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EARLY STAGES OF METAL-Si (Ge) COUMPOUND FORMATION : A FIM STUDY OF INTERFACIAL
ATOMIC STRUCTURES
Hong Liu, T. Tsong
To cite this version:
Hong Liu, T. Tsong. EARLY STAGES OF METAL-Si (Ge) COUMPOUND FORMATION : A FIM
STUDY OF INTERFACIAL ATOMIC STRUCTURES. Journal de Physique Colloques, 1986, 47 (C7),
pp.C7-327-C7-332. �10.1051/jphyscol:1986756�. �jpa-00225951�
EARLY STAGES OF METAL-Si (Ge) COUMPOUND FORMATION : A FIM STUDY OF INTERFACIAL ATOMIC STRUCTURES
H.F. LIU, H.M. L I U and T.T. TSONG
Physics Department, The Pemsylvania State University, University Park, PA 16802, U.S.A.
Abstract
-
Early stages of metal-Si (Ge) compounds growth and the interface atomic structures have been studied with FIM. Single crystal NiSi2 and CoSi2 films can be grown on the [ I l l ] oriented Si tip in UHV. F r m the axial symme- try change, either identical or 180" change, the interfaces are identified to be either A-type or B-type. Atomic structures of silicide overlayers g r m on either metal or silicon tip surfaces have also been studied. Very well develop- ed silicide overlayer structures have been observed for RhSi (~hGe) g r m on Rh(OO1),
or RhSi(RhGe)/Rh(001), and other systems such as IrSi(IrGe)/Ir(OOl), RhSi/Si(lZO), PtSi/Si(l20) and IrSi/Si(lZO), In almost every case, two differ- ent types of interface, or overlayer, structure have been observed.1. INTRODUCTION
It is now well recognized that Schottky barrier height Gan significantly depend on the atomic structure at the metal-semiconductor interfaces/l,2/. Transition metal- semiconductor compound films have been widely used as contacts and Schottky barriers in very large scale integrated circuits. The interface is highly reactive. The strong material reaction changes the stoichiometry, atomic structure and chemical bonding at the interface. Questions asking the interface atomic structures are extremely difficult to answer for most metal-semiconductor systems.
Field ion microscope (FIM) is capable of imaging and field evaporating individual surface atoms. With appropriate modification, the FIM can play an important role in the study of metal-semiconductor cmpound formation and interface atomic structures /3,4/. Our systematic study stemmed £rom the earlier success of the FIM study of single Si atom surface diffusion and Si adsorption layer superstructure formation on W surfaces/S/. In this study, the experiment is very difficult to do and the data analysis is challenging, the results to be presented here are still preliminary.
II. MPERIMENTAL RESULTS AND DISCUSSION
Si samples (very sharp tip) were made £rom rod, cut f r m Si crystal, by mechanical polishing and electrochemical etching. When the vacuum of the FIM chamber reaches the low 10-l0 Torr range, the Si tip is field evaporated at 1OOK in Hz, He or Ne
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986756
C7-328 JOURNAL DE PHYSIQUE
image gas until a spherical, clean surface is obtained. We find that field evapora- tion of Si atoms is not very smooth and the Si surface developed by field evapora- tion is never as perfect as surfaces of metal tips. However, a definitive identifi- cation of crystal planes is possible especially when the surface is being slowly field evaporated. Fig. l(a) shows a 60K Ne ion image of a [ I l l ] oriented Si tip.
At such low temperature, it is necessary to illuminate the sample with a beam of light to reduce the resistivity of the Si tip by the photoconductivity effect. The illumination enhances the image magnification by 5-10% and considerably improves the image quality and reduces the best image voltage. Thin films of NiSi2 were grown in situ epitaxially on Si surfaces by vapor deposition method. The 25K Ne ion image of the NiSi2 films, shown in Fig. l(b), have the same three-fold symmetry as the Si sub- strate, as can be seen by comparing Figs. l(a) and (bl. A computer simulation of the image was carried out by assuming that the hemispherical crystal is pure NiSi2 epitaxially gr- on the Si(ll1) plane with type-B interface. A computer simulated image, tip radius is lOOa and the shell thickness is 0.0533a where a is the Si bond length, is shown in (cl, and a simulated image of NiSi2 epitaxially grown on the Si(ll1) plane with B-type interface is shown in (dl. The tip radius is 220 a and
Fig. 1.
b o t h show t h e same t h r e e - f o l d 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 FI and t h e computer s i m u l a t e d images of t h e o r i g i n a l S i t i p . The c l o s e resemblance between t h e simulat- ed image and a l 1 t h e F I images t a k e n a f t e r layer-by-layer f i e l d e v a p o r a t i o n suggest- ed t h a t t h e e p i t a x i a l l y grown NiSi2 f i l m i s a s i n g l e c r y s t a l and t h a t NiSi2 i s g r a m 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 al1 d i r e c t i o n s , although t h e i n t e r f a c e s i n d i r e c t i o n s o t h e r t h a n t h e s i [ l l l I may n o t match p e r f e c t l y .
Our FIM s t u d y of up t o t e n d i f f e r e n t m e t a l s i l i c i d e f i l m s g r a m on e i t h e r m e t a l o r S i s u b s t r a t e s shows t h a t o n l y t h e metal atoms g i v e r i s e t o b r i g h t f i e l d i o n images, S i atoms i n t h e compounds appear much dimmer o r a r e not imaged a t a l l . The symmetry of t h e image should only show t h e f c c s t r u c t u r e of t h e N i s u b l a t t i c e . Two t y p e s of p l a n e s e x i s t i n NiSi2 c r y s t a l . The fundamental p l a n e c o n s i s t s of only one s p e c i e s of a t m , whereas s u p e r s t r u c t u r e p l a n e c o n t a i n s both S i and N i atoms. We f i n d t h a t t h e N i l a y e r of t h e (111) NiSi2 can be r e s o l v e d i n t h e FIM, a s shown i n Fig. 2 , which is t a k e n a t 25K w i t h Ne; i n ( a ) t h e S i s u b s t r a t e is made from a S i rod and i n ( b ) from a S i whisker. Good images of t h e (111) S i l a y e r cannot be o b t a i n e d y e t . Two pure N i l a y e r s with hexagonal s t r u c t u r e a r e followed by two S i l a y e r s . The s t r u c t u r e of t h e (111) NiSi2 p l a n e s and t h e i r packing sequence w e have observed a g r e e w e l l w i t h t h e NiSi2 c r y s t a l . NiSi2 and CoSi2 f i l m s e p i t a x i a l l y grown on S i t i p s u r f a c e s w i t h A- t y p e i n t e r f a c e have a l s o been observed.
Fig. 2.
I r , Rh and P t s i l i c i d e s were a l s o grown on metal t i p s u r f a c e s . D i s t i n c t i v e s t a g e s of g r a v t h have been observed. D e s p i t e l a r g e l a t t i c e mismatch of t h e compound t o t h e s u b s t r a t e i n t h e s e non-epitaxy systems, two d i s t i n c t i v e t y p e s of ordered FI image s t r u c t u r e s have been observed a t t h e I r S i ( I r G e ) / I r ( O O l ) and RhSi/Rh(001) i n t e r f a c e s . I n t h e I r - S i system, one t y p e of t h e image s t r u c t u r e r e s e m b l i n g t h e ~ ( 2 x 2 ) o v e r l a y e r s t r u c t u r e of t h e s u b s t r a t e can be observed on t h e I r ( 0 0 1 ) , a s shown i n Fig. 3 , which i s t a k e n a t 15K w i t h He; I r ( 0 0 1 ) s u b s t r a t e s t r u c t u r e i s shown i n t h e upper l e f t cor- n e r . A s t h e p l a n e s i z e i s g r a d u a l l y reduced by f i e l d e v a p o r a t i o n , t h e s q u a r e u n i t ce11 of t h e s u p e r s t r u c t u r e is r e l a x e d s l i g h t l y i n t o a rhombic shape w i t h an a n g l e of
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Fig. 3 .
-78k2". Underneath t h i s t o p I r l a y e r , l i e s a dim S i l a y e r , whose s t r u c t u r e i s n o t r e s o l v e d . The dim S i l a y e r and t h e r h o m b i c s t r u c t u r e d Ir l a y e r is one t y p e type) of t h e i n t e r f a c e , o r t h e o v e r l a y e r , s t r u c t u r e .
Another t y p e of t h e image h a s a r e c t a n g u l a r s t r u c t u r e , which is s i m i l a r t o t h e image shown i n Fig. 4 (15K H e i o n images) f o r t h e Ir-Ge system. A f t e r a p p r o p r i a t e correc- t i o n t o t h e image d i s t o r t i o n due t o t h e t i p g e a n e t r y i s considered, t h e s i d e s of t h e r e c t a n g u l a r u n i t ce11 a r e e s t i m a t e d t o b e -4.5k0.5 8 and -6.0k0.5 w i t h a r a t i o of -1.4f0.1. The l o n g e r and t h e s h o r t e r s i d e s l i e along t h e I r [ 0 1 0 ] and [IO01 d i r e c
t i o n s , r e s p e c t i v e l y . T h i s s t r u c t u r e i s s t a b l e . I n t h e f i e l d e v a p o r a t i o n p r o c e s s , no s u r f a c e r e l a x a t i o n h a s been observed. The b r i g h t I r l a y e r and t h e two dim S i l a y e r s l y i n g on t o p of t h e I r ( 0 0 1 ) form a n o t h e r t y p e (B-type) of t h e i n t e r f a c e , o r t h e o v e r l a y e r , s t r u c t u r e .
An a c c u r a t e i d e n t i f i c a t i o n of t h e s u r f a c e atomic s t r u c t u r e of an a l l o y from FI image a l o n e is o f t e n v e r y d i f f i c u l t because of image d i s t o r t i o n and non-uniform magnifica- t i o n of t h e FI image. The a n a l y s i s p r e s e n t e d h e r e is only preliminary. However, t h e s u r f a c e p l a n e s of I r S i can b e i d e n t i f i e d on t h e b a s i s of t h e atomic s t r u c t u r e and t h e expected image p a t t e r n of t h e c r y s t a l p l a n e s of I r S i c r y s t a l . I r S i has t h e orthorhombic MnP s t r u c t u r e w i t h ao=5.558 A, b0=3.211 A, and co=6.273 8. The atomic and image s t r u c t u r e s of v a r i o u s c r y s t a l p l a n e s of I r S i c r y s t a l have been worked o u t . Only t h e (011) I r l a y e r of I r S i c l o s e l y resembles t h e rhombic image s t r u c t u r e . The a n g l e i n t h e rhombic s t r u c t u r e i s 76.54'. whereas t h e images, shown i n F i g . 3 g i v e a n a n g l e of -78+2'. T h e i r s i z e s a r e a l s o c l o s e t o one another. Thus we b e l i w e t h a t t h e r e l a x e d (rhombic) s t r u c t u r e resembles t h e (011) I r l a y e r s of I r S i c r y s t a l . We cannot y e t i d e n t i f y t h e r e c t a n g u l a r image s t r u c t u r e i n terms of t h e I r S i c r y s t a l plane.
of atomic l a y e r s were f i e l d evaporated. T h i s o b s e r v a t i o n i n d i c a t e s t h a t ordered i n t e r f a c e atomic s t r u c t u r e s do e x i s t i n t h e non-epitaxy I r S i / I r ( 0 0 1 ) system.
Germanium can a l s o r e a c t w i t h I r t o form IrGe w i t h orthorhombic MnP s t r u c t u r e . Almost i d e n t i c a l t y p e s of t h e ordered i n t e r f a c e atomic c o n f i g u r a t i o n have been observed a t t h e I r G e / I r ( 0 0 1 ) i n t e r f a c e . The r e c t a n g u l a r image p a t t e r n of I r G e observed a t t h e I r ( 0 0 1 ) i s shown i n F i g . 4. The s i d e s of t h e r e c t a n g u l a r u n i t c e l l s a r e e s t i m a t e d t o b e "4.2'0.5 A and -6 .5I0.5 A w i t h t h e s i d e s r a t i o being -1.6+0.1.
The e x a c t s t r u c t u r e of t h i s l a y e r is n o t y e t c o r r e l a t e d t o IrGe c r y s t a l plane. The image s t r u c t u r e of t h e t y p e A i n t e r f a c e s t r u c t u r e of Ir-Ge system i s s i m i l a r t o t h a t shown i n Fig. 3. The r e l a x e d image s t r u c t u r e (rhombic) i s b e l i e v e d t o b e produced by t h e (011) I r l a y e r of t h e IrGe c r y s t a l on t h e b a s i s of both t h e p a t t e r n of t h e atomic s t r u c t u r e s and t h e expected s p a t i a l r e s o l u t i o n of t h e FIM.
Fig. 4 .
Rh and I r have s i m i l a r £cc c r y s t a l s t r u c t u r e , and w i t h S i and Ge t h e y c a n form com- pounds which have orthorhomkic MnP s t r u c t u r e . However, t h e i n t e r f a c e atomic s t r u c - t u r e s observed a t t h e Rh(001) a r e d i f f e r e n t £rom t h a t of t h e I r c o u n t e r p a r t s . Two l a y e r s of RhSi were formed on t h e Rh(001) plane. One t y p e of t h e image s t r u c t u r e resembles t h e ~ ( 2 x 2 ) o v e r l a y e r s t r u c t u r e and t h e s q u a r e u n i t ce11 can r e l a x i n t o a rhombic a s t h e p l a n e s i z e s h r i n k s by f i e l d evaporation. By s i m i l a r argument, we b e l i e v e t h a t t h e r e l a x e d s t r u c t u r e corresponds t o t h e (011) Rh l a y e r of orthorhombic RhSi. Another t y p e of t h e s t r u c t u r e shows a s t a b l e ~ ( 2 x 2 ) s t r u c t u r e , which is b e l i e v e d t o c o r r e l a t e t o t h e (001) c u b i c RhSi s u p e r s t r u c t u r e plane. I n both c a s e s , t h e s t r u c t u r e s of t h e dim S i l a y e r l y i n g underneath t h e b r i g h t image l a y e r were not resolved.
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I n summary, t h i n f i l m s of s i n g l e c r y s t a l NiSi2 and CoSi2 have been grown e p i t a x i a l l y o n t h e [ I l l ] o r i e n t e d S i t i p s u r f a c e s i n UHV. The a x i a l symmetry o f t h e s i l i c i d e images c a n b e c o n t r o l l e d t o be e i t h e r i d e n t i c a l t o o r t o b e r o t a t e d 180' w i t h r e s p e c t t o t h a t o f t h e s u b s t r a t e , c o r r e s p o n d i n g t o type-A o r type-B i n t e r f a c e . Temperature dependent a t o m i c s t r u c t u r e s have been o b s e r v e d a t I r S i ( I r G e ) / I r ( O O l ) a n d RhSi/Rh(Oll) i n t e r f a c e s . O r d e r e d a t o m i c s t r u c t u r e s have a l s o been o b s e r v e d a t I r S i / I r ( O l l ) , RhGe/Rh(Oll) and a few o t h e r metal-semiconductor i n t e r f a c e s . W e have a l s o o b s e r v e d v e r y w e l l o r d e r e d o v e r - l a y e r s of s i l i c i d e s of Rh, P t , and I r grown on S i ( 1 2 0 ) p l a n e s /6/. Again two d i f f e r e n t t y p e s o f image s t r u c t u r e s have been o b s e r v e d f o r a l 1 t h e s e systems. I n g e n e r a l £rom FIM images a l o n e t h e e x a c t s t r u c t u r e of t h e i n t e r f a c e can- n o t b e d e t e r m i n e d w i t h complete c e r t a i n t y b e c a u s e of t h e i n t r i n s i c l i m i t a t i o n s of t h e FIM. W e hope t h a t t h e i n t e r e s t i n g o b s e r v a t i o n s we have made i n t h i s FIM s t u d y w i l l s t i m u l a t e f u r t h e r i n v e s t i g a t i o n s w i t h o t h e r t e c h n i q u e s , e s p e c i a l l y w i t h d i f f r a c - t i o n t e c h n i q u e s .
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