LASER AND ELECTRON BEAM ENHANCED CRYSTALLIZATION OF Si AND Ge

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LASER AND ELECTRON BEAM ENHANCED CRYSTALLIZATION OF Si AND Ge

R. Andrew, M. Wautelet

To cite this version:

R. Andrew, M. Wautelet. LASER AND ELECTRON BEAM ENHANCED CRYSTALLIZA- TION OF Si AND Ge. Journal de Physique Colloques, 1983, 44 (C5), pp.C5-171-C5-174.

�10.1051/jphyscol:1983526�. �jpa-00223111�

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JOURNAL DE PHYSIQUE

Colloque C5, suppl6ment au nOIO, Tome 44, octobre 1983 page C5-171

LASER AND ELECTRON BEAM ENHANCED CRYSTALLIZATION OF Si AND Ge

R. Andrew and M. Wautelet

IRIS Mons, Faculte's des Sciences, Universite' de Z'Etat, Av. Maistriau 23, 7000 Mons, Belgium

RSsumd - On montre que l e taux de n u c l 6 a t i o n e t l a v i t e s s e de c r o i s s a n c e c r i s - t a l l i n e s o n t t r s s fortement a c c r u s sous i r r a d i a t i o n s l a s e r e t S l e c t r o n i q u e . Ceci e s t i n t e r p r S t b comme r s s u l t a n t d'une s o m e d ' e f f e t s thermiques e t a t h e r - miques, q u i modifient l e s paramstres de l a d i f f u s i o n d e s v e c t e u r s de l a t r a n s - formation amorphe-cristal, 2 s a v o i r l e s l i a i s o n s pendantes.

A b s t r a c t - ^Iti s shown t h a t l a s e r and e l e c t r o n beam i r r a d i a t i o n of amorphous Ge and S i f i l m s r e s u l t s i n much l a r g e r n u c l e a t i o n r a t e and growth v e l o c i t y than under thermal annealing c o n d i t i o n s . This i s i n t e r p r e t e d a s due t o a com- b i n a t i o n of thermal plus non-thermal mechanisms which modify t h e parameters involved i n t h e d i f f u s i o n of t h e v e c t o r of t h e amorphous t o c r y s t a l t r a n s f o r - mation, i . e . t h e dangling bonds.

INTRODUCTION

We s t u d y h e r e l a s e r and e l e c t r o n beam enhanced amorphous t o c r y s t a l l i n e t r a n s i t i o n s i n which no i n t e r m e d i a t e m e l t i n g i s involved. Under t h e s e c o n d i t i o n s c r y s t a l growth v e l o c i t i e s can be many o r d e r s of magnitude g r e a t e r t h a n i n conventional thermal annea- l i n g . Moreover, s i n c e t h e enhancement of growth r a t e i s n o t accompanied t o t h e same degree by a corresponding i n c r e a s e i n n u c l e a t i o n r a t e , t h e dimensions of t h e c r y s t a l s produced a r e t y p i c a l l y g r e a t e r than would o t h e r w i s e be t h e c a s e .

Whatever t h e c r y s t a l l i z a t i o n p r o c e s s , however, it presumes a s t a g e of n u c l e a t i o n o r formation of c r y s t a l s e e d s which n e c e s s a r i l y precedes t h e i r growth, and n u c l e a t i o n r a t e s , i . e . s i t e s cm3 s e c , have been measured f o r both S i and Ge by many a u t h o r s . I t i s t h e purpose of t h i s paper t o d e s c r i b e some v e r y simple l a s e r and e l e c t r o n beam annealing experiments from which we can deduce n u c l e a t i o n r a t e s which a r e s e v e r a l o r d e r s of magnitude g r e a t e r t h a n any v a l u e s ever r e p o r t e d f o r thermal a n n e a l i n g , and which a r e t h u s l e s s than wholly c o n s i s t e n t w i t h a p u r e l y thermal model of such beam induced e f f e c t s .

We show t h a t a combination of thermal and non-thermal e f f e c t s i s r e q u i r e d t o i n t e r - p r e t t h e d a t a and advance a model where t h e r o l e of t h e dangling bonds i n t h e amor- p h o u s - c r y s t a l l i n e t r a n s i t i o n i s taken i n t o account. The observed enhancement of t h e n u c l e a t i o n p r o c e s s i s l i n k e d t o v a r i a t i o n s of bond l e n g t h and a n g l e s around t h e D.B.

which a r e caused by e l e c t r o n i c e x c i t a t i o n of t h e l a t t e r , l e a d i n g t o a n i n c r e a s e i n t h e i r entropy of m i g r a t i o n .

EXPERIMENTAL Film p r e p a r a t i o n .

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1000 t h i c k Ge and 2400 A t h i c k S i f i l m s were condensed onto f r e s h l y cleaved NaCl c r y s t a l s maintained around room temperature, by thermal (Ge) o r E-gun ( S i ) evapora- t i o n of hligh p u r i t y Ge o r S i i n 10-6 - 10-8 t o r r vacuum. Films were f l o a t e d o f f onto Cu TEM g r i d s w i t h 80y h o l e s i z e .

Laser i r r a d i a t i o n .

Exposure of t h e f i l m s t o pulsed l a s e r i r r a d i a t i o n above a c e r t a i n energy t h r e s h o l d r e s u l t s i n t h e appearance of c r y s t a l l i z e d zones. The form of t h e zones, which have been observed by many a u t h o r s , s u g g e s t r a p i d outward growth from a c e n t r a l p o i n t . Since h e r e we wish t o c o n s i d e r only t h e n u c l e a t i o n p r o c e s s we can s e t a lower l i m i t t o t h e n u c l e a t i o n r a t e by simple counting t h e number of c r y s t a l l i z e d zones w i t h i n

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983526

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C5-172 JOURNAL DE PHYSIQUE

a g i v e n f i l m volume and i n t r o d u c i n g t h e p u l s e d u r a t i o n o r , i n o r d e r t o c o n s i d e r t h e r - mal e f f e c t s , t h e time f o r which t h e s e l a t t e r w i l l p e r s i s t . Using both chopped ~ r + and p u l s e d dye l a s e r s we have measured t h e t h r e s h o l d energy f o r t h e f o r m a t i o n of c r y s t a l l i z e d z o n e s a s a f u n c t i o n o f l a s e r p u l s e d u r a t i o n . F o r p u l s e d u r a t i o n small compa- r e d t o t h e time over which s i g n i f i c a n t thermal l o s s e s can occur t h e t h r e s h o l d energy i s c o n s t a n t , whereas f o r long p u l s e d u r a t i o n thermal e q u i l i b r i u m i s e s t a b l i s h e d d u r i n g t h e l a s e r p u l s e and t h e t h r e s h o l d power i s c o n s t a n t . The t r a n s i t i o n b e t - ween t h e s e two regimes i s shown t o be - ²^x10-5 s f o r Ge and 4 x 10-5 f o r S i , i n good agreement with h e a t flow c a l c u l a t i o n which assume t h e Cu g r i d b a r s t o a c t a s an e f f i c i e n t h e a t s i n k .

The maximum temperature a t t a i n e d can be c a l c u l a t e d r a t h e r p r e c i s e l y f o r t h e c o n s t a n t energy regime by measuring t h e t h r e s h o l d energy c o r r e c t e d f o r r e f l e c t i o n t r a n s m i s s i o n , and i n t r o d u c i n g t h e s p e c i f i c h e a t . We f i n d Tmax i n t h e range 400 - 500" C f o r both Ge and S i , a temperature which w i l l b e maintained f o r approximately t h e t r a n s i t i o n times noted above.

The minimum n u c l e a t i o n r a t e i n t h i s regime can a l s o be e a s i l y c a l c u l a t e d by assuming j u s t one ( g e n e r a l l y t h e r e a r e 2-5) c r y s t a l l i z e d zone p e r g r i d square t o be produced i n t h i s time. We o b t a i n R = 1014 - 1015 cm-3 s e c , which should be compared t o t h e r - mal ( s t e a d y s t a t e v a l u e s near 400°C of R = 105 - cm-3 s-1 f o r S i [ 1] o r

R = 107 s-I f o r Ge [ 2 ] o r with t h e maximum of R v s T i . e . R = 1011 cmA3 s-l n e a r 800°C f o r S i [ 31 .

It i s e v i d e n t t h a t while t h e r e i s a thermal e f f e c t p r e s e n t , o t h e r mechanisms f o r t h e observed n u c l e a t i o n r a t e enhancement must a l s o be c o n s i d e r e d .

Ele_c_tr_on-b_eam-

Grid mounted specimens of Ge 350 were looded i n t o t h e t r a n s m i s s i o n e l e c t r o n micros- cope. I n i t i a l l y amorphous, and s t a b l e under normal viewing c o n d i t i o n s , such specimens could be r e a d i l v c r y s t a l l i z e d by b r i e f l y boosting t h e beam power t o a much h i g h e r l e v e l . The c r y s t a l l i t e s s o formed were t y p i c a l l y 200 d i a ( r a t h e r l a r g e r f o r a more gradual i n c r e a s e i n beam power towards t h r e s h o l d ) , could be formed i n a time d i f f i c u l t t o e s t i m a t e p r e c i s e l y b u t <<ls, and were a p p a r e n t l y formed i n a uniform f a s h i o n w i t h i n t h e i r r a d i a t e d a r e a r a t h e r t h a n by growth of a m i c r o s c r y s t a l l i z e d zone i n t o t h e amorphous m a t e r i a l . The whole c r y s t a l l i z e d zone had e x a c t l y t h e geometry and s i z e of t h e beam spot, i e normally c i r c u l a r , even when performed a t t h e very edge of t h e f i l m near t o a g r i d b a r . Though t h e r e w i l l be a s i g n i f i c a n t h e a t i n g e f f e c t under t h e beam s p o t t h e s e o b s e r v a t i o n s do n o t suggest a simple thermal e x p l a n a t i o n and, a s b e f o r e , we may r e a d i l y c a l c u l a t e R i n t h i s c a s e >_ 1017 cm-3 s-1.

DISCUSSION

The experiments d e s c r i b e d show t h a t , phenomenologically speaking a t l e a s t , t h e nuclea- t i o n r a t e s which can be obtained i n beam i r r a d i a t e d amorphous Ge and S i simply do n o t f i t t h e a v a i l a b l e d a t a f o r t h e r m a l l y induced n u c l e a t i o n . We may n o t e h e r e t h a t f a c - t o r s such a s t h e h e a t of c r y s t a l l i z a t i o n which can produce explosive-type c r y s t a l l i - z a t i o n i n Ge and o t h e r m a t e r i a l s should n o t i n t e r v e n e h e r e , s i n c e we a r e concerned only with t h e n u c l e a t i o n event coming b e f o r e , b u t evidenced by, c r y s t a l growth.

I n o r d e r t o i n t e r p r e t t h e s e r e s u l t s we d e v i a t e a l i t t l e way i n t o t h e fundamental n a t u r e of t h e n u c l e a t i o n e v e n t .

The amorphous-crystalline t r a n s i t i o n , when i t t a k e s p l a c e i n t h e s o l i d phase, i m p l i e s t h e r e l o c a t i o n of some of t h e c o v a l e n t bonds, and t h i s l a t t e r most e a s i l y occurs v i a t h e i n t e r m e d i a r y of a m i g r a t i n g dangling bond. The b a s i c s t e p h e r e i s t o break a bond nearby and t o t i e up t h e o r i g i n a l d a n g l i n g bond w i t h one h a l f of t h e broken bond, whose remaining h a l f now becomes t h e d . b . , but i n a new p o s i t i o n . I t i s n o t d i f f i - c u l t t o s e e t h a t t h e m a t e r i a l can be r e s t r u c t u r e d i n t h i s way but i t i s r a t h e r l e s s easy t o s e e why we should g e t a c r y s t a l a s r e s u l t , a t l e a s t i n terms of bond r e l o c a - t i o n . P a r t of t h e problem i s t h e l a c k of a good d e s c r i p t i o n of t h e s t r u c t u r e surrounding a d . b . , and indeed t h e l a c k of a t o l e r a b l y f i n i t e d e s c r i p t i o n of t h e whole amorphous s t r u c t u r e i t s e l f . I n a model t o b e d e s c r i b e d i n more d e t a i l elsewhere we have found i t u s e f u l t o i n t r o d u c e t h e concept of t o p o l o g i c a l l y "good" and "bad" r e g i o n s of c r y s t a l . The former we d e f i n e a s any assembly of atoms which a r e bonded t o g e t h e r i n a manner t o p o l o g i c a l l y e q u i v a l e n t t o normal good c r y s t a l ( i . e . t h e assembly could, w i t h no i n t e r n a l bond r e l o c a t i o n , b e i n c o r p o r a t e d p e r f e c t l y i n t o a s u i t a b l y s i z e d h o l e i n a p e r f e c t c r y s t a l ) , even though t h e atom c o r e p o s i t i o n s may n o t correspond a t a l l t o t h o s e i n a c r y s t a l owing t o bond s t r e t c h i n g , bending and t w i s t i n g . Note t h a t t h e a d d i t i o n of one wrongly connected atom t o our r e g i o n of "good" c r y s t a l

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r e n d e r s t h e whole l o t "bad", and t h a t t h e r e f o r e t h e c l a s s i f i c a t i o n of a r e g i o n of s t r u c t u r e depends very much on where we make t h e c u t s ; any r e g i o n of "bad" w i l l in- c l u d e s m a l l e r sub-regions of "good". We p o s t u l a t e t h a t t h e l i m i t s t o bond deforma- t i o n r e s t r i c t t h e s i z e of ''bad" c r y s t a l t h a t can be b u i l t without i n t r o d u c i n g d . b P s , and we d e s c r i b e t h e amorphous s t r u c t u r e a s c o n s i s t i n g of interwoven r e g i o n s of "good"

and "bad" c r y s t a l p l u s d . b g s . Now i t i s n o t d i f f i c u l t t o show t h a t with t h e s e d e f i - n i t i o n s t h e f o l l o w i n g a r e consequences :

a ) a l l d . b g s a r e connected t o "bad1' c r y s t a l i n some d i r e c t i o n , and t o "good" i n some o t h e r .

b) It i s i m p o s s i b l e t o embed a s i n g l e d.b. and i t s a s s o c i a t e d "bad" c r y s t a l i n t o

"good" c r y s t a l without i n t r o d u c i n g a n o t h e r ( o r any odd number o f ) d .b . on t h e j o i n i n g s u r f ace.

i . e .

c ) A l l d . b l s a r e connected t o o t h e r s by t h e i r "bad" c r y s t a l .

We can a l s o show t h a t most d . b l s a r e a s s o c i a t e d w i t h a vacancy o r , t o be more p r e c i s e , w i t h h a l f a vacancy, e x c e p t t h a t t h e i n t e r a t o m i c space r e p r e s e n t e d by t h i s h a l f vacancy i s n o t c o n c e n t r a t e d a t t h e d.b. i t s e l f but i s spread o u t over t h e a d j a c e n t "bad"

c r y s t a l . I n o t h e r words, t h e "bad" c r y s t a l a d j o i n i n g a d.b. i s l e s s dense t h a n t h e

"good". Note t h a t t h i s i s a p a r t of t h e reason why t h e a-phase i s o v e r a l l l e s s dense t h a n t h e c r y s t a l l i n e one, b u t it i s n o t he whole s t o r y s i n c e , depending on t h e method of p r e p a r a t i o n , t h e amorphous v e r s i o n of v a c a n c i e s , u n a s s o c i a t e d w i t h d . b g s , o r even small v o i d s tend t o be i n c o r p o r a t e d i n t o t h e s t r u c t u r e . We now suppose, q u i t e s i m - p l y , t h a t t h e p r e f e r r e d (though not n e c e s s a r i l y r e s u l t a n t ) d i r e c t i o n of motion of a m i g r a t i n g d.b. w i l l be down t h e d e n s i t y g r a d i e n t .

Since we know t h a t t h e j o i n between "good" and "bad" c r y s t a l i n t h i s r e g i o n must con- t a i n a ( i . e , *) d.b. t h e n it n e c e s s a r i l y f o l l o w s t h a t when t h e d.b. moves i n t o i t s '

"bad" c r y s t a l t h e n i t must l e a v e "good" c r y s t a l behind i t , and a s t h e volume of "good"

c r y s t a l i n c r e a s e s so t h e bonds s t a r t t o r e l a x towards t h e i r normal l e n g t h s and o r i e n - t a t i o n s , t h e atoms t a k e up p o s i t i o n s corresponding t o t h o s e i n t h e c r y s t a l , and t h e

"good" c r y s t a l becomes c r y s t a l . We can e a s i l y envisage a c r i t i c a l s i z e of c r y s - t a l l i n e zone c r e a t e d i n t h i s way which becomes e f f e c t i v e l y i n v i o l a t e t o d i s r u p t i o n by o t h e r wandering d . b 7 s and a s such corresponds t o t h e a c t b i r t h , o r n u c l e a t i o n . The n u c l e a t i o n e v e n t , then, i s dependent on t h e a b i l i t y of t h e DBs t o m i g r a t e , which i n t u r n depends on t h e l o c a l bond d i s t o r t i o n and a v a i l a b l e pathways f o r e n e r g e t i c a l l y

~ o s s i b l e bond r e l o c a t i o n . Thermodynamically, t h e m i g r a t i o n p r o b a b i l i t y i s r e l a t e d w i t h t h e energy of m i g r a t i o n : E, = H,,, - T.Sm, where H, and S,,, a r e t h e corresponding e n t h a l p y and e n t r o p y , r e s p e c t i v e l y . Hm i s r e l a t e d t o t h e energy n e c e s s a r y t o break bonds d u r i n g t h e m i g r a t i o n e v e n t [ 4 ] , while Sm i s a f u n c t i o n of t h e i n t e r a t o m i c f o r c e c o n s t a n t s between t h e d e f e c t atom and i t s n e a r e s t neighbours [ 5 ] . I t t u r n s o u t t h a t a s o f t e n i n g of t h e f o r c e c o n s t a n t s (by e l o n g a t i o n of t h e bonds) l e a d s t o an i n c r e a s e of S,.

O p t i c a l e x c i t a t i o n of amorphous S i and Ge f i l m s l e a d s t o t h e appearance of v e r y long- l i v e d m e t a s t a b l e s t a t e s [ 6 ] , due t o t h e presence of o p t i c a l l y e x c i t e d DB s t a t e s i n t h e form e i t h e r of p a i r s of charged DB's (DB' - DB-) ( s e l f - t r a p p e d e x c i t o n s [ 71 ), o r dehybridized sp2 - pz DBs [ 8 ] . The c o n f i g u r a t i o n a l p r o p e r t i e s of t h e s e e x c i t e d s t a t e s a r e d i f f e r e n t from t h o s e of t h e fundamental s t a t e s , so t h a t t h e i r m i g r a t i o n p r o p e r t i e s a r e d i f f e r e n t [ 4 ] . I n p a r t i c u l a r , DB- s i t e s and sp2 - pz ones have l a r g e r m i g r a t i o n e n t r o p i e s t h a n DB and DB+ s i t e s , i . e . t h e i r m i g r a t i o n i s enhanced [ 4 ] . Another mechanism i s a l s o p o s s i b l e , r e l a t e d t o t h e recombination process a t t h e DB s i t e . J u s t a f t e r o p t i c a l e x c i t a t i o n , t h e DB s i t e and i t s surrounding decays r a p i d l y v i a emission of phonons. The same i s t r u e during t h e d e - e x c i t a t i o n of t h e m e t a s t a b l e s t a t e t o t h e fundamental one. I f t h e e m i t t e d phonons remain l o c a l i z e d n e a r t h e DB s i t e f o r a s u f f i c i e n t l y l o n g time (due, f o r i n s t a n c e , t o t h e l a c k of p e r i o d i c i t y ) , t h e v i b r a t i o n a l energy of t h e system would be i n c r e a s e d . Moreover, some t r a n s - f e r of movement i s p o s s i b l e t o some v i b r a t i o n a l modes p a r a l l e l t o t h e m i g r a t i o n d i r e c - t i o n , s i n c e any s h i f t of t h e atom p e r p e n d i c u l a r l y t o t h e DB o r b i t a l e f f e c t s t h e geometry of a l l bonds around t h e c a v i t y and, t h e r e f o r e , modifies s l i g h t l y t h e o r b i t a l o v e r l a p s and, then, t h e e n e r g y of t h e bonds. These c o n d i t i o n s a r e known t o reduce Em by a s u b s t a n t i a l amount, ER [ 9 ] . This c a l l e d recombination-enhanced d i f f u s i o n . A numerical e v a l u a t i o n of ER 1s u n f o r t u n a t e l y n o t meaningful, due t o t h e number of unknown parameters i n t h e a-phase ( f o r c e c o n s t a n t s , bond a n g l e s and l e n g t h i n t h e c a v i t y , phonon spectrum, . . ^.).

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CONCLUSIONS

A l t o g e t h e r , we have shown t h a t l a s e r and e l e c t r o n i r r a d i a t i o n enhance t h e p r o c e s s of n u c l e a t i o n i n S i and Ge f i l m s . T h i s i s proposed t o b e due t o a combination of t h e r - mal and non-thermal e f f e c t s a c t i n g on t h e v e c t o r s of t h e amorphous t o c r y s t a l t r a n - s i t i o n , i . e . t h e d a n g l i n g bonds.

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