EFFECT OF FAST SOLIDIFICATION ON IMPURITY TRAPPING AND AMORPHOUS FORMATION IN Si

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EFFECT OF FAST SOLIDIFICATION ON IMPURITY TRAPPING AND AMORPHOUS FORMATION IN Si

P. Baeri

To cite this version:

P. Baeri. EFFECT OF FAST SOLIDIFICATION ON IMPURITY TRAPPING AND AMOR- PHOUS FORMATION IN Si. Journal de Physique Colloques, 1983, 44 (C5), pp.C5-157-C5-170.

�10.1051/jphyscol:1983525�. �jpa-00223106�

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JOURNAL DE PHYSIQUE

Colloque C5, supplkment au nOIO, Tome 44, octobre 1983 page C5-157

EFFECT OF FAST S O L I D I F I C A T I O N ON IMPURITY TRAPPING AND AMORPHOUS FORMATION I N S i

P. Baeri

I s t i t u t o Dipartimentale d i F i s i c a , U n i v e r s i t d d i Catania, Corso I t a Z i u 57, I 95129 Catunia, I t a l y

Resume - Les aspects p r i n c i p a u x du modele thermique de l l " a n n e a l i n g " des semi- conducteurs par l a s e r o n t PtP e t u d i e s en regardant en p a r t i c u l i e r l e s parame- t r e s concernant l a v i t e s s e de l ' i n t e r f a c e l i q u i d e - s o l i d e pendant l a phase de s o l i d i f i c a t i o n . On a mis en evidence l e s c o n d i t i o n s experimentales sous l e s - q u e l l e s on peut c o n t r a l e r c e t t e v i t e s s e e n t r e quelque cm/s e t 100 m/s. En p a r t i c u l i e r on montre, comment l a segregation e t l e " t r a p p i n g " des impuretes dependent de c e t t e v i t e s s e 3 p a r t i r de mesures de c a n a l i s a t i o n e t backscatte- r i n g sur des c i b l e s de s i l i c i u m implantees e t i r r a d i e e s par l a s e r . On a mis en evidence l ' e f f e t de l a d i f f u s i v i t e dans l a phase s o l i d e sur l e c o e f f i c i e n t de segregation 2 des v i t e s s e s t r e s elevees, en conparant l e t r a p p i n g dans l e S i e t dans Gede l a m&me impurete. Les r e s u l t a t s concernant l ' a m o r p h i s a t i o n du Si par i r r a d i a t i o n breve (picoseconde) au l a s e r o n t e t @ mis en r e l a t i o n avec l e s c a l c u l s de v i t e s s e de l ' i n t e r f a c e l i q u i d e - s o l i d e . Dans c e t t e comparaison on a employe un modele p l u s complique dans l e q u e l on a i n t r o d u i t l ' e l a r g i s s e - ment de l a d i s t r i b u t i o n de l ' e n e r g i e deposee due 3 l a d i f f u s i o n des e l e c t r o n s - lacunes

.

A b s t r a c t

-

The main features o f t h e thermal model f o r t h e semiconductor l a - s e r annealing are reviewed w i t h t h e main emphasis t o t h e parameters which a f - f e c t t h e l i q u i d s o l i d i n t e r f a c e v e l o c i t y d u r i n g t h e s o l i d f i c a t i o n . The e x p e r i - mental c o n d i t i o n s c o n t r o l l i n g t h i s v e l o c i t y are p o i n t e d o u t and several exam- p l e s which cover a broad range o f values, from few cm/s up t o 100 m/s a r e shown.

The v e l o c i t y dependence o f t h e i l l l p u r i t y segregation and t r a p p i n g i s shown as e x p e r i m e n t a l l y determined by channeling b a c k s c a t t e r i n g measurements o f S i im- p l a n t e d and i r r a d i a t e d samples. The t r a p p i n g behaviour a t t h e l i m i t i n g condi- t i o n o f very h i g h and very low speed i s discussed a l s o i n con n e c t i o n w i t h t h e c e l l u l a r s t r u c t u r e a r i s i n g from i n t e r f a c e i n s t a b i l i t y . The r o l e of t h e s o l i d phase d i f f u s i v i t y on t h e segregation c o e f f i c i e n t a t very h i g h speed i s a l s o evidenced by means o f several examples i n c l udinq t h e comparison between t r a p p i n g i n S i and Ge o f t h e same i m p u r i t y . Results on t h e formation of amorphous s i l i c o n l a y e r s by very f a s t s o l i d i f i c a t i o n a f t e r picosecond l a s e r i r r a - d i a t i o n a r e a l s o presented i n comparison w i t h c a l c u l a t i o n o f t h e s o l i d - l i q u i d i n t e r f a c e v e l o c i t y . I n t h i s case a more complex model, which take i n t o account t h e smearing o u t of t h e deposited energy d i s t r i b u t i o n by f r e e c a r r i e r genera- t i o n and d i f f u s i o n i s required.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983525

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JOURNAL DE PHYSIQUE

INTRODUCTION

Most o f t h e phenomenq r e l a t i n g t h e l a s e r annealing o f S i l i c c n are s t r o n g l y a f f e c t e d by t h e r a t e o f c o o l i n g which f o l l o w s t h e m e l t i n g o f a t h i n l a y e r o f t h e sample a f - t e r t h e l a s e r i r r a d i a t i o n . Among t h i s phenomena, t h e t r a p p i n g o f i m p u r i t i e s a t c o n c e n t r a t i o n h i g h e r than t h e maximum e q u i l i b r i u m s o l u b i l i t y , t h e s u r f a c e segregation of these i m p u r i t i e s , t h e f o r m a t i o n o f p r e c i p i t a t e s i n c e l l u l a r s t r u c t u r e , t h e f o r - mation o f metastable compounds and t h e formation o f a metastable amorphous phase.

B a s i c a l l y , from t h e p o i n t o f view o f simple thermal model /1,2,3/, a t h i n l a y e r o f t h e sample i s heated and melted f o l l o w i n g t h e i r r a d i a t i o n o f a l a s e r p u l s e o f s u i - t a b l e energy d e n s i t y and t i m e duration; a f t e r t h e s w i t c h o f f o f t h e l a s e r i t r e s o l i - d i f y w i t h a p l a n a r s o l i d - l i q u i d i n t e r f a c e which moves a t a given speed v toward t h e sample s u r f a c e (see f i g . 1 )

SOLIDIFICATION VELOCITY

From t h e energy conservation i t f o l l o w s t h a t

Fig.1. Schematic r e p r e s e n t a t i o n o f t h e

where J i s t h e heat f l u x a t t h e interface,Ki t h e thermal c o n d u c t i v i t y a t t h e i n t e r f a c e ,

g /

i s t h e temperature g r a d i e n t a t t h e i n t e r f a c e i n t h e s o l i d , H i s t h e m e l t i n g dx i

PULSE ON PULSE oFF

XTL

enthalpy, p i s t h e mass d e n s i t y and v t h e l i q u i d - s o l i d i n t e r f a c e v e l o c i t y . Another term t a k i n g i n t o account t h e temperature g r a d i e n t i n t h e l i q u i d l a y e r should be i n c l u d e d i n equation ( 1 ) b u t i t can be neglected due t o t h e much lower value

m e l t i n g and s o l i f i c a t i o n o f a surface l a y e r induced by a l a s e r pulse.

- dT i n the l i q u i d i n most p r a c t i c a l cases /4/.

dx

D e t a i l e d n u ~ e r i c a l c a l c u l a t i o n s which i n c l u d e a l s o t h e temperature g r s d i e n t i n t h e l i q u i d as w e l l as t h e temperature and sample s t r u c t u r e dependence o f a l l t h e t h e r - mal and o p t i c a l parameters o f t h e m a t e r i a l , can be e a s i l y performed and g i v e as r e s u l t t h e k i n e t i c o f s o l i d - l i q u i d i n t e r f a c e b o t h d u r i n g t h e m e l t i n g and t h e s o l i - d i f i c a t i o n . But,as a f i r s t approximation,from e q . ( l ) one f i n d t h a t t h e main parameter which a f f e c t t h e value o f v i s t h e heat f l u x t h r o u g h t t h e temperature g r a d i e n t i n t h e s o l i d . T h i s l a s t , i n t v r n , can be approximated, e s p e c i a l l y f o r i r r a d i a t i o n w i t h energy d e n s i t y very close t o t h e m e l t i n g t h r e ~ h o l d ~ w i t h the temperature gra- d i e n t b u i l d e d up b ~ / t h e h e a t i n g pulse. T h i s temperature p r o f i l e i s detertnined by t h e c o m p e t i t i o n between t h e r a t e o f t h e energy d e p o s i t i o n and t h e r a t e o f heat d i f f u s i o n . At t h e end o f t h e pulse, whose d u r a t i o n i s T, t h e l a s e r energy i s deposited w i t h i n .

the absorption l e n g t h a-I o r t h e heat d i f f u s i o n l e n g t h JZD-r, D being t h e (average) thermal d i f f u s i v i t y , d e p e n d i n g on which o f these two parameters i s l a r g e r .

The s o l i d i f i c a t i o n v e l o c i t y i s thendetermined by t h e f o l l o w i n g parameters f o r d i f f e - r e n t regimes :

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1) pulse duration: when J% >a-' i .e. long pulse o r highly absorbed l a s e r wavelength. In t h i s case the temperature gradierlt i s about T,/J% and so the velo- c i t y increases with the inverse of the square root of T .

U.) absorption c o e f f i c i e n t : when ~5 < C 1 i .e. short pulse or s l i g h t l y absorbed l a s e r wavelength In t h i s case t h e temperature gradient i s about Tm-ole and so the ve-

l o c i t y i s almost independent on T b u t increases with a.

m)

energy density: The cases I ) and U) a r e valid near the melting threshold, increa sing the energy density above t h i s value generally one obtain a reduction of the velocity which in most cases i s inversally proportional t o the energy density.

R) substrate temperature: In the s i l i c o n case t h e heat diffusion c o e f f i c i e n t D i s strongly dependent on temperature so t h e velocity can be controlled by varying the substrate temperature. This e f f e c t althought was the f i r s t one used the change the R-s i n t e r f a c e velocity, leads t o lower variation than the other three.

The l i g h t absorption c o e f f i c i n t f o r Si czn range from l o 6 cm-' i n the case of U . V .

i r r a d i a t i o n t o 10 cm-l i n the case of Neodimium 1.06 um wavelength, the time duration of a l a s e r pulse can be varied from some hundred us in f r e e running mode down t o some ps. Due t o t h i s great range of variation of the parameterja and T , using d i f f e r e n t available l a s e r s , t h e heat flux J a t t h e i n t e r f a c e can be varied by three order of magnitude from 10' t o 10' watt/crnz leading t o a velocity ranging from a10-I up t o a 100 m/s

TABLE I

Fig.2. Melt f r o n t position vs. time f o r several d i f f e r e n t i r r a d i a t i o n s in Si as described in t a b l e I . As zero time has been taken the s t a r t of the s o l i d i f i c s t i o n in a l l cases.

In f i g . 2 and t a b l e I i s shown in the same scale t h e melt f r o n t position as function of the time f o r four d i f f e r e n t cases. Case a i s a f r e e running Neodimium l a s e r on amorphous s i l i c o n ; case b i s a Q-switched Nd l a s e r on c r y s t a l l i n e s i l i c o n pre-heated a t 300°C with an absorption length ranging from 100 urn ( a t 300°C) t o lpm near the melting pointand a diffusion length about 1.5 urn; case c i s the same of case b but on amorphous s i l i c o n with an absorption length of about 0.5 um;

case d i s an,ultraviolet (ruby 2" harmonic) 2.5 ns pulse with absorption length of few hundred A and a diffusion length of about 0.5 urn.

In the csse a , c and d the velocity i s mainly determined by the time duration of the pulse in case b t h e velocity i s determined by the l i g h t absorption length which in average i s about 10 urn. In the present example theoaverage velocity during s o l i f i - cation of a layer whose thickness ranges from 1000 A t o 1 urn i s 2 cm/s, 80 cm/s, 4 m/s, 15 m/s in t h e case a,b,c and d respectively.

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C5-160 JOURNAL DE PHYSIQUE

Some limitations,however,to t h e p o s s i b i l i t y o f o b t a i n i n g extremely h i g h v e l o c i t i e s may a r i s e from t h e f o l l o w i n g c o n s i d e r a t i o n s . For several tens o f m/s t h e tem- p e r a t u r e g r a d i e n t should be some 10' K/cm which can be reached o n l y i f t h e energy i s deposited and converted i n t o heat w i t h i n l o w 5 cm from t h e sample surface. Even i f both t h e d i f f u s i o n l e n g t h and adsorption l e n g t h are s m a l l e r than t h i s value, one may ask i f t h e energy o f t h e l a s e r p u l s e which i s p r i m a r l y s t o r e d i n t h e e l e c t r o n i c e x c i t a t i o n i s converted i n t o heat before t h e generated f r e e c a r r i e r s d i f f u s e more than lo-' cm. Assuming an ambipolar d i f f u s i o n c o e f f i c i e n t f o r f r e c a r r i e r s o f about 20 cm2/s /5/ t h e corresponding average t i m e needed f o r t h e energy s t o r e d i n e l e c t r o - n i c e x c i t a t i o n t o be converted i n t o heat i s about 10 ps.

To e x p l a i n w i t h an example t h i s p o i n t , i n Fig.3 i s represented t h e R-s i n t e r f a c e

0

0 5 10 15

Time (ns) N d 1 ~ 0 . 5 3 pm on Si-XTL

20 ps pulse

Fig.3. M e l t f r o n t p o s i t i o n f o r a 20 ps green p u l s e on c r y s t a l S i computed assuming the simple thermal model (T,~=O) (0.25 J/cm2)and i n c l u d i n g f r e e c a r r i e r generation and d i f f u s i o n w i t h a r e l a x a t i o n time o f 10 ps.

(0.7 J/cm2)

p o s i t i o n versus t i m e f o r a Neodimium 2" harmonic 20 ps p u l s e on S i c r y s t a l . The a b s o r p t i o n c o e f f i c i e n t f o r t h i s wavelength ranges from %lo4 a t room temperature t o 10' a t t h e m e l t i n g p o i n t so t h e absorption l e n g t h h$s an average value o f some 10-5cm. The heat d i f f u s i o n l e n g t h i s o n l y few hundred A. The corresponding s o l i d i - f i c a t i o n v e l o c i t y c a l c u l a t e d by t h e simple thermal model i s about 15 m/s (see curve l a b e l l e d TeoZO). F o l l o w i n g t h e computer model o f L i e t o l a and Gibson/6/ one can t a k e i n t o account a l s o t h e d i f f u s i o n o f f r e e c a r r i e r s b e f o r e t h e r m a l i z a t i o n . Assuming-a -r r e l a x a t i o n time dependent on f r e e c a r r i e r c o n c e n t r a t i o n as i n ref./6/,the curve l a

b e l l e d -ce0=lOps i s t h e c a l c u l a t e d m e l t f r o n t p o s i t i o n vs t i m e w i t h T = T ~ ~ ( ~ + ~ / ~ ~ ~ where p i s t h e c a r r i e r d e n s i t y and p,=

-

2x102' c a r r i e r s / c m 3 . The r e s u l t i n g v e l o c i -

t y i s slowed down a t about 5 m/s.

However,recent experiments /7/ have shown t h a t t h e r e l a x a t i o n t i m e i s l e s s than 1 ps and i n most casesthe simple thermal model can be used w i t h confidence. Moreover r e - cent t i m e r e s o l v e d transmictance measurement/8/ w i t h a probe beam a t 1.64 pm on a sample i r r a d i a t e d w i t h anul t r a v i o l e t picosecond l a s e r , c r e a t e d t h e c o n d i t i o n s f o r an extremely h i g h regrowth v e l o c i t y . Eoth heat d i f f u s i o n l e n g t h and a b s o r p t i o n l e n g t h a r e o f t h e o r d e r o f few hundred A and thus t h e v e l o c i t y computed on t h e b a s i s o f t h e simple thermal model should be o f t h e o r d e r o f hundred m/s.The experimental m e l t f r o n t k i n e t i c s e x t r a c t e d from t h e t r a n s m i s s i v i t y data show; f o r a 32 mJ/cm2 pulse, a t t h e beginning o f t h e s o l i d i f i c a t i o n , a v e l o c i t y value v e r y c l o s e t o 100 m/s although t h e average v e l o c i t y r e s u l t s about 30 m/s. T h i s data, again support t h e i d e a t h a t t h e average t i m e needed f o r t h e energy t o be converted from e l e c t r o n e x c i t a t i o n t o l a t t i c e v i b r a t i o n i s l e s s than 1 ps.

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Standard heat flow calculation describes t h e s o l i d i f i c a t i o n process a s occurring a t the equilibrium melting point. Moreover t o l e t t h e s o l i d i f i c a t i o n occur with a given r a t e , an interface undercooling must be included in order t o provide t h e required driving force. If t h i s undercooling i s taken into account the resulting calculated s o l i d i f i c a t i o n velocity will be smaller because the heat flux a t the in- t e r f a c e has t o balance t h e l a t e n t heat a r i s i n g from sol i d i f i c a t i o r ~ & has t o provide t h e required undercooling.

Accurate estimates f o r Si undercool ing do not e x i s t on t h i s s o l i d i f i c a t i o n velocity range. Heat flow calculation can be done t o include t h i s undercooling term /4/, b u t in order t o simplify them, a l i n e a r relationship between v and t h e required undercooling AT must be assumed: AT=Bv.

Following d i f f e r e n t authors/9,10/ a value of 5-15 K sm-I can be chosen f o r the para meter 13.

As an example in fig.4 i s reported a computed melt f r o n t positionas a function of

Using pulse l a s e r heating of semiconductor samples a new regime of crystal growth became open f o r experimental investigation. The process occurr f a r from thermodynamic equilibrium and a l l t h e physicalmodel of f a s t crystal growth can be rigorously tested also because of the large range of s o l i f i c a t i o n velocity t h a t can be explored as discussed i n the f i r s t part of t h i s work.

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The f i r s t e f f e c t studied as function of the R-s i n t e r f a c e velocity was the impuri- t i e s surface accumulation. After the complete s o l i d i f i c a t i o n of the i r r a d i a t e d sam ple implanted with some dopant, part of the impurity i s rejected on t h e sample surface.

According t o t h e c l a s s i c a l theory of normal freezing t h i s has been explained a s f o l fows: whilst the melt f r o n t comstoward the surface, the impurity p r o f i l e i s broa- dened by the diffusion in t h e l i q u i d , a fraction of dopant i s rejected by the advancing i n t e r f a c e i n the phase characterized by the higher s o l u b i l i t y , i . e . in the l i q u i d , and then accumulatesat t h e sample surface. A t the i n t e r f a c e t h e concentra- tions of impurities i n liquid c R a n d in the s o l i d cs will stay in a given r a t i o

C

K 2 K i s t h e equilibrium d i s t r i b u t i o n c o e f f i c i e n t defined as the r a t i o of the

0 CI' 0

concentration in the solid and t h e liquid phase determined by t h e phase diagram a t a fixed temperature close t o t h e melting point. B u t f a r from thermodynamic equilibrium, a t the very high speed reached i n t h e case of l a s e r i r r a d i a t i o n , the process of surface rejection of impurity can be described s t i l l in term of normal freezing but using instead of KO an interface d i s t r i b u t i o n c o e f f i c i e n t K1(v) which i s velocity dependent.

Usually KO i s much l e s s than unity and the very common r e s u l t a t high velocity reached by l a s e r annealing i s t h a t K' i s several order of magnitude greater than KO and depend strongly on the velocity. Fixed a l l the other parameters,the amount of impurity rejected on t h e sample surface increaseswith decreasing t h e i n t e r f a c e segregation c o e f f i c i e n t K ' . The amount of impurity which i s retained i n t h e s o l i d i s located into the l a t t i c e s i t e s of the s i l i c o n crystal a t a concentration f a r above the s o l i d s o l u b i l i t y l i m i t thus forming a supersaturated s o l i d solution.

To determine K1(v) one has t o compare t h e measured dopant p r o f i l e a f t e r l a s e r i r r a - diation with the r e s u l t s of a model calculation f o r dopant redistribution /3,12/

using K' as a f i t t i n g parameter. The k i n e t i c of t h e melt frontremain fixed by the i r r a d i a t i o n condition throught t h e heat flow calculations. As an example,in Fig.5 /13/ a r e shown two d i s t r i b u t i o n obtained a f t e r i r r a d i a t i o n of a 120 keV B i implanted Si (111) sample i r r a d i a t e d w i t h ruby l a s e r a t 30 ns, 1.5 J/cm2(up) and 100 ns, 2.5 J/cm2 (down).

In t h i s case the velocity of t h e liquid-solid i n t e r f a c e has been varied from 2.5 t o 0.8 m/s and the whole Bi p r o f i l e can be f i t t e d following t h e previously discussed model with an i n t e r f a c i a l d i s t r i b u t i o n coefficient K1=0.25 and K1=0.08 in the f i r s t and i n second case respectively.

The equilibrium d i s t r i b u t i o n c o e f f i c i e n t f o r Bismuth in s i l i c o n i s b=7 lo-' and the large value of K' w i t h respect t o KO as well a s i t s strong variation with t h e velocity, ref1 e c t s t h e high non equilibrium nature of l a s e r induced epitaxial growth As second example, f i g . 6 , shows depth p r o f i l e s of 150 keV In implanted in Si <loo>

and i r r a d i a t e d with 5ns ruby l a s e r pulse of d i f f e r e n t energy density. With increa- sing energy density one observes a deeper penetration of In t a i l s and a more pro- nuncied surface accumulation. A thicker molten layer i s produced and t h e s o l i d i f i - cation velocity decreases. The penetration of In t a i l s i s then explained i n term of diffusion in t h e liquid. The difference of surface accumulation must be due t o the dependence of K ' on s o l i d i f i c a t i o n velocity.

The calculated v e l o c i t i e s a r e 8.5, 5.5 and 3.5 m/s and the corresponding K ' values a r e 0.45,0.2 and 0.07 f o r i r r a d i a t i o n a t 0.44, 0.66 and 0.86 J / G ~ ' respectively.

As a l a s t example of d i f f e r e n t r e d i s t r i b u t i o n s due t o d i f f e r e n t s o l i d i f i c a t i o n r a t e s

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Fig.5. Bi depth profiles f o r ruby l a s e r i r r a d i a t i o n of Bi implanted (111) S i . Smooth curve are i n t e r f a c e segregation c o e f f i c i e n t ( K t ) f i t s .

Fia.6. Depth p r o f i l e s of 150 keV In implanted i n S i a f t e r i r r a d i a t i o n with 5 ns ruby l a s e r pulse a t several energy density

.

Fig.7 reports t h e depth profiles of Te implanted in Si and i r r a d i a t e d with a Nd l a s e r pulse a t 1.06 um. The lower part shows t h e p r o f i l e s a f t e r i r r a d i a t i o n with 2.0 ~/cm' of the implanted sample. The amount of surface accumulation i s s 20%, the calculated s o l i d i f i c a t i o n velocity i s about 3 m/s. If the implanted sample i s f i r s t l y thermally annealed so t h a t a s i n g l e crystal i s obtained before the Nd l a s e r i r r a d i a t i o n , the Te p r o f i l e reported i n the upper part of t h e figure,shows a consi- derably l a r g e r surface accumulation.

The small absorption c o e f f i c i e n t of the 1.06 pm wavelength in Si crystal gives r i s e

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Fig.8. Measured i n t e r f a c i a l segregation c o e f f i c i e n t f o r B i and I n i n Si(100) as f u n c t i o n o f t h e s o l i d i f i c a t i o n v e l o c i t y .

DEPTH (pm)

Fig.7. The depth p r o f i l e o f a Si(100) sample implanted w i t h 150 keV Te a f t e r i r r a d i a t i o n w i t h 30 ns Nd l a s e r p u l s e f o r a s i n g l e c r y s t a l (up) and w i t h 1500 A amorphous l a y e r (down).

t o a small temperature g r a d i e n t and t h e c a l c u l a t e d s o l i d i f i c a t i o n v e l o c i t y i s about 0.8 m/s. The corresponding K' f i t t i n g value a r e 20.5 and 0.03 f o r t h e h i g h e r and t h e lower v e l o c i t y r e s p e c t i v e l y . The experimental dependence o f K' on v has been measured f o r some dopants and r e s u l t s f o r In and B i a r e r e p o r t e d i n Fig.8.

I n p a r t i c u l a r f o r b o t h i m p u r i t i e s K' seems t o s a t u r a t e a t a value <1. From these and o t h e r /14/ a v a i l a b l e l i t e r a t u r e data one can argue t h a t K1 depends on v e l o c i t y as s c h e m a t i c a l l y i l l u s t r a t e d i n Fig.9. A c r i t i c a l v a l u e separates t h e low v e l o c i - t y range, where Ki%Ko and t h e h i g h v e l o c i t y range where Ki>YKo and i t s value s a t u r a - t e s a t a K' v e r y c l o s e t o u n i t y . The determination o f vcrit and K1 a r e c r u c i a l p o i n t s which any t h e o r y has t o deal w i t h . s a t

A t t h e present any determination o f K' through t h e i m p u r i t y p r o f i l e has been done by means o f t h e b a c k s c a t t e r i n g technique. Due t o t h e l i m i t a t i o n i n t h e s e n s i t i v i t y t o v e r y low i m p u r i t y c o n c e n t r a t i o n r e t a i n e d i n t h e sample and i n t h e depth r e s o l u t i o n of t h e a n a l y s i s , t h e determination o f v e r y low value o f K'(<10-2) o r v e r y c l o s e t o t h e u n i t y (>0.6) i s prevented and o t h e r techniques w i t h h i g h e r s e n s i t i v i t y and b e t - t e r depth r e s o l u t i o n should be used f o r these purposes.

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Fig.10. C e l l s t r u c t u r e a r i s i n g a f t e r

V c r ~ t

r I L . -

^log^{v ( a}^u ^sampleruby l a s e r implanted i r r a d i a t i o n w i t h I n i n (Courtesy a S i (100) o f A.G.Cullisl

Fig.9.Schematic r e p r e s e n t a t i o n o f t h e v e l o c i t y dependence o f t h e i n t e r f a c i a l d i s t r i b u t i o n c o e f f i c i e n t .

At t h e present i t i s s t i l l an open question i f t h e K' value r e a l l y s a t u r a t e t o a value l e s s t h e u n i t y o r t o 1 i . e . i f a l l t h e implanted i m p u r i t i e s can be r e t a i n e d i n t h e sample a f t e r l a s e r annealing. On t h e o t h e r hand, l a c k o f measurementrpre- vent t o understand t h e exact behaviour o f K' a t v e r y low v e l o c i t y i . e . when K ' s t a r t s t o d e v i a t e from Ko,because f o r K1<O.O1 more than 95% o f implanted irnpuritie, = a r e r e j e c t e d o u t o f t h e sample and i t i s hard, by b a c k s c a t t e r i n u measurement,to d i s t i n - guish between d i f f e r e n t values o f K' through t h e few percent o f r e t a i n e d dopant.

The increase o f s o l i d i f i c a t i o n v e l o c i t y gives r i s e s t o an i n c r e a s e o f dopant concen- t r a t i o n r e t a i n e d i n s u b s t i t u t i o n a l l a t t i c e s i t e s . However t h e maximum s o l u b i l i t y seems t o be l i m i t e d by t h r e e mechanism: i n t e r f a c e s t a b i l i t y , mechanical s t r a i n and thermodynamic l i m i t . The f i r s t one i s t h e most common l i m i t t o s u p e r s a t u r a t i o n and we w e l l discuss i t b r i e f l y . The c o n d i t i o n under which a f l a t R-s can proceed

t h e t e r m on l e f t i s t h e temperature g r a d i e n t i n t h e l i q u i d c l o s e t o t h e i n t e r f a c e and t h a t on t h e r i g h t i s t h e g r a d i e n t o f s o l i d i f i c a t i o n temperature t a k i n g i n t o account t h e dopant c o n c e n t r a t i o n i n t h e l i q u i d near t h e i n t e r f a c e and t h e l i q u i d u s curve o f t h e phase diagram. Experiments /16/ have been performed i n I n implanted i n S i by changing e i t h e r t h e t o t a l f l u e n c e o r t h e R-s i n t e r f a c e v e t o c i t y . It has been shown t h a t c e l l s t r u c t u r e s o f I n p r e c i p a t e s (Fig.10) a r e formed when t h e c o n d i t i o n given by ( 2 ) i s reached. The s i z e o f t h e c e l l can be a l s o described w i t h i n t h e c l a r s i c a l model.

ORGR

versus v /17/. The step shape o f t h i s I n Fig.11 i s r e p o r t e d t h e f u n c t i o n - m c 1-K'

-

curve i s mainly determined by t h e v e l o z i t y dependence o f K' assuming t h a t K' satura- t e s a t a value l e s s than u n i t y a t l a r g e v e l o c i t i e s . This s t e p l i k e curve i n t e r s e c t s t h e slope 1 l i n e i n t h r e e p o i n t s evidencing f o u r regicns:region I where r e j e c t e d im- p u r i t i e s have a t i m e l o n g enough t o d i f f u s e i n t h e l i q u i d , g e n e r a t i n g a small s o l i d i - f i c a t i o n temperature g r a d i e n t and then a f l a t i n t e r f a c e can propagate. I n c r e a s i n g t h e s o l i d i f i c a t i o n v e l o c i t y t h e r e j e c t e d i m o u r i t i e s form a narrow r e g i o n w i t h l a r g e

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4

Fig.11. Schematic r e p r e s e n t a t i o n o f the f o u r d i f f e r e n t v e l o c i t y regions a l t e r n a - t i n g t h e c o n d i t i o n s f o r t h e growth o f a XTL CELLS

- ' "

^x ^L ^S good s i n g l e c r y s t a l t o those f o r c o n s t i - t u t i o n a l supercooling t o occur.

/

log v

c o n c e n t r a t i o n g r a d i e n t f u l f i l 1 in g c o n d i t i o n s f o r c o n s t i t u t i o n a l supercooling and a c e l l s t r u c t u r e s t a r t s t o develope. The t r a n s i t i o n from r e g i o n I t o r e g i o n I1 occurs a t low v e l o c i t i e s and has been i n v e s t i g a t e d i n d e t a i l s i n c l a s s i c a l metal l u r - g i c a l works. The c e l l s i z e i s o f t h e o r d e r o f t h e coherence l e n g t h i .e. t0-'+10-~cm.

A f u r t h e r increase i n t h e v e l o c i t y changes d r a s t i c a l l y t h e i n t e r f a c i a l segregation c o e f f i c i e n t thus r e j e c t i n g i n t h e l i q u i d a l e s s amount o f i m p u r i t i e s . I n r e g i o n 111 then a f l a t i n t e r f a c e can propagate and due t o t h e l a r g e value o f K1 a l a r g e amount o f i m p u r i t i e s w i l l be i n c o r p o r a t e d i n t h e growing s o l i d , p h a s e . I f t h e v e l o c i t y i s increased t o h i g h e r values and K t < l , t h e c o n c e n t r a t i o n g r a d i e n t s t a r t s t o b u i l d up again i n a narrower coherence l e n g t h . C o n s t i t u t i o n supercooling can then occur again and t h e developed c e l l s t r u c t u r e w i l l be r a t h e r small. T.E.M. observation i n I n implanted S i i n d i c a t e d a c e l l s i z e o f t h e order o f 10-100 nm.

Several t h e o r i e s /18,19,20,21/ have been developed t o describe non e q u i l i b r i u m dopant segregation a t a f a s t moving t - s i n t e r f a c e . It i s f a r from t h e purpose o f t h i s work t o review them and t h e reader i s r e f e r r e d t o t h e o r i g i n a l papers o r t o r e c e n t review /17/.

The common f e a t u r e t o these models i s a q u a n t i t y Di d e f i n e d as t h e i m p u r i t y d i f f u - s i o n c o e f f i c i e n t i n t h e near i n t e r f a c e region. I t has been shown /22/ t h a t assuming D ~ = ( D ~ D & ) ) being Ds and DQ t h e d i f f u s i o n c o e f f i c i e n t s i n t h e s o l i d and l i q u i d phase r e s p e c t i v e l y , b o t h a t t h e m e l t i n g p o i n t , i t i s p o s s i b l e t o p r e d i c t i f a given impu- r i t y w i l l be trapped o r r e j e c t e d on t h e surface. I f v =Di/X,v,being X t h e i n t e r f a - ce thickness,(i.e. few monolayers) t h e i m p u r i t y i s fas-fer than t h e i n t e r f a c e and w i l l accumulate i n t h e h i g h s o l u b i l i t y phase, i .e. t h e l i q u i d , t h e f i n a l r e s u l t w i l l be a K' value very c l o s e t o t h e e q u i l i b r i u m KO value. I f vc=Di/Xsv then t h e impu- r i t y w i l l be frozen by t h e f a s t advancing s o l i d phase and K' w i l l reach a value v e r y c l o s e t o u n i t y

.

F o l l o w i n g t h i s simple i d e a i t i s e a s i l y understood t h a t f a s t d i f f u s e r s ( l i k e Zn,Fe,Cu) i n s i l i c o n which have a d i f f u s i o n c o e f f i c i e n t i n t h e s o l i d phase a t temperature c l o - se t o t h e m e l t i n g p o i n t o f t h e o r d e r o f 10-640-4 cm2/s cannot be trapped because t h e i r c r i t i c a l v e l o c i t y v w i l l be o f t h e o r d e r o f 10-100 m/s, w h i l s t slow d i f f u - sers (1 i ke B i ,In ,Te) w h i t 6 a d i f f u s i o n c o e f f i c i e n t ' o f t h e o r d e r o f 1 0 - ~ ~ + 1 0 - ~ 0 ~ m 2 / ~ , have a c r i t i c a l v e l o c i t y v o f t h e o r d e r o f I t 1 0 cm/s,so we a r e e a s i l y i n c o n d i t i o n o f having a K t value much g r e a t e r then t h e e q u i l i b r i u m C K value.

0

The d i f f u s i o n mechanism i n S i and Ge o f almost a l l t h e dopants i s t h e same, t h e S i

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and Ge c r y s t a l a r e c h a r a c t e r i z e d by t h e same l a t t i c e s t r u c t u r e so t h e dopant t r a p - ped i n S i w i l l be trapped i n Ge. A n o t i c e a b l e exception i s Zn which i s a f a s t i n - t e r s t i t i a l d i f f u s e r i n S i ( ~ ~ = 8 ~ 1 0 - ~ c m ' / s a t 1690K) and a slow s u b s t i t u t i o n a l d i f f u - s e r i n Ge (Ds=lO-l2 cm2/s a t 1210).

I n f i g . 1 2 /13/ i s r e p o r t e d t h e depth p r o f i l e o f Zn i n S i measured by b a c k s c a t t e r i n g technique b e f o r e and a f t e r t h e i r r a d i a t i o n o f an implanted sample by means o f a 30 ns Nd l a s e r a t 2.10 J/cm2. The pulsed annealing causes t h e accumulation o f a l l

Fig.12. Depth p r o f i l e s o f 80 keV Zn implanted i n S i b e f o r e ( 0 ) and a f t e r (0) l a s e r i r r a d i a t i o n .

Fig.13. Ge and Zn K, l i n e s induced by 2.0 MeV He+ f o r random ( a ) and a l i g n e d ( 0 ) incidence on t h e implanted and l a - s e r i r r a d i a t e d c r y s t a l .

ENERGY (keV)

t h e implanted Zn a t t h e sample s u r f a c e and t h e r e d i s t r i b u t j o n can be described by any K1<lO-'. Q u i t e d i f f e r e n t r e s u l t s a r e obtained i n Ge a f t e r i r r a d i a t i o n a t 1.15 J/cmZ. The Zn depth p r o f i l e cannot be measured by b a c k s c a t t e r i n g technique due

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t o t h e presence o f t h e heavier Ge s u b s t r a t e . Fig.13 shows t h e X r a y K l i n e s o f both Ge and Zn induced by 2.0 MeV ~ e + f o r a l i g n e d and random incidence? The y i e l d are 8% and 40% f o r Ge and Zn r e s p e c t i v e l y . T h i s i n d i c a t e a good r e c r y s t a l l i z a t i o n o f t h e Ge and t h a t a p a r t o f o f Zn i s n o t l o c a t e d and probably i t i s r e i e r + o A t h e surface, b u t about 60% o f t h e implanted dose has been r e t a i n e d i n s u b s t i t u t i o n a l l a t t i c e s i t e s o f t h e r e c r y s t a l l i z e d Ge.

For t h e c a l c u l a t e d v e l o c i t y which i s about 3 m/s i n b o t h S i and Ge case,one can e s t i - mate a segregation c o e f f i c i e n t K1d.3+0.5 b u t w i t h l a r g e uncertanhies. Although t h e KO value o f Zn i n S i and Ge i s t h e same (4.10-') i t s i n t e r f a c e segregation coef- f i c i e n t i n t h e m/s, d i f f e r n o t i c e a b l e i n t h e two substrates according t o t h e d i f f e - r e n t i s o l i d phase d i f f u s i v i t i e s .

SII-ICON AMORPHIZATION

Although f o r f a s t d i f f u s e r s measurements o f K t a t much h i g h e r v e l c o i t y would be of g r e a t i n t e r e s t , t h e y cannot be performed, It has been shown i n f a c t ; t h a t f o r so- l i f i c a t i o n v e l o c i t i e s approaching 15 m/s amorphous m a t e r i a l i s obtained. Under t h i s c o n d i t i o n t h e usual d e f i n i t i o n o f K' loses i t s s i g n i f i c a n c e .

Amorphous l a y e r have been produced i n f a c t by i r r a d i a t i o n o f c r y s t a l s i l i c o n e i t h e r u s i n g extremely s h o r t p u l s e o r v e r y t h i n absorption length,both c o n d i t i o n s l e a d i n g t o f a s t v e l o c i t y (see f o r example f i g . 3 and 4 ) .

I n a r e c e n t work, amorphitation was observed and t h e s o l i d f i c a t i o n v e l o c i t y was a l - so measured by means o f t r a n s i e n t conductance measurement /11/ an S i l i c o n s i n g l e c r y s t a l ( 1 0 0 ) and (111) o r i e n t e d i r r a d i a t e by 2.5 ns p u l s e a t 0.347 um wavelength.

The m e l t f r o n t k i n e t i c s c l o s e l y agree w i t h t h e one computed on t h e b a s i s of t h e thermal model, t h e v e l o c i t y does n o t show any appreciable r e d u c t i o n i n p r o x i m i t y o f t h e maximum m e l t f r o n t p e n e t r a t i o n as showed i n t h e dashed curve o f Fig.4,sup- p o r t i n g t h e idea t h a t t h e undercooling should be much l e s s than t h e 15 K/ms used i n t h e previous c a l c u l a t i o n s .

I n f i g u r e 14 t h e measured regrowth v e l o c i t y i s shown as f u n c t i o n o f t h e maximum m e l t

25

F i g . 14. Regrowth v e l o c i t y versus me1 t

2 ²⁰ depth f o r pulses U.V. i r r a d i a t i o n mea-

C E

-

sured by t r a n s i e n t conductance measure-

2 - ¹⁵ ments. S o l i d data p o i n t s i n d i c a t e t h a t

0

- 0 a

amorphous S i forms from t h e melt.

>

f 10 ( f r o m Ref.11)

e I

: 0, ⁵

1 I I I I

50 100 150 200 250

Melt depth (nm)

depth. The black dots r e f e r t o samplesin which i t has been produced theamorphouslay er. Amaximum R-s i n t e r f a c e v e l o c i t y o f 15 m/s then e x i s t s f o r t h e regrowth of a s i n g l e S i c r y s t a l from t h e m e l t i n t h e (100) d i r e c t i o n .

A even lower l i m i t i n g value has been found i n t h e case o f (111) o r i e n t e d substrate.

A maximum allowed v e l o c i t y f o r t h e c r y s t a l regrowth o f 10 m/s was found,moreover

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ENERGY (MeV)

I

the (1 11) oriented regrown crystal contains microtwins lamel l a e u n t i l t h e velocity of the R-s interface i s slowed down to 6 m/s/24/. There i s s t i l l an open question r e l a t i n g t o the amorphous layer formation. In the present cases /11,24/ the amorphous layer thickness was much l e s s then the molten thickness suggesting t h a t there a r e some kinetics limitations on t h e nucleation of t h e amorphous phase. Further informationscan be obtained from a comparison between the amorphous formation in a B i doped and i n an updoped Si crystal following t h e i r r a d i a t i o n with a ps Wd(X/2) l a s e r . The impurities i n t h e liquid should a c t as heterogeneous nucleation centers f o r the amorphous phase.

-

_,_0-

-

3 U

In fig.15 a r e showii t h e aligned spectrum f o r 2 MeV He i incident on Si (111) samples i r r a d i a t e d w i t h a 0.2 J/cm2, 20ps pulse a t A=0.53 doped with 2 ~ 1 0 ~ ~ Bi atoms/cm3 (A) and undoped. The surface peak corresponds t o a highly disorder or amorphous material. The average thickness of the disordered layer i s 45 nm f o r the doped sam ple and 25 nm f o r the undoped sample respectively. In any case a thermal annealing a t 550" f o r 30 m i n causes t h e epitaxial recrystallization of the l a s e r quenched am01 phous layer both i n the B i doped and i n the undoped sample.

SI (111) ~.ld%N/cm'

Depth(nm) i

~d 30ps h-053prn Fig .IS. 2.0 MeV He backscattering spec-

60 40 29 ?

o 20 J / c ~ ~ trum obtained from Si crystal irradiated

CONCLUSION

In conclusion the p o s s i b i l i t y of controlling within a broad range of values the quenching r a t e t h a t can be obtained by means of t h e l a s e r i r r a d i a t i o n represents a powerful tcol t o investigate in detai 1 the non equi 1 i brium thermodynamics r e i a - t i n g the phase t r a n s i t i o n s induced i n a semiconductors with o r withcut impurities.

Much have been done b u t s t i l l more work i s needed t o investigate t h e solute trapping behaviour a t very high velocity close t o the amorphization threshold as well as in the range of lower velocity when the segregation coefficient s t a r t s t o deviate from i t s equilibrium value. Some important questions r e l a t i n g the nucleation of t h e amorphous phase from the melt a r e s t i l l not understood and will be surely c l a r i f y in the r e a r feature.

Re~olut~on with 0.2 J/cm2 30 ps Nd (X/2) pulse (A)

3

-

Bi doped, ( n ) undoped

S

W_ >

0 z 0 5 -

'2 5 i

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C5-170 JOURNAL DE PHYSIQUE REFERENCES

1) P.Baeri,S.U.Campisano,G.Foti and E.Rimini, J.Appl.Phys. - 50,788(1979) 2) R.F.Wood and G.E.Giles: Phys.Rev.B 3,2923(1981)

3) P.Baeri,S.U.Campisano in "Laser annealing of semiconductors" ed. by J.W.Mayer and J.N.Poate (Academic Press, New York 1982) Chap.4

4) P.Baeri in "Laser and Electron Beam Interaction with Solids" ed by B.

.

Appleton and G.K.Ce1ler (North Holland Amsterdam 1982)p.151 5) P.J.Price, Phil .Mag. - 46,1252(1955)

6) A.Lietola and J.E.Gibbons, J.Appl.Phys. - 53,3207(1982)

7) R.Yen, J.M.Liu, M.Kurz, N.Bloemberger, Appl.Phys. A27,153(1982) -

8) P.M.Bucsbaum and J.Bokor, Phys.Rev.Lett. in press.

9) K.A.Jackson, referred as private comunication by W.L.Brown in "Laser and Electron-Beam Solid Interactions and Material Processing"

ed. by J.F.Gibbons, L.D.Mess and T.W.Sigmon (North Holland, Amsterdam 1981) p.1 10) R.F.Wood, Proceeding of MRS Boston meating 1982

1 1 ) M.O.Thompson,J.W.Mayer,A.G.Cullis,M.C.Webber, N.G.Chew and J.M.Poate, D.C.Jacobson, Phys.Rev.Lett. in press

12) C.W.Whi te,S.R.Wilson ,B.R.Appleton ,F.W.Young Jr. : J.Appl .Phys. - 51 738(1980) 13) P.Baeri,G.Foti,J.M.Poate,S.U.Campisano,A.G.Cullis~ Appl .Phys.Lett.

2

326(1980) 14) C.W.White,D.M.Zehener,S.U.Campisano,A.G.Cullis in "Surface Modification and

Alloying" ed by J.M.Poate and G.Foti (Plenum,N.Y. 1982)Chap.4 15) W.W.Mul1 ins ,R.F.Sekarka : J.App1 .Phys. - 35 444(1964)

16) A.G.Cullis,D.T.Hurle,H.C.Webber,N.G.Chew,J.M.Poate,P.Baeri and G.Foti:

Appl .Phys .Lett. - 38 642(1981)

17) S .U.Campisano , ^Appl

.

^Phys

.

^A30- ^195(1983)

18) R.F.Wood: Appl.Phys.Lett. 37,302 (1980) 19) M.J.Aziz, J.Appl.Phys. - 53 1158(1982) 20) R.N.Hal1: J.Phys.Chem.

-

57 836(1953)

21) J.C.Brice: "The growth of crystal from the melt1' (North Holland Amsterdam 1975) p.66

22) B.K.Jindall,W.A.Tiller: J.Chem. Phys. - 49 4632 (1968) 23) S.U.Campisano and P.Baeri, Appl.Phys.Lett. in press

24) A.G.Cullis, H.C.Webber,N.G.Chew,J.M.Poate and P.Baeri, Phys.Rev.Lett. - 49,219 (1982)

25) S.U.Campisano,P.Baeri,Zhang Jing-Ping,E.Rimini and A.M.Malvezzi: Appl.Phys.

Lett. in press.