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Luminescence of fluorine doped silica glass
Journal of Non-Crystalline Solids 332 (2003) 219–228 www.elsevier.com/locate/jnoncrysol
Luminescence of fluorine doped silica glass A.N. Trukhin a
a,*
, J. Jansons a, T.A. Ermolenko b, I.I. Cheremisin
b
Solid State Physics Institute, University of Latvia, Kengaraga 8, LV-1063 Riga, Latvia b Institute of Silicate Chemistry, RAS, St-Petersburg, Russia Received 18 February 2003
Abstract The role of fluorine doping on silica properties was studied by luminescence methods. Non-doped samples of the same preparation technology possess an absorption band at 7.6 eV on the level of 2 cm1 . A trace of this band in the fluorine-doped sample is on the level of 0.1 cm1 . In both samples 7.6 eV photons as well as ionizing irradiation (X-ray, electron beam) excite photoluminescence of so-called oxygen deficient centers with a blue (2.7 eV) and a UV band (4.4 eV). The luminescence of the fluorine doped sample increases with dose many times from the initial low level for the same excitation. Also, thermally stimulated luminescence appears after irradiation. The energetic yield under ionizing irradiation of induced luminescence is the same level as reference samples. The decay kinetics of cathodoluminescence show that the blue band decays faster and the UV band decays more slowly than was known for the oxygen deficient center. It is concluded that structural imperfections, responsible for the absorption band at 7.6 eV, remain similar in silica glasses during preparation; however fluorine changes the electronic transition nature and, therefore, the absorption band is of low intensity. Such imperfections, passivated with fluorine, interact with electronic excitation produced by radiation. Transient changes of imperfections geometric and electronic configurations take place with probable removal of fluorine, and that provides a growth of the luminescence centers even at low temperature and changes the decay relatively to intra-center luminescence. Therefore the fluorine-doped sample is similar to the nondoped samples through corresponding recombination luminescence of oxygen deficient centers. Ó 2003 Elsevier B.V. All rights reserved. PACS: 78.55.)Hx; 42.70.Ce
1. Introduction The fluorine-doped silica raises interest due to significant improvement of the optical transmission in the VUV range, which allows improvement of the silica application. The role of fluorine needs
*
Corresponding author. Tel.: +371-7 260 686; fax: +371-7 132 778. E-mail address: [email protected] (A.N. Trukhin).
detailed studies. It was found [1], that absorption band at 7.6 eV, almost absent in fluorine doped sample, could be restored to the usual level of slightly oxygen deficient samples by heating at 1000 °C during several days in helium atmosphere. In Ref. [1] the effect was explained as creation of Si–Si bonds from Si–F and collection F2 molecules in interstitial under influence of high temperature. Another explanation of this effect could be as evaporation of fluorine from defect responsible for the band at 7.6 eV and collection as F2 molecules
0022-3093/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2003.09.014
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in interstitial. The band could be restored without deep structural transformations. This point of view means lack of significant structural changes during thermal treatments. This controversy could be checked by detection of defects at room or even low temperatures, where significant structural changes less possible. The luminescence is one of convenient methods for that. It allows one to detect dynamic process of defects transformation during excitation. We have chosen X-ray, cathodo and photoexcitation for detection of luminescence defect creation from precursors. Let us remember that defects, after definitions, are structural imperfections detectable spectrally, or by other means. Precursors of defects also are structural imperfections, for which electronic transitions are covered by intrinsic absorption. Therefore such precursors could be detected only after their transformation to the defects. The absorption band at 7.6 eV is related to oxygen deficiency (oxygen deficient center (I) or ODC(I)) [1–4]. Another oxygen deficient center is lone twofold-coordinated silicon center or ODC(II) in dry silica [5]. One center gives two luminescence bands: blue one at 2.7 eV, due to triplet–singlet transitions and UV one at 4.4 eV due to singlet–singlet transitions. In the case of ODCD(II) center the blue luminescence time constant is 10 ms and that of UV band is 4.5 ns [5]. The relation between these bands is not stable and is depending on many parameters. In ODC(II) conversion processes from excited singlet level to triplet determine this relation. In ODC(I) this relation additionally is affected by samples preparation history (see, for example, Refs. [1–4]). The nature of the ODC(I) and correspondingly of the absorption band at 7.6 eV is the object of different interpretations. One of them is oxygen vacancy in short-range order with tetrahedron structure, for example, [1,2]. Another is interpretation of this band as oxygen deficiency in octahedron short-range order [3,4]. Let remind that glass structure could be imagined as many different kinds of structural motifs of short-range order and each motif could correspond to material ability exist in crystalline polymorph modifications. Silicon dioxide is characterized by huge number of polymorph
modifications. The main tetrahedron structure in short-range order of silicon dioxide provides equal possibilities for defect structure in crystal and glass. Indeed, the most investigated E0 -center possesses very similar properties in crystalline aquartz and silica glass. Contrary situation with other defects, and, for example the oxygen deficient center with corresponding luminescence bands cannot be observed in a-quartz crystal. Only after crystal converting to amorphous state by neutron irradiation were similar defects detected [6]. On the other hand, localized states of wide gap oxide glasses [4,7] were determined as minority structural modifications in short-range order of a glass. The most significant situation is for germanium dioxide case. The germanium dioxide (strongly oxidized) glass possesses two intrinsic absorption tails: one the same as GeO2 crystal with a-quartz structure at 6 eV and another at 4.5 eV as rutile-type structured crystal [8]. The last determines luminescence of localized states [8,9]. The oxygen deficient GeO2 glass reveals similar ODC(II) luminescence as silica glass doped with germanium [10]. The situation in glassy SiO2 about role of polymorph modifications is not so definitive. Seems, analogous absorption of octahedron corresponding motifs is situated at higher energy than that of tetrahedron motifs. Nevertheless, the case of oxygen deficiency is similar for both kinds of material SiO2 and GeO2 [4]. So, we bind the 7.6 eV absorption band in silica with octahedron motifs in short-range order, which are affected by oxygen deficiency [3,4]. We propose that fluorine passivation just changes electronic states of those localized states modified by oxygen deficiency and the intensity of absorption band at 7.6 eV becomes reduced. Changed electronic transitions could correspond to higher energy above the intrinsic absorption threshold. The concentration of such states is depending on preparation prehistory and should be similar in samples with and without of fluorine. The luminescence of oxygen deficient silica was studied in many publications [2–5,7–11]. The creation of luminescence oxygen deficient centers from precursors was proved by study of dose de-
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pendences under electron beam excitation [12–14], and X-ray excitation [15,16]. The process of luminescence excitation under ionizing irradiation is very complex and involves recombination of previously created centers, which living time is from few nanoseconds to infinity (the last corresponds to permanently existing induced centers). So, in the case when distorted structure passivated with fluorine (structural imperfections) exists, we should observe the same luminescence as in fluorine-free sample, when we could provide dynamic release of the fluorine, permanent or transient. Therefore now we compare luminescence properties of pure silica with and without fluorine.
2. Experimental The sample for investigation was pure silica (the level of metallic impurities is about 106 wt%) with and without doping of fluorine. The samples were produced as KC-4B (or the same KS-4V) dry silica with difference that F-doping was realized by fusing in pure SiF4 atmosphere. The concentration of fluorine in doped sample is on the level of 0.1 wt%. The detected intensity of OH group absorption is on the level of 7.5 ppm in doped sample. However, no red luminescence was detected in the sample under X-ray irradiation at 80 K; therefore the influence of OH group on processes could be accounted as negligible. Photoluminescence experiment was performed with the use of deuterium continuous discharge source as well as of spark discharge in helium– hydrogen gas mixture. In the last case, the pulses with 6 ns duration on half height of intensity were obtained, however with a long (several hundreds of ns) tail. The 0.5 m Seya–Namioka vacuum monochromator with toroidal concave grating was used. The sample was kept in cryostat with nitrogen screen, which allows one to avoid condensation of contamination film on the sample surface at low temperatures. Cathodoluminescence (CL) spectra and time resolved experiments were conducted by means of pulsed electron beam equipment with electron energies 6 keV, 0.2–1 ls pulse duration. The fast cut of the pulse (about 1 ns) provides possibility to
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measure fast kinetics. From these data we deduced the luminescence kinetics. An X-ray tube with W anticathode (15 mA, 40 kV) was used to study luminescence under ionizing radiation (details of experiment in Ref. [17]). Luminescence was analyzed by a grating monochromator and detected by a photomultiplier. The decay kinetics were recorded with a 256channel analyzer. The measured curves are presented in figures as received therefore they reflect the level of errors. The sodium salicylate double recrystallized was used as reference standard for photoluminescence measurements. Its luminescence quantum yield is about 0.5 independent of photon energy from about 4–2000 eV and, consequently, it used to correct excitation curves on spectral distribution of excitation light. Nevertheless, the luminescence intensity is presented in arbitrary units because it was not our task to determine its absolute quantum yield. When intensities are compared, then presented data were measured in the same geometry and the same conditions.
3. Results The absorption spectra of studied samples are presented in Fig. 1. It could be seen that fluorinedoped sample possesses good optical transparency in the range of 7.6 eV (the trace of absorption is on the level of 0.1 cm for possible band there); however this absorption band is well defined in the reference-undoped sample. Absorption changes with vary of the temperature with check of Urbach approximation for sample 5-mm-thick are presented in Fig. 2. It is seen that Urbach approximation is holding for fluorine glass, as it was known for non-doped glass [18,19] (see Fig. 3); however the form of Urbach rule in the case of fluorine doped silica corresponds to Ôglass-likeÕ [20,21] with parallel shift of absorption in semi-log scale rather than usual for silica glass Ôcrystal-likeÕ (Fig. 3) with cross-point. The intrinsic absorption of 5-mm-thick sample obeys to Urbach rule in the form usual for disordered materials or Ôglass-likeÕ: a ¼ I0 exp½AE þ T =T1 ;
ð1Þ
A.N. Trukhin et al. / Journal of Non-Crystalline Solids 332 (2003) 219–228 ASORPTION COEFFICIENT (cm-1)
222 12 10
Absorption spectrum of silica glass with fluorine compared with undoped sample T=290 K
8 6
without F
4 2
F
0 5
6
7
8
PHOTON ENERGY (eV)
ln Absorption coefficient (cm-1)
Fig. 1. Optical absorption spectra of silica glass samples with fluorine (F) and without fluorine doping, both made by KC-4B technology.
14 12
Urbach in silica glass with fluorine
10 8 6 4 2
405 K
0 520 K -2 -4
343 K 297 K
7.6
7.8
8.0
8.2
8.4
8.6
8.8
PHOTON ENERGY (eV)
ln Absorption coefficient (cm-1)
Fig. 2. Urbach approximation of absorption spectrum of the fluorine-doped silica. Points – result of measurements, lines – calculation after expression (1).
15
Urbach in pure silica glass (light deficit of O)
10 5 0
370 K 410 K
7.6
7.8
295 K
8.0 8.2 8.4 8.6 PHOTON ENERGY (eV)
8.8
Fig. 3. Urbach approximation of absorption spectrum of the fluorine non-doped silica. Points – result of measurements, lines – calculation after expression (2).
where A is a parameter of slope ( 7 eV1 for F-doped sample) and T1 ( 123 K for F-doped sample) is a characteristic temperature. The E is photon energy. The semi-logarithmic spectrum moves parallel to itself with change of the temperature. The Ôcrystal-likeÕ Urbach rule corresponds, as was explained by Toyozawa [22], to that of creation of the momentarily self-trapped exciton (STE): a ¼ a0 exp½rðE0 EÞ=kT ;
ð2Þ
r ¼ r0 2kT =hx tanh hx=2kT :
ð3Þ
The parameters a0 , E0 correspond to position of the cross-point, r is slope parameter, r0 parameter is reciprocal of exciton–phonon interaction strength, hx is the energy of phonon. The obtained parameters for reference-undoped sample are in good agreement with previously published (a0 106 cm1 , E0 8:7 eV for reference sample, Fig. 3). So, the fluorine doping strongly affects even host material electronic states. However, theoretical explanation of Ôglass-likeÕ is far from the crystal case. We were able to detect photoluminescence in fluorine-doped sample under continuous excitation, in spite of low level of absorption (Fig. 4). It is need to underline that in reference sample, containing well detectable absorption band at 7.6 eV, the same luminescence could be excited by photons with this energy (7.6 eV), Fig. 4. Two broad luminescence bands, blue and UV are detected at room and low (77 K) temperatures in both studied samples. Corresponding excitation spectra are presented in Fig. 4. The high energy cutoff in excitation band well corresponds to the position of Urbach threshold. The shift to high energy at low temperature corresponds to position of absorption threshold at low temperature. So, at the range of Urbach-tail no specific photoluminescence was detected. The excited luminescence corresponds to extrinsic absorption and to host material defects. In reference-undoped sample different relation between two bands was observed than in fluorine-doped sample. The UV band intensity in fluorine-doped sample prevails the blue band, whereas in non-doped sample blue band prevail UV band, Fig. 4. So, in both cases the main
Photoluminescence of silica glass with fluorine PLE, 4.4 eV hvexc =7.7 eV PL, T=77 K
8
T=77 K
6
hvexc=7 eV
4 x10 293 K
2 0
without F
2
3
4
5 6 7 PHOTON ENERGY (eV)
8
9
Fig. 4. Optical absorption spectra of silica glass samples with fluorine (F) and without fluorine doping, both made by KC-4B technology. Photoluminescence measurements of the fluorinedoped silica. Undoped sampleÕs data are presented for comparison. The dispersion of points reflects measurement errors.
LUMINESCENCE INTENSITY (arb.units)
luminescence bands are due to oxygen deficient defects. The dose kinetics of UV band growth during irradiation by 7.7 eV photons are presented in Fig. 5. When detection-step time is 1 s, the growth is about three times from initial level. Low yield of luminescence does not allow to measure dose kinetics with shorter detection time that to resolve initial growth. Thus, the luminescence centers are produced by irradiation. A decrease followed shows also on the radiation destruction of created center and on the complicated processes taken place during irradiation. We were trying to measure decay kinetics of luminescence in fluorine doped sample with the low intensity spark pulsed
700 600 500 400 300 200
Dose dependence of 7.7 eV excited photoluminescence (band at 4.4 eV) of fluorine-doped silica glass sample. T=80 K
100 0
0
500
1000
1500
TIME (s)
Fig. 5. Photoluminescence dose dependence of the fluorinedoped silica. The dispersion of points reflects measurement errors.
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light source. Experiment was realized with photon counting system. The decay curves could be collected during many hours, that means high sensitivity of the system. Nevertheless, we failed to get a measurable signal. Lack of signal under spark excitation and existence of luminescence under continuous excitation means lack of center excitable in intra-center process. Therefore the luminescence could not be excited in pure intra-center process because there is no luminescence centers. They appear only under continuous irradiation, probably in multi-steps process. The case is very similar to luminescence properties of hydrogen treated oxygen-deficient samples [12], where the luminescence centers, existing before treatment, are destroyed by hydrogen and renew only under irradiation at low temperature because of transient removal of hydrogen from defect and followed restoration of the luminescence ability. Use of ionizing irradiation reveals luminescence yield (energetic yield – emitted energy per absorbed energy) of fluorine doped sample on the level of ordinary pure silica. In Fig. 6 the X-ray excited luminescence of compared samples are presented. The spectral contents are similar, as well as levels of luminescence intensities are the same (even in the case of F-doped sample is higher). In the visible spectral range there could be influence from STE luminescence at low temperatures [17] but the existence of the UV band in fluorine-doped sample unequivocally shows on luminescence center appearance under ionizing LUMINESCENCE INTENSITY (arb.units)
10
absorption 293 K
Photoluminescence intensity (arb.units) Absorption coefficient (cm-1)
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25 X-ray excited spectra of silica samples.
20 F, 80 K
15 10 F, 180 K
5 no F,80 K no F, 180 K
0 2
3
4
5
PHOTON ENERGY (eV)
Fig. 6. X-ray excited spectra of silica glass samples with fluorine (F) and without fluorine doping (no F) at different temperatures.
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LUMINESCENCE INTENSITY (arb.units)
irradiation analogously to irradiation by 7.7 eV photons. So, in spite of low level of absorption and expected on that low imperfection concentration, the luminescence of fluorine-doped sample reveals the same quantity of imperfection taken part in radiative recombination (of the same nature as in fluorine non-doped sample) at low temperature, when possible transformation of defects could not be related to serious changes of imperfection structure. Only ions or atoms not involved in network of glass (mobile ions) [23] could be responsible for imperfections transformation to luminescence defects. The dose dependences under X-ray irradiation are presented in Fig. 7. It could be seen that luminescence in both samples performs complicated creation–destruction kinetics. Initially both bands, blue and UV, grow from almost zero level (in this case of sufficient level of yield we were able to use minimal detection step 0.1 s), and then, after few minutes of irradiation, they have a decrease in both kind of samples followed with increase in half-hour time of irradiation. This situation is also analogous to the case of photoexcitation. That also shows on small influence of STE luminescence, which should appear under irradiation without delay. Such kinetics, with so-call turnaround behavior, were observed for irradiation of silicon dioxide film with electron beam; see for example Refs. [13,14]. That was explained before
2.0 F, 4.4 eV
1.5
F, 2.7 eV :10
1.0
no F, 4.4 eV
0.5 0.0
Dose dependence of x-ray excited luminescence of silica glass samples. T=80 K
0
500
1000
1500
2000
2500
3000
TIME (s)
Fig. 7. X-ray excited dose dependences of luminescence intensities for blue (2.7 eV) and UV (4.4 eV) bands of silica glass samples with fluorine (F) and without fluorine (no F) doping. The lines are drawn as a guide for the eye. The dispersion of points reflects measurement errors.
as creation–destruction of the defects from precursors with participation of some mobile ions (interstitial oxygen, hydrogen, chlorine and, in our case, also fluorine) [23]. Initially, luminescence intensity is almost zero and that could correspond to low concentration of luminescence centers and, therefore, there is a creation process. The creation process is related to luminescence centersÕ appearance from so-call precursors or others words localized states of a glass, existing because of disordered state. The band at 7.6 eV is related to localized states modified by oxygen deficiency [3,4,11]. Fluorine, probably, changes electronic transitions of localized states and absorption band at 7.6 eV could not be recorded directly by absorption measurement, however through luminescence center revealing it could be detected on excitation spectra. If we stop irradiation, then some part of non-recombined traps remains, and we observe thermally stimulated luminescence (Fig. 8). The position of the TSL peaks well corresponds to the self-trapped holes thermal release [24]. Then we can conclude that STH are connected to the areas of localized states. The luminescent content of TSL corresponds to luminescence related to oxygen deficiency (the blue and UV bands). Then there is no difference in luminescence of silica both doped and non-doped with fluorine. In both cases we see luminescence connected to oxygen deficiency. The fact that there is no ODC(I) band in
LUMINESCENCE INTENSITY (arb.units)
224
75 Thermally stimulated luminescence of of x-ray irradiated fluorine-doped silica samples.
50 2.7 eV
25
4.4 eV
0 50
100
150
200
250
300
TEMPERATURE (K)
Fig. 8. Thermally stimulated luminescence curves for blue (2.7 eV) and UV (4.4 eV) bands of silica glass samples with fluorine (F) doping. Sample was irradiated during 2 h at 80 K with X-ray. The dispersion of points reflects measurement errors.
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CL decay (blue band) kinetics of silica glasses, T = 80 K
4
10
t-1.2
}
3
t-0.3
}
10
sample with fluorine
pure sample
2
10
1
10
0
10
-2
10
-1
10
0
1
10 10 TIME (µs)
2
10
10 CL (UV band) decay kinetics of silica glasses T = 80 K
9 8 7 6
cut off beam pulse
pure
5
10000 CL INTENSITY (arb.units)
t-0.6
}
CL INTENSITY (arb.units)
5
10
decay becomes once again t0:3 , which usually corresponds to a initiation of another recombination process, which we, actually, could not detect due to apparatus limitation in slow part of decay. We explain such rich kinetic by removal, transportation on different distance and localization on different wells of ions, where fluorine is expected to be principal. We expect that removal of fluorine from the precursor restores ability of defect participates in recombination processes with luminescence. Fluorine or other ions captured nearby are working as quenching factor, diminishing ÔnaturalÕ lifetime. We did not detect ÔnormalÕ intracenter decay; then recombination takes place in transient center of luminescence, that is in center created for short time, which could not be destroyed in excited state by nearest ions or atoms. The decay of reference sample is similar, but power law is a bit different. Then similar processes take place in reference-undoped sample however with different mobile ions participation. The decay kinetics curves for UV band are compared in Fig. 10. In this case the decay slower than intra-center process is determined mainly by recombination parameters. Surrounding perturbations are influencing in very short time. The UV band of ODC(II) is very fast and is about 4.5 ns of non-perturbed by surrounding transitions. In CL we also observe a fast decay, which is probably ln LUMINESCENCE INTENSITY (arb.units)
F-doped sample, but we still have luminescence as we have this band, show that fluorine does not change structure of ODC(I) or as we start call it as localized states modified by oxygen deficiency [3,4,7,11]. Just we have changed electronic transitions in this defect by presence of the fluorine. Interaction of this modified by fluorine localized states with electronic excitations lead to transient or permanent remove of the fluorine and renew of the luminescence center as transient or permanent centers. In low temperature we have mainly transient changes in experiment actually performed. Permanent changes detected by TSL are of low intensity then created defects could not be detected by absorption method. The data of blue band decay kinetics studies under electron beam excitation are presented in Fig. 9. The decay is significantly faster than 10 ms and is not exponential. Therefore there is significant quenching influence of surrounding on the living time of triplet luminescence. Kinetics curves are very complicated and there are some differences between fluorine-doped and reference sample. However, we observe in both samples at least three processes. The first one is fast taken place in few nanoseconds after cutoff of excitation beam. We could not detect parameters of such process because of apparatus limitations. There could be essential influence of STE luminescence. Then a transition process takes place with power law t0:6 preceding decay with t1:2 , probably corresponding to tunnel recombination of pairs. Then
225
UV band decay T=290 K
1000 100
t-1 10
4
1
0.01
0.1
3
1 TIME (µs)
10
2 1 0 0.0
fluorine containing
0.5
1.0
1.5
2.0
2.5
TIME (µs)
3
10
Fig. 9. Cathodoluminescence (blue band) decay kinetics curves of silica glass samples with fluorine and without fluorine doping. The curves are measured in different ranges of time and sewed.
Fig. 10. Cathodoluminescence (UV band) decay kinetics curves of silica glass samples with fluorine and without fluorine doping. Insertion present decay in slower time range. The straight lines correspond to hyperbolic approximation with least square fit method. The dispersion of points reflects measurement errors.
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related to luminescence center transitions. Then a slow decay is observed with t1 power law independently on temperature and equal for both samples under comparison (F-doped and non-doped), and it could be explained only as recombination luminescence, probably of tunneling nature. That is in good agreement with explanation of blue band decay. Therefore we have evidences of recombination pairÕs creation also from short-time kinetics measurements.
4. Discussion The influence of fluorine doping on absorption of silica previously obtained in Ref. [1] is confirmed. Fluorine not only diminishes strongly the absorption band at 7.6 eV but significantly affects intrinsic absorption tail of silica. It is shifted to higher energy that 5 mm sample possesses higher transmission of light than 1 mm sample both produced by the same technology. Even the nature of electron–phonon interaction of the electronic states of the intrinsic absorption tail is changed from crystal-like in fluorine-free sample to glasslike in fluorine-doped sample. The studies of intrinsic absorption of fluorine doped silica need to be more detailed with thinner samples, but we can say already that even a small amount of fluorine strongly affects the states of silica. The main result is in observation of usual luminescence of oxygen deficient center in fluorine doped silica sample in spite of very low intensity of ODC(I) absorption band at 7.6 eV excited there. The luminescence content is the same as for fluorine-free sample. The low intensity of this absorption band correlates with the fact that initial concentration of luminescence center is close to zero. They appear only under continuous irradiation of low intensity. Influence of laser excitation should be studied. The process of luminescence excitation is complicated. The lack of luminescence detection in pure intra-center excitation of fluorine doped sample (excitation with short pulses and sensitive photon counting of decay kinetics) shows that luminescence centers do not exist before specific processes of creation. The luminescence centers are created during continuous
irradiation with 7.6 eV photons as well as with Xray and electron beam irradiation. The luminescence then could be excited in recombination process. The TSL observation directly shows on that. The TSL corresponds to recombination of long-living defects. The steady-state luminescence as well as decay after pulsed electron beam excitation corresponds to recombination of shortliving defects. The decay kinetics measured under pulsed electron beam show that there is wide collection of recombination rates. We have possibility to compare decay kinetics of the blue and UV bands in fluorine doped sample with that of both bands excited in pure intra-center process known from literature for fluorine-free sample (blue – 10 ms, UV – 4.5 ns, Ref. [5]). The blue band decay obtained now is strongly non-exponential and significantly faster than pure intra-center process. So, some process of limitation of life time in corresponding excited state will appear as shortening decay kinetics. That could be in the case when luminescence center is destroyed during recombination process. At low temperatures this could be possible when a defect, divided to several parts during excitation with one of them as the luminescence center, recovers back to initial state because of recombination of these parts. During such recombination the luminescence center goes through excited states. The variety of surrounding, where mobile parts could be trapped, explains different stages of decay of the blue band. The decay of UV band in pure intra-center transition is very fast, 4.5 ns, and then it could help in detection of slowest than 4.5 ns processes. Therefore the observation of slow decay kinetics of the UV band directly shows on recombination kinetics rates. Significant is prevailing of UV band in fluorinedoped sample with respect to undoped sample under photoexcitation. That could be explained with luminescence center destruction during recombination. Indeed, the fast UV luminescence has time for radiative transition, whereas longliving blue luminescence radiative transitions are limited by recombination rate ruining luminescence center and, therefore, diminished in intensity.
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Let us consider the creation processes of luminescence centers during irradiation with ionizing radiation. When a sample is excited by ionizing radiation, then complicated processes of absorbed energy relaxation from high energy non-elementary electronic excitations to elementary ones and energy transport by the last precede to the act of luminescence of a center. In fluorine-doped sample, where the imperfections may be represented with intrinsic localized states mainly, the interaction of the electronic excitations takes place with those localized states modified with nearest fluorine ions or atoms. The situation is similar to oxygen surplus sample, where the localized states exists as some structural motifs in short-range order of non-tetrahedron structure but their absorption is at higher energy than intrinsic absorption of silica. In fluorine-free, nevertheless with some level of oxygen deficiency, these localized states are presented openly with absorption band at 7.6 eV. The electronic excitations interact with localized states provoking photochemical dissolving with creation of non-mobile part and mobile parts. The mobile parts could be trapped on wells and could be removed from these wells. This mobile part in fluorine doped sample certainly could be fluorine together with other possible mobile ions or atoms. In fluorine free sample that could be oxygen or other ions or atoms modifying localized states. The luminescence observed in fluorine doped sample under photoexcitation in the range of 7.6 eV corresponds to luminescence center creation due to light absorption in trace of the absorption band there and then, corresponds to ÔnormalÕ excitation of this band as in fluorine-free sample [11]. That means many steps transformations of imperfection with photodissociation and recombination processes. The blue band and UV bands, as in previous papers [3,4,7,11], were attributed to luminescence of the twofold-coordinated silicon center also created by ionizing radiation and excited in complicated (fast and slow components) recombination process. The complexity of the decay kinetics could be also attributed to the complexity of defects taken part in electronic processes. It was obtained that lone twofold-coordinated center in-
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teracts with electronic excitation inefficiently [3,4]. Interaction with electronic excitation is realized via a complex defect responsible for the absorption band at 7.6 eV and the luminescence of the twofold-coordinated silicon center appears when reconstructions in the complex defect take place. These reconstructions may be produced by photoexcitation or by capturing of charge carriers of both types within complex defect. The luminescence appears from short-living twofold-coordinated silicon center disturbed by nearest surrounding, giving different shape of the UV band in luminescence excited in recombination process from intra-center excited lone center [5]. That also explains different decay under cathodoexcitation and photoexcitation [17]. This model is valid for explanation of the data obtained for fluorine-doped silica. Therefore the observed permanent recovery of 7.6 eV band under heating of fluorine-doped silica [1] is supported also in radiation processes with possibility of recreation of corresponding defect in transient process. We obtained data for room temperatures and even low temperature, when deep reconstructions in network are not possible, then structural imperfections, to which fluorine is connected, are not changed. Only ions or/and atoms could be released and transported to different from initial position place. On that show similar recombination luminescence observation for F-doped and non-doped samples. So, the structure of glass responsible for localized due to disorder states works as adsorbent of oxygen deficiency and dopants. Interaction of such complexes with electronic excitations provide rich picture of radiation processes. This model needs deep investigation.
5. Conclusions In fluorine doped silica, which main property is absence of the absorption band at 7.6 eV, the corresponding imperfections still take place and manifest themselves through recombination luminescence of oxygen deficient centers (blue and UV band). It is concluded that under ionizing irradiation removal and transportation of ions such as
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fluorine is possible and transient restoration of defect takes place both at room and liquid nitrogen temperatures. Acknowledgements This work was supported by the grants 01.0822 of the Scientific Society of Latvia. The authors thank Dr V. Halilov for determination of OH group concentration. References [1] K. Awazu, H. Kawazoe, K. Muta, J. Appl. Phys. 69 (1991) 4183. [2] H. Imai, K. Arai, H. Imagawa, H. Hosono, Y. Abe, Phys. Rev. B 38 (1988) 12772. [3] A.N. Trukhin, H.-J. Fitting, J. Non-Cryst. Solids 248 (1999) 49. [4] A.N. Trukhin, in: D. Griscom, G. Pacchioni, L. Skuja (Eds.), NATO Science Series. II Mathematics, Physics and Chemistry, 17th course, International School of Solid State Physics, vol. 2, 2000, p. 235. [5] L.N. Skuja, A.N. Streletsky, A.B. Pakovich, Solid State Commun. 50 (1984) 1069. [6] C.M. Gee, M. Kastner, J. Non-Cryst. Solids 40 (1980) 577. [7] A.N. Trukhin, J. Non-Cryst. Solids 189 (1995) 1.
[8] A.N. Trukhin, P.A. Kulis, J. Non-Cryst. Solids 188 (1995) 125. [9] A.N. Trukhin, J. Non-Cryst. Solids 189 (1995) 291. [10] L.N. Skuja, Phys. Stat. Sol. (a) 114 (1989) 731. [11] A.N. Trukhin, L.N. Skuja, A.G. Boganov, V.S. Rudenko, J. Non-Cryst. Solids 149 (1992) 96. [12] A.N. Trukhin, H.-J. Fitting, T. Barfels, A. von Czarnovski, J. Non-Cryst. Solids 260 (1999) 132. [13] M. Goldberg, A.N. Trukhin, H.-J. Fitting, Mater. Sci. Eng., B 42 (1996) 293. [14] M. Goldberg, H.-J. Fitting, A.N. Trukhin, J. Non-Cryst. Solids 220 (1997) 69. [15] I. Vıtols, J. Zak ß is, A. Silin ßs, A. Trukhin, Opt. Spectrosc. 30 (1971) 115. [16] A.J. Miller, R.G. Leisure, W.R. Austin, J. Appl. Phys. 86 (1999) 2042. [17] A.N. Trukhin, M. Goldberg, J. Jansons, H.-J. Fitting, I.A. Tale, J. Non-Cryst. Solids 223 (1998) 114. [18] A. Truhins, A. Boganovs, I. Godmanis, Sov. Phys. Chem. Solids 6 (1980) 507. [19] I.T. Godmanis, A.N. Trukhin, K. Hubner, Phys. Stat. Sol. (b) 116 (1983) 279. [20] L.B. Glebov, PhD thesis, University of Leningrad, 1975, p. 22. [21] B.L. Gelmont, V.I. Perel, I.N. Jassievich, Sov. Solid State Phys. 25 (1983) 727. [22] Y.A. Toyozawa, Technical Report, ISSP A 1 (1964) 119. [23] H.-J. Fitting, T. Barfels, A.N. Trukhin, B. Schmidt, A. Gulans, A. von Czarnowski, J. Non-Cryst. Solids 303 (2002) 218. [24] D.L. Griscom, J. Non-Cryst. Solids 149 (1992) 137.
Luminescence of fluorine doped silica glass A.N. Trukhin a
a,*
, J. Jansons a, T.A. Ermolenko b, I.I. Cheremisin
b
Solid State Physics Institute, University of Latvia, Kengaraga 8, LV-1063 Riga, Latvia b Institute of Silicate Chemistry, RAS, St-Petersburg, Russia Received 18 February 2003
Abstract The role of fluorine doping on silica properties was studied by luminescence methods. Non-doped samples of the same preparation technology possess an absorption band at 7.6 eV on the level of 2 cm1 . A trace of this band in the fluorine-doped sample is on the level of 0.1 cm1 . In both samples 7.6 eV photons as well as ionizing irradiation (X-ray, electron beam) excite photoluminescence of so-called oxygen deficient centers with a blue (2.7 eV) and a UV band (4.4 eV). The luminescence of the fluorine doped sample increases with dose many times from the initial low level for the same excitation. Also, thermally stimulated luminescence appears after irradiation. The energetic yield under ionizing irradiation of induced luminescence is the same level as reference samples. The decay kinetics of cathodoluminescence show that the blue band decays faster and the UV band decays more slowly than was known for the oxygen deficient center. It is concluded that structural imperfections, responsible for the absorption band at 7.6 eV, remain similar in silica glasses during preparation; however fluorine changes the electronic transition nature and, therefore, the absorption band is of low intensity. Such imperfections, passivated with fluorine, interact with electronic excitation produced by radiation. Transient changes of imperfections geometric and electronic configurations take place with probable removal of fluorine, and that provides a growth of the luminescence centers even at low temperature and changes the decay relatively to intra-center luminescence. Therefore the fluorine-doped sample is similar to the nondoped samples through corresponding recombination luminescence of oxygen deficient centers. Ó 2003 Elsevier B.V. All rights reserved. PACS: 78.55.)Hx; 42.70.Ce
1. Introduction The fluorine-doped silica raises interest due to significant improvement of the optical transmission in the VUV range, which allows improvement of the silica application. The role of fluorine needs
*
Corresponding author. Tel.: +371-7 260 686; fax: +371-7 132 778. E-mail address: [email protected] (A.N. Trukhin).
detailed studies. It was found [1], that absorption band at 7.6 eV, almost absent in fluorine doped sample, could be restored to the usual level of slightly oxygen deficient samples by heating at 1000 °C during several days in helium atmosphere. In Ref. [1] the effect was explained as creation of Si–Si bonds from Si–F and collection F2 molecules in interstitial under influence of high temperature. Another explanation of this effect could be as evaporation of fluorine from defect responsible for the band at 7.6 eV and collection as F2 molecules
0022-3093/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2003.09.014
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in interstitial. The band could be restored without deep structural transformations. This point of view means lack of significant structural changes during thermal treatments. This controversy could be checked by detection of defects at room or even low temperatures, where significant structural changes less possible. The luminescence is one of convenient methods for that. It allows one to detect dynamic process of defects transformation during excitation. We have chosen X-ray, cathodo and photoexcitation for detection of luminescence defect creation from precursors. Let us remember that defects, after definitions, are structural imperfections detectable spectrally, or by other means. Precursors of defects also are structural imperfections, for which electronic transitions are covered by intrinsic absorption. Therefore such precursors could be detected only after their transformation to the defects. The absorption band at 7.6 eV is related to oxygen deficiency (oxygen deficient center (I) or ODC(I)) [1–4]. Another oxygen deficient center is lone twofold-coordinated silicon center or ODC(II) in dry silica [5]. One center gives two luminescence bands: blue one at 2.7 eV, due to triplet–singlet transitions and UV one at 4.4 eV due to singlet–singlet transitions. In the case of ODCD(II) center the blue luminescence time constant is 10 ms and that of UV band is 4.5 ns [5]. The relation between these bands is not stable and is depending on many parameters. In ODC(II) conversion processes from excited singlet level to triplet determine this relation. In ODC(I) this relation additionally is affected by samples preparation history (see, for example, Refs. [1–4]). The nature of the ODC(I) and correspondingly of the absorption band at 7.6 eV is the object of different interpretations. One of them is oxygen vacancy in short-range order with tetrahedron structure, for example, [1,2]. Another is interpretation of this band as oxygen deficiency in octahedron short-range order [3,4]. Let remind that glass structure could be imagined as many different kinds of structural motifs of short-range order and each motif could correspond to material ability exist in crystalline polymorph modifications. Silicon dioxide is characterized by huge number of polymorph
modifications. The main tetrahedron structure in short-range order of silicon dioxide provides equal possibilities for defect structure in crystal and glass. Indeed, the most investigated E0 -center possesses very similar properties in crystalline aquartz and silica glass. Contrary situation with other defects, and, for example the oxygen deficient center with corresponding luminescence bands cannot be observed in a-quartz crystal. Only after crystal converting to amorphous state by neutron irradiation were similar defects detected [6]. On the other hand, localized states of wide gap oxide glasses [4,7] were determined as minority structural modifications in short-range order of a glass. The most significant situation is for germanium dioxide case. The germanium dioxide (strongly oxidized) glass possesses two intrinsic absorption tails: one the same as GeO2 crystal with a-quartz structure at 6 eV and another at 4.5 eV as rutile-type structured crystal [8]. The last determines luminescence of localized states [8,9]. The oxygen deficient GeO2 glass reveals similar ODC(II) luminescence as silica glass doped with germanium [10]. The situation in glassy SiO2 about role of polymorph modifications is not so definitive. Seems, analogous absorption of octahedron corresponding motifs is situated at higher energy than that of tetrahedron motifs. Nevertheless, the case of oxygen deficiency is similar for both kinds of material SiO2 and GeO2 [4]. So, we bind the 7.6 eV absorption band in silica with octahedron motifs in short-range order, which are affected by oxygen deficiency [3,4]. We propose that fluorine passivation just changes electronic states of those localized states modified by oxygen deficiency and the intensity of absorption band at 7.6 eV becomes reduced. Changed electronic transitions could correspond to higher energy above the intrinsic absorption threshold. The concentration of such states is depending on preparation prehistory and should be similar in samples with and without of fluorine. The luminescence of oxygen deficient silica was studied in many publications [2–5,7–11]. The creation of luminescence oxygen deficient centers from precursors was proved by study of dose de-
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pendences under electron beam excitation [12–14], and X-ray excitation [15,16]. The process of luminescence excitation under ionizing irradiation is very complex and involves recombination of previously created centers, which living time is from few nanoseconds to infinity (the last corresponds to permanently existing induced centers). So, in the case when distorted structure passivated with fluorine (structural imperfections) exists, we should observe the same luminescence as in fluorine-free sample, when we could provide dynamic release of the fluorine, permanent or transient. Therefore now we compare luminescence properties of pure silica with and without fluorine.
2. Experimental The sample for investigation was pure silica (the level of metallic impurities is about 106 wt%) with and without doping of fluorine. The samples were produced as KC-4B (or the same KS-4V) dry silica with difference that F-doping was realized by fusing in pure SiF4 atmosphere. The concentration of fluorine in doped sample is on the level of 0.1 wt%. The detected intensity of OH group absorption is on the level of 7.5 ppm in doped sample. However, no red luminescence was detected in the sample under X-ray irradiation at 80 K; therefore the influence of OH group on processes could be accounted as negligible. Photoluminescence experiment was performed with the use of deuterium continuous discharge source as well as of spark discharge in helium– hydrogen gas mixture. In the last case, the pulses with 6 ns duration on half height of intensity were obtained, however with a long (several hundreds of ns) tail. The 0.5 m Seya–Namioka vacuum monochromator with toroidal concave grating was used. The sample was kept in cryostat with nitrogen screen, which allows one to avoid condensation of contamination film on the sample surface at low temperatures. Cathodoluminescence (CL) spectra and time resolved experiments were conducted by means of pulsed electron beam equipment with electron energies 6 keV, 0.2–1 ls pulse duration. The fast cut of the pulse (about 1 ns) provides possibility to
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measure fast kinetics. From these data we deduced the luminescence kinetics. An X-ray tube with W anticathode (15 mA, 40 kV) was used to study luminescence under ionizing radiation (details of experiment in Ref. [17]). Luminescence was analyzed by a grating monochromator and detected by a photomultiplier. The decay kinetics were recorded with a 256channel analyzer. The measured curves are presented in figures as received therefore they reflect the level of errors. The sodium salicylate double recrystallized was used as reference standard for photoluminescence measurements. Its luminescence quantum yield is about 0.5 independent of photon energy from about 4–2000 eV and, consequently, it used to correct excitation curves on spectral distribution of excitation light. Nevertheless, the luminescence intensity is presented in arbitrary units because it was not our task to determine its absolute quantum yield. When intensities are compared, then presented data were measured in the same geometry and the same conditions.
3. Results The absorption spectra of studied samples are presented in Fig. 1. It could be seen that fluorinedoped sample possesses good optical transparency in the range of 7.6 eV (the trace of absorption is on the level of 0.1 cm for possible band there); however this absorption band is well defined in the reference-undoped sample. Absorption changes with vary of the temperature with check of Urbach approximation for sample 5-mm-thick are presented in Fig. 2. It is seen that Urbach approximation is holding for fluorine glass, as it was known for non-doped glass [18,19] (see Fig. 3); however the form of Urbach rule in the case of fluorine doped silica corresponds to Ôglass-likeÕ [20,21] with parallel shift of absorption in semi-log scale rather than usual for silica glass Ôcrystal-likeÕ (Fig. 3) with cross-point. The intrinsic absorption of 5-mm-thick sample obeys to Urbach rule in the form usual for disordered materials or Ôglass-likeÕ: a ¼ I0 exp½AE þ T =T1 ;
ð1Þ
A.N. Trukhin et al. / Journal of Non-Crystalline Solids 332 (2003) 219–228 ASORPTION COEFFICIENT (cm-1)
222 12 10
Absorption spectrum of silica glass with fluorine compared with undoped sample T=290 K
8 6
without F
4 2
F
0 5
6
7
8
PHOTON ENERGY (eV)
ln Absorption coefficient (cm-1)
Fig. 1. Optical absorption spectra of silica glass samples with fluorine (F) and without fluorine doping, both made by KC-4B technology.
14 12
Urbach in silica glass with fluorine
10 8 6 4 2
405 K
0 520 K -2 -4
343 K 297 K
7.6
7.8
8.0
8.2
8.4
8.6
8.8
PHOTON ENERGY (eV)
ln Absorption coefficient (cm-1)
Fig. 2. Urbach approximation of absorption spectrum of the fluorine-doped silica. Points – result of measurements, lines – calculation after expression (1).
15
Urbach in pure silica glass (light deficit of O)
10 5 0
370 K 410 K
7.6
7.8
295 K
8.0 8.2 8.4 8.6 PHOTON ENERGY (eV)
8.8
Fig. 3. Urbach approximation of absorption spectrum of the fluorine non-doped silica. Points – result of measurements, lines – calculation after expression (2).
where A is a parameter of slope ( 7 eV1 for F-doped sample) and T1 ( 123 K for F-doped sample) is a characteristic temperature. The E is photon energy. The semi-logarithmic spectrum moves parallel to itself with change of the temperature. The Ôcrystal-likeÕ Urbach rule corresponds, as was explained by Toyozawa [22], to that of creation of the momentarily self-trapped exciton (STE): a ¼ a0 exp½rðE0 EÞ=kT ;
ð2Þ
r ¼ r0 2kT =hx tanh hx=2kT :
ð3Þ
The parameters a0 , E0 correspond to position of the cross-point, r is slope parameter, r0 parameter is reciprocal of exciton–phonon interaction strength, hx is the energy of phonon. The obtained parameters for reference-undoped sample are in good agreement with previously published (a0 106 cm1 , E0 8:7 eV for reference sample, Fig. 3). So, the fluorine doping strongly affects even host material electronic states. However, theoretical explanation of Ôglass-likeÕ is far from the crystal case. We were able to detect photoluminescence in fluorine-doped sample under continuous excitation, in spite of low level of absorption (Fig. 4). It is need to underline that in reference sample, containing well detectable absorption band at 7.6 eV, the same luminescence could be excited by photons with this energy (7.6 eV), Fig. 4. Two broad luminescence bands, blue and UV are detected at room and low (77 K) temperatures in both studied samples. Corresponding excitation spectra are presented in Fig. 4. The high energy cutoff in excitation band well corresponds to the position of Urbach threshold. The shift to high energy at low temperature corresponds to position of absorption threshold at low temperature. So, at the range of Urbach-tail no specific photoluminescence was detected. The excited luminescence corresponds to extrinsic absorption and to host material defects. In reference-undoped sample different relation between two bands was observed than in fluorine-doped sample. The UV band intensity in fluorine-doped sample prevails the blue band, whereas in non-doped sample blue band prevail UV band, Fig. 4. So, in both cases the main
Photoluminescence of silica glass with fluorine PLE, 4.4 eV hvexc =7.7 eV PL, T=77 K
8
T=77 K
6
hvexc=7 eV
4 x10 293 K
2 0
without F
2
3
4
5 6 7 PHOTON ENERGY (eV)
8
9
Fig. 4. Optical absorption spectra of silica glass samples with fluorine (F) and without fluorine doping, both made by KC-4B technology. Photoluminescence measurements of the fluorinedoped silica. Undoped sampleÕs data are presented for comparison. The dispersion of points reflects measurement errors.
LUMINESCENCE INTENSITY (arb.units)
luminescence bands are due to oxygen deficient defects. The dose kinetics of UV band growth during irradiation by 7.7 eV photons are presented in Fig. 5. When detection-step time is 1 s, the growth is about three times from initial level. Low yield of luminescence does not allow to measure dose kinetics with shorter detection time that to resolve initial growth. Thus, the luminescence centers are produced by irradiation. A decrease followed shows also on the radiation destruction of created center and on the complicated processes taken place during irradiation. We were trying to measure decay kinetics of luminescence in fluorine doped sample with the low intensity spark pulsed
700 600 500 400 300 200
Dose dependence of 7.7 eV excited photoluminescence (band at 4.4 eV) of fluorine-doped silica glass sample. T=80 K
100 0
0
500
1000
1500
TIME (s)
Fig. 5. Photoluminescence dose dependence of the fluorinedoped silica. The dispersion of points reflects measurement errors.
223
light source. Experiment was realized with photon counting system. The decay curves could be collected during many hours, that means high sensitivity of the system. Nevertheless, we failed to get a measurable signal. Lack of signal under spark excitation and existence of luminescence under continuous excitation means lack of center excitable in intra-center process. Therefore the luminescence could not be excited in pure intra-center process because there is no luminescence centers. They appear only under continuous irradiation, probably in multi-steps process. The case is very similar to luminescence properties of hydrogen treated oxygen-deficient samples [12], where the luminescence centers, existing before treatment, are destroyed by hydrogen and renew only under irradiation at low temperature because of transient removal of hydrogen from defect and followed restoration of the luminescence ability. Use of ionizing irradiation reveals luminescence yield (energetic yield – emitted energy per absorbed energy) of fluorine doped sample on the level of ordinary pure silica. In Fig. 6 the X-ray excited luminescence of compared samples are presented. The spectral contents are similar, as well as levels of luminescence intensities are the same (even in the case of F-doped sample is higher). In the visible spectral range there could be influence from STE luminescence at low temperatures [17] but the existence of the UV band in fluorine-doped sample unequivocally shows on luminescence center appearance under ionizing LUMINESCENCE INTENSITY (arb.units)
10
absorption 293 K
Photoluminescence intensity (arb.units) Absorption coefficient (cm-1)
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25 X-ray excited spectra of silica samples.
20 F, 80 K
15 10 F, 180 K
5 no F,80 K no F, 180 K
0 2
3
4
5
PHOTON ENERGY (eV)
Fig. 6. X-ray excited spectra of silica glass samples with fluorine (F) and without fluorine doping (no F) at different temperatures.
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LUMINESCENCE INTENSITY (arb.units)
irradiation analogously to irradiation by 7.7 eV photons. So, in spite of low level of absorption and expected on that low imperfection concentration, the luminescence of fluorine-doped sample reveals the same quantity of imperfection taken part in radiative recombination (of the same nature as in fluorine non-doped sample) at low temperature, when possible transformation of defects could not be related to serious changes of imperfection structure. Only ions or atoms not involved in network of glass (mobile ions) [23] could be responsible for imperfections transformation to luminescence defects. The dose dependences under X-ray irradiation are presented in Fig. 7. It could be seen that luminescence in both samples performs complicated creation–destruction kinetics. Initially both bands, blue and UV, grow from almost zero level (in this case of sufficient level of yield we were able to use minimal detection step 0.1 s), and then, after few minutes of irradiation, they have a decrease in both kind of samples followed with increase in half-hour time of irradiation. This situation is also analogous to the case of photoexcitation. That also shows on small influence of STE luminescence, which should appear under irradiation without delay. Such kinetics, with so-call turnaround behavior, were observed for irradiation of silicon dioxide film with electron beam; see for example Refs. [13,14]. That was explained before
2.0 F, 4.4 eV
1.5
F, 2.7 eV :10
1.0
no F, 4.4 eV
0.5 0.0
Dose dependence of x-ray excited luminescence of silica glass samples. T=80 K
0
500
1000
1500
2000
2500
3000
TIME (s)
Fig. 7. X-ray excited dose dependences of luminescence intensities for blue (2.7 eV) and UV (4.4 eV) bands of silica glass samples with fluorine (F) and without fluorine (no F) doping. The lines are drawn as a guide for the eye. The dispersion of points reflects measurement errors.
as creation–destruction of the defects from precursors with participation of some mobile ions (interstitial oxygen, hydrogen, chlorine and, in our case, also fluorine) [23]. Initially, luminescence intensity is almost zero and that could correspond to low concentration of luminescence centers and, therefore, there is a creation process. The creation process is related to luminescence centersÕ appearance from so-call precursors or others words localized states of a glass, existing because of disordered state. The band at 7.6 eV is related to localized states modified by oxygen deficiency [3,4,11]. Fluorine, probably, changes electronic transitions of localized states and absorption band at 7.6 eV could not be recorded directly by absorption measurement, however through luminescence center revealing it could be detected on excitation spectra. If we stop irradiation, then some part of non-recombined traps remains, and we observe thermally stimulated luminescence (Fig. 8). The position of the TSL peaks well corresponds to the self-trapped holes thermal release [24]. Then we can conclude that STH are connected to the areas of localized states. The luminescent content of TSL corresponds to luminescence related to oxygen deficiency (the blue and UV bands). Then there is no difference in luminescence of silica both doped and non-doped with fluorine. In both cases we see luminescence connected to oxygen deficiency. The fact that there is no ODC(I) band in
LUMINESCENCE INTENSITY (arb.units)
224
75 Thermally stimulated luminescence of of x-ray irradiated fluorine-doped silica samples.
50 2.7 eV
25
4.4 eV
0 50
100
150
200
250
300
TEMPERATURE (K)
Fig. 8. Thermally stimulated luminescence curves for blue (2.7 eV) and UV (4.4 eV) bands of silica glass samples with fluorine (F) doping. Sample was irradiated during 2 h at 80 K with X-ray. The dispersion of points reflects measurement errors.
A.N. Trukhin et al. / Journal of Non-Crystalline Solids 332 (2003) 219–228
CL decay (blue band) kinetics of silica glasses, T = 80 K
4
10
t-1.2
}
3
t-0.3
}
10
sample with fluorine
pure sample
2
10
1
10
0
10
-2
10
-1
10
0
1
10 10 TIME (µs)
2
10
10 CL (UV band) decay kinetics of silica glasses T = 80 K
9 8 7 6
cut off beam pulse
pure
5
10000 CL INTENSITY (arb.units)
t-0.6
}
CL INTENSITY (arb.units)
5
10
decay becomes once again t0:3 , which usually corresponds to a initiation of another recombination process, which we, actually, could not detect due to apparatus limitation in slow part of decay. We explain such rich kinetic by removal, transportation on different distance and localization on different wells of ions, where fluorine is expected to be principal. We expect that removal of fluorine from the precursor restores ability of defect participates in recombination processes with luminescence. Fluorine or other ions captured nearby are working as quenching factor, diminishing ÔnaturalÕ lifetime. We did not detect ÔnormalÕ intracenter decay; then recombination takes place in transient center of luminescence, that is in center created for short time, which could not be destroyed in excited state by nearest ions or atoms. The decay of reference sample is similar, but power law is a bit different. Then similar processes take place in reference-undoped sample however with different mobile ions participation. The decay kinetics curves for UV band are compared in Fig. 10. In this case the decay slower than intra-center process is determined mainly by recombination parameters. Surrounding perturbations are influencing in very short time. The UV band of ODC(II) is very fast and is about 4.5 ns of non-perturbed by surrounding transitions. In CL we also observe a fast decay, which is probably ln LUMINESCENCE INTENSITY (arb.units)
F-doped sample, but we still have luminescence as we have this band, show that fluorine does not change structure of ODC(I) or as we start call it as localized states modified by oxygen deficiency [3,4,7,11]. Just we have changed electronic transitions in this defect by presence of the fluorine. Interaction of this modified by fluorine localized states with electronic excitations lead to transient or permanent remove of the fluorine and renew of the luminescence center as transient or permanent centers. In low temperature we have mainly transient changes in experiment actually performed. Permanent changes detected by TSL are of low intensity then created defects could not be detected by absorption method. The data of blue band decay kinetics studies under electron beam excitation are presented in Fig. 9. The decay is significantly faster than 10 ms and is not exponential. Therefore there is significant quenching influence of surrounding on the living time of triplet luminescence. Kinetics curves are very complicated and there are some differences between fluorine-doped and reference sample. However, we observe in both samples at least three processes. The first one is fast taken place in few nanoseconds after cutoff of excitation beam. We could not detect parameters of such process because of apparatus limitations. There could be essential influence of STE luminescence. Then a transition process takes place with power law t0:6 preceding decay with t1:2 , probably corresponding to tunnel recombination of pairs. Then
225
UV band decay T=290 K
1000 100
t-1 10
4
1
0.01
0.1
3
1 TIME (µs)
10
2 1 0 0.0
fluorine containing
0.5
1.0
1.5
2.0
2.5
TIME (µs)
3
10
Fig. 9. Cathodoluminescence (blue band) decay kinetics curves of silica glass samples with fluorine and without fluorine doping. The curves are measured in different ranges of time and sewed.
Fig. 10. Cathodoluminescence (UV band) decay kinetics curves of silica glass samples with fluorine and without fluorine doping. Insertion present decay in slower time range. The straight lines correspond to hyperbolic approximation with least square fit method. The dispersion of points reflects measurement errors.
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related to luminescence center transitions. Then a slow decay is observed with t1 power law independently on temperature and equal for both samples under comparison (F-doped and non-doped), and it could be explained only as recombination luminescence, probably of tunneling nature. That is in good agreement with explanation of blue band decay. Therefore we have evidences of recombination pairÕs creation also from short-time kinetics measurements.
4. Discussion The influence of fluorine doping on absorption of silica previously obtained in Ref. [1] is confirmed. Fluorine not only diminishes strongly the absorption band at 7.6 eV but significantly affects intrinsic absorption tail of silica. It is shifted to higher energy that 5 mm sample possesses higher transmission of light than 1 mm sample both produced by the same technology. Even the nature of electron–phonon interaction of the electronic states of the intrinsic absorption tail is changed from crystal-like in fluorine-free sample to glasslike in fluorine-doped sample. The studies of intrinsic absorption of fluorine doped silica need to be more detailed with thinner samples, but we can say already that even a small amount of fluorine strongly affects the states of silica. The main result is in observation of usual luminescence of oxygen deficient center in fluorine doped silica sample in spite of very low intensity of ODC(I) absorption band at 7.6 eV excited there. The luminescence content is the same as for fluorine-free sample. The low intensity of this absorption band correlates with the fact that initial concentration of luminescence center is close to zero. They appear only under continuous irradiation of low intensity. Influence of laser excitation should be studied. The process of luminescence excitation is complicated. The lack of luminescence detection in pure intra-center excitation of fluorine doped sample (excitation with short pulses and sensitive photon counting of decay kinetics) shows that luminescence centers do not exist before specific processes of creation. The luminescence centers are created during continuous
irradiation with 7.6 eV photons as well as with Xray and electron beam irradiation. The luminescence then could be excited in recombination process. The TSL observation directly shows on that. The TSL corresponds to recombination of long-living defects. The steady-state luminescence as well as decay after pulsed electron beam excitation corresponds to recombination of shortliving defects. The decay kinetics measured under pulsed electron beam show that there is wide collection of recombination rates. We have possibility to compare decay kinetics of the blue and UV bands in fluorine doped sample with that of both bands excited in pure intra-center process known from literature for fluorine-free sample (blue – 10 ms, UV – 4.5 ns, Ref. [5]). The blue band decay obtained now is strongly non-exponential and significantly faster than pure intra-center process. So, some process of limitation of life time in corresponding excited state will appear as shortening decay kinetics. That could be in the case when luminescence center is destroyed during recombination process. At low temperatures this could be possible when a defect, divided to several parts during excitation with one of them as the luminescence center, recovers back to initial state because of recombination of these parts. During such recombination the luminescence center goes through excited states. The variety of surrounding, where mobile parts could be trapped, explains different stages of decay of the blue band. The decay of UV band in pure intra-center transition is very fast, 4.5 ns, and then it could help in detection of slowest than 4.5 ns processes. Therefore the observation of slow decay kinetics of the UV band directly shows on recombination kinetics rates. Significant is prevailing of UV band in fluorinedoped sample with respect to undoped sample under photoexcitation. That could be explained with luminescence center destruction during recombination. Indeed, the fast UV luminescence has time for radiative transition, whereas longliving blue luminescence radiative transitions are limited by recombination rate ruining luminescence center and, therefore, diminished in intensity.
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Let us consider the creation processes of luminescence centers during irradiation with ionizing radiation. When a sample is excited by ionizing radiation, then complicated processes of absorbed energy relaxation from high energy non-elementary electronic excitations to elementary ones and energy transport by the last precede to the act of luminescence of a center. In fluorine-doped sample, where the imperfections may be represented with intrinsic localized states mainly, the interaction of the electronic excitations takes place with those localized states modified with nearest fluorine ions or atoms. The situation is similar to oxygen surplus sample, where the localized states exists as some structural motifs in short-range order of non-tetrahedron structure but their absorption is at higher energy than intrinsic absorption of silica. In fluorine-free, nevertheless with some level of oxygen deficiency, these localized states are presented openly with absorption band at 7.6 eV. The electronic excitations interact with localized states provoking photochemical dissolving with creation of non-mobile part and mobile parts. The mobile parts could be trapped on wells and could be removed from these wells. This mobile part in fluorine doped sample certainly could be fluorine together with other possible mobile ions or atoms. In fluorine free sample that could be oxygen or other ions or atoms modifying localized states. The luminescence observed in fluorine doped sample under photoexcitation in the range of 7.6 eV corresponds to luminescence center creation due to light absorption in trace of the absorption band there and then, corresponds to ÔnormalÕ excitation of this band as in fluorine-free sample [11]. That means many steps transformations of imperfection with photodissociation and recombination processes. The blue band and UV bands, as in previous papers [3,4,7,11], were attributed to luminescence of the twofold-coordinated silicon center also created by ionizing radiation and excited in complicated (fast and slow components) recombination process. The complexity of the decay kinetics could be also attributed to the complexity of defects taken part in electronic processes. It was obtained that lone twofold-coordinated center in-
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teracts with electronic excitation inefficiently [3,4]. Interaction with electronic excitation is realized via a complex defect responsible for the absorption band at 7.6 eV and the luminescence of the twofold-coordinated silicon center appears when reconstructions in the complex defect take place. These reconstructions may be produced by photoexcitation or by capturing of charge carriers of both types within complex defect. The luminescence appears from short-living twofold-coordinated silicon center disturbed by nearest surrounding, giving different shape of the UV band in luminescence excited in recombination process from intra-center excited lone center [5]. That also explains different decay under cathodoexcitation and photoexcitation [17]. This model is valid for explanation of the data obtained for fluorine-doped silica. Therefore the observed permanent recovery of 7.6 eV band under heating of fluorine-doped silica [1] is supported also in radiation processes with possibility of recreation of corresponding defect in transient process. We obtained data for room temperatures and even low temperature, when deep reconstructions in network are not possible, then structural imperfections, to which fluorine is connected, are not changed. Only ions or/and atoms could be released and transported to different from initial position place. On that show similar recombination luminescence observation for F-doped and non-doped samples. So, the structure of glass responsible for localized due to disorder states works as adsorbent of oxygen deficiency and dopants. Interaction of such complexes with electronic excitations provide rich picture of radiation processes. This model needs deep investigation.
5. Conclusions In fluorine doped silica, which main property is absence of the absorption band at 7.6 eV, the corresponding imperfections still take place and manifest themselves through recombination luminescence of oxygen deficient centers (blue and UV band). It is concluded that under ionizing irradiation removal and transportation of ions such as
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fluorine is possible and transient restoration of defect takes place both at room and liquid nitrogen temperatures. Acknowledgements This work was supported by the grants 01.0822 of the Scientific Society of Latvia. The authors thank Dr V. Halilov for determination of OH group concentration. References [1] K. Awazu, H. Kawazoe, K. Muta, J. Appl. Phys. 69 (1991) 4183. [2] H. Imai, K. Arai, H. Imagawa, H. Hosono, Y. Abe, Phys. Rev. B 38 (1988) 12772. [3] A.N. Trukhin, H.-J. Fitting, J. Non-Cryst. Solids 248 (1999) 49. [4] A.N. Trukhin, in: D. Griscom, G. Pacchioni, L. Skuja (Eds.), NATO Science Series. II Mathematics, Physics and Chemistry, 17th course, International School of Solid State Physics, vol. 2, 2000, p. 235. [5] L.N. Skuja, A.N. Streletsky, A.B. Pakovich, Solid State Commun. 50 (1984) 1069. [6] C.M. Gee, M. Kastner, J. Non-Cryst. Solids 40 (1980) 577. [7] A.N. Trukhin, J. Non-Cryst. Solids 189 (1995) 1.
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