ORIENTATION OF AN IRON FILM FORMED ON A SPHERICAL COPPER SUBSTRATE : AN FIM STUDY

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ORIENTATION OF AN IRON FILM FORMED ON A SPHERICAL COPPER SUBSTRATE : AN FIM

STUDY

M. Wada, S. Uda, M. Kato

To cite this version:

M. Wada, S. Uda, M. Kato. ORIENTATION OF AN IRON FILM FORMED ON A SPHERICAL

COPPER SUBSTRATE : AN FIM STUDY. Journal de Physique Colloques, 1987, 48 (C6), pp.C6-65-

C6-70. �10.1051/jphyscol:1987611�. �jpa-00226814�

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JOURNAL DE PHYSIQUE

Colloque C6, suppl6ment au n O 1 l , Tome 48, novembre 1987

ORIENTATION OF AN IRON FILM FORMED ON A SPHERICAL COPPER SUBSTRATE

:

AN FIM STUDY

M. Wada, S. ~ d a * and M. Kato

Department of Materials Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 227 Japan

Abstract- Fe film of about 5 nm thick was formed on the field-evaporated tip surface of FIM specimen of Cu and the orientation relationship between the film and the substrate Cu was examined from the FIM images. It was

shown that on the [Ill] oriented tip surface of Cu, Fe film showed K u r d j ~ o v - Sachs orientation relationship predominantly on the [loll, [I101 and [Oll]

zones of the Cu substrate.

I

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INTRODUCTION

It has been shown that an initially fcc film of Fe formed on Cu substrate transforms to bcc as the film thickness increases beyond a critical value (.-2nm), and the orientation relationship between the transformed film and the substrate has been examined[l,2]. In these studies using the electron diffraction technique, Fe films are formed on flat low-index planes such as (100) or (111) of Cu. In the present study, Fe was vapor-deposited on a field-evaporated tip surface of an FIM specimen of Cu and the orientation relationship between the transformed Fe film and substrate was examined from the FIM images. The field-evaporated tip surface of an FIM

specimen is roughly spherical and various atomic planes are exposed on the surface of a single specimen. It is quite likely that the transformation of the initially coherent fcc Fe film to bcc is influenced by the underlying surface planes of the substrate and the present method seems to be quite suited to examine the effect of the substrate surface on the transformation of the Fe film.

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EXPERIMENTAL

In this study, the substrate on which Fe was vapor-deposited was a tip of a Cu FIM specimen. A Cu wire (99.999%) of 0.3 mm in diameter, annealed for I hour at 1273 K, was used for the FIM specimen after electropolishing one end in 10 % H3P04 solution kept at- 273 K. FIM images of the Cu tip surface were obtained with the lmaging gas of neon at -2x10-~~a. The specimen temperature during the observation was "20 K and the background pressure of the FIM chamber was "IO-~P~. By a careful field-evaporation, a clear image of the smooth and nearly spherical substrate surface wasobtained on the whole region of the image screen, 75 mm in diameter and "70 mm away from the specimen tip. The FIM chamber was equipped with a movable Fe evaporator near the specimen tip and Fe was resistively heated and evaporated on the field-evaporated Cu surface without moving the tip in the chamber. A coiled wire of 99.9 % Fe was used as the Fe source.

*presently at TOYOTA MOTOR CO.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987611

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When Fe was deposited on the Cu tip kept at -20 K, the obtained FIM image consisted of randomly distributed bright spots. By changing the tip temperature during the Fe deposition, it was found that the clear images of Fe layer could be obtained when the tip temperature during the deposition was above -573 K. However, a heating of the specimen before the deposition to such a high temperature caused a change in surface morphology and deteriorated the original clear substrate image on which many high-index planes had been recognized. Therefore, Fe was first deposited on the substrate at 20 K and then the specimen was heated to about 573 K while continuing the Fe deposition. This procedure produced a reasonably good crystalline image of the deposited Fe layer. The thickness of the Fe layer was at least 5 nm, which was estimated by counting a number of field-evaporated .1211) planes of the bcc Fe layer.

An attempt was made to observe an image of a thinner Fe layer, possibly an fcclayer, by reducing the deposition time. As the applied voltage was increased, however, the thin Fe layer easily field-evaporated together with the substrate Cu surface layer before a clear image of Fe layer was obtained.

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RESULTS AND DISCUSSION

Figure la shows a neon image of the substrate Cu surface and the major atomic planes in the figure are indexed in Fig. lb. Figure 2 is the image obtained after the Fe deposition followed by a slight field-evaporation. In the uppermost region, the image of substrite Cu is seen. By a further field-evaporation, the substrate area increased and the boundary on the image between the Cu substrate and Fe film moved downwards as shown in Fig. 3. As the field-evaporation proceeds, thickness of the Fe layer decreases. However, neither apparent change of the positions of the top atomic planes nor an indication of a structural changeof theFe layer was noticed.

In Fig. 4, positions of the top atomic planes of the Fe layer, recognized on the images in Figs. 2 and 3 are schematically shown by dashed circles together with the positions of the top planes of the substrate. Indices without subscripts are for the substrate planes. Lines I and I1 are the boundaries of the substrate and the Fe layer images in Figs. 2 and 3, respectively. It can be seen in Fig. 3 that there is a low-index plane of Fe parallel to the (111) plane of substrate. The center of this dominant ring pattern on Fe coincides with that of the (111) substraterings.

Other atomic planes of Fe layer are also seen in Figs. 2 and 3. From the size of the planes and the relative ~ositions of

Fig. la. FIM image of Cu substrate obtained at 20 K. Applied voltage 12 kV

the planes, three types of planes can be - in Fig. 4. If the Fe layer is an

Fig. lb. Indexes of the atomic planes shown in Fig. la.

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F i g u r e 2. FIM image of Fe-deposited Cu specimen o b t a i n e d a t 20 K. Applied v o l i s 12 kV. I n t h e uppermost r e g i o n , Cu s u b s t r a t e i s seen. The whole s u b s t r a t e image i s shown i n F i g . l a .

Figure 3. The image of t h e shown i n F i g . 2 a f t e r a f i e 1 The boundary between t h e Cu Fe l a y e r moved downwards.

same specimen .d-evaporation.

s u b s t r a t e and

e p i t a x i a l l y - g r o w n c o h e r e n t f c c c r y s t a l , a l l t h e p o s i t i o n s of t h e Fe p l a n e s s h o u l d c o i n c i d e w i t h t h o s e of t h e s u b s t r a t e . T h i s i s n o t observed i n Fig. 4. S i n c e t h e r e i s n o r e a s o n t o have an i n c o h e r e n t f c c f i l m on Cu s u b s t r a t e , we c o n s i d e r t h a t t h e Fe l a y e r i n F i g s . 2 and 3 has t h e bcc s t r u c t u r e . The observed most dominant Fe p l a n e p a r a l l e l t o t h e of s u b s t r a t e i s most l i k e l y t h e ( l l O ) b p l a n e , j u d g i n g by t h e FIM image of p u r e Fe shown i n Fig. 5. The s u b s c r i p t s f and b d e n o t e fcc and b c c , o r t h e s u b s t r a t e and t h e Fe l a y e r , r e s p e c t i v e l y . The second dominant p l a n e s , l a b e l e d A i n Fig. 4 , a r e t h e 12111 t y p e p l a n e s of bcc, s i n c e t h e a n g l e between t h e (110) b and t h e A p l a n e s i s c l o s e t o 30°. (The a n g l e between and {3111f i s 29.5' and t h r e e A p l a n e s i n Fig. 4 a r e n e a r l y p a r a l l e l t o {311If.) ( 1 3 0 1 ~ p l a n e s of Fe show a s i m i l a r appearance t o t h a t of f2111b on t h e image.

However, t h e a n g l e between ( l l O ) b and (130Ib i s 26.6O and i t i s d i f f i c u l t t o a s s i g n some o f A p l a n e s t o 1130)

.

^It^IS r e a s o n a b l e t o c o n s i d e r , from t h e r e l a t i v e p o s i t i o n s of ( l l O ) b and 18111~ p l a n e s , t h a t p l a n e s

B

a r e of t h e { l l l l b t y p e , p l a n e s C of t h e {3321b t y p e and p l a n e D of {321Ib type.

It i s c l e a r from Fig. 4 t h a t t h e Fe l a y e r on t h e n e a r l y s p h e r i c a l Cu s u b s t r a t e i s n o t a c o n t i n u o u s s i n g l e c r y s t a l f i l m i n t h e observed a r e a , s i n c e t h e symmetry around t h e ( l l O ) b p o l e i s q u i t e d i f f e r e n t from t h a t of b c c s t r u c t u r e . It h a s been r e p o r t e d [ 1 , 2 ] t h a t Fe f i l m formed on Cu h a s t h e Kurdjumov-Sachs (KS), Nishiyama, o r P i t s c h

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F i g u r e 5a.

F i g u r e 4. P o s i t i o n s of t h e top atomic p l a n e s of Cu s u b s t r a t e and Fe o v e r l a y e r determined by FIM images i n F i g s . 1, 2 and 3 .

F i g u r e 5b.

FIM image of a pure Fe a t 20 K. 1ndexes of t h e atomic planes shown i n

Applied v o l t a g e i s 6 kV. Fig. 5a.

o r i e n t a t i o n r e l a t i o n s h i p depending on t h e s u b s t r a t e o r i e n t a t i o n . They concluded t h a t such o r i e n t a t i o n r e l a t i o n s h i p s were r e a l i z e d when t h e i n i t i a l l y f c c Fe f i l m had transformed m a r t e n s i t i c a l l y t o bcc a s t h e t h i c k n e s s of t h e f i l m i n c r e a s e d . I n t h e p r e s e n t experiment, s u b s t r a t e s u r f a c e i s n e a r l y s p h e r i c a l and v a r i o u s atomic p l a n e s a r e exposed on t h e s u r f a c e . The f a c t t h a t t h e observed Fe f i l m i s not a continuous s i n g l e c r y s t a l i n d i c a t e s a s t r o n g e f f e c t of s u r f a c e o r i e n t a t i o n on t h e transformatdon of t h e Fe l a y e r . Because of t h e symmetry of Fe planes around t h e

( l l O ) b i n Fig. 4 , i t i s reasonable t o c o n s i d e r t h a t t h e c o n d i t i o n ( l l O ) b / / ( l l l ) f i s common t o a l l t h e r e g i o n s of t h e Fe l a y e r

.

Let u s c o n s i d e r t h e KS o r i e n t a t i o n r e l a t i o n s h i p . For t h e (111) / / ( l l O ) b , t h e r e a r e s i x v a r i a n t s s a t i s f y i n g t h e KS r e l a t i o n s h i p , which we c a l l &s-1 t o KS-6 here. They a r e s p e c i f i e d by t h e r e l a t i o n s ;

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Figure 6. R e l a t i v e p o s i t i o n s of f c c p o l e s (open symbols) and bcc p o l e s ( s o l i d symbols) s a t i s f y i n g t h e KS o r i e n t a t i o n r e l a t i o n s h i p . Arrows i n d i c a t e t h e corresponding p l a n e s between f c c and bcc a f t e r t h e Bain deformation.

KS-I.; [ i o ~ i ~ / / [ i l i ~ ~ , KS-2; [ o i l i f / / [ i l i i b , KS-3; [ 1 i 0 ] ~ / / [ i l i i ~ , KS-4; [ i o i ~ ~ / / [ T l l ] ~ , KS-5; [ o l i l f / / [ i l i l b , KS-6; [ i i ~ ] ~ / / [ i l l ] ~ . F i g u r e 6 shows r e l a t i v e p o s i t i o n s of f c c p o l e s (open symbols) and bcc p o l e s ( s o l i d symbols) of p l a n e s s a t i s f y i n g t h e KS-1 o r i e n t a t i o n r e l a t i o n s h i p on t h e s t e r e o g r a p h i c p r o j e c t i o n . The corresponding p l a n e s between bcc and f c c c r y s t a l s a f t e r t h e Bain deformation[3] can be w r i t t e n f o r KS-l(a1so a p p l i c a b l e t o KS-2) a s ,

This correspondence i s i n d i c a t e d by arrows i n Fig. 6. A s d i s c u s s e d above, we have considered t h a t t h e A-planes il Fig. 4 are-of t h e ( 2 1 1 ) ~ type. I n Fig. 4 , a l l t h e A planes a r e on t h e [ l o l l f , [110If and [011If zones. On t h e o t h e r hand, f o r t h e

-

KS-1 o r i e n t a t i o n r e l a t i o n s h i p i n F i g . 6 only ( 2 1 i ) b and (121)b a r e on t h e <llO>b zones. In F i g . 6 , i f we r o t a t e bcc p o l e s ~ n t i c l o c k w i s e by cos-1(-1/3) which i s

about 109.5" around [110Ib, (211)b and (121)b come on t h e [ l o l l f zone. ( N ~ w , ( l l l ) ~ i s t o t h e r i g h t of (131) .) T h i s - r o t a t i o n r e s u l t s i n t h e KS-4 r e l a t i o n and t h e

[ T o l l f zone c o i n c i d e s wigh t h e [ i l l ] zone. Therefore, i t can b e s a i d t h a t t h e KS-1 o r KS-4 i s s a t i s f i e d only n e a r t h e [ T o l l f zone. S i m i l a r l y , KS-3 o r ES-6 i s s a t i s f i e d n e a r t h e [110If zone and KS-2 o r KS-5 i s s a t i s f i e d n e a r t h e [011If zone i n Fig. 4. Thus, t h e Fe l a y e r observed h e r e i s t h e combination of s i x KS v a r i a n t s w i t h ( l l l ) f / / ( l l O ) b .

As

shown above, KS-1 r e l a t i o n i s s a t i s f i e d only n e a r t h e [ T o l l f zone w i t h which t h e [111Ib zone c o i n c i d e s . The arrows i n Fig. 6 show t h e movement of p o l e s by t h e Bain deformation. I t i s n o t i c e d t h a t t h e d i r e c t i o n of arrows of a l l t h e planes on t h e [ l o l l f zone a r e p a r a l l e l t o t h e zone l i n e connecting t h e (O1O)f, and (lO1)f poles: -This means t h a t a l l t h e f c c p l a n e s on t h e [ l o l l f zone become bcc p l a n e s on t h e [&11lb zone a f t e r t h e t r a n s f o r m a t i o n by t h e Bain deformation, and t h e [ l o l l f and [ l l l l b d i r e c t i o n s s t a y p a r a l l e l . This t y p e of correspondenceof fcc-b_cc p l a n e s o c c u r s on t h e [ l O l l f zone only f o r t h e KS-1 o r KS-4 r e l a t i o n . On t h e [ l l O l f zone,

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KS-3 o r KS-6 g i v e s t h i s correspondence and on t h e [ u I T ] ~ zone KS-2 o r KS-5. If one c o n s i d e r t h e hard-sphere model of f c c and bcc c r y d t a l s , t h e most d e n s e l y connected atomic c h a i n s w i l l b e found a l o n g t h e <110>f and d i r e c t i o n s f o r f c c and bcc, r e s p e c t i v e l y , and t h e d i s t a n c e between t h e atoms i n t h e c h a i n

is

t h e same along t h e

<llO>f and i f t h e s i z e of s p h e r e s i s t h e same. On t h e [ l o l l zone r e g i o n , f o r example, one of t h e c h a i n axes, [ l o l l f , i s contained i n t h e Fe l a y e r and t h i s i s a l s o p a r a l l e l t o t h e Cu s u b s t r a t e s u r f a c e b e f o r e t h g t r a n s f o r m a t i o n of t h e Fe l g y e r t o bcc. I n o t h e r words, c h a i n s of atoms a l o n g [ l o l l f i n t h e Fe l a y e r on t h e

[ l o l l f

zone do n o t p e n e t r a t e i h e s u b s t r a t e . A f t e r t h e t r a n s f o r m a t i o n , t h e [ r o l l f c h a i n s of Fe atoms become [111Ib c h a i n s w i t h o u t changing t h e i r d i r e c t i o n and they a r e s t i l l contained i n t h e l a y e r . Moreover, t h e l e n g t h of t h e c h a i n s o r t h e d i s t a n c e between atoms i n t h e c h a i n along t h i s d i r e c t i o n does n o t change. The d i s t a n c e s between atoms a l o n g <ill> of b c c Fe and a l o n g <110> of f c c Cu a r e q u i t e s i m i l a r i f one c o n s i d e r t h e l a t t i c e c o n s t a n t s of 0.361 nm and 0.286 nm for-Cu and bcc Fe, r e s p e c t i v e l y . Thus t h e t r a n s f o r m a t i o n of t h e Fe l a y e r on t h e [ l o l l f zone i s accomplished simply by t h e rearrangement of t h e [ l o l l f c h a i n s p a r a l l e l t o t h e Fe l a y e r and t h e s u b s t r a t e . The p r e s e n t r e s u l t s showed t h a t t h e KS o r i e n t a t i o n r e l a t i o n s h i p was predominantly n o t i c e d i n r e g i o n s on t h e [ l o l l f , [110If and [OITlf zones. This i n d i c a t e s t h a t t h e t r a n s f o r m a t i o n of Fe f i l m i s more f a v o r a b l e i n r e g i o n s c o n t a i n i g <110>f atomic c h a i n s which do no change t h e i r o r i e n t a t i o n a f t e r t h e Bain deformation.

REFERENCES

[ I ] Olsen, G. H. and J e s s e r , W. A., Acta m e t a l l .

2

(1971) 1009.

[ 2 ] Kato, M., Fukase, S , , Sato, A. and Mori, T., Acta m e t a l l .

2

(1986) 1179.

[3] f o r example, Nishiyama, Z., M a r t e n s i t i c Transformation, Academic P r e s s , New York (1977).