PHONON SOFTENING IN ULTRA HEAVILY DOPED Si AND Ge

HAL Id: jpa-00223116

https://hal.archives-ouvertes.fr/jpa-00223116

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.

PHONON SOFTENING IN ULTRA HEAVILY DOPED Si AND Ge

A. Compaan, G. Contreras, M. Cardona, A. Axmann

To cite this version:

A. Compaan, G. Contreras, M. Cardona, A. Axmann. PHONON SOFTENING IN ULTRA HEAV- ILY DOPED Si AND Ge. Journal de Physique Colloques, 1983, 44 (C5), pp.C5-197-C5-201.

�10.1051/jphyscol:1983531�. �jpa-00223116�

JOURNAL DE PHYSIQUE

Colloque C5, suppl6ment au nO1O, Tome 44, octobre 1983 page C5-197

PHONON SOFTENING I N ULTRA HEAVILY DOPED Si AND Ge A. cornpaan+, G. Contreras ++ , M. Cardona aqd A. Axmann

Max-PZanck-Institut far FestkOrperforschung, Heisenbergstr.

7000 S t u t t g a r t

F.R.G.

r ~ r a u n k o f e r - ~ n s t i t u t far Angewandte F e s t k ~ r p e r p h y s i k , Eckerstr.

7800 Freiburg,

Resume - Un r e c u i t l a s e r XeCl 2 impulsions (308 nm, 10 nsec, %0,8J/cm 2 ) a etO u t i l i s e pour o b t e n i r des p r o f i l s de concentration p l a t s a p r e s f o r t e implan- t a t i o n ionique de Ga, B, P e t As 21 des e n e r g i e s a l l a n t j u s q u ' a 350 kev.

L ' i n f l u e n c e de l a concentration de p o r t e u r s l i b r e s s u r l a frequence du phonon de c e n t r e de zone a ete e t u d i e e par s p e c t r o s c o p i e Raman d l ' a i d e des r a i e s l a s e r continues v i o l e t t e s e t ul t r a - v i o l e t t e s , a f i n de s ' a s s u r e r que s e u l e s l e s regions implantees sont concernees.

avons d e t e c t & une reduction de la frequence du phonon Raman d'environ 10 cm-1 pour Si:As (Ne = 3

1021 cm-3) e t d ' e n v i r o n 20 cm-1 pour Si:B (Np = 1

1021 cm-3).

Abstract - Multiple pulse annealing with a XeCl l a s e r (308 nm, lonsec, s Q . 8 J 1 been used t o produce f l a t concentration p r o f i l e s f o l lowing heavy implantations of Ga, B, P , and As a t e n e r g i e s up t o 350 keV.

The i n f l u e n c e of t h e f r e e c a r r i e r c o n c e n t r a t i o n on t h e zone c e n t e r phonon f r e - quency has been s t u d i e d by Raman s c a t t e r i n g with v i o l e t and u l t r a - v i o l e t

l a s e r l i n e s t o ensure t h a t only t h e implanted region was sampled. We f i n d a s o f t e n i n g of t h e zone c e n t e r o t i c mode i n Si :As (Ne

3

1021 cm-3) of 10 cm-1 and f o r S i : a (Np = 1

1051 cm-3) a s o f t e n i n g of

20 cm-1.

High dose implantation followed by pulsed l a s e r annealing has t h e demonstrated ca- p a b i l i t y of producing s u b s t i t u t i o n a l dopant concentrations i n Si f a r exceeding normal s o l i d s o l u b i l i t y l i m i t s / I / . The p r o p e r t i e s of t h i s u l t r a heavily doped Si a r e of g r e a t i n t e r e s t s i n c e they provide t e s t s f o r a wide v a r i e t y o f phenomena such a s carrier-phonon s c a t t e r i n g a t high d e n s i t i e s , band gap renormalization due t o dense c a r r i e r systems, and c a r r i e r recombination processes a t high d e n s i t y . How- ever, t h e p r o p e r t i e s of such heavily doped Si have thus f a r received remarkably l i t t l e a t t e n t i o n . In t h i s paper we d e s c r i b e r e s u l t s on t h e s o f t e n i n g o r renormali- z a t i o n of t h e zone c e n t e r o p t i c phonon energy a s a consequence of i n t e r a c t i o n with t h e dense c a r r i e r ( e l e c t r o n o r hole) system i n t h e s e m a t e r i a l s . Other r e l a t e d papers d i s c u s s luminescence s t u d i e s i n heavily doped Si and Ge / 2 / , e l l i p s o m e t r i c s t u d i e s of broadening and s h i f t s i n t h e e l e c t r o n i c s t r u c t u r e /3/ and Raman s t u d i e s of l o c a l i z e d v i b r a t i o n a l modes of t h e dopant atoms / 4 / . The samples used i n t h i s study come from a v a r i e t y of sources ( s e e acknowledgements), however t h e majority of samples were implanted a t t h e Fraunhofer I n s t i t u t e (Freiburg) following prepara- t i o n a t t h e Max Planck I n s t i t u t e from commercially grown, pure s i n g l e c r y s t a l boules. The ion beam was provided by a High Voltage Engineering 350 keV implanter equipped with a hollow-cathode ion source. The focused ion beam a f t e r electromag- n e t i c mass s e p a r a t i o n was swept e l e c t r o s t a t i c a l l y t o achieve a uniform implantation over a 5 x 5 cm 2 a r e a . The t o t a l dose was determined by i n t e g r a t i n g t h e ion beam c u r r e n t . The dose r a t e s were about

~A/cm 2 . The wafers were misaligned under an angle of 7 degrees between t h e normal of t h e wafer s u r f a c e and t h e i n c i d e n t beam t o 'permanent address

von Humbolt Foundation Fellow, on leave from Kansas State University, Dept. of Physics, Cardwell Hall, Manhattan, Kansas 66506, U.S.A.

++permanent address

DAAD Fellow, on leave from E.S.F.M.-I.P.N., Mkxico,

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983531

C5-198

JOURNAL DE PHYSIQUE

a v o i d c h a n n e l l i n g . The source f e e d m a t e r i a l s a r e d e s c r i b e d elsewhere / 5 / .

I o n i m p l a n t a t i o n a t a s i n g l e energy r e s u l t s i n a r o u g h l y Gaussian d e p t h d i s t r i b u t i o n o f t h e i m p l a n t e d s p e c i e s . T h i s s e v e r e l y c o m p l i c a t e s m e a n i n g f u l s t u d y o f t h e ma- t e r i a l p r o p e r t i e s even a f t e r s u c c e s s f u l oven o r cw l a s e r a n n e a l i n g /6,7/. However, p u l s e d l a s e r a n n e a l i n g p e r m i t s a c o n s i d e r a b l e amount of dopant r e d i s t r i b u t i o n when performed a t l a s e r e n e r g i e s s i g n i f i c a n t l y above t h e a n n e a l i n g t h r e s h o l d /I/. I t has r e c e n t l y been shown /8/ t h a t p u l s e d a n n e a l i n g w i t h

t i p l e 1 a s e r p u l s e s can p r o v i d e an e s s e n t i a l l y f l a t c o n c e n t r a t i o n p r o f i l e i n t h e r e g i o n which has been i o n i m p l a n t e d . The work of r e f . 8 was performed w i t h a r u b y l a s e r which s u f f e r s f r o m t h e d i s a d v a n t a g e t h a t t h e a b s o r p t i o n o f t h e f i r s t p u l s e i n t h e i o n i m p l a n t e d amor- phous l a y e r ( i n S i ) i s much h i g h e r t h a n t h e a b s o r p t i o n o f t h e second and subsequent p u l s e s w h i c h see r e c r y s t a l l i z e d m a t e r i a l .

I n t h i s work we have performed m u l t i p l e p u l s e a n n e a l i n g w i t h a XeCl l a s e r (Lambda Physi k model EMG 100) w i t h A

308 nm. T h i s wavelength p r o v i d e s a n e a r l y i d e a l match between t h e a b s o r p t i o n c o e f f i c i e n t s of amorphous and c r y s t a l l i n e S i . Thus we

n

have used c o n s t a n t p u l s e e n e r g i e s ( t y p i c a l l y 0.8 t o 1.2 ~ / c m ~ ) and r e p e t i t i o n r a t e s o f 1-4 Hz w h i l e r a s t e r scanning t h e sample t o i n s u r e t h a t each p o i n t was annealed w i t h t y p i c a l l y f i v e p u l s e s . The beam was weakly focussed t o an a r e a o f about 500 x 1500 ym.

C a r r i e r d e n s i t i e s i n t h e i m p l a n t e d l a y e r were o b t a i n e d v i a i n f r a r e d r e f l e c t i v i t y . We have used t h e l o c a t i o n o f t h e r e f l e c t i v i t y minimum t o g e t h e r w i t h t h e e x p e r i m e n t a l r e s u l t s f o r b u l k samples t a b u l a t e d by F i s t u l / 9 / and t h e r e c e n t s t u d y b y Miyao e t a l . / l o / on i o n i m p l a n t e d S i .

- - ^{h = 4880 A}

=

- c

V) Z W I-

z

Z

a

1

I a

-200 800

F i g . 1 - Raman s p e c t r a o b t a i n e d from i o n i m p l a n t e d , p u l s e d l a s e r annealed samples.

I n each case t h e h i g h e r f r e q u e n c y peak of t h e d o u b l e t shown i n t h e i n s e t i s t h e 520 cm-I Raman l i n e o f t h e p u r e s u b s t r a t e . The l o w e r energy peak a r i s e s f r o m t h e h e a v i l y doped r e g i o n .

I o n i m p l a n t a t i o n foll'owed b y p u l s e d l a s e r a n n e a l i n g c r e a t e s a doped s u r f a c e l a y e r

o n l y 0 . 1 t o 0.3 ym deep. I n s i l i c o n t h i s p r e s e n t s m a j o r d i f f i c u l t i e s f o r Raman

s t u d i e s w i t h t h e u s u a l l a s e r f r e q u e n c i e s , hu 2.7 eV, because t h e l i g h t probes

w e l l i n t o t h e b u l k o f t h e sample / 7 / . The r e q u i r e d d e c o n v o l u t i o n o v e r t h e dopant

p r o f i l e and t h e bulk response severely l i m i t t h e accuracy of t h e r e s u l t s . Therefore we have used i n t h i s work primarily t h e v i o l e t (A

406.7 and 413.1 nm) and UV(X

356.4 and 350.7 nm) l i n e s of a Kr l a s e r . The corresponding Raman probe depths i n pure Si a r e /11/ ( 2 a ) - '

6C0 8 ^{and 50} 8 f o r t h e v i o l e t and uv l i n e s , r e s p e c t i v e l y . To i l l u s t r a t e t h i s behavior we present s p e c t r a i n Fig. 1 obtained f o r wavelengths where t h e l a s e r p e n e t r a t e s s l i g h t l y i n t o t h e pure bulk m a t e r i a l . Comparison of t h e s e s p e c t r a with o t h e r s obtained a t s h o r t e r wavelengths where t h e absorption depth i s considerably l e s s show t h a t the t r a n s i t i o n zone between bulk and t h e heavily doped l a y e r has n e g l i g i b l e influence on t h e s p e c t r a . This i s c o n s i s t e n t with t h e ob- s e r v a t i o n t h a t f o r m u l t i p l e pulse l a s e r annealing t h i s zone i s very narrow / 8 / . The influence of f r e e c a r r i e r s on phonon frequencies and phonon l i f e t i m e s has been e x t e n s i v e l y s t u d i e d i n t h e l a s t several y e a r s by Raman s c a t t e r i n g /12,13,14/, acou- s t i c measurements /15/, and neutron s c a t t e r i n g /16/ i n bul

doped s e m i c o n ~ u c t o r s .

The i n t e r a c t i o n of t h e Raman phonon with t h e continuum of e l e c t r o n i c e x c i t a t i o n s gives r i s e t o an asymmetric broadened and s h i f t e d Raman l i n e (Fano l i n e shape).

However, t h e asymmetry becomes n e g l i g i b l y small a s t h e l a s e r frequency approaches t h e resonance f o r d i r e c t e l e c t r o n i c band-to-band t r a n s i t i o n s . For Si t h e v i o l e t and uv l i n e s a r e c l o s e t o t h e EA, El d i r e c t gap and hence t h e asymmetry i s very

. -

small compared t o t h e broadening which occurs i n t h e extremely heavily doped samples which a r e t h e s u b j e c t of t h i s study. Thus we have avoided a d e t a i l e d a n a l y s i s of t h e Fano l i n e shape parameters and p r e s e n t t h e s h i f t s of t h e Raman l i n e s which a r e d i r e c t l y observed.

bulk doped (Chandrasekhar. et al ,1980 )

% implanted. laser annealed

I

/

1

/ et al.

1978

A w - n

I

1019 1020 10 21 l o z 2

CARRIER CONCENTRATION ( ~ r n - ~ 1

Fig. 2 - S h i f t of t h e Raman l i n e i n doped S i . Data on bulk doped samples a r e from r e f s . 12, 13, and 14. S o l i d curve f o r p-type Si i s from r e f . 16. Dashed curve show- ing l i n e a r dependence on concentration s e r v e s only a s an overall reference f o r t h e n-type data.

In Fig. 2 a r e displayed our r e s u l t s f o r t h e s h i f t s of t h e Raman l i n e i n Si f o r s e - veral of t h e implanted samples we have s t u d i e d . In a d d i t i o n we have p l o t t e d r e s u l t s from previous s t u d i e s on bulk doped samples,

In t h e case of p-type Si (B-doped) t h e s o f t e n i n g o r phonon s e l f energy a r i s e s from

C5-200 J O U R N A L DE PHYSIQUE

i n t e r a c t i o n w i t h t h e spectrum o f s i n g l e p a r t i c l e e l e c t r o n i c e x c i t a t i o n s w i t h i n t h e heavy h o l e , l i g h t h o l e and s p i n - o r b i t - s p l i t v a l e n c e b ~ n d s . The s o l i d c u r v e i n t h e f i g u r e i s reproduced f r o m t h e work o f Lawaetz /17/ who has d e s c r i b e d i n d e t a i l t h e b e h a v i o r f o r p - t y p e m a t e r i a l s . C l e a r l y t h e t h e o r y r e p r e s e n t s q u i t e w e l l t h e t r e n d s o f t h e d a t a a l t h o u g h t h e observed s o f t e n i n g appears t o b e s i g n i f i c a n t l y l a r g e r t h a n p r e d i c t e d a t h i g h d e n s i t i e s .

I n t h e case o f n - t y p e S i (As-doped) s o f t e n i n g a r i s e s f r o m phonon i n t e r a c t i o n w i t h i n t e r b a n d c a r r i e r e x c i t a t i o n s between t h e al and a2, bands i n t h e v i c i n i t y o f t h e c o n d u c t i o n and v a l l e y near t h e B r i l l o u i n zone boundary i n t h e (100) directionLlIn t h e h e a v i e s t bulk-doped sample p r e v i o u s l y s t u d i e d a s h i f t o f o n l y 2.3 + 0.2 cm had been observed /13/. I n t h e much more h e a v i l y doped samples o b t a i n e d by p u l s e d l a s e r a n n e a l i n g t h e observed s h i f t s i n c r e a s e r a p i d l y ( r o u g h l y p r o p o r t i o n a l t o t h e e l e c t r o n c o n c e n t r a t i o n as shown by t h e dashed l i n e i n F i g . 2 ) . O p t i c phonon r e n o r - m a l i z a t i o n i n n - t y p e S i has been d e s c r i b e d t h e o r e t i c a l l y i n r e f . 14. The a n a l y s i s y i e l d s a f u n c t i o n a l dependence on c o n c e n t r a t i o n a p p r o x i m a t e l y 1 i ke ~z~~ i n t h e h i g h d e n s i t y l i m i t and l i k e Ne f o r small d e n s i t i e s .

The comparison o f bulk-doped w i t h i o n implanted, p u l s e d l a s e r annealed samples must be t r e a t e d w i t h c a u t i o n , however, s i n c e t h e e f f e c t s o f dopant-induced l a t t i c e con- s t a n t changes a r e d i f f e r e n t f o r t h e two t y p e s o f samples: I n t h e e p i t a x i a l r e g r o w t h o f t h e PLA m a t e r i a l t h e doped l a y e r i s f r e e t o a d j u s t o n l y i n t h e d i r e c t i o n normal t o t h e s u r f a c e / l a / . P r e l i m i n a r y a n a l y s i s i n d i c a t e s t h e e f f e c t s a r e s i g n i f i c a n t , e s p e c i a l l y f o r t h e B-doped S i .

F i n a l l y , we n o t e h e r e t h a t Biswas and Ambegaokar /19/ have r e c e n t l y c a l c u l a t e d pho- non f r e q u e n c y changes i n p h o t o e x c i t e d S i . They f i n d , f o r n

p

7 x 1021 , an o p t i c phonon s o f t e n i n g o f 4 0 cm-l. Comparisons between p h o t o e x c i t e d and doped ma- t e r i a l m u s t be done w i t h care, however t h e r e s u l t s h e r e i n d i c a t e c o n s i d e r a b l y l a r g e r phonon s h i f t s t h a n p r e d i c t e d i n r e f . 18.

F o r phonon s o f t e n i n g i n p - t y p e Ge h e a v i l y i m p l a n t e d w i t h Ga and A l , o u r r e s u l t s i n - d i c a t e o p t i c phonon s o f t e n i n g as l a r g e as 6% f o r c a r r i e r d e n s i t i e s i n t h e lo2' cm-3 range.

ACKNOWLEDGEMFIITS

We a r e i n d e b t e d t o C.W. White (Oak Ridge) f o r s u p p l y i n g two o f t h e most h e a v i l y im- p l a n t e d samples shown here. P r e l i m i n a r y r e s u l t s were o b t a i n e d f r o m p u l s e d r u b y l a s e r annealed s p o t s on t h e s e samples; t h e d a t a shown h e r e a r e f r o m excimer annealed r e - g i o n s . O t h e r samples were k i n d l y s u p p l i e d by B. Clow (Western E l e c t r i c ) and H.

Ryssel ( F r a u n h o f e r I n s t . , M u n i c h ) . Thanks a r e a l s o due t o Messrs. G . K i s e l a f o r sample p r e p a r a t i o n , H. H i r t , P. Wurster, and M. Siemers f o r t e c h n i c a l a s s i s t a n c e . We a r e especially g r a t e f u l t o T.P. M a r t i n f o r use o f t h e excimer l a s e r and t o H. B r e i t s c h w e r d t f o r i n f r a r e d analyses.

REFERENCES

1. WHITE C.W., APPLETO14 B.R., STRITZKER B., ZEHNER D.M., and WILSON S.R., i n L a s e r and Electron-Beam S o l i d I n t e r a c t i o n s and M a t e r i a l s Processing, ed. by Gibbons, Hess, and Sigmon ( N o r t h H o l l a n d , New York, 1981) p. 59.

2. CONTRERAS G., COMPAAIJ A., WAGNER 3 , CARDONA M, and AXMANN A. (paper, these p r o - ceedings); WAGNER J., COMPAAN A., and AXMANN A. (paper, t h e s e p r o c e e d i n g s ) . 3. VIIJA L., COMPAAtI A., CARDONA M., and AXMANN A. (paper, t h e s e p r o c e e d i n g s ) . 4. COIITRERP5 G., COMPAAN A., and AXPIANN A. (paper, t h e s e p r o c e e d i n g s ) .

5. See, e.g., A. Axmann, Source Feed M a t e r i a l s i n I o n Beam Technology i n I m p u r i t y

Doping Processes i n S i l i c o n , e d i t e d by F.F.Y. Wang, N o r t h - H o l l a n d P u b l i s h i n g

Company, 1981.

I d . 19.

PANLIK M., i n Laser and Electron-Beam I n t e r a c t i o n s w i t h S o l i d s , Appleton and C e l l e r , eds. (North Holland, Amsterdam 1982) p. 579.

EI~GSTROIII H. and BATES J.B., J. Appl. Phys. 50 (1979) 2921.

HILL C., BUTLER A.L.. and DALY J .A., O ~ . C i t r r e f . 6,

579.

FISTUL v.I., ~ e a v i l ~ ~ ~ o ~ e d ~ e m i c o n d u c ~ l e n u m , New York, 1969).

MIYAO M., MOT N., and TOKUYAMA T., 0 p . c i t . r e f . 1, p. 163.

ASPNES D.E. a : z T k ~ i A ~ E ~ : h v s . Rev. B 27 I19831 985.

See, e.g., CHANDRASEKHAR M., CH~NDRASEKHARTI.R., GRIMSDITCH M., and CARDONA M.

Phys. Rev. B 22 (1980) 4825 and references t h e r e i n .

JOUAHI~E M., T X s i s , U n i v e r s i t y o f P a r i s (1975, unpubl i s h o d ) .

CHANDRASEKHAR M., RENUCCI J .B., and CARDONA M., Phys. Rev. B 17 (1978) 1623.

FJELOLY T.A., CERDEIRA F., and CARDONA M., Phys. Rev. B 8 (1973) 4723.

PINTSCHOVIUS L., VERGES J.A., and CARDONA P., Phys. ~ e v . ~ 26 (1982) 5658.

LAWAETZ P., The I n f l u e n c e o f Holes on t h e Phonon Spectrum o f Semiconductors, Tech. Univ. Denmark 1978, unpublished).

LARSON B.C., WHITE C!W., and APPLETON B.R., Appl . Phys. L e t t . 32 (1978) 801.

BISWAS R. and APBEGAOKAR V., Phys. Rev. B 26 (1982) 1980.