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ELSEVIER Surface Science 357-35 8 (1996) 555-559s u r f c e s c i e n c e

H 2 TP D s tudy on the difference in the grow th k inet ics betweenS i ll 4 a n d S i 2 H 6 G S M B E

M . S u e m i ts u , H . N a k a z a w a , N . M i y a m o t oResearch Institute o f Electrical Communication Toho ku University 2- 1- 1 Katahima Sendal 980 Japan

Received 22 A ugus t 1995; accepted for pub lication 1 N ovem ber 1995

Abstract

Hydrogen desorption kinetics from a Si(100) surface grown b y gas-source molecular beam epitaxy (GSM BE) using either silaneor disilane has bee n studied w ith temperature-programmed-desorption (TPD) measurements. For each source gas, a series of TP Dspectra was obtained from surfaces quenched from the growth at various temperatures. The T PD spectra from disilane-GSMBE-grown surfaces showed no shift in the p eak temperature, while those from the silane-grown surfaces showed a peak shift towardshigher temperatures by 25C at hydrogen coverages below 0.3 ML . T his shift, indicative of the presence of higher-order desorptionkinetics, suggests a more isolated configuration of the adsorbed hydrogen o n the silane-grown surfaces. Difference in the growthkinetics between silane and disilane is also discussed.

1 I n t r o d u c t i o n

B o t h s i l a n e ( S i l l 4 ) a n d d i s i l a n e ( S i z H 6 ) a r e t h ec o m m o n S i - b e a r i n g s o u r c e g a s e s e q u a l l y u s e d i nS i c h e m i c a l v a p o r d e p o s i t i o n o r S i g a s - s o u r c em o l e c u l a r b e a m e p i ta x y . T h e y , h o w e v e r , s h o w d i s -s i m i l a r i t i e s i n t h e i r t e m p e r a t u r e - d e p e n d e n c e o r t h ed o p i n g e f fe c t o f th e g r o w t h r a te ; r e a c t i v e s t ic k i n gc o e ff i ci e n t ( R S C ) o f d i s i la n e i s a b o u t t w o o r d e r s o fm a g n i t u d e h i g h e r t h a n t h a t o f s i l a n e 1- 1,2 ], w h i l ed o p i n g w i t h p h o s p h i n e s i g n i f i c a n t l y r e d u c e s t h ep o l y - si l ic o n g r o w t h r a t e b y s i la n e, b u t n o t s o m u c hby d i s i l a n e [-3 ] .

O n e o f t h e m e c h a n i s m s u n d e r l y i n g t h e d i f f er e n tg r o w t h b e h a v i o r i s t h e d i f f e r e n t a d s o r p t i o n s t a t eo f t h e s o u r c e g a s es . A d s o r p t i o n s t a t e is r e s p o n s i b l ef o r b o t h t h e a d s o r p t i o n r a t e o f t h e s o u r c e g a s

*Corresponding author. F ax : +81 22 2175482; e-mail:Nakazawa@miyamoto.riec.tohoku:ac.jp.

m o l e c u l e a n d t h e h y d r o g e n d e s o r p t i o n r a t e t h e r e -f r om . A g o o d e x a m p l e o f th e f o r m e r is th e n u m b e ro f r e q u i r e d a d s o r p t i o n s it es f o r th e g r o w t h . T h eh i g h R S C o f d i si l an e i s p a r t l y u n d e r s t o o d i n t e r m so f i t s fe w e r ( t w o ) n e c e s s a r y a d s o r p t i o n s i te s 1 - 4 ] a sc o m p a r e d t o t h a t o f s i l a n e (f o u r ) [ -5 ,6 ] .

T h e i n fl u en c e o f th e a d s o r p t i o n s t a t e o nh y d r o g e n d e s o r p t i o n c a n b e b e s t d e sc r ib e d i n t e r m so f t h e r e a c t i o n o r d e r o f t h e p r o c e s s , I n g e n e r a l , iti s w e l l e s t a b l i s h e d t h a t t h e h y d r o g e n d e s o r p t i o nf r o m S i (1 0 0 ) 2 x 1 m o n o h y d r i d e p h a s e f o l lo w s af i r s t - o r d e r r e a c t i o n 1-7 ] . T h e p r e p a i r e d m o d e l [, 7 ] ,i n w h i c h t h e d o u b l y o c c u p i e d d i m e r ( D O D ) a c tsa s t h e d e s o r p t i o n p r e c u r s o r , h a s b e e n w i d e l ya c c e p t e d t o a c c o u n t f o r t h e f i r s t o r d e r r e a c t i o n .H o w e v e r , i f t h e a r r a n g e m e n t o f su r fa c e h y d r o g e ni s s u c h t h a t i t i n t e r f e r e s w i t h t h e i r a s s o c i a t i o np r o c e ss f o r f o r m i n g D O D s , t h e d e s o r p t i o n w o u l db e o f h i g h e r o r d e r k i n e ti c s. T h e a i m o f t h i s p a p e ri s t o o b t a i n i n s i g h t o n t h e d i ff e re n c e i n t h e g r o w t h

0039-6028/96/ 15.00 C opyrigh t 1996 Elsevier Science B.V. All rights reservedP I I S0039-6028 (96) 00222-1

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556 M. Suemitsu et aL /Surface Science 357-358 1996) 555-559k i n e t i c s b e t w e e n t h e t w o s o u r c e g a s e s t h r o u g hs t u d ie s o n t h e h y d r o g e n d e s o r p t i o n k i n e t i c s o f t h eg r o w n s u rf a c es .

2 Exper imentE x p e r i m e n t s w e r e m a d e i n a n a l u m i n u m - a l l o y

u l t r a h i g h v a c u u m c h a m b e r w h o s e b a s e p r e s s u r ew a s b e l o w 1 .0 x 10 - 9 T o r r . S a m p l e s w e r e b o r o n -d o p e d S i 1 0 0 ) p = 2 0 - 5 0 f ~. c m ) w a f e r s w h i c h w e r ec u t t o 5 x 3 3 m m 2 . P r i o r t o i n t r o d u c t i o n t o t h ec h a m b e r , s a m p l e s w e r e c h e m i c a l l y c l e a n e d b y1 H z S O 2 : l H 2 0 z a n d 1 H F : 2 0 H z O f o l l o w e d b y a 1m i n u l t r a v i o l e t o z o n e c l e a n i n g . S a m p l e s w e r e r e s i s -t i v e ly h e a t e d i n v a c u o a n d w e r e d e g a s s e d a t 7 0 0 Cf o r 1 5 r a i n a n d f l a s h a n n e a l e d a t 1 2 0 0 C f o r 3 0 st o o b t a i n t h e c l e a n 2 1 + 1 x 2 r e c o n s t r u c t e d s u r -f a c e b y r e f l e c ti o n h i g h e n e r g y e l e c t r o n d i f f r a c t io n

R H E E D ) .G r o w t h w a s c o n d u c t e d b y u s i n g e i t h e r

2 . 0 x 1 0 - 4 T o r r s i la n e o r 1 .0 x 1 0 - 5 T o r r d i s il a n e .T h e d i f f e r e nt c h o i c e o f t h e p r e s s u r e w a s t o o b t a i ns i m i la r h y d r o g e n c o v e r a g e a f t e r 1 0 r a i n o f s t a t io n -a r y g r o w t h . A f t e r 1 0 ra i n o f t h e g r o w t h , t h e g r o w i n gs u rf a ce w a s q u e n c h e d b y s t o p p in g b o t h t h e s o u r c eg a s a n d t h e s a m p l e c u r r e n t s u p p l i e s . A s r e p o r t e dp r e v i o u s l y [ 8 ] , t h e a m o u n t o f t h e q u e n c h e d s u r f a c eh y d r o g e n s d e c r e a s e s w i t h i n c r e a s i n g g r o w t h t e m -p e r a t u r e , w h i c h w a s u t i l i z e d t o c o n t r o l t h e f o r m e rq u a n t i t y .

T h e r e a c t i o n o r d e r n o f t h e h y d r o g e n d e s o r p t i o np r o c e s s w a s o b t a i n e d f r o m a s e r ie s o f t e m p e r a t u r e -p r o g r a m m e d d e s o r p t i o n T P D ) m e a su r e m e n ts , i nw h i c h t h e q u e n c h e d h y d r o g e n c o v e r a g e 0 w a sv a r ie d ; th e T P D p e a k t e m p e r a t u r e T p is i n d e p e n -d e n t o f 0 w h e n n= > i , w h i l e i t s h i ft s t o w a r d sh i g h e r t e m p e r a t u r e s w i t h d e c r e a s i n g 0 w h e n n > I .T P D m e a s u r em e n t s w e r e c o n d u c t e d a t m e=2H 2 ) b y u si n g a q u a d r u p o l e m a s s s p e c t r o m e t e r

w i t h a r a m p r a t e o f 5 . 7 C /s .

3 Resu lts and discussionF i gs . 1 a n d 2 a r e s e ri es o f T P D s p e c t r a o b t a i n e d

f r o m t h e g r o w n s u r f a c e s u s i n g s i l a n e a n d d i s i l a n e ,r e s p e c t i v e l y . A l l t h e s p e c t r a p r e s e n t a s in g l e,

v

090

I I IS i H 4 / S i ( 1 0 0 )P s i H 4 = 2 . 0 x l 0 - 4 T o r r . 4 5 0 o c/ / ~ J ( 0 . 8 3 M E )/ ', \ / 5 0 0y

/ / ~ " ~ 5 2 0 7 )I 1 ' , , ~ . /6 0 0/ / / / ~ \ ~ " ( 0 . 1 3 )/ / 2 t '__ 2 ~ \ J 6 5 0

4 0 0 6 0 0l - e m p e r a t u r e C )F ig . 1 . H y d ro g e n c o v e ra g e d e p e n d en c e o f H 2 T P D s p e c t ru mfrom SiH 4-GS M BE grown Si 100) su rface.

. m - .

c

c o0 3( 2

I iS i 2 H 6 / S i ( 1 0 0 )" P S i 2 H 6 = 1 . 0 x l0 - 5 T o r ri

C. ~ 4 5 0/ ~ ( 0 . 7 7 M L )/ ~ / 5 0 0/ i . . ~ 0 .4 4 )

/ / I f \ / 5 5 00 . 2 6 )/ / ~ \ / 6 0 0/ / / / : \ ~ . . . " " ( 0 . 1 1 )/ / / ; e-N~,L~ 6 5 0' ( 0 . 0 7 )6 ( ) 00 0T e m p e r a t u r e ( C )

F ig . 2 . H y d ro g e n c o v e ra g e de p e n d e n c e o f H 2 T P D s p e c t ru mfrom Si2H6-G SMB E grown Si 100) su rface.

s o - c a ll e d f ll p e a k a t a r o u n d 5 0 0 C , w h i c h c a n b ea t t r ib u t e d t o h y d r o g e n d e s o r p t i o n f r o m t h e m o n o -h y d r i d e p h a se . T h i s fa c t, to g e t h e r w i t h o u r R H E E Do b s e r v a t i o n o f t h e 2 1 re c o n s t r u c t i o n , i n d i c a t e st h a t t h e h y d r o g e n d e s o r p t i o n s r e l e v a n t t o t h ep r e s e n t s t u d y a r e a l l f r o m t h e S i 1 0 0 ) - 2 x 1 m o n o -h y d r i d e p h a s e .

I n s p i t e o f t h i s i d e n t i c a l o r i g i n o f t h e s u r f a c eh y d r o g e n , t h e s i l a n e - a n d t h e d i s i l a n e - g r o w n s u r -f a c e s p r e s e n t q u i t e a c o n t r a s t i n t h e i r T P D p e a k

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M . S u e m i t s u e t a l . / S u r f a c e S c ie n c e 3 5 7 - 3 5 8 1 9 9 6 ) 5 5 5 - 5 5 9 557behavior. For the silane-grown surface Fig. 1), thespectrum presents a Tp shift by more than 25Cfor hydrogen coverages below 0.3 ML. Since thelower coverage surfaces correspond to a growth athigher temperatures, where the surface migrationsare enhanced, the presence of the shift indeedindicates the rate-limitation by a higher orderprocess in the hydrogen desorption. In fact, ourprevious analysis [8] on the identical systemyielded n= 1.59 for the reaction order. Quite con-trary to this, the surface quenched from disilanegrowth Fig. 2) presented no Tp shift down to 0 =0.1 ML. This shows that the hydrogen desorptionfrom disilane-grown surfaces is dominated by afirst order process.

This difference in the reaction order can be bestinterpreted as being caused by the differenthydrogen association rate at the surface to formthe desorption precursor, DOD. One possiblefactor that is responsible for the surface associationprocess is the surface morphology or the surfaceroughness. As far as the atomic force microscopyand Nomarski optical microscopy are concerned,however, no differences were observed betweensilane-and disilane-grown surfaces. Therefore, wefocus here to a nother more likely factor, the surfacehydrogen arrangement. If the hydrogen atoms fromthe source gas molecules are readily paired asDOD just after the adsorption, the desorptionkinetics should be of first order. On the otherhand, if the adsorbed hydrogen atoms are sittingseparately as-Sill2 or singly occupied dimersSOD), the desorption kinetics should be morelimited by higher order processes.In order to proceed further, detailed knowledgeon the adsorption state of silane and disilane is

indispensable. The initial stage of disilane adsorp-tion onto Si 100) has been rather well clarified byFourier-transformed infrared absorption [9],electron energy loss spectroscopy [10], orstatic secondary ion-mass spectrometry [11],which all presents a common view that disilaneutilizes two neighboring dangling bonds to formtwo adjacent-Sill3 adsorbates Figs. 3a and 3b).They are then combined together to form DODsFig. 3c).In cont rast to the well established unders tandingof the Si 100)/disilane system, studies on the silane

b )

c ) 1

d ) ]

Fig. 3. Model of Si2H6 adsorption on Si 100) during SizH6-GSMBE: a) before adsorption, b) just after the adsorption,c) the second stage after the adsorption and d)surfaceduring growth.

adsorption process have been insufficient. Thenumber of required adsorption sites itself has beenan issue. Our contribution, however, is the finding[ 5,6] that a silane molecule most probably requiresfour adjacent adsorption sites on its adsorptionFigs. 4a and 4b). This conclusion has its basis onour observation that the growth rate shows afourth power dependence on the free site densityof the surface.From this knowledge on the initial stages of thesilane and disilane adsorption, the difference in thehydrogen desorption kinetics between the twosource gases is explained as follows. On disilane-grown surfaces Fig. 3d), the two Si adatoms from

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558 M . S u e m i t s u e t a l . / S u r f a c e S c i e n c e 3 5 7 - 3 5 8 1 9 9 6 ) 5 5 5 - 5 5 9

a ) , A

b )

c )

d )

Fig. 4. M ode l of Si l l4 adsorption on Si 100) during SiH4-GSM BE: a) before adsorption, b) just a fter the adsorption,c) the se cond stage after the a dsorpt ion and d) surfaceduring grow th.a s o u r c e ga s m o l e c u l e a d s o r b a s a n a d a t o m d i m e r .T h e s i x h y d r o g e n s o f t h e m o l e c u l e a r e m o s t l yf o r m i n g D O D s , a p a r t o f w h i c h h a s h o w e v e rd e s o r b e d d u e t o t h e e l e v a te d t e m p e r a t u r e o f t h eg r o w i n g s u r f a c e . T h i s e x c l u s i v e p r e s e n c e o f s u r f a c eh y d r o g e n s a s D O D s i s r e s p o n s i b l e f o r t h e f i r s to r d e r d e s o r p t i o n k i n e ti c s o f t h e h y d r o g e n s o n t h ed i s i l a n e - g r o w n s u r f a c e .

T h e s i l a n e - g r o w n s u r f a c e d if f e r s f r o m t h e d i s i-l a n e - g r o w n s u r f a c e b y t h e f a c t t h a t t h e s o u r c e g a sm o l e c u l e p r e s e n t s o n l y o n e S i a d a t o m . I n t h em o d e l i l l u s tr a t e d i n F ig . 4 b , th e a d s o r b a t e p r e c u r -s o rs a r e t w o a t o m i c h y d r o g e n s a n d a = S i l l 2 . T h et w o a t o m i c h y d r o g e n s m a y r e a d il y b e c o m b i n e d

t o f o r m a D O D , b u t t h e = S i l l 2 s ta r ts m i g r a ti n go n t h e s u r f a c e F i g . 4 c ). O n t h e g r o w i n g s u r f a c eF ig . 4 d ) , a p a r t o f th e D O D s m a y h a v e d e s o r b e d

a n d t h e = S i ll 2 p r e c u r s o r s a r e p r e d o m i n a n t l y l ef t.I n o r d e r f o r t h e h y d r o g e n s o f th i s = S i ll 2 t o d e s o r b ,t h e p r e c u r s o r m u s t m i g r a te o n t h e s u r f a c e t o f o r ma n a d a t o m d i m e r o r D O D . T h e h i g h e r - o r d e rh y d r o g e n d e s o r p t i o n k i n e t i c s f r o m t h e s i l a n e -g r o w n s u r f a c e s i s u n d e r s t o o d f r o m t h e s p a r s e l yd i s tr i b u te d = S i l l 2 . H e r e w e h a v e a s s u m e d t h a tt h e t w o h y d r o g e n s o f a ~ - S iH 2 p r e c u r s o r b y t h e m -s e lv e s a r e h a r d t o d e s o r b a s H 2 a s e v i d e n c e d f r o mt h e a b s e n c e o f t h e f12 p e a k i n t h e T P D s p e c t r aFig. 1).

F i n a l l y , le t u s b r i e f ly d i s c u ss t h e h y d r o g e nd e s o r p t i o n f r o m t h e a t o m i c - h y d r o g e n - a d s o r b e dS i 1 0 0 ) s u rf a c e. F r o m t h e a r g u m e n t a b o v e , o n em i g h t r a i s e a q u e s t i o n a s t o w h y a n a t o m i c -h y d r o g e n d o s e d S i 1 0 0 ) 2 x 1 s u r fa c e p r e s e n ts af i r s t - o r d e r d e s o r p t i o n , o r n o T p s h i f t , w h e n m o s to f t h e s u r fa c e h y d r o g e n a t o m s a r e c o n s i d e r e d t ob e a d s o r b i n g r a n d o m l y . F o r t h i s , a r e c e n t T P Do b s e r v a t i o n b y W i d d r a e t a l . i s i n s t r u c t i v e [ 1 2 ] .T h e S i 1 0 0 ) s u r f a c e d o s e d w i t h a t o m i c h y d r o g e n s ,t o o u r s u r p ri se , s h o w e d a d o m i n a n c e o f D O D sf o r a s u b s t r a te t e m p e r a t u r e a s l o w a s 1 50 K . T h es u p e r t h e r m a l e n e r g y o f t h e i m p in g i n g a t o m i ch y d r o g e n w a s p r o p o s e d t o a c c o u n t f o r th e n o n -t h e r m a l l y a c t i v a t e d s u r f a c e m i g r a t i o n .

T h e p r e s e n t a d s o r p t i o n m o d e l c a n a l s o e x p l a i nt h e d i f f e r e n t R S C f o r t h e t w o s o u r c e g a s e s . F i r s t l y ,d i s il a n e is m o r e r e a d i l y d i ss o c i a t e d d u r i n g a d s o r p -t io n . T h i s i s so b e c a u s e t h e e n e r g y r e q u i r e d t ob r e a k t h e S i - S i b o n d 1 . 80 e V ) t o d i ss o c i a te d is i-l a n e i s m u c h l o w e r t h a n t h e o n e r e q u i r e d t o b r e a kt h e S i - H b o n d 3 . 0 5 e V ) to d i ss o c i a te s il an e .S e c o n d l y , t h e a d a t o m s f r o m d i s i la n e a r e m o r er e a d i l y s t a b i l i z e d a t t h e s u r f a c e b y f o r m i n g a na d a t o m d i m e r , w h i l e t h e a d a t o m s f r o m s i l an e m u s tm i g r a t e to f i n d a c o u n t e r p a r t t o f o r m a n a d a t o md i m e r . T h i r d l y , t h e s u r f a c e h y d r o g e n d e s o r p t i o np r o c e s s i s m o r e e n h a n c e d o n d i s i l a n e - g r o w ns u r faces .

T o s u m m a r i z e , t h e d i f f er e n c e i n t h e g r o w t h k i n e t -i cs b e t w e e n s i l a n e - a n d d i s i la n e - G S M B E o n S i 1 0 0 )s u r f a c e s h a s b e e n i n v e s t i g a t e d , b y u s i n g t h eh y d r o g e n d e s o r p t i o n p r o c e s s f ro m t h e g r o w n s u r-f a c e a s a p r o b e . T h e o b s e r v e d h i g h e r - o r d e r d e s o r p -

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M . S u e m i t s u e t a l. / S u r f a c e S c ie n c e 3 5 7 - 3 5 8 1 9 9 6 ) 5 5 5 - 5 5 9 559tion kinetics for the silane-grown surface incontrast to the first-order one for the disilane-grown surface has been given the interpretationthat the surface hydrogens are more sparsely dis-tributed on the silane-grown surface. A model hasbeen proposed which successfully explains thedifferent distribution of the surface hydrogens interms of the different content of Si atoms in thesource gas molecule.

eferences

[ 1 ] K. W eaner , S . Butzke , S . Rad e laa r and P . Ba lk , J . Crys ta lGrowth 136 1994) 322 .[2 ] M. S ue mit su , K . - J . K im, H . N a k a z a w a a n d N . Miy a mo to ,Appl. Surf. Sci. , to be published.

1 -3] S . Nakayama, H. Yonezawa and J . Muro ta , Jpn . J . Appl .Phys. Par t 2 23 1984) 493.[4 ] K. Weaner , S . Bntzke , S . Rade laa r and P . Ba lk, J . Crys ta lGrowth 136 1994) 338 .[5 ] K . - J . K im, M . S u e mit su , M. Y a ma n a k a a n dN. M iyam oto , Appl . Phys . Le t t . 62 1993) 3461 .[6 ] F . H i ro s e , M. S u e mi t s u a n d N . M iy a mo to , J . A p p l . P h y s .70 1991) 580.

1-7] J .J . Bo land, P hys. Rev. Lett. 67 1991) 1539.[8 ] M. S u e mit su , K . - J. K im a n d N . M iy a mo to , P h y s . R e v . B4 1994) 48.[9 ] A.C. Dil lon , M.B. Robinson and S .M. George , Surf . Sc i .Lett. 295 1993) L998.

[10 ] Y. Suda , D. Lubben , T . M otto ka an d J .E . Greene , J . Vac .Sci. Techn ol. A 8 1990) 61.1 -11] S .M. Ga tes an d C.M . Chiang , Chem . Phys . Le t t . I841991) 448.[12] W. Widdra , S . I . Yi , R . Maboudian , G.A.D. Briggs andW.H. Weinberg , Phys .