- Email: [email protected]
Localized states in germanate glasses. Ultraviolet absorption tail of crystalline and glassy germanium dioxide and alkali germanate
]OURNA~OF
ELSEVIER
Journal of Non-Crystalline Solids 188 (1995) 125-129
Localized states in germanate glasses. Ultraviolet absorption tail of crystalline and glassy germanium dioxide and alkali germanate Anatoly N. Truldfin *, Pgteris Kfilis Institute of Solid State Physics, University of Latvia, 8 Kengaraga St., L V-1063 Riga, Latvia Received 21 July 1994; revised manuscript received 4 January 1995
Abstract The optical absorption of crystalline and glassy germanate were measured with a double-beam instrument. Pure GeO2, Li2GeO 3 crystal, pure GeO2, sodium germanate and lanthanum germanate glasses were studied. Glass samples were prepared by melting and some by evaporation during melting. Melting conditions were normal and oxidizing. Intrinsic absorption tails of a GeO 2 crystal with a structure of a-quartz and in a Li2GeO 3 crystal are situated at 6 eV. Two absorption thresholds were observed in a GeO 2 glass: one is situated at 6 + 0.2 eV, the other at 4.5 + 0.2 eV. The intensity of the latter increases in sodium germanate glasses and corresponds to the intrinsic absorption threshold of a tetragonal GeO 2 crystal. This threshold is known from the literature. The absorption spectra and their thermal dependences are well approximated by the Urbach rule in two forms. The threshold at 6 eV in all the materials studied obeys the Urbach-Toyozawa rule. The threshold at 4.5 eV agrees with the so called 'glass' type rule.
1. Introduction The ultraviolet absorption tail of a material is an important parameter of the electronic structure. The comparison between these parameters in both crystal and glass can give information about the peculiarities of the electronic structure in the glassy state. GeO 2 and related materials have not been intensely studied from this point of view (see Refs. [1,2] and references therein). The task of this work is to investigate the optical absorption in a GeO z crystal and glass as well as in an alkali germanate crystal and glass. Such a comparison also provides information about the role of modifier ions in alkali germanate glasses.
* Corresponding author. Tel: + 371-2 260 686. Telefax: + 371-2 260 543. E-mail: [email protected].
It is known that the intrinsic absorption, o~, and its temperature dependence in many crystals obeys the so called Urbach rule: a = ot0 exp[ - o - ( e 0 - E ) / k T ] ,
(1)
o" = o'oZkT/h to tanh( h t o / 2 k T ) .
(2)
The Urbach rule is in a form which corresponds to a model of this rule introduced by Toyozawa [3] in the case of a strong exciton-phonon interaction. The slope of the absorption spectrum changes with temperature. Thus there is a cross-point in the semilogarithmic plots of In a versus E. The parameters, a 0 and E0, are coordinates of the cross-point and they are close to the maximum of the first intrinsic absorption band. The parameter, tro, is inversely proportional to the strength of the exciton-phonon interaction. The parameter, h to, is an effective phonon energy.
0022-3093/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved
SSDI 0 0 2 2 - 3 0 9 3 ( 9 5 ) 0 0 0 9 4 - 1
126
A.N. Trukhin, P. K~lis /Journal of Non-CrystaUine Solids 188 (1995) 125-129
In the case of glassy materials, for example As2Se 3 [4] or Na20 "3SiO 2 [5], the intrinsic absorption tail agrees with the 'glassy' modification of the Urbach rule, where no cross-point can be found. The slope of the spectra does not change with temperature. The semilogarithmic spectra move parallel to each other with change of temperature:
a = I o exp[ AE + T/r~],
(3)
where A is a parameter of the slope and T~ is a characteristic temperature. There is no consensus on the mechanism of a rule of form (3). However, this problem has been analyzed in many works (see for example Refs. [6,7]). As a qualitative interpretation, the theory of Ref. [8] seems to be more convenient. Here the optical transition takes place between adiabatic potentials, having different vibration frequency for ground and excited states. Thus the Urbach rule in a crystal is explained. In the case of a glassy modification, these adiabatic potentials belong to the ensemble of structurally non-equivalent sites, probably affected by soft modes. From previous investigations it is known that in the case of silica the 'glassy' modification of the Urbach role does not work, but in the 'crystal' case Eqs. (1) and (2) are realized [9]. The results of the present work show that in the case of GeO2 glasses both cases are realized.
2. Experimental The samples investigated were various GeO 2 glasses. A sample of a thin GeO 2 glass can be obtained by condensing GeO 2 vapour on a substrate. As a substrate a silica plate with dimensions 10 X 10 X 1 mm 3 was used, which covered a crucible with melted GeO 2. The samples obtained were 0.1 and 0.2 mm thick. To achieve these thicknesses the main sample was melted in a crucible for different lengths of time. As an example of alkali germanate the sodium germanate glasses, melted under strongly oxidizing conditions, were used. The samples of sodium and lanthanum germanate glasses were melted under oxidizing conditions. They were of four compositions: 15Na20-85GeO 2 (labelled 15 Na), 25Na20.75GeO 2 (labelled 25 Na), 33Na2067GeO 2 (labelled 33 Na), 8La203 • 92GeO 2 (labelled 8 La). As corresponding crystals we possess GeO 2
and Li2GeO 3. We consider the Li2GeO 3 crystal to be convenient for comparison with alkali germanate glasses, because its structure [10] contains a plane of the GeO4 tetrahedra and can be used when comparing with germanium dioxide and sodium germanate glasses as well as with the GeO 2 crystal. The GeO 2 crystals were the same as in Ref. [2]. A thin layer, isostructural to a-quartz, was grown by the hydrothermal method on a quartz plate. The thickness of the layer was about 0.08 mm. The sample had dimension 10 X 10 mm 2. The optical absorption in the temperature range 5-300 K was measured with a helium cryostat and a vacuum monochromator. In the range 300-700 K, the measurements were realized on a thermostat and a spectrophotometer Specord M40. Its high precision determined the quality of the recorded spectra. The absolute errors of measurements were determined by the sample quality. The GeO 2 crystal had a translucent appearance. Thin films of the GeO 2 glass were not of good optical quality. The bulk samples of the germanate glass were of higher optical quality.
3. Results The absorption spectra of some samples are presented in Fig. 1. It can be seen that the samples of GeO 2 possess a strong absorption, which takes place near 6 eV both for crystal and glass. The intrinsic nature of the absorption threshold at 6 eV has been determined [2] for crystalline GeO 2. At energies less t., ~sorptionspectro ,,.~u~10
- C~O~-cnmtol - ~02-glo~
3
A
. - U20e03/ ry~o/ /
1
PHOTONENEMY (ev) Fig. 1. Absorption spectra of germanate crystals and glasses. T = 293 K.
A.N. Trukhin, P. K~lis /Journal o[Non-Crystalline Solids 188 (1995) 125-129 -
•
-
i
-
i
-
•
-
|
-
•
•
i
•
•
-
-
0.6
Porometem of Urboch rule: E,= 6.651-0.2 eV; a0=5.10s cm-1 ~ P "
#
i
-
|
-
I
-
i
-
i
-
-
I
0.5 0.4. lb 0.3
c8 6
-
(squores) ond t h C , ~ (circles) erystoln i(T)(j>orometer of Urbach rule) ne - best fit
tol
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i
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0.2
T9
0.1
4
....
.
.
.
*
.
•
.
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.
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.
.
100200300400500000 Temperature(K)
5.4 5.6 5.8 6.0 6.2 6.4 6,6 6.8 Photon Energy (eV) Fig. 2. Temperature dependences o f absorption spectra at intrinsic absorption tail of GeO 2 crystal. Points - measured data; lines approximation with the Urbach rule (1). The background determined at the lowest temperature (4.5 K) was subtracted from each spectrum after measurements.
Fig. 4. Temperature dependences of the Urbach rule parameter, tr, for GeO 2 and Li2GeO 3 crystals. Points - o" values calculated after (1); lines - approximation by Eq. (2). For GeO 2 crystal the parameters are: cro = 0.58, hto 0 = 56 meV; for Li2GeO 3 crystal the parameters are: o-0 = 0.62, hto 0 = 61 meV.
than the threshold, there is an additional tail of absorption which, in the case of sodium germanate glasses, has a higher intensity ( a > 1000 cm-1). In the LizGeO 3 crystal the energy of the intrinsic threshold is the same as in pure crystalline GeO 2. For both these crystals the temperature dependence of the absorption spectra at the intrinsic threshold obeys the Urbach rule in Eqs. (1) and (2) and has practically identical values of all the parameters: E o = 6.6 eV, a o = 3 × 10 6 c m - l , tr° __ 0.6 and h to
= 60 meV (Figs. 2-4). The parameters were obtained by least-squares fits of In a versus E corresponding to Eq. (1) and of or versus T after Eq. (2). In the case of the GeO 2 glass thin film, the absorption of the intrinsic threshold also depends on the temperature in the form approximated by the
1 2
i
.
•
.
•
.
•
.
i
C-eO2 glasses obsorpUon spectra T1-300 K 61"=50
.
. .
,-- 8 "t
/////#//t
//,¢/
"[HI;.1/A ,0t
/////
41-
":; |
4.0 0
5.0
5.5
6.0
. l l ~ i i~" .I I I l l l l m ' "
t .
6.5
7.0
Photon Energy (eV) Fig. 3. Temperature dependences of absorption spectra at intrinsic absorption tail of LizGeO 3 crystal. Points - measured data; lines - approximation with the Urbach rule (1). The background was subtracted analogously with the data of Fig. 2.
.
..........
l'' •
*
.
j
.
|
.
•
4.5 5.0 5.5 6.0 Photon Energy (eV)
.
|
6.5
Fig. 5. Temperature dependences of absorption spectra at intrinsic absorption tail of GeO 2 glass thin film. Points - measured data; lines - approximation with the Urbach rule (1). T 1 curve was measured at 300 K, then the sample was heated to 700 K and measurements with a cool-down followed. In the range h v > 5.5 there is no essential influence of such a procedure on data. A certain annealing effect takes place for the lower energy. The background was subtracted analogously with the data of Fig. 2.
A.N. Trukhin, P. Kfilis /Journal of Non-Crystalline Solids 188 (1995) 125-129
128
~rT1
'
!
I
4.0 4.5 5.0 Photon Energy (eV)
I
|
5.5
Fig. 6. Temperature dependences of absorption spectra at intrinsic absorption tail of Na203GeO 2 glass. Points - measured data; lines - exponential approximation with Eq. (3). The background was subtracted analogously with data of Fig. 2.
Urbach rule in Eqs. (1) and (2) with cross-point coordinates close to those of the crystals. The absorption of the long-wavelength tail in bulk samples of pure GeO 2 and alkali germanate also depends on the temperature, but is approximated by Eq. (3) (Figs. 5 and 6), because a least-squares fit of In a versus E gives a series of parallel straight lines. The values of the parameters, A and T1, of Eq. (3) are presented in Table 1.
4. Discussion The main results are as follows. (1) The positions of the intrinsic absorption threshold in the GeO 2 glass and the GeO 2 crystal with the a-quartz structure were determined. Approximately the same value was found in both materials. The result is in good agreement with the data [11,12] on GeO 2 glass, but disagrees with Refs. [13,14]. Table 1 Parameters A and T1 Sample
A ( ± 0 . 2 eV -1)
T1 ( ± 5 K)
8 La 15 Na 25 Na 33 Na GeO 2
5.5 7.5 8.7 8.5 5.7
213 185 105 120 231
(2) The temperature dependence of the spectra in the range of the threshold is described by the Urbach rule in the form modified by Toyozawa [3]. The key to this form is in the exciton-phonon interaction with exciton self-trapping. The exciton self-trapping was reported in Ref. [2] for the GeO 2 crystal and the result is in good agreement with previous data. (3) The coincidence of the intrinsic threshold behavior in the GeO 2 and Li2GeO 3 crystal may be due to similar electronic states. Then, the self-trapped exciton could be found in the Li2GeO 3 crystal. Thus we conclude that GeO 4 tetrahedra are responsible for the electronic transition, which forms the absorption tail. The transition corresponding to the lithiumoxygen sublattice may be situated in a higher-energy range. (4) The absorption tail below the intrinsic absorption threshold is amplified by sodium ions. The position of this absorption tail agrees with the intrinsic absorption threshold of tetragonal GeO 2 crystal, previously studied in Ref. [15]. Probably, in a short range the glass structure has not only the GeO 4 tetrahedra but also a certain concentration of a sixfold-coordinated germanium corresponding to a rutile structure. In sodium germanate glasses, the intensity of absorption at threshold increases by several order of magnitude, due to the larger concentration of sixfold-coordinated germanium. This opinion is in agreement with the crystallization experiments [12], where it was observed that alkali ions stimulate crystallization of the glass in the 'rutile' structure. The ionic modifiers have an influence on the position of absorption and a significant influence on the thermal behaviour through the effect on the 'glasslike' Urbach rule parameters, A and T1, in Eq. (3) which are given in Table 1. The physical meanings of these parameters are not known. The concentration of the states corresponding to the low-energy tail can be estimated from the absorption coefficient: it is about 100 cm-1. The oscillator strength of the transition to these states can be very small, because even for the tetragonal GeO 2 crystal the absorption coefficient in this range does not exceed 1000 cm -a. Thus, these states in pure GeO 2 cannot be ascribed to a defect state. The question remains - is this absorption due to a 'rutile' phase or is it just the absorption due to the structure naturally incorporated into a glass network? This question will
A.N. Trukhin, P. l~lis /Journal of Non-Crystalline Solids 188 (1995) 125-129
be investigated by a luminescence method in a future paper.
129
grant LF9000 of ISF. The authors thank Dr E. Raaben (oxidized alkali germanate), Dr A. Veispals (thin films of pure GeO 2 glass) and Dr G. Liberts (Li2GeO 3 crystal) for the samples provided.
5. Conclusions The intrinsic absorption thresholds are situated in crystalline GeO 2 (a-quartz) and Li2GeO 3 at an energy near 6 eV. The spectral and thermal dependences of the absorption at the threshold obey the U r b a c h - T o y o z a w a rule with the parameters: E 0 = 6.6 eV, a 0 = 3 × 10 6 cm -1, 0 - 0 = 0 . 6 and h t o = 60 meV. The intrinsic absorption threshold of the germania glass is also near 6 eV and it also obeys the Urbach-Toyozawa rule with values of the parameters similar to those of the crystals. The identity of these values is determined by an electronic transition due to the GeO 4 tetrahedra. An additional absorption threshold in germania glass at 4.2 + 0.2 eV is basic in alkali germanate glasses. It obeys the 'glass-like' Urbach rule. This absorption threshold is ascribed to the electronic transitions in the 'rutile' type or sixfold-coordinated germanium structural motifs. Electronic transitions of the modifier ions are situated at higher energy and are in the range of electronic transitions due to the fourfold- and sixfold-coordinated germanium. This work was supported in part by grants 93.656 and 93.659 of the Scientific Society of Latvia and by
References [1] L. Pajasova, Czech. J. Phys. B19 (1969) 1265. [2] A.N. Trukhin, Solid State Commun. 85 (1993) 723. [3] Y.A. Toyozawa, Techn. Report. ISSP A1 (1964) 119. [4] J.R. Zakis and H. Fritzsche, Phys. Status Solidi (b)64 (1974) 123. [5] L.B. Glebov and M.N. Tolstoi, Fiz. Khim. Stekla (Soy. Phys. Chem Glasses) 1 (1975) 239. [6] I.Z. Kostadinov, J. Phys. C10 (1977) L263. [7] D.J. Dunstan, J. Phys. C30 (1982) L419. [8] B.L. Gelmont, V.I. Perel and I.N. Jassievich, Sov. Solid State Phys. 25 (1983) 727. [9] I.T. Godmanis, A.N. Trukhin and K. Hubner, Phys. Status Solidi (b)l16 (1983) 279. [10] A.N. Lazarev and A.P. Mirgorodsky,Neorg. Mater. 12 (1973) 2159. [11] R.A. Weeks and T. Purcell, J. Chem. Phys. 43 (1965) 483. [12] H. Bohm, Phys. Chem. Glass. 11 (1970) 177. [13] J. Beynon, M. Samanoudyand S. Al-any, J. Mater. Sci. Lett. 8 (1989) 786. [14] C. Hogarth and M. Nadeem, Phys. Status Solidi (a)68 (1981) K181. [15] M. Stapelbroek and B.D. Evans, Solid State Commun. 25 (1978) 959.
ELSEVIER
Journal of Non-Crystalline Solids 188 (1995) 125-129
Localized states in germanate glasses. Ultraviolet absorption tail of crystalline and glassy germanium dioxide and alkali germanate Anatoly N. Truldfin *, Pgteris Kfilis Institute of Solid State Physics, University of Latvia, 8 Kengaraga St., L V-1063 Riga, Latvia Received 21 July 1994; revised manuscript received 4 January 1995
Abstract The optical absorption of crystalline and glassy germanate were measured with a double-beam instrument. Pure GeO2, Li2GeO 3 crystal, pure GeO2, sodium germanate and lanthanum germanate glasses were studied. Glass samples were prepared by melting and some by evaporation during melting. Melting conditions were normal and oxidizing. Intrinsic absorption tails of a GeO 2 crystal with a structure of a-quartz and in a Li2GeO 3 crystal are situated at 6 eV. Two absorption thresholds were observed in a GeO 2 glass: one is situated at 6 + 0.2 eV, the other at 4.5 + 0.2 eV. The intensity of the latter increases in sodium germanate glasses and corresponds to the intrinsic absorption threshold of a tetragonal GeO 2 crystal. This threshold is known from the literature. The absorption spectra and their thermal dependences are well approximated by the Urbach rule in two forms. The threshold at 6 eV in all the materials studied obeys the Urbach-Toyozawa rule. The threshold at 4.5 eV agrees with the so called 'glass' type rule.
1. Introduction The ultraviolet absorption tail of a material is an important parameter of the electronic structure. The comparison between these parameters in both crystal and glass can give information about the peculiarities of the electronic structure in the glassy state. GeO 2 and related materials have not been intensely studied from this point of view (see Refs. [1,2] and references therein). The task of this work is to investigate the optical absorption in a GeO z crystal and glass as well as in an alkali germanate crystal and glass. Such a comparison also provides information about the role of modifier ions in alkali germanate glasses.
* Corresponding author. Tel: + 371-2 260 686. Telefax: + 371-2 260 543. E-mail: [email protected].
It is known that the intrinsic absorption, o~, and its temperature dependence in many crystals obeys the so called Urbach rule: a = ot0 exp[ - o - ( e 0 - E ) / k T ] ,
(1)
o" = o'oZkT/h to tanh( h t o / 2 k T ) .
(2)
The Urbach rule is in a form which corresponds to a model of this rule introduced by Toyozawa [3] in the case of a strong exciton-phonon interaction. The slope of the absorption spectrum changes with temperature. Thus there is a cross-point in the semilogarithmic plots of In a versus E. The parameters, a 0 and E0, are coordinates of the cross-point and they are close to the maximum of the first intrinsic absorption band. The parameter, tro, is inversely proportional to the strength of the exciton-phonon interaction. The parameter, h to, is an effective phonon energy.
0022-3093/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved
SSDI 0 0 2 2 - 3 0 9 3 ( 9 5 ) 0 0 0 9 4 - 1
126
A.N. Trukhin, P. K~lis /Journal of Non-CrystaUine Solids 188 (1995) 125-129
In the case of glassy materials, for example As2Se 3 [4] or Na20 "3SiO 2 [5], the intrinsic absorption tail agrees with the 'glassy' modification of the Urbach rule, where no cross-point can be found. The slope of the spectra does not change with temperature. The semilogarithmic spectra move parallel to each other with change of temperature:
a = I o exp[ AE + T/r~],
(3)
where A is a parameter of the slope and T~ is a characteristic temperature. There is no consensus on the mechanism of a rule of form (3). However, this problem has been analyzed in many works (see for example Refs. [6,7]). As a qualitative interpretation, the theory of Ref. [8] seems to be more convenient. Here the optical transition takes place between adiabatic potentials, having different vibration frequency for ground and excited states. Thus the Urbach rule in a crystal is explained. In the case of a glassy modification, these adiabatic potentials belong to the ensemble of structurally non-equivalent sites, probably affected by soft modes. From previous investigations it is known that in the case of silica the 'glassy' modification of the Urbach role does not work, but in the 'crystal' case Eqs. (1) and (2) are realized [9]. The results of the present work show that in the case of GeO2 glasses both cases are realized.
2. Experimental The samples investigated were various GeO 2 glasses. A sample of a thin GeO 2 glass can be obtained by condensing GeO 2 vapour on a substrate. As a substrate a silica plate with dimensions 10 X 10 X 1 mm 3 was used, which covered a crucible with melted GeO 2. The samples obtained were 0.1 and 0.2 mm thick. To achieve these thicknesses the main sample was melted in a crucible for different lengths of time. As an example of alkali germanate the sodium germanate glasses, melted under strongly oxidizing conditions, were used. The samples of sodium and lanthanum germanate glasses were melted under oxidizing conditions. They were of four compositions: 15Na20-85GeO 2 (labelled 15 Na), 25Na20.75GeO 2 (labelled 25 Na), 33Na2067GeO 2 (labelled 33 Na), 8La203 • 92GeO 2 (labelled 8 La). As corresponding crystals we possess GeO 2
and Li2GeO 3. We consider the Li2GeO 3 crystal to be convenient for comparison with alkali germanate glasses, because its structure [10] contains a plane of the GeO4 tetrahedra and can be used when comparing with germanium dioxide and sodium germanate glasses as well as with the GeO 2 crystal. The GeO 2 crystals were the same as in Ref. [2]. A thin layer, isostructural to a-quartz, was grown by the hydrothermal method on a quartz plate. The thickness of the layer was about 0.08 mm. The sample had dimension 10 X 10 mm 2. The optical absorption in the temperature range 5-300 K was measured with a helium cryostat and a vacuum monochromator. In the range 300-700 K, the measurements were realized on a thermostat and a spectrophotometer Specord M40. Its high precision determined the quality of the recorded spectra. The absolute errors of measurements were determined by the sample quality. The GeO 2 crystal had a translucent appearance. Thin films of the GeO 2 glass were not of good optical quality. The bulk samples of the germanate glass were of higher optical quality.
3. Results The absorption spectra of some samples are presented in Fig. 1. It can be seen that the samples of GeO 2 possess a strong absorption, which takes place near 6 eV both for crystal and glass. The intrinsic nature of the absorption threshold at 6 eV has been determined [2] for crystalline GeO 2. At energies less t., ~sorptionspectro ,,.~u~10
- C~O~-cnmtol - ~02-glo~
3
A
. - U20e03/ ry~o/ /
1
PHOTONENEMY (ev) Fig. 1. Absorption spectra of germanate crystals and glasses. T = 293 K.
A.N. Trukhin, P. K~lis /Journal o[Non-Crystalline Solids 188 (1995) 125-129 -
•
-
i
-
i
-
•
-
|
-
•
•
i
•
•
-
-
0.6
Porometem of Urboch rule: E,= 6.651-0.2 eV; a0=5.10s cm-1 ~ P "
#
i
-
|
-
I
-
i
-
i
-
-
I
0.5 0.4. lb 0.3
c8 6
-
(squores) ond t h C , ~ (circles) erystoln i(T)(j>orometer of Urbach rule) ne - best fit
tol
16
i
127
0.2
T9
0.1
4
....
.
.
.
*
.
•
.
|
.
•
.
.
100200300400500000 Temperature(K)
5.4 5.6 5.8 6.0 6.2 6.4 6,6 6.8 Photon Energy (eV) Fig. 2. Temperature dependences o f absorption spectra at intrinsic absorption tail of GeO 2 crystal. Points - measured data; lines approximation with the Urbach rule (1). The background determined at the lowest temperature (4.5 K) was subtracted from each spectrum after measurements.
Fig. 4. Temperature dependences of the Urbach rule parameter, tr, for GeO 2 and Li2GeO 3 crystals. Points - o" values calculated after (1); lines - approximation by Eq. (2). For GeO 2 crystal the parameters are: cro = 0.58, hto 0 = 56 meV; for Li2GeO 3 crystal the parameters are: o-0 = 0.62, hto 0 = 61 meV.
than the threshold, there is an additional tail of absorption which, in the case of sodium germanate glasses, has a higher intensity ( a > 1000 cm-1). In the LizGeO 3 crystal the energy of the intrinsic threshold is the same as in pure crystalline GeO 2. For both these crystals the temperature dependence of the absorption spectra at the intrinsic threshold obeys the Urbach rule in Eqs. (1) and (2) and has practically identical values of all the parameters: E o = 6.6 eV, a o = 3 × 10 6 c m - l , tr° __ 0.6 and h to
= 60 meV (Figs. 2-4). The parameters were obtained by least-squares fits of In a versus E corresponding to Eq. (1) and of or versus T after Eq. (2). In the case of the GeO 2 glass thin film, the absorption of the intrinsic threshold also depends on the temperature in the form approximated by the
1 2
i
.
•
.
•
.
•
.
i
C-eO2 glasses obsorpUon spectra T1-300 K 61"=50
.
. .
,-- 8 "t
/////#//t
//,¢/
"[HI;.1/A ,0t
/////
41-
":; |
4.0 0
5.0
5.5
6.0
. l l ~ i i~" .I I I l l l l m ' "
t .
6.5
7.0
Photon Energy (eV) Fig. 3. Temperature dependences of absorption spectra at intrinsic absorption tail of LizGeO 3 crystal. Points - measured data; lines - approximation with the Urbach rule (1). The background was subtracted analogously with the data of Fig. 2.
.
..........
l'' •
*
.
j
.
|
.
•
4.5 5.0 5.5 6.0 Photon Energy (eV)
.
|
6.5
Fig. 5. Temperature dependences of absorption spectra at intrinsic absorption tail of GeO 2 glass thin film. Points - measured data; lines - approximation with the Urbach rule (1). T 1 curve was measured at 300 K, then the sample was heated to 700 K and measurements with a cool-down followed. In the range h v > 5.5 there is no essential influence of such a procedure on data. A certain annealing effect takes place for the lower energy. The background was subtracted analogously with the data of Fig. 2.
A.N. Trukhin, P. Kfilis /Journal of Non-Crystalline Solids 188 (1995) 125-129
128
~rT1
'
!
I
4.0 4.5 5.0 Photon Energy (eV)
I
|
5.5
Fig. 6. Temperature dependences of absorption spectra at intrinsic absorption tail of Na203GeO 2 glass. Points - measured data; lines - exponential approximation with Eq. (3). The background was subtracted analogously with data of Fig. 2.
Urbach rule in Eqs. (1) and (2) with cross-point coordinates close to those of the crystals. The absorption of the long-wavelength tail in bulk samples of pure GeO 2 and alkali germanate also depends on the temperature, but is approximated by Eq. (3) (Figs. 5 and 6), because a least-squares fit of In a versus E gives a series of parallel straight lines. The values of the parameters, A and T1, of Eq. (3) are presented in Table 1.
4. Discussion The main results are as follows. (1) The positions of the intrinsic absorption threshold in the GeO 2 glass and the GeO 2 crystal with the a-quartz structure were determined. Approximately the same value was found in both materials. The result is in good agreement with the data [11,12] on GeO 2 glass, but disagrees with Refs. [13,14]. Table 1 Parameters A and T1 Sample
A ( ± 0 . 2 eV -1)
T1 ( ± 5 K)
8 La 15 Na 25 Na 33 Na GeO 2
5.5 7.5 8.7 8.5 5.7
213 185 105 120 231
(2) The temperature dependence of the spectra in the range of the threshold is described by the Urbach rule in the form modified by Toyozawa [3]. The key to this form is in the exciton-phonon interaction with exciton self-trapping. The exciton self-trapping was reported in Ref. [2] for the GeO 2 crystal and the result is in good agreement with previous data. (3) The coincidence of the intrinsic threshold behavior in the GeO 2 and Li2GeO 3 crystal may be due to similar electronic states. Then, the self-trapped exciton could be found in the Li2GeO 3 crystal. Thus we conclude that GeO 4 tetrahedra are responsible for the electronic transition, which forms the absorption tail. The transition corresponding to the lithiumoxygen sublattice may be situated in a higher-energy range. (4) The absorption tail below the intrinsic absorption threshold is amplified by sodium ions. The position of this absorption tail agrees with the intrinsic absorption threshold of tetragonal GeO 2 crystal, previously studied in Ref. [15]. Probably, in a short range the glass structure has not only the GeO 4 tetrahedra but also a certain concentration of a sixfold-coordinated germanium corresponding to a rutile structure. In sodium germanate glasses, the intensity of absorption at threshold increases by several order of magnitude, due to the larger concentration of sixfold-coordinated germanium. This opinion is in agreement with the crystallization experiments [12], where it was observed that alkali ions stimulate crystallization of the glass in the 'rutile' structure. The ionic modifiers have an influence on the position of absorption and a significant influence on the thermal behaviour through the effect on the 'glasslike' Urbach rule parameters, A and T1, in Eq. (3) which are given in Table 1. The physical meanings of these parameters are not known. The concentration of the states corresponding to the low-energy tail can be estimated from the absorption coefficient: it is about 100 cm-1. The oscillator strength of the transition to these states can be very small, because even for the tetragonal GeO 2 crystal the absorption coefficient in this range does not exceed 1000 cm -a. Thus, these states in pure GeO 2 cannot be ascribed to a defect state. The question remains - is this absorption due to a 'rutile' phase or is it just the absorption due to the structure naturally incorporated into a glass network? This question will
A.N. Trukhin, P. l~lis /Journal of Non-Crystalline Solids 188 (1995) 125-129
be investigated by a luminescence method in a future paper.
129
grant LF9000 of ISF. The authors thank Dr E. Raaben (oxidized alkali germanate), Dr A. Veispals (thin films of pure GeO 2 glass) and Dr G. Liberts (Li2GeO 3 crystal) for the samples provided.
5. Conclusions The intrinsic absorption thresholds are situated in crystalline GeO 2 (a-quartz) and Li2GeO 3 at an energy near 6 eV. The spectral and thermal dependences of the absorption at the threshold obey the U r b a c h - T o y o z a w a rule with the parameters: E 0 = 6.6 eV, a 0 = 3 × 10 6 cm -1, 0 - 0 = 0 . 6 and h t o = 60 meV. The intrinsic absorption threshold of the germania glass is also near 6 eV and it also obeys the Urbach-Toyozawa rule with values of the parameters similar to those of the crystals. The identity of these values is determined by an electronic transition due to the GeO 4 tetrahedra. An additional absorption threshold in germania glass at 4.2 + 0.2 eV is basic in alkali germanate glasses. It obeys the 'glass-like' Urbach rule. This absorption threshold is ascribed to the electronic transitions in the 'rutile' type or sixfold-coordinated germanium structural motifs. Electronic transitions of the modifier ions are situated at higher energy and are in the range of electronic transitions due to the fourfold- and sixfold-coordinated germanium. This work was supported in part by grants 93.656 and 93.659 of the Scientific Society of Latvia and by
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