THE NATURE OF INTERMEDIATE RANGE ORDER IN Si:F:H:(P) ALLOY SYSTEMS

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THE NATURE OF INTERMEDIATE RANGE ORDER IN Si:F:H:(P) ALLOY SYSTEMS

R. Tsu, S. Chao, M. Izu, S. Ovshinsky, G. Jan, F. Pollak

To cite this version:

R. Tsu, S. Chao, M. Izu, S. Ovshinsky, G. Jan, et al.. THE NATURE OF INTERMEDIATE RANGE

ORDER IN Si:F:H:(P) ALLOY SYSTEMS. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-

269-C4-272. �10.1051/jphyscol:1981457�. �jpa-00220914�

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JOURNAL DE PHYSIQUE

CoZZoque C4, supple'ment a u nOIO, Tome 42, o c t o b r e 1981 page C4-269

THE NATURE OF INTERMEDIATE RANGE ORDER I N S1:F:H: ( P ) ALLOY SYSTEMS R. Tsu, S.S. Chao, M. Izu, S .R. Ovshinsky, G . J . .Jan* and F.H. P o l l a k +$

Energy C o n v e r s i o n D e v i c e s , Inc., T r o y , Michigan 48084, U. S. A .

*

BrookZyn CoZZege o f CUNY, BrookZyn, New Y o r k 11210, U.S.A.

Abstract.- Our i n v e s t i g a t i o n o f i n t r i n s i c - S i : F : H a l l o y s have shown them t o be t o t a l l y amorphous w i t h no d e t e c t a b l e i n t e r m e d i a t e range order o r m i c r o c r y s t a l - l i n e phase using TEM, and Raman techniques. We r e p o r t our i n v e s t i g a t i o n o f h i g h l y P-doped Si:F:H a l l o y s having an i n t e r m e d i a t e range order. The char- a c t e r i s t i c l e n g t h o f t h i s o r d e r i s 20 t o 60A. Transmission e l e c t r o n micro- scopy i s used t o corroborate t h e p a r t i c l e size, and t h e volume f r a c t i o n of c r y s t a l l i n i t y i n a two-phase system, determined by Raman s c a t t e r i n g . Trans- mission e l e c t r o n d i f f r a c t i o n i n d i c a t e s t h a t t h e s t r u c t u r e i s s i m i l a r t o m i c r o - c r y s t a l s . U n l i k e c o n d u c t i v i t y , e l e c t r o r e f l e c t a n c e response does n o t depend on t h e c r i t i c a l volume f r a c t i o n o f c r y s t a l l i n i t y i n a p e r c o l a t i o n process. The h i g h c o n d u c t i v i t y and low o p t i c a l a b s o r p t i o n f o r these P-doped Si:F:H a l l o y s a r e i d e a l p r o p e r t i e s f o r c o n t a c t l a y e r s i n an a-Si:F:H s o l a r c e l l .

I n t r o d u c t i o n . - Previously, we have reported, i n h e a v i l y As- o r P-doped SI:F:H a l l o y systems, t h e appearance o f a Raman peak l y i n g i n t e r m e d i a t e between 522 cm 1 f o r c-Si, and 480 cm-1 f o r amorphous-Si(l), ( 2 ) . Whenever such Raman peak i s observed, electrolyte-electroreflectance (EER) peaks appear around 2 eV, t o g e t h e r w i t h those associated w i t h c-Si a t 3.4 eV and 4.5 eV. We have explained these observations i n terms o f an i n t e r m e d i a t e range order o r a " m i c r o c r y s t a l l i n e phase". Nowe we have found s i m i l a r observations i n moderately P-doped samples. On glass substrates EER may be observed when t h e volume f r a c t i o n o f c r y s t a l l i n i t y has passed 0.16, t h e c r i t i c a l d e n s i t y , ,,p, i n p e r c o l a t i o n processes (3), ( 4 ) . However, on s t a i n l e s s

u s

s t e e l substrates, EER has been observed f o r

per

< 0.16, i n d i c a t i n g t h a t u n l i k e con- d u c t i v i t y , EER r e q u i r e s o n l y t h e existence o f r e l a t i v e l y sharp e l e c t r o n i c d e n s i t y o f s t a t e s . Extensive s t r u c t u r a l s t u d i e s i n d i c a t e t h a t t h i s i n t e r m e d i a t e o r d e r i s d i s t i n c t from amorphous s t a t e and i s s i m i l a r t o t h e m i c r o c r y s t a l l i n e s t a t e discussed by Moss and Graczyk ( 5 ) . Transmission e l e c t r o n d i f f r a c t i o n shows t h a t t h e f i r s t , second and t h i r d r i n g s c o i n c i d e w i t h those o f ( I l l ) , (220) and (311) o f c r y s t a l l i n e s i icon.

B

The average p a r t i c l e s i z e determined from Raman peak p o s i t i o n g i v e s 20 t o 60

.

P a r t i c l e s i z e s i n t h i s range o r even smaller have been r e p o r t e d by Goodman (7), however, s u b s t a n t i a l amounts o f oxygen were i n t r o d u c e d i n s i l i c o n . Therefore, our samples o f f e r a unique o p p o r t u n i t y f o r t h e i n v e s t i g a t i o n of small p a r t i c l e and g r a i n boundary e f f e c t s .

Subsequent t o o u r r e p o r t on t h e As-doped Si:F:H a l l o y system ( 8 ) , Tanaka a l . ( 9 ) r e p o r t e d t h e o b s e r v a t i o n o f a c r y s t a l l i n e phase i n h i g h l y P-doped Si:F:H.

-

Applying h i g h e r RF-power, Hamasaki

u.

(10) has produced c r y s t a l 1 iz a t i o n i n P- doped a-Si:H. I t i s i m p o r t a n t f o r us t o p o i n t o u t t h a t t h e occurrence o f c r y s t a l - l i n i t y i n f a i r l y h e a v i l y n-doped r e g i o n i s a c t u a l l y d e s i r a b l e f o r s o l a r c e l l pur- poses because o f higher e l e c t r i c a l c o n d u c t i v i t y and lower o p t i c a l absorption. The l a t t e r p r o p e r t y a l l o w s maximum l i g h t transmission through t h e c o n t a c t t o t h e i n - t r i n s i c Si:F:H which i s w i t h o u t t r a c e o f c r y s t a l l i n i t y .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981457

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.C4-270 JOURNAL DE PHYSIQUE

Fig. 1: Raman l i n e w i d t h and p a r t i c l e s i z e (dashed) versus frequency.

Results and Discussions.- Our S i :F:H(P) samples a r e prepared by a glow discharge decomposition o f SiF4 and Hz. Phosphorus doping i s i n t r o d u c e d by m i x i n g pH3 w i t h t h e gas m i x t u r e having t h e r a t i o o f pH3 t o SiF4 between 20 t o 500 ppm. Composition i s determined by Auger, SIMS and resonant n u c l e a r r e a c t i o n (11). T y p i c a l l y , our f i l m s c o n t a i n 1-4% o f F, 4-6% o f H, and 1-10% o f P. F i g . 1 shows t h e measured Raman l i n e w i d t h Aw versus t h e Raman frequency. Note t h a t t h e c r y s t a l l i n e Raman peak a t 522 cm-1 f o r S i i s s h i f t e d down t o peak p o s i t i o n as low as 508 cm-1 due t o small p a r t i c l e s i z e e f f e c t s o r p o s s i b l y a l s o by s t r a i n . On t h e o t h e r hand, t h e amorphous peak, marked a, has a l i n e w i d t h o f % 80 cm-1 centered a t 480 cm-1. The s p u t t e r e d a-Si has n o t i c e a b l y broader l i n e w i d t h and lower Raman frequency. Also p l o t t e d i n Fig. 1 i s t h e p a r t i c l e s i z e R versus frequency, obtained from R ^%2 ~ / k u s i n g t h e phonon d i s p e r s i o n curve from neutron s c a t t e r i n g (12). Thus we foynd t h a t t h e p a r t i c l e s i z e o f o u r m i c r o c r y s t a l l i n e i s i n t h e range o f 20R t o 6 0 ~ . ~t i s s i g - n i f i c a n t t h a t no Raman peak has been found between 483 cm-I and 508 cm-1, i n d i c a t i n g t h e a b s e n ~ e o f a continuous t r a n s i t i o n .

F i g u r e 2 shows a t y p i c a l TED, (a); and TEM, Fig. 2:

(b). Note t h a t t h e second amorphous r i n g l i e s between t h e (220) and (31 1) o f c-Si.

The d i f f r a c t i o n p a t t e r n i s a superposi- t i o n o f c-Si and a-Si. I n t h e b r i g h t f i e l d TEM micrograph ( b ) , t h e r e appear c l u s t e r s o f few hundred anstroms. However w i t h i n each c l u s t e r , t e r e are f i n e s t r u c t u r e s o f t h e o r d e r o f 50

B .

Therefore, t h e p a r t i c l e s i z e i n Fig. 1 corresponds t o t h e f i n e s t r u c t u r e s i n TEM.

TED 2a, showing t h e ( I l l ) , (220) and (311) r i n g s of. c-Si. Note t h e amorphous second r i n g l i e s between (220) and (311).

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The Raman spectrum o f a t y p i c a l Si :F:H(P % 1%) sample, shown i n Fig. 3, may be resolved i n t o an amorphous component, marked

a,

and a c r y s t a l l i n e component having a peak a t 519.5 cm-l. The plasma l i n e o f t h e A r - l a s e r a t 19750 cm-1 i s l e f t i n d u r i n g the measurements as an accurate c a l i b r a t i o n f o r t h e Raman frequency. The volume f r a c t i o n o f c r y s t a l l i n i t y f o r t h i s sample i s found t o be, p,, ^%0.25. Since t h e c r i t i c a l d e n s i t y i n p e r c o l a t i o n processes f o r these samples wa; determined t o be 0.16 (3), t h i s sample i s h i g h l y conductive having o % 4 (ficm)-1, and shows EER r e - sponse i n s p i t e o f t h e use o f a glass s u b s t r a t e . Note t h a t t h i s two-phase behavior was n o t seen on t h e samples r e p o r t e d i n Ref (1) and Ref (2), because i n our previous

C h l A I I & l r '

'R' I

R A M A N FREQUENCY ( c m - I )

Fig. 3: Raman i n t e n s i t y versus Raman frequency i n cm-l. P o r t i o n marked a i s due t o t h e amorphous p a r t o f t h e f i l m w h i l e t h e c r y s t a l 1 i n e peak i s marked 519.5.

The volume f r a c t i o n o f c r y s t a l l i n i t y a t t h e c r i t i c a l d e n s i t y f o r p e r c o l a t i o n i s found t o be 0.16.

i n v e s t i g a t i o n s , we d e a l t w i t h e s s e n t i a l l y single-phase " m i c r o c r y s t a l s " , having pv much higher. From t h e p r e p a r a t i o n p o i n t o f view, i n a d d i t i o n t o lower phosphorus, contents, these two-phase samples a r e g e n e r a l l y t h i n n e r , having thicknesses% 600A.

P a r t o f our present success i n i d e n t i f y i n g t h e n a t u r e o f these f i l m s i s due t o t h e extreme s e n s i t i v i t y o f Raman s c a t t e r i g g . We a r e a b l e t o produce a good Raman spectrum o f a f i l m no t h i c k e r than 200A. These t h i n n e r f i l m s a l s o y i e l d b e t - t e r transmission e l e c t r o n micrographs and d i f f r a c t i o n r i n g s .

Fig. 4 shows t h e EER response o f two groups o f samples, one on glass sub- s t r a t e s , and t h e o t h e r on s t a i n l e s s s t e e l substrates. For t h e former, EER i s s t r o n g f o r pv = 0.24 taken a t 3 ^ct i m e constant; and weak f o r pv = 0.2 which r e q u i r e d a 30 s t i m e constant t o show up t h e usual 3.4 eV s t r u c t u r e . For pv = 0.08, which i s below t h e p e r c o l a t i o n l i m i t , no EER response was observed. On t h e o t h e r hand, f o r t h e l a t t e r samples, EER response i s s t r o n g f o r b o t h samples, pv = 0.17 and 0.08, above and below t h e c r i t i c a l d e n s i t y f o r a p e r c o l a t i o n process. Note t h a t t h e usual s t r u c t u r e near 2 eV, 3.4 eV and 4.5 eV a r e e s s e n t i a l l y s i m i l a r t o those r e p o r t e d by us p r e v i o u s l y . The d i f f e r e n c e between t h e two groups can be understood. I n o r d e r t o apply a h i g h e l e c t r i c f i e l d f o r these samples on glass substrates, one of t h e two e l e c t r o d e s was on t h e f i l m i t s e l f t h e r e f o r e r e q u i r i n g c o n d u c t i v i t y ; whereas, on s t a i n l e s s s t e e l , t h e metal serves as one o f t h e two e l e c t r o d e s which does away t h e need f o r h i g h e r c o n d u c t i v i t y . Therefore our r e s u l t s i n d i c a t e t h a t EER response, u n l i k e c o n d u c t i v i t y , does n o t depend on p e r c o l a t i o n processes. I n f a c t , as l o n g as t h e r e e x i s t s a s u f f i c i e n t amount o f m i c r o c r y s t a l s , EER response should appear whenever a h i g h e l e c t r i c f i e l d can be a p p l i e d .

Conclusions.- To summarize, we have now s p e c i f i e d i n d e t a i l t h e n a t u r e of t h e i n t e r g e d i a t e range order i n our S i :F:H(P) samples. The range i s approximately 20 t o 60A, having TED s i m i l a r t o m i c r o c r y s t a l s . The combination o f Raman s c a t t e r i n g and EER provides us means t o make d e t a i l e d studies. The minimum d e t e c t a b i l i t y o f

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JOURNAL DE PHYSIQUE

trace crystallinity apears to be about 2% crystallinity. On the other hand, EER may ultimately be a very sensitive tool to de- tect trace crystallinity. Unlike Raman measurements, at present EER does not give a quantitative determination of crystallinity.

However, the possibility of using EER to study the electronic structure of these extremely small particles should be very valuable.

Acknowledgments.- We like to thank ARC0 for their support.

F.H.Pollak and G.J.Jan also wish to acknowledge the support of PSC/BHE grant 13106.

Fig.4 EER spectra of two types of samples: Top, on glass;

and bottom, on stainless steel subtrates. The electrolyte was 0.001N solu- tion of KC1. Modulating frequency is between 17 to 3 4 5 6 36 Hz. The time-constant

PHOTON ENERGY ( e V ) used was 3s, however, 30s

was used for weak and noisy responses.

References.-

(1) R. TSU, M.Izu, S.R.Ovshinsky, and F.H.Pollak, Solid State Cornm.36, 817 (1980)

(2) R.Tsu, M. Izu, V.Cannella, S.R.Ovshinsky, G.J.Jan,and F.H.Pollak, Proc. 15th Int. Conf. Semicond., Kyoto, 1980. J.Phys. Soc. Japan 49, Suppl A, 1249(1980)

(3) "Critical Density in Percolation Process:Volume Fraction of crys- tallinityl',R.Tsu, J Gonzalez-Hernandez,S.C.Lee,S.S.Chao, and K.

Tanaka, to be publish.

(4) H.Scher and R-Zallen, J.Chem.Phys. =,3759(1970)

(5) S.C.Moss and J.F.Graczyk, Phys. Rev. Lett. 23, 1167(1969) (6) Z-Iqbal, A.P.Webb, and S.Veprek, ~ ~ ~ ~ 3 6 , 1 6 3 n 9 8 0 ) (7) A.M.Goodman, 1nst.Phys.Conf. Ser. 43,m5(1979)

(8) R. Tsu, M.Izu,S.R.Ovshinsky, and ~ x . ~ o l l a k , Bull Am-Phys. Soc.25, 295 (1980)

(9) K.Tanaka, K.Nakagawa, A. Matsuda, lI.MatsumuraI H. Yamamoto, S.Ya- masaki, H.Okushi, and S.Iizima, Proc. 12th Conf. Solid State De- vices, 1980, Tokyo.

(1O)T. ~amasaki, H.Kurata, M.Hirose, and Y.Osaka, A P L , Z , 1084(1980) (ll)M.H.Brodsky, M.A.Frisch,J.F.ZieglertandW.A.Lanford,APLr~,56l

(1977)

(12)G.Dollingt Elastic Scattering of Neutron,Symp.Chalk River,2,37 (1972)