HAL Id: jpa-00223114
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Submitted on 1 Jan 1983
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PICOSECOND LASER ANNEALING OF
IMPLANTED Si AND GaAs : A COMPARATIVE STUDY WITH A RAMAN MICROPROBE
J. Sapriel, Y. Nissim, J. Oudar
To cite this version:
J. Sapriel, Y. Nissim, J. Oudar. PICOSECOND LASER ANNEALING OF IMPLANTED Si AND
GaAs : A COMPARATIVE STUDY WITH A RAMAN MICROPROBE. Journal de Physique Collo-
ques, 1983, 44 (C5), pp.C5-187-C5-191. �10.1051/jphyscol:1983529�. �jpa-00223114�
PICOSECOND L A S E R A N N E A L I N G OF I M P L A N T E D S i AND GaAs : A C O M P A R A T I V E STUDY W I T H A RAMAN MICROPROBE
J . Sapriel, Y.I. Nissim and J.L. Oudar
C.N.E.T. Laboratoire de Bagnem*, 196 rue de Paris, 92220 Bagnem, France Resume
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Les transformations r e s u l t a n t de l ' i r r a d i a t i o n par laser picose- condedu Si e t de GaAs rendu amorphe par implantation sont Ptudi6es. I1 r6- s u l t e de l ' i r r a d i a t i o n dans l e s deux materiaux l a formation d'une tache mu1 tiannulaire. L16tude de ces taches e s t f a i t e 'a 1' aide d'une microsonde Raman ayant une r6solution s p a t i a l e de 1 p. Les taches sont constituees d'une alternance d'anneaux c r i s t a l l i n s e t amorphe dans l e cas du silicium.Dans l e cas de GaAs l e s t r a n s i t i o n s sont plus attenu6es entre des anneaux a tendance c r i s t a l l i n e e t des anneaux 'a tendance amorphe. Un processus de fusion-resolidification multiple pendant l a duree de l ' i r r a d i a t i o n pourrait expl iquer l a formation de ces structures.
Abstract
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The transformation of implanted amorphous Si and GaAs induced by a single pul se of a picosecond laser i s studied. In both materials a clear mu1 tiannul ar pattern was produced by the i r r a d i a t i o n . The d i f f e r e n t pat- terns have been investigated by scanning the surface with a 1 p spatial resolution Raman microprobe. Sharp t r a n s i t i o n s between amorphous and c r i s - t a l l i n e rings are observed for Si. In GaAs the t r a n s i t i o n s are smoother beetween near1 y amorphous and nearly crystal1 ine rings. A mu1 t i p l e me1 t i n g - resol i d i f i c a t i o n process within the laser pulse duration could explain the formation of these patterns.INTRODUCTION
When a pulsed beam i s used t o anneal implanted semiconductors, i t i s l i k e l y t h a t melting of a surface layer under i r r a d i a t i o n followed by a l i q u i d phase epitaxial regrowth occurs. However there i s a threshold in the speed of the r e s o l i d i f i c a t i o n process above which the c r i s t a l l i n e structure does not recover, resulting in amor- phous material formation. Resol i d i f i c a t i o n r a t e s above t h i s thresh01 d can be obtai
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ned in semiconductors with extremely short pulses (picosecond 111) or with laser pulses of extremely high absorption ( U . V . l i g h t / 2 / ) .
The annealing of implanted amorphized silicon and GaAs with a picosecond Nd:YAG l a s e r i s reported here. Raman scattering measurements were performed to investigate the structure of an annealed spot obtained by a single pulse i r r a d i a t i o n . Spatial resolution up t o one micron was obtained with a microprobe. The resulting structure z f the annealed spots i s composed of concentric rings made by an alternation of crystal 1 ine 1 i ke" and "amorphous 1 ike" material
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These patterns do not correspond t o a continuous change of structure but to a periodic structure. Since the laser pul ses have a gaussian spatial and temporal p r o f i l e , a more complex me1 ting-sol i d i-
f i c a t i o n process than the one presented for single crystal s i l i c o n i r r a d i a t i o n /3/
i s proposed here.
SAMPLE PREPARATION, ANNEALING AND RAMAN SCATTERING SET UP
Single crystal s i l i c o n and gallium arsenide (100) oriented were implanted with heavy ions in order t o create a 1000 A thick amorphous layer. For the s i l i c o n
*Laboratoire associ6 au C.N.R.S. (LA 250)
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983529
JOURNAL DE PHYSIQUE
substrates AS+ i o n s were u t i l i z e d a t a dose o f l x 1 0 ~ ~ / c m ~ and a t an energy o f 100keV. Te ions a t a dose o f lx1015/cm2 and a t an energy o f 250keV were implanted i n t h e GaAs substrates. The annealing was c a r r i e d o u t w i t h a mode locked Nd:YAG l a s e r described i n r e f / 4 / . S i n g l e pulses o f average d u r a t i o n 27 psec a t 1.06 p can be e x t r a c t e d from t h i s system. The sample was placed a t the focal p o i n t o f a one meter f o c a l l e n g t h lens. Each pulse focused through the l e n s had a gaussian p r o f i l e (spa- t i a l and temporal) w i t h a diameter o f 450 pm a t the l / e i n t e n s i t y p o i n t s . The l a s e r was operated a t a low r e p e t i t i o n r a t e i n combination w i t h a s h u t t e r , so t h a t t h e
sample c o u l d be trans1 ated between each p u l se.
Raman s c a t t e r i n g i s a v e r s a t i l e and non d e s t r u c t i v e t o o l f o r t h i n f i l m c h a r a c t e r i z a - t i o n . The whole surface o f a sample can be analyzed w i t h a s p a t i a l r e s o l u t i o n o f t h e focused analyzing beam. From 1 ig h t absorption considerations, depth p r o f i l e s can be a1 so i n v e s t i g a t e d by t h i s technique. Single c r y s t a l s can be e a s i l y d i s t i n g u i s h e d from p o l y c r y s t a l l i n e or amorphous semiconductors as they a l l g i v e r i s e t o q u i t e d i f f e r e n t Raman spectra. Furthermore the presence o f s t r a i n and damages i n c r y s t a l - l i n e m a t e r i a l can be i d e n t i f i e d by a change i n the shape and p o s i t i o n o f the Raman peaks. Raman measurements were per formed i n t h e b a c k s c a t t e r i n g arrangement. A con- v e n t i o n a l Raman s e t up was f i r s t used t o study the annealed semiconductors and t o perform p o l a r i z e d 1 ig h t s c a t t e r i n g i n v e s t i g a t i o n s . The s p a t i a l r e s o l u t i o n was subse- q u e n t l y improved by the use o f a Raman microprobe described i n r e f [ 5 ] w i t h a r e s o l u - t i o n o f 1 pm.
ANNEALING OF SILICON
The r e s u l t s presented i n t h i s s e c t i o n are described i n r e f / 4 / and /5/, b u t t h e r e l e - vant p r o p e r t i e s are summarized here as a background f o r comparison w i t h the r e s u l t s obtained on GaAs and f o r the modeling. A very sharp r e c r y s t a l l i z a t i o n p a t t e r n was obtained a t 1.06 pm j u s t below t h e t h r e s h o l d o f l a s e r induced damage ( E = 2 ~ / c m ~ ) . The p a t t e r n obtained under these c o n d i t i o n s i s shown i n t h e Nomar s k i o p t i c a l micrograph o f Fig.1. T h i s p a t t e r n was scanned along the arrows w i t h a Raman microprobe. Crys- t a l l i n e s i l i c o n e x h i b i t s one Raman a c t i v e peak r e l a t e d t o t h e i n t e r a c t i o n o f a pho- t o n induced e l e c t r o n - h o l e p a i r w i t h t h e zone-center o p t i c a l phonon whose frequency i s 521 on-l. T h i s peak i s taken as an i n t e n s i t y reference for a l l the annealed r e - gions i n v e s t i g a t e d . The reduced Raman i n t e n s i t y ( r = I
,1/I ,,,,
c r y s t a l ) o f the 521 cm-' l i n e i s then p l o t t e d as a f u n c t i o n o f p o s i t i o n i n F i g u r e 1. The thickness o f r e s i d u a l non c r y s t a l l i n e l a y e r s can be deduced from t h i s r a t i o for a given e x i t i n g wavelength w i t h absorption depth c o n s i d e r a t i o n /4/. These r e s u l t s combined w i t h p o l a r i z e d 1 ig h t measurement have a1 1 owed t o o b t a i n a d e t a i l 1 ed d e s c r i p t i o n ( l a t e r a l as w e l l as i n depth) o f the mu1 t i a n n u a l p a t t e r n . T h i s i s presented i n the t a b l e below :E = 2 ~ / c m ~ h = 1 . 0 6 p m
- Centr a1 r e g i o n : s t r e s s f r e e (100) s i n g l e c r y s t a l
diameter = 180 p
F i r s t r i n g : amorphous
w i d t h = 30 pn thickness = 100 A Second r i n g : s t r e s s f r e e (100) s i n g l e c r y s t a l
w i d t h = 25 p
T h i r d r i n g : amorphous
w i d t h = 20 p thickness = 330 A Fourth r i n g : n o t i d e n t i f i e d
F i n a l l y i t i s important t o mention t h a t w i t h i n the r e s o l u t i o n o f the microprobe, no t r a n s i t i o n r e g i o n between the r i n g s could be observed.
f o r ~=2J/cm-* and h=1.06p : Nomarski o p t i c a l micrograph o f the annealed area and spa- t i a l e v o l u t i o n o f the i n t e n - s i t y o f t h e 521cm-l l i n e o f s i 1 ic o n obtained w i t h t h e mi
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croprobe.
FREQUENCY SHIFT ltm-'I
Fig.2 : Raman spectra o f the d i f f e r e n t regions o f a picosecond l a s e r annealed i o n implanted amorphized GaAs spot f o r E=1.5J/~m-~ and h = 1.06 pm.
L o ' A d '
ioo'
FREQUENCY SHIFT (cm-'1
Fig.3 : Raman spectra o f the d i f f e r e n t regions o f a picosecond l a s e r annealed i o n imp1 anted amorphized GaAs spot f o r
~ = 0 . 8 5 ~ / c m - ~ and = 1.06 w.
JOURNAL DE PHYSIQUE
ANNEALING OF GaAs
Similar experiments were carried out with an imp1 anted GaAs substrate. Sing1 e pul se annealing from the same laser a t 1.06 pm resulted in a multiannular pattern. The resulting optical micrographs are a l e s s contrasted, indicating a d i f f e r e n t crystal reconstruction within the rings of the annealed area. The best optical contrast was obtained above the damage threshold of the substrate. The Raman microprobe was u t i - lized t o investigate a spot annealed a t 1.5 J/cm2 which was degraded a t the center.
Crystalline GaAs exhibits two Raman active peaks related t o the interaction of a photon induced el ectron-hol e pair with the zone-center optical phonons whose f r e - quency are 292 cm-l for the LO mode and 267 cm-l for the TO mode. The Raman s c a t t e - ring were performed in the experimental arrangement Z ( X Y )Z on samples oriented per
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pendicular t o Z. In t h i s backscattering configuration only the LO mode i s allowed.
This i s i l l u s t r a t e d in the spectrum ( A ) of Fig.2 obtained from a single crystal reference. All the spectra presented in t h i s figure are taken in the same experimen- tal conditions. The probing l i g h t i s the green l i n e (h=5145 8) of an argon ion 1 aser
.
The f i r s t ring surrounding the damaged center i s essentially recrystallized.As can be seen from the spectrum ( B ) of Fig.2, the LO peak intensity i s about 30 % lower than for a perfect crystal and i s downshifted by 2cm-1. Furthermore a very small peak appears a t 267 cm-I which i s assigned to the TO mode and could be a con- sequence of a s l i g h t disorder in t h i s recrystallized area. When the Raman microprobe i s translated outside the pattern the spectrum ( E l of the s t a r t i n g amorphous mate- r i a l i s obtained. I t r e f l e c t s the one-phonon density of s t a t e of the crystal broa- dened by a convolution w i t h a Gaussian factor / 6 / . The spectra ( C ) and (0) of Fig.2 obtained from the other rings of the pattern are c l e a r l y intermediate s t a t e s between c r y s t a l l i n e ( B ) and amorphous (El s t a t e s . This could be the signature of a polycrys- t a l l ine material. The low frequency peak of ( C ) and ( D ) are TO-like and their high frequency peak LO-like. The f i r s t one i s a signature of disorder and the second one i s a consequence of order. The i n t e n s i t y r a t i o between t h i s two peaks i s then s i g n i - f i c a n t of the degree of disorder. With these considerations, the region (0) i s more ordered than the region ( C ) . To summarize the study of t h i s annealed pattern one can notice t h a t i t i s formed of an a1 ternation of rather ordered and rather disordered rings.
When the incident energy i s reduced to 0.85 J/cm2 a t 1.06 pn wavelength, a similar annular recrystal 1 i z a t i on pattern is obtained with no visible damage a t the center.
the spectra resulting from the Raman microprobe scan are shown in Fig.3. The calcu- l a t i o n of the r a t i o between the LO l i k e and TO l i k e i n t e n s i t y peak permits t o con- clude t h a t the central region ( A ) i s amorphous-like, the f i r s t ring ( B ) i s ordered and the second one ( C ) disordered. The order in region ( B ) i s accentuated by the up- s h i f t of the LO-like peak towards i t s single crystal position. For t h i s incident energy (E=0.85 J/cm2), the central region i s amorphous l i k e rather than crystal1 i n e as was determined a t higher energy (E=1.5 J/cm2). This indicates t h a t the diameter of the rings can be controlled with the energy of the annealing beam l i k e in s i l i c o n /3/.
DISCUSSION AND CONCLUSION
The annealing of Si and GaAs w i t h a picosecond laser reveals strong differences in the resulting material s t a t e . The differences between these two semiconductors in the threshold resol i d i f i c a t i o n speed for the l a t t i c e to reconstruct as well as the d i f f i c u l t y for the compound semiconductor to recrystal 1 i ze epitaxial 1 y are respon- sib1 e for these structural differences. However the annealed patterns display simi
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l a r tendancy to form multiannular structures with more than one amorphous or amor- phous-like ring. Since the semiconductor substrates are good heat sink, no l a t e r a l variation of heat can account for the formation of these patterns. Therefore these annular structures cannot be explained within the resol i d i fication process.
To investigate further the physics of the phenomenon, multiple shots from the pico- second laser with energy of 2 J/cm2 each were sent on a stationnary implanted
t h e f i r s t pulse will r e s u l t in selective absorption w i t h higher absorption in the amorphous regions. This experiment shows t h a t more energy has been deposited in the r e c r y s t a l l i z e d center and second ring a1 though the laser beam i s perfectly gaussian.
Fig.4 : Nomarski optical m i - graph of a multiple shot pico- second annealed, ion imp1 anted amor phized s i l icon substrate.
A possible explanation of the multiannular structure could be a mu1 t i p l e melting- resol i d i f i c a t i o n process during the pulse duration leading t o the superposition of basic structures (crystal 1 ine center surrounded by an amorphous ring) of d i f f e r e n t sizes. The higher r e f l e c t i v i t y of the molten layer could provide enough shielding from the laser l i g h t to prevent any further energy deposition before r e s o l i d i f i c a - t i o n . Recently very high r e s o l i d i f i c a t i o n speeds were measured a f t e r melting with a picosecond laser /7/. These measurements support t h i s model.
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