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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 16, Iss. 7 — Mar. 31, 2008
  • pp: 4895–4900
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Twofold coordinated Ge defects induced by gamma-ray irradiation in Ge-doped SiO2

A. Alessi, S. Agnello, F. M. Gelardi, S. Grandi, A. Magistris, and R. Boscaino  »View Author Affiliations


Optics Express, Vol. 16, Issue 7, pp. 4895-4900 (2008)
http://dx.doi.org/10.1364/OE.16.004895


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Abstract

We report an experimental study by photoluminescence, optical absorption and Electron Paramagnetic Resonance measurements on the effects of exposure of Ge-doped amorphous SiO2 to γ ray radiation at room temperature. We have evidenced that irradiation at doses of the order of 1 MGy is able to generate Ge-related defects, recognizable from their optical properties as twofold coordinated Ge centers. Until now, such centers, responsible for photosensitivity of Ge-doped SiO2, have been induced only in synthesis procedures of materials. The found result evidences a role played by γ radiation in generating photosensitive defects and could furnish a novel basis for photosensitive pattern writing through ionizing radiation.

© 2008 Optical Society of America

1. Introduction

In this work we report on the generation of stable GLPC by exposure of Ge-doped silica to γ-ray at room temperature. Our results could furnish a basis for novel devices employing ionizing radiation for the preparation of spatial structures on target materials for successive UV writing, a point that has been found to be relevant for novel optics applications [12

12. M. A. Vincenti, S. Almaviva, R. M. Montereali, H. J. Kalinowski, and R. N. Nogueira, “Permanent luminescent micropatterns photoinduced by low-power ultraviolet irradiation in lithium fluoride,” Appl. Phys. Lett. 89, 241125 1–3 (2006). [CrossRef]

].

2. Experimental procedures

The measurements reported here were done on two sol-gel Ge doped silica samples, and in particular the sample named A has Ge content of 104 part per million (ppm) molar, while the sample named B has a content of 103 ppm mol. The investigated materials are produced by a mixture of TEOS (tetra-etil-orthosilicate) and TEOG (tetra-etil-orthogermanate). The starting aerogels have been densificated using two different temperature routes. In particular, sample A was heated up to 1200°C with a rate of 0.3°C/min and then it was kept at this temperature for 0.75 hours, finally, it was cooled to room temperature. In the heating stage a flux of O2+N2 was used up to 800°C, then it was changed to normal atmosphere for the remaining part of the temperature route. The sample B was heated up to 1150 °C with a rate of 0.3 °C/min, and it was kept to this temperature for a time of 24 hours, and finally it was cooled to room temperature. During the heating a flux of O2 was used up to 700 °C, and then, after half an hour in He, the atmosphere was switched to vacuum (~10-8 atm).

3. Results and discussion

In the sample A no PL and EPR activities were found before the γ-ray irradiation, as well as no evidence of the B band was found in the OA spectrum (Fig. 1(a)). The sample was then γ-ray irradiated up to 1 MGy exposing it to a 60Co source at room temperature.

Fig. 1. (a). OA spectra of sample A before and after γ irradiation to 1 MGy and that of the sample B are reported; (b) PL spectra under excitation energy 5.0 eV acquired on sample A γ-irradiated at 1 MGy and on sample B, normalized to the maximum at ~3.2 eV.

As reported in Fig. 1(a), after irradiation, the absorption spectrum shows the appearance of a signal in the ultraviolet range. A photoluminescence excited at 5.0 eV is also found, whose spectrum, reported in Fig. 1(b), shows two luminescence bands centered at ~3.2 eV and 4.3 eV. It is important to underline that this latter band is clearly evidenced after the correction of the PL for the absorption coefficient value in the 3.5-4.8 eV spectral range (shown in Fig. 1(a)).

In Fig. 2(a), the PL spectra in the range 2.5–3.7 eV of irradiated sample A recorded using different excitation energies (5.51, 5.00, 4.81, 4.70 eV) are shown. These spectra evidence spectral changes of the 3.2 eV band compatible with already reported data for GLPC centers induced during materials synthesis [4

4. M. Leone, S. Agnello, R. Boscaino, M. Cannas, and F. M. Gelardi, “Optical absorption, luminescence, and ESR spectral properties of point defects in silica,” in Silicon-based Materials and Devices: Properties and Devices, H. S. Nalwa, ed., (Academic Press, San Diego, 2001).

]. A more detailed comparison is reported in Fig. 2(b), where the variations of the 3.2 eV maximum position and of its FWHM (inset) as a function of the excitation energy (Eexc) are shown for irradiated sample A and as grown sample B.

Fig. 2. (a). normalized PL spectra of 3.2 eV band recorded on sample A γ irradiated at 1 MGy exciting at different energies, (b) variation of the peak position as function of excitation energy: (●) 1 MGy irradiated sample A, (○) sample B; in the inset the dependence of FWHM on excitation energy is reported: (●) 1 MGy irradiated sample A, (○) sample B.

The peak position of the 3.2 eV band of the irradiated sample A changes with the Eexc in the same way of the non irradiated sample B, in particular it increases almost monotonically on increasing Eexc. At variance, the FWHM in the investigated materials depend on Eexc in different ways. In details, in sample B it increases monotonically with Eexc, while in the irradiated sample A the FWHM has lower dependence changing only from 0.44 to 0.48 eV (with an error of ~ 0.01 eV). The correction procedure for the high absorption in irradiated sample A gives a large error of the α E band peak position avoiding the accurate investigation of its dependence on excitation energy.

Fig. 3. PL emission at 3.2 eV as a function of time under pulsed excitation at 4.9 eV: (□) experimental data of 1 MGy irradiated sample A, (○) experimental data of sample B, the reported data are normalized to the maximum emission; the full lines represent best fits.

To further compare the PL activity of the irradiated sample A with that of sample B we investigated the PL time decay of the 3.2 eV band, under pulsed excitation at 4.9 eV. In Fig. 3, the ln(I(t)/I0) (I(t) indicating the PL intensity as a function of time and I0 the initial intensity), is reported. For both samples the decay is a single exponential and the estimated lifetimes are τ=(97±2) µs in the irradiated sample A, and τ=(109±2) µs in the sample B, this latter being in good agreement with the value reported by Skuja [5

5. L. Skuja, “Isoelectronic series of twofold coordinated Si, Ge, and Sn atoms in glassy SiO2: a luminescence study,” J. Non Cryst. Sol. 149, 77–95 (1992). [CrossRef]

].

Fig. 4. (a). EPR doublet (dashed line, magnified by a factor 200) associated to the H(II) defect as detected in sample A irradiated at 1 MGy; the central part of the spectrum (continuous line), reporting the region near g=2, is due to the superposition of Ge(1) and E’-Ge centers; (b). H(II) centers concentration as a function of the accumulated dose in sample A.

The signal [13

13. J. Vitko Jr, “ESR studies of hydrogen hyperfine spectra in irradiated vitreous silica,” J. Appl. Phys. 49, 5530–5535 (1978). [CrossRef]

, 14

14. K. Awazu, K. Muta, and H. Kawazoe, “Formation mechanism of hydrogen-associated defect with an 11.9 mT doublet in electron spin resonance and red luminescence in 9SiO2:GeO2 fibers,” J. Appl. Phys 74, 2237–2240 (1993). [CrossRef]

] of H (II) defect [5

5. L. Skuja, “Isoelectronic series of twofold coordinated Si, Ge, and Sn atoms in glassy SiO2: a luminescence study,” J. Non Cryst. Sol. 149, 77–95 (1992). [CrossRef]

] is present in the EPR spectra of irradiated sample A, as shown in Fig. 4(a), and is characterized by a doublet split by ~118 G. The H (II) center is a product of GLPC and it results from the reaction with a H atom [5

5. L. Skuja, “Isoelectronic series of twofold coordinated Si, Ge, and Sn atoms in glassy SiO2: a luminescence study,” J. Non Cryst. Sol. 149, 77–95 (1992). [CrossRef]

, 14

14. K. Awazu, K. Muta, and H. Kawazoe, “Formation mechanism of hydrogen-associated defect with an 11.9 mT doublet in electron spin resonance and red luminescence in 9SiO2:GeO2 fibers,” J. Appl. Phys 74, 2237–2240 (1993). [CrossRef]

]. In particular the H atom binds itself with one of the two electrons of the lone pair, giving the structure =Ge-H. This kind of defect is paramagnetic, and its EPR spectrum consists of two components generated by the hyperfine interaction of the electron spin with the nuclear spin of the H atom [5

5. L. Skuja, “Isoelectronic series of twofold coordinated Si, Ge, and Sn atoms in glassy SiO2: a luminescence study,” J. Non Cryst. Sol. 149, 77–95 (1992). [CrossRef]

, 13

13. J. Vitko Jr, “ESR studies of hydrogen hyperfine spectra in irradiated vitreous silica,” J. Appl. Phys. 49, 5530–5535 (1978). [CrossRef]

]. The central part of the spectrum reported in Fig. 4(a) shows the region near g=2 and evidences the presence of Ge(1) and E’-Ge centers [2

2. G. Pacchioni, L. Skuja, and D. L. Griscom, Defects in SiO2 and related dielectrics: Science and Technology, (Kluwer Academic Publishers, Dordrecht, 2000). [PubMed]

, 3

3. V. B. Neustruev, “Colour centres in germanosilicate glass and optical fibres,” J. Phys. Condens. Matter 6, 6901–6936 (1994). [CrossRef]

]. In Fig. 4(b), the concentration growth of the H (II) centers as a function of the accumulated dose is shown as determined using the signal of both the hyperfine lines after every irradiation step. It is important to note that the EPR signal of H (II) defect has been detected at minor dose values than the PL signal of GLPC. This happens because of the higher sensitivity of the EPR measurement technique and for the presence, in 3-6 eV range, of other OA active centers that subtract light to GLPC ones.

From the comparison of the here reported data we can argue that the γ irradiation has induced GLPC centers in sample A. In fact, we observe the characteristic two PL bands centered at ~3.2 eV and ~4.3 eV with a ratio between them very similar to that of PL bands linked to GLPC centers produced during sample synthesis (native centers) and observed also in natural silica. At the same time the peak position of 3.2 eV induced PL component depends on excitation energy similarly to the native one, whereas its FWHM shows lower dependence. This difference can be attributed to different effects of the defect environment [4

4. M. Leone, S. Agnello, R. Boscaino, M. Cannas, and F. M. Gelardi, “Optical absorption, luminescence, and ESR spectral properties of point defects in silica,” in Silicon-based Materials and Devices: Properties and Devices, H. S. Nalwa, ed., (Academic Press, San Diego, 2001).

, 5

5. L. Skuja, “Isoelectronic series of twofold coordinated Si, Ge, and Sn atoms in glassy SiO2: a luminescence study,” J. Non Cryst. Sol. 149, 77–95 (1992). [CrossRef]

]. From the time decay data we can argue that the same cause originates the small differences in lifetime of native and irradiation induced GLPC defects. The different values could be due, for example, to a more efficient non radiative decay channel connecting T1 to S0 in the induced defect. The generation of GLPC in sample A is further supported by the experimental detection of the paramagnetic defect H(II) generated by a reaction of this defect with an H atom [4

4. M. Leone, S. Agnello, R. Boscaino, M. Cannas, and F. M. Gelardi, “Optical absorption, luminescence, and ESR spectral properties of point defects in silica,” in Silicon-based Materials and Devices: Properties and Devices, H. S. Nalwa, ed., (Academic Press, San Diego, 2001).

, 5

5. L. Skuja, “Isoelectronic series of twofold coordinated Si, Ge, and Sn atoms in glassy SiO2: a luminescence study,” J. Non Cryst. Sol. 149, 77–95 (1992). [CrossRef]

].

On the basis of these considerations, the GLPC generation process by ionizing radiation in our sample A, having about 1000 ppm by weight of OH and no detectable H2, can be attributed to oxygen displacements without directly involving the H species. In this context, also large differences in OH content between sol-gel and MCVD or VAD materials could not be relevant for the generation of GLPC by ionizing radiation.

4. Conclusions

We have studied the effects of gamma radiation in sol-gel Ge-doped samples, and we have evidenced the presence of induced GLPC defects. This finding shows the occurrence of a relevant photosensitivity process in Ge-doped silica as GLPC are responsible for refractive index change. In addition, our results showing the possibility to induce GLPC with ionizing radiation, suggest that such radiation could be used to spatially activate the amorphous system enabling to write a photosensitive pattern through for example electron or X ray beams.

Acknowledgments

We thank people of the LAMP laboratory at the Department of Physical and Astronomical Sciences of the University of Palermo (http://www.fisica.unipa.it//amorphous) for useful discussions and comments. The authors would like to thank E. Calderaro and A. Parlato of the Nuclear Engineering Department of the University of Palermo for taking care of γ ray irradiation in the IGS-3 irradiator. Partial financial support by P.O.R. Sicilia 2000/2006 Misura 3.15-Sottoazione C and technical assistance by G. Napoli and G. Tricomi are gratefully acknowledged.

References and links

1.

K. O. Hill, Y. Fujii, D. C. Johnson, and B. S. Kawasaki, “Photosensitivity in optical fiber waveguides: Application to reflection filter fabrication,” Appl. Phys. Lett. 32, 647–649 (1978). [CrossRef]

2.

G. Pacchioni, L. Skuja, and D. L. Griscom, Defects in SiO2 and related dielectrics: Science and Technology, (Kluwer Academic Publishers, Dordrecht, 2000). [PubMed]

3.

V. B. Neustruev, “Colour centres in germanosilicate glass and optical fibres,” J. Phys. Condens. Matter 6, 6901–6936 (1994). [CrossRef]

4.

M. Leone, S. Agnello, R. Boscaino, M. Cannas, and F. M. Gelardi, “Optical absorption, luminescence, and ESR spectral properties of point defects in silica,” in Silicon-based Materials and Devices: Properties and Devices, H. S. Nalwa, ed., (Academic Press, San Diego, 2001).

5.

L. Skuja, “Isoelectronic series of twofold coordinated Si, Ge, and Sn atoms in glassy SiO2: a luminescence study,” J. Non Cryst. Sol. 149, 77–95 (1992). [CrossRef]

6.

K. Awazu, H. Kawazoe, and M. Yamane, “Simultaneous generation of optical absorption bands 5.14 and 0.452 eV in 9 SiO2:GeO2 glasses heated under H2 atmosphere,” J. Appl. Phys 68, 2713–2718 (1990). [CrossRef]

7.

M. Fujimaki, T. Watanabe, T. Katoh, T. Kasahara, N. Miyazaki, Y. Ohki, and H. Nishikawa, “Structures and generation mechanisms of paramagnetic centers and absorption bands responsible for Ge-doped SiO2 optical-fiber gratings,” Phys. Rev. B 57, 3920–3926 (1998). [CrossRef]

8.

F. Messina, S. Agnello, R. Boscaino, M. Cannas, S. Grandi, and E. Quartarone, “Optical properties of Ge-oxygen deficient centers embedded in silica films,” J. Non Cryst. Sol. 353, 670–673 (2007). [CrossRef]

9.

J. Albert, B. Malo, F. Bilodeau, D. C. Johnson, K. O. Hill, Y. Hibino, and M. Kawachi, “Photosensitivity in Ge-doped silica optical waveguides and fibers with 193-nm light from ArF excimer laser,” Opt. Lett. 19, 387–389 (1994). [PubMed]

10.

M. Lancry, B. Poumellec, P. Niay, M. Douay, P. Cordier, and C. Depecker, “VUV and IR absorption spectra induced in H2-loaded and UV hyper-sensitized standard germanosilicate preform plates through exposure to ArF laser light,” J. Non Cryst. Sol. 351, 3773–3783 (2005). [CrossRef]

11.

A. J. Cohen and H. L. Smith, “Ultraviolet and infrared absorption of fused germania,” J. Phys. Chem. Solids 7, 301–306 (1958). [CrossRef]

12.

M. A. Vincenti, S. Almaviva, R. M. Montereali, H. J. Kalinowski, and R. N. Nogueira, “Permanent luminescent micropatterns photoinduced by low-power ultraviolet irradiation in lithium fluoride,” Appl. Phys. Lett. 89, 241125 1–3 (2006). [CrossRef]

13.

J. Vitko Jr, “ESR studies of hydrogen hyperfine spectra in irradiated vitreous silica,” J. Appl. Phys. 49, 5530–5535 (1978). [CrossRef]

14.

K. Awazu, K. Muta, and H. Kawazoe, “Formation mechanism of hydrogen-associated defect with an 11.9 mT doublet in electron spin resonance and red luminescence in 9SiO2:GeO2 fibers,” J. Appl. Phys 74, 2237–2240 (1993). [CrossRef]

15.

J. Albert, B. Malo, K. O. Hill, D. C. Johnson, J. L. Brebner, and R. Leonelli, “Refractive-index changes in fused silica produced by heavy-ion implantation followed by photobleaching,” Opt. Lett. 17, 1652–1654 (1992). [CrossRef] [PubMed]

16.

J. M. Verhaegen, J. L. Brebner, L. B. Allard, and J. Albert, “Ion implantation-induced strong photosensitivity in high-purity fused silica: Correlation of index changes with VUV color centers,” Appl. Phys. Lett. 68, 3084–3086 (1996). [CrossRef]

17.

J. Albert, K. O. Hill, B. Malo, D. C. Johnson, J. L. Brebner, Y. B. Trudeau, and G. Kajrys, “Formation and bleaching of strong ultraviolet absorption bands in germanium implanted synthetic fused silica,” Appl. Phys. Lett. 60, 148–150 (1992). [CrossRef]

18.

Y. Sakurai, K. Nagasawa, H Nishikawa, and Y. Ohki, “Point defects in high purity silica induced by high-dose gamma irradiation,” J. Appl. Phys. 75,1372–1377 (1994). [CrossRef]

19.

S. Agnello and L. Nuccio, “Thermal stability of gamma-irradiation-induced oxygen-deficient centers in silica,” Phys. Rev. B 73, 115203 1–6 (2006). [CrossRef]

OCIS Codes
(160.2220) Materials : Defect-center materials
(160.2750) Materials : Glass and other amorphous materials
(350.5610) Other areas of optics : Radiation
(060.3738) Fiber optics and optical communications : Fiber Bragg gratings, photosensitivity
(160.5335) Materials : Photosensitive materials

ToC Category:
Materials

History
Original Manuscript: February 19, 2008
Revised Manuscript: March 18, 2008
Manuscript Accepted: March 18, 2008
Published: March 26, 2008

Citation
A. Alessi, S. Agnello, F. M. Gelardi, S. Grandi, A. Magistris, and R. Boscaino, "Twofold co-ordinated Ge defects induced by gamma-ray irradiation in Ge-doped SiO2," Opt. Express 16, 4895-4900 (2008)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-16-7-4895


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References

  1. K. O. Hill, Y. Fujii, D. C. Johnson, and B. S. Kawasaki, "Photosensitivity in optical fiber waveguides: Application to reflection filter fabrication," Appl. Phys. Lett. 32, 647-649 (1978). [CrossRef]
  2. G. Pacchioni, L. Skuja, and D. L. Griscom, Defects in SiO2 and related dielectrics: Science and Technology, (Kluwer Academic Publishers, Dordrecht, 2000). [PubMed]
  3. V. B. Neustruev, "Colour centres in germanosilicate glass and optical fibres," J. Phys. Condens. Matter 6, 6901-6936 (1994). [CrossRef]
  4. M. Leone, S. Agnello, R. Boscaino, M. Cannas, and F. M. Gelardi, "Optical absorption, luminescence, and ESR spectral properties of point defects in silica," in Silicon-based Materials and Devices: Properties and Devices, H. S. Nalwa, ed., (Academic Press, San Diego, 2001).
  5. L. Skuja, "Isoelectronic series of twofold coordinated Si, Ge, and Sn atoms in glassy SiO2: a luminescence study," J. Non Cryst. Sol. 149, 77-95 (1992). [CrossRef]
  6. K. Awazu, H. Kawazoe, and M. Yamane, "Simultaneous generation of optical absorption bands 5.14 and 0.452 eV in 9 SiO2:GeO2 glasses heated under H2 atmosphere," J. Appl. Phys 68, 2713-2718 (1990). [CrossRef]
  7. M. Fujimaki, T. Watanabe, T. Katoh, T. Kasahara, N. Miyazaki, Y. Ohki, and H. Nishikawa, "Structures and generation mechanisms of paramagnetic centers and absorption bands responsible for Ge-doped SiO2 optical-fiber gratings," Phys. Rev. B 57, 3920-3926 (1998). [CrossRef]
  8. F. Messina, S. Agnello, R. Boscaino, M. Cannas, S. Grandi, and E. Quartarone, "Optical properties of Ge-oxygen deficient centers embedded in silica films," J. Non Cryst. Sol. 353, 670-673 (2007). [CrossRef]
  9. J. Albert, B. Malo, F. Bilodeau, D. C. Johnson, K. O. Hill, Y. Hibino, and M. Kawachi, "Photosensitivity in Ge-doped silica optical waveguides and fibers with 193-nm light from ArF excimer laser," Opt. Lett. 19, 387-389 (1994). [PubMed]
  10. M. Lancry, B. Poumellec, P. Niay, M. Douay, P. Cordier, and C. Depecker, "VUV and IR absorption spectra induced in H2-loaded and UV hyper-sensitized standard germanosilicate preform plates through exposure to ArF laser light," J. Non Cryst. Sol. 351, 3773-3783 (2005). [CrossRef]
  11. A. J. Cohen and H. L. Smith, "Ultraviolet and infrared absorption of fused germania," J. Phys. Chem. Solids 7, 301-306 (1958). [CrossRef]
  12. M. A. Vincenti, S. Almaviva, R. M. Montereali, H. J. Kalinowski, and R. N. Nogueira, "Permanent luminescent micropatterns photoinduced by low-power ultraviolet irradiation in lithium fluoride," Appl. Phys. Lett. 89, 241125 1-3 (2006). [CrossRef]
  13. J. Vitko Jr, "ESR studies of hydrogen hyperfine spectra in irradiated vitreous silica," J. Appl. Phys. 49, 5530-5535 (1978). [CrossRef]
  14. K. Awazu, K. Muta, and H. Kawazoe, "Formation mechanism of hydrogen-associated defect with an 11.9 mT doublet in electron spin resonance and red luminescence in 9SiO2:GeO2 fibers," J. Appl. Phys 74, 2237-2240 (1993). [CrossRef]
  15. J. Albert, B. Malo, K. O. Hill, D. C. Johnson, J. L. Brebner, and R. Leonelli, "Refractive-index changes in fused silica produced by heavy-ion implantation followed by photobleaching," Opt. Lett. 17, 1652-1654 (1992). [CrossRef] [PubMed]
  16. J. M. Verhaegen, J. L. Brebner, L. B. Allard, and J. Albert, "Ion implantation-induced strong photosensitivity in high-purity fused silica: Correlation of index changes with VUV color centers," Appl. Phys. Lett. 68, 3084-3086 (1996). [CrossRef]
  17. J. Albert, K. O. Hill, B. Malo, D. C. Johnson, J. L. Brebner, Y. B. Trudeau, and G. Kajrys, "Formation and bleaching of strong ultraviolet absorption bands in germanium implanted synthetic fused silica," Appl. Phys. Lett. 60, 148-150 (1992). [CrossRef]
  18. Y. Sakurai, K. Nagasawa, H, Nishikawa, and Y. Ohki, "Point defects in high purity silica induced by high-dose gamma irradiation," J. Appl. Phys. 75, 1372 -1377 (1994). [CrossRef]
  19. S. Agnello and L. Nuccio, "Thermal stability of gamma-irradiation-induced oxygen-deficient centers in silica," Phys. Rev. B 73, 115203 1-6 (2006). [CrossRef]

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