DYNAMICS OF LASER ANNEALING OF α-GaAs

HAL Id: jpa-00223101

https://hal.archives-ouvertes.fr/jpa-00223101

Submitted on 1 Jan 1983

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

DYNAMICS OF LASER ANNEALING OF α-GaAs

W. Marine, J. Marfaing, P. Mathiez, F. Salvan

To cite this version:

W. Marine, J. Marfaing, P. Mathiez, F. Salvan. DYNAMICS OF LASER ANNEALING OF α-GaAs.

Journal de Physique Colloques, 1983, 44 (C5), pp.C5-123-C5-127. �10.1051/jphyscol:1983520�. �jpa-

00223101�

JOURNAL DE PHYSIQUE

Colloque C5, suppl6ment au nO1O, Tome 44, octobre 1983 page C5-123

DYNAMICS OF LASER A N N E A L I N G OF a-GaAs

W. Marine, J. Marfaing, P. Mathiez and F . Salvan

FacuZt6 des Sciences de Luminy, Dipa~tement de physique*, Case 901, 23288 Marse'iZZe Ceder 9, France

RGsum6.- Nous montrons l ' i n f l u e n c e de l a d i f f u s i o n de p o r t e u r s l i b r e s s u r l e chauffa- ge de GaAs e t mesurons l a v i t e s s e de s o l i d i f i c a t i o n .

A b s t r a c t . - We show t h e i n f l u e n c e of f r e e c a r r i e r d i f f u s i o n on t h e h e a t i n g of GaAs and perform t h e measurements of 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 .

Usually t h e r e a l time behavior of t h e l a s e r a n n e a l i n g process h a s been s t u d i e d by r e f l e c t a n c e and t r a n s m i t t a n c e measurements, b u t t h e s e measurements only g i v e t h e f u l l d u r a t i o n of t h e phase of high r e f l e c t i v i t y and a r e i n s e n s i t i v e t o t h e dynamics of t h e h e a t i n g and t h e r e s o l i d i f i c a t i o n .

I n t h i s paper we use t h e t r a n s i e n t g r a t i n g method f o r i n v e s t i g a t i n g dynamics of l a s e r annealing. We r e p o r t r e s u l t s on b o t h s e l f - d i f f r a c t i o n and CW l a s e r probe d i f f r a c t i o n measurements on t r a n s i e n t g r a t i n g s produced i n a-GaAs f i l m s under high e x c i t a t i o n power. We show t h a t t h e f r e e c a r r i e r d i f f u s i o n i s a predominant p r o c e s s i n l a s e r h e a t i n g f o r ns s c a l e and we p r e s e n t t h e f i r s t measurements of 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 of GaAs.

Experimental r e s u l t s . - I n our experiments we used a-GaAs f i l m s 0.8 and 0.5 pm t h i c k d e p o s i t e d by RF s p u t t e r i n g onto a l a s s s u b s t r a t e . The v a l u e s of t h e a b s o r p t i o n c o e f f i c i e n t ( a = 8.104 cm-l ; 4.105 cm-l f o r X = 0.63 pm) and t h e r e f r a c t i v e index (R = 4.6 ; 4.1) f o r a and c-GaAs r e s p e c t i v e l y were determined by e l l i p s o m e t r i c measurements. For e x c i t a t i o n we used a Q-switched ruby l a s e r , w i t h a p u l s e d u r a t i o n T L = 40 ns (FWHM)

.

The l a s e r probes were CW A r and He-Ne l a s e r s . The experimental arrangement was d e t a i l e d i n [ I ] .

The minimum power d e n s i t y a t which c r y s t a l l i z a t i o n occurs was observed a t W = 3.5 MW/c&. Above t h i s t h r e s h o l d c r y s t a l l i t e s 1 t o 3 p l a r g e a c c o r d i n g t o t h e e x c i t a t i o n energy were examinated with o p t i c a l microscopy and scanning e l e c t r o n microscopy. The width of c r y s t a l l i z e d zone extended about 10 pm. Laser damage i n t h e zone c e n t r e was observed a t W = 15 MW/cm2. For W > 15 MW/cm2 v a p o r i z a t i o n of GaAs occurred.

Figure l a shows t h e t y p i c a l o s c i l l o s c o p e t r a c e s of both f i r s t o r d e r s e l f - d i f f r a c t e d s i g n a l S and e x c i t a t i o n p u l s e L. One can n o t e t h a t f o r W < 7.5 M W / C ~ ~ ,

S reproduces t h e shape of t h e e x c i t a t i o n s i g n a l without any d i s c o n t i n u i t y (SP-CR).

I n t h e power range 7.5 MW/cm2 < W < 15 NWlcm2 (LP-CR), one o b s e r v e s , b e s i d e s a maximum, a d i p caused by a d i s c o n t i n u i t y i n t h e S s i g n a l . This d i p occurs a t s h o r t e r time t l when one i n c r e a s e s W.

When t h e g r a t i n g d i f f r a c t i o n was checked v i a t h e CW probe, t h e d i f f r a c t e d l i g h t p r e s e n t e d t h e time e v o l u t i o n behaviour i n d i c a t e d i n f i g u r e l b . For W < 7.5 MW/cm*, no d i p was observed. For 7.5 MW/cm2 < W < 15 MW/cm2, a d i s c o n t i n u i t y occurred a t time t 2 and under t h e same e x c i t a t i o n c o n d i t i o n s , w i t h i n t h e l i m i t s of our r e s o l u - t i o n t 2 = t i . We observed d i f f r a c t e d i n t e n s i t y a t l o n g e r times a f t e r t h e e x c i t a t i o n p u l s e and i n t h e whole range where c r y s t a l l i z a t i o n was p r e s e n t ; t h e d i f f r a c t e d , s i g n a l s t a b i l i z e d a t a c o n s t a n t v a l u e a t times of t h e o r d e r of a few 100 n s . This long time s i g n a l corresponds t o t h e p e r i o d i c a l l y amorphous-crystallized zones c r e a t e d by t h e i r r a d i a t i o n .

"ERA CNRS 373

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983520

C5-124 JOURNAL DE PHYSIQUE

Fig. 1 . ( a ) The o s c i l l o s c o p e t r a c e s of t h e s e l f - d i f f r a c t e d s i g n a l S and (b) CW l a s e r probe.- L i s t h e l a s e r p u l s e s i g n a l .

Discussion.- The amplitude t r a n s m i s s i o n f u n c t i o n of t h e amplitude-phase g r a t i n g i s given by

T(X) = exp(-a) exp[ (a+iO) cos 2n XI (1)

T(X) can b e expanded i n a F o u r i e r s e r i e s and t h e d i f f r a c t e d i n t e n s i t y i n n t h o r d e r i s t h e n w r i t t e n a s

In = (I-R) I ( t ) x .-ad

1

ⁱⁿ Jn (O+ia)

I

² (2)

where 1 ( t ) * i s t h e i n t e n s i t y of t h e probe p u l s e , Jn i s t h e n t h o r d e r B e s s e l f u n c t i o n of t h e f i r s t k i n d , 2@ = 2TAnz*/fi and 2a = ~T~AKZ*/A a r e t h e f u l l modulation depth of r e s p e c t i v e l y t h e phase and amplitude, d and Z* a r e t h e f u l l and a c t i v e t h i c k n e s s of t h e sample and

X

i s t h e l a s e r wavelength ; A i s t h e g r a t i n g p e r i o d .

I n o u r experiments, d u r i n g t h e e x c i t a t i o n p u l s e , we observe t h e s u p e r p o s i t i o n i n time of two g r a t i n g s : b e f o r e t h e d i p t < t l f r e e c a r r i e r g r a t i n g . (.FCG) and thermal g r a t i n g (TG), a f t e r t h e d i p t > t l g r a t i n g due t o appearance t o t h e l i q u i d phase (LG) and (TG).

I n a r e c e n t paper, we have shown [ l ] t h a t

i ) t h e appearance of t h e d i s c o n t i n u i t y i n t h e d i f f r a c t e d s i g n a l i s connected with a r a p i d change i n t h e o p t i c a l c o n s t a n t s , e s p e c i a l l y i n t h e r e f r a c t i v e index which i s y i e l d e d by t h e s o l i d - l i q u i d phase t r a n s i t i o n ,

i i ) t h e t r a n s i e n t g r a t i n g i n time t < t i i s a phase g r a t i n g

(41

> a ) ,

i i i ) under high e x c i t a t i o n (W > 7.5 ~ / c m 2 ) t h e process i s r e l a t e d t o a l i q u i d phase c r y s t a l l i z a t i o n , and under weak r a t e of e x c i t a t i o n (3.5 MW/cd < W < 7 . 5 MW/cm2) t h e c r y s t a l l i z a t i o n proceeds without m e l t i n g .

I n t h e l a s e r a n n e a l i n g experiments, the coupling of t h e l a s e r beam energy with t h e l a t t i c e i s d e s c r i b e d by t h e e q u a t i o n s f o r l a t t i c e temperature T1 and t h e c a r r i e r s c o n c e n t r a t i o n N

a T 1

* a

a T 1

pc(T ₁ ) --

a t

= G (N,T,,Z,t) +

- a z

[K(T ¹ )

-1 az

^{( 3 )}

where G* and G a r e r e s p e c t i v e l y t h e h e a t and t h e f r e e c a r r i e r s g e n e r a t i o n r a t e , K i s t h e thermal c o n d u c t i v i t y , p and c a r e t h e mass d e n s i t y and t h e s p e c i f i c h e a t respec- t i v e l y ; T i s t h e f r e e c a r r i e r l i f e t i m e and Da t h e ambipolar d i f f u s i o n c o e f f i c i e n t . Numerous c a l c u l a t i o n s have been performed assuming t h a t t h e l a s e r energy i s t r a n s - f e r r e d t o t h e l a t t i c e i n t h e same region i n which i t i s i n i t i a l l y absorbed [2,31.

B u t . i n t h e dense h o t - c a r r i e r s system, t h e p r o c e s s of c a r r i e r d i f f u s i o n can p l a y important r o l e s and i t l e a d s t o i n c r e a s e t h e s i z e of t h e h e a t e d volume and it can s i g n i f i c a n t l y reduce t h e h e a t r a t e .

Free c a r r i e r d i f f u s i o n : I n t h e dynamic t h e o r y of dense laser-induced plasma 141

...

Yoffa g i v e s e x p r e s s i o n f o r t h e s t e a d y s t a t e d e n s i t y of t h e f r e e c a r r i e r s . The l a t t i c e h e a t i n g r a t e i s , t h e r e f o r e , given by

x

blw

~a

z

exec-"0,

= ( ~ ) ~ e . s s

(x)

⁼ ~ ~ ( l - ~ ) a

T C

(La) -1 exp(-

x)

- (La) 2 -1

where

blw

is t h e energy of phonon emission, -re i s t h e phonon emission time due t o c o l l i s i o n s o f t h e p h o t o e x c i t e d e l e c t r o n s with l a t t i c e and L =

1 4

~ ( E / l w ) ] ~ / ~ i s t h e e f f e c t i v e d i f f u s i o n l e n g t h of t h e f r e e c a r r i e r s . Da = 2 K B T e ( ~ e ~ h / m e ~ h X + {-re)

h

*

i s t h e ambipolar d i f f u s i v i t y i n terms of e l e c t r o n ( h o l e ) mass me(%). Yoffa used t h e energy c o n s e r v a t i o n c o n d i t i o n and assumed t h a t t h e energy p e r c a r r i e r E / N , t h e temperature of e l e c t r o n Te and -re a r e weakly dependent on N. For t h e e x c i t a t i o n l e v e l s which a r e used, t h e photogenerated plasma i s degenerated and simple Boltzmann model p r e d i c t s t h a t t h e d i f f u s i o n c o e f f i c i e n t h a s a s t r o n g dependence on N.

Young e t a1.151 have t r e a t e d t h i s problem t a k i n g i n t o account manybody e f f e c t s i n t h e d e n s i t y dependence of t h e ambipolar d i f f u s i v i t y and have p r e d i c t e d enhance o f t h e d i f f u s i o n c o e f f i c i e n t . On t h e o t h e r hand, t h e d i f f u s i o n c o e f f i c i e n t i s known t o d e c r e a s e with i n c r e a s i n g l a t t i c e temperature. Likewise, t h e band gap d e c r e a s e s with i n c r e a s i n g l a t t i c e temperature again l e a d i n g t o c o n f i n e t h e plasma [61. It i s c l e a r t h a t t h i s problem i s p o t e n t i a l l y more complicated t h a n admitted by d i f f e r e n t t h e o r i e s .

I n our experiments we observe t h e appearance of t h e l i q u i d phase under high e x c i t a t i o n W > 7.5 MW/cm2 i n time t i . I n o r d e r t o go f u r t h e r i n o u r i n t e r p r e t a t i o n , we c a l c u l a t e t h e absorbed energy Eth i n t h e GaAs sample between t h e beginning of l a s e r p u l s e and time t l . We d e f i n e t h e m e l t i n g t h r e s h o l d as :

t l

Eth = (I-R)J Y ( t ) d t where To and Y ( t ) a r e t h e r a d i a t i o n i n t e n s i t y and t h e time dependence z f t h e e x c i t a t i o n p u l s e .

Experimental p o i n t s

(e)

v e r s u s t h e e x c i t a t i o n power d e n s i t y (W) a r e shown i n Fig. 2. On t h e same f i g u r e , we give t h e r e s u l t s of t h e numerical c a l c u l a t i o n s of t h e m e l t i n g t h r e s h o l d (Eth). It has been performed from e q u a t i o n (2) and t a k i n g i n t o account e q . ( 5 ) . The dashed curves a r e t h e r e s u l t s f o r d i f f e r e n t v a l u e s o f t h e para- meter aL, and t h e d o t t e d curves cor,respond t o a l i n e a r dependence of L on W i n t h e form aL = BW-l

,

where f3 i s c o n s t a n t .

i/cm2

Fig. 2. Melting t h r e s h o l d v e r s u s a.

p=q.~o-

⁷ power e x c i t a t i o n s .

b. p

=

3.6.10- 7 ⁽⁺⁾ experimental ~ o i n t s . The dashed and d o t t e d curves a r e t h e r e s u l t s of t h e numerical c a l c u l a t i o n s from eq. ( 2 ) . aL=2

- - - I - _

I n t h e c a l c u l a t i o n s we have used t h e following GaAs arameters : p = 5.31 g ~ c m - ~ ; T

-

^{1.511 K ; R = 0.1 71}

^-

^X(T)

⁼

0.425 ( T / ? J O O ) - ~ . ~ ~/cmK

I81

;

CV)

⁼

^{0.335 +}

^8.09

10-i(T-400) J/gK for T

>

400

K 191.

We n o t i c e a good agreement with t h e o r e t i c a l c a l c u l a t i o n s performed f o r L % W and t h e experimental r e s u l t s o b t a i n e d from s e l f d i f f r a c t e d measurements. These

C5-126

JOURNAL

DE PHYSIQUE

r e s u l t s c l e a r l y show t h a t under high e x c i t a t i o n s t h e f r e e c a r r i e r s d i f f u s i o n i s a predominant process i n l a s e r h e a t i n g of GaAs f o r ns s c a l e . A t high c a r r i e r s densi- t i e s e-h l i f e t i m e i s aoverned by Auger recombination

- ^. -

T = l/yN2 w i t h

*

y = 1031 cm6 s - I . The maximum f r e e c a r r i e r s c o n c e n t r a t i o n N can be o b t a i n e d from e q u a t i o n dN/dt = G

-

yN3 = 0 , where t h e g e n e r a t i o n r a t e G i s given by (I-R)W/hvZ and Z i s t h e depth where energy i s r e l e a s e d . The simple e s t i m a t i o n s under our expe- r i m e n t a l c o n d i t i o n s (W = 7.5 t o 15 MW/cm2) show t h a t t h e change of t h e power of e x c i t a t i o n by a f a c t o r of two l e a d s t o t h e change of D, i n a f a c t o r ⁵ 7.6 and we can t h u s expect t h e value % 40-50 cm2 sec-1 f o r Da under c o n c e n t r a t i o n of o r d e r

3-5 1020 cm-3.

V e l o c i t y o f s o l i d i f i c a t i o n : The t y p i c a l shape of t h e d i f f r a c t e d s i g n a l of CW l a s e r i s shown i n F i g . l b . The time between t h e beginning of t h e d i f f r a c t e d s i g n a l and Imax corresponds t o t h e h e a t i n g of sample w h i l e a t time t i a s o l i d - l i q u i d t r a n - s i t i o n o c c u r s . The decay time TL corresponds t o t h e p r o c e s s of s o l i d i f i c a t i o n and formation t o t h e p e r i o d i c a l l y amorphous-crystallized zones. I n t h i s p a r t of t h e s i g n a l , we observe a weak p e r i o d i c modulation which corresponds t o a change i n t h e o p t i c a l c o n s t a n t s a f t e r t h e t r a n s f o r m a t i o n l i q u i d t o s o l i d (analogously w i t h t h e e x i s t e n c e of t h e d i p ) and a change i n t h e c r y s t a l l i n e o p t i c a l c o n s t a n t s then bounded t o t h e c o o l i n g of t h e c r y s t a l l i t e s . A f t e r a long time (1-2 us) we observe s t a b i l i z a - t i o n of t h e s i g n a l . I t corresponds t o t h e uniform temperature d i s t r i b u t i o n i n t h e c r y s t a l l i z e d zones. The measurements of t h e decay time TL provides an a c c u r a t e measurements of t h e f u l l melt d u r a t i o n and g i v e s t h e f u l l d u r a t i o n of t h e s o l i d i f i - c a t i o n . The f i g u r e 3 shows t h e dependence of TL v e r s u s power e x c i t a t i o n . I n t h e time t > t l , i . e . a f t e r m e l t i n g we observe a s t r o n g i n c r e a s e of t h e a b s o r p t i o n c o e f f i c i e n t Aa 1 5 1 6 cm-!. It i s due t o t h e f a c t t h a t l i g h t i s completely absorbed i n a t h i c k - ness of % 200 A. I n t h i s c a s e we can c o n s i d e r TG a s pure amplitude g r a t i n g s .

From e q . ( 2 ) t h e e f f i c i e n c y of d i f f r a c t i o n i n minus f i r s t o r d e r i s d e s c r i b e d by

where I l ( a ) i s t h e modified B e s s e l f u n c t i o n of t h e f i r s t k i n d .

S a t u r a t i o n of t h e d i f f r a c t e d s i g n a l e f f i c i e n c y which we observe under t h e e x c i t a t i o n power 12.5 MW/cm2 g i v e s t h e proof t h a t complete m e l t i n g occurs i n t h e f u l l t h i c k n e s s of t h e sample. So, v i a n o r m a l i z a t i o n on t h e f u l l t h i c k n e s s of t h e sample from t h e measurements of t h e e f f i c i e n c y , we determine t h e maximum propagation of t h e m e l t i n g f r o n t i n t h e Z d i r e c t i o n , i . e . t h e t h i c k n e s s of t h e sample. The s l o p e of t h e melted depth v e r s u s time i s t h e v e l o c i t y of t h e s o l i d i f i c a t i o n . F i g u r e 4 shows t h e dependence of t h e v e l o c i t y v e r s u s t h e e x c i t a t i o n power. For t h e power range < 7.5 M W / C ~ ~ (SPCR), t h e v e l o c i t y i s r e l a t i v e l y c o n s t a n t ^% 1-1.2 m / s . When t h e m e l t i n g occurs (LPCR), t h e growth v e l o c i t y h a s a jump and i s about

1 .SO f 0.05 m/s n e a r W = 7.5 MW/cm2 and l i n e a r l y s c a l e s with i n c i d e n t power up t o 2.95-3 m/s where ahe melted depth i s maximum.

Fig. 3. The decay time t r a n s i e n t g r a t i n g v e r s u s power e x c i t a t i o n .

The s o l i d i f i c a t i o n v e l o c i t y (V) i s given by t h e r a t e of d i s s i p a t i o n of t h e l a t e n t h e a t (AH) [ 101

AHpV = K (-) aT a T

s az s -

^{K~ (Z)L}

where K S and K a r e t h e thermal c o n d u c t i v i t y of t h e s o l i d and l i q u i d phase. The parameter whick governs t h e melt v e l o c i t y i s t h e temperature g r a d i e n t i n t h e s o l i d phase and t h e regrowth v e l o c i t y i s t h e r e s u l t of two c o m p e t i t i v e p r o c e s s e s i n t h e h e a t flow. The presence of t h e support ( g l a s s s u b s t r a t e ) causes l a r g e temperature g r a d i e n t s a t t h e i n t e r f a c e GaAs-substrate and l e a d s t o i n c r e a s e t h e regrowth velo- c i t y ; but low thermal c o n d u c t i v i t y of t h e s u p p o r t reduces t h e h e a t flow i n t o t h e sample, reducing t h e g r a d i e n t ' s e f f e c t . Under h i g h e r e x c i t a t i o n , t h e temperature changes t h e thermal c o n d u c t i v i t y of t h e g l a s s s u b s t r a t e (Kg(300) 2. 1.3 W/mK and Kg(lOOO) 2. 2 W / m K [ I l l ) , t h i s e f f e c t i s important and l e a d s t o an i n c r e a s e of t h e regrowth v e l o c i t y .

F i g . 4. The v e l o c i t y of s o l i d i f i c a t i o n of GaAs v e r s u s power e x c i t a t i o n .

I n summary, we have p r e s e n t e d t r a n s i e n t g r a t i n g t e c h n i c s t o study t h e dynamic of pulsed l a s e r a n n e a l i n g on a-GaAs. The s e l f - d i f f r a c t i o n experimental r e s u l t s show t h a t t h e f r e e c a r r i e r d i f f u s i o n i s a predominant process i n l a s e r h e a t i n g f o r ns s c a l e . Time-resolved measurements of t h e g r a t i n g decay time and e f f i c i e n c y allows d i r e c t c a l c u l a t i o n s of t h e melted depths and 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 . These measurements g i v e d e t a i l e d informations on t h e dynamics of t h e l a s e r annealing.

References

I . MARINE, W., MARFAING, J . and SALVAN, F., J. p h y s i q u e L e t t r e s

5

(1983) L 271.

2. WOOD, R.F. and GILES, G . E . , P h y s . R e v . B 23 (1981) 2923.

3. BAERI, P . , CAMPISANO, S.U., FOTI, G. and

EMINI,E.,

^J. ~ p p l . ~ h y s .

2

(1979) 788.

4. YOFFA, E.J., P h y s . Rev. B

21

(1980) 2415.

5. YOUNG, J.F. and VAN DRIEL, H.M., p h y s . R e v . B

2

(1982) 2147.

6. VAN VEUHZEN,J.A. and WAUTELET, M., P h y s . R e v . B 23 (1981) 5543.

7. NEUBERGER, M., Handbook o f E l e c t r o n i c ater rials - F ~ / ~ l e n u r n , New-York 1970)V01.2.

8. AMITH, A., KUDMAN, I. and STEIGMELER, E.F., P h y s . Rev. A

138

(1965) 1270.

9. LIGHTER, B.D. and SOMMELET, P., T r a n s . M e t a l l . S o c . AIME

245

(1969) 1021.

10. BAERI, P . , p . 151 i n L a s e r and E l e c t r o n - B e a m I n t e r a c t i o n s w i t h S o l i d s , ed. by B.R. APPLETON and G.K. CELLER, North-Holland, New-York 1982.

11. TOULOUKIAN, Y.S. and BUYCO, E.H., T h e r m o p h y s i c a l p r o p e r t i e s o f M a t t e r (IFI/Plenum, New-York 1970) Vo1.5.