fesi transtition

8/13/2019 fesi transtition

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Applied Surface Science 70/71 (1993) 578-582North-Holland pplied

surf ce science

St ruc tura l phase t rans i t ion dur ing he te roep i tax ia l g rowtho f i ron s i l i c ide s on S i l l l )

J . A l v a r e z , A . L . V f l z q u e z d e P a r g a , J .J . H i n a r e j o s , J . d e l a F i g u e r a , E . G . M i c h e l , C . O c a la n d R . M i r a n d aDepartamento de Fisica de la Materia Condensada Universidad Autdnoma de Madrid 28049 Madrid Spain

Received 4 September 1992; accepted for publication 11 October 1992

We rep ort here on a novel metal- semi cond ucto r phase tra nsition in FeSiz, induc ed by epitaxy on Si(111). The tr ansition occursat co nstant temp erat ure as a function of the thickness of a silicide film with constant composition, epitaxially grown on S i(ll l). Thephase transition consists in a change of the silicide structure from y-FeSi2, a metallic phase with the highly symmetric fluoritestructure, t o/3-Fe Si 2, a semiconductin g phase with the low symmetry orthorhombic struc ture.

T h e s t u d y o f m e t a s t a b l e , c r y s ta l l in e p h a s e s h a sa t t r a c t e d m u c h a t t e n t i o n r e c e n t l y. T h i s h a s b e e ne x e m p l i f ie d b y th e r e n e w e d i n t e r e s t in m e t a s t a b l em a g n e t i c f i l m s s u c h a s f c c C o [ 1 ] o r b c c C o [ 2 ] .T h e s e p h a s e s h a v e b e e n s t a b i l i z e d a t r o o m t e m -p e r a t u r e b y e p i t a x i a l g r o w t h o n t o s u i t a b l e s u b -

s t r a t e s . T h e i r m a g n e t i c p r o p e r t i e s a r e a f f e c t e db e c a u s e t h e s t r u c t u r a l e n e rg y d i f f e r e n c e b e t w e e nt h e d i f f e r e n t p h a s e s i s o f t h e s a m e o r d e r o fm a g n i t u d e a s t h e m a g n e t i c e n e rg y d i f f e r e n c e .H o w e v e r , t h e d i f f e r e n c e i n s tr u c t u r a l e n e rg y b e -t w e e n c o m p a c t s t r u c t u r e s s u c h a s f c c a n d h c p C oi s r a t h e r s m a l l a n d b o t h a r e m e t a ls . A m o r er a d i c a l s t r u c t u r a l p h a s e t r a n s i t i o n ( e . g . f r o m c u b i cd i a m o n d t o d i s t o r t e d o r t h o r h o m b i c ) m a y i n v o l v el a rg e r d i f f e r e n c e s i n e n e rg y, a l l o w i n g f o r a m o r es e v e r e c h a n g e i n p r o p e r t i e s , f o r e x a m p l e f r o m am e t a l t o a s e m i c o n d u c t o r. O n e s u c h c a s e i s t h es e m i c o n d u c t o r S i w h i c h c o n v e r t s a t h i g h h y d r o -s t a t i c p r e s s u r e ( 11 . 3 - 1 2 . 5 G P a ) o r c l o s e t o p l a s t i ci n d e n t a t i o n s i n t o a m e t a l w i t h t h e / 3 - t i n s t r u c t u r e[31.

D i s il ic i d e s f o r m e d b y t h e m a g n e t i c m e t a l s C oa n d N i ( C o S i a , N i S i 2 ) c ry s t a l li z e i n t h e f l u o r i t es t r u c t u r e [ 4]. O n e m a y e x p e c t t h a t t h e t h i r d m e m -b e r o f t h e f a m i l y, n a m e l y F e S i 2 , w o u l d a l s o c r y s-t a l l i z e i n t h e f l u o r i t e s t r u c t u r e . T h e F e - S i b u l k

p h a s e d i a g r a m , h o w e v e r , s h o w s t h a t s u c h a p h a s ed o e s n o t e x i s t a t a m b i e n t p r e s s u r e . T h e e l e c t r o n i cs t r u c t u r e o f a n h y p o t h e t i c a l p h a s e w i t h a F e S i 2c o m p o s i t i o n a n d a f l u o r i t e c r y s t a l l i n e s t r u c t u r eh a s b e e n c a l c u l a t e d b y C h r i s t e n s e n [5 ]. A c c o r d i n gt o h i s c a l c u l a t i o n i t w o u l d b e a m e t a l l i c c o m -

p o u n d w i t h h i g h d e n s i t y o f s t a t e s ( D O S ) a t t h eF e r m i l e v e l . T h e r e f o r e , i t w o u l d b e p r o n e t os t r u c t u r a l p h a s e t r a n s i t i o n s d i m i n i s h i n g t h e e n -e rg y a s s o c i a t e d w i t h a h i g h D O S a t E F. I n f a c t ,n a t u r e s h o w s t h a t t h e p h a s e s t a b l e a t RT a n da t m o s p h e r i c p r e s s u r e i s / 3 - F e S i 2 , a s e m i c o n d u c t -i n g p h a s e w i t h a n o r t h o r h o m b i c s t r u c t u r e ( a =9 .86 A, b = 7 .79 ,~ , c = 7 .83 A ) , wh ich m ay b ec o n s i d e r e d a s a d i s t o r t e d f l u o r i t e s t r u c t u r e [ 6 ] .T h e r e f o r e , it s t o t a l e n e rg y m u s t b e l o w e r t h a nt h a t o f y - F e S i 2 . T h e c a l c u l a t i o n s i n d i c a t e t h a tYo-FeSi2 wo uld hav e a l a t t i ce co ns ta n t o f a = 5 .389A [ 5 ] . T h e l o w m i s m a t c h w i t h t h e l a t t i c e p a r a m e -t e r o f S i ( 5 . 4 3 . ~ ) s u g g e s t s t h a t t h e g a m m a p h a s ec o u l d b e s t a b i li z e d b y e p i t a x i a l g r o w t h o n S i (111 ),s p e c i a l ly if o n e c o n s i d e r s t h a t a l a rg e i n t e r f a c i a le n e rg y m u s t b e a s s o c ia t e d w i th t h e i m p o r t a n tl a tt ic e m i s m a t c h o f / 3 - F e S i 2 w i t h S i ( l l l ) . I n f a c t ,O n d a e t a l . [ 7] i d e n t i f i e d f o r t h e f i r s t t i m e y - F e S i 2a s a t h i n f i l m g r o w n O n S i ( 111 ) . T h e y r e p o r t e d a ni r r e v e r s i b l e p h a s e t r a n s i t i o n t o t h e / 3 - p h a s e in -

0169-4332/ 93/ 06.00 1993 - Elsevier Science Publishers B.V. All rights reserved


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J . A l v a r e z e t al . / H e t e r o e p i t a x i a l g r o w t h o f i r o n s i li c id e s o n S i l l l ) 579

d u c e d b y t h e r m a l a n n e a l i n g [7 ], p r o v i n g t h a t t h ef l u o r it e s t r u c t u r e o f F e S i 2 w a s i n d e e d m e t a s t a b l e .A c c o r d i n g l y, y - F e S i 2 m u s t l i e a t a l o c al m i n i m u mo f t h e t o t a l e n e r g y, h i g h e r, h o w e v e r , t h a n t h e o n ec o r r e s p o n d i n g t o f l - F e S i 2 . D u r i n g e p i t a x i a l

g r o w t h o f y - F e S i 2 , e a c h a d d i t i o n a l l a y e r s h o u l da d d s o m e e x c e s s e n e r g y ( w it h r e s p e c t t o / 3 - F e S i 2 )i n s u c h a w a y t h a t t h i s e n e r g y w o u l d s u r p a s s , fo ra g i v e n t h i c k n e s s , t h e i n t e r f a c i a l s t r a i n e n e rg yg a i n e d . A t t h i s p o i n t t h e f i l m m u s t s p o n t a n e o u s l yc o n v e r t i n t o t h e m o r e s t a b l e / 3 - F eS i 2 p h a s e . W er e p o r t o n t h e f i rs t o b s e r v a t i o n o f t h i s p h a s e t r a n -s i ti o n a s a fu n c t i o n o f t h i c k n e s s a t c o n s t a n t t e m -p e r a t u r e . T h e e x p e r i m e n t s h a v e b e e n c a r r i e d o u ti n t w o d i f f e r e n t u l t r a h i g h v a c u u m ( U H V ) c h a m -b e r s p r e v i o u s l y d e s c r i b e d [ 8 , 9 ] .

A s a p a r t o f a n e x t e n s iv e r e s e a r c h p r o j e c t , t h eF e - S i p h a s e d i a g r a m h a s b e e n e x p l o r e d [10 ] a n ds e v e r a l m e t h o d s t o p r e p a r e a n y d e s i r e d i r o n s i li -c i d e h a v e b e e n d e t e r m i n e d [ 8 - 1 4 ]. I n p a r ti c u l a r ,s i l i c i d e s c a n b e g r o w n b y e v a p o r a t i n g F e o n ah e a t e d S i s u b s t r a t e a t a f i x e d t e m p e r a t u r e , s e -l e c t e d t o p r o v i d e a g i v e n c o m p o s i t i o n o f t h e s i li -c i d e [8 ,1 0]. T h e m e t h o d h a s b e e n t e r m e d r e a c t i v ed e p o s i ti o n e p i t a xy ( R D E ) . W e h a v e d e m o n -s t r a te d e l s e w h e r e th a t R D E a t t e m p e r a t u r e s b e -t w e e n 5 5 0 a n d 7 0 0 C r e s u l t s i n f o r m a t i o n o fc o m p o u n d s w i t h F e S i 2 s t o i c h i o m e t r y [8 ,1 2]. O n

t h e o t h e r h a n d , i r o n s il ic id e s c a n b e p r e p a r e d b yF e e v a p o r a t i o n a t RT a n d a n n e a li n g . T h i s m e t h o di s k n o w n a s s o l id p h a s e e p i t a x y ( S P E ) [ 13 ].

T h e u p p e r p a n e l o f fi g. 1 s h o w s t h e i n t e n s i t yr a ti o o f F e 3 p / S i 2 p c o r e l ev e ls m e a s u r e d w i t hX - r a y p h o t o e m i s s i o n s p e c t ro s c o p y ( X P S ) d u r i n ge v a p o r a t i o n o f F e o n S i ( l l l ) a t 41 0 C ( o p e n d o t s )a n d 6 6 5 C ( f i l l e d d o t s ) , r e s p e c t i v e l y. T h e e v a p o r a -t io n r a t e w a s in th e r a n g e o f 0 . 5 -1 M L / 1 0 s. A t4 1 0 C t h e r a t i o o f i n t e n s i t i e s q u i c k l y s a t u r a t e s a ta v a l u e o f 0 .4 5 . F u r t h e r d e p o s i t i o n o f F e k e e p st h e F e / S i r a t i o c o n s t a n t , i n d i c a t i n g t h e f o r m a t i o no f a s t a b l e c o m p o u n d w i t h w e l l - d e f i n e d s t o i-c h i o m e t r y. T h e r a t i o o f 0 .4 5 p r o v e s t h a t t h e c o m -p o u n d f o r m e d is F e S i2 , a s c o n c lu d e d b y c o m p a r i -s o n t o p r e v i o u s c a l i b r a t i o n s [ 8 , 1 2 ] . A s i m i l a r, a l -b e i t s l o w e r e v o l u t i o n i s o b s e r v e d d u r i n g e v a p o r a -t i o n a t 6 6 5 C. I n s p i t e o f t h e f a c t t h a t t h e c h e m i -c a l c o m p o s i t i o n o f th e f i l m d o e s n o t c h a n g e , t h ev a l e n c e b a n d D O S o f t h e c o m p o u n d c h a n g e s

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F i g . 1. T h e u p p e r p a n e l r e p r o d u c e s t h e r a t i o o f i n t e n s i t i e s o fFe3p and Si2p core levels (excited with Mg Ka radiation)during evapo ration of Fe on Si(111) at 410C (open do ts) and665C (filled dots), respectively. The low er panel reproducesUPS spectra of the valence band measured at points A and B.They reflect the 2)


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580 J . Alv are z e t a l . / Heteroep i taxia l grow th of i ron s il ic ides o n Si l l )

d i f f r a c t i o n ( L E E D ) a n d s c a n n i n g t u n n e l i n g m i -c r o s c o p y ( S T M ) [ 14 ]. T h e U P S s p e c t r u m ( B ) a f t e rt h e t r a n s i t i o n , r e f l e c t s a s e m i c o n d u c t i n g s a m p l ep - d o p e d a s g r o w n . A c c o r d i n g l y, t h e F e r m i l e v el isf i xe d to t h e t o p o f t h e v a l e n c e b a n d . P e a k s a r e

o b s e r v e d a t 0 . 8 , 1 . 3 , a n d 1 . 6 e V. T h i s s p e c t r u mh a s b e e n i d e n t i f i e d a s d u e t o / 3 - F e S i 2 , a s e m i c o n -d u c t i n g p h a s e w i t h a n o r t h o r h o m b i c s t r u c t u r e[8,101.

T h e L E E D p a t t e r n o b se r v ed d ur in g R D E a t6 6 5 C ( a f t e r t h e t r a n s i t i o n ) , h o w e v e r, i n d i c a t e st h a t t h e o b t a i n e d f l - F e S i 2 f i l m i s p o l y c r y s t a l l i n e .I n f a c t , X - r a y d i f f r a c t i o n s t u d i e s o f t h i c k f i l m sp r e p a r e d b y R D E a t 6 00 C c o n f i r m t h ei r p o ly -c r y s t a l l i n e c h a r a c t e r a n d s h o w t h a t t h e y c o n t a i n/ 3 - F e S i 2 ( l l 0 ) a n d ( 1 01 ) f a ce s w i th r e c t a n g u l a r u n i tce l l s o f 7 .79 .~ x 12 .59 A a nd f l -FeS 2 (100 ) and(010) faces wi th sq ua re 7 .79 A 7 .83 A un i t ce l l s .T h e f r a c t i o n (1 0 0 ) / ( 11 0 ) i n c r e a s e s w i t h t h e d e p o -s i t i o n t e m p e r a t u r e . A t 6 6 5 C , t h e ( 1 0 0 ) v a r i e t y isp r e d o m i n a n t , w h i l e a t 4 1 0 C t h e ( 11 0 ) v a r i e t y i sm o r e a b u n d a n t [ 1 6 ] . F i g . 2 s h o w s S T M i m a g e sr e c o r d e d a f t e r R D E d e p o s i ti o n o f F e o n S i ( l l l )a t 6 6 5 C , w h i c h r e v e a l t h e a t o m i c a r r a n g e m e n t o ft h e f i l m u n d e r t h e s e c o n d i t io n s . M o s t o f t h es u r f a c e i s c o v e r e d b y c r y s t a l l i t e s h a v i n g t h e s u r -f a c e s t r u c t u r e s h o w n i n f i g . 2 a . I t c o n s i s t s o f ac o m p l e x a r r a n g e m e n t o f a t o m s f o r m i n g s t r i p e s

h u n d r e d s o f ,~ l o n g a n d 3 0 - 5 0 , ~ w i d e . C r y s t al -l it e s sh o w i n g t h i s k i n d o f s t r u c t u r e s c o r r e s p o n dm o s t p r o b a b l y t o t h e f l - F eS i z p h a s e . T h e i r s e m i -c o n d u c t i n g c h a r a c t e r i s i n d e e d c o n f i r m e d b y t h eS T M i m a g e s r e p r o d u c e d i n f ig . 3 , r e c o r d e d a t th e

s a m e s u r f a c e l o c a t i o n w i t h d i f f e r e n t s a m p l e b i a s( i n m a g n i t u d e a n d s i g n ) . T h e r e a r e c l e a r c h a n g e si n t h e a s p e c t o f t h e a t o m i c p r o t r u s i o n s v i s u a l i ze d .T h i s i s i n a g r e e m e n t w i t h t h e g e n e r a l f i n d i n g t h a ts e m i c o n d u c t i n g s u r f a c e s d i s p l a y S T M f e a t u r e st h a t c h a n g e n o t a b l y w i t h t h e s a m p l e b i a s, s i nc ee m p t y a n d f i l le d d a n g l i n g b o n d s a r e l o c a t e d o nd i f f e r e n t s p a t i a l p o s i t i o n s , n o t n e c e s s a r i l y r e l a t e dt o t h e a t o m i c p o s i t i o n s [1 5]. A s e x e m p l i f i e d b y f ig .3 , t h e i m a g e s c h a n g e i n t h i s c a s e i n s u c h a c o m -p l e x f a sh i o n w i t h t u n n e l i n g b i a s t h a t a d e t a i l e da t o m i c m o d e l f o r t h e s u r f a c e s t r u c t u r e o f th e/ 3 -F e S i z p h a s e h a s n o t y e t b e e n o b t a i n e d .

F i n a l l y, i t s h o u l d b e m e n t i o n e d t h a t t h e s t r u c -t u r a l p h a s e t r a n s i t i o n r e p o r t e d h e r e e x h i b i t s t h ef a m i l i a r p h e n o m e n o n o f p h a s e c o e x i s t e n c e , i n d i-c a t i n g t h a t i t i s a f i r s t - o r d e r m e t a s t a b l e - t o - s t a b l ep h a s e t r a n s i t i o n , s i m i l a r t o t h e 1 x 1 - 5 x 1 t r a n s i -t i o n o f c l e a n I r ( 1 0 0 ) [1 7] . T h e p h a s e c o e x i s t e n c ei s p r o v e d b y t h e S T M i m a g e r e p r o d u c e d i n f ig .2 b , w h i c h s h o w s a s m a l l c r y s t a l l i t e ( 2 0 0 . ~ w i d e )d i s p l a y i n g t h e c h a r a c t e r i s t i c h e x a g o n a l a r r a n g e -m e n t o f Si a d a t o m s w i t h t h e 2 x 2 r e c o n s t r u c t i o n

Fig. 2. (a) STM ima ge of the surface of /3-FeSi2 grown by RDE at 665C. The size of the imag e s 250 A x 250 .~ and it wasrecorded at V = - 1.32 V. Th e g rey scale covers 3.2 A from w hite to black; (b) STM imageo(200 A x 200 ,~) of a crystallite

7-FeSi 2 coexistingwith/3-FeSi 2 on a terrace o f Si(111). The grey scale covers 4.0 A. Th e b ias was 1.69 V.


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J . A lv a re z e t al . / He t e roep i t a x i a l g rowth o f i r on s il i c ide s on S i l l l ) 581

F i g . 3 . S e q u e n c e o f c o n s t a n t - c u r r e n t S T M i m a g e s o f t h e s a m e s u r f a c e p o s i t i o n f o r f l - F e S i 2 g r o w n b y R D E o n S i ( 111 ) . T h e a r r o wi n d i c a t e s a d e f e c t t h a t m a y b e u s e d t o g u i d e t h e e y e . T h e i m a g e s w e r e r e c o r d e d w i t h s a m p l e b i a s o f ( a ) 2 .8 5 V; ( b ) 1 .5 6 V; (c ) 0 . 6V; ( d ) - 1 . 8 V; ( e ) - 1 . 2 V an d ( f ) - 0 . 53 V, r e sp ec t i v e l y. Th e s i z e o f t he im a g e s i s 140 , ~ x 140 ,~ . Th e t o t a l g r e y s ca l e cove r s 2 .0 , ~


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582 J . Alv are z e t a l. / Hetero epi taxia l growth o f i ron si lic ides on Si l l )

of y-F eSi 2(ll l) [9]. It constitutes a minority rep-resentation of the fluorite phase. A hint of theatomistic mechanism of the phase transition (tobe reported elsewhere) is given by the noticeableincrease of longitudinal defects detected on topof these crystallites [15].

In conclusion we have demonstrated that 3'-FeSi 2 is metastable and that an irreversible struc-tural phase transition occurs to the/3 phase as afunction of the film thickness. Previously, thistransition had only been observed upon thermalannealing [7,16]. The structural phase transitionproduces a metal-semiconductor transition. It in-volves phase coexistence and leads to a polycrys-talline phase. The driving force of the transitionis the reduced bulk energy of the semiconducting

phase. The countervailing energy cost is associ-ated to the larger misfit of the /3 versus the 3'phase.

Financial support by the CICYT MAT 91-0176-CE and by ESPRI T BRA 3026 are grate-fully acknowledged.

e f e r e n c e s

[1] C.M. Schneider, P. Bressler, P. Schuster, J . Kirschner,J .J . de Miguel and R. Miranda, Phys. Rev. Let t . 64 (1990)1059.

[2] G. Pr inz, J . Magn. Magn. Mater. 100 (1991) 469.[3] See, for example: R.W. Cahn, Nature 357 (1992) 645.[4] H. von K~inel , R. Stalder, H. Sirr inghaus, N. Onda and J .

Henz, Appl. Surf. Sci. 53 (1991) 196.[5] N.E. Chris tensen , Phys. Rev. B 42 (1990) 7148.[6] M.C. B ost and J .E. M aha n, J . Appl . Phys. 58 (1985) 2696.[7] N. Onda et a l . , Helv. Phys. Acta 64 (1991) 197;

N. Onda, J . Henz, E. Mii l ler, K. M~ider and H. vonK[inel, Appl. Surf. Sci. 56-58 (1992) 421.

[8] J . Alvarez, J .J . Hinarejos , E.G. Michel , G.R. Castro andR. Miranda, Phys. Rev. B 45 (1992) 14042.

[9] A.L. V~izquez de Parga, J . de la Figuera, C. Ocal and R.Miranda, Europhys. Let t . 18 (1992) 595.

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fesi transtition

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