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The correlation of the 7.6 eV optical absorption band in pure fused silicon dioxide with twofold-coordinated silicon
Journal of Non-Crystalline Solids 149 (1992) 96-101 North-Holland
~
] O U R N A L OF
The correlation of the 7.6 eV optical absorption band in pure fused silicon dioxide with twofold-coordinated silicon A . N . T r u k h i n a n d L.N. S k u j a Institute of Solid State Physics, University of Latvia, 8 Kengaraga St, 226063 Riga, Latvia
A.G. B o g a n o v a n d V.S. R u d e n k o Institute of Silicate Chemistry, Academy of Sciences of Russia, 2 Makarova St, 199064 St Petersburg, Russia
Received 14 August 1991 Revised manuscript received 5 February 1992
The optical absorption band at 7.6 eV, which appears in oxygen deficient pure silica, does not correlate with any ESR signal in non-irradiated samples. Longlasting illumination at 80 K in the range of its absorption leads to an increase of the absorption band at 5 eV. Subsequent heating to 290 K restores the initial absorption. These data can be explained as photodissociation and thermal recreation of a complex defect containing a twofold-coordinated silicon defect. This complex defect is responsible for the 7.6 eV absorption band.
1. Introduction The properties of the 7.6 eV absorption band in silica have been studied since the 1960s [1]. In ref. [2] it was found that in non-irradiated silica of types I and II, it is possible to excite in this band the same luminescence as in the absorption band at 5 eV (bands at 2.7-3.1 eV and at 4.4 eV). T h e n in ref. [3] it was found that two different centers, one intrinsic and another G e related, give luminescence in these ranges after excitation in the bands at 5 eV and at 7.6 eV. Later it was shown that the absorption band at 5 eV is due to twofold-coordinated silicon in pure samples [4] and twofold-coordinated G e in doped samples [5]. The intrinsic luminescence appears in irradiated and non-irradiated pure samples after excitation at 7.6 eV [2]. These results were confirmed by later investigations [6]. In ref. [7], correlation between the peroxy radical E S R signal and the irradiation-induced 7.6 eV band was found and the absorption was ascribed to the peroxy radical. However, it cannot explain the absence of the
E S R signal in non-irradiated samples where this absorption band also appears. It was found [8] that the relation between the 7.6 and 5 eV bands is not a one-to-one relationship in spite of the fact that both are detected in oxygen deficient specimens. We confirm the results of ref. [8]. The initial samples in whose spectra the 7.6 eV absorption band is observed and the band at 5 eV is hardly noticeable were studied. In the present work, we have found that a band at 5 eV increases after illumination at 7.6 eV. We try to explain this effect and suggest a model for the 7.6 eV band in our samples.
2. Experimental procedure The spectra of optical absorption, photoluminescence and its excitation, as well as luminescence decay kinetics and t e m p e r a t u r e dependences of all the measured luminescent properties have been measured. The photoconductivity was also measured. Samples of fused silica of
0022-3093/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
A.N. Trukhin et al. / Correlation of 7.6 eV optical absorption band in silicon dioxide
four types were investigated. The trade marks of the samples were: KI (type I); KV (type II); KU-1, Corning 7940 (type III); and KUVI, Suprasil W1, KS-4V (type IV). The last sample was given particular attention. To analyze luminescence we used a grating monochromator, MDR-12, and to analyze absorption and excitation, a 0.5 m vacuum monochromator with a toroidal grating was used. A pulsed discharge in a H e - H 2 mixture was used to measure kinetics, the pulse duration being approximately 10 ns at half height. A deuterium discharge lamp was used for continuous illumination measurements. The samples were placed in a liquid helium cryostat. LiF windows were used on the excitation axes and type III fused silica windows for luminescence measurements. We used photomultipliers with S-20 photocathodes. The experiments were computer controlled. For photoconductivity measurements, we used a VA-J-52 electrometer.
3. Results
The optical absorption spectra of types III and IV silica glass samples are presented in fig. 1. It can be seen that the absorption band in the range near the fundamental absorption tail is observed in all the samples and that the KS-4V samples
97
4C
=c30 .2o
E20 2
I,° 3
4 hv(eV)
,-~
Fig. 2. Photoluminescence spectra of KS-4V (155) observed at 80 K under excitation with 7.6 eV photons• 1, spectrum of an initial sample; 2, after irradiation during 1 h with the same photon energy.
have the best transparency in the short wavelength range and a well defined band at 7.6 eV. It is important to mention that the band at 5 eV is also observed in these samples but its intensity is small. It can, however, be observed in sufficiently thick samples or by means of luminescence excitation spectra of the luminescence band at 4.4 eV (see below). For non-irradiated samples, the photoluminescence bands at 4.4 eV and 2.7 eV can be excited
o
2
/
/
/ /o
/
C /
0
/
1
o/
¢
E
T 5
6 - -
?
8
hv(eV) --'--l-
Fig. 1. Comparison of optical absorption spectra at room temperature in KS-4V silica glasses of different sample treatments: 135, untreated; 155, treated in CI atmosphere; 161, treated in oxygen atmosphere. W1, sample of Suprasil W1; 7940, Coming 7940.
/
/g /
7, --o~
1'0
( cnTll
'
2'0 m
Fig. 3. Dependence of photoluminescence intensity of the band at 4.4 eV on the intensity of the optical absorption band at 7.6 eV. Different points correspond to different samples of the KS-4V glass. The excitation photon energy is 7.6 eV. Points at 20 cm -1 belong to samples treated in hydrogen atmosphere at 1500 K.
A.N. Trukhin et al. / Correlation of 7.6 eV optical absorption band in silicon dioxide
98
T
15
T(K)
E ~-5
|
,
3
|
-
,
h~leV)
4
---
Fig. 4. Photolumincscence spectra of KS-4V (155) under excitation with 5.2 eV photons. ], spectrum of an initial sample; 2, after irradiation during ! h with the same photon energy.
T= 80 K.
in the range of the 7.6 eV absorption band as shown in fig. 2. Direct relation between the intensities of the luminescence band at 4.4 eV and the absorption band at 7.6 eV (fig. 3) for untreated samples has been established with the exception of the samples treated in hydrogen atmosphere at 1200°C. This treatment leads to an increase of absorption in the 7.6 eV band with parallel decrease of luminescence intensity. Analogous treatment in an oxygen atmosphere diminishes the 7.6 eV absorption band. The intensity of 4.4 eV luminescence increases with time of continuous excitation in the range of the 7.6 eV band (figs. 2, 4 and 5). While this
!
6
7
hvleV)
-
Fig. 6. The excitation spectrum of thermostimulated luminescence (TL) in KS-4V (155). Each point is measured after 1 h excitation at 80 K. The inset shows the example of TL peaks after excitation with 7.6 eV photons at 80 K (1) and 4.5 K (2).
effect is stronger at low temperatures, it is also observed at room temperature (RT). After illumination, a significant increase in the intensity of the 4.4 eV band excited at 5 eV is observed (fig. 4). Subsequent heating of the irradiated sample leads to thermostimulated luminescence (TL) in which only one luminescence band is observed at 2.7 eV. There is no evidence of a luminescence band at 4.4 eV. After heating, the initial situation is restored. The TL peak is very broad ranging from RT to 10 K. The TL peak excitation spectrum (fig. 6) starts from the 7.6 eV band and ranges to almost the fundamental absorption tail. Thus, it clearly results from the 7.6 eV band and is not related to the fundamental absorption.
f "
.-_=ze C
4. Discussion
025
/,/
oE
23
1'2
2t(min}36
=
Fig. 5. The kinetics of photoluminescence at 4.4 eV under continuous excitation with 7.6 eV photons. Solid line: sample of KS-4V treated in hydrogen atmosphere. Dashed line: in CI atmosphere. T = 80 K.
The photoluminescence bands at 2.7 eV and 4.4 eV excited at 7.6 eV may well be due to twofold-coordinated silicon, but it is not clear why the absorption band at 5 eV is so weak in the non-treated samples. On the other hand, the band at 4.4 eV occurred in KS-4V samples also after special doping with alkali ions [9] and its excitation band is situated in the vicinity of 7.6 eV, but there is no band at 5 eV. The eases of twofold-co-
A.N. Trukhin et aL / Correlation of 7.6 eV optical absorption band in silicon dioxide
KY-1-Ge
1
100C '"
I
63 100
go
•
.°.
'
100
0 0
500 1000 tins) Fig. 7. The comparison of photoluminescence decay kinetics (the range of 4.4 eV) of a Ge-activated sample (KY-1-Ge), Li-activated KS-4V (63) and unactivated KS-4V (135). Excitation at 7.6 eV with light of spark discharge. T = 80 K.
ordinated silicon and alkali ions can be separated by means of luminescence decay kinetics measurements (fig. 7). It is known that the twofoldcoordinated silicon has fast kinetics (a few ns) for the 4.4 eV band due to the singlet-singlet transitions, while the 4.4 eV luminescence band in samples doped with alkali ions has slow decay kinetics (non-exponential, lasting for about a few hundred p.s, analogous to the host material luminescence of alkali silicate glasses [10]). The measurements show fast decay kinetics of the photoluminescence at 4.4 eV excited at 7.6 eV (fig. 7) in undoped samples and this effect proves that this luminescence is due to the twofold-coordinated silicon and not to traces of alkali ions (the concentrations of which are estimated to be ~ 10 -6 mass%). Treatment in hydrogen leads to an increase of intensity of the 7.6 eV absorption band. In some cases the intensity of the 4.4 eV luminescence band decreases. Obviously, hydrogen affects radiative transitions in some way. This effect has been known since the early 1970s [2] in the case of types I and II silicas. In the type II samples, the intensity of the 4.4 eV band is less than in type I samples. However, after excitation at 8 eV, both types have the same intensity of this photoluminescence band. Such an effect was explained as photo-stimulated hydrogen removal from the luminescence center in the type II silica and in this way it became possible to avoid the hydrogen quenching effect. We have observed the same effect in hydrogen-treated samples of KS-4V. So
99
the process is similar to photo-stimulated molecular dissociation with the stabilization of products in the structure. It is possible that these products are neutral because we did not find any photoconductivity signal proportional to the intensity of the 7.6 eV band. The retrieval of the molecule is due to the thermally activated release of products from localization sites. The spectrum of these products may be very broad and the width of the TL peak in fig. 6 is consistent with this hypothesis. The absence of the 4.4 eV luminescence band in TL is consistent with the possibility that this luminescence is quenched by the removable part of the defect. The formation the 5 eV absorption band after illumination at 7.6 eV is, we suggest, related to the formation of a single twofold-coordinated silicon center. As it should be for the twofold-coordinated silicon, the band at 2.7 eV is not excited at low temperatures with 5 eV photons. In the spectrum in fig. 4, there is no band in the range of 2.7 eV, but it appears after excitation with photons with higher energy even at low temperatures. In the case of a single twofold-coordinated silicon, the band at 2.7 eV due to the tripletsinglet transitions has a slow exponential decay with a time constant equal to 10 ms [4]. However, the photoluminescence band at 2.7 eV excited at 7.6 eV has a non-exponential decay (fig. 8) unlike the exponential decay previously observed for a single center [4]. The observed increase of inten-
41" . . . . .
4t . . . .
";;:'~'.~:'-'.'--./='.
...... ...t
.............
1 2 3 4 5 6 7 8 9 10 11 12 t(ms)
Fig. 8. The comparison of luminescence decay kinetics for the band at 2.7 eV excited with a spark source (7.6 eV) in KS-4V glass (134-136).
100
A.N. Trukhin et al. / Correlation of 7.6 eV optical absorption band in silicon dioxide
sity of the luminescence band at 2.7 eV, the increase the the 4.4 eV band intensity, as well as its decay which is in the same time range as that for single twofold-coordinated silicon may serve as a basis for the interpretation of the 2.7 eV band as the result of triplet-singlet transition in the twofold-coordinated silicon incorporated into a complex defect. It is important that the total quantity of the twofold-coordinated silicon does not change during irradiation and the formation of the 5 eV band which is due to single twofold-coordinated silicon. This model was deduced by comparing the intensity of luminescence, determined by integration of the decay curves after pulsed excitation, measured before and after extended irradiation at 7.6 eV, for both luminescence bands. In spite of the luminescence intensity increase under continuous excitation, the pulsed luminescence intensity does not change when the pulsed excitation energy is 7.6 eV. It is clear, however, that the pulsed luminescence intensity at 5 eV increases after continuous irradiation at 7.6 eV. Thus we assume the existence of two processes of photodissociation of a complex defect, one fast and one slow. During the fast process, the complex defect containing an immobile twofold-coordinated silicon and a mobile part which quenches the singlet-singlet luminescence, after absorption of a photon with the energy of about 7.6 eV, is excited to an antibonding state and the distance between both parts increases. Then the excited twofold-coordinated silicon relaxes to singlet or triplet states from which a radiative transition can take place. The lifetime of the latter is sensitive to the distance between the twofold-coordinated silicon and mobile part. After emission of a photon, the complex defect returns to the initial position. The same is the slow process but it has an additional stage of photo- or thermostimulated migration of the mobile part of the complex defect and its subsequent stabilization in the structure. The smaller intensity of the TL peak after excitation at 4.5 K than after excitation at 80 K possibly points to a decrease of migration of the mobile part. The same results show that for the complex defect the retrieval process is also thermostimulated, and the wide temperature
range of the TL peak is due to the existence of many possible routes for migration and stabilization of the mobile part of the complex defect. The mobile part of the complex defect may be hydrogen a n d / o r chlorine. It is well known [2] that hydrogen suppresses the band at 5 eV and it is possible that chlorine can do the same. The mobile part of this complex defect stimulates a singlet-triplet conversion in the excited state and thus quenches singlet-singlet luminescence of the twofold coordinated silicon. A sufficiently strong triplet-singlet luminescence makes it possible to use the method of optically detected magnetic resonance for investigation of this complex defect.
5. Conclusions
The photoluminescence bands at 2.7 eV and at 4.4 eV excited in the 7.6 eV absorption band, which are observed in untreated samples, are due to twofold-coordinated silicon incorporated into a complex defect. The non-exponential decay of the 2.7 eV photoluminescence excited in the 7.6 eV absorption band can be explained as originating from an intersection of a long-lived triplet state with its surroundings. A complex defect containing twofold-coordinated silicon and a mobile part of some type is responsible for the 7.6 eV absorption band. The mobile part may be hydrogen or chlorine. Illumination at 7.6 eV leads to the complex defect photolysis with formation of a single twofold-coordinated silicon center and a mobile part which migrates in a phonon-assisted process out of the complex defect.
References [1] C.M. Nelson and R.A. Weeks, J. Appl. Phys. 32 (!961) 883. [2] J.R. Zakis and A.N. Trukhin. Utch. Zap. Latv. Univ. (Scientific Reports of the University of Latvia) 182 (1973) 50. [3] A.N. Trukhin, A.G. Boganov and A.M. Praulinsh, Fiz. Khim. Stekla (Sov. Phys. Chem. Glasses) 6 (1979) 346.
A.N. Trukhin et al. / Correlation of 7.6 eV optical absorption band in silicon dioxide
[4] L.N. Skuja, A.N., Streletsky and A.B. Pakovich, Solid State Commun. 50 (1984) 1069. [5] L.N. Skuja, A.N. Trukhin and A.E. Plaudis, Phys. Status Solidi A84 (1984) K153. [6] C.M. Gee and M. Kastner, Phys. Rev. Lett. 42 (1979) 1765. [7] E.J. Friebele, D.L. Griscom and M. Stapelbroek, Phys. Rev. Lett. 42 (1979) 1346.
101
[8] R. Tohmon, H. Mizuno, Y. Ohki, K. Sasagane, K. Nagasawa and Y. Hama, Phys. Rev. 39 (1989) 1337. [9] A.N. Trukhin, Phys. Status Solidi A93 (1986) K185. [10] A.N. Trukhin, L.B. Glebov and M.N. Tolstoi, Fiz. Khim. Stekl. (Soy. Phys. Chem. Glasses) 14 (1988) 547.
~
] O U R N A L OF
The correlation of the 7.6 eV optical absorption band in pure fused silicon dioxide with twofold-coordinated silicon A . N . T r u k h i n a n d L.N. S k u j a Institute of Solid State Physics, University of Latvia, 8 Kengaraga St, 226063 Riga, Latvia
A.G. B o g a n o v a n d V.S. R u d e n k o Institute of Silicate Chemistry, Academy of Sciences of Russia, 2 Makarova St, 199064 St Petersburg, Russia
Received 14 August 1991 Revised manuscript received 5 February 1992
The optical absorption band at 7.6 eV, which appears in oxygen deficient pure silica, does not correlate with any ESR signal in non-irradiated samples. Longlasting illumination at 80 K in the range of its absorption leads to an increase of the absorption band at 5 eV. Subsequent heating to 290 K restores the initial absorption. These data can be explained as photodissociation and thermal recreation of a complex defect containing a twofold-coordinated silicon defect. This complex defect is responsible for the 7.6 eV absorption band.
1. Introduction The properties of the 7.6 eV absorption band in silica have been studied since the 1960s [1]. In ref. [2] it was found that in non-irradiated silica of types I and II, it is possible to excite in this band the same luminescence as in the absorption band at 5 eV (bands at 2.7-3.1 eV and at 4.4 eV). T h e n in ref. [3] it was found that two different centers, one intrinsic and another G e related, give luminescence in these ranges after excitation in the bands at 5 eV and at 7.6 eV. Later it was shown that the absorption band at 5 eV is due to twofold-coordinated silicon in pure samples [4] and twofold-coordinated G e in doped samples [5]. The intrinsic luminescence appears in irradiated and non-irradiated pure samples after excitation at 7.6 eV [2]. These results were confirmed by later investigations [6]. In ref. [7], correlation between the peroxy radical E S R signal and the irradiation-induced 7.6 eV band was found and the absorption was ascribed to the peroxy radical. However, it cannot explain the absence of the
E S R signal in non-irradiated samples where this absorption band also appears. It was found [8] that the relation between the 7.6 and 5 eV bands is not a one-to-one relationship in spite of the fact that both are detected in oxygen deficient specimens. We confirm the results of ref. [8]. The initial samples in whose spectra the 7.6 eV absorption band is observed and the band at 5 eV is hardly noticeable were studied. In the present work, we have found that a band at 5 eV increases after illumination at 7.6 eV. We try to explain this effect and suggest a model for the 7.6 eV band in our samples.
2. Experimental procedure The spectra of optical absorption, photoluminescence and its excitation, as well as luminescence decay kinetics and t e m p e r a t u r e dependences of all the measured luminescent properties have been measured. The photoconductivity was also measured. Samples of fused silica of
0022-3093/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
A.N. Trukhin et al. / Correlation of 7.6 eV optical absorption band in silicon dioxide
four types were investigated. The trade marks of the samples were: KI (type I); KV (type II); KU-1, Corning 7940 (type III); and KUVI, Suprasil W1, KS-4V (type IV). The last sample was given particular attention. To analyze luminescence we used a grating monochromator, MDR-12, and to analyze absorption and excitation, a 0.5 m vacuum monochromator with a toroidal grating was used. A pulsed discharge in a H e - H 2 mixture was used to measure kinetics, the pulse duration being approximately 10 ns at half height. A deuterium discharge lamp was used for continuous illumination measurements. The samples were placed in a liquid helium cryostat. LiF windows were used on the excitation axes and type III fused silica windows for luminescence measurements. We used photomultipliers with S-20 photocathodes. The experiments were computer controlled. For photoconductivity measurements, we used a VA-J-52 electrometer.
3. Results
The optical absorption spectra of types III and IV silica glass samples are presented in fig. 1. It can be seen that the absorption band in the range near the fundamental absorption tail is observed in all the samples and that the KS-4V samples
97
4C
=c30 .2o
E20 2
I,° 3
4 hv(eV)
,-~
Fig. 2. Photoluminescence spectra of KS-4V (155) observed at 80 K under excitation with 7.6 eV photons• 1, spectrum of an initial sample; 2, after irradiation during 1 h with the same photon energy.
have the best transparency in the short wavelength range and a well defined band at 7.6 eV. It is important to mention that the band at 5 eV is also observed in these samples but its intensity is small. It can, however, be observed in sufficiently thick samples or by means of luminescence excitation spectra of the luminescence band at 4.4 eV (see below). For non-irradiated samples, the photoluminescence bands at 4.4 eV and 2.7 eV can be excited
o
2
/
/
/ /o
/
C /
0
/
1
o/
¢
E
T 5
6 - -
?
8
hv(eV) --'--l-
Fig. 1. Comparison of optical absorption spectra at room temperature in KS-4V silica glasses of different sample treatments: 135, untreated; 155, treated in CI atmosphere; 161, treated in oxygen atmosphere. W1, sample of Suprasil W1; 7940, Coming 7940.
/
/g /
7, --o~
1'0
( cnTll
'
2'0 m
Fig. 3. Dependence of photoluminescence intensity of the band at 4.4 eV on the intensity of the optical absorption band at 7.6 eV. Different points correspond to different samples of the KS-4V glass. The excitation photon energy is 7.6 eV. Points at 20 cm -1 belong to samples treated in hydrogen atmosphere at 1500 K.
A.N. Trukhin et al. / Correlation of 7.6 eV optical absorption band in silicon dioxide
98
T
15
T(K)
E ~-5
|
,
3
|
-
,
h~leV)
4
---
Fig. 4. Photolumincscence spectra of KS-4V (155) under excitation with 5.2 eV photons. ], spectrum of an initial sample; 2, after irradiation during ! h with the same photon energy.
T= 80 K.
in the range of the 7.6 eV absorption band as shown in fig. 2. Direct relation between the intensities of the luminescence band at 4.4 eV and the absorption band at 7.6 eV (fig. 3) for untreated samples has been established with the exception of the samples treated in hydrogen atmosphere at 1200°C. This treatment leads to an increase of absorption in the 7.6 eV band with parallel decrease of luminescence intensity. Analogous treatment in an oxygen atmosphere diminishes the 7.6 eV absorption band. The intensity of 4.4 eV luminescence increases with time of continuous excitation in the range of the 7.6 eV band (figs. 2, 4 and 5). While this
!
6
7
hvleV)
-
Fig. 6. The excitation spectrum of thermostimulated luminescence (TL) in KS-4V (155). Each point is measured after 1 h excitation at 80 K. The inset shows the example of TL peaks after excitation with 7.6 eV photons at 80 K (1) and 4.5 K (2).
effect is stronger at low temperatures, it is also observed at room temperature (RT). After illumination, a significant increase in the intensity of the 4.4 eV band excited at 5 eV is observed (fig. 4). Subsequent heating of the irradiated sample leads to thermostimulated luminescence (TL) in which only one luminescence band is observed at 2.7 eV. There is no evidence of a luminescence band at 4.4 eV. After heating, the initial situation is restored. The TL peak is very broad ranging from RT to 10 K. The TL peak excitation spectrum (fig. 6) starts from the 7.6 eV band and ranges to almost the fundamental absorption tail. Thus, it clearly results from the 7.6 eV band and is not related to the fundamental absorption.
f "
.-_=ze C
4. Discussion
025
/,/
oE
23
1'2
2t(min}36
=
Fig. 5. The kinetics of photoluminescence at 4.4 eV under continuous excitation with 7.6 eV photons. Solid line: sample of KS-4V treated in hydrogen atmosphere. Dashed line: in CI atmosphere. T = 80 K.
The photoluminescence bands at 2.7 eV and 4.4 eV excited at 7.6 eV may well be due to twofold-coordinated silicon, but it is not clear why the absorption band at 5 eV is so weak in the non-treated samples. On the other hand, the band at 4.4 eV occurred in KS-4V samples also after special doping with alkali ions [9] and its excitation band is situated in the vicinity of 7.6 eV, but there is no band at 5 eV. The eases of twofold-co-
A.N. Trukhin et aL / Correlation of 7.6 eV optical absorption band in silicon dioxide
KY-1-Ge
1
100C '"
I
63 100
go
•
.°.
'
100
0 0
500 1000 tins) Fig. 7. The comparison of photoluminescence decay kinetics (the range of 4.4 eV) of a Ge-activated sample (KY-1-Ge), Li-activated KS-4V (63) and unactivated KS-4V (135). Excitation at 7.6 eV with light of spark discharge. T = 80 K.
ordinated silicon and alkali ions can be separated by means of luminescence decay kinetics measurements (fig. 7). It is known that the twofoldcoordinated silicon has fast kinetics (a few ns) for the 4.4 eV band due to the singlet-singlet transitions, while the 4.4 eV luminescence band in samples doped with alkali ions has slow decay kinetics (non-exponential, lasting for about a few hundred p.s, analogous to the host material luminescence of alkali silicate glasses [10]). The measurements show fast decay kinetics of the photoluminescence at 4.4 eV excited at 7.6 eV (fig. 7) in undoped samples and this effect proves that this luminescence is due to the twofold-coordinated silicon and not to traces of alkali ions (the concentrations of which are estimated to be ~ 10 -6 mass%). Treatment in hydrogen leads to an increase of intensity of the 7.6 eV absorption band. In some cases the intensity of the 4.4 eV luminescence band decreases. Obviously, hydrogen affects radiative transitions in some way. This effect has been known since the early 1970s [2] in the case of types I and II silicas. In the type II samples, the intensity of the 4.4 eV band is less than in type I samples. However, after excitation at 8 eV, both types have the same intensity of this photoluminescence band. Such an effect was explained as photo-stimulated hydrogen removal from the luminescence center in the type II silica and in this way it became possible to avoid the hydrogen quenching effect. We have observed the same effect in hydrogen-treated samples of KS-4V. So
99
the process is similar to photo-stimulated molecular dissociation with the stabilization of products in the structure. It is possible that these products are neutral because we did not find any photoconductivity signal proportional to the intensity of the 7.6 eV band. The retrieval of the molecule is due to the thermally activated release of products from localization sites. The spectrum of these products may be very broad and the width of the TL peak in fig. 6 is consistent with this hypothesis. The absence of the 4.4 eV luminescence band in TL is consistent with the possibility that this luminescence is quenched by the removable part of the defect. The formation the 5 eV absorption band after illumination at 7.6 eV is, we suggest, related to the formation of a single twofold-coordinated silicon center. As it should be for the twofold-coordinated silicon, the band at 2.7 eV is not excited at low temperatures with 5 eV photons. In the spectrum in fig. 4, there is no band in the range of 2.7 eV, but it appears after excitation with photons with higher energy even at low temperatures. In the case of a single twofold-coordinated silicon, the band at 2.7 eV due to the tripletsinglet transitions has a slow exponential decay with a time constant equal to 10 ms [4]. However, the photoluminescence band at 2.7 eV excited at 7.6 eV has a non-exponential decay (fig. 8) unlike the exponential decay previously observed for a single center [4]. The observed increase of inten-
41" . . . . .
4t . . . .
";;:'~'.~:'-'.'--./='.
...... ...t
.............
1 2 3 4 5 6 7 8 9 10 11 12 t(ms)
Fig. 8. The comparison of luminescence decay kinetics for the band at 2.7 eV excited with a spark source (7.6 eV) in KS-4V glass (134-136).
100
A.N. Trukhin et al. / Correlation of 7.6 eV optical absorption band in silicon dioxide
sity of the luminescence band at 2.7 eV, the increase the the 4.4 eV band intensity, as well as its decay which is in the same time range as that for single twofold-coordinated silicon may serve as a basis for the interpretation of the 2.7 eV band as the result of triplet-singlet transition in the twofold-coordinated silicon incorporated into a complex defect. It is important that the total quantity of the twofold-coordinated silicon does not change during irradiation and the formation of the 5 eV band which is due to single twofold-coordinated silicon. This model was deduced by comparing the intensity of luminescence, determined by integration of the decay curves after pulsed excitation, measured before and after extended irradiation at 7.6 eV, for both luminescence bands. In spite of the luminescence intensity increase under continuous excitation, the pulsed luminescence intensity does not change when the pulsed excitation energy is 7.6 eV. It is clear, however, that the pulsed luminescence intensity at 5 eV increases after continuous irradiation at 7.6 eV. Thus we assume the existence of two processes of photodissociation of a complex defect, one fast and one slow. During the fast process, the complex defect containing an immobile twofold-coordinated silicon and a mobile part which quenches the singlet-singlet luminescence, after absorption of a photon with the energy of about 7.6 eV, is excited to an antibonding state and the distance between both parts increases. Then the excited twofold-coordinated silicon relaxes to singlet or triplet states from which a radiative transition can take place. The lifetime of the latter is sensitive to the distance between the twofold-coordinated silicon and mobile part. After emission of a photon, the complex defect returns to the initial position. The same is the slow process but it has an additional stage of photo- or thermostimulated migration of the mobile part of the complex defect and its subsequent stabilization in the structure. The smaller intensity of the TL peak after excitation at 4.5 K than after excitation at 80 K possibly points to a decrease of migration of the mobile part. The same results show that for the complex defect the retrieval process is also thermostimulated, and the wide temperature
range of the TL peak is due to the existence of many possible routes for migration and stabilization of the mobile part of the complex defect. The mobile part of the complex defect may be hydrogen a n d / o r chlorine. It is well known [2] that hydrogen suppresses the band at 5 eV and it is possible that chlorine can do the same. The mobile part of this complex defect stimulates a singlet-triplet conversion in the excited state and thus quenches singlet-singlet luminescence of the twofold coordinated silicon. A sufficiently strong triplet-singlet luminescence makes it possible to use the method of optically detected magnetic resonance for investigation of this complex defect.
5. Conclusions
The photoluminescence bands at 2.7 eV and at 4.4 eV excited in the 7.6 eV absorption band, which are observed in untreated samples, are due to twofold-coordinated silicon incorporated into a complex defect. The non-exponential decay of the 2.7 eV photoluminescence excited in the 7.6 eV absorption band can be explained as originating from an intersection of a long-lived triplet state with its surroundings. A complex defect containing twofold-coordinated silicon and a mobile part of some type is responsible for the 7.6 eV absorption band. The mobile part may be hydrogen or chlorine. Illumination at 7.6 eV leads to the complex defect photolysis with formation of a single twofold-coordinated silicon center and a mobile part which migrates in a phonon-assisted process out of the complex defect.
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A.N. Trukhin et al. / Correlation of 7.6 eV optical absorption band in silicon dioxide
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