- Email: [email protected]
Blue emission in mesoporous silica excited by synchrotron radiation
Optical Materials 27 (2005) 958–961 www.elsevier.com/locate/optmat
Blue emission in mesoporous silica excited by synchrotron radiation A. Anedda, C.M. Carbonaro *, F. Clemente, R. Corpino, P.C. Ricci Dipartimento di Fisica, Universita` of Cagliari, and INFM, UdR-CA, S.P. n° 8, Km 0.7, 09042 Monserrato, Cagliari, Italy Available online 8 October 2004
Abstract The optical properties of porous silica were studied by means of synchrotron radiation. We investigated the spectral properties of the blue photoluminescence band as a function of the porosity of the samples. In the whole set of samples the blue band is centered at about 2.8 eV, with main excitations at 5.0 and 6.0 eV. Time decay analysis indicates the contemporary presence of two contributions of photoluminescence. Ó 2004 Elsevier B.V. All rights reserved. PACS: 78.30.Ly; 78.47.+p; 78.55. m; 78.55.Mb Keywords: Porous silica; Photoluminescence; Sol–gel; Silanols
1. Introduction The scaling improvement of silicon technology requires the investigation of Si and SiO2 nanometric layers. Indeed intermetal insulation can be realized by porous silica films [1] and the circuit integration miniaturization process leads to gates whose nanometric thickness is comparable to the skeleton walls of porous silica [2]. In addition it has been shown that the photoluminescence (PL) of porous silica closely resembles the one observed in oxidized porous silicon [3]. Thus, sol– gel synthesized mesoporous silica is a suitable candidate to study the optical properties of thin SiO2 layers and its relation with the chemical and physical properties of the surface. The absorption spectrum of porous silica shows a main absorption band in the 4.7–6.5 eV range [4]. A composite emission in the 4.5–2.5 eV range is observed
by exciting at 5.6 eV, with main contributions at about 3.7 and 2.8 eV [4–6]. It is presently accepted that the PL observed in porous silica originates from surface centers, as the efficiency and emission range are affected by the chemical and physical structure of the porous silica surface [3,5–11]. While the contribution at 3.7 eV has been recently ascribed to OH-related surface defects [6,10], the attribution of the 2.8 eV component is still open. The aim of this work is the analysis of the blue PL of mesoporous silica in samples with different specific surface values and different pore diameters. By exciting with synchrotron radiation we investigated the optical properties of the emitting center to assess its possible correlation with the surface conditions of the samples. The recorded data allow us to picture the optical levels of the centers and to estimate the decay time of the observed emission.
2. Experimental * Corresponding author. Tel.: +39 07 6754823/4755; fax: +39 07 0510171. E-mail addresses: [email protected], carlo.maria.carbonaro @dsf.unica.it (C.M. Carbonaro).
0925-3467/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2004.08.043
Measurements were performed on sol–gel synthesized porous silica monoliths produced by Geltech Inc. (US). Investigated samples have pore diameter ranging from
A. Anedda et al. / Optical Materials 27 (2005) 958–961
959
Table 1 Characteristics of the porous samples: U = pore diameter (5% standard deviation); PV = pore volume; SSA = specific surface area; D = density U (nm)
PV (cm3 g 1)
SSA (m2 g 1)
D (g cm 3)
A B C D
3.2 5.5 7.5 18.2
0.488 0.741 1.000 1.208
594 540 525 264
1.2 0.9 0.7 0.6
It has been reported that the blue PL is excited at 5 and 6 eV [7]. The emission spectra excited at 4.96 eV are reported in Fig. 1 for each set of samples; the spectra were arbitrarily normalized to their maximum for a better comparison. Two PL bands are detected, the larger one centered at about 3.7 eV and the smaller one peaked at about 2.8 eV. The relative intensity of the blue band depends on the samples and it is the largest in the B ones where the two contributions are spectrally resolved. The PLE spectra of the 2.77 eV emission are reported in Fig. 2. The main excitation channel is peaked around 5.0 eV for the whole set of samples; other excitation channels are centered at about 6.0 and 7.0 eV. The details of the main excitation channel depend on the samples: apart from the 18.2 nm samples where a large and no resolved excitation band is detected from 4.5 to 6.0 eV, two peaks at 4.8 and 5.2 eV can be detected. The relative intensity of these two excitation peaks changes in the different samples. In particular the contribution at 4.8 eV is better resolved in B samples. Decay times of the 2.77 eV emission were recorded for different excitation energies (data are vertically shifted to avoid
PL Intensity (arb. units) PL Intensity (arb. units)
3. Results
PL Intensity (arb. units)
3.2 to 18.2 nm, details of porosimetric data are reported in Table 1. PL and excitation of PL (PLE) spectra were recorded at the SUPERLUMI experimental station on the I beamline of the HASYLAB synchrotron laboratories at Desy (Hamburg). Samples were shined by the pulsed excitation light of the synchrotron radiation (SR). The PL signal was dispersed by a 0.5 m Czerny–Turner monochromator (emission bandwidth of 16 nm) and detected in the 1.5–5.0 eV energy range with a photomultiplier (Valvo XP2020Q). The PLE measurements were performed in the 4–10 eV energy range with 0.3 nm of bandwidth. Excitation spectra were corrected for the spectral efficiency of the excitation source. PL and PLE spectra were recorded under multi-bunch operation and detected with an integral time window of 190 ns correlated to the SR pulses. Single-bunch operation was applied to gather decay time in the ns domain by means of 1024 channels to scan the 192 ns interval time between adjacent pulses (pulse width of 0.2 ns [12]).
PL Intensity (arb. units)
Sample
1
Eexc=4.96 eV 3.2 nm
0.8 0.6 0.4 0.2 0 1
Eexc=4.96 eV 5.5 nm
0.8 0.6 0.4 0.2 0 1
Eexc=4.96 eV 7.5 nm
0.8 0.6 0.4 0.2 0 1
Eexc=4.96 eV 18.2 nm
0.8 0.6 0.4 0.2 0
2
2.5
3
3.5
4
4.5
5
Emission energy (eV) Fig. 1. Normalized PL spectra with excitation at 4.96 eV of porous silica with different pore diameters.
their overlap). In Fig. 3a we compared the decay time of the blue band excited at 5.0 eV for the different samples. In Fig. 3b the decay times of the blue band excited at 5.0, 5.4 and 6.5 eV for the A samples are reported. A non-single exponential law is displayed by the semi-logarithmic scale in both figures. A simulation of the
A. Anedda et al. / Optical Materials 27 (2005) 958–961 1010
Eem=2.77 eV
1
3.2 nm
2.8 eV PL amplitude (arb. units)
0.6 0.4 0.2 0 1
Eem=2.77 eV 5.5 nm
0.8
(a)
Eexc = 4.96 eV
0.8
0.6
108
3.2 nm
106
5.5 nm
104 7.5 nm
102
18.2 nm
0.4 100
0.2
E =2.77 eV em
7.5 nm
0.8 0.6 0.4 0.2 0 1
Eem=2.77 eV 18.2 nm
0.8
107 Eexc = 5.39 eV 105
103
Eexc = 6.53 eV
0
5
10
15
20
25
30
35
40
Time (ns)
0.4 0.2
4.5
Eexc = 4.96 eV
101
0.6
0
(b)
3.2 nm samples
109
0 1
2.8 eV PL amplitude (arb. units)
PL Intensity (arb. units)
PL Intensity (arb. units)
PL Intensity (arb. units)
PL Intensity (arb. units)
960
5
5.5
6
6.5
7
7.5
8
Excitation energy (eV) Fig. 2. Normalized PLE spectra for the emission at 2.77 eV in porous silica with different pore diameters.
reported data was successfully achieved by fitting the data with two exponential decays with life time s1 2.4 and s2 > 20 ns (Eexc = 4.96 eV). Fitting details are reported in Table 2.
4. Discussion The photoluminescence properties of mesoporous silica are technologically interesting for their possible applications in photonics in the optical window of blue and UV emitting devices. In addition the similarities of the optical emission in porous silica and silicon [3] calls for a possible common explanation of the observed physical properties. Indeed some models based on impurities surface centers, like H-and C-related centers, have
Fig. 3. 2.8 eV PL decay time with excitations at 4.96 eV in porous silica with different pore diameters (a); 2.8 eV PL decay time in 3.2 nm porous silica samples excited at 4.96, 5.39 and 6.53 eV (b). The data are vertically shifted to avoid their overlap.
already been proposed for porous silicon and silica [5,11,13,14]. In this perspective we focused our attention on the blue emission observed in mesoporous silica under UV excitation. The aim of the present investigation was to research a correlation between the optical emission and the porosity characteristics of the examined samples. The analysis of samples with different pore diameters and related physical parameters like density, pore volume and specific surface area (Table 1) displayed the presence of the blue emission in the whole set of samples. However the PL intensity with respect to the contribution detected at higher energies (UV PL band) does not correlate with the reported parameters: the relative intensity of the blue band is sample dependent (Fig. 1). The investigation of the PLE spectra also indicates a sample dependence of the observed emission: even though the same excitation channels can be detected for all the samples their relative contributions in terms of intensity and width depend on the sample
A. Anedda et al. / Optical Materials 27 (2005) 958–961
961
Table 2 Fitting data of the decay times with two exponential decays at an excitation energy of 4.96 eV. Results at different excitation energies are reported for the A samples
s1 (ns) s2 (ns)
A
B
C
D
Eexc = 4.96 eV
Eexc = 5.39 eV
Eexc = 6.53 eV
2.4 29.5
2.3 34.5
2.2 23.8
2.4 32.6
2.4 29.5
1.8 21.2
1.7 20.7
(Fig. 2). The analysis of the decay time of the blue PL band in samples with different pore diameters and at different excitation energies indicates the presence of two emitting centers: by fitting the data with two exponential decays we obtained two lifetimes s1 2.4 and s2 > 25 ns. According to the fitting results two emitting centers should be responsible of the detected PL. As regards the attribution of the observed emission, the huge surface-to-volume ratio suggests a surface related model, which in principle could be able to explain the sample dependence of the reported data. Indeed it has been reported that the blue emission disappear by heating the samples at 923 K [7]. By considering that thermal treatment causes the shrinking of the porous samples, a porosity dependence of the blue emission should be hypothesized. However the analysis reported here of the PL spectra in samples with different porosity does not show any correlation between the blue emission and the porosity sample parameters. Thus the sample dependence of the blue band intensity supports the attribution to surface centers whose contribution depends on the details of the synthesis procedure. Indeed the differences in the optical properties of the samples could be related to the different chemical and physical morphology of the surface, the details of which should be further investigated.
5. Conclusions We have presented the study of the blue photoluminescence observed in porous silica as a function of the porosity sample parameters. The reported data show that this emission is sample dependent and the analysis of the decay time suggests the presence of two different emitting centers.
Acknowledgements We thank M. Kirm of the G. Zimmerer group and A. Paleari for the SR experimental time at DESY (Hamburg). This study has been supported by a national research project (PRIN2002) of MIUR (Ministero dellÕIstruzione, dellÕUniversita` e della Ricerca) and by INFM (Istituto Nazionale per la Fisica della Materia) of Italy.
References [1] J.K. Hong, H.S. Yang, M.H. Jo, H.H. Park, S.Y. Choi, Thin Solid Films 308 & 309 (1997) 495. [2] M.A. Garcy´a-Sanchez, A. Campero, J. Non-Cryst. Solids 296 (2001) 50. [3] G.G. Qin, J. Lin, J.Q. Duan, G.Q. Yao, Appl. Phys. Lett. 69 (1996) 1689. [4] A. Anedda, C.M. Carbonaro, F. Clemente, R. Corpino, F. Raga, A. Serpi, J. Non-Cryst. Solids 322 (2003) 95. [5] Y.D. Glinka, S.-H. Lin, L.P. Hwang, Y.-T. Chen, Appl. Phys. Lett. 77 (2000) 3968. [6] A. Anedda, C.M. Carbonaro, F. Clemente, R. Corpino, S. Grandi, P. Mustarelli, A. Magistris, J. Non-Cryst. Solids 322 (2003) 68. [7] N. Chiodini, F. Meinardi, F. Morazzoni, A. Paleari, R. Scotti, D. Di Martino, Appl. Phys. Lett. 76 (2000) 3209. [8] Y.D. Glinka, S.N. Naumenko, V.M. Ogenko, A.A. Chuiko, Opt. Spectrosc. (USSR) 71 (1992) 250. [9] Y.D. Glinka, S.-H. Lin, Y.-T. Chen, Phys. Rev. B 62 (2000) 4733. [10] B. Yao, H. Shi, X. Zhang, L. Zhang, Appl. Phys. Lett. 78 (2001) 174. [11] Y.D. Glinka, S.-H. Lin, Y.-T. Chen, Appl. Phys. Lett. 75 (1999) 778. [12] G. Zimmerer, Nucl. Instrum. Meth. A 308 (1991) 178. [13] J.L. Gole, D.A. Dixon, Phys. Rev. B 57 (1998) 12002. [14] A.G. Cullis, L.T. Canham, P.D.J. Calcott, J. Appl. Phys. 82 (1997) 909.
Blue emission in mesoporous silica excited by synchrotron radiation A. Anedda, C.M. Carbonaro *, F. Clemente, R. Corpino, P.C. Ricci Dipartimento di Fisica, Universita` of Cagliari, and INFM, UdR-CA, S.P. n° 8, Km 0.7, 09042 Monserrato, Cagliari, Italy Available online 8 October 2004
Abstract The optical properties of porous silica were studied by means of synchrotron radiation. We investigated the spectral properties of the blue photoluminescence band as a function of the porosity of the samples. In the whole set of samples the blue band is centered at about 2.8 eV, with main excitations at 5.0 and 6.0 eV. Time decay analysis indicates the contemporary presence of two contributions of photoluminescence. Ó 2004 Elsevier B.V. All rights reserved. PACS: 78.30.Ly; 78.47.+p; 78.55. m; 78.55.Mb Keywords: Porous silica; Photoluminescence; Sol–gel; Silanols
1. Introduction The scaling improvement of silicon technology requires the investigation of Si and SiO2 nanometric layers. Indeed intermetal insulation can be realized by porous silica films [1] and the circuit integration miniaturization process leads to gates whose nanometric thickness is comparable to the skeleton walls of porous silica [2]. In addition it has been shown that the photoluminescence (PL) of porous silica closely resembles the one observed in oxidized porous silicon [3]. Thus, sol– gel synthesized mesoporous silica is a suitable candidate to study the optical properties of thin SiO2 layers and its relation with the chemical and physical properties of the surface. The absorption spectrum of porous silica shows a main absorption band in the 4.7–6.5 eV range [4]. A composite emission in the 4.5–2.5 eV range is observed
by exciting at 5.6 eV, with main contributions at about 3.7 and 2.8 eV [4–6]. It is presently accepted that the PL observed in porous silica originates from surface centers, as the efficiency and emission range are affected by the chemical and physical structure of the porous silica surface [3,5–11]. While the contribution at 3.7 eV has been recently ascribed to OH-related surface defects [6,10], the attribution of the 2.8 eV component is still open. The aim of this work is the analysis of the blue PL of mesoporous silica in samples with different specific surface values and different pore diameters. By exciting with synchrotron radiation we investigated the optical properties of the emitting center to assess its possible correlation with the surface conditions of the samples. The recorded data allow us to picture the optical levels of the centers and to estimate the decay time of the observed emission.
2. Experimental * Corresponding author. Tel.: +39 07 6754823/4755; fax: +39 07 0510171. E-mail addresses: [email protected], carlo.maria.carbonaro @dsf.unica.it (C.M. Carbonaro).
0925-3467/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2004.08.043
Measurements were performed on sol–gel synthesized porous silica monoliths produced by Geltech Inc. (US). Investigated samples have pore diameter ranging from
A. Anedda et al. / Optical Materials 27 (2005) 958–961
959
Table 1 Characteristics of the porous samples: U = pore diameter (5% standard deviation); PV = pore volume; SSA = specific surface area; D = density U (nm)
PV (cm3 g 1)
SSA (m2 g 1)
D (g cm 3)
A B C D
3.2 5.5 7.5 18.2
0.488 0.741 1.000 1.208
594 540 525 264
1.2 0.9 0.7 0.6
It has been reported that the blue PL is excited at 5 and 6 eV [7]. The emission spectra excited at 4.96 eV are reported in Fig. 1 for each set of samples; the spectra were arbitrarily normalized to their maximum for a better comparison. Two PL bands are detected, the larger one centered at about 3.7 eV and the smaller one peaked at about 2.8 eV. The relative intensity of the blue band depends on the samples and it is the largest in the B ones where the two contributions are spectrally resolved. The PLE spectra of the 2.77 eV emission are reported in Fig. 2. The main excitation channel is peaked around 5.0 eV for the whole set of samples; other excitation channels are centered at about 6.0 and 7.0 eV. The details of the main excitation channel depend on the samples: apart from the 18.2 nm samples where a large and no resolved excitation band is detected from 4.5 to 6.0 eV, two peaks at 4.8 and 5.2 eV can be detected. The relative intensity of these two excitation peaks changes in the different samples. In particular the contribution at 4.8 eV is better resolved in B samples. Decay times of the 2.77 eV emission were recorded for different excitation energies (data are vertically shifted to avoid
PL Intensity (arb. units) PL Intensity (arb. units)
3. Results
PL Intensity (arb. units)
3.2 to 18.2 nm, details of porosimetric data are reported in Table 1. PL and excitation of PL (PLE) spectra were recorded at the SUPERLUMI experimental station on the I beamline of the HASYLAB synchrotron laboratories at Desy (Hamburg). Samples were shined by the pulsed excitation light of the synchrotron radiation (SR). The PL signal was dispersed by a 0.5 m Czerny–Turner monochromator (emission bandwidth of 16 nm) and detected in the 1.5–5.0 eV energy range with a photomultiplier (Valvo XP2020Q). The PLE measurements were performed in the 4–10 eV energy range with 0.3 nm of bandwidth. Excitation spectra were corrected for the spectral efficiency of the excitation source. PL and PLE spectra were recorded under multi-bunch operation and detected with an integral time window of 190 ns correlated to the SR pulses. Single-bunch operation was applied to gather decay time in the ns domain by means of 1024 channels to scan the 192 ns interval time between adjacent pulses (pulse width of 0.2 ns [12]).
PL Intensity (arb. units)
Sample
1
Eexc=4.96 eV 3.2 nm
0.8 0.6 0.4 0.2 0 1
Eexc=4.96 eV 5.5 nm
0.8 0.6 0.4 0.2 0 1
Eexc=4.96 eV 7.5 nm
0.8 0.6 0.4 0.2 0 1
Eexc=4.96 eV 18.2 nm
0.8 0.6 0.4 0.2 0
2
2.5
3
3.5
4
4.5
5
Emission energy (eV) Fig. 1. Normalized PL spectra with excitation at 4.96 eV of porous silica with different pore diameters.
their overlap). In Fig. 3a we compared the decay time of the blue band excited at 5.0 eV for the different samples. In Fig. 3b the decay times of the blue band excited at 5.0, 5.4 and 6.5 eV for the A samples are reported. A non-single exponential law is displayed by the semi-logarithmic scale in both figures. A simulation of the
A. Anedda et al. / Optical Materials 27 (2005) 958–961 1010
Eem=2.77 eV
1
3.2 nm
2.8 eV PL amplitude (arb. units)
0.6 0.4 0.2 0 1
Eem=2.77 eV 5.5 nm
0.8
(a)
Eexc = 4.96 eV
0.8
0.6
108
3.2 nm
106
5.5 nm
104 7.5 nm
102
18.2 nm
0.4 100
0.2
E =2.77 eV em
7.5 nm
0.8 0.6 0.4 0.2 0 1
Eem=2.77 eV 18.2 nm
0.8
107 Eexc = 5.39 eV 105
103
Eexc = 6.53 eV
0
5
10
15
20
25
30
35
40
Time (ns)
0.4 0.2
4.5
Eexc = 4.96 eV
101
0.6
0
(b)
3.2 nm samples
109
0 1
2.8 eV PL amplitude (arb. units)
PL Intensity (arb. units)
PL Intensity (arb. units)
PL Intensity (arb. units)
PL Intensity (arb. units)
960
5
5.5
6
6.5
7
7.5
8
Excitation energy (eV) Fig. 2. Normalized PLE spectra for the emission at 2.77 eV in porous silica with different pore diameters.
reported data was successfully achieved by fitting the data with two exponential decays with life time s1 2.4 and s2 > 20 ns (Eexc = 4.96 eV). Fitting details are reported in Table 2.
4. Discussion The photoluminescence properties of mesoporous silica are technologically interesting for their possible applications in photonics in the optical window of blue and UV emitting devices. In addition the similarities of the optical emission in porous silica and silicon [3] calls for a possible common explanation of the observed physical properties. Indeed some models based on impurities surface centers, like H-and C-related centers, have
Fig. 3. 2.8 eV PL decay time with excitations at 4.96 eV in porous silica with different pore diameters (a); 2.8 eV PL decay time in 3.2 nm porous silica samples excited at 4.96, 5.39 and 6.53 eV (b). The data are vertically shifted to avoid their overlap.
already been proposed for porous silicon and silica [5,11,13,14]. In this perspective we focused our attention on the blue emission observed in mesoporous silica under UV excitation. The aim of the present investigation was to research a correlation between the optical emission and the porosity characteristics of the examined samples. The analysis of samples with different pore diameters and related physical parameters like density, pore volume and specific surface area (Table 1) displayed the presence of the blue emission in the whole set of samples. However the PL intensity with respect to the contribution detected at higher energies (UV PL band) does not correlate with the reported parameters: the relative intensity of the blue band is sample dependent (Fig. 1). The investigation of the PLE spectra also indicates a sample dependence of the observed emission: even though the same excitation channels can be detected for all the samples their relative contributions in terms of intensity and width depend on the sample
A. Anedda et al. / Optical Materials 27 (2005) 958–961
961
Table 2 Fitting data of the decay times with two exponential decays at an excitation energy of 4.96 eV. Results at different excitation energies are reported for the A samples
s1 (ns) s2 (ns)
A
B
C
D
Eexc = 4.96 eV
Eexc = 5.39 eV
Eexc = 6.53 eV
2.4 29.5
2.3 34.5
2.2 23.8
2.4 32.6
2.4 29.5
1.8 21.2
1.7 20.7
(Fig. 2). The analysis of the decay time of the blue PL band in samples with different pore diameters and at different excitation energies indicates the presence of two emitting centers: by fitting the data with two exponential decays we obtained two lifetimes s1 2.4 and s2 > 25 ns. According to the fitting results two emitting centers should be responsible of the detected PL. As regards the attribution of the observed emission, the huge surface-to-volume ratio suggests a surface related model, which in principle could be able to explain the sample dependence of the reported data. Indeed it has been reported that the blue emission disappear by heating the samples at 923 K [7]. By considering that thermal treatment causes the shrinking of the porous samples, a porosity dependence of the blue emission should be hypothesized. However the analysis reported here of the PL spectra in samples with different porosity does not show any correlation between the blue emission and the porosity sample parameters. Thus the sample dependence of the blue band intensity supports the attribution to surface centers whose contribution depends on the details of the synthesis procedure. Indeed the differences in the optical properties of the samples could be related to the different chemical and physical morphology of the surface, the details of which should be further investigated.
5. Conclusions We have presented the study of the blue photoluminescence observed in porous silica as a function of the porosity sample parameters. The reported data show that this emission is sample dependent and the analysis of the decay time suggests the presence of two different emitting centers.
Acknowledgements We thank M. Kirm of the G. Zimmerer group and A. Paleari for the SR experimental time at DESY (Hamburg). This study has been supported by a national research project (PRIN2002) of MIUR (Ministero dellÕIstruzione, dellÕUniversita` e della Ricerca) and by INFM (Istituto Nazionale per la Fisica della Materia) of Italy.
References [1] J.K. Hong, H.S. Yang, M.H. Jo, H.H. Park, S.Y. Choi, Thin Solid Films 308 & 309 (1997) 495. [2] M.A. Garcy´a-Sanchez, A. Campero, J. Non-Cryst. Solids 296 (2001) 50. [3] G.G. Qin, J. Lin, J.Q. Duan, G.Q. Yao, Appl. Phys. Lett. 69 (1996) 1689. [4] A. Anedda, C.M. Carbonaro, F. Clemente, R. Corpino, F. Raga, A. Serpi, J. Non-Cryst. Solids 322 (2003) 95. [5] Y.D. Glinka, S.-H. Lin, L.P. Hwang, Y.-T. Chen, Appl. Phys. Lett. 77 (2000) 3968. [6] A. Anedda, C.M. Carbonaro, F. Clemente, R. Corpino, S. Grandi, P. Mustarelli, A. Magistris, J. Non-Cryst. Solids 322 (2003) 68. [7] N. Chiodini, F. Meinardi, F. Morazzoni, A. Paleari, R. Scotti, D. Di Martino, Appl. Phys. Lett. 76 (2000) 3209. [8] Y.D. Glinka, S.N. Naumenko, V.M. Ogenko, A.A. Chuiko, Opt. Spectrosc. (USSR) 71 (1992) 250. [9] Y.D. Glinka, S.-H. Lin, Y.-T. Chen, Phys. Rev. B 62 (2000) 4733. [10] B. Yao, H. Shi, X. Zhang, L. Zhang, Appl. Phys. Lett. 78 (2001) 174. [11] Y.D. Glinka, S.-H. Lin, Y.-T. Chen, Appl. Phys. Lett. 75 (1999) 778. [12] G. Zimmerer, Nucl. Instrum. Meth. A 308 (1991) 178. [13] J.L. Gole, D.A. Dixon, Phys. Rev. B 57 (1998) 12002. [14] A.G. Cullis, L.T. Canham, P.D.J. Calcott, J. Appl. Phys. 82 (1997) 909.