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Heavy-ion and electron irradiation effects in vitreous silica
Journal of Non-Crystalline Solids 44 (1981) 321-330 North-Holland Publishing Company
321
HEAVY-ION AND ELECTRON IRRADIATION EFFECTS IN VITREOUS SILICA M. ANTONINI *, P. CAMAGNI, A. M A N A R A and L. MORO Joint Research Centre, 21020 Ispra, Varese, Italy Received 23 October 1979 Revised manuscript received 8 August 1980
Amorphous silica samples have been bombarded with Ni+6 ions of 46.5 MeV to produce a large number of atomic displacements up to about 1 dpa. Other samples with the same impurity content have been irradiated with electrons of 1.5 MeV. Optical absorption spectra induced with these treatments, in the range 190 to 2000 nm, have been compared, focussing the attention on the two prominent bands at 215 and 245 nm observed in the heavy-ion irradiated samples. While the band at 215 rim, identified as the E'I coulour centre, is present after irradiations with both types of particles, the so-called B2 band at 245 nm is totally absent after electron bombardment and therefore it can be attributed to direct atomic displacements. Conversely, the concentration of E'I does not correlate with dpa values; the origin of these centres must therefore be sought in other processes, such as ionization and strained bonds.
1. Introduction The effects o f radiation damage in amorphous silica have been studied b y many authors, with special attention to structural variations [ 1 - 4 ] and defects properties [5,6]. The latter, in particular, have been probed in a number o f colour centre and paramagnetic resonance investigations, covering a variety o f experiments with neutrons, electrons, light ions, X- or "r-rays [ 7 - 1 5 ] . Irradiation effects from low-energy heavy ions have also been reported [16]. In the above studies, particular efforts have been made to identify the primary defects produced b y various forms o f irradiation and to inquire into their formation mechanisms. However, a certain controversy still exists on this subject, due to the lack o f systematic experiments aiming at comparing the two basic aspects o f atomic displacement and b o n d ionization. In order to tackle this problem, it is necessary to use irradiating conditions capable o f affording a substantial increase in the fraction o f displacement processes, as suggested by Arnold [16].
* Also: Gruppo Nazionale Struttura della Materia and lstituto di Fisica dell'Universita, 41100 Modena, Italy. 0 0 2 2 - 3 0 9 3 / 8 1 / 0 0 0 0 - 0 0 0 0 / $ 0 2 . 5 0 © North-Holland
322
M. Antonini et al. / Irradiation effects in vitreous silica
Following this consideration, we have undertaken experiments with heavy ions of high energy, as a means to fulfill the twofold requirement of large penetration depths in the material and higher share of direct displacements over other dissipation events. The specific damage thus obtained was then monitored following the relative yields of two representative centres: (i) the B2 centre, which is supposedly associated with oxygen vacancies originated in direct atomic displacements [ 16,17] and (ii) the E'I centre, which, being one of the dominant products with all types of irradiation, might be ascribed either to atomic displacements [18], or to bond ionizations [10,19,20]. The results on colour centre production are described and compared with those obtained after electron irradiation.
2. Experiments and results Our SiO2 samples were in Pursil-Ultra or Tetrasil-E from Quartz & Silice (France). The two materials differ in maximum OH- content, which is 5 and 200 ppm, respectively. Metallic impurities do not exceed a fraction of ppm. Both qualities are highly transparent in the explored wavelength range, with minor differences in the near u.v. Typical sample dimensions were 5 × 5 mm, with a thickness of 0.2 or 1 mm. Two distinct values of thickness were adopted in order to compare specimens subjected to different conditions of radiation heating. Some of the samples were coated with a 100 A Ag layer to reduce thermal gradients and to prevent space charge and breakdown. They were mounted onto a water-cooled sample holder, using a conductive adhesive. The temperature of the rear surface was approximately 15°C; the temperature of the irradiated surface was monitored by a pyrometric method, indicating that an upper limit of ca. 150°C was not overcome during irradiation. Irradiations were performed in the Variable Energy Cyclotron at AERE (Harwell), using the same facility adopted in a previous work [21 ], first described by Worth [22]. The irradiatiom source was a collimated beam of Ni ÷6 ions with an impinging energy of 46.5 MeV. The irradiation sequences were performed in a rocking mode, which was programmed to obtain total doses going from 0.05 to 1 dpa over an average effective depth of about 11/~m, at beam current densities in the range 4 to 12 × 1011 ions/cm 2 sec. The rocking angles and corresponding doses were computed using the Harwell version of computer code E-DEP-1 [23], simulating the depth distribution of deposited energy, A value E D = 25 eV was assumed in this computation for the displacement energy, in the absence of more stringent experimental or theoretical values for vitreous SiO2, Such a figure is intermediate between those appropriate to crystalline oxides (e.g. MgO [24] and A12Oa [11]) and crystalline quartz [25]. The absorption measurements were made with a Cary 14 spectrophotometer in
323
M. Antonini et al. /Irradiation effects in vitreous silica
0.1 DPA Ag COATED SAMPLE UNCOATED SAMPLE
0.8
/ 0.6 B2
E'I
I
" /I /
0.4"
0.2"
PHOTON ENERGY(¢V)
Fig. 1. Change in optical density induced by irradiation with 46.5 MeV Ni+6 ions in amorphous SiO2 : 0.1 dpa. The full line corresponds to a sample coated with a 100 A silver layer.
the wavelength range between 190 and 2400 nm. The sample mount was designed to house an irradiated sample besides a clear unirradiated sample, so as to allow their alternate positioning within the exploring beam. In this way net coloration spectra were easily taken, minimizing uncertainties due to inaccurate replica of geometrical and reflectance conditions. Figs. I and 2 show typical coloration spectra induced in identical SiO2 samples irradiated at total doses o f 0.1 and 1 dpa. In each figure an uncoated and a silvercoated specimen are directly compared. As a common feature two major absorption bands are observed in the near u.v. at 5.06 eV and 5.76 eV; both are superimposed on a strong absorption tail coming from the more distant u.v. The arrows indicate the position of B2 and E'1 bands as found by previous authors, after irradiations with neutrons and ions of different mass and energy [7,16]. In the main the spectra give sufficient evidence that the two prominent u.v. peaks coincide in all cases with the quoted bands as far as location or shape is concerned; moreover, each of them remains o f comparable strength regardless o f a tenfold difference in total dose. The same qualitative features were observed in all our experiments with heavy ions, irrespective o f sample material (Pursil or Tetrasil) and sample thickness. In particular, saturation was always found to occur even at the lowest doses employed (0.05
324
M. A n t o n i n i et al. / Irradiation effects in vitreous silica 1.2. 1 DPA Ag COATED SAMPLE 1 ......
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Fig. 2. Change in optical density induced by irradiation with 46.5 MeV Ni÷6 ions in amorphous SiO2 : 1 dpa. Adopted symbols are the same as in fig. 1.
dpa). These results are not obscured by peculiar coating effects seen in fig. 2, i.e. the appearance of a broad absorption band in the visible and the increase of the u.v. background, which can be observed in coated samples at the highest doses. The visible absorption is probably due to colloid formation coming from forced imbedding of silver atoms into the matrix, under the influence of primary collisions at the surface [26]. This band does not seem to affect the growth of intrinsic centres, and will not be discussed further, as well as other minor structures detected in the spectrum. However, varying proportions of this band and of the u.v. tail make it difficult to assess the half-widths and intensities of the two major bands concerned. For this reason the observed spectra were subjected to analytic decomposition only in cases where the coating effect was absent or moderate and restricting the analysis to the range above 4 eV. The procedure consisted in fitting experimental spectra with computed spectra which were generated as a superposition of the following components: - an unknown background, two Gaussian-shaped bands of unknown width and intensity located at the peak -
M. Antonini et al. /Irradiation effects in vitreous silica
325
0.8 DOSE = 0.5 DPA o
EXPERIMENTAL POINTS
--
BEST FIT
....
COMPUTED SPECTRAL COMPONENTS
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7 /
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./ •
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B2 and E'l and one Lorentzian tail E. energies of the B: and E'l bands, a Gaussian or, alternatively, a Lorentzian-shaped band of unknown intensity, with position between 7.4 and 7.6 eV and FWHM between 0.8 and 1.2 eV. The latter component should represent the undetected structure giving rise to the u.v. tail, which we tentatively identify with the so-called E band. A twofold possibility was considered for the shape of this band, on account of the fact that the corresponding absorption is found in irradiated [7,15] as well as in unirradiated [27] material, so that its excitonic character cannot be excluded. The specific choice, however, turns out to be practically irrelevant to the determination of B2 and E'I parameters, within our experimental accuracy. The computerized best-fit was actually carried out keeping four of the unknows as free parameters and treating the remaining ones as trial parameters, in different orders. The procedure was applied to all the recorded spectra, with the exception of those belonging to coated samples at the highest dose. An example illustrating the quality of best-fit is given in fig. 3. With a similar procedure the B2 and E'I bands could be resolved for each experiment. The results are given in table 1 summarizing typical intensity values obtained at various doses for the two components. The evaluations of dpa were carried out with the help of the computer program already quoted. The band strengths given in table 1 are expressed either as peak absorption -
326
M. Antonini et al. /Irradiation effects in vitreous silica
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~
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M. A n t o n i n i et al. /Irradiation effects in vitreous silica
!327
coefficients a in cm -~ (derived from optical densities with reference to an average coloration depth of 11/am), or as concentration numbers in cm -3. The latter are obtained assuming a tentative value [ = 0.5 for the oscillator strength of the B2 centre, and the suggested value f = 0.14 for the E'a centre [28]. With this choice one may think that B2 concentrations are not overestimated. In order to have complementary information on the energy dissipation mechanisms, irradiations were also performed in the Van de Graaff electron accelerator available at the Joint Research Centre of Ispra, at an energy of 1.5 MeV and current
Hectl'0ns/cm
1.S
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Fig. 4. Change in optical density after 1.5 MeV electron irradiation of SiO 2 at different temperatures and doses.
328
M. A n t o n i n i et al. / Irradiation effects in vitreous silica
densities between 1.5 and 5/JA cm -2. The samples used in this case were of Tetrasil-E with a thickness of 3 mm. As a consequence of radiation heating, a maximum temperature of about 140°C was reached during the irradiations. By proper combination of beam current and cooling conditions, the irradiation temperature could be kept as low as 40°C. The results of electron experiments are summarized in fig. 4, giving the net coloration spectra obtained with different treatments. As one can notice, the B2 band is absent and the general u.v. background is remarkably reduced. The most prominent effect is the growth of a large E'~ band with a distinct low-energy shoulder at 4.6-4.8 eV, which is not detected in the ion-irradiated samples. This structure might be the so-called Do band found by previous authors [9,13] after 7-irradiation and theoretically discussed by Mott [29]. In the absence of a detailed spectral separation, no quantitative assessments can be made concerning the behaviour of this band. However, preliminary observations seem to indicate that its growth and thermal annealing take place in parallel with those of E'~. Inspection of fig. 4 gives also a picture of dose dependence which is strongly different for experiments conducted at different irradiation temperature. The intensity of the E'1 band at 140°C shows saturated values that are much smaller than those obtained for comparable or smaller doses during irradiation at 40°C. At the lower temperature, the dose dependence is sublinear, with a tendency to saturation at the highest doses. Data concerning non-saturating electron experiments are summarized in the second part of table 1. The evaluation of dpa was carried out using semi-emepirical expressions of the energy loss over the actual range of 3 mmm [30]. The strengths of E'I were taken from the spectra without the help of best-fit procedures, owing to the clean structure of the background and to the large size of this band.
3. Discussion
When examining the results reported in the previous section, a few considerations can be made. First the remarkable yield of the B2 band in the heavy ions case agrees very well with the fact that the amount of energy dissipated in nuclear collisions by the heavy ions was very large, in a rato 105 : 1 with respect to electrons (see table 1). This seems to prove that this band is originated by direct nuclear displacements, as suggested by other authors [16,17]. In order to make such a statement more definite, an assessment of the direct dependence between B: concentrations and dpa values would be appropriate. This implies the extension of our experiment to lower doses, to avoid saturation. Preliminary results obtained after irradiation at 0.01 dpa indicate that at this level saturation begins to be incomplete. A similar correlation might be attempted for the E'~ band. However, one soon notices from table 1 that the concentrations obtained in the heavy ion and in the electron cases differ significantly less than would be suggested by the above quoted
M. Antonini et al. / Irradiation effects in vitreous silica
329
dpa ratio. Furthermore, the number of E'~ centres produced in electron irradiations appears systematically higher than the corresponding number of nuclear displacements. This is especially evident at the lower doses: for instance to a calculated dpa of about 2.5 X 10 -7 corresponds a fractional concentration of 4 × 10 -6 E'I centres. It seems therfore that either the assumed value of the displacement energy is a large overestimate or that the E'1 band is related to several mechanisms, in which direct displacements play a limited role. In the latter case, no direct dependence is to be expected between the defect concentration and dpa and one should look for a more general connection with total energy loss, prior to onset of saturation. Such connection might be tested in parallel for E'~ and Do, in view of their apparent correlation, so as to monitor the effects of ionization and possibly to clarify the role of bond rupture and rearrangement which has been suggested by certain authors [ 10,19,20]. Further information on this point could probably be obtained by exploring the far u.v. region up to 10 eV, whose structure seems remarkably different for the two types of impinging particles.
References [1] W. Primak, The Compacted States of Vitreous Silica (Gordon and Breach, New York, 1975). [2] R. Brtickner, J. Non-Cryst. Solids 5 (1971) 177. [3] E.P. Eernisse, J. Appl. Phys. 45 (1974) 167. [4] B. Bates, R.W. Hendricks and L.A. Shaffer, J. Chem. Phys. 51 (1974) 4163. [5] F. Lell, N.J. Kreidl and J.R. Hensler, Progress Ceram. Sci. 4 (1966) 1. [6] D.L. Griscom, Defects and Their Structure in Non-Metallic Solids, ed. B. Handerson and A.E. Hughes (Plenum Press, New York, 1975) p. 323. [7] E.W.J. Mitchell and E.G.S. Paige. Phil. Mag. 1 (1965) 1085. [8] R.A. Weeks, J. Appl. Phys. 27 (1956) 1376; R.A. Weeks and C.M. Nelson, J. Appl. Phys. 31 (1960) 1555. [9] P.W. Levy, J. Phys. Chem. Solids 13 (1960) 287. [10] C.M. Nelson and J.H. Crawford, J. Phys. Chem. Solids 13 (1960) 296. [11] W.D. Compton and G.W. Arnold, Discuss. Faraday Soc. (G.B.) 31 (1961) 130. [12] D.L. Griscom, E.J. Friebele and G.H. Sigel, Solid State Comm. 15 (1974) 479; also, Proc. Xlth Int. Congress on Glass, Vol. 1 (1977) p. 3. [13] E.J. Friebele, D.L. Griscom and G.H. Sigel, J. Non-Cryst. Solids (1977) 154. [14] D.L. Griscom, Proc. Int. Conf. The Physics of SiO2 and its Interfaces, ed. S.T. Pantelides (Pergamon Press, 1978) p. 232. [15] M. Stapelbroek, D.L. Griscom, E.J. Friebele and G.H. Siegel, J. Non-Crystalline Solids 32 (1979) 313. [16] G.W. Arnold, IEEE Trans. Nucl. Sci. NS-20 (1973) 220. [17] A.J. Cohen, Phys. Rev. 105 (1977) 1151. [18] F.J. Feigl, W.B. Fowler and K.L. Yip, Solid State Commun. 14 (1974) 225. [19] A.R. Ruffa, Phys. Rev. Lett. 25 (1970) 650. [20] G.N. Greaves, p. 268 of the proceedings quoted in ref. [14]. [21] M. Antonini, A. Manara and P. Lensi, p. 316 of the proceedings quoted in ref. [14]. [22] J.W. Worth, The Use of Cyclotron in Chemistry, Metallurgy and Biology (Butterworths, London, 1969).
330 [23] [24] [25] [26] [27]
M. Antonini et al. / Irradiation effects in vitreous silica
M.D. Matthews, AERE-R 7805 (1974). J.V. Sharp and D. Rumsby, Rad. Effects 17 (1973) 65. Gopal Das and T.E. Mitchell, Rad. Effects 23 (1974) 49. G.W. Arnold and J.A. Borders, 1. Appl. Phys. 48 (1977) 1488. A. Appleton, F. Chiranjivi and M. Jafaripur-Ghazvini, p. 94 of the proceedings quoted in ref. [14]. [28 ] R.A. Weeks and E. Sonder, Paramagnetic Resonance II, ed. W. Low (Academic Press, New York, 1963) p. 869. [29] N.F. Mort, Adv. Phys. 26 (1977) 363. [30] W. Corbett, Solid State Phys., Suppl. 7 (Pergamon Press, 1966).
321
HEAVY-ION AND ELECTRON IRRADIATION EFFECTS IN VITREOUS SILICA M. ANTONINI *, P. CAMAGNI, A. M A N A R A and L. MORO Joint Research Centre, 21020 Ispra, Varese, Italy Received 23 October 1979 Revised manuscript received 8 August 1980
Amorphous silica samples have been bombarded with Ni+6 ions of 46.5 MeV to produce a large number of atomic displacements up to about 1 dpa. Other samples with the same impurity content have been irradiated with electrons of 1.5 MeV. Optical absorption spectra induced with these treatments, in the range 190 to 2000 nm, have been compared, focussing the attention on the two prominent bands at 215 and 245 nm observed in the heavy-ion irradiated samples. While the band at 215 rim, identified as the E'I coulour centre, is present after irradiations with both types of particles, the so-called B2 band at 245 nm is totally absent after electron bombardment and therefore it can be attributed to direct atomic displacements. Conversely, the concentration of E'I does not correlate with dpa values; the origin of these centres must therefore be sought in other processes, such as ionization and strained bonds.
1. Introduction The effects o f radiation damage in amorphous silica have been studied b y many authors, with special attention to structural variations [ 1 - 4 ] and defects properties [5,6]. The latter, in particular, have been probed in a number o f colour centre and paramagnetic resonance investigations, covering a variety o f experiments with neutrons, electrons, light ions, X- or "r-rays [ 7 - 1 5 ] . Irradiation effects from low-energy heavy ions have also been reported [16]. In the above studies, particular efforts have been made to identify the primary defects produced b y various forms o f irradiation and to inquire into their formation mechanisms. However, a certain controversy still exists on this subject, due to the lack o f systematic experiments aiming at comparing the two basic aspects o f atomic displacement and b o n d ionization. In order to tackle this problem, it is necessary to use irradiating conditions capable o f affording a substantial increase in the fraction o f displacement processes, as suggested by Arnold [16].
* Also: Gruppo Nazionale Struttura della Materia and lstituto di Fisica dell'Universita, 41100 Modena, Italy. 0 0 2 2 - 3 0 9 3 / 8 1 / 0 0 0 0 - 0 0 0 0 / $ 0 2 . 5 0 © North-Holland
322
M. Antonini et al. / Irradiation effects in vitreous silica
Following this consideration, we have undertaken experiments with heavy ions of high energy, as a means to fulfill the twofold requirement of large penetration depths in the material and higher share of direct displacements over other dissipation events. The specific damage thus obtained was then monitored following the relative yields of two representative centres: (i) the B2 centre, which is supposedly associated with oxygen vacancies originated in direct atomic displacements [ 16,17] and (ii) the E'I centre, which, being one of the dominant products with all types of irradiation, might be ascribed either to atomic displacements [18], or to bond ionizations [10,19,20]. The results on colour centre production are described and compared with those obtained after electron irradiation.
2. Experiments and results Our SiO2 samples were in Pursil-Ultra or Tetrasil-E from Quartz & Silice (France). The two materials differ in maximum OH- content, which is 5 and 200 ppm, respectively. Metallic impurities do not exceed a fraction of ppm. Both qualities are highly transparent in the explored wavelength range, with minor differences in the near u.v. Typical sample dimensions were 5 × 5 mm, with a thickness of 0.2 or 1 mm. Two distinct values of thickness were adopted in order to compare specimens subjected to different conditions of radiation heating. Some of the samples were coated with a 100 A Ag layer to reduce thermal gradients and to prevent space charge and breakdown. They were mounted onto a water-cooled sample holder, using a conductive adhesive. The temperature of the rear surface was approximately 15°C; the temperature of the irradiated surface was monitored by a pyrometric method, indicating that an upper limit of ca. 150°C was not overcome during irradiation. Irradiations were performed in the Variable Energy Cyclotron at AERE (Harwell), using the same facility adopted in a previous work [21 ], first described by Worth [22]. The irradiatiom source was a collimated beam of Ni ÷6 ions with an impinging energy of 46.5 MeV. The irradiation sequences were performed in a rocking mode, which was programmed to obtain total doses going from 0.05 to 1 dpa over an average effective depth of about 11/~m, at beam current densities in the range 4 to 12 × 1011 ions/cm 2 sec. The rocking angles and corresponding doses were computed using the Harwell version of computer code E-DEP-1 [23], simulating the depth distribution of deposited energy, A value E D = 25 eV was assumed in this computation for the displacement energy, in the absence of more stringent experimental or theoretical values for vitreous SiO2, Such a figure is intermediate between those appropriate to crystalline oxides (e.g. MgO [24] and A12Oa [11]) and crystalline quartz [25]. The absorption measurements were made with a Cary 14 spectrophotometer in
323
M. Antonini et al. /Irradiation effects in vitreous silica
0.1 DPA Ag COATED SAMPLE UNCOATED SAMPLE
0.8
/ 0.6 B2
E'I
I
" /I /
0.4"
0.2"
PHOTON ENERGY(¢V)
Fig. 1. Change in optical density induced by irradiation with 46.5 MeV Ni+6 ions in amorphous SiO2 : 0.1 dpa. The full line corresponds to a sample coated with a 100 A silver layer.
the wavelength range between 190 and 2400 nm. The sample mount was designed to house an irradiated sample besides a clear unirradiated sample, so as to allow their alternate positioning within the exploring beam. In this way net coloration spectra were easily taken, minimizing uncertainties due to inaccurate replica of geometrical and reflectance conditions. Figs. I and 2 show typical coloration spectra induced in identical SiO2 samples irradiated at total doses o f 0.1 and 1 dpa. In each figure an uncoated and a silvercoated specimen are directly compared. As a common feature two major absorption bands are observed in the near u.v. at 5.06 eV and 5.76 eV; both are superimposed on a strong absorption tail coming from the more distant u.v. The arrows indicate the position of B2 and E'1 bands as found by previous authors, after irradiations with neutrons and ions of different mass and energy [7,16]. In the main the spectra give sufficient evidence that the two prominent u.v. peaks coincide in all cases with the quoted bands as far as location or shape is concerned; moreover, each of them remains o f comparable strength regardless o f a tenfold difference in total dose. The same qualitative features were observed in all our experiments with heavy ions, irrespective o f sample material (Pursil or Tetrasil) and sample thickness. In particular, saturation was always found to occur even at the lowest doses employed (0.05
324
M. A n t o n i n i et al. / Irradiation effects in vitreous silica 1.2. 1 DPA Ag COATED SAMPLE 1 ......
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,
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/ llll I
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Fig. 2. Change in optical density induced by irradiation with 46.5 MeV Ni÷6 ions in amorphous SiO2 : 1 dpa. Adopted symbols are the same as in fig. 1.
dpa). These results are not obscured by peculiar coating effects seen in fig. 2, i.e. the appearance of a broad absorption band in the visible and the increase of the u.v. background, which can be observed in coated samples at the highest doses. The visible absorption is probably due to colloid formation coming from forced imbedding of silver atoms into the matrix, under the influence of primary collisions at the surface [26]. This band does not seem to affect the growth of intrinsic centres, and will not be discussed further, as well as other minor structures detected in the spectrum. However, varying proportions of this band and of the u.v. tail make it difficult to assess the half-widths and intensities of the two major bands concerned. For this reason the observed spectra were subjected to analytic decomposition only in cases where the coating effect was absent or moderate and restricting the analysis to the range above 4 eV. The procedure consisted in fitting experimental spectra with computed spectra which were generated as a superposition of the following components: - an unknown background, two Gaussian-shaped bands of unknown width and intensity located at the peak -
M. Antonini et al. /Irradiation effects in vitreous silica
325
0.8 DOSE = 0.5 DPA o
EXPERIMENTAL POINTS
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/
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./ •
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Fig. 3. Typical best-fit of colouration spectra of ion-irradiated SiO2, with two Gaussian bands
B2 and E'l and one Lorentzian tail E. energies of the B: and E'l bands, a Gaussian or, alternatively, a Lorentzian-shaped band of unknown intensity, with position between 7.4 and 7.6 eV and FWHM between 0.8 and 1.2 eV. The latter component should represent the undetected structure giving rise to the u.v. tail, which we tentatively identify with the so-called E band. A twofold possibility was considered for the shape of this band, on account of the fact that the corresponding absorption is found in irradiated [7,15] as well as in unirradiated [27] material, so that its excitonic character cannot be excluded. The specific choice, however, turns out to be practically irrelevant to the determination of B2 and E'I parameters, within our experimental accuracy. The computerized best-fit was actually carried out keeping four of the unknows as free parameters and treating the remaining ones as trial parameters, in different orders. The procedure was applied to all the recorded spectra, with the exception of those belonging to coated samples at the highest dose. An example illustrating the quality of best-fit is given in fig. 3. With a similar procedure the B2 and E'I bands could be resolved for each experiment. The results are given in table 1 summarizing typical intensity values obtained at various doses for the two components. The evaluations of dpa were carried out with the help of the computer program already quoted. The band strengths given in table 1 are expressed either as peak absorption -
326
M. Antonini et al. /Irradiation effects in vitreous silica
0
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M. A n t o n i n i et al. /Irradiation effects in vitreous silica
!327
coefficients a in cm -~ (derived from optical densities with reference to an average coloration depth of 11/am), or as concentration numbers in cm -3. The latter are obtained assuming a tentative value [ = 0.5 for the oscillator strength of the B2 centre, and the suggested value f = 0.14 for the E'a centre [28]. With this choice one may think that B2 concentrations are not overestimated. In order to have complementary information on the energy dissipation mechanisms, irradiations were also performed in the Van de Graaff electron accelerator available at the Joint Research Centre of Ispra, at an energy of 1.5 MeV and current
Hectl'0ns/cm
1.S
z)
0,55 x 10 '6
1.10 x I0" o="
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8.00 x 10" 5.50 x I0 'e G.60 x 10 ~
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Fig. 4. Change in optical density after 1.5 MeV electron irradiation of SiO 2 at different temperatures and doses.
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M. A n t o n i n i et al. / Irradiation effects in vitreous silica
densities between 1.5 and 5/JA cm -2. The samples used in this case were of Tetrasil-E with a thickness of 3 mm. As a consequence of radiation heating, a maximum temperature of about 140°C was reached during the irradiations. By proper combination of beam current and cooling conditions, the irradiation temperature could be kept as low as 40°C. The results of electron experiments are summarized in fig. 4, giving the net coloration spectra obtained with different treatments. As one can notice, the B2 band is absent and the general u.v. background is remarkably reduced. The most prominent effect is the growth of a large E'~ band with a distinct low-energy shoulder at 4.6-4.8 eV, which is not detected in the ion-irradiated samples. This structure might be the so-called Do band found by previous authors [9,13] after 7-irradiation and theoretically discussed by Mott [29]. In the absence of a detailed spectral separation, no quantitative assessments can be made concerning the behaviour of this band. However, preliminary observations seem to indicate that its growth and thermal annealing take place in parallel with those of E'~. Inspection of fig. 4 gives also a picture of dose dependence which is strongly different for experiments conducted at different irradiation temperature. The intensity of the E'1 band at 140°C shows saturated values that are much smaller than those obtained for comparable or smaller doses during irradiation at 40°C. At the lower temperature, the dose dependence is sublinear, with a tendency to saturation at the highest doses. Data concerning non-saturating electron experiments are summarized in the second part of table 1. The evaluation of dpa was carried out using semi-emepirical expressions of the energy loss over the actual range of 3 mmm [30]. The strengths of E'I were taken from the spectra without the help of best-fit procedures, owing to the clean structure of the background and to the large size of this band.
3. Discussion
When examining the results reported in the previous section, a few considerations can be made. First the remarkable yield of the B2 band in the heavy ions case agrees very well with the fact that the amount of energy dissipated in nuclear collisions by the heavy ions was very large, in a rato 105 : 1 with respect to electrons (see table 1). This seems to prove that this band is originated by direct nuclear displacements, as suggested by other authors [16,17]. In order to make such a statement more definite, an assessment of the direct dependence between B: concentrations and dpa values would be appropriate. This implies the extension of our experiment to lower doses, to avoid saturation. Preliminary results obtained after irradiation at 0.01 dpa indicate that at this level saturation begins to be incomplete. A similar correlation might be attempted for the E'~ band. However, one soon notices from table 1 that the concentrations obtained in the heavy ion and in the electron cases differ significantly less than would be suggested by the above quoted
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dpa ratio. Furthermore, the number of E'~ centres produced in electron irradiations appears systematically higher than the corresponding number of nuclear displacements. This is especially evident at the lower doses: for instance to a calculated dpa of about 2.5 X 10 -7 corresponds a fractional concentration of 4 × 10 -6 E'I centres. It seems therfore that either the assumed value of the displacement energy is a large overestimate or that the E'1 band is related to several mechanisms, in which direct displacements play a limited role. In the latter case, no direct dependence is to be expected between the defect concentration and dpa and one should look for a more general connection with total energy loss, prior to onset of saturation. Such connection might be tested in parallel for E'~ and Do, in view of their apparent correlation, so as to monitor the effects of ionization and possibly to clarify the role of bond rupture and rearrangement which has been suggested by certain authors [ 10,19,20]. Further information on this point could probably be obtained by exploring the far u.v. region up to 10 eV, whose structure seems remarkably different for the two types of impinging particles.
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M.D. Matthews, AERE-R 7805 (1974). J.V. Sharp and D. Rumsby, Rad. Effects 17 (1973) 65. Gopal Das and T.E. Mitchell, Rad. Effects 23 (1974) 49. G.W. Arnold and J.A. Borders, 1. Appl. Phys. 48 (1977) 1488. A. Appleton, F. Chiranjivi and M. Jafaripur-Ghazvini, p. 94 of the proceedings quoted in ref. [14]. [28 ] R.A. Weeks and E. Sonder, Paramagnetic Resonance II, ed. W. Low (Academic Press, New York, 1963) p. 869. [29] N.F. Mort, Adv. Phys. 26 (1977) 363. [30] W. Corbett, Solid State Phys., Suppl. 7 (Pergamon Press, 1966).