STRUCTURE OF AMORPHOUS SOLID INTERFACES USING COMPOSITIONALLY MODULATED SUPERLATTICES

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STRUCTURE OF AMORPHOUS SOLID INTERFACES USING COMPOSITIONALLY

MODULATED SUPERLATTICES

P. Persans, A. Ruppert, B. Abeles, T. Tiedje, H. Stasiewski

To cite this version:

P. Persans, A. Ruppert, B. Abeles, T. Tiedje, H. Stasiewski. STRUCTURE OF AMORPHOUS

SOLID INTERFACES USING COMPOSITIONALLY MODULATED SUPERLATTICES. Journal

de Physique Colloques, 1985, 46 (C8), pp.C8-597-C8-601. �10.1051/jphyscol:1985895�. �jpa-00225248�

JOURNAL DE PHYSIQUE

Colloque C8, supplement au n°12, Tome 46, decembre 1985 page C8-597

STRUCTURE OF AMORPHOUS SOLID INTERFACES USING COMPOSITIONALLY MODULATED SUPERLATTICES

P.D. P e r s a n s , A . F . R u p p e r t , B. A b e l e s , T. T i e d j e and H. S t a s i e w s k i

Corporate Research Science Laboratories, Exxon Research and Engineering Company, Annandale, N.J. 08801, U.S.A.

ABSTRACT - We demonstrate the use of amorphous s u p e r l a t t i c e s t r u c t u r e s combined w i t h q u a n t i t a t i v e Raman spectroscopy t o study the extent of i n t e r m i x i n g at as-grown a-Si:H/a-Ge:H s o l i d - s o l i d i n t e r f a c e s . We f i n d t h a t the i n t e r f a c e can be described by ~ one monolayer of randomly mixed Si and Ge bounded by pure m a t e r i a l s .

INTRODUCTION

The recent discovery t h a t amorphous m a t e r i a l s can be prepared i n p e r i o d i c s t r u c t u r e s ( s u p e r l a t t i c e s ) w i t h a high density of r e p r o d u c i b l e i n t e r f a c e s p r o v i d e s , / 1 - 5 / i n combination w i t h conventional bulk

s p e c t r o s c o p i e s , a powerful new t o o l f o r the study of amorphous

i n t e r f a c e s . S u p e r l a t t i c e s w i t h repeat distances d

as small as 8A have been deposited by plasma-enhanced chemical vapor d e p o s i t i o n , thereby i n c r e a s i n g experimental s e n s i t i v i t y t o i n t e r f a c e s by orders of magnitude.

In t h i s paper we demonstrate the use of s u p e r l a t t i c e s t r u c t u r e s combined w i t h v i b r a t i o n a l Raman s c a t t e r i n g t o study the extent of atomic i n t e r m i x i n g

i n as-deposited a-Si:H/a-Ge:H m a t e r i a l s .

The a-Si/a-Ge s u p e r l a t t i c e system i s nearly i d e a l f o r the development of new t o o l s t o study i n t e r f a c e s . The s t r u c t u r e of the i n t e r f a c e i s expected t o be one of the simplest because both m a t e r i a l s have t e t r a h e d r a l random network s t r u c t u r e / 6 / and t h e i r bond lengths d i f f e r by less than 5%. In p r i n c i p l e , an i n t e r f a c e model can be constructed through which a smooth surface can be drawn which separates a-Si from a-Ge and i n which the network d i s t o r t i o n s on e i t h e r side of the surface are minor compared t o normal bulk network f l u c t u a t i o n s . This i d e a l p i c t u r e e s t a b l i s h e s a t h e o r e t i c a l basis f o r comparison t o measurements. This system i s

i n t e r e s t i n g because the s t r u c t u r e of real as-grown i n t e r f a c e s may be much more complex. The growth process f o r amorphous t e t r a h e d r a l t h i n f i l m s i s a n o n - e q u i l i b r i u m vapor d e p o s i t i o n process which may y i e l d a growth surface which i s rough on an atomic s c a l e . Atomic rearrangement t o r e l i e v e s t r a i n and thermal i n t e r d i f f u s i o n may also lead t o extensive .atomic m i x i n g .

I d e n t i f i c a t i o n of the i n t e r f a c e component of network v i b r a t i o n a l spectra i s aided by the f a c t t h a t the v i b r a t i o n a l f o r c e constants f o r Ge- Ge, S i - S i and Si-Ge bonds are nearly e q u a l / 7 / whereas p a i r masses are very d i f f e r e n t and t h e r e f o r e t h e i r o p t i c - l i k e modes are w e l l - s e p a r a t e d .

Heteropolar Si-Ge i n t e r f a c e bonds have an o p t i c - l i k e mode centered at

RESUME - L'état de mélange à l'interface solide-solide d'alliages amorphes Si-H/Ge-H est déterminé par spectrométrie Raman sur des échantillons à struc- ture de superrëseaux. L'interface paraît être une simple monocouche de mé- lange Si-Ge séparant les matériaux purs de part et d'autre.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985895

C8-598 JOURNAL D E PHYSIQUE

370 cm'l compared t o 470 cm-' f o r Si-Si and 270 cm-' f o r Ge-Ge

bonds./8,9/ I n a l l o y s , t h e r e l a t i v e mode i n t e n s i t i e s are d i r e c t l y r e l a t e d t o t h e number o f bonds o f each type i n t h e network./9/

EXPERIMENTAL DETAILS

M u l t i l a y e r s t r u c t u r e s (- 1 pm t o t a l t h i c k n e s s ) were grown by p e r i o d i c a l l y s w i t c h i n g gases i n a low pressure plasma-assisted chemical vapor d e p o s i t i o n system w h i l e m a i n t a i n i n g t h e plasma. Amorphous s i l i c o n sub-layers were grown from SiH4 and amorphous germanium sub-layers were grown from 10:l H2:GeH4 mixture. The average f i l m composition f o r t h e s e r i e s reported here was h e l d constant by h o l d i n g sub-layer growth times i n constant p r o p o r t i o n . The i n d i v i d u a l growth r a t e s were - ^0.7%/s f o r a-Ge:H and - ^0.%/s f o r a-Si :H l a y e r s r e s p e c t i v e l y , determined from b u l k f i l m s . The average growth timelmonolayer was - 3 sec whereas t h e gas residence t i m e was < 1 sec. D e t a i l e d growth c o n d i t i o n s , i n f r a r e d absorption, o p t i c a l absorption, x-ray reflectance/2,10/ and transmission m i c r o s c o p y / l l / s t u d i e s have been r e p o r t e d e l sewhere.

Raman s p e c t r a were taken i n near-backscattering geometry u s i n g t h e Kr 586% l i n e . Stokes s c a t t e r e d l i g h t w i t h p o l a r i z a t i o n perpendicular t o t h a t o f t h e l a s e r was analyzed w i t h a Spex 1877B spectrograph a l p detected w i t h a multichannel d e t e c t o r w i t h spect a1 e s o l u t i o n o f - ^{8 cm} . Laser power was kept below 100 m W over 5 x cmr t o avoid i r r e v e r s i b l e sample h e a t i n g e f f e c t s . We estimate t h e sample temperature d u r i n g i l l u m i n a t i o n t o be 330 + 10K.

RESULTS

I n Fig. 1 we show t h e reduced Raman spectrum i n t h e range 200-500 cm-I f o r a-Ge:H, a-Si :H, a-Si Ge 45:H and several s u p e r l a t t i c e samples.

Reduced spectra are r e l a t z a to'measured Stokes s h i f t e d Ra an spectra b 1

phonon occupation and resonance terms, I = 1 , r ( r - ^{to)-' [n(w) + 11-}

where I i s t h e reduced spectrum, I i s t h e measureb spectrum, r~ i s t h e l a s e r energy, u i s t h e Stokes s if?, and n ( r ) = [fxp (kT/hu)-11 P I n t h e a l l o y we observe Si-Si (470 cm' ), Si-Ge (370 cm- ) and Ge-Ge (270 cm-l) bands w i t h t h e Si-Ge mode -2x stronger than e i t h e r Si-Si o r Ge-Ge as expected f o r a random a l l o y . I n t h e s u p e r l a t t i c e spectra a s u p e r p o s i t i o n o f t h e a-Si:H, a-Ge:H and a l l o y spectra i s seen.

I n order t o q u a n t i f y t h e number o f h e t e r o p o l a r bonds a t t h e i n t e r f a c e we represent t h e i n t e r f a c e as an a1 l o y - l i k e r e g i o n and analyse t h e

s u p e r l a t t i c e spectra as t h e l i n e a r s u p e r p o s i t i o n o f t h e measured a-Ge:H, a-Si :H, and a-Si 55 Ge 45:H spectra i n Fig. 1 which we use as standards.

Since i 1 lu m i n a t i 6 n and'coll e c t i o n o p t i c s were unchanged, t h e data can be scaled t o t h e volume o r depth Probed by c a l c u l a t i n g s i g n a l per u n i t volume o f t h e standard samples. The e f f e c t i v e depth probed i s determined by t h e o p t i c a l b s o r p t i o n co f f i c i e n t a a t 568% and i s given by ( 2 a ) - I , ( 2 asi )-I and ( 2 .A)-' and t h e s i g n a l per volume i s given by ( p a 1).

( 2 as1 I S . ) and ( 2 I A ) f o r germanium, s i 1 ic o n and a1 l o y respecF~ve?y .

Thus, we f i t t h e super1 a t t i c e spectra q u a n t i t a t i v e l y using t h e r e l a t i o n : ISL = L s i ( 2 aSi I s i ) + LGe ( 2 a~~ I G e )

LA ( 2 aA I A )

where t h e L ' s a r e a d j u s t a b l e parameters corresponding t o t h e e f f e c t i v e t h i c k n e s s o f each standard necessary t o f i t t h e Raman data. The absorption c o e f f i c i e n t s determined by transmission meas rements on t h e s t a dard f i l m s

!

were aGe = 3.3 x l o 5 cm-l, ysi = 7 x l o 4 c K y and ah = 1.2 x 10 cm-I a t

568%. An example of t h e f i t t o a dr = 244 sample i s shown i n Fig. 2.

I n Table 1 we give t h e e f f e c t i v e L's f o r several s u p e r l a t t i c e samples

w i t h dr from 84 t o 1604. An estimate o f t h e f i t t i n g e r r o r s are given a t

A - Difference

0 - ^'v-' -

200

300 400

Raman Shift (cm

Raman Shift (cm - 1)

F i g . 1 - Reduced Raman s p e c t r a f o r a-Si :H, a-Ge:H, a-Si 5 Ge , :H and s e v e r a l s u p e i ? a t t i c e safip?es w i t h r e p e a t d i s t a n c e dr as marked.

F i g . 2 - Reduced Raman spectrum o f a dr = 24 A s u p e r l a t t i c e sample a1 ong w i t h component s p e c t r a w i t h L ' s as g i v e n i n Table 1.

F i g . 3 - Volume f r a c t i o n s fGe and fA o f germanium and a1 l o y

r e s p e c t i v e l y , determined by Raman 2

s c a t t e r i n g as discussed i n t h e

t e x t , p l o t t e d a g a i n s t i n v e r s e

r e p e a t d i s t a n c e .

JOURNAL DE PHYSIQUE

Table 1 - F i t t i n g parameters f o r decomposition o f s u p e r l a t t i ce spectra i n t o standard a-Si :H, a-Ge:H and a1 l o y spectra.

dr(A) 8 16 24 32 48 96 160 E r r o r

t h e bottom o f each column i n t h e t a b l e . I n a d d i t i o n t o f i t t i n g e r r o r s , systematic e r r o r s o f + 5% o f each L may be i n t r o d u c e d by e r r o r i n t h e measurement o f a f o r %e standards. E r r o r s due t o d r i f t s i n l a s e r power are e l i m i n a t e d by t a k i n g t h e r a t i o o f each component L t o t h e t o t a l Raman depth LT = L + L + LA.and u s i n g t h e volume f r a c t i o n f f o r c a l c u l a t i o n s , f o r example Fee ^- ?L/L~ i s t e volume f r a c t i o n o f a-Ge:H. I n Fig. 3 we p l o t fGe and FA i g a 1 n s t (dr)-'. I f t h e i n t e r f a c e width i s independent o f l a y e r repeat d i s t a n c e we expect a l l t h r e e volume f r a c t i o n s t o be s t r a i g h t 1 in e s i n dr-l. Because o f t h e r e l a t i v e smallness o f t h e raw s i g n a l from a- Si :H, e r r o r s i n f are s u b s t a n t i a l l y l a r g e r t h e r e f o r e we concentrate on fGe and fA. The TIne through f A should i n t e r s e c t f = 1 f o r d = 2d where d i s t h e e f f e c t i v e w i d t h o f t h e i n t e r f a c e as represented by {he a l f o y . Tke l i n e through fGe i s expected t o i n t e r s e c t f = 0 f o r dr =

dA (rSi + r G e ) / r G e where rsi and r~ are t h e growth r a t e s f o r a-Si:H and a- Ge:H r e s p e c t i v e l y . From fGe we f i n s dA = 5.2 + ¹ A and from fA we f i n d dA

= 5.9 21 A.

DISCUSSION

The e f f e c t i v e a l l o y thickness dA i s t h i n enough (5.54) t h a t bonds l i n k i n g t h e a l l o y l a y e r t o pure regions must be considered when

i n t e r p r e t i n g t h e data. I n order t o f a c i l i t a t e i n t e r p r e t a t i o n we convert t h e e f f e c t i v e dA t o Si-Ge i n t e r f a c e bond d e n s i t y a by m u l t i p l y i n g dA by t h e b u l k d e n s i t y o f bonds 2

and by t h e f r a c t i o n o f those bonds which a r e heteropolar. For a random a l l o y l a y e r o = 4 x ( l - x ) d where

i s t h e atom d e n s i t y and t h e a1 i s represented as a-Six Gel- F r d 5.54, x =

-9 4 ⁼

.55 and

= 5 x 1 0 " ~ m - ~ we f i n d o = 2.6 + .5 x 18 cm . We note t h a t t h i s value i s i n s e n s i t i v e t o x f o r 0.4 < x-< 0.6).

The number o f bonds across an a r b i t r a r y plane i n a random

t e t r a h e d r a l l y bonded s o l i d i s estimated/l2/ t o be r p = 1.3 x 1015 cm-2 where r = 2.44 i s t h e bond length. For a random a l l o y l a y e r w i t h two i n t e r f a c e s t o pure m a t e r i a l s t h e sum o f t h e d e n s i t i e s o f Si-Ge bonds across t h e two i n t e r f a c e s i s s t i l l r

but h e t e r o p o l a r bonds w i t h i n t h e l a y e r now c o n t r i b u t e 2x(1-x) o~ where a8 i s t h e number o f bonds w i t h i n t h e l a y e r . For x = 0.4 < x <.6 we f i n d t h a t a random l a y e r 2.4 - 3.N t h i c k i s c o n s i s t e n t w i t h t h e measured data. This i n d i c a t e s t h a t mixing and i n t e r d i f f u s i o n d u r i n g growth i s n e g l i g i b l e .

We have demonstrated t h e u t i l i t y o f v i b r a t i o n a l Raman s c a t t e r i n g

combined w i t h s u p e r l a t t i c e s t r u c t u r a l c o n t r o l t o s t u bonding a t amorphous

sol i d - s o l i d i n t e r f a c e s . S e n s i t i v i t y t o l e s s than 10% heteropol a r bonds

per square centimeter has been demonstrated f o r t h e Si-Ge system. We f e e l

t h a t t h i s approach opens up new p o s s i b i l i t i e s f o r t h e study o f growth and re1 a x a t i o n processes i n disordered m a t e r i a l s .

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