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Liquid phase epitaxial growth of indium substituted magnesium ferrite films
Mat. Res. Bull. Vol. 12, pp. 707-715,1977. Umt~ ~a~s.
LIQUID
PHASE
EPITAXIAL
GROWTH
FERRITE
Pergamon Press, ~ c .
OF I N D I U M
SUBSTITUTED
P~ntedinthe
MAGNESIUM
FILMS
P.J.M. van der S t r a t e n and R. M e t s e l a a r L a b o r a t o r y of P h y s i c a l C h e m i s t r y , U n i v e r s i t y of T e c h n o l o g y , E i n d h o v e n , The N e t h e r l a n d s .
(Received May 20, 1977; Communicated by A. Rabenau)
ABSTRACT S i n g l e crystal films of M g ( F e , l n ) 2 0 4 have b e e n grown by m e a n s of l i q u i d - p h a s e - e p i t a x y on ( 1 0 0 ) - M g O s u b s t r a t e s u s i n g a P b O - B 2 0 3 - F e 2 0 3 solvent. The final p o l i s h i n g of the s u b s t r a t e s a p p e a r e d to be very i m p o r t a n t in c o n n e c t i o n w i t h the n e c e s s a r y large s u p e r s a t u r a t i o n of the flux. E l e c t r o n m i c r o p r o b e a n a l y s i s r e v e a l e d Pt and high Pb c o n t e n t s in our films. The easy axis of m a g n e t i z a t i o n is in-plane, p r o b a b l y due to the c o m p r e s s i v e s t r a i n of the films c a u s e d by the d i f f e r e n c e in thermal e x p a n s i o n of MgO and M g ( F e , l n ) 2 0 4 .
Introduction M a g n e t i c films have a t t r a c t e d m u c h i n t e r e s t b e c a u s e of their p o t e n t i a l a p p l i c a b i l i t y in m a g n e t i c and m a g n e t o - o p t i c devices. M o s t s t u d i e s c o n c e r n the l i q u i d - p h a s e - e p i t a x i a l (LPE) g r o w t h of g a r n e t films (I) and r e c e n t l y the LPE g r o w t h of h e x a g o n a l f e r r i t e s has been r e p o r t e d (2,3). C o n c e r n i n g the gro~h of spinel f e r r i t e s most work, so far, has b e e n done by m e a n s of c h e m i c a l vapor d e p o s i t i o n (CVD). In v i e w of the very limited a m o u n t of LPE spinel s t u d i e s (4,5) we have s t a r t e d a p r o g r a m to i n v e s t i g a t e the g r o w t h of spinel ferrite films. In this w o r k we w a n t to r e p o r t on the g r o w t h c o n d i t i o n s of i n d i u m s u b s t i t u t e d m a g n e s i u m f e r r i t e by m e a n s of LPE. A b a s i c r e q u i r e m e n t for h e t e r o - e p i t a x i a l g r o w t h is a close m a t c h of lattice s p a c i n g b e t w e e n o v e r g r o w t h and s u b s t r a t e . For the g r o w t h of spinel f e r r i t e s n o n - m a g n e t i c c o m p o u n d s w i t h spinel s t r u c t u r e e g M e G a 0 ( M e = N i , M g , Z n , C u ) w o u l d be "" 24 s u i t a b l e as s u b s t r a t e s . H o w e v e r , untll now, apart from M g A I 2 0 4 ,
707
708
P.J.M.
VAN DER STRATEN, et al.
Vol. 12, No. 7
no s i n g l e c r y s t a l s of a d e q u a t e size have b e e n a v a i l a b l e . For most f e r r i t e s the use of M g A I g O A (a = 8.09 ~) as a s u b s t r a t e is not a t t r a c t i v e since the l ~ t ~ i c e ° m i s m a t c h is too large. I n s t e a d , w e have used M g O w h i c h is c o m m e r c i a l l y a v a i l a b l e . M a g n e s i a has a l a t t i c e c o n s t a n t of 4.21 ~, and e i g h t unit cells fit p e r f e c t l y to a s p i n e l cell w i t h a = 8.42 ~. If one is i n t e r e s t e d in films w h i c h can s u s t a i n ° m a g n e t i c b u b b l e s t h e r e are a d d i t i o n a l r e q u i r e m e n t s b o t h to the s u b s t r a t e and the o v e r g r o w t h itself. For this p u r p o s e the f i l m m u s t have a u n i a x i a l anisotropy K w i t h the p r e f e r r e d axis p e r p e n d i c u l a r to the p l a n e of t h e U f i l m . The u n i a x i a l a n i s o t r o p y f i e l d H k = K u / 2 M s has to be l a r g e r than the d e m a g n e t i z a t i o n f i e l d 4~M s. A low value of the s a t u r a t i o n m a g n e t i z a t i o n M s is h e l p f u l in this r e s p e c t . The a n i s o t r o p y can be e i t h e r g r o w t h - i n d u c e d or s t r e s s induced. We h a v e f o c u s s e d our a t t e n t i o n to M g F e 2 0 . as o v e r g r o w t h and M g O as s u b s t r a t e . The l a t t i c e c o n s t a n t of 4MgFe~OI, (high t e m p e r a t u r e form) is a = 8.39 ~, c l o s e to the v a l ~ e ~ o f 8.42 for e i g h t unit cells o~ MgO. A b e t t e r fit of l a t t i c e c o n s t a n t s b e t w e e n f i l m and s u b s t r a t e can be a c c o m p l i s h e d by s u b s t i t u t i o n of In, Sn or Ti in the film. A d i s a d v a n t a g e of M g O as s u b s t r a t e is its r a t h e r h i g h t h e r m a l e x p a n s i o n c o e f f i c i e n t (6) c o m p a r e d w i t h s p i n e l f e r r i t e s . A l s o in this r e g a r d M g F e 0 approaches 2 4 M g O the b e s t (7). C o m p a r e d to o t h e r s p i n e l f e r r l t e s the s a t u r a tion moment, of MgFe_Ot.. 2 is r e l a t i v e l y low (4~M s ~ 1500 G), and g r o w t h l n d u c e d u n l a x l a ~ a n i s o t r o p y of f l u x - g r o w n b u l k c r y s t a l s of A l - s u b s t i t u t e d M g F e 2 0 4 has b e e n r e p o r t e d (8). Experimental The t h e r m a l e x p a n s i o n of M g O and In, Sn or Ti s u b s t i t u t e d M g F e 2 0 4 has b e e n d e t e r m i n e d u s i n g h i g h t e m p e r a t u r e d i f f r a c t o m e t r y . S u b s t i t u t i o n is a c c o m p l i s h e d in such away that the r o o m t e m p e r a t u r e l a t t i c e c o n s t a n t s are a b o u t twice the v a l u e of MgO. F r o m the r e s u l t s , s u m m a r i z e d in T a b l e I, it can be c o n c l u d e d that the b e s t t h e r m a l e x p a n s i o n fit w i t h M g O is o b t a i n e d for I n - s u b s t i t u t e d M g F e O . The a c c u r a c y of the d e t e r m i n e d l a t t i c e c o n s t a n t s is + 0 . 0 0 5 4~. TABLE
I
T e m p e r a t u r e d e p e n d e D c e of l a t t i c e p a r a m e t e r s (~) e x p r e s s e d as a = a + bT + cT z (T in °C) of M g O and In, Sn ~r Ti s u b s t i t u ~ e a M g F e 2 0 4 (20 - 1200°C).
compound
a
b x
!05
c x
108
O
MgO
4.212
5.44
0.41
M g F e l . 8 1 1 n 0 . ;904
8.425
7.53
1.77
8.428
7.86
0.83
8.424
3.98
4.18
Mgl.18Fel.64Sn0.
1804
Mgl.50Fel.00Ti0.5004
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We h a v e u s e d a P b O - B 2 0 _ F e 2 0 3 f l u x for the LPE g r o w t h of m a g n e s i u m f e r r i t e . Fig. ! g i v e s a s u r v e y of l i t e r a t u r e d a t a on s o l v e n t c o m p o s i t i o n s u s e d for LPE g r o w t h of g a r n e t films and for f l u x g r o w t h of bulk MgFe20 ~ crystals (8, 9-15). In the f i g u g e the r e l e v a n t p a r t of the p s e u d o - t e r n a r y p h a s e d i a g r a m is shown. The composition of the s o l v e n t u s e d in our LPE s t u d y of M g F e 2 0 4 films, as i n d i c a t e d by p o l n t H in Fig. |, is ! PbO : 0.081B^O 3 : 0.035 Fe^O ( m o l e s ) . ~o this s o l v e n ~ ~ . 0 6 6 FIG. 1 m o l e s of p r e - r e a c t e d MgFel.78 Part of the ternary phase diaIn_ ~^O_ p o w d e r was a d d e d u.zz 4 g r a m P b O F e 2 0 B 2 0 . Solvent per one m o l e of PbO. The f l u x compositions ~n mo~ % u s e d for constituents w e r e m i x e d and f l u x g r o w t h and s o l u b i l i t y premelted in a 75 ml Pt crus t u d i e s of M g F e 2 0 4 and for cible. A c t u a l l y l% PbO was LPE g a r n e t s t u d l e s . r e p l a c e d by PbO_ for p r o t e c A ref. |0h B ref. If; C ref. 8; t i o n of the p l a t i n u m (16). D ref. 12; E ref. 13; F ref. 9; I s o t h e r m a l d i p p i n g of the subG ref. I4; H ref. I5, this w o r k s t r a t e s is p e r f o r m e d a c c o r d i n g to s t a n d a r d LPE g a r n e t p r o c e d u r e s in a v e r t i c a l 3 - z o n e r e s i s t a n c e furnace. M g O s i n g l e c r y s t a l s w e r e o b t a i n e d f r o m S p i c e r Ltd., E n g l a n ~ The c r y s t a l s w e r e X - r a y o r i e n t e d w i t h i n 0.5 ° and cut into ( | 0 0 ) w a f e r s , w i t h a d i a m e t e r of 12 mm and a t h i c k n e s s of 0.8 mm. The s l i c e s w e r e p o l i s h e d w i t h d i a m o n d p a s t e s w i t h s u c c e s s i v e l y d e c r e a s i n g p a r t i c l e s i z e s r a n g i n g f r o m 30 to 0.25 ~m. As w i l l be s h o w n later, a f i n a l t r e a t m e n t w i t h S y t o n - W - 3 0 was necessary. P r i o r to the f i l m g r o w t h the s u b s t r a t e s w e r e c l e a n e d w i t h o r g a n i c s o l v e n t s . A f t e r g r o w t h t e r m i n a t i o n , the a d h e r i n g f l u x d r o p l e t s are d i s s o l v e d in a m i x t u r e of hot d i l u t e a c e t i c a c i d w i t h n i t r i c acid. M e a s u r e m e n t s of the l a t t i c e c o n s t a n t m i s m a t c h are m a d e by s c a n n i n g the ( 8 0 0 ) - s p i n e l r e f l e c t i o n and the ( 4 0 0 ) - M g O r e f l e c t i o n w i t h F e - K ~ r a d i a t i o n . The m i s f i t is o b t a i n e d f r o m the r e l a t i o n Aa/a
=
I - v I + 9
A8 tan @"
Here
Aa
= as
- af,
is
the
difference
b e t w e e n the u n s t r a i n e d lattice parameters of s u b s t r a t e and film, @ is the B r a g g r e f l e c t i o n a n g l e and 9 is the P o i s s o n r a t i o , w h i c h is a s s u m e d to be 0.25 (17). F i l m t h i c k n e s s is m e a s u r e d by g r i n d i n g a s p h e r i c a l hole, j u s t t h r o u g h the film, u s i n g a 40 m m steel b a l l c o a t e d w i t h d i a m o n d p o l i s h i n g compofind and m e a s u r i n g the d i a m e t e r of the c o n c e n t r i c c i r c l e s f o r m e d by the f i l m s u r f a c e s . R e s u l t s w e r e c h e c k e d by i n t e r f e r o m e t r i c measurements.
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P . J . M . V A N D E R S T R A T E N , et al.
Vol. 12, No.7
Results
A f t e r s t i r r i n g the melt for about 100 h at 1000°C all solid p a r t i c l e s were d i s s o l v e d , but after c o o l i n g to about 900°C tiny c r y s t a l s could be o b s e r v e d f l o a t i n g on top of the melt. These c r y s t a l s had an o c t a h e d r a l shape and could be i d e n t i f i e d as s l i g h t l y In, Pb and Pt s u b s t i t u t e d M g F e 2 0 ~a ; 8.425 + 0.005 A). E l e c t r o n m i c r o p r o b e a n a l y s i s i n d i c a t e ~g inclusions in some of these crystals. C o n t i n u o u s s t i r r i n g of the melt at e l e v a t e d t e m p e r a t u r e s d i m i n i s h e d the a m o u n t of solid p a r t i c l e s at lower t e m p e r a t u r e s but the p a r t i c l e s n e v e r d i s a p p e a r e d c o m p l e t e l y . O b v i o u s l y some s p o n t a n e o u s n u c l e a t i o n occurs d u r i n g c o o l i n g b e l o w the l i q u i d u s t e m p e r a t u r e of the melt. F r o m our first LPE e x p e r i m e n t s two f e a t u r e s b e c a m e clear. F i r s t l y a rather large s u p e r s a t u r a t i o n of the flux is n e c e s s a r y to n u c l e a t e film growth, I' and s e c o n d l y the surface • i i f i n i s h of the Mg0 s u b s t r a t e s is e x t r e m e l y important. We i c o n s i d e r the latter point in m I"I; more detail. At t e m p e r a t u r e s I i b e t w e e n 900°C and 850°C a b a d l y a d h e r e n t o v e r g r o w t h was o b t a i n e d on the d i a m o n d p o l i s I | hed s u b s t r a t e s , w h i l e on the I . t syton p o l i s h e d s u b s t r a t e s no ,10pro g r o w t h could be observed. At lower t e m p e r a t u r e s we a c h i e v e ~ FIG. 2 a firmly a d h e r e n t film on the diamond polished substrates. Island g r o w t h on a syton poThis o v e r g r o w t h , h o w e v e r , lished MgO s u b s t r a t e , g r o w t h looked very rough and opaque. t e m p e r a t u r e 808°C. F i l m g r o w t h on the syton polished substrates started only at t e m p e r a t u r e s b e l o w ' D / 0B a p p r o x i m a t e l y 780°C. O f t e n D i jl island g r o w t h was o b s e r v e d 111 [] (Fig. 2) w i t h c r y s t a l l i t e s in /I 0.6 I the form of i d e n t i c a l l y o r i e n E [] z/ ':D., //'I 2 Q ted t r u n c a t e d p y r a m i d e s w i t h ~' 0.~ basal edges along d i r e c t i o n s . At t e m p e r a t u r e s " 5mrn,O r p m b e t w e e n 675°C and 750°C a 0.2 o closed, smooth and t r a n s p a rent, red c o l o r e d o v e r g r o w t h I i I I / I was obtained. S o l i d i f i e d 725 750 775 800 8 2 5 ~°C a d h e r e n t flux d r o p l e t s locally spoiled the o t h e r w i s e FIG. 3 c o m p l e t e l y shiny surfaces. As shown in Fig. 3 the g r o w t h G r o w t h rates as a f u n c t i o n of rates showed a quite d i f f e g r o w t h t e m p e r a t u r e for a diarent t e m p e r a t u r e d e p e n d e n c e mond p o l i s h e d s u b s t r a t e (dasfor the syton and d i a m o n d hed line) and for a syton pop o l i s h e d s u b s t r a t e s . On the lished s u b s t r a t e (solid line). diamond polished substrates
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Vol. 12, No.7
MAGNESIUM FERRITE FILMS
FIG.
4A
FIG,
711
4B
S t e r e o s c a n p i c t u r e of a f i l m g r o w n on a d i a m o n d p o l i s h e d s u b s t r a t e (a) or on a s y t o n p o l i s h e d s u b s t r a t e (b).
the g r o w t h r a t e d e c r e a s e ~ l i n e a r l y f r o m 0.9 ~ m / m i n at 8 5 0 ° C to less than 0.1 ~ m / m i n at 700°C. For the s y t o n p o l i s h e d subs t r a t e s the g r o w t h rate s h o w e d a m a x i m u m of a b o u t 0.5 p m / m i n n e a r 7 6 0 ° C ( d i p p i n g time 5 m i n . ) . The d i f f e r e n c e in g r o w t h b e h a v i o r m u s t be due to the s u r f a c e m o r p h o l o g y of the s u b s t r a tes. S t e r e o s c a n p i c t u r e s r e v e a l a m u c h s m o o t h e r M g O s u r f a c e for the s y t o n p o l i s h e d s u b s t r a t e s c o m p a r e d w i t h the d i a m o n d p o l i s h e ~ s u b s t r a t e s . This d i f f e r e n c e in s u r f a c e m o r p h o l o g y is c l e a r l y r e f l e c t e d in the r e s p e c t i v e f i l m s u r f a c e s . Fig. 4a s h o w s the s u r f a c e of a r o u g h , o p a q u e f i l m g r o w n on a d i a m o n d p o l i s h e s subs t r a t e . The p i c t u r e s h o w s a r a t h e r o p e n s t r u c t u r e , l o o k i n g like a crocket-work, with needles I f i i pointing along direct i o n s . Fig. 4b s h o w s the s u r ._c 0.6 ,125rpm face of a t r a n s p a r e n t f i l m E g r o w n on a s y t o n p o l i s h e d subE s t r a t e . The s t r u c t u r e is m o r e d e n s e and r a t h e r flat in this min,Orp~ case. The s u r f a c e m o r p h o l o g y of a r e a s w i t h a f t e r - g r o w t h on 0.2 an o t h e r w i s e s m o o t h s u r f a c e is o i d e n t i c a l w i t h that s h o w n in I_ o~ Fig. 4a. I I I I Fig. 5 s h o w s the g r o w t h r a t e 725 750 775 800 .--,-° C of f i l m s g r o w n on s y t o n p o l i s hed MgO, by m e a n s of h o r i z o n FIG. 5 tal d i p p i n g at a r o t a t i o n G r o w t h rate as a f u n c t i o n of s p e e d of 125 rpm, or by v e r t i g r o w t h t e m p e r a t u r e of f i l m s cal d i p p i n g . R o t a t i o n of the g r o w n on s y t o n p o l i s h e d MgO. s u b s t r a t e d u r i n g the g r o w t h C ircles, vertically dipped; produces a much smoother overtriangles, horizontally dipg r o w t h and less c o n t a m i n a t i o n ped w i t h s u b s t r a t e r o t a t i on of the s u r f a c e by f l u x d r o p 125 rpm.
~ ~
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VAN DER STRATEN, et al.
Vol. 12, No. 7
lets. The f l a t n e s s of the h o r i z o n t a l l y d i p p e d f i l m was I-2 f r i n g e s in N a - D light, c o m p a r e d w i t h 5-I0 f r i n g e s for the films g r o w n by v e r t i c a l d i p p i n g . T h e r e also is a d i f f e r e n t t e m p e r a t u r e d e p e n d e n c e on the g r o w t h rates for the two cases s h o w n in the figure. X-ray
analysis
X-ray diffraction p a t t e r n s of the films w e r e i n v e s t i g a t e d by m e a n s of a S i e m e n s t e x t u r e g o n i o m e t e r . P o l e f i g u r e s of one or m o r e forms of l a t t i c e p l a n e s can be o b t a i n e d of b o t h s u b s t r a t e a~d f i l m by a p r o p e r s e l e c t i o n of the B r a g g a n g l e 0. The { I l l } p o l e s of M g O c a n n o t be d i s t i n g u i s h e d f r o m the { 2 2 2 } - s p i n e l poles, but the { 2 2 0 } - p o l e s of the s p i n e l a p p e a r alone. F r o m the r e s u l ting p o l e f i g u r e we c o n c l u d e that the films are s i n g l e c r y s t a l line. For the films g r o w n on s y t o n p o l i s h e d s u b s t r a t e s the < 1 0 0 > ~ i r e c t i o n s of f i l m and s u b s t r a t e c o i n c i d e . H o w e v e r , the o p a q u e films g r o w n on d i a m o n d p o l i s hed M g O s h o w a d i f f e r e n c e I-2 ° 0 rprn b e t w e e n the < 1 0 0 > d i r e c t i o n s olO f -0.02 of f i l m and s u b s t r a t e . Due to this d i f f e r e n c e the m i s f i t _..-o f AI o<%' c a n n o t be m e a s u r e d b e c a u s e M g O ~ 125rpm and s p i n e l r e f l e c t i o n c a n n o t ..,,.=0.03 be a c h i e v e d for the same p l a Ii$, I / telet s e t t i n g in the d i f f r a c tion equipment. The u n s t r a i n e d m i s f i t Aa = 1 I I I a s - af, b e t w e e n s u b s t r a t e and 725 750 775 800--,IC film, is p l o t t e d in Fig. 6 as FIG. 6 a f u n c t i o n of g r o w t h t e m p e r a ture Tg for films g r o w n on syM i s f i t , a ( s u b s t r a t e ) - a (film), ton p o l i s h e d MgO. All films as a f u n c t i o n of g r o w t h t e m p e are in c o m p r e s s i o n i.e. Aa < 0. rature. Circles, vertically For films g r o w n w i t h o u t r o t a ~ipped; triangles, horizontalt i o n the m i s f i t i n c r e a s e s f r o m ly d i p p e d w i t h s u b s t r a t e roAa = - 0 . 0 1 9 for Tg = 800 ° to Aa t a t i o n 125 rpm; d i p p i n g time = - 0 . 0 3 6 A for T~ = 730°C. For 15 min. films g r o w n u n a e r r o t a t x o n the m i s f i t is a l w a y s 0.01 larger. Due to the d i f f e r e n c e in t h e r m a l e x p a n s i o n c o e f f i c i e n t of s u b s t r a t e and film, a zero m i s f i t at the g r o w t h t e m p e r a t u r e leads to a f i l m u n d e r c o m p r e s s i o n at room temperature. For T. = 8 0 0 ° C the r e s u l t i n g r o o m t e m p e r a t u r e m i s f i t is Aa = - 0 . 0 1 7 0 A~ w h i l e for Tg = 7 0 0 ° C Aa = - 0 . 0 1 6 5 ~. F r o m Fig. 6 it f o l l o w s that the films are c l o s e to zero m i s f i t or s l i g h t l y in c o m p r e s s i o n at Tg. The h i g h e r c o m p r e s s i v e strain of the films g r o w n u n d e r r o t a t i o n is p r o b a b l y r e l a t e d to the h i g h e r g r o w t h rate. i
i
i
i
-o.o4
Microprobe
analysis
Generally differences in l a t t i c e c o n s t a n t s are c a u s e d by d i f f e r e n c e s in c o m p o s i t i o n . F r o m s t u d i e s on LPE g r o w n g a r n e t films it is k n o w n that a p p r e c i a b l e a m o u n t s of Pb can be i n c o r p o r a t e d (18). I n d e e d , x - r a y f l u o r e s c e n c e measurements r e v e a l e d the p r e s e n c e of a large a m o u n t of Pb in our s p i n e l films. In o r d e r to o b t a i n m o r e q u a n t i t a t i v e information a b o u t the c h e m i c a l
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MAGNESIUM FERRITE FILMS
c o m p o s i t l o n we have p e r f o r m e a an e l e c t r o n m i c r o p r o b e analysis. The m e a s u r e m e n t s w e r e p e r f o r m e a u s i n g a SEM II A e l e c t r o n p r o b e m i c r o - a n a l y s e r . The c o n c e n t r a tions w e r e c a l c u l a t e d w i t h the aid of a c o m p u t e r p r o g r a m u s i n g m e a s u r e d i n t e n s i t i e s of pure Fe, In, Pb, Pt and MgO as standards. To o p t i m i z e the e x p e r i m e n t a l c o n d i t i o n s and to check the a c c u r a c y of the r e s u l t s s t a n d a r d s of s i n t e r e d M g F e 2 _ x InxO 4 (x=0, 0.1, 0.2, 0.3) w e r e m e a s u r e d . For the determinationof Mg a low a c c e l e r a t i n g voltage is p r e f e r r e d , h o w e v e r , e s p e c i a l l y for Pb a h i g h voltage has to be applied. The best c o m p r o m i s e was 20kV. At this v o l t a g e the d e p t h of the a n a l y s e d r e g i o n in the f i l m is 1.25 pm. F r o m m e a s u r e m e n t s on the c e r a m i c s t a n d a r d s the following a c c u r a c y was o b t a i n e a (atoms per f o r m u l a unit) : Mg ! 0.2 (due to high a b s o r p t i o n c o r r e c t i o n ) , Fe + 0.05, In + 0.02. The e s t i m a t e d a c c u r a c y of Pb and Pt is + 0.01. All films w e r e found to be Pb and Pt substituated MgFe~ In O., w i t h x = 0.09 - 0 . l ~ ? X A s X s ~ o w n in Fig. 7 the Pt c o n t e n t is about 0.05, w i t h s l i g h t l y i n c r e a s i n g c o n c e n t r a t i o n at lower g r o w t h t e m p e r a t u r e s . The Pb c o n t e n t is s t r o n g l y d e p e n d e n t on the g r o w t h p a r a m e t e r s . At the low e s t g r o w t h t e m p e r a t u r e s the c o n c e n t r a t i o n i n c r e a s e s to about 0.2 atoms per f o r m u l a unit. So far w~ have a n a l y s e d the c o m p o s i t i o n of our films n o r m a l to the plane of the platelet w i t h an a n a l y s e d d e p t h of 1.25 Hm. In order to study the c o m p o s i t i o n as f u n c t i o n of f i l m t h i c k n e s s we have grown a 27 Hm thick f i l m (60 min, ;25 rpm, 760°C). A n a l y s i s are p e r f o r m e d on the b r o k e n and p o l i s h e d platelet n o r m a l to the g r o w t h dir e c t i o n w i t h steps of 2.8 ~m f r o m s u b s t r a t e to film surface.
718
i
,
0.20
,
:
l
i
Pb
0.15 E
"" 0.10
o~ 0.05
7'2s
7 ,o
7';s
FIG.
8bo--- '°c
7
A n a l y s e d Pt and Pb c o n t e n t in the films vs g r o w t h t e m p e r a t u re. O p e n symbols, v e r t i c a l l y aipped; full symbols, h o r i z o n tally d i p p e d w i t h s u b s t r a t e r o t a t i o n 125 rpm; d i p p i n g time rain. r
0.15
u ' ~ ~ O
0
.-=, C
o o.lo
o\
0.05
Pt
0
2'0 distance above substrate (prn)
FIG. C o n c e n t r a t i o n vs d e p t h in a 27 Hm thick film, h o r i z o n t a l l y aipped with substrate rotation 125 rpm for 60 min. at 760°C.
714
P.J.M. VAN DER STRATEN,
et al.
Vol. 12, No. 7
As can be seen f r o m Fig. 8 the Pb and In c o n t e n t s d e c r e a s e w h e n g o i n g to the f i l m s u r f a c e w h i l e the Pt c o n t e n t s l i g h t l y i n c r e a ses. T h e s e c o m p o s i t i o n a l variations are p r o b a b l y r e l a t e d to c h a n g e s in g r o w t h w i t h d i p p i n g time. Magnetic
parameters
The s a t u r a t i o n m a g n e t i z a t i o n of the films was m e a s u r e d w i t h a vibrating sample magnetometer. The 4~M s v a l u e s d e c r e a s e c o n t i n u o u s l y f r o m 2000 ~ 100 G a u s s for a f i l m g r o w n at 790°C to 1300 G a u s s for a f i l m g r o w n at 724°C. R o t a t i o n of the s u b s t r a t e leads to a b o u t I0% lower v a l u e s of 4~M s. M a g n e t i c t o r q u e m e a s u r e m e n t s r e v e a l e d that the e a s y axes of m a g n e t i z a t i o n are i n - p l a n e . In a c c o r d a n c e w i t h this r e s u l t no s t r i p e d o m a i n s w e r e o b s e r v e d . Discussion We h a v e s e e n that a r a t h e r h i g h s u p e r s a t u r a t i o n of the f l u x (AT ~ 2 0 0 o c ) is n e c e s s a r y to o b t a i n f i l m g r o w t h on ( 1 0 0 ) - s y t o n p o l i s h e d M g O s u b s t r a t e s . An e x p l a n a t i o n can be f o u n d in the h i g h e r s u r f a c e free e n e r g y of a ( 1 0 0 ) - p l a n e of M g O c o m p a r e d to a ( I I I ) - p l a n e . F i l m g r o w t h on ( I l l ) - M g O s t a r t s a b o u t 100°C h i g h e r than on ( 1 0 0 ) - M g O s u b s t r a t e s (19), so w h e n an i n s u f f i c i e n t l y polished (IO0) s u b s t r a t e is d i p p e d at a t e m p e r a t u r e b e t w e e n these saturation temperatures, i n i t i a l g r o w t h can o c c u r on t h o s e p a r t s of the s u r f a c e w h i c h h a v e a lower s u r f a c e free e n e r g y (20). This e x p l a i n s w h y g r o w t h on d i a m o n d p o l i s h e d s u b s t r a t e s s t a r t s at a higher temperature than on s y t o n p o l i s h e d s u b s t r a t e s . As a r e s u l t films on d i a m o n d p o l i s h e d s u b s t r a t e s h a v e an i n f e r i o r q u a l i t y and < I 0 0 > d i r e c t i o n s of f i l m and s u b s t r a t e can e v e n m a k e a small a n g l e w i t h e a c h other. We find an a p p r e c i a b l e drop of the s a t u r a t i o n m a g n e t i z a t i o n with decreasing growth temperature. This e f f e c t can be a t t r i b u ted to the i n c r e a s i n g Pb c o n c e n t r a t i o n . The lead ions are p r o bably substituted at o c t a h e d r a l sites, w h i c h gives a d e c r e a s e of 4~M s. At the same time the C u r i e t e m p e r a t u r e d e c r e a s e s due to the d e c r e a s i n g m a g n e t i c i n t e r a c t i o n and this leads to a f u r t h e r l o w e r i n g of the m a g n e t i z a t i o n . The i n f l u e n c e of Pb is c l e a r l y p r o v e d by the e f f e c t of s u b s t r a t e r o t a t i o n . F i n a l l y the e q u i l i brium distribution of Mg ions also d e p e n d s on the t e m p e r a t u r e . At lower t e m p e r a t u r e s m o r e Mg is p r e s e n t at o c t a h e d r a l sites, and this also c o n t r i b u t e s to the l o w e r i n g of 4~M s. So far we did not s u c c e e d in g r o w i n g films w i t h the p r e f e r r e d m a g n e t i z a t i o n direction perpendicular to the f i l m p l a n e . This is p r o b a b l y due to the c o m p r e s s i v e s t r e s s in c o m b i n a t i o n w i t h a n e g a t i v e m a g n e tostriction c o n s t a n t ~ , n n of the f e r r i t e . A s s u m i n g Y o u n g ' s mod u l u s E ~ 2 x I0 L a y n / ¢ m 2 , a m i s f i t Aa = - 0 . 0 2 ~ c o r r e s p o n d s to a s t r e s s ~ ~ -6 x 109 d y n / c m 2. If we f u r t h e r take 4~M s 1500 G and ~ I O 0 ~ - I 0 - 5 , the s t r e s s i n d u c e d a n i s o t r o p y f i e l d is H s ~ r e s s ~ - 1 5 0 0 Oe. A c c o r d i n g to B o r r e l l i (8) the g r o w t h - i n d u cea a n l s o t r o p y in b u l k c r y s t a l s is of the o r d e r H ro t + I 0 0 0 Oe. If H r o w t h has a s i m i l a r m a g n i t u d e in t~e ~ 1 ~ m s a s l i g h t l y p o s i t i v e m l s ~ i t is n e c e s s a r y . We h a v e t r i e d to a c h i e v e this by a d d i n g small a m o u n t s of e i t h e r A I 2 0 3 or G a 2 0 3 to the flux. H o w e v e r , the r e s u l t i n g films a l w a y s s h o w e d light c o l o r e d , b r o a d , s t r a i g h t lines a l o n g < I 0 0 > d i r e c t i o n s as soon as Aa > - 0 . 0 1 4 . This m e a n s that due to the o c c u r e n c e of some s t r e s s
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MAGNESIUM FERRITE FILMS
r e l i e v i n g m e c h a n i s m no p e r f e c t films + 0 . 0 0 3 at the g r o w t h t e m p e r a t u r e .
could be grown w i t h
715
Aa >
Conclusions We have shown that it is p o s s i b l e to grow e p i t a x i a l films of m a g n e s i u m - i n d i u m f e r r i t e on (I00) MgO s u b s t r a t e s by m e a n s of the LPE m e t h o d . To grow p e r f e c t films a good s u r f a c e f i n i s h of the s u b s t r a t e s is n e c e s s a r y . A serious d i s a d v a n t a g e of MgO is the large d i f f e r e n c e in thermal e x p a n s i o n c o m p a r e d w i t h the ferrite. As a c o n s e q u e n c e good films are always in c o m p r e s s i o n at r o o m temperature. Acknowledgements The authors w o u l d like to thank J.M. R o b e r t s o n and P.F. B o n g e r s of P h i l i p s R e s e a r c h L a b o r a t o r i e s , E i n a h o v e n for their a a v i c e s and support; H.A. L o g m a n s of the same l a b o r a t o r y p e r f o r m e d the m a g n e t i c m e a s u r e m e n t s . J . W . G . A . V r o l i j k of our l a b o r a t o r y is t h a n k e a for a s s i s t a n c e w i t h the e l e c t r o n m i c r o p r o b e c a l c u l a t i o n s and H. de Jonge Baas for t e c h n i c a l a s s i s t a n c e . References I. H.J. L e v i n s t e i n , S. Licht, R.W. L a n s d o r f and S.L. Blank, Appl. Phys. L e t t e r s 19, 486 (1971). 2. F.S. S t e a r n s and H.L. Glass, Mat. Res. Bull. IO, 1255 (1975) 3. F.S. S t e a r n s ana H.L. Glass, Mat. Res. Bull. I__~I, 1319 (1976). 4. R.J. G a m b i n o , J. Appl. Phys. 38, 1129 (1967). 5. D.A. Herman, R.L. White, R.S. F e i g e l s o n , B.L. M a t t e s and H.W. Swarts, A.I.P. Conf. Proc. 24, 580 (1975). 6 B.M. Dutta, J. Sci. Res. B a n a r a s H i n d u Univ. 15, 80 (1964-1965). 7 R.E. Carter, J. Am. Cer. Soc. 42, 324 (1959). 8. N.F. B o r r e l l i , J. Appl. Phys. 46, 430 (1975). 9 J . E . Davies, E.A.D. W h i t e and J.D.C. Wood, J. C r y s t a l G r o w t h 27, 227 (1974). l0 T. M a t s u y a m a , Y. N a g a t a and K. Ohta, J a p a n J. Appl. Phys. 14, 727 (1975). [I E.V. R y b a l s k a y a , T.G. P e t r o v and A.G. ~itova, Soviet P h y s i c s - C r y s t a l l o g r a p h y 15, 958 (1971). 12 H.L. Glass and M.T. E l l i o t t , J. C r y s t a l G r o w t h 27, 253 (1974). 13 A.E. M o r g a n , J. C r y s t a l G r o w t h 27, 266 (1974). 14. R. Ghez and E.A. Giess, J. C r y s t a l G r o w t h 27, 221 (1974). 15. J.M. R o b e r t s o n , We thank J.M. R o b e r t s o n for his advice c o n c e r n i n g the flux c o m p o s i t i o n . 16. B.M. W a n k l y n , J. C r y s t a l G r o w t h ~, 323 (1969). 17. D.J. Dumin, J. E l e c t r o c h e m . Soc. 114, 749 (1967). 18. J.E. Davies, E.A. Giess, J.D. K u p t s i s and W. Reuter, J. C r y s t a l G r o w t h 36, 1971 (1976). 19. J.M. R o b e r t s o n , p r i v a t e c o m m u n i c a t i o n . 20. I. Kasai and D.W. Bassett, J. C r y s t a l G r o w t h 27, 215 (1974).
LIQUID
PHASE
EPITAXIAL
GROWTH
FERRITE
Pergamon Press, ~ c .
OF I N D I U M
SUBSTITUTED
P~ntedinthe
MAGNESIUM
FILMS
P.J.M. van der S t r a t e n and R. M e t s e l a a r L a b o r a t o r y of P h y s i c a l C h e m i s t r y , U n i v e r s i t y of T e c h n o l o g y , E i n d h o v e n , The N e t h e r l a n d s .
(Received May 20, 1977; Communicated by A. Rabenau)
ABSTRACT S i n g l e crystal films of M g ( F e , l n ) 2 0 4 have b e e n grown by m e a n s of l i q u i d - p h a s e - e p i t a x y on ( 1 0 0 ) - M g O s u b s t r a t e s u s i n g a P b O - B 2 0 3 - F e 2 0 3 solvent. The final p o l i s h i n g of the s u b s t r a t e s a p p e a r e d to be very i m p o r t a n t in c o n n e c t i o n w i t h the n e c e s s a r y large s u p e r s a t u r a t i o n of the flux. E l e c t r o n m i c r o p r o b e a n a l y s i s r e v e a l e d Pt and high Pb c o n t e n t s in our films. The easy axis of m a g n e t i z a t i o n is in-plane, p r o b a b l y due to the c o m p r e s s i v e s t r a i n of the films c a u s e d by the d i f f e r e n c e in thermal e x p a n s i o n of MgO and M g ( F e , l n ) 2 0 4 .
Introduction M a g n e t i c films have a t t r a c t e d m u c h i n t e r e s t b e c a u s e of their p o t e n t i a l a p p l i c a b i l i t y in m a g n e t i c and m a g n e t o - o p t i c devices. M o s t s t u d i e s c o n c e r n the l i q u i d - p h a s e - e p i t a x i a l (LPE) g r o w t h of g a r n e t films (I) and r e c e n t l y the LPE g r o w t h of h e x a g o n a l f e r r i t e s has been r e p o r t e d (2,3). C o n c e r n i n g the gro~h of spinel f e r r i t e s most work, so far, has b e e n done by m e a n s of c h e m i c a l vapor d e p o s i t i o n (CVD). In v i e w of the very limited a m o u n t of LPE spinel s t u d i e s (4,5) we have s t a r t e d a p r o g r a m to i n v e s t i g a t e the g r o w t h of spinel ferrite films. In this w o r k we w a n t to r e p o r t on the g r o w t h c o n d i t i o n s of i n d i u m s u b s t i t u t e d m a g n e s i u m f e r r i t e by m e a n s of LPE. A b a s i c r e q u i r e m e n t for h e t e r o - e p i t a x i a l g r o w t h is a close m a t c h of lattice s p a c i n g b e t w e e n o v e r g r o w t h and s u b s t r a t e . For the g r o w t h of spinel f e r r i t e s n o n - m a g n e t i c c o m p o u n d s w i t h spinel s t r u c t u r e e g M e G a 0 ( M e = N i , M g , Z n , C u ) w o u l d be "" 24 s u i t a b l e as s u b s t r a t e s . H o w e v e r , untll now, apart from M g A I 2 0 4 ,
707
708
P.J.M.
VAN DER STRATEN, et al.
Vol. 12, No. 7
no s i n g l e c r y s t a l s of a d e q u a t e size have b e e n a v a i l a b l e . For most f e r r i t e s the use of M g A I g O A (a = 8.09 ~) as a s u b s t r a t e is not a t t r a c t i v e since the l ~ t ~ i c e ° m i s m a t c h is too large. I n s t e a d , w e have used M g O w h i c h is c o m m e r c i a l l y a v a i l a b l e . M a g n e s i a has a l a t t i c e c o n s t a n t of 4.21 ~, and e i g h t unit cells fit p e r f e c t l y to a s p i n e l cell w i t h a = 8.42 ~. If one is i n t e r e s t e d in films w h i c h can s u s t a i n ° m a g n e t i c b u b b l e s t h e r e are a d d i t i o n a l r e q u i r e m e n t s b o t h to the s u b s t r a t e and the o v e r g r o w t h itself. For this p u r p o s e the f i l m m u s t have a u n i a x i a l anisotropy K w i t h the p r e f e r r e d axis p e r p e n d i c u l a r to the p l a n e of t h e U f i l m . The u n i a x i a l a n i s o t r o p y f i e l d H k = K u / 2 M s has to be l a r g e r than the d e m a g n e t i z a t i o n f i e l d 4~M s. A low value of the s a t u r a t i o n m a g n e t i z a t i o n M s is h e l p f u l in this r e s p e c t . The a n i s o t r o p y can be e i t h e r g r o w t h - i n d u c e d or s t r e s s induced. We h a v e f o c u s s e d our a t t e n t i o n to M g F e 2 0 . as o v e r g r o w t h and M g O as s u b s t r a t e . The l a t t i c e c o n s t a n t of 4MgFe~OI, (high t e m p e r a t u r e form) is a = 8.39 ~, c l o s e to the v a l ~ e ~ o f 8.42 for e i g h t unit cells o~ MgO. A b e t t e r fit of l a t t i c e c o n s t a n t s b e t w e e n f i l m and s u b s t r a t e can be a c c o m p l i s h e d by s u b s t i t u t i o n of In, Sn or Ti in the film. A d i s a d v a n t a g e of M g O as s u b s t r a t e is its r a t h e r h i g h t h e r m a l e x p a n s i o n c o e f f i c i e n t (6) c o m p a r e d w i t h s p i n e l f e r r i t e s . A l s o in this r e g a r d M g F e 0 approaches 2 4 M g O the b e s t (7). C o m p a r e d to o t h e r s p i n e l f e r r l t e s the s a t u r a tion moment, of MgFe_Ot.. 2 is r e l a t i v e l y low (4~M s ~ 1500 G), and g r o w t h l n d u c e d u n l a x l a ~ a n i s o t r o p y of f l u x - g r o w n b u l k c r y s t a l s of A l - s u b s t i t u t e d M g F e 2 0 4 has b e e n r e p o r t e d (8). Experimental The t h e r m a l e x p a n s i o n of M g O and In, Sn or Ti s u b s t i t u t e d M g F e 2 0 4 has b e e n d e t e r m i n e d u s i n g h i g h t e m p e r a t u r e d i f f r a c t o m e t r y . S u b s t i t u t i o n is a c c o m p l i s h e d in such away that the r o o m t e m p e r a t u r e l a t t i c e c o n s t a n t s are a b o u t twice the v a l u e of MgO. F r o m the r e s u l t s , s u m m a r i z e d in T a b l e I, it can be c o n c l u d e d that the b e s t t h e r m a l e x p a n s i o n fit w i t h M g O is o b t a i n e d for I n - s u b s t i t u t e d M g F e O . The a c c u r a c y of the d e t e r m i n e d l a t t i c e c o n s t a n t s is + 0 . 0 0 5 4~. TABLE
I
T e m p e r a t u r e d e p e n d e D c e of l a t t i c e p a r a m e t e r s (~) e x p r e s s e d as a = a + bT + cT z (T in °C) of M g O and In, Sn ~r Ti s u b s t i t u ~ e a M g F e 2 0 4 (20 - 1200°C).
compound
a
b x
!05
c x
108
O
MgO
4.212
5.44
0.41
M g F e l . 8 1 1 n 0 . ;904
8.425
7.53
1.77
8.428
7.86
0.83
8.424
3.98
4.18
Mgl.18Fel.64Sn0.
1804
Mgl.50Fel.00Ti0.5004
Vol. 12, No. ?
MAGNESIUM
FERBITE
FILMS
709
We h a v e u s e d a P b O - B 2 0 _ F e 2 0 3 f l u x for the LPE g r o w t h of m a g n e s i u m f e r r i t e . Fig. ! g i v e s a s u r v e y of l i t e r a t u r e d a t a on s o l v e n t c o m p o s i t i o n s u s e d for LPE g r o w t h of g a r n e t films and for f l u x g r o w t h of bulk MgFe20 ~ crystals (8, 9-15). In the f i g u g e the r e l e v a n t p a r t of the p s e u d o - t e r n a r y p h a s e d i a g r a m is shown. The composition of the s o l v e n t u s e d in our LPE s t u d y of M g F e 2 0 4 films, as i n d i c a t e d by p o l n t H in Fig. |, is ! PbO : 0.081B^O 3 : 0.035 Fe^O ( m o l e s ) . ~o this s o l v e n ~ ~ . 0 6 6 FIG. 1 m o l e s of p r e - r e a c t e d MgFel.78 Part of the ternary phase diaIn_ ~^O_ p o w d e r was a d d e d u.zz 4 g r a m P b O F e 2 0 B 2 0 . Solvent per one m o l e of PbO. The f l u x compositions ~n mo~ % u s e d for constituents w e r e m i x e d and f l u x g r o w t h and s o l u b i l i t y premelted in a 75 ml Pt crus t u d i e s of M g F e 2 0 4 and for cible. A c t u a l l y l% PbO was LPE g a r n e t s t u d l e s . r e p l a c e d by PbO_ for p r o t e c A ref. |0h B ref. If; C ref. 8; t i o n of the p l a t i n u m (16). D ref. 12; E ref. 13; F ref. 9; I s o t h e r m a l d i p p i n g of the subG ref. I4; H ref. I5, this w o r k s t r a t e s is p e r f o r m e d a c c o r d i n g to s t a n d a r d LPE g a r n e t p r o c e d u r e s in a v e r t i c a l 3 - z o n e r e s i s t a n c e furnace. M g O s i n g l e c r y s t a l s w e r e o b t a i n e d f r o m S p i c e r Ltd., E n g l a n ~ The c r y s t a l s w e r e X - r a y o r i e n t e d w i t h i n 0.5 ° and cut into ( | 0 0 ) w a f e r s , w i t h a d i a m e t e r of 12 mm and a t h i c k n e s s of 0.8 mm. The s l i c e s w e r e p o l i s h e d w i t h d i a m o n d p a s t e s w i t h s u c c e s s i v e l y d e c r e a s i n g p a r t i c l e s i z e s r a n g i n g f r o m 30 to 0.25 ~m. As w i l l be s h o w n later, a f i n a l t r e a t m e n t w i t h S y t o n - W - 3 0 was necessary. P r i o r to the f i l m g r o w t h the s u b s t r a t e s w e r e c l e a n e d w i t h o r g a n i c s o l v e n t s . A f t e r g r o w t h t e r m i n a t i o n , the a d h e r i n g f l u x d r o p l e t s are d i s s o l v e d in a m i x t u r e of hot d i l u t e a c e t i c a c i d w i t h n i t r i c acid. M e a s u r e m e n t s of the l a t t i c e c o n s t a n t m i s m a t c h are m a d e by s c a n n i n g the ( 8 0 0 ) - s p i n e l r e f l e c t i o n and the ( 4 0 0 ) - M g O r e f l e c t i o n w i t h F e - K ~ r a d i a t i o n . The m i s f i t is o b t a i n e d f r o m the r e l a t i o n Aa/a
=
I - v I + 9
A8 tan @"
Here
Aa
= as
- af,
is
the
difference
b e t w e e n the u n s t r a i n e d lattice parameters of s u b s t r a t e and film, @ is the B r a g g r e f l e c t i o n a n g l e and 9 is the P o i s s o n r a t i o , w h i c h is a s s u m e d to be 0.25 (17). F i l m t h i c k n e s s is m e a s u r e d by g r i n d i n g a s p h e r i c a l hole, j u s t t h r o u g h the film, u s i n g a 40 m m steel b a l l c o a t e d w i t h d i a m o n d p o l i s h i n g compofind and m e a s u r i n g the d i a m e t e r of the c o n c e n t r i c c i r c l e s f o r m e d by the f i l m s u r f a c e s . R e s u l t s w e r e c h e c k e d by i n t e r f e r o m e t r i c measurements.
710
P . J . M . V A N D E R S T R A T E N , et al.
Vol. 12, No.7
Results
A f t e r s t i r r i n g the melt for about 100 h at 1000°C all solid p a r t i c l e s were d i s s o l v e d , but after c o o l i n g to about 900°C tiny c r y s t a l s could be o b s e r v e d f l o a t i n g on top of the melt. These c r y s t a l s had an o c t a h e d r a l shape and could be i d e n t i f i e d as s l i g h t l y In, Pb and Pt s u b s t i t u t e d M g F e 2 0 ~a ; 8.425 + 0.005 A). E l e c t r o n m i c r o p r o b e a n a l y s i s i n d i c a t e ~g inclusions in some of these crystals. C o n t i n u o u s s t i r r i n g of the melt at e l e v a t e d t e m p e r a t u r e s d i m i n i s h e d the a m o u n t of solid p a r t i c l e s at lower t e m p e r a t u r e s but the p a r t i c l e s n e v e r d i s a p p e a r e d c o m p l e t e l y . O b v i o u s l y some s p o n t a n e o u s n u c l e a t i o n occurs d u r i n g c o o l i n g b e l o w the l i q u i d u s t e m p e r a t u r e of the melt. F r o m our first LPE e x p e r i m e n t s two f e a t u r e s b e c a m e clear. F i r s t l y a rather large s u p e r s a t u r a t i o n of the flux is n e c e s s a r y to n u c l e a t e film growth, I' and s e c o n d l y the surface • i i f i n i s h of the Mg0 s u b s t r a t e s is e x t r e m e l y important. We i c o n s i d e r the latter point in m I"I; more detail. At t e m p e r a t u r e s I i b e t w e e n 900°C and 850°C a b a d l y a d h e r e n t o v e r g r o w t h was o b t a i n e d on the d i a m o n d p o l i s I | hed s u b s t r a t e s , w h i l e on the I . t syton p o l i s h e d s u b s t r a t e s no ,10pro g r o w t h could be observed. At lower t e m p e r a t u r e s we a c h i e v e ~ FIG. 2 a firmly a d h e r e n t film on the diamond polished substrates. Island g r o w t h on a syton poThis o v e r g r o w t h , h o w e v e r , lished MgO s u b s t r a t e , g r o w t h looked very rough and opaque. t e m p e r a t u r e 808°C. F i l m g r o w t h on the syton polished substrates started only at t e m p e r a t u r e s b e l o w ' D / 0B a p p r o x i m a t e l y 780°C. O f t e n D i jl island g r o w t h was o b s e r v e d 111 [] (Fig. 2) w i t h c r y s t a l l i t e s in /I 0.6 I the form of i d e n t i c a l l y o r i e n E [] z/ ':D., //'I 2 Q ted t r u n c a t e d p y r a m i d e s w i t h ~' 0.~ basal edges along
.%| - I I
Vol. 12, No.7
MAGNESIUM FERRITE FILMS
FIG.
4A
FIG,
711
4B
S t e r e o s c a n p i c t u r e of a f i l m g r o w n on a d i a m o n d p o l i s h e d s u b s t r a t e (a) or on a s y t o n p o l i s h e d s u b s t r a t e (b).
the g r o w t h r a t e d e c r e a s e ~ l i n e a r l y f r o m 0.9 ~ m / m i n at 8 5 0 ° C to less than 0.1 ~ m / m i n at 700°C. For the s y t o n p o l i s h e d subs t r a t e s the g r o w t h rate s h o w e d a m a x i m u m of a b o u t 0.5 p m / m i n n e a r 7 6 0 ° C ( d i p p i n g time 5 m i n . ) . The d i f f e r e n c e in g r o w t h b e h a v i o r m u s t be due to the s u r f a c e m o r p h o l o g y of the s u b s t r a tes. S t e r e o s c a n p i c t u r e s r e v e a l a m u c h s m o o t h e r M g O s u r f a c e for the s y t o n p o l i s h e d s u b s t r a t e s c o m p a r e d w i t h the d i a m o n d p o l i s h e ~ s u b s t r a t e s . This d i f f e r e n c e in s u r f a c e m o r p h o l o g y is c l e a r l y r e f l e c t e d in the r e s p e c t i v e f i l m s u r f a c e s . Fig. 4a s h o w s the s u r f a c e of a r o u g h , o p a q u e f i l m g r o w n on a d i a m o n d p o l i s h e s subs t r a t e . The p i c t u r e s h o w s a r a t h e r o p e n s t r u c t u r e , l o o k i n g like a crocket-work, with needles I f i i pointing along
~ ~
712
P.J.M.
VAN DER STRATEN, et al.
Vol. 12, No. 7
lets. The f l a t n e s s of the h o r i z o n t a l l y d i p p e d f i l m was I-2 f r i n g e s in N a - D light, c o m p a r e d w i t h 5-I0 f r i n g e s for the films g r o w n by v e r t i c a l d i p p i n g . T h e r e also is a d i f f e r e n t t e m p e r a t u r e d e p e n d e n c e on the g r o w t h rates for the two cases s h o w n in the figure. X-ray
analysis
X-ray diffraction p a t t e r n s of the films w e r e i n v e s t i g a t e d by m e a n s of a S i e m e n s t e x t u r e g o n i o m e t e r . P o l e f i g u r e s of one or m o r e forms of l a t t i c e p l a n e s can be o b t a i n e d of b o t h s u b s t r a t e a~d f i l m by a p r o p e r s e l e c t i o n of the B r a g g a n g l e 0. The { I l l } p o l e s of M g O c a n n o t be d i s t i n g u i s h e d f r o m the { 2 2 2 } - s p i n e l poles, but the { 2 2 0 } - p o l e s of the s p i n e l a p p e a r alone. F r o m the r e s u l ting p o l e f i g u r e we c o n c l u d e that the films are s i n g l e c r y s t a l line. For the films g r o w n on s y t o n p o l i s h e d s u b s t r a t e s the < 1 0 0 > ~ i r e c t i o n s of f i l m and s u b s t r a t e c o i n c i d e . H o w e v e r , the o p a q u e films g r o w n on d i a m o n d p o l i s hed M g O s h o w a d i f f e r e n c e I-2 ° 0 rprn b e t w e e n the < 1 0 0 > d i r e c t i o n s olO f -0.02 of f i l m and s u b s t r a t e . Due to this d i f f e r e n c e the m i s f i t _..-o f AI o<%' c a n n o t be m e a s u r e d b e c a u s e M g O ~ 125rpm and s p i n e l r e f l e c t i o n c a n n o t ..,,.=0.03 be a c h i e v e d for the same p l a Ii$, I / telet s e t t i n g in the d i f f r a c tion equipment. The u n s t r a i n e d m i s f i t Aa = 1 I I I a s - af, b e t w e e n s u b s t r a t e and 725 750 775 800--,IC film, is p l o t t e d in Fig. 6 as FIG. 6 a f u n c t i o n of g r o w t h t e m p e r a ture Tg for films g r o w n on syM i s f i t , a ( s u b s t r a t e ) - a (film), ton p o l i s h e d MgO. All films as a f u n c t i o n of g r o w t h t e m p e are in c o m p r e s s i o n i.e. Aa < 0. rature. Circles, vertically For films g r o w n w i t h o u t r o t a ~ipped; triangles, horizontalt i o n the m i s f i t i n c r e a s e s f r o m ly d i p p e d w i t h s u b s t r a t e roAa = - 0 . 0 1 9 for Tg = 800 ° to Aa t a t i o n 125 rpm; d i p p i n g time = - 0 . 0 3 6 A for T~ = 730°C. For 15 min. films g r o w n u n a e r r o t a t x o n the m i s f i t is a l w a y s 0.01 larger. Due to the d i f f e r e n c e in t h e r m a l e x p a n s i o n c o e f f i c i e n t of s u b s t r a t e and film, a zero m i s f i t at the g r o w t h t e m p e r a t u r e leads to a f i l m u n d e r c o m p r e s s i o n at room temperature. For T. = 8 0 0 ° C the r e s u l t i n g r o o m t e m p e r a t u r e m i s f i t is Aa = - 0 . 0 1 7 0 A~ w h i l e for Tg = 7 0 0 ° C Aa = - 0 . 0 1 6 5 ~. F r o m Fig. 6 it f o l l o w s that the films are c l o s e to zero m i s f i t or s l i g h t l y in c o m p r e s s i o n at Tg. The h i g h e r c o m p r e s s i v e strain of the films g r o w n u n d e r r o t a t i o n is p r o b a b l y r e l a t e d to the h i g h e r g r o w t h rate. i
i
i
i
-o.o4
Microprobe
analysis
Generally differences in l a t t i c e c o n s t a n t s are c a u s e d by d i f f e r e n c e s in c o m p o s i t i o n . F r o m s t u d i e s on LPE g r o w n g a r n e t films it is k n o w n that a p p r e c i a b l e a m o u n t s of Pb can be i n c o r p o r a t e d (18). I n d e e d , x - r a y f l u o r e s c e n c e measurements r e v e a l e d the p r e s e n c e of a large a m o u n t of Pb in our s p i n e l films. In o r d e r to o b t a i n m o r e q u a n t i t a t i v e information a b o u t the c h e m i c a l
Vol. 12, No. 7
MAGNESIUM FERRITE FILMS
c o m p o s i t l o n we have p e r f o r m e a an e l e c t r o n m i c r o p r o b e analysis. The m e a s u r e m e n t s w e r e p e r f o r m e a u s i n g a SEM II A e l e c t r o n p r o b e m i c r o - a n a l y s e r . The c o n c e n t r a tions w e r e c a l c u l a t e d w i t h the aid of a c o m p u t e r p r o g r a m u s i n g m e a s u r e d i n t e n s i t i e s of pure Fe, In, Pb, Pt and MgO as standards. To o p t i m i z e the e x p e r i m e n t a l c o n d i t i o n s and to check the a c c u r a c y of the r e s u l t s s t a n d a r d s of s i n t e r e d M g F e 2 _ x InxO 4 (x=0, 0.1, 0.2, 0.3) w e r e m e a s u r e d . For the determinationof Mg a low a c c e l e r a t i n g voltage is p r e f e r r e d , h o w e v e r , e s p e c i a l l y for Pb a h i g h voltage has to be applied. The best c o m p r o m i s e was 20kV. At this v o l t a g e the d e p t h of the a n a l y s e d r e g i o n in the f i l m is 1.25 pm. F r o m m e a s u r e m e n t s on the c e r a m i c s t a n d a r d s the following a c c u r a c y was o b t a i n e a (atoms per f o r m u l a unit) : Mg ! 0.2 (due to high a b s o r p t i o n c o r r e c t i o n ) , Fe + 0.05, In + 0.02. The e s t i m a t e d a c c u r a c y of Pb and Pt is + 0.01. All films w e r e found to be Pb and Pt substituated MgFe~ In O., w i t h x = 0.09 - 0 . l ~ ? X A s X s ~ o w n in Fig. 7 the Pt c o n t e n t is about 0.05, w i t h s l i g h t l y i n c r e a s i n g c o n c e n t r a t i o n at lower g r o w t h t e m p e r a t u r e s . The Pb c o n t e n t is s t r o n g l y d e p e n d e n t on the g r o w t h p a r a m e t e r s . At the low e s t g r o w t h t e m p e r a t u r e s the c o n c e n t r a t i o n i n c r e a s e s to about 0.2 atoms per f o r m u l a unit. So far w~ have a n a l y s e d the c o m p o s i t i o n of our films n o r m a l to the plane of the platelet w i t h an a n a l y s e d d e p t h of 1.25 Hm. In order to study the c o m p o s i t i o n as f u n c t i o n of f i l m t h i c k n e s s we have grown a 27 Hm thick f i l m (60 min, ;25 rpm, 760°C). A n a l y s i s are p e r f o r m e d on the b r o k e n and p o l i s h e d platelet n o r m a l to the g r o w t h dir e c t i o n w i t h steps of 2.8 ~m f r o m s u b s t r a t e to film surface.
718
i
,
0.20
,
:
l
i
Pb
0.15 E
"" 0.10
o~ 0.05
7'2s
7 ,o
7';s
FIG.
8bo--- '°c
7
A n a l y s e d Pt and Pb c o n t e n t in the films vs g r o w t h t e m p e r a t u re. O p e n symbols, v e r t i c a l l y aipped; full symbols, h o r i z o n tally d i p p e d w i t h s u b s t r a t e r o t a t i o n 125 rpm; d i p p i n g time rain. r
0.15
u ' ~ ~ O
0
.-=, C
o o.lo
o\
0.05
Pt
0
2'0 distance above substrate (prn)
FIG. C o n c e n t r a t i o n vs d e p t h in a 27 Hm thick film, h o r i z o n t a l l y aipped with substrate rotation 125 rpm for 60 min. at 760°C.
714
P.J.M. VAN DER STRATEN,
et al.
Vol. 12, No. 7
As can be seen f r o m Fig. 8 the Pb and In c o n t e n t s d e c r e a s e w h e n g o i n g to the f i l m s u r f a c e w h i l e the Pt c o n t e n t s l i g h t l y i n c r e a ses. T h e s e c o m p o s i t i o n a l variations are p r o b a b l y r e l a t e d to c h a n g e s in g r o w t h w i t h d i p p i n g time. Magnetic
parameters
The s a t u r a t i o n m a g n e t i z a t i o n of the films was m e a s u r e d w i t h a vibrating sample magnetometer. The 4~M s v a l u e s d e c r e a s e c o n t i n u o u s l y f r o m 2000 ~ 100 G a u s s for a f i l m g r o w n at 790°C to 1300 G a u s s for a f i l m g r o w n at 724°C. R o t a t i o n of the s u b s t r a t e leads to a b o u t I0% lower v a l u e s of 4~M s. M a g n e t i c t o r q u e m e a s u r e m e n t s r e v e a l e d that the e a s y axes of m a g n e t i z a t i o n are i n - p l a n e . In a c c o r d a n c e w i t h this r e s u l t no s t r i p e d o m a i n s w e r e o b s e r v e d . Discussion We h a v e s e e n that a r a t h e r h i g h s u p e r s a t u r a t i o n of the f l u x (AT ~ 2 0 0 o c ) is n e c e s s a r y to o b t a i n f i l m g r o w t h on ( 1 0 0 ) - s y t o n p o l i s h e d M g O s u b s t r a t e s . An e x p l a n a t i o n can be f o u n d in the h i g h e r s u r f a c e free e n e r g y of a ( 1 0 0 ) - p l a n e of M g O c o m p a r e d to a ( I I I ) - p l a n e . F i l m g r o w t h on ( I l l ) - M g O s t a r t s a b o u t 100°C h i g h e r than on ( 1 0 0 ) - M g O s u b s t r a t e s (19), so w h e n an i n s u f f i c i e n t l y polished (IO0) s u b s t r a t e is d i p p e d at a t e m p e r a t u r e b e t w e e n these saturation temperatures, i n i t i a l g r o w t h can o c c u r on t h o s e p a r t s of the s u r f a c e w h i c h h a v e a lower s u r f a c e free e n e r g y (20). This e x p l a i n s w h y g r o w t h on d i a m o n d p o l i s h e d s u b s t r a t e s s t a r t s at a higher temperature than on s y t o n p o l i s h e d s u b s t r a t e s . As a r e s u l t films on d i a m o n d p o l i s h e d s u b s t r a t e s h a v e an i n f e r i o r q u a l i t y and < I 0 0 > d i r e c t i o n s of f i l m and s u b s t r a t e can e v e n m a k e a small a n g l e w i t h e a c h other. We find an a p p r e c i a b l e drop of the s a t u r a t i o n m a g n e t i z a t i o n with decreasing growth temperature. This e f f e c t can be a t t r i b u ted to the i n c r e a s i n g Pb c o n c e n t r a t i o n . The lead ions are p r o bably substituted at o c t a h e d r a l sites, w h i c h gives a d e c r e a s e of 4~M s. At the same time the C u r i e t e m p e r a t u r e d e c r e a s e s due to the d e c r e a s i n g m a g n e t i c i n t e r a c t i o n and this leads to a f u r t h e r l o w e r i n g of the m a g n e t i z a t i o n . The i n f l u e n c e of Pb is c l e a r l y p r o v e d by the e f f e c t of s u b s t r a t e r o t a t i o n . F i n a l l y the e q u i l i brium distribution of Mg ions also d e p e n d s on the t e m p e r a t u r e . At lower t e m p e r a t u r e s m o r e Mg is p r e s e n t at o c t a h e d r a l sites, and this also c o n t r i b u t e s to the l o w e r i n g of 4~M s. So far we did not s u c c e e d in g r o w i n g films w i t h the p r e f e r r e d m a g n e t i z a t i o n direction perpendicular to the f i l m p l a n e . This is p r o b a b l y due to the c o m p r e s s i v e s t r e s s in c o m b i n a t i o n w i t h a n e g a t i v e m a g n e tostriction c o n s t a n t ~ , n n of the f e r r i t e . A s s u m i n g Y o u n g ' s mod u l u s E ~ 2 x I0 L a y n / ¢ m 2 , a m i s f i t Aa = - 0 . 0 2 ~ c o r r e s p o n d s to a s t r e s s ~ ~ -6 x 109 d y n / c m 2. If we f u r t h e r take 4~M s 1500 G and ~ I O 0 ~ - I 0 - 5 , the s t r e s s i n d u c e d a n i s o t r o p y f i e l d is H s ~ r e s s ~ - 1 5 0 0 Oe. A c c o r d i n g to B o r r e l l i (8) the g r o w t h - i n d u cea a n l s o t r o p y in b u l k c r y s t a l s is of the o r d e r H ro t + I 0 0 0 Oe. If H r o w t h has a s i m i l a r m a g n i t u d e in t~e ~ 1 ~ m s a s l i g h t l y p o s i t i v e m l s ~ i t is n e c e s s a r y . We h a v e t r i e d to a c h i e v e this by a d d i n g small a m o u n t s of e i t h e r A I 2 0 3 or G a 2 0 3 to the flux. H o w e v e r , the r e s u l t i n g films a l w a y s s h o w e d light c o l o r e d , b r o a d , s t r a i g h t lines a l o n g < I 0 0 > d i r e c t i o n s as soon as Aa > - 0 . 0 1 4 . This m e a n s that due to the o c c u r e n c e of some s t r e s s
Vol. 12, No. 7
MAGNESIUM FERRITE FILMS
r e l i e v i n g m e c h a n i s m no p e r f e c t films + 0 . 0 0 3 at the g r o w t h t e m p e r a t u r e .
could be grown w i t h
715
Aa >
Conclusions We have shown that it is p o s s i b l e to grow e p i t a x i a l films of m a g n e s i u m - i n d i u m f e r r i t e on (I00) MgO s u b s t r a t e s by m e a n s of the LPE m e t h o d . To grow p e r f e c t films a good s u r f a c e f i n i s h of the s u b s t r a t e s is n e c e s s a r y . A serious d i s a d v a n t a g e of MgO is the large d i f f e r e n c e in thermal e x p a n s i o n c o m p a r e d w i t h the ferrite. As a c o n s e q u e n c e good films are always in c o m p r e s s i o n at r o o m temperature. Acknowledgements The authors w o u l d like to thank J.M. R o b e r t s o n and P.F. B o n g e r s of P h i l i p s R e s e a r c h L a b o r a t o r i e s , E i n a h o v e n for their a a v i c e s and support; H.A. L o g m a n s of the same l a b o r a t o r y p e r f o r m e d the m a g n e t i c m e a s u r e m e n t s . J . W . G . A . V r o l i j k of our l a b o r a t o r y is t h a n k e a for a s s i s t a n c e w i t h the e l e c t r o n m i c r o p r o b e c a l c u l a t i o n s and H. de Jonge Baas for t e c h n i c a l a s s i s t a n c e . References I. H.J. L e v i n s t e i n , S. Licht, R.W. L a n s d o r f and S.L. Blank, Appl. Phys. L e t t e r s 19, 486 (1971). 2. F.S. S t e a r n s and H.L. Glass, Mat. Res. Bull. IO, 1255 (1975) 3. F.S. S t e a r n s ana H.L. Glass, Mat. Res. Bull. I__~I, 1319 (1976). 4. R.J. G a m b i n o , J. Appl. Phys. 38, 1129 (1967). 5. D.A. Herman, R.L. White, R.S. F e i g e l s o n , B.L. M a t t e s and H.W. Swarts, A.I.P. Conf. Proc. 24, 580 (1975). 6 B.M. Dutta, J. Sci. Res. B a n a r a s H i n d u Univ. 15, 80 (1964-1965). 7 R.E. Carter, J. Am. Cer. Soc. 42, 324 (1959). 8. N.F. B o r r e l l i , J. Appl. Phys. 46, 430 (1975). 9 J . E . Davies, E.A.D. W h i t e and J.D.C. Wood, J. C r y s t a l G r o w t h 27, 227 (1974). l0 T. M a t s u y a m a , Y. N a g a t a and K. Ohta, J a p a n J. Appl. Phys. 14, 727 (1975). [I E.V. R y b a l s k a y a , T.G. P e t r o v and A.G. ~itova, Soviet P h y s i c s - C r y s t a l l o g r a p h y 15, 958 (1971). 12 H.L. Glass and M.T. E l l i o t t , J. C r y s t a l G r o w t h 27, 253 (1974). 13 A.E. M o r g a n , J. C r y s t a l G r o w t h 27, 266 (1974). 14. R. Ghez and E.A. Giess, J. C r y s t a l G r o w t h 27, 221 (1974). 15. J.M. R o b e r t s o n , We thank J.M. R o b e r t s o n for his advice c o n c e r n i n g the flux c o m p o s i t i o n . 16. B.M. W a n k l y n , J. C r y s t a l G r o w t h ~, 323 (1969). 17. D.J. Dumin, J. E l e c t r o c h e m . Soc. 114, 749 (1967). 18. J.E. Davies, E.A. Giess, J.D. K u p t s i s and W. Reuter, J. C r y s t a l G r o w t h 36, 1971 (1976). 19. J.M. R o b e r t s o n , p r i v a t e c o m m u n i c a t i o n . 20. I. Kasai and D.W. Bassett, J. C r y s t a l G r o w t h 27, 215 (1974).