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Effect of copper on the local structure of GeSe2Cux probed by Raman spectroscopy
Journal of Non-Crystalline Solids 328 (2003) 40–47 www.elsevier.com/locate/jnoncrysol
Effect of copper on the local structure of GeSe2Cux probed by Raman spectroscopy Mary M.J. Tecklenburg a,*, Elisabeth Larsen a, Bogdan Lita b, Didarul Islam Qadir b,* a
Department of Chemistry, Central Michigan University, Mt. Pleasant, MI 48859, USA Department of Physics, Central Michigan University, Mt. Pleasant, MI 48859, USA
b
Received 19 June 2002; received in revised form 30 May 2003
Abstract Glassy GeSe2 is known to exhibit photodarkening that is extinguished by the presence of copper. Bulk glasses of GeSe2 Cux (0 < x < 0:16), prepared by melt-quenching, were studied by Raman spectroscopy to ascertain the effect of copper on the local structure of the glass. Decreases were observed in relative areas of the A1c (218 cm1 ) and Ge–Ge (178 cm1 ) peaks relative to the A1 (201 cm1 ) for compositions of less than x ¼ 0:05 copper but changed very little at higher compositions. The position of the A1 and A1c peaks decreased by 2 cm1 in low copper compositions. The density of the samples also increased sharply below x ¼ 0:05 and more gradually above x ¼ 0:05. The copper is seen as affecting the defects (Ge–Ge bonds) in the stoichiometric germanium–selenium glass but not affecting the tetrahedral Ge(Se1=2 )4 chains. Ó 2003 Elsevier B.V. All rights reserved. PACS: 78.30.Ly; 61.43.Fs; 61.72.Ww
1. Introduction Chalcogenide glasses like GeSe2 and As2 Se3 are known to be very good covalently bonded glass formers. The dominant feature of the network is fourfold coordinated Ge. The structure of glassy GeSe2 has been a well-discussed area of research
*
Corresponding authors. Tel.: +1-989 774 3078 (Mary M.J. Tecklenburg), +1-989 774 3336 (D.I. Qadir); fax: +1-989 774 3883 (Mary M.J. Tecklenburg). E-mail addresses: [email protected] (M.M.J. Tecklenburg), [email protected] (D.I. Qadir).
although the issue of the detailed structure of the glass network is still not quite resolved. The unresolved issues pertain to how the Ge(Se1=2 )4 tetrahedral features are joined and how defects like Se–Se and Ge–Ge bonds are distributed in the glassy network. The structural issue becomes murky when different metal additives participate in glass formation. Chalcogenide glasses have some interesting optical properties including photodarkening. It has been seen that addition of metal additives, at low concentrations, can cause the photodarkening property of the glass to be absent [1]. In an earlier work, we found that addition of copper had such
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M.M.J. Tecklenburg et al. / Journal of Non-Crystalline Solids 328 (2003) 40–47
an effect [2]. This work was initiated to determine how added copper influenced the structure of the GeSe2 glassy network. Discernment of a correlation between structure and photodarkening may contribute to an understanding about the origins of photodarkening in these glasses. Raman scattering experiments were used to study the effect of Cu on the local structure of glassy GeSe2 . Fig. 1 shows the Raman spectra of glassy GeSe2 between 100 and 300 cm1 . The peaks have been assigned differently by competing structural models, namely the molecular cluster network (MCN) model [3] and the stochastic random network (SRN) [4] model. Subsequent theoretical work [5–7] leads us to assign the dominant (A1 ) peak at 201 cm1 to the symmetric stretch mode of the corner-sharing tetrahedral Ge sites and the adjacent (A1c ) peak at 218 cm1 to the ring breathing of the four-membered ring of the edge-sharing tetrahedral Ge sites. The Ge–Ge bonded site is seen as a small peak at 178 cm1 that is assigned to an ethane-like structure, Ge2 (Se1=2 )6 [3,5,7]. Two small broad peaks in the GeSe2 spectra at 240 and 265 cm1 have previously been assigned to either the T2 mode (antisymmetric stretch) of the Ge(Se1=2 )4 tetrahedra [3,8,9] or to Se–Se stretches [4,10]. Alloying germanium–selenium glasses with Ag [11], Sn [12] or P, Sb, or Bi [13] changes the relative
Fig. 1. Raman spectrum of GeSe2 with vibrational assignments ( ¼ Se, d ¼ Ge). Spectral conditions: laser 30 mW at 647 nm, 0.5 cm1 stepsize and 1 s detector integration time per point. ¼ Krþ laser plasma line.
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intensities of the A1 and A1c peaks in the Raman spectra. The metal additive may replace Ge in the tetrahedral sites (as tin does in Ge1x Snx Se2 glasses [12]) or bond with excess Se (as does P, Sb, and Bi in Ge0:2 Se0:8y Xy glasses [1,13] and Ag in (Ge0:25 Se0:75 )1y Agy glasses [11]). The alloying of copper reduces photodarkening and increases the density, refractive index, and the optical gap of (Ge0:95 Sn0:05 Se2 )1x Cux thin films [2]. In this work, we monitored the relative changes in the Raman spectra of glasses with increasing molar fractions of Cu. Since the backbone of this glass network is the corner-sharing tetrahedral Gesite, we analyzed our data by comparing the Raman intensity of other sites to the intensity of these A1 sites. We studied samples of GeSe2 Cux with x varying from 0 to 0.16.
2. Experimental Bulk glasses of GeSe2 Cux were prepared from pure elements (99.999% or better) with copper content between x ¼ 0:00 and 0.16. The elements were vacuum-sealed in small diameter quartz tubes, heated to 1000 °C for 48 h. During this time the tube was frequently inverted, and finally quenched in ice water. The samples were then annealed at approximately 50 °C below the glass transition temperature of each glass for 24 h. Densities were measured by displacing distilled water using a digital analytical balance. Raman spectra were collected on an ISA HG2S one-meter double monochromator with a PMT detector (thermoelectrically cooled) and a Krþ laser. The samples were analyzed in the sealed quartz tubes in the 135° back-scattering geometry. The laser wavelength was 647.1 nm and the power at the sample was 30 mW, well below the onset of photocrystallization (observed at 150 mW or greater). The instrument slits were set to give a bandpass of 4 cm1 . Stokes spectra were collected from 50 to 300 cm1 at 0.5 cm1 intervals with integration times of 1–2 s. For the samples containing copper, seven to ten spectra were collected at different spots on the sample and averaged. This was done to prevent photocrystallization due to prolonged exposure of the same spot to laser light.
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The wavenumber axis was calibrated with a plasma emission line from the Kr ion laser [14] that appeared at 233.4 cm1 on the Raman-shifted scale. The spectrum was offset to zero at its lowest point and Gaussian curves were fit to each spectrum to determine the peak position and area. Measurement error was estimated from five independently made samples at one composition (x ¼ 0:06) as the range in peak position or area.
3. Results The molar volume (from the density measurements) of bulk glass GeSe2 Cux as a function of x is shown in Fig. 2. For low Cu concentrations (x < 0:05), the molar volume decreases sharply with increasing copper content. The rate of decrease slows down for higher Cu contents. The Raman spectra of glassy GeSe2 Cux at several compositions (intensity normalized to the A1 peak of GeSe2 ) are shown in Fig. 3. As copper content increases the intensity of the A1c mode (the edge-sharing Ge site) decreases relative to the A1 mode (corner-sharing Ge site). This is accompanied by a decrease in the Ge–Ge peak at 178 and at 265 cm1 . The smaller peak at 243 cm1 in GeSe2
Fig. 3. Raman spectra of GeSe2 Cux as copper composition changes; (a) x ¼ 0:0, (b) x ¼ 0:02, (c) x ¼ 0:04, (d) x ¼ 0:08, (e) x ¼ 0:10, (f) x ¼ 0:14. Conditions are the same as in Fig. 1 and each spectrum is an average of 7–10 scans. ¼ Krþ laser plasma line.
Fig. 4. Raman peak position (in wavenumbers) of the A1 mode as a function of copper content, x, in GeSe2 Cux .
Fig. 2. Density (d) of GeSe2 Cux as a function of copper content, x, (right axis). Also plotted as molar volume (j) normalized to 1.0 at x ¼ 0 (left axis).
remains as a difficult to distinguish shoulder on the 265 cm1 peak. The positions of the A1 and A1c peaks decrease by 2 cm1 (Fig. 4) when the smallest amount of copper is added (x ¼ 0:02) and then decrease more slowly as the copper content further increases. Fig. 5 is a plot of the ratio of the intensities of the A1c to the A1 peaks versus x (normalized to x ¼ 0). The ratio decreases sharply between x ¼ 0
M.M.J. Tecklenburg et al. / Journal of Non-Crystalline Solids 328 (2003) 40–47
Fig. 5. Ratio of the peak areas of the A1c to A1 mode as a function of copper content, x, in the Raman spectra of GeSe2 Cux .
and 0.04 and then decreases further for x > 0:10. For 0:04 < x < 0:10 the ratio remains unchanged. On studying the corresponding changes in other peaks, it is seen that the relative intensity of the peak at 178 cm1 also decreases from x ¼ 0 to 0.04. The change is small at higher copper content
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Fig. 7. Area of the A1 peak versus copper content, x, in the Raman spectra of GeSe2 Cux . The y-axis has a break in order to include the point at x ¼ 0 which is 25 times more intense than the next point at x ¼ 0:02.
(Fig. 6). The intensity of the 265 cm1 mode increases relative to the Al peak from x ¼ 0 to 0.04 (Fig. 6) but with no clear trend for higher compositions. We found that above x ¼ 0:06 the relative intensity of the 265 cm1 peak did not yield consistently reproducible results for samples of the same composition due to sample inhomogeneity, possibly arising from phase separation [12]. The copper content also impacts the total scattering intensity of the Raman spectrum. Comparing the areas of the A1 peak as copper content increases, Fig. 7 shows a prominent decrease in the Raman scattering intensity from x ¼ 0 to 0.04 and then a gradual decrease at x > 0:04.
4. Discussion
Fig. 6. Peak area ratios of the Ge–Ge (d) and Se–Se (j) modes divided by the area of the A1 mode as a function of copper content, x, in the Raman spectra of GeSe2 Cux . At compositions above x ¼ 0:06 the intensity data was inconsistent.
All of the measured properties show a change in the trends with copper content around x ¼ 0:05 (molar volume, Raman peak wavenumber, Raman intensity). A sharp rise or decline from x ¼ 0 to nearly 0.05 is followed by very gradual change above that composition. A parallel trend is also
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observed in the optical gap and refractive index as Cu content increases [2]. In our study the effect of copper on glassy GeSe2 appears to be limited to compositions with less than x ¼ 0:05. Presence of additional Cu impacts the glass forming ability and sample homogeneity as was reported for copper doped [15] and tin doped [12,16] germanium diselenide glasses. In this study we found that with higher copper than x ¼ 0:05, sample inhomogeneity makes identifying reproducible trends increasingly difficult. It is evident that at low concentrations the Cu does have a significant effect on the germanium– selenium glass network. The results of the density measurements show a reduction of the molar volume by approximately 10% at x ¼ 0:06 (Fig. 2). Such a large drop in molar volume cannot be accounted for by simple substitution of Cu in place of Ge, and clearly points toward a rearrangement of the glass network. The sharp decrease in the molar volume of the glasses with increased copper content for low copper concentrations implies that copper goes into the glass network as a network modifier and not simply to occupy Ge sites. Results of Raman measurements point toward the same conclusion. A substitution of Cu for Ge in the network would increase the amount of Ge available for Ge–Ge bonding but the opposite is seen as the 178 cm1 peak decreases (Fig. 6). The sharp drop in the relative intensity of the A1c peak seen at low copper concentrations (x < 0:05) show that the modification of the glass network arises at the cost of sites associated with the A1c peak, edgesharing sites. The assignment of Raman spectra above 230 cm1 is complicated by over lapping modes. There are several vibrations expected in this region. The highest energy vibration of a tetrahedron is the m3 antisymmetric stretch of T2 symmetry which has weak intensity in the Raman and strong in the infrared. Infrared spectra of GeSe2 show a very strong peak at 258 cm1 and a weaker one at 310 cm1 [17,18]. These are assigned to a split pair of T2 modes of the Ge(Se1=2 )4 tetrahedra. In addition, a shoulder grows in the IR spectra at 285 cm1 as the composition of Gex Se1x becomes Ge rich (up to x ¼ 0:4) [18]. A 285 cm1 band is also seen in the Raman spectra of Ge0:4 Se0:6 and is assigned to the
T2 -like antisymmetric Ge–Ge stretch [13]. Finally, amorphous selenium has a peak at 255 cm1 and a shoulder at 235 cm1 that have been assigned to A1 vibrations of non-parallel and parallel Se–Se chains [10] or Se8 rings [4]. In GeSe2 all three of these types of modes (Se–Se, Ge–Ge, and Ge(Se1=2 )4 tetrahedra) contribute to the very broad and weak Raman scattering intensity from 230 to 320 cm1 . To which mode do we then assign the increase observed at 260–265 cm1 as Cu is added? The intensity of the T2 mode of the Ge(Se1=2 )4 tetrahedra is not expected to change relative to the A1 intensity from the same site. The antisymmetric Ge–Ge mode should be weaker in Raman spectra than the symmetric mode at 178 cm1 . Since the symmetric mode decreases as Cu content increases the rise at 265 cm1 cannot be assigned to Ge–Ge sites. The A1 Se–Se modes are strongly allowed in the Raman spectra so we assign the increase in intensity at 265 cm1 in copper containing alloys to Se–Se modes. Fig. 6 shows that when x < 0:05 the relative intensity of the 178 cm1 peak (Ge–Ge sites) decreases and the 265 cm1 peak (Se–Se sites) increases. Difficulty arises when one tries to account for necessary atoms in the possible bonding arrangements of the modified network. If Ge–Ge sites are fewer, and if they do not bond to Cu, this would necessitate more Se atoms to be engaged to form tetrahedral Ge. Since we see that the Se–Se site intensity increases, rather than decreases, it implies that Cu engages the extra Ge atoms. For each tetrahedral Ge(Se1=2 )4 unit a Cu atom precludes from forming, the copper based structure is richer in one Ge and two Se atoms. For example, one could envisage the formation of Cu3 Ge, which would free up two Se atoms, resulting in an increased fractional occurrence of Se–Se type features in the network. The direct anti-correlation between the relative intensities of the 178 cm1 Ge–Ge sites and the 265 cm1 Se–Se sites at low Cu content (x ¼ 0 to 0.04) is in keeping with this scenario. The decrease in the A1c and Ge–Ge peaks as the Se–Se peak increases is the same pattern that is seen in the Raman spectra [4] of Gex Se1x as x decreases from 0.33 to 0.30. As the Gex Se1x glass becomes selenium-rich the edge-sharing Ge sites
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Table 1 Comparison of the Raman shifts (cm1 ) and relative intensities of GeSe2 Cux and Gex Se1x Composition
Ge–Ge
A1
A1c
Se–Se
GeSe2 GeSe2 Cu0:02 GeSe2 Cu0:04
178.5 (0.14) 179.3 (0.05) 177.3 (0.03)
201.3 (1.00) 199.4 (1.00) 199.2 (1.00)
217.9 (0.40) 215.7 (0.38) 216.5 (0.29)
268.7 (0.09) 263.6 (0.35) 264.0 (0.41)
Ge0:33 Se0:67 Ge0:30 Se0:70 Ge0:25 Se0:75
178.5 (0.18) 178.0 (0.03) – (0.0)
201.2 (1.00) 198.2 (1.00) 196.1 (1.00)
218.0 (0.43) 216.6 (0.36) 214.9 (0.30)
269.5 (0.32) 267.5 (0.42) 261.9 (0.79)
The intensities, peak area relative to the A1 peak area, are in parentheses. The Gex Se1x data are from Table 2 in [4]. (Note: GeSe2 is the same as Ge0:33 Se0:67 .)
decrease (216 cm1 ), the Se–Se sites increase (265 cm1 ), and the Ge–Ge sites disappear (178 cm1 ). In Table 1 Raman peak wavenumbers and relative intensities of GeSe2 Cux are compared with that of Gex Se1x . There is a remarkable similarity in the trends in the peak wavenumbers and relative intensities as copper content increases in GeSe2 Cux and as selenium content increases in Gex Se1x . The GeSe2 crystal (a, high temperature form) has a layered structure with sheets consisting of chains of Ge(Se1=2 )4 tetrahedral corner-sharing sites crosslinked by edge-sharing tetrahedra. The melt-quenched glass is a distortion of the crystal structure that retains the same elements of edgesharing and corner-sharing sites. As the Gex Se1x glass composition becomes selenium-rich the crosslinking edge-sharing sites decrease and Se–Se bonding increases. In our Cu doped glasses we see the same decrease in the edge-sharing tetrahedra and increase in Se–Se bonding as though the glass is becoming selenium-rich. This can only happen if the Ge is less available for bonding. Apparently small amounts of Cu in amorphous GeSe2 interact with the germanium leaving the rest of the glass selenium-rich. One may consider why the Ge–Ge and Se–Se sites arise at all in the stoichiometric composition GeSe2 glass. The Raman spectrum of the crystal [3] shows no bands at 178 and 265 cm1 . However, the very fast cooling in the melt-quenched glass does not allow the attainment of perfect chemical order. Local site excess of Ge or Se leads to the formation of a small number of Se–Se and Ge–Ge bonds. The Ge–Ge bonds have been assigned to
ethane-like Ge2 (Se1=2 )6 structures [3,7]. The presence of Cu greatly reduces the formation of Ge–Ge bonds and decreases the fraction of Ge in edgesharing tetrahedra, which leads to an increase in Se–Se bonding. The concentration of Ge–Ge sites in GeSe2 was recently calculated from a simulated Raman spectrum [7]. From the calculated absolute intensities and the experimental Raman intensities the relative concentrations of the Ge sites were determined. They found the fraction of edge-sharing, corner-sharing, and ethane-like (Ge–Ge) sites to be 0.58, 0.33, and 0.09, respectively. In a neutron diffraction study [19] the fraction of the number of Ge–Ge bonds was determined to be 0.04. The fraction of Ge–Ge sites is approximately the same as the copper content where most properties change slope, around x ¼ 0:05. It appears that when enough Cu is added to restrain the formation of most of the Ge–Ge sites, additional Cu has little effect on the local structure of the GeSe2 glass. At compositions of x > 0:05 the excess copper may intercalate into voids in the glass which are present as 16-membered rings associated with two neighboring edge-sharing Ge sites. Calculations [5] show that such features and other large voids are likely to abound in melt-quenched glassy GeSe2 . In a mixed chalcogenide glass, Cux (Ge0:125 As0:25 Se0:625 )1x , a decrease in Tg for x < 0:02 was interpreted as Cu atoms distorting the GeSe2 tetrahedra and acting as a plasticiser in interlayer positions [20]. Copper certainly interacts with Se atoms in the glass. Through coordinate bonds with the lonepair electrons on Se, copper may bond with a
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coordination number up to 4. Studies on Cu photodoped GeSe2 films show that Cu 3d states do mix to some degree with Se 4p states [21,22]. The authors propose one covalent and three coordinate bonds. The coordinate bonding, however, does not break the bonds of the Ge(Se1=2 )4 tetrahedra and so has a minimal effect on the Raman spectra. No additional bands are seen in the Raman spectra as the Cu concentration increases. The IR spectrum of photodoped GeSe2 Cu0:75 [23], where a much higher metal concentration is achieved than by alloying, does show a shift in the weaker of the two T2 peaks but no new peaks arise. A Raman signal for Cu–Se may not be observed because the bonding is weak coordinate type or because the frequency may overlap other strong features in the spectrum. Other physical evidence for metal binding in chalcogenide glasses is found in X-ray (XPS and XES) and optical spectroscopic studies. Increasing Cu or Ag shifts the binding energies [24–26] of Se, As, and Ge in (As2 Se3 )1x Mx and (GeSe2 )1x Mx where M ¼ Ag or Cu. The results indicate that electrons are transferred from the metal atoms to As and Se for As2 Se3 and to Ge and Se for GeSe2 . An X-ray analysis [27] of (GeSe3 )1x Agx showed that Ag atoms interact mainly with Se atoms leaving the Ge(Se1=2 )4 tetrahedra intact but break the connectivity of the chains. In the first examples, metal additives in stoichiometric glass, metal– Ge bonding is indicated. The latter example is a selenium-rich glass with no Ge–Ge bonding present. The lack of metal–Ge bonding is due to the stability of the Ge(Se1=2 )4 tetrahedra. In a recent DFT calculation on glassy GeSe2 that stability was quantified. The ethane-like fragment (Se3 Ge– GeSe3 ) binding energy per bond [28] was 0.09 eV lower than the binding energy per bond of the corner-sharing fragment. So the stability of the corner-sharing Ge tetrahedra prevents disruption by metal additives while the metal perturbs the less energetically favored Ge–Ge sites. The small, but very reproducible, 2 cm1 downshift in the energy of the A1 corner-sharing vibration from 201 to 199 cm1 as the Cu content is increased to x ¼ 0:04 is probably due to a subtle change in structure. One possibility is the reduction of the crosslinking as the edge-sharing sites
decrease leaving the corner-sharing tetrahedral chains more flexible. The reduced angle strain could cause the reduced frequency of the A1 vibration. The calculated A1 frequency [7] of glassy GeSe2 is 195, 6 cm1 less than observed experimentally. However, a small fragment was used to represent the corner-sharing site, two linked tetrahedra, so the frequency is for an isolated tetrahedral chain rather than one adjacent to a corner-sharing site. A larger fragment connected by corner-sharing sites had A1 modes at 200 and 195 cm1 [28]. So a reduction in frequency of the A1 vibration is the predicted result when the A1c intensity (edge-sharing sites) decreases. Studies of photodarkening in Ge–Se glasses show that photodarkening is correlated with the Se–Se lone pair p-orbitals forming the top of the valance band [29,30]. Also, it has been proposed that photodarkening results from medium order structural changes upon illumination [29]. It is seen that added copper in chalcogenide glasses inhibits photodarkening. If the former is the cause of photodarkening, it needs to be reconciled with our experimental results that adding copper increases the percent occurrence of Se–Se bonds. A possible explanation may be derived from the findings of Fritzsche [31] suggesting that copper, when incorporated into chalcogenide glasses, acts as recombination centers for transient excitons that initiate photoinduced changes. Our Raman results address the impact of added Cu on local site changes to GeSe2 leaving the issue of copperÕs effect on photodarkening still to be resolved.
5. Conclusion Our study shows how a metal additive, Cu, affects the glass formation of stoichiometric GeSe2 . Below x ¼ 0:05 the copper composition is correlated with shifts in density and Raman band position and intensity. The copper does not disrupt the tetrahedral Ge(Se1=2 )4 chains but does reduce the glass defects (Ge–Ge bonds) and the chainlinking edge-sharing Ge sites. Above x ¼ 0:05 the additional copper has no effect on the very stable fourfold coordinate germanium–selenium bonding.
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Effect of copper on the local structure of GeSe2Cux probed by Raman spectroscopy Mary M.J. Tecklenburg a,*, Elisabeth Larsen a, Bogdan Lita b, Didarul Islam Qadir b,* a
Department of Chemistry, Central Michigan University, Mt. Pleasant, MI 48859, USA Department of Physics, Central Michigan University, Mt. Pleasant, MI 48859, USA
b
Received 19 June 2002; received in revised form 30 May 2003
Abstract Glassy GeSe2 is known to exhibit photodarkening that is extinguished by the presence of copper. Bulk glasses of GeSe2 Cux (0 < x < 0:16), prepared by melt-quenching, were studied by Raman spectroscopy to ascertain the effect of copper on the local structure of the glass. Decreases were observed in relative areas of the A1c (218 cm1 ) and Ge–Ge (178 cm1 ) peaks relative to the A1 (201 cm1 ) for compositions of less than x ¼ 0:05 copper but changed very little at higher compositions. The position of the A1 and A1c peaks decreased by 2 cm1 in low copper compositions. The density of the samples also increased sharply below x ¼ 0:05 and more gradually above x ¼ 0:05. The copper is seen as affecting the defects (Ge–Ge bonds) in the stoichiometric germanium–selenium glass but not affecting the tetrahedral Ge(Se1=2 )4 chains. Ó 2003 Elsevier B.V. All rights reserved. PACS: 78.30.Ly; 61.43.Fs; 61.72.Ww
1. Introduction Chalcogenide glasses like GeSe2 and As2 Se3 are known to be very good covalently bonded glass formers. The dominant feature of the network is fourfold coordinated Ge. The structure of glassy GeSe2 has been a well-discussed area of research
*
Corresponding authors. Tel.: +1-989 774 3078 (Mary M.J. Tecklenburg), +1-989 774 3336 (D.I. Qadir); fax: +1-989 774 3883 (Mary M.J. Tecklenburg). E-mail addresses: [email protected] (M.M.J. Tecklenburg), [email protected] (D.I. Qadir).
although the issue of the detailed structure of the glass network is still not quite resolved. The unresolved issues pertain to how the Ge(Se1=2 )4 tetrahedral features are joined and how defects like Se–Se and Ge–Ge bonds are distributed in the glassy network. The structural issue becomes murky when different metal additives participate in glass formation. Chalcogenide glasses have some interesting optical properties including photodarkening. It has been seen that addition of metal additives, at low concentrations, can cause the photodarkening property of the glass to be absent [1]. In an earlier work, we found that addition of copper had such
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M.M.J. Tecklenburg et al. / Journal of Non-Crystalline Solids 328 (2003) 40–47
an effect [2]. This work was initiated to determine how added copper influenced the structure of the GeSe2 glassy network. Discernment of a correlation between structure and photodarkening may contribute to an understanding about the origins of photodarkening in these glasses. Raman scattering experiments were used to study the effect of Cu on the local structure of glassy GeSe2 . Fig. 1 shows the Raman spectra of glassy GeSe2 between 100 and 300 cm1 . The peaks have been assigned differently by competing structural models, namely the molecular cluster network (MCN) model [3] and the stochastic random network (SRN) [4] model. Subsequent theoretical work [5–7] leads us to assign the dominant (A1 ) peak at 201 cm1 to the symmetric stretch mode of the corner-sharing tetrahedral Ge sites and the adjacent (A1c ) peak at 218 cm1 to the ring breathing of the four-membered ring of the edge-sharing tetrahedral Ge sites. The Ge–Ge bonded site is seen as a small peak at 178 cm1 that is assigned to an ethane-like structure, Ge2 (Se1=2 )6 [3,5,7]. Two small broad peaks in the GeSe2 spectra at 240 and 265 cm1 have previously been assigned to either the T2 mode (antisymmetric stretch) of the Ge(Se1=2 )4 tetrahedra [3,8,9] or to Se–Se stretches [4,10]. Alloying germanium–selenium glasses with Ag [11], Sn [12] or P, Sb, or Bi [13] changes the relative
Fig. 1. Raman spectrum of GeSe2 with vibrational assignments ( ¼ Se, d ¼ Ge). Spectral conditions: laser 30 mW at 647 nm, 0.5 cm1 stepsize and 1 s detector integration time per point. ¼ Krþ laser plasma line.
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intensities of the A1 and A1c peaks in the Raman spectra. The metal additive may replace Ge in the tetrahedral sites (as tin does in Ge1x Snx Se2 glasses [12]) or bond with excess Se (as does P, Sb, and Bi in Ge0:2 Se0:8y Xy glasses [1,13] and Ag in (Ge0:25 Se0:75 )1y Agy glasses [11]). The alloying of copper reduces photodarkening and increases the density, refractive index, and the optical gap of (Ge0:95 Sn0:05 Se2 )1x Cux thin films [2]. In this work, we monitored the relative changes in the Raman spectra of glasses with increasing molar fractions of Cu. Since the backbone of this glass network is the corner-sharing tetrahedral Gesite, we analyzed our data by comparing the Raman intensity of other sites to the intensity of these A1 sites. We studied samples of GeSe2 Cux with x varying from 0 to 0.16.
2. Experimental Bulk glasses of GeSe2 Cux were prepared from pure elements (99.999% or better) with copper content between x ¼ 0:00 and 0.16. The elements were vacuum-sealed in small diameter quartz tubes, heated to 1000 °C for 48 h. During this time the tube was frequently inverted, and finally quenched in ice water. The samples were then annealed at approximately 50 °C below the glass transition temperature of each glass for 24 h. Densities were measured by displacing distilled water using a digital analytical balance. Raman spectra were collected on an ISA HG2S one-meter double monochromator with a PMT detector (thermoelectrically cooled) and a Krþ laser. The samples were analyzed in the sealed quartz tubes in the 135° back-scattering geometry. The laser wavelength was 647.1 nm and the power at the sample was 30 mW, well below the onset of photocrystallization (observed at 150 mW or greater). The instrument slits were set to give a bandpass of 4 cm1 . Stokes spectra were collected from 50 to 300 cm1 at 0.5 cm1 intervals with integration times of 1–2 s. For the samples containing copper, seven to ten spectra were collected at different spots on the sample and averaged. This was done to prevent photocrystallization due to prolonged exposure of the same spot to laser light.
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The wavenumber axis was calibrated with a plasma emission line from the Kr ion laser [14] that appeared at 233.4 cm1 on the Raman-shifted scale. The spectrum was offset to zero at its lowest point and Gaussian curves were fit to each spectrum to determine the peak position and area. Measurement error was estimated from five independently made samples at one composition (x ¼ 0:06) as the range in peak position or area.
3. Results The molar volume (from the density measurements) of bulk glass GeSe2 Cux as a function of x is shown in Fig. 2. For low Cu concentrations (x < 0:05), the molar volume decreases sharply with increasing copper content. The rate of decrease slows down for higher Cu contents. The Raman spectra of glassy GeSe2 Cux at several compositions (intensity normalized to the A1 peak of GeSe2 ) are shown in Fig. 3. As copper content increases the intensity of the A1c mode (the edge-sharing Ge site) decreases relative to the A1 mode (corner-sharing Ge site). This is accompanied by a decrease in the Ge–Ge peak at 178 and at 265 cm1 . The smaller peak at 243 cm1 in GeSe2
Fig. 3. Raman spectra of GeSe2 Cux as copper composition changes; (a) x ¼ 0:0, (b) x ¼ 0:02, (c) x ¼ 0:04, (d) x ¼ 0:08, (e) x ¼ 0:10, (f) x ¼ 0:14. Conditions are the same as in Fig. 1 and each spectrum is an average of 7–10 scans. ¼ Krþ laser plasma line.
Fig. 4. Raman peak position (in wavenumbers) of the A1 mode as a function of copper content, x, in GeSe2 Cux .
Fig. 2. Density (d) of GeSe2 Cux as a function of copper content, x, (right axis). Also plotted as molar volume (j) normalized to 1.0 at x ¼ 0 (left axis).
remains as a difficult to distinguish shoulder on the 265 cm1 peak. The positions of the A1 and A1c peaks decrease by 2 cm1 (Fig. 4) when the smallest amount of copper is added (x ¼ 0:02) and then decrease more slowly as the copper content further increases. Fig. 5 is a plot of the ratio of the intensities of the A1c to the A1 peaks versus x (normalized to x ¼ 0). The ratio decreases sharply between x ¼ 0
M.M.J. Tecklenburg et al. / Journal of Non-Crystalline Solids 328 (2003) 40–47
Fig. 5. Ratio of the peak areas of the A1c to A1 mode as a function of copper content, x, in the Raman spectra of GeSe2 Cux .
and 0.04 and then decreases further for x > 0:10. For 0:04 < x < 0:10 the ratio remains unchanged. On studying the corresponding changes in other peaks, it is seen that the relative intensity of the peak at 178 cm1 also decreases from x ¼ 0 to 0.04. The change is small at higher copper content
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Fig. 7. Area of the A1 peak versus copper content, x, in the Raman spectra of GeSe2 Cux . The y-axis has a break in order to include the point at x ¼ 0 which is 25 times more intense than the next point at x ¼ 0:02.
(Fig. 6). The intensity of the 265 cm1 mode increases relative to the Al peak from x ¼ 0 to 0.04 (Fig. 6) but with no clear trend for higher compositions. We found that above x ¼ 0:06 the relative intensity of the 265 cm1 peak did not yield consistently reproducible results for samples of the same composition due to sample inhomogeneity, possibly arising from phase separation [12]. The copper content also impacts the total scattering intensity of the Raman spectrum. Comparing the areas of the A1 peak as copper content increases, Fig. 7 shows a prominent decrease in the Raman scattering intensity from x ¼ 0 to 0.04 and then a gradual decrease at x > 0:04.
4. Discussion
Fig. 6. Peak area ratios of the Ge–Ge (d) and Se–Se (j) modes divided by the area of the A1 mode as a function of copper content, x, in the Raman spectra of GeSe2 Cux . At compositions above x ¼ 0:06 the intensity data was inconsistent.
All of the measured properties show a change in the trends with copper content around x ¼ 0:05 (molar volume, Raman peak wavenumber, Raman intensity). A sharp rise or decline from x ¼ 0 to nearly 0.05 is followed by very gradual change above that composition. A parallel trend is also
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observed in the optical gap and refractive index as Cu content increases [2]. In our study the effect of copper on glassy GeSe2 appears to be limited to compositions with less than x ¼ 0:05. Presence of additional Cu impacts the glass forming ability and sample homogeneity as was reported for copper doped [15] and tin doped [12,16] germanium diselenide glasses. In this study we found that with higher copper than x ¼ 0:05, sample inhomogeneity makes identifying reproducible trends increasingly difficult. It is evident that at low concentrations the Cu does have a significant effect on the germanium– selenium glass network. The results of the density measurements show a reduction of the molar volume by approximately 10% at x ¼ 0:06 (Fig. 2). Such a large drop in molar volume cannot be accounted for by simple substitution of Cu in place of Ge, and clearly points toward a rearrangement of the glass network. The sharp decrease in the molar volume of the glasses with increased copper content for low copper concentrations implies that copper goes into the glass network as a network modifier and not simply to occupy Ge sites. Results of Raman measurements point toward the same conclusion. A substitution of Cu for Ge in the network would increase the amount of Ge available for Ge–Ge bonding but the opposite is seen as the 178 cm1 peak decreases (Fig. 6). The sharp drop in the relative intensity of the A1c peak seen at low copper concentrations (x < 0:05) show that the modification of the glass network arises at the cost of sites associated with the A1c peak, edgesharing sites. The assignment of Raman spectra above 230 cm1 is complicated by over lapping modes. There are several vibrations expected in this region. The highest energy vibration of a tetrahedron is the m3 antisymmetric stretch of T2 symmetry which has weak intensity in the Raman and strong in the infrared. Infrared spectra of GeSe2 show a very strong peak at 258 cm1 and a weaker one at 310 cm1 [17,18]. These are assigned to a split pair of T2 modes of the Ge(Se1=2 )4 tetrahedra. In addition, a shoulder grows in the IR spectra at 285 cm1 as the composition of Gex Se1x becomes Ge rich (up to x ¼ 0:4) [18]. A 285 cm1 band is also seen in the Raman spectra of Ge0:4 Se0:6 and is assigned to the
T2 -like antisymmetric Ge–Ge stretch [13]. Finally, amorphous selenium has a peak at 255 cm1 and a shoulder at 235 cm1 that have been assigned to A1 vibrations of non-parallel and parallel Se–Se chains [10] or Se8 rings [4]. In GeSe2 all three of these types of modes (Se–Se, Ge–Ge, and Ge(Se1=2 )4 tetrahedra) contribute to the very broad and weak Raman scattering intensity from 230 to 320 cm1 . To which mode do we then assign the increase observed at 260–265 cm1 as Cu is added? The intensity of the T2 mode of the Ge(Se1=2 )4 tetrahedra is not expected to change relative to the A1 intensity from the same site. The antisymmetric Ge–Ge mode should be weaker in Raman spectra than the symmetric mode at 178 cm1 . Since the symmetric mode decreases as Cu content increases the rise at 265 cm1 cannot be assigned to Ge–Ge sites. The A1 Se–Se modes are strongly allowed in the Raman spectra so we assign the increase in intensity at 265 cm1 in copper containing alloys to Se–Se modes. Fig. 6 shows that when x < 0:05 the relative intensity of the 178 cm1 peak (Ge–Ge sites) decreases and the 265 cm1 peak (Se–Se sites) increases. Difficulty arises when one tries to account for necessary atoms in the possible bonding arrangements of the modified network. If Ge–Ge sites are fewer, and if they do not bond to Cu, this would necessitate more Se atoms to be engaged to form tetrahedral Ge. Since we see that the Se–Se site intensity increases, rather than decreases, it implies that Cu engages the extra Ge atoms. For each tetrahedral Ge(Se1=2 )4 unit a Cu atom precludes from forming, the copper based structure is richer in one Ge and two Se atoms. For example, one could envisage the formation of Cu3 Ge, which would free up two Se atoms, resulting in an increased fractional occurrence of Se–Se type features in the network. The direct anti-correlation between the relative intensities of the 178 cm1 Ge–Ge sites and the 265 cm1 Se–Se sites at low Cu content (x ¼ 0 to 0.04) is in keeping with this scenario. The decrease in the A1c and Ge–Ge peaks as the Se–Se peak increases is the same pattern that is seen in the Raman spectra [4] of Gex Se1x as x decreases from 0.33 to 0.30. As the Gex Se1x glass becomes selenium-rich the edge-sharing Ge sites
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Table 1 Comparison of the Raman shifts (cm1 ) and relative intensities of GeSe2 Cux and Gex Se1x Composition
Ge–Ge
A1
A1c
Se–Se
GeSe2 GeSe2 Cu0:02 GeSe2 Cu0:04
178.5 (0.14) 179.3 (0.05) 177.3 (0.03)
201.3 (1.00) 199.4 (1.00) 199.2 (1.00)
217.9 (0.40) 215.7 (0.38) 216.5 (0.29)
268.7 (0.09) 263.6 (0.35) 264.0 (0.41)
Ge0:33 Se0:67 Ge0:30 Se0:70 Ge0:25 Se0:75
178.5 (0.18) 178.0 (0.03) – (0.0)
201.2 (1.00) 198.2 (1.00) 196.1 (1.00)
218.0 (0.43) 216.6 (0.36) 214.9 (0.30)
269.5 (0.32) 267.5 (0.42) 261.9 (0.79)
The intensities, peak area relative to the A1 peak area, are in parentheses. The Gex Se1x data are from Table 2 in [4]. (Note: GeSe2 is the same as Ge0:33 Se0:67 .)
decrease (216 cm1 ), the Se–Se sites increase (265 cm1 ), and the Ge–Ge sites disappear (178 cm1 ). In Table 1 Raman peak wavenumbers and relative intensities of GeSe2 Cux are compared with that of Gex Se1x . There is a remarkable similarity in the trends in the peak wavenumbers and relative intensities as copper content increases in GeSe2 Cux and as selenium content increases in Gex Se1x . The GeSe2 crystal (a, high temperature form) has a layered structure with sheets consisting of chains of Ge(Se1=2 )4 tetrahedral corner-sharing sites crosslinked by edge-sharing tetrahedra. The melt-quenched glass is a distortion of the crystal structure that retains the same elements of edgesharing and corner-sharing sites. As the Gex Se1x glass composition becomes selenium-rich the crosslinking edge-sharing sites decrease and Se–Se bonding increases. In our Cu doped glasses we see the same decrease in the edge-sharing tetrahedra and increase in Se–Se bonding as though the glass is becoming selenium-rich. This can only happen if the Ge is less available for bonding. Apparently small amounts of Cu in amorphous GeSe2 interact with the germanium leaving the rest of the glass selenium-rich. One may consider why the Ge–Ge and Se–Se sites arise at all in the stoichiometric composition GeSe2 glass. The Raman spectrum of the crystal [3] shows no bands at 178 and 265 cm1 . However, the very fast cooling in the melt-quenched glass does not allow the attainment of perfect chemical order. Local site excess of Ge or Se leads to the formation of a small number of Se–Se and Ge–Ge bonds. The Ge–Ge bonds have been assigned to
ethane-like Ge2 (Se1=2 )6 structures [3,7]. The presence of Cu greatly reduces the formation of Ge–Ge bonds and decreases the fraction of Ge in edgesharing tetrahedra, which leads to an increase in Se–Se bonding. The concentration of Ge–Ge sites in GeSe2 was recently calculated from a simulated Raman spectrum [7]. From the calculated absolute intensities and the experimental Raman intensities the relative concentrations of the Ge sites were determined. They found the fraction of edge-sharing, corner-sharing, and ethane-like (Ge–Ge) sites to be 0.58, 0.33, and 0.09, respectively. In a neutron diffraction study [19] the fraction of the number of Ge–Ge bonds was determined to be 0.04. The fraction of Ge–Ge sites is approximately the same as the copper content where most properties change slope, around x ¼ 0:05. It appears that when enough Cu is added to restrain the formation of most of the Ge–Ge sites, additional Cu has little effect on the local structure of the GeSe2 glass. At compositions of x > 0:05 the excess copper may intercalate into voids in the glass which are present as 16-membered rings associated with two neighboring edge-sharing Ge sites. Calculations [5] show that such features and other large voids are likely to abound in melt-quenched glassy GeSe2 . In a mixed chalcogenide glass, Cux (Ge0:125 As0:25 Se0:625 )1x , a decrease in Tg for x < 0:02 was interpreted as Cu atoms distorting the GeSe2 tetrahedra and acting as a plasticiser in interlayer positions [20]. Copper certainly interacts with Se atoms in the glass. Through coordinate bonds with the lonepair electrons on Se, copper may bond with a
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coordination number up to 4. Studies on Cu photodoped GeSe2 films show that Cu 3d states do mix to some degree with Se 4p states [21,22]. The authors propose one covalent and three coordinate bonds. The coordinate bonding, however, does not break the bonds of the Ge(Se1=2 )4 tetrahedra and so has a minimal effect on the Raman spectra. No additional bands are seen in the Raman spectra as the Cu concentration increases. The IR spectrum of photodoped GeSe2 Cu0:75 [23], where a much higher metal concentration is achieved than by alloying, does show a shift in the weaker of the two T2 peaks but no new peaks arise. A Raman signal for Cu–Se may not be observed because the bonding is weak coordinate type or because the frequency may overlap other strong features in the spectrum. Other physical evidence for metal binding in chalcogenide glasses is found in X-ray (XPS and XES) and optical spectroscopic studies. Increasing Cu or Ag shifts the binding energies [24–26] of Se, As, and Ge in (As2 Se3 )1x Mx and (GeSe2 )1x Mx where M ¼ Ag or Cu. The results indicate that electrons are transferred from the metal atoms to As and Se for As2 Se3 and to Ge and Se for GeSe2 . An X-ray analysis [27] of (GeSe3 )1x Agx showed that Ag atoms interact mainly with Se atoms leaving the Ge(Se1=2 )4 tetrahedra intact but break the connectivity of the chains. In the first examples, metal additives in stoichiometric glass, metal– Ge bonding is indicated. The latter example is a selenium-rich glass with no Ge–Ge bonding present. The lack of metal–Ge bonding is due to the stability of the Ge(Se1=2 )4 tetrahedra. In a recent DFT calculation on glassy GeSe2 that stability was quantified. The ethane-like fragment (Se3 Ge– GeSe3 ) binding energy per bond [28] was 0.09 eV lower than the binding energy per bond of the corner-sharing fragment. So the stability of the corner-sharing Ge tetrahedra prevents disruption by metal additives while the metal perturbs the less energetically favored Ge–Ge sites. The small, but very reproducible, 2 cm1 downshift in the energy of the A1 corner-sharing vibration from 201 to 199 cm1 as the Cu content is increased to x ¼ 0:04 is probably due to a subtle change in structure. One possibility is the reduction of the crosslinking as the edge-sharing sites
decrease leaving the corner-sharing tetrahedral chains more flexible. The reduced angle strain could cause the reduced frequency of the A1 vibration. The calculated A1 frequency [7] of glassy GeSe2 is 195, 6 cm1 less than observed experimentally. However, a small fragment was used to represent the corner-sharing site, two linked tetrahedra, so the frequency is for an isolated tetrahedral chain rather than one adjacent to a corner-sharing site. A larger fragment connected by corner-sharing sites had A1 modes at 200 and 195 cm1 [28]. So a reduction in frequency of the A1 vibration is the predicted result when the A1c intensity (edge-sharing sites) decreases. Studies of photodarkening in Ge–Se glasses show that photodarkening is correlated with the Se–Se lone pair p-orbitals forming the top of the valance band [29,30]. Also, it has been proposed that photodarkening results from medium order structural changes upon illumination [29]. It is seen that added copper in chalcogenide glasses inhibits photodarkening. If the former is the cause of photodarkening, it needs to be reconciled with our experimental results that adding copper increases the percent occurrence of Se–Se bonds. A possible explanation may be derived from the findings of Fritzsche [31] suggesting that copper, when incorporated into chalcogenide glasses, acts as recombination centers for transient excitons that initiate photoinduced changes. Our Raman results address the impact of added Cu on local site changes to GeSe2 leaving the issue of copperÕs effect on photodarkening still to be resolved.
5. Conclusion Our study shows how a metal additive, Cu, affects the glass formation of stoichiometric GeSe2 . Below x ¼ 0:05 the copper composition is correlated with shifts in density and Raman band position and intensity. The copper does not disrupt the tetrahedral Ge(Se1=2 )4 chains but does reduce the glass defects (Ge–Ge bonds) and the chainlinking edge-sharing Ge sites. Above x ¼ 0:05 the additional copper has no effect on the very stable fourfold coordinate germanium–selenium bonding.
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