INFERIOR ELECTRONIC PROPERTIES OF RF-SPUTTERED a-Si : H FILMS WITH ONLY THE 2000-cm-1 IR ABSORPTION BAND

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INFERIOR ELECTRONIC PROPERTIES OF RF-SPUTTERED a-Si : H FILMS WITH ONLY THE

2000-cm-1 IR ABSORPTION BAND

S. Oguz, D. Paul, J. Blake, R. Collins, A. Lachter, B. Yacobi, W. Paul

To cite this version:

S. Oguz, D. Paul, J. Blake, R. Collins, A. Lachter, et al.. INFERIOR ELECTRONIC PROPERTIES OF RF-SPUTTERED a-Si : H FILMS WITH ONLY THE 2000-cm-1 IR ABSORPTION BAND.

Journal de Physique Colloques, 1981, 42 (C4), pp.C4-679-C4-682. �10.1051/jphyscol:19814150�. �jpa-

00220773�

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JOURIiAL DE PhYSIQUE

Co LLoque C4, supp LbmenL au nOIO, Tcme 4 2 , u c t o b r e 190 7

INFERIOR ELECTRONIC PROPERTIES OF RF-SPUTTERED a - S i : H F I L F S WITH ONLY THE 2 0 0 0 - c m - ~ I R ABSORPTION BAND

S. Ogut, D.K. Paul, J. Blake, R.W. Collins, A. L a c h t e r , B.G. Yacobi and W. Paul

D i v i s i o n o f Applied Scie.rces, Harvard U n i v e r s i t y , Cambridge, P?/; O Z I Y d , U.S.F.

ABSTRACT: The I R v i b r a t i o n a l a b s o r p t i o n l s p e c t r a o f a-Si :H f i l m s show two bands a t 2000- and 2100-cm-

,

a t t r i b u t e d t f Sit1 s t r e t c h v i b r a t i o n s . The widespread a s s o c i a t i o n o f t h e 2000-cm- band w i t h u s e f u l p h o t o e l e c t r o n i c p r o p e r t i e s i s t e s t e d . It i s f o u n d t h a t s p u t t e r e d a-Si:H f i l m s d e p o s i t e d c l o s e t o t h e t a r g e t d i s p l a y o n l y t h e 2000-cm- band, b u t t h e s e f i l m s a1 so have much d e t e r i o r a t e d p r o p e r t i e s . It i s proposed t h a t b o t h r e s u l t s a r e caused by i n c r e a s e d t a r g e t - p a r t i c l e bombardment a t c l o s e d i s t a n c e s . Regardless o f t h e mechanism i n v o l v e d , t h e s e r e s u l t s a r e p r e s e n t e d a?

d e f i n i t i v e e v i d e n c e a g a i n s t t h e u n i q u e a s s o c i a t i o n o f t h e 2000-cm- band w i t h u s e f u l p h o t o e l e c t r o n i c p r o p e r t i e s .

INTRODUCTION: Much c o n s i d e r a t i o n has been g i v e n t o t h e r e l a t i v f . i n t e n s i t i e s o f t h e SiH v i b r a t i o n a l a b s o r p t i o n bands a t 2000- and 2100-cm- i n a-Si:H f i l m s . Based on t h e o b s e r v a t i o n t h a t thelglow d i s c h a r g e f i l m s which have t h e b e s t d e v i c e p r o p e r t i e s show o n l y t h e 2000-cm- band, and t h a t s p u t t e r e d f i l m ~ , ~ w h i c h o f t e n have l e s s d e s i r a b l e p r o p e r t i e s , u s u a l l y have a dominant 2100-cm- band, a h i g h 2000/2100 r a t i o has o f t e n been u n i q u e l y a ~ s o c i a t e d w i t h h i g h qua1 i t y a-Si

:H

f i l m s . Consequently, e l i m i n a t i o n o f t h e 2100-cm- band i n s p u t t e r e d a-Si :H has b e ~ n sought and, indeed, f i l m s w i t h o n l y t h e 2000-cm- band have been r e p o r t e d

.

N o t i c e a b l e i s t h e f a c t , however, t h a t none o f t h e s e r e s u l t s was accompanied by r e p o r t s o f t h e expected improvements i n p h o t o e l e c t r o n i c p r o p e r t i e s . We have now c a r r i e d o u t IR v i b r a t i o n a l a b s o r p t i o n , o p t i c a l abso t i o n , dark c o n d u c t i v i t y , p h o t o c o n d u c t i v i t y , and photoluminescence measurements on a-Si:H samples produced TP by p l a c i n g s u b s t r a t e s a t d i f f e r e n t p o s i t i o n s i n t h e s p u t t e r i n g plasma. f i n d t h a t t h e r a t i o o f t h e i n t e g r a t e d i n t e n s i t i e s o f t h e 2000- and 2100-cm-Yebands, A(2000)/A(2100), i n c r e a s e s r a p i d l y w i t h d e c r e a s i n g t a r g e t - s u b s t r a t e d i s ance, w i t h f i l m s c l o s e s t t o t h e t a r g e t d i s p l a y i n g e s s e n t i a l l y o n l y t h e 2 0 0 0 - c i f band. We a l s o f i n d t h a t f i l m s prepared c l o s e t o t h e t a r g e t have much d e t e r i o r a t e d p h o t o e l e c t r o n i c p r o p e r t i e s . We p r e s e n t t h se r e s u l t s as d e f i n i t i v e e v i d e n c e a g a i n s t t h e u n i q u e a s s o c i a t i o n o f t h e 2000-cm-I band w i t h u s e f u l p h o t o e l e c t r o n i c p r o p e r t i e s . We propose t h a t t h e h i g h A(2000)/A(2100) r a t i o i s a c t u a l l y induced by t a r g e t - p a r t i c l e bombardment, which a t t h e same t i m e i s r e s p o n s i b l e f o r t h e i n f e r i o r p r o p e r t i e s o f t h e s e f i l m s .

EXPERIMENTAL: We made a s e r i e s oft d e p o s i t i o n s where s u b s t r a t e s were p l a c e d on d i f f e r e n t h e i g h t g l a s s " p e d e s t a l s " on t h e s u b s $ r a t e p l a t f o r m o f t h e RF s p u t t e r i n g system. W i t h t h i s arrangement we prepared a-Si:H f i l m s w i t h d i f f e r e n t t a r g e t - s u b s t r a t e s e p a r a t i o n s s i m u l t a n e o u s l y , and w i t h o u t changing t h e b a s i c geometry o f t h e plasma c a v i t y . The s u b s t r a t e p l a t f o r m was k e p t a t 200° C, b u t t h e t e m p e r a t u r e o f t h e s u b s t r a t e s on t h e p e d e s t a l s was n o t c o n t r o l l e d . O t h e r parameters o f t h e d e p o s i t i o n were: RF power = 200 W; p a r t i a l p r e s s u r e o f hydrogen = 0.8 mT and o f argon = 5 mT.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814150

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C4-680 J O U R N A L DE PHYSIQUE

1.0 1.5 2.0 2.5 3.0 3.5

Target- Substrate Distance(inches)

F i g . 1: E p g y

Ey*

'pi:

Wl~'

^(;T)130

^{R j}

and t h e r a t i o s A(2000)/A(w)

A 2100 /A(W A 2000 /A 2100 p l o t t e d vs. t a r g e t - s u b s t r a t e d i s t a n c e .

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1.6 - - - - target-subst. dist. = 3.5 in.

- ^.\ ^;? ^{- -} target-subst. dist. = I . 0 in.

1.2 - _I ' _\ ⁱ ^\.

- _I _\i _1.

X

-

I

!

0.8 -

0

\

Y

_u - \

I . _\ _1.

0.4 - ^/ ^/

^\

- ,'.I.

^\^\

^\

/ /

¹

:.<- ^\.

¹

0.0 ^I

1900 2000 2100 2200 2300

F i g . 2: IR s t r e t c h bands o f f i l m s d e p o s i t e d on t h e s u b s t r a t e p l a t f o r m and one i n c h away f r o m t h e t a r g e t .

RESULTS: Fig. 1 shows t h e parameters o b t a i n e d f r o m t h e c h a r a c t e r i z a t i o n a l measurements p l o t t e d vs. t h e t a r g e t - ~ u b s t r a t e ~ s e p a ~ t i o n : E t h e energy a t 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 i e n t a e q u a l s 10 cm- ; E,, c o n d u c t i v i t y a c t i v a t i o n energy; IpL, t h e r e l a t i v e photoluminescence i n t e n s i t y ; W t h e w i d t h of t h e photolumi nescence peak; aRT, t h e room t e m p e r a t u r e dark eb;ductivity;

( ~ 1 ) ~ . t h e mobi 1 it y - 1 i f e t i r n e p r o d u c t f o r e l e c t r o n s d e r i v e d f rom p h o t o c o n d u c t i v i t y measurementsil A(2000)/A(W), A(2100)/A(W), t h e i n t e g r a t e d i n t e n s i t i e s o f the-$000- and 2100-cm bands n o r m a l l z e d t o t h e i n t e g r a t e d i n t e n s i t y o f t h e 640-cm wag band, A ( W ) , (assumed t o b e p r o p o r t i o n a l t o t h e t o t a l H c o n t e n t ) , and t h e A(2000)/A(2100) r a t i o . It i s observed t h a t E E I and ( U T ) decrease r a p i d l y , a f t e r i n i t i a l s l i g h t increases, as thg4'tar!Lt-gbbstrate P e p a r a t i o n decreases, w h i l e W and a increase. The IR parameters show monotonic changes w i t h t h e t a r g e t - s u b s f b a t e di!$ance: A(2100)/A(W) vanishes f o r samples c l o s e s t t o t h e t a r g e t and A(2000)/A(2100) i n c r e a s e s s i g n i f i c a n t l y . The I R s t r e t c h bands o f two f i l m s , one on t h e s u b s t r a t e p l a t f o r m and one 1 i n c h away f r o m t h e t a r g e t , a r e shown i n Fig.2.

DISCUSSION: From Fig.1 i t i s seen t h a t t h e decrease i n t h e t a r g e t - s u b s t r a t e s e p a r a t i o n l e a d s t o an i n c r e a s e i n t h e A(2000)/A(2100) r a t i o which i s p a r a l l e l e d by d e t e r i o r a t i o n o f t h e p h o t o e l e c t r o n i c p r o p e r t i e s . The d e c r e a s i n g E i n d i c a t e s a n a r r o w i n g gap, w h i l e t h e i n c r e a s i n g W suggests b r o a d e n i n g bans4edges. The quenching of t h e photoluminescence, r e d u c t i g h o f ( u ~ ) f o r e l e c t r o n s and t h e i n c r e a s e i n t h e d a r k c o n d u c t i v i t y a r e a l l syaPtomatic o f h i g h d e f e c t c o n c e n t r a t i o n s and a l a r g e d e n s i t y o f s t a t e s i n t h e gap i n f i l m s d e p o s j t e d c l o s e t o t h e t a r g e t . It i s e s s e n t i a l t o m e n t i o n t h a t t h e hydrogen c o n t e n t ,c d r o p s f r o m 14 at.% f o r samples on t h e s u b s t r a t e p l a t f o r m t o 9 at.% f o r t h e ones ":losest t o t h e t a r g e t and t h i s change i n cH i s

not

s u f f i c i e n t t o e x p l a i n t h e observed changes i n t h e p r o p e r t i e s .

It i s known t h a t secondary e l e c t r o n s and n e g a t i v e i o n s e m i t t e d from t h e t a r g e t 4 d u r i n g s p u t t e r i n g can be a c c e l e r a t e d t o v e r y h i g h v e l o c i t i e s as t h e y f a l l t h r o u g h t h e dark space. These p a r t i c l e s can bombard t h e growing f i l m s u r f a c e w i t h c o n s i d e r a b l e e n e r g i e s i f t h e t a r g e t - s u b s t r a t e d i s t a n c e i s t o o s m a l l o r t h e t o t a l

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C4-682 JOURNAL DE PHYSIQUE

gas pressure i s too low t o allow thermalization before impact. We believe t h a t t h i s t a r g e t - p a r t i c l e bombardment i s the cause 50r t h e d e t e r i o r a t i o n of t h e properties with decreasing target-substrate distance

.

Based on t h e r e s u l t s of lie+-ion bombardment studies6 where i t was shown t h a t bombardment induced damage enhanced t h e i n t e n s i t i e s of c e r t a i n

I R

bands, we conjecture t h a t t h e elimination of t h e 2100-cm- band and t h e increase i n t h e A(2000)/A(2100) r a t i o with decreasing target-substrate separation i s a l s o caused by increasing t a r g e t - p a r t i c l e bombardment a t closer distances. We suppose t h a t t h e bombardment during t h e growth of t h e film promotes H incorporation into s i t e s associated with t h e 2000-cm- band.

A close look a t t h e sample preparation conditions i n t h e reports on sputtered f i l m with only t h e 2000-cm- band reveals t h a t t h i s was achieved through high RF power levels o r by placing jubstrates a short distance away from t h e t a r g e t , j u s t outside t h e dark space

.

Thus both methods Ieem t o rely on increased t a r g e t - p a r t i c l e bombardment f o r eliminating t h e 2100- band.

Regardless of t h e exact mechanisms involved, however, t h e experimental observations of t h i s st_ufly provide d e f i n i t i v e evidence against t h e unique association of t h e 2000-cm band with useful photoelectroni c properties.

This work was supported by J o i n t Services Electronics Program under contract number N00014-75-c-0648 and by National Science Foundation under contract number NSF-DMR78-10014.

References.-

1. F.R. J e f f r e y , H.R. Shanks, G.C. Danielson, J. Non-Cryst. Solids

35-36,

261 (1980)

F.R. J e f f r e y , H.R. Shanks, G.S. Danielson, J. Appl. Phys. 50, 7034(1979) 2. Details of preparation and measurement techniques have been given i n

W. Paul and D.A. Anderson, review, Solar Energy Materials, t o be published 3. The c a l i b r a t i o n of the hydrogen content, c

-

N (N +N .)x100, t o

A(") was deduced from a comparison of ti ev8iut~;n,'nu~lear reaction and

IR measurements on s p u t t e r deposited films.

4. Handbook of Thin Film Technolo

,

(L.I. Maissel and R. Glang eds., kGraw Hin-

m , - m F

^4-11

5. The i n i t i a l increases i n E

, ^E,,

I and (UT) can be explained i n terms of s l i g h t l y more fav8fable e f h c t i v e g r d t h temperatures t h a t these films, which a r e thermally isolated f r a n t h e temperature control led s u b s t r a t e platform, experience due t o plasma heating.