ELECTRICAL AND OPTICAL PROPERTIES OF AMORPHOUS TIN OXIDE

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ELECTRICAL AND OPTICAL PROPERTIES OF AMORPHOUS TIN OXIDE

I. Chambouleyron, C. Constantino, D. Jousse, R. Assumpção, R. Brenzikofer

To cite this version:

I. Chambouleyron, C. Constantino, D. Jousse, R. Assumpção, R. Brenzikofer. ELECTRICAL AND

OPTICAL PROPERTIES OF AMORPHOUS TIN OXIDE. Journal de Physique Colloques, 1981, 42

(C4), pp.C4-1009-C4-1012. �10.1051/jphyscol:19814220�. �jpa-00220850�

ELECTRICAL AND O P T I C A L PROPERTIES OF AMORPHOUS T I N OXIDE

I. Chambouleyron, C. Constantinox, D. Jousse**, R. Assumpq;io and R. Brenzikofer.

Physics I n s t i t u t e , UNICLMP, C . P. 1270, Campinas, S . P., 13.100 Brazil

ABSTRACT.- Thin layers of tin oxide have been deposited by the chemical spray method. From X-ray diffraction, Hall effect and optical measurements we

conclude that films deposited at temperatures lower than 3000C are amorphous.

Transport and optical data are given for both polycrystalline and amorphous materials.

INTRODUCTION.- Electrical and optical properties and structure of single crystal tin dioxide have been investigated both theoretically and experimentally. Films grown under different conditions and with different methods have been extensively studied too. To the authors knowledge no such properties have been reported yet on amorphous tin oxide. We present here preliminary results on the influence of deposition temperature on the X-ray diffraction patterns, Hall mobility and carrier concentration and the optical absorption in the fundamental edge region of the spectrum. The properties of the films are strongly dependent on annealing processes in air or vacuum. Changes of several orders of magnitude in resistivity have been reported to occur in annealed samples. In this paper we report measurements on "as grown" samples only. The time elapsed while deposition is made also influences the layer properties.

FILM GROWTH AND STRUCTURAL PROPERTIES.- SnOx layers were grown on glass and quartz flat substrates. All films were obtained by the chemical spray method using ethanol as a solvent in the sprayed solution. The substrate temperature Ts at which we deposited our samples was allowed to vary between 260 and 4400C. Between those limits films of relatively good transparency were obtained, although with important differences in their structural, electrical and optical properties. Typical

deposition parameters are as follows: carrier gas (nitrogen) flow: 7 e/min;

deposition rate: 1 &/min. The sprayed solution volume is always 25

d.

Sv.bstrate temperature was continuously monitored with a chrome1 alumel thermocouple. The experimental set-up is essentially the same as given in (1).

Tin dioxide crystallizes in the tetragonal structure with coordination number 6:3. The ideal reaction for the pyrolitic decomposition of stannic chloride is

SnCe4 + 2H20 -+ Sn02 + 4 H a

This is likely to happen at temperatures higher than 5000C and the

composition of the films is not very far from stoichiometry. When the temperature is lowered the reaction is incomplete. Full oxidation of tin does not occur and atomic chlorine and hydrogen are included into the lattice. Secondary Ion Mass Spectroscopy and Auger electron spectroscopy of layers grown at 2600C show a large concentration of chlorine that amounts to a few atomic percent (Chambouleyron and Farias

,

unpublished)

.

Optical microscopy shows that low temperature deposited films are somewhat

*

Supported by FAPESP;

**

On leave from CNRS, France.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814220

C4-1010 JOURNAL DE PHYSIQUE

inhomogeneous. As t h e s u b s t r a t e t e m p e r a t u r e i n c r e a s e s homogeneity i m p r o v e s . I t i s p o s s i b l e t h a t when d e p o s i t i o n t e m p e r a t u r e i s low, s m a l l adatom s u r f a c e m o b i l i t y and a l a r g e c o n c e n t r a t i o n o f f o r e i g n atoms produce a h i g h l y inhomogeneous m a t e r i a l .

X-ray d i f f r a c t i o n p a t t e r n s were o b t a i n e d w i t h a 1 2 KW Rigaku R o t a f l e x r o t a t i n g anode d i f f r a c t o r n e t e r . They g i v e i n d i c a t i o n s o f t h e d e g r e e o f

c r y s t a l l i n i t y and were u s e d among o t h e r c r i t e r i a , t o d e t e c t t h e amorphous n a t u r e o f t h e f i l m s . I t a p p e a r s t h a t d e p o s i t i o n t e m p e r a t u r e s lower t h a n around 3000 p r o d u c e f i l m s w i t h a v e r y poor d e g r e e c o r t h e a b s e n c e o f c r y s t a l l i n i t y . On t h e grounds o f t h e i r 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 we b e l i e v e t h e s e f i l m s a r e amorphous.

TRANSPORT PROPERTIES.- E l e c t r i c a l c o n d u c t i v i t y and H a l l e f f e c t measurements were performed on 0.5-1 urn t h i c k l a y e r s u s i n g t h e Van d e r Pauw geometry. Some s a m p l e s h a v e a l s o been c h a r a c t e r i z e d by v a r y i n g t h e measurement t e m p e r a t u r e from l i q u i d n i t r o g e n t o room t e m p e r a t u r e .

The c o n d u c t i v i t y r e a c h e s a r e m a r k a b l e p l a t e a u v a l u e o f 130 (R cm)-' a t 3000K i n t h e T r a n g e {3000C, 4 0 0 0 ~ } . C o n d u c t i v i t y d e c r e a s e s d r a s t i c a l l y of two o r d e r s o f magnitudg f o r T l o w e r t h a n 3000C.

The s i g n o f t h e H a l l p o t e n t i a l i s t h e same f o r a l l s a m p l e s and i n d i c a t e a n-type c o n d u c t i v i t y . F i g u r e 1 shows t h e d e n s i t y o f f r e e c a r r i e r s n a s deduced from t h e u s u a l H a l l c o n s t a n t f o r m u l a = -I./ne t h a t we assume t o a p p l y i n a l l m a t e r i a l s . The e x p e r i m e n t a l d i f f i c u l t y i n e r m i n i n g t h e s m a l l H a l l p o t e n t i a l s (10-50vV) l e a d t o i m p o r t a n t u n c e r t a i n t i e s f o r n e s p e c i a l l y i n t h e c a s e o f h i g h e r r e s i s t i v i t y s a m p l e s . A l l m a t e r i a l s a r c e i t h e r h i g h l y d e g e n e r a t e d (3000C<Ts

,<

440%) o r l i g h t l y d e g e n e r a t e d (T 62800C). Assuming a v a l u e of 0 . 3 5 % ( 2 ) f o r t h e e l e c t r o n i c d e n s i t y of s t a t e s e f f e E t i v e mass and a p a r a b o l i c band model, t h e e f f e c t i v e d e n s i t y o f s t a t e s a t t h e bottom o f t h e c o n d u c t i o n band i s ~ ~ - 5 x 1 0 ' ~ c m - ~ i n c r y s t a l Sn02.

F i g . 1

-

H a l l c a r r i e r c o n c e n t r a t i o n s v e r s u s F i g . 2

-

E l e c t r o n H a l l s u b s t r a t e t e m p e r a t u r e o f amorphous (A) and m o b i l i t y v e r s u s s u b s t r a t e temp2 p o l y c r y s t a l l i n e (P) t i n o x i d e f o r v a r i o u s r a t u r e o f some s a m p l e s o f f i g . 1 . s o l u t i o n f l u x e s ( 0 i n t e r m i t t e n t 1 . 5 d / m i n ,

r c o n t . 4 d l m i n , A c o n t . l & / m i n . ) .

H a l l m o b i l i t i e s UH a r e r e p r e s e n t e d i n f i g . 2 . A s t h e d e p o s i t i o n t e m p e r a t u r e g o e s down below 300°C, t h e m o b i l i t y d r o p s a n

o r d e r o f magnitude from t h e u s u a l p o l y c r y s t a l l i n e v a l u e o f 10-15 cm2/v.s.'This d r o p g i v e s some e v i d e n c e o f a t r a n s i t i o n towards a n amorphous s t a t e . Measured v a l u e s

Friedman (4) t h e v a l u e o f UH i s e x p e c t e d t o be s m a l l e r t h a n t h e c o n d u c t i v i t y m o b i l i t y i n e x t e n d e d s t a t e s d c . L e t u s remark t h a t a n u n d e r e s t i m a t i o n of

uc

from H a l l measurements l e a d t o a n o v e r e s t i m a t i o n o f n . T h i s would imply t h a t t h e n v a l u e measured f o r o u r amorphous s a m p l e s shown i n f i g u r e 1 do n o t c o r r e s p o n d t o t r u e c a r r i e r c o n c e n t r a t i o n s . A c t u a l v a l u e s may be n e a r t o o r lower t h a n t h e 1 0 ' ~ c m - ~ d e g e n e r a c y v a l u e . I n d e e d , t e m p e r a t u r e dependence of c o n d u c t i v i t y performed on two m a t e r i a l s p r e p a r e d a t 320°C and 260°C e n l i g h t e n s t h i s p o i n t . As shown i n f i g u r e ? c o n d u c t i v i t y i n p o l y c r y s t a l l i n e t i n o x i d e (Ts=3200C)does n o t v3ry w i t h t e m p e r a t u r e a s e x p e c t e d f o r a s e m i c o n d u c t o r i n which t h e Fcrmi l e v e l l i e s f a r i n s i d e t h e c o n d u c t i o n band. On t h e c o n t r a r y , t h e amorphous t i n o x i d e c o n d u c t i v i t y i n c r e a s e s w i t h t e m p e r a t u r e which s u g g e s t s a t f i r s t g l a n c e t h a t t h e Fermi l e v e l l i e s below t h e c o n d u c t i o n m o b i l i t y e d g e Ec. I n o u r p r e l i m i n a r y e x p e r i m e n t s no a c t i v a t i o n e n e r g y

h a s been c l e a r l y d e t e c t e d a t h i g h

U (Rcm)-' t e m p e r a t u r e s . However a l i n e a r

L'

^{1 -1- 1-1 - F 7-}

+ i

dependence o f l o g a v s T - ~ / ~ i s o b t a i n e d a t t e m p e r a t u r e s lower t h a n n=1.1 x l o 2 0 c m - 3 2500K. T h i s would i n d i c a t e a h o p p i n g

c o n d u c t i o n p r o c e s s t h r o u g h l o c a l i z e d

1 0 2 ^{t + + + +} s t a t e s around t h e Fermi l e v e l ( 3 ) .

t L 1

A F e r m i - l e v e l s i t u a t e d v e r y n e a r o f E,

-

a c c o u n t s f o r t h e g r e a t s e n s i b i l i t y o f

I

t h e e l e c t r o n d e n s i t i e s t o p r e p a r a t i o n

l 0

l-

c o n d i t i o n s when Ts<3000C ( f i g u r e 1 ) .

1

I n d e e d , t h e c r e a t i o n of d e f e c t o r i m p u r i t y s t a t e s i n t h e gap o c c u r i n g a t

+ n=5.8 x 1 0 1 8 c m - 3

I:

^{+ +} low Ts, a s e v i d e n c e d from o p t i c a l l

measurements i s most p r o b a b l y

+

_I

r e s p o n s i b l e f o r t h e downwards s h i f t o f

+ ttie F e r m i - l e v e l .

!

⁺ i

-l OPTTcfi ABSowTIoN

.-

O p t i c a l

1 l L L - t r a n s m i s s i o n and r e f l e c t i u a were

10- ⁰

'

^O 1 0 3 / ~ ( ~ - ' ) measured w i t h Z e i s s DMC-25 and

Beckmann DK 2 A s p e c t r o p h o t o m e t e r s i n F i g . 3 - DC C o n d u c t i v i t y of t h e U. ,V., v i s i b l e and n e a r I . R . r a n g e amorphous (T =2600C) and poly c r i s t a l l i n e of t h e s p e c t r u m . From i n t e r f e r e n c e

(T =3200C) m i t e r i a l s v e r s u s 1 / T . p a t t e r n s we c o u l d e s t i m a t e t h e t h i c k n e s s o f t h e f i l m s . Mechanical T a l y s t e p measurements c o n f i r m e d t h e o p t i c a l l y deduced v a l u e s . A b s o r p t i o n c o e f f i c e n t s were c a l c u l a t e d from t r a n s m i s s i o n and r e f l e c t i o n measurements. I n d e p e n d e n t v a l u e s were o b t a i n e d from f i l m s i d e n t i c a l e x c e p t f o r t h i c k n e s s u s i n g t h e r e l a t i o n

C ( = - I 1

e n

-

A t I 2

w h e r e A t = t 2 - t l i s t h e f i l m t h i c k n e s s d i f f e r e n c e a n d 1 1 i s t h e t r a n s m i s s i o n t h r o u g h t h e t h i n n e r l a y e r .

F i g u r e 4 s h o w s a l o g a r i t h m i c p l o t o f a b s o r p t i o n c o e f f i c i e n t v s p h o t o n e n e r g y f o r s a m p l e s g r o w n a t 3 2 0 0 a n d 2 8 0 0 C , c o m p a r e d t o s i n g l e c r y s t a l m a t e r i a l ( 5 , 6 ) . T h e a b s o r p t i o n c o e f f i c i e n t o f s p r a y e d t i n o x i d e l a y e r s s h o w s s o m e i m p o r t a n t d i f f e r e n c e s w i t h r e s p e c t t o c r y s t a l l i n e m a t e r i a l . On t h e o n e h a n d ^C( i s h i g h e r f o r a m o r p h o u s m a t e r i a l i n t h e w h o l e r a n g e . On t h e o t h e r h a n d b o t h m a t e r i a l s s h o w a n a b s o r p t i o n c o e f f i c i e n t n e a r t h e f u n d a m e n t a l e d g e t h a t h a s a n e x p o n e n t i a l t y p e d e p e n d e n c e o n p h o t o n e n e r g y . As t h e d e p o s i t i o n t e m p e r a t u r e i n c r e a s e s a n d t h e m a t e r i a l b e c o m e s m o r e o r d e r e d t h e a b s o r p t i o n e d g e e v o l v e s t o w a r d s a c u r v e h a v i n g t w o d i f f e r e n t r e g i o n s , a n e x p o n e n t i a l s h a p e o n

C4-1012 JOURNAL DE PHYSIQUE

the low photon energy side and a parabolic type dependence at high photon energies.

An absorption coefficient that increases exponentially with photon energy has been measured on a great number of crystalline and in almost all amorphous

semiconductors.

*

I

*

amorphous

* 4

Ts=2800C

. .*/ * I

*

^/

* ^{** j}

4

* '

* *

single

I

No single explanation has been accepted for this behaviour. Our preliminary results indicate however that two contributions might be responsible for the observed shape. One is an electric field broadening of the absorption edge (Franz-Keldysh effetc). The internal field could arise in principle from charged impurity states. The exponential behaviour might also be due to electronic transitions between tails of states in the band edge that fall off exponentially with energy. This interpretation is consistent with the exaerimentally observed fact that higher deposition temperatures give lower absorption in the tails. With increasing deposition temperature two mechanisms should occur: impurity atom

concentration (Cl and H) and deviations from stoichiometry (incomplete oxidation of tin) decrease and crystalline order is improved. Both mechanisms "clean" the forbidden band.

1

l /

L_- - ^--

^1-.!Fig.4 - Absorption coefficient versus

2 3 4 photon energy for amorphous, poly-

crystalline and single crystal tin PI3OTON EKERGY (eV) oxide.

ACKNOWLEDGEMENTS.- We thank I. Torriani and M. Fantini for x-ray diffraction

measurements. This work has been partially supported by Companhia ~nergstica de S ~ O Paulo.

REFERENCES

1.. I. CHAPlBOULEYRON and E. SAUCEDO, Solar Energy Materials, 1, (1979), 299.

2 . NAGASAWA, SHIONOYA and MAKISHIMA, J. Phys. Soc., Japan,

25

(1965), 1093.

3. N. F. MOTT and E. A. DAVIS, Electronic Process in Non-Crystalline Materials, Clarendon Press, Oxford, (1979).

4. L. FRIEDYAN, J. Non-Crystalline Solids, 6, (1971), 329.

5. S. REDDAWAY and D. WRIGHT, Brit. J. hppiT Phys. 16 (1965), 195.

6. SUMMIT, MARLEY and BORELLI, J. Phys .& Chem. solids,

2

(19641, 1465.