PREPARATION AND CHARACTERIZATION OF Ge-Si ALLOYS CARRIED OUT BY MOCVD

(1)

HAL Id: jpa-00229624

https://hal.archives-ouvertes.fr/jpa-00229624

Submitted on 1 Jan 1989

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.

PREPARATION AND CHARACTERIZATION OF Ge-Si ALLOYS CARRIED OUT BY MOCVD

A. Reynes, C. Dufor, P. Mazerolles, R. Morancho

To cite this version:

A. Reynes, C. Dufor, P. Mazerolles, R. Morancho. PREPARATION AND CHARACTERIZATION

OF Ge-Si ALLOYS CARRIED OUT BY MOCVD. Journal de Physique Colloques, 1989, 50 (C5),

pp.C5-757-C5-764. �10.1051/jphyscol:1989591�. �jpa-00229624�

(2)

JOURNAL D E PHYSIQUE

Colloque C5, supplement au n05, Tome 50, mai 1989

PREPARATION AND CHARACTERIZATION OF Ge-Si ALLOYS CARRIED OUT B Y MOCVD

A. REYNES, C. DUFOR*, P. MAZEROLLES* and R. MORANCHO

Laboratoire de Cristallochimie Reactivite et Protection des Materiaux, CNRS UA-445, Institut National Polytechnique de Toulouse, Ecole

Nationale Superieure de Chimie de Toulouse, 118 Route de Narbonne, F-31077 Toulouse Cedex, France

" ~ a b o r a t o i r e des Organometalliques, CNRS UA-477, Universite Paul Sabatier, 118 Route de N a r b o ~ e , F-31077 Toulouse Cedex, France.

Resume

-

L ' e l a b o r a t i o n d ' a l l i a q e s Si-Ge p a r dep8t chimique en phase vapeur a Bt6 e n t r e p r i s e

a

p a r t i r de deux composes organometalliques de formule H3Si-(CH2I2-GeH3 e t H ~ S ~ - ( C H ~ ) ~ - G ~ H ~ . Le comportement thermique de ces composes e s t e t u d i e p a r des analyses de l a phase gazeuse de decomposition e t du materiau s o l i d e . Les couches obte-

nues sont compos6es de s i l i c i u m , de germanium e t ne contiennent pas de carbone. En f o n c t i o n de ces r e s u l t a t s e t de c a r a c t e r i s a t i o n s physico-chimiques des precurseurs e t du materiau une d i s c u s s i o n nous amene proposer une s t r u c t u r e du materiau.

A b s t r a c t - The p r e p a r a t i o n o f Ge-Si a l l o y s by chemical vapour d e p o s i t i o n has been undertaken u s i n g two organometal l i c compounds, H3Si-(CH2)2-GeH3 and H3Si -(CH2I3-GeH3.

T h e i r thermal behaviour was i n v e s t i g a t e d by a n a l y s i s o f t h e gaseous products o f decom- p o s i t i o n and o f t h e s o l i d m a t e r i a l . The t h i n coatings a r e composed o f s i l i c o n , germanium and do n o t c o n t a i n carbon. According t o these r e s u l t s and physico-chemical charac- t e r i z a t i o n s o f t h e precursors and t h e products, we a r e l e a d t o propose a s t r u c t u r e f o r t h e sol i d m a t e r i a l .

1

- INTRODUCTION

The p r e p a r a t i o n and t h e p r o p e r t i e s o f amorphous a l l o y s s u c h a s a Sil-xCx/1,2,3/, a Gel-xCx/4/ and a Sil-xGex/5,7,8,9/ have been e x t e n s i v e l y s t u d i e d because they present i n t e r e s t i n g semiconducting p r o p e r t i e s which a r e u s e f u l i n s o l a r c e l l s o r photosensors.These amorphous m a t e r i a l s present advantages w i t h r e s p e c t t o t h e i r chemical composition and s t r u c - t u r e . I n p a r t i c u l a r , according t o t h e parameter X , i t i s p o s s i b l e t o vary f o r each k i n d o f a l l o y t h e value o f t h e o p t i c a l clrap w i t h i n a r e l e v a n t range. Floreover, i n previous s t u d i e s o f these m a t e r i a l s , various methods o f p r e p a r a t i o n a r e used t o c o n t r o l t h e X v a l u e and t h e s t r u c t u r e t o achieve t h e b e s t p r o p e r t i e s . Two methods a r e : plasma a s s i s t e d chemical vapour deposition, and chemical vapourdeposition. I n each o f these methods t h e p r e p a r a t i o n o f b i n a r y a l l o y s i s c a r r i e d o u t u s i n g two precursors which c o n t a i n t h e elements c o n s t i t u t i v e o f the m a t e r i a l . I n t h e case o f s p u t t e r i n g t h e precursors a r e s o l i d w h i l e i n Plasma Assisted Ghemical Vapour D e p o s i t i o n (P.A.C.V.D.) and C.V.D. the precursors a r e gaseous. For instance i n t h e p r e p a r a t i o n o f a Si,-,GeX a l l o y s , t h e s p u t t e r i n q method used germanium and s i l i c o n t a r g e t s / l O / and P.A.C.V.D. and C.V.D. use sermane (GeH4) and s i l a n e (SiH4) as precursors.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989591

(3)

c5-758 JOURNAL DE PHYSIQUE

Nevertheless, i n t h e case o f thermal C.V.D. i t i s n o t easy t o c o n t r o l t h e composition X w i t h t h e experimental c o n d i t i o n s e s p e c i a l l y when t h e precursors have very d i f f e r e n t temperatures o f decomposition. With t h e aim t o used o n l y one p r e c u r s o r which i s a b l e t o c a r r y toge,ther a l l t h e elements o f t h e m a t e r i a l , we have undertaken a program t o study t h e behaviour o f organo- m e t a l l i c precursors. I n preceding work /3,5/ we have a l r e a d y obtained Si,-xCx and Gel-xCx a l l o y s u s i n g organometallics. We r e p o r t here t h e extension o f t h i s work t o t h e e l a b o r a t i o n o f Ge-Si and Ge-Si-C s l l o y s s t a r t i n g from one orpanometallic Drecursor.

2

- SELECTION AND SYNTHESIS OF PRECURSORS

2-1

-

Precursor S e l e c t i o n

The choice o f t h e precursors has been d i r e c t e d by t h e goal o f p r e p a r i n g Si-Ge-C a l l o y s whose composition i s determined by the composition o f t h e s t a r t i n g precursor. Two organome- t a l l i c s compounds (0.F1.).

have been synthesized which have t h e f o l l o w i n g f e a t u r e s :

-

a l l t h e carbon atoms a r e l o c a t e d between t h e s i l i c o n and the germanium atoms.

-

t h e number o f carbon atoms i s d i f f e r e n t , two atoms f o r t h e molecule A and t h r e e atoms f o r the molecule B.

-

t h e bonds energies o f t h e Si-C and Ge-C a r e h i g h and q u i t e s i m i l a r (90 Kcal.mo1-I f o r Si-C i n Si(MeI4 ; 76 ~ c a l .mol-l f o r Ge-C i n G e ( ~ e ) ~ ) /l!/.

-

under optimal c o n d i t i o n s o f p y r o l y s i s we a n t i c i p a t e t h e d e p o s i t i o n o f t h e elements i n t h e c e n t r a l chains, Si-C-C-Ge and Si-C-C-C-Ge.

2-2

-

Synthesis and c h a r a c t e r i z a t i o n o f t h e precursors

The precursors have both been synthesized according t o PETROV and a l . /12/. The compounds a r e obtained i n d i b u t y l e t h e r then p u r i f i e d by d i s t i l l a t i o n . T h e i r p u r i t y was v e r i f i e d by

N.M.R.

and Mass Spectrometry. I n t a b l e I a r e r e p o r t e d t h e mass spectra obtained f o r t h e two molecules A and B. The fragments produced by e l e c t r o n impact (70 ev) d i s p l a y t h e weakness o f some bonds. The most i n t e n s e i o n s (I = 1000) correspond t o t h e formula H3Si (CH*); a t m/e = 59 f o r t h e molecule A and H ~ S ~ ( C H ~ ) ' m/e = 45 f o r t h e molecule B which a r i s e f r o m breaking o f t h e bondsa and 6 t o t h e germanium atom. The weaker bond energy o f Ge-C i n Ge-CH2 weather than t h a t o f Si-C i n SiCH2 can e x p l a i n t h e presence and t h e importance o f these fragments.

(4)

Table I : I n t e n s i t i e s o f t h e p r i n c i p a l peaks coming from t h e mass spectra under e l e c t r o n bombardement a t 70 ev.

3

-

EXPERIMENTAL

H3Si

-

CH2

-

CH2

-

GeH3 ( A )

The apparatus used t o prepare these deposits i s a c o l d w a l l r e a c t o r which has been a l r e a d y described elsewhere /13/. The gaseous phase coming f r o m t h e decomposition e n t e r s a gas phase chromatograph f o r separation and a n a l y s i s .

The p y r o l y s i s o f t h e molecules A and B have been performed a t atmospheric pressure.

The c a r r i e r gas was helium N55 and t h e t o t a l f l o w r a t e was 10 l h - l . Three temperatures o f decomposition were used (425, 450, 475 "C) and t h e temperature o f t h e precursor source was r e g u l a t e d i n order t o have a molar f r a c t i o n x = I O - ~ .

P r i n c i p a l f e a t u r e s H3~i(cH2);

H3si+

H3Ge(cH2);

H Ge+

H3Si

-

^CH2

-

CH2

-

CH2

-

GeH3 (B)

4

-

^RESULTS

4-1

-

Gas phase a n a l y s i s m e 59 3 1 103 7 5

I n t e n s i t y

1000 700 197 140 P r i n c i p a l

f e a t u r e s H3Si (CH);

H3Si ( c H ~ ) ~ H3Ge(cH2)i H3Ge(cH2);

The decomposition o f each molecule as a f u n c t i o n o f t h e temperature o f t h e s u b s t r a t e has been f o l l o w e d by gas chromatography. The f i g u r e 1 shows t h e chromatogram o f gas phase o f decomposition coming from t h e p y r o l y s i s ofmolecule A. The minimum temperatures o f decomposi- t i o n were determined by t h e decrease o f t h e chromatographic s i g n a l o f t h e precursor o r by t h e appearance o f new s i g n a l s which i n d i c a t e s t h e beginning o f t h e decomposition /6/. These temperatures were r e s p e c t i v e l y 325°C f o r A and 350°C f o r B. I n t h e i n v e s t i g a t e d range o f temperaturesthe gases o f decomposition were ethylene and a c e t y l e n e f o r A and propene f o r B.

T h e i r r e l a t i v e presence i n t h e gas phase i s temperature dependent, t h e percentage o f decom- p o s i t i o n o f t h e precursors i s displayed i n f i g u r e 2 f o r t h e molecules A and B.

I n t e n s i t y

1000 521 172 120

m

e

45 73 116 105

(5)

JOURNAL DE PHYSIQUE

s t a r t

$-l -85

RT AREA AREA %

1.85 8319 17.26

2.41 1664 3.45

4.50 38188 79.24

F i g . 1 : Chromatogram o f molecule A. Fig. 2 : Percentage o f decomposition according t o t h e temperature.

4-2

-

S o l i d phase

The s o l i d phase has been deposited on s i l i c a f o r t h e study o f o p t i c a l p r o p e r t i e s , on s i l i c o n f o r i n f r a r e d absorption, and on alumina f o r a n a l y s i s by energy d i s p e r s i o n o f X Rays.

I n t h e case o f t h i n f i l m s , t h e s i g n a l ( A l . Ka l i n e ) coming from t h e s u b s t r a t e w i l l n o t i n t e r a c t w i t h t h e l i n e s e m i t t e d by t h e elements o f t h e c o a t i n g .

X.P.S. a n a l y s i s has been used t o i d e n t i f y t h e elements i n c l u d e d i n t h e f i l m s and t o determine by observation o f chemical s h i f t s t h e n a t u r e o f t h e d i f f e r e n t bonds. The samples were examined b e f o r e and a f t e r s p u t t e r i n g by ~ r ' . The A1 Ka monochromatized X-ray l i n e

(1486.6 eV) was used. On t h e surface, i n a d d i t i o n t o s i l i c o n and germanium, we n o t e t h e presence o f oxygen and a small peak o f carbon. A f t e r i o n i c bombardment, t h e oxygen peak decreases and t h e carbon peak disappears. By comparison w i t h m o n o c r y s t a l l i n e germanium and s i l i c o n standard samples, we do n o t f i n d any chemical s h i f t o f t h e germanium peaks w h i l e t h e s i l i c o n d i s p l a y s a s h i f t towards t h e h i g h e r energies. Moreover t h e peaks 2p o f t h e s i l i c o n a r e broad ( w i d t h a t h a l f maximum = 3.6 eV) compared t o t h e s i l i c o n standard sample (1.2 eV) f i g u r e (3a and 3b). T h i s broadness may i n d i c a t e t h a t t h e s i 1 ic o n may be i n d i f f e r e n t chemical forms t h a t can be r e l a t e d t o t h e presence o f oxygen. On t h e o t h e r hand, t h e decrease o f t h e oxygen i n t e n s i t y a f t e r s p u t t e r i n g shows t h a t oxydation may occur subsequent t o chemical vapor d e p o s i t i o n r e a c t i o n when the samples a r e i n a i r . I n any case, t h e must i n t e r e s t i n g f e a t u r e o f t h i s study i s t h e absence o f carbon d e s p i t e o f t h e use o f organometallic precursors.

(6)

Fig. 3 : X-ray photoelectron spectroscopy of the Si2p peak in the sample carried out at 450°C from the molecule (a) in monocrystalline silicon (b).

(7)

JOURNAL DE PHYSIQUE

The examination o f t h e surfaces of t h e samples prepared according t o t h e experimental c o n d i t i o n s g i v e n i n t a b l e I1 shows o n d u l a t i o n s a r i s i n g f r o m g r a i n s having a mean dimension o f 1.4 pm ( f i g u r e 4 ) . When t h e temperature o f d e p o s i t i o n increases we n o t e a morphological e v o l u t i o n , t h e g r a i n s become s m a l l e r = 0.7 pm a t 450°C and 0.3 pm a t 500°C. A s i m i l a r exami- n a t i o n o f t h e samples prepared f r o m molecule B shows t h e same e v o l u t i o n . The E.D.X. q u a n t i - t a t i v e analyses are d i s p l a y e d i n t a b l e 11. I n t h e two cases (molecules A and B ) t h e s i l i c o n percentage increases w i t h t h e temperature o f decomposition.

Table I 1 : P r i n c i p a l r e s u l t s o f t h e thermal decomposition o f t h e molecules A and B.

H3Si

-

CH2

-

CH2

-

Ge H3 (A)

Fig. 4 : Microphotography o f t h e surface o f a sample prepared a t 425OC.

Temperature

"C

425 450 475

% decomposition

45 65 72

H3Si

-

^CH2

-

CH2

-

CH2

-

Ge H3 (5) gases o f

decomposition

ETHYLENE ACETYLENE

425 450 475

27 37 5 2

PROPENE

GAP eV 1.26 1.02 1.09 E D A X Analysis

% Ge 60 40 40

50 40 30

% S i

40 6 0 60

50 60 70

-

(8)

I n f r a r e d a b s o r p t i o n spectra have been performed t o i n d e n t i f y p o s s i b l e GeH and SiH bonds whose presence would be due t h e low temperature o f decomposition.. The r e s u l t s obtained w i t h a FTIR P e r k i n Elmer 1710 do n o t show hydrogen. Meanwhile, we can see t h e c h a r a c t e r i s t i c f e a t u r e s of a Si-0 bond which i s i n agreement w i t h t h e X.P.S. studies. The o p t i c a l gaps were determined on coatings obtained from decomposition o f t h e molecule A from t h e r e l a t i o n o f MOTT and DAVIS /14/. The values r e p o r t e d i n t h e t a b l e I 1 show a weak o p t i c a l gap v a r i a t i o n as a f u n c t i o n of t h e temperature o f d e p o s i t i o n and the s i l i c o n percentage.

5

-

DISCUSSION

-

CONCLUSION

The r e s u l t s obtained from t h e mass spectrometry o f precursor, t h e gaseous phase ana- l y s i s a f t e r p y r o l y s i s and t h e s o l i d phase a n a l y s i s p e r m i t us t o propose a s t r u c t u r e o f t h e s o l i d m a t e r i a l and t o understand i t s formation.

The s t a b i l i t y o f t h e i o n i c fragments i d e n t i f i e d by mass spectrometry may h e l p t o d e f i n e the d i f f e r e n t steps o f the decomposition process. Since t h e molar f r a c t i o n i s small

( IO-~) we expect t h a t i n t e r m o l e c u l a r i n t e r a c t i o n i s l e s s probable and, as a consequence, t h a t t h e mechanism of p y r o l y s i s w i l l be i n t r a m o l e c u l a r /15/. The p r i n c i p a l i o n i c species.

i d e n t i f i e d i n t h e mass spectra, H3Si ( c H ~ ) ; and s~H;, f o r molecule A, H3Si ( C H ~ ) ' ,

H3Si ( c H ~ ) ~ and s~H;, f o r molecule B, show t h a t t h e C-Ge bonds a r e e a s i l y broken. Moreover t h e h i g h i n t e n s i t i e s o f t h e peaks S ~ H ; species i n t h e mass spectra ( 5 0 % for t h e molecule A ; 42 % f o r t h e molecule B ) suggest t h a t t h e Si-C bond i s cleaved w i t h a h i g h p r o b a b i l i t y . The gaseous phase a n a l y s i s i s c o n s i s t a n t w i t h t h i s conclusion since ethylene, acetylene and propene come r e s p e c t i v e l y from t h e p y r o l y s i s o f molecule A and B. The c e n t r a l c h a i n between t h e s i l i c o n and germanium atoms i n t h e two molecules being e l i m i n a t e d we can p o s t u l a t e t h a t t h e c o a t i n g i s b u i l t from two d i f f e r e n t e n t i t i e s whfch a r i s e from t h e gaseous phase.

The o p t i c a l gap values observed ( 1 eV

-

1.3 eV) a r e i n c o n t r a d i c t i o n w i t h those p r e - v i o u s l y observed f o r d i f f e r e n t silicon-germanium a l l o y s c o n t a i n i n g s i m i l a r germanium per- centages, obtained by s p u t t e r i n g o r P.A.C.V.D. g e n e r a l l y t h e o p t i c a l gaps o f such m a t e r i a l s increase as a f u n c t i o n o f t h e percentage o f s i l i c o n i n t h e s o l i d . I n o u r case, we observe a v a r i a t i o n i n t h e opposite d i r e c t i o n . This observation leads us t o t h i n k t h a t t h e m a t e r i a l deposited from the A and B molecules c o u l d be heterogeneous and c o u l d be due t o t h e presence o f Ge c l u s t e r s and Si c l u s t e r s . As a cbnsequence i n t h e a b s o r p t i o n spectra recorded t o determine t h e o p t i c a l gaps, t h e germanium absorbs a t low frequencies w h i l e s i l i c o n absorbs a t h i g h frequencies. According t o t h i s process, t h e a b s o r p t i o n spectra d i s p l a y s preferen- t i a l l y t h e o p t i c a l a b s o r p t i o n o f germanium.

From these p r e l i m i n a r y experiments, we can draw some conclusions :

i

-

the goal displayed ~t th e b e g i n r l ~ n ~ 6 f t h e paper, i.e. keen t h e c e n t r a l chain o f t h e molecules SiC2Ge and SiC3Ge has n o t been a t t a i n e d .

ii - a proposal regarding t h e n a t u r e o f t h e s o l i d e product i s made from t h e a n a l y s i s o f t h e gaseous phase o f decomposition and o f t h e s o l i d phase.

(9)

C5-764

JOURNAL DE PHYSIQUE

iii

-

a f t e r these r e s u l t s on t h e study o f h e t e r o d i n u c l e a r organometall i c compounds i t would be u s e f u l t o modify t h e s t r e n g t h of t h e bonds between t h e m e t a l l i c atoms and t h e b r i d - g i n g e n t i t i e s . For example, i n o u r case, a strengthening o f t h e Ge-C and S i - C bonds seems d e s i r a b l e . A c t u a l l y , our work i s o r i e n t e d towards t h e synthesis o f molecules p r e s e n t i n g t h e f o l l o w i n g s t r u c t u r e s :

i n which t h e double o r t r i p l e bonds may have a s t a b i l i z i n g e f f e c t on t h e metal-carbon bonds by an e l e c t r o n t r a n s f e r t o f t h e V e l e c t r o n o f t h e m u l t i p l e bond t o t h e d o r b i t a l s o f t h e metal atoms /16/.

REFERENCES

/l/ Anderson D.A. and Spear, W.E., P h i l . Mag.

-

35 (1977) 17.

/2/ Sussman, R.S. and Ogden, R., Phi1

.

^Mag.

^-

44 (1981) 1.

/3/ Gerault, J.P., Morancho, R. and Constant, G., P h i l . Mag.

849

(1984) 11.

/4/ Catherine, Y. and Turban, G., Thin S o l i d Films

-

70 (1980) 101-104.

/5/ Gau, S.G. and Weinberger, B.R., Appl

.

Phys. L e t t e r

39

( l Q81 ) 5.

/6/ Morancho, R., Reynes, A., and S e f i a n i , N., J. Organ. Chem. 328 (1987) 49-59.

/7/ Saito, N., Aoki, K., Sannomiya, H. and Yamaguchi, T., Thin Sol i d Films 115 (1984), 253-262.

/8/ Crete1 l a y M.C. and Gregory, J .A., J. Electrochem. Soc.

129

(1982) 2850-2855.

/9/ W i l l iam, P. i n fondamental Phys. o f Amorphous Semiconductors

5

(1981) 72.

/10/ Morimotot, Kataoka, T., Kumeda, M. and Shimizu, T., P h i l . Mag.

B50

(1984) 517-537.

/11/ Richard, A. and Jackson, J. Org. Chem.

166

(1979) 17-19.

/ l 2 1 Petrov, A.D., Mironov, V.F. and Dzhurinskaya, N.G., Dokl. Akad. Nauk. SSSR. 128 (1088) 302.

/13/ D e l p e r i e r , B., Vinante, C. and Morancho, R., J. A n a l y t i c and A p p l i e d P y r o l y s i s

-

13 (1388) 141-149.

/14/ Davis, E.A. and Mott, N.F., P h i l . Mag. B44 (1981) 285.

/15/ Litzow, M.R. and Spalding, T.R., Mass. Spectrometry o f I n o r g a n i c and Organometallic compounds., El s e v i e r ( 1973).

/16/ Flathis-NoSl, R., Mazerolles, P. and Mathis, F., B u l l . Soc. Chim. France,

314

(1961) 1955-1 966.