PHASE SEPARATION IN AMORPHOUS Au17Si83 ALLOY

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PHASE SEPARATION IN AMORPHOUS Au17Si83

ALLOY

M. Audier, P. Guyot, J. Simon, N. Valignat

To cite this version:

PHASE SEPARATION IN AMORPHOUS A u1 7S i g3 ALLOY M. A u d i e r , P . Guyot, J . P . Simon and N. V a l i g n a t

Laboratoire de Thermodynamique et Physiao-Chimie Métallurgiques, ENSEEG, Domaine Universitaire, B.P. 75, 38402 Saint Martin d'Hères, France

Résumé - Dans une étude antérieure d'alliages amorphes Au Si /!/ il a été annoncé, sur la base d'une interprétation de résultats diffraction électronique, que les alliages amorphes AuSi, riches en Si (x<0,45), peuvent être considérés comme biphasés. Le présent travail, réalisé sur un alliage amorphe Au Si préparé par vapo-déposition sous ultravide, confirme cet état, grâce à des observations en microscopie électronique CTEM et STEM.

Abstract - A previous study by electron diffraction of the Au Si amorphous alloys /l/ indicated that Si-rich AuSi amorphous alloys (x < 0.45) are a mixture of two amorphous phases. The present study evidences directly through electron microscopy (STEM and CTEM) observations such an unmixed state in an amorphous Au Si alloy, prepared by vapor deposition.

1 / 83

I - INTRODUCTION

Among the different cases where an unmixing state has been suspected in amorphous metallic alloys, the results reported on the AuSi system appear to be the most convincing /l/ : using electron diffraction the structure functions of amorphous Au Si alloys (from x=0 to x=0.8) appear to be, for x<0.45, the superimposition of two structure functions, one corresponding to pure amorphous silicon, the other corresponding to a Au rich AuSi amorphous phase. Direct confirmation of such an unmixed state in the amorphous AuSi system is given in this paper from electron microscopy observations, through various imaging and diffraction techniques. The samples Au Si (30nm thick) have been supplied by G.Marchal /!/. They have been observed on two electron microscopes, a Scanning Transmission Electron Microscope VGHB501 and a Conventional Transmission Electron Microscope JEOL 200CX.

II- UNMIXED AMORPHOUS STATE

The bright field (B.F.) micrographs exhibit a contrast variation with a wavelength of ca.5nm (fig.l and 2) which can be attributed to the unmixed state with an interconnected or percolated morphology. High resolution CTEM images display the "orange peal" contrast characteristic of the transfer function of the objective lens for an amorphous thin foil (fig.5 right part). The overall amorphous structure is confirmed on the diffraction patterns obtained in CTEM and STEM (where the filtering of the inelastic scattered electrons cleans the pattern,at small angle) in fig.4a. Since there is a large difference in atomic number (Z) between Au(79) and Si(14), the Z-contrast technique can be used / 2 / : in STEM, the annular dark field (ADF) signal is composed of electrons scattered between JZf = 22 mrad and $ = 360 mrad. Such a signal is predominantly elastic (Rutherford scattering) and therefore displays a strong dependence on the atomic number Z. In the absence of thickness fluctuations, the observed contrast in "BF" and "ADF" images of fig.l and 2 can be interpreted as a two phases system where Au-rich phase appears white and Si-rich phase dark in the "ADF" image. This Z-dependence can also been observed on the serie of "DF" CTEM

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images (Fig.2) in which the objective aperture has been moved at increasing angles (the contrast related to the relative variation of the squared elastic scattering factors for Au and Si increases with the scattering angle

$

measured at the centre of the objective aperture, as shown in the plot (IAu-ISi)/ISi vs sin

P(

/ A in fig.2). However, in order to ascertain that such a "DF" contrast is not due to thickness fluctuations, a couple of "ADF" and filtered "BF" on the Si plasmon has been realized (fig.3). The Si- rich phase appears white in the Si plasmon filtered image and corresponds to the dark contrast of the "ADF" image. Moreover a stereographic study, performed in classical "BF-CTEM" conditions, confirms that the contrast of ca.5nm is due to a bulk percolated morphology and not to a surface phenomenom.

Two other compositions (Au Si and A u ~ ~ have been observed by CTEM S ~ ~ ~ ) : as for the AU Si alloy, a simif& zzFi contrast was observed for the A&fi73 sample, but not wi# t% A U ~ ~ S ~ sample which appeared to be homogeneous. results are in agreement with the 2;pothetical free-energy diagram proposed by Mangin et a1 for the Au Si amorphous system (see fig.1 of ref.1).

x 1-x

I11

-

PARTIALLY CRYSTALLIZED STATE

It has been reported that during the crystallization of amorphous Au Si alloys, Si curiously crystallizes first (for x

<

0.45) /1,3/. The

crystallizes

sl~ccon and the remaining amorphous (AuSi) phase are shown in fig.4 and 5. The electron diffraction patterns (STEM and CTEM) taken within a partially crystallized domain (fig.4b) present the dotted rings (Ill), (220) and (311) characteristic of Si nanocrystals and a blurred ring characteristic of the remaining amorphous (AuSi) phase. It is interesting to note that the Si crystals embedded in the remaining (AuSi) amorphous phase have the same shape and size as the initial amorphous Si domains (fig.5). Such a Si-crystallization appears to extend fairly isotropically in the plane of the thin foil (fig.4).

Another interesting feature is observed at the interface between the amorphous matrix and the partially crystallized region, either at low or high resolution (fig.4 or 5)

: this border is crystallized and appears, after performing a local XEDS analysis to be rich in Au, although it has not been fully identified (see diffraction pattern of fig.5). It is the same thing for the central core of partially crystallized domain (fig.4). Such an Au-rich crystalline phase has not been previously observed /1/ but that is may be specific to our crystallization treatment (e.g. by electron irradiation).

IV - CONCLUSION

Direct STEM and CTEM observations of vapor deposited amorphous AuSi alloy thin foils have confirmed the existence of the unmixing in two amorphous phases, one being close to pure amorphous Si and the other one being close to a metallic glass. Further work is in progress (J.L. VERGER-GAUGRY et al. to be published) for a better understanding of this two phases system (separate measurement of their structure functions, precise description of their morphologies in term of percolation ...)

ACKNOWLEDGEMENTS : The authors would like to thank G.Marcha1 for supplying the AuSi specimen and J.M.Penisson and A.Bourret for allowing the use of their microscope JEOL 200CX.

REFERENCES

/1/ Mangin, Ph., Marchal, G., Mourey, C. and Janot, Chr., Phys.Rev. B21, (1980) 3047. /2/ Treacy, M.M.J., J.Microsc.Spectrosc.Electron. 7, (1982) 511.

b r i g h t field-(ADF) a n n u l a r dark

'

f i e l d .

FIG.2 :-Variation of (IAu-ISi)/tSi vs.sine/h and (superimposed) posi- t i o n s of t h e annular d e t e c t o r f o r STEM "ADF" image and of t h e objec-

-

t i v e a p e r t u r e f o r CTEM "DF" images I

vs. sine/X.

-"DF" CTEM imaqes a t increa-

2

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FIG.4 : BF-CTEM image of the p a r t i a l l y c r y s t a l l i z e d Au , S i B 3 a l l o y and corresponding diffraction patterns (=EM and CTEM) of l a ) f u l l y amor- phous a l l o y and (b) c r y s t a l l i z e d S i l i c o n .

FIG.5 : HREM images of f u l l y amorphous a l l o y and c r y s t a l l i z e d S i embed-