AMORPHOUS TRANSITION METAL OXIDES
J. Livage
To cite this version:
J. Livage. AMORPHOUS TRANSITION METAL OXIDES. Journal de Physique Colloques,
1981, 42 (C4), pp.C4-981-C4-992. <10.1051/jphyscol:19814215>. <jpa-00220844>
HAL Id: jpa-00220844
https://hal.archives-ouvertes.fr/jpa-00220844
Submitted on 1 Jan 1981
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JOURNAL DE PHYSIQUE
page C4-981
Colloque C4, suppl&ment a u nO1O, Tome 42, o c t o b r e 1981
AMORPHOUS TRANSITION METAL OXIDES
J . Livage
S p e c t r o c h i m i e du Solide (LA 3021, T.44
4, p l a c e J u s s i e u , 75230 Paris, France
Abstract
-
U n i v e r s i t d P i e r r e e t Marie Curie,
.-
The semiconducting p r o p e r t i e s o f T r a n s i t i o n : l e t a l Oxide (T.M.O.)
glasses
have been known f c r almost 30 y e a r s , s i n c e t'ne p i o n e e r i n g work o f Denton e t a i i n
1954 ( 1 ) . They have been e x t e n s i v e l y s t u d i e d and r e c e n t l y reviewed by Wackenzie
e t a 1 ( 2 ) ( 3 ) . T h e l r s p e c i f i c p r o p e r t i e s a r i s e from t h e f a c t t h a t t r a n s i t i o n m e t a l
i o n s may e x h i b i t s e v e r a l v a l e n c e s t a t e s ( v 4 + - ~ 5 + ,W ~ + - I @ )
so t h a t e l e c t r o n transf e r from low t o high v a l e n c e s t a t e s can t a k e p l a c e . T h i s e l e c t r o n t r a n s f e r c a n be
e i t h e r o p t i c a l l y o r t h e r m a l l y a c t i v a t e d and T.M.O. g l a s s e s w i l l e x h i b i t b o t h
o p t i c a l and e l e c t r i c a l p r o p e r t i e s .
E l e c t r o n exchanges i n mixed v a l e n c e compounds a r e commonly d e s c r i b e d by
u s i n g t h e diagramn shown on f i g u r e ( 1 ) . Such a diagrarnm g i v e s t h e p o t e n t i a l energy
o f an e l e c t r o n hopping between two m e t a l l i c i o n s A and B a s a f u n c t i o n o f a s i n g l e
c o n f i g u r a t i o n a l parameter q = q -60, c o r r e s p o n d i n g t o an a n t i s y m m e t r i c combination
a
o f t h e v i b r a t i o n a l modes o f
s i t e s A anu B ( 4 ) ( 5 ) .
The e l e c t r o n c a n be t h e r m a l l y t r a n s f e r e d from one s i t e t,o t h e o t h e r w i t h an a c t i v a t i c n energy E
= W g i v e n by t h e h e i g h t o f t h e p o t e n t i a l energy b a r r i e r s c p a r a t i n g t h e two
t h c o n f i g u r a t i o n s . It c a n a l s o be o p t i c a l l y e x c i t e d w i t h o u t moving
t h e i o n s ( i . e . a c c o r d i n g t o tt.e Franck-Condon p r i n c i p l e ) w i t h a n energy hv.
According t o t h e harmonic a p p r o x i m a t i o n , t h e a c t i v a t i o n e n e r g i e s o f t h e two process e s a r e r e l a t e d by 4 E
= 2
a s shown by Hush ( 6 ) .
th
opt
P i g . 1 - P o t e n t i a l e n e r g y diagram o f a n e l e c t r o n hopping between
two m e t a l l i c s i t e s .
a ) w i t h o u t any e l e c t r o n i c i n t e r a c t i o n J = 0
b ) with an e l e c t r o n i c i n t e r a c t i o n J # 0
E l e c t r o n exchange between A and 3 i o n s i s a c t u a l l y o n l y
o v e r l a p between t h e wave f u n c t i o n s Q A and Q i s d i f f e r e n t from
'the c r o s s i n g p o i n t
n i c i n t e r a c t i o n removes t h e degeneracy a t
energy c u r v e s l e a d i n g t o two energy l e v e l s separated. by 2J ( J =
The t h e r m a l a c t i v a t i o r i energy i s jhus lowered (Eth = W - J ) , and
t h e r m a l p r o c e s s e s a r e r e l a t e d by 2 E
= E
+ J (I+).
opt
th
possible i f the
zero. This electroof the potential
transfer integral).
t h e o p t i c a l and
Mixed v e l e n c e compounds w i l l e x h i b i t d i f f e r e n t b e h a v i o r s a c c o r d i n g t o t h e
v a l u e o f J , l e a d i n g t o t h e w e l l known c l a s s i f i c a t i o n s w z e s t e d by Robin and Day(7).
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814215
C4-982
JOURNAL DE PHYSIQUE
KO exchange occurs when J = 0, valence states remain localized and the corresponding
compounds behave as insulators (class I). Electron hopping between localized states
is observed when J is small (class 11). These compounds usually exhibit a deep blue
color due to intervalence transfer (photochromism of amorphous WO ) together with
semiconducting properties via a hopping process (vanadate glasses)? Delocalized
states occur when J becomes large enough (class 111) leading to metallic conductivity as in the well known tungsten bronzes Iqa NO
X 3'
d-overlap is usually quite small in T.M.O. glasses, so that most of these
compounds belong to the class 11. The most thoroughly investigated systems are the
vanadate glasses for their semiconducting properties (8)(9)and the amorphous WO
thin films for their electrochromic behavior (10)( 1 1 ) These materials are usual?y
obtained as bulk glasses by adding a glass former or as thin film by sputtering or
vacuum evaporation. Their properties will be reviewed in the first two parts of
this paper. Amorphous T.M.O. such as V 0 or WO have recently been obtained as
"gels" by chemical polymerization (12)?
aain interest arises from the fact
that they can easily be deposited onto a substrate giving semiconducting layers of
large area (13), leading to new applications in the field of amorphous T.M.O. (14).
These mixed valence gels have mainly been studied by the french "groupe des gels"
during the past two years. They exhibit optical and electrical properties arising
from electron transfer between transition metal. ions. The main results will be
reviewed in the last part of this paper.
.
heir
-
SEVICONDUCTING PROPERTIES OF T.M.O. GLASSES
T.M.O. glasses can be obtained by quenching from the melt. A glass former
has to be added to the molten oxide and most studies are dealing with binary systems based on P 0 : P 0 -V 0 (16-lg), P205-WO (20) P 0 -MOO (21). Many other
CUO,~
':~~O,~COO,
NiO,. . ) . They
studied ( F ~ o ,Tia
T.M.O. have bee2 'also2
Other glass formers such as
have been recently reviewed by Mackenzie et a1
Te02 or ge02 can also been used (15)(22). Despite the fact that conduction in T.M.O.
malnly depends on electron hopping between metallic ions, the nature
glasses
of the glass former appears to be very important too (22). The electrical conductivity of V 0 -TeO glasses for instance is 3 orders of magnitude greater ( I o - ~ Q-'
2
cm-1 at
5room temperature) than that of V 0 -P 0 glasses containing similar
5have also been reported to
vanadium concentrations (15). V 0 -ge02 glasgez
(34).
exhibit greater conductivities
' '
(5;.
.
'
Amorphous VsOI1 WOI or MOO thin films can be obtained by vacuum evaporation
(23)(24), r.f. sput e lng 25)(~6)~0r C.V.D. (27). Small ribbons of amorphous V 0
2 5
have been obtained by splat-cooling from the melt using the roller-quenching
method previously developped for metallic glasses ( 2 8 ) .
Two localization processes occur in T.M.O. glasses. The first one arises
from the strong interaction of the unpaired electron with the polar network. It
leads to a polarization of the lattice and a displacement of the oxygen ions around
the low valence transition metal ion. In T.M.O. glasses, these distorsions are
usually limited to the nearest neighbours and the unpaired electron becomes selftrapped in its own potential well. The electron and its accompanying polarization
cloudmay be treated as a pseudo-particle called a "small polaron". Present treatments of small polarons in semiconducting T.M.O. glasses are based mainly on the
theoretical work of Mott (28) and Austin and Mott ( h ) . According to these anthors,
the small-polaron binding energy W in an ionic lattice can be expressed as :
W = e2/2 c rp ; rp is the small 'polaron radius. cp = ( I/E, - 1 / ~ ~ ) -where
~ , Es
respectively the static and high-frequency dielectric constants
P and E- ape
of the
glass. The semiconducting properties of T.M.O. glasses are due to thermally activated hopping of small polarons between metallic sites, as shown on figure
(2). Let us suppose that the unpaired electron is initially trapped on site A
(fig. 2a). The hopping process will be thermally activated by the vibrations of the
lattice. The smallest activation energy corresponds to the configuration (2b), when
two adjacent wells have the same depth. It can be shown that the energy W necessary
to produce such a configuration corresponds to half the polaron binding Henergy :
WH = 1 W (4). In the state (2b), the electron can tunnel from A to B leading to a
'lowering of the potential energy in ~(3b).Typical values for WH in T.M.O.
Fig.2
-
P o l a r i z a t i o n w e l l s f o r two
t r a n s i t i o n a e t a l ions during
t h e s n a l l p o l a r o n hopping i n a
M O g l a s s ( a f t e r Mott ( 1 1 ) )
W
= e
2
2
/E
p
r
2
: Polaron binding
energy
a : b e f o r e hopping,
b : thermally activated
t r a n s i ~ i o ns t a t e
c : a f t e r hopping
A
B
range jetween 0 . 2 end 0.6 CV ( 2 ) .
A second l c c a l i z a t i o n p r o c e s s o c c u r s i n T.M.O. g l a s s e s . It a r i s e s from t h e
s t r u c t u r a l d i s o r d e r and c o r r e s p o n d s t o t h e w e l l known "Anderson l o c a l i z a t i o n " .
An a d d i t i o n a l t e r m WD , c o r r e s p o n d i n g t o t h e random d i s t r i b u t i o n o f t h e p o t e n t i a l
energy among t h e
' i r a n s i t i o r ! n e t a l s i t e s , h a s t o , b e t a k e n i n t o a c c o u n t . The
a c t i v a t i o n energy f o r hopping t h e n becomes W = W + L W i n t h e h i g h t e m p e r a t u r e
li
2 U
regime ( 4 ) .
Following MoLt ( h ) , t h e d . c . e l e c t r i c a l c o n d u c t i v i . t y i n T.M.O.
usually expressed a s :
voe?
U = c ( l - c ) exp(-2ai3) exp-(W/kT)
( 1)
g l a s s e s is
,<,,,,
where : v i s a phonon Yrequency r e l a t e d t o t h e llebye temperature 0 by hVo = k0.
R i s t h e 'average hoppir~g,d i s t a n c e , c i s t h e r a t i o o r low v a l e n c e s t a t e s r e p o r t e d
t o t h e t o t a l 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 i o n s ; a i s t h e r a t e o f t h e wave
f u n c t i o n decay. I t c c r r e s p o n d s t o a t u n n e l l i n g t r a n s f e r .
A s shown by e q ( l ) , t h e c o n d u c t j . v i t y depends on t h e c o n c e n t r a t i o n c o f low
v a l e n c e t r a r i o i t i o n m e t a l i o n s . I t s h o u l d go t h r o u g h a m a x i m f o r c = 0 . 5 . Such a
henavior i s a l m o s t never observed and r e g o r t e d maximum f o r V 0 g l a s s e s l i e between
0.1 and 0 . 2 ( l 5 ) ( 1 7 ) . Various e x p l a n a t i o n s have been prcpose?i,5taking i n t o account
t r a p p i n g e f f e c t s ( i 7 ) o r p o l a r o n - p o l a r o n i n t e r a c t i o n s ( 6 ) ( 9 ) . I t seems morc p r o b a b l e
Lhat t h e observed d i s c r e p a n c i e s a r i s e from phase s e p a r a t i o n i n t h e g l a s s , b u t
e x p e r i m e n t a l evidence o f t h i s e f f e c t i s n o t abundant ( 3 0 ) .
T!!e importance OS t h e t u n n e l l i n g term e x p ( - 2 a ~ ) i s n o t o b v i o u s . Sayer and
Mansingi; (8) have shown t h a t i t was a l m o s t c o n s t a n t f o r WO -i' 0 and V 0 -P 0
3 2 5
g l a s s e s . Wether t h i s r e s u l t can be e x t r a p o l a t e d t o o t h e r T.M.O. g l a s s e z 5is2 5a
s u b j e c t o f controversy.Many a t t e n p t s h a v e b e ? n made t o c a l c u l a t e CI and r e p o r t e d
v a l u e s r a n g e octween 0.14 and 4 A ( 2 ) .
The most d i f f i c u l t problem i n t h e a n a l y s i s o f c o n d u c t i o n p r o c e s s e s i n T.X.O.
g l a s s e s , would b e t o s e p a r a t e t h e measured a c t i v a t i o n energy W i n t o a p o l a r o n term
W. ar.2 a d i s o r d e r term WD. According t o Schnakerlgerg ( 3 1 ) , a t high t e m p e r a t u r e
('~>0/2), t h e small p o l a r o n hopping i s a c t i v a t e d by a n o p t i c a l multiphonon p r o c e s s
1
and t h e a c t i v a t i o n energy i s g i v e n by W = W + - W
As t h e t e m p e r a t u r e i s lowered,
I1
t h e phonon spectrum f r e e z e s o u t and t h e o b s e r v e d 2 a c ? i v a t i o n energy c o n t i n u o u s l y
d e c r e a s e s . An a c o u s t i c a l phonon a s s i s t e d hopping t a k e s p l a c e a t low t e m p e r a t u r e
1
(T<0/1)and t h e a c t i v a t i o n energy becomes W = - W A non l i n e a r p l o t o f l n ( u T )
2 g l i i s e s i s reported on f i g . ( 3 ) .
v e r s u s T-l t y p i c a l o f s m a l l p o l a r o n s i n T.M.O.
.
JOURNAL DE PHYSIQUE
F i g . 3 - Temperature dependence
of the conductivity of
amorphous V205 o b t a i n e d
by s p l a t c o o l l n g ( 2 8 ) .
I n t h e l i m i t W , A u s t i n and Gamble ( 3 2 ) s t a t e d t h a t h' = WD, b u t Mott ( 2 9 )
p o i n t e d o u t t h a t a t v e r y low t e m p e r a t u r e , t h e observed v a l u e o f W s h o u l d approach
D
z e r o and a v a r i a b l e r a n g e hopping p r o c e s s s h o u l d t a k e p l a c e .
Due t o t h e h i g h r e s i s t i v i t y o f T.M.O. g l a s s e s , t h e low t e m p e r a t u r e regime can u s u a l l y n o t b e observed and c o n d u c t i v i t y measurements have r a r e l y been performed below
77K. An e x a c t e s t i m a t i o n of W i n t h e n r a t h e r d i f f i c u l t . An e v a l u a t i o n c a n be made
from :[iller-Abrahams t h e o r y ( 3 3 ) f o r i m p u r i t y c o n d u c t i o n i n doped and compensated
R
K i s a constant
semiconductors. The d i s o r d e r t e r m i s g i v e n by W = K ~ ~ / E ? where
o f o r d e r 0 . 3 . Such a c a l c u l a t i o n l e a d s t o W "0.y eV i n t 6 e c a s e o f V 0 -P 0
g l a s s e s ( 1 8 ) . One o f t h e b e s t method f o r
D e s t i m a t i n g WD would b e
S
'from
low t e m p e r a t u r e measurements o f t h e t h e r m o e l e c t r i c power a s r e p o r t e d f o r V20 -Te02
g l a s s e s where a v a l u e o f WD = 0.02 eV h a s been found ( 1 5 ) . D i r e c t
of
W at v e r y low t e m p e r a t u r e (below 4 0 ~ )have r e c e n t l y been performed by B u l l o t e t a1
(?3) o n amorphous V20 l a y e r s d e p o s i t e d from g e l s . They w i l l be d e s c r i b e d l a t e r i n
t h e s e c t i o n devoted tz T.M.O. g e l s .
measure men?.^
T a b l e I summarizes some o f t h e main d a t a s p u b l i s h e d i n t h e l i t e r a t u r e f o r
g l a s s e s . As c a n b e seem t h e c o n d i t i o n s f o r a p p l y i n g s m a l l p o l a r o n t h e o r y ,
< R , a r e u s u a l l y f u l f i l l e d and t h e electron-phonon c o u p l i n g p a r a m e t e r y i s
quitePhigh (y>10).
T.M.O.
T.M.0 g i a s s
V 0 -P 02 5 2 2
V 0 -Te02
2 5
WO3 -P205
MOO -P O
i
3 2 5
TiO-PO
2 2 5
W
(ev)
W
a(?)
D(ev)
0.29-0.42
0.1-0.4
0.25-0.34
0.02
0.29-0.35
0.1
0.05
1.7
0.5-0.69
0.48-0.54
2.1-2.9
2.9-4
ref.
8,18,19
0.97
15
l .2
2.h-2.8
35
1-2
0.45-0.8
36
37
-
ELECTROClIROMISM IN AMORPXOUS T.M.O. THIN FILMS
Photochromism and electrochromism of amorphous T.M.O. have received much
attention during the last decade. Most of the studies are dealing with amorphous
WO thin films obtained by vacuum evaporation or r.f. sputtering. Such films could
3 considerable application in digital display devices
have
(10). The first paper was
published in 1973 by Deb (38) who reported that amorphous WO3 could exhibit two
stable states. One is transparent and highly resistive while the other is deep blue
and less resistive. Coloration can be obtaincd by optical irradiation, application
of an electric field, or proton injection from an electrolyte ( 1 1 ) .
Photochromism occurs when the film is U.V. irradiated (hv>3.4 eV) for several hours (38). The blue color develops gradually and grows nore intecse with prolonged exposure (39)(40). This process is not reversible and the blue coloration
remains after exposure.
Electrochromism is performed by application of an electric field of about
10 V/cm. The blue color first appears at the cathode and then propagates toward
the anode. This coloration persists after the current has been removed, the transparent state being restored by inversing the polarity of the current. The blue
color then migrates back and disappears into the anode while new coloration begins
at the cathode (38).
These two processes are rather long and cannot therefore be applied for
display devices. Most of the studies are now dealing with electrochemichromism
(10). The WO thin film is placed into contact of a suitable electrolyte, usually
3
and acid aqueous
solution or gel (41 ) , in an electrochemical cell (figure 4).
Fig. 4
-
Electrochemichromism of
amorphous T.M.O. thin films.
Coloration WO
counter electrode
I
3
+ If
+ + e-
->
H WO
X 3
(blue)
(transparent)
Bleaching HxWo3
+
-> W 0 3 + H
+ e
electrolyte
(blue)
(transparent)
The application of a negative potential (>0.7 V) causes the WO film to turn deep
blue within 0.5-1.0 seconds. The blue state remains after the 3voltage has been
removed but the initial transparent state can be restored by applying a reverse
+ 0.8 V potential. The coloring and bleaching cycles have been repeated more than
106 times without significant degradation (41). Non aqueous solvents (propylene
carbonate) with LiC104 electrolyte can also been used, but the coloring process is
slower owing to the lower rate of diffusion of ~ i +
(40)(42). Display devices based
on solid state electrolytes have recently been proposed (43).
C4-986
JOURNAL DE PHYSIQUE
I t i s now well. a d m i t t e d t h a t t h e b l u e c o l o r a t i o n o f u o r p h o u s U0 t h i n f i l m s
3
a r i s e s f r o m a b r o a d a b s o r p t i o n band c c n t e r c d a r o u n d 900 nm ( 1 . 3 8 e V ) ( k 3 ) . But a
controversy remains about t h e d e s c r i p t i o n o f t h e c o l o r a t i o n and bleaching p r o c e s s e s
i n t h e s e f i l m s . S p e c t r o s c o p i c e x p e r i m e n t s (=S,
ESH
) show t h a t Lpon c o l o r a t i o n
i s o l a t e d W?+ i o n s a r e formed w h i l e t h e y d i s a p p e a r upon ' c l e a c h i n g ( 4 3 ) ( 4 4 ) . Those
o b s e r v a t i o n s r u l e o u t t h e f i r s t mechanism p r o p o s e d by Deb, s u g g e s t i n g t h c f o r m a t i o n
of F c o l o r c e n t e r ( 3 8 ) .
...
Infra-red
e x p e r i m e n t s show
t h e presence o f water molecules adsorbed on
UO, t h i n f i l m s . Therefore Char~gc t a1 ( 1 0 ) su6:gested t h e f o r m a t i o n o f nons t o f c h i o m e t r i c V10
3
a c c o r d i n g t o t h e f o l l o w i n g mechanism :
The c u r r e n t l y a d o p t e d model f o r e l e c t r o l y t i c c o l o r a t i o r . i s t h a t o f a d o u b l e i n j e c t i o n p r o c e s s i n which t h e n e g a t i v e c h a r g e c a r r i e r s a r c e l e c t r o n s w h i l e IY+ i o n s
( M = H,Li) a r e t h e c h a r g e compensators ( 1 0 ) ( 3 9 ) . A t u n g s t e n brorize v o u l d t h e n be
formed by p r o t o n i n j e c t i o n d u r i n g t h e c o l o r a t i o n p r o c c s s ( 4 6 - 9 )
6+ .
lons.
Thc b l u e c o l o r a t i o n a r i s e s from i n t e r v a l e n c e t r a n s f e r s between W?+ a n d W
The small p o l a r o n model c o u l d a p p l i e d t o t h e s e b l u e ainorphcus t h i n f i l m s .
E l e c t r i c a l c o r i d u c t i v i t y measurerncnts show t h a t t h e y e x h i b i t s e m i c o n d u c t i n g p r o p e r t i e s d u e t o a h o p p i n g p r o c e s s betweer] l o c a l i z e d s t a t e s . C o n d ! ~ c t i v i t y d e p e n d s on
t h e number o f c h a r g e c a r r i e r s and t h e a c t i v a t i o n e n e r g y d e c r c a s c s s s t h e n m b e r
o f i n j e c t e d p r o t o n s i n c r e a s e s . Faughnan e v e n found a n i n s u l a t o r - m e t a l t r a n s i t i o n
a t X = 0 . 3 2 when h y d r o g e n c o r i c e n t r a t i o n i n c r e a s e s from 0 t o 0 . 5 ( 5 0 ) .
A n o t h e r d e s c r i p t i o r ~o f t h e c o l o r e d a n d b l e a c h e d s t a t e s h a s b e e n r e c e n t l y
pushed f o r w a r d by D e n e u v i l l e e t a1 ( 5 1 ) . They snowed t h a t c o l o r a t i o n c f t r a n s p a r e n t
v i r g i n WO f i l m s c o u l d t)c o b t a i n c d w i t h o u t a n y p r o t o n i n j e c t i o n , by a n n e a l i n g u n d e r
O films t o
vacuum o r 3 u . v . i r r a d i a t i o n . A c c o r d i n g t o t h c s e a u t h o r s , t h e a b i l i t y o f W
u n d e r g o 3.V. c o l o r a t i o n r e q u i r e s b o t h a d e f i c i e n c y o f oxygcn a n d t h e p r e s 2 n c e o f
hydrogen i n t h e i r t r a n s p a r e n t s t a t e . The c o l o r a t i o n was assumed t o o r i g i n a t e from
l o c a l t r a n s f e r o f hydrogen from p a s s i v e t c n c t i v e s i t e s i n t h e m a t e r i c l . A b i p o l a r o n model was t h e n p r o p o s e d suggesting p a i r e d d i a m a g n e t i c ~ 5 ~ ioris ( a s s e e n by
X-ray p h o t o e m i s s i o n ) i n t h e t r a n s p a r e n t s t a t e and i s o l a t e d v5+ p a r a m a g n e t i c i o n s
i n t h e b l u e one.
D e s p i t e t h e c o n t r o v e r s y a b o u t t h e c o l o r a t i o n nechanion; i.n morphous t h i n
f i l m s , t h e i r almost i n s t a n t a n e o u s r e v e r s i b l e coloratiorl-bleaching p o s s i b i l i t i e s open
c o n s i d e r a b l e a p p l i c a t i o n s as d i g i t a l d i s p l a y d e v i c c s .
They a r e i n e x p e n s i v e a n d c o m p a c t , a n d c a n b e e a s i l y s e e n i n b r i g h t s u n l i g h t .
C o l o r a t i o n o c c u r s most e f f i c i e n t l y when t h e f i l m a r e a n o r p h o u s r a t h e r t h a n c r y s t a l l i n e . I t a p p e a r s a l s o t o be s e n s i t i v c t o m b i e n t c o n d i t i o n s , mainly t h e presence
o f m o i s t u r e a n d t h e t e m p e r a t u r e ( a maximum i s o b s e r v e d arourid 80°C).
- T3ANSITION METAL OXIDE GELS
...
Amorphous T.M.O. (V 0
W
O , MOO
) h a v e r e c e n t l y been o b t a i n e d as g e l s
by c h e m i c a l p o l y m e r i z a t i o n
5 i 1 2 ) ? l'his3polymerization o c c u r s through a condensat i o n p r o c e s s i n which w a t e r m o l e c u l e s a r e e l i m i n a t e d . Vanadium p e n t o x i d e g e l s f o r
i n s t a n c e c a n b e formed by p o l y m e r i z a t i o n o f v a n a d i c a c i d s a c c o r d i n g t o t h e f o l l o -
Polyvanaciic a c i d s o l u t i o n s a r e o b t a i n e ? by i o n e x c n z n g e i n a r e s i n e from
sodium m e t a v a n a d a t e s o l u t i o n s ( 5 2 ) . The f r e s h l y p r e p a r e d a c i d i s y e l l o w c o l o r e d
and d e c a c o n d e n s e d . High polymers a r e s p o n t a n c o u s l y formed at room t e m p e r a t u r e .
Dark r e d c o l l o i d a l s o l u t i o n s o r g e l s a r e o b t a i n e d , d e p e n d i n g o n t h e vanadium conc e n t r a t i o n C. C e l a t i o n o c c u r s f o r C>0.1 "01.1-'. O t h e r s mcthods o f p r e p a r a t i o n h a v e
been d e v e l o p p e d . V 0 g e l s c a n a l s o been o b t a i n e d by q u e n c h i n g t h e m o l t e n o x i d e
d i r e c t l y i n t o wate? 'or by h y d r o l y s i s o f v a n a d i c e s t e r s ( 1 3 ) .
A g e l i s a n i n t e r m e d i a t e s t a t e o f t h e m a t t e r , l y i n g between s o l i d s a n d
l i q u i d s . Vanadium p e n t o x i d e g e l s f o r i n s t a n c e a r e made o f e n t a n g l e d p o l y m e r i c
vanadium-oxygen f i b r e s , s t r o n g l y a s s o c i a t e d w i t h w a t e r m o l e c u l e s .
E l e c t r o n m i c r o g r a p h o f a g e l shows t h a t t h e s e f i b r e s a r e a b o u t 1 m i c r o n l o n g a n d
5 0 t o 100 A wide f i g . ( 5 ) .
Fig. 5 - Electron micrograph o f
a V20 g e l showing t h e
fibrozs texture of t h i s
i n o r g a n i c polymer.
The main i n t e r e s t o f t h e s e g e l s i s t h a t t h e y behave l i k e a p a i n t i n g and c a n
be e a s i l y d e p o s i t e d o r sprayed o n t o g l a s s o r polymeric s u b s t r a t e s g i v i n g l a y e r s o f
l a r g e a e r a . A f t e r d r y i n g , r a t h e r h a r d c o a t i n g s a r e o b t a i n e d t h e t h i c k n e s s o f which
r a n g e s from 500 t o 10.000 X.
T h e s e T.M.O. g e l s a r e m i x e d - v a l e n c e compounds. They e x h i b i t o p t i c a l a n d
e l e c t r i c a l p r o p e r t i e s d u e t o e l e c t r o n t r a n s f e r between m e t a l l i c i o n s . T h e s e prop e r t i e s have been s t u d i e d d u r i n g t h e l a s t two y e a r s by f r e n c h g r o u p s . We s h a l i
f o c u s h e r e o n l y o n t h e s e m i c o n d u c t i n g p r o p e r t i e s o f amorphous V205 l a y e r s , b u t
electrochromisn o f W
O g e l s is a l s o under study.
3
JOURNAL DE PHYSIQUE
- SEiflICONDUCTING COATINGS
The semiconducting p r o p e r t i e s o f amorphous V 0 l a y e r s d e p o s i t e d from g e l s
have been s t u d i e d by B u l l o t e t a 1 ( 1 3 ) . L i n e a r c u r r e n z - q o l t a g e c h a r a c t e r i s t i c s were
o b t a i n e d and t h e t e m p e r a t u r e dependence o f l n ( a T ) between 300;: and 20i; i s shown on
figure (6).
Temperature dependence o f a:
V 0 layer deposited
fro;
( 13).
T h i s c u r v e i s t y p i c a l o f s m a l l p o l a r o n conduction i n amorphous T.i.1.O. T'ne room
t e m p e r a t u r e c o n d u c t i v i t y a p p e a r s t o be s u r p r i n s i n g l y h i g h , around 1 2-1 cm-l.
Such a v a l u e i s much g r e a t e r t h a n 1;hose a l r e a d y r e p o r t e d f o r V 0 based g l a s s e s
and even f o r c r y s t a l l i n e V 0 ( T a b l e 11). The meagured conduct?v?ty d o e s n o t
depend on t h e t h i c k n e s s o f 2 t z e l a y e r (600 t o 104 A ) , s o t h a t we may a s c e r t a i n t h a t
we a r e d e a l i n g w i t h b u l k c o n d u c t i v i t y r a t h e r t h a n s u r f a c e e f f e c t s .
T a b l e I1
3001;
(Q-' cm-1 )
Crystalline V 0
2 5
Vapor d e p o s i t e d V205
r.f.sputteredV0
2 5
V 0 -P 0 g l a s s e s
2 5 2 5
V 0 -Te02 g l a s s e s
2 5
V 0 gels
2 5
10-2- 0-4
w(ev)
3eference
0.21
53
o - ~
0.66
23
IO-~-IO-~
0.7
25
0.32-0.44
17
0.25-0.32
15
1
10-~-10-~
1o
-~
1
0.17
13
The r e a s o n o f such a high c o n d u c t i v i t y i s n o t y e t c l e a r l y understood. It
4 p a r t l y a r i s e s from t h e high V + / V r a t i o ( a b o u t 0 . 0 6 ) . A p o s s i b l e i o n i c c o n d u c t i v i t y
due t o w a t e r had t o be examined, t h e r e f o r e , t h e l a y e r h a s been a n n e a l e d i n a i r a t
160°c f o r 16 h i n o r d e r t o remove p h y s i c a l l y adsorbed w e t e r . The c o n d u c t i v i t y o f
t h e a n n e a l e d sample d e c r e a s e d i r r e v e r s i b l y by a f a c t o r o f 1,5. The a u t h o r s t h e n
concluded t h a t t h e high c o n d u c t i v i t y of t h e V 0 g e l was not due t o an i o n i c conduct i o n of water, but r a t h e r t o t h e i n t r i n s i c na$u?e o f t h e m a t e r i a l ( 1 3 ) .
Such, a high c o n d u c t i v i t y , t o g e t h e r w i t h t h e p o s s i b i l i t y of making e a s i l y
l a y e r s o f l a r g e a e r a l e d an i n d u s t r i a l company t o p a t e n t t h e s e g e l s i n o r d e r t o use
t h e n a s a n t i s t a t i c c o a t i n g s d e p o s i t e d on t h e back of photographic f i l m s ( 1 4 ) .
Such c o a t i n g s appear t o e x h i b i t a much b e t t e r behavior toward moisture t h a n t h e
usual polymeric a n t i s t a t i n g c o a t i n g s ( 5 4 ) .
The high c o n d u c t i v i t y of t h e amorphous V205 l a y e r s deposited from g e l s
allowed B u l l o t e t al ( 5 5 ) t o perform c o n d u c t i v i t y measurements down t o 28ii f i g . ( 6 ) .
They could t h e n reach t h e low temperature regime ( ~ < 0 / 4 where,
)
according t o
Schnakenberg ( 3 1 ) an a c o u s t i c a l phonon a s s i s t e d hopping p r e v a i l s , l e a d i n g t o an
W
A l i n e a r p l o t i s a c t u a l l y found below 4 3 ~ ,g i v i n g t h e
a c t i v a t i o n energy W =
f i r s t d i r e c t measureme& Dof WD i n amorphous T.M.O. (WDhO.l elr).
4+
E l e c t r o n m o b i l i t y i n V 0 g e l s has a l s o been s t u d i e d by E.S.R. A well r e s o l v e d V
hyperfine s t r u c t u r e is2 5observed up t o room temperature f i g . ( 7 ) , showing t h a t t h e
charge c a r r i e r s a r e l o c a l i z e d on vanadium s i t e s . Their hopping frequency remains
smaller t h a n t h e ESR l i n e w i d t h ( i . e . vh<lOO MHZ) up t o 300K.
.
Fig. 7
-
.
l
The most s t r i k i n g f e a t u r e of t h e
ESR s p e c t r a of V 0 g e l s comes
from t h e i r s t r o n g 5frequency
dependence f i g . ( 7 ) . T h i s has been
a t t r i b u t e d t o a d i s t r i b u t i o n of g
values among t h e d i f f e r e n t vanadium s i t e s , a r i s i n g from f l u c t u a t i o n of t h e l o c a l c r y s t a l f i e l d A
( 2 8 ) . These two q u a n t i t i e s a r e
r e l a t e d by g = g (l-nX/A) where
g = 2.0023, X i g t h e s p i n - o r b i t
c$upling c o n s t a n t and n a paramet e r depending on t h e ground s t a t e
wave f u n c t i o n and t h e magnetic
f i e l d o r i e n t a t i o n . An a n a l y s i s
of t h e ESR l i n e w i d t h a s a funct i o n o f t h e microwave frequency
(hv = L OH) l e d t o a v a l u e of t h e
crystal f i e l d fluctuations
dA = 0.08 eV. These random f l u c tuations contribute t o t h e disorder term WD involved i n t h e
a c t i v a t i o n energy f o r hopping
and t h e bA value found by ESR
appears t o be i n good agreement
with t h e W value deduced from
conductivi?y measurement S.
SWITCHING DEVICES
Non d e s t r u c t i v e breakdown,otherwise c a l l e d switching has been observed i n
a wide v a r i e t y of amorphous m a t e r i a l s s i n c e it was discovered i n 1959 by Ovshinsky
i n chalcogenide g l a s s e s ( 5 6 ) . The s u b j e c t has been reviewed r e c e n t l y by F r i t z s c h e
( 5 7 ) . Under t h e l o o s e l y d e f i n e d name o f "switching" a v a r i e t y of phenomena a r e
referred t o :
a ) negative r e s i s t a n c e device, b ) negative r e s i s t a n c e with memory, c ) switching,
d) switching with memory ( 5 7 ) .
They a l l occur when t h e v o l t a g e V a p p l i e d a c r o s s a t h i n f i l m exceeds a t h r e s h o l d
value Vt. A t y p i c a l Intensity-Voltage c h a r a c t e r i s t i c o f a switching device ( t y p e c )
i s shown on f i g u r e ( 8 ) . For V<V
t h e device i s i n t h e high r e s i s t a n c e "OFF" s t a t e
while f o r V>Vt it switches t o F;e low r e s i s t a n c e "ON" s t a t e . When t h e c u r r e n t i s
JOURNAL DE PHYSIQUE
C4-990
d e c r e a s e d below t h c h o l d i n g c u r r e n t l , t h e d e v i c e s w i t c h e s back t o i t s o r i g i a 1 OFF
s t a t e . Under p u l s e c o n d i t i o n s , some Hamorphoils a l l o y s have been o p e r a t e d 1 0 l e t i m e s
without f a i l u r e .
On state '
\
X
\
Off state
"t
Potential (V)
- rlg. 8 -7.
S w i t c h i n g i n amorphous semiconductors
The t h e o r i c a l d e s c r i p t i o n o f
s w i t c h i n g h a s been s u b j e c t o f much
c o n t r o v e r s y o v e r t h e p a s t few
y e a r s . Following some a u t h o r s ,
s w i t c h i n g i s due t o t h e r m a l i n s t a b i l i t y ( 5 7 ) whereas o t h e r s p u t
forward a double i n j e c t i o n phenomenon ( 5 8 ) ( 5 9 ) . S w i t c h i n g h a s been
observed i n o x i d e g l a s s e s ( 6 0 ) and
i n p a r t i c u l a r i n T.M.0 g l a s s e s
c o n t a i n i n g W , V , .Q, Fe and ~ ~ ( 6 1 ) .
V 0 -P 0 g l a s s e s o f v a r i o u s com2 5 . 2 5
p o s x t l o n s were found t o have v e r y
s t a b l e s w i t c h i n g v o l t a g e s and low
h o l d i n g c u r r e n t s ( 6 2 ) ( 6 3 ) , whereas
CuO-V 0 -P 0 g l a s s e s were found
capab?e5
50f p a s s i n g a c u r r e n t
o f 1 A i n t h e ON s t a t e w i t h o u t
p h y s i c a l damage ( 6 4 ) . I n such
d e v i c e s , t h e OFF t o ON r e s i s t a n c e
r a i o t y p i c a l l y r a n g e s between
,OS-107. Depending o n t h e a p p l i e d
power, V 0 -P 0 g l a s s e s were
shown t o
5exhibit e i t h e r
t h e r e s h o l d o r memory s w i t c h i n g
(65)(66).
Up t o now, s w i t c h i n g i n pure amorphous V 0 h a s not been r e p o r t e d .
T h e r e f o r e , B u l l o t e t al, t e s t e d t h e s w i t c h i n g c a p2a b5l l .l t y o f amorphous V 0 l a y e r s
2 5
d e p o s i t e d from g e l s . Samples about 1 IJrn t h i c k were d e p o s i t e d o n t o microscope
slides
and g o l d e l e c t r o d e s 100 urn a p a r t were vacuum e v a p o r a t e d i n a c o p l a n a r geometry
fig. (9).
I
G l a s s substratc
(/
Fig. 9
S w i t c h i n g e f f e c t i n a l a y e r o f amorphous V 0
2 5
d e p o s i t e d from g e l .
A d . c . v o l t a g e was a p p l i e d t h r o u g h a c u r r e n t - l i m i t i n g r e s i s t o r R
The v o l t a g e d r o p
vx a c r o s s t h e sample and t h e v o l t a g e d r o p V a c r o s s a s e r i e s o f L ' r e s i s t o r s R were
i s shown on f i g ( 9 ) :
r e c o r d e d on a xy r e c o r d e r . A t y p i c a l I-V ' c h a r a c t e r i s t i c
S w i t c h i n g o c c u r s f o r V r a n g i n g between 10 and 20 v o l t s , t h e t h e r e s h o l d v o l t a g e
d e c r e a s i n g when t h e C t r a t i o ( C = v4+/v) i n c r e a s e s . No s w i t c h i n g i s o b t a i n e d f o r
C>0.06. The h o l d i n g c u r r e n t v a r i e s between 1.6 and 2.5 mA. Some o f t h e s e d e v i c e s
were c a p a b l e o f p a s s i n g c u r r e n t s up t o 50 mA. Upon a p p l i c a t i o n of a 50 1Iz a . c .
v o l t a g e , a s t a b l e c h a r a c t e r i s t i c was obtained even a f t e r s e v e r a l days of continuous
operation. These t h r e s h o l d v o l t a g e decreases l i n e a r l y when t h e temperature.increases,switching disappears above 350K.For a c = 0.01 sample, Vt = 1 1 v o l t s a t room
temperature and reaches a maximum v a l u e a t 2 6 0 ~ .A p l a t e a u i s observed f o r lower
temperature.
- CONCLUSION
Amorphous T r a n s i t i o n Metal Oxides e x h i b i t a wide v a r i e t y o f p r o p e r t i e s .
Semiconductivity electrochromism and switching e f f e c t have been reviewed h e r e ,
but many o t h e r a s p e c t s could be i n t e r e s t i n g too. Lithium containing t u n g s t a t e
cm-' a t
and molybdate g l a s s e s e x h i b i t very high i o n i c c o n d u c t i v i t i e s ( 10-5
300K), much g r e a t e r t h a n t h e corresponding c r y s t a l l i n e phases ( 6 7 ) . They could
be used a s r e v e r s i b l e cathodes i n l i t h i u m b a t t e r i e s . Some semiconducting t r a n s i t i c n Metal Oxide such a s Ti02, Wl.
o r V205 a r e found t o be good candidates f o r
3
t h e p h o t o e l e c t r o l y s i s of water (8 ( 6 8 ) .
A s i n t h e c a s e of s i l i c o n , inexpensive amorphous photoelectrodes could
compete with s i n g l e c r y s t a l s . T.M.0 c o l l o i d a l suspensions appear t o be very promising ( 6 9 ) . Magnetic p r o p e r t i e s of amorphous T.M.0 a r e a l s o e x t e n s i v e l y s t u d i e d ,
and from a chemical p o i n t of view, most of t h e t r a n s i t i o n m e t a l oxides a r e k n o w
t o be very good c a t a l y s t s , o f t e n used i n t h e i r amorphous s t a t e by t h e chemical
inciustry.
Another g r e a t advantage of amorphous T.M.0 i s t h a t t h e y can be obtained
i n d i f f e r e n t s t a t e s : bulk g l a s s e s , t h i n filnis, powders o r g e l s . Their composition
can be v a r i e d smoothly over a wide s c a l e , allowing an o ~ t i m i s a t i o nof t h e m a t e r i a l
according t o t h e planned a p p l i c a t i o n .
-
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