CONDUCTIVITY TRANSIENTS OF Si AND InSb UNDER INTENSE LASER EXCITATION

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CONDUCTIVITY TRANSIENTS OF Si AND InSb UNDER INTENSE LASER EXCITATION

E. Kreutz, G. Treusch, W. Zimmer

To cite this version:

E. Kreutz, G. Treusch, W. Zimmer. CONDUCTIVITY TRANSIENTS OF Si AND InSb UNDER INTENSE LASER EXCITATION. Journal de Physique Colloques, 1983, 44 (C5), pp.C5-111-C5-118.

�10.1051/jphyscol:1983518�. �jpa-00223099�

JOURNAL DE PHYSIQUE

Colloque C5, suppl6ment au nclO, Tome 44, octobre 1983 page C5-111

CONDUCTIVITY TRANSIENTS OF S i AND I n S b UNDER INTENSE LASER EXCITATION

E.W. K r e u t z , G . T r e u s c h and W. Zimmer

I n s t i t u t fW Angewandte Physik, Technische HochschuZe Darmstadt, SchZoBgartenstr. 7 , D 6100 Darmstadt, F.R.G.

Resume - Nous avons mesur6 l a c o n d u c t i v i t e G l e c t r i q u e de specimens monocri- s t a l l i n s de S i e t InSb durant l ' i r r a d i a t i o n par un faisceau l a s e r p u i s s a n t . Les surfaces des c r i s t a u x n l G t a i e n t pas specialement t r a i t e e s e t l e s mesures o n t & t @ f a i t e s avec une grande r e s o l u t i o n temporelle. On a Gtudie l ' i n f l u e n c e des parametres temperature, c o n c e n t r a t i o n des p o r t e u r s e t p r e s s i o n atmosphfz- r i q u e . Les p o s s i b l e s i n f l u e n c e s sur l e t r a i t e m e n t des mat@riaux par l a s e r sont d i s c u t & e s .

A b s t r a c t

-

Time r e s o l v e d measurements o f e l e c t r i c a l c o n d u c t i v i t y have been performed d u r i n g i n t e n s e l a s e r e x c i t a t i o n o f s i n g l e - c r y s t a l l i n e S i and InSb specimens w i t h o x i d i z e d surfaces. The i n f l u e n c e o f temperature, c a r r i e r con- c e n t r a t i o n , and pressure o f t h e surrounding atmosphere was studied. The r e l e - vance t o m a t e r i a l s processing I s discussed.

The thermal m e l t i n g and t h e nonthermal plasma annealing have been proposed as t h e fundamental p h y s i c a l mechanisms o f pulsed l a s e r h e a t i n g o f semiconductors /I ,2/.

Time-resolved measurements o f Raman s c a t t e r i n g /3,4/, o p t i c a l r e f l e c t i v i t y /5 t o 7/, synchrotron X-ray d i f f r a c t i o n /8/, t r a n s p o r t p r o p e r t i e s /9,10/ as w e l l as time-of- f l i g h t measurements o f evaporated atoms /11/ have been r e p o r t e d i n o r d e r t o i n v e s t i - gate t h e r e a l - t i m e behaviour o f pulsed-laser annealing.

2 9

During m a t e r i a l s processing h i g h i n t e n s i t y l a s e r r a d i a t i o n ( l o 6 < I(W/cm ) < 10 ) n e c e s s a r i l y i s used mainly removing t h e m a t e r i a l i n vaporized form. C h a r a c t e r i s t i c phenomena associated w i t h t h e processing a r e the intensity-dependent absorption and t h e plasma i g n i t i o n by gas breakdown / I 2 t o 14/ o r t a r g e t breakdown / I 5 t o 17/. The i n t e r a c t i o n o f t h e l a s e r beam w i t h t h e expanding plasma as w e l l as t h e feedback t o t h e molten working area are o f h i g h complexity t h a t t h e determination o f t h e ne- cessary l a s e r parameters f o r m a t e r i a l processing i s no longer a s t r a i g h t forward absorptance o f l a s e r r a d i a t i o n and heat f l o w c a l c u l a t i o n o f t h e m a t e r i a l .

Time r e s o l v e d t r a n s i e n t s o f e l e c t r i c a l c o n d u c t i v i t y have been measured as a f u n c t i o n o f temperature and c a r r i e r c o n c e n t r a t i o n d u r i n g i n t e n s e l a s e r e x c i t a t i o n o f s i n g l e c r y s t a l Si and InSb w i t h r e a l surfaces t o determine t h e m e l t d u r a t i o n and t h e dyna- mics o f the m e l t i n g and r e s o l i d i f i c a t i o n processes. O p t i c a l microscopy gives addi- t i o n a l i n f o r m a t i o n on t h e m e l t depth and on t h e changes o f surface morphology. The influence o f pressure and composition o f t h e surrounding atmosphere was s t u d i e d t o i n v e s t i g a t e t h e suppression o f t h e vapour plasma and t h e enhanced c o u p l i n g t o t h e processing m a t e r i a l .

1. Experimental

We used Si ( 1 11 ) wafers o f (0.003 t o 1)

n

cm grown by t h e Czochralski method. The InSb (11 ) wafers are prepared from p-type (excess e l e c t r o n c o n c e n t r a t i o n about

l o I 5 ern-$)

s i n g l e c r y s t a l s grown i n t h e 1211

(

d i r e c t i o n by t h e Bridgman technique.

The samples were r e c t a n g u l a r slabs ( 8 t o 9 mm i n length, 1 t o 5 mm i n width, 0.02 t o 0.7 mm i n t h i c k n e s s ) as prepared by s e c t i o n i n g w i t h a w i r e saw and by mechanical p o l i s h i n g w i t h S i c powder i n d i s t i l l e d water and e t c h i n g e i t h e r i n CP6 /18/ f o r Si o r i n CP4A /18/ f o r InSb. The e t c h i n g was stopped by f l o o d i n g w i t h d i s t i l l e d water, which was removed w i t h f i l t e r paper. The o x i d a t i o n s t a t e i n v o l v e d i n t h e d i s s o l u - t i o n r e s u l t e d i n surfacescovered by a n a t u r a l oxide /19/.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983518

JOURNAL

DE PHYSIQUE

S t a i n l e s s s t e e l wedges accomplished c u r r e n t c o n t a c t s spread over t h e w i d t h o f t h e samples, tantalum sheets ( t h i c k n e s s 0.1 mm) p o t e n t i a l probes over t h e e n t i r e t h i c k - ness, which a r e a t l e a s t a q u a r t e r o f t h e specimen l e n g t h away from t h e end surfa- ces. This arrangement was e a s i l y s u f f i c i e n t o f render e f f e c t s due t o shortening by end c o n t a c t s n e g l i g i b l e and t o minimise e r r o r due t o any c a r r i e r i n j e c t i o n o r diffu- s i o n o f i m p u r i t i e s from t h e end contacts. The ohmic n a t u r e o f t h e c u r r e n t contacts, which was f r e q u e n t l y checked, was confirmed by t h e l i n e a r i t y o f t h e I - V characte- r i s t i c s . The samples were mounted on a molybdenum b l o c k i n a v a r i a b l e temperature (77 t o 400 K) c r y o s t a t , which was p a r t o f a s t a i n l e s s s t e e l vacuum chamber w i t h gas i n l e t system pumped by a turbomolecular pump.

The e l e c t r i c a l c o n d u c t i v i t y a was measured e i t h e r i n t h e constant v o l t a g e mode of o p e r a t i o n f o r high-ohmic samples o r i n t h e constant c u r r e n t mode f o r low-ohmic samp- l e s . The c u r r e n t through t h e samples was obtained. from t h e v o l t a g e across t h e v o l t a - ge probes and a l o a d r e s i s t o r f e d d i r e c t l y i n t o a t r a n s i e n t d i g i t a l analyzer. Four- point-probe measurements showed t h a t t h e c o n t a c t r e s i s t a n c e i s small compared t o t h e r e s i s t a n c e o f t h e sample g i v i n g no i n d i c a t i o n f o r a considerable i n f l u e n c e . I r r a d i a t i o n was p r o v i d e d by a Nd: YAG l a s e r o f TEMoo mode a t a wave-length o f 1.06 pm. Square pulses are formed w i t h a Pockels c e l l o u t s i d e t h e resonator g i v i n g w i t h t h e p u l s e d u r a t i o n (25 < T, (ps) < 400) and t h e l a s e r power (240 < P(W) < 300)

L 7 2

a l a s e r r a d i a t i o n i n t e n s i t y

I

< 10 W/cm

.

With a Pockels c e l l w i t h i n t h e resonator Q-switched l a s e r pulses o f n e a r l y Gaussian p r o f i l e are generated a t a pulse l e n g t h o f about 40 ns f u l l w i d t h a t h a l f maximum (FHWM) y i e l d i n g a maximum i n t e n s i t y

Imx = 6 x 10 w/cm2. The i n c i d e n t l a s e r energy was v a r i e d by n e u t r a l d e n s i t y f i l - 9 t e r s t o ensure t h a t t h e l a s e r p u l s e shape was independent o f energy t o avoid any i n f l u e n c e on t h e m e l t i n g behaviour /20/. The spot s i z e a t the sample surface was measured by scanning t h e l a s e r beam w i t h a wedge i n combination w i t h a photodiode.

2. Experimental Results

The p r i n c i p a l f e a t u r e s o f a l l t h e measured Si and InSb specimens q u a l i t a t i v e l y are s i m i l a r w i t h i n t h e range o f c a r r i e r concentrations i n v e s t i g a t e d . Repeated measure- ments over several runs y i e l d e d r e p r o d u c i b l e r e s u l t s w i t h i n t h e experimental uncer- t a i n i t i e s c o n s i d e r i n g the changes o f e l e c t r i c a l c o n d u c t i v i t y d u r i n g storage i n an ambient atmosphere /21,22/. The drawings show t h e photon induced changes AU of e l e c t r i c a l c o n d u c t i v i t y normalized t o t h e maximum value Aamax a t

Imax

c o r r e c t i n g f o r t h e c a r r i e r losses by the sample geometry and arrangement f o r i r r a d i a t i o n (Sec- t i o n 1).

Aa/namax i s p l o t t e d i n Fig. 1 versus time f o r v a r i o u s l a s e r r a d i a t i o n i n t e n s i t i e s . The changes o f c o n d u c t i v i t y r e f l e c t almost t h e shape o f t h e l a s e r p u l s e f o r low i n - t e n s i t i e s . u increases s t e e p l y a t t h e onset o f t h e l a s e r p u l s e and decreases w i t h nearTy t h e same r a t e a f t e r passing a maximum. Above a c r i t i c a l i n t e n s i t y Ic t h e ab- r u p t increase o f c o n d u c t i v i t y i s f o l l o w e d by a n e a r l y s a t u r a t i o n r e g i o n (Fig. 1').

This enhanced c o n d u c t i v i t y remained constant u n t i l i t decreases t o t h e i n i t i a l value. As the r a d i a t i o n i n t e n s i t y i s increased above Ic, t h e d u r a t i o n o f t h e f l a t p o r t i o n increased. T h i s i s i l l u s t r a t e d i n F i g . 2, where t h e d u r a t i o n T, o f enhanced c o n d u c t i v i t y i s p l o t t e d as a f u n c t i o n o f t h e square o f t h e l a s e r r a d i a t i o n i n t e n - s i t y .

a

Z 0 0

z u TIME

t Ips1

Fig.

1

- Normalized e l e c t r i c a l c o n d u c t i v i t y a s a function of time a t p

=

bar

9 2

( 0 0.2, A 0.5, A 0.9, ⁰ 1.8, m 4.6 x 10

W/cm

).

Fig, 2 -- Duration

o f

t h e enhanced c o n d u c t i v i t y versus square

o f t h e l a s e r r a d i a t i o n

i n t e n s i t y a t bar.

C5-114 JOURNAL DE PHYSIQUE

Fig. 3 shows t h e normalized c o n d u c t i v i t y change a s a function o f l a s e r r a d i a t i o n i n t e n s i t y . A t Ic

t h e

c o n d u c t i v i t y change i n c r e a s e a b r u p t l y by an o r d e r of magnitude.

lo8 109

LASER INTENSITY I [ ~ c r n - ' ]

Fig.

3

- Normalized e l e c t r i c a l c o n d u c t i v i t y a s a function of l a s e r r a d i a t i o n i n t e n - s i t y

( n

o 4 x

A

r 10' b a r ) . The arrows i n d i c a t e t h e l a s e r r a d i a t i o n i n t e n s i t y f o r o p t i c a l micrographs.

8 2

For I

>

5

x

10 W/cm Au/aomaX s a t u r a t e s . Ic depends mainly on t h e pressure (Fig. 3)

and composition of t h e surrounding atmosphere /23/ and l e s s pronounced on c a r r i e r

concentration. I, becomes higher with i n c r e a s i n g pressure i n combination with a

simultaneous broadening of t h e t h r e s h o l d ( F i g . 3).

The v i s u a l i z a t i o n o f t h e i n t e r a c t i n g zone e x h i b i t s plasma i g n i t i o n . The behaviour o f t h i s plasma i n vacuum was d r a m a t i c a l l y d i f f e r e n t from t h a t o f t h e a i r plasma.

The plasma i n vacuum o f n e a r l y s p h e r i c a l symmetry i s confined s t r o n g l y t o the sur- face r e g i o n propagating a t l e a s t about 2 mm i n t o t h e vacuum. The propagating d i - stance of t h e a i r plasma w i t h approximately c y l i n d r i c a l symmetry increases t o seve- r a l mm's a t low pressures. The t r a n s i t i o n from one t o t h e o t h e r w i t h decreasing a i r pressure was abrupt.

The exposed surfaces show damaged regions (Fin. 3) w i t h the observed e f f e c t s /24/

d e p e n d i n g o n t h e t y p e o f c r y s l t a 1 , t h e s u r f a c e c r y s t a l l o g r a p h y , and t h e s u r f a c e s t a t e . A t v e r y low i n t e n s i t i e s c r a c k i n g appears w i t h t h e l i n e s making v e r y d e f i n i t e angles w i t h each other. I n c r e a s i n g t h e l a s e r i n t e n s i t y a c r a t e r w i t h c i r c u l a r symmetry i s formed showing beyond

I r

deep holes. W i t h i n t h e s a t u r a t i o n r e g i o n o f AU/AU,,,~~ t h e s u r f a c e i s i r r e g u l a r l y molten w i t h pronounced hydrodynamic e f f e c t s near Imax, 3. Discussion

Generation and r e l a x a t i o n processes change the energy d i s t r i b u t i o n o f t h e c a r r i e r s i n a semiconductor. During t h e a t t e n u a t i o n o f t h e i n c i d e n t l a s e r beam f r e e c a r r i e r ab- s o r p t i o n , i o n i z a t i o n o f l o c a l i z e d i m p u r i t i e s w i t h i n t h e gap, d i r e c t and i n d i r e c t i n t e r b a n d and i n t r a b a n d t r a n s i t i o n s may b u i l d up a s u b s t a n t i a l d e n s i t y o f e l e c t r o n s and holes v i a one- o r m u l t i p h o t o n absorption, t u n n e l i n g mechanisms and impact i o n i - z a t i o n . The r e l a x a t i o n o f t h e energy s t o r e d i n t h e c a r r i e r system d u r i n g e x c i t a t i o n may occur v i a c a r r i e r - c a r r i e r c o l l i s i o n s , recombination processes, and c a r r i e r -

l a t t i c e c o l l i s i o n s . N e g l e c t i n p e f f e c t s o f c a r r i e r d i f f u s i o n t h e d e t a i l e d a n a l y s i s o f t h e l i g h t induced n u ' s /23/ has shown t h a t t h e r e l a x a t i o n o f t h e e l e c t r o n energy towards e q u i l i b r i u m and f o r t h e t r a n s f e r t o the l a t t i c e i s m a i n l y due t o i m p u r i t y - , band-band- and Auger recombination. The comparison w i t h c a l c u l a t i o n s on t h e basis o f c o n t i n u i t y equation y i e l d e d /23/ t h a t t h e most probable mechanism o f recombina-

t i 6 a t h i g h r a t e s o f i n j e c t i o n i s t h e Auger process g i v i n g i n d i c a t i o n f o r a h i g h c a r r i e r c o n c e n t r a t i o n i n t h e s u r f a c e r e g i o n probably an e l e c t r o n

-

h o l e plasma /I ,2/. For beam i n t e n s i t i e s around 10 bI/cm2 e x c i t e d c a r r i e r s are created a t a r a t e 8 o f s - l ~ m - ~ . A l a r g e d e n s i t y o f h o t c a r r i e r s t h e r e f o r e b u i l d s up before t h e l a t t i c e i s appreciably heated s u p p o r t i n g t h e considerations above. During c a r r i e r l a t t i c e c o l l i s i o n s phonons are generated h e a t i n g t h e l a t t i c e .

7 2

For low i n t e n s i t i e s (I < 5 x 10 b!/cm ) t h e ohonon d e n s i t y seems o n l y s u f f i c i e n t t o produce l o c a l l y an increase o f temperature r e s u l t i n g i n i n t e r n a l stresses which lead t o f r a c t u r e s along cleavage planes /25/. This regime consequently i s n o t e f f e c t i v e i n mass removal f o r m a t e r i a l s processing i n agreement w i t h t h e observed l a s e r i n f l u e n c e d s u r f a c e zone (Fig. 3). These observations i m p l y no t r a n s i e n t chan- ges o f t h e time-dependent r e f l e c t a n c e measured simultaneously

/lo/.

The observed

A D ' S are produced by p h o t o c o n d u c t i v i t y governed i n magnitude and d u r a t i o n by t h e c o n c e n t r a t i o n and l i f e t i m e i n t h e S i as we11 as by

I

and r L .

For i n t e n s i t i e s around Ic t h e phonon d e n s i t y i s s u f f i c i e n t l y h i g h t o r i s e conside- r a b l y t h e temperature. Magnitude, sign, d u r a t i o n and i l l u m i n a t i o n i n t e n s i t y depen- dence o f t h e l a s e r induced AU (Fig. 1 ) i n d i c a t e t h e m e l t i n g o f t h e S i . The d u r a t i o n r C t o the f l a t - t o p p o r t i o n o f AU consequently represents t h e l i q u i d u s t i m e o f t h e surface. The absorbed l a s e r energy i s s t o r e d i n t h e heat c a p a c i t y o f t h e h o t s o l i d and i s m a i n l y d i s t r i b u t e d over t h e thickness o f t h e molten s u r f a c e l a y e r which i s governed by d i f f u s i o n processes v i a t h e f l o w of heat. Accordin? t o / 2 6 / T~ i s given

by .2

where C i s t h e heat capacity, D i s t h e thermal d i f f u s i v i t y , and Tm and T are t h e

C5-116 JOURNAL DE PHYSIQUE

m e l t i n g and t h e ambient temperatures. I n t h e l i m i t o f I l a r g e compared t o t h e t h r e s - h o l d

I

f o r m e l t i n g , t h e absorbed enerFy p e r u n i t area i s expected t o be p r o p o r t i o - n a l ~ , ~ e ~ n . ( 1 ) p r e d i c t s t h e observed q u a d r a t i c dependence ( F i g . 2 ) n e g l e c t i n g t h e heat o f f u s i o n . Since m o l t e n S i i s m e t a l l i c , ^CT i n c r e a s e s upon m e l t i n g ( F i g . 1 ) accompanied by a s i m i l a r i n c r e a s e i n t b e o p t i c a l r e f l e c t i v i t y

/ l o / .

The c o n d u c t i v i t y changes o r i g i n a t e f r o m t h e c o n d u c t i v i t y t h r o u g h t h e m o l t e n S i s u r f a c e l a s e r . The n o ' s decay towards t h e i n i t i a l v a l u e as t h e c a r r i e r s recombine and t h e s u r f a c e l a y e r r e s o l d i f i e s . An a n a l y t i c a l model /27/, w h i c h i s based on a heat f l o w c a l c u l a t i o n , r e p r e s e n t s t h e t i m e and i n t e n s i t y dependence o f t h e c o n d u c t i v i t y changes by s o l i d h e a t i n g , m e l t i n g and b o i l i n g o f t h e i r r a d i a t e d s o l i d s u p p o r t i n g t h e c o n s i d e r a t i o n s above. T h i s model a l s o accounts f o r m a t e r i a l removing p r o c e s s i n g such as d r i l l i n g and c u t t i n g as t o be seen f r o m t h e h o l e f o r m a t i o n ( F i g . 3) d u r i n g h i g h i n t e n s i t y l a s e r i r r a d i a t i o n .

The s a t u r a t i o n o f t h e auv i n vacuum and t h e smooth i n c r e a s e o f nua i n a i r ( F i g . 3 )

-

i n d i c a t e f o r

I

>

I c

t h e i n t e r f e r i n g i n f l u e n c e o f a plasma, w h i c h i s generated by gas breakdown /28/ e i t h e r i n t h e s u r r o u n d i n g gas atmosphere

/ I 2

t o 14/ o r i n t h e evapo- r a t e d t a r g e t m a t e r i a l / I 5 t o 17/. F i r s t f r e e e l e c t r o n s l i b e r a t e d from t h e S i s u r f a c e g a i n energy f r o m t h e l a s e r photons by i n v e r s e bremsstrahlung. The i g n i t i o n c r i t e r i o n i s t h a t t h e r a t e o f energy g a i n must exceed l o s s e s t o e l a s t i c c o l l i s i o n s t h a t t h e k i n e t i c energy o f t h e e l e c t r o n s reaches t h e i o n i z a t i o n l i m i t and t h e low d e n s i t y o f f r e e e l e c t r o n s be cascade m u l t i p l i e d i n t o t h e h i g h d e n s i t y o f a plasma /29/. S t a r - t i n g f r o m r a t e e q u a t i o n s f o r e l e c t r o n d e n s i t y , momentum and energy and n e g l e c t i n g l o s s e s by d i f f u s i o n and i n e l a s t i c s c a t t e r i n g processes /30/ y d e l d s i n t h e l i m i t o f c l a s s i c a l r f - a o o r o x i m a t i o n o f t h e i n v e r s e bremsstrahluna breakdown i n t e n s i t i e s o f

. . -

l o 7

<

I B

(w/cm2) < 10 c o r r e s p o n d i n g t o measured d a t a ( F i g . 3 ) . The pressure-depen- 9 d e n t d i f f e r e n c e s i n aoa and auV m i g h t be e x p l a i n e d by t h e decay o f t h e plasma pressure. The plasma d e n s i t y decreases r a p i d l y w i t h t h e p r o p a g a t i o n speed v o f t h e

D plasma /31/ depending on t h e d e n s i t y of t h e s u r r o u n d i n ? atmosphere. I n vacuum v

P i s h i g h e r r e s u l t i n g i n an o p t i c a l l y t h i n plasma i n f r o n t o f t h e t a r g e t . I n a i r t h e l o w e r p r o p a g a t i o n speed y i e l d s an o p t i c a l l y dense plasma s h i e l d i n g t h e t a r g e t from t h e i n c i d e n t i n g l a s e r r a d i a t i o n . The o p t i c a l energy s t o r e d i n t h e t a r g e t consequent- l y i s h i g h e r i n vacuum t h a t AD, > ao2 ( F i g . 3 ) . The o p t i c a l micrographs (Fig. 3 )

-

r e f l e c t t h e s t r o n g i n t e r a c t i o n o f t h e dens'e plasma w i t h t h e t a r g e t . The o n l y adequa- t e means o f suppression o f t h e a i r plasma i g n i t i o n i s by p u t t i n g t h e specimens under vacuum p r e v e n t i n g i g n i t i o n a t h i g h l a s e r i n t e n s i t i e s w h i c h a r e r e q u i r e d f o r mate- r i a l s processing. Time-resolved i n v e s t i g a t i o n s o f t h e o p t i c a l r e f l e c t i v i t y a r e under way /32/ m o n i t o r i n g i n combination w i t h a s t r e a k camera t h e plasma f o r m a t i o n as a f u n c t i o n o f pressure t o c o n f i r m t h e s e c o n s i d e r a t i o n s . These measurements s h o u l d a l s o d e c i s i v e l y a l l o w t o determine t h e development o f combustion o r d e t o n a t i o n waves du- r i n g l a s e r i r r a d i a t i o n r e d u c i n g s u r f a c e heating.

4. Summary

The c a r r i e r c r e a t i o n and c a r r i e r h e a t i n g o c c u r by t h e g e n e r a t i o n of e l e c t r o n - h o l e p a i r s and t h e e x c i t a t i o n o f e l e c t r o n and h o l e t r a n s i t i o n s w i t h i n t h e s i n g l e bands v i a i n t e r b a n d - and f r e e c a r r i e r a b s o r p t i o n . I m p u r i t y - , band-band- and Auger recom- b i n a t i o n a r e t h e mechanisms /23/ f o r t h e r e l a x a t i o n o f t h e e l e c t r o n energy towards e q u i l i b r i u m and f o r t h e t r a n s f e r t o t h e l a t t i c e g i v i n g i n d i c a t i o n f o r an e l e c t r o n - h o l e plasma.

A thermal model i n c l u d i n g h e a t i n p , m e l t i n g and b o i l i n g e x p l a i n s t h e k i n e t i c s o f t h e c o n d u c t i v i t y changes a t h i g h i n t e n s i t i e s r e l a t e d t o l a s e r m a t e r i a l s processing. The p r e s s u r e dependence o f t h e plasma i g n i t i o n governs t h e l a s e r r a d i a t i o n i n t e n s i t i e s o p t i m i z i n g t h e c o u p l i n g o f t h e o p t i c a l energy i n t o t h e s o l i d .

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