ANNEALING AND HYDROGENATION BEHAVIOUR OF EVAPORATED AND SPUTTERED HIGH-PURITY AMORPHOUS SILICON FILMS

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ANNEALING AND HYDROGENATION

BEHAVIOUR OF EVAPORATED AND SPUTTERED HIGH-PURITY AMORPHOUS SILICON FILMS

N. Kniffler, W. Müller, J. Pirrung, N. Hänisch, B. Schröder, J. Geiger

To cite this version:

N. Kniffler, W. Müller, J. Pirrung, N. Hänisch, B. Schröder, et al.. ANNEALING AND HY- DROGENATION BEHAVIOUR OF EVAPORATED AND SPUTTERED HIGH-PURITY AMOR- PHOUS SILICON FILMS. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-811-C4-814.

�10.1051/jphyscol:19814178�. �jpa-00220803�

ColZoque C4, supple'ment au nO1O, Tome 4 2 , oetohre 7981 page C4-81 I

ANNEALING AND HYDROGENATION BEHAVIOUR OF EVAPORATED AND SPUTTERED HIGH-PURITY AMORPHOUS S I L I C O N F I L M S

N . K n i f f l e r , W . b l . M L ' l l e r , J.M. P i r r u n g , N. H a n i s c h , B. S c h r o d e r a n d J . G e i g e r

A b s t r a c t . - Amorphous s i l i c o n f i l m s have been prepared by u l t r a - c l e a n evapora-

--

t i o n and s p u t t e r t e c h n i q u e e n c l o s i n g d i f f e r e n t amounts of hydrogen i n t o t h e f i l m s . The d a r k c o n d u c t i v i t y o f b o t h t y p e s o f f i l m s can be reduced f r o m

(ncm)-I as d e p o s i t e d t o v a l u e s s m a l l e r t h a n 4 . 1 0 - l 1 ( n c m ) - I i f t h e f i l m s c o n t a i n enough hydrogen and a r e annealed t o a b o u t 600 K. C l o s e t o t h e

a n n e a l i n g minimum o f t h e d a r k c o n d u c t i v i t y a maximum o f p h o t o c o n d u c t i v i t y i s observed.

I n t r o d u c t _ i o n . - I t i s w e l l known t h a t t h e i n c o r p o r a t i o n o f hydrogen i n t o amorphous s i 1 i c o n f i i s d r a s t i c a l l y reduces t h e c o n c e n t r a t i o n o f t h e danql i n g bond s t a t e s and makes an e f f i c i e n t d o p i n g o f t h e m a t e r i a l f e a s i b l e . W h i l e a-Si f i l m s pvepared by glow d i s c h a r g e ( g d ) decomposition o f s i l a n e always c o n t a i n s s u f f i c i e n t l y hydrogen, f o r t h e o t h e r a-Si p r e p a r a t i o n t e c h n i q u e s a h y d r o g e n a t i o n o f t h e f i l m s has t o be p r o v i d e d s e p a r a t e l y . F i r s t Paul e t a l . /1/ produced f i l m s w h i c h can be doped b y r e - a c t i v e s p u t t e r i n g i n a hydrogen-argon m i x t u r e . Other a u t h o r s / 2 , 3 / r e p o r t e d a b o u t a subsequent h y d r o g e n a t i o n by h e a t d i f f u s i o n o f a t o m i c hydrogen i n t o t h e f i l m . M i l l e r e t a l . / 4 / added atomic hydrogen d u r i n g t h e e v a p o r a t i o n process and were a b l e t o r e - duce t h e room temperature d a r k c o n d u c t i v i t y ^{O D} f r o m 10- ⁺ t o 10-9 ( ~ c m ) - ' a f t e r a h e a t t r e a t m e n t . I n a p r e v i o u s paper / 5 / we a l r e a d y p o i n t e d o u t t h a t s t r u c t u r a l r e - l a x a t i o n processes i n u l t r a - c l e a n a-Si f i l m s may d i m i n i s h t h e d e n s i t y of t h e dang- l i n g bond s t a t e s c o n s i d e r a b l y . We d i d n o t succeed, however, h i t h e r t o t o reduce O D

a t room t e m p e r a t u r e much below ( n c m ) - l .

I n t h i s paper we p r e s e n t measurements on a-Si f i l m s evaporated i n a m o l e c u l a r hydrogen beam, where t h e h y d r o g e n a t i o n t a k e s p l a c e d u r i n g e v a p o r a t i o n and i n a f o l l o w i n g a n n e a l i n g t r e a t m e n t . The h y d r o g e n a t i o n and a n n e a l i n g b e h a v i o u r o f t h e s e f i l m s a r e compared w i t h t h o s e prepared b y r e a c t i v e s p u t t e r i n g .

Experimental procedures.- The a-Si f i l m s a r e p r e p a r e d i n UHV based e v a p o r a t i o n and s p u t t e r equipments. The e v a p o r a t i o n system c o n s i s t s o f two separated chambers f o r t h e e-gun s o u r c e and f o r t h e s u b s t r a t e , r e s p e c t i v e l y . A r e s i d u a l gas p r e s s u r e o f l e s s than 1 0 - l o t o r r c o u l d b e m a i n t a i n e d d u r i n g e v a p o r a t i o n w i t h a r a t e of a p p r o x i - m a t e l y 4 R/s. S u b s t r a t e h e a t t r e a t m e n t was p o s s i b l e between 10 K and 800 K as w e l l as i n - s i t u d a r k c o n d u c t i v i t y measurements. A m o l e c u l a r hydrogen beam c o l l i m a t e d by a m u l t i c h a n n e l n o z z l e c o u l d be d i r e c t e d t o t h e s u b s t r a t e . Much c a r e has been t a k e n t o a v o i d any c o n t a m i n a t i o n o f t h e s u b s t r a t e and o f t h e system by t h e gas i n l e t sys- tem. F o r t h e UHV s p u t t e r s y s t e n a commercial rf s p u t t e r gun was c o n v e r t e d i n t o an UHV c o m p a t i b l e v e r s i o n . An A r o r Hz p a r t i a l p r e s s u r e up t o 10-3 t o r r c o u l d be ad- m i t t e d w i t h o u t a l t e r i n g t h e r e s i d u a l gas background f o r more t h a n a f a c t o r of two.

S p u t t e r r a t e s o f 1-4 81's a t a d i s t a n c e o f 25 crn a r e o b t a i n e d by u s i n g a n A r w o r k i n g p r e s s u r e o f a b o u t t o r r .

P h o t o c o n d u c t i v i t y , t h e r m o e l e c t r i c power, and i r a b s o r p t i o n measurements c o u l d be c a r r i e d o u t i n s e p a r a t e systems.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814178

C4-8 12 JOURNAL DE PHYSIQUE

R e s u l t s and d i s c u s s i o n . - The room temperature d a r k c o n d u c t i v i t y i s shown i n F i g . 1 f o r v a r i o u s a-Si f i l m s as a f u n c t i o n of t h e a n n e a l i n g temperatures T A .

F i g . 1: Room temperature d a r k c o n d u c t i - ^{I I I I I I I}

v i t y f o r t y p i c a l amorphous s i l i c o n f i l m s prepared b y e v a p o r a t i o n o r s p u t t e r tech-

-

n i q u e as a f u n c t i o n o f t h e a n n e a l i n g

5

temperature. TS i s t h e s u b s t r a t e tem-

C

p e r a t u r e d u r i n g p r e p a r a t i o n o f t h e films, ^{- L} P H ~ i s t h e e f f e c t i v e p a r t i a l hydrogen ² pressure, po t h e r e s i d u a l gas p r e s s u r e

%

( w i t h o u t Hz and A r ) d u r i n g e v a p o r a t i o n

- 6

o r s p u t t e r i n g . I

The uppermost c u r v e o f F i g . 1 r e p r e s e n t s t h e

!!

t y p i c a l a n n e a l i n g behaviour o f " n o t as c l e a n "

5

f i l m s prepared by s p u t t e r o r e v a p o r a t i o n tech-

-'-

n i q u e w i t h o u t hydrogen. The c u r v e beneath

shows t h e r e d u c t i o n o f ^OD f o r a n n e a l i n g tem-

a

p e r a t u r e s up t o 600 K, caused by r e l a x a t i o n 0

processes i n t h e c l e a n f i l m . Two f a c t s c o u l d ^-I0 -po=10-8 n o t r e a l l y be c l e a r e d up f o r t h i s f i l m : F i r s t ,

2

t h e c o n d u c t i v i t y minimum around TA = 600 K m i g h t be deeper. The e x p e r i m e n t a l d a t a a r e t o o

scarce i n t h i s temperature range. Second, i t -12 ^{1 1 1 1 1 ~ ~}

i s s t i l l open, whether t h e c o n d u c t i v i t y m i n i - 300 500 700 900K mum i s o n l y due t o r e l a x a t i o n processes b e t -

ween s i l i c o n atoms themselves, o r t h e v e r y low ANNEALING TEMPERATURE hydrogen background i n these f i l m s i s a l r e a d y

capable t o p a s s i v a t e a c o n s i d e r a b l e p a r t o f t h e d e f e c t s t a t e s . Under t h e same p r e - p a r a t i o n c o n d i t i o n an e f f e c t i v e p a r t i a l hydrogen p r e s s u r e o f 2.10-6 t o r r i s a l r e a d y s u f f i c i e n t t o move t h e OD-minimum below 10-9 ( n c m ) - l . aD v a l u e s as l o w as 3 - l o - "

(ncm)-I can be achieved by i n c r e a s i n g t h e e f f e c t i v e hydrogen p r e s s u r e t o

l o e 4

t o r r . T h i s v a l u e a l m o s t reaches t h e oD = 1 0 - l 2 (nun)-1 minimum o b t a i n e d by r e a c t i v e s p u t t e r i n g i n an A r

+

^Hz m i x t u r e .

The s t r o n g decrease o f ^OD o f i n hydrogen evaporated samples i s a t t r i b u t e d t o an a c t i v a t i o n o f t h e m o l e c u l a r hydrogen, t h a t i s d i s s o c i a t e d and s a t u r a t e s t h e dang- l i n g bonds o f s i l i c o n , combined w i t h s t r u c t u r a l r e l a x a t i o n . F i g . 2 shows t h e con- t e n t o f ' a c t i v a t e d ' hydrogen measured

5

L

P)

- 3

'5 .

0 N

c

- 2

1

1 by t h e o p t i c a l a b s o r p t i o n o f t h e Si-H

wagging v i b r a t i o n a t 640 cm-1 f o l l o w i n g Fang e t al. /6/.

.--.

1.0

S

F i g . 2: Hydrogen c o n c e n t r a t i o n o f

;

' a c t i v a t e d 1 hydrogen i n an evapo-

-

r a t e d a-Si sample as a f u n c t i o n z 0.75 o f t h e a n n e a l i n g temperature. The

c o n c e n t r a t i o n was determined by 4 i r a b s o r p t i o n o f t h e wagging v i -

b r a t i o n a l band z _{W 0.5}

0

According t o F i g . 2 a c e r t a i n amount o f hydrogen i s a l r e a d y bound i n t h e f r e s h - l y evaporated a - S i f i l m . With i n c r e a s -

5

i n g a n n e a l i n g temperature t h e concen- a 0.25 t r a t i o n o f a c t i v e hydrogen f i r s t i n -

&

creases. I t reaches a maximum w i t h o >

0.75% a t 500 K, a t 600 K t h e hydrogen x c o n c e n t r a t i o n drops t o about 0.6%. The

minimum o f OD i s always observed around 300 ^I LOO 500 600 700 800 900 K 600 K. A t t h i s temperature, which i s

a l s o f a v o u r a b l e f o r t h e p r e p a r a t i o n o f ANNEALING TEMPERATURE

I

,

^{I I I I I}

- -

a

-

Si : H on KBr

= torr

-

-

-

- b

-

I I I I I I

t h e h e l p o f a c l u s t e r model. A t higher annealing temperatures u a t room temperature r i s e s r a p i d e l y f o r hydrogenated and non-hydrogenated samples.

T R ~

s r i s e may be a t t r i b u t e d t o t h e l o s s o f hydrogen and simultaneously t o a r e g e n e r a t i o n o f d e f e c t s . The t h e r m o - e l e c t r i c power becomes l a r g e r and negative, what i n d i c a t e s a s h i f t o f t h e Fermi l e v e l toward t o t h e conduction band. I f t h e r e g e n e r a t i o n o f defects i s r e s p o n s i b l e f o r the r i s e o f UD f o r TA

>

600 K a t a l l , the regenerated d e f e c t d i s t r i - b u t i o n can n o t be symmetrically t o t h e midgap.

The temperature dependence o f t h e dark con-

d u c t i v i t y UD presented i n F i g . 3 w i t h TA as para- TEMPERATURE meters shows t h a t v a r i a b l e range hopping t r a n s p o r t 575 375 275 175

K

i s dominating f o r low TA values a t low and mo- d e r a t e temperatures, w h i l e a c t i v a t e d t r a n s p o r t i n extended s t a t e s takes p l a c e f o r TA

-

^{600 K} ( a c t i - v a t i o n energy EA

-

0.8 eV f o r curve 4) and a t h i g h temperatures.

Fig. 3: Temperature dependence o f t h e dark

5

m t i v i t y o f an a-Si f i l m evaporated i n hydrogen f o r d i f f e r e n t annealing tempera- t u r e s TA.

A comparison o f the annealing behaviour o f the dark and p h o t o c o n d u c t i v i t y o f evaporated and sputtered a-Si f i l m s i n F i g . 4 shows t h a t m i n i - mal values o f UD almost c o r r e l a t e w i t h maximal values o f opt,. The two maxima o f oph occuring i n e i t h e r cases may be due t o two d i f f e r e n t photo- Q

conduction pathes. The p h o t o c o n d u c t i v i t y r e p o r t e d 10-10

-

here i s n o t as h i g h as obtained i n gd-prepared /8/ and s p u t t e r e d / 9 / samples by o t h e r authors.

An o p t i m i z a t i o n o f t h e p h o t o c o n d u c t i v i t y i s s t i l l 10-I]-,

, , , , , -

t o be c a r r i e d o u t . 2 3 1 5 6 7 ~ ~ '

F i n a l l y , F i g . 5 i l l u l s t r a t e s t h e decrease o f t h e room temperature c o n d u c t i v i t y UD w i t h i n - creasing hydrogen p a r t i a l pressure.

Fig. 4: The annealing behaviour o f t h e pho- t o c o n d u c t i v i ty UP

( i l l u m i n a t i o n w i t h A = 633 nm, 2 mW/cm2, 6-1015 p h o t o n ~ - c m - ~ . s - ~ ) o f evaporated and rf- s p u t t e r e d a-Si f i l m s w i t h h i g h hydrogen

p a r t i a l pressure com- pared t o t h e v a r i a - t i o n o f t h e dark con- d u c t i v i t y UD measured a t room temperature.

-2 -6

5

C a-Si:H,50nm

- E

- 7 EVAPORATED RF -SPUTTERED

C 3

a LL

- 8 I W C

5

- 9

0 (L C a

,

-10 v P

0

S -I1

0 U

ANNEALING TEMPERATURE

C4-8 14 JOURNAL DE PHYSIQUE

I I

-2 kc- - - 6, for p , , ~ - F i g . 5: Room temperature dark c o n d u c t i v i t y

- - .

^torr

versus p a r t i a l hydrogen pressure f o r a-Si

E

samples prepared by r f - s p u t t e r i n g (po i s t h e

,

^-L

r e s i d u a l gas pressure except PAr and p ~ 2 ) . _w

LT 3

w

z

- 6 - Conclusion.- An e f f e c t j v e hydrogenation o f a-Si- I

E

f i l m s i s p o s s i b l e by u l t r a - c l e a n evaporation ^I technique i n a

H,

beam and subsequent annealing

E

^{- 8 -}

t o 600 K s i m i l a r as by r e a c t i v e s p u t t e r technique.

Dark c o n d u c t i v i t y values as low as 3.10-l1 ( a m ) - I a f o r evaporation and 1 0 - l 2 ( ~ c m ) - l fo r s p u t t e r i n g o a r e c o r r e l a t e d w i t h a maximum o f p h o t o c o n d u c t i v i t y .

00

^-I0-

S

-

-12

-

I I I

10-6 10-3

P H ~ (fort)

Acknowledgement.- The authors would l i k e t o thank t h e Bundesministerium f u r Forschung und Technologie f o r t h e f i n a n c i a l support.

References

/1/ PAUL W., LEWIS A.J., CONNELL G.A.N., MOUSTAKAS T.D., Sol i d S t a t e Comm. - 20, (1976) 969

/2/ KAPLAN D., SOL N., VELASCO G., THOMAS P.A.

,

Appl .\'Phys. L e t t .

2

(1978) 440 /3/ JANG J., KANG J.H., LEE C., J. Non. C r y s t . S o l i d s

35-36,

(1980) 313

/4/ MILLER D.L., LUTZ H., WIESEMANN H., ROCK E., GLOWSKY A.K., RAMAMOORTHY S., STROUGIN M., J. Appl. Phys.

-

49, (1978) 6192

/5/ SCHRODER B., GEIGER J., KNIFFLER N., MOLLER H.W., S t a t u s b e r i c h t Sonnenenergie, Bundeministerium f u r Forschung und Technologie, VDI Verlag 1980, S. 991

/6/ FANG C.J., GRUNTZ K. J., LEY L., CARDONA M., DEMOND F.J., MOLLER G., KALBITZER

S.,

J. Non-Cryst. S o l i d s

35-36

(1980) 255

/ 7 / MATTES B.L. i n " P o l y c r y s t a l l i n e and Amorphous Thin Films and Devices", ed. by L.C. Kasmerski, Acad. Press, New York 1980, p . 1

/8/ PLXTTNER R.D., KROHLER W.W., S t a t u s b e r i c h t Sonnenenergie, Bundeministerium f u r Forschung und Technologie, VDI Verlag 1980, S. 943

/9/ See f o r example: ANDERSON D.A., MODDEL G., PAESLER M.A., PAUL W., J. Vac. S c i Technology 16 (1979) 906