ARSENIC SEGREGATION IN POLYSILICON AND AT THE POLY/SINGLE CRYSTALLINE SILICON INTERFACE

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ARSENIC SEGREGATION IN POLYSILICON AND

AT THE POLY/SINGLE CRYSTALLINE SILICON

INTERFACE

H. Oppolzer, W. Eckers, H. Schaber

To cite this version:

H. Oppolzer, W. Eckers, H. Schaber. ARSENIC SEGREGATION IN POLYSILICON AND AT THE

POLY/SINGLE CRYSTALLINE SILICON INTERFACE. Journal de Physique Colloques, 1985, 46

(C4), pp.C4-523-C4-528. �10.1051/jphyscol:1985458�. �jpa-00224709�

ARSENIC SEGREGATION IN POLYSILICON AND A T THE POLY/SINGLE CRYSTALLINE SILICON INTERFACE

H. Oppolzer, W. Eckers and H. Schaber

Siemens AG, Research Laboratories, Munich, F.R.G.

Résumé La ségrégation de l'arsenic aux joints de grains des films de silicium polycristallin fortement dopés a été comparée à la ségrégation de l'arsenic à l'interface du silicium polycrystallin avec le substrat en silicium et avec le SiO„. Des échantillons ayant subi divers traitements de surface avant le dépôt de silicium polycristallin ont été étudiés à l'aide du microscope électronique à transmission, sur des sections transversales; ces sections minces ont égale-ment été utilisées pour mesurer les profils de l'arsenic aux interfaces, par microanalyse aux rayons X à l'aide d'un microscope électronique à transmission et à balayage avec une résolution spatiale élevée ( < 10 nm). La quantité de la ségrégation de l'arsenic aux joints des grains était environ 10 cm" , ce qui correspond approximativement à une monocouche. La quantité trouvée était pratiquement la même aux interfaces avec le substrat de silicium (avec différents oxydes aux interfaces) et avec le Si0„. On a estimé que la largeur de la zone à concentration accrue d'arsenic était interferieure à 5 nm. Abstract Arsenic segregation at the grain boundaries of highly doped poly-silicon films was compared to the arsenic segregation at the interface of polysilicon to the silicon substrate as well as to Si0„. Samples with various surface treatments prior to polysilicon deposition were studied by TEM of thin cross sections, which were also used to measure the arsenic profiles across the interface by x-ray microanalysis in a scanning TEM with high spatial resolution ( < 10 nm)« The amount of arsenic segregation at the grain boundaries was about 10 cm" which corresponds approximately to one mono-layer. About the same amount was also found at the various interfaces to the silicon substrate (with different interfacial oxides) and to Si0„. The width of the zone with increased arsenic concentration was estimated to be < 5 nm.

1. Introduction

Thin films of polycrystalline silicon (polysilicon) are widely used in integrated circuit technology. In MOS circuits, low resistivity of the highly doped polysilicon is essential. The higher resistivity of phosphorus and arsenic doped polysilicon com-pared to single crystalline silicon with similar doping concentrations was shown to be due not only to a reduction in effective carrier concentration by carrier trapping at the grain boundaries, but also to segregation of the dopant atoms at the grain bound-aries where they are supposed to be electrically inactive /1,2/. This arsenic segrega-tion was recently measured directly by x-ray microanalysis in a scanning TEM (STEM) /3/. In modern bipolar processes, polysilicon is deposited directly onto the single crystal silicon substrate. Arsenic doped polysilicon films then serve as diffusion source and contacting layers for shallow emitters. Depending on the surface treatment prior to polysilicon deposition different interfacial oxides are formed which control the dopant diffusion into the silicon substrate and can also have a favourable influence on the electrical bipolar transistor parameters /4,5/. Arsenic segregation at this interface was confirmed by depth profiling techniques /5,6/ and was also correlated to the morphology of the interfacial oxides, i.e. whether they were broken up during annealing or remained continuous /7,8/.

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I t was t h e aim o f t h e p r e s e n t work t o measure t h e a r s e n i c s e g r e g a t i o n a t t h e p o l y s i - l i c o n / s i l i c o n i n t e r f a c e f o r v a r i o u s i n t e r f a c i a l o x i d e s and t o compare

it

t o t h e s e g r e g a t i o n a t t h e i n t e r f a c e p o l y s i l i c o n / S i O a s w e l l a s a t g r a i n b o u n d a r i e s w i t h i n t h e p o l y s i l i c o n . By TEM o b s e r v a t i o n o f t h i n & o s s s e c t i o n s through t h e samples t h e i n t e r f a c i a l o x i d e s and changes i n t h e m i c r o s t r u c t u r e o f t h e p o l y s i l i c o n c o u l d b e s t u d i e d d i r e c t l y a s a f u n c t i o n o f f i l m depth. The c r o s s s e c t i o n s were a l s o used t o measure t h e a r s e n i c p r o f i l e s a c r o s s t h e i n t e r f a c e s by x-ray m i c r o a n a l y s i s i n a STEM with high s p a t i a l r e s o l u t i o n .

2. Experimental

P o l y s i l i c o n f i l m s were d e p o s i t e d on (100) s i l i c o n w a f e r s by s t a n d a r d low-pressure chemical vapour d e p o s i t i o n (LPCVD) a t l g tem e r a t u r e o f 620 OC, and a r s e n i c doped by i o n i m p l a n t a t i o n with a dose o f 2 . 10 cm-'. Subsequent a n n e a l i n g was performed a t 950 OC. P r i o r t o p o l y s i l i c o n d e p o s i t i o n v a r i o u s s i l i c o n s u r f a c e t r e a t m e n t s were a p p l i e d : ( a ) e t c h i n g i n d i l u t e d h y d r o f l u o r i c a c i d

(HF

d i p , sample l ) , and ( b ) growth of a t h i n t h e r m a l o x i d e with a t h i c k n e s s o f a b o u t 3 nm (sample 2 ) . To s t u d y t h e poly- s i l i c o n / S i 0 2 i n t e r f a c e a 60 nm t h i c k t h e r m a l o x i d e was used below t h e p o l y s i l i c o n (sample 3 ) .

Cross s e c t i o n a l TEM specimens were p r e p a r e d a s d e s c r i b e d i n /9/ and i n v e s t i g a t e d a t 200 kV beam v o l t a g e u s i n g a JEOL 200 CX microscope. For x-ray m i c r o a n a l y s i s a STEM equipped w i t h a f i e l d e m i s s i o n gun (Siemens ELMISKOP ST LOO F) was used

/lo/.

T h i s i n s t r u m e n t p r o v i d e s a n e l e c t r o n beam w i t h a d i a m e t e r o f

1 t o 2 nm t o g e t h e r w i t h a

high c u r r e n t of

1

nA. The s m a l l d i a m e t e r approches c l o s l y t h e

l i m i t o f a p o i n t

s o u r c e beam s o t h a t t h e s p a t i a l r e s o l u t i o n is p r i m a r i l y determined by beam broad- e n i n g due t o e l a s t i c e l e c t r o n s c a t t e r i n g i n t h e f o i l . Table I g i v e s v a l u e s f o r t h e beam broadening i n s i l i c o n f o r v a r i o u s specimen t h i c k n e s s e s , c a l c u l a t e d by a s i n g l e s c a t t e r i n g model /11/. The v a l u e s show t h a t a s p a t i a l r e s o l u t i o n o f

<

1 0 nm c a n only be o b t a i n e d f o r a specimen t h i c k n e s s o f

<

100 nm. The high beam c u r r e n t pro- vided by t h e f i e l d e m i s s i o n gun and e f f e c t i v e x-ray c o l l e c t i o n a l l o w s t o d e t e c t a minimum mass f r a c t i o n o f 0.05 L a l s o on such t h i n specimens w i t h i n r e a s o n a b l e coun- t i n g t i m e s (e.g. 50 s ) . A b e t t e r i n s i g h t i n t o t h e s p a t i a l v a r i a t i o n o f x-ray pro- d u c t i o n i n t h e specimen can be o b t a i n e d by Monte C a r l o s i m u l a t i o n o f e l e c t r o n t r a - j e c t o r i e s /12/. I n t a b l e I 1 t h e f r a c t i o n o f t h e t o t a l x-ray i n t e n s i t y is g i v e n which is produced w i t h i n c y l i n d e r s o f d i a m e t e r d by a p o i n t e l e c t r o n s o u r c e i n a 100 nm t t i i c k aluminum f i l m . The v a l u e s f o r aluminum s h o u l d b e n e a r l y i d e n t i c a l t o t h o s e f o r s i l i c o n . The d i a m e t e r of a c y l i n d e r c o n t a i n i n g 90 7; o f t h e e x c i t e d x-rays is u s u a l l y t a k e n a s t h e s p a t i a l r e s o l u t i o n f o r x-ray m i c r o a n a l y s i s and a g r e e s w e l l w i t h t h e beam broadening c a l c u l a t e d from e l a s t i c s c a t t e r i n g . When measuring s e g r e g a t i o n a t a n i n t e r f a c e (which must be p a r a l l e l t o t h e e l e c t r o n beam) one h a s t o c o n s i d e r t h a t p o s i t i o n i n g o f t h e e l e c t r o n beam a s w e l l as'specimen d r i f t is very c r i t i c a l s i n c e most o f t h e x-rays a r e g e n e r a t e d w i t h i n a c y l i n d e r o f a few nm i n diameter. The t a i l o f t h e x-ray p r o d u c t i o n d i s t r i b u t i o n , on t h e o t h e r hand, can produce a x-ray s i g n a l a t measurement p o i n t s very c l o s e t o t h e i n t e r f a c e . For most o f t h e a r s e n i c p r o f i l e measurements,computer c o n t r o l l e d p o s i t i o n i n g o f t h e e l e c t r o n beam a l o n g a row o f p o i n t s a c r o s s t h e i n t e r f a c e was used.

Specimen

t h i c k n e s s /nm/ 40 100 200 400 I c y l / I t o t 0.5 0.8 0.9 0.95

beam d /nm/ 1 5 9 1 4

broading /nm/ 2 8 22 64

Table 11: F r a c t i o n o f t o t a l x-ray Table I : beam b r o a d i n g i n s i l i c o n , i n t e n s i t y ( I cyl/Itot) produced w i t h i n

afterl/

c y l i n d e r o f d i a m e t e r d , a f t e r /12/

3.

R e s u l t s

Grain boundary s e g r e g a t i o n : To measure t h e a r s e n i c s e g r e g a t i o n a t g r a i n b o u n d a r i e s a TEM specimen t h i n n e d from t h e back o f t h e wafer was used s i n c e most o f t h e g r a i n b o u n d a r i e s a r e p e r p e n d i c u l a r t o t h e f i l m p l a n e . Fig. l a shows t h e p o l y s i l i c o n f i l m o f sample 2 (on 3 nm t h i c k t h e r m a l o x i d e ) a f t e r a r s e n i c i m p l a n t a t i o n and a n n e a l i n g .

4 -4

.

-

20 10 0 10 20 n m

b

d i s t a n c e f r o m g r a i n b o u n d a r y

F i g . 1 (a) TEM image o f p o l y s i l i c o n f i l m doped w i t h 8

l o z 0

a r s e n i c (sample 2), ( b ) A r s e n i c p r o f i l e a c r o s s g r a i n boundary ( l i k e a t A i n F i g . ( a ) ) .

A t g r a i n boundaries which were p a r a l l e l t o t h e e l e c t r o n beam ( l i k e a t A i n F i g . l a ) t h e a r s e n i c p r o f i l e was measured u s i n g 5 nm d i s t a n c e o f t h e measurement p o i n t s ( F i g . l b ) . A t t h e g r a i n boundary t h e a r s e n i c c o n c e n t r a t i o n i s h i g h e r by a f a c t o r o f about

3 compared t o t h e g r a i n volume. The v e r t i c a l b a r o f t h e c r o s s i n c l u d e d i n F i g . l b g i v e s t h e e r r o r due t o c o u n t i n g s t a t i s t i c s i n t h e p o l y s i l i c o n , t h e h o r i z o n t a l bar shows t h e amount o f beam broadening i n t h e specimen. Since t h e i n c r e a s e o f t h e a r s e n i c c o n c e n t r a t i o n a t 5 nm d i s t a n c e from t h e g r a i n boundary stems m o s t l y from t h e t a i l of t h e x-ray p r o d u c t i o n d i s t r i b u t i o n , t h e w i d t h o f t h e zone a t t h e g r a i n boundary w i t h i n c r e a s e d a r s e n i c c o n c e n t r a t i o n must be l e s s t h a n 5 nm.

The amount o f i m p u r i t y s e g r e g a t i o n i s u s u a l l y g i v e n i n terms o f f r a c t i o n s o f a mono- l a y e r assuming t h a t a l l i m p u r i t i e s s i t r i g h t a t t h e g r a i n boundary. To measure t h i s q u a n t i t y , t h e i n c r e a s e i n a r s e n i c c o n c e n t r a t i o n when a n a l y z i n g a d e f i n e d specimen volume c o n t a i n i n g a g r a i n boundary and a l s o r e g i o n s o f b o t h a d j a c e n t g r a i n s , was compared t o t h e f y n c e n i r a t i o n i n t h e g r a i n volume ( w i t h o u t g r a i n boundary). Values of 0.7

- 1.5

-

lol5

cm- a r s e n i c atoms a t t h e g r a i n boundary were found. The mean- v a l u e o f 1.1

.

10 cmm2 corresponds approximately t o one monolayer.

Seqreqation a t p o l y s i l i c o n / s i l i c o n i n t e r f a c e : F i g . 2 shows TEM c r o s s s e c t i o n s through t h e samples i n v e s t i g a t e d . I n sample 1 ( w i t h HF-dip p r i o r t o p o l y s i l i c o n d e p o s i t i o n ) some areas remained p o l y c r y s t a l l i n e ( r i g h t i n F i g . 2a) whereas o t h e r areas regrew e p i t a x i a l l y as can be seen from t h e t h i c k n e s s f r i n g e s ( l e f t i n F i g . 2a). I n b o t h cases t h e i n t e r f a c i a l o x i d e was broken up i n t o s m a l l p a r t i c l e s ( F i g . 2b). These a r e seen b e s t i n t h e e p i t a x i a l r e g i o n s when t h e i n t e r f a c e i s t i l t e d ( F i g . 2c). F i g . 2d shows a h i g h r e s o l u t i o n TEM image o f sample 2 w i t h an approx. 3 nm t h i c k t h e r m a l o x i d e between t h e p o l y s i l i c o n ( t o p , showing ( 1 1 1 ) - l a t t i c e f r i n g e s ) and t h e s i l i c o n s u b s t r a t e (bottom). T h i s i n t e r f a c i a l o x i d e d i d n o t break up d u r i n g annealing. T y p i c a l a r s e n i c p r o f i l e s o f samples 1 and 2 e x t e n d i n g from t h e o x i d e on t o p o f t h e p o l y s i l i c o n down t o t h e s u b s t r a t e a r e p l o t t e d i n F i g . 3. Arsenic segregates a t b o t h i n t e r f a c e s o f t h e p o l y s i l i c o n . More d e t a i l e d measurements o f t h e p o l y s i l i c o n / s i l i c o n i n t e r f a c e (Fig. 4) w i l l be discussed below. The a r s e n i c c o n c e n t r a t i o n a t t h e upper p o l y s i l i c o n i n t e r f a c e o f sample 1 s h o u l d be s i m i l a r t o t h a t o f sample 2. Probably t h i s i n t e r f a c e was n o t p a r a l l e l t o t h e e l e c t r o n beam f o r sample 1 due t o p o l y s i l i c o n surface roughness and c o u l d t h e r e f o r e n o t be measured p r e c i s e l y . The a r s e n i c concen- t r a t i o n i n t h e p o l y s i l i c o n i s lower f o r sample 1 t h a n f o r sample 2 s i n c e a r s e n i c c o u l d d i f f u s e more e a s i l y i n t o t h e s u b s t r a t e a f t e r b r e a k i n g up o f t h e i n t e r f a c i a l oxide. The t h r e s h o l d t i m e f o r t h i s b r e a k i n g up /7/ i n sample 1, however, v a r i e d l o c a l l y so t h a t i n some r e g i o n s w i t h s h o r t e r t h r e s h o l d t i m e t h e p o l y s i l i c o n c o u l d regrow e p i -

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Fig.

2

TEM

cross sections through polysilicon films on silicon o f sample

1

((a) , ( b ) ,

(c); interfacial oxide is broken up) and sample

2

( ( d ) ;

3

nm thick thermal oxide).

Fig.

3

Arsenic profiles of samples 1 and 2 extending from top surface into the sili- con substrate.

Fig. 4 shows a r s e n i c p r o f i l e s across t h e lower p o l y s i l i c o n i n t e r f a c e measured w i t h h i g h s p a t i a l r e s o l u t i o n (5 nm d i s t a n c e o f measurement p o i n t s ) . For comparison w i t h t h e p o l y s i l i c o n / s i l i c o n i n t e r f a c e o f samples 1 and 2 t h e p o l y s i l i c o n / S i O i n t e r f a c e o f sample 3 was i n c l u d e d (Fig. 4c). I n a l l samples t h e maximum arSPnicg8ncentrations a t t h e i n t e r f a c e s were s i m i l a r and ranged between 1.7 and 2.5 ' 1 0 cm

.

Whereas i n sample 1 th e a r s e n i c c o n c e n t r a t i o n i n t h e s u b s t r a t e j u s t below t h e i n t e r f a c e i s n e a r l y e p y a l

59

t h a t i n t h e p o l y s i l i c o n (Fig. 4a), i t drops below t h e d e t e c t i o n l i m i t o f 4 ' 1 0 cm a t a d i s t a n c e o f 10 nm from i n t e r f a c e i n t h e s u b s t r a t e o f sample 2 (Fig. 4b). The same decrease i n a r s e n i c c o n c e n t r a t i o n was found i n t h e SiO o f sample 3 (Fig. 4c). For t h e measurement p o i n t s 5 nm away from t h e i n t e r f a c g s , t h e same holds as mentioned above f o r t h e g r a i n boundary measurements, i.e. t h a t t h e t a i l o f t h e x-ray p r o d u c t i o n d i s t r i b u t i o n c o n t r i b u t e s t o t h e a r s e n i c x-ray i n t e n - s i t y . This i s e s p e c i a l l y r e l e v a n t f o r t h e low a r s e n i c c o n c e n t r a t i o n values i n t h e s i l i c o n resp. SiD o f Figs. 4b,c, so t h a t t h e w i d t h o f t h e r e g i o n i n which t h e a r - s e n i c c o n c e n t r a t i g n decreases very s t e e p l y i s probably

<

5 nm.

b

I I

.

poly-Si I Si poly-si

I

si poly-si

I

Si02

+

det. det.

limit . limit

-..-L7-

20 10 0 10 20 20 10 0 10 20 20 10 0 10 20 distance from interface [nm]

Fig. 4 Arsenic p r o f i l e s across p o l y s i l i c o n / s i l i c o n i n t e r f a c e of sample 1 (a) and sample 2 (b) and p o l y s i l i c o n / S i 0 2 i n t e r f a c e o f sample 3 ( c ) .

4. Discussion

A f t e r a f i r s t h i g h temperature anneal t o s t a b i l i z e t h e g r a i n s t r u c t u r e , t h e a r s e n i c segregation a t g r a i n boundaries was shown i n /1/ and /3/ t o i n c r e a s e w i t h decreasing temperature o f a second anneal. I n t h e present work o n l y a s i n g l e s t e p anneal Was used. D u r i n g annealing a t 950 t h e g r a i n s i z e o f sample 2 (Fig. 1) w i t h a t o t a l a r s e n i c c o n c e n t r a t i o n o f

8

~ 1 0 ' ~ c i 3 increased from about 50 m t o 0.3 p*l. The v a r i a t i o n i n t h e amount o f a r s e n i c segregation (0.7

-

1.5 10" cm-'1 can be explained by d i f f e r e n c e s i n t h e g r a i n boundary s t r u c t u r e which cause d i f f e r e n t segf5gati9n behaviour. From t h e amount o f a r s e n i c segregation (mean value o f 1.1.10 cm ) and t h e g r a i n size, t h e f r a c t i o n o f t h e a r s e n i c atoms trapped a t t h e g r a i n boundaries can be calculated. Assuming cube shaped g r a i n s and t h e same amount o f segregation a t t h e t o p and bottom i n t e r f a c e s o f t h e p o l y s i l i c o n as a t $be s f g i n boundaries (which w i l l be j u s t i f i e d below), t h i s f r a c t i o n amounts t o 2.5 ' 1 0 cm

.

This means t h a t 30 % o f a l l a r s e n i c atoms w i l l be trapped a t t h e g r a i n boundaries where they a r e supposed t o be e l e c t r i c a l l y i n a c t i v e . T h i s value agrees f a i r l y w e l l w i t h a v a l u e o f 40

X

estimated by e x t r a p o l a t i n g data given i n /1/ t o h i g h e r concentrations.

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DE PHYSIQUE

When one compares t h e a r s e n i c p r o f i l e s o f F i g . 4 measured w i t h h i g h s p a t i a l reso- l u t i o n , i t i s s u r p r i s i n g t&t t h 3 range o f maximum c o n c e n t r a t i o n s a t t h e v a r i o u s i n t e r f a c e s (1.7 t o 2.5 . 1 0 cm- ) i s about e q u a l f o r a l l samples. The major p a r t o f t h e s c a t t e r i n g o f t h e s e maxima i s p r o b a b l y due t o t h e measurement technique, i.e. s e n s i t i v i t y t o beam p o s i t i o n i n g and specimen d r i f t as mentioned above. The a r s e n i c maxima a t t h e v a r i o u s i n t e r f a c e s r e a c h t h e ~ a m 5 ~ l e v e j as t h e a r s e n i c maxima a t t h e g r a i n boundaries w i t h i n t h e p o l y s i l i c o n (2.4 * 10 cm I n F l g . l b ) . From t h i s i t can be concluded t h a t a l s o t h e amount o f s e g r e g a t i o n i s about equal, i.e., t h a t approximately one monolayer i s segregated a l s o a t t h e v a r i o u s i n t e r f a c e s . By Auger depth p r o f i l i n g of h e a v i l y phosphorus doped and t h e r m a l l y o x i d i z e d s i l i c o n

i t was shown t h a t t h e phosphorus p i l e up a t t h e SiO / s i l i c o n i n t e r f a c e does n o t depend on o x i d e t h i c k n e s s a f t e r some o x i d a t i o n /13/? Since a r s e n i c s e g r e g a t i o n behaviour i s s i m i l a r t o t h a t o f phosphorus /1/ these r e s u l t s can be compared t o t h o s e of t h e p r e s e n t work. The upper i n t e r f a c e o f t h e p o l y s i l i c o n t o t h e t h e r m a l o x i e on t o p o f i t showed t h e same maximum a r s e n i c c o n c e n t r a t i o n s (up t o 2.5 - 1 0 2 1 cmQ) as t h e l o w e r i n t e r f a c e t o , e.g., t h e t h i c k o x i d e (sample 3 ) . T h i s agrees w e l l w i t h t h e o b s e r v a t i o n i n /13/ t h a t t h e p i l e up a f t e r prolonged o x i d a t i o n was t h e same as o b t a i n e d by a n n e a l i n g a f t e r a v e r y s h o r t o x i d a t i o n , i n d i c a t i n g t h a t t h e segrega- t i o n behaviour i s a p r o p e r t y o f t h e i n t e r f a c e i t s e l f . T h i s argument r e l a t i n g t h e s e g r e g a t i o n t o a sharp minimum o f t h e chemical p o t e n t i a l a t t h e i n t e r f a c e a l s o a l l o w s t o e x p l a i n t h a t t h e same a r s e n i c p r o f i l e was observed f o r t h e very t h i n o x i d e o f sample 2 and t h e t h i c k o x i d e o f sample 3 (Figs. 4b,c). F o r b o t h o f t h e s e samples w i t h a continuous o x i d e t h e a r s e n i c p i l e up must be l o c a t e d a t t h e l o w e r p o l y s i l i c o n i n t e r f a c e (and p r o b a b l y i n t h e p o l y s i l i c o n a d j a c e n t t o i t ) s i n c e t h e a r s e n i c p r o f i l e s a r e very asymmetric w i t h a s t e e p decrease i n a r s e n i c c o n c e n t r a t i o n i n t h e s u b s t r a t e resp. Si02.

For sample 1 where t h e i n t e r f a c i a l o x i d e i s broken up i n t o i n d i v i d u a l p a r t i c l e s , t h e s i t u a t i o n i s more complicated. By t h i s b r e a k i n g up t h e i n t e r f a c e area p o l y s i l i c o n / SiG i s reduced. Since t h e a r s e n i c p r o f i l e , however, i s n e a r l y symmetrical ( t h e doping l e v g l s on b o t h s i d e s o f t h e i n t e r f a c e a r e about equal), n o t o n l y t h e i n t e r f a c e t o t h e p o l y s i l i c o n b u t t h e whole s u r f a c e o f t h e o x i d e p a r t i c l e s can a c t as s i n k f o r a r s e n i c s e g r e g a t i o n t h u s i n c r e a s i n g t h e e f f e c t i v e i n t e r f a c e area. I n t h e p o l y c r y s t a l - l i n e r e g i o n s o f sample 1 t h e r e i s a d d i t i o n a l s e g r e g a t i o n a t t h e g r a i n boundaries t o t h e s u b s t r a t e between t h e o x i d e p a r t i c l e s . The d i f f e r e n c e i n s e g r e g a t i o n compared t o t h e e p i t a x i a l l y regrown r e g i o n i s under i n v e s t i g a t i o n .

Acknowledgements: The a u t o r s w i s h t o thank V. Huber f o r TEM specimen p r e p a r a t i o n and p a r t o f t h e TEM work and t h e i r c o l l e a g u e s from t h e technology l a b s f o r sample f a b r i c a t i o n . T h i s work has been supported under t h e T e c h n i c a l Program o f t h e F e d e r a l Department o f Research and Technology o f t h e Fed. Rep. Germany. The a u t h o r s a l o n e a r e r e s p o n s i b l e f o r t h e content.

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