Nucleation and growth of a bcc Fe phase deposited on a single crystal (001) Cu film

Scripta METALLURGICA et M A T E R I A L I A

Vol. 25, p p . 6 6 3 - 6 6 8 , 1991 P r i n t e d in the U.S.A.

Pergamon

Press

plc

NUCLEATION AND G R O W T H OF A BCC Fe PHASE D E P O S I T E D ON A SINGLE CRYSTAL (001) Cu F I L M

Center

for M a t e r i a l s

J. Koike Science, Los Alamos N a t i o n a l Los Alamos, NM 87545.

Laboratory,

( R e c e i v e d N o v e m b e r 26, 1990) ( R e v i s e d D e c e m b e r 28, 1990) Introduction As a thin film o v e r l a y e r grows on a substrate with a different structure, the overlayer initially adopts the s u b s t r a t e structure and s u b s e q u e n t l y transforms to an e q u i l i b r i u m bulk structure. Such a growth c h a r a c t e r i s t i c has been extensively studied in Fe/Cu bicrystals. An Fe o v e r l a y e r grown on a Cu s u b s t r a t e is known to have the fcc s t r u c t u r e up to a t h i c k n e s s of 2 nm [i], w h e r e a s a t h i c k e r Fe overlayer consists of s u b m i c r o m e t e r grains of the b c c structure [2]. The orientation relationship between bcc-Fe and f c c - C u has b e e n r e p o r t e d in a relatively thick Fe film and was found to consist of the N i s h i y a m a (N), KurdjumovSacks (KS), or Pitsch (P), d e p e n d i n g on the o r i e n t a t i o n of the s u b s t r a t e surface [2]. However, previous s t u d i e s have not e x p l a i n e d how the bcc s t r u c t u r e nucleates or how the observed s u b m i c r o m e t e r p o l y c r y s t a l l i n e grains form. The p r e s e n t work was u n d e r t a k e n to p r o v i d e an u n d e r s t a n d i n g of these two points. T r a n s m i s s i o n e l e c t r o n m i c r o s c o p y (TEM) was used to study F e / C u bicrystals as the Fe t h i c k n e s s was v a r i e d systematically. ~Analysis of m o i r 4 fringes, which are caused by s u p e r p o s i t i o n of d i f f e r e n t structures, e n a b l e d us to determine the orientation r e l a t i o n s h i p b e t w e e n the very thin Fe layer and the Cu substrate. We show that a single v a r i a n t of the P o r i e n t a t i o n relationship, w h i c h accompanies atomic r e a r r a n g e m e n t p a r a l l e l to the interface, p r e d o m i n a t e s at the nucleation stage of the bcc structure. N u c l e a t i o n of other v a r i a n t s of P, N, and KS occurs with increasing Fe t h i c k n e s s a n d causes the f o r m a t i o n of the s u b m i c r o m e t e r bcc grains.

Experimental

Procedure

Fe/Cu bicrystals were p r e p a r e d in a UHV e l e c t r o n - g u n d e p o s i t i o n system with a base p r e s s u r e of 3 x 10 -6 Pa. A quartz o s c i l l a t o r m o n i t o r e d f i l m t h i c k n e s s and e v a p o r a t i o n rate. A rock salt s u b s t r a t e was p r e p a r e d by c l e a v i n g it in air and imme d i a t e l y p l a c i n g it in a v a c u u m system. Pure Cu (99.999%) was d e p o s i t e d onto the substrate h e a t e d at 430 °C at an e v a p o r a t i o n rate of 0.3 nm/s to a thickness of 200 nm. This c o n d i t i o n e n s u r e d the formation of a u n i f o r m single crystalline layer with [001] p a r a l l e l to the growth direction. A f t e r cooling the film to room temperature, pure Fe (99.998%) was d e p o s i t e d onto the single c r y s t a l Cu film at the same e v a p o r a t i o n rate of 0.3 nm/s to t h i c k n e s s e s v a r i e d from 2.0 to 15.0 nm. The p r e s s u r e d u r i n g d e p o s i t i o n was the upper 10 -5 Pa range for Cu and the upper 10 -6 Pa range for Fe. The F e / C u b i c r y s t a l was s u b s e q u e n t l y r e m o v e d from the rock salt substrate by d i s s o l v i n g it in d i s t i l l e d water. The r e m o v e d film was floated onto a Cu grid for f o l l o w i n g TEM examination. The p l a n - v i e w m i c r o s t r u c t u r e of the Fe/Cu bicrystal was i n v e s t i g a t e d at room temperature with a Philips CM-30 operated at 300 kV.

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Fe D E P O S I T E D ON Cu

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Analvsis of the Moire Fringes When two crystalline films with different structural p a r a m e t e r s overlap each other along the i n c i d e n t - e l e c t r o n - b e a m direction, some d i f f r a c t i o n spots from the overlayer may appear in close p r o x i m i t y to those from the u n d e r l y i n g layer. When the film is sufficiently t h i c k to give rise to m u l t i p l e scattering, additional diffraction spots appear a r o u n d the t r a n s m i t t e d b e a m at the same relative position as the two adjacent d i f f r a c t i o n spots. In the b r i g h t - f i e l d image, interference between the additional spots and the t r a n s m i t t e d b e a m creates p e r i o d i c intensity variations, known as moir~ fringes. The spacing and direction of the moir~ fringes can be calculated by k i n e m a t i c a l - d i f f r a c t i o n t h e o r y if the c r y s t a l l i n e structure and orientation relationship of the two o v e r l a p p i n g crystals are known [3]. Possible orientation r e l a t i o n s h i p s between bcc-Fe and fcc-Cu are 24 variants of K u r d J u m o v - S a c k s (KS), 12 of N i s h i y a m a (N), and 12 of P i t s c h (P) . These relationships are listed in Table 1 in an a b b r e v i a t e d form. On the basis of these o r i e n t a t i o n relationships, the e x p e c t e d d i f f r a c t i o n p a t t e r n s were calculated, using the Diffract computer code [4]. Shape effects of the thin Fe film were taken into account for intensity calculation. The p o s i t i o n s of c l o s e l y located spots in the c a l c u l a t e d diffraction p a t t e r n yield the spacings of the c o r r e s p o n d i n g lattice planes and the angle between them, w h i c h are then used for the calculation of the moir~ fringes. Table 2 shows the c a l c u l a t e d direction, e x p r e s s e d as the angle from the < I i 0 > directions, and the s p a c i n g of the m o i r ~ f r i n g e s when the Fe/Cu bicrystal is observed in p l a n - v i e w where the Cu (001) p l a n e is p e r p e n d i c u l a r to the i n c i d e n t b e a m d i r e c t i o n . The n u m b e r s in p a r e n t h e s e s c o r r e s p o n d to the additional moir~ fringes when the Cu (001) plane is i n c l i n e d by 5 ° from the above configuration. This table is used later to analyze the moir~ fringes. Results and D~scussion B r i g h t - f i e l d images and c o r r e s p o n d i n g d i f f r a c t i o n p a t t e r n s are shown in Figure 1 for various t h i c k n e s s e s of the Fe overlayer. At an Fe thickness of 2 nm, no misfit dislocations are o b s e r v e d in the image. The d i f f r a c t i o n p a t t e r n shows spots o n l y from the fcc structure. At 2.3 nm, m i s f i t d i s l o c a t i o n s are observed along the [ll0]f and the [[10]f directions. When the Fe t h i c k n e s s increases to 5.0 nm, the misfit d i s l o c a t i o n d e n s i t y increases and the d i f f r a c t i o n p a t t e r n clearly shows a d d i t i o n a l spots in the v i c i n i t y of the Cu d i f f r a c t i o n spots. These additional diffraction spots are consistent with previous o b s e r v a t i o n s [1,2], and are a t t r i b u t e d to the f o r m a t i o n of a bcc phase. At 15.0 nm, s u b m i c r o m e t e r grains of a d i a m e t e r of a p p r o x i m a t e l y 5 to 25 nm a p p e a r in the image. The additional intensities in the diffraction p a t t e r n are much stronger than those for 5.0 nm. Figure 2 shows b r i g h t - f i e l d images at a h i g h e r m a g n i f i c a t i o n . At 2.0 nm, no n o t i c e a b l e features are o b s e r v e d in the image. At 2.3 nm, fringes are observed along the [ll0]f and [~10]f d i r e c t i o n s with a s p a c i n g of a p p r o x i m a t e l y 1.4 nm. Table 2 indicates that these fringes are due to the f o r m a t i o n of a bcc Fe phase with the P* o r i e n t a t i o n relationship. Notice that the fringe contrast covers only a part of the image. W h e n the Fe t h i c k n e s s increases to 5.0 nm, almost all areas exhibit fringe contrast along various directions with various spacings. Using the c a l c u l a t e d d i r e c t i o n and spacing in Table 2, we can explain a p p r o x i m a t e l y 70% of the f r i n g e sets by the f o r m a t i o n of the b c c p h a s e w i t h KS, N, P, and P* orientation r e l a t i o n s h i p s when we d i s r e g a r d an a n g u l a r d e v i a t i o n from the values in Table 2 of ± 2 ° and a s p a c i n g d e v i a t i o n of ± 0.05 nm. Such d e v i a t i o n s are possible if the severe elastic strain is a c c u m u l a t e d in those areas. Among the 70% of the fringe sets, 20% were e x p l a i n e d by the n u m b e r in parentheses.

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Fe D E P O S I T E D ON Cu

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Thus, Fig. 2 indicates that increasing Fe thickness results in the structural change in the Fe o v e r l a y e r from fcc to bcc, i n i t i a l l y w i t h the p r e f e r e n t i a l P* o r i e n t a t i o n r e l a t i o n s h i p and l a t e r w i t h v a r i o u s o r i e n t a t i o n r e l a t i o n s h i p s . Formation of the bcc nuclei with various o r i e n t a t i o n r e l a t i o n s h i p s appears to be the origin for the s u b m i c r o m e t e r p o l y c r y s t a l l i n e grains o b s e r v e d in the thicker Fe overlayer (see Fig. i) . This is because the bcc nuclei d i a m e t e r s shown in Fig. 2 (5 to 15 nm) fall i n t o the same r a n g e as d i a m e t e r s of the s u b m i c r o m e t e r polycrystalline grains shown in Fig. 1 (5 to 25 nm). We now discuss the formation of the p r e f e r e n t i a l P* orientation relationship in terms of the relative m a g n i t u d e b e t w e e n the strain p a r a l l e l to the interface (the b i a x i a l strain) a n d the t h i c k n e s s - d e p e n d e n t strain p e r p e n d i c u l a r to the interface (the p e r p e n d i c u l a r strain). When the bcc structure forms, lattice misfit between bcc and fcc s t r u c t u r e s induces a b i a x i a l strain at the interface. The p e r p e n d i c u l a r strain is also induced t h r o u g h the Poisson effect. However, for a very thin bcc overlayer, this p e r p e n d i c u l a r strain is most likely relaxed at the surface. The residual biaxial strain appears to be relaxed by the formation of the P* orientation relationship for the following reasons. Figure 3 shows the lattice m a t c h i n g for b c c - F e and fcc-Cu [2], derived from the Bain lattice c o r r e s p o n d e n c e [5] and the i n v a r i a n t line condition [6] which p r e s u m a b l y give a m i n i m u m e n e r g y c o n f i g u r a t i o n . In the figure, the m a t c h i n g lattice planes, atomic configuration and o r i e n t a t i o n relationships at the i n t e r f a c e are given for t h r e e d i f f e r e n t fcc i n t e r f a c e s , (lll)f, (101)f, and (001)f. The figure indicates that the P* o r i e n t a t i o n r e l a t i o n s h i p is expected on the (001)f interface, and that the structural change from fcc to bcc is realized mainly by atomic r e a r r a n g e m e n t p a r a l l e l to the interface plane. Therefore, the P* orientation relationship appears to be the most effective in relaxing the biaxial strain p a r a l l e l to the (001)f interface, and for this reason, p r e f e r e n t i a l l y forms at the nucleation stage in a very thin bcc overlayer. W i t h an i n c r e a s e in the o v e r l a y e r t h i c k n e s s , o n l y a small part of the p e r p e n d i c u l a r strain will be relaxed at the surface, while a significant part will be a c c u m u l a t e d in the a t o m i c layers b e l o w the surface. Hence, a p e r p e n d i c u l a r component of relaxation becomes necessary. This appears to be accomplished by the formation of the KS, N, and P o r i e n t a t i o n r e l a t i o n s h i p s . For these orientation relationships, the m a t c h i n g p l a n e s are i n c l i n e d to the (001)f interface. It is easy to see that the i n c l i n e d m a t c h i n g planes have a p e r p e n d i c u l a r component of relaxation because p r o j e c t i o n of the strain r e l a x a t i o n in the m a t c h i n g planes to the p e r p e n d i c u l a r d i r e c t i o n has a n o n - z e r o value. Therefore, with an increase in the o v e r l a y e r thickness, the formation of KS, N, and P o r i e n t a t i o n relationships is n e c e s s a r y to relax the p e r p e n d i c u l a r strain. On the o t h e r hand, the biaxial component of relaxation becomes smaller for the inclined m a t c h i n g plane than that for the p a r a l l e l case in the P* o r i e n t a t i o n r e l a t i o n s h i p . This seems to be compensated for by the increasing number of misfit dislocations when the overlayer thickness increases as shown in Figure i. Summarv Fe was deposited on a single crystalline Cu thin film. N u c l e a t i o n of the bcc structure and subsequent microstructural evolution in the Fe f i l m were investigated. As the Fe thickness increases, the Fe structure changes from fcc to bcc. At the nucleation stage of the bcc structure, the P* o r i e n t a t i o n relationship is p r e f e r r e d to relax the biaxial strain. W i t h an increase in the Fe thickness, other o r i e n t a t i o n r e l a t i o n s h i p s form to relax the p e r p e n d i c u l a r strain as well as the b i a x i a l strain, c a u s i n g the f o r m a t i o n of a s u b m i c r o m e t e r p o l y c r y s t a l l i n e layer.

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Acknowleduement The author gratefully a c k n o w l e d g e s M. N a s t a s i for use of his e v a p o r a t o r and for fruitful discussion. I wish to thank D. M, Parkin for his continuous support and encouragement, and T. E. Mitchell for c r i t i c a l l y reading the manuscript. This work was performed under the auspices of the U. S. Department of Energy. References I. W. A. Jesser and J. W. Matthews, Philos. Mag., 15, 1097 (1967). 2. M. Kato, S. Fukase, A. Sato, and T. Mori, Acta metall., 34, 1179 (1986). 3. P. Hirsch, A. Howie, R. B. Nicholson, D. W. Pashley, and M. J. Whelan, Electron Microscopy of Thin Crystals, 2nd. Ed., p.343, Robert E. Krieger Publishing Co., Inc., Malabar, Florida (1977). 4. Microdev Software Inc., P. O. Box 2302, Evergreen, CO 80439. 5. E. C. Bain, Trans. Metall. Soc. AIME., 70, 25 (1924). 6. U. Dahmen, Acta metall., 30, 63 (1982).

TABLE 1 E x p e c t e d o r i e n t a t i o n r e l a t i o n s h i p s (OR) b e t w e e n a b c c - F e and an fcc-Cu. The Pitsch o r i e n t a t i o n r e l a t i o n s h i p is g r o u p e d in two, P and P*, in terms of the surface orientation of Cu. Complete description can be found in Ref. 2.

OR

Parallel Planes

KS

{lll}f // {ll0}b

<10[>f // b

24

N

{lll}f // {ll0}b

f // b

12

P

(100)f //

(ll0)b

<011>f // b

4

(010)f //

(ll0)b

<101>f // b

4

(001)f //

(ll0)b

f // b

4

P*

Parallel Directions

Total Variants

TABLE 2 The d i r e c t i o n and the s p a c i n g orientation relationships (OR).

Angle 45 o (45 (42 36 (14 9 7

Spacing 1.67 nm 0 69 1 66 0 92 1 20 1 30 0 90

of

the

moir~

fringes

expected

O R

Angle

Spacing

O R

P P ) N ) KS N ) KS KS

6 o 4 ( 3 2 1 0 0

0.93 nm 0.56 1.38 1.15 0.54 1.18 1.39

P* P* KS) N KS N P*

for

various

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FIG.

1

Fe D E P O S I T E D ON Cu

667

Variation of the m i c r o s t r u c t u r e in Fe/Cu bicrystals with increasing the Fe thickness, which is i n d i c a t e d in the upper left corner of each figure.

Fe DEPOSITED

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FIG. 2

ON Cu

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Variation of the Fe/Cu microstructure at a higher magnification. The Fe layer thickness is indicated in the upper left corner of each figure.

Bain lattice correspondence

{111}f//{llO}b

{101}f//{001}b

{101}f//{ll2}b

{001}f//{ll0} b

P

P*

Atom arrangement

Corresponding 0 R

FIG. 3

KS

N

The lattice matching between bcc-Fe and fcc-Cu for the given Cu interfaces [2]. Open and filled circles represent Cu and Fe atoms, respectively.

3