- Email: [email protected]
Proton-induced changes of optical properties and defect formation in quartz glasses
Nuclear Instruments and Methods in Physics ResearchB 127/128 (1997) 497-502
NIIIMIB
BOMB-s
Withkhbtnlah&btome
ELSEVIER
Proton-induced changes of optical properties and defect formation in quartz glasses R.R. Gulamova
*,
E.M. Gasanov, R. Alimov
Institute of Nuclear Physics, Vlugbek 702132, Tashkent, Uzbekistan
Abstract Optical absorption and luminescence of three quartz glass types (KI, KV, KU-l) with different impurities exposed to proton fluences of 1013-1016 protons/cm’ were studied. The role of ionisation and elastic energy losses resulting in defect formations along the proton track was considered. On a larger part of the track, for proton energies Ep 2 5 MeV, defect creation is due to ionisation energy losses and the colour and luminescence centers are mainly formed by means of recharge of native defects. As a result on this part of the track the numbers of the [=Si-0 Al=] and E’-centers grow as the proton fluence increases. For proton energies to Ep < 5 MeV the creation of structural defects, like displaced atoms and their vacancies, dominates by means of elastic atom collisions with protons and recoil atoms. This leads to intensive generation of E’-centers and to a more rapid increase of absorption at 215 nm in all glasses and to a destruction of the alumina-alkaline centers in Kl glasses. At the end of the proton track the transformation of [=Si-1, [=Si-O-1 and [=Si-O-Al=] centers into [=Si-HI, [=Si-OH] and [=Si-OH Al=] centers is probable, because the free bonds can be occupied by the stopping protons.
1. Introduction The radiation modification of materials became a perspective method for the purposeful changes of material properties. Owing to the short penetration depth ion beams are especially perspective for studying material surface layers and improvement of the surface properties, as for example, increase of corrosive hardness, ion implantation of semiconductors, etc. For successful change of material properties it is necessary to know the nature of radiationinduced defects and the defect formation mechanisms as a function of radiation type, absorbed energy and initial defects of the irradiated materials. Highly energetic protons passing through materials are known to lose their energy by ionisation and excitation of electrons along the proton track. After lowering the proton energy to a threshold value, depending on the irradiated material, defects are created by elastic atom-proton collisions. Owing to spatial separation of elastic and inelastic energy losses along the proton track the investigation of materials affected by high-energy protons may be useful for studying the role of ionisation and elastic energy losses in radiation-induced defect formation.
Corresponding author. Tel.: (3712) 616474, fax: (3712) 642590, e-mail: [email protected] l
In this paper the nature of defects, the mechanisms of defect formation and the role of ionisation and elastic energy losses am presented. Changes of optical properties of quartz glasses along the proton track are studied.
2. Materials and experimental techniques Three quartz glass types with different impurities were chosen for investigation. The glasses of KI type contain alkaline metal (Na) impurities of 10-3-10-z%, the glasses KV contain Na impurities of 10-3-10-4% and OH-groups of lo-‘%, and the glasses KU- 1 contain 0.1% OH-groups. Irradiation of samples was performed on the U-150 Cyclotron with proton fluences of 10’3-10’6 protons/cm*, a proton flux of 109-10” gorotons/cm* s and proton energies of lo-18 MeV, in Co y-ray sources at dose rates of 20-40 Gy/s and in the reactor WWR-S with fluences of 10’3-10’9 neutrons/cm*. Optical absorption spectra have been measured on the spectrophotometers models “Hitachi” and “Specord MW’, and the infrared (IR) absorption spectra were measured on the spectrophotometer model UR-20. The photoluminescence (PL) spectra have been measured with fluorophotometry attachment at a Hitachi spectrophotometer. The absorbed energy in irradiated samples was calculated according to the formulas described in [l]. The
0168-583X/97/$17.00 0 1997Elsevier Science B.V. All rights reserved. PII 30168-583X(96)01 124-X
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498
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lnstr. and Meth. in Phys. Res. B 127/ I28 (1997) 497-502
proton track length 1 in SiO, was calculated from the table data in [2]. For separate estimation of the ionisation and elastic losses contributing to the radiation-induced defect formation, samples with different thickness T (7 > 1 and r < I) and different proton energies were used. The thickness of samples varied from 1 to 2.5 mm and the proton energies were lo- 18 MeV.
3. Experimental
results
Irradiation of quartz glasses with protons leads to the arise of the same absorption bands with A,, = 550, 300 and 215 nm and of the same luminescence bands with A = 285,396, 450-470,520 and 660 nm as in the cases oyy- and n-y-irradiation. But accumulation kinetics of the related colour and luminescence centers are different and depend on the radiation type and on the initial defects of samples. The dose dependencies of optical density COD) at 215 nm for glasses KU-l with T> I exposed by y-, n-yand proton ( Ep = 18 MeV) radiations are presented in Fig. la, and those for glasses KV exposed by neutrons and
1.00
1
0.80
I-
;I ,,,,&;;,__, ,,,j lE+4
lE+5
lE*6
lE+7
lE+E
Absorbeddoze,Gy
Fig. 1. The OD at 215 nm vs. the absorbed dose and the proton fluence of the y-, n-y- and proton-exposed glasses KU- 1 (a) and KV (b): I = y-exposed samples KU-l; 2 = n--y-exposed samples KU-I and KV; 3 = KU-l and KV glasses exposed by protons with E, = 18 MeV; 4 = KV glasses exposed by protons with E, = 14 MeV. The samples thickness T = 2 mm (T 2 1). 1 Gy = 1.67X 10s proton/cm2 = 1.35X10” neutron/cm2.
09
._.-__! 0.0
04
0.8
1.2
1.6
2.0
Track length, mm
Fig. 2. The OD at 550 nm (2) and 215 (2) vs. the proton track length of quartz glasses KI (T = 2 mm) exposed by protons with EP = 18 MeV and F = 5 X lOI protons/cm*.
protons with Ep= 14 (I= 1.3 mm) and 18 MeV (1=2 mm) are shown in Fig. 1b. Fig. 1 demonstrates the influence of different energy transfer mechanisms on the formation kinetics of E’-centers absorbing at 2 15 nm. At the first stage of irradiation (proton fluences F < 5 X lOI protons/cm2) the E’-center formation kinetics for the y- and proton-exposed samples are close to each other. After further irradiation the E’-center formation kinetics of the proton-exposed samples become close to that of the n-yexposed samples. The OD at 215 nm in proton-exposed samples grows proportional to the proton fluence, similarly to the n-y-exposed samples. However, the E’-center formation rate in the proton-exposed samples KV and KU-l (Fig. 1, lines 3) is lower than that in the n-y-exposed samples (Fig. 1, lines 2). For proton-exposed samples KV the E’-center formation rate is higher than that in glasses KU- 1. The lowering of the proton energy to 14 MeV leads also to a decrease of the E’-center formation rate (Fig. 1b, line 4). The changes of the OD at 215 and 550 nm along the proton track are presented in Fig. 2 for glasses KI exposed by proton fluences of 5 X lOI protons/cm2. As shown in Fig. 2, at the end of the proton track, where the elastic losses prevail, the OD at 550 nm decreases and the OD at 215 nm increases. The absorption band with A, = 550 nm is known to be due to [=Si-0 Al=] hole centers, the precursors of which are the alumina-alkaline centers [3,4]. Consequently, at the end of the proton track (1> 1.7 mm) the destruction of the [-Si-0 Al=] centers by elastic proton-atom collisions dominates. Simultaneously, this process leads to an increase in the El-center concentration. The radiation-induced defect creation is accompanied by changes of the PL spectra. Luminescence bands of the
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Fig. 3. The initial (I) and proton-induced (Z-4) PL spectra of glasses KV and KI: I = initial spectrum, KV; 2 = KV, irradiated by protons with Ep = 18 MeV; 3 = KV, thermal treated at T = 400°C and irradiated by protons with E, = 14 MeV; 4 = KI, thermal treated at T = 400°C and irradiated by protons with Ep=14MeV. initial PL spectra with A_ - 285 and 396 MI (A,,, = 250 nm) are observed in the glasses KI and KV. After irradia-
tion of these glasses by protons with I$, = 18 MeV the intensities of the initial bands decrease till their disappearance, and new bands with A,, = 450-470 and 660 nm (A,,, = 260 nm> appear in the PL spectra.
499
According to literature data the band at 280-285 nm is attributed to the [ESi-Sis] defects (neutral oxygen vacancies) [5] and the band with A,, = 660 nm is due to nonbridging oxygen atoms [61. There is no agreement about the nature of the bands with A_ - 470 and 396 nm. The band at 470 nm was related to the luminescence of oxygen vacancies [7] and two-fold coordinated silicons [8]. This luminescence at 460-470 run also arises in the recombination processes. For the luminescence at 3% run the models of oxygen vacancy (Amosov et al.), impurity of Ge (Carino-Carina) and impurity of Fe were proposed [9]. In Fig. 3 the PL spectra of the initial and irradiated samples KV ( Ep = 18 and 14 MeV) and the irradiated samples KI (E, = 14 MeV) are presented. For initial and irradiated glasses KI (Ep - 18 MeV) the PL spectra are similar to the PL spectra of glasses KV (lines 1 and 2), only the band at 450 nm is shifted to 470 nm and the intensities of the maxima at 285, 470 and 660 nm are higher than in glasses KV. Therefore, these spectra are not shown in Fig. 3. The PL spectra of glasses KU-l contain a wide weak band with A,, = 396 nm ( Aexc= 250 nm). However, after irradiation of these samples by protons with E = 18 MeV the same bands at 660 and 450 nm are found & the PL spectra as in those of glasses KV and KI. The samples KI and KV exposed by protons with Ep = 14 MeV were preliminary thermal treated at T400°C during 15 min (Fig. 3, lines 3 and 4). As seen from Fig. 3, the lowering of the proton energy leads to a shift of the maximum from 450-470 to 420 nm and to the decrease of the band at 660 nm in glasses KI. In glasses KV the band at 660 nm cannot be observed. The change of the
Fig. 4. The IR spectra of the initial (1, 4) and proton-exposed(2, 3, 5) glasses KV and KU-l: I = initial spectrum, KV; 2 = KV, irradiated by F = 5 X 10’4protons/cm2; 3 = KV, irradiated by F = 5 X lOI protons/cm 2; 4 = initial spectrum, KU-l; 5 = KU-l, irradiated by F = 5 X lOI protons/cm2.
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PL spectra may be caused by the increase of elastic losses, resulting in defect formation and by a growth of proton concentration in the exposed layer. The thermal treatment at T- 400°C is not enough for annealing the centers luminous at 470 and 660 nm. For irradiated glasses KI ( Ep = 18 MeV) the luminescence bands at 470 and 660 nm were observed after annealing at the same temperature. Irradiated KV glasses show a band near 520 nm. The nature of the band at 520 nm is not reliable defined yet. It is supposed in [lO,l 1] that the luminescence at 470 and 520 nm of the n-y-exposed samples are due to the interstitial O-- and O;-ions. For clearing up the hydrogen’s role in the radiation-induced defect formation infrared (IR) absorption spectra of glasses KI, KV and KU-l were taken. The presence of hydrogen centers like Si-H and Si-OH causes the appearance of absorption bands at 2270 and 3690 cm-’ in the IK-spectra. The IR-spectra of the initial and proton-exposed samples KV and KU-l are presented in Fig. 4. The IR-spectra of the initial samples of all types of glasses show a wide band with maximum at 2270 cm-‘. As known from the literature, the absorption bands of the overtone of the own vibration of the [=Si-0-Si=] chains and of the valent vibration of the [ =Si-H] bonds appear in this IR-region [ 121. In the hydrogen-containing glasses KV and KU-l the absorption band at 3690 cm-’ additionally appears due to the valent vibration of [=Si-OH] groups. The proton irradiation causes noticeable changes of the IR-spectra, depending on initial defects of glasses. In glasses KV and KI the intensities of the absorption bands at 2270 and 3690 cm-’ decrease and in samples KU-l the as the proton fluence grows. one at 3690 cm-’ Increases . Probably these results may be explained by the competing processes of the destruction of the Si-H and Si-OH bonds and creation of new Si-H and Si-OH bonds taking place in quartz glasses during the proton irradiation. For all types of the proton-exposed glasses a shift of the maximum of band from 2270 to 2230 cm-i is observed.
defects at E, > 5 MeV is hardly probable. As it has been calculated for the y-exposed quartz glasses, the oxygen atom displacement by elastic collision with a secondary electron is possible at electron energies E > 200 keV [ 141. For nonbridging oxygen this electron energy is lower. The maximal energy transferred by moving protons to electrons varies from 11 keV ( Ep = 5 MeV) to 47 keV (E, = 18 MeV). Evidently, this energy is not enough for displacement of bridging and nonbridging oxygen atoms by means of elastic electron-atom collisions. Creation of structural defects by means of elastic collisions of atoms with protons and recoil atoms dominates after lowering the proton energy to E, < 5 MeV. At the end of a proton track the processes of transformation of centers are probable because of occupation of free bonds by the stopping protons. The combination of these processes and the initial structure defects of glasses determine the observed changes of the optical properties. The peculiarities of proton interaction with quartz glasses may explain the E’-center accumuiation kinetics. In samples with r < I, in which ionisation losses prevail, and in the first stage of proton irradiation in samples with r > 1, in which the defects are created by both ionisation and elastic losses, the colour and luminescence centers form at the localizations of charges on the initial defects. This is why the E’-center formation kinetics are analogous in y- and proton-exposed samples. As the proton fluence grows, the elastic collisions contributing to defect formation rises in samples with T> 1 and the colour and luminescence centers form on both the native and proton-induced defects similarly to the n-y-exposed samples (Fig. 1). Based on the experimental results and literature data it may be supposed that at proton irradiation the following main processes take place in quartz glasses. When the glasses KI and KV containing the metallic impurities are irradiated by protons with E, = 18 MeV, hole [=Si-0 Al=] centers are effectively created at a larger part of the track according to the following reaction:
4. Discussion
[=Si - 0 - - MAI=] 5 [-Si - oAl=]
The observed experimental results may be explained by different mechanisms of the proton energy transfer to quartz glasses. As protons pass through materials they lose their energy by elastic and inelastic interactions with glasses. In [13] the threshold value of 5 MeV for the proton energy was calculated. Above this energy the ionisation losses dominate and below it the proton energy is transferred by elastic collisions. The track length of protons with this energy is equal to 0.24 mm. Consequently, for protons with E, = 18 MeV passing through quartz glasses the ionisation losses prevail on a larger part of the proton track and the colour and luminescence centers mainly form by means of recharge of the native defects like the effect of r-irradiation. Formation of new structural
As a result the OD at 550 nm, the maximum of the thermoluminescence (‘IL) band at 460 nm and the TL peak at 360°C increase as the proton fluence grows up to - lOI protons/cm’ [13,14]. This reaction prevails in samples with r < I and at irradiation by high-energy protons. Simultaneously to this reaction the process of destruction of the [-Si-0 Al=] centers occurs because of nonbridging oxygen displacement at proton-nonbridging oxygen atom collisions similar to neutron and high-dose yirradiation [ 141. This process occurs mainly at the end of the proton track when E, < 5 MeV and prevails in samples with r > I. The decrease of the OD at 550 nm and the simultaneous increase in the OD maximum at 215 nm at
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the end of the proton track at I > 1.7 mm seen in Fig. 2 may be explained by this destruction mechanism of this hole center as suggested earlier for the high-dose y-irradiation [ 141.The results represented in Fig. 2 confinn that the nonbridging oxygen is displaced by elastic collisions with protons or with the secondary electrons at the y-irradiation. At the end of proton track the transformation of the [=Si-0-NaAl=] centers into [=Si-O-H Al=] centers is probable because of the substitution of Na+ ions by stopping protons prevailing at the irradiation by the lowenergy protons and in samples with r> 1. The decrease of the OD at 550 nm in samples exposed to protons with E, = 10 (Z = 0.7 mm) and 14 MeV [I31 and the appearance of the weak band at 3690 cm-’ in the IR spectra when the samples are irradiated by F > lOI protons/cm2 may be caused by this process. The destruction of regular [=Si-0-Sic] bonds at the elastic collisions with protons according to reaction: SSi-O-SiE
5 GSi-O+Siz
takes place in all types of glasses and prevails in samples with r > 1. The formation of the nonbridging oxygen atoms and the three-fold coordinated silicon resulting from this reaction may be responsible for the appearance of the PL band with maximum at 660 nm and growth of the absorption band at 215 nm in all glasses. Probably the decrease of the IR absorption band at 2270 cm - ’ in glasses KI is due to this reaction. At the end of the proton track the formation of the [&i-H] and [&i-OH] bonds can occur because the free bonds [=Si-] and [&i-O-_] are occupied by the slowered down protons: +i+H
5 ESi-H
mSi-O+H
> =Si-OH
These processes are especially effective in samples with large concentrations of H- and OH-groups and at irradiation by low-energy protons. The decrease of the absorption at 215 nm in samples KV and KU-l exposed by protons with Er = 10 and 14 MeV may be explained by these reactions (Fig. 1). They also may be responsible for decrease of the luminescence intensity at 660 nm in glasses KI and for its disappearance in glasses KV exposed by protons with E, = 14 MeV (Fig. 3). In the hydrogen-containing glasses KV and KU-l destruction of [=Si-OH] and [-Si-H] bonds occur at the elastic collisions with protons side by side with creation of [=Si-OH] and [-Si-HI centers. Analysis of the IR spectra of glasses KI, KV and KU- 1 allows us to suppose domination of destruction of the hydrogen bonds in glasses KV, which leads to decrease of concentration of the hydrogen bonds and to lowering of the IR band at 2270 and 3690 cm-’ as the proton fluence grows. In glasses KU-l containing more OH-groups than KV-glasses the formation of
501
the [=Si-OH] and [=Si-H] bonds prevails, leading to the decrease of the E-center formation rate (Fig. 1) and to increase of the hydrogen centers and to growth of the absorption at 3690 cm - ‘. Based on the shift of the LR band at 2270-2230 cm-’ in proton-exposed samples it may be supposed that the aggregation of defects changing the mutual arrangement of atoms occurs at proton irradiation in all glasses similar to the n--y-irradiation. Unlike the n-y-irradiation where the aggregation begins at fluences F > 10” neutrons/cm2 while at proton irradiation it begins already at F > 5 X 10i3-1 X lOI protons/cm2.
Conclusions On the part of the proton track with proton energies of E,, > 5 MeV the defect creation is due to ionisation energy losses and the colour and luminescence centers are mainly formed by means of recharge of the native defects. With decreasing of the proton energy to E, < 5 MeV the creation of structural defects by means of elastic atom-proton collisions dominates at the end of the proton track. The occupation of free bonds by the stopping protons occurs at the end of the proton track, leading to transformation of [=Si-01, [=Si] and [+.i-0 Al=] centers into [=Si-OH], [=Si-H] and [=Si-0-HAI=] centers. The destruction of regular [=Si-0-Sic] and defect [=Si-0 Al=], [=Si-OH] and [=Si-H] centers by elastic collisions with protons and center transformations determine the change of optical properties of quartz glasses.
References
[ll S.M. Brekhovskikh, Yu.N. Viktorova, Yu.L. Grinshten and L.M. Laada, in: Bases of Material Sciences of Glasses and Ceramics (Stroyizdat, Moscow, 1971) p, 255. I21Q.F. Nemech and Yu.V. Gofman, in: Reference book on Nuclear Physics (Naukova Dumka, Kiev, 1975) p. 415. [31 A.R. Silin and A.N. Trukhin, in: The Point Defects and Elementary Excitations in Crystalline and Noncrystalline SiO, (Zinatne, Riga, 1985). ]41 A.V. Amosov, G.T. Petrovskii and D.M. Yudin, Dokl. Akad. Nauk S.S.S.R. I88 (1969) 1020.
[51 A.V. Amosov
and G.T. Petrovskii, Dokl. Akad. Nauk S.S.S.R. 268 1 (1983) 66. [61A.R. Silin, L.N. Skuja, A.N. Shendrik, Fiz. Khim. Stekla 4, 4 ( 1978) 405. [71 A.N. Trukhin and L.N. Skuja, Fiz. Khim. Stekloobr. Sist. 63 (1975) 36. 181L.N. Skuja, A.N. Streletsky and A.N. Pakovich, Solid State Commun. 50 12 (1984) 1089. [91 A.N. Trukhin, A.G. Boganov, et al., Fiz. Khim. Stekla 5 (1979) 3.
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[lo] C.E. Jones and D. Embree, J. Appl. Phys. 47 12 (1976) 5365.
[l 11B. Sanaev, G.I. Ikramov, SW. Usmanova, et al., Fiz. Khim. Stekla 3 4 (1977) 376. [I21 A.N. Lazarev, in: The Oscillation Spectra and The Silicate Structure (Nedra, Leningrad, 1968), p. 217.
in Phys. Res. B 127/
128 (1997) 497-502
[13] R.R. Gulamova and E.M. Gasanov, Proc. 9th Int. Conf. on Ion Beam Modification of Materials, Canberra, A.C.T., 199.5, 965. 1141R.R. Gulamova, E.M. Gasanov and E.V. Sazonova, Phys. Status Solidi A 135 (1993) 109.
NIIIMIB
BOMB-s
Withkhbtnlah&btome
ELSEVIER
Proton-induced changes of optical properties and defect formation in quartz glasses R.R. Gulamova
*,
E.M. Gasanov, R. Alimov
Institute of Nuclear Physics, Vlugbek 702132, Tashkent, Uzbekistan
Abstract Optical absorption and luminescence of three quartz glass types (KI, KV, KU-l) with different impurities exposed to proton fluences of 1013-1016 protons/cm’ were studied. The role of ionisation and elastic energy losses resulting in defect formations along the proton track was considered. On a larger part of the track, for proton energies Ep 2 5 MeV, defect creation is due to ionisation energy losses and the colour and luminescence centers are mainly formed by means of recharge of native defects. As a result on this part of the track the numbers of the [=Si-0 Al=] and E’-centers grow as the proton fluence increases. For proton energies to Ep < 5 MeV the creation of structural defects, like displaced atoms and their vacancies, dominates by means of elastic atom collisions with protons and recoil atoms. This leads to intensive generation of E’-centers and to a more rapid increase of absorption at 215 nm in all glasses and to a destruction of the alumina-alkaline centers in Kl glasses. At the end of the proton track the transformation of [=Si-1, [=Si-O-1 and [=Si-O-Al=] centers into [=Si-HI, [=Si-OH] and [=Si-OH Al=] centers is probable, because the free bonds can be occupied by the stopping protons.
1. Introduction The radiation modification of materials became a perspective method for the purposeful changes of material properties. Owing to the short penetration depth ion beams are especially perspective for studying material surface layers and improvement of the surface properties, as for example, increase of corrosive hardness, ion implantation of semiconductors, etc. For successful change of material properties it is necessary to know the nature of radiationinduced defects and the defect formation mechanisms as a function of radiation type, absorbed energy and initial defects of the irradiated materials. Highly energetic protons passing through materials are known to lose their energy by ionisation and excitation of electrons along the proton track. After lowering the proton energy to a threshold value, depending on the irradiated material, defects are created by elastic atom-proton collisions. Owing to spatial separation of elastic and inelastic energy losses along the proton track the investigation of materials affected by high-energy protons may be useful for studying the role of ionisation and elastic energy losses in radiation-induced defect formation.
Corresponding author. Tel.: (3712) 616474, fax: (3712) 642590, e-mail: [email protected] l
In this paper the nature of defects, the mechanisms of defect formation and the role of ionisation and elastic energy losses am presented. Changes of optical properties of quartz glasses along the proton track are studied.
2. Materials and experimental techniques Three quartz glass types with different impurities were chosen for investigation. The glasses of KI type contain alkaline metal (Na) impurities of 10-3-10-z%, the glasses KV contain Na impurities of 10-3-10-4% and OH-groups of lo-‘%, and the glasses KU- 1 contain 0.1% OH-groups. Irradiation of samples was performed on the U-150 Cyclotron with proton fluences of 10’3-10’6 protons/cm*, a proton flux of 109-10” gorotons/cm* s and proton energies of lo-18 MeV, in Co y-ray sources at dose rates of 20-40 Gy/s and in the reactor WWR-S with fluences of 10’3-10’9 neutrons/cm*. Optical absorption spectra have been measured on the spectrophotometers models “Hitachi” and “Specord MW’, and the infrared (IR) absorption spectra were measured on the spectrophotometer model UR-20. The photoluminescence (PL) spectra have been measured with fluorophotometry attachment at a Hitachi spectrophotometer. The absorbed energy in irradiated samples was calculated according to the formulas described in [l]. The
0168-583X/97/$17.00 0 1997Elsevier Science B.V. All rights reserved. PII 30168-583X(96)01 124-X
Bib. PROCESSINGOPTICAL MATERIALS
498
R.R. Guiamova et ai./Nucl.
lnstr. and Meth. in Phys. Res. B 127/ I28 (1997) 497-502
proton track length 1 in SiO, was calculated from the table data in [2]. For separate estimation of the ionisation and elastic losses contributing to the radiation-induced defect formation, samples with different thickness T (7 > 1 and r < I) and different proton energies were used. The thickness of samples varied from 1 to 2.5 mm and the proton energies were lo- 18 MeV.
3. Experimental
results
Irradiation of quartz glasses with protons leads to the arise of the same absorption bands with A,, = 550, 300 and 215 nm and of the same luminescence bands with A = 285,396, 450-470,520 and 660 nm as in the cases oyy- and n-y-irradiation. But accumulation kinetics of the related colour and luminescence centers are different and depend on the radiation type and on the initial defects of samples. The dose dependencies of optical density COD) at 215 nm for glasses KU-l with T> I exposed by y-, n-yand proton ( Ep = 18 MeV) radiations are presented in Fig. la, and those for glasses KV exposed by neutrons and
1.00
1
0.80
I-
;I ,,,,&;;,__, ,,,j lE+4
lE+5
lE*6
lE+7
lE+E
Absorbeddoze,Gy
Fig. 1. The OD at 215 nm vs. the absorbed dose and the proton fluence of the y-, n-y- and proton-exposed glasses KU- 1 (a) and KV (b): I = y-exposed samples KU-l; 2 = n--y-exposed samples KU-I and KV; 3 = KU-l and KV glasses exposed by protons with E, = 18 MeV; 4 = KV glasses exposed by protons with E, = 14 MeV. The samples thickness T = 2 mm (T 2 1). 1 Gy = 1.67X 10s proton/cm2 = 1.35X10” neutron/cm2.
09
._.-__! 0.0
04
0.8
1.2
1.6
2.0
Track length, mm
Fig. 2. The OD at 550 nm (2) and 215 (2) vs. the proton track length of quartz glasses KI (T = 2 mm) exposed by protons with EP = 18 MeV and F = 5 X lOI protons/cm*.
protons with Ep= 14 (I= 1.3 mm) and 18 MeV (1=2 mm) are shown in Fig. 1b. Fig. 1 demonstrates the influence of different energy transfer mechanisms on the formation kinetics of E’-centers absorbing at 2 15 nm. At the first stage of irradiation (proton fluences F < 5 X lOI protons/cm2) the E’-center formation kinetics for the y- and proton-exposed samples are close to each other. After further irradiation the E’-center formation kinetics of the proton-exposed samples become close to that of the n-yexposed samples. The OD at 215 nm in proton-exposed samples grows proportional to the proton fluence, similarly to the n-y-exposed samples. However, the E’-center formation rate in the proton-exposed samples KV and KU-l (Fig. 1, lines 3) is lower than that in the n-y-exposed samples (Fig. 1, lines 2). For proton-exposed samples KV the E’-center formation rate is higher than that in glasses KU- 1. The lowering of the proton energy to 14 MeV leads also to a decrease of the E’-center formation rate (Fig. 1b, line 4). The changes of the OD at 215 and 550 nm along the proton track are presented in Fig. 2 for glasses KI exposed by proton fluences of 5 X lOI protons/cm2. As shown in Fig. 2, at the end of the proton track, where the elastic losses prevail, the OD at 550 nm decreases and the OD at 215 nm increases. The absorption band with A, = 550 nm is known to be due to [=Si-0 Al=] hole centers, the precursors of which are the alumina-alkaline centers [3,4]. Consequently, at the end of the proton track (1> 1.7 mm) the destruction of the [-Si-0 Al=] centers by elastic proton-atom collisions dominates. Simultaneously, this process leads to an increase in the El-center concentration. The radiation-induced defect creation is accompanied by changes of the PL spectra. Luminescence bands of the
RR. Gulamova et al./Nucl.
3w
350
4cQ
450 so0 550 Wave length, nm
6M)
650
In&. and Meth. in Phys. Res. B 127/ 128 (1997) 497-502
7w
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Fig. 3. The initial (I) and proton-induced (Z-4) PL spectra of glasses KV and KI: I = initial spectrum, KV; 2 = KV, irradiated by protons with Ep = 18 MeV; 3 = KV, thermal treated at T = 400°C and irradiated by protons with E, = 14 MeV; 4 = KI, thermal treated at T = 400°C and irradiated by protons with Ep=14MeV. initial PL spectra with A_ - 285 and 396 MI (A,,, = 250 nm) are observed in the glasses KI and KV. After irradia-
tion of these glasses by protons with I$, = 18 MeV the intensities of the initial bands decrease till their disappearance, and new bands with A,, = 450-470 and 660 nm (A,,, = 260 nm> appear in the PL spectra.
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According to literature data the band at 280-285 nm is attributed to the [ESi-Sis] defects (neutral oxygen vacancies) [5] and the band with A,, = 660 nm is due to nonbridging oxygen atoms [61. There is no agreement about the nature of the bands with A_ - 470 and 396 nm. The band at 470 nm was related to the luminescence of oxygen vacancies [7] and two-fold coordinated silicons [8]. This luminescence at 460-470 run also arises in the recombination processes. For the luminescence at 3% run the models of oxygen vacancy (Amosov et al.), impurity of Ge (Carino-Carina) and impurity of Fe were proposed [9]. In Fig. 3 the PL spectra of the initial and irradiated samples KV ( Ep = 18 and 14 MeV) and the irradiated samples KI (E, = 14 MeV) are presented. For initial and irradiated glasses KI (Ep - 18 MeV) the PL spectra are similar to the PL spectra of glasses KV (lines 1 and 2), only the band at 450 nm is shifted to 470 nm and the intensities of the maxima at 285, 470 and 660 nm are higher than in glasses KV. Therefore, these spectra are not shown in Fig. 3. The PL spectra of glasses KU-l contain a wide weak band with A,, = 396 nm ( Aexc= 250 nm). However, after irradiation of these samples by protons with E = 18 MeV the same bands at 660 and 450 nm are found & the PL spectra as in those of glasses KV and KI. The samples KI and KV exposed by protons with Ep = 14 MeV were preliminary thermal treated at T400°C during 15 min (Fig. 3, lines 3 and 4). As seen from Fig. 3, the lowering of the proton energy leads to a shift of the maximum from 450-470 to 420 nm and to the decrease of the band at 660 nm in glasses KI. In glasses KV the band at 660 nm cannot be observed. The change of the
Fig. 4. The IR spectra of the initial (1, 4) and proton-exposed(2, 3, 5) glasses KV and KU-l: I = initial spectrum, KV; 2 = KV, irradiated by F = 5 X 10’4protons/cm2; 3 = KV, irradiated by F = 5 X lOI protons/cm 2; 4 = initial spectrum, KU-l; 5 = KU-l, irradiated by F = 5 X lOI protons/cm2.
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PL spectra may be caused by the increase of elastic losses, resulting in defect formation and by a growth of proton concentration in the exposed layer. The thermal treatment at T- 400°C is not enough for annealing the centers luminous at 470 and 660 nm. For irradiated glasses KI ( Ep = 18 MeV) the luminescence bands at 470 and 660 nm were observed after annealing at the same temperature. Irradiated KV glasses show a band near 520 nm. The nature of the band at 520 nm is not reliable defined yet. It is supposed in [lO,l 1] that the luminescence at 470 and 520 nm of the n-y-exposed samples are due to the interstitial O-- and O;-ions. For clearing up the hydrogen’s role in the radiation-induced defect formation infrared (IR) absorption spectra of glasses KI, KV and KU-l were taken. The presence of hydrogen centers like Si-H and Si-OH causes the appearance of absorption bands at 2270 and 3690 cm-’ in the IK-spectra. The IR-spectra of the initial and proton-exposed samples KV and KU-l are presented in Fig. 4. The IR-spectra of the initial samples of all types of glasses show a wide band with maximum at 2270 cm-‘. As known from the literature, the absorption bands of the overtone of the own vibration of the [=Si-0-Si=] chains and of the valent vibration of the [ =Si-H] bonds appear in this IR-region [ 121. In the hydrogen-containing glasses KV and KU-l the absorption band at 3690 cm-’ additionally appears due to the valent vibration of [=Si-OH] groups. The proton irradiation causes noticeable changes of the IR-spectra, depending on initial defects of glasses. In glasses KV and KI the intensities of the absorption bands at 2270 and 3690 cm-’ decrease and in samples KU-l the as the proton fluence grows. one at 3690 cm-’ Increases . Probably these results may be explained by the competing processes of the destruction of the Si-H and Si-OH bonds and creation of new Si-H and Si-OH bonds taking place in quartz glasses during the proton irradiation. For all types of the proton-exposed glasses a shift of the maximum of band from 2270 to 2230 cm-i is observed.
defects at E, > 5 MeV is hardly probable. As it has been calculated for the y-exposed quartz glasses, the oxygen atom displacement by elastic collision with a secondary electron is possible at electron energies E > 200 keV [ 141. For nonbridging oxygen this electron energy is lower. The maximal energy transferred by moving protons to electrons varies from 11 keV ( Ep = 5 MeV) to 47 keV (E, = 18 MeV). Evidently, this energy is not enough for displacement of bridging and nonbridging oxygen atoms by means of elastic electron-atom collisions. Creation of structural defects by means of elastic collisions of atoms with protons and recoil atoms dominates after lowering the proton energy to E, < 5 MeV. At the end of a proton track the processes of transformation of centers are probable because of occupation of free bonds by the stopping protons. The combination of these processes and the initial structure defects of glasses determine the observed changes of the optical properties. The peculiarities of proton interaction with quartz glasses may explain the E’-center accumuiation kinetics. In samples with r < I, in which ionisation losses prevail, and in the first stage of proton irradiation in samples with r > 1, in which the defects are created by both ionisation and elastic losses, the colour and luminescence centers form at the localizations of charges on the initial defects. This is why the E’-center formation kinetics are analogous in y- and proton-exposed samples. As the proton fluence grows, the elastic collisions contributing to defect formation rises in samples with T> 1 and the colour and luminescence centers form on both the native and proton-induced defects similarly to the n-y-exposed samples (Fig. 1). Based on the experimental results and literature data it may be supposed that at proton irradiation the following main processes take place in quartz glasses. When the glasses KI and KV containing the metallic impurities are irradiated by protons with E, = 18 MeV, hole [=Si-0 Al=] centers are effectively created at a larger part of the track according to the following reaction:
4. Discussion
[=Si - 0 - - MAI=] 5 [-Si - oAl=]
The observed experimental results may be explained by different mechanisms of the proton energy transfer to quartz glasses. As protons pass through materials they lose their energy by elastic and inelastic interactions with glasses. In [13] the threshold value of 5 MeV for the proton energy was calculated. Above this energy the ionisation losses dominate and below it the proton energy is transferred by elastic collisions. The track length of protons with this energy is equal to 0.24 mm. Consequently, for protons with E, = 18 MeV passing through quartz glasses the ionisation losses prevail on a larger part of the proton track and the colour and luminescence centers mainly form by means of recharge of the native defects like the effect of r-irradiation. Formation of new structural
As a result the OD at 550 nm, the maximum of the thermoluminescence (‘IL) band at 460 nm and the TL peak at 360°C increase as the proton fluence grows up to - lOI protons/cm’ [13,14]. This reaction prevails in samples with r < I and at irradiation by high-energy protons. Simultaneously to this reaction the process of destruction of the [-Si-0 Al=] centers occurs because of nonbridging oxygen displacement at proton-nonbridging oxygen atom collisions similar to neutron and high-dose yirradiation [ 141. This process occurs mainly at the end of the proton track when E, < 5 MeV and prevails in samples with r > I. The decrease of the OD at 550 nm and the simultaneous increase in the OD maximum at 215 nm at
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the end of the proton track at I > 1.7 mm seen in Fig. 2 may be explained by this destruction mechanism of this hole center as suggested earlier for the high-dose y-irradiation [ 141.The results represented in Fig. 2 confinn that the nonbridging oxygen is displaced by elastic collisions with protons or with the secondary electrons at the y-irradiation. At the end of proton track the transformation of the [=Si-0-NaAl=] centers into [=Si-O-H Al=] centers is probable because of the substitution of Na+ ions by stopping protons prevailing at the irradiation by the lowenergy protons and in samples with r> 1. The decrease of the OD at 550 nm in samples exposed to protons with E, = 10 (Z = 0.7 mm) and 14 MeV [I31 and the appearance of the weak band at 3690 cm-’ in the IR spectra when the samples are irradiated by F > lOI protons/cm2 may be caused by this process. The destruction of regular [=Si-0-Sic] bonds at the elastic collisions with protons according to reaction: SSi-O-SiE
5 GSi-O+Siz
takes place in all types of glasses and prevails in samples with r > 1. The formation of the nonbridging oxygen atoms and the three-fold coordinated silicon resulting from this reaction may be responsible for the appearance of the PL band with maximum at 660 nm and growth of the absorption band at 215 nm in all glasses. Probably the decrease of the IR absorption band at 2270 cm - ’ in glasses KI is due to this reaction. At the end of the proton track the formation of the [&i-H] and [&i-OH] bonds can occur because the free bonds [=Si-] and [&i-O-_] are occupied by the slowered down protons: +i+H
5 ESi-H
mSi-O+H
> =Si-OH
These processes are especially effective in samples with large concentrations of H- and OH-groups and at irradiation by low-energy protons. The decrease of the absorption at 215 nm in samples KV and KU-l exposed by protons with Er = 10 and 14 MeV may be explained by these reactions (Fig. 1). They also may be responsible for decrease of the luminescence intensity at 660 nm in glasses KI and for its disappearance in glasses KV exposed by protons with E, = 14 MeV (Fig. 3). In the hydrogen-containing glasses KV and KU-l destruction of [=Si-OH] and [-Si-H] bonds occur at the elastic collisions with protons side by side with creation of [=Si-OH] and [-Si-HI centers. Analysis of the IR spectra of glasses KI, KV and KU- 1 allows us to suppose domination of destruction of the hydrogen bonds in glasses KV, which leads to decrease of concentration of the hydrogen bonds and to lowering of the IR band at 2270 and 3690 cm-’ as the proton fluence grows. In glasses KU-l containing more OH-groups than KV-glasses the formation of
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the [=Si-OH] and [=Si-H] bonds prevails, leading to the decrease of the E-center formation rate (Fig. 1) and to increase of the hydrogen centers and to growth of the absorption at 3690 cm - ‘. Based on the shift of the LR band at 2270-2230 cm-’ in proton-exposed samples it may be supposed that the aggregation of defects changing the mutual arrangement of atoms occurs at proton irradiation in all glasses similar to the n--y-irradiation. Unlike the n-y-irradiation where the aggregation begins at fluences F > 10” neutrons/cm2 while at proton irradiation it begins already at F > 5 X 10i3-1 X lOI protons/cm2.
Conclusions On the part of the proton track with proton energies of E,, > 5 MeV the defect creation is due to ionisation energy losses and the colour and luminescence centers are mainly formed by means of recharge of the native defects. With decreasing of the proton energy to E, < 5 MeV the creation of structural defects by means of elastic atom-proton collisions dominates at the end of the proton track. The occupation of free bonds by the stopping protons occurs at the end of the proton track, leading to transformation of [=Si-01, [=Si] and [+.i-0 Al=] centers into [=Si-OH], [=Si-H] and [=Si-0-HAI=] centers. The destruction of regular [=Si-0-Sic] and defect [=Si-0 Al=], [=Si-OH] and [=Si-H] centers by elastic collisions with protons and center transformations determine the change of optical properties of quartz glasses.
References
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