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A study of the PL emission mechanisms in silica glass by considering the growth of the PL
Journal of Non-Crystalline Solids 291 (2001) 86±92
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A study of the PL emission mechanisms in silica glass by considering the growth of the PL Yuryo Sakurai a,*, Kaya Nagasawa b a
Department of Electrical, Electronics and Media Engineering, Shonan Institute of Technology, 1-1-25 Tujido-Nishikaigan, Fujisawa, Kanagawa 251-8511, Japan b Department of System and Communication Engineering, Shonan Institute of Technology, 1-1-25 Tujido-Nishikaigan, Fujisawa, Kanagawa 251-8511, Japan Received 3 October 2000; received in revised form 12 April 2001
Abstract We studied the photoluminescence (PL) emission mechanisms in silica glass by considering the growth of the PL curve. The most interesting part of this work was the study of the time required to attain maximum PL intensity. This time is due to some transfer mechanism (e.g., charge-transfer or an energy-transfer transition between energy donor defect and energy acceptor defect). We observed PL bands at 1.5, 1.75, 1.9, 2.25, 2.7, 3.08, 3.15 and 4.3 eV. Although the time required to attain maximum PL intensity for the 1.75, 1.9 and 2.25 eV PL bands excited by visible light was less than the laser pulse width [full width at half maximum FWHM 10 ns], the time required to attain maximum PL intensity for the 1.5, 1.9, 2.7, 3.08 and 3.15 eV bands excited by ultraviolet light was greater than the laser pulse width. These results indicate the possibility that some transfer mechanism is included in the PL mechanism for the 1.5, 1.9, 2.7, 3.08 and 3.15 eV bands excited by ultraviolet light. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction Optical absorption, photoluminescence (PL), electron spin resonance (ESR), and other methods have been used to study the structural defects in silica glass [1]. The PL originating in the oxygen vacancy (O3 BSi±SiBO3 ) [2±4] and the non-bridging oxygen hole center (NBOHC, O3 BSi±O ", where " represents an unpaired electron) [5±11] have been studied in depth using the peak energy, full width at half maximum (FWHM) and decay time s1 .
* Corresponding author. Tel.: +81-466 34 4111; fax: +81-466 35 8897. E-mail address: [email protected] (Y. Sakurai).
The growth and decay curves of the PL (intensity) are dierent and provide dierent information. Generally, the PL decay time has been de®ned as the time for the steady-state PL intensity to decay to 1=e, or 0.368, of the original intensity, i.e., I t I 0 exp t=s1 (t is the time and I t is the intensity or number of photons emitted per second at t). Numerous studies have reported the decay time for the PL band in silica glass when the glass is exposed to both visible and UV light. However, there has been little work reported concerning the time required to attain maximum PL intensity. For example, Tohmon et al. [8] have reported a 1.9 eV PL band excited by the 4.8 eV excitation band in various types of silica glass. We suggest that the time required to attain maximum PL intensity is very helpful in understanding the
0022-3093/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 0 8 1 1 - 0
Y. Sakurai, K. Nagasawa / Journal of Non-Crystalline Solids 291 (2001) 86±92
details of the PL mechanism in silica glass. This growth time suggests that there is either a time delay due to charge-transfer [11] or an energytransfer transition between energy donor defect and energy acceptor defect [8,9]. Therefore, we have studied the time required to attain maximum PL intensity for the 1.5 [12,13], 1.75 [14], 1.9, 2.25 [15], 2.7 [16], 3.08 [17], 3.15 [17] and 4.3 eV [16] PL bands.
2. Experiment The samples used in this study are listed in Table 1. All samples were bulk silica glass prepared by chemical vapor deposition (CVD) soot remelting methods and by plasma CVD methods [16]. All measurements were made at room temperature (outside the cryostat) or in a Cryo Kelvin helium ¯ow cryostat with temperature adjustable between 290 and 20 K. The PL was measured using an Nd:YAG laser [532 nm (2.33 eV), 355 nm (3.49 eV) and 266 nm (4.66 eV), laser pulse width (FWHM 10 ns)] as the excitation source. The PL was observed with a 1/4-m monochromator and a multichannel analyzer control system (200±1000 nm). The PL decay was measured by observing the decay of the PL after pulsed excitation by the Nd:YAG laser. The output current from the photomultiplier produces a voltage drop across a load resistor R 50 X to 1 MX, and this voltage is recorded with a digital oscilloscope. This terminal load resistance aects all observed curves. When the photocurrent decreases and the load resistance has to be increased, so that the voltage can be observed on the oscilloscope,
87
the behavior of the emission intensity is obscured because of the time delay introduced by the load resistance. On all light emissions, the terminal load resistance was gradually changed from 50 X to 1 MX. The eect of the load resistance on the PL curves was examined. The PL intensity curve was observed at the same instants of time as the exciting curve. This technique was used so that the time for the PL to reach maximum intensity could be measured with respect to the rise time of the exciting radiation. The ®rst question is whether the PL intensity reaches its peak before the exciting pulse reaches its peak or after the exciting pulse reaches its peak. In our system the times are not accurate because of the time lags introduced by the terminal load resistance. Therefore, the experiment was carried out from the viewpoint of whether a mechanism leading to a gently increasing PL response exists, rather than making numerical measurements. The experiments conducted were repeated several times to obtain a PL intensity variance of less than 5%. c-irradiation was carried out at room temperature in air using a 60 Co source with a dose rate of 10 kGy/h up to a total dose of 10 MGy.
3. Results and discussion We observed PL bands at 1.5, 1.75, 1.9, 2.25, 2.7, 3.08, 3.15 and 4.3 eV. The PL intensity curves at the 1.75, 1.9 and 2.25 eV PL bands excited by visible light were equivalent to the ones observed using the ultraviolet exciting light. In the case of the 1.5, 1.9, 2.7, 3.08 and 3.15 eV bands excited by ultraviolet light, there was an increase in the time for the curves to reach maximum strength.
Table 1 Category, manufacturing methods, and impurities of the samples used for the experiments Sample name A1 P S1 S4
Category Oxygen de®cient, PO2 1:0% Oxygen surplus Unknown B2 b) Oxygen de®cient
Manufacturing method Ar plasma Ar O2 plasma CVD soot remelting CVD soot remelting
PO2 : Oxygen partial pressure during the synthesis. CVD: chemical vapor deposition.
Impurity (ppm) Cl
OH
12 000 370 0.3 Free
Free 0.6 200 6.0
88
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3.1. Eect of the terminal load resistance We measured the extinction curve on all emission intensities and exciting lights using 50, 100, 200, 300, 500, 1k, 5k, 10k and 1 MX in order to clarify existence of the time delay to reach maximum strength. There was a gentle increase with increasing termination resistance, when we observed a delay in the light emission. As a result, the delay was generated within the time in which the exciting light reaches maximum and the time in which the light emission reaches maximum. For small termination resistance, it was necessary to increase the termination resistance to observe the existence of the delay. At either termination resistance, the delay was about 10 ns, within the time in which the exciting light reached maximum. Therefore, the time for the light emission to reach maximum is greater than this time (10 ns), when there is a delay. In this paper, the result of measuring at a termination resistance of 1 MX was shown in order to clarify the existence of the delay. We regard it as obtaining information on the existence of the delay in the build-up, though accurate numerical values were not obtained from this experiment. 3.2. 1.5 eV PL band Fig. 1 shows the emission decay of the 1.5 eV PL peak (FWHM 0:2 eV, s1 2 ms [15] excited by the 3.49 eV Nd:YAG laser R 1 MX in unirradiated sample P at 100 K. This PL is not observed with 2.33 and 4.66 eV excitations. As shown in Fig. 1 there is a gentle increase in the 1.5 eV PL intensity curve. In previous papers [12,13], we suggested that the 1.5 eV PL was associated with peroxy linkage (POL; O3 BSi±O±O±SiBO3 ). Because of the ®ne structure, we assumed that molecular vibrations in¯uence the PL spectrum [13]. If this light emission originates from POL, the vibration of the O±O bond is the most likely source of the ®ne structure. The gently increasing portion of the curve of intensity vs. time for the 1.5 eV PL is reminiscent of the temporal behavior of the atmospheric O2 b X `day glow' bonds produced by energy-transfer from O 1 D to O2 [18]. The
Fig. 1. Time decay of the 1.5 eV PL in unirradiated sample P excited by 3.49 eV at 100 K R 1 MX.
oxygen seems to be involved in the generation of the light emission, because this light emission is strong in the high oxygen content sample. This fact is the basis for our conjecture that the generation of the light emission is similar to the above-mentioned energy-transfer mechanism. Although the details of the generation of this light emission are uncertain, some energy-transfer mechanism in which the oxygen is involved seems likely. 3.3. 1.9 eV PL band We measured the extinction curve for exciting light (4.66 eV) and 1.9 eV light emission using the 50 X resistor (see Fig. 2). The time delay to reach maximum strength in both was about 20 ns, as shown in Fig. 2. This value is similar to the value
Fig. 2. Time decay of the 1.9 eV PL in 10 MGy c-irradiated sample A1 excited by 4.66 eV at 290 K R 50 X.
Y. Sakurai, K. Nagasawa / Journal of Non-Crystalline Solids 291 (2001) 86±92
(<20 ns) reported in [19]. Concerning the values reported in [3,20], these delay values seem to be caused by other factors. Since the 1.9 eV PL intensity under 2.33 eV photo-excitation (50 X) is weak, the measurement was dicult. Therefore, it was not possible to examine the dierence between the time to reach maximum strength for the 2.33 and 4.66 eV photo-excitation. The examination of this time dierence would be possible with stronger PL intensity. When the termination resistance was increased it was dicult to obtain an accurate rise time because of the eect of the time constant. But we came to the conclusion that verifying the existence of the delay was possible. Therefore, we determined the purpose of this experiment to be to obtain useful information for clarifying the emission mechanism from the build-up part of the curve of extinction of emission intensity. We emphasize the existence of the delay phenomenon and leave the numerical examination to future work. Figs. 3(a) and (b) show the 1.9 eV PL peaks (FWHM 0:2 eV, s1 14 ls) [14] excited at 2.33 (a) and 4.66 eV (b) by the Nd:YAG laser R 1 MX in 10 MGy c-irradiated sample P at 290 K. The decreasing part of the curve observed using the 4.66 eV exciting light is due to the resistance value. In Fig. 3(a) there is no time lag between the 2.33 eV excitation curve and the 1.9 eV PL, though in Fig. 3(b) there is a time lag between the 4.66 eV excitation and the 1.9 eV PL. From the results shown in Figs. 3(a) and (b), it is
89
clear that the PL mechanism is dierent in these two cases. Now look to the dierence in the increasing part of the PL curve for both excitations. The model of the 1.9 eV PL mechanism has been reported previously [8,11,21]. In the case of visible light excitation, the 1.9 eV PL is caused by direct excitation of the emitting species (NBOHC). The 1.9 eV PL and 2.0 eV absorption transition occurs due to the charge-transfer transition in NBOHC between the half-®lled non-bonding 2pp orbital of the non-bridging oxygen atom and the lone-pair 2p orbital of one of the ligand oxygens [11]. Skuja [11] proposed that the 1.9 eV PL excited by UV light is associated with the dipole moment transition in SiO4 (NBOHC). Based on this model it appears that the gentle increase in the PL curve due to ultraviolet excitation is due to the dipole moment transition. This gently increasing portion of the 1.9 eV emission curve is not observed for visible light excitation, because there is no dipole moment transition. The decreasing portion of the curve is due to energy-transfer mechanisms that occur with both excitations. Therefore, the emission lifetimes for both are equivalent. The dierence between the increasing portion of the curve for both excitations can be explained using the Skuja model [11]. When the 1.9 eV emission mechanism is clari®ed, it will be easier to understand the dierences between the increasing portions of the curves for both excitations.
Fig. 3. Time decay of the 1.9 eV PL in 10 MGy c-irradiated sample A1 excited by 2.33 (a) and 4.66 eV (b) at 290 K R 1 MX.
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Tohmon et al. [8] have suggested that the 1.9 eV PL excited by UV light is associated with an energy-transfer transition between an energy donor defect (4.8 eV absorption band) and an energy acceptor defect (NBOHC). Considering this model, we examined the slight dierence in the increasing part of the excitation curve for both excitations [see Figs. 3(a) and (b)]. We assume that the gently increasing portion of the curve of the ultraviolet radiation excitation is due to energytransfer transitions between energy donor defect (4.8 eV absorption band) and energy acceptor defect (NBOHC) [see Fig. 3(b)]. The gently increasing portion of the curve of the 1.9 eV PL intensity is not observed for visible light excitation (see Fig. 3(a)). In the case of the ozone model [21], oxygen molecules (atoms) are formed in the ozone dissociation process. Considering this model there is the possibility that the generation of light emission is similar to the above-mentioned energy-transfer mechanism [18]. In the case of the 1.9 eV PL emission we suggest that transfer mechanisms such as the dipole moment transition, energy-transfer transition between energy donor defect (4.8 eV absorption band) and energy acceptor defect (NBOHC), and the energytransfer of the oxygen molecule (atom) are possible. Our experiments indicate that all of these mechanisms can occur in PL.
Fig. 4. Time decay of the 2.7 eV PL in 10 MGy c-irradiated sample A1 excited by 4.66 eV at 290 K R 1 MX.
3.5. 3.08 and 3.15 eV PL bands Fig. 5 shows the 3.08 intensity peaks (FWHM 0:4 eV; s1 100 ls [17] and 3.15 eV (FWHM 0:4 eV; s1 10 ls [17] PL excited at 4.66 eV by the Nd:YAG laser in unirradiated samples S1 and S4 at 290 K. Fig. 4 clearly shows a gently increasing portion for both the 3.08 and 3.15 eV PL intensity curves. In a previous paper [17], we proposed that the mechanism for the 3.08 and 3.15 eV PL bands is a thermally activated radiative process for the 4.66 eV excitation. This conclusion resulted from our
3.4. 2.7 eV PL band Fig. 4 shows the 2.7 eV PL peak (FWHM 0:4 eV, s1 10 ms) [3] excited at 4.66 eV (b) by the Nd:YAG laser R 1 MX in 10 MGy c-irradiated sample A1 at 290 K. Fig. 3 clearly shows a gently increasing portion for this 2.7 eV PL intensity curve. Tohmon et al. [3] have suggested that the 2.7 eV PL is due to a triplet-to-ground state transition of a neutral oxygen vacancy defect (O3 BSi±SiBO3 ). This model likely accounts for the gently increasing portion of the curve of the 2.7 eV emission intensity because the times involved are comparable to times for the change of electron spin.
Fig. 5. Time decay of the 3.08 and 3.15 eV PL in unirradiated samples S1 and S4 excited by 4.66 eV at 290 K R 1 MX.
Y. Sakurai, K. Nagasawa / Journal of Non-Crystalline Solids 291 (2001) 86±92
study of the low temperature behavior of the PL intensity, speci®cally that both PL intensities increased incrementally with rising temperature due to the thermally activated inter-system crossing from site to site. From this fact and the results shown in Fig. 5, we assume that the gently increasing portion of the curve of both emission intensities is due to the charge-trap process. We used a terminal load resistance. There was no examination of the in¯uence of this resistance until we reached the larger emission intensities. At the larger intensities, the terminal load resistance did in¯uence the emission intensity measurements. We have not made a detailed study of the in¯uence of the load resistance on the measured intensities. By changing the terminal load resistance, we were able to examine the behavior of the PL intensity curves from initiation up to their maximum. Their behavior indicates that some transfer was taking place during this time interval. The emission intensity curves from initiation up to their maximum can be expressed as I t I 01 exp t=s2 . Knowing both s1 and s2 is necessary in order to understand how the emission intensity changes. It is dicult to understand the details of the underlying mechanisms producing the (shape of) PL intensity curves as they rise to their maximums. Accurate information on the rise of the PL curves requires direct observation of the PL growth, separate from the in¯uence of the terminal load resistance. This is a future problem. Our studies of the time required to attain maximum PL intensity in silica glass will be helpful in further work leading to a full understanding of the mechanism of luminescence. In this paper, the result of measuring at a termination resistance of 1 MX was shown in order to clarify the existence of the delay. We regard these measurements as obtaining information on the existence of the delay in the build-up, though accurate numerical values of the delay were obtained from this experiment. At present, it is also uncertain as to the cause of the delay, because the mechanism has not been clari®ed for most light emission. These are future problems, including a measuring method for the build-up time. We emphasized the existence of the delay phenomenon
91
and leave the numerical examination to future work.
4. Conclusion In order to investigate the mechanism of PL in silica glass, we measured the time required to attain maximum PL intensity for various PL bands in silica glass samples. These results indicate the possibility that charge-transfer and trap, energytransfer, and triplet excitation processes are included in the PL mechanism for the 1.5, 1.9, 2.7, 3.08 and 3.15 eV bands. Previous work oriented toward understanding the PL mechanism has been based on trying to understand what leads to the decreasing portion of the PL intensity curves. In this paper we have attempted to analyze the PL mechanism based on both the increasing and decreasing portions of the PL intensity curves. Our work is summarized as follows. 1. The time for the emission intensity to reach maximum was 10 ns (laser pulse width) or less when the ¯uorescence emission lifetime was 10 6 to 10 12 s. 2. The time for the emission intensity to reach maximum was over 10 ns (laser pulse width) when the phosphorescence radiation lifetime was 10 2 to 10 5 s. 3. The behavior of the PL intensity until it reaches maximum can be expressed as I t I 01 exp t=s2 . The exception is the 1.9 eV emission intensity curve obtained for visible light excitation. 4. A clear correlation does not exist between the behavior of the increasing part of the emission intensity curves and the emission lifetime. 5. We propose that in order to understand the emission mechanism it is necessary to study not only the lifetime but also how the PL intensity increases to its maximum.
Acknowledgements The authors would like to thank Professors Yoshimasa Hama and Yoshimichi Ohki of
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Waseda University, and Dr Hiroyuki Nishikawa for helpful discussions. References [1] D.L. Griscom, J. Ceram. Soc. Jpn. 99 (1991) 923. [2] R. Tohmon, H. Mizuno, Y. Ohki, K. Sasagane, K. Nagasawa, Y. Hama, Phys. Rev. B 39 (1989) 1337. [3] R. Tohmon, Y. Shimogaichi, H. Mizuno, Y. Ohki, K. Nagasawa, Y. Hama, Phys. Rev. Lett. 62 (1989) 1388. [4] H. Nishikawa, E. Watanabe, D. Ito, Y. Ohki, Phys. Rev. Lett. 72 (1994) 2101. [5] L.N. Skuja, A.R. Silin, Phys. Stat. Sol. A 56 (1979) K11. [6] G.H. Sigel, M.J. Marrone, J. Non-Cryst. Solids 45 (1981) 235. [7] J.H. Stathis, M.A. Kastner, Philos. Mag. B 49 (1984) 357. [8] R. Tohmon, Y. Shimogaichi, S. Munekuni, Y. Ohki, Y. Hama, K. Nagasawa, Appl. Phys. Lett. 54 (1989) 1650.
[9] S. Munekuni, T. Yamanaka, Y. Shimogaichi, R. Tohmon, Y. Ohki, K. Nagasawa, Y. Hama, J. Appl. Phys. 68 (1990) 1212. [10] H. Nishikawa, T. Shiroyama, R. Nakamura, Y. Ohki, K. Nagasawa, Y. Hama, Phys. Rev. B 45 (1992) 586. [11] L. Skuja, J. Non-Cryst. Solids 179 (1994) 51. [12] Y. Sakurai, K. Nagasawa, J. Non-Cryst. Solids 261 (2000) 21. [13] Y. Sakurai, J. Non-Cryst. Solids 276 (2000) 159. [14] Y. Sakurai, K. Nagasawa, H. Nishikawa, Y. Ohki, J. Appl. Phys. 86 (1999) 370. [15] Y. Sakurai, K. Nagasawa, J. Appl. Phys. 86 (1999) 1377. [16] Y. Sakurai, K. Nagasawa, H. Nishikawa, Y. Ohki, J. Appl. Phys. 75 (1994) 1372. [17] Y. Sakurai, J. Non-Cryst. Solids 271 (2000) 218. [18] L.C. Lee, T.G. Slanger, J. Chem. Phys. 69 (1978) 4053. [19] A. Anedda, G. Bongiovanni, M. Cannas, F. Congiu, A. Mura, M. Martini, J. Appl. Phys. 74 (1993) 3993. [20] M. Gallagher, U. Osterberg, Appl. Phys. Lett. 63 (1993) 2987. [21] K. Awazu, H. Kawazoe, J. Appl. Phys. 68 (1990) 3584.
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A study of the PL emission mechanisms in silica glass by considering the growth of the PL Yuryo Sakurai a,*, Kaya Nagasawa b a
Department of Electrical, Electronics and Media Engineering, Shonan Institute of Technology, 1-1-25 Tujido-Nishikaigan, Fujisawa, Kanagawa 251-8511, Japan b Department of System and Communication Engineering, Shonan Institute of Technology, 1-1-25 Tujido-Nishikaigan, Fujisawa, Kanagawa 251-8511, Japan Received 3 October 2000; received in revised form 12 April 2001
Abstract We studied the photoluminescence (PL) emission mechanisms in silica glass by considering the growth of the PL curve. The most interesting part of this work was the study of the time required to attain maximum PL intensity. This time is due to some transfer mechanism (e.g., charge-transfer or an energy-transfer transition between energy donor defect and energy acceptor defect). We observed PL bands at 1.5, 1.75, 1.9, 2.25, 2.7, 3.08, 3.15 and 4.3 eV. Although the time required to attain maximum PL intensity for the 1.75, 1.9 and 2.25 eV PL bands excited by visible light was less than the laser pulse width [full width at half maximum FWHM 10 ns], the time required to attain maximum PL intensity for the 1.5, 1.9, 2.7, 3.08 and 3.15 eV bands excited by ultraviolet light was greater than the laser pulse width. These results indicate the possibility that some transfer mechanism is included in the PL mechanism for the 1.5, 1.9, 2.7, 3.08 and 3.15 eV bands excited by ultraviolet light. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction Optical absorption, photoluminescence (PL), electron spin resonance (ESR), and other methods have been used to study the structural defects in silica glass [1]. The PL originating in the oxygen vacancy (O3 BSi±SiBO3 ) [2±4] and the non-bridging oxygen hole center (NBOHC, O3 BSi±O ", where " represents an unpaired electron) [5±11] have been studied in depth using the peak energy, full width at half maximum (FWHM) and decay time s1 .
* Corresponding author. Tel.: +81-466 34 4111; fax: +81-466 35 8897. E-mail address: [email protected] (Y. Sakurai).
The growth and decay curves of the PL (intensity) are dierent and provide dierent information. Generally, the PL decay time has been de®ned as the time for the steady-state PL intensity to decay to 1=e, or 0.368, of the original intensity, i.e., I t I 0 exp t=s1 (t is the time and I t is the intensity or number of photons emitted per second at t). Numerous studies have reported the decay time for the PL band in silica glass when the glass is exposed to both visible and UV light. However, there has been little work reported concerning the time required to attain maximum PL intensity. For example, Tohmon et al. [8] have reported a 1.9 eV PL band excited by the 4.8 eV excitation band in various types of silica glass. We suggest that the time required to attain maximum PL intensity is very helpful in understanding the
0022-3093/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 0 8 1 1 - 0
Y. Sakurai, K. Nagasawa / Journal of Non-Crystalline Solids 291 (2001) 86±92
details of the PL mechanism in silica glass. This growth time suggests that there is either a time delay due to charge-transfer [11] or an energytransfer transition between energy donor defect and energy acceptor defect [8,9]. Therefore, we have studied the time required to attain maximum PL intensity for the 1.5 [12,13], 1.75 [14], 1.9, 2.25 [15], 2.7 [16], 3.08 [17], 3.15 [17] and 4.3 eV [16] PL bands.
2. Experiment The samples used in this study are listed in Table 1. All samples were bulk silica glass prepared by chemical vapor deposition (CVD) soot remelting methods and by plasma CVD methods [16]. All measurements were made at room temperature (outside the cryostat) or in a Cryo Kelvin helium ¯ow cryostat with temperature adjustable between 290 and 20 K. The PL was measured using an Nd:YAG laser [532 nm (2.33 eV), 355 nm (3.49 eV) and 266 nm (4.66 eV), laser pulse width (FWHM 10 ns)] as the excitation source. The PL was observed with a 1/4-m monochromator and a multichannel analyzer control system (200±1000 nm). The PL decay was measured by observing the decay of the PL after pulsed excitation by the Nd:YAG laser. The output current from the photomultiplier produces a voltage drop across a load resistor R 50 X to 1 MX, and this voltage is recorded with a digital oscilloscope. This terminal load resistance aects all observed curves. When the photocurrent decreases and the load resistance has to be increased, so that the voltage can be observed on the oscilloscope,
87
the behavior of the emission intensity is obscured because of the time delay introduced by the load resistance. On all light emissions, the terminal load resistance was gradually changed from 50 X to 1 MX. The eect of the load resistance on the PL curves was examined. The PL intensity curve was observed at the same instants of time as the exciting curve. This technique was used so that the time for the PL to reach maximum intensity could be measured with respect to the rise time of the exciting radiation. The ®rst question is whether the PL intensity reaches its peak before the exciting pulse reaches its peak or after the exciting pulse reaches its peak. In our system the times are not accurate because of the time lags introduced by the terminal load resistance. Therefore, the experiment was carried out from the viewpoint of whether a mechanism leading to a gently increasing PL response exists, rather than making numerical measurements. The experiments conducted were repeated several times to obtain a PL intensity variance of less than 5%. c-irradiation was carried out at room temperature in air using a 60 Co source with a dose rate of 10 kGy/h up to a total dose of 10 MGy.
3. Results and discussion We observed PL bands at 1.5, 1.75, 1.9, 2.25, 2.7, 3.08, 3.15 and 4.3 eV. The PL intensity curves at the 1.75, 1.9 and 2.25 eV PL bands excited by visible light were equivalent to the ones observed using the ultraviolet exciting light. In the case of the 1.5, 1.9, 2.7, 3.08 and 3.15 eV bands excited by ultraviolet light, there was an increase in the time for the curves to reach maximum strength.
Table 1 Category, manufacturing methods, and impurities of the samples used for the experiments Sample name A1 P S1 S4
Category Oxygen de®cient, PO2 1:0% Oxygen surplus Unknown B2 b) Oxygen de®cient
Manufacturing method Ar plasma Ar O2 plasma CVD soot remelting CVD soot remelting
PO2 : Oxygen partial pressure during the synthesis. CVD: chemical vapor deposition.
Impurity (ppm) Cl
OH
12 000 370 0.3 Free
Free 0.6 200 6.0
88
Y. Sakurai, K. Nagasawa / Journal of Non-Crystalline Solids 291 (2001) 86±92
3.1. Eect of the terminal load resistance We measured the extinction curve on all emission intensities and exciting lights using 50, 100, 200, 300, 500, 1k, 5k, 10k and 1 MX in order to clarify existence of the time delay to reach maximum strength. There was a gentle increase with increasing termination resistance, when we observed a delay in the light emission. As a result, the delay was generated within the time in which the exciting light reaches maximum and the time in which the light emission reaches maximum. For small termination resistance, it was necessary to increase the termination resistance to observe the existence of the delay. At either termination resistance, the delay was about 10 ns, within the time in which the exciting light reached maximum. Therefore, the time for the light emission to reach maximum is greater than this time (10 ns), when there is a delay. In this paper, the result of measuring at a termination resistance of 1 MX was shown in order to clarify the existence of the delay. We regard it as obtaining information on the existence of the delay in the build-up, though accurate numerical values were not obtained from this experiment. 3.2. 1.5 eV PL band Fig. 1 shows the emission decay of the 1.5 eV PL peak (FWHM 0:2 eV, s1 2 ms [15] excited by the 3.49 eV Nd:YAG laser R 1 MX in unirradiated sample P at 100 K. This PL is not observed with 2.33 and 4.66 eV excitations. As shown in Fig. 1 there is a gentle increase in the 1.5 eV PL intensity curve. In previous papers [12,13], we suggested that the 1.5 eV PL was associated with peroxy linkage (POL; O3 BSi±O±O±SiBO3 ). Because of the ®ne structure, we assumed that molecular vibrations in¯uence the PL spectrum [13]. If this light emission originates from POL, the vibration of the O±O bond is the most likely source of the ®ne structure. The gently increasing portion of the curve of intensity vs. time for the 1.5 eV PL is reminiscent of the temporal behavior of the atmospheric O2 b X `day glow' bonds produced by energy-transfer from O 1 D to O2 [18]. The
Fig. 1. Time decay of the 1.5 eV PL in unirradiated sample P excited by 3.49 eV at 100 K R 1 MX.
oxygen seems to be involved in the generation of the light emission, because this light emission is strong in the high oxygen content sample. This fact is the basis for our conjecture that the generation of the light emission is similar to the above-mentioned energy-transfer mechanism. Although the details of the generation of this light emission are uncertain, some energy-transfer mechanism in which the oxygen is involved seems likely. 3.3. 1.9 eV PL band We measured the extinction curve for exciting light (4.66 eV) and 1.9 eV light emission using the 50 X resistor (see Fig. 2). The time delay to reach maximum strength in both was about 20 ns, as shown in Fig. 2. This value is similar to the value
Fig. 2. Time decay of the 1.9 eV PL in 10 MGy c-irradiated sample A1 excited by 4.66 eV at 290 K R 50 X.
Y. Sakurai, K. Nagasawa / Journal of Non-Crystalline Solids 291 (2001) 86±92
(<20 ns) reported in [19]. Concerning the values reported in [3,20], these delay values seem to be caused by other factors. Since the 1.9 eV PL intensity under 2.33 eV photo-excitation (50 X) is weak, the measurement was dicult. Therefore, it was not possible to examine the dierence between the time to reach maximum strength for the 2.33 and 4.66 eV photo-excitation. The examination of this time dierence would be possible with stronger PL intensity. When the termination resistance was increased it was dicult to obtain an accurate rise time because of the eect of the time constant. But we came to the conclusion that verifying the existence of the delay was possible. Therefore, we determined the purpose of this experiment to be to obtain useful information for clarifying the emission mechanism from the build-up part of the curve of extinction of emission intensity. We emphasize the existence of the delay phenomenon and leave the numerical examination to future work. Figs. 3(a) and (b) show the 1.9 eV PL peaks (FWHM 0:2 eV, s1 14 ls) [14] excited at 2.33 (a) and 4.66 eV (b) by the Nd:YAG laser R 1 MX in 10 MGy c-irradiated sample P at 290 K. The decreasing part of the curve observed using the 4.66 eV exciting light is due to the resistance value. In Fig. 3(a) there is no time lag between the 2.33 eV excitation curve and the 1.9 eV PL, though in Fig. 3(b) there is a time lag between the 4.66 eV excitation and the 1.9 eV PL. From the results shown in Figs. 3(a) and (b), it is
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clear that the PL mechanism is dierent in these two cases. Now look to the dierence in the increasing part of the PL curve for both excitations. The model of the 1.9 eV PL mechanism has been reported previously [8,11,21]. In the case of visible light excitation, the 1.9 eV PL is caused by direct excitation of the emitting species (NBOHC). The 1.9 eV PL and 2.0 eV absorption transition occurs due to the charge-transfer transition in NBOHC between the half-®lled non-bonding 2pp orbital of the non-bridging oxygen atom and the lone-pair 2p orbital of one of the ligand oxygens [11]. Skuja [11] proposed that the 1.9 eV PL excited by UV light is associated with the dipole moment transition in SiO4 (NBOHC). Based on this model it appears that the gentle increase in the PL curve due to ultraviolet excitation is due to the dipole moment transition. This gently increasing portion of the 1.9 eV emission curve is not observed for visible light excitation, because there is no dipole moment transition. The decreasing portion of the curve is due to energy-transfer mechanisms that occur with both excitations. Therefore, the emission lifetimes for both are equivalent. The dierence between the increasing portion of the curve for both excitations can be explained using the Skuja model [11]. When the 1.9 eV emission mechanism is clari®ed, it will be easier to understand the dierences between the increasing portions of the curves for both excitations.
Fig. 3. Time decay of the 1.9 eV PL in 10 MGy c-irradiated sample A1 excited by 2.33 (a) and 4.66 eV (b) at 290 K R 1 MX.
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Tohmon et al. [8] have suggested that the 1.9 eV PL excited by UV light is associated with an energy-transfer transition between an energy donor defect (4.8 eV absorption band) and an energy acceptor defect (NBOHC). Considering this model, we examined the slight dierence in the increasing part of the excitation curve for both excitations [see Figs. 3(a) and (b)]. We assume that the gently increasing portion of the curve of the ultraviolet radiation excitation is due to energytransfer transitions between energy donor defect (4.8 eV absorption band) and energy acceptor defect (NBOHC) [see Fig. 3(b)]. The gently increasing portion of the curve of the 1.9 eV PL intensity is not observed for visible light excitation (see Fig. 3(a)). In the case of the ozone model [21], oxygen molecules (atoms) are formed in the ozone dissociation process. Considering this model there is the possibility that the generation of light emission is similar to the above-mentioned energy-transfer mechanism [18]. In the case of the 1.9 eV PL emission we suggest that transfer mechanisms such as the dipole moment transition, energy-transfer transition between energy donor defect (4.8 eV absorption band) and energy acceptor defect (NBOHC), and the energytransfer of the oxygen molecule (atom) are possible. Our experiments indicate that all of these mechanisms can occur in PL.
Fig. 4. Time decay of the 2.7 eV PL in 10 MGy c-irradiated sample A1 excited by 4.66 eV at 290 K R 1 MX.
3.5. 3.08 and 3.15 eV PL bands Fig. 5 shows the 3.08 intensity peaks (FWHM 0:4 eV; s1 100 ls [17] and 3.15 eV (FWHM 0:4 eV; s1 10 ls [17] PL excited at 4.66 eV by the Nd:YAG laser in unirradiated samples S1 and S4 at 290 K. Fig. 4 clearly shows a gently increasing portion for both the 3.08 and 3.15 eV PL intensity curves. In a previous paper [17], we proposed that the mechanism for the 3.08 and 3.15 eV PL bands is a thermally activated radiative process for the 4.66 eV excitation. This conclusion resulted from our
3.4. 2.7 eV PL band Fig. 4 shows the 2.7 eV PL peak (FWHM 0:4 eV, s1 10 ms) [3] excited at 4.66 eV (b) by the Nd:YAG laser R 1 MX in 10 MGy c-irradiated sample A1 at 290 K. Fig. 3 clearly shows a gently increasing portion for this 2.7 eV PL intensity curve. Tohmon et al. [3] have suggested that the 2.7 eV PL is due to a triplet-to-ground state transition of a neutral oxygen vacancy defect (O3 BSi±SiBO3 ). This model likely accounts for the gently increasing portion of the curve of the 2.7 eV emission intensity because the times involved are comparable to times for the change of electron spin.
Fig. 5. Time decay of the 3.08 and 3.15 eV PL in unirradiated samples S1 and S4 excited by 4.66 eV at 290 K R 1 MX.
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study of the low temperature behavior of the PL intensity, speci®cally that both PL intensities increased incrementally with rising temperature due to the thermally activated inter-system crossing from site to site. From this fact and the results shown in Fig. 5, we assume that the gently increasing portion of the curve of both emission intensities is due to the charge-trap process. We used a terminal load resistance. There was no examination of the in¯uence of this resistance until we reached the larger emission intensities. At the larger intensities, the terminal load resistance did in¯uence the emission intensity measurements. We have not made a detailed study of the in¯uence of the load resistance on the measured intensities. By changing the terminal load resistance, we were able to examine the behavior of the PL intensity curves from initiation up to their maximum. Their behavior indicates that some transfer was taking place during this time interval. The emission intensity curves from initiation up to their maximum can be expressed as I t I 01 exp t=s2 . Knowing both s1 and s2 is necessary in order to understand how the emission intensity changes. It is dicult to understand the details of the underlying mechanisms producing the (shape of) PL intensity curves as they rise to their maximums. Accurate information on the rise of the PL curves requires direct observation of the PL growth, separate from the in¯uence of the terminal load resistance. This is a future problem. Our studies of the time required to attain maximum PL intensity in silica glass will be helpful in further work leading to a full understanding of the mechanism of luminescence. In this paper, the result of measuring at a termination resistance of 1 MX was shown in order to clarify the existence of the delay. We regard these measurements as obtaining information on the existence of the delay in the build-up, though accurate numerical values of the delay were obtained from this experiment. At present, it is also uncertain as to the cause of the delay, because the mechanism has not been clari®ed for most light emission. These are future problems, including a measuring method for the build-up time. We emphasized the existence of the delay phenomenon
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and leave the numerical examination to future work.
4. Conclusion In order to investigate the mechanism of PL in silica glass, we measured the time required to attain maximum PL intensity for various PL bands in silica glass samples. These results indicate the possibility that charge-transfer and trap, energytransfer, and triplet excitation processes are included in the PL mechanism for the 1.5, 1.9, 2.7, 3.08 and 3.15 eV bands. Previous work oriented toward understanding the PL mechanism has been based on trying to understand what leads to the decreasing portion of the PL intensity curves. In this paper we have attempted to analyze the PL mechanism based on both the increasing and decreasing portions of the PL intensity curves. Our work is summarized as follows. 1. The time for the emission intensity to reach maximum was 10 ns (laser pulse width) or less when the ¯uorescence emission lifetime was 10 6 to 10 12 s. 2. The time for the emission intensity to reach maximum was over 10 ns (laser pulse width) when the phosphorescence radiation lifetime was 10 2 to 10 5 s. 3. The behavior of the PL intensity until it reaches maximum can be expressed as I t I 01 exp t=s2 . The exception is the 1.9 eV emission intensity curve obtained for visible light excitation. 4. A clear correlation does not exist between the behavior of the increasing part of the emission intensity curves and the emission lifetime. 5. We propose that in order to understand the emission mechanism it is necessary to study not only the lifetime but also how the PL intensity increases to its maximum.
Acknowledgements The authors would like to thank Professors Yoshimasa Hama and Yoshimichi Ohki of
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Waseda University, and Dr Hiroyuki Nishikawa for helpful discussions. References [1] D.L. Griscom, J. Ceram. Soc. Jpn. 99 (1991) 923. [2] R. Tohmon, H. Mizuno, Y. Ohki, K. Sasagane, K. Nagasawa, Y. Hama, Phys. Rev. B 39 (1989) 1337. [3] R. Tohmon, Y. Shimogaichi, H. Mizuno, Y. Ohki, K. Nagasawa, Y. Hama, Phys. Rev. Lett. 62 (1989) 1388. [4] H. Nishikawa, E. Watanabe, D. Ito, Y. Ohki, Phys. Rev. Lett. 72 (1994) 2101. [5] L.N. Skuja, A.R. Silin, Phys. Stat. Sol. A 56 (1979) K11. [6] G.H. Sigel, M.J. Marrone, J. Non-Cryst. Solids 45 (1981) 235. [7] J.H. Stathis, M.A. Kastner, Philos. Mag. B 49 (1984) 357. [8] R. Tohmon, Y. Shimogaichi, S. Munekuni, Y. Ohki, Y. Hama, K. Nagasawa, Appl. Phys. Lett. 54 (1989) 1650.
[9] S. Munekuni, T. Yamanaka, Y. Shimogaichi, R. Tohmon, Y. Ohki, K. Nagasawa, Y. Hama, J. Appl. Phys. 68 (1990) 1212. [10] H. Nishikawa, T. Shiroyama, R. Nakamura, Y. Ohki, K. Nagasawa, Y. Hama, Phys. Rev. B 45 (1992) 586. [11] L. Skuja, J. Non-Cryst. Solids 179 (1994) 51. [12] Y. Sakurai, K. Nagasawa, J. Non-Cryst. Solids 261 (2000) 21. [13] Y. Sakurai, J. Non-Cryst. Solids 276 (2000) 159. [14] Y. Sakurai, K. Nagasawa, H. Nishikawa, Y. Ohki, J. Appl. Phys. 86 (1999) 370. [15] Y. Sakurai, K. Nagasawa, J. Appl. Phys. 86 (1999) 1377. [16] Y. Sakurai, K. Nagasawa, H. Nishikawa, Y. Ohki, J. Appl. Phys. 75 (1994) 1372. [17] Y. Sakurai, J. Non-Cryst. Solids 271 (2000) 218. [18] L.C. Lee, T.G. Slanger, J. Chem. Phys. 69 (1978) 4053. [19] A. Anedda, G. Bongiovanni, M. Cannas, F. Congiu, A. Mura, M. Martini, J. Appl. Phys. 74 (1993) 3993. [20] M. Gallagher, U. Osterberg, Appl. Phys. Lett. 63 (1993) 2987. [21] K. Awazu, H. Kawazoe, J. Appl. Phys. 68 (1990) 3584.