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DIFFUSION KINETICS IN GOLD-AMORPHOUS GeTe4 THIN FILMS
J. Mackowski, M. Bendali, P. Normandon, P. Kumurdjian
To cite this version:
J. Mackowski, M. Bendali, P. Normandon, P. Kumurdjian. DIFFUSION KINETICS IN GOLD-
AMORPHOUS GeTe4 THIN FILMS. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-975-C4-
978. �10.1051/jphyscol:19814214�. �jpa-00220843�
JOURNAL DE PHYSIQUE
CoZZoque
C4,suppZ6men-t a u n
'10,lbme 42, o c t o b r e 1981 page
C 4 - 9 7 5D I F F U S I O N KINETICS IN GOLD-AMORPHOUS
GeTe4THIN F I L M S
J.M. Mackowski, M . B e n d a l i , P . Normandon and P. ~ u m u r d j i a n *
I n s t i t u t de Physique NuclGaire (and
I l j 2 P 3 ) ,Universite' Lyon-I,
43,Bd du I 1 Novembre 1918 - 63622 PLZleurbanne Ceder, France
* C . E . A .
BruyGres-le-ChBteL, C.E.A., B.P. 561, 92542 t4ontrouge Cedez, France
ABSTRACT - Tne main p r o c e s s e s which o c c u r d u r i n g t h e g o l d f a s t d i f f u s i o n i n t o amorphous GeTeq f i l m s a r e f o l l o w e d by RL3S measurements, f o r a n n e a l i n g s c a r r i e d o u t a t & m p e r a t u r e s below Tg ( 1 4 0 ° C ) . From t h e p r o f i l e s o b t a i n e d , s t r i k i n g l y s i m i l a r t o g r a i n - b o u n d a r y (G.B.) p r o f i l e s , we e x t r a c t : t h e d i f f u s i o n c o e f f i - c i e n t s f o r l o w - c o n c e n t r a t i o n k i n e t i c s u s i n g s p e c i f i c G.B. t h e o r i e s . Then we p r e s e n t a model b a s e d on r e c e n t t r e n d s i n amorphous s t r u c t u r e s . T h i s m o d e l i s a l s o c o n s i s t e n t w i t h e x p e r i m e n t a l d a t a o b t a i n e d from complementary c h a r a c - t e r i z a t i o n s a s e l e c t r o n microscopy X-ray a n d t r a n s p o r t p r o p e r t i e s .
INTRODUCTION - I n e a r l i e r p a p e r s [ 1 , 2 ] we r e p o r t e d on t h e e l e c t r i c a l d e g r a d a t i o n s k i - n e t i c s i n d u c e d by t h e m e t a l e l e c t r o d e d i f f u s i o n . The o b s e r v e d l o w - t e m p e r a t u r e f a s t - d i f f u s i o n e x h i b i t s u n u s u a l k i n e t i c s c h a n g e s a s s o c i a t e d w i t h t h e t e m p e r a t u r e r i s e . I n t h i s p a p e r we s h o u l d l i k e t o p r e s e n t and d i s c u s s new r e s u l t s o n t h e d i f f u s i o n p r o c e s - s e s i n i t i a t e d by a i r o r vacuum a n r i e a l i n g s . Knowledge on t h e metal-amorphous t h i n c o u - 2 l e s d i f f u s i o n i s n e c e s s a r y i n d e e d t o improve t h e r e l i a b i l i t y o f amorphous d e v i c e s . EXPEFLIMENTAL
r i n q system.
- The d i f f u s i o n c o u p l e s a r e e l a b o r a t e d i n a m m u l t i t a r g e t d i o d e s p u t t e - The f i n a l t h i c k n e s s e s o f t h e d e p o s i t s , from 0.14 t o 1.1 IJ m o f GeTeq from
i
o f g o l d , a r e c o n t r o l l e d u s i n g Tencor a p p a r a t u s . A f t e r a i r o r vacuum i s o t h e r m a l a n n e a l i n g s a t Room T e m p e r a t u r e( m ) ,
80°C, 100°C a n d 120°C, s a m p l e s a r e t h e n a n a l y s e d by He-ion R.B.S. i n t h e 3.7-8 MeV e n e r g y r a n g e .F u r t h e r i n v e s t i g a t i o n s a r e c a r r i e d o u t by X-ray d i f f r a c t i o n o f d i f f u s i o n s t u - d i e s a s w e l l a s e l e c t r i c a l r e s i s t i v i t y measurements, a l l s u p p o r t e d by TEM o r SEM ob- s e r v a t i o n s .
DIFFUSION RESULTS - The p r o f i l e s o b t a i n e d from t h i c k ( 1 . 1 Pm GeTe ) c o n f i g u r a t i o n s ( f i g . IF) c l e a r l y d i s p l a y G.B.-depth dependance. So we u s e i n this p a r t a p p r o p r i a t e 4 c l a s s i c a l p o l y c r y s t a l l i n e d i f f u s i o n models and n o t a t i o n s .
For b u l k GeTe4 and i n t h i s t e m p e r a t u r e r a n g e , t h e volume d i f f u s i o n c o e f f i c i e n t (D1) i s n e a r l y n e g l i g e a b l e and i n no c a s e can e x p l a i n t h e h i g h g o l d - c o n c e n t r a t i o n l e - v e l s r e a c h e d i n o u r f i l m s . T h a t i s why we a d o p t t h e Whipple-based " d e f e c t - e n h a n c e d "
d i f f u s i o n f o r m a l i s m ( f i g . 3 a ) i n which t h e s o l u t e m i g r a t e s from G.B. t h r o u g h r e g i o n s w i t h h i g h d e n s i t y o f d e f e c t s c h a r a c t e r i z e d by t h e p s e u d o - l a t t i c e d i f f u s i o n c o e f f i c i e n t D ' 1 [ 3 ] . Assuming a n i n f i n i t e v a l u e f o r t h e Soundary d i f f u s i o n c o e f f i c i e n t (Db) one g e t s f l a t G.B. p r o f i l e s . The " p l a t e a u r i s e " r a t e , f o l l o w i n g a t o - 5 law, g i v e s t h e D ' 1 e x p r e s s i o n : D'? = K c2
/:
wher: K i s a g e o m e t r i c a l f a c t o r and C t h e p l a t e a u concen- t r a t i o n . T h i n c o u p l e s (1400 ~ / 4 0 0 A) a r e s p e c i a l l y d e s i g n e d i n o r d e r t o g i v e a b e t t e r a d j u s t m e n t t o t h e Db i n f i n i t e c o n d i t i o n , s o we o b t a i n f o r a i r and f o r vacuum annea- l i n a s :A i r vacuum :
For g o l d c o n c e n t r a t i o n above t o 10 a t . % , t h e model i s n o l o n g e r a p p r o p r i a t e , s i n c e t h e t i m e deperidance t h e n s t a r t s t o d e c r e a s e ( f i g . 4 ) .
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814214
JOURNAL DE PHYSIQUE
According t o Gilmer and F a r r e l l ' s t h i n f i l m s f o r m u l a t i o n [41
,
we o b t a i n t h e r e a l v a l u e s f o r G.B. d i f f u s i v i t i e s ( f i g . 2 ) :-11 cm3*s-1 A i r : ~ 6= 2.9 10 ~ b ~
QbA = 0.7 eV
-13 3 - 1 V a c u u m : ~ G ~ b ~ = 1.5 10 cm . S
QbV = 0.55 eV
6 s
l2O.c 100.C macr -
IF i g u r e 1
-
Gold d i f f u s i o n p r o f i l e s i n amorphous CeTeq t h i n - f i l m s . The c u r v e F is o b t a i n e d f o r a t h i c k e rc o n f i g u r a t i o n . 2.6 2 8
F i g u r e 2
-
A r r h e n i u s p l o t s o f "defect-enhanced"( D ' l ) and Gilmer a n d F a r r e l l ' s GB d i f f u s i o n c o e f f i c i e n t s . (.,C) : vacuum a n n e a l s ; ( b , d ) :
a i r a n n e a l s r (-
-
-) : e x t r a p o l a t e d d a t a from o u r low-T r e s i s t i v i t y measurements.POLYCRYSTAL
INTERGRANULAR
F i g u r e 3
-
a ) P o l y c r y s t a l l i n e "defect-enhanced" d i f f u s l o n p r o c e s s e s . b ) Amorphous c o r r e s p o n d i n g p r o c e s s e s i n o u r " g l o b u l a r " model,i n c l u d i n g f o r m a t i o n o f t h e c-AuTe,., compound.
D I S C U S S I O N
-
The p r e v i o u s v a l u e s must be r e l a t e d t o a system f r e e o f s t r u c t u r a l c h a n - g e s , a s g r a i n growth o r phase t r a n s i t i o n : t h a t indeed o c c u r s d u r i n g t h e t o - 5 low- c o n c e n t r a t i o n dependance, dependance confirmed by t h e e l e c t r i c a l measurements ( f i g . 6 a ) .Now t h e problem i s how t o connect t h a t c r y s t a l l i n e p o i n t of view w i t h t h e amoqhous s i t u a t i o n . That i s achieved by t h e proposed model summarized i n t h e f i g . 3 b .
The main f e a t u r e s o f t h i s model c o n s i s t i n t h r e e b a s i c a l l y d i f f e r e n t c o m p e t i t i n g me- chanism.
Figure S
-
Growth k i n e t i c s o f a c-AuTe,., l a y e r (thick- n e s s E ) during t h e Au/Te pdycrystalline i r t e r d i f f u s i o n .a ) A f i r s t very f a s t d i f f u s i o n Db f i l l s t h e i n t e r g r a n u l a r o r intercolumnar ( i n t e r g l o - b u l a r ) s p a c e s ( f i g . 7 a , b , c ) f o r which a g e n e r a l agreement seems t o e x i s t [ 5 , 6 , 7 , 8 1 . I n o u r c a s e , t h e mean s i z e of t h e s e observed s t r u c t u r e s i s c l o s e t o L = 300
X,
t h i s va- l u e b e i n g used f o r numerical c a l c u l a t i o n s [ 3 1 . A t R.T.,only t h i s p r o c e s s o c c u r s , a s shown by t h e f l a t p r o f i l e ( c ) i n f i g . 1 and l e a d s t o a mean s o l u t e c o n c e n t r a t i o n a b o u t 5 a t . % . T h i s v a l u e can be s i g n i f i c a n t l y reduced by s t r u c t u r a l thermal r e l a x a t i o n be- f o r e g o l d d e p o s i t i o n .b) The secondary, somewhat s l o w e r , easy-
p a t h d i f f u s i o n i s e q u i v a l e n t t o a D'1 c a l - 40 l e d Do (open p o r o s i t y ) and f i l l s up t o
10 a$.% w i t h o u t any X-ray evidence f o r phase t r a n s i t i o n s ( f i g . 4 : T = 8 0 ° c ) . I t t a k e s p l a c e i n t h e l a r g e h i g h - s t r e s s e d r e - S 30 g i o n s l o c a t e d i n t h e " g l o b u l a r " r e g i o n s .
Z
I n a r e c e n t p a p e r , P h i l l i p s [ 5 ] d e s c r i b e s t h e s e r e g i o n s a s i n c l u d i n g a more o r l e s s connected network o f p o r o s i t i e s . I n o u r W c a s e , t h e tendancy o f Te atoms t o e a s i l y t-a
s e g r e g a t e a t s u r f a c e s u r e l y c o n t r i b u t e s t o 10 make t h o s e porous r e g i o n s Te-rich. RBS ,O s t u d i e s o f t h e oxygen p r o f i l e e v o l u t i o n
d u r i n g a i r o r vacuum a n n e a l s show t h a t
t h i s element, p r e f e r e n t i a l l y s i t u a t e d a t
0
t h e two p e r t u r b e d i n t e r f a c e s , a l s o d i f f u -
0
12 3
45
s e s n o t i c e a b l y from t h e " h e a r t " of t h e TIME^ (h0.5) f i l m . A "de-screnning e f f e c t " due t o t h e
d e p a r t u r e o f oxygen atoms from t h o s e Te- Figure 4
-
Time dependance o f the Au concentration r i c h (in fact ~ ~ - 0 regions may ex- l e v e l s , obtained by averaging over the window o f plain that the vacuum-kinetics are faster F i g . 1 . V : vacuum anneals ; A : a i r anneals t h a n t h e a i r ones by an o r d e r o f magnitu-de ( f i g . 2 ) .
C ) L a s t l y , a t c o n c e n t r a t i o n s above 10 a t . % , a multiphase d i f f u s i o n Dac o c c u r s when
d e t e c t a b l e amounts o f t h e non-equilibrium l a c u n a r c r y s . 2 a l l i n e AuTela7 compound a r e formed, t h u s c r e a t i n g new amorphous-crystal i n t e r f a c e s . I t s k i n e t i c s , corresponding t o t h e r i g h t p a r t o f t h e c u r v e s of t h e f i g . 4 f o l l o w s
t a
(12,<
0.5) laws.The AuTel-7 compounds i s i d e n t i f i e d by X- r a y experiments and e l e c t r o n d i f f r a c t i o n microscopy ( f i g . 7d)
.
Using A U / T ~ t h i n f i l m d i f f u s i o n d a t a ( f i g . 5 ) , we can ex- p l a i n t h e c u r v e s a s p e c t s i n r e s p e c t t o t h e temperature : t h e n u c l e a t i o n and growth r a t e s o f t h e compound a r e d r a s t i c a l l y d i f - f e r e n t : a t low temperature (80°C) i t s f o r - mation i s reduced and t h e s a t u r a t i o n l e v e l i s t y p i c a l l y t h e g o l d c o n t e n t of t h e two f i r s t t y p e s of s h o r t - c i r c u i t s (C 10 a t . % ) , whereas a t h i g h temperature (120°C) t h e 102 compound formation i s f a s t e s t ( X 501, l e a - d i n g t o complete i n t e r d i f f u s i o n (C=
30 a t .%)I
W
Te 1 8 0 0 ~ SATURATION LEVEL103---
-
-
-
//I-
&
-
-
and r e a c t i o n ( f i g . 7 d ) . T h i s compound, used I I
i n a b a r r i e r c o n f i g u r a t i o n , improves t h e li- loo 10' 10' Time ( h '
a3
f e t i m e o f t h e c o n t a c t by a f a c t o r 20 t o 100, v a r y i n g t h e compound t h i c k n e s s ( f i g . 6 b ) .
C4-978 JOURNAL DE PHYSIQUE
Figure 7
-
a ) m n microsection showing an a - ~ e ~ e q / ~ u i n i t i a l d e p o s i t b)SEM observatlon of .an a-GeTe4 f i l m on a v i t r e o u s C subs-
t r a t e
C ) SEM o b s e r v a t i o n o f an a-CeTeq f i l m on a NaCX s u b s t r a t e
d ) n n observation o f a n a - G e ~ e q / ~ u u l t r a - t h i n system a f t e r cou@lete i n t e r d i f fusion.
Figure 6
-
P a s u l t s from e l e c t r i c a l measurements showing :a ) t h e dependance o f t h e r e l a t i v e changes i n r e s i s t a n c e corresponding to s h o r t times conside- r e d a t higher temperatures.
b) t h e e l e c t r i c a l degradation r a t e T c21 of t h e
1.4 pm-thick GeTeq/2501( gold system.
.
1 : without any b a r r i e r ; 2 : w i t h a 800 A AuTele7 b a r r i e r ; 3 : with AUTel.-] r e p l a c i n g t h e Au e l e c t r o d e .
CONCLUSION
-
Adapted from p o l y c r y s t a l l i n e "defect-enhanced" d i f f u s i o n model, a t h r e e - s t e p d i f f u s i o n model is proposed t o t a k e i n t o account t h e g o l d d i f f u s i o n i n t o t h e amorphous GeTe4 t h i n f i l m s . The d i s c u s s i o n p o i n t s o u t t h e rough e s t i m a t i o n s o f U g o l d - c a p a c i t i e s " of t h e i n t e r - g l o b u l a r s p a c e s and t h e open p o r o s i t y of t h e s p u t t e r e d GeTeq m a t e r i a l , a s w e l l a s the oxygen e f f e c t .The model, e x c e p t f o r t h e multiphase d i f f u s i o n type which i s t y p i c a l l y due t o t h e low thermal s t a b i l i t y of t h e GeTeq m a t e r i a l , may d e s c r i b e s s u c c e s s f u l l y t h e m e - t a l d i f f u s i o n behaviour of g l o b u l a r amorphous f i l m s .
Work supported by t h e CEA under C o n t r a c t
#
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