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Photodarkening in Ge3Se17 glass
Journal of Non-Crystalline Solids 274 (2000) 23±29
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Photodarkening in Ge3Se17 glass Craig R. Schardt a,*, Joseph H. Simmons a, Pierre Lucas b, Lydia Le Neindre c, Jacques Lucas c a
Department of Materials Science and Engineering, University of Florida, P.O. Box 116400, Gainesville, FL 32611, USA b Department of Chemistry, Arizona State University, Tempe, AZ 85287, USA c Laboratoire de Verres et Ceramiques, University of Rennes, Rennes, France
Abstract In this paper, we present measurements of the kinetics of photodarkening in Ge3 Se17 glass. The photodarkening is induced with sub-bandgap light and observed as transmission changes through a 1 mm thick sample of the glass. Both transient nonlinear and permanent processes are detected with these measurements. We consider the various sources that could lead to changes in sample optical properties and suggest that changes in electronic structure are the most reasonable explanation for the observed photodarkening. Comparison of the results with a model of light induced changes in amorphous selenium leads to an explanation for these eects based on the formation of dynamical bonds during illumination. Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction 1.1. Eects of dopants and photo-induced defects The purpose of this paper is to describe optical processes associated with photo-induced defects in glasses. In this area, glasses are unique and present a wealth of potential physical processes, as well as a wealth of unique applications. This paper will present novel results from studies of germanium selenide glasses with less than 33% Ge content and the optical processes associated with photoinduced defects. Photosensitivity in glasses can be either transient or permanent. It has been observed in a wide variety of glasses, most notably oxygen de®cient GeO2 ±SiO2 glasses [1] and chalcogenides [2]. The
* Corresponding author. Tel.: +1-352 392 6679; fax: +1-352 392 1751.
process is best understood in oxygen de®cient GeO2 ±SiO2 glasses where it is permanent, unless the glass is heated to near the glass transition. The oxygen de®ciency generally causes oxygen vacancies about the Ge atoms [3]. This defect has an absorption band near 242 nm. Exposure to ultraviolet (UV) or visible light excites free carriers either by single-photon or two-photon processes. The hole carriers are subsequently trapped at the Ge oxygen vacancy centers turning them into E0 centers with an absorption band near 202 nm [3]. This decrease (bleaching) of the 242 nm band and growth of the 202 nm band causes an associated change in refractive index [3]. The combined bleaching and growth processes can produce either a positive or negative index change [4]. At larger incident light intensities (sucient to form a Mott density of electrons [5]), the electric ®eld or lattice heating will relax the glass structure irreversibly and produce density changes. These generally produce a positive index change.
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 ( 0 0 ) 0 0 2 0 0 - 3
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C.R. Schardt et al. / Journal of Non-Crystalline Solids 274 (2000) 23±29
The photosensitivity process in amorphous chalcogenides is less understood and appears to consist of transient, recoverable, and permanent changes. The transient changes are those which disappear as soon as the optical excitation is removed from the sample, while the recoverable changes are those which can be completely removed by annealing at temperatures lower than the glass-transition temperature, Tg . Changes that cannot be removed by annealing have so far only been observed in chalcogenide glass ®lms [6]. In addition to photodarkening, optical anisotropy can be developed with polarized light, so that the ratio of absorption in the direction of polarization to absorption in the direction perpendicular to the direction of polarization decreases [6]. Recent studies on amorphous selenium by Kolobov et al. [7], have examined extended X-ray absorption ®ne structure (EXAFS) data measured in the presence of illumination with energies greater than the bandgap. The results show that the Se±Se bond length is not changed by the illumination, however, both the variance of the Se±Se correlation peak and the coordination of Se atoms increase under illumination. The change in coordination is small (less than 5% increase in the number of threefold-coordinated selenium) and reversible, disappearing when the light is removed. The eect of the light exposure appears to be the transient formation of threefold-coordinated Se pairs called dynamical bonds (DB). The decay of these DB leads to changes in structure and the formation of valence-alternation pairs (VAP) as follows [7]: 8 0 > > 2C2 original ÿ 0 < 0 0 0 2C2 2hm ! 2C2 ! C3 ÿ C3 ! 2C2 new > > : ÿ C1 C 3: The subscript represents the coordination number of the chalcogen atom and the superscript represents the charge of the ion. The absorption of a photon creates an excited chalcogen atom. Two excited atoms can bond together forming a dynamical bond. The decay of the dynamical bond can lead to three possible states; the structure can return to its original con®guration, it can become a new con®guration, or it can become a VAP. The
broadening of the Se±Se correlation peak is permanent, and is interpreted by the authors as an increase in disorder in the glass [7]. Recent X-ray photoelectron spectroscopy (XPS) by Jain et al. [8] reveals that, in As50 Se50 glasses, light exposure produces permanent changes in the chemical environments of the constituent atoms. In addition to the permanent changes, a transient change in the XPS spectra of a bulk glass sample is observed during illumination of the sample with sub-bandgap light. The transient changes are observed in the valence band of the glass and indicate a light-induced change in the density of states in the valence (bonding electrons) band. This observation may be an eect of DB on the electronic structure of the chalcogenide glass. 1.2. Model for photosensitivity of Se containing glasses The states at the top of the valence band of Se are formed by the Se lone pair electrons [9]. Under illumination, an electron from one of these states is photoexcited into the conduction band, leaving a hole trapped on the Se atom. Two such Se atoms can form a new bond as long as the atoms are on neighboring chains, and the holes have opposite spins [7]. The distance between Se atoms on neighboring chains is about 0.368 nm, which is almost identical to the second nearest neighbor distance within chains [10]. This con®guration is the dynamical bond suggested by Kolobov. In their model, they suggest that the DB exist until the light excitation is removed, at which time they recombine by returning to the original structure or by creating increased structural disorder and VAP. We suggest that the kinetics of photodarkening can be explained by this model if we view the DB as continuously forming and decaying while the material is photoexcited. The formation of DB requires the excitation of two electrons, and the presence of these bonds should alter the valence band of the chalcogenide glass. This alteration should have a transient non-linear absorption observable when the glass is initially illuminated. The rate of formation of the transient absorption
C.R. Schardt et al. / Journal of Non-Crystalline Solids 274 (2000) 23±29
should be related to the equilibrium between creation and decay of DB. When these bonds decay to their original state, the a-Se should show no reversible change in absorption. However, when they decay to VAP or form new bonding arrangements with increased disorder the glass will have reversible and possibly irreversible absorption and index changes. The rate of darkening should be indicative of the rate at which dynamical bond decay produces changes in the structure. When the light is removed, all of the DB recombine and any absorption due to their presence disappears. Any recombination which changes the local structure though will be frozen in until the glass is annealed near Tg . We assume that the glass formed by thermal cooling or annealing will adopt a relaxed state in which the distribution of Se±Se bonds will be relaxed. Photoexcitation and de-excitation are likely to form a less relaxed state, resulting in a larger distribution of Se±Se bonds. This distribution should produce a net expansion of the glass structure and a permanent decrease in refractive index. What is not known at present is the role of atoms such as As or Ge with greater coordination than Se. These atoms appear to stabilize the darkening process at room temperature, possibly by reducing the structural freedom of the lattice. If this is the case, they should also reduce the formation of DB by hindering the reorientation of Se atoms. We have chosen to study Ge±Se glasses because little is known about the role of Ge in the photodarkening process. Most of the research into photodarkening in chalcogenide glass has been on As containing glasses [13]. While these studies have provided signi®cant insight into the fundamental processes of photodarkening, it is still not well understood. Comparing the photodarkening of glasses containing Ge with that of glasses containing As should further understanding of the mechanisms of photo-induced changes in chalcogenide glasses. In addition, waveguides have been fabricated in a quaternary glass containing Ge, As, S, and Se [11]. Understanding the role of Ge in such a system is important for the future of such devices. It should be easier to isolate the effects of Ge in a simple binary composition and
25
then apply this knowledge to the more complicated multi-component glasses. The possibility of gaining new information about the photodarkening process and the practical importance of germanium as a component of chalcogenide glass devices makes the germanium selenium system worthy of study. 2. Experimental detail Gex Se1ÿx glasses were formed from 99.999% purity raw materials further puri®ed by sublimation at 250°C. The glasses were melted in dehydrated silica ampoules at 600°C. The ampoules were rocked during melting to improve homogeneity. The ampoules were air cooled and the silica was cut to release Gex Se1ÿx rods approximately 1 cm diameter by 10 cm long. The rods were sliced into 1 mm thick disks and both faces were polished ¯at and parallel. No further processing is done to the samples after they are polished. Because of the manner in which these glasses are made, the samples are in a well-annealed state. Data for the composition Ge3 Se17 will be presented here. This composition has Tg 126°C. The experiments were carried out on bulk samples, not ®lms. This sample geometry minimizes deposition and surface related eects. The samples were 1 mm thick, too thick to permit direct measurement of the Tauc gap [12], however data by Oe et al. [13], for ®lms about 1mm thick indicate that Eg 1:95 eV (636 nm) for this stoichiometry. The kinetics of photodarkening were observed by recording the change in transmission of a subbandgap light source as a function of exposure time and illumination intensity. The same beam was used for inducing darkening and measuring the darkening. The samples were exposed to linearly polarized light from a titanium±sapphire laser operating at 800 nm in the pulsed mode with 150 fs pulses at a repetition rate of 76 MHz. Sample transmission was measured with Si photodetectors and a charge-coupled detector (CCD) camera. The incident power on the sample was controlled by neutral-density glass ®lters placed in the beam before the lens. The laser was brought to a focus by a 150 mm focal length lens. For
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C.R. Schardt et al. / Journal of Non-Crystalline Solids 274 (2000) 23±29
measurements of the transmission change during photodarkening, the sample was placed 20 mm behind the focal point. In this geometry, the spot size on the sample is estimated to be 0.35 mm in radius and the peak intensity of a 1 mW beam was about 0.5 W/cm2 . The data reported here were collected with laser powers of 20±40 mW total power. The peak intensities are therefore approximately of 10±20 W/cm2 . Data were collected every second from the beginning of exposure with the aid of a computer data-acquisition system. Unless otherwise noted, each darkening run was started at a dierent (unexposed) location on a sample. In one set of experiments a 50% mechanical chopper running at 15 Hz was placed in the beam path. Under these conditions, the average power on the sample is halved but the intensity of the electric ®eld is unchanged.
Fig. 2. Change in relative transmittance. Light exposure is interrupted for 5 min and restarted at cumulative exposure times of 300, 600 and 1200 s.
3. Results Fig. 1 shows the change in transmittance observed as a function of exposure time for two dierent powers. Two distinct response functions are evident from the data of Fig. 2 which show the eect of arresting and restarting the exposure to the light. The curves show a partial recovery of transparency which is lost upon re-exposure to the light. Fig. 3 shows the time dependence of trans-
Fig. 1. Photodarkening of Ge3 Se17 glass as a function of time and laser power.
Fig. 3. Photodarkening as measured by transmittance as a function of time. The open symbols represent the condition where the light is chopped as described in the text. ML stands for mode-locked pulses. The circles represent an increase in power as shown in the key. The triangles represent a return to the original power.
mittance for chopped and unchopped light with power increased and decreased once the darkening has reached saturation. The curves from 0 to 1200 s represent the initial darkening of two dierent spots on the sample. At 1200 s, the experiment was stopped, the laser power increased, and then the experiment restarted. At 1600 s the experiment again was stopped, the laser power returned to itÕs original level and the experiment restarted. The spot exposed to chopped light had a slower dark-
C.R. Schardt et al. / Journal of Non-Crystalline Solids 274 (2000) 23±29
ening response than the spot exposed continuously indicating that the darkening process is related to the ¯uence. A reduced transmittance is observed when both darkened spots are exposed to larger power. This change is power dependent and transient since the transmittance returns to its previous value when the power is reduced. After darkening has reached saturation, no further darkening occurs even when the sample is exposed to larger powers. Because the transmittance was measured with respect to the beam intensity by a double beam method, the error in the transmittance data was less than 2% for exposure of the same region of the glass. Variance from region to region was about 15%, however, the data had the same relative changes (less than 5%) if the spot remained ®xed in the glass. 4. Discussion Fig. 2 shows that the photodarkening eect results from the sum of at least two independent processes, one faster and reversible in the absence of illumination and the other permanent. Both of these changes may consist of more than one process. Because the transmission is measured during exposure, we cannot directly separate the recoverable and transient eects, however Figs. 2 and 3 provide evidence that both transient and permanent eects are present in this material. The initial rapid decrease in transmission is caused by a transient darkening processes. The decrease happens at the start of every exposure and the magnitude seems to be independent of the amount of darkening already present in the sample. The slower change appears to be cumulative from one exposure to the next. This change is associated with the recoverable and permanent changes present in the sample. The transmission change measured is the sum of multiple simultaneous processes. These can be broken down to the following four mechanisms as shown in the following equation: Da Dap q Dap T Dar r Dat eÿ ;
27
where ap refers to permanent absorbance changes and ar refers to recoverable absorbance changes and at refers to a transient absorbance change. The terms are functions of dierent variables for de®ning the state of the glass. q represents the density of the glass, T the temperature of the illuminated region, r the structural disorder, and eÿ is the electronic con®guration. The ®rst term is due to irreversible changes in the density of the glass. Measurements of the glass after exposure shows an increase in volume in the illuminated region and a permanent decrease in refractive index. This term appears to be the largest of the changes observed in the glass with exposure to light. Its magnitude would be proportional to ¯uence as observed in Fig. 1. The second term in the equation is a permanent change resulting from thermal annealing of the glass during exposure. Stokes/anti-Stokes Raman measurements [14] of the Ge±Se and the Se±Se vibrations at 196 and 256 cmÿ1 under the same exposure conditions as the darkening experiments limit the temperature change to less than 5°C. Therefore, this term is negligible. The lack of temperature increase would also rule out the possibility of any transient thermal changes appearing in these measurements. The third term represents the recoverable absorbance changes associated with increased structural disorder and formation of VAP. This darkening is associated with a decrease in the bandgap energy [15]. Such a decrease in bandgap energy should cause an increase in the index of refraction. Both the rate of darkening and the saturation appear to be dependent on the intensity of the inducing light. The fourth term represents the transient change in absorbance caused by the presence of photoexcited electrons in the conduction band and possibly by the presence of DB. Based on the studies of Kolobov et al. [7], the faster, reversible process appears to be associated with the excitation of lone pair electrons and the formation of DB. This eect is power dependent, thus transmittance change is proportional to the photon density to the ®rst power (typical of a two-photon process), consistent with the dynamical bond formation model. It is responsible for the change in
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C.R. Schardt et al. / Journal of Non-Crystalline Solids 274 (2000) 23±29
transmittance with increasing and decreasing power shown in Fig. 3. Since this eect also increases absorption, it should cause a transient, positive change in the index of refraction. Overall, we have observed a negative change in the index of refraction as measured by a decrease in the sample re¯ectivity during darkening. This observed decrease in the refractive index in the exposed region appears to contradict the predicted change, speci®cally, in a simple model, any process which decreases the bandgap should increase the index of refraction for wavelengths of light less energetic than the bandgap. Despite these positive contributions to the index of refraction, the overall decrease in the index of refraction is brought about by the much larger eect of the permanent density change. This decrease in density decreases the refractive index enough to overwhelm the smaller positive contributions from the transient and recoverable darkening processes. It is interesting to compare the photo-induced processes in Ge3 Se17 with those in GeO2 ±SiO2 glasses. In the chalcogenide glass, the photosensitivity results from excitation of band-edge defects (lone pair or valence electrons), while the oxide glass photosensitivity results from alteration of color centers deeper in the bandgap. Structural changes occur in both materials, however, the threshold powers in the oxide glass are much larger than those in the chalcogenide. In the chalcogenide glass, electronic excitation (since it occurs near the band edge) is directly coupled to bonding alterations and rearrangement of the structure. In the oxide glass, the electronic excitations can be developed independently of the structural alteration of the glass. Both processes lead to permanent photo-induced changes. However, the chalcogenide glass has a transient photosensitivity from the electronic excitations, while the same eect appears to be smaller, if present, in the oxide glasses. One major dierence in structure may be the role of Ge. Additional concentrations of Ge should stien the chalcogenide network and may increase the threshold for permanent photodarkening. Clearly, this area of research is interesting since it begins to supply clues for the unexpected ability of light to alter the
structure of glass without heating to the glass transition temperature. 5. Conclusions Measurements of photodarkening in Ge3 Se17 glass show both a transient and a permanent eect. Both changes increase the absorbance of the glass. The permanent photodarkening is associated with a permanent increase in sample volume in the exposed region and also by recoverable changes in the defect concentration and bonding. Based on similarity to EXAFS results obtained on amorphous Se, it appears that the transient eect is associated with excitation of Se lone pair electrons to form VAP in which some Se atoms reach a coordination of three while others decrease to one. Upon removal of the light, the free carriers recombine and alter the structure in the process to produce a lower density material. This lower density material has a greater distribution of bond lengths and angles which leads to a broadening of the band tail and a decrease in the energy of the UV edge. The transient photodarkening is linearly dependent on optical intensity and corresponds to a large optical non-linearity in both the absorption and refractive index. References [1] K.O. Hill, Y. Fujii, D.C. Johnson, B.S. Kawasaki, Appl. Phys. Lett. 32 (1978) 647. [2] G. Pfeier, M.A. Paesler, S.C. Agarwal, J. Non-Cryst. Solids 130 (1991) 111. [3] K.D. Simmons, J.H. Simmons, Appl. Phys. Lett. 66 (1995) 2104. [4] K.D. Simmons, G.I. Stegeman, B.G. Potter, J.H. Simmons, J. Non-Cryst. Solids 179 (1994) 254. [5] N.F. Mott, Metal-Insulator Transitions, Taylor and Francis, London, 1990, p. 22. [6] H. Fritzsche, Philos. Mag. B 68 (1993) 561. [7] A.V. Kolobov, H. Oyanagi, Ka. Tanaka, Ke. Tanaka, Phys. Rev. B 55 (1997) 726. [8] H. Jain, S. Krishnaswami, A.C. Miller, P. Krecmer, S.R. Elliot, M. Vlcek, these Proceedings, p. 115. [9] A.V. Kolobov, P. Kostikov, S.S. Lantratova, V.M. Lyubin, Sov. Phys. Solid State 33 (1991) 444. [10] H. Richter, J. Non-Cryst. Solids 8±10 (1972) 388. [11] S. Ramachandran, S.G. Bishop, Appl. Phys. Lett. 74 (1999) 13.
C.R. Schardt et al. / Journal of Non-Crystalline Solids 274 (2000) 23±29 [12] J. Tauc, R. Grigorovici, A. Vancu, Phys. Status Solidi 15 (1966) 627. [13] K. Oe, Y. Toyoshima, H. Nagai, J. Non-Cryst. Solids 20 (1976) 405.
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[14] P. Tronc, M. Bensoussan, A. Branac, Phys. Rev. B 8 (1973) 5947. [15] L. Tich y, H. Ticha, P. Nagels, R. Callaerts, J. Non-Cryst. Solids 240 (1998) 177.
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Photodarkening in Ge3Se17 glass Craig R. Schardt a,*, Joseph H. Simmons a, Pierre Lucas b, Lydia Le Neindre c, Jacques Lucas c a
Department of Materials Science and Engineering, University of Florida, P.O. Box 116400, Gainesville, FL 32611, USA b Department of Chemistry, Arizona State University, Tempe, AZ 85287, USA c Laboratoire de Verres et Ceramiques, University of Rennes, Rennes, France
Abstract In this paper, we present measurements of the kinetics of photodarkening in Ge3 Se17 glass. The photodarkening is induced with sub-bandgap light and observed as transmission changes through a 1 mm thick sample of the glass. Both transient nonlinear and permanent processes are detected with these measurements. We consider the various sources that could lead to changes in sample optical properties and suggest that changes in electronic structure are the most reasonable explanation for the observed photodarkening. Comparison of the results with a model of light induced changes in amorphous selenium leads to an explanation for these eects based on the formation of dynamical bonds during illumination. Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction 1.1. Eects of dopants and photo-induced defects The purpose of this paper is to describe optical processes associated with photo-induced defects in glasses. In this area, glasses are unique and present a wealth of potential physical processes, as well as a wealth of unique applications. This paper will present novel results from studies of germanium selenide glasses with less than 33% Ge content and the optical processes associated with photoinduced defects. Photosensitivity in glasses can be either transient or permanent. It has been observed in a wide variety of glasses, most notably oxygen de®cient GeO2 ±SiO2 glasses [1] and chalcogenides [2]. The
* Corresponding author. Tel.: +1-352 392 6679; fax: +1-352 392 1751.
process is best understood in oxygen de®cient GeO2 ±SiO2 glasses where it is permanent, unless the glass is heated to near the glass transition. The oxygen de®ciency generally causes oxygen vacancies about the Ge atoms [3]. This defect has an absorption band near 242 nm. Exposure to ultraviolet (UV) or visible light excites free carriers either by single-photon or two-photon processes. The hole carriers are subsequently trapped at the Ge oxygen vacancy centers turning them into E0 centers with an absorption band near 202 nm [3]. This decrease (bleaching) of the 242 nm band and growth of the 202 nm band causes an associated change in refractive index [3]. The combined bleaching and growth processes can produce either a positive or negative index change [4]. At larger incident light intensities (sucient to form a Mott density of electrons [5]), the electric ®eld or lattice heating will relax the glass structure irreversibly and produce density changes. These generally produce a positive index change.
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 ( 0 0 ) 0 0 2 0 0 - 3
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C.R. Schardt et al. / Journal of Non-Crystalline Solids 274 (2000) 23±29
The photosensitivity process in amorphous chalcogenides is less understood and appears to consist of transient, recoverable, and permanent changes. The transient changes are those which disappear as soon as the optical excitation is removed from the sample, while the recoverable changes are those which can be completely removed by annealing at temperatures lower than the glass-transition temperature, Tg . Changes that cannot be removed by annealing have so far only been observed in chalcogenide glass ®lms [6]. In addition to photodarkening, optical anisotropy can be developed with polarized light, so that the ratio of absorption in the direction of polarization to absorption in the direction perpendicular to the direction of polarization decreases [6]. Recent studies on amorphous selenium by Kolobov et al. [7], have examined extended X-ray absorption ®ne structure (EXAFS) data measured in the presence of illumination with energies greater than the bandgap. The results show that the Se±Se bond length is not changed by the illumination, however, both the variance of the Se±Se correlation peak and the coordination of Se atoms increase under illumination. The change in coordination is small (less than 5% increase in the number of threefold-coordinated selenium) and reversible, disappearing when the light is removed. The eect of the light exposure appears to be the transient formation of threefold-coordinated Se pairs called dynamical bonds (DB). The decay of these DB leads to changes in structure and the formation of valence-alternation pairs (VAP) as follows [7]: 8 0 > > 2C2 original ÿ 0 < 0 0 0 2C2 2hm ! 2C2 ! C3 ÿ C3 ! 2C2 new > > : ÿ C1 C 3: The subscript represents the coordination number of the chalcogen atom and the superscript represents the charge of the ion. The absorption of a photon creates an excited chalcogen atom. Two excited atoms can bond together forming a dynamical bond. The decay of the dynamical bond can lead to three possible states; the structure can return to its original con®guration, it can become a new con®guration, or it can become a VAP. The
broadening of the Se±Se correlation peak is permanent, and is interpreted by the authors as an increase in disorder in the glass [7]. Recent X-ray photoelectron spectroscopy (XPS) by Jain et al. [8] reveals that, in As50 Se50 glasses, light exposure produces permanent changes in the chemical environments of the constituent atoms. In addition to the permanent changes, a transient change in the XPS spectra of a bulk glass sample is observed during illumination of the sample with sub-bandgap light. The transient changes are observed in the valence band of the glass and indicate a light-induced change in the density of states in the valence (bonding electrons) band. This observation may be an eect of DB on the electronic structure of the chalcogenide glass. 1.2. Model for photosensitivity of Se containing glasses The states at the top of the valence band of Se are formed by the Se lone pair electrons [9]. Under illumination, an electron from one of these states is photoexcited into the conduction band, leaving a hole trapped on the Se atom. Two such Se atoms can form a new bond as long as the atoms are on neighboring chains, and the holes have opposite spins [7]. The distance between Se atoms on neighboring chains is about 0.368 nm, which is almost identical to the second nearest neighbor distance within chains [10]. This con®guration is the dynamical bond suggested by Kolobov. In their model, they suggest that the DB exist until the light excitation is removed, at which time they recombine by returning to the original structure or by creating increased structural disorder and VAP. We suggest that the kinetics of photodarkening can be explained by this model if we view the DB as continuously forming and decaying while the material is photoexcited. The formation of DB requires the excitation of two electrons, and the presence of these bonds should alter the valence band of the chalcogenide glass. This alteration should have a transient non-linear absorption observable when the glass is initially illuminated. The rate of formation of the transient absorption
C.R. Schardt et al. / Journal of Non-Crystalline Solids 274 (2000) 23±29
should be related to the equilibrium between creation and decay of DB. When these bonds decay to their original state, the a-Se should show no reversible change in absorption. However, when they decay to VAP or form new bonding arrangements with increased disorder the glass will have reversible and possibly irreversible absorption and index changes. The rate of darkening should be indicative of the rate at which dynamical bond decay produces changes in the structure. When the light is removed, all of the DB recombine and any absorption due to their presence disappears. Any recombination which changes the local structure though will be frozen in until the glass is annealed near Tg . We assume that the glass formed by thermal cooling or annealing will adopt a relaxed state in which the distribution of Se±Se bonds will be relaxed. Photoexcitation and de-excitation are likely to form a less relaxed state, resulting in a larger distribution of Se±Se bonds. This distribution should produce a net expansion of the glass structure and a permanent decrease in refractive index. What is not known at present is the role of atoms such as As or Ge with greater coordination than Se. These atoms appear to stabilize the darkening process at room temperature, possibly by reducing the structural freedom of the lattice. If this is the case, they should also reduce the formation of DB by hindering the reorientation of Se atoms. We have chosen to study Ge±Se glasses because little is known about the role of Ge in the photodarkening process. Most of the research into photodarkening in chalcogenide glass has been on As containing glasses [13]. While these studies have provided signi®cant insight into the fundamental processes of photodarkening, it is still not well understood. Comparing the photodarkening of glasses containing Ge with that of glasses containing As should further understanding of the mechanisms of photo-induced changes in chalcogenide glasses. In addition, waveguides have been fabricated in a quaternary glass containing Ge, As, S, and Se [11]. Understanding the role of Ge in such a system is important for the future of such devices. It should be easier to isolate the effects of Ge in a simple binary composition and
25
then apply this knowledge to the more complicated multi-component glasses. The possibility of gaining new information about the photodarkening process and the practical importance of germanium as a component of chalcogenide glass devices makes the germanium selenium system worthy of study. 2. Experimental detail Gex Se1ÿx glasses were formed from 99.999% purity raw materials further puri®ed by sublimation at 250°C. The glasses were melted in dehydrated silica ampoules at 600°C. The ampoules were rocked during melting to improve homogeneity. The ampoules were air cooled and the silica was cut to release Gex Se1ÿx rods approximately 1 cm diameter by 10 cm long. The rods were sliced into 1 mm thick disks and both faces were polished ¯at and parallel. No further processing is done to the samples after they are polished. Because of the manner in which these glasses are made, the samples are in a well-annealed state. Data for the composition Ge3 Se17 will be presented here. This composition has Tg 126°C. The experiments were carried out on bulk samples, not ®lms. This sample geometry minimizes deposition and surface related eects. The samples were 1 mm thick, too thick to permit direct measurement of the Tauc gap [12], however data by Oe et al. [13], for ®lms about 1mm thick indicate that Eg 1:95 eV (636 nm) for this stoichiometry. The kinetics of photodarkening were observed by recording the change in transmission of a subbandgap light source as a function of exposure time and illumination intensity. The same beam was used for inducing darkening and measuring the darkening. The samples were exposed to linearly polarized light from a titanium±sapphire laser operating at 800 nm in the pulsed mode with 150 fs pulses at a repetition rate of 76 MHz. Sample transmission was measured with Si photodetectors and a charge-coupled detector (CCD) camera. The incident power on the sample was controlled by neutral-density glass ®lters placed in the beam before the lens. The laser was brought to a focus by a 150 mm focal length lens. For
26
C.R. Schardt et al. / Journal of Non-Crystalline Solids 274 (2000) 23±29
measurements of the transmission change during photodarkening, the sample was placed 20 mm behind the focal point. In this geometry, the spot size on the sample is estimated to be 0.35 mm in radius and the peak intensity of a 1 mW beam was about 0.5 W/cm2 . The data reported here were collected with laser powers of 20±40 mW total power. The peak intensities are therefore approximately of 10±20 W/cm2 . Data were collected every second from the beginning of exposure with the aid of a computer data-acquisition system. Unless otherwise noted, each darkening run was started at a dierent (unexposed) location on a sample. In one set of experiments a 50% mechanical chopper running at 15 Hz was placed in the beam path. Under these conditions, the average power on the sample is halved but the intensity of the electric ®eld is unchanged.
Fig. 2. Change in relative transmittance. Light exposure is interrupted for 5 min and restarted at cumulative exposure times of 300, 600 and 1200 s.
3. Results Fig. 1 shows the change in transmittance observed as a function of exposure time for two dierent powers. Two distinct response functions are evident from the data of Fig. 2 which show the eect of arresting and restarting the exposure to the light. The curves show a partial recovery of transparency which is lost upon re-exposure to the light. Fig. 3 shows the time dependence of trans-
Fig. 1. Photodarkening of Ge3 Se17 glass as a function of time and laser power.
Fig. 3. Photodarkening as measured by transmittance as a function of time. The open symbols represent the condition where the light is chopped as described in the text. ML stands for mode-locked pulses. The circles represent an increase in power as shown in the key. The triangles represent a return to the original power.
mittance for chopped and unchopped light with power increased and decreased once the darkening has reached saturation. The curves from 0 to 1200 s represent the initial darkening of two dierent spots on the sample. At 1200 s, the experiment was stopped, the laser power increased, and then the experiment restarted. At 1600 s the experiment again was stopped, the laser power returned to itÕs original level and the experiment restarted. The spot exposed to chopped light had a slower dark-
C.R. Schardt et al. / Journal of Non-Crystalline Solids 274 (2000) 23±29
ening response than the spot exposed continuously indicating that the darkening process is related to the ¯uence. A reduced transmittance is observed when both darkened spots are exposed to larger power. This change is power dependent and transient since the transmittance returns to its previous value when the power is reduced. After darkening has reached saturation, no further darkening occurs even when the sample is exposed to larger powers. Because the transmittance was measured with respect to the beam intensity by a double beam method, the error in the transmittance data was less than 2% for exposure of the same region of the glass. Variance from region to region was about 15%, however, the data had the same relative changes (less than 5%) if the spot remained ®xed in the glass. 4. Discussion Fig. 2 shows that the photodarkening eect results from the sum of at least two independent processes, one faster and reversible in the absence of illumination and the other permanent. Both of these changes may consist of more than one process. Because the transmission is measured during exposure, we cannot directly separate the recoverable and transient eects, however Figs. 2 and 3 provide evidence that both transient and permanent eects are present in this material. The initial rapid decrease in transmission is caused by a transient darkening processes. The decrease happens at the start of every exposure and the magnitude seems to be independent of the amount of darkening already present in the sample. The slower change appears to be cumulative from one exposure to the next. This change is associated with the recoverable and permanent changes present in the sample. The transmission change measured is the sum of multiple simultaneous processes. These can be broken down to the following four mechanisms as shown in the following equation: Da Dap q Dap T Dar r Dat eÿ ;
27
where ap refers to permanent absorbance changes and ar refers to recoverable absorbance changes and at refers to a transient absorbance change. The terms are functions of dierent variables for de®ning the state of the glass. q represents the density of the glass, T the temperature of the illuminated region, r the structural disorder, and eÿ is the electronic con®guration. The ®rst term is due to irreversible changes in the density of the glass. Measurements of the glass after exposure shows an increase in volume in the illuminated region and a permanent decrease in refractive index. This term appears to be the largest of the changes observed in the glass with exposure to light. Its magnitude would be proportional to ¯uence as observed in Fig. 1. The second term in the equation is a permanent change resulting from thermal annealing of the glass during exposure. Stokes/anti-Stokes Raman measurements [14] of the Ge±Se and the Se±Se vibrations at 196 and 256 cmÿ1 under the same exposure conditions as the darkening experiments limit the temperature change to less than 5°C. Therefore, this term is negligible. The lack of temperature increase would also rule out the possibility of any transient thermal changes appearing in these measurements. The third term represents the recoverable absorbance changes associated with increased structural disorder and formation of VAP. This darkening is associated with a decrease in the bandgap energy [15]. Such a decrease in bandgap energy should cause an increase in the index of refraction. Both the rate of darkening and the saturation appear to be dependent on the intensity of the inducing light. The fourth term represents the transient change in absorbance caused by the presence of photoexcited electrons in the conduction band and possibly by the presence of DB. Based on the studies of Kolobov et al. [7], the faster, reversible process appears to be associated with the excitation of lone pair electrons and the formation of DB. This eect is power dependent, thus transmittance change is proportional to the photon density to the ®rst power (typical of a two-photon process), consistent with the dynamical bond formation model. It is responsible for the change in
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C.R. Schardt et al. / Journal of Non-Crystalline Solids 274 (2000) 23±29
transmittance with increasing and decreasing power shown in Fig. 3. Since this eect also increases absorption, it should cause a transient, positive change in the index of refraction. Overall, we have observed a negative change in the index of refraction as measured by a decrease in the sample re¯ectivity during darkening. This observed decrease in the refractive index in the exposed region appears to contradict the predicted change, speci®cally, in a simple model, any process which decreases the bandgap should increase the index of refraction for wavelengths of light less energetic than the bandgap. Despite these positive contributions to the index of refraction, the overall decrease in the index of refraction is brought about by the much larger eect of the permanent density change. This decrease in density decreases the refractive index enough to overwhelm the smaller positive contributions from the transient and recoverable darkening processes. It is interesting to compare the photo-induced processes in Ge3 Se17 with those in GeO2 ±SiO2 glasses. In the chalcogenide glass, the photosensitivity results from excitation of band-edge defects (lone pair or valence electrons), while the oxide glass photosensitivity results from alteration of color centers deeper in the bandgap. Structural changes occur in both materials, however, the threshold powers in the oxide glass are much larger than those in the chalcogenide. In the chalcogenide glass, electronic excitation (since it occurs near the band edge) is directly coupled to bonding alterations and rearrangement of the structure. In the oxide glass, the electronic excitations can be developed independently of the structural alteration of the glass. Both processes lead to permanent photo-induced changes. However, the chalcogenide glass has a transient photosensitivity from the electronic excitations, while the same eect appears to be smaller, if present, in the oxide glasses. One major dierence in structure may be the role of Ge. Additional concentrations of Ge should stien the chalcogenide network and may increase the threshold for permanent photodarkening. Clearly, this area of research is interesting since it begins to supply clues for the unexpected ability of light to alter the
structure of glass without heating to the glass transition temperature. 5. Conclusions Measurements of photodarkening in Ge3 Se17 glass show both a transient and a permanent eect. Both changes increase the absorbance of the glass. The permanent photodarkening is associated with a permanent increase in sample volume in the exposed region and also by recoverable changes in the defect concentration and bonding. Based on similarity to EXAFS results obtained on amorphous Se, it appears that the transient eect is associated with excitation of Se lone pair electrons to form VAP in which some Se atoms reach a coordination of three while others decrease to one. Upon removal of the light, the free carriers recombine and alter the structure in the process to produce a lower density material. This lower density material has a greater distribution of bond lengths and angles which leads to a broadening of the band tail and a decrease in the energy of the UV edge. The transient photodarkening is linearly dependent on optical intensity and corresponds to a large optical non-linearity in both the absorption and refractive index. References [1] K.O. Hill, Y. Fujii, D.C. Johnson, B.S. Kawasaki, Appl. Phys. Lett. 32 (1978) 647. [2] G. Pfeier, M.A. Paesler, S.C. Agarwal, J. Non-Cryst. Solids 130 (1991) 111. [3] K.D. Simmons, J.H. Simmons, Appl. Phys. Lett. 66 (1995) 2104. [4] K.D. Simmons, G.I. Stegeman, B.G. Potter, J.H. Simmons, J. Non-Cryst. Solids 179 (1994) 254. [5] N.F. Mott, Metal-Insulator Transitions, Taylor and Francis, London, 1990, p. 22. [6] H. Fritzsche, Philos. Mag. B 68 (1993) 561. [7] A.V. Kolobov, H. Oyanagi, Ka. Tanaka, Ke. Tanaka, Phys. Rev. B 55 (1997) 726. [8] H. Jain, S. Krishnaswami, A.C. Miller, P. Krecmer, S.R. Elliot, M. Vlcek, these Proceedings, p. 115. [9] A.V. Kolobov, P. Kostikov, S.S. Lantratova, V.M. Lyubin, Sov. Phys. Solid State 33 (1991) 444. [10] H. Richter, J. Non-Cryst. Solids 8±10 (1972) 388. [11] S. Ramachandran, S.G. Bishop, Appl. Phys. Lett. 74 (1999) 13.
C.R. Schardt et al. / Journal of Non-Crystalline Solids 274 (2000) 23±29 [12] J. Tauc, R. Grigorovici, A. Vancu, Phys. Status Solidi 15 (1966) 627. [13] K. Oe, Y. Toyoshima, H. Nagai, J. Non-Cryst. Solids 20 (1976) 405.
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[14] P. Tronc, M. Bensoussan, A. Branac, Phys. Rev. B 8 (1973) 5947. [15] L. Tich y, H. Ticha, P. Nagels, R. Callaerts, J. Non-Cryst. Solids 240 (1998) 177.