IN-SITU TEM OF THE AMORPHIZATION REACTION IN Al-Pt MULTILAYERS

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IN-SITU TEM OF THE AMORPHIZATION REACTION IN Al-Pt MULTILAYERS

B. Blanpain, J.-M. Legresy, J. Mayer

To cite this version:

B. Blanpain, J.-M. Legresy, J. Mayer. IN-SITU TEM OF THE AMORPHIZATION REAC- TION IN Al-Pt MULTILAYERS. Journal de Physique Colloques, 1990, 51 (C4), pp.C4-131-C4-137.

�10.1051/jphyscol:1990415�. �jpa-00230775�

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COLLOQUE DE PHYSIQUE

Colloque C4, suppl6ment au n014, Tome 51, 15 juillet 1990

IN-SITU TEM OF THE AMORPHIZATION REACTION IN AI-Pt MULTILAYERS

B. BLANPAIN, J.-M. LEGRESY* and J.W. MAYER

Department o f Materials Science, Bard Hall, Cornell u n i v e r s i t y , Ithaca, YY 14853, U . S . A .

Centre de Recherches de Voreppe, B P . 2 7 , F-38340 Voreppe, France

Rksumk - La reaction d'amorphisation d'kchantillons multicouches A1-Pt est observ6e in-sik par microscopie Bectronique & transmission. Nous montrons que la reaction d'amorphisation commence a basse tempkrature (150°C 8. 200°C). En termes de composition, la phase amorphe est situee entreles compos6s intermktalliques AlzlPts et A13Pt2. La rbction d'amorphisation dans le systBme A1-Pt est discut6e en termes des criteres de stabilitk thermodynamique, cinktiques et structuraux, proposks pour les systkmes oh une rkaction d'amorphisation en phase solide est observke.

Abstract

-

The amorphization reaction of A1-Pt multilayer samples is monitored during in-situ transmission electron microscopy. It is shown that the amorphization reaction starts at low temperatures (150°C to 200°C). The compositional region of the amorphous phase is estimated to be situated between the AlzlPts and the A13Pt2 compounds. The occurrence of a solid state amorphization reaction in the A1-Pt system is discussed in relation to the thermodynamic, kinetic and structural stability criteria proposed for systems in which solid state amorphization reactions are observed.

1- Introduction

Schwarz and Johnson [l] first reported the amorphization by interdiffusion between crystalline metallic layers in the La-Au system, and Herd et al [2] showed that an amorphous silicide is formed in the reaction between Rh and amorphous Si. The criteria proposed by Johnson for solid state amorphization (31 are:

a large and negative heat of mixing, anomalous diffusion and low mobility for one of the elements in the amorphous phase. In addition the Egami criterion [4], which determines the composition region of an amorphous alloy, predicts an amorphous alloy for a substantial difference in the atomic radii of the two elements involved.

We show here the solid state amorphization between crystalline layers of two fcc metals AI and Pt, which do not meet all the mentioned criteria: their atomic radii are comparable and anomalous diffusion is absent. Up to now little work has been done on AI-Pt amorphous alloys. Hung et al. [5] have reported the formation of an amorphous phase after room temperature ion beam mixing of A1-Pt thin iilms. In previous work [6], we observed a dissolution reaction of A1 into a coevaporated amorphous A1-Pt layer. We also noted the presence of an amorphous layer between a bilayer of A1 and P t in the as-deposited state.

Subsequently we have shown that the amorphous layer grows during low temperature (150°C to 220°C) annealing, consuming the crystalline A1 and Pt layers [7]. In separate work, Bordeaux and Yavari have demonstrated the amorphization reaction in A1-Pt multilayers prepared by cold-rolling techniques [8]. In this paper we expand on our previous report [7] of the amorphization reaction in A1-Pt multilayers.

2- Experimental Methods

The AI-Pt multilayers were deposited by electron beam evaporation in a cryogenically pumped system.

For a typical evaporation, the base pressure was 1 X 1 0 - ~ Torr and remained in the 1oW7 Tom range during evaporation. Layers of A1 and Pt were sequentially evaporated with deposition rates of 1.0 nm.sec-l for A1 and 0.15 nm.sec-l for Pt. The sample configuration consists of five layers: three aluminurn layers are separated by two platinum layers. The sample thickness was subject to the restriction of transparency for transmission electron microscopy (TEM) work. Each P t layer thickness was kept constant around 10 nm and we adjusted the total AI layer thickness to obtain the required overall composition, with the middle A1 layer chosen to be thicker. Rutherford backscattering (RBS) was used to check composition and iilm thickness. We used both cleaved sodium-chloride crystals and thermally oxidized Si wafers as substrates.

The films on the sodium-chloride substrates were floated off in deionized water onto Cu grids for in-situ

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990415

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COLLOQUE DE PHYSIQUE

Table 1: Summary of the relevant information of the AI-Pt multilayer samples A, B, C.

TEM annealing and TEM observation. The films on the oxidized Si wafers were used in Auger Electron Spectroscopy (AES) profiling and X-ray diffractometry (XRD).

sample

3- Results

In this section we will discuss the results of experiments on three samples with different overall composition (see table 1).

The composition of the samples was analyzed by RBS (Fig. 1). Since the AI peak is superimposed on the signal of the sodium-chloride substrate, a subtraction of the chlorine peak is necessary to integrate the counts of the A1 peak. After background subtraction the Al/Pt ratio was determined using the RUMP [g]

RBS analysis program.

Energy (MeV)

70, ^1.4

,

^1.6l ^1.8I ^2.0I ^2.2l ^2.4I ^2.61 1

M 360

M 270

CY

+

^{P t} ^X²³⁰

RBS composition

(at.% P t )

Channel

amorphization reaction products

60 50

sputtertime (min)

Figure 1: RBS spectrum of sample B in the as- Figure 2: AES depth profile of the AI-Pt mul- deposited state on a NaCl substrate. tilayer structure (sample B) on oxidized Si.

crystallization temperature

("c)

dominant crystallization

phase

- -

l '

Pt peak

1

I

-

Tilt 7'

2.79 MeV ~ e ~ +

-

I

\

250 300 350 400 450 500

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The statistical error in the fraction determination

(f

4 at.%) is mainly due to the background subtraction.

We also expect the RBS measurement to slightly underestimate the effective P t atomic fraction. Namely, RBS includes the A1 atoms in the surface oxide which are not available for the amorphization reaction. A summary of the composition data is given in Table 1. Since the RBS analysis geometry was not optimized for maximum depth resolution, only a minor dip is apparent in the Pt signal. However, the layered structure of the samples becomes obvious from the AES depth profile (Fig. 2). With the AES profiling technique, we can also measure the oxygen impurity level in the film. In the initial stage of the depth profile the presence of oxygen arises from a surface Al-oxide layer. Once the Ar-ions have sputtered through the surface-oxide, the oxygen level drops rapidly to a level of around lat% throughout the multilayer structure.

The TEM specimens were annealed in-situ in a JEOL 200CX microscope using a heating stage. Micro- graphs were taken in three different stages of the thermal history of the samples: in the as-deposited state, in the amorphized state and in the crystallized state. We first discuss the micrographs of the sample B (Fig. 3). The bright field image of the as-deposited state (Fig. 3a) show the superimposition of A1 and Pt crystals. The A1 grains (200-500 nm) are larger than the P t grains (50-100 nm). The corresponding diffraction patterns (Fig. 3a) are the typical diffraction rings for the fcc structure. The A1 rings are inside the P t rings and are also easily recognized by their spotty appearance, as a result of the bigger A1 grains.

In addition, diffuse rings originating from the amorphous phase are already discernible. This confirms the observation that already, in the as-deposited state, the amorphous phase has formed at the interface between the A1 and P t layers [6].

The amorphized state was reached by heating the samples in-situ in the TEM (dT/dt ^E5OC) and holding them at 200°C for 15 min. Am~rphization starts noticeably around 150°C. At 200°C the reaction is fast, so we can assume that the amorphization reaction is completed after the 15 min period at this temperature.

Sample B (Fig. 3b) has been almost completely amorphized, as is indicated by the bright field picture together with the SAD pattern. Some crystallites remain, but their presence is minute and there is no evidence of a sharp crystalline diffraction ring. In distinction to the complete amorphization of sample B, sample A (Fig. 4b) and C (Fig. 5b) still show clev patches of crystalline regions. In sample A, the crystals give rise to a clear A1 diffraction ring in addition to the diffuse rings of the amorphous phase. In sample B, the remaining crystals are fcc Pt, as evidenced by the corresponding diffraction pattern

.

We then conclude that sample A is on the A1 rich side and sample C on the P t rich side of the compositional region of the amorphous phase. Only sample B is in the composition range of the amorphous phase.

The samples were then further heated above 400°C. The approximate crystallization temperatures of the different samples are listed in table 1. The trend is toward lower crystallization temperature for increasing Pt content. The dominant crystallization phase are A121Pt8 for sample A (Fig. 4c), AlzPt for sample B (Fig. 3c) and A13Pt2 for sample C (Fig. 5c). The amorphization is due to interdiffusion as is illustrated by the Auger depth profile of sample B in the amorphized state (Fig. 6). The seperate A1 and Pt layers that were evident in the AES depth profile of the as-deposited sample, have become intermixed in the amorphized state. These results let us conclude that SSA is present in the A1-Pt system for compositions located between the compositions of the equilibrium compounds AlzlPts and ALPt3 (Fig. 7).

4- Discussion

The A1-Pt system has 8 intermetallic compounds [10], a definite indication of a large negative heat of mixing [ll]. This is confirmed by using the Miedema calculation [ll]. For a 1:l solid solution of A1 and Pt, one obtains a value of -55 kJ/mole-atom. With the inclusion of the chemical short range ordering the heat of mixing becomes -82 kJ/mole-atom. This high driving force for interdiffusion is considered to be a necessary requirement for amorphization 131.

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C4-134 COLLOQUE DE PHYSIQUE

sine/h (A-')

Figure 3: Results of the in-situ annealing experiment of sample B (see table 1) in a TEM: (a) as-deposited state, (b) amorphized state after 15 min. at 200°C, and (c) after crystallization upto 400°C. On the right are the bright field images of the sample in the different stages. On the left side we show the microdensitometer traces of the corresponding electron diffraction patterns.

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Figure 4: Rescllts of the in-situ annealing experiment of sample A (see table 1) in a TEM:

(a) as-deposited state, (b) partialIy amorphized state after 15 min. at 200°C, and (c) after crystallization upto 400°C. On the right are the bright field images of the sample in the different stages.

On the left side we show the corresponding electron diffraction patterns.

Figure 5: Results of the in-situ annealing experiment of sample C (see table 1) in a TEM:

(a) as-deposited state, (b) partially amorphized state after 15 min. at 200°C, and ( c ) after crystal- Iization upto 400°C. On the right are the bright field images of the sample in the different stages.

On the left side we show the corresponding electron diffraction patterns.

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COLLOQUE DE PHYSIQUE

sputtertime (min)

-

SSA

Atomic percentage Pt Figure 6: AES depth profile of the A1-Pt multi- Figure 7: The Al-rich side of the A1-Pt phase di-

agram with the estimated compositional region layer (sample B) in the amorphized condition.

of the amorphous alloy.

The proposed kinetic criteria for SSA are anomalous diffusion (e.g as in the case of Ni in Zr) and a very low mobility of one of the elements in the amorphous phase. We have evidence that A1 is the dominant moving specie during the amorphization reaction, while the P t mobility in the amorphous alloy is low enough to prevent nucleation at 200°C. Namely, the low temperature dissolution of A1 in an coevaporated amorphous AI-Pt alloy [6] shows that A1 diffusion in the amorphous phase is significant at these temperatures. Although the A1 diffusion is fast, the mobility of P t in the amorphous layer has to be very small, otherwise there would be no kinetic barrier for nucleation of the crystalline phase. Also we have found that in AI-Pt bilayers [l21 void formation in the AI layer is taking place at the amorphization temperature, before any sign of the first crystalline phase formation. This void formation in the AI layer during reaction with P t has been extensively described by Colgan [l31 for the formation of the Al3Pt2 compound in A1-Pt bilayer structures. The void formation in the A1-Pt reaction, although even more pronounced, is very reminiscent of the void formation in the Ni layer during the amorphization reaction in the Ni-Zr system as has been frequently reported (e.g. [14]). The voids have been explained as a result of the dominant diffusion of the Ni atoms. We can thus conclude that Al is the fast diffuser and that P t has a low mobility in the amorphous alloy. Although A1 seems to be the fast diffuser in the amorphous alloy, it is not known to be an anomalous diffuser in Pt[15]: the A1 impurity diffusion and the Pt self-diffusion coefficients are within the same order of magnitude over a wide temperature range. A1 and Pt are very close in atomic diameters. Therefore structural stability criteria like the Egarni criterion [4] fail to predict a compositional region for the A1-Pt system.

That not all of the proposed criteria for SSA hold in the case of the AI-Pt system indicate that SSA is even a more general phenomenon in solid state reactions than was first believed.

5- Conclusion

We have shown that a solid state amorphization reaction occurs between crystalline A1 and P t layers during low temperature vacuum annealing. The amorphous alloy region is situated between the equilibrium A21Pts and the A13Pt2 compounds. The properties of the binary AI-Pt system are in agreement with some of the proposed criteria for systems showing SSA: it has a high and negative heat of mixing and Pt has a low mobility in the amorphous AI-Pt alloy. However, this system does not show anomalous diffusion and has only a very small difference in atomic size between the two elements.

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6- Acknowledgements

The evaporations were performed at the National Nanofabrication Facility a t Cornell. The Electron Microscopy was done at the Cornell Materials Science Center Microscopy Facility. We thank L. Rathbun for making the Auger depth profiling. Part of the work was supported through a grant of NSF. B.B. is grateful to the Materials Science Center of Cornell University for a travel grant.

References

[l] R.B. Schwarz and W.L. Johnson, Phys. Rev. Lett.

a

(1983) 415.

[2] S.R. Herd, K.N. Tu, K.Y. Ahn, Appl. Phys. Lett.

3

(1983) 597. 597(1983) [S] W.L. Johnson, Progress in Materials Science

3

(1986) 81.

[4] T. Egami and Y. Waseda, J. Non-Cryst. Sol.

64

(1984) 113.

[5] L.S. Hung, M. Nastasi, J. Guylai, J.W. Mayer, Appl. Phys. Lett. (1983) 672.

[6] J.M. Legresy, B. Blanpain, J.W. Mayer, J. Mater. Res.

3

(1988) 884.

[7] B. Blanpain, L.H. Allen, J.-M. Legresy and J.W. Mayer, Phys. Rev. B

39

(1989) 13067.

[g] F. Bordeaux and A.R. Yavari. J. Appl. Phys., in press.

[g] L.R. Doolittle, Nucl. Instr. Meth. B 9 (1985) 344.

[l01 A.J. McAlister and D.J. Kahan, Bull. Alloy Phase Diagrams, Z(1986) 83.

[l11 A.R. Miedema, P.F. de Chtitel and F.R. de Boer, Physica B

100

(1980) 1.

[l21 J.M. Legresy, B. Blanpain and J.W. Mayer, unpublished results.

[l31 E.G. Colgan, C.-Y. Li and J.W. Mayer, J. Mater. Res.

2

(1987)557.

[l41 S.B. Newcornbe and K.N. Tu, Appl. Phys. Lett.

B

(1986) 1436.

[l51 D. Berger, K. Schwarz, Neue Huette 23 (1978) 210 and G. Rein, M. Mehrer, F. Maier, Phys. Stat.

Sol. A

45

(1978) 253.