- Email: [email protected]
Correlation between the 2.7 eV and 4.3 eV photoluminescence bands in silica glass
LETTER TO THE EDITOR
Journal of Non-Crystalline Solids 352 (2006) 2917–2920 www.elsevier.com/locate/jnoncrysol
Letter to the Editor
Correlation between the 2.7 eV and 4.3 eV photoluminescence bands in silica glass Yuryo Sakurai
*
Department of Electrical, Electronics and Media Engineering, Shonan Institute of Technology, 1-1-25 Tujido-Nishikaigan, Fujisawa, Kanagawa 251-8511, Japan Received 22 August 2005; received in revised form 6 February 2006 Available online 5 June 2006
Abstract Our previous studies have reported the excitation energy dependence of the 2.7 and 4.3 eV photoluminescence (PL) bands in oxygen deficient silica glass at low temperature (20 K). An oxygen vacancy (O3„SiASi„O3) was thought to be the origin of the two PL bands. In order to verify the origin of the 2.7 and 4.3 eV PL bands in silica glass, we measured the PL band of various thermally heat treated silica glasses. In the sample after heat treatment, we did not observe the 4.3 eV PL band, though we did observe the 2.7 eV PL band. These results suggest that these two PL bands do not have a common origin. 2006 Elsevier B.V. All rights reserved. PACS: 61.43.Fs; 61.72.Ji; 78.55.Hx Keywords: Luminescence; Silica; Defects; Short-range order
1. Introduction Optical absorption bands at 5.0 and 7.6 eV, and photoluminescence (PL) bands at 2.7 and 4.3 eV have been associated with oxygen deficiency in high-purity silica glass [1–5]. To date, two structural models have been proposed for these absorption and PL bands; an oxygen vacancy (O3„SiASi„O3, 5.0 and 7.6 eV absorption bands) [1–6] and a twofold coordinated silicon (O2@Si:) [6–8]. There is a strong consensus in the literature that PL in the 4.2– 4.4 eV range arises from oxygen vacancy centers [9]. Generally, the 2.7 eV and 4.3 eV PL intensities are strong in the oxygen-deficiency-type sample, and they are observed together. Therefore, it is thought that the origin of the two PL bands is identical, i.e., the two PL bands are transitions from the excited state to the ground state of the oxygen vacancy.
On the other hand, there is a report that the origin of the two PL bands is not identical. Tohmon et al. [2,3] has shown the relationship between 2.7 and 4.4 eV PL bands and oxygen vacancy by an ab initio molecular-orbital calculation using the cluster (HO)3SiASi(OH)3. Though verification of the existence of the 4.4 eV PL band failed, the existence of the 2.7 eV PL band and the two absorption bands was verified [2,3]. Itoh et al. [10] conclude that the 2.7 eV PL band was the recombination of the self-trapped excitation. The purpose of this article is to examine whether the origin of the 2.7 and 4.3 eV PL bands is identical. Based on work reported in our previous papers [11–13] we have continued to investigate the thermal heat treatment characteristics of the 2.7 eV and 4.3 eV PL bands in silica glass excited by UV light. 2. Experiment
*
Tel.: +81 466 34 4111; fax: +81 466 35 8897. E-mail address: [email protected]
0022-3093/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.02.089
The samples used in this study are listed in Table 1. The lower detection limit value of impurity for the samples
LETTER TO THE EDITOR
2918
Y. Sakurai / Journal of Non-Crystalline Solids 352 (2006) 2917–2920
Table 1 Category, manufacturing methods, and impurities of the samples used for the experiments Sample name
Category
Manufacturing method
Impurity (ppm) Cl
OH
P D S1 S2 S3 S4
Oxygen surplus High-OH Unknown (B2b) Oxygen deficient Oxygen deficient Oxygen deficient
Ar + O2 plasma Flame hydrolysis CVD soot remelting CVD soot remelting CVD soot remelting CVD soot remelting
370 ND 0.3 0.2 0.3 ND
0.6 1000 200 ND ND 6.0
CVD: Chemical vapor deposition. ND: Not detected.
shown in Table 1 is 1 ppm or less. All samples were bulk amorphous SiO2 prepared by the plasma (Ar + O2) chemical vapor deposition (CVD), flame hydrolysis, and CVD soot remelting methods. All of the sample (20 · 20 · 20 mm) surfaces including the sides were polished. The PL was detected by a monochromator (Jobin-Yvon, HR250) equipped with a multichannel detector (Atago Max-3000). The PL excitation was with a Nd:YAG laser [266 nm (4.66 eV), laser pulse width (FWHM: 10 ns)] light source. The time-resolved PL spectra were measured with a pulse generator (Stanford Research System, DG535) that controls the delay times (td) following the laser pulse, and the exposure times (te). Generally, td = 0 s is when the light emission begins. However, it was difficult to accurately determine the time when light emission started with the measuring instruments used in this experiment. High precision measuring instruments are necessary in order to accurately determine when light emission starts. In this experiment we standardized the value of td as the time between when the sample was exposed to the exciting light and the start of spectroscopy. This can be controlled by the output waveform of the pulse generator and timing of the observation of the spectrum. First, various adjustment were carried out, while the excit-
ing light and output waveform of the pulse generator were observed with the digital oscilloscope. In this way it was possible to adjust the timing. Next, measurement of the spectrum of the exciting light was carried out, while td was adjusted. The result of this work was that we were able to determine the (20 ns) error in the response time of the gate circuit of the multichannel detector. It was difficult to remove the effect of this error. In the spectroscopy of the PL band with emission lifetime over 100 ns, the effect of this error can be disregarded. However, the effect of this error cannot be disregarded in the spectroscopy of the PL band with emission lifetime of 10 ns or less. Therefore, the measurement was carried out, when td was observed in ns order. The PL decay was measured by observing the decay of the PL after a pulsed excitation with the Nd:YAG laser. The voltage across a resistor (R = 50 X 1 MX) carrying the output current from a photomultiplier (Hamamatsu, E2762) was recorded with a digital oscilloscope (Tektronix, 2440). All measurements were made at 290 K. PL spectra were normalized to the spectral response of the detection system using a standard tungsten lamp. The melting processing was carried out using an H2/O2 burner in atmosphere. It was easy to melt the angular part of the sample in comparison with the plane part. The temperature during the melting is uncertain. The melting was maintained until an area sufficient for the light emission observation was obtained. The irradiation of the YAG laser light to the melted part of the sample that was melted was controlled by the slit. The experiments were repeated several times to obtain a PL intensity variance of less than 5%. 3. Results and discussion Fig. 1 shows the spectra of the non treated oxygen-deficient-type samples (a) S3 and (b) S4 excited at 4.66 eV by the Nd:YAG laser at 290 K measured at td < 10 ns and te = 10 ms. A PL band with peak energy near 3.1 eV and full width at half maximum (FWHM) of 0.44 eV was
Fig. 1. Time-resolved PL spectra of the non-treated oxygen-deficient-type samples (a) S3 and (b) S4 excited at 4.66 eV by the Nd:YAG laser at 290 K measured at td < 10 ns and te = 10 ms at 290 K.
LETTER TO THE EDITOR
Y. Sakurai / Journal of Non-Crystalline Solids 352 (2006) 2917–2920
observed. The peak energy (3.15 eV lifetime, s: 10 ls) of sample S4 is slightly larger than that for sample S3 (3.08 eV, s: 110 ls). The peak energy and FWHM, of the PL spectrum for sample S3, is similar to those observed for the B2b type sample S1 [14]. The 2.7 eV PL band (FWHM: 0.4 eV, s: 10 ms) is generally observed in the oxygen deficient type sample. It is difficult to confirm the whole spectrum of the 4.3 eV PL band because the PL intensity is very weak and overlaps the exciting light (4.66 eV). The component that appeared near the 4.0 eV is a part of the 4.3 eV PL band. Fig. 2 shows the spectra of the heat treated oxygen-deficient-type samples S3 and S4 excited at 4.66 eV by the Nd: YAG laser at 290 K measured at td < 10 ns and te = 10 ms. In sample S3, only the 2.7 eV PL band was observed. On the other hand, in sample S4, the 2.7 and 3.15 eV PL bands
Fig. 2. Time-resolved PL spectra of the heat treated oxygen-deficient-type samples S3 and S4 excited at 4.66 eV by the Nd:YAG laser at 290 K measured at td < 10 ns and te = 10 ms at 290 K.
Fig. 3. Time decay of the heat treated oxygen-deficient-type samples S3 and S4 excited at 4.66 eV by the Nd:YAG laser at 290 K.
2919
were observed. The 4.3 eV PL band was not observed in either sample. Fig. 3 shows the emission decay of the 2.7 eV PL peak excited at 4.66 eV for the heat treated oxygen-deficient-type samples S3 and S4 excited at 290 K (R = 1 MX). As shown in Fig. 3, the decay time at 290 K was estimated from the slope of the curve to be 10 ms. The curve follows a single exponential [exp(t/s)], and the lifetime of the 2.7 eV PL band was equivalent to the numerical value (10 ms) previously reported. At the outset of our work we assumed that the PL band would not be observed because the point defects (origin of the PL emission), which exist in the samples, disappears with heat treatment. However, the PL band was observed. We also noticed that the PL bands of the 2.7 and 4.3 eV were not observed together. The first time that we observed that the 2.7 and 4.3 eV bands did not occur together we thought that we may have made a mistake. There is a situation in which they cannot be detected, and that is when the numerical value of td is decreased, because the lifetime of the 4.3 eV PL band is short, a few ns [12]. Therefore, we looked for these two bands at various td. The result of these measurements was that the 4.3 eV PL band could not be detected. Though the 2.7 eV PL band was also observed in samples S1, S2 and D, the 4.3 eV PL band was not. In sample P, two PL bands were not observed. Except for this experiment, the two PL bands were observed together. Therefore, the observation was carried out carefully when the 4.3 eV PL band was hard to observe in the presence of the 2.7 eV PL band. It was possible to confirm the existence of the 4.3 eV PL band in the presence of the 2.7 eV PL band by adjusting td, even if the 4.3 eV PL band could not be detected by setting td. Therefore observation of the 2.7 eV PL band was made very carefully when the 4.3 eV band was hard to observe. The ratio of 2.7 eV PL intensity to 4.3 eV PL intensity for D2 lamp excitation was about 1:4. Therefore, the spectrum intensity ratio of about 1:4 is obtained as the ratio of 2.7 eV PL intensity and 4.3 eV PL intensity. If it is, then td * 0 s. The PL intensity drops to about 1/10 after about 10 ns, after light is emitted, if the 4.3 eV PL emission lifetime is 5 ns [12]. Generally, the PL emission lifetime has been defined 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/s) (t is the time and I(t) is the intensity, or number of photons emitted per second at t). The strength becomes about 1/10, when the life observed 5 ns emission intensity at td = 10 ns, i.e., I(td) becomes I(0) exp(10 ns/5 ns) = I(0) · 0.135) as s = 5 ns and t = td = 10 ns. It is possible to experientially estimate the value of td from these results. It is important to observe the 2.7 eV PL band and exciting light in Fig. 2. If the 4.3 eV PL band exists, the tail of this band can be detected near 4.0 eV, even if it overlaps with the exciting light. We took the non-existence of this tail as evidence that the 4.3 eV PL band did not exist. Prior to this work we believed that the origin of the 2.7 and 4.3 eV PL bands was identical. Based on these latest
LETTER TO THE EDITOR
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Y. Sakurai / Journal of Non-Crystalline Solids 352 (2006) 2917–2920
results we believe that the origin of the two PL bands is not identical. If our observed results are correct, it is necessary to reexamine the origin of the 2.7 and 4.3 eV PL bands. We believe there is a low probability that the observed 2.7 and 3.15 eV PL bands are associated with impurities because we used samples with impurity concentrations of under 1 ppm, i.e., the origin of the observed 2.7 and 3.15 eV PL [14] are not metal impurities but most likely oxygen-deficient-associated defect centers. The origin of 3.1 eV light emission from sample S3 seems to be a structural defect in which an OH radical is involved. The intensity ratio of 3.1 eV and 2.7 eV light emission with heat treatment was examined. The intensity ratio of the two light emissions was equivalent before the heat treatment. In sample S3, the intensity ratio of the two light emissions changes greatly, and in sample S4, there is a small change. Considering this result, the structural defect in which an OH radical in sample S3 was involved seemed to decompose with heat treatment. In the case of sample S4, the 3.1 eV light emission seemed to increase with respect to the 2.7 eV light emission with heat treatment. The structure of the origin of these light emissions is unknown at this time. Tohmon et al. [2,3] has concluded that they succeeded in demonstrating the oxygen vacancy as the origin of the 2.7 eV PL band and failed in demonstrating the oxygen vacancy as the origin of the 4.4 eV PL band. This conclusion is based on the assumption that the origin of the two PL bands is identical. Based on the our observations, we conclude that their calculated result was correct, i. e., oxygen vacancy is not the origin of the 4.3 eV PL band. Our results reported in this paper support the observations reported in Refs. [2,3]. If the oxygen vacancy is the origin of the 2.7 eV PL band, we can assume that the desorption of the oxygen from the SiAOASi bond was generated by the melting (SiAOASi ! SiASi + O). Based on the above information the oxygen vacancy is proposed as the origin of the 2.7 eV PL band, while it is not considered as the origin of the 4.3 eV PL band. There is the E 0 center (the 5.8 eV absorption band) as a point defect which has not been examined as a possible PL emission origin in silica glass. Therefore, it is necessary to examine the relation between the 4.3 eV PL band and the E 0 center. The E 0 center is one of the oxygen deficient type defects, and there is a high probability of this defect forming with the oxygen vacancy. Therefore, it is important to make a sample with only the E 0 center. The results of our work increase the need to clarify whether the E 0 center is
connected with the origin of generation of the 4.3 PL band. In order to realize it, it is necessary to make the sample which conducted the special processing [15]. This is a problem for further study. 4. Conclusion In summary, we measured the PL properties of the sample on which we carried out the melting processing. Our results indicate that there is a high probability that the origin of the 2.7 and 4.3 eV PL bands is not identical. Though the melting mechanism could not be clarified, this study should be useful in clarifying the relation of the PL band to oxygen vacancy, and in understanding the structural defects in silica glass. Acknowledgements The authors would like to thank Professor Kaya Nagasawa of Shonan Institute of Technology, Professors Yoshimasa Hama and Yoshimichi Ohki of Waseda University, and Assistant Professor Hiroyuki Nishikawa of shibaura Institute of Technology for help during the course of this work. References [1] H. Imai, K. Arai, H. Imagawa, H. Hosono, Y. Abe, Phys. Rev. B 38 (1988) 12772. [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. Hosono, Y. Abe, H. Imagawa, H. Imai, K. Arai, Phys. Rev. B 44 (1991) 12043. [5] L.N. Skuja, J. Non-Cryst. Solids 149 (1992) 77. [6] L.N. Skuja, J. Non-Cryst. Solids 167 (1994) 229. [7] G.W. Arnold, IEEE Trans. Nucl. Sci. NS-20 (1973) 220. [8] L.N. Skuja, A.R. Silin, J. Mares, Phys. Stat. Sol. A 50 (1978) K149. [9] L.N. Skuja, A.N. Streletsky, A.B. Pakovich, Sol. Stat. Comm. 50 (1984) 1069. [10] C. Itoh, K. Tanimura, N. Itoh, J. Phys. C 21 (1988) 4693. [11] Y. Sakurai, K. Nagasawa, H. Nishikawa, Y. Ohki, J. Appl. Phys. 75 (1994) 1372. [12] H. Nishikawa, Y. Miyake, E. Watanabe, D. Ito, K.S. Seol, Y. Ohki, K. Ishii, Y. Sakurai, K. Nagasawa, J Non-Cryst. Solids 222 (1997) 221. [13] Y. Sakurai, K. Nagasawa, J. Non-Cryst. Solids 290 (2001) 189. [14] Y. Sakurai, J. Non-Cryst. Solids 271 (1997) 218. [15] M.A. Stevens-Kalceff, J. Wong, J Appl. Phys. 97 (2005) 113519.
Journal of Non-Crystalline Solids 352 (2006) 2917–2920 www.elsevier.com/locate/jnoncrysol
Letter to the Editor
Correlation between the 2.7 eV and 4.3 eV photoluminescence bands in silica glass Yuryo Sakurai
*
Department of Electrical, Electronics and Media Engineering, Shonan Institute of Technology, 1-1-25 Tujido-Nishikaigan, Fujisawa, Kanagawa 251-8511, Japan Received 22 August 2005; received in revised form 6 February 2006 Available online 5 June 2006
Abstract Our previous studies have reported the excitation energy dependence of the 2.7 and 4.3 eV photoluminescence (PL) bands in oxygen deficient silica glass at low temperature (20 K). An oxygen vacancy (O3„SiASi„O3) was thought to be the origin of the two PL bands. In order to verify the origin of the 2.7 and 4.3 eV PL bands in silica glass, we measured the PL band of various thermally heat treated silica glasses. In the sample after heat treatment, we did not observe the 4.3 eV PL band, though we did observe the 2.7 eV PL band. These results suggest that these two PL bands do not have a common origin. 2006 Elsevier B.V. All rights reserved. PACS: 61.43.Fs; 61.72.Ji; 78.55.Hx Keywords: Luminescence; Silica; Defects; Short-range order
1. Introduction Optical absorption bands at 5.0 and 7.6 eV, and photoluminescence (PL) bands at 2.7 and 4.3 eV have been associated with oxygen deficiency in high-purity silica glass [1–5]. To date, two structural models have been proposed for these absorption and PL bands; an oxygen vacancy (O3„SiASi„O3, 5.0 and 7.6 eV absorption bands) [1–6] and a twofold coordinated silicon (O2@Si:) [6–8]. There is a strong consensus in the literature that PL in the 4.2– 4.4 eV range arises from oxygen vacancy centers [9]. Generally, the 2.7 eV and 4.3 eV PL intensities are strong in the oxygen-deficiency-type sample, and they are observed together. Therefore, it is thought that the origin of the two PL bands is identical, i.e., the two PL bands are transitions from the excited state to the ground state of the oxygen vacancy.
On the other hand, there is a report that the origin of the two PL bands is not identical. Tohmon et al. [2,3] has shown the relationship between 2.7 and 4.4 eV PL bands and oxygen vacancy by an ab initio molecular-orbital calculation using the cluster (HO)3SiASi(OH)3. Though verification of the existence of the 4.4 eV PL band failed, the existence of the 2.7 eV PL band and the two absorption bands was verified [2,3]. Itoh et al. [10] conclude that the 2.7 eV PL band was the recombination of the self-trapped excitation. The purpose of this article is to examine whether the origin of the 2.7 and 4.3 eV PL bands is identical. Based on work reported in our previous papers [11–13] we have continued to investigate the thermal heat treatment characteristics of the 2.7 eV and 4.3 eV PL bands in silica glass excited by UV light. 2. Experiment
*
Tel.: +81 466 34 4111; fax: +81 466 35 8897. E-mail address: [email protected]
0022-3093/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.02.089
The samples used in this study are listed in Table 1. The lower detection limit value of impurity for the samples
LETTER TO THE EDITOR
2918
Y. Sakurai / Journal of Non-Crystalline Solids 352 (2006) 2917–2920
Table 1 Category, manufacturing methods, and impurities of the samples used for the experiments Sample name
Category
Manufacturing method
Impurity (ppm) Cl
OH
P D S1 S2 S3 S4
Oxygen surplus High-OH Unknown (B2b) Oxygen deficient Oxygen deficient Oxygen deficient
Ar + O2 plasma Flame hydrolysis CVD soot remelting CVD soot remelting CVD soot remelting CVD soot remelting
370 ND 0.3 0.2 0.3 ND
0.6 1000 200 ND ND 6.0
CVD: Chemical vapor deposition. ND: Not detected.
shown in Table 1 is 1 ppm or less. All samples were bulk amorphous SiO2 prepared by the plasma (Ar + O2) chemical vapor deposition (CVD), flame hydrolysis, and CVD soot remelting methods. All of the sample (20 · 20 · 20 mm) surfaces including the sides were polished. The PL was detected by a monochromator (Jobin-Yvon, HR250) equipped with a multichannel detector (Atago Max-3000). The PL excitation was with a Nd:YAG laser [266 nm (4.66 eV), laser pulse width (FWHM: 10 ns)] light source. The time-resolved PL spectra were measured with a pulse generator (Stanford Research System, DG535) that controls the delay times (td) following the laser pulse, and the exposure times (te). Generally, td = 0 s is when the light emission begins. However, it was difficult to accurately determine the time when light emission started with the measuring instruments used in this experiment. High precision measuring instruments are necessary in order to accurately determine when light emission starts. In this experiment we standardized the value of td as the time between when the sample was exposed to the exciting light and the start of spectroscopy. This can be controlled by the output waveform of the pulse generator and timing of the observation of the spectrum. First, various adjustment were carried out, while the excit-
ing light and output waveform of the pulse generator were observed with the digital oscilloscope. In this way it was possible to adjust the timing. Next, measurement of the spectrum of the exciting light was carried out, while td was adjusted. The result of this work was that we were able to determine the (20 ns) error in the response time of the gate circuit of the multichannel detector. It was difficult to remove the effect of this error. In the spectroscopy of the PL band with emission lifetime over 100 ns, the effect of this error can be disregarded. However, the effect of this error cannot be disregarded in the spectroscopy of the PL band with emission lifetime of 10 ns or less. Therefore, the measurement was carried out, when td was observed in ns order. The PL decay was measured by observing the decay of the PL after a pulsed excitation with the Nd:YAG laser. The voltage across a resistor (R = 50 X 1 MX) carrying the output current from a photomultiplier (Hamamatsu, E2762) was recorded with a digital oscilloscope (Tektronix, 2440). All measurements were made at 290 K. PL spectra were normalized to the spectral response of the detection system using a standard tungsten lamp. The melting processing was carried out using an H2/O2 burner in atmosphere. It was easy to melt the angular part of the sample in comparison with the plane part. The temperature during the melting is uncertain. The melting was maintained until an area sufficient for the light emission observation was obtained. The irradiation of the YAG laser light to the melted part of the sample that was melted was controlled by the slit. The experiments were repeated several times to obtain a PL intensity variance of less than 5%. 3. Results and discussion Fig. 1 shows the spectra of the non treated oxygen-deficient-type samples (a) S3 and (b) S4 excited at 4.66 eV by the Nd:YAG laser at 290 K measured at td < 10 ns and te = 10 ms. A PL band with peak energy near 3.1 eV and full width at half maximum (FWHM) of 0.44 eV was
Fig. 1. Time-resolved PL spectra of the non-treated oxygen-deficient-type samples (a) S3 and (b) S4 excited at 4.66 eV by the Nd:YAG laser at 290 K measured at td < 10 ns and te = 10 ms at 290 K.
LETTER TO THE EDITOR
Y. Sakurai / Journal of Non-Crystalline Solids 352 (2006) 2917–2920
observed. The peak energy (3.15 eV lifetime, s: 10 ls) of sample S4 is slightly larger than that for sample S3 (3.08 eV, s: 110 ls). The peak energy and FWHM, of the PL spectrum for sample S3, is similar to those observed for the B2b type sample S1 [14]. The 2.7 eV PL band (FWHM: 0.4 eV, s: 10 ms) is generally observed in the oxygen deficient type sample. It is difficult to confirm the whole spectrum of the 4.3 eV PL band because the PL intensity is very weak and overlaps the exciting light (4.66 eV). The component that appeared near the 4.0 eV is a part of the 4.3 eV PL band. Fig. 2 shows the spectra of the heat treated oxygen-deficient-type samples S3 and S4 excited at 4.66 eV by the Nd: YAG laser at 290 K measured at td < 10 ns and te = 10 ms. In sample S3, only the 2.7 eV PL band was observed. On the other hand, in sample S4, the 2.7 and 3.15 eV PL bands
Fig. 2. Time-resolved PL spectra of the heat treated oxygen-deficient-type samples S3 and S4 excited at 4.66 eV by the Nd:YAG laser at 290 K measured at td < 10 ns and te = 10 ms at 290 K.
Fig. 3. Time decay of the heat treated oxygen-deficient-type samples S3 and S4 excited at 4.66 eV by the Nd:YAG laser at 290 K.
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were observed. The 4.3 eV PL band was not observed in either sample. Fig. 3 shows the emission decay of the 2.7 eV PL peak excited at 4.66 eV for the heat treated oxygen-deficient-type samples S3 and S4 excited at 290 K (R = 1 MX). As shown in Fig. 3, the decay time at 290 K was estimated from the slope of the curve to be 10 ms. The curve follows a single exponential [exp(t/s)], and the lifetime of the 2.7 eV PL band was equivalent to the numerical value (10 ms) previously reported. At the outset of our work we assumed that the PL band would not be observed because the point defects (origin of the PL emission), which exist in the samples, disappears with heat treatment. However, the PL band was observed. We also noticed that the PL bands of the 2.7 and 4.3 eV were not observed together. The first time that we observed that the 2.7 and 4.3 eV bands did not occur together we thought that we may have made a mistake. There is a situation in which they cannot be detected, and that is when the numerical value of td is decreased, because the lifetime of the 4.3 eV PL band is short, a few ns [12]. Therefore, we looked for these two bands at various td. The result of these measurements was that the 4.3 eV PL band could not be detected. Though the 2.7 eV PL band was also observed in samples S1, S2 and D, the 4.3 eV PL band was not. In sample P, two PL bands were not observed. Except for this experiment, the two PL bands were observed together. Therefore, the observation was carried out carefully when the 4.3 eV PL band was hard to observe in the presence of the 2.7 eV PL band. It was possible to confirm the existence of the 4.3 eV PL band in the presence of the 2.7 eV PL band by adjusting td, even if the 4.3 eV PL band could not be detected by setting td. Therefore observation of the 2.7 eV PL band was made very carefully when the 4.3 eV band was hard to observe. The ratio of 2.7 eV PL intensity to 4.3 eV PL intensity for D2 lamp excitation was about 1:4. Therefore, the spectrum intensity ratio of about 1:4 is obtained as the ratio of 2.7 eV PL intensity and 4.3 eV PL intensity. If it is, then td * 0 s. The PL intensity drops to about 1/10 after about 10 ns, after light is emitted, if the 4.3 eV PL emission lifetime is 5 ns [12]. Generally, the PL emission lifetime has been defined 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/s) (t is the time and I(t) is the intensity, or number of photons emitted per second at t). The strength becomes about 1/10, when the life observed 5 ns emission intensity at td = 10 ns, i.e., I(td) becomes I(0) exp(10 ns/5 ns) = I(0) · 0.135) as s = 5 ns and t = td = 10 ns. It is possible to experientially estimate the value of td from these results. It is important to observe the 2.7 eV PL band and exciting light in Fig. 2. If the 4.3 eV PL band exists, the tail of this band can be detected near 4.0 eV, even if it overlaps with the exciting light. We took the non-existence of this tail as evidence that the 4.3 eV PL band did not exist. Prior to this work we believed that the origin of the 2.7 and 4.3 eV PL bands was identical. Based on these latest
LETTER TO THE EDITOR
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results we believe that the origin of the two PL bands is not identical. If our observed results are correct, it is necessary to reexamine the origin of the 2.7 and 4.3 eV PL bands. We believe there is a low probability that the observed 2.7 and 3.15 eV PL bands are associated with impurities because we used samples with impurity concentrations of under 1 ppm, i.e., the origin of the observed 2.7 and 3.15 eV PL [14] are not metal impurities but most likely oxygen-deficient-associated defect centers. The origin of 3.1 eV light emission from sample S3 seems to be a structural defect in which an OH radical is involved. The intensity ratio of 3.1 eV and 2.7 eV light emission with heat treatment was examined. The intensity ratio of the two light emissions was equivalent before the heat treatment. In sample S3, the intensity ratio of the two light emissions changes greatly, and in sample S4, there is a small change. Considering this result, the structural defect in which an OH radical in sample S3 was involved seemed to decompose with heat treatment. In the case of sample S4, the 3.1 eV light emission seemed to increase with respect to the 2.7 eV light emission with heat treatment. The structure of the origin of these light emissions is unknown at this time. Tohmon et al. [2,3] has concluded that they succeeded in demonstrating the oxygen vacancy as the origin of the 2.7 eV PL band and failed in demonstrating the oxygen vacancy as the origin of the 4.4 eV PL band. This conclusion is based on the assumption that the origin of the two PL bands is identical. Based on the our observations, we conclude that their calculated result was correct, i. e., oxygen vacancy is not the origin of the 4.3 eV PL band. Our results reported in this paper support the observations reported in Refs. [2,3]. If the oxygen vacancy is the origin of the 2.7 eV PL band, we can assume that the desorption of the oxygen from the SiAOASi bond was generated by the melting (SiAOASi ! SiASi + O). Based on the above information the oxygen vacancy is proposed as the origin of the 2.7 eV PL band, while it is not considered as the origin of the 4.3 eV PL band. There is the E 0 center (the 5.8 eV absorption band) as a point defect which has not been examined as a possible PL emission origin in silica glass. Therefore, it is necessary to examine the relation between the 4.3 eV PL band and the E 0 center. The E 0 center is one of the oxygen deficient type defects, and there is a high probability of this defect forming with the oxygen vacancy. Therefore, it is important to make a sample with only the E 0 center. The results of our work increase the need to clarify whether the E 0 center is
connected with the origin of generation of the 4.3 PL band. In order to realize it, it is necessary to make the sample which conducted the special processing [15]. This is a problem for further study. 4. Conclusion In summary, we measured the PL properties of the sample on which we carried out the melting processing. Our results indicate that there is a high probability that the origin of the 2.7 and 4.3 eV PL bands is not identical. Though the melting mechanism could not be clarified, this study should be useful in clarifying the relation of the PL band to oxygen vacancy, and in understanding the structural defects in silica glass. Acknowledgements The authors would like to thank Professor Kaya Nagasawa of Shonan Institute of Technology, Professors Yoshimasa Hama and Yoshimichi Ohki of Waseda University, and Assistant Professor Hiroyuki Nishikawa of shibaura Institute of Technology for help during the course of this work. References [1] H. Imai, K. Arai, H. Imagawa, H. Hosono, Y. Abe, Phys. Rev. B 38 (1988) 12772. [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. Hosono, Y. Abe, H. Imagawa, H. Imai, K. Arai, Phys. Rev. B 44 (1991) 12043. [5] L.N. Skuja, J. Non-Cryst. Solids 149 (1992) 77. [6] L.N. Skuja, J. Non-Cryst. Solids 167 (1994) 229. [7] G.W. Arnold, IEEE Trans. Nucl. Sci. NS-20 (1973) 220. [8] L.N. Skuja, A.R. Silin, J. Mares, Phys. Stat. Sol. A 50 (1978) K149. [9] L.N. Skuja, A.N. Streletsky, A.B. Pakovich, Sol. Stat. Comm. 50 (1984) 1069. [10] C. Itoh, K. Tanimura, N. Itoh, J. Phys. C 21 (1988) 4693. [11] Y. Sakurai, K. Nagasawa, H. Nishikawa, Y. Ohki, J. Appl. Phys. 75 (1994) 1372. [12] H. Nishikawa, Y. Miyake, E. Watanabe, D. Ito, K.S. Seol, Y. Ohki, K. Ishii, Y. Sakurai, K. Nagasawa, J Non-Cryst. Solids 222 (1997) 221. [13] Y. Sakurai, K. Nagasawa, J. Non-Cryst. Solids 290 (2001) 189. [14] Y. Sakurai, J. Non-Cryst. Solids 271 (1997) 218. [15] M.A. Stevens-Kalceff, J. Wong, J Appl. Phys. 97 (2005) 113519.