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Post-irradiation kinetics of UV laser induced defects in silica
Nuclear Instruments and Methods in Physics Research B 191 (2002) 401–405 www.elsevier.com/locate/nimb
Post-irradiation kinetics of UV laser induced defects in silica M. Cannas *, S. Agnello, R. Boscaino, S. Costa, F.M. Gelardi Ist. Naionale di Fisica della Mat. and Dipartimento di Scienze Fisiche ed Astronomiche dell’Universita, via Archirafi 36, I-90123 Palermo, Italy
Abstract We report an experimental study on post-irradiation kinetics of point defects generated in natural silica (a-SiO2 ) by UV photons (266 nm) of a Nd:YAG pulsed laser. Isothermal time dependencies of the UV-induced centers were investigated by electron spin resonance spectra recorded at different delays from the laser exposure. Our measurements evidenced two distinct processes active for some hours after the irradiation: the partial annealing of the E0c centers (BSi ) and the increase of the H(II) defects (@Ge AH). The results are discussed in the framework of the models concerning the diffusion and the recombination of atomic and molecular species occurring in the SiO2 matrix. New insights on both the reduction of E0c centers and the growth of H(II) centers are obtained, suggesting a crucial role of the diffusing molecular hydrogen in the structural changes of defects observed in the silica specimens after the UV laser exposure. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Silica; Paramagnetic defects; UV irradiation; Isothermal annealing
1. Introduction The effect of UV laser irradiation in inducing local structural changes (point defects) in wideband gap insulators like amorphous SiO2 (9 eV) is currently an attractive research field owing to the large utilization of silica-based optical components for laser applications [1]. It is well accepted that the generation of defects by UV photons takes place from the transformation of native precursor centers [2–4]. Different channels can activate these conversion processes: directly by the bond cleavage at the precursor site or indi-
*
Corresponding author. Tel.: +39-091-6234220; fax: +39091-6162461. E-mail address: cannas@fisica.unipa.it (M. Cannas).
rectly by trapping of atomic or molecular diffusing species released by the UV radiation. The electron spin resonance (ESR) spectroscopy is one of the most useful tools to investigate UV-induced transformations involving paramagnetic defects, i.e. centers having an unpaired electron. Indeed, as the signals observed in the ESR spectra are often a fingerprint of paramagnetic centers with a well-defined structural model, they could provide information on the nature and the kinetics of structural changes occurring at the sites of these defects in silica matrix [5]. To this aim, in this work we have investigated the effect of UV exposure by a Nd:YAG laser on samples of natural silica, looking at their ESR properties at different delays from the irradiation. Our analysis concerns with the post-irradiation kinetics of two different paramagnetic defects which are induced by UV photons in these silica
0168-583X/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 2 ) 0 0 5 5 1 - 7
402
M. Cannas et al. / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 401–405
specimens: the E0c [6,7] and the H(II) [8,9] centers. The first is generally ascribed to an unpaired electron spin on a Si atom bonded with three oxygen (BSi ), the second is associated with the hyperfine interaction between an electron spin on a Ge atom and a hydrogen nucleus (@Ge AH) giving rise to a doublet with a split of 11.8 mT. Our goal is to elucidate the role of diffusing species in the silica matrix in reacting with these UV-induced centers and in controlling their concentration after the laser irradiation.
2. Experimental method Two natural silica samples, slab shaped with dimensions 5 mm 5 mm 1 mm, were utilized in our experiments: a type I dry, OH content 10 ppm by weight, Infrasil I301 (I301) and a type II wet, OH content 150 ppm, Herasil 1 (H1). Both materials have a nominal Ge concentration of 1 ppm by weight due to the natural contamination of the unfused quartz used in the manufacturing [10]. Before the irradiation, these samples were characterized by the presence of the B-optical activity, consisting in an absorption band at 5.1 eV and two related emission at 3.1 and 4.2 eV that are almost universally associated with a twofold coordinated Ge [11], whereas no ESR signals were observed [12,13]. The UV exposure with 266 nm photons was done at room temperature by the IV harmonic generation of the pulsed radiation of a Nd:YAG laser. Our samples were irradiated with 103 laser pulses at a repetition rate of 10 Hz, each pulse having energy density of 41 mJ/cm2 and 5 ns duration. Isothermal time dependencies at room temperature of the UV-induced paramagnetic centers were investigated by ESR measurements performed by a Bruker EMX spectrometer at 9.8 GHz (X-band), at different delay times from the laser exposure. The E0c resonance line was detected with microwave power P ¼ 0:8 lW and a 100 kHz modulation field of peak-to-peak amplitude Bm ¼ 0:01 mT whereas the ESR signal of H(II) centers was measured with P ¼ 3:2 mW and Bm ¼ 0:4 mT at 100 kHz. In all ESR spectra the scan rate and the time constant of the noise filter were ad-
justed to minimize the ratio of their acquisition time with respect to the delay from the laser irradiation. 3. Results The UV rays effect on the ESR activity is reported in Fig. 1 as detected in our sample I301 at different time delays from the exposure of 1000 laser shots. E0c center resonance lines, recorded 20 min and 20 h after the irradiation, are shown in the central part of the spectrum. We note that the lower noise level of the spectrum after 20 h is due to the use of a filter with time constant four times larger. The comparison between the two ESR profiles evidences that the E0c center signal decreases by a factor 3 on increasing the time delay. In the same figure, the lateral structures split by 11.8 mT identify the H(II) centers whose generation is associated with the partial bleaching of the B-optical activity [13]. Contrary to E0c center, the amplitude of the 11.8 mT doublet increases on increasing the delay. We stress that the same kinetics features were also experienced in the ESR spectra detected in the H1 sample after UV laser exposure.
Fig. 1. ESR spectra detected at room temperature in the I301 sample irradiated by 103 Nd:YAG laser pulses (at 266 nm, pulse duration 5 ns, pulse rate 10 Hz, fluence 41 mJ/cm2 per pulse) showing the 11.8 mT doublet and the E0 center signal recorded at different delays from the laser exposure.
M. Cannas et al. / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 401–405
403
To point out the post-irradiation kinetics of induced E0c and H(II) paramagnetic defects, we report the dependence of the peak-to-peak height of the corresponding ESR signals as a function of the delay from the laser exposure. The kinetics of the E0c centers signal in the I301 and H1 samples is shown in Fig. 2(a) and (b), respectively, for the time interval following the UV irradiation, 103 –106 s. In the I301 sample, the E0c centers ESR amplitude decreases on increasing the delay in a time range 6105 s and tends to saturate down to 40% of the value recorded at 103 s. The partial annealing of E0c centers is also evident in the H1 sample but quantitatively different if compared to I301 for both a lower value of the ESR signal and a more pronounced attenuation down to 20%.
Fig. 3. Amplitude of the 11.8 mT ESR doublet signal plotted versus the time delay from the laser exposure in the samples I301 (a) and H1 (b).
In Fig. 3 it is shown the variation of the 11.8 mT doublet as detected as well in I301 (a) and H1 (b). In both samples the H(II) related ESR signal increases in a time range 6 105 s, after that it tends to a constant value almost comparable for both samples. In this case, we note that the relative growth is more marked in the I301 sample than in the H1 one, about the 200% and the 150% of the value detected at 103 s, respectively.
4. Discussion and conclusions Fig. 2. Kinetics of the E0 centers ESR signal as a function of the time delay from the laser irradiation in the samples I301 (a) and H1 (b).
The effectiveness of the UV exposure by a Nd:YAG laser in inducing paramagnetic centers in silica was evidenced in our ESR experiments
404
M. Cannas et al. / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 401–405
performed in two representative natural samples differing for their OH content. Two kinds of defects are induced by the UV irradiation: E0c and H(II) centers, the latter generated by the photobleaching of the native B-centers. The analysis of the ESR spectra detected at different time delays from the UV laser exposure shows a post-irradiation variation of both related signals effective for 105 s and the observed kinetics can be summarized by the following points: (i) partial annealing of the E0c centers, more pronounced in the sample having higher OH concentration, (ii) growth of the H(II) defects, by a factor 1.5–2 in the investigated time range. To build an understanding of these post-irradiation kinetics, we start from the same interval in which they are completed (105 s). This similarity suggests that the concentration change of both defects are governed by a common process. In this respect, on the basis of the H related structure of the H(II) centers, we can infer that the hydrogen is a good candidate for the occurrence of the amplitude changes of the ESR signals observed in our experiments. Atomic hydrogen H0 can be produced upon UV exposure by radiolysis of O–H or Si–H bonds [14– 19]. In the temperature range close 300 K, owing to the high mobility and reactivity of H0 , the hydrogen can diffuse in the silica matrix in the molecular form resulting from the dimerization, H0 þ H0 ! H2 [20,21]. Different researchers have hypothesized that H2 plays a crucial role in passivating and creating defects in silica [2,14–24]. In particular, it has been proposed that the reduction of E0c centers arises from the interaction of diffusing H2 at the sites of these defects following the reaction [18,22–24]: BSi þ H2 ! BSiAH þ H0
ð1Þ
while the growth of H(II) centers has been associated with the interaction between H2 and a twofold coordinated Ge [9]: @Ge þ H2 ! @Ge AH þ H0
ð2Þ
Thanks to Eqs. (1) and (2) we can qualitatively interpret the post-irradiation kinetics of ESR active centers evidenced in our results. As regards the dependence of the kinetics of the ESR signals on the OH concentration, we note that in a previous paper [13] we showed that the conversion rate from B to H(II) centers, induced by c rays, was found to be proportional to the OH concentration of a given sample. Even if the conversion processes involved in the present investigation are quite different, as they do not occur under the effect of ionizing radiation, it is reasonably to suppose that the OH groups play an indirect role in affecting the post-irradiation kinetics, at least as sources of atomic hydrogen available for the formation of the H2 molecules involved in the reactions (1) and (2). A more quantitative analysis of the reported results, based on both the mobility and the concentration of H2 , and also on the OH and SiH groups present in various silica specimens, is necessary to account for the rate and the extent of variation of paramagnetic centers in our UV irradiated silica samples. Work is in progress to make deeper these aspects.
Acknowledgements The authors wish to thank Professor M. Leone, L. Skuja and A. Trukhin for stimulating discussion. Technical assistance by Mr. G. Lapis is also acknowledged. This work is a part of research project supported by the Ministero Italiano della Ricerca Scientifica e Tecnologica, Rome, Italy. One of the authors (M. Cannas) was partially supported by the Excellence Center CAMART, Institute of Solid State Physics, University of Latvia, agreement no. 12.
References [1] H. Hosono, K. Kawamura, in: G. Pacchioni, L. Skuja, D.L. Griscom (Eds.), Defects in SiO2 and related dieletrics: science and technology, Kluwer Academic Publishers, Dordrecht, 2000. [2] D.L. Griscom, J. Non-Cryst. Solids 73 (1985) 51. [3] H. Nishikawa, R. Nakamura, R. Tohomon, Y. Ohki, Phys. Rev. B 41 (1990) 7828.
M. Cannas et al. / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 401–405 [4] H. Imai, H. Hirashima, J. Non-Cryst. Solids 179 (1994) 202. [5] D.L. Griscom, in: G. Pacchioni, L. Skuja, D.L. Griscom (Eds.), Defects in SiO2 and related dieletrics: science and technology, Kluwer Academic Publishers, Dordrecht, 2000. [6] R.A. Weeks, C.M. Nelson, J. Appl. Phys. 31 (1960) 1555. [7] D.L. Griscom, E.J. Friebele, G.H. Siegel Jr., Solid State Commun. 15 (1974) 479. [8] J. Vitko, J. Appl. Phys. 49 (1978) 3511. [9] V.A. Radtsig, A.A. Bobyshev, Phys. Stat. Sol. (b) 133 (1986) 621. [10] G. Hetherington, K.H. Jack, M.W. Ramsay, Phys. Chem. Glasses 6 (1965) 6. [11] L.N. Skuja, J. Non-Cryst. Solids 239 (1998) 16. [12] M. Leone, S. Agnello, R. Boscaino, M. Cannas, F.M. Gelardi, Phys. Rev. B 60 (1999) 11475.
405
[13] S. Agnello, R. Boscaino, M. Cannas, F.M. Gelardi, M. Leone, Phys. Rev. B 61 (2000) 1946. [14] J.E. Shelby, J. Appl. Phys. 50 (1979) 3702. [15] T.E. Tsai, D.L. Griscom, E.J. Friebele, Phys. Rev. B 40 (1989) 6374. [16] D.L. Griscom, J. Ceram. Soc. Jpn. 99 (1991) 923. [17] R.A.B. Devine, Jpn. J. Appl. Phys. 31 (1992) 4411. [18] H. Imai, K. Arai, H. Hosono, Y. Abe, T. Arai, H. Imagawa, Phys. Rev. B 44 (1991) 4812. [19] H. Imai, H. Hirashima, J. Non-Cryst. Solids 179 (1994) 202. [20] D.L. Griscom, J. Non-Cryst. Solids 68 (1984) 301. [21] L. Verdi, A. Miotello, Phys. Rev. B 47 (1993) 14187. [22] D.L. Griscom, J. Appl. Phys. 58 (1985) 2524. [23] A.H. Edwards, J.A. Pickard, R.E. Stahlbush, J. NonCryst. Solids 179 (1994) 148. [24] V.A. Radtsig, J. Non-Cryst. Solids 239 (1998) 49.
Post-irradiation kinetics of UV laser induced defects in silica M. Cannas *, S. Agnello, R. Boscaino, S. Costa, F.M. Gelardi Ist. Naionale di Fisica della Mat. and Dipartimento di Scienze Fisiche ed Astronomiche dell’Universita, via Archirafi 36, I-90123 Palermo, Italy
Abstract We report an experimental study on post-irradiation kinetics of point defects generated in natural silica (a-SiO2 ) by UV photons (266 nm) of a Nd:YAG pulsed laser. Isothermal time dependencies of the UV-induced centers were investigated by electron spin resonance spectra recorded at different delays from the laser exposure. Our measurements evidenced two distinct processes active for some hours after the irradiation: the partial annealing of the E0c centers (BSi ) and the increase of the H(II) defects (@Ge AH). The results are discussed in the framework of the models concerning the diffusion and the recombination of atomic and molecular species occurring in the SiO2 matrix. New insights on both the reduction of E0c centers and the growth of H(II) centers are obtained, suggesting a crucial role of the diffusing molecular hydrogen in the structural changes of defects observed in the silica specimens after the UV laser exposure. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Silica; Paramagnetic defects; UV irradiation; Isothermal annealing
1. Introduction The effect of UV laser irradiation in inducing local structural changes (point defects) in wideband gap insulators like amorphous SiO2 (9 eV) is currently an attractive research field owing to the large utilization of silica-based optical components for laser applications [1]. It is well accepted that the generation of defects by UV photons takes place from the transformation of native precursor centers [2–4]. Different channels can activate these conversion processes: directly by the bond cleavage at the precursor site or indi-
*
Corresponding author. Tel.: +39-091-6234220; fax: +39091-6162461. E-mail address: cannas@fisica.unipa.it (M. Cannas).
rectly by trapping of atomic or molecular diffusing species released by the UV radiation. The electron spin resonance (ESR) spectroscopy is one of the most useful tools to investigate UV-induced transformations involving paramagnetic defects, i.e. centers having an unpaired electron. Indeed, as the signals observed in the ESR spectra are often a fingerprint of paramagnetic centers with a well-defined structural model, they could provide information on the nature and the kinetics of structural changes occurring at the sites of these defects in silica matrix [5]. To this aim, in this work we have investigated the effect of UV exposure by a Nd:YAG laser on samples of natural silica, looking at their ESR properties at different delays from the irradiation. Our analysis concerns with the post-irradiation kinetics of two different paramagnetic defects which are induced by UV photons in these silica
0168-583X/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 2 ) 0 0 5 5 1 - 7
402
M. Cannas et al. / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 401–405
specimens: the E0c [6,7] and the H(II) [8,9] centers. The first is generally ascribed to an unpaired electron spin on a Si atom bonded with three oxygen (BSi ), the second is associated with the hyperfine interaction between an electron spin on a Ge atom and a hydrogen nucleus (@Ge AH) giving rise to a doublet with a split of 11.8 mT. Our goal is to elucidate the role of diffusing species in the silica matrix in reacting with these UV-induced centers and in controlling their concentration after the laser irradiation.
2. Experimental method Two natural silica samples, slab shaped with dimensions 5 mm 5 mm 1 mm, were utilized in our experiments: a type I dry, OH content 10 ppm by weight, Infrasil I301 (I301) and a type II wet, OH content 150 ppm, Herasil 1 (H1). Both materials have a nominal Ge concentration of 1 ppm by weight due to the natural contamination of the unfused quartz used in the manufacturing [10]. Before the irradiation, these samples were characterized by the presence of the B-optical activity, consisting in an absorption band at 5.1 eV and two related emission at 3.1 and 4.2 eV that are almost universally associated with a twofold coordinated Ge [11], whereas no ESR signals were observed [12,13]. The UV exposure with 266 nm photons was done at room temperature by the IV harmonic generation of the pulsed radiation of a Nd:YAG laser. Our samples were irradiated with 103 laser pulses at a repetition rate of 10 Hz, each pulse having energy density of 41 mJ/cm2 and 5 ns duration. Isothermal time dependencies at room temperature of the UV-induced paramagnetic centers were investigated by ESR measurements performed by a Bruker EMX spectrometer at 9.8 GHz (X-band), at different delay times from the laser exposure. The E0c resonance line was detected with microwave power P ¼ 0:8 lW and a 100 kHz modulation field of peak-to-peak amplitude Bm ¼ 0:01 mT whereas the ESR signal of H(II) centers was measured with P ¼ 3:2 mW and Bm ¼ 0:4 mT at 100 kHz. In all ESR spectra the scan rate and the time constant of the noise filter were ad-
justed to minimize the ratio of their acquisition time with respect to the delay from the laser irradiation. 3. Results The UV rays effect on the ESR activity is reported in Fig. 1 as detected in our sample I301 at different time delays from the exposure of 1000 laser shots. E0c center resonance lines, recorded 20 min and 20 h after the irradiation, are shown in the central part of the spectrum. We note that the lower noise level of the spectrum after 20 h is due to the use of a filter with time constant four times larger. The comparison between the two ESR profiles evidences that the E0c center signal decreases by a factor 3 on increasing the time delay. In the same figure, the lateral structures split by 11.8 mT identify the H(II) centers whose generation is associated with the partial bleaching of the B-optical activity [13]. Contrary to E0c center, the amplitude of the 11.8 mT doublet increases on increasing the delay. We stress that the same kinetics features were also experienced in the ESR spectra detected in the H1 sample after UV laser exposure.
Fig. 1. ESR spectra detected at room temperature in the I301 sample irradiated by 103 Nd:YAG laser pulses (at 266 nm, pulse duration 5 ns, pulse rate 10 Hz, fluence 41 mJ/cm2 per pulse) showing the 11.8 mT doublet and the E0 center signal recorded at different delays from the laser exposure.
M. Cannas et al. / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 401–405
403
To point out the post-irradiation kinetics of induced E0c and H(II) paramagnetic defects, we report the dependence of the peak-to-peak height of the corresponding ESR signals as a function of the delay from the laser exposure. The kinetics of the E0c centers signal in the I301 and H1 samples is shown in Fig. 2(a) and (b), respectively, for the time interval following the UV irradiation, 103 –106 s. In the I301 sample, the E0c centers ESR amplitude decreases on increasing the delay in a time range 6105 s and tends to saturate down to 40% of the value recorded at 103 s. The partial annealing of E0c centers is also evident in the H1 sample but quantitatively different if compared to I301 for both a lower value of the ESR signal and a more pronounced attenuation down to 20%.
Fig. 3. Amplitude of the 11.8 mT ESR doublet signal plotted versus the time delay from the laser exposure in the samples I301 (a) and H1 (b).
In Fig. 3 it is shown the variation of the 11.8 mT doublet as detected as well in I301 (a) and H1 (b). In both samples the H(II) related ESR signal increases in a time range 6 105 s, after that it tends to a constant value almost comparable for both samples. In this case, we note that the relative growth is more marked in the I301 sample than in the H1 one, about the 200% and the 150% of the value detected at 103 s, respectively.
4. Discussion and conclusions Fig. 2. Kinetics of the E0 centers ESR signal as a function of the time delay from the laser irradiation in the samples I301 (a) and H1 (b).
The effectiveness of the UV exposure by a Nd:YAG laser in inducing paramagnetic centers in silica was evidenced in our ESR experiments
404
M. Cannas et al. / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 401–405
performed in two representative natural samples differing for their OH content. Two kinds of defects are induced by the UV irradiation: E0c and H(II) centers, the latter generated by the photobleaching of the native B-centers. The analysis of the ESR spectra detected at different time delays from the UV laser exposure shows a post-irradiation variation of both related signals effective for 105 s and the observed kinetics can be summarized by the following points: (i) partial annealing of the E0c centers, more pronounced in the sample having higher OH concentration, (ii) growth of the H(II) defects, by a factor 1.5–2 in the investigated time range. To build an understanding of these post-irradiation kinetics, we start from the same interval in which they are completed (105 s). This similarity suggests that the concentration change of both defects are governed by a common process. In this respect, on the basis of the H related structure of the H(II) centers, we can infer that the hydrogen is a good candidate for the occurrence of the amplitude changes of the ESR signals observed in our experiments. Atomic hydrogen H0 can be produced upon UV exposure by radiolysis of O–H or Si–H bonds [14– 19]. In the temperature range close 300 K, owing to the high mobility and reactivity of H0 , the hydrogen can diffuse in the silica matrix in the molecular form resulting from the dimerization, H0 þ H0 ! H2 [20,21]. Different researchers have hypothesized that H2 plays a crucial role in passivating and creating defects in silica [2,14–24]. In particular, it has been proposed that the reduction of E0c centers arises from the interaction of diffusing H2 at the sites of these defects following the reaction [18,22–24]: BSi þ H2 ! BSiAH þ H0
ð1Þ
while the growth of H(II) centers has been associated with the interaction between H2 and a twofold coordinated Ge [9]: @Ge þ H2 ! @Ge AH þ H0
ð2Þ
Thanks to Eqs. (1) and (2) we can qualitatively interpret the post-irradiation kinetics of ESR active centers evidenced in our results. As regards the dependence of the kinetics of the ESR signals on the OH concentration, we note that in a previous paper [13] we showed that the conversion rate from B to H(II) centers, induced by c rays, was found to be proportional to the OH concentration of a given sample. Even if the conversion processes involved in the present investigation are quite different, as they do not occur under the effect of ionizing radiation, it is reasonably to suppose that the OH groups play an indirect role in affecting the post-irradiation kinetics, at least as sources of atomic hydrogen available for the formation of the H2 molecules involved in the reactions (1) and (2). A more quantitative analysis of the reported results, based on both the mobility and the concentration of H2 , and also on the OH and SiH groups present in various silica specimens, is necessary to account for the rate and the extent of variation of paramagnetic centers in our UV irradiated silica samples. Work is in progress to make deeper these aspects.
Acknowledgements The authors wish to thank Professor M. Leone, L. Skuja and A. Trukhin for stimulating discussion. Technical assistance by Mr. G. Lapis is also acknowledged. This work is a part of research project supported by the Ministero Italiano della Ricerca Scientifica e Tecnologica, Rome, Italy. One of the authors (M. Cannas) was partially supported by the Excellence Center CAMART, Institute of Solid State Physics, University of Latvia, agreement no. 12.
References [1] H. Hosono, K. Kawamura, in: G. Pacchioni, L. Skuja, D.L. Griscom (Eds.), Defects in SiO2 and related dieletrics: science and technology, Kluwer Academic Publishers, Dordrecht, 2000. [2] D.L. Griscom, J. Non-Cryst. Solids 73 (1985) 51. [3] H. Nishikawa, R. Nakamura, R. Tohomon, Y. Ohki, Phys. Rev. B 41 (1990) 7828.
M. Cannas et al. / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 401–405 [4] H. Imai, H. Hirashima, J. Non-Cryst. Solids 179 (1994) 202. [5] D.L. Griscom, in: G. Pacchioni, L. Skuja, D.L. Griscom (Eds.), Defects in SiO2 and related dieletrics: science and technology, Kluwer Academic Publishers, Dordrecht, 2000. [6] R.A. Weeks, C.M. Nelson, J. Appl. Phys. 31 (1960) 1555. [7] D.L. Griscom, E.J. Friebele, G.H. Siegel Jr., Solid State Commun. 15 (1974) 479. [8] J. Vitko, J. Appl. Phys. 49 (1978) 3511. [9] V.A. Radtsig, A.A. Bobyshev, Phys. Stat. Sol. (b) 133 (1986) 621. [10] G. Hetherington, K.H. Jack, M.W. Ramsay, Phys. Chem. Glasses 6 (1965) 6. [11] L.N. Skuja, J. Non-Cryst. Solids 239 (1998) 16. [12] M. Leone, S. Agnello, R. Boscaino, M. Cannas, F.M. Gelardi, Phys. Rev. B 60 (1999) 11475.
405
[13] S. Agnello, R. Boscaino, M. Cannas, F.M. Gelardi, M. Leone, Phys. Rev. B 61 (2000) 1946. [14] J.E. Shelby, J. Appl. Phys. 50 (1979) 3702. [15] T.E. Tsai, D.L. Griscom, E.J. Friebele, Phys. Rev. B 40 (1989) 6374. [16] D.L. Griscom, J. Ceram. Soc. Jpn. 99 (1991) 923. [17] R.A.B. Devine, Jpn. J. Appl. Phys. 31 (1992) 4411. [18] H. Imai, K. Arai, H. Hosono, Y. Abe, T. Arai, H. Imagawa, Phys. Rev. B 44 (1991) 4812. [19] H. Imai, H. Hirashima, J. Non-Cryst. Solids 179 (1994) 202. [20] D.L. Griscom, J. Non-Cryst. Solids 68 (1984) 301. [21] L. Verdi, A. Miotello, Phys. Rev. B 47 (1993) 14187. [22] D.L. Griscom, J. Appl. Phys. 58 (1985) 2524. [23] A.H. Edwards, J.A. Pickard, R.E. Stahlbush, J. NonCryst. Solids 179 (1994) 148. [24] V.A. Radtsig, J. Non-Cryst. Solids 239 (1998) 49.