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The Raman spectra of defects in neutron bombarded and Ge-rich vitreous GeO2
Journal of Non-Crystalline Solids 40 (1980) 527-533 © North-Holland Publishing Company
THE RAMAN SPECTRA OF DEFECTS IN NEUTRON BOMBARDED AND Ge-RICH VITREOUS GeO2 Frank L. GALEENER Xerox Palo Alto Research Center, Palo Alto, California 94304, USA
This paper reports the polarized Raman spectra of three forms of vitreous GeO2 : the pure glass, neutron irradiated pure glass and an unirradiated Ge-rich glass of composition Ge 1.102. The data reveals that the line seen at 520 cm-l in the pure glass is due to a network defect that is not a Ge-Ge bond and very probably also not an O-O bond. Comparison with spectra of fused silica suggests that the 606 cm-1 defect line seen in v-SiO2 is not due to Si-Si or O-O bonds.
1. Introduction The nature of network defects in glasses is of great scientific interest and technological importance, see e.g. [ 1 ]. This is especially so in vitreous (v-)SiO2 because it is the glass which has been most extensively studied by scientists (for an overview see ref. [2] and for a recent survey of scientific problems see ref. [3]), and most widely exploited by technologists. Microscopic defects play an important role in limiting the use o f v-SiO2 as an insulator in MOS integrated circuitry and other electronic applications, see e.g. ref. [4]. Raman spectra of v-GeO2 are reported and used to shed light on the nature of a defect in v-SiO2, which is referred to as the 606 cm -~ Raman active defect. In fig. 1 is shown the polarized Raman spectra of v-SiO2 and v-GeO2. These and other spectra presented in this paper were obtained under conditions as described in refs. [5] and [6]. The principal features labelled B, R, TO and LO are properties of the continuous random AX2 network as demonstrated in detail in refs. [7] and [8]. The glasses are known to be 4 - 2 tetrahedral networks with similar bridging oxygen bond angles [8]. The line marked D at 606 cm -1 in the spectrum ofv-SiOz was first ascribed to a defect in the SiO2 network by Stolen et al. (SKK) [9] who observed that it alone increased dramatically after the sample was bombarded by neutrons with energies > 1 0 keV. This line was further studied by Bates et al. [10], who showed its evolution with increasing high energy neutron exposure~ and were tempted to assign it to a localized mode of the non-bridging oxygen defect, denoted -zSi+ - O - S i =, where -= indicates bonding to three separate bridging oxygen atoms. This interpretation 527
528
F.L. Galeener / The Raman spectra of defects i n . . . 0.4
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i
i
i
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D
\1 o l 0
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,~
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500 1000 WAVENUMBER. W, cm -1
(XIO) I J
L 1500
Fig. 1. The polarized Raman spectra'of pure bulk vitreous SiO 2 and GeO2. D marks the defect lines discussed in this paper, 606 cm -1 for v-SiO2 and 520 cm-1 for v-GeO2 .
was supported in subsequent work by Stolen and Walrafen (SW) [11] who studied the 606 cm -1 defect line as a function of water content and thermal history. In an independent study of water-free and water-containing commercial v-SiO2, Galeener and Geils (GG) [6] found the same OH dependence o f the 606 cm -~ line as did SW. In particular, it was observed that the 606 cm -1 line was smaller in commercially available samples of Suprasil having higher OH content. Galeener and Geils also reported a 520 cm -1 "bump" in the HH Raman spectrum of v-GeO2 that decreased in samples with higher water content. For this reason, and because of its position relative to other features in the spectrum, GG inferred that the feature marked D in the lower half of fig. 1 was due to a defect in the v-GeO2 network, presumably analogousto the 606 cm -1 defect in v-SiO2. Assignment of these D lines to a non-bridging oxygen defect has been clouded by subsequent events. In follow-up work on the GG paper, Galeener et al. [12] showed that the 606 cm -~ defect line strength is independent of water content and that the observed variations in intensity were due to inequivalent thermal histories
F.L. Galeener / The Raman spectra-of defects i n . . .
529
of the v-SiO2 samples. (In a detailed study, Mikkelsen and Galeener [13] showed how the 606 cm -1 line strength depends on fictive temperature, annealing time, the rapidity of quench from annealing temperatures, and the viscosity of the glass). Moreover, Laughlin and co-workers [14] have made cluster Bethe lattice calculations for v-SiO2 on the bails of which they note that the 606 cm -1 defect may be due to Si-Si bonds. By analogy with the intimate valence alteration pair (IVAP) models now popular in studies of chalcogenide glasses [15], Lucovsky (L) [ 16] has asserted that the 606 cm -1 line is due to an IVAP involving a 3-bonded oxygen and a dangling oxygen. These models suffer from great uncertainty because they are mainly based on the coincidence of 606 cm -1 with frequencies that are calculated using theories of unproven accuracy involving adjustable parameters that are given ad-hoc values. In fact, there are additional defect models that give frequencies close to 606 cm -t , as shown, e.g. by SW. Until better theoretical methods are available I have taken the point of view that the D lines are best identified by methods which are as empirical as possible; hence the present study. The purpose of the present paper is to demonstrate empirically that it is unlikely that the 606 cm -1 line is due to Si-Si bonds or O - O bonds. The plan was to obtain bulk samples of Si-rich v-SiO2 which would be expected to contain large numbers of Si-Si bonds and small numbers of O - O bonds. Since I was unable to prepare suitable Si-rich material, or find it elsewhere, I have chosen to carry out a parallel study on the 520 cm -1 D line of v-GeO2. 2. Neutron bombarded v-GeO2 In fig. 2 is shown the polarized Raman spectra of unirradiated and irradiated v-GeO2. The irradiated material was obtained from Prof. A.J. Leadbetter (University of Exeter, Great Britain) who had subjected it to a total flux of 1021 fast /
i
~
~
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VITREOUS GeO2
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1021 FAST NEUTRONS/cm2 -- UNIRRADIATED
rr
1
g
/r'
\A
,
,55
500
1000
1500
WAVENUMBER, W, cm-1
Fig. 2. A comparison of the polarized Raman spectra of neutron irradiated and unirradiated pure v-GeO2. Note the substantial increase near 520 cm -1 in the irradiated material.
530
F.L. Galeener / The Raman spectra of defects i n . . .
neutrons cm -2. The most striking change upon bombardment is an increase in the HH Raman scattering in the vicinity of the 520 cm -1 D line. This identifies the D line as a "defect" induced by neutron damage. This fact and the fact that 520 cm -~ lies between the R line and the next higher frequency T O - L O pair strongly suggests that the 520 cm -x D line is of the same origin as the 606 cm -1 line in v-SiO2. Another clear observation is that the R line moves to higher frequency (from ~420 to ~430 cm -1) while the high frequency T O - L O pair moves to lower frequency, by ~7 cm -1 . A similar effect has been reported in v-SiO~ by Simon [17] and SKK [9]. It is known that v-SiO2 is compacted (made more dense) by neutron bombardment, and the simplest model for this is that the average value of the S i O - S i angle is reduced without change in the average value of the S i - O distance, for an overview see ref. [18]. Galeener [8] has shown that under these conditions the change in bridging oxygen angle A0 can be estimated by AO = ( 2 ~ 4 A w 4 M / a sin 0 ) ,
(1)
based on a central force model due to Sen and Thorpe [ 19]. Here, M is the mass of the oxygen atom, a is the central force constant, 0 is the average G e - O - G e angle and ~4 is taken to be the center value of the high frequency LO mode (973 cm-1). Using Aco4 = - 7 cm -1 and other values given in table 3 of ref. [8], one finds that Aco = - 2 . 3 ° . Thus I conclude from the Raman spectral changes that v-GeO2 is compacted by 102~ neutrons cm -2 to an extent that can be accounted for by a 2 o - 3 ° reduction in the G e - O - G e angle. When we think about the extensive network damage likely to be caused by bombardment with neutrons from 10 keV to 2 MeV, we realize the danger of using intuition to argue that a particular simple defect is most likely to appear in dominant numbers. Certainly bonds will be broken, but do they heal? It seems likely that most do; but what fraction? Does the material heated by the passage of the neutron segregate? Does it quench into a high temperature form, partially microcrystalline? The answers are not known. What we do know in the present context is that neutron irradiation of v-GeO2 produces defects which give a polarized Raman signal at the same position (520 cm -1) as defects which already exist in the unirradiated glass. It is reasonable to assume that the defects causing the 520 cm -1 lines in irradiated and unirradiated materials are the same, although the irradiated material may contain additional Raman inactive defects in substantial numbers. At this stage one cannot argue against the D line representing G e - G e bonds simply because one feels that broken-bond defects are the more likely result of neutron irradiation.
3. Ge-rich v-Ge02 On the other hand, we can argue that a Ge-rich sample of v-Ge02 almost certainly contains G e - G e bonds, which do not exist in the perfect continuous
531
F.L. Galeener / The Raman spectra.of defects i n . . . t z..-a 0
3
I I I RAMAN SPECTRA
I
I
I
I
I
I
425 I
I
- -
I I I VITREOUS GERMANIA Gel.~O 2
c~ cc
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,
9
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I 500
865
.~----~ I
~
"-.q.~.I~"'.I~L-..J__J__L_J_ 1000
WAVENUMER,
1500
W, cm -1
Fig. 3. A comparison o f the HH Raman spectra o f pure and Ge-rich v-GeO2. Note the absence of increased scattering near 520 cm -l in the Ge 1.102.
random network for v-GeO2. If the material is phase segregated into Ge particles in a v-GeO2 matrix there will certainly be G e - G e bonds. If it is not phase separated, G e - G e bonds must also exist, interspersed throughout the network. If the 520 cm -~ D line is due to a small number of G e - G e bonds in the nominally stoichiometric pure glass, then this line should increase markedly in a deliberately Ge-rich glass.
The solid line in fig. 3 is the HH Raman spectrum of a Ge-rich sample of vitreous "GeO2" with composition ~Ge1.102. This material was generously supplied by Dr. J.P. De Neufville (Exxon Research Labs, Linden, New Jersey). The sample was prepared in thin disk form as described in the paper by De Neufville and Turnbull [20] ; it was one of the samples that showed no evidence of phase separation by the tests described therein. The HV spectra are not shown in fig. 3, for clarity. As before, the dashed line is for pure v-GeO2. There is no change in the 520 cm -1 D line for 10% excess Ge. This is evident when one realizes that the Ge1.102 spectrum rides on the edge of a broad luminescence line that peaks above 1500 cm-1; approximate subtraction of that luminescence line from the Ge1.102 spectrum causes it to coincide with the GeO2 spectrum from ~425 to ~600 cm -1. Thus I conclude that the 520 cm -1 D line is not due to G e - G e bonds in pure v-GeO2. We can also argue that the availability of excess Ge atoms in a Ge-rich sample is likely to greatly reduce the number of O - O bonds if any exist in the stoichiometric material. Since the 520 cm -~ line did not decrease, I also conclude that the D line in v-GeO2 is not due to O - O bonds, either in the form of a dioxy-bridge = G e O - O - G e =, or a peroxy-radical - = G e - O - O - . In fig. 3 it is shown that there are changes in the spectrum due to excess Ge.
532
F.L. Galeener / The Raman spectra o f defects i n . . .
Principal among these is a change in the frequencies and relative strengths of the high frequency T O - L O pair, and the introduction of new scattering strength at frequencies below ~-400 cm -1. Qualitatively, these are as would be expected from the introduction of units like -=Ge-Ge =, since the vibrational frequencies of elemental germanium, see e.g. ref. [21] are below 400 cm -~ , and the high frequency modes in v-GeO2 are known to be caused by an antisymmetric stretch of the bridging oxygen atom [8], which is absent at the -=Ge-Ge= sites. If the Ge were segregated into Ge particles there would be little change in the high frequency portion of the spectrum. Therefore the Raman spectrum is consistent with the conclusion of De Neufville and Turnbull that the sample is n o t phase-separated.
4. Conclusions It has been shown by experiment that v-Ge02 contains a network "defect" that exhibits a Raman active line at ~ 5 2 0 cm -1 . This line grows with fast neutron bombardment, but is unchanged in Ge-rich material of composition Ge1.102. The line is thus shown not to arise from G e - G e or O - O bonds. Since this line occurs in a similar region of the spectrum as does the 606 cm -1 D line in v-SiO2, and behaves similarly under neutron bombardment, it is highly probable that the D lines in v-GeO2 and v-Si02 have the same (analogous) structural origin. It is therefore concluded that the 606 cm -1 line in v-SiO 2 is almost certainly n o t due to Si-Si or O O bonds.
Acknowledgments I am grateful to Prof. A.J. Leadbetter and Dr. J.P. De Neufville for giving me samples that enabled this work to be done. I am also grateful to Mr. W.J. Mosby for his skilled assistance in obtaining the Raman spectra.
References [ 1 ] D.L. Griscom, J. Non-Crystalline Solids 31 (1978) 241, and refs. therein. [2] R.H. Doremus, Glass Science (Wiley, New York, 1973). [3 ] The physics ofSiO z and Its Interfaces, ed. S.T. Pantelides (Pergamon, New York, 1978). [4] B.E. Deal, J. Electrochem. Soc. 121 (1974) 198C. [5] F.L. Galeener and G. Lucovsky, Phys. Rev. Letters 37 (1976) 1474. [6] F.L. Galeener and R.H. Geils, in: The structure of Non-Crystalline Materials, ed. P.H. Gaskell (Taylor and Francis, London, 1977) p. 223. [7] F.L. Galeener, in: Lattice Dynamics, ed. M. Balkanski (Flammarion, Paris, 1978) p. 345. [8] F.L. Galeener, Phys. Rev. B19 (1979) 4292; Bull. Am. Phys. Soc. 23 (1978) 338. [9] R.H. Stolen, J.T. Krause and C.R. Kurkjian, Disc. Faraday Soc. 50 (1970) 103.
F.L. Galeener / The Raman spectra of defects i n . . . [10] [11 ] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
533
J.B. Bates, R.W. Hendrick and L.B. Shaffer, J. Chem. Phys. 61 (1974) 4163. R.H. Stolen and G.E. Walrafen, J. Chem. Phys. 64 (1976) 2623. F.L. Galeener, J.C. Mikkelsen, Jr. and N.M. Johnson, in ref. [3] p. 284. J.C. Mikkelsen, Jr. and F.L. Galeener, J. Non-Crystalline Solids 37 (1980) 71. R.B. Laughlin, J.D. Joannopoulos, C.A. Murray, K.J. Hartnett and T.J. Greytak, Phys. Rev. Letters 40 (1978) 461. M. Kastner, J. Non-Crystalline Solids 31 (1978) 223. G. Lucovsky, Phil. Mag. B39 (1979) 513. I. Simon, J. Am. Ceram. Soc. 40 (1957) 150. W. Primak, The Compacted States of Vitreous Silica (Gordon and Breach, New York, 1975). P.N. Sen and M.F. Thorpe, Phys. Rev. B15 (1977) 4030. J.P. De Neufville and D. Turnbull, Disc. Faraday Soc. 50 (1970) 182. J.S. Lannin, in: Amorphous and Liquid Semiconductors, Vol. 2, eds. J. Stuke and W. Brenig (Taylor and Francis, London, 1974) p. 1245.
THE RAMAN SPECTRA OF DEFECTS IN NEUTRON BOMBARDED AND Ge-RICH VITREOUS GeO2 Frank L. GALEENER Xerox Palo Alto Research Center, Palo Alto, California 94304, USA
This paper reports the polarized Raman spectra of three forms of vitreous GeO2 : the pure glass, neutron irradiated pure glass and an unirradiated Ge-rich glass of composition Ge 1.102. The data reveals that the line seen at 520 cm-l in the pure glass is due to a network defect that is not a Ge-Ge bond and very probably also not an O-O bond. Comparison with spectra of fused silica suggests that the 606 cm-1 defect line seen in v-SiO2 is not due to Si-Si or O-O bonds.
1. Introduction The nature of network defects in glasses is of great scientific interest and technological importance, see e.g. [ 1 ]. This is especially so in vitreous (v-)SiO2 because it is the glass which has been most extensively studied by scientists (for an overview see ref. [2] and for a recent survey of scientific problems see ref. [3]), and most widely exploited by technologists. Microscopic defects play an important role in limiting the use o f v-SiO2 as an insulator in MOS integrated circuitry and other electronic applications, see e.g. ref. [4]. Raman spectra of v-GeO2 are reported and used to shed light on the nature of a defect in v-SiO2, which is referred to as the 606 cm -~ Raman active defect. In fig. 1 is shown the polarized Raman spectra of v-SiO2 and v-GeO2. These and other spectra presented in this paper were obtained under conditions as described in refs. [5] and [6]. The principal features labelled B, R, TO and LO are properties of the continuous random AX2 network as demonstrated in detail in refs. [7] and [8]. The glasses are known to be 4 - 2 tetrahedral networks with similar bridging oxygen bond angles [8]. The line marked D at 606 cm -1 in the spectrum ofv-SiOz was first ascribed to a defect in the SiO2 network by Stolen et al. (SKK) [9] who observed that it alone increased dramatically after the sample was bombarded by neutrons with energies > 1 0 keV. This line was further studied by Bates et al. [10], who showed its evolution with increasing high energy neutron exposure~ and were tempted to assign it to a localized mode of the non-bridging oxygen defect, denoted -zSi+ - O - S i =, where -= indicates bonding to three separate bridging oxygen atoms. This interpretation 527
528
F.L. Galeener / The Raman spectra of defects i n . . . 0.4
I
a z
0
W 0.3
Z
I
~
I
RAMAN SPECTRA H H ~ "
R /
I
I
I
I
!
I
I
LO
I
]
I
VITREOUS
V ~
SiO 2
TO
0.2
0 Z
g ._1 _1
HV
o.o
~
3
r
i
i
RAMAN SPECTRA
2
i i R
i
i
i
[
i
I
I
[
I
VITREOUS (~
\
GeO 2
D
\1 o l 0
0
,~
\1~ "°
A
,o
500 1000 WAVENUMBER. W, cm -1
(XIO) I J
L 1500
Fig. 1. The polarized Raman spectra'of pure bulk vitreous SiO 2 and GeO2. D marks the defect lines discussed in this paper, 606 cm -1 for v-SiO2 and 520 cm-1 for v-GeO2 .
was supported in subsequent work by Stolen and Walrafen (SW) [11] who studied the 606 cm -1 defect line as a function of water content and thermal history. In an independent study of water-free and water-containing commercial v-SiO2, Galeener and Geils (GG) [6] found the same OH dependence o f the 606 cm -~ line as did SW. In particular, it was observed that the 606 cm -1 line was smaller in commercially available samples of Suprasil having higher OH content. Galeener and Geils also reported a 520 cm -1 "bump" in the HH Raman spectrum of v-GeO2 that decreased in samples with higher water content. For this reason, and because of its position relative to other features in the spectrum, GG inferred that the feature marked D in the lower half of fig. 1 was due to a defect in the v-GeO2 network, presumably analogousto the 606 cm -1 defect in v-SiO2. Assignment of these D lines to a non-bridging oxygen defect has been clouded by subsequent events. In follow-up work on the GG paper, Galeener et al. [12] showed that the 606 cm -~ defect line strength is independent of water content and that the observed variations in intensity were due to inequivalent thermal histories
F.L. Galeener / The Raman spectra-of defects i n . . .
529
of the v-SiO2 samples. (In a detailed study, Mikkelsen and Galeener [13] showed how the 606 cm -1 line strength depends on fictive temperature, annealing time, the rapidity of quench from annealing temperatures, and the viscosity of the glass). Moreover, Laughlin and co-workers [14] have made cluster Bethe lattice calculations for v-SiO2 on the bails of which they note that the 606 cm -1 defect may be due to Si-Si bonds. By analogy with the intimate valence alteration pair (IVAP) models now popular in studies of chalcogenide glasses [15], Lucovsky (L) [ 16] has asserted that the 606 cm -1 line is due to an IVAP involving a 3-bonded oxygen and a dangling oxygen. These models suffer from great uncertainty because they are mainly based on the coincidence of 606 cm -1 with frequencies that are calculated using theories of unproven accuracy involving adjustable parameters that are given ad-hoc values. In fact, there are additional defect models that give frequencies close to 606 cm -t , as shown, e.g. by SW. Until better theoretical methods are available I have taken the point of view that the D lines are best identified by methods which are as empirical as possible; hence the present study. The purpose of the present paper is to demonstrate empirically that it is unlikely that the 606 cm -1 line is due to Si-Si bonds or O - O bonds. The plan was to obtain bulk samples of Si-rich v-SiO2 which would be expected to contain large numbers of Si-Si bonds and small numbers of O - O bonds. Since I was unable to prepare suitable Si-rich material, or find it elsewhere, I have chosen to carry out a parallel study on the 520 cm -1 D line of v-GeO2. 2. Neutron bombarded v-GeO2 In fig. 2 is shown the polarized Raman spectra of unirradiated and irradiated v-GeO2. The irradiated material was obtained from Prof. A.J. Leadbetter (University of Exeter, Great Britain) who had subjected it to a total flux of 1021 fast /
i
~
~
Ik~,3o
[
i
]
I
[
I-- RAMAN ~ [ - SPECTRA ~ /
zQ
2 ~"
I
I
I
VITREOUS GeO2
I~/
I
-
1021 FAST NEUTRONS/cm2 -- UNIRRADIATED
rr
1
g
/r'
\A
,
,55
500
1000
1500
WAVENUMBER, W, cm-1
Fig. 2. A comparison of the polarized Raman spectra of neutron irradiated and unirradiated pure v-GeO2. Note the substantial increase near 520 cm -1 in the irradiated material.
530
F.L. Galeener / The Raman spectra of defects i n . . .
neutrons cm -2. The most striking change upon bombardment is an increase in the HH Raman scattering in the vicinity of the 520 cm -1 D line. This identifies the D line as a "defect" induced by neutron damage. This fact and the fact that 520 cm -~ lies between the R line and the next higher frequency T O - L O pair strongly suggests that the 520 cm -x D line is of the same origin as the 606 cm -1 line in v-SiO2. Another clear observation is that the R line moves to higher frequency (from ~420 to ~430 cm -1) while the high frequency T O - L O pair moves to lower frequency, by ~7 cm -1 . A similar effect has been reported in v-SiO~ by Simon [17] and SKK [9]. It is known that v-SiO2 is compacted (made more dense) by neutron bombardment, and the simplest model for this is that the average value of the S i O - S i angle is reduced without change in the average value of the S i - O distance, for an overview see ref. [18]. Galeener [8] has shown that under these conditions the change in bridging oxygen angle A0 can be estimated by AO = ( 2 ~ 4 A w 4 M / a sin 0 ) ,
(1)
based on a central force model due to Sen and Thorpe [ 19]. Here, M is the mass of the oxygen atom, a is the central force constant, 0 is the average G e - O - G e angle and ~4 is taken to be the center value of the high frequency LO mode (973 cm-1). Using Aco4 = - 7 cm -1 and other values given in table 3 of ref. [8], one finds that Aco = - 2 . 3 ° . Thus I conclude from the Raman spectral changes that v-GeO2 is compacted by 102~ neutrons cm -2 to an extent that can be accounted for by a 2 o - 3 ° reduction in the G e - O - G e angle. When we think about the extensive network damage likely to be caused by bombardment with neutrons from 10 keV to 2 MeV, we realize the danger of using intuition to argue that a particular simple defect is most likely to appear in dominant numbers. Certainly bonds will be broken, but do they heal? It seems likely that most do; but what fraction? Does the material heated by the passage of the neutron segregate? Does it quench into a high temperature form, partially microcrystalline? The answers are not known. What we do know in the present context is that neutron irradiation of v-GeO2 produces defects which give a polarized Raman signal at the same position (520 cm -1) as defects which already exist in the unirradiated glass. It is reasonable to assume that the defects causing the 520 cm -1 lines in irradiated and unirradiated materials are the same, although the irradiated material may contain additional Raman inactive defects in substantial numbers. At this stage one cannot argue against the D line representing G e - G e bonds simply because one feels that broken-bond defects are the more likely result of neutron irradiation.
3. Ge-rich v-Ge02 On the other hand, we can argue that a Ge-rich sample of v-Ge02 almost certainly contains G e - G e bonds, which do not exist in the perfect continuous
531
F.L. Galeener / The Raman spectra.of defects i n . . . t z..-a 0
3
I I I RAMAN SPECTRA
I
I
I
I
I
I
425 I
I
- -
I I I VITREOUS GERMANIA Gel.~O 2
c~ cc
I
GeO2
~ 2 O z O_ 1-
!
\\
,
9
I
I
I
I
I 500
865
.~----~ I
~
"-.q.~.I~"'.I~L-..J__J__L_J_ 1000
WAVENUMER,
1500
W, cm -1
Fig. 3. A comparison o f the HH Raman spectra o f pure and Ge-rich v-GeO2. Note the absence of increased scattering near 520 cm -l in the Ge 1.102.
random network for v-GeO2. If the material is phase segregated into Ge particles in a v-GeO2 matrix there will certainly be G e - G e bonds. If it is not phase separated, G e - G e bonds must also exist, interspersed throughout the network. If the 520 cm -~ D line is due to a small number of G e - G e bonds in the nominally stoichiometric pure glass, then this line should increase markedly in a deliberately Ge-rich glass.
The solid line in fig. 3 is the HH Raman spectrum of a Ge-rich sample of vitreous "GeO2" with composition ~Ge1.102. This material was generously supplied by Dr. J.P. De Neufville (Exxon Research Labs, Linden, New Jersey). The sample was prepared in thin disk form as described in the paper by De Neufville and Turnbull [20] ; it was one of the samples that showed no evidence of phase separation by the tests described therein. The HV spectra are not shown in fig. 3, for clarity. As before, the dashed line is for pure v-GeO2. There is no change in the 520 cm -1 D line for 10% excess Ge. This is evident when one realizes that the Ge1.102 spectrum rides on the edge of a broad luminescence line that peaks above 1500 cm-1; approximate subtraction of that luminescence line from the Ge1.102 spectrum causes it to coincide with the GeO2 spectrum from ~425 to ~600 cm -1. Thus I conclude that the 520 cm -1 D line is not due to G e - G e bonds in pure v-GeO2. We can also argue that the availability of excess Ge atoms in a Ge-rich sample is likely to greatly reduce the number of O - O bonds if any exist in the stoichiometric material. Since the 520 cm -~ line did not decrease, I also conclude that the D line in v-GeO2 is not due to O - O bonds, either in the form of a dioxy-bridge = G e O - O - G e =, or a peroxy-radical - = G e - O - O - . In fig. 3 it is shown that there are changes in the spectrum due to excess Ge.
532
F.L. Galeener / The Raman spectra o f defects i n . . .
Principal among these is a change in the frequencies and relative strengths of the high frequency T O - L O pair, and the introduction of new scattering strength at frequencies below ~-400 cm -1. Qualitatively, these are as would be expected from the introduction of units like -=Ge-Ge =, since the vibrational frequencies of elemental germanium, see e.g. ref. [21] are below 400 cm -~ , and the high frequency modes in v-GeO2 are known to be caused by an antisymmetric stretch of the bridging oxygen atom [8], which is absent at the -=Ge-Ge= sites. If the Ge were segregated into Ge particles there would be little change in the high frequency portion of the spectrum. Therefore the Raman spectrum is consistent with the conclusion of De Neufville and Turnbull that the sample is n o t phase-separated.
4. Conclusions It has been shown by experiment that v-Ge02 contains a network "defect" that exhibits a Raman active line at ~ 5 2 0 cm -1 . This line grows with fast neutron bombardment, but is unchanged in Ge-rich material of composition Ge1.102. The line is thus shown not to arise from G e - G e or O - O bonds. Since this line occurs in a similar region of the spectrum as does the 606 cm -1 D line in v-SiO2, and behaves similarly under neutron bombardment, it is highly probable that the D lines in v-GeO2 and v-Si02 have the same (analogous) structural origin. It is therefore concluded that the 606 cm -1 line in v-SiO 2 is almost certainly n o t due to Si-Si or O O bonds.
Acknowledgments I am grateful to Prof. A.J. Leadbetter and Dr. J.P. De Neufville for giving me samples that enabled this work to be done. I am also grateful to Mr. W.J. Mosby for his skilled assistance in obtaining the Raman spectra.
References [ 1 ] D.L. Griscom, J. Non-Crystalline Solids 31 (1978) 241, and refs. therein. [2] R.H. Doremus, Glass Science (Wiley, New York, 1973). [3 ] The physics ofSiO z and Its Interfaces, ed. S.T. Pantelides (Pergamon, New York, 1978). [4] B.E. Deal, J. Electrochem. Soc. 121 (1974) 198C. [5] F.L. Galeener and G. Lucovsky, Phys. Rev. Letters 37 (1976) 1474. [6] F.L. Galeener and R.H. Geils, in: The structure of Non-Crystalline Materials, ed. P.H. Gaskell (Taylor and Francis, London, 1977) p. 223. [7] F.L. Galeener, in: Lattice Dynamics, ed. M. Balkanski (Flammarion, Paris, 1978) p. 345. [8] F.L. Galeener, Phys. Rev. B19 (1979) 4292; Bull. Am. Phys. Soc. 23 (1978) 338. [9] R.H. Stolen, J.T. Krause and C.R. Kurkjian, Disc. Faraday Soc. 50 (1970) 103.
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