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GROWTH OF THIN SINGLE CRYSTAL NiSi2 FILMS OF Si SURFACES, A FIELD ION MICROSCOPE
STUDY
Hong Liu, T. Tsong
To cite this version:
Hong Liu, T. Tsong. GROWTH OF THIN SINGLE CRYSTAL NiSi2 FILMS OF Si SURFACES, A FIELD ION MICROSCOPE STUDY. Journal de Physique Colloques, 1986, 47 (C2), pp.C2-315-C2- 319. �10.1051/jphyscol:1986248�. �jpa-00225682�
GROWTH OF THIN SINGLE CRYSTAL Nisi, FILMS OF Si SURFACES, A FIELD ION MICROSCOPE STUDY
H.F. LIU, H.M. LIU* and T.T. TSONG
Physics Department, The Pennsylvania State University, University Park, PA 16802, U.S.A.
A b s t r a c t - Thin s i n g l e c r y s t a l N i s i 2 f i l m s have been grown e p i t a x i a l l y o n t h e [l111 o r i e n t e d S i t i p s u r f a c e i n UHV. A 180' change i n t h e a x i a l symmetry i s found f o r t h e f i e l d i o n images t a k e n b e f o r e and a f t e r t h e growth of t h e s i l i - c i d e l a y e r s . From t h i s o b s e r v a t i o n and a computer s i m u l a t i o n of t h e f i e l d
ion images of A- and B-type i n t e r f a c e s we conclude t h a t t h e Si-Nisi2 i n t e r - f a c e formed has t h e B-type s t r u c t u r e .
I. INTRODUCTION
Metal s i l i c i d e t h i n f i l m s grown on S i s u b s t r a t e s a r e widely used i n semiconductor in- d u s t r y a s ohmic o r Schottky c o n t a c t s . 1 The formation of Schottky b a r r i e r a t t h e metal-semiconductor i n t e r f a c e s h a s been a s u b j e c t of i n t e n s e i n t e r e s t f o r over t h i r t y
year^.^-^ I t is now c l e a r t h a t e l e c t r i c a l p r o p e r t i e s of m e t a l s i l i c i d e - s i l i c o n con- t a c t s such a s t h e Schottky b a r r i e r h e i g h t can depend on t h e chemistry and t h e atomic s t r u c t u r e of t h e s i l i c i d e - s i l i c o n i n t e r f a c e . Nickel s i l i c i d e - s i l i c o n system i s one of %he h o s t s t u d i e d systems because N i s i Z has a s i m i l a r l a t t i c e s t r u c t u r e (CaF2) t o s i l i c o n and t h e l a t t i c e m i s f i t (-1.2%) to S i is small. Thin f i l m s of s i n g l e c r y s t a l N i s i Z can be grown w i t h high degree of p e r f e c t i o n on s i l i c o n under u l t r a high vacuum
(UHV) c o n d i t i o n s . The o r i e n t a t i o n of t h e N i s i 2 f i l m s grown on t h e S i ( l l 1 ) s u r f a c e can be c o n t r o l l e d by t h e c o n d i t i o n s of t h e i n i t i a l N i d e p o s i t i o n t o be e i t h e r i d e n t i - c a l t o t h a t of t h e s u b s t r a t e , type A i n t e r f a c e , o r t o be r o t a t e d 180- about t h e sur- f a c e normal d i r e c t i o n with t h e s u b s t r a t e , o r type B i n t e r f a ~ e . ~
Atom-probe and f i e l d i o n microscope6 h a s been s u c c e s s f u l l y employed t o s t u d y t h e com- p o s i t i o n a l v a r i a t i o n and atomic s t r u c t u r e s of WSi2-W and N i s i l i c i d e - S i interface^.^^^
Four e a r l y s t a g e s of growth of WSi2 on t h e W s u r f a c e have been i d e n t i f i e d . ' The t h i r d s t a g e is an e p i t a x i a l growth o f t h i n s i l i c i d e l a y e r s on t h e (001) p l a n e s of t h e W e m i t t e r s u r f a c e . The phase boundary between s i l i c i d e and t h e s u b s t r a t e is found t o be very sharp.
Here we w i l l r e p o r t t h e growth of n e a r l y p e r f e c t s i n g l e c r y s t a l N i s i 2 f i l m s on S i t i p s u r f a c e s i n UHV by vapor d e p o s i t i o n method. A 180' change i n t h e a x i a l symmetry is found f o r t h e f i e l d i o n images taken b e f o r e and a f t e r t h e growth of t h e t h i n s i l i c i d e l a y e r s . From t h i s symmetry change and a ccinputer s i m u l a t i o n of t h e f i e l d i o n images we conclude t h a t t h e Nisi2-Si i n t e r f a c e has t h e B-type s t r u c t u r e . We f i n d t h a t S i atoms, though v i s i b l e , appear much dimmer i n t h e f i e l d ion image, and t h e q u a l i t y o f t h e Ne i o n image is good enough t o r e v e a l t h e atomic s t r u c t u r e of t h e N i l a y e r of t h e
(111) plane. The f i l m s a r e f r e e of g r a i n boundaries.
* v i s i t i n g s c h o l a r f r o m D a l i a n Marine C o l l e g e , D a l i a n , C h i n a
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986248
JOURNAL DE PHYSIQUE
S i l i c o n t i p s a r e prepared both from S i whiskers and from r o d s c u t from a [l111 o r i - ented S i c r y s t a l by e l e c t r o c h e m i c a l e t c h i n g i n HN03-HI? mixed s o l u t i o n . Before it i s put i n t o t h e FIM, i t i s f i r s t g i v e n an "RCA c l e a n " t o remove t h e o x i d e l a y e r s . The FIM is equipped w i t h a Displex R e f r i g e r a t o r s o t h a t t h e t i p temperature c a n be v a r i e d from -15 K t o 300 K. Before an experiment, t h e system is s u b j e c t e d t o a l i g h t bake o u t t o achieve a vacuum i n t h e low 10-10 Torr range. The S i t i p is t h e n f i e l d evapo- r a t e d a t 100 K i n e i t h e r H2 o r Ne image g a s u n t i l a s p h e r i c a l , c l e a n S i s u r f a c e is obtained. We f i n d t h a t f i e l d e v a p o r a t i o n of S i atoms i s n o t very smooth and t h e S i s u r f a c e developed by f i e l d e v a p o r a t i o n i s never a s p e r f e c t a s s u r f a c e s of m e t a l t i p s . However, a d e f i n i t i v e i d e n t i f i c a t i o n of c r y s t a l p l a n e s is p o s s i b l e e s p e c i a l l y when
FIG. 1. ( a ) A 60 K neon f i e l d i o n image FIG. 2. ( a ) A 25 K neon i o n image of t h e of a [l111 o r i e n t e d S i t i p . (b) A com- e x p i t a x i a l l y grown N i s i 2 l a y e r s on t h e same p u t e r simulated image of t h e same S i t i p . S i t i p . (b) A computer simulated image of a The t i p r a d i u s is taken t q be 200 a and s i n g l e c r y s t a l N i s i 2 e p i t a x i a l l y grown on t h e s h e l l t h i c k n e s s is 0.0533 a where a t h e S i (111) p l a n e with t h e B-type i n t e r f a c e . is t h e bond l e n g t h of t h e S i l a t t i c e . The t i p r a d i u s is 220 a and t h e s h e l l t h i c k -
n e s s i s 0.05 a where a is t h e l e n g t h of t h e Si-Ni bond.
t h e vapor d e p o s i t i o n , t h e t i p is annealed i n s i t u a t 500°C f o r 5 minutes to induce e p i t a x i a l s i l i c i d e formation. The t i p is t h e n imaged a t 25 K i n neon a f t e r c a r e f u l f i e l d evaporation. The f i e l d i o n image q u a l i t y is improved c o n s i d e r a b l y a s shown i n Fig. 2 ( a ) . We f i n d t h a t a t such a low temperature it is necessary t o i l l u m i n a t e t h e sample with a l i g h t beam t o reduce t h e r e s i s t i v i t y of t h e S i t i p by t h e photo con- d u c t i v i t y e f f e c t . N e images o f t h e N i s i 2 f i l m s , shown i n Fig. 2 ( a ) have t h e same
3-fold symmetry a s t h e S i image. However, we f i n d t h a t t h e N i s i 2 image is r o t a t e d 180' with r e s p e c t t o t h e image of t h e s i l i c o n s u b s t r a t e a s c a n be s e e n by comparing Fig. l ( a ) with Fig. 2 ( a ) . To e x p l a i n t h i s o b s e r v a t i o n , we r e s o r t t o a computer simu- l a t i o n of t h e f i e l d i o n images.
The shape of a f i e l d i o n e m i t t e r is approximated by an [l111 o r i e n t e d cone with a hemispherical t i p s 9 The computer s i m u l a t i o n of t h e f i e l d ion image is based on a t h i n s h e l l model.1° For a pure s i l i c o n t i p , which has t h e diamond s t r u c t u r e , t h e simulated image a g r e e s v e r y w e l l with t h e f i e l d i o n image a s shown i n Fig. l ( b ) . N i s i 2 h a s a c u b i c C1 s t r u c t u r e 1 1 a s shown i n Fig. 3. S i n c e S i atoms appear t o be
FIG. 3. The c u b i c C 1 s t r u c t u r e of N i s i 2 .
much dimmer o r not imaged a t a11,12 f o r t h e computer s i m u l a t i o n , we assume t h a t o n l y N i atoms a r e imaged i n t h e FIM. The simulated f i e l d i o n images of t h e NiSi2, shown i n Fig. 2 ( b ) , is o b t a i n e d by r e p l a c i n g t h e hemispherical p a r t of t h e S i t i p with a s i n g l e c r y s t a l N i s i 2 e p i t a x i a l l y grown on t h e S i ( l l 1 ) p l a n e w i t h a type B i n t e r f a c e . A s c a n be s e e n by camparing Figs. 2 ( a ) , t h e simulated image a g r e e s very w e l l with t h e f i e l d ion image, both show t h e same 3-fold symmetry and t h e same c r y s t a l f a c e t s . The symmetry a x i s is, however, r o t a t e d 180. with r e s p e c t t o t h a t of both t h e F1 image and t h e computer simulated image of t h e o r i g i n a l S i t i p . The f i e l d i o n images of s i l i c i d e l a y e r s show no o b s e r v a b l e g r a i n boundary.
The N i l a y e r s d e p o s i t e d on t h e S i s u r f a c e is only about 50 l a y e r s i n t h i c k n e s s , and t h e growth of N i s i 2 f i l m s s t a r t s simultaneously from t h e v a r i o u s f a c e t s of t h e hemi- s p h e r i c a l s u r f a c e o f t h e S i t i p . The c l o s e resemblence between t h e simulated image and a l l t h e f i e l d ion images taken by layer-by-layer f i e l d e v a p o r a t i o n s u g g e s t s two s i g n i f i c a n t f a c t s . F i r s t , t h e e p i t a x i a l l y grown N i s i 2 is a s i n g l e c r y s t a l . Second, t h e N i s i 2 i s grown a s a c o n t i n u a t i o n of t h e S i s u b s t r a t e i n a l l d i r e c t i o n s , o t h e r w i s e a c o n s i d e r a b l e d e t e r i o r a t i o n of t h e images should occur. Tung e t a l . f i n d t h a t S i
[l111 is t h e s t r o n g l y p r e f e r r e d o r i e n t a t i o n f o r t h e e p i t a x i a l growth.13 Our observa- t i o n s u g g e s t s t h a t i n o t h e r d i r e c t i o n s , e p i t a x i a l growth can a l s o occur, although t h e i n t e r f a c e s may n o t be p e r f e c t .
JOURNAL DE PHYSIQUE
* * *
a * .. . 543A 1 . . l * .
+%A + 4 543A +
( 1 1 1 ) (1001 Atomic 8 Image Struc.
a Fundamental Ptanes
(110) (110)
Atomic Struc. Image Struc.
b Superstructure Plane
FIG. 4. ( a ) The a t o m i c s t r u c t u r e s o f a few f u n d a m e n t a l p l a n e s o f N i s i 2 . ( b ) The a t o m i c and e x p e c t e d image s t r u c t u r e s o f a few s u - p e r l a t t i c e p l a n e s o f N i s i 2 .
FIG. 5. A s c h e m a t i c diagram showing t h e 3-D a t o m i c a r - rangements o f f o u r (111) l a y e r s o f NiSi2.
FIG 6. Two neon i o n images o f t h e n i c k e l l a y e r o f t h e N i s i 2
(111) p l a n e . N i atoms a r e w e l l r e s o l v e d i n t h e images which show a h e x a g o n a l s t r u c t u r e .
show t h e £cc s t r u c t u r e o f t h e Ni s u b l a t t i c e . Fig. 4 shows t h e a t o m i c s t r u c t u r e s and t h e e x p e c t e d image s t r u c t u r e s o f some low i n d e x p l a n e s . Two t y p e s o f p l a n e s e x i s t . Each l a y e r i n t h e f u n d a m e n t a l p l a n e c o n s i s t s o f o n l y o n e a t o m i c s p e c i e s . An example
is t h e (111) p l a n e where t h e a t o m i c s t r u c t u r e i s h e x a g o n a l , and between two p u r e N i l a y e r s a r e two p u r e S i l a y e r s o f t h e same s t r u c t u r e a s shown i n F i g . 5. I n s u p e r - l a t t i c e p l a n e s , e a c h l a y e r c o n t a i n s 1 / 3 o f N i atoms and 2/3 o f S i atoms. We f i n d t h a t t h e N i l a y e r i n t h e (111) f u n d a m e n t a l p l a n e c a n b e r e s o l v e d i n t h e f i e l d i o n image a s shown i n F i g . 6. Although S i l a y e r s c a n a l s o be s e e n , i n g e n e r a l much dimmer, t h e i r f i e l d e v a p o r a t i o n is n o t a s smooth, and good images c a n n o t y e t b e o b t a i n e d .
Our FIM s t u d y o f s i l i c i d e - S i i n t e r f a c e is o n l y p r e l i m i n a r y . We f i n d a d e f e c t f r e e S i t i p t o be v e r y d i f f i c u l t t o p r e p a r e . N e v e r t h e l e s s , f u r t h e r s t u d y o f t h e s t r u c t u r e of t h e i n t e r f a c e w i t h a few m o n o l a y e r s o r submonolayer o f s i l i c i d e grown o n b o t h a S i and a m e t a l s u r f a c e a p p e a r s worthwhile. W e have a l s o found i n t e r e s t i n g image s t r u c - t u r e s f o r s i l i c i d e s grown o n m e t a l s u r f a c e s s u c h a s N i , P t , I r and W. Our a t t e m p t i n t h i s d i r e c t i o n is c o n t i n u i n g and r e s u l t s w i l l b e r e p o r t e d l a t e r .
T h i s work was s u p p o r t e d by NSF under g r a n t number DMR-8308057.
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