Doping Behavior of Implanted Magnesium in Silicon

Doping Behavior of Implanted Magnesium in Silicon

H. Sigmund and D. WeiG

F r a u n h o f e r - I n s t i t u t f u r F e s t k b r p e r t e c h n o l o g i e , P a u l - G e r h a r d t - A l l e e 4 2 , D-8000 Mlinchen 6 0 , F e d . Rep. o f Germany

A b s t r a c t

M g - i m p l a n t e d l a y e r s (5 x I0lk t o 5 x 1 01 5 cm"2) w h i c h a r e a n n e a l e d a t low t e m p e r a t u r e s (500°C t o 600°C) show h i g h n-type c o n d u c t i v i t y . C o n t r a r y t o t h e u s u a l d o p i n g elements i n s i l i c o n , t h e sheet c a r r i e r c o n c e n t r a t i o n shows a sharp d e c r e a s e w i t h i n c r e a s i n g a n n e a l i n g t e m p e r a t u r e s ; minimum s h e e t r e s i s t i v i t i e s o f 800 Q/Q were o b t a i n e d a f t e r a " t h e r m a l " a n n e a l i n g s t e p a t 550*C f o r 30 m i n . From p r o f i l e measurements ( H a l l e f f e c t and S I M S ) , i t i s c o n c l u d e d t h a t t h e i n t e r s t i t i a l s o l u b i l i t y f o r Mg a t 500*C i n t h e s e samples i s as h i g h as 1 x 1 01 8 cm"3. At t e m p e r a t u r e s above 700°C t h e e l e c t r i c a l l y a c t i v e Mg c o n c e n t r a t i o n i s r a p i d l y g e t t e r e d by t h e damage l a y e r and Mg p r e c i p i t a t e s , and becomes e l e c t r i c a l l y i n a c t i v e . A model f o r t h e s e g r e g a t i o n b e h a v i o r o f Mg i n s i l i c o n i s d i s c u s s e d b r i e f l y .

1. I n t r o d u c t i o n

I t i s known, based on a b s o r p t i o n and ESR measurements made on M g - d i f f u s e d s i l i c o n samples, t h a t i n t e r s t i t i a l l y d i s s o l v e d Mg i n s i l i c o n forms a d o u b l e donor w i t h t h e donor l e v e l s E - = 0.11 eV and E = 0.25 eV [ 1 , 2 J . The p r e p a r a t i o n o f t h e s e samples w a s^uA c c o m p l i s h e d by samlwich d i f f u s i o n a t 1200°C. The_samples showed a v e r y low i n t e r s t i t i a l s o l u b i l i t y

(» 1 0^{1 5} c m "³) , as r e v e a l e d by r e s i s t i v i t y measurements [ l ] . Measurements of t h e t o t a l l y d i s s o l v e d Mg were not made i n t h e s e e x p e r i m e n t s . R e c e n t l y p e r f o r m e d i n v e s t i g a t i o n s on t h e s e g r e g a t i o n and s o l u b i l i t y o f Mg i n s i l i - con a t h i g h t e m p e r a t u r e s (950°C t o 1200°C), by a t o m i c - a b s o r p t i o n s p e c t r o s - copy (AAS) as w e l l as by s e c o n d a r y - i o n mass s p e c t r o s c o p y j S I M S ) , have r e v e a l e d a h i g h s o l u b i l i t y o f Mg (4 x 1 0^{1 8} t o 1.5 x 1 0^{l 5}c m ³) [ 3 ] . The samples were p r e p a r e d by s p e c i a l l i q u i d - p h a s e e p i t a x y ; t h e e p i - l a y e r s showed c o n d u c t i v i t y v a l u e s s i m i l a r t o t h e d i f f u s e d samples. In o r d e r t o i n v e s t i g a t e t h e s e g r e g a t i o n and d o p i n g b e h a v i o r o f Mg i n s i l i c o n a t l o w e r t e m p e r a t u r e s , t h e s e e x p e r i m e n t s w i t h i m p l a n t e d Mg were performed. The i s o - t h e r m a l a n n e a l i n g and d i f f u s i o n b e h a v i o r i n t h e t e m p e r a t u r e range between 450°C and 950°C was i n v e s t i g a t e d t h r o u g h SIMS and H a l l - e f f e c t measure- ments.

2. E x p e r i m e n t a l P r o c e d u r e

Samples w i t h p a t t e r n s o f t h e van-der-Pauw t y p e [ 4 ] were made u s i n g t h e p l a n a r t e c h n i q u e ( F i g . 1 ) . An i m p o r t a n t a s p e c t o f t h e van-der-Pauw s t r u c t u - re i s r e p r e s e n t e d by t h e A s - i m p l a n t e d c o n t a c t s , which ensure ohmic beha- 473

ru.„ ... .r

^ Si - w a f e r

^ Rear side c o n t a c t (B)

F i g . 1. Van-der-Pauw p a t t e r n

F i g . 2. T h e o r e t i c a l ( c o n t i n o u s l i n e ) and measured (SIMS) Mg d i s t r i b u t i o n o f a 150 keV i m p l a n t a t i o n . I t can be seen t h a t t h e Mg p e n e t r a t e s more d e e p l y i n t o the c r y s t a l than would be e x p e c t e d f r o m t h e g a u s s i a n d i s t r i b u - t i o n w i t h Rp and

t o B i e r s a c k [9]

ARp v a l u e s a c c o r d i n g

Mg : Si E * 150 keV N^{q 3} 5 x I 0^{l 5}c m -²

— G a u s s i a n A a s i m p l a n t e d

( S I M S )

.4 6 .8 Depth (urn)

v i o r even at h i g h a n n e a l i n g t e m p e r a t u r e s . Boron i m p l a n t a t i o n i n t o t h e back s u r f a c e ( u n p o l i s h e d ) improved t h e c o n t a c t on t h e r e v e r s e s i d e , w h i c h i s u s e f u l f o r t h e a n o d i c s t r i p p i n g t e c h n i q u e and measurement of c u r r e n t - v o l t a - ge c h a r a c t e r i s t i c s . The i m p l a n t e d c o n t a c t s were a n n e a l e d f o r 30 min a t 900°C. Samples p r e p a r e d i n t h i s manner were i m p l a n t e d w i t h Mg i n t o t h e p o l i s h e d s u r f a c e at room t e m p e r a t u r e . Data on s t a r t i n g m a t e r i a l and t h e i m p l a n t a t i o n p a r a m e t e r s a r e l i s t e d i n T a b l e 1. To a v o i d c h a n n e l i n g , t h e a n g l e of i n c i d e n c e was 7° r e l a t i v e t o t h e (111) c r y s t a l d i r e c t i o n . F i g u r e 2 shows t h e t h e o r e t i c a l and measured (SIMS) d i s t r i b u t i o n o f a 150 keV Mg i m p l a n t a t i o n i n t o S i . A dose o f 5 x 1 0^{1 5} cm"² i s assumed t o r e a c h t h e amorphous dose, w h i l e 5 x 10lk cm"2 i s below t h i s dose [ 5 ] . A f t e r i m p l a n - t a t i o n , t h e s l i c e s were c u t i n t o q u a d r a t i c w a f e r s c o n t a i n i n g one v a n - d e r - Pauw p a t t e r n each ( F i g . 1 ) . I s o t h e r m a l a n n e a l i n g between 400°C and 800°C was c a r r i e d out i n a n i t r o g e n atmosphere. Time i n t e r v a l s from 7.5 t o 90 min were u s e d ; c o n t i n o u s 6 h i s o t h e r m a l a n n e a l i n g was a p p l i e d t o some s a m p l e s . The e f f e c t i v e c a r r i e r d e n s i t y N and t h e e f f e c t i v e m o b i l i t y u i s g i v e n by N ^{f f} = r/(e-R ) a n r f ' g " = R / ( r - p ) , where R^u . i §Tt h e measured s h e e ^ H a l l c o e f f i t f i n t and p et n e s h d e r r e s i s t i v i t y . F o l l o - w i n g t h e u s u a l p r a c t i c e , t h e s c a t t e r i n g f a c t o r r i s a p p r o x i m a t e d by u n i t y .

I n t e r p r e t a t i o n o f t h e e f f e c t i v e v a l u e s a r e d i s c u s s e d i n d e t a i l by Baron e t a l . [ 6 ] . The measurements were c a r r i e d out at room t e m p e r a t u r e . To a v o i d e r r o r s due t o m a g n e t o e l e c t r i c e f f e c t s ( N e r n s t , E t t i n g h a u s e n , R i g h i - L e d u c , e t c . ) , c u r r e n t and v o l t a g e probes were c y c l i c a l l y exchanged [ 7 ] . T h e r e f o -

T a b l e 1: M a t e r i a l and i m p l a n t a t i o n d a t a S i w a f e r s , Wacker F z - S i

O r i e n t a t i o n (111)

R e s i s t i v i t y 50 Qcm, p-type one s i d e p o l i s h e d

Doses 5 x 1 01 H and 5 x 1 01 5c m "

Energy 80 and 150 keV R 0.139 and 0.238 urn AR 0.053 and 0.0829 urn

P 1

r e , p and R a r e a v e r a g e v a l u e s e x t r a c t e d from t h e s e c u r r e n t and v o l t a - ge measurement's*. A n o d i c s t r i p p i n g t e c h n i q u e [ 8 ] was used f o r d e t e r m i n a t i o n of t h e number o f c a r r i e r s as a f u n c t i o n of depth. The a p p l i e d e l e c t r o l y t e was NMA (0.025g KN0~ d i s s o l v e d i n 10 ml H O + 500 ml N-methylacetamide );

t h e f o r m i n g v o l t a g e was 120 V. The t h i c i c n e s s of t h e removed l a y e r was d e t e r m i n e d m e c h a n i c a l l y , t h e a v e r a g e o f one s t e p was 26 ± 2nm.

3. E x p e r i m e n t a l R e s u l t s

The i s o t h e r m a l a n n e a l i n g e x p e r i m e n t s showed t h a t Mg-implanted samples d i s - p l a y a q u i t e d i f f e r e n t d o p i n g b e h a v i o r than t h a t f o r i m p l a n t a t i o n w i t h t h e o r d i n a r y d o p i n g e l e m e n t s . The r e s u l t s a r e shown i n F i g . 3, where t h e e f f e c t i v e e a r n e r d e n s i t y N j ^s p l o t t e d a g a i n s t t h e a n n e a l i n g t i m e . A l l samples w i t h a dose o f 5 *^{e T}l^T0^{1 5} cm"² showed a d e c r e a s i n g sheet c a r r i e r c o n c e n t r a t i o n w i t h i n c r e a s i n g a n n e a l i n g t i m e (> 7.5 m i n ) , u n t i l they r e - ached a n e a r l y c o n s t a n t v a l u e f o r a c e r t a i n t e m p e r a t u r e . In samples w i t h a dose of 5 x I0^lk cm"², t h e sheet c a r r i e r c o n c e n t r a t i o n at low t e m p e r a t u r e s (500°C, 550°C) f i r s t i n c r e a s e s , then reaches a maximum, and f i n a l l y drops t o n e a r l y t h e same v a l u e as i n t h e amorphous sample. F i g u r e 3 shows t h a t a t e m p e r a t u r e - d e p e n d e n t , q u a s i - e q u i 1 i b r i u m sheet c a r r i e r c o n c e n t r a t i o n i s e s t a b l i s h e d , t h e l e v e l of w h i c h drops d r a s t i c a l l y w i t h i n c r e a s i n g annea- l i n g t i m e . A good example o f t h i s c h a r a c t e r i s t i c i s r e p r e s e n t e d by t h e 600°C i s o t h e r m a l c u r v e s , where t h e e q u i l i b r i u m ^g^ v a l u e i s reached a f t e r a n n e a l i n g f o r c a . 3 h. The 800°C i s o t h e r m s show a n e a r l y c o n s t a n t sheet c a r r i e r d e n s i t y v a l u e over the whole i n v e s t i g a t e d range - t h e N~ ~*

d e c r e a s e o b v i o u s l y o c c u r s so q u i c k l y t h a t i t cannot be o b s e r v e d i n t h e s e

; Mg : Si

Time (min) ^T < ^c>

F i g . 3. I s o t h e r m a l a n n e a l i n g c u r - F i g . 4. E f f e c t i v e c a r r i e r c o n c e n t r a - ves f o r f o u r d i f f e r e n t t i o n (N ) and e f f e c t i v e t e m p e r a t u r e s

m o b i 1 i t y* \ i l

)

i s o t h e r m a l a n n e a l i n g

measurements. The n o n - c o n s t a n t b e h a v i o r o f t h e 550°C i s o t h e r m s i s assumed t o r e s u l t from a measurement e r r o r due t o non-ohmic c o n t a c t s , w h i c h can cause N f f v a l u e s which a r e t o low. F i g u r e 4 shows t h e e f f e c t i v e c a r r i e r d e n s i t y^sH f^{f r} and t h e e f f e c t i v e m o b i l i t y v e r s u s a n n e a l i n g t e m p e r a t u r e a f t e r 6 lr»rs£fthermal a n n e a l i n g f o r a t e m p e r a t u r e range from 400*C up t o 800°C. Samples i m p l a n t e d w i t h a dose o f 5 x 1 0^{1 5} cm"² c o u l d o n l y be measured a t an a n n e a l i n g t e m p e r a t u r e o f 500"C and above, d u e _ t o poor r e c r y s t a l 1 i z a t i o n at l o w e r t e m p e r a t u r e s . I m p l a n t s w i t h 5 x 1 01 5 cm"2 a l r e a - dy showed a h i g h e f f e c t i v e s h e e t c a r r i e r d e n s i t y (about 10% e l e c t r i c a l l y a c t i v e ) at an a n n e a l i n g t e m p e r a t u r e o f 400°C. F i g u r e 5 shows c u r r e n t - v o l t a - ge c h a r a c t e r i s t i c s under r e v e r s e b i a s , whereby t h e a n n e a l i n g t e m p e r a t u r e s are t h e v a r i a b l e p a r a m e t e r . The p l o t t e d c u r v e s show a s t e e p r e d u c t i o n o f l e a k a g e c u r r e n t between 450*C and 500°C a n n e a l i n g t e m p e r a t u r e , w h i l e a t h i g h e r a n n e a l i n g t e m p e r a t u r e s t h e drop o c c u r s much more g r a d u a l l y . The t e m p e r a t u r e range from 400°C up t o 550°C i s c h a r a c t e r i z e d by a s t e e p

i n c r e a s e i n t h e e f f e c t i v e m o b i l i t i e s u due t o t h e r e c r y s t a l 1 i z a t i o n w i t h i n c r e a s i n g a n n e a l i n g t e m p e r a t u r e eH n t h i s range. At t e m p e r a t u r e s h i g h e r t h a n 550*C, t h e m o b i l i t y v a l u e s a r e a l m o s t i n agreement w i t h I r v i n ' s d a t a [ 1 0 ] . M o b i l i t i e s i n l a y e r s w i t h a dose of 5 x 1 0^{1 5} cm"² l i e below t h e v a l u e s o f samples i m p l a n t e d w i t h a dose o f 5 x I0^lk cm"². The e f f e c t i v e s h e e t c a r r i e r c o n c e n t r a t i o n s d e c r e a s e a l m o s t e x p o n e n t i a l l y o v e r t h e e n t i r e i n v e s t i g a t e d range. The measured N$ v a l u e i s o n l y s l i g h t l y dependent on t h e i m p l a n t a t i o n dose. I t s h o u l d be mentioned t h a t i n our e x p e r i m e n t s t h e e f f e c t i v e c a r r i e r d e n s i t y i s not r e v e r s i b l e . T h i s means t h a t an e s t a b l i s h e d v a l u e o f N^{£ f f} f o r an a n n e a l e d sample cannot be i n c r e a s e d a g a i n by a n n e a l i n g i t a t a Tower t e m p e r a t u r e .

100-

E u

<

rio-

^ISQ°C (6h)

5 0 0° C( 6 h ) »

•

x 5 5 0° C( 6 h) y ^ *

J T^ ^ 7 0 0° C( 6 h )

J 1 1 1 1 1 ll 1 ' • • ' ' • > ' '

0.1 1 10 Reverse b i a s (V)

Fig._5. R e v e r s e c u r r e n t - v o l t a g e c h a r a c - t e r i s t i c s f o r d i f f e r e n t a n n e a l i n g t e m p e r a t u r e s (dose: 5 x 1 01 5

cm"³, 150 keV)

„ Mg (as i m p l a n t e d ) A Mg : Si E s ISO keV Nⁿ* 5 x l 0^{l 5}c m -²

Carrier density 500°C ( 6 h )

• Mobility 5 0 0 ° C (6h)

Mg (total) : 5 0 0 ° C ( 6 h ) o

F i g . 6. Doping p r o f i l e s and m o b i l i t i e s

( uo f fJ o f a sample i s o t h e r m a l a n n e a l e d

.4 .6 .3 Depth (jim)

ShSwA i s t h e t o t a l Mg d i s t r i b u t i o n (SIMS) and t h e measured c a r r i e r d i s t r i b u t i o n

(at room t e m p e r a t u r e ) i n c o m p a r i s o n t o t h e c a l c u l a t e d i n t e r s t i t i a l c o n c e n t r a t i o n o f Mg.

D i s t r i b u t i o n s o f c a r r i e r s , d e t e r m i n e d by H a l l - e f f e c t and s h e e t - r e s i s t i t i v i - ty measurements combined w i t h a n o d i c s t r i p p i n g , a r e compared t o SIMS p r o f i - l e s . With t h e SIMS t e c h n i q u e t h e t o t a l amount o f Mg i n t h e sample can be d e t e r m i n e d . In F i g . 6. t h e t o t a l Mg d i s t r i b u t i o n i n a sample - i m p l a n t e d w i t h a dose o f 5 x 1 01 5 cm"2 (150 keV) - b e f o r e and a f t e r a 6 h a n n e a l i n g a t 500°C i s p l o t t e d . An i m p o r t a n t f i n d i n g i s t h a t a f t e r a n n e a l i n g t h e Mg remains i n t h e sample t o a l a r g e e x t e n t ( 7 5 % ) . The o t h e r 25% o f t h e Mg i s assumed t o be a t t h e s u r f a c e p r o b a b l y as MgO. The s m a l l peak a t a c o n c e n t - r a t i o n o f 1.5 x 1 0^{1 5} cm"² s u g g e s t s a s l i g h t d i f f u s i o n i n t o t h e c r y s t a l . F i g u r e 6 a l s o shows t h e number o f c a r r i e r s and t h e m o b i l i t y as a f u n c t i o n of d e p t h . A l l data were o b t a i n e d a f t e r 6 h a n n e a l i n g a t 500°C. I t i s seen c l e a r l y t h a t o n l y a s m a l l p e r c e n t a g e o f t h e t o t a l Mg atoms i s e l e c t r i c a l l y a c t i v e . Brooks [ 1 1 ] has shown t h a t t h e a v e r a g e number ( f ) o f e l e c t r o n s o c c u p y i n g a d o u b l e donor i s g i v e n by:

f = l + L l+ 4 e x p ( E^{D 2}- E^F) / k T] / l+ 4 e x p ( E^F- E^{D 1}) / k T J

where E and E a r e t h e f i r s t and second donor l e v e l s , r e s p e c t i v e l y . The Fermi l e v e l E can be o b t a i n e d ( i f E i s s e v e r a l kT below t h e bottom edge of t h e c o n d u c t i o n band E ) from n E N e x p ( ( E -E )/kT) w i t h N b e i n g t h e e f f e c t i v e d e n s i t y o f statfes i n t h e con8uction^Fba9id. Thus, t h e⁰ c o n c e n t r a - t i o n o f Mg i n t e r s t i t i a l s (Mg ) can be c a l c u l a t e d from Eq (1) and t h e e x p r e s s i o n : Mg. = n / ( 2 - f ) . In F i g . 7, t h e average number o f e l e c t r o n s p e r Mg. atom t r a n s f e r r e d t o t h e c o n d u c t i o n band i s p l o t t e d v e r s u s t h e Mg.

c o n c e n t r a t i o n f o r s e v e r a l t e m p e r a t u r e s . I t s h o u l d be noted t h a t t e m p e r a t u - re-dependent H a l l - e f f e c t and s h e e t - r e s i s t i v i t y measurements i n d i c a t e i n - c r e a s i n g c a r r i e r d e n s i t i y w i t h i n c r e a s i n g t e m p e r a t u r e ( F i g . 8 ) .

F i g . 7. P r o b a b i l i t y o f i o n - i z a t i o n o f i n t e r s t i t i a l Mg i n dependence o f t h e Mg d o p i n g l e v e l

Mg-interstitial cone.

The c o n c e n t r a t i o n o f i n t e r s t i t i a l Mg atoms, d e t e r m i n e d by Eq. (1) from t h e measured e l e c t r o n d e n s i t y i s p l o t t e d i n F i g . 6 (dashed l i n e ) . The t o t a l Mg c o n c e n t r a t i o n i s g r e a t e r t h a n t h e i n t e r s t i t i a l c o n c e n t r a t i o n by f a c t o r o f 6 i n t h e t a i l and by a f a c t o r o f 17 a t t h e maximum o f t h e Mg - i n t e r s t i - t i a l d i s t r i b u t i o n . The r a t i o i n c r e a s e s s h a r p l y as one comes c l o s e r t o t h e s u r f a c e . The c a r r i e r p r o f i l e c o u l d o n l y be d e t e r m i n e d a t a depth o f 0.25 urn and above, where t h e c o n c e n t r a t i o n i n c r e a s e s r a p i d l y . I n c r e a s e o f t h e

100

7 ( ° C )

80 60 AO 20

Mg : Si E s 150 keV Nⁿ= 5 x l O^{l 5}c m -²

2.7 2.8 23 3 3.1 1 0 0 0 / T ( K_ 1)

Mg ( a s implanted) A

Mg : Si E s 150 keV N^Q= 5 x l 0^{1 5}c m "²

3.2 3.3 3.4,

F i g . 8. Temperature dependence o f t h e e f f e c t i v e c a r r i e r c o n c e n t r a t i o n

( NS o f f^{5 a n}d m o b i l i ty ( M^{a f}- )

(5§0^eC l 6 h) ^{e f f}

^Mg (total)

• " 8 0 0 ° C (1h)

•

Mg ( a s implanted) A Mg : Si E = 150 keV Nn s 5 x I 0l 5c m "2

A .6

Depth (jim)

1.2 F i g . 9. D o p i n g p r o f i l e o f Mg (SIMS) a s i m p l a n t e d and a f t e r i s o t h e r m a l a n - n e a l i n g (800°C, 1 h ) ; a l s o shown i s t h e measured peak o f t h e c a r r i e r p r o f i l e

F i g .

1.0 2.0 Depth (Mm)

3.0

10. Doping p r o f i l e (SIMS) o f Mg a f t e r an i s o t h e r m a l a n n e a l i n g a t 950'C, 30 m i n .

c a r r i e r c o n c e n t r a t i o n i s accompanied by i n c r e a s i n g m o b i l i t i e s . In F i g . 9 t h e t o t a l Mg c o n c e n t r a t i o n and measured e l e c t r o n c o n c e n t r a t i o n a f t e r 1 h a n n e a l i n g a t 800°C i s shown. A l s o i n t h i s c a s e , 75% o f t h e i m p l a n t e d Mg remains i n t h e sample. At t h i s a n n e a l i n g t e m p e r a t u r e , Mg does not d i f f u s e i n t o t h e c r y s t a l , but a p p r o a c h e s t h e G a u s s i a n i m p l a n t a t i o n d i s t r i b u t i o n . I t can t h e r e f o r e be c o n c l u d e d t h a t t h e Mg i s g e t t e r e d by t h e

damage r e g i o n or by Mg p r e c i p i t a t e s . T h i s peak i n carrier c o n c e n t r a t i o n l i e s c l o s e r t o the s u r f a c e (0.29 urn) t h a n t h e maximum of the c a r r i e r d i s t r i b u t i o n a f t e r a 6h a n n e a l i n g at 500°C. T h i s s u g g e s t s t h a t the damage l a y e r becomes t h i n n e r and t h a t t h e c r y s t a l r e c r y s t a l 1 i z e s from the b u l k . The maximum of the e l e c t r o n d e n s i t y now has a v a l u e of about 2 x 1 0^{1 6} cm"³. An i n t e r e s t i n g phenomenon i s o b s e r v e d when the a n n e a l i n g t e m p e r a t u r e i s i n c r e a s e d t o 950°C. F i g u r e 10 shows SIMS p r o f i l e s b e f o r e and a f t e r a 30 min a n n e a l i n g at 950*C, a t e m p e r a t u r e which l i e s s l i g h t l y above t h e e u t e c - t i c p o i n t . Mg d i f f u s e s f a s t i n t o t h e c r y s t a l a t a c o n c e n t r a t i o n of about 2 x 1 01 8 cm"3, and c o u l d be measured even at a depth of 15 urn. H a l l measurements on samples a n n e a l e d at t e m p e r a t u r e s h i g h e r than 800°C c o u l d not be p e r f o r m e d .

4. D i s c u s s i o n

In o r d e r t o e x p l a i n the a n n e a l i n g and d o p i n g b e h a v i o r of i m p l a n t e d Mg i n s i l i c o n , t h e f o l l o w i n g s e g r e g a t i o n model i s p r o p o s e d . A c c o r d i n g t o t h i s model, t h e i m p l a n t e d depth i s d i v i d e d i n t o two r e g i o n s (A and B) as d e p i c t e d i n F i g . 11. Region A can be c h a r a c t e r i z e d by low c a r r i e r c o n c e n t r a t i o n (* 5 x 1 0^{1 5} cm'³) low H a l l m o b i l i t i e s , whereas the t o t a l Mg c o n c e n t r a t i o n i n t h i s r e g i o n i s s e v e r a l o r d e r s of magnitude above t h e measured e l e c t r o n c o n c e n t r a t i o n . Region B shows h i g h e l e c t r o n c o n c e n t r a - t i o n s w i t h h i g h m o b i l i t y v a l u e s ( c l o s e t o bulk m o b i l i t i e s ) ; a h i g h f r a c - t i o n of t h e t o t a l Mg c o n c e n t r a t i o n i s e l e c t r i c a l l y a c t i v e . Because s u b s t i - t u t i o n a l Mg (Mg ) i n s i l i c o n p r o b a b l y forms a deep a c c e p t o r l e v e l ( p r e l i m i - nary DLTS measurements have r e v e a l e d a l e v e l at 0.55 e V ) , a s t r o n g compen- s a t i o n s h o u l d o c c u r i f t h e number o f s u b s t i t u t i o n a l Mg atoms exceeds the c o n c e n t r a t i o n of i n t e r s t i t i a l Mg atoms. The i m p l a n t e d samples showed n- t y p e c o n d u c t i v i t y i n a l l c a s e s . I t i s t h e r e f o r e very l i k e l y t h a t at lower a n n e a l i n g t e m p e r a t u r e s , Mg. atoms r e a c t w i t h Mg atoms and form n e u t r a l c o m p l e x e s : Mg. + Mg = (Mg1 • Mg.).

F i g . 11. Model f o r t h e a n n e a l i n g b e h a v i o r of i m p l a n t e d Mg i n s i l i c o n

D e p t h

479

The complexes (Mg • Mg.) a r e i s o e l e c t r o n i c , so t h e y s h o u l d have no i n f l u - ence on t h e e l e c t r o n c o n c e n t r a t i o n . At h i g h e r t e m p e r a t u r e s , t h e complexes i n r e g i o n B d i s s o c i a t e . F u r t h e r m o r e , t h e s u b s t i t u t i o n a l Mg atoms r e a c t w i t h S i i n t e r s t i t i a l s ( I ) a c c o r d i n g t o t h e " K i c k - o u t " mechanism [ 1 2 ] : Mg + I = Mg.. The t o t a l r e a c t i o n between S i i n t e r s t i t i a l s and t h e complexes w o u l d t h e n be:

(Mg^$ • Mg.) + I = 2 Mg.. (2)

Between r e g i o n s A and B, t h e r e i s a s h a r p g r a d i e n t i n t h e i n t e r s t i t i a l Mg c o n c e n t r a t i o n ( s . F i g . 11). T h i s c o n c e n t r a t i o n g r a d i e n t i s much s t e e p e r t h e n t h e g r a d i e n t i n t o t h e b u l k . Thus, at h i g h e r t e m p e r a t u r e s , Mg. atoms d i f f u s e r a p i d l y i n t o r e g i o n A, and t h e Mg c o n c e n t r a t i o n i s i n c r e a s e d . I t seems v e r y p r o b a b l e t h a t Mg s i l i c i d e c l u s t e r s a r e formed i n r e g i o n A. A s u i t a b l e r e a c t i o n may be g i v e n i n the f o l l o w i n g form: P + Mg. = P . + I , where P i s a M g - s i l i c i d e p r e c i p i t a t i o n w i t h n Mg atom's. Base*d o nⁿt T i i s assumption,^{1 1} t h e e x p e r i m e n t a l l y o b s e r v e d a n n e a l i n g k i n e t i c s o f F i g . 3 may be d e s c r i b e d a n a l y t i c a l l y [ 3 ] . At t e m p e r a t u r e s between 900*C and 950*C, t h e e q u i l i b r i u m p o i n t o f t h e p r e c i p i t a t i o n r e a c t i o n i s s h i f t e d t o t h e l e f t s i d e o f E q . ( 2 ) , and Mg atoms d i f f u s e i n t o t h e b u l k r a p i d l y . T h i s d r a s t i c change i n t h e b e h a v i o r o f r e g i o n A o c c u r s i n a very s m a l l t e m p e r a t u r e i n t e r v a l , so t h a t i s very p r o b a b l e t h a t t h e s t r u c t u r e o f t h e p r e c i p i t a t e s i s changed between 900°C and 9 5 0^-C .

We a r e i n d e b t e d t o Mr. K r a n z / I F T f o r p e r f o r m i n g t h e i m p l a n t a t i o n o f t h e s a m p l e s .

R e f e r e n c e s

1. R.K. F r a n k s , J.B. R o b e r t s o n : S o l . S t a t e Comm. j>, 479 (1967) 2. L.T. Ho, A.K. Ramadas: Phys. Rev. 35, 462 (1972)

3. H. Sigmund, R. B r a u n g a r t . Ch. H b p F l , D. Weift: R e s e a r c h R e p o r t NT 846 (1981) FRG

4. L . J . van der Pauw: P h i l i p s Res. R e p t s . 13, 1 (1958)

5. H. R y s s e l , I . Ruge: I o n e n i m p l a n t a t i o n , " S t u t t g a r t ( 1 9 7 8 ) , p. 33

6. R. B a r o n , G.A. S h i f r i n , O.J. M a r s h , J.W. Mayer: J . A p p l . Phys. 40, 3702 (1969)

7. H.H. W i e d e r , C h a r a c t e r i s a t i o n o f E p i t a x i a l S e m i c o n d u c t o r F i l m s , ed. by H. K r e s s e l , N.Y. (1976)

8. A. Manara, A. O s t i d i c h , G. P e d r o l i , G. R e s t e l l i : T h i n S o l i d F i l m s 8, 359 (1971)

9. J.P. B i e r s a c k : H a h n - M e i t n e r - R e p o r t , HMI-B334 ( 1 9 8 0 ) : p r o j e c t e d ranges o f t h e most common i o n - t a r g e t c o m b i n a t i o n s a r e p u b l i s h e d i n [ 5 ] .

10. S.M. Sze, J.C. I r v i n : S o l i d - S t a t e E l e c t r o n i c s 11, 599 (1968)

11. H. B r o o k s : Advances i n E l e c t r o n i c s and E l e c t r o n P h y s i c s , Vol 7. N.Y.

(1955)

12. A. S e e g e r : phys. s t a t . s o l . ( a ) 61, 521 (1980)