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Photodarkening in glassy As2S3
Journal of Non-Crystalline Solids 266±269 (2000) 929±932
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Photodarkening in glassy As2S3 P. Hari a, T. Su b, P.C. Taylor b,*, P.L. Kuhns c, W.G. Moulton c, N.S. Sullivan d a
b
Department of Physics, Vanderbilt University, Nashville, TN 37235, USA Department of Physics, University of Utah, Salt Lake City, UT 84112, USA c National High Magnetic Field Laboratory, Tallahassee, FL 32306, USA d Department of Physics, University of Florida, Gainesville, FL 32611, USA
Abstract Nuclear magnetic resonance (NMR) of 75 As at 17 T has been employed to study photodarkening (the shift of the optical absorption edge to lower energies after excitation with light of energy near the optical band edge) in glassy As2 S3 . After irradiation at 514.5 nm for 230 h with 170 mW/cm2 the average asymmetry parameter of the electric ®eld gradient (EFG) tensor increases from about 0:09 0:01 to about 0:12 0:01. This change is reversible on annealing at 200°C for 1.75 h. An increase in the asymmetry parameter implies an increase in the departure from cylindrical symmetry in the bonding at the arsenic pyramidal sites. Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Photodarkening, or the shift of the optical absorption edge to smaller energies after excitation with light whose energy is near that of the optical band edge, has been studied in many chalcogenide glasses for many years [1]. Over the years many models have been proposed to account for this eect [1], but most of them fail because they do not pertain to essentially all the atomic sites in the glass. A decrease (red shift) of the optical absorption edge implies a decrease in the energy gap, which must involve the vast majority of the atoms. Previous 75 As nuclear quadrupole resonance (NQR) experiments have shown that there is no change in the maximum component of the electric ®eld gradient (EFG) at the arsenic sites on photodarkening [1]. Since the EFG is determined by the bonding electrons (as they aect the po-
*
Corresponding author. Fax: +1-801 581 4246/4801. E-mail address: [email protected] (P.C. Taylor).
larization of the core electrons at each As nucleus), this result means that there are no gross bonding changes directly associated with the photodarkening process. In this paper we show that subtle changes in asymmetry of the As pyramidal sites account for the photodarkening eect. These changes are subtle enough that they would be dicult to observe by the usual diraction techniques normally employed for studying the structure of glasses. In the stoichiometric arsenic chalcogenide glasses, such as As2 S3 and As2 Se3 , the arsenic atoms are bonded to three chalcogen atoms in a pyramidal con®guration [1,2]. Previous high ®eld (17 T) nuclear magnetic resonance (NMR) studies have shown that these pyramidal As sites are nearly symmetric in both the crystalline and glassy phases [2,3]. 2. Experimental High ®eld ( up to 27 T) 75 As NMR measurements were performed using a 24.5 T dc magnet
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 8 6 8 - 6
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P. Hari et al. / Journal of Non-Crystalline Solids 266±269 (2000) 929±932
and a 30 T dc magnet. The pulsed NMR spectrometer was operated at about 125 MHz, a frequency which corresponds to a Zeeman ®eld of approximately 17 T for 75 As. The amplitude of the spin echo following a 90±180° Hahn echo sequence was plotted as a function of magnetic ®eld to produce the NMR powder pattern. All NMR measurements were performed at 77 K. Details are available elsewhere [2,3]. Photodarkening was accomplished by irradiation of the samples at 300 K in air for approximately 230 h. The irradiation source was an Ar ion laser operating at 514.5 nm with about 170 mW/cm2 at the sample. Annealing of the photodarkening was accomplished by heating the sample in a furnace at 230°C for 1.75 h. Samples of glassy As2 S3 were made by cutting and polishing cylinders of approximately 2 cm diameter to a thickness of approximately 200 lm. At this thickness saturated photodarkening occurs only in the ®rst few microns of the sample, but some photodarkening extends throughout the ®lm [4]. Although smaller photon energies could be used to provide greater penetration of the light into the sample, the eciency of the photodarkening process is decreased.
the EFG tensor normalized to the maximum component. Since the EFG tensor is traceless, the parameter, g, varies between 0 for axial symmetry and 1 for maximum asymmetry. Once the most probable values of q and g have been determined, the NMR and NQR lineshapes for glassy As2 S3 can be ®t by assuming that the NQR lineshape re¯ects primarily a variation in q and that, for simplicity, each value of q has the same value of g. In the following sections we will relax this restriction to place limits on the distributions of g that are consistent with the NMR and NQR measurements after photodarkening. All the ®ts to the NQR lineshape, which do not change on photodarkening, are accurate within the experimental error. Since this lineshape is featureless and insensitive to g, we will not show the NQR results in this paper. 4. Experimental results The high ®eld NMR lineshape at 77 K for 75 As in glassy As2 S3 is shown in Fig. 1. The solid circles are the experimental measurements and the solid
3. Theoretical background Previous studies [2,3] have shown that by combining the NQR and NMR experiments one can obtain an accurate ®t to the experimental data for both crystalline and glassy arsenic chalcogenides. For glassy As2 S3 this procedure involves using the peak frequency of the NQR line and the separation of the two divergences for the central transition (M 1=2 $ M ÿ1=2, where M is the nuclear spin projection quantum number) of the high ®eld NMR to obtain two equations from which to extract the two unknowns, the quadrupole coupling constant, e2 Qq/h and the asymmetry parameter, g. In these parameters e is the electronic charge, Q the quadrupole moment of the As nucleus, q the maximum component of the diagonalized EFG at the As nuclear site, h PlanckÕs constant and g is the absolute value of the dierence between the remaining two components of
Fig. 1. 75 As high ®eld NMR absorption at 77 K in glassy As2 S3 . Filled circles are experimental data and solid lines are theoretical ®ts as explained in the text. Experimental error in the data is 1% of the maximum intensity.
P. Hari et al. / Journal of Non-Crystalline Solids 266±269 (2000) 929±932
line is a ®t to the data to be described in the next section. The error in the data is represented by the scatter in the data points and is approximately 1% of the maximum. For the purposes of this paper we concentrate on the most distinctive features in this lineshape, the two peaks, or divergences, near 16 and 19 T in Fig. 1. These two features are the most sensitive to small changes in asymmetry. After the sample was irradiated with band-gap light as described in Section 2, the high ®eld NMR lineshape at 77 K for 75 As in glassy As2 S3 is shown in Fig. 2. Here also the solid circles are the experimental data and the solid line a ®t to the data. Errors are the same as in the case of Fig. 1. At ®rst glance the spectrum in Fig. 2 looks very similar to the one before photodarkening shown in Fig. 1. However, there is an important, subtle dierence to be discussed in the next section. This dierence is the relative heights of the two divergences in Figs. 1 and 2. In Fig. 1 the low-®eld peak is smaller than the high-®eld peak while in Fig. 2 this trend is reversed. The spectrum in Fig. 2 reverts to that shown in Fig. 1 after annealing at 200°C for 1.75 h.
931
5. Discussion Although the dierences between the spectra in Figs. 1 and 2 are subtle, they are reproducible, and the changes progress monotonically with the irradiation time. The spectra were ®t using a computer simulation that calculated the power pattern taking the quadrupole interaction as a second order perturbation on the Zeeman interaction [2,3]. A distribution of quadrupolar coupling constants was obtained from the measured NQR lineshape [2,3]. In the ®ts displayed in Figs. 1 and 2 every e2 Qq/h was assumed to have the same g. The curve in Fig. 1 was calculated for g 0:09 and that in Fig. 2 for g 0:12. Fits were calculated for dierent gs, and the error in both cases is estimated to be 0.01. It is unrealistic to assume that g does not vary from site to site, so we considered the eect that variations of g had on the above conclusion. A symmetric, Gaussian distribution in g about g 0:09 does not reproduce the spectrum in Fig. 2 no matter what the width is. A symmetric, Gaussian distribution in g about g 0:12 ®ts the spectrum in Fig. 2 within experimental error provided that the Gaussian width is less than about Dg 0:1. From these two results we conclude that the average asymmetry of the As pyramidal sites increases on photodarkening, and that photodarkening is not explained by an increase in the width of the distribution of asymmetries at the As sites that keeps the average asymmetry constant. 6. Summary The average asymmetry at a pyramidal arsenic bonding site in glassy As2 S3 increases after photodarkening. Acknowledgements
Fig. 2. 75 As high ®eld NMR absorption at 77 K in glassy As2 S3 after irradiation at 300 K with light of wavelength 514.5 nm for 230 h at 170 mW/cm2 intensity on the sample. Filled circles are experimental data and solid lines are theoretical ®ts as explained in the text. Experimental error in the data is 1% of the maximum intensity.
Work performed at the National High Magnetic Field Laboratory (NHMFL) was supported by NSF under grant number DMR 9527035, and work performed at the University of Utah was supported by NSF under grant number DMR 9704946. One of the authors (P.H.) gratefully
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P. Hari et al. / Journal of Non-Crystalline Solids 266±269 (2000) 929±932
acknowledges a travel grant from the users fund at the NHMFL. References [1] U. Strom, W.M. Pontuschka, D.J. Treacy, P.C. Taylor, J. Phys. Soc. Jpn. 49 (Suppl. A) (1980) 1155.
[2] P. Hari, P.C. Taylor, A. Kleinhammes, P.L. Kuhns, W.G. Moulton, N.S. Sullivan, Solid State Commun. 104 (1997) 669. [3] P.C. Taylor, P. Hari, A. Kleinhammes, P.L. Kuhns, W.G. Moulton, N.S. Sullivan, J. Non-Cryst. Solids 227±230 (1998) 770. [4] S. Ducharme, J. Hautala, P.C. Taylor, Phys. Rev. B 41 (1990) 12 250.
www.elsevier.com/locate/jnoncrysol
Photodarkening in glassy As2S3 P. Hari a, T. Su b, P.C. Taylor b,*, P.L. Kuhns c, W.G. Moulton c, N.S. Sullivan d a
b
Department of Physics, Vanderbilt University, Nashville, TN 37235, USA Department of Physics, University of Utah, Salt Lake City, UT 84112, USA c National High Magnetic Field Laboratory, Tallahassee, FL 32306, USA d Department of Physics, University of Florida, Gainesville, FL 32611, USA
Abstract Nuclear magnetic resonance (NMR) of 75 As at 17 T has been employed to study photodarkening (the shift of the optical absorption edge to lower energies after excitation with light of energy near the optical band edge) in glassy As2 S3 . After irradiation at 514.5 nm for 230 h with 170 mW/cm2 the average asymmetry parameter of the electric ®eld gradient (EFG) tensor increases from about 0:09 0:01 to about 0:12 0:01. This change is reversible on annealing at 200°C for 1.75 h. An increase in the asymmetry parameter implies an increase in the departure from cylindrical symmetry in the bonding at the arsenic pyramidal sites. Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Photodarkening, or the shift of the optical absorption edge to smaller energies after excitation with light whose energy is near that of the optical band edge, has been studied in many chalcogenide glasses for many years [1]. Over the years many models have been proposed to account for this eect [1], but most of them fail because they do not pertain to essentially all the atomic sites in the glass. A decrease (red shift) of the optical absorption edge implies a decrease in the energy gap, which must involve the vast majority of the atoms. Previous 75 As nuclear quadrupole resonance (NQR) experiments have shown that there is no change in the maximum component of the electric ®eld gradient (EFG) at the arsenic sites on photodarkening [1]. Since the EFG is determined by the bonding electrons (as they aect the po-
*
Corresponding author. Fax: +1-801 581 4246/4801. E-mail address: [email protected] (P.C. Taylor).
larization of the core electrons at each As nucleus), this result means that there are no gross bonding changes directly associated with the photodarkening process. In this paper we show that subtle changes in asymmetry of the As pyramidal sites account for the photodarkening eect. These changes are subtle enough that they would be dicult to observe by the usual diraction techniques normally employed for studying the structure of glasses. In the stoichiometric arsenic chalcogenide glasses, such as As2 S3 and As2 Se3 , the arsenic atoms are bonded to three chalcogen atoms in a pyramidal con®guration [1,2]. Previous high ®eld (17 T) nuclear magnetic resonance (NMR) studies have shown that these pyramidal As sites are nearly symmetric in both the crystalline and glassy phases [2,3]. 2. Experimental High ®eld ( up to 27 T) 75 As NMR measurements were performed using a 24.5 T dc magnet
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 8 6 8 - 6
930
P. Hari et al. / Journal of Non-Crystalline Solids 266±269 (2000) 929±932
and a 30 T dc magnet. The pulsed NMR spectrometer was operated at about 125 MHz, a frequency which corresponds to a Zeeman ®eld of approximately 17 T for 75 As. The amplitude of the spin echo following a 90±180° Hahn echo sequence was plotted as a function of magnetic ®eld to produce the NMR powder pattern. All NMR measurements were performed at 77 K. Details are available elsewhere [2,3]. Photodarkening was accomplished by irradiation of the samples at 300 K in air for approximately 230 h. The irradiation source was an Ar ion laser operating at 514.5 nm with about 170 mW/cm2 at the sample. Annealing of the photodarkening was accomplished by heating the sample in a furnace at 230°C for 1.75 h. Samples of glassy As2 S3 were made by cutting and polishing cylinders of approximately 2 cm diameter to a thickness of approximately 200 lm. At this thickness saturated photodarkening occurs only in the ®rst few microns of the sample, but some photodarkening extends throughout the ®lm [4]. Although smaller photon energies could be used to provide greater penetration of the light into the sample, the eciency of the photodarkening process is decreased.
the EFG tensor normalized to the maximum component. Since the EFG tensor is traceless, the parameter, g, varies between 0 for axial symmetry and 1 for maximum asymmetry. Once the most probable values of q and g have been determined, the NMR and NQR lineshapes for glassy As2 S3 can be ®t by assuming that the NQR lineshape re¯ects primarily a variation in q and that, for simplicity, each value of q has the same value of g. In the following sections we will relax this restriction to place limits on the distributions of g that are consistent with the NMR and NQR measurements after photodarkening. All the ®ts to the NQR lineshape, which do not change on photodarkening, are accurate within the experimental error. Since this lineshape is featureless and insensitive to g, we will not show the NQR results in this paper. 4. Experimental results The high ®eld NMR lineshape at 77 K for 75 As in glassy As2 S3 is shown in Fig. 1. The solid circles are the experimental measurements and the solid
3. Theoretical background Previous studies [2,3] have shown that by combining the NQR and NMR experiments one can obtain an accurate ®t to the experimental data for both crystalline and glassy arsenic chalcogenides. For glassy As2 S3 this procedure involves using the peak frequency of the NQR line and the separation of the two divergences for the central transition (M 1=2 $ M ÿ1=2, where M is the nuclear spin projection quantum number) of the high ®eld NMR to obtain two equations from which to extract the two unknowns, the quadrupole coupling constant, e2 Qq/h and the asymmetry parameter, g. In these parameters e is the electronic charge, Q the quadrupole moment of the As nucleus, q the maximum component of the diagonalized EFG at the As nuclear site, h PlanckÕs constant and g is the absolute value of the dierence between the remaining two components of
Fig. 1. 75 As high ®eld NMR absorption at 77 K in glassy As2 S3 . Filled circles are experimental data and solid lines are theoretical ®ts as explained in the text. Experimental error in the data is 1% of the maximum intensity.
P. Hari et al. / Journal of Non-Crystalline Solids 266±269 (2000) 929±932
line is a ®t to the data to be described in the next section. The error in the data is represented by the scatter in the data points and is approximately 1% of the maximum. For the purposes of this paper we concentrate on the most distinctive features in this lineshape, the two peaks, or divergences, near 16 and 19 T in Fig. 1. These two features are the most sensitive to small changes in asymmetry. After the sample was irradiated with band-gap light as described in Section 2, the high ®eld NMR lineshape at 77 K for 75 As in glassy As2 S3 is shown in Fig. 2. Here also the solid circles are the experimental data and the solid line a ®t to the data. Errors are the same as in the case of Fig. 1. At ®rst glance the spectrum in Fig. 2 looks very similar to the one before photodarkening shown in Fig. 1. However, there is an important, subtle dierence to be discussed in the next section. This dierence is the relative heights of the two divergences in Figs. 1 and 2. In Fig. 1 the low-®eld peak is smaller than the high-®eld peak while in Fig. 2 this trend is reversed. The spectrum in Fig. 2 reverts to that shown in Fig. 1 after annealing at 200°C for 1.75 h.
931
5. Discussion Although the dierences between the spectra in Figs. 1 and 2 are subtle, they are reproducible, and the changes progress monotonically with the irradiation time. The spectra were ®t using a computer simulation that calculated the power pattern taking the quadrupole interaction as a second order perturbation on the Zeeman interaction [2,3]. A distribution of quadrupolar coupling constants was obtained from the measured NQR lineshape [2,3]. In the ®ts displayed in Figs. 1 and 2 every e2 Qq/h was assumed to have the same g. The curve in Fig. 1 was calculated for g 0:09 and that in Fig. 2 for g 0:12. Fits were calculated for dierent gs, and the error in both cases is estimated to be 0.01. It is unrealistic to assume that g does not vary from site to site, so we considered the eect that variations of g had on the above conclusion. A symmetric, Gaussian distribution in g about g 0:09 does not reproduce the spectrum in Fig. 2 no matter what the width is. A symmetric, Gaussian distribution in g about g 0:12 ®ts the spectrum in Fig. 2 within experimental error provided that the Gaussian width is less than about Dg 0:1. From these two results we conclude that the average asymmetry of the As pyramidal sites increases on photodarkening, and that photodarkening is not explained by an increase in the width of the distribution of asymmetries at the As sites that keeps the average asymmetry constant. 6. Summary The average asymmetry at a pyramidal arsenic bonding site in glassy As2 S3 increases after photodarkening. Acknowledgements
Fig. 2. 75 As high ®eld NMR absorption at 77 K in glassy As2 S3 after irradiation at 300 K with light of wavelength 514.5 nm for 230 h at 170 mW/cm2 intensity on the sample. Filled circles are experimental data and solid lines are theoretical ®ts as explained in the text. Experimental error in the data is 1% of the maximum intensity.
Work performed at the National High Magnetic Field Laboratory (NHMFL) was supported by NSF under grant number DMR 9527035, and work performed at the University of Utah was supported by NSF under grant number DMR 9704946. One of the authors (P.H.) gratefully
932
P. Hari et al. / Journal of Non-Crystalline Solids 266±269 (2000) 929±932
acknowledges a travel grant from the users fund at the NHMFL. References [1] U. Strom, W.M. Pontuschka, D.J. Treacy, P.C. Taylor, J. Phys. Soc. Jpn. 49 (Suppl. A) (1980) 1155.
[2] P. Hari, P.C. Taylor, A. Kleinhammes, P.L. Kuhns, W.G. Moulton, N.S. Sullivan, Solid State Commun. 104 (1997) 669. [3] P.C. Taylor, P. Hari, A. Kleinhammes, P.L. Kuhns, W.G. Moulton, N.S. Sullivan, J. Non-Cryst. Solids 227±230 (1998) 770. [4] S. Ducharme, J. Hautala, P.C. Taylor, Phys. Rev. B 41 (1990) 12 250.