hassan 1988
TRANSCRIPT
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J O U R N A L O F M A T E R I A L S S C I E N C E 2 3 ( 1 9 88 ) 2 5 0 0 - 2 5 0 4
A s t u d y o f t h e s t r u c t u r a l , e l e c t r i c a l a n d o p t i c a l
p r o p e r t ie s o f c o p p e r t e l lu r i u m o x i d e g la ss es
M . A . H A S S A N , C . A . H O G A R T H
Physics Department, Brun el University, Uxbr idge, M iddlesex, UK
The resu l t s o f an i nves t i ga t i on o f some proper t i es o f CuO-TeOz g lasses a re repor ted . X- ray
d i f f rac t i on , d i f fe r en t i a l s c ann i ng c a l o r im e t r y and op t i c a l ab s o r p t i on m eas u r em en t s a r e d i s -
c us s ed w i t h r es pec t t o t he c om pos i t ions . D .c . c o nd uc t i v i t y is m eas u r ed i n t he t em pe r a t u r e
r ange 293 t o 4 53 K and d i s c us s ed i n t e r m s o f s m a l l po l a r on t heo r y .
1.
I n t r o d u c t i o n
Several transition metal oxides when heated with
glass-forming substances such as PzOs, TeO2 and
GeO 2, etc., form glasses on quenching the melt. The
loss of oxygen from the melt produces a proportion of
lower valency transition metal ions, and electrical
conduction in these glasses occurs by electron hopp ing
from an ion in the low valency state (e.g. Cu +) tran-
sition metal to a n ion in the higher valency state
C u ~ + ) [ 1 1 .
Differential thermal analysis, X-ray diffraction and
electrical conductivity for CuO-TeO2 glasses have
been studied by Ivanova e t a l . [2]. They foun d that for
all the compositions examined, the dependence of
electrical conduct ivity, cr, as a function o f reciprocal
temperature (l/T) yields different activation energies
at low and high temperatures. Electrical conductivity
measurements for glasses of the ternary system CuO-
V2Os-TeO 2 have been investigated by Dimitriev e t a l .
[3]. They noted that the temperature dependence of
the conductance is better described by the Arrhenius
equation above 100~ and that the calculated acti-
vat ion energy varies within the limits 0.4 to 0.8 eV.
The physical parameters obtained from measure-
ments such as X-ray diffraction, differential scan-
ning calorimetry (DSC), and optical absorpt ion are
discussed with respect to the compositions. The con-
ductivity was measured for semiconducting copper
tellurium oxide glasses. The present studies show that
the conductivityof CuO-TeO2 glasses is slightly higher
than that of the corresponding CuO-P205 glasses [4].
2 . E x p e r i m e n t a l w o r k
Six glasses in the CuO-TeO 2 system with 15, 25, 30,
35, 40, and 50 mol % CuO were prepared by melting
the oxides in alumina crucibles for 30min a t 700 to
900 ~ C. The electrical measurements were made by
standard techniques. Evaporated gold electrodes were
found to give ohmic contacts for fields up to at least
103 V cm-~ and were used in al l the electrical measure-
ments. Thin glass films, which are necessary for optical
absorption measurements, were obtained by blowing
techniques. Specimens in the thickness range 2.5 to
6.25 m were obtained as measured using a Sigma
comparator. All thin films were unannealed when
used for optical'measurements. The optical absorp-
tion measurements were made on thin blown films at
room temperature in the wavelength range 200 to
900 nm.
3 . R e s u l t s a n d d i s c u s s i o n
3 . 1 . X - r ay d i f f r ac t i on
X-ray diffraction is a quite useful technique because it
is possible to detect readily crystals in a glassy matrix
if the crystals are of dimensions greater than typically
100 nm [5]. The X-ray diffraction pattern of an amorph-
ous material is distinctly different from that of crystal-
line materia l and consists of a few broad diffuse haloes
15 nlol % CuO
25 m01% Cu0
1 d i f f r a c t i on o f t h re e
igure
X - r a y
p a t t e r n s
sa mpl e s o f c op pe r - t e l l u r i um ox i de g l as ses .
2500 0 0 2 2 - 2 4 6 1 / 8 8 $ 0 3 .0 0 + . 12
9 1988 Chapman and Hall Ltd.
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T A B L E I E x o t h e r m a l a n d e n d o t h e r m a l t e m p e r a t u r e t r a n s it i o n
p e a k s f o r s o m e g l a s s s a m p l e s u s i n g D S C i n t h e ra n g e 5 0 t o 6 0 0 ~
C u O E n d o t h e r m a l E x o t h e r m a l - A H
( m o i % ) t e m p ( ~ t e m p ( ~ ( j g - l )
15 356 410 88 .26
436 472 45 .30
25 352 396 14.79
428 476 54 .24
T A B L E I I C h e m i c a l c o m p o s i t i o n an d d e r iv e d pa r a m e t e r s o f
C u O - T e O 2 g l as s es
Cu O W E e , ( = 2 W ) Eop~ E o
( m o l %) ( e V ) ( e V ) ( e V ) ( e V )
15 0.8 1.60 - -
25 0.74 1.48 2.75 0.33
30 0.69 1.38 2.68 0.34
35 0.66 1.32 2.62 0.37
40 0.60 1.20 2.55 0.49
50 0.53 1.06 2.30 0.75
r a t h e r t h a n s h a r p r i n g s . A l l g l a s s s a m p l e s w e r e t e s t e d
a n d t h e r e s u l t s s h o w e d t h e a b s e n c e o f c r y s t a l li n e
c h a r a c t e r i s t i c s . F i g . 1 s h o w s t y p i c a l X - r a y d i f f r a c t i o n
p a t t e r n s f o r t h r e e c o m p o s i t i o n s , r e c o r d e d w i t h a
P h i l l i p s t y p e P W 1 0 5 0 d i f f r a c t o m e t e r , u s i n g a c o p p e r
t u b e a n d n i c k e l f i l t e r .
3.2. Thermal analysis
I n t h i s i n v e s t i g a t i o n t h e d i f f e r e n t i a l s c a n n i n g c a l o r -
i m e t r y ( D S C ) t e c h n i q u e w a s u s e d . A n i m p o r t a n t u s e
o f D S C i n g l as s e s i s t o m e a s u r e t h e g l a s s t r a n s i t i o n
t e m p e r a t u r e , T g . A s a m p l e a n d a n i n e r t r e f e r e n c e a r e
u s e d a n d t h e c e l l i s d e s i g n e d t o e n a b l e a d i r e c t q u a n -
t i t a t i v e m e a s u r e m e n t f o r t h e e n t h a l p y c h a n g e s t h a t
o c c u r i n a s a m p l e a s a f u n c t i o n o f e it h e r t e m p e r a t u r e
o r t i m e t o b e m a d e .
T h e t h e r m a l s t a b i l i t y o f t h e g l a ss e s w a s s t u d i e d
i n a P e r k i n E l m e r D S C 7 e q u i p m e n t f o r t w o c o m p o -
s i t i o n s i n t h e t e m p e r a t u r e r a n g e 3 1 3 to 8 7 3 K . T h e
h e a t f l ow t o t h e s a m p l e w a s m e a s u r e d u n d e r t h e r m a l l y
c o n t r o l l e d c o n d i t i o n s . D S C g a v e m u c h m o r e s e n si ti v e
r e s u l t s s h o w i n g t h a t t h e t r a n s i t i o n s s t a r t e d a t a l o w e r
t e m p e r a t u r e t h a n e x p e c t e d . F i g . 2 s h o w s t y p i c a l c u r v e s
o f th e t r a n s i t i o n s a n d T a b l e I g i v e s a s u m m a r y f o r t h e
g l a ss e n d o t h e r m a l p e a k v a l u e s , e x o t h e r m a l p e a k v a l u e s
a n d t h e a m o u n t o f h e a t a b s o r b e d b y t h e g la s s i n t h e
e n d o t h e r m a l t r a n s i t i o n s .
3 .3 . Optica l absorpt ion
A m a i n f e a t u r e o f t h e a b s o r p t io n e d g e o f a m o r p h o u s
s e m i c o n d u c t o r s , p a r t i c u l a r l y a t t h e l o w e r v a l u e s o f
a b s o r p t i o n c o e f f ic i e n t , i s a n e x p o n e n t i a l i n c r e a s e o f
t h e a b s o r p t i o n c o e f fi c ie n t e ( c o) w i t h p h o t o n e n e r g y
(h ~o ) i n a c c o r d a n c e w i t h a n e m p i r i c a l r e l a t i o n d u e t o
Urbach [6 ]
c~ = ~o ex p
( h c o / E o )
(1)
wh ere c~0 i s a co ns ta n t , E0 i s the U rba ch ene rgy wh ich
i n d i c a t e s t h e w i d t h o f t h e b a n d t a i l s o f t h e l o c a l i z e d
s t a te s a n d m i s t h e a n g u l a r f r e q u e n c y o f t h e r a d i a t i o n .
O p t i c a l a b s o r p t i o n m e a s u r e m e n t s w e r e m a d e a s
a f u n c t i o n o f p h o t o n e n e r g y a t r o o m t e m p e r a t u r e .
F i g . 3 s h o w s t h e a b s o r b a n c e i n a r b i t r a r y u n i t s a s a
f u n c t i o n o f t h e w a v e l e n g t h f o r g l a s s fi l m s o f d i ff e r e n t
c o m p o s i t i o n s .
T h e a b s o r p t i o n c o e f f i c ie n t c~ (co) c a n b e d e t e r m i n e d
n e a r t h e e d g e f r o m t h e f o r m u l a
1 I 0
~ ( c ~ ) = ~
tn 7
( 2 )
w h e r e d i s t h e t h i c k n e s s o f t h e s a m p l e , I 0 a n d I
a r e t h e i n t e n s i t i e s o f t h e i n c i d e n t a n d t r a n s m i t t e d
b e a m s , r e s p e c t iv e l y . T h e m o s t s a t i s f a c t o r y re s u l t s w e r e
o b t a i n e d b y p l o t t i n g t h e q u a n t i t y ( ~ 0 ) ) 1/2 a s a func-
t i o n o f ( hc o) a s s u g g e s t e d b y D a v i s a n d M o t t [ 7] . F o r
a b s o r p t i o n b y i n d i r e c t tr a n s i t io n s t h e e q u a t i o n t a k e s
t h e f o r m
~ ( c o ) = A ( h c o - E o p t ) 2 / h c o
(3)
w h e r e A i s a c o n s t a n t , E o p t i s t h e b a n d g a p a n d t h i s
40 -
30-
E
g
2 0
10-
~
.d
I I I
100 300
Tempe rature ( o C)
s ~ o
Figure 2 T y p i c a l D S C c u r v e s f o r t w o
c o p p e r - t e l l u r i u m o x i d e g l a s s e s . ( - - - )
2 5 m o I % Cu O , ( ) 1 5 t o o l % Cu O .
2501
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/
" / ~ . . _ I 9
=0.5, / .
- ]
~ i /
.~
o
. a ]
1 . 0 / ,
/ , ' /
I .
, /
I.
1.5.
4 0 0 5 0 0 6 0 0 7 0 0
Wavelength (nrn)
Figure 3 Absorban ce as a function of wav elength for three samp les
of copper-tel lurium oxide glasses. ( ) 25 mol % C uO , ( - - - )
35 mol % CuO , ( - . . . . . . ) 50 tool % CuO.
r e l a t i o n a p p l i e s t o m a n y o x i d e g l a ss e s , p a r t i c u l a r l y a t
t h e h i g h e r v a l u e s o f a b s o r p t i o n c o e ff ic i en t . F i g s 4 a n d
5 s h o w p l o t s o f ( e hc o) 1/2 a g a i n s t h e ) a n d t h e e q u i v a l e n t
U r b a c h p l o t i s p r e s e n t e d i n F i g s 6 a n d 7 in w h i c h t h e
a b s o r p t i o n c o e f f i c ie n t s a r e p l o t t e d a s f u n c t i o n s o f hc o
180
1 6 0
T
5140
3
120
100
I n l
(e)
/ I t I I
2 . 0 2 .5 3 . 0 3 . 5
r
l
/
/
/ a / ~ J
/ 1 o i
/ ~ / i
Photon energy eV)
F i g u r e 4 (~hco) /2 plotted against hco for three samples o f co ppe r-
te l lu r ium oxide g lasses. (a ) 50mo1% CuO, 2 .5#m; (b) 35m o1%
CuO, 2 .7#m; (c ) 25mo1% CuO, 2 .6#m.
(o) (b)
100 -
r a
>o
' E 8 0
6 O
/
/ /
/ /
I
I I I I
2.0 2.5 3 . 0 3,5
Pho ton e nergy (eV)
Figure 5 c~hc@/2
p lo t ted aga ins t he ) fo r two samp les o f copper -
te l lu r ium ox ide g lassesof the same th ickness, 6 .25 f i re. (a) 40 tool %
CuO, (b) 30 tool % CuO.
f o r d i f fe r en t c o m p o s i t io n s o f C u O - T e O 2 g l as se s . T h e
v a l u e s o f E ov d e t e r m i n e d f r o m F i g s 4 a n d 5 b y e x t r a -
p o l a t i n g t h e l in e a r p a r t s o f th e c u r v e s to (0~(D ) 1/2 = 0,
a n d t h e v a l u e s o f E 0 w h i c h a r e d e t e r m i n e d f r o m
F i g s 6 a n d 7 b y u s i n g E q u a t i o n 1 a r e t a b u l a t e d i n
T a b l e I I . I t is c l e a r f r o m F i g . 8 t h a t t h e o p t i c a l g a p E o p ,
d e c r e a s e s a s th e p r o p o r t i o n o f tr a n s i t i o n m e t a l o x i d e
C u O i s i n c r e a s e d , b u t o n t h e o t h e r h a n d t h e w i d t h o f
t h e b a n d t a i ls o f th e l o c a l i z e d s t a t e s is f o u n d t o
i n c r e a s e w i t h i n c r e a s i n g C u O c o n t e n t , a s s h o w n i n
F i g . 9 . T h e v a l u e o f E op t i s c o n s i d e r a b l y l a r g e r t h a n
t w i c e t h e e l e c t r i c a l a c t i v a t i o n e n e r g y , W .
3 . 4 . D . c .
c o n d u c t i v i t y
T h e d . c . e l e ct r ic a l c o n d u c t i v i t y o f C u O - T e O 2 g l a s s e s
w a s m e a s u r e d i n t h e t e m p e r a t u r e r a n g e 2 9 3 t o 45 3 K .
A l l m e a s u r e m e n t s w e r e t a k e n m o r e t h a n o n c e . T h e
9.0
'E
v
8.0
(a).
~
c)
I I I
2.0 2.5 3.0 3.5
Pho ton energy (eV)
Figure 6 Ln ~ plotted against he) for the three sa mples of Fig. 4.
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8~-
JE
2.0
( b )
I I I
2.5 3.0 3.5
Photon energy (eV)
Figure 7 L n e p l o t t e d a ga i ns t he ) fo r t he t w o sa mpl e s o f F i g . 5 .
d . c . c o n d u c t i v i t y , aa .. . i n c r e a s e s w i t h t e m p e r a t u r e a n d
a l s o w i t h t h e i n c re a s e in t h e c o n c e n t r a t i o n o f t r a n s i t i o n
m e t a l o x i d e C u O i n t h e g l as s , a s s h o w n i n F i g . 1 0 . A l l
g l as se s s h o w a s m o o t h v a r i a t i o n o f c o n d u c t i v i t y w i th
( 1 0 0 0 / T ) K - ~ . T h e d . c. c o n d u c t i v i t y i s a n e g a t i v e
e x p o n e n t i a l f u n c t i o n o f i n v er s e t e m p e r a t u r e a n d c a n
b e e x p r e s s e d b y t h e u s u a l e q u a t i o n
O-d.c. = a0 e x p - (4 )
w h e r e W i s t h e a c t i v a t i o n e n e r g y .
T h e a c t i v a t i o n e n e r g i e s w e r e c a l c u l a t e d f r o m t h e
s lopes o f the log ~d .c . aga in s t
I/T
c u r v e s a n d w e r e
f o u n d t o d e c r e a s e a s th e c o n c e n t r a t i o n o f t ra n s i t i o n
m e t a l o x i d e C u O i n c r e a s e s i n t h e g l a s s , a s s h o w n i n
F i g . I I . T h e v a l u e s o f t h e a c t i v a t i o n e n e r g y , W , a b o v e
r o o m t e m p e r a t u r e a r e t y p i c a l l y 0 .6 1 t o 0 .8 e V a n d a r e
2.8
2.6
~o
2.4
2.2
25 35 45
Cu0 con ten t ( mot % }
Figure 8 Fop
p l o t t e d a g a i n s t C u O c o n t e n t f o r c o p p e r - t e l l u r i u m
oxide glasses.
0.~
0.6
>=
0.4
0.2
I I
25 35 45
CuO content {tool o/o)
Figure 9 Eo p l o t t e d a g a i n s t C u O c o n t e n t f o r c o p p e r - t e l l u r i u m o x i d e
glasses.
c o n s i d e r a b l y l e s s t h a n h a l f t h e c a l c u l a t e d v a l u e s o f t h e
o p t i c a l g a p , a s s h o w n i n T a b l e I I .
T h e d . c . e l e c tr i c a l c o n d u c t i v i t y a n d t h e s h a p e o f th e
c u r r e n t - v o l t a g e c h a r a c t e r i st i c s w e r e s e n s it i ve t o t e m -
p e r a t u r e . T h e g l a s s e s s h o w o h m i c b e h a v i o u r a t l o w
f ie lds , a s s how n in F ig . 12 . As can b e s een f rom
F i g . 1 3 , th e r e s i s t a n c e o f C u O - T e O 2 g l a s s es i s i n d e -
p e n d e n t o f ti m e . T h i s c o u l d b e t a k e n a s e v id e n c e t h a t
t h e p o l a r i z a t i o n e f f e c t i n t h e s e g l a ss e s is m i n i m a l o r
a b s e n t a n d t h e e l e c t r ic a l c o n d u c t i o n w o u l d i n t h i s c a s e
b e d u e t o t r a n s p o r t o f e l e c tr o n s r a t h e r t h a n i o n s.
I 0 -6 -
•E
0-7
T
10 a
8
10-9
10-10 I ~ t
2.0 2.5 3.0 3.5
1000 /T (K -1 )
Figure 10 C o n d u c t i v i t y a s a f u n c t i o n o f i n v e r se t e m p e r a t u r e f o r f o u r
s a m p l e s o f c o p p e r - t e l l u r i u m o x i d e g la ss . ( a ) 1 5 m o 1 % C u O ,
(b ) 25 t oo l % C uO , ( c ) 30 t oo l % C uO , (d ) 40 t oo l % C uO.
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1.0
05
I 1 I i
15 25 35 ~5
CuO content (rno[ O/o)
Figure l l V a r i a t i o n o f a c t i v a ti o n e n e r g y , W , w it h C u O c o n t e n t o f
c o p p e r - t e l l u r i u m o x i d e g l a s s .
4 . D i s c u s s i o n
I t i s o f i n t e re s t t o d i s c u s s t h e f o l l o w i n g p o i n t s .
( i) T h e s h i f t o f t h e u l t r a v i o l e t a b s o r p t i o n b a n d t o
l o n g e r w a v e l e n g t h w i t h t h e i n c r e a s i n g C u O c o n t e n t .
( ii ) T h e v a r i a t i o n o f E o pt w i t h e l e c t ri c a l a c t i v a t i o n
e n e r g y .
(i ii ) T h e k i n d o f c o n d u c t i o n m e c h a n i s m w h i c h i s
l i k e l y to b e i n v o l v e d i n t h e s e m a t e r i a l s .
( i ) S t e v e l s [ 8 ] s u g g e s t s t h a t t h e m o v e m e n t o f t h e
u l t r a v i o l e t a b s o r p t i o n b a n d t o l o n g e r w a v e l e n g t h s
c o r r e s p o n d s t o a t r a n s i t i o n f r o m t h e n o n - b r i d g i n g
o x y g e n s w h i c h b i n d a n e x c i t e d e l e c t r o n l e s s t i g h t l y
t h a n a b r i d g i n g o x y g e n . A s c a n b e s e e n f r o m F i g . 3 th e
s h i f t o f t h e u l t r a v i o l e t a b s o r p t i o n b a n d t o l o n g e r
w a v e l e n g t h s w i t h i n c r e a s i n g C u O c o n t e n t c o u l d b e
e x p l a i n e d i n t h e s a m e w a y . T h u s t h e r e s u l t o f F i g . 8
s h o w i n g t h e v a r i a t i o n o f E op~ w i t h c o m p o s i t i o n c a n b e
e x p l a i n e d b y s u g g e s t in g t h a t t h e n o n - b r i d g i n g o x y g e n
(d)
_ _ c )
(b)
10-9 (a)
10-10
10- 1
10-12 I
10-13 [ i I
1 o 1o o 1o o o
Applied voltoge V )
Figure 12 V I c h a r a c t e ri s t ic s a t r o o m t e m p e r a t u r e o f t h e f o u r
s a m p l e s o f F i g . 1 0.
2 5 0 4
1 0 1 1
1 0 1 0
c
c ~
1 0 9
- - 2 0 C
5 0 ~ C
S
z ~- -- e ~ - e _- e
80 ~ C
1 2 0 ~
1 0 8 I l
I
1 2 3
T im e h )
Figure 13
Variation of resistance with tim e for a 30mo1% CuO
sample at different emperatures.
i o n c o n t e n t i n c r e a s e s w i t h i n c r e a s i n g C u O c o n t e n t ,
s h i f t i n g t h e b a n d e d g e t o l o w e r e n e r g i e s a n d t h u s
c o n s e q u e n t l y l e a d i n g t o a d e c r e a s e i n E o p t .
( ii ) As c an be seen f i ' om Tab le I I , th e so -ca l led
o p t i c a l g a p ( Eo pt ) i s t h r e e t o f o u r t i m e s l a r g e r t h a n t h e
h i g h - t e m p e r a t u r e a c t i v a t i o n e n e r g y . T h i s i s c o n s i s t e n t
w i t h t h e r e s u lt s o f m a n y w o r k e r s o n o x i d e a n d c h a l-
c o g e n i d e g l a s se s w h i c h s h o w t h a t t h e e l e c t r i c a l e n e r g y
g a p , s o m e t i m e s r e g a r d e d a s t w i c e t h e a c t i v a t i o n e n e r g y ,
W , is le s s t h a n t h e v a l u e o f th e o p t i c a l g a p o n t h e s a m e
g l a ss . T h e r e s u l t s s u g g e s t t h a t t h e e l e c t r o n i c a c t i v a t i o n
i s n o t a c r o s s t h e w h o l e m o b i l i t y g a p b u t i s p o s s i b l y
f r o m o n e o r m o r e t r a p p i n g l e v e l s t o t h e c o n d u c t i o n
b a n d o r f r o m b o n d i n g s t a t e s t o a t r a p p i n g l e v e l .
( ii i) T h e l o g o- a g a i n s t r e c i p r o c a l t e m p e r a t u r e p l o t
d i s p l a y s l in e a r b e h a v i o u r w h i c h i s t y p i c a l o f t h e s m a l l
p o l a r o n h o p p i n g m e c h a n i s m e x h i b i t e d b y 3 d a n d 4 d
t r a n s i t i o n m e t a l o x i d e g l a s s e s .
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