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TIME RESOLVED RAMAN SPECTRA DURING PULSED LASER HEATING OF SILICON
G. Wartmann, D. von der Linde
To cite this version:
G. Wartmann, D. von der Linde. TIME RESOLVED RAMAN SPECTRA DURING PULSED LASER HEATING OF SILICON. Journal de Physique Colloques, 1983, 44 (C5), pp.C5-107-C5-110.
�10.1051/jphyscol:1983517�. �jpa-00223098�
TIME RESOLVED RAMAN SPECTRA DURING PULSED LASER HEATING OF SILICON
G. Wartmann and D. von der Linde
UniversitCit Essen, Fachbereich Physik, 4300 Essen 1, F.R.G.
Resume
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On a e t u d i e l e s spectres de diffusion Raman du phonon optiquer25
m i c i u m c r i s t a l l in pendant 1
'
excitation intense par une impul sion 1 aser de 10 ns. Ces spectres donnent evidence que l'echauffage l a s e r n ' e s t pas uniforme e t que pendant une periode de t r a n s i t i o n de 10 ns des regionsso- l i d e s e t liquides coexistent l a surface.Abstract
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The Raman spectrum of ther,,
optical phonon in silicon during intense excitation by a 10 ns l a s e r pulse i s investigated with a time reso- lution of 2 ns. These spectra provide evidence t h a t non-uniform heating takes place and t h a t during a t r a n s i t i o n period of about 10 ns solid and liquid surface areas coexist.1. Introduction
Over the past several years the fundamental physical mechanisms of pulsed l a s e r annealing of semiconductors have been the subject of an active discussion. The bulk of both experimental and theoretical work supports a simple thermal model in which the l a s e r pul se f i r s t me1 t s the material, and subsequently an epitaxial regrowth process takes place. On the other hand, several experiments were reported /1-5/ in which spontaneous Stokes and anti-Stokes Raman scattering was used to measure the changes of the l a t t i c e temperature of silicon di~ring and a f t e r exposure t o an in- tense l a s e r heating pulse. The r e s u l t s of these Zaman experiments appear t o be in disagreement with the thermal melt hypothesis. The l a t t i c e temperature inferred from the anti-Stokes/Stokes r a t i o was always fomd t o be well below the melting point of silicon. In addition, Raman scattering corresponding to the
rZ5
zone cen- t e r optical phonon of s i l i c o n was also observed during the high r e f l e c t ~ v i t y phase.A sudden jump of the optical r e f l e c t i v i t y and the subsequent h i g h r e f l e c t i v i t y phase i s generally considered to indicate a sol id-to-1 iquid phase t r a n s i t i o n . The high r e f l e c t i v i t y phase i s a t t r i b u t e d to a thin surface layer of laser-molten me- t a l l i c silicon which i s not Raman active. Inadequate spatial and temporal resolu- t i o n and problems with properly correcting the measured Stokes/anti-Stokes r a t i o s f o r the temperature dependence of the optical properties and the Raman scattering cross sections /6/ have been ruled out /4,5/ as an explanation of the inconsistency of the Raman experiments with the thermal melt model.
In our previous work /3,4/ the temporal evolution of the Stokes/anti-Stokes r a t i o of spectrally integrated Raman scattering was measured. In t h i s report we present r e s u l t s of detailed measurements of time- and frequency-resolved Raman scattering.
These new data indicate t h a t a s p a t i a l l y non-uniform temperature distribution might be a possible explanation of the existing discrepancies.
2. Experimental
In our experiments we use l a s e r pulses of
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lOns duration from a single frequency, single transverse mode, passively Q-swi tched Nd-YAG l a s e r . The second harmonic of the l a s e r pulses (532nm) serves both f o r laser-heating and Raman scattering. Good spatial resolution in the direction parallel t o the surface i s achieved by imagingArticle published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983517
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c i r c u l a r a p e r t u r e i n f r o n t o f t h e s l i t passes o n l y s c a t t e r e d l i g h t o r i g i n a t i n g f r o m w i t h i n a r a d i u s c o r r e s p o n d i n g t o t h e 90 % o f maximum i n t e n s i t y c o n t o u r o f t h e hea- t i n g beam. The energy d e n s i t y v a r i e s smoothly over t h e a c t i v e s u r f a c e area , t h e t o t a l v a r i a t i o n b e i n g l e s s than 10 %.
The samples a r e (100) o r i e n t e d c r y s t a l 1 i n e s i l i c o n w a f e r s which a r e r a s t e r - scanned t o a v o i d mu1 t i p l e exposure.
Raman l i g h t i s d e t e c t e d w i t h a p h o t o m u l t i p l i e r tube h a v i n g a t i m e r e s o l u t i o n o f a b o u t 211s. The i n d i v i d u a l p h o t o m u l t i p l i e r p u l s e s a r e r e c o r d e d and processed by a computer c o n t r o l 1 ed waveform d i g i t i z e r . By accumulating a s u f f i c i a n t l y 1 a r g e r num- b e r o f s i n g l e photon events t h e temporal p r o f i l e o f t h e average Raman s c a t t e r i n g i n t e n s i t y can be r e c o n s t r u c t e d . The t i m e - r e s o l ved s p e c t r a a r e o b t a i n e d by measuring t h e temporal Raman p r o f i l e f o r d i f f e r e n t frequency p o s i t i o n s a c r o s s t h e spect, d l r e g i o n o f i n t e r e s t . The r e c o r d i n g frequency bandwidth corresponded t o 15 c l r < - l . D u r i n g t h e Raman measurements t h e o n s e t and d u r a t i o n o f t h e h i g h r e f l e c t i v i t y phase a r e m o n i t o r e d by simultaneous measurements o f t h e o p t i c a l r e f l e c t i v i t y ( 1 n s t i m e r e s o l u t i o n ) . F o r i n s t a n c e , t h e d u r a t i o n o f t h e h i g h r e f l e c t i v i t y phase f o r a h e a t i n g p u l s e o f 0.6 ~ / c m * was measured t o be 20 ns.
F i g . 1
Stokes s i g n a l a t 526 cm-' as a f u n c t i o n o f t i m e ( s o l i d c u r v e ) . D o t t e d c u r v e : l a - s e r h e a t i n g p u l s e . Dashed c u r v e : Stokes s c a t t e r i n g e f f i e n c y . Arrow: p o s i t i o n o f t h e r e f l e c t i v i t y jump.
3. R e s u l t s and D i s c u s s i o n
As an e x a m p l e o f t h e m e a s u r e d temporal p r o f i l e s t h e Stokes s i g n a l a t 526 cm-l i s shown by t h e s o l i d c u r v e o f F i g . 1. The h e a t i n g p u l s e i s g i v e n by t h e d o t t e d l i n e f o r comparison. The arrow marks t h e o n s e t o f t h e h i g h r e f l e c t i v i t y phase. The dashed l i n e shows t h e r a t i o o f Stokes t o l a s e r i n t e n s i t y which i s n o r m a l i z e d t o u n i t y a t t h e f r o n t o f t h e pulse. The f o l l o w i n g p o i n t s should be n o t i c e d :
( i ) The dashed l i n e i n d i c a t e s a d r a s t i c decrease o f t h e s c a t t e r i n g e f f i c i e n c y w i t h time;
( i i ) The s c a t t e r i n g e f f i c i e n c y passes smoothly i n t o t h e h i g h r e f l e c t i v i t y phase w i t h o u t any d i s c o n t i n u i t y ;
( i i i ) Raman s c a t t e r i n g c o n t i n u e s d u r i n g t h e h i g h r e f l e c t i v i t y phase.
Stokes s p e c t r a f o r e x c i t a t i o n w i t h 0.6 J/cm2 o f 532 nm l i g h t a r e d e p i c t e d i n F i g . 2. Curves 1 t o 4 r e p r e s e n t a sequence o f t i m e - r e s o l v e d Raman s p e c t r a w i t h a 4 n s t i m e increment. The i n s e r t i n d i c a t e s t h e temporal p o s i t i o n s w i t h r e s p e c t t o t h e e x c i t a t i o n pul se. F o r a l l c u r v e s t h e s c a t t e r i n g i n t e n s i t y i s n o r m a l i z e d w i t h r e s p e c t t o t h e i n s t a n t a n e o u s i n t e n s i t y o f t h e e x c i t a t i o n p u l s e .
sents the Raman spectrum j u s t before the r e f l e c t i v i t y jump. Comparing w i t h spectrum 1, the f o l l o w i n g changes can be noticed:
( i ) A decrease o f the peak s c a t t e r i n g i n t e n s i t y by o n l y about 20 %;
( i i ) A small increase o f the w i d t h and a small l i n e s h i f t t o lower frequencies;
( i i i ) An asymmetric shape w i t h a very pronounced 1 ow frequency t a i 1.
It i s i n t e r e s t i n g t o compare spectrum 2 w i t h t h e dashed curve which represents t h e expected Raman spectrum o f a s i l i c o n c r y s t a l w i t h a uniform temperature c l o s e t o the me1 t i n g p o i n t (about 1600 K). The w i d t h and the s h i f t o f the Raman spectrum representing a h o t c r y s t a l were obtained by e x t r a p o l a t i o n o f a v a i l a b l e 1 it e r a t u r e data /7/ t o a temperature o f 1600 K. I n a d d i t i o n t o the s h i f t and the increase o f t h e l i n e w i d t h a strong decrease o f t h e s c a t t e r i n g i n t e n s i t y t o l e s s than 10 % o f t h e room temperature value (curve 1) i s expected. T h i s e f f e c t i s due t o the i n - crease o f the o p t i c a l absorption w i t h temperature a t 532 nm /8/ which l e a d s t o a dramatic decrease o f the s c a t t e r i n g volume when the s i l i c o n c r y s t a l i s heated up.
Comparison o f curve 2 w i t h the dashed l i n e shows t h a t the a c t u a l Raman spectrum i n t h e v i c i n i t y o f the r e f l e c t i v i t y jump i s q u i t e d i f f e r e n t from the spectrum corre- sponding t o a uniform temperature o f
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1600 K. One i s t h e r e f o r e l e a d t o assume t h a t t h e c r y s t a l surface has a non-uniform temperature d i s t r i b u t i o n w i t h c o e x i s t i n g h o t and c o l d areas. The l a t t e r very s t r o n g l y dominate i n the Raman process g i v i n g a spectrum w i t h only a minor s h i f t and. broadening. The h o t areas, on the o t h e r hand, c o n t r i b u t e very l i t t l e t o the t o t a l Raman s c a t t e r i n g being r e s p o n s i b l e f o r the weak 1 ow frequency t a i 1.
FREQUENCY SHIFT
[cm-'I
Fig. 2
Raman Stokes spectra o f l a s e r heated s i - l i c o n . I n s e r t : l a s e r h e a t i n g p u l s e show- i n g t h e temporal p o s i t i o n o f t h e f o u r spectra. Arrow: onset o f the h i g h r e - f l e c t i v i t y phase. Dashed curve: expect- ed Raman spectrum o f s i l i c o n w i t h a u n i - form temperature o f 1600 KA1
Spectral r e s o l u t i o n : 20 cm
.
I t i s important t o emphasize t h a t we do n o t expect the non-uniformity o f t h e tempe- r a t u r e t o be simply caused by s p a t i a l l y non-uniform i l l u m i n a t i o n , because g r e a t care was exercised t o ensure a u n i f o r m s p a t i a l energy d i s t r i b u t i o n o f the l a s e r beam. Rather, we assume t h a t some i n s t a b i l i t y o f the h e a t i n g process i s responsible f o r the inhomogeniety of the surface temperature. Non-uniform h e a t i n g would pre- sumably l e a d t o a m e l t i n g process i n which molten and s o l i d m a t e r i a l c o e x i s t d u r i n g a c e r t a i n t r a n s i t i o n period. This would e x p l a i n the r e s i d u a l Raman d u r i n g t h e h i g h r e f l e c t i v i t y phase.
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Com a r i s o n o f curve 2 and 3 i n Fig. 2 shows t h a t the maximum o f t h e spectrum a t 520 cm-' decreases r a p i d l y a f t e r t h e onset o f the h i g h r e f l e c t i v i t y phase, whereas very l i t t l e change i s observed i n the low frequency t a i l . Curve 4 f i n a l l y represents the Raman spectrum a f t e r 8 ns where the s c a t t e r i n g i n t e n s i t y i s approaching the d e t e c t i o n l i m i t . During the remaining p a r t o f the pulse Raman s c a t t e r i n g i s too weak t o be detected.
The gradual disappearance o f Raman s c a t t e r i n g c o u l d be explained as being due t o an increase o f molten areas a t the expense o f sol i d areas. Complete coverage o f t h e surface by molten s i l i c o n would then take about 10 ns t o develop.
Attempts were made t o f i n d p o s i t i v e p r o o f o f a s p a t i a l temperature non-uniformity.
D i f f r a c t i o n experiments showed t h a t t h e r e are no t r a n s i e n t r i p p l e phenomena 191.
Permanent surface r i p p l e s can be generated o n l y by m u l t i p l e exposure o f the same c r y s t a l surface area. However, we discovered t h a t t h e r e i s a dramatic t r a n s i e n t enhancement o f d i f f u s e Rayleigh s c a t t e r i n g when the t r a n s i t i o n t o the h i g h r e f l e c - t i v i t y phase takes place. T h i s observation p o i n t s t o i r r e g u l a r s p a t i a l inhomoge- n i e t i e s r a t h e r than p e r i o d i c surface s t r u c t u r e s .
4. Conclusion
Time-resolved Raman spectra p r o v i d e evidence t h a t d u r i n g pulsed l a s e r h e a t i n g o f s i l i c o n t h e surface temperature i s n o t uniform. Because o f the very strong b i a s o f t h e Raman method i n favour o f c o l d surface areas, phonon temperatures i n f e r r e d from the anti-Stokes/Stokes i n t e n s i t y r a t i o o f 1 aser- heated s i l ic o n are expected t o l e a d t o an underestimate o f the l a t t i c e temperature.
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