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Submitted on 1 Jan 1981
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PULSED RAMAN MEASUREMENTS OF
INHIBITED ELECTRON-PHONON COUPLING AT
HIGH PLASMA DENSITIES IN SILICON
A. Compaan, H. Lo, M. Lee, A. Aydinli
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C6, suppZ6ment au n012, Tome 42, ddcembre 1981 page C6-453
PULSED RAMAN MEASUREMENTS OF INHIBITED ELECTRON-PHONON COUPLING AT HIGH
PLASMA D E N S I T I E S I N S I L I C O N
A. Cornpaan, H.W. Lo, M.C. Lee a n d A . A y d i n l i
Department of Physics, Kansas S t a t e U n i u e r s i t y , Manhattan, Kansas 66506, U.S.A.
Abstract. -2A 15 nsec duration, frequency doubled Hd:YAG l a s e r a t power densi- t i e s near 1 J/cm has been used t o create high plasma densities in silicon a t room temperature. Phonon Ra~nan scattering generated by a delayed 7 nsec dye l a s e r probe pulse shows l a t t i c e temperatures f a r below the s i l i c o n melting temperature of 1412°C. The r e s u l t s suggest the presence of a dense-plasma-induced phase transi- t i o n with a greatly inhibited e l e c t r o n - l a t t i c e coupling.
For l a s e r energy d e n s i t i e s of 1 3/cmL a t A = 532 nm in s i l i c o n the absorbed photon density a t t h e surface i s -2 X loz2 anh3, assuming t h a t t h e normal room temperature absorption c o e f f i c i e n t i s unchanged. Most absorbed photons are expect- ed t o c r e a t e electron-hole pairs across t h e energy gap and thus photoexcited c a r r i - e r d e n s i t i e s well in excess of lo2' may be expected f o r nanosecond duration l a s e r pulses. This i s a density regime f a r beyond t h a t which i s accessible, e.g., in t n e study of electron-hole droplets in s i l i c o n and i s a regime where many
2
unusual e f f e c t s nay be expected. The range of power densities near 1 J/cm i s also appropriate f o r l a s e r annealing of ion implanted s i l i c o n and i t has been widely believed t h a t t h i s annealing i s effected by melting of the surface layer and rapid epitaxial r e c r y s t a l l i z a t i o n . A simple calculation using the specific heat shows t h a t t h e deposited energy should be s u f f i c i e n t t o melt t h e s i l i c o n provided t h e energy i s immediately thermalized t o l a t t i c e system. Furthermore the occur- rence of a t r a n s i e n t r e f l e c t i v i t y r i s e t o a value close t o t h a t of (metallic) mol- ten s i l i c o n circurostantially supports a melting hypothesis (see Fig. l ) . tlowever, we have perf~rr~ied two independent measurements which show t h a t t h e normal e q u i l i - briur,~ nolten s t a t e i s not produced under these circumstances. The f i r s t i s a transmission measurement which shows t h a t the extinction c o e f f i c i e n t in the near IR i s f a r below t h a t of molten s i l i c o n .
'
The second, a measurement of o p t i c phonon occupation f a c t o r s by Raman scattering,' shows t h a t t h e peak l a t t i c e temperature r i s e can account f o r only a small f r a c t i o n of the deposited l a s e r energy.The optical transmission data were obtained using f o r excitation a broad band N2- pumped dye l a s e r focussed t o 200 pm diameter and f o r probes e i t h e r a He-Ne l a s e r ( A = 1152 nm) o r a second pulsed dye l a s e r focussed t o -30 diameter. The tine-resolved transmission through a 0.6 urn thick silicon-on-sapphire sample i s shown in Fig. 1. The transmitted signal displays primarily the e f f e c t s of t h e r e f l e c t i v i t y change with l i t t l e absorption apparent.
JOURNAL DE PHYSIQUE
TIME TIME
Fig. 1. Time resolved r e f l e c t i v i t y (R) and transniission (T) of 0.6 pm SOS f o r 1152 nm probe. Peak of the 8-nsec exci- t a t i o n pulse occurs on t h e f i r s t hori- zontal t i c k nark.
PHOTON ENERGY lev)
Fig. 2. Induced absorption c o e f f i c i e n t (dots) measured 25 nsec a f t e r pulsed excitation and compared with the curves f o r amorphous and c r y s t a l l i n e s i l i c o n . The spectral dependence of the extinction c o e f f i c i e n t was obtained with t h e tunable pulsed dye l a s e r (7 nsec pulse) delayed by 25 ns (Fig. 2 ) . The data show an abrupt increase in absorption above -1.1 eV which i s d i r e c t l y contrary t o t h a t expected of any metallic s t a t e which should exhibit high extinction coefficients f o r a l l energies below the plasma edge. We have recently demonstrated, via a d i r e c t Kraniers
-
Kronig analysis t h a t the absorption shown in Fig. 1 i s s u f f i c i e n t t o explain t h e r e f l e c t i v i t y r i s e seen in the inset.Ranan measurements of phonon occupation f a c t o r s were performed using a s i m i l a r excite/probe configuration. In our most recent measurements we have used a frequency doubled Nd:YAG beam
(A
= 532 nm) focused t o -1mm on t h e sample and a A = 405 nm Raman probe pulse focussed t o 250 r\m diameter w i t h a variable electronic delay. Figure 3 shows the o p t i c phonon temperature as a function of probe delay2
f o r an excitation energy of 0.8 J/cm
.
Note t h a t t h e peak temperature observed ifiimediately a f t e r r e c r y s t a l l i z a t i o n is completed (110 nsec) i s l e s s than 400°C. The cooling behavior i s consistent with t h a t expected of a thin surface layer using the known thermal d i f f u s i v i t y of c r y s t a l l i n e s i l i c o n a t these temperatures. How- ever, we f i n d no observable Raman l i n e during t h e period of high r e f l e c t i v i t y (-10- 80 nsec);L.s.,
any sharp f e a t u r e between 200 cm-' and 700 cm-' must be less than1 5% a s intense a s t h e usual Raman l i n e a t 520 cm-
.
Fig. 3. Temperature r i s e vs. probe pulse delay. 600 500.- ..-. '2 -400
t
E 300-
:
2200 I-" 100 0would a l s o explain t h e slow r a t e of energy t r a n s f e r from the photoexcited c a r r i e r s t o the l i t t i c e . I t i s a l s o possible t h a t under intense excitation a s t r u c t u r a l phase t r a n s i t i o n occurs t o a disordered, possibly f l u i d , s t a t e . 3 In t h i s case only a very broad, density-of-states Raman spectrum would be expected which could e a s i l y be two orders of magnitude l e s s intense than the usual sharp Raman line. However such a disordered s t a t e could not be the usual molten s t a t e of s i l i c o n since Fig. 2 shows the optical properties t o be much d i f f e r e n t and Fig. 3 shows t h e peak l a t t i c e temperature t o be f a r below the 141Z°C melting point of s i l i c o n .
In e i t h e r case we believe the low l a t t i c e temperature immediately following t h e high r e f l e c t i v i t y phase gives strong evidence t h a t the usually f a s t (-10 12
1
sec- ) e l e c t r o n - l a t t i c e relaxation has been inhibited. We suggest t h i s may a r i s e from screening e f f e c t s of a dense photo-excited plasma on the deformation potential c a r r i e r - l a t t i c e coupling which prevent rapid energy t r a n s f e r t o t h e phonon system. 3
The financial support of t h e U.S. Office of Naval Research under Contract No. N00014-80C-0419 i s g r a t e f u l l y acknowledged. 1 I I I I I I I 1 Excitation 5 3 2 nm (0.8 J/cm2)
-
Probe 405 nrn (0.05 J/crn2)-
-
--
.
-.
- - I I I I I I I I I1. M. C. Lee, H . W. Lo, A. Aydinli and A. Compaan, Appl. Phys. Lett.
38,
499(1 981) ; A. Aydinl i
,
H. W. Lo, M. C. Lee and A. Compaan, Phys. Rev. Lett.46,
1640 (1981).0 100 200 300 400 500 600 700 800 900 1000
Probe Delay (nsec)
2 . H. W. Lo and A. Compaan, Phys. Rev. Lett.
