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A propagation mechanism for chemical vapor deposited β-Sic whiskers
Scripta
METALLURGICA
A PROPAGATION
Vol. 21, pp. 637-638, 1987 P r i n t e d in the U.S.A.
MECHANISM
National
FOR CHEMICAL
P e r g a m o n J o u r n a l s , Ltd. All rights r e s e r v e d
VAPOR DEPOSITED
~-SIC
WHISKERS
D.J. C h e n g , W.J. Shy5 r, and M.H. H o n D e p a r t m e n t of M a t e r i a l s E n g i n e e r i n g , Cheng Kung University, Tainan, Taiwan, R.0.C. R e c e i v e d D e c e m b e r 3, 1986) R e v i s e d F e b r u a r y ii, 1987) Introduction
I
W h i s k e r s are c r y s t a l s w i t h e x t r e m e n o n - e q u i l i b r i u m forms; The v a r i o u s w h i s k e r s t u d i e s i n d i c a t e d that g r o w t h r e q u i r e s a d e p o s i t i o n s i n k at the w h i s k e r tip w h i c h w o u l d f u n c t i o n at s u p e r s a t u r a t i o n s a p p a r e n t l y too l o w f o r s i g n i f i c a n t g r o w t h at other regions. T y p i c a l l y t h e s e p a p e r s c o n t a i n a d i s c u s s i o n of two p r i n c i p a l w h i s k e r g r o w t h m e c h a n i s m s : that of the d i s l o c a t i o n m e c h a n i s m b y S e a r s (1;2) and the v a p o r l i q u i d - s o l i d (VLS) m e c h a n i s m b y W a g n e r and E l l i s (3,4). H o w e v e r , i n m a n y cases the m e c h a n i s m of w h i s k e r g r o w t h and its i n f l u e n c e on the m o r p h o l o g y is not yet clear. I n the past, s i l i c o n c a r b i d e w h i s k e r s of d i f f e r e n t c r y s t a l l o g r a p h i c s t r u c t u r e s w i t h i m p u r i t y s e e d e d s u b s t r a t e s h a v e b e e n grown; i n these cases g r o w t h is a t t r i b u t e d to the V L S m e c h a n i s m (5,6). In this paper, a p o s s i b l e p r o p a g a t i o n m e c h a n i s m of c h e m i c a l v a p o r d e p o s i t e d (CVD) ~ - S i C w h i s k e r w i t h o u t i m p u r i t y seed is d i s c u s s e d i n r e l a t i o n to o b s e r v a t i o n s . The r e s u l t s i n d i c a t e that a r e e n t r a n t t w i n m e c h a n i s m f o r ~ - S i C w h i s k e r g r o w t h m a y operate. Crystal
Growth
The g r o w t h of ~ - S i C w h i s k e r s b y t h e r m a l d e c o m p o s i t i o n of m e t h y l t r i c h l o r o s i lane (MTS) in a h y d r o g e n f l o w s y s t e m was c a r r i e d out in a c o n v e n t i o n a l h o t - w a l l type h o r i z o n t a l g r a p h i t e r e a c t o r tube~ p r o v i d e d w i t h t u n g s t e n r e s i s t a n t h e a t i n g . T h e s u b s t r a t e , a c l e a n g r a p h i t e plate, was p l a c e d on the r e g i o n of u n i f o r m temperature. T h e s u b s t r a t e t e m p e r a t u r e e x a m i n e d in this s t u d y was m a i n t a i n e d at 1 8 2 3 K a n d the t o t a l gas p r e s s u r e i n the r e a c t i o n c h a m b e r was 0.93 MPa. With a c o n t r o l l e d m o l e r a t i o of MTS to h y d r o g e n a n d a total f l o w at a p r e d e t e r m i n e d rate f o r the d i f f u s i o n l i m i t e d r e a c t i o n , s i l i c o n c a r b i d e w h i s k e r s of the cubic type w e r e grown. F i g u r e I s h o w s a t y p i c a l s c a n n i n g e l e c t r o n m i c r o s c o p e (SEM) p h o t o m i c r o g r a p h of C V D ~ - S i C w h i s k e r s . The whiskers had a transparent yellow-green appearance a n d at t h e i r tip s u r f a c e s h o w e d a p r o n o u n c e d l y f a e e t e d f e a t u r e s . The exact c o n d i t i o n s u n d e r w h i c h w h i s k e r s f o r m are not w e l l - d e f i n e d . They generally appear on g r o w t h s m a d e at h i g h e r d e p o s i t i o n t e m p e r a t u r e and v e r y l o w M T S c o n c e n t r a t i o n . I n d i v i d u a l w h i s k e r s , I to 40 ~ m i n d i a m e t e r w i t h l e n g t h s u p to I cm, are formed. T h e i r sizes i n c r e a s e w i t h the MTS c o n c e n t r a t i o n u n t i l the s u p e r s a t u r a t i o n e x c e e d s a c r i t i c a l v a l u e a n d the w h i s k e r s are r e p l a c e d b y d e n d r i t i c c r y s t a l s (7). The fi:nal c r y s t a l m o r p h o l o g y d e v e l o p s i n the i n i t i a l g r o w t h p r o c e s s w i t h o u t m u c h s u b s e q u e n t m o r p h o l o g i c a l change. F i g u r e 2 shows the m o r p h o l o g y of the i n i t i a l stage of C V D ~ - S i C w h i s k e r g r o w t h . E x a m i n a t i o n of t h e s e c r y s t a l s s h o w e d that t h e y c o n t a i n e d i n t e r s e c t i n g t w i n c o n f i g u r a t i o n s in the d i r e c t i o n of g r o w t h w h i c h r e s u l t e d i n the m o s t f a v o u r a b l e and c e a s e l e s s g r o w t h sites. F i g u r e 3 shows the v a r i o u s t y p i c a l m o r p h o l o g i e s of the C V D ~ - S i C w h i s k e r s . R e - e n t r a n t g r o o v e s and v a r i o u s m o r p h o l o g i e s r e s u l t i n g f r o m the t w i n n i n g p l a n e (8,9) c a n c l e a n l y be seen. F r o m the a b o v e o b s e r v a t i o n s , all these f e a t u r e s were t a k e n to i n d i c a t e that the t w i n p l a n e r e e n t r a n t edge m e c h a n i s m (10) is r e s p o n s i b l e for C V D E - S i C w h i s k e r growth.
637 0 0 3 6 - 9 7 4 8 / 8 7 $3.00 + .00 C o p y r i g h t (c) 1987 P e r g a m o n J o u r n a l s
Ltd.
638
VAPOR DEPOSITED SIC WHISKERS
Vol.
21, No. 5
Reference I. 2. 3. 4. 5. 6. 7. 8. 9. 10.
G.W. Sears, Acta Met. I, 457(1953) G.W. Sears, Acta Met. 3, 361(1955 ) R.S. Wagner and W.C Ellis, Appl. Phys. Letters, 4, 89(1964) R.S. Wagner and W.C~ Ellis, Trans. Met. Soc. AIME 233, 1053(1965) I. Berman and C.E. Ryan, J. Crystal Growth 9, 314(1971) G.A. Bootsma, W.F. Knippenberg and G. Verspui, J. Crystal Growth, 11, 297(1971) D.J. Cheng, W.J. Shyy and M.H, Hen, Scripta Met., 20, 1587(1986) J.W. Faust, Jr. and H.F. John, J. Phys. Chem, Solids 25, 1407(1964) J.F. Hamilton and L.E. Brady, J. Appl. Phys., 35, 414(1964) D.R. Hamilton and R.G. Seidensticker, J. Appl. Phys., 31, 1165(1960)
i (a) FIG.1.
FIG.2.
i%!i~?!i Typical SEM photomicrograph
(b) of CVD fl-SIC whisker
The morphology Of initial stage of CVD ~ - S i C whisker growth.
(~b)
~i~ ¸
~
f
FIG.3. (3a)
The variously typical morphology of the CVD fl -SiC whisker.
METALLURGICA
A PROPAGATION
Vol. 21, pp. 637-638, 1987 P r i n t e d in the U.S.A.
MECHANISM
National
FOR CHEMICAL
P e r g a m o n J o u r n a l s , Ltd. All rights r e s e r v e d
VAPOR DEPOSITED
~-SIC
WHISKERS
D.J. C h e n g , W.J. Shy5 r, and M.H. H o n D e p a r t m e n t of M a t e r i a l s E n g i n e e r i n g , Cheng Kung University, Tainan, Taiwan, R.0.C. R e c e i v e d D e c e m b e r 3, 1986) R e v i s e d F e b r u a r y ii, 1987) Introduction
I
W h i s k e r s are c r y s t a l s w i t h e x t r e m e n o n - e q u i l i b r i u m forms; The v a r i o u s w h i s k e r s t u d i e s i n d i c a t e d that g r o w t h r e q u i r e s a d e p o s i t i o n s i n k at the w h i s k e r tip w h i c h w o u l d f u n c t i o n at s u p e r s a t u r a t i o n s a p p a r e n t l y too l o w f o r s i g n i f i c a n t g r o w t h at other regions. T y p i c a l l y t h e s e p a p e r s c o n t a i n a d i s c u s s i o n of two p r i n c i p a l w h i s k e r g r o w t h m e c h a n i s m s : that of the d i s l o c a t i o n m e c h a n i s m b y S e a r s (1;2) and the v a p o r l i q u i d - s o l i d (VLS) m e c h a n i s m b y W a g n e r and E l l i s (3,4). H o w e v e r , i n m a n y cases the m e c h a n i s m of w h i s k e r g r o w t h and its i n f l u e n c e on the m o r p h o l o g y is not yet clear. I n the past, s i l i c o n c a r b i d e w h i s k e r s of d i f f e r e n t c r y s t a l l o g r a p h i c s t r u c t u r e s w i t h i m p u r i t y s e e d e d s u b s t r a t e s h a v e b e e n grown; i n these cases g r o w t h is a t t r i b u t e d to the V L S m e c h a n i s m (5,6). In this paper, a p o s s i b l e p r o p a g a t i o n m e c h a n i s m of c h e m i c a l v a p o r d e p o s i t e d (CVD) ~ - S i C w h i s k e r w i t h o u t i m p u r i t y seed is d i s c u s s e d i n r e l a t i o n to o b s e r v a t i o n s . The r e s u l t s i n d i c a t e that a r e e n t r a n t t w i n m e c h a n i s m f o r ~ - S i C w h i s k e r g r o w t h m a y operate. Crystal
Growth
The g r o w t h of ~ - S i C w h i s k e r s b y t h e r m a l d e c o m p o s i t i o n of m e t h y l t r i c h l o r o s i lane (MTS) in a h y d r o g e n f l o w s y s t e m was c a r r i e d out in a c o n v e n t i o n a l h o t - w a l l type h o r i z o n t a l g r a p h i t e r e a c t o r tube~ p r o v i d e d w i t h t u n g s t e n r e s i s t a n t h e a t i n g . T h e s u b s t r a t e , a c l e a n g r a p h i t e plate, was p l a c e d on the r e g i o n of u n i f o r m temperature. T h e s u b s t r a t e t e m p e r a t u r e e x a m i n e d in this s t u d y was m a i n t a i n e d at 1 8 2 3 K a n d the t o t a l gas p r e s s u r e i n the r e a c t i o n c h a m b e r was 0.93 MPa. With a c o n t r o l l e d m o l e r a t i o of MTS to h y d r o g e n a n d a total f l o w at a p r e d e t e r m i n e d rate f o r the d i f f u s i o n l i m i t e d r e a c t i o n , s i l i c o n c a r b i d e w h i s k e r s of the cubic type w e r e grown. F i g u r e I s h o w s a t y p i c a l s c a n n i n g e l e c t r o n m i c r o s c o p e (SEM) p h o t o m i c r o g r a p h of C V D ~ - S i C w h i s k e r s . The whiskers had a transparent yellow-green appearance a n d at t h e i r tip s u r f a c e s h o w e d a p r o n o u n c e d l y f a e e t e d f e a t u r e s . The exact c o n d i t i o n s u n d e r w h i c h w h i s k e r s f o r m are not w e l l - d e f i n e d . They generally appear on g r o w t h s m a d e at h i g h e r d e p o s i t i o n t e m p e r a t u r e and v e r y l o w M T S c o n c e n t r a t i o n . I n d i v i d u a l w h i s k e r s , I to 40 ~ m i n d i a m e t e r w i t h l e n g t h s u p to I cm, are formed. T h e i r sizes i n c r e a s e w i t h the MTS c o n c e n t r a t i o n u n t i l the s u p e r s a t u r a t i o n e x c e e d s a c r i t i c a l v a l u e a n d the w h i s k e r s are r e p l a c e d b y d e n d r i t i c c r y s t a l s (7). The fi:nal c r y s t a l m o r p h o l o g y d e v e l o p s i n the i n i t i a l g r o w t h p r o c e s s w i t h o u t m u c h s u b s e q u e n t m o r p h o l o g i c a l change. F i g u r e 2 shows the m o r p h o l o g y of the i n i t i a l stage of C V D ~ - S i C w h i s k e r g r o w t h . E x a m i n a t i o n of t h e s e c r y s t a l s s h o w e d that t h e y c o n t a i n e d i n t e r s e c t i n g t w i n c o n f i g u r a t i o n s in the d i r e c t i o n of g r o w t h w h i c h r e s u l t e d i n the m o s t f a v o u r a b l e and c e a s e l e s s g r o w t h sites. F i g u r e 3 shows the v a r i o u s t y p i c a l m o r p h o l o g i e s of the C V D ~ - S i C w h i s k e r s . R e - e n t r a n t g r o o v e s and v a r i o u s m o r p h o l o g i e s r e s u l t i n g f r o m the t w i n n i n g p l a n e (8,9) c a n c l e a n l y be seen. F r o m the a b o v e o b s e r v a t i o n s , all these f e a t u r e s were t a k e n to i n d i c a t e that the t w i n p l a n e r e e n t r a n t edge m e c h a n i s m (10) is r e s p o n s i b l e for C V D E - S i C w h i s k e r growth.
637 0 0 3 6 - 9 7 4 8 / 8 7 $3.00 + .00 C o p y r i g h t (c) 1987 P e r g a m o n J o u r n a l s
Ltd.
638
VAPOR DEPOSITED SIC WHISKERS
Vol.
21, No. 5
Reference I. 2. 3. 4. 5. 6. 7. 8. 9. 10.
G.W. Sears, Acta Met. I, 457(1953) G.W. Sears, Acta Met. 3, 361(1955 ) R.S. Wagner and W.C Ellis, Appl. Phys. Letters, 4, 89(1964) R.S. Wagner and W.C~ Ellis, Trans. Met. Soc. AIME 233, 1053(1965) I. Berman and C.E. Ryan, J. Crystal Growth 9, 314(1971) G.A. Bootsma, W.F. Knippenberg and G. Verspui, J. Crystal Growth, 11, 297(1971) D.J. Cheng, W.J. Shyy and M.H, Hen, Scripta Met., 20, 1587(1986) J.W. Faust, Jr. and H.F. John, J. Phys. Chem, Solids 25, 1407(1964) J.F. Hamilton and L.E. Brady, J. Appl. Phys., 35, 414(1964) D.R. Hamilton and R.G. Seidensticker, J. Appl. Phys., 31, 1165(1960)
i (a) FIG.1.
FIG.2.
i%!i~?!i Typical SEM photomicrograph
(b) of CVD fl-SIC whisker
The morphology Of initial stage of CVD ~ - S i C whisker growth.
(~b)
~i~ ¸
~
f
FIG.3. (3a)
The variously typical morphology of the CVD fl -SiC whisker.