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

Transcription

1 INSITU TEM OF THE AMORPHIZATION REACTION IN AlPt MULTILAYERS B. Blanpain, J.M. Legresy, J. Mayer To cite this version: B. Blanpain, J.M. Legresy, J. Mayer. INSITU TEM OF THE AMORPHIZATION REAC TION IN AlPt MULTILAYERS. Journal de Physique Colloques, 1990, 51 (C4), pp.c4131 C4137. < /jphyscol: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1990 HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published 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.

2 COLLOQUE DE PHYSIQUE Colloque C4, suppl6ment au n014, Tome 51, 15 juillet 1990 INSITU TEM OF THE AMORPHIZATION REACTION IN AIPt MULTILAYERS B. BLANPAIN, J.M. LEGRESY* and J.W. MAYER Department of Materials Science, Bard Hall, Cornell university, Ithaca, YY 14853, U.S.A. Centre de Recherches de Voreppe, BP. 27, F38340 Voreppe, France 1 Introduction Rksumk La reaction d'amorphisation d'kchantillons multicouches A1Pt est observ6e insik par microscopie Bectronique & transmission. Nous montrons que la reaction d'amorphisation commence a basse tempkrature (150 C C). En termes de composition, la phase amorphe est situee entreles compos6s intermktalliques AlzlPts et A13Pt2. La rbction d'amorphisation dans le systbme A1Pt 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 A1Pt multilayer samples is monitored during insitu 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 A1Pt system is discussed in relation to the thermodynamic, kinetic and structural stability criteria proposed for systems in which solid state amorphization reactions are observed. Schwarz and Johnson [l] first reported the amorphization by interdiffusion between crystalline metallic layers in the LaAu 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 AIPt amorphous alloys. Hung et al. [5] have reported the formation of an amorphous phase after room temperature ion beam mixing of A1Pt thin iilms. In previous work [6], we observed a dissolution reaction of A1 into a coevaporated amorphous A1Pt layer. We also noted the presence of an amorphous layer between a bilayer of A1 and Pt in the asdeposited 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 A1Pt multilayers prepared by coldrolling techniques [8]. In this paper we expand on our previous report [7] of the amorphization reaction in A1Pt multilayers. 2 Experimental Methods The AIPt multilayers were deposited by electron beam evaporation in a cryogenically pumped system. For a typical evaporation, the base pressure was 1 X 10~ 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.secl for A1 and 0.15 nm.secl 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 Pt 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 sodiumchloride crystals and thermally oxidized Si wafers as substrates. The films on the sodiumchloride substrates were floated off in deionized water onto Cu grids for insitu Article published online by EDP Sciences and available at

3 COLLOQUE DE PHYSIQUE Table 1: Summary of the relevant information of the AIPt multilayer samples A, B, C. sample RBS composition (at.% Pt) amorphization reaction products crystallization temperature ("c) dominant crystallization phase M 360 M 270 CY + Pt X 230 TEM annealing and TEM observation. The films on the oxidized Si wafers were used in Auger Electron Spectroscopy (AES) profiling and Xray diffractometry (XRD). 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 sodiumchloride 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) ,, l I I l I 1 1 Tilt 7' 2.79 MeV ~ e ~ + l' Pt peak \ Channel Figure 1: RBS spectrum of sample B in the asdeposited state on a NaCl substrate. I 1 I sputtertime (min) Figure 2: AES depth profile of the AIPt multilayer structure (sample B) on oxidized Si.

4 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 Pt 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 Aloxide layer. Once the Arions have sputtered through the surfaceoxide, the oxygen level drops rapidly to a level of around lat% throughout the multilayer structure. The TEM specimens were annealed insitu in a JEOL 200CX microscope using a heating stage. Micrographs were taken in three different stages of the thermal history of the samples: in the asdeposited 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 asdeposited state (Fig. 3a) show the superimposition of A1 and Pt crystals. The A1 grains ( nm) are larger than the Pt grains (50100 nm). The corresponding diffraction patterns (Fig. 3a) are the typical diffraction rings for the fcc structure. The A1 rings are inside the Pt 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 asdeposited state, the amorphous phase has formed at the interface between the A1 and Pt layers [6]. The amorphized state was reached by heating the samples insitu in the TEM (dt/dt E 5OC) 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 Pt 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 asdeposited sample, have become intermixed in the amorphized state. These results let us conclude that SSA is present in the A1Pt system for compositions located between the compositions of the equilibrium compounds AlzlPts and ALPt3 (Fig. 7). 4 Discussion The A1Pt 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/moleatom. With the inclusion of the chemical short range ordering the heat of mixing becomes 82 kj/moleatom. This high driving force for interdiffusion is considered to be a necessary requirement for amorphization 131.

5 C4134 COLLOQUE DE PHYSIQUE sine/h (A') Figure 3: Results of the insitu annealing experiment of sample B (see table 1) in a TEM: (a) asdeposited 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.

6 Figure 4: Rescllts of the insitu annealing experiment of sample A (see table 1) in a TEM: (a) asdeposited 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 insitu annealing experiment of sample C (see table 1) in a TEM: (a) asdeposited 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.

7 COLLOQUE DE PHYSIQUE SSA sputtertime (min) Figure 6: AES depth profile of the A1Pt multilayer (sample B) in the amorphized condition. Atomic percentage Pt Figure 7: The Alrich side of the A1Pt phase diagram with the estimated compositional region 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 Pt 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 AIPt alloy [6] shows that A1 diffusion in the amorphous phase is significant at these temperatures. Although the A1 diffusion is fast, the mobility of Pt 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 AIPt 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 Pt has been extensively described by Colgan [l31 for the formation of the Al3Pt2 compound in A1Pt bilayer structures. The void formation in the A1Pt reaction, although even more pronounced, is very reminiscent of the void formation in the Ni layer during the amorphization reaction in the NiZr 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 Pt 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 selfdiffusion 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 A1Pt system. That not all of the proposed criteria for SSA hold in the case of the AIPt 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 Pt 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 AIPt 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 AIPt alloy. However, this system does not show anomalous diffusion and has only a very small difference in atomic size between the two elements.

8 6 Acknowledgements The evaporations were performed at the National Nanofabrication Facility at 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) (1983) [S] W.L. Johnson, Progress in Materials Science 3 (1986) 81. [4] T. Egami and Y. Waseda, J. NonCryst. 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) [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) [l51 D. Berger, K. Schwarz, Neue Huette 23 (1978) 210 and G. Rein, M. Mehrer, F. Maier, Phys. Stat. Sol. A 45 (1978) 253.