge buildup on the sample surface. R o o m temperature implantation was made with magnetically mass-analyzed 1H, 19F, and 35C1 ions. The beam current densities were in the range of 2 x 1 0 l° to 2.5×1011 ions cm -2 s -1 for F and C1 ions, and 1 to 3.3 × 1012 ions cm -2 s -1 for protons. An aluminum mask restricted the beam b o m b a r d m e n t area of the sample to be 10 × 5 m m 2. The detailed experimental method has been described in another paper [17]. Some samples to be compared were irradiated with 0.633 MeV y-rays from 13VCs. ESR spectra were measured at room temperature using a Varian E-4 spectrometer and Bruker ER 220D-SRC spectrometer, because measurements at liquid nitrogen temperature showed no significant gain in signal over the room tempera-

Examples of experimental lineshapes obtained at X-band frequency are presented in fig. 1. It is clear that the spectra of the E~ defects in V-SiO 2 induced with 0.633 MeV y-rays, 2 MeV protons, and 17 MeV F and 17 MeV C1 ions at low ion fluences ( < 1012 particles c m - 2 ) , respectively, are similar. They are "double-humped" lineshapes with a mean g factor of 2.0005 _+ 0.0004 and a peak-to-peak linewidth of (2.4 _+ 0.2) G. In crystalline quartz the measured g factor for the E~ defect is g . = 2.0004 and gat = 2.0018 with a linewidth ranging between 0.07 and 0.5 G [19]. Assuming simple averaging of the anisotropic g factor, the mean g and linewidth are 2.0008 and 2.4 G, respectively. They are close to our experimental result in amorphous SiO 2. The ESR spectra with "double-humped" shapes have an inhomogeneously broadened resonance with a Gaussian shape due to inhomogeneities in the dc laboratory magnetic field, unresolved hyperfine interactions with neighboring shells of magnetic nuclei, or statistical distributions in s p i n - H a m i l t o n i a n parameters, and can be simulated by computer

Shi Chengru et al. / Radiation damage in vitreousfused silica

~ /

"~1 1X1017cm-2 '/

C1 ions than for 17 MeV F ions, since heavier ions lose more energy and produce more defects (below saturation) during an ion's passage through the SiO 2. The linewidth and lineshape variations in E~ defect spectra with ion fluence can be explained by the combined homogeneous and inhomogeneous broadening [23].

3.6XlOTrad

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33;2.0 .......

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3.2. Dependence of the E l defect density on implantation ion fluence

/k

1

'

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Fig. 1. ESR spectra of E~ defects in V-SiO2 created by: (a) 2 MeV protons, 1 x1017, 1 X1016, 1 x l0 ~5 particles cm-2; (b) 0.633 MeV "f-rays,3.6 X 107 rad; (c) 17 MeV F ions, 5 x 1014, ] X 1013, 5 X 1012, 1 X ]012 particles cm 2; (d) 17 M e V C1 ions, 1 × 10~4, 1 × 1013, 1.5 × 1012, 5 x 10u particles cm -2.

using g-value distributions [20]. For higher ion fluences the lineshapes of spectra of E~ defects vary and broaden as shown in fig. 1. The peak-topeak width changes are, for example, 2.4 _+ 0.1, 2.6 _+ 0.1, and 2.8 _ 0.1 for 17 MeV C1 ion fluences 5 × 1011, 1 × 1013, and 1 x 1014 particles cm -2, respectively. The latter effect results from a homogeneously broadened resonance with a Lorentizian shape. Its width is determined by dipole-dipole interactions between like spins, spin-lattice relaxation, interaction with the rf radiation field, or exchange or motional effects [21]. If spin-spin interactions are dominant, the theory of Kittel and Abrahams [22] gives the resonance half width as A H L = 5.3gt~BN,

87

(1)

where g is the g factor, ~B is the Bohr magneton, and N is the volume concentration of unpaired spins. For a typical defect density of about 20 x 1019cm-3, eq. (1) gives an upper limit estimate of the homogeneous broadening of 1.8 G. Eq. (1) can be used to explain that at the same ion fluence the experimental lineshape changes more for 17 MeV

The dependence of the E~ defect density and saturation feature on ion fluence have been investigated with ESR for 1H, 19F, and 35C1 ion implantation. The temperature of the b o m b a r d e d surface was measured by a thermocouple attached to an implanted area of a simulation sample. After bombarding for one and half hours at an ion current density of 0.83 /~A cm -2 for 2 MeV protons, the surface equilibrium temperature was (53 + 3)°C. All samples were implanted below this ion power density. Therefore, the annealing effect due to ion beam heating could be neglected. For simplicity in summarizing the experimental results we introduce the average yield of the E; defects per unit energy: Y= D/Ep = o/(IX

E,),

(2)

where D is the defect volume density, o = D X Rp is defined as the defect "surface density", Rp is the damage depth in SiO2, Ep = ( I × E i ) / R p is the total deposited energy density, I is the ion fluence, and E i is the ion energy. Fig. 2 shows the dependence of the E~ density on the ion fluence for 2 MeV protons in the range of 1 × 1014 to 2 X 1017 particles cm 2, corresponding to a total deposited energy of about 1 × 1023 keV cm 3 (the projected range in SiO 2 is about 42 /am), and the saturation defect density of about 4.1 × 1019 E~ cm 3. The measured E~ defect density increases linearly with ion fluence up to about 5 × 1015 particles cm -2, giving rise to the yield of 4.2 E~ MeV l, much higher than 1.4 E 1 MeV -1 for 70 keV H ion-implanted thermal SiO 2 reported by Devine [24], while at 2.5 × 1016 particles cm 2 we have an average yield of 1.9 E; MeV -1, close to the value of 2.1 E~ MeV-1 estimated by Antonini et al. with UV optical absorption and within the

Shi Chengru et aL / Radiation damage in oitreous fused silica

88

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experimental error quoted [15]. The reason for the difference between this work and the data given by Devine et al. may be that the former is an average value over the proton range, caused by electronic and nuclear processes (near the end of the ion projected range), the latter mainly results from ionization effects. Fig. 3 displays the variation of E~ defect density as a function of ion fluence for 17 MeV F and C1 ions. All measurements were performed at a beam current density below 30 nA cm 2. The results shown in fig. 3 indicate that for 17 MeV C1 ions the number of E~ defects created saturates at an ion fluence of 1 × 1014 particles cm -2, giving rise to an E~ defect density of about 2.1 × 1 0 1 9 E ~ cm -3, corresponding to a total deposited energy density of about 2.5 × 1021 keV cm -3 (the projected range is about 7/~m). The average yield of E; defects in the linear part of the rising curve in the range of ion fluence 5 x 10 ]a to 1 x 1012 particles cm -2 is estimated to be approximately 120 E~ MeV -1 For 17 MeV F ions the E~ defect density increase has a linear relation to ion fluence in the range of 5 × 1011 to 5 × 1012 particles cm -2, giving rise to an average yield of about 96 E~ MeV-1. At an ion fluence of about 5 × 1014 particles c m - 2 , the E~ defect density reaches a maxim u m of 1 . 9 × 1019 E~ cm -3, requiring a total deposited energy density of about 6 × 1021 keV cm 3 (the projected range is about 14 /~m). In

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Fig. 3. The variation of measured E~ defect density in V-SiO 2 produced by 17 MeV CI and 17 MeV F with ion fluence. The experimental data for 150 keV Ar-implanted thermal SiO2 taken from Devine [23] are also presented.

contrast, for 150 keV Ar-ion implanted thermal SiO 2, the E~ defect density saturates at an ion fluence of about 1 x 1013 particles cm -2, giving rise to a defect density of about 1.5 x 1019 E ; cm -3, corresponding to a total deposited energy density of about 1 × 102° keV cm -3 and an estimated average yield of E; defects of approximately 347 E~ MeV-1 [23].

3.3. Dependence of E 1' defect production on implantation energy and ion flux The variation of the surface density of E~ defects with implantation energy in the range of 50 to 150 keV for Ar ions has been investigated [23]. It was reported that it increased proportionally to implantation energy for the given ion fluences in the range of 1 × 1011 to 1 × 10 ~5 particles cm 2. Fig. 4 shows our measured results of the surface density of E~ defects as a function of ion fluence for 17, 7 and 3 MeV F ions, respectively. The measured average yield of E~ defects are estimated to be 96, 116 and 158 E~ MeV 1 for 1 7 , 7 and 3 MeV ions, respectively. In fig. 5 we plot the average yield of E~ defects as a function of ion energy in the range of 1-17 MeV for F and C1 ions at an ion fluence 3 × 1012 particles c m 2. As reported [23] for low energy heavy ions the relation between the surface density of E; defects and ion

Shi Chengru et al. / Radiation damage in vitreous fused silica

89

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energy is linear for a given ion fluence. This means that the average yield is constant. Whereas, in the energy range of 1 - 8 MeV for C1 and F ions the measured average defect yields decrease greatly in the same manner with increasing implantation energy. However, in the 8-17 MeV energy range they are nearly constant with increasing ion energy. The damage caused by ion-implantation is correlated with the ion energy loss processes. At ion velocities large compared to the Bohr velocity, the stopping power is dominated by losses to the

electronic system of the material. At low ion velocity the cross-section for nuclear collisions becomes large. The nuclear collisions lead to direct atomic displacements, and their energy transfer is much more effective by a factor of at least 1000 than that of electronic (ionizing) processes. The relative effectiveness of the nuclear process in producing physical property changes in fused silica is much larger than for the electronic process [25]. The experimental data shown in fig. 5 show that the efficiency of E~ defect creation for low energy ion-implantation is much higher than for high energy ion-implantation. The influence of the ion flux on measured damage in 40 keV heavy-ion implanted GaAs crystals has been reported [26,27]. But this effect was not observed in 1.54 MeV and 2.65 MeV and 2.65 MeV As-ion-implanted silicon crystals [28]. We have studied this influence in 17 MeV CI ion-implanted V-SiO 2. In order to keep the temperature of the bombarded area below 50° C, intermittent implantation was adopted when the ion beam current was higher than 80 nA. The beam bombardment time was 30 s and the interval time between two bombardments was 5 min. The results are displayed in fig. 6 for 17 MeV C1 ion implantation with fixed ion fluences of 1 × 10 t3 and 1 × 10 a4 particles cm 2. It is indicated that there is no influence of ion flux on the E~ defect creation, as long as the implantation temperature is the same in MeV ion implanted V - S i O 2.

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Shi Chengru et at,. / Radiation damage in vitreous fused silica

90

a.m. a.r.

200 C a

300 C

b 400 C

500 C

500 C

Fig. 7. Typical example of a change in the ESR spectrum during isochronal annealing (annealing time = 10 min). (a) 17 MeV F ions, 3 x 1014 particles cm 2; (b) 17 MeV C1 ions, 1 x 1014 particles cm -2.

3.4. Annealing We have investigated the annealing of E~ defects in g - s i o 2 induced by MeV heavy ions 2 MeV protons, and 0.633 MeV 7-ray irradiation, respectively. Fig. 7 presents the lineshape changes of ESR spectra with annealing temperature. Note that for MeV heavy ion implantation the experimental lineshapes vary with increasing annealing temperature from broadened lineshapes to "double-humped" ones. This is due to the E~ defect density decreasing with increasing annealing temperature and is consistent with the results of MeV heavy ion implantation at different ion fluences as shown in fig. 1. Fig. 8 shows the results of 20 min annealing of E~ defects for 17 MeV C1, 17 MeV F, 2 MeV H, and 0.633 MeV 7-ray irradiated V-SiO 2, respectively. It can be seen that E~ defects in V-SiO 2 induced by 0.633 MeV ~,-rays decay more readily than those induced by all MeV ions except protons, and they decay more slowly for 17 MeV F ions with an ion fluence of 3 x 10 ]4 particles c m - 2 than for 17 MeV C1 ions with an ion fluence of 1 X 1013 particles cm 2. The experimental results show that the annealing of E~ defects depends on the implanted ion fluence, and E~ defects created by either "c-ray or MeV heavy ion irradiation are found to anneal at a unique stage, and bleach after annealing at about 450 ° C. The annealing of E~ defects in MeV ion-implanted SiO 2 is a more complicated process which is still

unclear. For high defect densities, i.e., - 1 0 1 9 cm -3 induced by ion implantation, several mechanisms may be operative simultaneously in the annealing [29]. For 2 MeV proton implantation, the experimental results reveal its special annealing behavior. For example, for the proton fluence of - 1 x 1016 H + cm 2 ( - I X 1019 H c m - 3 n e a r the end of the proton range) E~ defects bleach at annealing temperatures above 600°C. Although the high efficiency of protons in producing E~ defects was explained in terms of H - O interactions, causing the rupture of oxygen bonding and an increase of dangling bonds at silicon sites [30],

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x

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500

400 ANNEAL

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Fig. 8. ESR isochronal anneal results for E~ defects in V-SiO 2. zx 17 MeV C1 ions, 1 x 1014 particles cm 2; [] 17 MeV F ions, 3x1014 particles c m - 2 ; • 17 MeV C1 ions, 1 xl013 particles c m - 2 ; 0 2 MeV protons, 1 x l 0 1 6 particles c m - 2 ; x 0.633 MeV y-rays, 3.6 X 107 rad.

Shi Chengru et aL / Radiation damage in vitreous fused silica

91

08

|

area

I0

d

Fig. 9. Dependence of surface cracks on V-SiO 2 as a function of 17 meV CI ion fluence ( × 1013 particles cm 2). The beam current density was below 20 n A c m 2. Pictures were taken at the corners of the bombarded areas with an optical microscope (focal length, 2.5; magnification, 43). Their dimensions are all 1663 p,m wide by 2171 ~ m high. Picture d shows the surface depression for subsequent higher ion fluence. Picture e is a photograph of the specimen with m a x i m u m crack density. Picture b was taken at a corner of the bombarded area of this specimen.

whether the effects of protons are the same in these two cases is unknown.

3.5. Surface cracking We have observed the surface cracking on VSiO 2 induced by MeV heavy ion implantation. Fig. 9 shows the pictures of surface cracking on V-SiO: at different ion fluences for 17 MeV C1 ions with a 20 nA c m - 2 current density. Pictures were taken at the the corners of the bombarded areas. The crack density increases with ion fluence. Cracks are readily observed at about 5 x 1012 particles cm 2, their density reaches a m a x i m u m at about 1 × 1013 particles cm -2, then decreases with in-

creasing ion fluence. Cracks disappear at about 1 × 1014 particles cm -2. In fig. 9, picture d shows the surface depression for subsequent higher ion fluence. In this case volume compaction occurs, but no cracks exist. For 17 MeV F ion implantation, the m a x i m u m crack density occurs at about 5 × 1013 particles c m - 2 , for subsequent higher ion fluences the cracks close up, too. No surface cracks were observed for 2 MeV proton implantation in the ion fluence range of 1 × 1014 tO 2 x 1017 protons cm 2. We reported these results in detail in another paper [17]. It is generally considered that the glass structure of fused silica is damaged by two different mechanisms for the deposition of energy into elec-

92

Shi Chengru et al. / Radiation damage in uitreous fused silica

tronic (ionizing) processes and into nuclear (elastic collision) processes. The first type of damage requires the order of 1023 keV cm --3 energy deposition for saturation, and the defects have a single stage thermal anneal [31]. The second is a more complex damage process, requiring the order of 10 2° keV cm -3 energy deposition for saturation [32]. In the measurements of refractive index change by 1.8 MeV proton irradiation, it was claimed that the damage produced by ionization could be separated from the direct displacement events near the end of the ion projected range. The nuclear events at the proton range induced saturation by an ion fluence of 1 x 1017 particles cm-2; whereas saturation of the ionization-induced change required the passage of greater than 2 × 1017 particles cm -2, corresponding to about ] X 1 0 23 keV cm -3 [33]. In studies of average tension stress versus ionizing dose in SiO 2 layers irradiated with low energy electrons the experimental data showed that the average tension stress saturated at a deposited energy density of about 2 × 1 0 23 keV c m - 3 [31]. In this work, for 2 MeV proton implantation the E~ defect density reaches a maximum at an ion fluence of about 2 × 10 iv particles cm -2, corresponding to a deposited energy of about 1 x 1023 keV cm -3, in agreement with the results of the refractive index change in S i O 2 irradiated with 1.8 MeV protons [33]. The efficiency of the atomic collision-induced index change was approximately 200 times that of the ionization-induced index change [33]. The compaction data gave rise to a factor of 70 [34]. Our experimental results also indicated that the efficiency of nuclear collisions to create the E~ defect is much higher than for electronic processes. For MeV heavy-ion implantation, our experimental results show that the defects saturate at the order of 1021 keV Cm -3 energy deposition. The ion fluence at which the defect density reaches a maximum decreases with increasing ion mass, while the average yield of the E~ defect increases as the ion mass increases at a given ion energy for C1 and F ions as shown in fig. 3. The processes of MeV heavy ion-induced damage in SiO 2 are different from simple ionization or nuclear collision processes. Watson and Tombrello [35] have shown that a fast heavy ion passing through a variety of

dielectric materials loses its energy primarily through the electronic excitation and ionization of atoms along its path. The highly excited electronic system modifies the interatomic lattice potential there. That is, the nuclei in this region experience a nonzero potential gradient, thereby transferring significant energy to the lattice; thus, most of the electronically deposited energy is eventually coupled into atomic motion. This transfer of energy is sufficiently well localized spatially and temporally to produce the extended region of atomic displacement which characterizes the registration of heavy ion tracks. The defect production and annihilation should be responsible for the ion track generation process. During the damage buildup, three regions may be identified: (1) the defects produced with an individual ion track, corresponding to the linear part of the curve in fig. 3; (2) the defects resulting when ion tracks partly overlap, corresponding to the transition region of the curve; (3) defect saturation, occurring when the cores of ion track overlap completely. The surface cracking occurs in the transition region of the damage creation. The defect density saturates and the maximum of crack density occurs at different ion fluences for C1 and F ions. This is approximately in agreement with the difference in track radii expected for C1 and F ions. For low energy heavy ion implantation, the energy loss is dominated by the nuclear collision process, causing direct atom displacement. The lateral straggling is much bigger; for example, the lateral straggling radius is approximately 240 ,~ [23], thus the cascade overlap is clearly present at an ion fluence of about 1011 cm 2. This makes the damage saturate easily.

4. Summary Our experimental results can be summarized in the following: (1) The ESR spectra of E~ defects in V-SiO2 induced by MeV heavy ion implantation at low ion fluences are similar to those created by y-ray or proton irradiation. They have a "doublehumped" lineshape with a mean g factor of 2.0005 _+ 0.0004. Broadening of the ESR spectra of

Shi Chengru et al. / Radiation damage in vitreous fused silica

E~ defects at high ion fluences is argued to result from spin-spin interactions due to the increase in their density. (2) For 2 MeV proton implantation, the E~ defect density reaches a maximum (about 4.1 x 1019 c m 3) at an ion fluence of about 2 x 1017 particles cm 2, corresponding to a deposited energy density of about 1 x 10 23 keV cm -s. The estimated average yield of E~ defects is approximately 4.2 MeV -1. The E~ defect production is dominated by electronic processes. (3) For MeV heavy ion implantation, the defect density saturates requiring a deposited energy density of the order of 10 21 keV cm -3. The ion fluence at which the E~ defect density reaches a maximum decreases with increasing ion mass, and with decreasing ion energy. The average yield of E~ defects increases as the ion energy decreases, and as the ion mass increases. The mechanism involved may be different from the simple electronic and nuclear processes. The production and annihilation of defects should be related to the generation process of ion tracks. (4) Under our experimental conditions we did not observe the influence of ion flux on the production of E~ defects. (5) The E~ defects created either by y-ray or MeV ion irradiation, other than protons, are found to anneal at a unique stage and bleach at temperatures above 450 ° C. The annealing behaviors are dependent on the ion fluence. For 2 MeV proton implantation, the annealing appears to have a special feature, namely, the E~ defects decay more slowly, and bleach at temperatures above 600 o C. The mechanism of this behavior is unclear and is reserved for further study. (6) The surface cracking of V - S i O 2 has been observed for MeV C1 and F ion implantation. The crack density increases with ion fluence, reaches a maximum, and then declines. The cracks close up in the defect saturation region. N o crack was observed for 2 MeV proton implantation in the ion fluence range of 1 X 1014 to 2 x 10 iv protons CITI 2.

The authors are grateful to Stephen L. Witt for help in the ESR measurements. Some of the ESR measurements were performed at the Department

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of Biophysics, University of Illinois; we would like to thank Chen Kai for all help given there.

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