SOLUBILITY LIMIT OF DOPANTS IN SILICON IRRADIATED BY RUBY LASER

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SOLUBILITY LIMIT OF DOPANTS IN SILICON

IRRADIATED BY RUBY LASER

E. Fogarassy, R. Stuck, J. Grob, A. Grob, P. Siffert

To cite this version:

S O L U B I L I T Y L I M I T OF DODANTS I N S I L I C O N IRRADIATED BY RUBY LASER

F o g a r a s s y , E . , S t u c k , R . , Grob, J . J . , Grob, A . and S i f f e r t , P .

C e n t r e d e R e c h e r c h e s N u c l B a i r e s , Groupe d e P h y s i q u e e t A p p l i c a t i o n s d e s S m i c o n d u c t e u r s , 67037

S t r a s b o u r g - C e d e x , France.

A b s t r a c t . - The s o l u b i l i t y of s e v e r a l d o p a n t s (Sb, Ga, B i , I n ) i n l a s e r t r e a t e d s i l i c o n h a s been i n v e s t i g a t e d . The do:>ants were i n t r o d u c e d by vacuum d e ~ o s i t i o n followed by a ruby l a s e r i r r a d i a t i o n . T h e i r s o l u b i l i t y was determined by Ruther- f o r d b a c k s c a t t e r i n g spectrometrlr measurements i n c h a n n e l i n g and random c o n d i t i o n s . I n a l l c a s e s a s o l u b i l i t y l i m i t C h i g h e r t h a n t h e e q u i l i b r i u m s o l u b i l i t y was found and a s i m p l e c o r r e l a t i o n wi??fi t h e e q u i l i b r i u m d i s t r i b u t i o n c o e f f i c i e n t k

c o u l d b e e s t a b l i s h e d : C' I n t r o d u c t i o n . - I t i s now w e l l e s t a b l i s h e d t h a t t h e s o l u b i l i t y of a d o p a n t i n a semi- conductor c a n b e s i g n i f i c a n t l y i n c r e a s e d b:~ i r r a d i a t i o n w i t h a h i g h e n e r g y l a s e r o r e l e c t r o n beam. I n s i l i c o n , f o r example, t h i s e f f e c t h a s been r e p o r t e d f o r antimony / 1 , 2 / , g a l l i u m /3/, p l a t i n u m / 4 / , a r s e n i c / 5 / , and i n some c a s e s t h e i n c r e a s e c a n b e of seve- r a l o r d e r s of magnitude. I t a ~ p e a r e d i n t e - r e s t i n g t o s e e i f t h e r e i s a s o l u b i l i t y li- m i t of t h e d o p a n t a f t e r such a t r e a t m e n t . T h e r e f o r e , we p r e p a r e d h e a v i l y d o ~ e d s a m ~ l e s of s i l i c o n by vacuum d e p o s i t i o n of t h e do- p a n t followed by ruby l a s e r a n n e a l i n g /6,7/. The c o n c e n t r a t i o n o f d o p a n t i n t r o d u c e d i n s u b s t i t u t i o n a l s i t e and t h u s t h e s o l u b i l i t y were measured u s i n g R u t h e r f o r d B a c k s c a t t e - r i n g S p e c t r o m e t r y (RBS). T h e r e f o r e , o u r ex- p e r i m e n t s were r e s t r i c t e d t o s h a l l o w d o p a n t s h e a v i e r t h a n s i l i c o n ; Sb, Ga, B i , I n . F o r a l l e l e m e n t s a s o l u b i l i t y l i m i t CmR indepen- d e n t on t h e l a s e r e n e r g y was found. The va- l u e s o b t a i n e d were compared t o t h e maximum s o l u b i l i t i e s found by o t h e r a u t h o r s u s i n g d i f f e r e n t doping methods and l a s e r t r e a t - - ments i n o r d e r t o s e e i f t h e s o l u b i l i t y li-

m i t found i s r e a l l y i n d e p e n d e n t o f t h e s e pa- r a m e t e r s . F i n a l l y t h e y have been p l o t t e d ag- a i n s t s e v e r a l p a r a m e t e r s . L i k e f o r t h e e q u i - l i b r i u m s o l u b i l i t y l i m i t a s i m p l e c o r r e l a - t i o n w i t h t h e e q u i l i b r i u m d i s t r i b u t i r o n

*

VJork performed under COPIES c o n t r a c t .

c o e f f i c i e n t was found

.

1 . Experimental c o n d i t i o n s . - 1.1. Samples.- *-wples used i n t h i s work were 300 pm t h i c k < I l l > o r i e n t e d s l i c e s o f s i l i c o n c u t from a boron o r ~ h o s p h o r u s doped C z o c h r a l s k i c r y s t a l ( 1 . 5

-

3Q.cm r e s i s t i v i t y ) . The sam-

ples were c h e m i c a l l y n o l i s h e d w i t h a w h i t e e t c h . Before d e p o s i t i o n of t h e d o p a n t t h e y were r i n s e d i n H F t o remove t h e s u r f a c e oxyde

.

The e v a p o r a t i o n chamber 1:ms numped down t o a p r e s s u r e below 1 , O x t o r r . The d o p a n t was e v a p o r a t e d by J o u l e e f f e c t . The f i l m t h i c k n e s s was monitored by a q u a r t z and t h e r a t e of d e p o s i t i o n was m a i n t a i n e d 0 below 2 A / s i n o r d e r t o avoid p r e s s u r e r i s e and sample h e a t i n g . The d e p o s i t e d t h i c k n e s s a s measured by t h e monitor ranged between

0

20 and 200 A . F o r t h e t h i c k e s t samples t h e s e v a l u e s were confirmed by e l e c t r o m e c h a n i c a l measurements performed w i t h a T a l y s t e p ap- p a r a t u s . The amount of d o p a n t d e ~ o s i t e d was measured i n a l l c a s e s by RBS and t h e v a l u e s

o b t a i n e d were g e n e r a l l y i n good agreement w i t h t h e i n d i c a t i o n s of t h e m o n i t o r .

1 . 2 L a s e r t r e a t m e n t . - The d o p a n t covered specimens were i r r a d i a t e d u s i n g t h e ampli- f i e d monomode o u t p u t of a p u l s e d r u b y l a s e r w i t h e n e r g y d e n s i t i e s i n t h e r a n g e 1.1 t o 2 ~ / c m ~ . The p u l s e d u r a t i o n t i m e was e q u a l t o

20 n s . The l a s e r s p o t was n e a r l y uniform o v e r i t s d i a m e t e r ( - Ian) and u s u a l l y one s i n g l e p u l s e was u s e d . A f t e r i r r a d i a t i o n t h e specimens were e t c h e d w i t h a p p r g p r i a t e

C4-42 JOURNAL DE PHYSIQUE m i x t u r e s i n o r d e r t o remove t h e d o p a n t which d i d n o t d i f f u s e i n t o t h e c r y s t a l . 1.3 D e t e r m i n a t i o n of d o p a n t c o n c e n t r a t i o n . - The c o n c e n t r a t i o n p r o f i l e s of t o t a l and i n t e r s t i t i a l d o p a n t was determined by RBS i n r a n d m and c h a n n e l i n g c o n d i t i o n s /8/. A 11

2 MeV ~ e + i o n beam ( g = 2mm) was used and t h e e n e r g y of t h e b a c k s c a t t e r e d p a r t i - c l e s was measured w i t h a c o o l e d s u r f a c e b a r - r i e r . T h i s arrangement allowed an e q u i v a l e n t

0

d e p t h r e s o l u t i o n of 200 A f o r S i . Some mea- surements were a l s o made by s e c o n d a r y I o n PIass S p e c t r o m e t r y (SItlS) which gave e s s e n - t i a l l y t h e same p r o f i l e s a s RBS.

2 . R e s u l t s . - F i g u r e 1 shov~s ty p i c a l s p e c t r a 0

o b t a i n e d f o r a d e ~ o s i t i o n of 4 0 A of a n t i - mony and one i r r a d i a t i o n w i t h a 2 ~ / c m ~ ~ 1 s . e .

Fig.1.- RBS spectra in channeling and random condi- tions of a Sb doped sample.

S i n c e t h e s u b s t i t u t i o n a l f r a c t i o n i s n e a r l y t h e same a l o n g a l l a x e s /7/ we can e s t i m a t e t h e s o l u b i l i t y from t h e d i f f e r e n c e s between t h e RBS S p e c t r a u n d e r random and c h a n n e l i n g c o n d i t i o n s . On f i g u r e 2 we have r e p o r t e d t h e maximum c o n c e n t r a t i o n o f d o p a n t a s w e l l a s t h e rnaxi- mum s o l u b i l i t y measured f o r v a r i o u s l a s e r e n e r g i e s and d e p o s i t e d t h i c l r n e s s o f a n t i - mony. I t a p p e a r s c l e a r l y t h a t t h e s o l u b i l i - t y r e a c h e s a l i m i t i n a l l c a s e s which i s n e a r l y i n d e p e n d e n t on t h e l a s e r e n e r g y and on t h e d e p o s i t e d t h i c k n e s s . The mezn v e l u e Zound i s 1.03 x 1021cm3. T h i s s o l u b i l i t y i s much h i g h e r t h e n t h e known e c p i l i b r i u m s o l u - b i l i t y l i m i t vrhic:~ i s 6 x 1 0 l ~ c m - ~ / 9 / . I I I I I I I 0 1 2 3 L 5 DEWSITED AMOUNT OF Sb (1016 = m i 2 )

Fig.2.- Variation of the solubility of antimony as a function of the deposited amount of Sb.

S i m i l a r e x p e r i m e n t s were performed w i t h o t h e r d o p a n t s . T a b l e I summarizes t h e r e s u - l t s o b t a i n e d a s w e l l a s t h e e q u i l i b r i u m li-

m i t a t T = 1200°C.

TABLE I

t h e r e s u l t s obtained by t h e two methods i s good. This i n d i c a t e s t h a t t h e s o l u b i l i t y li-

m i t i s n o t dependent on t h e way t h e dopant i s i n t r o d u c e d . TABLEAU I1 I The q u e s t i o n t o know i f t h e s o l u b i l i t y l i m i t depends on t h e l a s e r t r e a t m e n t i s more d i E f i c u l t t o answer, s i n c e no s o l u b i l i t y li- m i t measurements have been done w i t h o t h e r

l a s e r s o r e- beams.

However, t h e same high s u b s t i t u t i o n a l i t y has been r e p o r t e d f o r ruby l a s e r and Cln1

e l e c t r o n beam i r r a d i a t i o n of A s implanted Si/lO/. F u r t h e r m o r e ~ s i m i l a r r e s u l t s have been r e p o r t e d f o r CW Argon

Laser

and CWe-

h a m ' / l l / . Thus, although t h i s has t o be ca- r e f u l l y v e r i f i e d , i t appears t h a t t h e same s o l u b i l i t y can be expected f o r a l l l a s e r and e l e c t r o n beams used.

3ur experiments 1 x 1 0 ~ ~ c m - 3 9 x 1320 3 x 1 0 2 0 " 1 . 5 x 1 o 2 0 " of t h e s o l u b i l i t y l i m i t a g a i n s t Otiler Wara-. meters. One of t h e chosen parameters was t h e e q u i l i b r i u m d i s t x i b u t i o n c o e f f i c i e n t s i n c e F i s c h l e r /12/ h a s observed t h a t i n e q u i l i - brium c o n d i t i o n s , b o t h f o r s i l i c o n and ger- manium and f o r most i m p u r i t i e s t h e e x p e r i - mental maximum molar s o l u b i l i t y xm i s about one t e n t h o f t h e d i s t r i b u t i o n c o e f z i c i e n t ko a t t h e m e l t i n g p o i n t . Some b a s i s f o r t h i s dependence from thermodynamical considera- t i o n s h a s been given by S t a t z /13/. For i m - p u r i t i e s i n s i l i c o n t h e e x ~ e c t e d s o l i d so- l u b i l i t y should be White r e s u l t s /5/ 6 x 1021cm-3 1 . 3 x 1021

"

4.5 x 1 o 2 0

"

4 x 1020 1 x 1 0 2 0 It On f i g u r e 3 we have p o t t e d o u r v a l u e s of

Cma (and a l s o t h e maximum s o l u b i l i t i e s found by o t h e r a u t h o r s f o r a l a s e r p r o c e s s ) a g a i n s t ko. I t appears t h a t a simole c o r r e l a t i o n e x i s t s between Cma and ko.

'ma

= (8.6 x 1 0 ~ ~ ) k0.51cm-3. 0 I t should be n o t i c e d t h a t t h i s e q u a t i o n h o l d s I o r both group I11 and V elements and f o r ko v a l u e s r a n g i n g over about f o u r o r - d e r s of magnitude.

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s o l u b i l i t y limits have been a l s o

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C4-44 JOURNAL DE PHYSIQUE

c o r r e l a t e d t o t h e t e t r a e d r a l r a d i u s (Fig.4)

.

of t h e s i l i c o n m a t r i x can c o n v e r t from li-

Like f o r e q u i l i b r i u m c o n d i t i o n s two d i f f e - quid t o s o l i d w i t h o u t long d i s t a n c e motion, r e n t dependences a r e obtained f o r a c c e p t o r whereas t h e e x c e s s of dopant would have t o and donor i m p u r i t i e s . d i f f u s e over long d i s t a n c e s i n o r d e r t o be

f r o z e n i n t h e r a p i d l y moving i n t e r f a c e . Thus t h e s o l u b i l i t y of t h e dopant should depend on i t s m o b i l i t y a t t h e i n t e r f a c e and on t h e r e c r y s t a l l i z a t i o n v e l o c i t y . S t r i c t l y speaking t h e r e s u l t s p r e s e n t e d h e r e should t h e r e f o r e be v a l i d o n l y f o r ruby l a s e r s . Furthermore t h e f a c t t h a t t h e s o l u b i l i t y i s independent on t h e l a s e r energy can be e x p l a i n e d by c o n s i - d e r i n g t h a t t h e i n t e r f a c e v e l o c i t y i s n e a r l y independent on t h e l a s e r energy i n t h e range used h e r e /18/. F i n a l l y t h e e x i s t e n c e of a simple c o r r e l a t i o n between

Cml and ko f o r t h e dopants may be due t o t h e f a c t t h a t they have n e a r l y t h e same m o b i l i t y a t t h e i n t e r f a c e . This r e l a t i o n does probably n o t hold f o r a l l i m p u r i t i e s .

f

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4 . Conclusion.- This work demonstrates t h e

e x i s t e n c e of a s o l u b i l i t y l i m i t of dopants

0.8 1 1.2 1.1 1.6. i n ruby ' l a s e r t r e a t e d s i l i c o n . This l i m i t

TETRAEDRAL RADIUS (A) _{i s n e a r l y independent of doping c o n d i t i o n s}

qo1g

Fig.4.- variation of the solubi1it:y limit at thermal and l a s e r energy. A simple r e l a t i o n s h i p

equilibrium and after laser treatment as a function _{has been found between t h e s o l u b i l i t y li-}

of tetraedral radius.

n i t and t h e e q u i l i b r i u m d i s t r i b u t i o n coef-

\ ' , AL .\

-

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Laser treatment \

re--- Thermal equiLibr~um \\

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3 . Discussion.- Although t h e mecanism of l a - _{f i c i e n t .}

s e r annealing i s n o t y e t f i n a l l y e s t a b l i - _{I t allows t o p r e d i c t t h e s o l u b i l i t y li-}

-

shed /14/ we can t r y t o i n t e r p r e t our r e s u - _{m i t of any dopant and a l s o it may h e l p t o} Its u s i n g t h e thermal ( " m e l t i n g n ) model _{understand t h e fundamental mechanism of} /15,16/. _{t h e i n t e r a c t i o n of l a s e r w i t h s i l i c o n .}

T h i s model assumes t h a t t h e energy of

t h e i n c i d e n t photons i s converted i n t o h e a t References

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r a c t i o n s

and L a s e r P r o c e s s i n g , Boston

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