STRUCTURE AND LOCAL ORDER IN AMORPHOUS Si1-x SNx SEMI-CONDUCTOR ALLOYS

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STRUCTURE AND LOCAL ORDER IN

AMORPHOUS Si1-x SNx SEMI-CONDUCTOR

ALLOYS

M. Vergnat, M. Piecuch, J.-F. Geny, C. Mourey, G. Marchal, M. Gerl

To cite this version:

Colloque C8, supplirnent a u n012, Tome 46, d i c e m b r e 1985 _{page C8-287}

STRUCTURE AND LOCAL O R D E R I N AMORPHOUS Sil-, SNx

SEMI-CONDUCTOR ALLOYS

M. Vergnat, M. Piecuch, 3.-F. Geny, C . Mourey, G . Marchal and M. Gerl Laboratoire de Physique du S o l i d e , (U.A. au C . N.R.S. n o 2 5 5 / ,

Universite' de Nancy I , B.P. 239, 54506 Vandoeuvre ZDs Nancy cedex, France

R6sm.6

-

La structure atcnnique d'alliages m r p h e s Sil-x Sn, (x = 0 2 0,50) obtenus par d v a p o r a t i o n sous ultravide a Bt6 &tudide par diffraction Blec- tronique, s p e c t r d t r i e Paijssbauer e t mesures de densitB. Les alliages mntrent

une nette tendance

a

l'ordre, l e s atomes d16tain Bvitant de s e placer en p-

s i t i o n de proches w i s i n s pour des raisons d'encombrement st6rique. L'analyse des spectres Mijssbauer en fonction de l a t e r a t u r e donne des i n f o m t i o n s sur l e spectre de vibration des noyaux 9 ~ n .

Abstract

-

The atomic structure of co-evaporated amrphous Sil-x Sn, a l l o - s

has been studied by electron diffraction, fBssbauer spectroscopy and. density

measurements. The alloys show a tendency t o ordering, Sn atoms avoising to

be i n close contact. The analvsis of the temperature change of Mijssbauer spec- t r a provides sorne information about the vibration spectrum of

'

9 ~ n i n the

alloys.

The aim of this paper is to provide some information on the structure of Sil-, Snx

mrphous alloys by means of 6iffraction experiments, density and hyperfine

parame-

ters nkasurements, i n order to obtain a coherent picture of t h e i r atcanic structure. The random network &el of Polk /1/ and the m e 1 develop3 by Connell and Terkin /2/, based on tetrahedral units, account for the experimental observations mde i n

mrphous group

IV

semi-conductors using X-ray, electron and neutron diffraction,

namely /3/ :

i) the average number of nearest neighbours t o a given atom i s slightly smaller than four and the n. n. distznce i s close t o that observed i n the crystalline phase,

ii) the nmber of second. neighbows is close t o twelve, as

i n the crystal.

These d e l s have been used t o interpret the phvsical progerties of s m e seni-conduc-

t o r ccnpunds /4/ and it was interesting to investigate the structure of a IV-IV

amorphous alloy i n a large cornpasition range. The S i Sn system has been chosen be- cause it i s possible t o incorporate up t o 50 % of Sn i n mrphous horogeneous S i Sn alloys. Moreover the optical band gap continuously changes with ccmpsition, so that it i s possible, by monitoring the preparation conditions, t o fabricate layers or niultilayers t o be used. i n photovoltaic conversion devices o r i n imaging systems. S i Sn mrphous alloys, p r e p r e d by r f or dc sputtering, have been studied by VBri6 e t d l . / 5 , 6 / and by ; ~ ~ i l l i ~ o n and Deb / 7 / . This method of preparation leads t o the i n t r d u c t i o n of gaseous inpurities i n the alloys, t h a t may a l t e r t h e i r atomic and electronic structure. In order t o accurately control the purity of the samples, we have constructed an apparatus for preparing Si Sn amorphous alloys by co-evaporation i n a high vacuum / 8 / . Electron diffraction, MGssbauer spectrometry expzriments and

density masuremats have been made t o investigate the structure of the mrphous

films obtained by this procedure.

JOURNAL

DE

PHYSIQUE

I1

-

EXPEFSi'4ENTAL RESULTS

Sil-x Snx samples were z,reLpred i n an ultrahigh vacuum chamber (3 x

lo-*

Torr)

,

by sinrultaneous condensation of s i l i c o n and t i n on substrates held a t 77 K. Two quartz m n i t a r i n g systems were used t o control t h e evaporation r a t e of each constituent. For x < 0.5 perfectly homgeneous m r p h o u s alloys were obtained. I n t h e concentra- t i o n range 0.50 <

x

< 0.75 small 6

-

Sn c r y s t a l l i t e s are observed, by dark f i e l d electron microscow, i n a m t r i x t h a t rewains amorphous-For x > 0.75 t h e m r p h o u s phase disappears and a mixture of B

-

Sn and S i c r y s t a l l i t e s i s observed.

2. 1

-

Electron d i f f r a c t i o n

Electron d i f f r a c t i o n patterns have been obtained using a scanning high energy e l e c tron d i f f r a c t i o n apparatus /9/. A s i n -pure amrphous s i l i c o n o r g m i u m , t h e strue

t u r e factor exhibits two ~vell-defined peaks and a. t h i r d peak with a very small inten- s i t y . The r a t i o y = k2/kl of t h e values of k a t the f i r s t

two

maxima of S (k) conti- nuously changes from yl = 1.81 f o r me a-Si t o y2 = 1.63 f o r x = 0.57 (table 1).

This can be taken as an indication t h a t t h e atomic s t r u c t u r e of t h e alloys evolves f r m a Polk-like s t r u c t u r e f o r x = 0,

tmards

a structurewhereodd-rrenbered rings

are

excludqd f o r x = 0.5. A s

shcwn

in t a b l e I, e l m t a l semi-conductors are correctly described by the Polk model (y = 1.80

-

1.82) while t h e Connell-Temkin d e l

(y = 1.67

-

1.76) seems

mre

appropriate f o r m r p h o u s compounds.

Polk Connell-Temkin

&el &el

Model values

unrelaxed 1.80 1.67 a-Sil-x

relaxed 1.82 1.76 _{Present work} Snx

E x o e r ~ t a l values

Table I

-

The r a t i o y = k2/kl of the values of k a t the f i r s t and second d i f f r a c t i o n peaks ( f r m r e f . /3/)

.

F r m this discussion we can draw t h e conclusion t h a t i r ? S i Sn alloys, chemical and

s i z e constraints are large enough t o induce a well-characterized c h d c a l short ran- ge order. Sn atoms shacv a ten6ency t o be selectively surrounded by S i atoms so t h a t , as x increases, t h e number of odd-rmnbered rings decreases.

2.2

-

Mean interatomic distance

The m a n i n t e r a t m i c distance is determined a s f o l l w . The thickness of the

m r -

phous films i s measured by Tolanslq interferometry and t h e nu&er of atoms of each species t h a t are deposited per u n i t area is determined by t h e frequency s h i f t of t h e o s c i l l a t o r s . Assuming a tetrahedral netwogk it is possible to calculade d the avera- g e nearest neighbow distance ( f i g . 1) ; d i s found to vary l i n e a r l y with x from

d = 2.41

ic

pure a-Sir a figure 2.5 % larger than i n pure c-Si. V7emay therefore as- sums t h a t d would increase by 2.5 % when ( h y p t h e t i c a l ) c r y s t a l l i n e Sil, Sn?, alloys

the assumption that Sn atom avoid close contacts. :?it5 dSiSi = 2.35 A a s in c-Si, the experin-ental d.ata lead t o dSiSn = 2 . 5 3

A,

a figure 1.5 % smaller than

1/2 (dSisi + dsnsn)

.

Fig. 1

-

Variation with x of the average interatomic distance

3

i n a ~ r p h o u s SileX Snx alloys

2 . 3

-

~%ssbauer spectroscopy

Conventional transmission %ssbauer spectroscopy spectra have been obtained in the

tenperatwe range 4 . 2 K to 400

:<

i n alloys of different a q p s i t i o n s , using the ' ~ n resonant nucleus, with a 10 Ci Ba Sn O3 source. In mrphous alloys a single reso- nance l i n e i s observed, which f i t s accurately t o a Lorentzian of FVHM = 1.16

m.s-l .

This shows i) that there is only one s i t e for Sn atoms in our alloys, unlike in dc sputtered alloys /7/ ~Jhere the association with oxygen leads t o three lines, ii) Sn atoms tend t o avoi?. themselves because, i f the distribution of Sn atoms were randan, a broadened l i n e v~ould be observed.

The DebyeiValler and the second-order Doppler s h i f t have been masured as a function

of tempratme i n three diPferent alloys, to determine the dynamics of Sn atoms.

a

-

The t o t a l s h i f t of the absorption l i n e is made up of

two catpnents :

where IS, assured t o be independent of T I is the chemical s h i f t and the second t e r m i s due to the second-order Doppler effect. According to the conventional theory of l a t t i c e vibrations, <

v2

z can be written ;

JOURNAL

DE

PHYSIQUE

where Y is the heaviside function.

In order to determine &IS, we notice that the Debye band contributes by 3 kT/4mc to

6mes a t high temperature. A plot of

3

l

a

3

6

wE - y = s

+ - + -

m s 4 mc 8 mc

,6

9 3 coth

-

2kT

as a function of 1/T, extrapolated a t infinite tenprature, provides the value of

61s. The extrapolation process is not sensitive to the value of % ad the value

= 8 x 1013 s'-l has been selected for t h i s purpse. According to table I1 61s

1s observed to increase with x, which can be ascribed to the decrease of the number

of p-like electrons concentrated i n covalent bonds when the concentration of t i n increases.

V&en 61s i s determined, 512 variation of < v2 > with T can be obtained from eq. (1) (fig. 2 ) .

Fig. 2

-

Mean square q l i t u d e ( < x2 > ) and velocity ( < v2 > ) of l19sn nuclei i n S i 5 ~ Sn46 amorphous alloys.

b

-

The Debye-Waller factor i s determined from the Lamb-

Mijssbauer £-factor. The intensity factor E (0) i s defined as

1

(-1

-

1 (0)

E (0) = I (-)

where I (a) and I (0) are respectively the background intensity and the peak inten-

s i t y of the absorption line. It i s well k n m that E (0) can be written : -0.5 TA

E (0) = fs [l

-

e J, (i 0.5

TA)l

(4) where fs i s the recoilless fraction of the Ba Sn 03 source (fs = 0.6)

,

TA = n 0 0 f

where n

is

the number of resonant nuclei per unit surface, a, i s the

n?axir~l

reso- nance cross-section and J, i s the Bessel f n c t i o n of zero order.

Fram

the experimental absorption spectra we have determined f and the mean-square

amplitude of the oscillations of Sn nuclei from the equation

ted the Debye and Einstein frequencies defined by

eq.

(3) have been determined through a l e a s t square f i t t i n g of t h e experimmtal r e s u l t s to the theoretical expressions f o r

< x2 > and < v2 >.

I-

The values obtained f o r and % a r e recorded i n t a b l e 11.

Table I1

-

Values of t h e Debye frequency w~ and of the Einstein frequency f o r l x 9 s n i n S i Snx m r p h o u s alloys

1-x

Te f a c t t h a t

CiIs

increases with x can be ascribed to the decrease i n t h e number of p-like electrons wncentrated i n covalent bonds when the concentration of t i n in- creases /10,11/.

The values, obtained f o r wE a r e

consistent

with the Einstein frequency

9%

8.8 x1013s-I i n pure s i l i c o n /12/. The softening of the Debye frequency % when the concentration

of Sn increases can be attributed to the weakening of the force constants f o r l19sn

i n S i /12/ and to t h e presence of inpurity rides around w~ a 1.2 x

lo1

s-'

.

Detai- led calculations of t h e vibration spectra i n ordered Si50 Sn5o c r y s t a l l i n e alloys

a r e currently in progress, to obtain values of the interatomic force constants.

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2

(1971) 365.

Polk, D. E. and Boudreaux, D. S., Phys. Rev. Lett.,

2

(1973) 92. /2/ Connell, G. A., and TEnnkin, R. J., Phys. Rev. B

2

(197415323.

/3/ D i x m i e r , J., Gheorghiu, A. and Theye, M. L., J. Phys. C (1984)227.

/4/ Etherington, G., Wright, A. C . , Tqenzel, J. T., Dore, J. C., Clarke, J. H. and

Sinclair, R. N., J. Non-Cryst. Solids,

48

(1982) 265.

Sinclair, R. N., Desa, J. A. E., Etherington, G., Johnson, P. A. V. and lnlriqht,

A. C., J. Non-Cryst. Solids

42

(1980) 107.

/5/ V6ri6, C., Rochette, J. F. and Rebouillat, J. P., J. Phys. Paris

2

C 4 (1981) 667 /6/ V6ri6, C., Proc. 17th I. E. E. E., PVSC (to be published.).

/7/ FTilliamson, D. L. and Deb, S. K., J. Appl. Phys.

2

(1983) 2588.

/8/ Vergnat, M.

,

Thesis (1983) Nancy (unpublished)

.

Vergnat, M., Ma~chal, G., Piecuch, M., Gerl, M., Solid S t . C m . Z(Z (1984) 237. /9/ Vergn+, M., Piecuch, Y., Marchal, G., and G e r l , M., Philos. Mag.

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(1985) 327.

/lo/

Anton&ik, E

.

,

Phys

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Stat. Sol. (b)

2

(1977) 605.

/11/ VJeyer, G., Nylandsted-Larsen, A., Deutch, B. I., Andersen, J. V., and ~ n t o n d i k ,

E., Hyperfine Interactions, 1 (1975) 93.