THE ELECTRICAL AND OPTICAL PROPERTIES OF SODIUM GERMANATE GLASSES

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THE ELECTRICAL AND OPTICAL PROPERTIES OF SODIUM GERMANATE GLASSES

M. Khan, E. Khawaja

To cite this version:

M. Khan, E. Khawaja. THE ELECTRICAL AND OPTICAL PROPERTIES OF SODIUM GERMANATE GLASSES. Journal de Physique Colloques, 1982, 43 (C9), pp.C9-319-C9-322.

�10.1051/jphyscol:1982959�. �jpa-00222488�

JOURNAL DE PHYSIQUE

Colloque C9, supplement ecu n°12, Tome 43, decembre 1982 page C9-319

THE ELECTRICAL AND OPTICAL PROPERTIES OF SODIUM GERMANATE GLASSES

M. N. Khan and E. E. Khawaja

Department of Physics, University of Petroleum and Minerals, Dhahran, Saudi Arabia

Résumé.- On a mesuré la conductivité électrique d'une série d'échantillons du sys- tème Na.O - GeCL depuis l'ambiante jusqu'à 350°C. Les résultats sont interprétés en termes de gaps électriques qui décroissent linéairement avec la teneur en Na.O des verres. Les seuils d'absorption ont été mesurés et analysés en termes de transitions indirectes à travers un gap optique qui croît également linéairement avec Na.O. Ce- pendant, on a trouvé que le gap optique est environ le double du gap électrique correspondant.

Abstract.- The electrical conductivities of a series of specimens in the system.

Na.O-GeO„ were measured at temperatures ranging from room temperature to 350°C.

These results are interpreted in terms of electrical gaps, which decrease line- arly with the Na 0 content of the glasses.

Their absorption edges are measured and analysed in terms of forbidden non-direct transitions across an optical energy gap which is also found to in- crease linearly with the Na

0 content of the glasses. However, the optical gap was found roughly to be twice the corresponding electrical gap.

1. Introduction.- The conductivity of glasses and amorphous films has been the sub- ject of an extremely wide range of experimental and theoretical studies (1-4). It is clear even with materials devoid of long range order and with band tailing beyond the normal conduction and valence bands a forbidden gap exists and may be estimated from measurements of electrical conductivity as a function of temperature as well as from optical absorption measured as a function of photon energy.

In the present work the results of conductivity measurements with varying Na^O content in the system Na_0 - GeO. are reported. As the Na_0 content is increased the value of activation energy and hence the electrical gap decreases accordingly.

The optical absorption in the specimens was carefully measured. An analysis of the absorption near the absorption edge is presented and discussed in terms of a Davis and Mott. (5) model of the energy -levels in amorphous materials. The optical energy gap also decreases linearly with the increase in Na^O content.

2. Experimental.- The raw materials used for the preparation of the glasses were re- agent grade Na_CO (as a source for Na.O) and GeO-, both of purity 99.99 % and wei-e supplied by Fisher Scientific Company. The glass batch (compositions are given in table-1) was melted in platinium crucible using an electric furnace at temperatures from 1200°C to 1250°C for three hours depending upon the composition. The molten glass was casted on a stainless steel surface in the form of circular pallets of diameter 1 cm. For each composition more than two specimens were casted which vari- ed in thickness from 2 mm to 5 mm.

The glass samples were polished using diamond-paste down to minimum grit size of O.lym. Due to hygroscopic nature of germanium dioxide glass, acetone was used

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982959

C9-320 JOURNAL DE PHYSIQUE

i n t h e p r o c e s s of p o l i s h i n g . Gold was evaporated onto t h e f a c e s o f t h e specimen t o form a guarded e l e c t r o d e system which c o n s i s t e d of a main e l e c t r o d e o f 5 mm i n d i a - meter, a r i n g shaped guard e l e c t r o d e and an e l e c t r o d e o p p o s i t e . The temperature o f t h e sample was monitored u s i n g a chromel-alwnel thermocouple a t t a c h e d t o t h e sample.

For each composition ( t a b l e 1 ) t h e absorbance of two specimens o f d i f f e r e n t t h i c k n e s s e s was measured a t normal i n c i d e n c e u s i n g a P e r k i n Elmer 202 W / v i s i b l e spectrometer. Fig.2 i s an example o f t h e absorbance c u r v e s f o r t h e composition E

5 and samples t h i c k n e s s 3.30 mm and 2.09 mm.

Table - 1

Chemical Composition, e l e c t r i c a l and o p t i c a l energy gaps

GeO2 NazCos A c t i v a t i o n O p t i c a l

Melt con- con- Energy E g a p E o

No. t e n t t e n t (ev

(ev) -

( w t % ) (Lot%) -

E

Eo 100 0 1.00 - - .

l o - '

1 9 0 10 0.92 3.64

1 0 -11

3. R e s u l t s And Discussion

3.1. Time dependence of d i r e c t c u r r e n t s . - The time

dependence of c u r r e n t a t an a p p l i e d v o l t a g e o f 30V

was s t u d i e d f o r t h e specimens of d i f f e r e n t composit-

i o n s . The p o l a r i z a t i o n e f f e c t s were observed and

t h e r e s i s t i ; i t y of t h e specimens i n c r e a s e d with

time. T h i s s u g g e s t s t h a t t h e i o n i c c u r r e n t s a r e Fig'1:

dominant. Dominant i o n i c c u r r e n t s i n Na 0 - GeO ductivity versus

2 2 t e m ~ e r a t u r e f o r Na7O-GeO?

g l a s s e s h a s a l s o been r e p o r t e d by Evstropev e t g l a s s e s . a l . ( 6 ) .

3.2. d . c . c o n d u c t i v i t y . - Fig.1 shows t h e p l o t o f l o g a r i t h m i c c o n d u c t i v i t y a g a i n s t t h e r e c i p r o c a l o f a b s o l u t e temperature f o r a s e r i e s of g l a s s specimens. I n many t y p e s of i o n i c a l l y conducting o x i d e g l a s s e s , it i s well known t h a t t h e s e p l o t s a r e l i n e a r i n conformity with t h e following equation:

where a i s t h e c o n d u c t i v i t y o i s a c o n s t a n t f o r a g i v e n g l a s s , K i s t h e Boltzman c o n s t a n t , T i s a b s o l u t e temperature and AE i s t h e a c t i v a t i o n energy f o r conduction.

A l l t h e graphs shown h e r e c o n t a i n s t r a i g h t l i n e s , which a r e numbered i n ascending o r d e r a s t h e sodium i o n c o n c e n t r a t i o n i n t h e specimens i n c r e a s e s .

Table 1 l i s t s t h e d e r i v e d v a l u e s of t h e a c t i v a t i o n energy and r e l a t e s them t o composition and t o t h e measured v a l u e s o f o p t i c a l energy gap E

0 '

A p l o t of a c t i v a t i o n energy a g a i n s t Ge02 c o n c e n t r a t i o n i s shown i n Fig.4. I t i s c l e a r t h a t i n c r e a s i n g t h e NaO c o n t e n t causes a d e c r e a s e i n t h e a c t i v a t i o n energy

2

f o r i o n t r a n s p o r t . A t high sodium c o n c e n t r a t i o n s , it i s e a s i e r f o r t h e t r a n s p o r t o f

mobile i o n s t o t a k e p l a c e a t given t e m p e r a t u r e and e l e c t r i c f i e l d s t r e n g t h , and hence t h e i n c r e a s e i s equivalent conductance.

3 . 3 . O p t i c a l a b s o r p t i o n . - Fig.2 shows measurement o f absorbance (A) a g a i n s t wave- l e n g t h f o r two specimens with same composition E5 ( t a b l e 1 ) and t h i c k n e s s x1=3.30 mm

and x2=2.09 mm. The a b s o r p t i o n c o e f f i c i e n t a(X) which i s a f u n c t i o n of wavelengthX was c a l c u l a t e d from t h e f o l l o w i n g r e l a t i o n :

As (A) -A2(X)

a(),) = --- -- . . .

x, -x, (1)

I L

a(X) was c a l c u l a t e d f o r wave- l e n g t h s a t i n t e r v a l s o f 5 nm from Fig.2. This d a t a was t h e n analysed by t h e w e l l e s t a b l i s h - ed t h e o r y f o r t h e a b s o r p t i o n i n amorphous m a t e r i a l s given by Davis & Mott ( 5 ) . Accord- ing t o t h e t h e o r y K- conserva- t i o n r u l e breaks down i n amor- phous m a t e r i a l s and K i s n o t a good quantum number. F u r t h e r , i f t h e m a t r i x element f o r mom- entum v e c t o r f o r o p t i c a l t r a n s - i t i o n s i s taken c o n s t a n t ( t h i s assumption i s v a l i d i n t h e p r e s e n t c a s e a s t h e energy range concerned i s small) t h e n t h e a b s o r p t i o n can b e r e p r e s e n t e d by

where E i s t h e photon energy and Eo i s t h e o p t i c a l band gap, and C i s a c o n s t a n t .

Figure 3 shows p l o t s of ( a ~ ) % a g a i n s t E f o r t h e d i f - f e r e n t compositions of t h e g l a s s e s l i s t e d i n t a b l e 1. The p l o t s suggest t h a t t h e absorp- t i o n f o l l o w s equation ( 2 ) . Eo, t h e band gap f o r a l l t h e com- p o s i t i o n s s t u d i e d was o b t a i n e d by e x t r a p o l a t i n g t h e l i n e s t o c u t E- a x i s ( f i g u r e 3 ) . For an example, i n t h e c a s e of compos-

ZERO OFFSET

WAVELENGTH (nm)

Fig.2: Absorbance a s a f u n c t i o n o f wavelength f o r two samples of same composition E 5 ( t a b l e 1 ) . P l o t A l i s f o r sample t h i c k n e s s 3 . 3 nun and

f o r 2.09 mm.

PHOTON ENERGY (eV)

i t i o n E5, E was found t o bk

Fig.3: P l o t s of ( a ~ ) ' a g a i n s t E f o r d i f f e r e n t 3.33 eV and t h e corresponding

compositions ( t a b l e 1 ) . wavelength i s 372.4 nm. This

v a l u e of X i s i n d i c a t e d by an

arrow i n f i g . 2 . A l t e r n a t e l y , t h e a b s o r p t i o n edge may be d e f i n e d a s t h e photon energy a t which t h e a b s o r p t i o n s t a r t s i n c r e a s i n g from t h e z e r o v a l u e . T h i s i s equi- v a l e n t t o s a y i n g t h a t t h e a b s o r p t i o n edge i s t h e wavelength a t which t h e curves A1 and A? (Fig.2) s t a r t d e v i a t i n g from t h e i r c o n s t a n t v a l u e . T h i s v a l u e l i e s v e r y

-

c l o s e t o t h e v a l u e determined from t h e e q u a t i o n (2) and shown by a n a r r o r i n F i g . 2 .

A s i m i l a r agreement was reached i n a l l t h e f i v e compositions.

C9-3 22 JOURNAL DE PHYSIQUE

I t may b e mentioned t h a t i n t h e p r e s e n t work specimens of two t h i c k n e s s e s were s t u d i e d i n o r d e r t o e l i m i n a t e r e f r a c t i v e index term i n t h e formula f o r absorbance.

Otherwise s e p a r a t e measurement f o r t h e r e f r a c t i v e index was needed.

I n Fig.4 E i s p l o t t e d a g a i n s t specimen composition. I t i s c l e a r t h a t Eo decreases l i n e a r l y a s t h e Na 2 0 c o n t e n t i s i n c r e a s e d . E f o r 100% Ge02 i s e s t i m a t e d t o be 3.69 eV i n i t s amorphous s t a t e and a t room temperature.

4. Conclusion.-The e l e c t r i c a l c o n d u c t i v i t i e s and o p t i c a l a b s o r p t i o n measurements i n t h e system Na 0

GeO g l a s s e s

2 2

were c a r e f u l l y made. The

v a l u e s f o r e l e c t r i c a l energy

gaps were o b t a i n e d from con-

d u c t i v i t y d a t a . The o p t i c a l

energy gaps were determined

-

a c c u r a t e l y from t h e measured absorbance i n t h e specimens

of d i f f e r e n t t h i c k n e s s e s .

I t was found t h a t t h e .

e l e c t r i c a l and t h e o p t i c a l -

energy gap d e c r e a s e l i n e a r l y

with i n c r e a s i n g c o n t e n t of

Na,O i n t h e g l a s s e s . The

L

c o n d u c t i v i t y i n c r e a s e s with ^tc.

i n c r e a s i n g Na 0 c o n t e n t i n t h e

2 Fig.4: Dependence o f a c t i v a t i o n energy and

g l a s s e s . o p t i c a l band gap on g l a s s composition.

The o p t i c a l a b s o r p t i o n i n t h e g l a s s e s i s analysed i n terms o f n o n - d i r e c t t r a n s - i t i o n s a s formulated by Davis and Mott.

References

1) Mott,N.F. and Davi5,E.A. " E l e c t r o n i c Processes i n n o n - c r y s t a l l i n e m a t e r i a l s "

(Oxford U n i v e r s i t y P r e s s , Oxford 1971).

2) L i t t l e t o n , J . T . and Morey,G.W. "The E l e c t r i c a l P r o p e r t i e s of Glass",Wiley, New York, 1953.

3) Tallan,N.M. " E l e c t r i c a l c o n d u c t i v i t y i n ceramics and Glass P a r t 2", Marcel Dakker, Inc. 1974.

4) Myuller, Rudol F. " E l e c t r i c a l c o n d u c t i v i t y o f v i t r e o u s s u b s t a n c e s l ' t r a n s l a t e d from Russian by Drake,S. and Drake, C.F. C o n s u l t a n t s Bureau New York 1971.

5) Davis,E.A. and Mott,N.F. P h i l - Mag. 22 (1970) 903.

6) Evstropev,K.S., Pavlovskii,V.K. and Ivanov,A.O. The s t r u c t u r e o f g l a s s , Vo,.4 Ed. O.V. Mazurin C o n s u l t a n t s Bureau, New York (1965) 110.

Acknowledgement

The a u t h o r s wish t o acknowledge g r a t e f u l l y t h e f i n a n c i a l support f o r t h i s

i n v e s t i g a t i o n provided by t h e U n i v e r s i t y of Petroleum & Minerals1 Research

Committee.