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Correlation between 1.5 eV photoluminescence-band and 3.8 eV absorption band in silica glass
Journal of Non-Crystalline Solids 261 (2000) 21±27
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Correlation between 1.5 eV photoluminescence-band and 3.8 eV absorption band in silica glass Yuryo Sakurai *, Kaya Nagasawa Department of Electrical Engineering, Shonan Institute of Technology, 1-1-25 Tujido-Nishikaigan, Fujisawa, Kanagawa 251-8511, Japan Received 22 March 1999; received in revised form 23 September 1999
Abstract The 3.8 eV absorption band constitutes a characteristic feature of silica glass with excess oxygen. Since 1.5 eV PL band is formed in the presence of a 3.8 eV absorption band, we studied features of the PL near 1.5 eV to further understand the origin of 3.8 eV absorption band. The 1.5 eV PL band was found to have unique characteristics such as: (1) being the highest intensity PL band observed for an oxygen surplus sample with a 3.8 eV absorption band present, (2) having a long lifetime of several ms and (3) having a PL spectrum with hyper®ne structure. We also studied the relationship between 1.5 eV PL band and 3.8 eV absorption band. Two types of inter-related behaviors were observed in the intensity of 1.5 eV PL band and the amplitude of 3.8 eV absorption band. Using a thermal heat treatment experiment, we found a linear correlation between 1.5 eV PL band and 3.8 eV absorption band. Also, by comparing the eect of irradiation time on 3.8 eV absorption and 1.5 eV PL bands at 20 K, we found that an increase in the irradiation time caused a decrease in the amplitude of both 3.8 and 1.5 eV bands. Similarities were also observed in the spatial distribution of the two bands. The results obtained suggest that 3.8 eV absorption band plays a role in the formation of 1.5 eV PL band. Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Structural defects in amorphous SiO2 (a-SiO2 ) have been studied via optical absorption, photoluminescence (PL), and electron spin resonance [1]. Although the PL due to the oxygen vacancy (O3 ¹S±Si¹O3 ) [2±4] and in the non-bridging oxygen hole center (NBOHC, O3 ¹Si±O, where represents an unpaired electron) [5±15] has been investigated extensively, there are few papers about the PL band in excess oxygen states. For * Corresponding author. Tel.: +81-466 34 4111; fax: +81-466 35 8897. E-mail address: [email protected] (Y. Sakurai).
example, Awazu and Kawazoe [16] have reported a 1.9 eV PL band produced by ozone decomposition, and we have previously discussed a 2.3 eV PL band caused by a peroxy radical (POR, O3 ¹Si±O± O) [17]. Both the peroxy linkage (POL, O3 ¹Si± O±O±Si¹O3 ) [18] and the POR are oxygen surplus defects. The eects of the oxygen vacancy (the source of 2.7 and 4.3 eV PL bands) and the NBOHC (the source of 1.9 eV PL band) on the PL have been observed experimentally. However, the origin of 3.8 eV absorption band remains unclear. Since 1.5 eV PL band is formed in the presence of a 3.8 eV absorption band, we studied this relationship to further understand the origin of the 3.8 eV absorption band.
0022-3093/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 6 1 1 - 0
22
Y. Sakurai, K. Nagasawa / Journal of Non-Crystalline Solids 261 (2000) 21±27
2. Experiment Samples used in the present study are listed in Table 1. All samples were bulk a-SiO2 prepared by the plasma (Ar, O2 , and Ar + O2 ) chemical vapor deposition (CVD), ¯ame hydrolysis, and CVD soot remelting methods. Although samples A1±A4 were oxygen de®cient as a whole, the edge of the samples had a surplus of oxygen because the spatial distribution of oxygen was not uniform [19]. The oxygen partial pressure used in the synthesis of sample A2 (i.e., 1.5%) was the same as that required for normal commercial grade samples. Samples A3 (5%) and A4 (15%) were synthesized under excess oxygen pressure. Therefore, samples A3 and A4 were regarded as `oxygen de®-
cient + O2 ' [18]. Samples B1 and B3 was also an `oxygen de®cient + O2 ' type silica glass. Sample P was an oxygen-surplus-type silica glass with a 3.8 eV absorption band due to POL [18]. The samples were cut in a cylindrical shape with a diameter of 12 mm and a height of 10 mm. The cylindrical surface was a semi-circle or fan-shaped. All of the sample surfaces including the side face were mirror polished. PL measurements were made with a 1/4 meter-monochromator (320±1000 nm) equipped with a multichannel detector (200±1000 nm). The PL excitation of the samples was achieved with either the output of a He±Cd laser {325 nm (3.8 eV), 22.2 mW}, or the third harmonic of a Nd:YAG laser {355 nm (3.49 eV), 400 mJ/pulse}. Time-resolved PL spectra were measured with a
Table 1 Sample dependence of the 1.5 eV PL band excited by 3.49 eV photonsa Sample name A1 A2 A3 A4 AH1 AH2
Oxygen de®cient PO2 1.0% Oxygen de®cient PO2 1.5% Oxygen de®cient PO2 5.0% Oxygen de®cient PO2 15.0% High-OH (Oxygen de®cient + OH) High-OH (Oxygen de®cient + OH)
Manufacturing method
Impurity (ppm)
1.5 eV PL band (Normalized intensity)
Cl
OH
Ar plasma
12 000
Free
´
Ar plasma
3200
0.8
´
Ar plasma
1000
3.0
´
Ar plasma
400
3.3
(0.056)
Ar plasma +OH Ar plasma +OH
1600
20
´
340
500
´
Oxygen de®cient Mol. rate O2 :SiCl4 0.15:1 Oxygen de®cient Mol. rate O2 :SiCl4 1:1 High-OH (Oxygen de®cient + OH)
O2 plasma
1200
3.0
(0.069)
O2 plasma
340
3.0
(0.076)
O2 plasma +OH
310
200
(0.083)
P
Oxygen surplus
Ar + O2 plasma
370
0.6
(1)
D
High-OH
Flame hydrolysis
Free
1000
´
S1 S3 S4
Unknown (B2 b) Oxygen de®cient Oxygen de®cient
CVD soot remelting CVD soot remelting CVD soot remelting
0.3 0.3 Free
200 Free 6.0
´ ´ ´
B1
B3
BH1
a
Category
: observed; ´: not observed; PO2 : oxygen partial pressure during the synthesis; and CVD: chemical vapor deposition.
Y. Sakurai, K. Nagasawa / Journal of Non-Crystalline Solids 261 (2000) 21±27
pulse generator, which determines the delay time (Td ) after a pulsed excitation and exposure time (Te ). The PL decay was measured by observing the decay of the PL after a pulsed excitation by a Nd:YAG laser. In this experiment, the output current from a photomultiplier was dropped across an appropriate load resistor (1 MX) and was recorded with a digital oscilloscope. The PL spectra were corrected for the spectral response of the detection system. The absorption spectra in the visible-ultraviolet region were obtained with a double-beam UV-visible spectrometer. Measurements were carried out between room temperature and 20 K.
23
The PL spectrum of samples P and B3 excited by 3.49 eV photons at low temperature is shown in Fig. 1. We observed 1.5 eV PL spectrum after excitation with a He±Cd laser and a Nd:YAG laser at 532 nm (2.33 eV), 355 nm (3.49 eV) and 266 nm (4.66 eV). The 1.5 eV PL spectrum was not produced by illuminating the samples with a Nd:YAG laser at 2.33 and 4.66 eV. Table 1 shows a listing of measured samples excited by 3.49 eV photons at 20 K. The 1.5 eV PL
band is observed in those marked with an ` ' and is not observed for those marked with an `´'. From Table 1, 1.5 eV PL band was observed for samples prepared with an Ar plasma (A4), an O2 plasma (B1, B3, BH1), and an Ar + O2 plasma (P) but not observed for samples made with any of the other methods shown. The only spectrum with distinct hyper®ne structures was from sample P. The spectra of the other samples had weak PL intensities about 1/10 that of sample P, which made it dicult to detect any hyper®ne structures. The PL spectrum of the oxygen surplus sample P excited by 3.49 eV photons at low temperature is shown in Fig. 2. To avoid unwanted contamination of 1.5 eV PL band, the PL spectrum was measured with a Td 10 ms. The PL spectrum was characterized with a hyper®ne structure containing a total of seven peaks (P1±P7). The average peak separation was about 65 MeV. There are no reports of a PL spectrum with uniformly separated multiple peaks in silica glass except for those of Skuja [14,15] who found them originating from the NBOHC. The broad peak seen in Fig. 2 was determined to be 1.5 eV PL because the peak energy was 1.5 eV for T 6 100 K. Fig. 3(a) and (b) show the time-dependent attenuation curve of the PL intensity. (The result for the relative maximum P3 at 1.50 eV is shown in Fig. 2). In principle, measuring the time-dependent
Fig. 1. The PL spectra for samples P and B3 excited at 3.49 eV. The optical measuring interval was from 60 ns (Td ) to 10 ms (Te ) after excitation.
Fig. 2. Temperature dependence of the PL spectra for sample P excited at 3.49 eV. The optical measuring interval was from 10 ms (Td ) to 10 ms (Te ) after excitation.
3. Results 3.1. PL spectrum and lifetime
24
Y. Sakurai, K. Nagasawa / Journal of Non-Crystalline Solids 261 (2000) 21±27
2 ms in the phosphorescence region. As shown in Fig. 3(a), the time required to attain maximum PL intensity was about 0.1 ms (T 100 K), which is short when compared to the estimated lifetime. The luminescence intensity Ilumi with a radiative decay eciency g, can be given by Ilumi gIabs , where the Iabs is the light intensity absorbed by the sample with a speci®c absorption coecient and thickness. If the Iabs is constant, only the relative values of the g can be obtained. The lifetime of the 1.5 eV PL is given by s (k1 + k2 )ÿ1 , where the k1 and k2 are radiative and non-radiative decay rates, respectively. The value of s increases with decreasing k2 and decreasing temperature; i.e., g increases with decreasing temperature. Therefore, as shown in Figs. 2 and 3(a), the s and the PL intensity increased with decreasing temperature. 3.2. Correlation between the PL and the 3.8 eV absorption band Since 1.5 eV PL band is strongest in the oxygen surplus sample P, 1.5 eV PL band may be related to 3.8 eV absorption band. In fact, we did not observe 3.8 eV absorption band in the other samples. Based on these observations, we examined the relationship between the two bands using two dierent experiments: a thermal heat treatment experiment and a comparison of the eect of irradiation time on 3.8 eV absorption band and 1.5 eV at 20 K. Fig. 3. Semilogarithmic plot of the decay of the PL peak (labeled P3 in Fig. 2) in sample P (with excitation energy of 3.49 eV at 20 K) versus time: (a) 0±500 ms, (b) 0±12 ms.
attenuation curve of the PL intensity at ®xed energy is sucient for obtaining the lifetime of the PL intensity. However, the observed PL spectrum did not fully agree with the expected ideal spectrum. Using a longer time scale, the attenuation behavior (Fig. 3(b)) was observed as expected, but on a much shorter time scale (Fig. 3(a)). This indicated that the ®nite response time (the time required for the PL to reach equilibrium) aected the attenuation behavior. By extrapolating the curves in Fig. 3(b), the lifetime was estimated to be about
3.2.1. Thermal heat treatment The correlation between these two bands was investigated by a thermal heat treatment experiment. First, the PL spectrum and the amplitude of the 3.8 eV absorption band of an untreated sample (P) were measured at room temperature and at 20 K, respectively. Then, the sample was inserted into an electric furnace at a constant temperature of 900°C and removed after a ®nite period of time. The PL spectrum and the amplitude of 3.8 absorption band were repeated for a series of dierent cumulative heating time intervals. As shown in Fig. 4, the amplitude of 3.8 eV absorption band increased when (cumulative) heat was applied (annealing). If a similar change is observed in the intensity of 1.5 eV PL band after
Y. Sakurai, K. Nagasawa / Journal of Non-Crystalline Solids 261 (2000) 21±27
Fig. 4. Change in the optical absorption spectrum of sample P induced by heat treatment (at 900°C).
thermal treatment, a correlation between the two bands is likely. Fig. 5 shows the relationship between the amplitude of 3.8 eV absorption band and the intensity of 1.43 eV PL (i.e., the PL intensity of peak P3 in Fig. 2) obtained from the data cited above at 20 K. Fig. 5 represents a plot of the measured values from several samples. As shown in Fig. 5, a very good linear correlation between 1.5 eV PL band and 3.8 eV absorption band is found.
25
3.2.2. Eect of irradiation time on 3.8 eV absorption band and 1.5 eV PL band We further studied the relationship between 1.5 eV band and 3.8 eV absorption band by comparing the eect of irradiation time on 3.8 and 1.5 eV bands. The experiments were carried out in a system especially constructed to measure the PL and the absorption spectra while keeping the temperature constant at 20 K. One of the heat treated samples (P) was selected for this experiment. Therefore, the initial absorption in Figs. 4 and 6 was dierent. As shown in Fig. 6, an increase in the irradiation time caused a decrease in the amplitude of 3.8 eV absorption band at very low temperature (20 K). If a similar change in the intensity of 1.5 eV PL band is observed, a correlation is likely. As shown in Fig. 7, 1.5 eV PL band tended to decrease with increasing irradiation time; therefore, a correlation is likely. The amplitude of the 3.8 eV absorption band and 1.43 eV PL intensity were measured at varying irradiation times and plotted on a graph. Fig. 8 shows the relationship between the amplitude of 3.8 eV absorption band and the intensity of 1.43 eV PL obtained from the data cited above at 20 K. A very good linear correlation between 1.5 eV PL band and 3.8 eV absorption band was found. 3.3. Spatial distribution Finally, we studied the spatial distribution of 1.5 eV PL band and 3.8 eV absorption band. Each
Fig. 5. The relationship of 3.8 eV absorption band and 1.43 eV (labeled P2 in Fig. 2) PL intensity in sample P. Measurement reproducibility was within a 5% variance (see Section 3.4).
Fig. 6. Change in optical absorption spectra of sample P induced by dierent irradiation times, when excited at 3.8 eV at 20 K.
26
Y. Sakurai, K. Nagasawa / Journal of Non-Crystalline Solids 261 (2000) 21±27
Fig. 7. Change in the PL spectra of sample P at 20 K P induced by dierent irradiation times, when excited at 3.8 eV.
Fig. 9. The radial distribution of 1.43 eV (h) PL intensity and 3.8 eV absorption band (d) in sample P. Measurement reproducibility was within a 5% variance (see Section 3.4).
spatial distribution of 1.5 eV PL band and 3.8 eV absorption band could not be determined in the other samples because the intensity of the two bands were too weak. Fig. 9 shows the radial distributions of 1.43 eV PL (h) and 3.8 eV absorption band (d) in sample P as a function of position. The two results correlate very well qualitatively and fairly well quantitatively. As shown in Fig. 9, the tendency of distribution correlates well, and the intensity of both bands is greater at the outer edges than at the center of the sample. 3.4. Measurement reproducibility
Fig. 8. The relationship of 3.8 eV absorption band and 1.43 eV (labeled P2 in Fig. 2) PL intensity in sample P. Measurement reproducibility was within a 5% variance (see Section 3.4).
rod sample has a characteristic spatial distribution of defects [19]. Investigating the spatial distribution is very useful for establishing a relationship between the observed luminescence and 3.8 eV absorption band because we would expect to ®nd similarities in their radial distributions if a correlation does indeed exist. The radial distribution properties of 1.5 eV PL band and 3.8 eV absorption band of the rod-shaped sample P was measured using a He±Cd laser beam at 3.8 eV. The
The experiments conducted were repeated several times to obtain a PL intensity variance of less than 5% for the same heat-treatment times and temperatures. However due to the length of the experiments and the need to measure the same spot in the glass after each heat-treatment, it was not possible to investigate point-to-point variations in the sample or between samples. Consequently, while the measurement reproducibility is better than 5% in Figs. 5, 7 and 8, the sample to sample variation has not been measured. 4. Discussion By comparing the measured 1.5 eV PL intensities and 3.8 eV absorption band amplitudes (see
Y. Sakurai, K. Nagasawa / Journal of Non-Crystalline Solids 261 (2000) 21±27
Figs. 5, 8 and 9), we infer that the formation of 1.5 eV PL band is closely connected in origin with that of 3.8 eV absorption band. Peroxy linkage [18] and molecular chlorine (Cl2 ) [16] are currently proposed models for the origin of 3.8 eV absorption band. The PL in Cl2 is observed between 4.1 and 9.5 eV [20], but the existence of a PL band predicted by POL has never been demonstrated. This fact in combination with our results suggests that origin of 1.5 eV PL band is not Cl2 but POL. We determined that 1.5 eV PL spectrum with a distinct hyper®ne structure (the average peak separation was about 65 MeV.) appeared is only the oxygen surplus sample P with 3.8 eV absorption band (see Figs. 1 and 2), and also demonstrated that the PL band is strongly associated with 3.8 eV absorption band (see Table 1). From the observed results, we conclude that 1.5 eV PL band is directly related to 3.8 eV absorption band. Although the origin of 3.8 eV absorption band and the associated 1.5 eV PL band could not be determined from the results of this experiment, studies of 1.5 eV PL band in silica glass will be useful for further elucidating the origin (peroxy linkage [18], molecular chlorine [16], and other possibilities) of 3.8 eV absorption band. This is the subject for a future study, which will include a statistical analysis of the numerical values. 5. Conclusion The correlation between 1.5 eV PL band and 3.8 eV PL band has been established and is reported in detail for the ®rst time in this paper. The 1.5 eV PL band was also studied in this paper and is characterized as follows: 1. The peak energy was about 1.5 eV with a lifetime of several ms. 2. The band was observed when samples were excited by 3.49 and 3.8 eV photons but not when excited by 2.33 and 4.66 eV photons. 3. The band was observed in sample(s) prepared by the plasma deposition method and the strongest PL was observed in an excess-oxygen
27
silica glass sample with 3.8 eV absorption band also present. 4. The band was not observed at room temperature, but the PL spectrum with hyper®ne structures was found at suciently low temperatures. Acknowledgements The authors acknowledge Professors Yoshimasa Hama and Yoshimichi Ohki of Waseda University and Dr Hiroyuki Nishikawa of Tokyo Metropolitan University for helpful discussions. References [1] D.L. Griscom, J. Ceram. Soc. Japan 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] H. Nishikawa, T. Shiroyama, R. Nakamura, Y. Ohki, K. Nagasawa, Y. Hama, Phys. Rev. B 45 (1992) 586. [6] L.N. Skuja, A.R. Silin, Phys. Stat. Sol. A 56 (1979) K11. [7] G.H. Sigel, M.J. Marrone, J. Non-Cryst. Solids 45 (1981) 235. [8] J.H. Stathis, M.A. Kastner, Philos. Mag. B 49 (1984) 357. [9] J.H. Stathis, M.A. Kastner, Phys. Rev. B 35 (1987) 2972. [10] R.A.B. Devine, C. Fiori, J. Robertson, MRS Symposium Proceedings, vol. 61, Materials Research Society, Pittsburgh, PA, 1986, p. 177. [11] Y. Hibono, H. Hanafusa, J. Non-Cryst. Solids 107 (1988) 23. [12] R. Tohmon, Y. Shimogaichi, S. Munekuni, Y. Ohki, Y. Hama, K. Nagasawa, Appl. Phys. Lett. 54 (1989) 1650. [13] S. Munekuni, T. Yamanaka, Y. Shimogaichi, R. Tohmon, Y. Ohki, K. Nagasawa, Y. Hama, J. Appl. Phys. 68 (1990) 1212. [14] L. Skuja, Solid State Commun. 84 (1992) 613. [15] L. Skuja, J. Non-Cryst. Solids 179 (1994) 51. [16] K. Awazu, H. Kawazoe, J. Appl. Phys. 68 (1990) 3584. [17] Y. Sakurai, K. Nagasawa, J. Appl. Phys. 86 (1999) 1377. [18] H. Nishikawa, R. Tohmon, Y. Ohki, K. Nagasawa, Y. Hama, J. Appl. Phys. 65 (1989) 4672. [19] R. Tohmon, A. Ikeda, Y. Shimogaichi, S. Munekuni, Y. Ohki, K. Nagasawa, Y. Hama, J. Appl. Phys. 67 (1990) 1302. [20] S.D. Peyerimho, R.J. Buener, Chem. Phys. 57 (1981) 279.
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Correlation between 1.5 eV photoluminescence-band and 3.8 eV absorption band in silica glass Yuryo Sakurai *, Kaya Nagasawa Department of Electrical Engineering, Shonan Institute of Technology, 1-1-25 Tujido-Nishikaigan, Fujisawa, Kanagawa 251-8511, Japan Received 22 March 1999; received in revised form 23 September 1999
Abstract The 3.8 eV absorption band constitutes a characteristic feature of silica glass with excess oxygen. Since 1.5 eV PL band is formed in the presence of a 3.8 eV absorption band, we studied features of the PL near 1.5 eV to further understand the origin of 3.8 eV absorption band. The 1.5 eV PL band was found to have unique characteristics such as: (1) being the highest intensity PL band observed for an oxygen surplus sample with a 3.8 eV absorption band present, (2) having a long lifetime of several ms and (3) having a PL spectrum with hyper®ne structure. We also studied the relationship between 1.5 eV PL band and 3.8 eV absorption band. Two types of inter-related behaviors were observed in the intensity of 1.5 eV PL band and the amplitude of 3.8 eV absorption band. Using a thermal heat treatment experiment, we found a linear correlation between 1.5 eV PL band and 3.8 eV absorption band. Also, by comparing the eect of irradiation time on 3.8 eV absorption and 1.5 eV PL bands at 20 K, we found that an increase in the irradiation time caused a decrease in the amplitude of both 3.8 and 1.5 eV bands. Similarities were also observed in the spatial distribution of the two bands. The results obtained suggest that 3.8 eV absorption band plays a role in the formation of 1.5 eV PL band. Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Structural defects in amorphous SiO2 (a-SiO2 ) have been studied via optical absorption, photoluminescence (PL), and electron spin resonance [1]. Although the PL due to the oxygen vacancy (O3 ¹S±Si¹O3 ) [2±4] and in the non-bridging oxygen hole center (NBOHC, O3 ¹Si±O, where represents an unpaired electron) [5±15] has been investigated extensively, there are few papers about the PL band in excess oxygen states. For * Corresponding author. Tel.: +81-466 34 4111; fax: +81-466 35 8897. E-mail address: [email protected] (Y. Sakurai).
example, Awazu and Kawazoe [16] have reported a 1.9 eV PL band produced by ozone decomposition, and we have previously discussed a 2.3 eV PL band caused by a peroxy radical (POR, O3 ¹Si±O± O) [17]. Both the peroxy linkage (POL, O3 ¹Si± O±O±Si¹O3 ) [18] and the POR are oxygen surplus defects. The eects of the oxygen vacancy (the source of 2.7 and 4.3 eV PL bands) and the NBOHC (the source of 1.9 eV PL band) on the PL have been observed experimentally. However, the origin of 3.8 eV absorption band remains unclear. Since 1.5 eV PL band is formed in the presence of a 3.8 eV absorption band, we studied this relationship to further understand the origin of the 3.8 eV absorption band.
0022-3093/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 6 1 1 - 0
22
Y. Sakurai, K. Nagasawa / Journal of Non-Crystalline Solids 261 (2000) 21±27
2. Experiment Samples used in the present study are listed in Table 1. All samples were bulk a-SiO2 prepared by the plasma (Ar, O2 , and Ar + O2 ) chemical vapor deposition (CVD), ¯ame hydrolysis, and CVD soot remelting methods. Although samples A1±A4 were oxygen de®cient as a whole, the edge of the samples had a surplus of oxygen because the spatial distribution of oxygen was not uniform [19]. The oxygen partial pressure used in the synthesis of sample A2 (i.e., 1.5%) was the same as that required for normal commercial grade samples. Samples A3 (5%) and A4 (15%) were synthesized under excess oxygen pressure. Therefore, samples A3 and A4 were regarded as `oxygen de®-
cient + O2 ' [18]. Samples B1 and B3 was also an `oxygen de®cient + O2 ' type silica glass. Sample P was an oxygen-surplus-type silica glass with a 3.8 eV absorption band due to POL [18]. The samples were cut in a cylindrical shape with a diameter of 12 mm and a height of 10 mm. The cylindrical surface was a semi-circle or fan-shaped. All of the sample surfaces including the side face were mirror polished. PL measurements were made with a 1/4 meter-monochromator (320±1000 nm) equipped with a multichannel detector (200±1000 nm). The PL excitation of the samples was achieved with either the output of a He±Cd laser {325 nm (3.8 eV), 22.2 mW}, or the third harmonic of a Nd:YAG laser {355 nm (3.49 eV), 400 mJ/pulse}. Time-resolved PL spectra were measured with a
Table 1 Sample dependence of the 1.5 eV PL band excited by 3.49 eV photonsa Sample name A1 A2 A3 A4 AH1 AH2
Oxygen de®cient PO2 1.0% Oxygen de®cient PO2 1.5% Oxygen de®cient PO2 5.0% Oxygen de®cient PO2 15.0% High-OH (Oxygen de®cient + OH) High-OH (Oxygen de®cient + OH)
Manufacturing method
Impurity (ppm)
1.5 eV PL band (Normalized intensity)
Cl
OH
Ar plasma
12 000
Free
´
Ar plasma
3200
0.8
´
Ar plasma
1000
3.0
´
Ar plasma
400
3.3
(0.056)
Ar plasma +OH Ar plasma +OH
1600
20
´
340
500
´
Oxygen de®cient Mol. rate O2 :SiCl4 0.15:1 Oxygen de®cient Mol. rate O2 :SiCl4 1:1 High-OH (Oxygen de®cient + OH)
O2 plasma
1200
3.0
(0.069)
O2 plasma
340
3.0
(0.076)
O2 plasma +OH
310
200
(0.083)
P
Oxygen surplus
Ar + O2 plasma
370
0.6
(1)
D
High-OH
Flame hydrolysis
Free
1000
´
S1 S3 S4
Unknown (B2 b) Oxygen de®cient Oxygen de®cient
CVD soot remelting CVD soot remelting CVD soot remelting
0.3 0.3 Free
200 Free 6.0
´ ´ ´
B1
B3
BH1
a
Category
: observed; ´: not observed; PO2 : oxygen partial pressure during the synthesis; and CVD: chemical vapor deposition.
Y. Sakurai, K. Nagasawa / Journal of Non-Crystalline Solids 261 (2000) 21±27
pulse generator, which determines the delay time (Td ) after a pulsed excitation and exposure time (Te ). The PL decay was measured by observing the decay of the PL after a pulsed excitation by a Nd:YAG laser. In this experiment, the output current from a photomultiplier was dropped across an appropriate load resistor (1 MX) and was recorded with a digital oscilloscope. The PL spectra were corrected for the spectral response of the detection system. The absorption spectra in the visible-ultraviolet region were obtained with a double-beam UV-visible spectrometer. Measurements were carried out between room temperature and 20 K.
23
The PL spectrum of samples P and B3 excited by 3.49 eV photons at low temperature is shown in Fig. 1. We observed 1.5 eV PL spectrum after excitation with a He±Cd laser and a Nd:YAG laser at 532 nm (2.33 eV), 355 nm (3.49 eV) and 266 nm (4.66 eV). The 1.5 eV PL spectrum was not produced by illuminating the samples with a Nd:YAG laser at 2.33 and 4.66 eV. Table 1 shows a listing of measured samples excited by 3.49 eV photons at 20 K. The 1.5 eV PL
band is observed in those marked with an ` ' and is not observed for those marked with an `´'. From Table 1, 1.5 eV PL band was observed for samples prepared with an Ar plasma (A4), an O2 plasma (B1, B3, BH1), and an Ar + O2 plasma (P) but not observed for samples made with any of the other methods shown. The only spectrum with distinct hyper®ne structures was from sample P. The spectra of the other samples had weak PL intensities about 1/10 that of sample P, which made it dicult to detect any hyper®ne structures. The PL spectrum of the oxygen surplus sample P excited by 3.49 eV photons at low temperature is shown in Fig. 2. To avoid unwanted contamination of 1.5 eV PL band, the PL spectrum was measured with a Td 10 ms. The PL spectrum was characterized with a hyper®ne structure containing a total of seven peaks (P1±P7). The average peak separation was about 65 MeV. There are no reports of a PL spectrum with uniformly separated multiple peaks in silica glass except for those of Skuja [14,15] who found them originating from the NBOHC. The broad peak seen in Fig. 2 was determined to be 1.5 eV PL because the peak energy was 1.5 eV for T 6 100 K. Fig. 3(a) and (b) show the time-dependent attenuation curve of the PL intensity. (The result for the relative maximum P3 at 1.50 eV is shown in Fig. 2). In principle, measuring the time-dependent
Fig. 1. The PL spectra for samples P and B3 excited at 3.49 eV. The optical measuring interval was from 60 ns (Td ) to 10 ms (Te ) after excitation.
Fig. 2. Temperature dependence of the PL spectra for sample P excited at 3.49 eV. The optical measuring interval was from 10 ms (Td ) to 10 ms (Te ) after excitation.
3. Results 3.1. PL spectrum and lifetime
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Y. Sakurai, K. Nagasawa / Journal of Non-Crystalline Solids 261 (2000) 21±27
2 ms in the phosphorescence region. As shown in Fig. 3(a), the time required to attain maximum PL intensity was about 0.1 ms (T 100 K), which is short when compared to the estimated lifetime. The luminescence intensity Ilumi with a radiative decay eciency g, can be given by Ilumi gIabs , where the Iabs is the light intensity absorbed by the sample with a speci®c absorption coecient and thickness. If the Iabs is constant, only the relative values of the g can be obtained. The lifetime of the 1.5 eV PL is given by s (k1 + k2 )ÿ1 , where the k1 and k2 are radiative and non-radiative decay rates, respectively. The value of s increases with decreasing k2 and decreasing temperature; i.e., g increases with decreasing temperature. Therefore, as shown in Figs. 2 and 3(a), the s and the PL intensity increased with decreasing temperature. 3.2. Correlation between the PL and the 3.8 eV absorption band Since 1.5 eV PL band is strongest in the oxygen surplus sample P, 1.5 eV PL band may be related to 3.8 eV absorption band. In fact, we did not observe 3.8 eV absorption band in the other samples. Based on these observations, we examined the relationship between the two bands using two dierent experiments: a thermal heat treatment experiment and a comparison of the eect of irradiation time on 3.8 eV absorption band and 1.5 eV at 20 K. Fig. 3. Semilogarithmic plot of the decay of the PL peak (labeled P3 in Fig. 2) in sample P (with excitation energy of 3.49 eV at 20 K) versus time: (a) 0±500 ms, (b) 0±12 ms.
attenuation curve of the PL intensity at ®xed energy is sucient for obtaining the lifetime of the PL intensity. However, the observed PL spectrum did not fully agree with the expected ideal spectrum. Using a longer time scale, the attenuation behavior (Fig. 3(b)) was observed as expected, but on a much shorter time scale (Fig. 3(a)). This indicated that the ®nite response time (the time required for the PL to reach equilibrium) aected the attenuation behavior. By extrapolating the curves in Fig. 3(b), the lifetime was estimated to be about
3.2.1. Thermal heat treatment The correlation between these two bands was investigated by a thermal heat treatment experiment. First, the PL spectrum and the amplitude of the 3.8 eV absorption band of an untreated sample (P) were measured at room temperature and at 20 K, respectively. Then, the sample was inserted into an electric furnace at a constant temperature of 900°C and removed after a ®nite period of time. The PL spectrum and the amplitude of 3.8 absorption band were repeated for a series of dierent cumulative heating time intervals. As shown in Fig. 4, the amplitude of 3.8 eV absorption band increased when (cumulative) heat was applied (annealing). If a similar change is observed in the intensity of 1.5 eV PL band after
Y. Sakurai, K. Nagasawa / Journal of Non-Crystalline Solids 261 (2000) 21±27
Fig. 4. Change in the optical absorption spectrum of sample P induced by heat treatment (at 900°C).
thermal treatment, a correlation between the two bands is likely. Fig. 5 shows the relationship between the amplitude of 3.8 eV absorption band and the intensity of 1.43 eV PL (i.e., the PL intensity of peak P3 in Fig. 2) obtained from the data cited above at 20 K. Fig. 5 represents a plot of the measured values from several samples. As shown in Fig. 5, a very good linear correlation between 1.5 eV PL band and 3.8 eV absorption band is found.
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3.2.2. Eect of irradiation time on 3.8 eV absorption band and 1.5 eV PL band We further studied the relationship between 1.5 eV band and 3.8 eV absorption band by comparing the eect of irradiation time on 3.8 and 1.5 eV bands. The experiments were carried out in a system especially constructed to measure the PL and the absorption spectra while keeping the temperature constant at 20 K. One of the heat treated samples (P) was selected for this experiment. Therefore, the initial absorption in Figs. 4 and 6 was dierent. As shown in Fig. 6, an increase in the irradiation time caused a decrease in the amplitude of 3.8 eV absorption band at very low temperature (20 K). If a similar change in the intensity of 1.5 eV PL band is observed, a correlation is likely. As shown in Fig. 7, 1.5 eV PL band tended to decrease with increasing irradiation time; therefore, a correlation is likely. The amplitude of the 3.8 eV absorption band and 1.43 eV PL intensity were measured at varying irradiation times and plotted on a graph. Fig. 8 shows the relationship between the amplitude of 3.8 eV absorption band and the intensity of 1.43 eV PL obtained from the data cited above at 20 K. A very good linear correlation between 1.5 eV PL band and 3.8 eV absorption band was found. 3.3. Spatial distribution Finally, we studied the spatial distribution of 1.5 eV PL band and 3.8 eV absorption band. Each
Fig. 5. The relationship of 3.8 eV absorption band and 1.43 eV (labeled P2 in Fig. 2) PL intensity in sample P. Measurement reproducibility was within a 5% variance (see Section 3.4).
Fig. 6. Change in optical absorption spectra of sample P induced by dierent irradiation times, when excited at 3.8 eV at 20 K.
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Y. Sakurai, K. Nagasawa / Journal of Non-Crystalline Solids 261 (2000) 21±27
Fig. 7. Change in the PL spectra of sample P at 20 K P induced by dierent irradiation times, when excited at 3.8 eV.
Fig. 9. The radial distribution of 1.43 eV (h) PL intensity and 3.8 eV absorption band (d) in sample P. Measurement reproducibility was within a 5% variance (see Section 3.4).
spatial distribution of 1.5 eV PL band and 3.8 eV absorption band could not be determined in the other samples because the intensity of the two bands were too weak. Fig. 9 shows the radial distributions of 1.43 eV PL (h) and 3.8 eV absorption band (d) in sample P as a function of position. The two results correlate very well qualitatively and fairly well quantitatively. As shown in Fig. 9, the tendency of distribution correlates well, and the intensity of both bands is greater at the outer edges than at the center of the sample. 3.4. Measurement reproducibility
Fig. 8. The relationship of 3.8 eV absorption band and 1.43 eV (labeled P2 in Fig. 2) PL intensity in sample P. Measurement reproducibility was within a 5% variance (see Section 3.4).
rod sample has a characteristic spatial distribution of defects [19]. Investigating the spatial distribution is very useful for establishing a relationship between the observed luminescence and 3.8 eV absorption band because we would expect to ®nd similarities in their radial distributions if a correlation does indeed exist. The radial distribution properties of 1.5 eV PL band and 3.8 eV absorption band of the rod-shaped sample P was measured using a He±Cd laser beam at 3.8 eV. The
The experiments conducted were repeated several times to obtain a PL intensity variance of less than 5% for the same heat-treatment times and temperatures. However due to the length of the experiments and the need to measure the same spot in the glass after each heat-treatment, it was not possible to investigate point-to-point variations in the sample or between samples. Consequently, while the measurement reproducibility is better than 5% in Figs. 5, 7 and 8, the sample to sample variation has not been measured. 4. Discussion By comparing the measured 1.5 eV PL intensities and 3.8 eV absorption band amplitudes (see
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Figs. 5, 8 and 9), we infer that the formation of 1.5 eV PL band is closely connected in origin with that of 3.8 eV absorption band. Peroxy linkage [18] and molecular chlorine (Cl2 ) [16] are currently proposed models for the origin of 3.8 eV absorption band. The PL in Cl2 is observed between 4.1 and 9.5 eV [20], but the existence of a PL band predicted by POL has never been demonstrated. This fact in combination with our results suggests that origin of 1.5 eV PL band is not Cl2 but POL. We determined that 1.5 eV PL spectrum with a distinct hyper®ne structure (the average peak separation was about 65 MeV.) appeared is only the oxygen surplus sample P with 3.8 eV absorption band (see Figs. 1 and 2), and also demonstrated that the PL band is strongly associated with 3.8 eV absorption band (see Table 1). From the observed results, we conclude that 1.5 eV PL band is directly related to 3.8 eV absorption band. Although the origin of 3.8 eV absorption band and the associated 1.5 eV PL band could not be determined from the results of this experiment, studies of 1.5 eV PL band in silica glass will be useful for further elucidating the origin (peroxy linkage [18], molecular chlorine [16], and other possibilities) of 3.8 eV absorption band. This is the subject for a future study, which will include a statistical analysis of the numerical values. 5. Conclusion The correlation between 1.5 eV PL band and 3.8 eV PL band has been established and is reported in detail for the ®rst time in this paper. The 1.5 eV PL band was also studied in this paper and is characterized as follows: 1. The peak energy was about 1.5 eV with a lifetime of several ms. 2. The band was observed when samples were excited by 3.49 and 3.8 eV photons but not when excited by 2.33 and 4.66 eV photons. 3. The band was observed in sample(s) prepared by the plasma deposition method and the strongest PL was observed in an excess-oxygen
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silica glass sample with 3.8 eV absorption band also present. 4. The band was not observed at room temperature, but the PL spectrum with hyper®ne structures was found at suciently low temperatures. Acknowledgements The authors acknowledge Professors Yoshimasa Hama and Yoshimichi Ohki of Waseda University and Dr Hiroyuki Nishikawa of Tokyo Metropolitan University for helpful discussions. References [1] D.L. Griscom, J. Ceram. Soc. Japan 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] H. Nishikawa, T. Shiroyama, R. Nakamura, Y. Ohki, K. Nagasawa, Y. Hama, Phys. Rev. B 45 (1992) 586. [6] L.N. Skuja, A.R. Silin, Phys. Stat. Sol. A 56 (1979) K11. [7] G.H. Sigel, M.J. Marrone, J. Non-Cryst. Solids 45 (1981) 235. [8] J.H. Stathis, M.A. Kastner, Philos. Mag. B 49 (1984) 357. [9] J.H. Stathis, M.A. Kastner, Phys. Rev. B 35 (1987) 2972. [10] R.A.B. Devine, C. Fiori, J. Robertson, MRS Symposium Proceedings, vol. 61, Materials Research Society, Pittsburgh, PA, 1986, p. 177. [11] Y. Hibono, H. Hanafusa, J. Non-Cryst. Solids 107 (1988) 23. [12] R. Tohmon, Y. Shimogaichi, S. Munekuni, Y. Ohki, Y. Hama, K. Nagasawa, Appl. Phys. Lett. 54 (1989) 1650. [13] S. Munekuni, T. Yamanaka, Y. Shimogaichi, R. Tohmon, Y. Ohki, K. Nagasawa, Y. Hama, J. Appl. Phys. 68 (1990) 1212. [14] L. Skuja, Solid State Commun. 84 (1992) 613. [15] L. Skuja, J. Non-Cryst. Solids 179 (1994) 51. [16] K. Awazu, H. Kawazoe, J. Appl. Phys. 68 (1990) 3584. [17] Y. Sakurai, K. Nagasawa, J. Appl. Phys. 86 (1999) 1377. [18] H. Nishikawa, R. Tohmon, Y. Ohki, K. Nagasawa, Y. Hama, J. Appl. Phys. 65 (1989) 4672. [19] R. Tohmon, A. Ikeda, Y. Shimogaichi, S. Munekuni, Y. Ohki, K. Nagasawa, Y. Hama, J. Appl. Phys. 67 (1990) 1302. [20] S.D. Peyerimho, R.J. Buener, Chem. Phys. 57 (1981) 279.