HAL Id: jpa-00220846
https://hal.archives-ouvertes.fr/jpa-00220846
Submitted on 1 Jan 1981
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
AMORPHOUS VANADIUM OXIDES BY CVD : PREPARATION, ELECTRICAL AND MAGNETIC
PROPERTIES
T. Szörényi, K. Bali, I. Hevesi
To cite this version:
T. Szörényi, K. Bali, I. Hevesi. AMORPHOUS VANADIUM OXIDES BY CVD : PREPARATION,
ELECTRICAL AND MAGNETIC PROPERTIES. Journal de Physique Colloques, 1981, 42 (C4),
pp.C4-997-C4-1000. �10.1051/jphyscol:19814217�. �jpa-00220846�
CoZZoque C4, supplGment au nOIO, Tome 42, octobre 2981 page C4-997
AMORPHOUS VANADIUM O X I D E S BY C V D : PREPARATION, ELECTRICAL A N D MAGNETIC PROPERTIES
T. SzGrSnyi, K. Bali and I. Hevesi
Research Group on Luminescence and Semiconductors, Hungarian Academy of Sciences, Szeged, Hungary
A b s t r a c t . - Chemical vapour d e p o s i t i o n of V O C 1 3 w i t h H20 i n con- t r o l l e d atmosphere i s a unique method i n p r o d u c i n g amorphous vanadium o x i d e s t h a t c o n t a i n o n l y V 5 + and V 4 + i o n s w i t h con- t i n u o u s l y v a r y i n g v a l e n c e r a t i o / c / between (0.02 and
=
1 . 0 . The room t e m p e r a t u r e d . c . c o n d u c t i v i t y of a s p r e p a r e d V 2 0 5 f i l m s r a n g e s between 10-5 and 10-3 ohm-'m-l and i s s t r o n g l y dependent on p r e p a r a t i o n c o n d i t i o n s . I n c r e a s i n g t h e v a l e n c e r a t i o t h e con- d u c t i v i t y f i r s t i n c r e a s e s t o %10-2 ohm-'m-l, t h e n d e c l i n e s t o 10-Q ohm-lm-l. E P R s p e c t r a i n d i c a t e t h a t t h e environment of V 4 + i o n s i s n o t i d e n t i c a l i n f i l m s of d i f f e r e n t c o m p o s i t i o n s .I n t r o d u c t i o n . - Conduction i n amorphous vanadium o x i d e s and semiconduc- t i n g v a n a d a t e g l a s s e s t a k e s p l a c e v i a e l e c t r o n t r a n s f e r between i o n s o f d i f f e r e n t v a l e n c e . T h e r e f o r e , t h e v a l e n c e r a t i o i s one o f t h e de- t e r m i n a n t f a c t o r s of e l e c t r i c a l c o n d u c t i v i t y . I n vanadium p h o s p h a t e g l a s s e s
-
a l t h o u g h t h e V 4 + / V t o t a l r a t i o can be v a r i e d i n wide range-
t h e m i c r o s t r u c t u r a l f e a t u r e s have v e r y s i g n i f i c a n t i n f l u e n c e on t h e e l e c t r i c a l p r o p e r t i e s [ 1 , 2 1 , and s o t h e s e a r e n o t i d e a l f o r t e s t i n g t h e c o n d u c t i v i t y model [ 3 ] . I n an a t t e m p t t o c l a r i f y t h e c o n d u c t i v i t y problem, s u c h a model m a t e r i a l i s needed / i / i n which t h e v a l e n c e r a - t i c c- be v a r i e d between t h e V2O5 and V204 l i m i t s and L i i l which i s d e v o i d of p e r t u r b i n g s t r u c t u r a l f e a t u r e s / e . g . , of phase s e p a r a t i o n / .
I n t h i s c o n t r i b u t e d p a p e r we r e p o r t on a n o v e l model m a t e r i a l which h a s been p r e p a r e d by chemical vapour d e p o s i t i o n /CVD/ improving o u r t e c h n o l o g y o r i g i n a l l y s u g g e s t e d f o r vanadium p e n t o x i d e p r e p a r a - t i o n [ 4 1 .
P r e p a r a t i o n and g e n e r a l c h a r a c t e r i z a t i o n o f t h e f i l m s . - Chemical va- pour d e p o s i t i o n /CVD/ proved t o b e a u s e f u l method f o r p r o d u c i n g n e a r s t o i c h i o m e t r i c amorphous vanadium p e n t o x i d e [ 4 ] . F o r g a s e o u s S02 r e - duces vanadium e x c l u s i v e l y t o t h e V 4 + s t a t e we i n v e s t i g a t e d CVD of VOC13 w i t h H20 i n SO2-containing atmosphere i n o r d e r t o o b t a i n amor- phous vanadium o x i d e f i l m s of changing V 4 + c o n t e n t . I n t r o d u c i n g d i f - f e r e n t amount o f SO2 i n t o t h e r e a c t i o n chamber we p r e p a r e d f i l m s of V 4 + c o n t e n t r a n g i n g from
=
1 % t o W 1 0 0 % . A c c o r d i n g l y , t h e c o l o u r of t h e f i l m s changed from o r a n g e - r e d / t y p i c a l f o r V205/ t h r o u g h g r e e n t o b l u e / n e a r V 2 0 4 / . To o u r knowledge t h i s method i s unique i n p r o d u c i n g vanadium o x i d e s t h a t c o n t a i n o n l y V 5 + and V 4 + i o n s w i t h c o n t i n u o u s l y changing v a l e n c e r a t i o c=V4+/Vtotal between < 0.02 and % 1 [5]. Each CVD f i l m i s w e l l s o l u b l e i n w a t e r . The p r e p a r a t i o n of h i g h l y reduced l a y e r s i s l i m i t e d by t h e i r i n c r e a s i n g s o l u b i l i t y w i t h i n c r e a s i n g V'+c o n t e n t i n t h e r e a c t i o n atmosphere.
R o u t i n e p o l a r i z a t i o n m i c r o s c o p i c e x a m i n a t i o n s v e r i f i e d t h a t t h e f i l m s were amorphous. D i f f r a c t i o n micrographs of t h e f i l m s gave broad- -band p a t t e r n c h a r a c t e r i s t i c of amorphous m a t e r i a l s . Rowever, i t h a s t o b e n o t e d t h a t even s h o r t e x p o s u r e / a few s e c o n d s / t o t h e e l e c t r o n
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814217
C4-998 JOURNAL DE PHYSIQUE
beam r e p e a t e d l y l e d t o c h a n g e s i n t h e p a t t e r n / d u e t o m e l t i n g , decom- p o s i t i o n / a n d s u b s e q u e n t l y a p o l y c r y s t a l l i n e p a t t e r n was o b s e r v e d . E P R r e s u l t s . - S i n c e t h e r e a r e o n l y V 4 + and V 5 + i o n s i n t h e f i l m s , t h e c h e m i c a l c o m p o s i t i o n c a n b e d e t e r m i n e d from t h e V 4 + c o n t e n t . The V 4 + c o n t e n t o f t h e s a m p l e s was m e a s u r e d by q u a n t i t a t i v e EPR u s i n g vana- dium p h o s p h a t e g l a s s s t a n d a r d s o f known w e i g h t a n d c h e m i c a l composi- t i o n /V4+ c o n t e n t / . Each s a m p l e w i l l s u b s e q u e n t l y b e s p e c i f i e d by i t s r e d u c e d vanadium r a t i o / c / . Beyond t h i s h i g h s e n s i t i v i t y s p i n concen- t r a t i o n measurement, e v e n a q u a l i t a t i v e a n a l y s i s o f EPR s p e c t r a c a n f u r n i s h a d d i t i o n a l i n f o r m a t i o n c o n c e r n i n g t h e e l e c t r o n i c s t r u c t u r e o f t h e c h a r g e c a r r i e r V 4 + i o n s and t h e V 4 + - V 4 + i n t e r a c t i o n [ 6 , 7 1 . Repre- s e n t a t i v e f i r s t d e r i v a t i v e EPR s p e c t r a a r e g i v e n i n F i g . 1. A s i t i s
F i g . 1: Room t e m p e r a t u r e EPR s p e c t r a o f vanadium o x i d e s : / a / c = 0 . 0 1 ; / b / c = 0 . 0 3 ; and / c / c = 0 . 4 0 .
e v i d e n t f r o m t h e c o l l a p s e o f t h e h y p e r f i n e s t r u c t u r e , t h e s p e c t r a l l i n e s h a p e s a r e d e t e r m i n e d by i n c r e a s i n g s p i n - s p i n i n t e r a c t i o n . d . c . c o n d u c t i v i t y d a t a . - E l e c t r i c a l c o n d u c t i v i t y o f t h e f i l m s was measured by t h e f o u r - p o i n t p r o b e method o f Van d e r Pauw
1 0 1
u s i n g p a i n t e d s i l v e r e l e c t r o d e s a t t h e c o r n e r s o f t h e s a m p l e s , t y p i c a l l y 14x14 mm2 i n a r e a . T h i c k n e s s e s 10.2-3.0 pm/ w e r e d e t e r m i n e d b y o p t i - c a l i n t e r f e r e n c e method. The r e s u l t s c a n b e summarized a s f o l l o w s :The c o n d u c t i v i t y o f a s p r e p a r e d vanadium p e n t o x i d e f i l m s / b r i g h t o r a n g e c o l o u r , c < 0 . 0 1 5 a n d f o r EPR s p e c t r u m s e e F i g . l a / r a n g e s b e t w e e n 10-5 a n d 10-3 ohm-'m-' a t room t e m p e r a t u r e and s t r o n g l y d e p e n d s on p r e p a r a t i o n c o n d i t i o n s , e s p e c i a l l y o n t h e w a t e r v a p o u r c o n t e n t o f t h e a t m o s p h e r e i n t h e r e a c t i o n chamber. When exposed t o a i r t h e c o n d u c t i v i t y m a r k e d l y i n c r e a s e s ; i n d r y a t m o s p h e r e t h e con- d u c t i v i t y f i r s t d e c r e a s e s , r e a c h e s a s t e a d y - s t a t e v a l u e which i s a l w a y s h i g h e r t h a n t h e i n i t i a l v a l u e , and t h e n s l o w l y i n c r e a s e s . D u r i n g p r o l o n g e d e x p o s u r e t o d r y a t m o s p h e r e t h e c o n d u c t i v i t y s t e a d i l y i n c r e a s e s a n d i n few d a y s r e a c h e s a l o n g - t e r m maximum c o n d u c t i v i t y o f
w 1 O W 2 ohm-'m-'. T h e s e f i l m s a r e l e m o n - y e l l o w i s h c o l o u r e d . The i n i t i a l and f i n a l c o n d u c t i v i t y v a l u e s a r e i n d e p e n d e n t o f f i l m t h i c k n e s s , b u t t h e r a t e o f c o n d u c t i v i t y c h a n g e s i s h i g h e r f o r t h i n n e r / c 1 pm/ f i l m s .
H e a t i n g t h e a s p r e p a r e d vanadium p e n t o x i d e f i l m s up t o max.
400 K t h e c o n d u c t i v i t y d i s p l a y e d t h e r m a l l y a c t i v a t e d b e h a v i o u r ; t h e a c t i v a t i o n e n e r g i e s , d e t e r m i n e d from t h e s l o p e o f t h e l n / a T / v s . 1 0 0 0 / T p l o t s , were 0.6-0.7 eV / F i g . 2., t r a c e l / . Above M 350 K t h e
the subsequent cooling cycle down to m250 K the activation energies varied between 0.3 and 0.5 eV /trace 2 1 , depending on how far the heating cycle proceeded. With repeated heat-ing-cooling cycles de- creasing activation energies could be measured /trace 3 1 . These find- i ~ s provide evidence that the amorphous Vz05 films prepared by CVD also show thermochromism [ g 1 from temperatures as low as 350 K.
F ? : Thermal formation of V205 films as followed by the temper- ature dependence of d.c. conduc- tivity.
Increasing the V4+ content of the material /c s 0.02, for typ- ical EPR spectrum see Fig. lb/ the conductivity of the as prepared oxides slightly increased / U W 1OW2 ohm-'m-'/. These films seemed to be much more stable in a few days run, as indicated by their constant colour and conductivity. Further reduction led to a slight decrease in conductivity; these films were very stable. The films of c > 0.5 were very hygroscopic; their weight and overall appearance drasti- cally changed in different atmospheres. It is interesting to note that during hydration the electrical conductivity / a w 10-U ohm-lm-'/
proved to be practically constant.
Summing up the d.c. conductivity data, the dominant changes occur at near Vz05 compositions /c < 0.051. Increasing the valence ratio from 0.05 to 0.6 the conductivity decreases from W 10-2 to x 10-' ohm-'m-'.
Discussion and conclusions.- CVD of VOCl, with HzO in reducing atmos- phere proved to be successful in producing amorphous vanadium oxides in a wide composition range. From a literature survey and our own data we have found that the R.T. conductivity of amorphous Vz05 pre- pared by different methods /vapor deposition [IOI, rf sputtering [ 5 ] , splat cooling [Ill and deposition from gels [l211 ranges from 10-7 to 0.3-1 ohm-'cm-'. This very large spread in conductivity can not be accounted for simply by changes in the valence ratio, for this can hardly be larger than two orders of magnitude [5,10-121. A careful re-examination of existing conductivity data shows that higher con- ductivity is always accompanied with lower activation energy. In this work we demonstrated that the environment of the V4+ ions is differ- ent in the films of different composition. These results suggest that in the analysis of conductivity both changes in charge carrier density /in our case expressed by the c ratio/ and changes in the mobility
C4- 1000 JOURNAL DE PHYSIQUE
have to be taken into account.
The above described CVD films are promising model materials which can help in elucidating the conductivity problem and clarifying catalytic and electrochemical properties of amorphous vanadium oxides.
References.
[ l ] EIURAWSKI, L., CHUNG, C. H. and MACKENZIE, J. D., J. Non-Crystal- line Solids
32
119791 91.[21 SZ6RENY1, T., WOJNAROVITS, I. and HEVESI, I., J. Non-Crystalline Solids
42
/1930/ 393.[3] AUSTIN, I. G. and YOTT, N. F., Advan. Phys. 13 /1969/ 41.
[41 SZGRENYI, T., BALI, K. and HEVESI, I., J. ~ o c ~ r y s t a l l i n e Solids 35-36 /19d0/ 1245.
[51 ~ B E R G , F. P. and BENKO, F. A., Philos. Mag. /1973/ 357.
[61 KAHN, A., LIVAGE, J. and COLLONGUES, R., Phys. Status Solidi
a26
/1974/ 175.[7] LIVAGE, J., PINEAU, P., LEROY, ?.I. C. and PIICHAUD, M - , Phys.
Status Solidi a39 119771 73.
[81 VAN DER PAUW,
TJ.,
Philips Res. Rep. 13 /l9561 1.[g] GAVRILYUK, A. I., REINOV, N. PI. and CHUDNOVSKII, F. A., Pis'ma V Zh. Tekh. F i z . /USSR/
2
119791 1227.[l01 KENNEDY, T. N., HAKIM, R. and MACKENZIE, J. D., Mater. Res. Bull.
2 119671 193.
[l11 ZIVAGE, J. and COLLONGUES, R., Mater. Sci. and Eng.
2
119761 297.[l21 BULLOT, J., GALLAIS, O., GAUTHIER, M. and LIVAGE, J., Appl. Phys.
Lett.
